Internet Engineering Task Force (IETF)                  R. Fielding, Ed.Request for Comments: 9110                                         AdobeSTD: 97                                               M. Nottingham, Ed.Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235,                    Fastly           7538, 7615, 7694                              J. Reschke, Ed.Updates: 3864                                                 greenbytesCategory: Standards Track                                      June 2022ISSN: 2070-1721                             HTTP SemanticsAbstract   The Hypertext Transfer Protocol (HTTP) is a stateless application-   level protocol for distributed, collaborative, hypertext information   systems.  This document describes the overall architecture of HTTP,   establishes common terminology, and defines aspects of the protocol   that are shared by all versions.  In this definition are core   protocol elements, extensibility mechanisms, and the "http" and   "https" Uniform Resource Identifier (URI) schemes.   This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232,   7233, 7235, 7538, 7615, 7694, and portions of 7230.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available in Section 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained at   https://www.rfc-editor.org/info/rfc9110.Copyright Notice   Copyright (c) 2022 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Revised BSD License text as described in Section 4.e of the   Trust Legal Provisions and are provided without warranty as described   in the Revised BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents   1.  Introduction     1.1.  Purpose     1.2.  History and Evolution     1.3.  Core Semantics     1.4.  Specifications Obsoleted by This Document   2.  Conformance     2.1.  Syntax Notation     2.2.  Requirements Notation     2.3.  Length Requirements     2.4.  Error Handling     2.5.  Protocol Version   3.  Terminology and Core Concepts     3.1.  Resources     3.2.  Representations     3.3.  Connections, Clients, and Servers     3.4.  Messages     3.5.  User Agents     3.6.  Origin Server     3.7.  Intermediaries     3.8.  Caches     3.9.  Example Message Exchange   4.  Identifiers in HTTP     4.1.  URI References     4.2.  HTTP-Related URI Schemes       4.2.1.  http URI Scheme       4.2.2.  https URI Scheme       4.2.3.  http(s) Normalization and Comparison       4.2.4.  Deprecation of userinfo in http(s) URIs       4.2.5.  http(s) References with Fragment Identifiers     4.3.  Authoritative Access       4.3.1.  URI Origin       4.3.2.  http Origins       4.3.3.  https Origins       4.3.4.  https Certificate Verification       4.3.5.  IP-ID Reference Identity   5.  Fields     5.1.  Field Names     5.2.  Field Lines and Combined Field Value     5.3.  Field Order     5.4.  Field Limits     5.5.  Field Values     5.6.  Common Rules for Defining Field Values       5.6.1.  Lists (#rule ABNF Extension)         5.6.1.1.  Sender Requirements         5.6.1.2.  Recipient Requirements       5.6.2.  Tokens       5.6.3.  Whitespace       5.6.4.  Quoted Strings       5.6.5.  Comments       5.6.6.  Parameters       5.6.7.  Date/Time Formats   6.  Message Abstraction     6.1.  Framing and Completeness     6.2.  Control Data     6.3.  Header Fields     6.4.  Content       6.4.1.  Content Semantics       6.4.2.  Identifying Content     6.5.  Trailer Fields       6.5.1.  Limitations on Use of Trailers       6.5.2.  Processing Trailer Fields     6.6.  Message Metadata       6.6.1.  Date       6.6.2.  Trailer   7.  Routing HTTP Messages     7.1.  Determining the Target Resource     7.2.  Host and :authority     7.3.  Routing Inbound Requests       7.3.1.  To a Cache       7.3.2.  To a Proxy       7.3.3.  To the Origin     7.4.  Rejecting Misdirected Requests     7.5.  Response Correlation     7.6.  Message Forwarding       7.6.1.  Connection       7.6.2.  Max-Forwards       7.6.3.  Via     7.7.  Message Transformations     7.8.  Upgrade   8.  Representation Data and Metadata     8.1.  Representation Data     8.2.  Representation Metadata     8.3.  Content-Type       8.3.1.  Media Type       8.3.2.  Charset       8.3.3.  Multipart Types     8.4.  Content-Encoding       8.4.1.  Content Codings         8.4.1.1.  Compress Coding         8.4.1.2.  Deflate Coding         8.4.1.3.  Gzip Coding     8.5.  Content-Language       8.5.1.  Language Tags     8.6.  Content-Length     8.7.  Content-Location     8.8.  Validator Fields       8.8.1.  Weak versus Strong       8.8.2.  Last-Modified         8.8.2.1.  Generation         8.8.2.2.  Comparison       8.8.3.  ETag         8.8.3.1.  Generation         8.8.3.2.  Comparison         8.8.3.3.  Example: Entity Tags Varying on Content-Negotiated                 Resources   9.  Methods     9.1.  Overview     9.2.  Common Method Properties       9.2.1.  Safe Methods       9.2.2.  Idempotent Methods       9.2.3.  Methods and Caching     9.3.  Method Definitions       9.3.1.  GET       9.3.2.  HEAD       9.3.3.  POST       9.3.4.  PUT       9.3.5.  DELETE       9.3.6.  CONNECT       9.3.7.  OPTIONS       9.3.8.  TRACE   10. Message Context     10.1.  Request Context Fields       10.1.1.  Expect       10.1.2.  From       10.1.3.  Referer       10.1.4.  TE       10.1.5.  User-Agent     10.2.  Response Context Fields       10.2.1.  Allow       10.2.2.  Location       10.2.3.  Retry-After       10.2.4.  Server   11. HTTP Authentication     11.1.  Authentication Scheme     11.2.  Authentication Parameters     11.3.  Challenge and Response     11.4.  Credentials     11.5.  Establishing a Protection Space (Realm)     11.6.  Authenticating Users to Origin Servers       11.6.1.  WWW-Authenticate       11.6.2.  Authorization       11.6.3.  Authentication-Info     11.7.  Authenticating Clients to Proxies       11.7.1.  Proxy-Authenticate       11.7.2.  Proxy-Authorization       11.7.3.  Proxy-Authentication-Info   12. Content Negotiation     12.1.  Proactive Negotiation     12.2.  Reactive Negotiation     12.3.  Request Content Negotiation     12.4.  Content Negotiation Field Features       12.4.1.  Absence       12.4.2.  Quality Values       12.4.3.  Wildcard Values     12.5.  Content Negotiation Fields       12.5.1.  Accept       12.5.2.  Accept-Charset       12.5.3.  Accept-Encoding       12.5.4.  Accept-Language       12.5.5.  Vary   13. Conditional Requests     13.1.  Preconditions       13.1.1.  If-Match       13.1.2.  If-None-Match       13.1.3.  If-Modified-Since       13.1.4.  If-Unmodified-Since       13.1.5.  If-Range     13.2.  Evaluation of Preconditions       13.2.1.  When to Evaluate       13.2.2.  Precedence of Preconditions   14. Range Requests     14.1.  Range Units       14.1.1.  Range Specifiers       14.1.2.  Byte Ranges     14.2.  Range     14.3.  Accept-Ranges     14.4.  Content-Range     14.5.  Partial PUT     14.6.  Media Type multipart/byteranges   15. Status Codes     15.1.  Overview of Status Codes     15.2.  Informational 1xx       15.2.1.  100 Continue       15.2.2.  101 Switching Protocols     15.3.  Successful 2xx       15.3.1.  200 OK       15.3.2.  201 Created       15.3.3.  202 Accepted       15.3.4.  203 Non-Authoritative Information       15.3.5.  204 No Content       15.3.6.  205 Reset Content       15.3.7.  206 Partial Content         15.3.7.1.  Single Part         15.3.7.2.  Multiple Parts         15.3.7.3.  Combining Parts     15.4.  Redirection 3xx       15.4.1.  300 Multiple Choices       15.4.2.  301 Moved Permanently       15.4.3.  302 Found       15.4.4.  303 See Other       15.4.5.  304 Not Modified       15.4.6.  305 Use Proxy       15.4.7.  306 (Unused)       15.4.8.  307 Temporary Redirect       15.4.9.  308 Permanent Redirect     15.5.  Client Error 4xx       15.5.1.  400 Bad Request       15.5.2.  401 Unauthorized       15.5.3.  402 Payment Required       15.5.4.  403 Forbidden       15.5.5.  404 Not Found       15.5.6.  405 Method Not Allowed       15.5.7.  406 Not Acceptable       15.5.8.  407 Proxy Authentication Required       15.5.9.  408 Request Timeout       15.5.10. 409 Conflict       15.5.11. 410 Gone       15.5.12. 411 Length Required       15.5.13. 412 Precondition Failed       15.5.14. 413 Content Too Large       15.5.15. 414 URI Too Long       15.5.16. 415 Unsupported Media Type       15.5.17. 416 Range Not Satisfiable       15.5.18. 417 Expectation Failed       15.5.19. 418 (Unused)       15.5.20. 421 Misdirected Request       15.5.21. 422 Unprocessable Content       15.5.22. 426 Upgrade Required     15.6.  Server Error 5xx       15.6.1.  500 Internal Server Error       15.6.2.  501 Not Implemented       15.6.3.  502 Bad Gateway       15.6.4.  503 Service Unavailable       15.6.5.  504 Gateway Timeout       15.6.6.  505 HTTP Version Not Supported   16. Extending HTTP     16.1.  Method Extensibility       16.1.1.  Method Registry       16.1.2.  Considerations for New Methods     16.2.  Status Code Extensibility       16.2.1.  Status Code Registry       16.2.2.  Considerations for New Status Codes     16.3.  Field Extensibility       16.3.1.  Field Name Registry       16.3.2.  Considerations for New Fields         16.3.2.1.  Considerations for New Field Names         16.3.2.2.  Considerations for New Field Values     16.4.  Authentication Scheme Extensibility       16.4.1.  Authentication Scheme Registry       16.4.2.  Considerations for New Authentication Schemes     16.5.  Range Unit Extensibility       16.5.1.  Range Unit Registry       16.5.2.  Considerations for New Range Units     16.6.  Content Coding Extensibility       16.6.1.  Content Coding Registry       16.6.2.  Considerations for New Content Codings     16.7.  Upgrade Token Registry   17. Security Considerations     17.1.  Establishing Authority     17.2.  Risks of Intermediaries     17.3.  Attacks Based on File and Path Names     17.4.  Attacks Based on Command, Code, or Query Injection     17.5.  Attacks via Protocol Element Length     17.6.  Attacks Using Shared-Dictionary Compression     17.7.  Disclosure of Personal Information     17.8.  Privacy of Server Log Information     17.9.  Disclosure of Sensitive Information in URIs     17.10. Application Handling of Field Names     17.11. Disclosure of Fragment after Redirects     17.12. Disclosure of Product Information     17.13. Browser Fingerprinting     17.14. Validator Retention     17.15. Denial-of-Service Attacks Using Range     17.16. Authentication Considerations       17.16.1.  Confidentiality of Credentials       17.16.2.  Credentials and Idle Clients       17.16.3.  Protection Spaces       17.16.4.  Additional Response Fields   18. IANA Considerations     18.1.  URI Scheme Registration     18.2.  Method Registration     18.3.  Status Code Registration     18.4.  Field Name Registration     18.5.  Authentication Scheme Registration     18.6.  Content Coding Registration     18.7.  Range Unit Registration     18.8.  Media Type Registration     18.9.  Port Registration     18.10. Upgrade Token Registration   19. References     19.1.  Normative References     19.2.  Informative References   Appendix A.  Collected ABNF   Appendix B.  Changes from Previous RFCs     B.1.  Changes from RFC 2818     B.2.  Changes from RFC 7230     B.3.  Changes from RFC 7231     B.4.  Changes from RFC 7232     B.5.  Changes from RFC 7233     B.6.  Changes from RFC 7235     B.7.  Changes from RFC 7538     B.8.  Changes from RFC 7615     B.9.  Changes from RFC 7694   Acknowledgements   Index   Authors' Addresses1.  Introduction1.1.  Purpose   The Hypertext Transfer Protocol (HTTP) is a family of stateless,   application-level, request/response protocols that share a generic   interface, extensible semantics, and self-descriptive messages to   enable flexible interaction with network-based hypertext information   systems.   HTTP hides the details of how a service is implemented by presenting   a uniform interface to clients that is independent of the types of   resources provided.  Likewise, servers do not need to be aware of   each client's purpose: a request can be considered in isolation   rather than being associated with a specific type of client or a   predetermined sequence of application steps.  This allows general-   purpose implementations to be used effectively in many different   contexts, reduces interaction complexity, and enables independent   evolution over time.   HTTP is also designed for use as an intermediation protocol, wherein   proxies and gateways can translate non-HTTP information systems into   a more generic interface.   One consequence of this flexibility is that the protocol cannot be   defined in terms of what occurs behind the interface.  Instead, we   are limited to defining the syntax of communication, the intent of   received communication, and the expected behavior of recipients.  If   the communication is considered in isolation, then successful actions   ought to be reflected in corresponding changes to the observable   interface provided by servers.  However, since multiple clients might   act in parallel and perhaps at cross-purposes, we cannot require that   such changes be observable beyond the scope of a single response.1.2.  History and Evolution   HTTP has been the primary information transfer protocol for the World   Wide Web since its introduction in 1990.  It began as a trivial   mechanism for low-latency requests, with a single method (GET) to   request transfer of a presumed hypertext document identified by a   given pathname.  As the Web grew, HTTP was extended to enclose   requests and responses within messages, transfer arbitrary data   formats using MIME-like media types, and route requests through   intermediaries.  These protocols were eventually defined as HTTP/0.9   and HTTP/1.0 (see [HTTP/1.0]).   HTTP/1.1 was designed to refine the protocol's features while   retaining compatibility with the existing text-based messaging   syntax, improving its interoperability, scalability, and robustness   across the Internet.  This included length-based data delimiters for   both fixed and dynamic (chunked) content, a consistent framework for   content negotiation, opaque validators for conditional requests,   cache controls for better cache consistency, range requests for   partial updates, and default persistent connections.  HTTP/1.1 was   introduced in 1995 and published on the Standards Track in 1997   [RFC2068], revised in 1999 [RFC2616], and revised again in 2014   ([RFC7230] through [RFC7235]).   HTTP/2 ([HTTP/2]) introduced a multiplexed session layer on top of   the existing TLS and TCP protocols for exchanging concurrent HTTP   messages with efficient field compression and server push.  HTTP/3   ([HTTP/3]) provides greater independence for concurrent messages by   using QUIC as a secure multiplexed transport over UDP instead of TCP.   All three major versions of HTTP rely on the semantics defined by   this document.  They have not obsoleted each other because each one   has specific benefits and limitations depending on the context of   use.  Implementations are expected to choose the most appropriate   transport and messaging syntax for their particular context.   This revision of HTTP separates the definition of semantics (this   document) and caching ([CACHING]) from the current HTTP/1.1 messaging   syntax ([HTTP/1.1]) to allow each major protocol version to progress   independently while referring to the same core semantics.1.3.  Core Semantics   HTTP provides a uniform interface for interacting with a resource   (Section 3.1) -- regardless of its type, nature, or implementation --   by sending messages that manipulate or transfer representations   (Section 3.2).   Each message is either a request or a response.  A client constructs   request messages that communicate its intentions and routes those   messages toward an identified origin server.  A server listens for   requests, parses each message received, interprets the message   semantics in relation to the identified target resource, and responds   to that request with one or more response messages.  The client   examines received responses to see if its intentions were carried   out, determining what to do next based on the status codes and   content received.   HTTP semantics include the intentions defined by each request method   (Section 9), extensions to those semantics that might be described in   request header fields, status codes that describe the response   (Section 15), and other control data and resource metadata that might   be given in response fields.   Semantics also include representation metadata that describe how   content is intended to be interpreted by a recipient, request header   fields that might influence content selection, and the various   selection algorithms that are collectively referred to as "content   negotiation" (Section 12).1.4.  Specifications Obsoleted by This Document   +============================================+===========+=====+   | Title                                      | Reference | See |   +============================================+===========+=====+   | HTTP Over TLS                              | [RFC2818] | B.1 |   +--------------------------------------------+-----------+-----+   | HTTP/1.1 Message Syntax and Routing [*]    | [RFC7230] | B.2 |   +--------------------------------------------+-----------+-----+   | HTTP/1.1 Semantics and Content             | [RFC7231] | B.3 |   +--------------------------------------------+-----------+-----+   | HTTP/1.1 Conditional Requests              | [RFC7232] | B.4 |   +--------------------------------------------+-----------+-----+   | HTTP/1.1 Range Requests                    | [RFC7233] | B.5 |   +--------------------------------------------+-----------+-----+   | HTTP/1.1 Authentication                    | [RFC7235] | B.6 |   +--------------------------------------------+-----------+-----+   | HTTP Status Code 308 (Permanent Redirect)  | [RFC7538] | B.7 |   +--------------------------------------------+-----------+-----+   | HTTP Authentication-Info and Proxy-        | [RFC7615] | B.8 |   | Authentication-Info Response Header Fields |           |     |   +--------------------------------------------+-----------+-----+   | HTTP Client-Initiated Content-Encoding     | [RFC7694] | B.9 |   +--------------------------------------------+-----------+-----+                               Table 1   [*] This document only obsoletes the portions of RFC 7230 that are   independent of the HTTP/1.1 messaging syntax and connection   management; the remaining bits of RFC 7230 are obsoleted by   "HTTP/1.1" [HTTP/1.1].2.  Conformance2.1.  Syntax Notation   This specification uses the Augmented Backus-Naur Form (ABNF)   notation of [RFC5234], extended with the notation for case-   sensitivity in strings defined in [RFC7405].   It also uses a list extension, defined in Section 5.6.1, that allows   for compact definition of comma-separated lists using a "#" operator   (similar to how the "*" operator indicates repetition).  Appendix A   shows the collected grammar with all list operators expanded to   standard ABNF notation.   As a convention, ABNF rule names prefixed with "obs-" denote obsolete   grammar rules that appear for historical reasons.   The following core rules are included by reference, as defined in   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF   (line feed), OCTET (any 8-bit sequence of data), SP (space), and   VCHAR (any visible US-ASCII character).   Section 5.6 defines some generic syntactic components for field   values.   This specification uses the terms "character", "character encoding   scheme", "charset", and "protocol element" as they are defined in   [RFC6365].2.2.  Requirements Notation   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described in   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.   This specification targets conformance criteria according to the role   of a participant in HTTP communication.  Hence, requirements are   placed on senders, recipients, clients, servers, user agents,   intermediaries, origin servers, proxies, gateways, or caches,   depending on what behavior is being constrained by the requirement.   Additional requirements are placed on implementations, resource   owners, and protocol element registrations when they apply beyond the   scope of a single communication.   The verb "generate" is used instead of "send" where a requirement   applies only to implementations that create the protocol element,   rather than an implementation that forwards a received element   downstream.   An implementation is considered conformant if it complies with all of   the requirements associated with the roles it partakes in HTTP.   A sender MUST NOT generate protocol elements that do not match the   grammar defined by the corresponding ABNF rules.  Within a given   message, a sender MUST NOT generate protocol elements or syntax   alternatives that are only allowed to be generated by participants in   other roles (i.e., a role that the sender does not have for that   message).   Conformance to HTTP includes both conformance to the particular   messaging syntax of the protocol version in use and conformance to   the semantics of protocol elements sent.  For example, a client that   claims conformance to HTTP/1.1 but fails to recognize the features   required of HTTP/1.1 recipients will fail to interoperate with   servers that adjust their responses in accordance with those claims.   Features that reflect user choices, such as content negotiation and   user-selected extensions, can impact application behavior beyond the   protocol stream; sending protocol elements that inaccurately reflect   a user's choices will confuse the user and inhibit choice.   When an implementation fails semantic conformance, recipients of that   implementation's messages will eventually develop workarounds to   adjust their behavior accordingly.  A recipient MAY employ such   workarounds while remaining conformant to this protocol if the   workarounds are limited to the implementations at fault.  For   example, servers often scan portions of the User-Agent field value,   and user agents often scan the Server field value, to adjust their   own behavior with respect to known bugs or poorly chosen defaults.2.3.  Length Requirements   A recipient SHOULD parse a received protocol element defensively,   with only marginal expectations that the element will conform to its   ABNF grammar and fit within a reasonable buffer size.   HTTP does not have specific length limitations for many of its   protocol elements because the lengths that might be appropriate will   vary widely, depending on the deployment context and purpose of the   implementation.  Hence, interoperability between senders and   recipients depends on shared expectations regarding what is a   reasonable length for each protocol element.  Furthermore, what is   commonly understood to be a reasonable length for some protocol   elements has changed over the course of the past three decades of   HTTP use and is expected to continue changing in the future.   At a minimum, a recipient MUST be able to parse and process protocol   element lengths that are at least as long as the values that it   generates for those same protocol elements in other messages.  For   example, an origin server that publishes very long URI references to   its own resources needs to be able to parse and process those same   references when received as a target URI.   Many received protocol elements are only parsed to the extent   necessary to identify and forward that element downstream.  For   example, an intermediary might parse a received field into its field   name and field value components, but then forward the field without   further parsing inside the field value.2.4.  Error Handling   A recipient MUST interpret a received protocol element according to   the semantics defined for it by this specification, including   extensions to this specification, unless the recipient has determined   (through experience or configuration) that the sender incorrectly   implements what is implied by those semantics.  For example, an   origin server might disregard the contents of a received   Accept-Encoding header field if inspection of the User-Agent header   field indicates a specific implementation version that is known to   fail on receipt of certain content codings.   Unless noted otherwise, a recipient MAY attempt to recover a usable   protocol element from an invalid construct.  HTTP does not define   specific error handling mechanisms except when they have a direct   impact on security, since different applications of the protocol   require different error handling strategies.  For example, a Web   browser might wish to transparently recover from a response where the   Location header field doesn't parse according to the ABNF, whereas a   systems control client might consider any form of error recovery to   be dangerous.   Some requests can be automatically retried by a client in the event   of an underlying connection failure, as described in Section 9.2.2.2.5.  Protocol Version   HTTP's version number consists of two decimal digits separated by a   "." (period or decimal point).  The first digit (major version)   indicates the messaging syntax, whereas the second digit (minor   version) indicates the highest minor version within that major   version to which the sender is conformant (able to understand for   future communication).   While HTTP's core semantics don't change between protocol versions,   their expression "on the wire" can change, and so the HTTP version   number changes when incompatible changes are made to the wire format.   Additionally, HTTP allows incremental, backwards-compatible changes   to be made to the protocol without changing its version through the   use of defined extension points (Section 16).   The protocol version as a whole indicates the sender's conformance   with the set of requirements laid out in that version's corresponding   specification(s).  For example, the version "HTTP/1.1" is defined by   the combined specifications of this document, "HTTP Caching"   [CACHING], and "HTTP/1.1" [HTTP/1.1].   HTTP's major version number is incremented when an incompatible   message syntax is introduced.  The minor number is incremented when   changes made to the protocol have the effect of adding to the message   semantics or implying additional capabilities of the sender.   The minor version advertises the sender's communication capabilities   even when the sender is only using a backwards-compatible subset of   the protocol, thereby letting the recipient know that more advanced   features can be used in response (by servers) or in future requests   (by clients).   When a major version of HTTP does not define any minor versions, the   minor version "0" is implied.  The "0" is used when referring to that   protocol within elements that require a minor version identifier.3.  Terminology and Core Concepts   HTTP was created for the World Wide Web (WWW) architecture and has   evolved over time to support the scalability needs of a worldwide   hypertext system.  Much of that architecture is reflected in the   terminology used to define HTTP.3.1.  Resources   The target of an HTTP request is called a "resource".  HTTP does not   limit the nature of a resource; it merely defines an interface that   might be used to interact with resources.  Most resources are   identified by a Uniform Resource Identifier (URI), as described in   Section 4.   One design goal of HTTP is to separate resource identification from   request semantics, which is made possible by vesting the request   semantics in the request method (Section 9) and a few request-   modifying header fields.  A resource cannot treat a request in a   manner inconsistent with the semantics of the method of the request.   For example, though the URI of a resource might imply semantics that   are not safe, a client can expect the resource to avoid actions that   are unsafe when processing a request with a safe method (see   Section 9.2.1).   HTTP relies upon the Uniform Resource Identifier (URI) standard [URI]   to indicate the target resource (Section 7.1) and relationships   between resources.3.2.  Representations   A "representation" is information that is intended to reflect a past,   current, or desired state of a given resource, in a format that can   be readily communicated via the protocol.  A representation consists   of a set of representation metadata and a potentially unbounded   stream of representation data (Section 8).   HTTP allows "information hiding" behind its uniform interface by   defining communication with respect to a transferable representation   of the resource state, rather than transferring the resource itself.   This allows the resource identified by a URI to be anything,   including temporal functions like "the current weather in Laguna   Beach", while potentially providing information that represents that   resource at the time a message is generated [REST].   The uniform interface is similar to a window through which one can   observe and act upon a thing only through the communication of   messages to an independent actor on the other side.  A shared   abstraction is needed to represent ("take the place of") the current   or desired state of that thing in our communications.  When a   representation is hypertext, it can provide both a representation of   the resource state and processing instructions that help guide the   recipient's future interactions.   A target resource might be provided with, or be capable of   generating, multiple representations that are each intended to   reflect the resource's current state.  An algorithm, usually based on   content negotiation (Section 12), would be used to select one of   those representations as being most applicable to a given request.   This "selected representation" provides the data and metadata for   evaluating conditional requests (Section 13) and constructing the   content for 200 (OK), 206 (Partial Content), and 304 (Not Modified)   responses to GET (Section 9.3.1).3.3.  Connections, Clients, and Servers   HTTP is a client/server protocol that operates over a reliable   transport- or session-layer "connection".   An HTTP "client" is a program that establishes a connection to a   server for the purpose of sending one or more HTTP requests.  An HTTP   "server" is a program that accepts connections in order to service   HTTP requests by sending HTTP responses.   The terms client and server refer only to the roles that these   programs perform for a particular connection.  The same program might   act as a client on some connections and a server on others.   HTTP is defined as a stateless protocol, meaning that each request   message's semantics can be understood in isolation, and that the   relationship between connections and messages on them has no impact   on the interpretation of those messages.  For example, a CONNECT   request (Section 9.3.6) or a request with the Upgrade header field   (Section 7.8) can occur at any time, not just in the first message on   a connection.  Many implementations depend on HTTP's stateless design   in order to reuse proxied connections or dynamically load balance   requests across multiple servers.   As a result, a server MUST NOT assume that two requests on the same   connection are from the same user agent unless the connection is   secured and specific to that agent.  Some non-standard HTTP   extensions (e.g., [RFC4559]) have been known to violate this   requirement, resulting in security and interoperability problems.3.4.  Messages   HTTP is a stateless request/response protocol for exchanging   "messages" across a connection.  The terms "sender" and "recipient"   refer to any implementation that sends or receives a given message,   respectively.   A client sends requests to a server in the form of a "request"   message with a method (Section 9) and request target (Section 7.1).   The request might also contain header fields (Section 6.3) for   request modifiers, client information, and representation metadata,   content (Section 6.4) intended for processing in accordance with the   method, and trailer fields (Section 6.5) to communicate information   collected while sending the content.   A server responds to a client's request by sending one or more   "response" messages, each including a status code (Section 15).  The   response might also contain header fields for server information,   resource metadata, and representation metadata, content to be   interpreted in accordance with the status code, and trailer fields to   communicate information collected while sending the content.3.5.  User Agents   The term "user agent" refers to any of the various client programs   that initiate a request.   The most familiar form of user agent is the general-purpose Web   browser, but that's only a small percentage of implementations.   Other common user agents include spiders (web-traversing robots),   command-line tools, billboard screens, household appliances, scales,   light bulbs, firmware update scripts, mobile apps, and communication   devices in a multitude of shapes and sizes.   Being a user agent does not imply that there is a human user directly   interacting with the software agent at the time of a request.  In   many cases, a user agent is installed or configured to run in the   background and save its results for later inspection (or save only a   subset of those results that might be interesting or erroneous).   Spiders, for example, are typically given a start URI and configured   to follow certain behavior while crawling the Web as a hypertext   graph.   Many user agents cannot, or choose not to, make interactive   suggestions to their user or provide adequate warning for security or   privacy concerns.  In the few cases where this specification requires   reporting of errors to the user, it is acceptable for such reporting   to only be observable in an error console or log file.  Likewise,   requirements that an automated action be confirmed by the user before   proceeding might be met via advance configuration choices, run-time   options, or simple avoidance of the unsafe action; confirmation does   not imply any specific user interface or interruption of normal   processing if the user has already made that choice.3.6.  Origin Server   The term "origin server" refers to a program that can originate   authoritative responses for a given target resource.   The most familiar form of origin server are large public websites.   However, like user agents being equated with browsers, it is easy to   be misled into thinking that all origin servers are alike.  Common   origin servers also include home automation units, configurable   networking components, office machines, autonomous robots, news   feeds, traffic cameras, real-time ad selectors, and video-on-demand   platforms.   Most HTTP communication consists of a retrieval request (GET) for a   representation of some resource identified by a URI.  In the simplest   case, this might be accomplished via a single bidirectional   connection (===) between the user agent (UA) and the origin server   (O).            request   >       UA ======================================= O                                   <   response                                  Figure 13.7.  Intermediaries   HTTP enables the use of intermediaries to satisfy requests through a   chain of connections.  There are three common forms of HTTP   "intermediary": proxy, gateway, and tunnel.  In some cases, a single   intermediary might act as an origin server, proxy, gateway, or   tunnel, switching behavior based on the nature of each request.            >             >             >             >       UA =========== A =========== B =========== C =========== O                  <             <             <             <                                  Figure 2   The figure above shows three intermediaries (A, B, and C) between the   user agent and origin server.  A request or response message that   travels the whole chain will pass through four separate connections.   Some HTTP communication options might apply only to the connection   with the nearest, non-tunnel neighbor, only to the endpoints of the   chain, or to all connections along the chain.  Although the diagram   is linear, each participant might be engaged in multiple,   simultaneous communications.  For example, B might be receiving   requests from many clients other than A, and/or forwarding requests   to servers other than C, at the same time that it is handling A's   request.  Likewise, later requests might be sent through a different   path of connections, often based on dynamic configuration for load   balancing.   The terms "upstream" and "downstream" are used to describe   directional requirements in relation to the message flow: all   messages flow from upstream to downstream.  The terms "inbound" and   "outbound" are used to describe directional requirements in relation   to the request route: inbound means "toward the origin server",   whereas outbound means "toward the user agent".   A "proxy" is a message-forwarding agent that is chosen by the client,   usually via local configuration rules, to receive requests for some   type(s) of absolute URI and attempt to satisfy those requests via   translation through the HTTP interface.  Some translations are   minimal, such as for proxy requests for "http" URIs, whereas other   requests might require translation to and from entirely different   application-level protocols.  Proxies are often used to group an   organization's HTTP requests through a common intermediary for the   sake of security services, annotation services, or shared caching.   Some proxies are designed to apply transformations to selected   messages or content while they are being forwarded, as described in   Section 7.7.   A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as   an origin server for the outbound connection but translates received   requests and forwards them inbound to another server or servers.   Gateways are often used to encapsulate legacy or untrusted   information services, to improve server performance through   "accelerator" caching, and to enable partitioning or load balancing   of HTTP services across multiple machines.   All HTTP requirements applicable to an origin server also apply to   the outbound communication of a gateway.  A gateway communicates with   inbound servers using any protocol that it desires, including private   extensions to HTTP that are outside the scope of this specification.   However, an HTTP-to-HTTP gateway that wishes to interoperate with   third-party HTTP servers needs to conform to user agent requirements   on the gateway's inbound connection.   A "tunnel" acts as a blind relay between two connections without   changing the messages.  Once active, a tunnel is not considered a   party to the HTTP communication, though the tunnel might have been   initiated by an HTTP request.  A tunnel ceases to exist when both   ends of the relayed connection are closed.  Tunnels are used to   extend a virtual connection through an intermediary, such as when   Transport Layer Security (TLS, [TLS13]) is used to establish   confidential communication through a shared firewall proxy.   The above categories for intermediary only consider those acting as   participants in the HTTP communication.  There are also   intermediaries that can act on lower layers of the network protocol   stack, filtering or redirecting HTTP traffic without the knowledge or   permission of message senders.  Network intermediaries are   indistinguishable (at a protocol level) from an on-path attacker,   often introducing security flaws or interoperability problems due to   mistakenly violating HTTP semantics.   For example, an "interception proxy" [RFC3040] (also commonly known   as a "transparent proxy" [RFC1919]) differs from an HTTP proxy   because it is not chosen by the client.  Instead, an interception   proxy filters or redirects outgoing TCP port 80 packets (and   occasionally other common port traffic).  Interception proxies are   commonly found on public network access points, as a means of   enforcing account subscription prior to allowing use of non-local   Internet services, and within corporate firewalls to enforce network   usage policies.3.8.  Caches   A "cache" is a local store of previous response messages and the   subsystem that controls its message storage, retrieval, and deletion.   A cache stores cacheable responses in order to reduce the response   time and network bandwidth consumption on future, equivalent   requests.  Any client or server MAY employ a cache, though a cache   cannot be used while acting as a tunnel.   The effect of a cache is that the request/response chain is shortened   if one of the participants along the chain has a cached response   applicable to that request.  The following illustrates the resulting   chain if B has a cached copy of an earlier response from O (via C)   for a request that has not been cached by UA or A.               >             >          UA =========== A =========== B - - - - - - C - - - - - - O                     <             <                                  Figure 3   A response is "cacheable" if a cache is allowed to store a copy of   the response message for use in answering subsequent requests.  Even   when a response is cacheable, there might be additional constraints   placed by the client or by the origin server on when that cached   response can be used for a particular request.  HTTP requirements for   cache behavior and cacheable responses are defined in [CACHING].   There is a wide variety of architectures and configurations of caches   deployed across the World Wide Web and inside large organizations.   These include national hierarchies of proxy caches to save bandwidth   and reduce latency, content delivery networks that use gateway   caching to optimize regional and global distribution of popular   sites, collaborative systems that broadcast or multicast cache   entries, archives of pre-fetched cache entries for use in off-line or   high-latency environments, and so on.3.9.  Example Message Exchange   The following example illustrates a typical HTTP/1.1 message exchange   for a GET request (Section 9.3.1) on the URI "http://www.example.com/   hello.txt":   Client request:   GET /hello.txt HTTP/1.1   User-Agent: curl/7.64.1   Host: www.example.com   Accept-Language: en, mi   Server response:   HTTP/1.1 200 OK   Date: Mon, 27 Jul 2009 12:28:53 GMT   Server: Apache   Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT   ETag: "34aa387-d-1568eb00"   Accept-Ranges: bytes   Content-Length: 51   Vary: Accept-Encoding   Content-Type: text/plain   Hello World! My content includes a trailing CRLF.4.  Identifiers in HTTP   Uniform Resource Identifiers (URIs) [URI] are used throughout HTTP as   the means for identifying resources (Section 3.1).4.1.  URI References   URI references are used to target requests, indicate redirects, and   define relationships.   The definitions of "URI-reference", "absolute-URI", "relative-part",   "authority", "port", "host", "path-abempty", "segment", and "query"   are adopted from the URI generic syntax.  An "absolute-path" rule is   defined for protocol elements that can contain a non-empty path   component.  (This rule differs slightly from the path-abempty rule of   RFC 3986, which allows for an empty path, and path-absolute rule,   which does not allow paths that begin with "//".)  A "partial-URI"   rule is defined for protocol elements that can contain a relative URI   but not a fragment component.     URI-reference = <URI-reference, see [URI], Section 4.1>     absolute-URI  = <absolute-URI, see [URI], Section 4.3>     relative-part = <relative-part, see [URI], Section 4.2>     authority     = <authority, see [URI], Section 3.2>     uri-host      = <host, see [URI], Section 3.2.2>     port          = <port, see [URI], Section 3.2.3>     path-abempty  = <path-abempty, see [URI], Section 3.3>     segment       = <segment, see [URI], Section 3.3>     query         = <query, see [URI], Section 3.4>     absolute-path = 1*( "/" segment )     partial-URI   = relative-part [ "?" query ]   Each protocol element in HTTP that allows a URI reference will   indicate in its ABNF production whether the element allows any form   of reference (URI-reference), only a URI in absolute form (absolute-   URI), only the path and optional query components (partial-URI), or   some combination of the above.  Unless otherwise indicated, URI   references are parsed relative to the target URI (Section 7.1).   It is RECOMMENDED that all senders and recipients support, at a   minimum, URIs with lengths of 8000 octets in protocol elements.  Note   that this implies some structures and on-wire representations (for   example, the request line in HTTP/1.1) will necessarily be larger in   some cases.4.2.  HTTP-Related URI Schemes   IANA maintains the registry of URI Schemes [BCP35] at   <https://www.iana.org/assignments/uri-schemes/>.  Although requests   might target any URI scheme, the following schemes are inherent to   HTTP servers:   +============+====================================+=========+   | URI Scheme | Description                        | Section |   +============+====================================+=========+   | http       | Hypertext Transfer Protocol        | 4.2.1   |   +------------+------------------------------------+---------+   | https      | Hypertext Transfer Protocol Secure | 4.2.2   |   +------------+------------------------------------+---------+                              Table 2   Note that the presence of an "http" or "https" URI does not imply   that there is always an HTTP server at the identified origin   listening for connections.  Anyone can mint a URI, whether or not a   server exists and whether or not that server currently maps that   identifier to a resource.  The delegated nature of registered names   and IP addresses creates a federated namespace whether or not an HTTP   server is present.4.2.1.  http URI Scheme   The "http" URI scheme is hereby defined for minting identifiers   within the hierarchical namespace governed by a potential HTTP origin   server listening for TCP ([TCP]) connections on a given port.     http-URI = "http" "://" authority path-abempty [ "?" query ]   The origin server for an "http" URI is identified by the authority   component, which includes a host identifier ([URI], Section 3.2.2)   and optional port number ([URI], Section 3.2.3).  If the port   subcomponent is empty or not given, TCP port 80 (the reserved port   for WWW services) is the default.  The origin determines who has the   right to respond authoritatively to requests that target the   identified resource, as defined in Section 4.3.2.   A sender MUST NOT generate an "http" URI with an empty host   identifier.  A recipient that processes such a URI reference MUST   reject it as invalid.   The hierarchical path component and optional query component identify   the target resource within that origin server's namespace.4.2.2.  https URI Scheme   The "https" URI scheme is hereby defined for minting identifiers   within the hierarchical namespace governed by a potential origin   server listening for TCP connections on a given port and capable of   establishing a TLS ([TLS13]) connection that has been secured for   HTTP communication.  In this context, "secured" specifically means   that the server has been authenticated as acting on behalf of the   identified authority and all HTTP communication with that server has   confidentiality and integrity protection that is acceptable to both   client and server.     https-URI = "https" "://" authority path-abempty [ "?" query ]   The origin server for an "https" URI is identified by the authority   component, which includes a host identifier ([URI], Section 3.2.2)   and optional port number ([URI], Section 3.2.3).  If the port   subcomponent is empty or not given, TCP port 443 (the reserved port   for HTTP over TLS) is the default.  The origin determines who has the   right to respond authoritatively to requests that target the   identified resource, as defined in Section 4.3.3.   A sender MUST NOT generate an "https" URI with an empty host   identifier.  A recipient that processes such a URI reference MUST   reject it as invalid.   The hierarchical path component and optional query component identify   the target resource within that origin server's namespace.   A client MUST ensure that its HTTP requests for an "https" resource   are secured, prior to being communicated, and that it only accepts   secured responses to those requests.  Note that the definition of   what cryptographic mechanisms are acceptable to client and server are   usually negotiated and can change over time.   Resources made available via the "https" scheme have no shared   identity with the "http" scheme.  They are distinct origins with   separate namespaces.  However, extensions to HTTP that are defined as   applying to all origins with the same host, such as the Cookie   protocol [COOKIE], allow information set by one service to impact   communication with other services within a matching group of host   domains.  Such extensions ought to be designed with great care to   prevent information obtained from a secured connection being   inadvertently exchanged within an unsecured context.4.2.3.  http(s) Normalization and Comparison   URIs with an "http" or "https" scheme are normalized and compared   according to the methods defined in Section 6 of [URI], using the   defaults described above for each scheme.   HTTP does not require the use of a specific method for determining   equivalence.  For example, a cache key might be compared as a simple   string, after syntax-based normalization, or after scheme-based   normalization.   Scheme-based normalization (Section 6.2.3 of [URI]) of "http" and   "https" URIs involves the following additional rules:   *  If the port is equal to the default port for a scheme, the normal      form is to omit the port subcomponent.   *  When not being used as the target of an OPTIONS request, an empty      path component is equivalent to an absolute path of "/", so the      normal form is to provide a path of "/" instead.   *  The scheme and host are case-insensitive and normally provided in      lowercase; all other components are compared in a case-sensitive      manner.   *  Characters other than those in the "reserved" set are equivalent      to their percent-encoded octets: the normal form is to not encode      them (see Sections 2.1 and 2.2 of [URI]).   For example, the following three URIs are equivalent:      http://example.com:80/~smith/home.html      http://EXAMPLE.com/%7Esmith/home.html      http://EXAMPLE.com:/%7esmith/home.html   Two HTTP URIs that are equivalent after normalization (using any   method) can be assumed to identify the same resource, and any HTTP   component MAY perform normalization.  As a result, distinct resources   SHOULD NOT be identified by HTTP URIs that are equivalent after   normalization (using any method defined in Section 6.2 of [URI]).4.2.4.  Deprecation of userinfo in http(s) URIs   The URI generic syntax for authority also includes a userinfo   subcomponent ([URI], Section 3.2.1) for including user authentication   information in the URI.  In that subcomponent, the use of the format   "user:password" is deprecated.   Some implementations make use of the userinfo component for internal   configuration of authentication information, such as within command   invocation options, configuration files, or bookmark lists, even   though such usage might expose a user identifier or password.   A sender MUST NOT generate the userinfo subcomponent (and its "@"   delimiter) when an "http" or "https" URI reference is generated   within a message as a target URI or field value.   Before making use of an "http" or "https" URI reference received from   an untrusted source, a recipient SHOULD parse for userinfo and treat   its presence as an error; it is likely being used to obscure the   authority for the sake of phishing attacks.4.2.5.  http(s) References with Fragment Identifiers   Fragment identifiers allow for indirect identification of a secondary   resource, independent of the URI scheme, as defined in Section 3.5 of   [URI].  Some protocol elements that refer to a URI allow inclusion of   a fragment, while others do not.  They are distinguished by use of   the ABNF rule for elements where fragment is allowed; otherwise, a   specific rule that excludes fragments is used.      |  *Note:* The fragment identifier component is not part of the      |  scheme definition for a URI scheme (see Section 4.3 of [URI]),      |  thus does not appear in the ABNF definitions for the "http" and      |  "https" URI schemes above.4.3.  Authoritative Access   Authoritative access refers to dereferencing a given identifier, for   the sake of access to the identified resource, in a way that the   client believes is authoritative (controlled by the resource owner).   The process for determining whether access is granted is defined by   the URI scheme and often uses data within the URI components, such as   the authority component when the generic syntax is used.  However,   authoritative access is not limited to the identified mechanism.   Section 4.3.1 defines the concept of an origin as an aid to such   uses, and the subsequent subsections explain how to establish that a   peer has the authority to represent an origin.   See Section 17.1 for security considerations related to establishing   authority.4.3.1.  URI Origin   The "origin" for a given URI is the triple of scheme, host, and port   after normalizing the scheme and host to lowercase and normalizing   the port to remove any leading zeros.  If port is elided from the   URI, the default port for that scheme is used.  For example, the URI      https://Example.Com/happy.js   would have the origin      { "https", "example.com", "443" }   which can also be described as the normalized URI prefix with port   always present:      https://example.com:443   Each origin defines its own namespace and controls how identifiers   within that namespace are mapped to resources.  In turn, how the   origin responds to valid requests, consistently over time, determines   the semantics that users will associate with a URI, and the   usefulness of those semantics is what ultimately transforms these   mechanisms into a resource for users to reference and access in the   future.   Two origins are distinct if they differ in scheme, host, or port.   Even when it can be verified that the same entity controls two   distinct origins, the two namespaces under those origins are distinct   unless explicitly aliased by a server authoritative for that origin.   Origin is also used within HTML and related Web protocols, beyond the   scope of this document, as described in [RFC6454].4.3.2.  http Origins   Although HTTP is independent of the transport protocol, the "http"   scheme (Section 4.2.1) is specific to associating authority with   whomever controls the origin server listening for TCP connections on   the indicated port of whatever host is identified within the   authority component.  This is a very weak sense of authority because   it depends on both client-specific name resolution mechanisms and   communication that might not be secured from an on-path attacker.   Nevertheless, it is a sufficient minimum for binding "http"   identifiers to an origin server for consistent resolution within a   trusted environment.   If the host identifier is provided as an IP address, the origin   server is the listener (if any) on the indicated TCP port at that IP   address.  If host is a registered name, the registered name is an   indirect identifier for use with a name resolution service, such as   DNS, to find an address for an appropriate origin server.   When an "http" URI is used within a context that calls for access to   the indicated resource, a client MAY attempt access by resolving the   host identifier to an IP address, establishing a TCP connection to   that address on the indicated port, and sending over that connection   an HTTP request message containing a request target that matches the   client's target URI (Section 7.1).   If the server responds to such a request with a non-interim HTTP   response message, as described in Section 15, then that response is   considered an authoritative answer to the client's request.   Note, however, that the above is not the only means for obtaining an   authoritative response, nor does it imply that an authoritative   response is always necessary (see [CACHING]).  For example, the Alt-   Svc header field [ALTSVC] allows an origin server to identify other   services that are also authoritative for that origin.  Access to   "http" identified resources might also be provided by protocols   outside the scope of this document.4.3.3.  https Origins   The "https" scheme (Section 4.2.2) associates authority based on the   ability of a server to use the private key corresponding to a   certificate that the client considers to be trustworthy for the   identified origin server.  The client usually relies upon a chain of   trust, conveyed from some prearranged or configured trust anchor, to   deem a certificate trustworthy (Section 4.3.4).   In HTTP/1.1 and earlier, a client will only attribute authority to a   server when they are communicating over a successfully established   and secured connection specifically to that URI origin's host.  The   connection establishment and certificate verification are used as   proof of authority.   In HTTP/2 and HTTP/3, a client will attribute authority to a server   when they are communicating over a successfully established and   secured connection if the URI origin's host matches any of the hosts   present in the server's certificate and the client believes that it   could open a connection to that host for that URI.  In practice, a   client will make a DNS query to check that the origin's host contains   the same server IP address as the established connection.  This   restriction can be removed by the origin server sending an equivalent   ORIGIN frame [RFC8336].   The request target's host and port value are passed within each HTTP   request, identifying the origin and distinguishing it from other   namespaces that might be controlled by the same server (Section 7.2).   It is the origin's responsibility to ensure that any services   provided with control over its certificate's private key are equally   responsible for managing the corresponding "https" namespaces or at   least prepared to reject requests that appear to have been   misdirected (Section 7.4).   An origin server might be unwilling to process requests for certain   target URIs even when they have the authority to do so.  For example,   when a host operates distinct services on different ports (e.g., 443   and 8000), checking the target URI at the origin server is necessary   (even after the connection has been secured) because a network   attacker might cause connections for one port to be received at some   other port.  Failing to check the target URI might allow such an   attacker to replace a response to one target URI (e.g.,   "https://example.com/foo") with a seemingly authoritative response   from the other port (e.g., "https://example.com:8000/foo").   Note that the "https" scheme does not rely on TCP and the connected   port number for associating authority, since both are outside the   secured communication and thus cannot be trusted as definitive.   Hence, the HTTP communication might take place over any channel that   has been secured, as defined in Section 4.2.2, including protocols   that don't use TCP.   When an "https" URI is used within a context that calls for access to   the indicated resource, a client MAY attempt access by resolving the   host identifier to an IP address, establishing a TCP connection to   that address on the indicated port, securing the connection end-to-   end by successfully initiating TLS over TCP with confidentiality and   integrity protection, and sending over that connection an HTTP   request message containing a request target that matches the client's   target URI (Section 7.1).   If the server responds to such a request with a non-interim HTTP   response message, as described in Section 15, then that response is   considered an authoritative answer to the client's request.   Note, however, that the above is not the only means for obtaining an   authoritative response, nor does it imply that an authoritative   response is always necessary (see [CACHING]).4.3.4.  https Certificate Verification   To establish a secured connection to dereference a URI, a client MUST   verify that the service's identity is an acceptable match for the   URI's origin server.  Certificate verification is used to prevent   server impersonation by an on-path attacker or by an attacker that   controls name resolution.  This process requires that a client be   configured with a set of trust anchors.   In general, a client MUST verify the service identity using the   verification process defined in Section 6 of [RFC6125].  The client   MUST construct a reference identity from the service's host: if the   host is a literal IP address (Section 4.3.5), the reference identity   is an IP-ID, otherwise the host is a name and the reference identity   is a DNS-ID.   A reference identity of type CN-ID MUST NOT be used by clients.  As   noted in Section 6.2.1 of [RFC6125], a reference identity of type CN-   ID might be used by older clients.   A client might be specially configured to accept an alternative form   of server identity verification.  For example, a client might be   connecting to a server whose address and hostname are dynamic, with   an expectation that the service will present a specific certificate   (or a certificate matching some externally defined reference   identity) rather than one matching the target URI's origin.   In special cases, it might be appropriate for a client to simply   ignore the server's identity, but it must be understood that this   leaves a connection open to active attack.   If the certificate is not valid for the target URI's origin, a user   agent MUST either obtain confirmation from the user before proceeding   (see Section 3.5) or terminate the connection with a bad certificate   error.  Automated clients MUST log the error to an appropriate audit   log (if available) and SHOULD terminate the connection (with a bad   certificate error).  Automated clients MAY provide a configuration   setting that disables this check, but MUST provide a setting which   enables it.4.3.5.  IP-ID Reference Identity   A server that is identified using an IP address literal in the "host"   field of an "https" URI has a reference identity of type IP-ID.  An   IP version 4 address uses the "IPv4address" ABNF rule, and an IP   version 6 address uses the "IP-literal" production with the   "IPv6address" option; see Section 3.2.2 of [URI].  A reference   identity of IP-ID contains the decoded bytes of the IP address.   An IP version 4 address is 4 octets, and an IP version 6 address is   16 octets.  Use of IP-ID is not defined for any other IP version.   The iPAddress choice in the certificate subjectAltName extension does   not explicitly include the IP version and so relies on the length of   the address to distinguish versions; see Section 4.2.1.6 of   [RFC5280].   A reference identity of type IP-ID matches if the address is   identical to an iPAddress value of the subjectAltName extension of   the certificate.5.  Fields   HTTP uses "fields" to provide data in the form of extensible name/   value pairs with a registered key namespace.  Fields are sent and   received within the header and trailer sections of messages   (Section 6).5.1.  Field Names   A field name labels the corresponding field value as having the   semantics defined by that name.  For example, the Date header field   is defined in Section 6.6.1 as containing the origination timestamp   for the message in which it appears.     field-name     = token   Field names are case-insensitive and ought to be registered within   the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see   Section 16.3.1.   The interpretation of a field does not change between minor versions   of the same major HTTP version, though the default behavior of a   recipient in the absence of such a field can change.  Unless   specified otherwise, fields are defined for all versions of HTTP.  In   particular, the Host and Connection fields ought to be recognized by   all HTTP implementations whether or not they advertise conformance   with HTTP/1.1.   New fields can be introduced without changing the protocol version if   their defined semantics allow them to be safely ignored by recipients   that do not recognize them; see Section 16.3.   A proxy MUST forward unrecognized header fields unless the field name   is listed in the Connection header field (Section 7.6.1) or the proxy   is specifically configured to block, or otherwise transform, such   fields.  Other recipients SHOULD ignore unrecognized header and   trailer fields.  Adhering to these requirements allows HTTP's   functionality to be extended without updating or removing deployed   intermediaries.5.2.  Field Lines and Combined Field Value   Field sections are composed of any number of "field lines", each with   a "field name" (see Section 5.1) identifying the field, and a "field   line value" that conveys data for that instance of the field.   When a field name is only present once in a section, the combined   "field value" for that field consists of the corresponding field line   value.  When a field name is repeated within a section, its combined   field value consists of the list of corresponding field line values   within that section, concatenated in order, with each field line   value separated by a comma.   For example, this section:   Example-Field: Foo, Bar   Example-Field: Baz   contains two field lines, both with the field name "Example-Field".   The first field line has a field line value of "Foo, Bar", while the   second field line value is "Baz".  The field value for "Example-   Field" is the list "Foo, Bar, Baz".5.3.  Field Order   A recipient MAY combine multiple field lines within a field section   that have the same field name into one field line, without changing   the semantics of the message, by appending each subsequent field line   value to the initial field line value in order, separated by a comma   (",") and optional whitespace (OWS, defined in Section 5.6.3).  For   consistency, use comma SP.   The order in which field lines with the same name are received is   therefore significant to the interpretation of the field value; a   proxy MUST NOT change the order of these field line values when   forwarding a message.   This means that, aside from the well-known exception noted below, a   sender MUST NOT generate multiple field lines with the same name in a   message (whether in the headers or trailers) or append a field line   when a field line of the same name already exists in the message,   unless that field's definition allows multiple field line values to   be recombined as a comma-separated list (i.e., at least one   alternative of the field's definition allows a comma-separated list,   such as an ABNF rule of #(values) defined in Section 5.6.1).      |  *Note:* In practice, the "Set-Cookie" header field ([COOKIE])      |  often appears in a response message across multiple field lines      |  and does not use the list syntax, violating the above      |  requirements on multiple field lines with the same field name.      |  Since it cannot be combined into a single field value,      |  recipients ought to handle "Set-Cookie" as a special case while      |  processing fields.  (See Appendix A.2.3 of [Kri2001] for      |  details.)   The order in which field lines with differing field names are   received in a section is not significant.  However, it is good   practice to send header fields that contain additional control data   first, such as Host on requests and Date on responses, so that   implementations can decide when not to handle a message as early as   possible.   A server MUST NOT apply a request to the target resource until it   receives the entire request header section, since later header field   lines might include conditionals, authentication credentials, or   deliberately misleading duplicate header fields that could impact   request processing.5.4.  Field Limits   HTTP does not place a predefined limit on the length of each field   line, field value, or on the length of a header or trailer section as   a whole, as described in Section 2.  Various ad hoc limitations on   individual lengths are found in practice, often depending on the   specific field's semantics.   A server that receives a request header field line, field value, or   set of fields larger than it wishes to process MUST respond with an   appropriate 4xx (Client Error) status code.  Ignoring such header   fields would increase the server's vulnerability to request smuggling   attacks (Section 11.2 of [HTTP/1.1]).   A client MAY discard or truncate received field lines that are larger   than the client wishes to process if the field semantics are such   that the dropped value(s) can be safely ignored without changing the   message framing or response semantics.5.5.  Field Values   HTTP field values consist of a sequence of characters in a format   defined by the field's grammar.  Each field's grammar is usually   defined using ABNF ([RFC5234]).     field-value    = *field-content     field-content  = field-vchar                      [ 1*( SP / HTAB / field-vchar ) field-vchar ]     field-vchar    = VCHAR / obs-text     obs-text       = %x80-FF   A field value does not include leading or trailing whitespace.  When   a specific version of HTTP allows such whitespace to appear in a   message, a field parsing implementation MUST exclude such whitespace   prior to evaluating the field value.   Field values are usually constrained to the range of US-ASCII   characters [USASCII].  Fields needing a greater range of characters   can use an encoding, such as the one defined in [RFC8187].   Historically, HTTP allowed field content with text in the ISO-8859-1   charset [ISO-8859-1], supporting other charsets only through use of   [RFC2047] encoding.  Specifications for newly defined fields SHOULD   limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB.   A recipient SHOULD treat other allowed octets in field content (i.e.,   obs-text) as opaque data.   Field values containing CR, LF, or NUL characters are invalid and   dangerous, due to the varying ways that implementations might parse   and interpret those characters; a recipient of CR, LF, or NUL within   a field value MUST either reject the message or replace each of those   characters with SP before further processing or forwarding of that   message.  Field values containing other CTL characters are also   invalid; however, recipients MAY retain such characters for the sake   of robustness when they appear within a safe context (e.g., an   application-specific quoted string that will not be processed by any   downstream HTTP parser).   Fields that only anticipate a single member as the field value are   referred to as "singleton fields".   Fields that allow multiple members as the field value are referred to   as "list-based fields".  The list operator extension of Section 5.6.1   is used as a common notation for defining field values that can   contain multiple members.   Because commas (",") are used as the delimiter between members, they   need to be treated with care if they are allowed as data within a   member.  This is true for both list-based and singleton fields, since   a singleton field might be erroneously sent with multiple members and   detecting such errors improves interoperability.  Fields that expect   to contain a comma within a member, such as within an HTTP-date or   URI-reference element, ought to be defined with delimiters around   that element to distinguish any comma within that data from potential   list separators.   For example, a textual date and a URI (either of which might contain   a comma) could be safely carried in list-based field values like   these:   Example-URIs: "http://example.com/a.html,foo",                 "http://without-a-comma.example.com/"   Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"   Note that double-quote delimiters are almost always used with the   quoted-string production (Section 5.6.4); using a different syntax   inside double-quotes will likely cause unnecessary confusion.   Many fields (such as Content-Type, defined in Section 8.3) use a   common syntax for parameters that allows both unquoted (token) and   quoted (quoted-string) syntax for a parameter value (Section 5.6.6).   Use of common syntax allows recipients to reuse existing parser   components.  When allowing both forms, the meaning of a parameter   value ought to be the same whether it was received as a token or a   quoted string.      |  *Note:* For defining field value syntax, this specification      |  uses an ABNF rule named after the field name to define the      |  allowed grammar for that field's value (after said value has      |  been extracted from the underlying messaging syntax and      |  multiple instances combined into a list).5.6.  Common Rules for Defining Field Values5.6.1.  Lists (#rule ABNF Extension)   A #rule extension to the ABNF rules of [RFC5234] is used to improve   readability in the definitions of some list-based field values.   A construct "#" is defined, similar to "*", for defining comma-   delimited lists of elements.  The full form is "<n>#<m>element"   indicating at least <n> and at most <m> elements, each separated by a   single comma (",") and optional whitespace (OWS, defined in   Section 5.6.3).5.6.1.1.  Sender Requirements   In any production that uses the list construct, a sender MUST NOT   generate empty list elements.  In other words, a sender has to   generate lists that satisfy the following syntax:     1#element => element *( OWS "," OWS element )   and:     #element => [ 1#element ]   and for n >= 1 and m > 1:     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )   Appendix A shows the collected ABNF for senders after the list   constructs have been expanded.5.6.1.2.  Recipient Requirements   Empty elements do not contribute to the count of elements present.  A   recipient MUST parse and ignore a reasonable number of empty list   elements: enough to handle common mistakes by senders that merge   values, but not so much that they could be used as a denial-of-   service mechanism.  In other words, a recipient MUST accept lists   that satisfy the following syntax:     #element => [ element ] *( OWS "," OWS [ element ] )   Note that because of the potential presence of empty list elements,   the RFC 5234 ABNF cannot enforce the cardinality of list elements,   and consequently all cases are mapped as if there was no cardinality   specified.   For example, given these ABNF productions:     example-list      = 1#example-list-elmt     example-list-elmt = token ; see Section 5.6.2   Then the following are valid values for example-list (not including   the double quotes, which are present for delimitation only):     "foo,bar"     "foo ,bar,"     "foo , ,bar,charlie"   In contrast, the following values would be invalid, since at least   one non-empty element is required by the example-list production:     ""     ","     ",   ,"5.6.2.  Tokens   Tokens are short textual identifiers that do not include whitespace   or delimiters.     token          = 1*tchar     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"                    / DIGIT / ALPHA                    ; any VCHAR, except delimiters   Many HTTP field values are defined using common syntax components,   separated by whitespace or specific delimiting characters.   Delimiters are chosen from the set of US-ASCII visual characters not   allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").5.6.3.  Whitespace   This specification uses three rules to denote the use of linear   whitespace: OWS (optional whitespace), RWS (required whitespace), and   BWS ("bad" whitespace).   The OWS rule is used where zero or more linear whitespace octets   might appear.  For protocol elements where optional whitespace is   preferred to improve readability, a sender SHOULD generate the   optional whitespace as a single SP; otherwise, a sender SHOULD NOT   generate optional whitespace except as needed to overwrite invalid or   unwanted protocol elements during in-place message filtering.   The RWS rule is used when at least one linear whitespace octet is   required to separate field tokens.  A sender SHOULD generate RWS as a   single SP.   OWS and RWS have the same semantics as a single SP.  Any content   known to be defined as OWS or RWS MAY be replaced with a single SP   before interpreting it or forwarding the message downstream.   The BWS rule is used where the grammar allows optional whitespace   only for historical reasons.  A sender MUST NOT generate BWS in   messages.  A recipient MUST parse for such bad whitespace and remove   it before interpreting the protocol element.   BWS has no semantics.  Any content known to be defined as BWS MAY be   removed before interpreting it or forwarding the message downstream.     OWS            = *( SP / HTAB )                    ; optional whitespace     RWS            = 1*( SP / HTAB )                    ; required whitespace     BWS            = OWS                    ; "bad" whitespace5.6.4.  Quoted Strings   A string of text is parsed as a single value if it is quoted using   double-quote marks.     quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE     qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text   The backslash octet ("\") can be used as a single-octet quoting   mechanism within quoted-string and comment constructs.  Recipients   that process the value of a quoted-string MUST handle a quoted-pair   as if it were replaced by the octet following the backslash.     quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )   A sender SHOULD NOT generate a quoted-pair in a quoted-string except   where necessary to quote DQUOTE and backslash octets occurring within   that string.  A sender SHOULD NOT generate a quoted-pair in a comment   except where necessary to quote parentheses ["(" and ")"] and   backslash octets occurring within that comment.5.6.5.  Comments   Comments can be included in some HTTP fields by surrounding the   comment text with parentheses.  Comments are only allowed in fields   containing "comment" as part of their field value definition.     comment        = "(" *( ctext / quoted-pair / comment ) ")"     ctext          = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text5.6.6.  Parameters   Parameters are instances of name/value pairs; they are often used in   field values as a common syntax for appending auxiliary information   to an item.  Each parameter is usually delimited by an immediately   preceding semicolon.     parameters      = *( OWS ";" OWS [ parameter ] )     parameter       = parameter-name "=" parameter-value     parameter-name  = token     parameter-value = ( token / quoted-string )   Parameter names are case-insensitive.  Parameter values might or   might not be case-sensitive, depending on the semantics of the   parameter name.  Examples of parameters and some equivalent forms can   be seen in media types (Section 8.3.1) and the Accept header field   (Section 12.5.1).   A parameter value that matches the token production can be   transmitted either as a token or within a quoted-string.  The quoted   and unquoted values are equivalent.      |  *Note:* Parameters do not allow whitespace (not even "bad"      |  whitespace) around the "=" character.5.6.7.  Date/Time Formats   Prior to 1995, there were three different formats commonly used by   servers to communicate timestamps.  For compatibility with old   implementations, all three are defined here.  The preferred format is   a fixed-length and single-zone subset of the date and time   specification used by the Internet Message Format [RFC5322].     HTTP-date    = IMF-fixdate / obs-date   An example of the preferred format is     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate   Examples of the two obsolete formats are     Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format     Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format   A recipient that parses a timestamp value in an HTTP field MUST   accept all three HTTP-date formats.  When a sender generates a field   that contains one or more timestamps defined as HTTP-date, the sender   MUST generate those timestamps in the IMF-fixdate format.   An HTTP-date value represents time as an instance of Coordinated   Universal Time (UTC).  The first two formats indicate UTC by the   three-letter abbreviation for Greenwich Mean Time, "GMT", a   predecessor of the UTC name; values in the asctime format are assumed   to be in UTC.   A "clock" is an implementation capable of providing a reasonable   approximation of the current instant in UTC.  A clock implementation   ought to use NTP ([RFC5905]), or some similar protocol, to   synchronize with UTC.   Preferred format:     IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT     ; fixed length/zone/capitalization subset of the format     ; see Section 3.3 of [RFC5322]     day-name     = %s"Mon" / %s"Tue" / %s"Wed"                  / %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"     date1        = day SP month SP year                  ; e.g., 02 Jun 1982     day          = 2DIGIT     month        = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"                  / %s"May" / %s"Jun" / %s"Jul" / %s"Aug"                  / %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"     year         = 4DIGIT     GMT          = %s"GMT"     time-of-day  = hour ":" minute ":" second                  ; 00:00:00 - 23:59:60 (leap second)     hour         = 2DIGIT     minute       = 2DIGIT     second       = 2DIGIT   Obsolete formats:     obs-date     = rfc850-date / asctime-date     rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT     date2        = day "-" month "-" 2DIGIT                  ; e.g., 02-Jun-82     day-name-l   = %s"Monday" / %s"Tuesday" / %s"Wednesday"                  / %s"Thursday" / %s"Friday" / %s"Saturday"                  / %s"Sunday"     asctime-date = day-name SP date3 SP time-of-day SP year     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))                  ; e.g., Jun  2   HTTP-date is case sensitive.  Note that Section 4.2 of [CACHING]   relaxes this for cache recipients.   A sender MUST NOT generate additional whitespace in an HTTP-date   beyond that specifically included as SP in the grammar.  The   semantics of day-name, day, month, year, and time-of-day are the same   as those defined for the Internet Message Format constructs with the   corresponding name ([RFC5322], Section 3.3).   Recipients of a timestamp value in rfc850-date format, which uses a   two-digit year, MUST interpret a timestamp that appears to be more   than 50 years in the future as representing the most recent year in   the past that had the same last two digits.   Recipients of timestamp values are encouraged to be robust in parsing   timestamps unless otherwise restricted by the field definition.  For   example, messages are occasionally forwarded over HTTP from a non-   HTTP source that might generate any of the date and time   specifications defined by the Internet Message Format.      |  *Note:* HTTP requirements for timestamp formats apply only to      |  their usage within the protocol stream.  Implementations are      |  not required to use these formats for user presentation,      |  request logging, etc.6.  Message Abstraction   Each major version of HTTP defines its own syntax for communicating   messages.  This section defines an abstract data type for HTTP   messages based on a generalization of those message characteristics,   common structure, and capacity for conveying semantics.  This   abstraction is used to define requirements on senders and recipients   that are independent of the HTTP version, such that a message in one   version can be relayed through other versions without changing its   meaning.   A "message" consists of the following:   *  control data to describe and route the message,   *  a headers lookup table of name/value pairs for extending that      control data and conveying additional information about the      sender, message, content, or context,   *  a potentially unbounded stream of content, and   *  a trailers lookup table of name/value pairs for communicating      information obtained while sending the content.   Framing and control data is sent first, followed by a header section   containing fields for the headers table.  When a message includes   content, the content is sent after the header section, potentially   followed by a trailer section that might contain fields for the   trailers table.   Messages are expected to be processed as a stream, wherein the   purpose of that stream and its continued processing is revealed while   being read.  Hence, control data describes what the recipient needs   to know immediately, header fields describe what needs to be known   before receiving content, the content (when present) presumably   contains what the recipient wants or needs to fulfill the message   semantics, and trailer fields provide optional metadata that was   unknown prior to sending the content.   Messages are intended to be "self-descriptive": everything a   recipient needs to know about the message can be determined by   looking at the message itself, after decoding or reconstituting parts   that have been compressed or elided in transit, without requiring an   understanding of the sender's current application state (established   via prior messages).  However, a client MUST retain knowledge of the   request when parsing, interpreting, or caching a corresponding   response.  For example, responses to the HEAD method look just like   the beginning of a response to GET but cannot be parsed in the same   manner.   Note that this message abstraction is a generalization across many   versions of HTTP, including features that might not be found in some   versions.  For example, trailers were introduced within the HTTP/1.1   chunked transfer coding as a trailer section after the content.  An   equivalent feature is present in HTTP/2 and HTTP/3 within the header   block that terminates each stream.6.1.  Framing and Completeness   Message framing indicates how each message begins and ends, such that   each message can be distinguished from other messages or noise on the   same connection.  Each major version of HTTP defines its own framing   mechanism.   HTTP/0.9 and early deployments of HTTP/1.0 used closure of the   underlying connection to end a response.  For backwards   compatibility, this implicit framing is also allowed in HTTP/1.1.   However, implicit framing can fail to distinguish an incomplete   response if the connection closes early.  For that reason, almost all   modern implementations use explicit framing in the form of length-   delimited sequences of message data.   A message is considered "complete" when all of the octets indicated   by its framing are available.  Note that, when no explicit framing is   used, a response message that is ended by the underlying connection's   close is considered complete even though it might be   indistinguishable from an incomplete response, unless a transport-   level error indicates that it is not complete.6.2.  Control Data   Messages start with control data that describe its primary purpose.   Request message control data includes a request method (Section 9),   request target (Section 7.1), and protocol version (Section 2.5).   Response message control data includes a status code (Section 15),   optional reason phrase, and protocol version.   In HTTP/1.1 ([HTTP/1.1]) and earlier, control data is sent as the   first line of a message.  In HTTP/2 ([HTTP/2]) and HTTP/3 ([HTTP/3]),   control data is sent as pseudo-header fields with a reserved name   prefix (e.g., ":authority").   Every HTTP message has a protocol version.  Depending on the version   in use, it might be identified within the message explicitly or   inferred by the connection over which the message is received.   Recipients use that version information to determine limitations or   potential for later communication with that sender.   When a message is forwarded by an intermediary, the protocol version   is updated to reflect the version used by that intermediary.  The Via   header field (Section 7.6.3) is used to communicate upstream protocol   information within a forwarded message.   A client SHOULD send a request version equal to the highest version   to which the client is conformant and whose major version is no   higher than the highest version supported by the server, if this is   known.  A client MUST NOT send a version to which it is not   conformant.   A client MAY send a lower request version if it is known that the   server incorrectly implements the HTTP specification, but only after   the client has attempted at least one normal request and determined   from the response status code or header fields (e.g., Server) that   the server improperly handles higher request versions.   A server SHOULD send a response version equal to the highest version   to which the server is conformant that has a major version less than   or equal to the one received in the request.  A server MUST NOT send   a version to which it is not conformant.  A server can send a 505   (HTTP Version Not Supported) response if it wishes, for any reason,   to refuse service of the client's major protocol version.   A recipient that receives a message with a major version number that   it implements and a minor version number higher than what it   implements SHOULD process the message as if it were in the highest   minor version within that major version to which the recipient is   conformant.  A recipient can assume that a message with a higher   minor version, when sent to a recipient that has not yet indicated   support for that higher version, is sufficiently backwards-compatible   to be safely processed by any implementation of the same major   version.6.3.  Header Fields   Fields (Section 5) that are sent or received before the content are   referred to as "header fields" (or just "headers", colloquially).   The "header section" of a message consists of a sequence of header   field lines.  Each header field might modify or extend message   semantics, describe the sender, define the content, or provide   additional context.      |  *Note:* We refer to named fields specifically as a "header      |  field" when they are only allowed to be sent in the header      |  section.6.4.  Content   HTTP messages often transfer a complete or partial representation as   the message "content": a stream of octets sent after the header   section, as delineated by the message framing.   This abstract definition of content reflects the data after it has   been extracted from the message framing.  For example, an HTTP/1.1   message body (Section 6 of [HTTP/1.1]) might consist of a stream of   data encoded with the chunked transfer coding -- a sequence of data   chunks, one zero-length chunk, and a trailer section -- whereas the   content of that same message includes only the data stream after the   transfer coding has been decoded; it does not include the chunk   lengths, chunked framing syntax, nor the trailer fields   (Section 6.5).      |  *Note:* Some field names have a "Content-" prefix.  This is an      |  informal convention; while some of these fields refer to the      |  content of the message, as defined above, others are scoped to      |  the selected representation (Section 3.2).  See the individual      |  field's definition to disambiguate.6.4.1.  Content Semantics   The purpose of content in a request is defined by the method   semantics (Section 9).   For example, a representation in the content of a PUT request   (Section 9.3.4) represents the desired state of the target resource   after the request is successfully applied, whereas a representation   in the content of a POST request (Section 9.3.3) represents   information to be processed by the target resource.   In a response, the content's purpose is defined by the request   method, response status code (Section 15), and response fields   describing that content.  For example, the content of a 200 (OK)   response to GET (Section 9.3.1) represents the current state of the   target resource, as observed at the time of the message origination   date (Section 6.6.1), whereas the content of the same status code in   a response to POST might represent either the processing result or   the new state of the target resource after applying the processing.   The content of a 206 (Partial Content) response to GET contains   either a single part of the selected representation or a multipart   message body containing multiple parts of that representation, as   described in Section 15.3.7.   Response messages with an error status code usually contain content   that represents the error condition, such that the content describes   the error state and what steps are suggested for resolving it.   Responses to the HEAD request method (Section 9.3.2) never include   content; the associated response header fields indicate only what   their values would have been if the request method had been GET   (Section 9.3.1).   2xx (Successful) responses to a CONNECT request method   (Section 9.3.6) switch the connection to tunnel mode instead of   having content.   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)   responses do not include content.   All other responses do include content, although that content might   be of zero length.6.4.2.  Identifying Content   When a complete or partial representation is transferred as message   content, it is often desirable for the sender to supply, or the   recipient to determine, an identifier for a resource corresponding to   that specific representation.  For example, a client making a GET   request on a resource for "the current weather report" might want an   identifier specific to the content returned (e.g., "weather report   for Laguna Beach at 20210720T1711").  This can be useful for sharing   or bookmarking content from resources that are expected to have   changing representations over time.   For a request message:   *  If the request has a Content-Location header field, then the      sender asserts that the content is a representation of the      resource identified by the Content-Location field value.  However,      such an assertion cannot be trusted unless it can be verified by      other means (not defined by this specification).  The information      might still be useful for revision history links.   *  Otherwise, the content is unidentified by HTTP, but a more      specific identifier might be supplied within the content itself.   For a response message, the following rules are applied in order   until a match is found:   1.  If the request method is HEAD or the response status code is 204       (No Content) or 304 (Not Modified), there is no content in the       response.   2.  If the request method is GET and the response status code is 200       (OK), the content is a representation of the target resource       (Section 7.1).   3.  If the request method is GET and the response status code is 203       (Non-Authoritative Information), the content is a potentially       modified or enhanced representation of the target resource as       provided by an intermediary.   4.  If the request method is GET and the response status code is 206       (Partial Content), the content is one or more parts of a       representation of the target resource.   5.  If the response has a Content-Location header field and its field       value is a reference to the same URI as the target URI, the       content is a representation of the target resource.   6.  If the response has a Content-Location header field and its field       value is a reference to a URI different from the target URI, then       the sender asserts that the content is a representation of the       resource identified by the Content-Location field value.       However, such an assertion cannot be trusted unless it can be       verified by other means (not defined by this specification).   7.  Otherwise, the content is unidentified by HTTP, but a more       specific identifier might be supplied within the content itself.6.5.  Trailer Fields   Fields (Section 5) that are located within a "trailer section" are   referred to as "trailer fields" (or just "trailers", colloquially).   Trailer fields can be useful for supplying message integrity checks,   digital signatures, delivery metrics, or post-processing status   information.   Trailer fields ought to be processed and stored separately from the   fields in the header section to avoid contradicting message semantics   known at the time the header section was complete.  The presence or   absence of certain header fields might impact choices made for the   routing or processing of the message as a whole before the trailers   are received; those choices cannot be unmade by the later discovery   of trailer fields.6.5.1.  Limitations on Use of Trailers   A trailer section is only possible when supported by the version of   HTTP in use and enabled by an explicit framing mechanism.  For   example, the chunked transfer coding in HTTP/1.1 allows a trailer   section to be sent after the content (Section 7.1.2 of [HTTP/1.1]).   Many fields cannot be processed outside the header section because   their evaluation is necessary prior to receiving the content, such as   those that describe message framing, routing, authentication, request   modifiers, response controls, or content format.  A sender MUST NOT   generate a trailer field unless the sender knows the corresponding   header field name's definition permits the field to be sent in   trailers.   Trailer fields can be difficult to process by intermediaries that   forward messages from one protocol version to another.  If the entire   message can be buffered in transit, some intermediaries could merge   trailer fields into the header section (as appropriate) before it is   forwarded.  However, in most cases, the trailers are simply   discarded.  A recipient MUST NOT merge a trailer field into a header   section unless the recipient understands the corresponding header   field definition and that definition explicitly permits and defines   how trailer field values can be safely merged.   The presence of the keyword "trailers" in the TE header field   (Section 10.1.4) of a request indicates that the client is willing to   accept trailer fields, on behalf of itself and any downstream   clients.  For requests from an intermediary, this implies that all   downstream clients are willing to accept trailer fields in the   forwarded response.  Note that the presence of "trailers" does not   mean that the client(s) will process any particular trailer field in   the response; only that the trailer section(s) will not be dropped by   any of the clients.   Because of the potential for trailer fields to be discarded in   transit, a server SHOULD NOT generate trailer fields that it believes   are necessary for the user agent to receive.6.5.2.  Processing Trailer Fields   The "Trailer" header field (Section 6.6.2) can be sent to indicate   fields likely to be sent in the trailer section, which allows   recipients to prepare for their receipt before processing the   content.  For example, this could be useful if a field name indicates   that a dynamic checksum should be calculated as the content is   received and then immediately checked upon receipt of the trailer   field value.   Like header fields, trailer fields with the same name are processed   in the order received; multiple trailer field lines with the same   name have the equivalent semantics as appending the multiple values   as a list of members.  Trailer fields that might be generated more   than once during a message MUST be defined as a list-based field even   if each member value is only processed once per field line received.   At the end of a message, a recipient MAY treat the set of received   trailer fields as a data structure of name/value pairs, similar to   (but separate from) the header fields.  Additional processing   expectations, if any, can be defined within the field specification   for a field intended for use in trailers.6.6.  Message Metadata   Fields that describe the message itself, such as when and how the   message has been generated, can appear in both requests and   responses.6.6.1.  Date   The "Date" header field represents the date and time at which the   message was originated, having the same semantics as the Origination   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The   field value is an HTTP-date, as defined in Section 5.6.7.     Date = HTTP-date   An example is   Date: Tue, 15 Nov 1994 08:12:31 GMT   A sender that generates a Date header field SHOULD generate its field   value as the best available approximation of the date and time of   message generation.  In theory, the date ought to represent the   moment just before generating the message content.  In practice, a   sender can generate the date value at any time during message   origination.   An origin server with a clock (as defined in Section 5.6.7) MUST   generate a Date header field in all 2xx (Successful), 3xx   (Redirection), and 4xx (Client Error) responses, and MAY generate a   Date header field in 1xx (Informational) and 5xx (Server Error)   responses.   An origin server without a clock MUST NOT generate a Date header   field.   A recipient with a clock that receives a response message without a   Date header field MUST record the time it was received and append a   corresponding Date header field to the message's header section if it   is cached or forwarded downstream.   A recipient with a clock that receives a response with an invalid   Date header field value MAY replace that value with the time that   response was received.   A user agent MAY send a Date header field in a request, though   generally will not do so unless it is believed to convey useful   information to the server.  For example, custom applications of HTTP   might convey a Date if the server is expected to adjust its   interpretation of the user's request based on differences between the   user agent and server clocks.6.6.2.  Trailer   The "Trailer" header field provides a list of field names that the   sender anticipates sending as trailer fields within that message.   This allows a recipient to prepare for receipt of the indicated   metadata before it starts processing the content.     Trailer = #field-name   For example, a sender might indicate that a signature will be   computed as the content is being streamed and provide the final   signature as a trailer field.  This allows a recipient to perform the   same check on the fly as it receives the content.   A sender that intends to generate one or more trailer fields in a   message SHOULD generate a Trailer header field in the header section   of that message to indicate which fields might be present in the   trailers.   If an intermediary discards the trailer section in transit, the   Trailer field could provide a hint of what metadata was lost, though   there is no guarantee that a sender of Trailer will always follow   through by sending the named fields.7.  Routing HTTP Messages   HTTP request message routing is determined by each client based on   the target resource, the client's proxy configuration, and   establishment or reuse of an inbound connection.  The corresponding   response routing follows the same connection chain back to the   client.7.1.  Determining the Target Resource   Although HTTP is used in a wide variety of applications, most clients   rely on the same resource identification mechanism and configuration   techniques as general-purpose Web browsers.  Even when communication   options are hard-coded in a client's configuration, we can think of   their combined effect as a URI reference (Section 4.1).   A URI reference is resolved to its absolute form in order to obtain   the "target URI".  The target URI excludes the reference's fragment   component, if any, since fragment identifiers are reserved for   client-side processing ([URI], Section 3.5).   To perform an action on a "target resource", the client sends a   request message containing enough components of its parsed target URI   to enable recipients to identify that same resource.  For historical   reasons, the parsed target URI components, collectively referred to   as the "request target", are sent within the message control data and   the Host header field (Section 7.2).   There are two unusual cases for which the request target components   are in a method-specific form:   *  For CONNECT (Section 9.3.6), the request target is the host name      and port number of the tunnel destination, separated by a colon.   *  For OPTIONS (Section 9.3.7), the request target can be a single      asterisk ("*").   See the respective method definitions for details.  These forms MUST   NOT be used with other methods.   Upon receipt of a client's request, a server reconstructs the target   URI from the received components in accordance with their local   configuration and incoming connection context.  This reconstruction   is specific to each major protocol version.  For example, Section 3.3   of [HTTP/1.1] defines how a server determines the target URI of an   HTTP/1.1 request.      |  *Note:* Previous specifications defined the recomposed target      |  URI as a distinct concept, the "effective request URI".7.2.  Host and :authority   The "Host" header field in a request provides the host and port   information from the target URI, enabling the origin server to   distinguish among resources while servicing requests for multiple   host names.   In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], the Host header field is, in   some cases, supplanted by the ":authority" pseudo-header field of a   request's control data.     Host = uri-host [ ":" port ] ; Section 4   The target URI's authority information is critical for handling a   request.  A user agent MUST generate a Host header field in a request   unless it sends that information as an ":authority" pseudo-header   field.  A user agent that sends Host SHOULD send it as the first   field in the header section of a request.   For example, a GET request to the origin server for   <http://www.example.org/pub/WWW/> would begin with:   GET /pub/WWW/ HTTP/1.1   Host: www.example.org   Since the host and port information acts as an application-level   routing mechanism, it is a frequent target for malware seeking to   poison a shared cache or redirect a request to an unintended server.   An interception proxy is particularly vulnerable if it relies on the   host and port information for redirecting requests to internal   servers, or for use as a cache key in a shared cache, without first   verifying that the intercepted connection is targeting a valid IP   address for that host.7.3.  Routing Inbound Requests   Once the target URI and its origin are determined, a client decides   whether a network request is necessary to accomplish the desired   semantics and, if so, where that request is to be directed.7.3.1.  To a Cache   If the client has a cache [CACHING] and the request can be satisfied   by it, then the request is usually directed there first.7.3.2.  To a Proxy   If the request is not satisfied by a cache, then a typical client   will check its configuration to determine whether a proxy is to be   used to satisfy the request.  Proxy configuration is implementation-   dependent, but is often based on URI prefix matching, selective   authority matching, or both, and the proxy itself is usually   identified by an "http" or "https" URI.   If an "http" or "https" proxy is applicable, the client connects   inbound by establishing (or reusing) a connection to that proxy and   then sending it an HTTP request message containing a request target   that matches the client's target URI.7.3.3.  To the Origin   If no proxy is applicable, a typical client will invoke a handler   routine (specific to the target URI's scheme) to obtain access to the   identified resource.  How that is accomplished is dependent on the   target URI scheme and defined by its associated specification.   Section 4.3.2 defines how to obtain access to an "http" resource by   establishing (or reusing) an inbound connection to the identified   origin server and then sending it an HTTP request message containing   a request target that matches the client's target URI.   Section 4.3.3 defines how to obtain access to an "https" resource by   establishing (or reusing) an inbound secured connection to an origin   server that is authoritative for the identified origin and then   sending it an HTTP request message containing a request target that   matches the client's target URI.7.4.  Rejecting Misdirected Requests   Once a request is received by a server and parsed sufficiently to   determine its target URI, the server decides whether to process the   request itself, forward the request to another server, redirect the   client to a different resource, respond with an error, or drop the   connection.  This decision can be influenced by anything about the   request or connection context, but is specifically directed at   whether the server has been configured to process requests for that   target URI and whether the connection context is appropriate for that   request.   For example, a request might have been misdirected, deliberately or   accidentally, such that the information within a received Host header   field differs from the connection's host or port.  If the connection   is from a trusted gateway, such inconsistency might be expected;   otherwise, it might indicate an attempt to bypass security filters,   trick the server into delivering non-public content, or poison a   cache.  See Section 17 for security considerations regarding message   routing.   Unless the connection is from a trusted gateway, an origin server   MUST reject a request if any scheme-specific requirements for the   target URI are not met.  In particular, a request for an "https"   resource MUST be rejected unless it has been received over a   connection that has been secured via a certificate valid for that   target URI's origin, as defined by Section 4.2.2.   The 421 (Misdirected Request) status code in a response indicates   that the origin server has rejected the request because it appears to   have been misdirected (Section 15.5.20).7.5.  Response Correlation   A connection might be used for multiple request/response exchanges.   The mechanism used to correlate between request and response messages   is version dependent; some versions of HTTP use implicit ordering of   messages, while others use an explicit identifier.   All responses, regardless of the status code (including interim   responses) can be sent at any time after a request is received, even   if the request is not yet complete.  A response can complete before   its corresponding request is complete (Section 6.1).  Likewise,   clients are not expected to wait any specific amount of time for a   response.  Clients (including intermediaries) might abandon a request   if the response is not received within a reasonable period of time.   A client that receives a response while it is still sending the   associated request SHOULD continue sending that request unless it   receives an explicit indication to the contrary (see, e.g.,   Section 9.5 of [HTTP/1.1] and Section 6.4 of [HTTP/2]).7.6.  Message Forwarding   As described in Section 3.7, intermediaries can serve a variety of   roles in the processing of HTTP requests and responses.  Some   intermediaries are used to improve performance or availability.   Others are used for access control or to filter content.  Since an   HTTP stream has characteristics similar to a pipe-and-filter   architecture, there are no inherent limits to the extent an   intermediary can enhance (or interfere) with either direction of the   stream.   Intermediaries are expected to forward messages even when protocol   elements are not recognized (e.g., new methods, status codes, or   field names) since that preserves extensibility for downstream   recipients.   An intermediary not acting as a tunnel MUST implement the Connection   header field, as specified in Section 7.6.1, and exclude fields from   being forwarded that are only intended for the incoming connection.   An intermediary MUST NOT forward a message to itself unless it is   protected from an infinite request loop.  In general, an intermediary   ought to recognize its own server names, including any aliases, local   variations, or literal IP addresses, and respond to such requests   directly.   An HTTP message can be parsed as a stream for incremental processing   or forwarding downstream.  However, senders and recipients cannot   rely on incremental delivery of partial messages, since some   implementations will buffer or delay message forwarding for the sake   of network efficiency, security checks, or content transformations.7.6.1.  Connection   The "Connection" header field allows the sender to list desired   control options for the current connection.     Connection        = #connection-option     connection-option = token   Connection options are case-insensitive.   When a field aside from Connection is used to supply control   information for or about the current connection, the sender MUST list   the corresponding field name within the Connection header field.   Note that some versions of HTTP prohibit the use of fields for such   information, and therefore do not allow the Connection field.   Intermediaries MUST parse a received Connection header field before a   message is forwarded and, for each connection-option in this field,   remove any header or trailer field(s) from the message with the same   name as the connection-option, and then remove the Connection header   field itself (or replace it with the intermediary's own control   options for the forwarded message).   Hence, the Connection header field provides a declarative way of   distinguishing fields that are only intended for the immediate   recipient ("hop-by-hop") from those fields that are intended for all   recipients on the chain ("end-to-end"), enabling the message to be   self-descriptive and allowing future connection-specific extensions   to be deployed without fear that they will be blindly forwarded by   older intermediaries.   Furthermore, intermediaries SHOULD remove or replace fields that are   known to require removal before forwarding, whether or not they   appear as a connection-option, after applying those fields'   semantics.  This includes but is not limited to:   *  Proxy-Connection (Appendix C.2.2 of [HTTP/1.1])   *  Keep-Alive (Section 19.7.1 of [RFC2068])   *  TE (Section 10.1.4)   *  Transfer-Encoding (Section 6.1 of [HTTP/1.1])   *  Upgrade (Section 7.8)   A sender MUST NOT send a connection option corresponding to a field   that is intended for all recipients of the content.  For example,   Cache-Control is never appropriate as a connection option   (Section 5.2 of [CACHING]).   Connection options do not always correspond to a field present in the   message, since a connection-specific field might not be needed if   there are no parameters associated with a connection option.  In   contrast, a connection-specific field received without a   corresponding connection option usually indicates that the field has   been improperly forwarded by an intermediary and ought to be ignored   by the recipient.   When defining a new connection option that does not correspond to a   field, specification authors ought to reserve the corresponding field   name anyway in order to avoid later collisions.  Such reserved field   names are registered in the "Hypertext Transfer Protocol (HTTP) Field   Name Registry" (Section 16.3.1).7.6.2.  Max-Forwards   The "Max-Forwards" header field provides a mechanism with the TRACE   (Section 9.3.8) and OPTIONS (Section 9.3.7) request methods to limit   the number of times that the request is forwarded by proxies.  This   can be useful when the client is attempting to trace a request that   appears to be failing or looping mid-chain.     Max-Forwards = 1*DIGIT   The Max-Forwards value is a decimal integer indicating the remaining   number of times this request message can be forwarded.   Each intermediary that receives a TRACE or OPTIONS request containing   a Max-Forwards header field MUST check and update its value prior to   forwarding the request.  If the received value is zero (0), the   intermediary MUST NOT forward the request; instead, the intermediary   MUST respond as the final recipient.  If the received Max-Forwards   value is greater than zero, the intermediary MUST generate an updated   Max-Forwards field in the forwarded message with a field value that   is the lesser of a) the received value decremented by one (1) or b)   the recipient's maximum supported value for Max-Forwards.   A recipient MAY ignore a Max-Forwards header field received with any   other request methods.7.6.3.  Via   The "Via" header field indicates the presence of intermediate   protocols and recipients between the user agent and the server (on   requests) or between the origin server and the client (on responses),   similar to the "Received" header field in email (Section 3.6.7 of   [RFC5322]).  Via can be used for tracking message forwards, avoiding   request loops, and identifying the protocol capabilities of senders   along the request/response chain.     Via = #( received-protocol RWS received-by [ RWS comment ] )     received-protocol = [ protocol-name "/" ] protocol-version                       ; see Section 7.8     received-by       = pseudonym [ ":" port ]     pseudonym         = token   Each member of the Via field value represents a proxy or gateway that   has forwarded the message.  Each intermediary appends its own   information about how the message was received, such that the end   result is ordered according to the sequence of forwarding recipients.   A proxy MUST send an appropriate Via header field, as described   below, in each message that it forwards.  An HTTP-to-HTTP gateway   MUST send an appropriate Via header field in each inbound request   message and MAY send a Via header field in forwarded response   messages.   For each intermediary, the received-protocol indicates the protocol   and protocol version used by the upstream sender of the message.   Hence, the Via field value records the advertised protocol   capabilities of the request/response chain such that they remain   visible to downstream recipients; this can be useful for determining   what backwards-incompatible features might be safe to use in   response, or within a later request, as described in Section 2.5.   For brevity, the protocol-name is omitted when the received protocol   is HTTP.   The received-by portion is normally the host and optional port number   of a recipient server or client that subsequently forwarded the   message.  However, if the real host is considered to be sensitive   information, a sender MAY replace it with a pseudonym.  If a port is   not provided, a recipient MAY interpret that as meaning it was   received on the default port, if any, for the received-protocol.   A sender MAY generate comments to identify the software of each   recipient, analogous to the User-Agent and Server header fields.   However, comments in Via are optional, and a recipient MAY remove   them prior to forwarding the message.   For example, a request message could be sent from an HTTP/1.0 user   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to   forward the request to a public proxy at p.example.net, which   completes the request by forwarding it to the origin server at   www.example.com.  The request received by www.example.com would then   have the following Via header field:   Via: 1.0 fred, 1.1 p.example.net   An intermediary used as a portal through a network firewall SHOULD   NOT forward the names and ports of hosts within the firewall region   unless it is explicitly enabled to do so.  If not enabled, such an   intermediary SHOULD replace each received-by host of any host behind   the firewall by an appropriate pseudonym for that host.   An intermediary MAY combine an ordered subsequence of Via header   field list members into a single member if the entries have identical   received-protocol values.  For example,   Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy   could be collapsed to   Via: 1.0 ricky, 1.1 mertz, 1.0 lucy   A sender SHOULD NOT combine multiple list members unless they are all   under the same organizational control and the hosts have already been   replaced by pseudonyms.  A sender MUST NOT combine members that have   different received-protocol values.7.7.  Message Transformations   Some intermediaries include features for transforming messages and   their content.  A proxy might, for example, convert between image   formats in order to save cache space or to reduce the amount of   traffic on a slow link.  However, operational problems might occur   when these transformations are applied to content intended for   critical applications, such as medical imaging or scientific data   analysis, particularly when integrity checks or digital signatures   are used to ensure that the content received is identical to the   original.   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is   designed or configured to modify messages in a semantically   meaningful way (i.e., modifications, beyond those required by normal   HTTP processing, that change the message in a way that would be   significant to the original sender or potentially significant to   downstream recipients).  For example, a transforming proxy might be   acting as a shared annotation server (modifying responses to include   references to a local annotation database), a malware filter, a   format transcoder, or a privacy filter.  Such transformations are   presumed to be desired by whichever client (or client organization)   chose the proxy.   If a proxy receives a target URI with a host name that is not a fully   qualified domain name, it MAY add its own domain to the host name it   received when forwarding the request.  A proxy MUST NOT change the   host name if the target URI contains a fully qualified domain name.   A proxy MUST NOT modify the "absolute-path" and "query" parts of the   received target URI when forwarding it to the next inbound server   except as required by that forwarding protocol.  For example, a proxy   forwarding a request to an origin server via HTTP/1.1 will replace an   empty path with "/" (Section 3.2.1 of [HTTP/1.1]) or "*"   (Section 3.2.4 of [HTTP/1.1]), depending on the request method.   A proxy MUST NOT transform the content (Section 6.4) of a response   message that contains a no-transform cache directive (Section 5.2.2.6   of [CACHING]).  Note that this does not apply to message   transformations that do not change the content, such as the addition   or removal of transfer codings (Section 7 of [HTTP/1.1]).   A proxy MAY transform the content of a message that does not contain   a no-transform cache directive.  A proxy that transforms the content   of a 200 (OK) response can inform downstream recipients that a   transformation has been applied by changing the response status code   to 203 (Non-Authoritative Information) (Section 15.3.4).   A proxy SHOULD NOT modify header fields that provide information   about the endpoints of the communication chain, the resource state,   or the selected representation (other than the content) unless the   field's definition specifically allows such modification or the   modification is deemed necessary for privacy or security.7.8.  Upgrade   The "Upgrade" header field is intended to provide a simple mechanism   for transitioning from HTTP/1.1 to some other protocol on the same   connection.   A client MAY send a list of protocol names in the Upgrade header   field of a request to invite the server to switch to one or more of   the named protocols, in order of descending preference, before   sending the final response.  A server MAY ignore a received Upgrade   header field if it wishes to continue using the current protocol on   that connection.  Upgrade cannot be used to insist on a protocol   change.     Upgrade          = #protocol     protocol         = protocol-name ["/" protocol-version]     protocol-name    = token     protocol-version = token   Although protocol names are registered with a preferred case,   recipients SHOULD use case-insensitive comparison when matching each   protocol-name to supported protocols.   A server that sends a 101 (Switching Protocols) response MUST send an   Upgrade header field to indicate the new protocol(s) to which the   connection is being switched; if multiple protocol layers are being   switched, the sender MUST list the protocols in layer-ascending   order.  A server MUST NOT switch to a protocol that was not indicated   by the client in the corresponding request's Upgrade header field.  A   server MAY choose to ignore the order of preference indicated by the   client and select the new protocol(s) based on other factors, such as   the nature of the request or the current load on the server.   A server that sends a 426 (Upgrade Required) response MUST send an   Upgrade header field to indicate the acceptable protocols, in order   of descending preference.   A server MAY send an Upgrade header field in any other response to   advertise that it implements support for upgrading to the listed   protocols, in order of descending preference, when appropriate for a   future request.   The following is a hypothetical example sent by a client:   GET /hello HTTP/1.1   Host: www.example.com   Connection: upgrade   Upgrade: websocket, IRC/6.9, RTA/x11   The capabilities and nature of the application-level communication   after the protocol change is entirely dependent upon the new   protocol(s) chosen.  However, immediately after sending the 101   (Switching Protocols) response, the server is expected to continue   responding to the original request as if it had received its   equivalent within the new protocol (i.e., the server still has an   outstanding request to satisfy after the protocol has been changed,   and is expected to do so without requiring the request to be   repeated).   For example, if the Upgrade header field is received in a GET request   and the server decides to switch protocols, it first responds with a   101 (Switching Protocols) message in HTTP/1.1 and then immediately   follows that with the new protocol's equivalent of a response to a   GET on the target resource.  This allows a connection to be upgraded   to protocols with the same semantics as HTTP without the latency cost   of an additional round trip.  A server MUST NOT switch protocols   unless the received message semantics can be honored by the new   protocol; an OPTIONS request can be honored by any protocol.   The following is an example response to the above hypothetical   request:   HTTP/1.1 101 Switching Protocols   Connection: upgrade   Upgrade: websocket   [... data stream switches to websocket with an appropriate response   (as defined by new protocol) to the "GET /hello" request ...]   A sender of Upgrade MUST also send an "Upgrade" connection option in   the Connection header field (Section 7.6.1) to inform intermediaries   not to forward this field.  A server that receives an Upgrade header   field in an HTTP/1.0 request MUST ignore that Upgrade field.   A client cannot begin using an upgraded protocol on the connection   until it has completely sent the request message (i.e., the client   can't change the protocol it is sending in the middle of a message).   If a server receives both an Upgrade and an Expect header field with   the "100-continue" expectation (Section 10.1.1), the server MUST send   a 100 (Continue) response before sending a 101 (Switching Protocols)   response.   The Upgrade header field only applies to switching protocols on top   of the existing connection; it cannot be used to switch the   underlying connection (transport) protocol, nor to switch the   existing communication to a different connection.  For those   purposes, it is more appropriate to use a 3xx (Redirection) response   (Section 15.4).   This specification only defines the protocol name "HTTP" for use by   the family of Hypertext Transfer Protocols, as defined by the HTTP   version rules of Section 2.5 and future updates to this   specification.  Additional protocol names ought to be registered   using the registration procedure defined in Section 16.7.8.  Representation Data and Metadata8.1.  Representation Data   The representation data associated with an HTTP message is either   provided as the content of the message or referred to by the message   semantics and the target URI.  The representation data is in a format   and encoding defined by the representation metadata header fields.   The data type of the representation data is determined via the header   fields Content-Type and Content-Encoding.  These define a two-layer,   ordered encoding model:     representation-data := Content-Encoding( Content-Type( data ) )8.2.  Representation Metadata   Representation header fields provide metadata about the   representation.  When a message includes content, the representation   header fields describe how to interpret that data.  In a response to   a HEAD request, the representation header fields describe the   representation data that would have been enclosed in the content if   the same request had been a GET.8.3.  Content-Type   The "Content-Type" header field indicates the media type of the   associated representation: either the representation enclosed in the   message content or the selected representation, as determined by the   message semantics.  The indicated media type defines both the data   format and how that data is intended to be processed by a recipient,   within the scope of the received message semantics, after any content   codings indicated by Content-Encoding are decoded.     Content-Type = media-type   Media types are defined in Section 8.3.1.  An example of the field is   Content-Type: text/html; charset=ISO-8859-4   A sender that generates a message containing content SHOULD generate   a Content-Type header field in that message unless the intended media   type of the enclosed representation is unknown to the sender.  If a   Content-Type header field is not present, the recipient MAY either   assume a media type of "application/octet-stream" ([RFC2046],   Section 4.5.1) or examine the data to determine its type.   In practice, resource owners do not always properly configure their   origin server to provide the correct Content-Type for a given   representation.  Some user agents examine the content and, in certain   cases, override the received type (for example, see [Sniffing]).   This "MIME sniffing" risks drawing incorrect conclusions about the   data, which might expose the user to additional security risks (e.g.,   "privilege escalation").  Furthermore, distinct media types often   share a common data format, differing only in how the data is   intended to be processed, which is impossible to distinguish by   inspecting the data alone.  When sniffing is implemented,   implementers are encouraged to provide a means for the user to   disable it.   Although Content-Type is defined as a singleton field, it is   sometimes incorrectly generated multiple times, resulting in a   combined field value that appears to be a list.  Recipients often   attempt to handle this error by using the last syntactically valid   member of the list, leading to potential interoperability and   security issues if different implementations have different error   handling behaviors.8.3.1.  Media Type   HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and   Accept (Section 12.5.1) header fields in order to provide open and   extensible data typing and type negotiation.  Media types define both   a data format and various processing models: how to process that data   in accordance with the message context.     media-type = type "/" subtype parameters     type       = token     subtype    = token   The type and subtype tokens are case-insensitive.   The type/subtype MAY be followed by semicolon-delimited parameters   (Section 5.6.6) in the form of name/value pairs.  The presence or   absence of a parameter might be significant to the processing of a   media type, depending on its definition within the media type   registry.  Parameter values might or might not be case-sensitive,   depending on the semantics of the parameter name.   For example, the following media types are equivalent in describing   HTML text data encoded in the UTF-8 character encoding scheme, but   the first is preferred for consistency (the "charset" parameter value   is defined as being case-insensitive in [RFC2046], Section 4.1.2):     text/html;charset=utf-8     Text/HTML;Charset="utf-8"     text/html; charset="utf-8"     text/html;charset=UTF-8   Media types ought to be registered with IANA according to the   procedures defined in [BCP13].8.3.2.  Charset   HTTP uses "charset" names to indicate or negotiate the character   encoding scheme ([RFC6365], Section 2) of a textual representation.   In the fields defined by this document, charset names appear either   in parameters (Content-Type), or, for Accept-Encoding, in the form of   a plain token.  In both cases, charset names are matched case-   insensitively.   Charset names ought to be registered in the IANA "Character Sets"   registry (<https://www.iana.org/assignments/character-sets>)   according to the procedures defined in Section 2 of [RFC2978].      |  *Note:* In theory, charset names are defined by the "mime-      |  charset" ABNF rule defined in Section 2.3 of [RFC2978] (as      |  corrected in [Err1912]).  That rule allows two characters that      |  are not included in "token" ("{" and "}"), but no charset name      |  registered at the time of this writing includes braces (see      |  [Err5433]).8.3.3.  Multipart Types   MIME provides for a number of "multipart" types -- encapsulations of   one or more representations within a single message body.  All   multipart types share a common syntax, as defined in Section 5.1.1 of   [RFC2046], and include a boundary parameter as part of the media type   value.  The message body is itself a protocol element; a sender MUST   generate only CRLF to represent line breaks between body parts.   HTTP message framing does not use the multipart boundary as an   indicator of message body length, though it might be used by   implementations that generate or process the content.  For example,   the "multipart/form-data" type is often used for carrying form data   in a request, as described in [RFC7578], and the "multipart/   byteranges" type is defined by this specification for use in some 206   (Partial Content) responses (see Section 15.3.7).8.4.  Content-Encoding   The "Content-Encoding" header field indicates what content codings   have been applied to the representation, beyond those inherent in the   media type, and thus what decoding mechanisms have to be applied in   order to obtain data in the media type referenced by the Content-Type   header field.  Content-Encoding is primarily used to allow a   representation's data to be compressed without losing the identity of   its underlying media type.     Content-Encoding = #content-coding   An example of its use is   Content-Encoding: gzip   If one or more encodings have been applied to a representation, the   sender that applied the encodings MUST generate a Content-Encoding   header field that lists the content codings in the order in which   they were applied.  Note that the coding named "identity" is reserved   for its special role in Accept-Encoding and thus SHOULD NOT be   included.   Additional information about the encoding parameters can be provided   by other header fields not defined by this specification.   Unlike Transfer-Encoding (Section 6.1 of [HTTP/1.1]), the codings   listed in Content-Encoding are a characteristic of the   representation; the representation is defined in terms of the coded   form, and all other metadata about the representation is about the   coded form unless otherwise noted in the metadata definition.   Typically, the representation is only decoded just prior to rendering   or analogous usage.   If the media type includes an inherent encoding, such as a data   format that is always compressed, then that encoding would not be   restated in Content-Encoding even if it happens to be the same   algorithm as one of the content codings.  Such a content coding would   only be listed if, for some bizarre reason, it is applied a second   time to form the representation.  Likewise, an origin server might   choose to publish the same data as multiple representations that   differ only in whether the coding is defined as part of Content-Type   or Content-Encoding, since some user agents will behave differently   in their handling of each response (e.g., open a "Save as ..." dialog   instead of automatic decompression and rendering of content).   An origin server MAY respond with a status code of 415 (Unsupported   Media Type) if a representation in the request message has a content   coding that is not acceptable.8.4.1.  Content Codings   Content coding values indicate an encoding transformation that has   been or can be applied to a representation.  Content codings are   primarily used to allow a representation to be compressed or   otherwise usefully transformed without losing the identity of its   underlying media type and without loss of information.  Frequently,   the representation is stored in coded form, transmitted directly, and   only decoded by the final recipient.     content-coding   = token   All content codings are case-insensitive and ought to be registered   within the "HTTP Content Coding Registry", as described in   Section 16.6   Content-coding values are used in the Accept-Encoding   (Section 12.5.3) and Content-Encoding (Section 8.4) header fields.8.4.1.1.  Compress Coding   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding   [Welch] that is commonly produced by the UNIX file compression   program "compress".  A recipient SHOULD consider "x-compress" to be   equivalent to "compress".8.4.1.2.  Deflate Coding   The "deflate" coding is a "zlib" data format [RFC1950] containing a   "deflate" compressed data stream [RFC1951] that uses a combination of   the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.      |  *Note:* Some non-conformant implementations send the "deflate"      |  compressed data without the zlib wrapper.8.4.1.3.  Gzip Coding   The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy   Check (CRC) that is commonly produced by the gzip file compression   program [RFC1952].  A recipient SHOULD consider "x-gzip" to be   equivalent to "gzip".8.5.  Content-Language   The "Content-Language" header field describes the natural language(s)   of the intended audience for the representation.  Note that this   might not be equivalent to all the languages used within the   representation.     Content-Language = #language-tag   Language tags are defined in Section 8.5.1.  The primary purpose of   Content-Language is to allow a user to identify and differentiate   representations according to the users' own preferred language.   Thus, if the content is intended only for a Danish-literate audience,   the appropriate field is   Content-Language: da   If no Content-Language is specified, the default is that the content   is intended for all language audiences.  This might mean that the   sender does not consider it to be specific to any natural language,   or that the sender does not know for which language it is intended.   Multiple languages MAY be listed for content that is intended for   multiple audiences.  For example, a rendition of the "Treaty of   Waitangi", presented simultaneously in the original Maori and English   versions, would call for   Content-Language: mi, en   However, just because multiple languages are present within a   representation does not mean that it is intended for multiple   linguistic audiences.  An example would be a beginner's language   primer, such as "A First Lesson in Latin", which is clearly intended   to be used by an English-literate audience.  In this case, the   Content-Language would properly only include "en".   Content-Language MAY be applied to any media type -- it is not   limited to textual documents.8.5.1.  Language Tags   A language tag, as defined in [RFC5646], identifies a natural   language spoken, written, or otherwise conveyed by human beings for   communication of information to other human beings.  Computer   languages are explicitly excluded.   HTTP uses language tags within the Accept-Language and   Content-Language header fields.  Accept-Language uses the broader   language-range production defined in Section 12.5.4, whereas   Content-Language uses the language-tag production defined below.     language-tag = <Language-Tag, see [RFC5646], Section 2.1>   A language tag is a sequence of one or more case-insensitive subtags,   each separated by a hyphen character ("-", %x2D).  In most cases, a   language tag consists of a primary language subtag that identifies a   broad family of related languages (e.g., "en" = English), which is   optionally followed by a series of subtags that refine or narrow that   language's range (e.g., "en-CA" = the variety of English as   communicated in Canada).  Whitespace is not allowed within a language   tag.  Example tags include:     fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN   See [RFC5646] for further information.8.6.  Content-Length   The "Content-Length" header field indicates the associated   representation's data length as a decimal non-negative integer number   of octets.  When transferring a representation as content, Content-   Length refers specifically to the amount of data enclosed so that it   can be used to delimit framing (e.g., Section 6.2 of [HTTP/1.1]).  In   other cases, Content-Length indicates the selected representation's   current length, which can be used by recipients to estimate transfer   time or to compare with previously stored representations.     Content-Length = 1*DIGIT   An example is   Content-Length: 3495   A user agent SHOULD send Content-Length in a request when the method   defines a meaning for enclosed content and it is not sending   Transfer-Encoding.  For example, a user agent normally sends Content-   Length in a POST request even when the value is 0 (indicating empty   content).  A user agent SHOULD NOT send a Content-Length header field   when the request message does not contain content and the method   semantics do not anticipate such data.   A server MAY send a Content-Length header field in a response to a   HEAD request (Section 9.3.2); a server MUST NOT send Content-Length   in such a response unless its field value equals the decimal number   of octets that would have been sent in the content of a response if   the same request had used the GET method.   A server MAY send a Content-Length header field in a 304 (Not   Modified) response to a conditional GET request (Section 15.4.5); a   server MUST NOT send Content-Length in such a response unless its   field value equals the decimal number of octets that would have been   sent in the content of a 200 (OK) response to the same request.   A server MUST NOT send a Content-Length header field in any response   with a status code of 1xx (Informational) or 204 (No Content).  A   server MUST NOT send a Content-Length header field in any 2xx   (Successful) response to a CONNECT request (Section 9.3.6).   Aside from the cases defined above, in the absence of Transfer-   Encoding, an origin server SHOULD send a Content-Length header field   when the content size is known prior to sending the complete header   section.  This will allow downstream recipients to measure transfer   progress, know when a received message is complete, and potentially   reuse the connection for additional requests.   Any Content-Length field value greater than or equal to zero is   valid.  Since there is no predefined limit to the length of content,   a recipient MUST anticipate potentially large decimal numerals and   prevent parsing errors due to integer conversion overflows or   precision loss due to integer conversion (Section 17.5).   Because Content-Length is used for message delimitation in HTTP/1.1,   its field value can impact how the message is parsed by downstream   recipients even when the immediate connection is not using HTTP/1.1.   If the message is forwarded by a downstream intermediary, a Content-   Length field value that is inconsistent with the received message   framing might cause a security failure due to request smuggling or   response splitting.   As a result, a sender MUST NOT forward a message with a Content-   Length header field value that is known to be incorrect.   Likewise, a sender MUST NOT forward a message with a Content-Length   header field value that does not match the ABNF above, with one   exception: a recipient of a Content-Length header field value   consisting of the same decimal value repeated as a comma-separated   list (e.g, "Content-Length: 42, 42") MAY either reject the message as   invalid or replace that invalid field value with a single instance of   the decimal value, since this likely indicates that a duplicate was   generated or combined by an upstream message processor.8.7.  Content-Location   The "Content-Location" header field references a URI that can be used   as an identifier for a specific resource corresponding to the   representation in this message's content.  In other words, if one   were to perform a GET request on this URI at the time of this   message's generation, then a 200 (OK) response would contain the same   representation that is enclosed as content in this message.     Content-Location = absolute-URI / partial-URI   The field value is either an absolute-URI or a partial-URI.  In the   latter case (Section 4), the referenced URI is relative to the target   URI ([URI], Section 5).   The Content-Location value is not a replacement for the target URI   (Section 7.1).  It is representation metadata.  It has the same   syntax and semantics as the header field of the same name defined for   MIME body parts in Section 4 of [RFC2557].  However, its appearance   in an HTTP message has some special implications for HTTP recipients.   If Content-Location is included in a 2xx (Successful) response   message and its value refers (after conversion to absolute form) to a   URI that is the same as the target URI, then the recipient MAY   consider the content to be a current representation of that resource   at the time indicated by the message origination date.  For a GET   (Section 9.3.1) or HEAD (Section 9.3.2) request, this is the same as   the default semantics when no Content-Location is provided by the   server.  For a state-changing request like PUT (Section 9.3.4) or   POST (Section 9.3.3), it implies that the server's response contains   the new representation of that resource, thereby distinguishing it   from representations that might only report about the action (e.g.,   "It worked!").  This allows authoring applications to update their   local copies without the need for a subsequent GET request.   If Content-Location is included in a 2xx (Successful) response   message and its field value refers to a URI that differs from the   target URI, then the origin server claims that the URI is an   identifier for a different resource corresponding to the enclosed   representation.  Such a claim can only be trusted if both identifiers   share the same resource owner, which cannot be programmatically   determined via HTTP.   *  For a response to a GET or HEAD request, this is an indication      that the target URI refers to a resource that is subject to      content negotiation and the Content-Location field value is a more      specific identifier for the selected representation.   *  For a 201 (Created) response to a state-changing method, a      Content-Location field value that is identical to the Location      field value indicates that this content is a current      representation of the newly created resource.   *  Otherwise, such a Content-Location indicates that this content is      a representation reporting on the requested action's status and      that the same report is available (for future access with GET) at      the given URI.  For example, a purchase transaction made via a      POST request might include a receipt document as the content of      the 200 (OK) response; the Content-Location field value provides      an identifier for retrieving a copy of that same receipt in the      future.   A user agent that sends Content-Location in a request message is   stating that its value refers to where the user agent originally   obtained the content of the enclosed representation (prior to any   modifications made by that user agent).  In other words, the user   agent is providing a back link to the source of the original   representation.   An origin server that receives a Content-Location field in a request   message MUST treat the information as transitory request context   rather than as metadata to be saved verbatim as part of the   representation.  An origin server MAY use that context to guide in   processing the request or to save it for other uses, such as within   source links or versioning metadata.  However, an origin server MUST   NOT use such context information to alter the request semantics.   For example, if a client makes a PUT request on a negotiated resource   and the origin server accepts that PUT (without redirection), then   the new state of that resource is expected to be consistent with the   one representation supplied in that PUT; the Content-Location cannot   be used as a form of reverse content selection identifier to update   only one of the negotiated representations.  If the user agent had   wanted the latter semantics, it would have applied the PUT directly   to the Content-Location URI.8.8.  Validator Fields   Resource metadata is referred to as a "validator" if it can be used   within a precondition (Section 13.1) to make a conditional request   (Section 13).  Validator fields convey a current validator for the   selected representation (Section 3.2).   In responses to safe requests, validator fields describe the selected   representation chosen by the origin server while handling the   response.  Note that, depending on the method and status code   semantics, the selected representation for a given response is not   necessarily the same as the representation enclosed as response   content.   In a successful response to a state-changing request, validator   fields describe the new representation that has replaced the prior   selected representation as a result of processing the request.   For example, an ETag field in a 201 (Created) response communicates   the entity tag of the newly created resource's representation, so   that the entity tag can be used as a validator in later conditional   requests to prevent the "lost update" problem.   This specification defines two forms of metadata that are commonly   used to observe resource state and test for preconditions:   modification dates (Section 8.8.2) and opaque entity tags   (Section 8.8.3).  Additional metadata that reflects resource state   has been defined by various extensions of HTTP, such as Web   Distributed Authoring and Versioning [WEBDAV], that are beyond the   scope of this specification.8.8.1.  Weak versus Strong   Validators come in two flavors: strong or weak.  Weak validators are   easy to generate but are far less useful for comparisons.  Strong   validators are ideal for comparisons but can be very difficult (and   occasionally impossible) to generate efficiently.  Rather than impose   that all forms of resource adhere to the same strength of validator,   HTTP exposes the type of validator in use and imposes restrictions on   when weak validators can be used as preconditions.   A "strong validator" is representation metadata that changes value   whenever a change occurs to the representation data that would be   observable in the content of a 200 (OK) response to GET.   A strong validator might change for reasons other than a change to   the representation data, such as when a semantically significant part   of the representation metadata is changed (e.g., Content-Type), but   it is in the best interests of the origin server to only change the   value when it is necessary to invalidate the stored responses held by   remote caches and authoring tools.   Cache entries might persist for arbitrarily long periods, regardless   of expiration times.  Thus, a cache might attempt to validate an   entry using a validator that it obtained in the distant past.  A   strong validator is unique across all versions of all representations   associated with a particular resource over time.  However, there is   no implication of uniqueness across representations of different   resources (i.e., the same strong validator might be in use for   representations of multiple resources at the same time and does not   imply that those representations are equivalent).   There are a variety of strong validators used in practice.  The best   are based on strict revision control, wherein each change to a   representation always results in a unique node name and revision   identifier being assigned before the representation is made   accessible to GET.  A collision-resistant hash function applied to   the representation data is also sufficient if the data is available   prior to the response header fields being sent and the digest does   not need to be recalculated every time a validation request is   received.  However, if a resource has distinct representations that   differ only in their metadata, such as might occur with content   negotiation over media types that happen to share the same data   format, then the origin server needs to incorporate additional   information in the validator to distinguish those representations.   In contrast, a "weak validator" is representation metadata that might   not change for every change to the representation data.  This   weakness might be due to limitations in how the value is calculated   (e.g., clock resolution), an inability to ensure uniqueness for all   possible representations of the resource, or a desire of the resource   owner to group representations by some self-determined set of   equivalency rather than unique sequences of data.   An origin server SHOULD change a weak entity tag whenever it   considers prior representations to be unacceptable as a substitute   for the current representation.  In other words, a weak entity tag   ought to change whenever the origin server wants caches to invalidate   old responses.   For example, the representation of a weather report that changes in   content every second, based on dynamic measurements, might be grouped   into sets of equivalent representations (from the origin server's   perspective) with the same weak validator in order to allow cached   representations to be valid for a reasonable period of time (perhaps   adjusted dynamically based on server load or weather quality).   Likewise, a representation's modification time, if defined with only   one-second resolution, might be a weak validator if it is possible   for the representation to be modified twice during a single second   and retrieved between those modifications.   Likewise, a validator is weak if it is shared by two or more   representations of a given resource at the same time, unless those   representations have identical representation data.  For example, if   the origin server sends the same validator for a representation with   a gzip content coding applied as it does for a representation with no   content coding, then that validator is weak.  However, two   simultaneous representations might share the same strong validator if   they differ only in the representation metadata, such as when two   different media types are available for the same representation data.   Strong validators are usable for all conditional requests, including   cache validation, partial content ranges, and "lost update"   avoidance.  Weak validators are only usable when the client does not   require exact equality with previously obtained representation data,   such as when validating a cache entry or limiting a web traversal to   recent changes.8.8.2.  Last-Modified   The "Last-Modified" header field in a response provides a timestamp   indicating the date and time at which the origin server believes the   selected representation was last modified, as determined at the   conclusion of handling the request.     Last-Modified = HTTP-date   An example of its use is   Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT8.8.2.1.  Generation   An origin server SHOULD send Last-Modified for any selected   representation for which a last modification date can be reasonably   and consistently determined, since its use in conditional requests   and evaluating cache freshness ([CACHING]) can substantially reduce   unnecessary transfers and significantly improve service availability   and scalability.   A representation is typically the sum of many parts behind the   resource interface.  The last-modified time would usually be the most   recent time that any of those parts were changed.  How that value is   determined for any given resource is an implementation detail beyond   the scope of this specification.   An origin server SHOULD obtain the Last-Modified value of the   representation as close as possible to the time that it generates the   Date field value for its response.  This allows a recipient to make   an accurate assessment of the representation's modification time,   especially if the representation changes near the time that the   response is generated.   An origin server with a clock (as defined in Section 5.6.7) MUST NOT   generate a Last-Modified date that is later than the server's time of   message origination (Date, Section 6.6.1).  If the last modification   time is derived from implementation-specific metadata that evaluates   to some time in the future, according to the origin server's clock,   then the origin server MUST replace that value with the message   origination date.  This prevents a future modification date from   having an adverse impact on cache validation.   An origin server without a clock MUST NOT generate a Last-Modified   date for a response unless that date value was assigned to the   resource by some other system (presumably one with a clock).8.8.2.2.  Comparison   A Last-Modified time, when used as a validator in a request, is   implicitly weak unless it is possible to deduce that it is strong,   using the following rules:   *  The validator is being compared by an origin server to the actual      current validator for the representation and,   *  That origin server reliably knows that the associated      representation did not change twice during the second covered by      the presented validator;   or   *  The validator is about to be used by a client in an      If-Modified-Since, If-Unmodified-Since, or If-Range header field,      because the client has a cache entry for the associated      representation, and   *  That cache entry includes a Date value which is at least one      second after the Last-Modified value and the client has reason to      believe that they were generated by the same clock or that there      is enough difference between the Last-Modified and Date values to      make clock synchronization issues unlikely;   or   *  The validator is being compared by an intermediate cache to the      validator stored in its cache entry for the representation, and   *  That cache entry includes a Date value which is at least one      second after the Last-Modified value and the cache has reason to      believe that they were generated by the same clock or that there      is enough difference between the Last-Modified and Date values to      make clock synchronization issues unlikely.   This method relies on the fact that if two different responses were   sent by the origin server during the same second, but both had the   same Last-Modified time, then at least one of those responses would   have a Date value equal to its Last-Modified time.8.8.3.  ETag   The "ETag" field in a response provides the current entity tag for   the selected representation, as determined at the conclusion of   handling the request.  An entity tag is an opaque validator for   differentiating between multiple representations of the same   resource, regardless of whether those multiple representations are   due to resource state changes over time, content negotiation   resulting in multiple representations being valid at the same time,   or both.  An entity tag consists of an opaque quoted string, possibly   prefixed by a weakness indicator.     ETag       = entity-tag     entity-tag = [ weak ] opaque-tag     weak       = %s"W/"     opaque-tag = DQUOTE *etagc DQUOTE     etagc      = %x21 / %x23-7E / obs-text                ; VCHAR except double quotes, plus obs-text      |  *Note:* Previously, opaque-tag was defined to be a quoted-      |  string ([RFC2616], Section 3.11); thus, some recipients might      |  perform backslash unescaping.  Servers therefore ought to avoid      |  backslash characters in entity tags.   An entity tag can be more reliable for validation than a modification   date in situations where it is inconvenient to store modification   dates, where the one-second resolution of HTTP-date values is not   sufficient, or where modification dates are not consistently   maintained.   Examples:   ETag: "xyzzy"   ETag: W/"xyzzy"   ETag: ""   An entity tag can be either a weak or strong validator, with strong   being the default.  If an origin server provides an entity tag for a   representation and the generation of that entity tag does not satisfy   all of the characteristics of a strong validator (Section 8.8.1),   then the origin server MUST mark the entity tag as weak by prefixing   its opaque value with "W/" (case-sensitive).   A sender MAY send the ETag field in a trailer section (see   Section 6.5).  However, since trailers are often ignored, it is   preferable to send ETag as a header field unless the entity tag is   generated while sending the content.8.8.3.1.  Generation   The principle behind entity tags is that only the service author   knows the implementation of a resource well enough to select the most   accurate and efficient validation mechanism for that resource, and   that any such mechanism can be mapped to a simple sequence of octets   for easy comparison.  Since the value is opaque, there is no need for   the client to be aware of how each entity tag is constructed.   For example, a resource that has implementation-specific versioning   applied to all changes might use an internal revision number, perhaps   combined with a variance identifier for content negotiation, to   accurately differentiate between representations.  Other   implementations might use a collision-resistant hash of   representation content, a combination of various file attributes, or   a modification timestamp that has sub-second resolution.   An origin server SHOULD send an ETag for any selected representation   for which detection of changes can be reasonably and consistently   determined, since the entity tag's use in conditional requests and   evaluating cache freshness ([CACHING]) can substantially reduce   unnecessary transfers and significantly improve service availability,   scalability, and reliability.8.8.3.2.  Comparison   There are two entity tag comparison functions, depending on whether   or not the comparison context allows the use of weak validators:   "Strong comparison":  two entity tags are equivalent if both are not      weak and their opaque-tags match character-by-character.   "Weak comparison":  two entity tags are equivalent if their opaque-      tags match character-by-character, regardless of either or both      being tagged as "weak".   The example below shows the results for a set of entity tag pairs and   both the weak and strong comparison function results:   +========+========+===================+=================+   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |   +========+========+===================+=================+   | W/"1"  | W/"1"  | no match          | match           |   +--------+--------+-------------------+-----------------+   | W/"1"  | W/"2"  | no match          | no match        |   +--------+--------+-------------------+-----------------+   | W/"1"  | "1"    | no match          | match           |   +--------+--------+-------------------+-----------------+   | "1"    | "1"    | match             | match           |   +--------+--------+-------------------+-----------------+                            Table 38.8.3.3.  Example: Entity Tags Varying on Content-Negotiated Resources   Consider a resource that is subject to content negotiation   (Section 12), and where the representations sent in response to a GET   request vary based on the Accept-Encoding request header field   (Section 12.5.3):   >> Request:   GET /index HTTP/1.1   Host: www.example.com   Accept-Encoding: gzip   In this case, the response might or might not use the gzip content   coding.  If it does not, the response might look like:   >> Response:   HTTP/1.1 200 OK   Date: Fri, 26 Mar 2010 00:05:00 GMT   ETag: "123-a"   Content-Length: 70   Vary: Accept-Encoding   Content-Type: text/plain   Hello World!   Hello World!   Hello World!   Hello World!   Hello World!   An alternative representation that does use gzip content coding would   be:   >> Response:   HTTP/1.1 200 OK   Date: Fri, 26 Mar 2010 00:05:00 GMT   ETag: "123-b"   Content-Length: 43   Vary: Accept-Encoding   Content-Type: text/plain   Content-Encoding: gzip   ...binary data...      |  *Note:* Content codings are a property of the representation      |  data, so a strong entity tag for a content-encoded      |  representation has to be distinct from the entity tag of an      |  unencoded representation to prevent potential conflicts during      |  cache updates and range requests.  In contrast, transfer      |  codings (Section 7 of [HTTP/1.1]) apply only during message      |  transfer and do not result in distinct entity tags.9.  Methods9.1.  Overview   The request method token is the primary source of request semantics;   it indicates the purpose for which the client has made this request   and what is expected by the client as a successful result.   The request method's semantics might be further specialized by the   semantics of some header fields when present in a request if those   additional semantics do not conflict with the method.  For example, a   client can send conditional request header fields (Section 13.1) to   make the requested action conditional on the current state of the   target resource.   HTTP is designed to be usable as an interface to distributed object   systems.  The request method invokes an action to be applied to a   target resource in much the same way that a remote method invocation   can be sent to an identified object.     method = token   The method token is case-sensitive because it might be used as a   gateway to object-based systems with case-sensitive method names.  By   convention, standardized methods are defined in all-uppercase US-   ASCII letters.   Unlike distributed objects, the standardized request methods in HTTP   are not resource-specific, since uniform interfaces provide for   better visibility and reuse in network-based systems [REST].  Once   defined, a standardized method ought to have the same semantics when   applied to any resource, though each resource determines for itself   whether those semantics are implemented or allowed.   This specification defines a number of standardized methods that are   commonly used in HTTP, as outlined by the following table.   +=========+============================================+=========+   | Method  | Description                                | Section |   | Name    |                                            |         |   +=========+============================================+=========+   | GET     | Transfer a current representation of the   | 9.3.1   |   |         | target resource.                           |         |   +---------+--------------------------------------------+---------+   | HEAD    | Same as GET, but do not transfer the       | 9.3.2   |   |         | response content.                          |         |   +---------+--------------------------------------------+---------+   | POST    | Perform resource-specific processing on    | 9.3.3   |   |         | the request content.                       |         |   +---------+--------------------------------------------+---------+   | PUT     | Replace all current representations of the | 9.3.4   |   |         | target resource with the request content.  |         |   +---------+--------------------------------------------+---------+   | DELETE  | Remove all current representations of the  | 9.3.5   |   |         | target resource.                           |         |   +---------+--------------------------------------------+---------+   | CONNECT | Establish a tunnel to the server           | 9.3.6   |   |         | identified by the target resource.         |         |   +---------+--------------------------------------------+---------+   | OPTIONS | Describe the communication options for the | 9.3.7   |   |         | target resource.                           |         |   +---------+--------------------------------------------+---------+   | TRACE   | Perform a message loop-back test along the | 9.3.8   |   |         | path to the target resource.               |         |   +---------+--------------------------------------------+---------+                                Table 4   All general-purpose servers MUST support the methods GET and HEAD.   All other methods are OPTIONAL.   The set of methods allowed by a target resource can be listed in an   Allow header field (Section 10.2.1).  However, the set of allowed   methods can change dynamically.  An origin server that receives a   request method that is unrecognized or not implemented SHOULD respond   with the 501 (Not Implemented) status code.  An origin server that   receives a request method that is recognized and implemented, but not   allowed for the target resource, SHOULD respond with the 405 (Method   Not Allowed) status code.   Additional methods, outside the scope of this specification, have   been specified for use in HTTP.  All such methods ought to be   registered within the "Hypertext Transfer Protocol (HTTP) Method   Registry", as described in Section 16.1.9.2.  Common Method Properties9.2.1.  Safe Methods   Request methods are considered "safe" if their defined semantics are   essentially read-only; i.e., the client does not request, and does   not expect, any state change on the origin server as a result of   applying a safe method to a target resource.  Likewise, reasonable   use of a safe method is not expected to cause any harm, loss of   property, or unusual burden on the origin server.   This definition of safe methods does not prevent an implementation   from including behavior that is potentially harmful, that is not   entirely read-only, or that causes side effects while invoking a safe   method.  What is important, however, is that the client did not   request that additional behavior and cannot be held accountable for   it.  For example, most servers append request information to access   log files at the completion of every response, regardless of the   method, and that is considered safe even though the log storage might   become full and cause the server to fail.  Likewise, a safe request   initiated by selecting an advertisement on the Web will often have   the side effect of charging an advertising account.   Of the request methods defined by this specification, the GET, HEAD,   OPTIONS, and TRACE methods are defined to be safe.   The purpose of distinguishing between safe and unsafe methods is to   allow automated retrieval processes (spiders) and cache performance   optimization (pre-fetching) to work without fear of causing harm.  In   addition, it allows a user agent to apply appropriate constraints on   the automated use of unsafe methods when processing potentially   untrusted content.   A user agent SHOULD distinguish between safe and unsafe methods when   presenting potential actions to a user, such that the user can be   made aware of an unsafe action before it is requested.   When a resource is constructed such that parameters within the target   URI have the effect of selecting an action, it is the resource   owner's responsibility to ensure that the action is consistent with   the request method semantics.  For example, it is common for Web-   based content editing software to use actions within query   parameters, such as "page?do=delete".  If the purpose of such a   resource is to perform an unsafe action, then the resource owner MUST   disable or disallow that action when it is accessed using a safe   request method.  Failure to do so will result in unfortunate side   effects when automated processes perform a GET on every URI reference   for the sake of link maintenance, pre-fetching, building a search   index, etc.9.2.2.  Idempotent Methods   A request method is considered "idempotent" if the intended effect on   the server of multiple identical requests with that method is the   same as the effect for a single such request.  Of the request methods   defined by this specification, PUT, DELETE, and safe request methods   are idempotent.   Like the definition of safe, the idempotent property only applies to   what has been requested by the user; a server is free to log each   request separately, retain a revision control history, or implement   other non-idempotent side effects for each idempotent request.   Idempotent methods are distinguished because the request can be   repeated automatically if a communication failure occurs before the   client is able to read the server's response.  For example, if a   client sends a PUT request and the underlying connection is closed   before any response is received, then the client can establish a new   connection and retry the idempotent request.  It knows that repeating   the request will have the same intended effect, even if the original   request succeeded, though the response might differ.   A client SHOULD NOT automatically retry a request with a non-   idempotent method unless it has some means to know that the request   semantics are actually idempotent, regardless of the method, or some   means to detect that the original request was never applied.   For example, a user agent can repeat a POST request automatically if   it knows (through design or configuration) that the request is safe   for that resource.  Likewise, a user agent designed specifically to   operate on a version control repository might be able to recover from   partial failure conditions by checking the target resource   revision(s) after a failed connection, reverting or fixing any   changes that were partially applied, and then automatically retrying   the requests that failed.   Some clients take a riskier approach and attempt to guess when an   automatic retry is possible.  For example, a client might   automatically retry a POST request if the underlying transport   connection closed before any part of a response is received,   particularly if an idle persistent connection was used.   A proxy MUST NOT automatically retry non-idempotent requests.  A   client SHOULD NOT automatically retry a failed automatic retry.9.2.3.  Methods and Caching   For a cache to store and use a response, the associated method needs   to explicitly allow caching and to detail under what conditions a   response can be used to satisfy subsequent requests; a method   definition that does not do so cannot be cached.  For additional   requirements see [CACHING].   This specification defines caching semantics for GET, HEAD, and POST,   although the overwhelming majority of cache implementations only   support GET and HEAD.9.3.  Method Definitions9.3.1.  GET   The GET method requests transfer of a current selected representation   for the target resource.  A successful response reflects the quality   of "sameness" identified by the target URI (Section 1.2.2 of [URI]).   Hence, retrieving identifiable information via HTTP is usually   performed by making a GET request on an identifier associated with   the potential for providing that information in a 200 (OK) response.   GET is the primary mechanism of information retrieval and the focus   of almost all performance optimizations.  Applications that produce a   URI for each important resource can benefit from those optimizations   while enabling their reuse by other applications, creating a network   effect that promotes further expansion of the Web.   It is tempting to think of resource identifiers as remote file system   pathnames and of representations as being a copy of the contents of   such files.  In fact, that is how many resources are implemented (see   Section 17.3 for related security considerations).  However, there   are no such limitations in practice.   The HTTP interface for a resource is just as likely to be implemented   as a tree of content objects, a programmatic view on various database   records, or a gateway to other information systems.  Even when the   URI mapping mechanism is tied to a file system, an origin server   might be configured to execute the files with the request as input   and send the output as the representation rather than transfer the   files directly.  Regardless, only the origin server needs to know how   each resource identifier corresponds to an implementation and how   that implementation manages to select and send a current   representation of the target resource.   A client can alter the semantics of GET to be a "range request",   requesting transfer of only some part(s) of the selected   representation, by sending a Range header field in the request   (Section 14.2).   Although request message framing is independent of the method used,   content received in a GET request has no generally defined semantics,   cannot alter the meaning or target of the request, and might lead   some implementations to reject the request and close the connection   because of its potential as a request smuggling attack (Section 11.2   of [HTTP/1.1]).  A client SHOULD NOT generate content in a GET   request unless it is made directly to an origin server that has   previously indicated, in or out of band, that such a request has a   purpose and will be adequately supported.  An origin server SHOULD   NOT rely on private agreements to receive content, since participants   in HTTP communication are often unaware of intermediaries along the   request chain.   The response to a GET request is cacheable; a cache MAY use it to   satisfy subsequent GET and HEAD requests unless otherwise indicated   by the Cache-Control header field (Section 5.2 of [CACHING]).   When information retrieval is performed with a mechanism that   constructs a target URI from user-provided information, such as the   query fields of a form using GET, potentially sensitive data might be   provided that would not be appropriate for disclosure within a URI   (see Section 17.9).  In some cases, the data can be filtered or   transformed such that it would not reveal such information.  In   others, particularly when there is no benefit from caching a   response, using the POST method (Section 9.3.3) instead of GET can   transmit such information in the request content rather than within   the target URI.9.3.2.  HEAD   The HEAD method is identical to GET except that the server MUST NOT   send content in the response.  HEAD is used to obtain metadata about   the selected representation without transferring its representation   data, often for the sake of testing hypertext links or finding recent   modifications.   The server SHOULD send the same header fields in response to a HEAD   request as it would have sent if the request method had been GET.   However, a server MAY omit header fields for which a value is   determined only while generating the content.  For example, some   servers buffer a dynamic response to GET until a minimum amount of   data is generated so that they can more efficiently delimit small   responses or make late decisions with regard to content selection.   Such a response to GET might contain Content-Length and Vary fields,   for example, that are not generated within a HEAD response.  These   minor inconsistencies are considered preferable to generating and   discarding the content for a HEAD request, since HEAD is usually   requested for the sake of efficiency.   Although request message framing is independent of the method used,   content received in a HEAD request has no generally defined   semantics, cannot alter the meaning or target of the request, and   might lead some implementations to reject the request and close the   connection because of its potential as a request smuggling attack   (Section 11.2 of [HTTP/1.1]).  A client SHOULD NOT generate content   in a HEAD request unless it is made directly to an origin server that   has previously indicated, in or out of band, that such a request has   a purpose and will be adequately supported.  An origin server SHOULD   NOT rely on private agreements to receive content, since participants   in HTTP communication are often unaware of intermediaries along the   request chain.   The response to a HEAD request is cacheable; a cache MAY use it to   satisfy subsequent HEAD requests unless otherwise indicated by the   Cache-Control header field (Section 5.2 of [CACHING]).  A HEAD   response might also affect previously cached responses to GET; see   Section 4.3.5 of [CACHING].9.3.3.  POST   The POST method requests that the target resource process the   representation enclosed in the request according to the resource's   own specific semantics.  For example, POST is used for the following   functions (among others):   *  Providing a block of data, such as the fields entered into an HTML      form, to a data-handling process;   *  Posting a message to a bulletin board, newsgroup, mailing list,      blog, or similar group of articles;   *  Creating a new resource that has yet to be identified by the      origin server; and   *  Appending data to a resource's existing representation(s).   An origin server indicates response semantics by choosing an   appropriate status code depending on the result of processing the   POST request; almost all of the status codes defined by this   specification could be received in a response to POST (the exceptions   being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not   Satisfiable)).   If one or more resources has been created on the origin server as a   result of successfully processing a POST request, the origin server   SHOULD send a 201 (Created) response containing a Location header   field that provides an identifier for the primary resource created   (Section 10.2.2) and a representation that describes the status of   the request while referring to the new resource(s).   Responses to POST requests are only cacheable when they include   explicit freshness information (see Section 4.2.1 of [CACHING]) and a   Content-Location header field that has the same value as the POST's   target URI (Section 8.7).  A cached POST response can be reused to   satisfy a later GET or HEAD request.  In contrast, a POST request   cannot be satisfied by a cached POST response because POST is   potentially unsafe; see Section 4 of [CACHING].   If the result of processing a POST would be equivalent to a   representation of an existing resource, an origin server MAY redirect   the user agent to that resource by sending a 303 (See Other) response   with the existing resource's identifier in the Location field.  This   has the benefits of providing the user agent a resource identifier   and transferring the representation via a method more amenable to   shared caching, though at the cost of an extra request if the user   agent does not already have the representation cached.9.3.4.  PUT   The PUT method requests that the state of the target resource be   created or replaced with the state defined by the representation   enclosed in the request message content.  A successful PUT of a given   representation would suggest that a subsequent GET on that same   target resource will result in an equivalent representation being   sent in a 200 (OK) response.  However, there is no guarantee that   such a state change will be observable, since the target resource   might be acted upon by other user agents in parallel, or might be   subject to dynamic processing by the origin server, before any   subsequent GET is received.  A successful response only implies that   the user agent's intent was achieved at the time of its processing by   the origin server.   If the target resource does not have a current representation and the   PUT successfully creates one, then the origin server MUST inform the   user agent by sending a 201 (Created) response.  If the target   resource does have a current representation and that representation   is successfully modified in accordance with the state of the enclosed   representation, then the origin server MUST send either a 200 (OK) or   a 204 (No Content) response to indicate successful completion of the   request.   An origin server SHOULD verify that the PUT representation is   consistent with its configured constraints for the target resource.   For example, if an origin server determines a resource's   representation metadata based on the URI, then the origin server   needs to ensure that the content received in a successful PUT request   is consistent with that metadata.  When a PUT representation is   inconsistent with the target resource, the origin server SHOULD   either make them consistent, by transforming the representation or   changing the resource configuration, or respond with an appropriate   error message containing sufficient information to explain why the   representation is unsuitable.  The 409 (Conflict) or 415 (Unsupported   Media Type) status codes are suggested, with the latter being   specific to constraints on Content-Type values.   For example, if the target resource is configured to always have a   Content-Type of "text/html" and the representation being PUT has a   Content-Type of "image/jpeg", the origin server ought to do one of:   a.  reconfigure the target resource to reflect the new media type;   b.  transform the PUT representation to a format consistent with that       of the resource before saving it as the new resource state; or,   c.  reject the request with a 415 (Unsupported Media Type) response       indicating that the target resource is limited to "text/html",       perhaps including a link to a different resource that would be a       suitable target for the new representation.   HTTP does not define exactly how a PUT method affects the state of an   origin server beyond what can be expressed by the intent of the user   agent request and the semantics of the origin server response.  It   does not define what a resource might be, in any sense of that word,   beyond the interface provided via HTTP.  It does not define how   resource state is "stored", nor how such storage might change as a   result of a change in resource state, nor how the origin server   translates resource state into representations.  Generally speaking,   all implementation details behind the resource interface are   intentionally hidden by the server.   This extends to how header and trailer fields are stored; while   common header fields like Content-Type will typically be stored and   returned upon subsequent GET requests, header and trailer field   handling is specific to the resource that received the request.  As a   result, an origin server SHOULD ignore unrecognized header and   trailer fields received in a PUT request (i.e., not save them as part   of the resource state).   An origin server MUST NOT send a validator field (Section 8.8), such   as an ETag or Last-Modified field, in a successful response to PUT   unless the request's representation data was saved without any   transformation applied to the content (i.e., the resource's new   representation data is identical to the content received in the PUT   request) and the validator field value reflects the new   representation.  This requirement allows a user agent to know when   the representation it sent (and retains in memory) is the result of   the PUT, and thus it doesn't need to be retrieved again from the   origin server.  The new validator(s) received in the response can be   used for future conditional requests in order to prevent accidental   overwrites (Section 13.1).   The fundamental difference between the POST and PUT methods is   highlighted by the different intent for the enclosed representation.   The target resource in a POST request is intended to handle the   enclosed representation according to the resource's own semantics,   whereas the enclosed representation in a PUT request is defined as   replacing the state of the target resource.  Hence, the intent of PUT   is idempotent and visible to intermediaries, even though the exact   effect is only known by the origin server.   Proper interpretation of a PUT request presumes that the user agent   knows which target resource is desired.  A service that selects a   proper URI on behalf of the client, after receiving a state-changing   request, SHOULD be implemented using the POST method rather than PUT.   If the origin server will not make the requested PUT state change to   the target resource and instead wishes to have it applied to a   different resource, such as when the resource has been moved to a   different URI, then the origin server MUST send an appropriate 3xx   (Redirection) response; the user agent MAY then make its own decision   regarding whether or not to redirect the request.   A PUT request applied to the target resource can have side effects on   other resources.  For example, an article might have a URI for   identifying "the current version" (a resource) that is separate from   the URIs identifying each particular version (different resources   that at one point shared the same state as the current version   resource).  A successful PUT request on "the current version" URI   might therefore create a new version resource in addition to changing   the state of the target resource, and might also cause links to be   added between the related resources.   Some origin servers support use of the Content-Range header field   (Section 14.4) as a request modifier to perform a partial PUT, as   described in Section 14.5.   Responses to the PUT method are not cacheable.  If a successful PUT   request passes through a cache that has one or more stored responses   for the target URI, those stored responses will be invalidated (see   Section 4.4 of [CACHING]).9.3.5.  DELETE   The DELETE method requests that the origin server remove the   association between the target resource and its current   functionality.  In effect, this method is similar to the "rm" command   in UNIX: it expresses a deletion operation on the URI mapping of the   origin server rather than an expectation that the previously   associated information be deleted.   If the target resource has one or more current representations, they   might or might not be destroyed by the origin server, and the   associated storage might or might not be reclaimed, depending   entirely on the nature of the resource and its implementation by the   origin server (which are beyond the scope of this specification).   Likewise, other implementation aspects of a resource might need to be   deactivated or archived as a result of a DELETE, such as database or   gateway connections.  In general, it is assumed that the origin   server will only allow DELETE on resources for which it has a   prescribed mechanism for accomplishing the deletion.   Relatively few resources allow the DELETE method -- its primary use   is for remote authoring environments, where the user has some   direction regarding its effect.  For example, a resource that was   previously created using a PUT request, or identified via the   Location header field after a 201 (Created) response to a POST   request, might allow a corresponding DELETE request to undo those   actions.  Similarly, custom user agent implementations that implement   an authoring function, such as revision control clients using HTTP   for remote operations, might use DELETE based on an assumption that   the server's URI space has been crafted to correspond to a version   repository.   If a DELETE method is successfully applied, the origin server SHOULD   send   *  a 202 (Accepted) status code if the action will likely succeed but      has not yet been enacted,   *  a 204 (No Content) status code if the action has been enacted and      no further information is to be supplied, or   *  a 200 (OK) status code if the action has been enacted and the      response message includes a representation describing the status.   Although request message framing is independent of the method used,   content received in a DELETE request has no generally defined   semantics, cannot alter the meaning or target of the request, and   might lead some implementations to reject the request and close the   connection because of its potential as a request smuggling attack   (Section 11.2 of [HTTP/1.1]).  A client SHOULD NOT generate content   in a DELETE request unless it is made directly to an origin server   that has previously indicated, in or out of band, that such a request   has a purpose and will be adequately supported.  An origin server   SHOULD NOT rely on private agreements to receive content, since   participants in HTTP communication are often unaware of   intermediaries along the request chain.   Responses to the DELETE method are not cacheable.  If a successful   DELETE request passes through a cache that has one or more stored   responses for the target URI, those stored responses will be   invalidated (see Section 4.4 of [CACHING]).9.3.6.  CONNECT   The CONNECT method requests that the recipient establish a tunnel to   the destination origin server identified by the request target and,   if successful, thereafter restrict its behavior to blind forwarding   of data, in both directions, until the tunnel is closed.  Tunnels are   commonly used to create an end-to-end virtual connection, through one   or more proxies, which can then be secured using TLS (Transport Layer   Security, [TLS13]).   CONNECT uses a special form of request target, unique to this method,   consisting of only the host and port number of the tunnel   destination, separated by a colon.  There is no default port; a   client MUST send the port number even if the CONNECT request is based   on a URI reference that contains an authority component with an   elided port (Section 4.1).  For example,   CONNECT server.example.com:80 HTTP/1.1   Host: server.example.com   A server MUST reject a CONNECT request that targets an empty or   invalid port number, typically by responding with a 400 (Bad Request)   status code.   Because CONNECT changes the request/response nature of an HTTP   connection, specific HTTP versions might have different ways of   mapping its semantics into the protocol's wire format.   CONNECT is intended for use in requests to a proxy.  The recipient   can establish a tunnel either by directly connecting to the server   identified by the request target or, if configured to use another   proxy, by forwarding the CONNECT request to the next inbound proxy.   An origin server MAY accept a CONNECT request, but most origin   servers do not implement CONNECT.   Any 2xx (Successful) response indicates that the sender (and all   inbound proxies) will switch to tunnel mode immediately after the   response header section; data received after that header section is   from the server identified by the request target.  Any response other   than a successful response indicates that the tunnel has not yet been   formed.   A tunnel is closed when a tunnel intermediary detects that either   side has closed its connection: the intermediary MUST attempt to send   any outstanding data that came from the closed side to the other   side, close both connections, and then discard any remaining data   left undelivered.   Proxy authentication might be used to establish the authority to   create a tunnel.  For example,   CONNECT server.example.com:443 HTTP/1.1   Host: server.example.com:443   Proxy-Authorization: basic aGVsbG86d29ybGQ=   There are significant risks in establishing a tunnel to arbitrary   servers, particularly when the destination is a well-known or   reserved TCP port that is not intended for Web traffic.  For example,   a CONNECT to "example.com:25" would suggest that the proxy connect to   the reserved port for SMTP traffic; if allowed, that could trick the   proxy into relaying spam email.  Proxies that support CONNECT SHOULD   restrict its use to a limited set of known ports or a configurable   list of safe request targets.   A server MUST NOT send any Transfer-Encoding or Content-Length header   fields in a 2xx (Successful) response to CONNECT.  A client MUST   ignore any Content-Length or Transfer-Encoding header fields received   in a successful response to CONNECT.   A CONNECT request message does not have content.  The interpretation   of data sent after the header section of the CONNECT request message   is specific to the version of HTTP in use.   Responses to the CONNECT method are not cacheable.9.3.7.  OPTIONS   The OPTIONS method requests information about the communication   options available for the target resource, at either the origin   server or an intervening intermediary.  This method allows a client   to determine the options and/or requirements associated with a   resource, or the capabilities of a server, without implying a   resource action.   An OPTIONS request with an asterisk ("*") as the request target   (Section 7.1) applies to the server in general rather than to a   specific resource.  Since a server's communication options typically   depend on the resource, the "*" request is only useful as a "ping" or   "no-op" type of method; it does nothing beyond allowing the client to   test the capabilities of the server.  For example, this can be used   to test a proxy for HTTP/1.1 conformance (or lack thereof).   If the request target is not an asterisk, the OPTIONS request applies   to the options that are available when communicating with the target   resource.   A server generating a successful response to OPTIONS SHOULD send any   header that might indicate optional features implemented by the   server and applicable to the target resource (e.g., Allow), including   potential extensions not defined by this specification.  The response   content, if any, might also describe the communication options in a   machine or human-readable representation.  A standard format for such   a representation is not defined by this specification, but might be   defined by future extensions to HTTP.   A client MAY send a Max-Forwards header field in an OPTIONS request   to target a specific recipient in the request chain (see   Section 7.6.2).  A proxy MUST NOT generate a Max-Forwards header   field while forwarding a request unless that request was received   with a Max-Forwards field.   A client that generates an OPTIONS request containing content MUST   send a valid Content-Type header field describing the representation   media type.  Note that this specification does not define any use for   such content.   Responses to the OPTIONS method are not cacheable.9.3.8.  TRACE   The TRACE method requests a remote, application-level loop-back of   the request message.  The final recipient of the request SHOULD   reflect the message received, excluding some fields described below,   back to the client as the content of a 200 (OK) response.  The   "message/http" format (Section 10.1 of [HTTP/1.1]) is one way to do   so.  The final recipient is either the origin server or the first   server to receive a Max-Forwards value of zero (0) in the request   (Section 7.6.2).   A client MUST NOT generate fields in a TRACE request containing   sensitive data that might be disclosed by the response.  For example,   it would be foolish for a user agent to send stored user credentials   (Section 11) or cookies [COOKIE] in a TRACE request.  The final   recipient of the request SHOULD exclude any request fields that are   likely to contain sensitive data when that recipient generates the   response content.   TRACE allows the client to see what is being received at the other   end of the request chain and use that data for testing or diagnostic   information.  The value of the Via header field (Section 7.6.3) is of   particular interest, since it acts as a trace of the request chain.   Use of the Max-Forwards header field allows the client to limit the   length of the request chain, which is useful for testing a chain of   proxies forwarding messages in an infinite loop.   A client MUST NOT send content in a TRACE request.   Responses to the TRACE method are not cacheable.10.  Message Context10.1.  Request Context Fields   The request header fields below provide additional information about   the request context, including information about the user, user   agent, and resource behind the request.10.1.1.  Expect   The "Expect" header field in a request indicates a certain set of   behaviors (expectations) that need to be supported by the server in   order to properly handle this request.     Expect =      #expectation     expectation = token [ "=" ( token / quoted-string ) parameters ]   The Expect field value is case-insensitive.   The only expectation defined by this specification is "100-continue"   (with no defined parameters).   A server that receives an Expect field value containing a member   other than 100-continue MAY respond with a 417 (Expectation Failed)   status code to indicate that the unexpected expectation cannot be   met.   A "100-continue" expectation informs recipients that the client is   about to send (presumably large) content in this request and wishes   to receive a 100 (Continue) interim response if the method, target   URI, and header fields are not sufficient to cause an immediate   success, redirect, or error response.  This allows the client to wait   for an indication that it is worthwhile to send the content before   actually doing so, which can improve efficiency when the data is huge   or when the client anticipates that an error is likely (e.g., when   sending a state-changing method, for the first time, without   previously verified authentication credentials).   For example, a request that begins with   PUT /somewhere/fun HTTP/1.1   Host: origin.example.com   Content-Type: video/h264   Content-Length: 1234567890987   Expect: 100-continue   allows the origin server to immediately respond with an error   message, such as 401 (Unauthorized) or 405 (Method Not Allowed),   before the client starts filling the pipes with an unnecessary data   transfer.   Requirements for clients:   *  A client MUST NOT generate a 100-continue expectation in a request      that does not include content.   *  A client that will wait for a 100 (Continue) response before      sending the request content MUST send an Expect header field      containing a 100-continue expectation.   *  A client that sends a 100-continue expectation is not required to      wait for any specific length of time; such a client MAY proceed to      send the content even if it has not yet received a response.      Furthermore, since 100 (Continue) responses cannot be sent through      an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an      indefinite period before sending the content.   *  A client that receives a 417 (Expectation Failed) status code in      response to a request containing a 100-continue expectation SHOULD      repeat that request without a 100-continue expectation, since the      417 response merely indicates that the response chain does not      support expectations (e.g., it passes through an HTTP/1.0 server).   Requirements for servers:   *  A server that receives a 100-continue expectation in an HTTP/1.0      request MUST ignore that expectation.   *  A server MAY omit sending a 100 (Continue) response if it has      already received some or all of the content for the corresponding      request, or if the framing indicates that there is no content.   *  A server that sends a 100 (Continue) response MUST ultimately send      a final status code, once it receives and processes the request      content, unless the connection is closed prematurely.   *  A server that responds with a final status code before reading the      entire request content SHOULD indicate whether it intends to close      the connection (e.g., see Section 9.6 of [HTTP/1.1]) or continue      reading the request content.   Upon receiving an HTTP/1.1 (or later) request that has a method,   target URI, and complete header section that contains a 100-continue   expectation and an indication that request content will follow, an   origin server MUST send either:   *  an immediate response with a final status code, if that status can      be determined by examining just the method, target URI, and header      fields, or   *  an immediate 100 (Continue) response to encourage the client to      send the request content.   The origin server MUST NOT wait for the content before sending the   100 (Continue) response.   Upon receiving an HTTP/1.1 (or later) request that has a method,   target URI, and complete header section that contains a 100-continue   expectation and indicates a request content will follow, a proxy MUST   either:   *  send an immediate response with a final status code, if that      status can be determined by examining just the method, target URI,      and header fields, or   *  forward the request toward the origin server by sending a      corresponding request-line and header section to the next inbound      server.   If the proxy believes (from configuration or past interaction) that   the next inbound server only supports HTTP/1.0, the proxy MAY   generate an immediate 100 (Continue) response to encourage the client   to begin sending the content.10.1.2.  From   The "From" header field contains an Internet email address for a   human user who controls the requesting user agent.  The address ought   to be machine-usable, as defined by "mailbox" in Section 3.4 of   [RFC5322]:     From    = mailbox     mailbox = <mailbox, see [RFC5322], Section 3.4>   An example is:   From: spider-admin@example.org   The From header field is rarely sent by non-robotic user agents.  A   user agent SHOULD NOT send a From header field without explicit   configuration by the user, since that might conflict with the user's   privacy interests or their site's security policy.   A robotic user agent SHOULD send a valid From header field so that   the person responsible for running the robot can be contacted if   problems occur on servers, such as if the robot is sending excessive,   unwanted, or invalid requests.   A server SHOULD NOT use the From header field for access control or   authentication, since its value is expected to be visible to anyone   receiving or observing the request and is often recorded within   logfiles and error reports without any expectation of privacy.10.1.3.  Referer   The "Referer" [sic] header field allows the user agent to specify a   URI reference for the resource from which the target URI was obtained   (i.e., the "referrer", though the field name is misspelled).  A user   agent MUST NOT include the fragment and userinfo components of the   URI reference [URI], if any, when generating the Referer field value.     Referer = absolute-URI / partial-URI   The field value is either an absolute-URI or a partial-URI.  In the   latter case (Section 4), the referenced URI is relative to the target   URI ([URI], Section 5).   The Referer header field allows servers to generate back-links to   other resources for simple analytics, logging, optimized caching,   etc.  It also allows obsolete or mistyped links to be found for   maintenance.  Some servers use the Referer header field as a means of   denying links from other sites (so-called "deep linking") or   restricting cross-site request forgery (CSRF), but not all requests   contain it.   Example:   Referer: http://www.example.org/hypertext/Overview.html   If the target URI was obtained from a source that does not have its   own URI (e.g., input from the user keyboard, or an entry within the   user's bookmarks/favorites), the user agent MUST either exclude the   Referer header field or send it with a value of "about:blank".   The Referer header field value need not convey the full URI of the   referring resource; a user agent MAY truncate parts other than the   referring origin.   The Referer header field has the potential to reveal information   about the request context or browsing history of the user, which is a   privacy concern if the referring resource's identifier reveals   personal information (such as an account name) or a resource that is   supposed to be confidential (such as behind a firewall or internal to   a secured service).  Most general-purpose user agents do not send the   Referer header field when the referring resource is a local "file" or   "data" URI.  A user agent SHOULD NOT send a Referer header field if   the referring resource was accessed with a secure protocol and the   request target has an origin differing from that of the referring   resource, unless the referring resource explicitly allows Referer to   be sent.  A user agent MUST NOT send a Referer header field in an   unsecured HTTP request if the referring resource was accessed with a   secure protocol.  See Section 17.9 for additional security   considerations.   Some intermediaries have been known to indiscriminately remove   Referer header fields from outgoing requests.  This has the   unfortunate side effect of interfering with protection against CSRF   attacks, which can be far more harmful to their users.   Intermediaries and user agent extensions that wish to limit   information disclosure in Referer ought to restrict their changes to   specific edits, such as replacing internal domain names with   pseudonyms or truncating the query and/or path components.  An   intermediary SHOULD NOT modify or delete the Referer header field   when the field value shares the same scheme and host as the target   URI.10.1.4.  TE   The "TE" header field describes capabilities of the client with   regard to transfer codings and trailer sections.   As described in Section 6.5, a TE field with a "trailers" member sent   in a request indicates that the client will not discard trailer   fields.   TE is also used within HTTP/1.1 to advise servers about which   transfer codings the client is able to accept in a response.  As of   publication, only HTTP/1.1 uses transfer codings (see Section 7 of   [HTTP/1.1]).   The TE field value is a list of members, with each member (aside from   "trailers") consisting of a transfer coding name token with an   optional weight indicating the client's relative preference for that   transfer coding (Section 12.4.2) and optional parameters for that   transfer coding.     TE                 = #t-codings     t-codings          = "trailers" / ( transfer-coding [ weight ] )     transfer-coding    = token *( OWS ";" OWS transfer-parameter )     transfer-parameter = token BWS "=" BWS ( token / quoted-string )   A sender of TE MUST also send a "TE" connection option within the   Connection header field (Section 7.6.1) to inform intermediaries not   to forward this field.10.1.5.  User-Agent   The "User-Agent" header field contains information about the user   agent originating the request, which is often used by servers to help   identify the scope of reported interoperability problems, to work   around or tailor responses to avoid particular user agent   limitations, and for analytics regarding browser or operating system   use.  A user agent SHOULD send a User-Agent header field in each   request unless specifically configured not to do so.     User-Agent = product *( RWS ( product / comment ) )   The User-Agent field value consists of one or more product   identifiers, each followed by zero or more comments (Section 5.6.5),   which together identify the user agent software and its significant   subproducts.  By convention, the product identifiers are listed in   decreasing order of their significance for identifying the user agent   software.  Each product identifier consists of a name and optional   version.     product         = token ["/" product-version]     product-version = token   A sender SHOULD limit generated product identifiers to what is   necessary to identify the product; a sender MUST NOT generate   advertising or other nonessential information within the product   identifier.  A sender SHOULD NOT generate information in   product-version that is not a version identifier (i.e., successive   versions of the same product name ought to differ only in the   product-version portion of the product identifier).   Example:   User-Agent: CERN-LineMode/2.15 libwww/2.17b3   A user agent SHOULD NOT generate a User-Agent header field containing   needlessly fine-grained detail and SHOULD limit the addition of   subproducts by third parties.  Overly long and detailed User-Agent   field values increase request latency and the risk of a user being   identified against their wishes ("fingerprinting").   Likewise, implementations are encouraged not to use the product   tokens of other implementations in order to declare compatibility   with them, as this circumvents the purpose of the field.  If a user   agent masquerades as a different user agent, recipients can assume   that the user intentionally desires to see responses tailored for   that identified user agent, even if they might not work as well for   the actual user agent being used.10.2.  Response Context Fields   The response header fields below provide additional information about   the response, beyond what is implied by the status code, including   information about the server, about the target resource, or about   related resources.10.2.1.  Allow   The "Allow" header field lists the set of methods advertised as   supported by the target resource.  The purpose of this field is   strictly to inform the recipient of valid request methods associated   with the resource.     Allow = #method   Example of use:   Allow: GET, HEAD, PUT   The actual set of allowed methods is defined by the origin server at   the time of each request.  An origin server MUST generate an Allow   header field in a 405 (Method Not Allowed) response and MAY do so in   any other response.  An empty Allow field value indicates that the   resource allows no methods, which might occur in a 405 response if   the resource has been temporarily disabled by configuration.   A proxy MUST NOT modify the Allow header field -- it does not need to   understand all of the indicated methods in order to handle them   according to the generic message handling rules.10.2.2.  Location   The "Location" header field is used in some responses to refer to a   specific resource in relation to the response.  The type of   relationship is defined by the combination of request method and   status code semantics.     Location = URI-reference   The field value consists of a single URI-reference.  When it has the   form of a relative reference ([URI], Section 4.2), the final value is   computed by resolving it against the target URI ([URI], Section 5).   For 201 (Created) responses, the Location value refers to the primary   resource created by the request.  For 3xx (Redirection) responses,   the Location value refers to the preferred target resource for   automatically redirecting the request.   If the Location value provided in a 3xx (Redirection) response does   not have a fragment component, a user agent MUST process the   redirection as if the value inherits the fragment component of the   URI reference used to generate the target URI (i.e., the redirection   inherits the original reference's fragment, if any).   For example, a GET request generated for the URI reference   "http://www.example.org/~tim" might result in a 303 (See Other)   response containing the header field:   Location: /People.html#tim   which suggests that the user agent redirect to   "http://www.example.org/People.html#tim"   Likewise, a GET request generated for the URI reference   "http://www.example.org/index.html#larry" might result in a 301   (Moved Permanently) response containing the header field:   Location: http://www.example.net/index.html   which suggests that the user agent redirect to   "http://www.example.net/index.html#larry", preserving the original   fragment identifier.   There are circumstances in which a fragment identifier in a Location   value would not be appropriate.  For example, the Location header   field in a 201 (Created) response is supposed to provide a URI that   is specific to the created resource.      |  *Note:* Some recipients attempt to recover from Location header      |  fields that are not valid URI references.  This specification      |  does not mandate or define such processing, but does allow it      |  for the sake of robustness.  A Location field value cannot      |  allow a list of members because the comma list separator is a      |  valid data character within a URI-reference.  If an invalid      |  message is sent with multiple Location field lines, a recipient      |  along the path might combine those field lines into one value.      |  Recovery of a valid Location field value from that situation is      |  difficult and not interoperable across implementations.      |  *Note:* The Content-Location header field (Section 8.7) differs      |  from Location in that the Content-Location refers to the most      |  specific resource corresponding to the enclosed representation.      |  It is therefore possible for a response to contain both the      |  Location and Content-Location header fields.10.2.3.  Retry-After   Servers send the "Retry-After" header field to indicate how long the   user agent ought to wait before making a follow-up request.  When   sent with a 503 (Service Unavailable) response, Retry-After indicates   how long the service is expected to be unavailable to the client.   When sent with any 3xx (Redirection) response, Retry-After indicates   the minimum time that the user agent is asked to wait before issuing   the redirected request.   The Retry-After field value can be either an HTTP-date or a number of   seconds to delay after receiving the response.     Retry-After = HTTP-date / delay-seconds   A delay-seconds value is a non-negative decimal integer, representing   time in seconds.     delay-seconds  = 1*DIGIT   Two examples of its use are   Retry-After: Fri, 31 Dec 1999 23:59:59 GMT   Retry-After: 120   In the latter example, the delay is 2 minutes.10.2.4.  Server   The "Server" header field contains information about the software   used by the origin server to handle the request, which is often used   by clients to help identify the scope of reported interoperability   problems, to work around or tailor requests to avoid particular   server limitations, and for analytics regarding server or operating   system use.  An origin server MAY generate a Server header field in   its responses.     Server = product *( RWS ( product / comment ) )   The Server header field value consists of one or more product   identifiers, each followed by zero or more comments (Section 5.6.5),   which together identify the origin server software and its   significant subproducts.  By convention, the product identifiers are   listed in decreasing order of their significance for identifying the   origin server software.  Each product identifier consists of a name   and optional version, as defined in Section 10.1.5.   Example:   Server: CERN/3.0 libwww/2.17   An origin server SHOULD NOT generate a Server header field containing   needlessly fine-grained detail and SHOULD limit the addition of   subproducts by third parties.  Overly long and detailed Server field   values increase response latency and potentially reveal internal   implementation details that might make it (slightly) easier for   attackers to find and exploit known security holes.11.  HTTP Authentication11.1.  Authentication Scheme   HTTP provides a general framework for access control and   authentication, via an extensible set of challenge-response   authentication schemes, which can be used by a server to challenge a   client request and by a client to provide authentication information.   It uses a case-insensitive token to identify the authentication   scheme:     auth-scheme    = token   Aside from the general framework, this document does not specify any   authentication schemes.  New and existing authentication schemes are   specified independently and ought to be registered within the   "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".   For example, the "basic" and "digest" authentication schemes are   defined by [RFC7617] and [RFC7616], respectively.11.2.  Authentication Parameters   The authentication scheme is followed by additional information   necessary for achieving authentication via that scheme as either a   comma-separated list of parameters or a single sequence of characters   capable of holding base64-encoded information.     token68        = 1*( ALPHA / DIGIT /                          "-" / "." / "_" / "~" / "+" / "/" ) *"="   The token68 syntax allows the 66 unreserved URI characters ([URI]),   plus a few others, so that it can hold a base64, base64url (URL and   filename safe alphabet), base32, or base16 (hex) encoding, with or   without padding, but excluding whitespace ([RFC4648]).   Authentication parameters are name/value pairs, where the name token   is matched case-insensitively and each parameter name MUST only occur   once per challenge.     auth-param     = token BWS "=" BWS ( token / quoted-string )   Parameter values can be expressed either as "token" or as "quoted-   string" (Section 5.6).  Authentication scheme definitions need to   accept both notations, both for senders and recipients, to allow   recipients to use generic parsing components regardless of the   authentication scheme.   For backwards compatibility, authentication scheme definitions can   restrict the format for senders to one of the two variants.  This can   be important when it is known that deployed implementations will fail   when encountering one of the two formats.11.3.  Challenge and Response   A 401 (Unauthorized) response message is used by an origin server to   challenge the authorization of a user agent, including a   WWW-Authenticate header field containing at least one challenge   applicable to the requested resource.   A 407 (Proxy Authentication Required) response message is used by a   proxy to challenge the authorization of a client, including a   Proxy-Authenticate header field containing at least one challenge   applicable to the proxy for the requested resource.     challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]      |  *Note:* Many clients fail to parse a challenge that contains an      |  unknown scheme.  A workaround for this problem is to list well-      |  supported schemes (such as "basic") first.   A user agent that wishes to authenticate itself with an origin server   -- usually, but not necessarily, after receiving a 401 (Unauthorized)   -- can do so by including an Authorization header field with the   request.   A client that wishes to authenticate itself with a proxy -- usually,   but not necessarily, after receiving a 407 (Proxy Authentication   Required) -- can do so by including a Proxy-Authorization header   field with the request.11.4.  Credentials   Both the Authorization field value and the Proxy-Authorization field   value contain the client's credentials for the realm of the resource   being requested, based upon a challenge received in a response   (possibly at some point in the past).  When creating their values,   the user agent ought to do so by selecting the challenge with what it   considers to be the most secure auth-scheme that it understands,   obtaining credentials from the user as appropriate.  Transmission of   credentials within header field values implies significant security   considerations regarding the confidentiality of the underlying   connection, as described in Section 17.16.1.     credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]   Upon receipt of a request for a protected resource that omits   credentials, contains invalid credentials (e.g., a bad password) or   partial credentials (e.g., when the authentication scheme requires   more than one round trip), an origin server SHOULD send a 401   (Unauthorized) response that contains a WWW-Authenticate header field   with at least one (possibly new) challenge applicable to the   requested resource.   Likewise, upon receipt of a request that omits proxy credentials or   contains invalid or partial proxy credentials, a proxy that requires   authentication SHOULD generate a 407 (Proxy Authentication Required)   response that contains a Proxy-Authenticate header field with at   least one (possibly new) challenge applicable to the proxy.   A server that receives valid credentials that are not adequate to   gain access ought to respond with the 403 (Forbidden) status code   (Section 15.5.4).   HTTP does not restrict applications to this simple challenge-response   framework for access authentication.  Additional mechanisms can be   used, such as authentication at the transport level or via message   encapsulation, and with additional header fields specifying   authentication information.  However, such additional mechanisms are   not defined by this specification.   Note that various custom mechanisms for user authentication use the   Set-Cookie and Cookie header fields, defined in [COOKIE], for passing   tokens related to authentication.11.5.  Establishing a Protection Space (Realm)   The "realm" authentication parameter is reserved for use by   authentication schemes that wish to indicate a scope of protection.   A "protection space" is defined by the origin (see Section 4.3.1) of   the server being accessed, in combination with the realm value if   present.  These realms allow the protected resources on a server to   be partitioned into a set of protection spaces, each with its own   authentication scheme and/or authorization database.  The realm value   is a string, generally assigned by the origin server, that can have   additional semantics specific to the authentication scheme.  Note   that a response can have multiple challenges with the same auth-   scheme but with different realms.   The protection space determines the domain over which credentials can   be automatically applied.  If a prior request has been authorized,   the user agent MAY reuse the same credentials for all other requests   within that protection space for a period of time determined by the   authentication scheme, parameters, and/or user preferences (such as a   configurable inactivity timeout).   The extent of a protection space, and therefore the requests to which   credentials might be automatically applied, is not necessarily known   to clients without additional information.  An authentication scheme   might define parameters that describe the extent of a protection   space.  Unless specifically allowed by the authentication scheme, a   single protection space cannot extend outside the scope of its   server.   For historical reasons, a sender MUST only generate the quoted-string   syntax.  Recipients might have to support both token and quoted-   string syntax for maximum interoperability with existing clients that   have been accepting both notations for a long time.11.6.  Authenticating Users to Origin Servers11.6.1.  WWW-Authenticate   The "WWW-Authenticate" response header field indicates the   authentication scheme(s) and parameters applicable to the target   resource.     WWW-Authenticate = #challenge   A server generating a 401 (Unauthorized) response MUST send a WWW-   Authenticate header field containing at least one challenge.  A   server MAY generate a WWW-Authenticate header field in other response   messages to indicate that supplying credentials (or different   credentials) might affect the response.   A proxy forwarding a response MUST NOT modify any WWW-Authenticate   header fields in that response.   User agents are advised to take special care in parsing the field   value, as it might contain more than one challenge, and each   challenge can contain a comma-separated list of authentication   parameters.  Furthermore, the header field itself can occur multiple   times.   For instance:   WWW-Authenticate: Basic realm="simple", Newauth realm="apps",                    type=1, title="Login to \"apps\""   This header field contains two challenges, one for the "Basic" scheme   with a realm value of "simple" and another for the "Newauth" scheme   with a realm value of "apps".  It also contains two additional   parameters, "type" and "title".   Some user agents do not recognize this form, however.  As a result,   sending a WWW-Authenticate field value with more than one member on   the same field line might not be interoperable.      |  *Note:* The challenge grammar production uses the list syntax      |  as well.  Therefore, a sequence of comma, whitespace, and comma      |  can be considered either as applying to the preceding      |  challenge, or to be an empty entry in the list of challenges.      |  In practice, this ambiguity does not affect the semantics of      |  the header field value and thus is harmless.11.6.2.  Authorization   The "Authorization" header field allows a user agent to authenticate   itself with an origin server -- usually, but not necessarily, after   receiving a 401 (Unauthorized) response.  Its value consists of   credentials containing the authentication information of the user   agent for the realm of the resource being requested.     Authorization = credentials   If a request is authenticated and a realm specified, the same   credentials are presumed to be valid for all other requests within   this realm (assuming that the authentication scheme itself does not   require otherwise, such as credentials that vary according to a   challenge value or using synchronized clocks).   A proxy forwarding a request MUST NOT modify any Authorization header   fields in that request.  See Section 3.5 of [CACHING] for details of   and requirements pertaining to handling of the Authorization header   field by HTTP caches.11.6.3.  Authentication-Info   HTTP authentication schemes can use the "Authentication-Info"   response field to communicate information after the client's   authentication credentials have been accepted.  This information can   include a finalization message from the server (e.g., it can contain   the server authentication).   The field value is a list of parameters (name/value pairs), using the   "auth-param" syntax defined in Section 11.3.  This specification only   describes the generic format; authentication schemes using   Authentication-Info will define the individual parameters.  The   "Digest" Authentication Scheme, for instance, defines multiple   parameters in Section 3.5 of [RFC7616].     Authentication-Info = #auth-param   The Authentication-Info field can be used in any HTTP response,   independently of request method and status code.  Its semantics are   defined by the authentication scheme indicated by the Authorization   header field (Section 11.6.2) of the corresponding request.   A proxy forwarding a response is not allowed to modify the field   value in any way.   Authentication-Info can be sent as a trailer field (Section 6.5) when   the authentication scheme explicitly allows this.11.7.  Authenticating Clients to Proxies11.7.1.  Proxy-Authenticate   The "Proxy-Authenticate" header field consists of at least one   challenge that indicates the authentication scheme(s) and parameters   applicable to the proxy for this request.  A proxy MUST send at least   one Proxy-Authenticate header field in each 407 (Proxy Authentication   Required) response that it generates.     Proxy-Authenticate = #challenge   Unlike WWW-Authenticate, the Proxy-Authenticate header field applies   only to the next outbound client on the response chain.  This is   because only the client that chose a given proxy is likely to have   the credentials necessary for authentication.  However, when multiple   proxies are used within the same administrative domain, such as   office and regional caching proxies within a large corporate network,   it is common for credentials to be generated by the user agent and   passed through the hierarchy until consumed.  Hence, in such a   configuration, it will appear as if Proxy-Authenticate is being   forwarded because each proxy will send the same challenge set.   Note that the parsing considerations for WWW-Authenticate apply to   this header field as well; see Section 11.6.1 for details.11.7.2.  Proxy-Authorization   The "Proxy-Authorization" header field allows the client to identify   itself (or its user) to a proxy that requires authentication.  Its   value consists of credentials containing the authentication   information of the client for the proxy and/or realm of the resource   being requested.     Proxy-Authorization = credentials   Unlike Authorization, the Proxy-Authorization header field applies   only to the next inbound proxy that demanded authentication using the   Proxy-Authenticate header field.  When multiple proxies are used in a   chain, the Proxy-Authorization header field is consumed by the first   inbound proxy that was expecting to receive credentials.  A proxy MAY   relay the credentials from the client request to the next proxy if   that is the mechanism by which the proxies cooperatively authenticate   a given request.11.7.3.  Proxy-Authentication-Info   The "Proxy-Authentication-Info" response header field is equivalent   to Authentication-Info, except that it applies to proxy   authentication (Section 11.3) and its semantics are defined by the   authentication scheme indicated by the Proxy-Authorization header   field (Section 11.7.2) of the corresponding request:     Proxy-Authentication-Info = #auth-param   However, unlike Authentication-Info, the Proxy-Authentication-Info   header field applies only to the next outbound client on the response   chain.  This is because only the client that chose a given proxy is   likely to have the credentials necessary for authentication.   However, when multiple proxies are used within the same   administrative domain, such as office and regional caching proxies   within a large corporate network, it is common for credentials to be   generated by the user agent and passed through the hierarchy until   consumed.  Hence, in such a configuration, it will appear as if   Proxy-Authentication-Info is being forwarded because each proxy will   send the same field value.   Proxy-Authentication-Info can be sent as a trailer field   (Section 6.5) when the authentication scheme explicitly allows this.12.  Content Negotiation   When responses convey content, whether indicating a success or an   error, the origin server often has different ways of representing   that information; for example, in different formats, languages, or   encodings.  Likewise, different users or user agents might have   differing capabilities, characteristics, or preferences that could   influence which representation, among those available, would be best   to deliver.  For this reason, HTTP provides mechanisms for content   negotiation.   This specification defines three patterns of content negotiation that   can be made visible within the protocol: "proactive" negotiation,   where the server selects the representation based upon the user   agent's stated preferences; "reactive" negotiation, where the server   provides a list of representations for the user agent to choose from;   and "request content" negotiation, where the user agent selects the   representation for a future request based upon the server's stated   preferences in past responses.   Other patterns of content negotiation include "conditional content",   where the representation consists of multiple parts that are   selectively rendered based on user agent parameters, "active   content", where the representation contains a script that makes   additional (more specific) requests based on the user agent   characteristics, and "Transparent Content Negotiation" ([RFC2295]),   where content selection is performed by an intermediary.  These   patterns are not mutually exclusive, and each has trade-offs in   applicability and practicality.   Note that, in all cases, HTTP is not aware of the resource semantics.   The consistency with which an origin server responds to requests,   over time and over the varying dimensions of content negotiation, and   thus the "sameness" of a resource's observed representations over   time, is determined entirely by whatever entity or algorithm selects   or generates those responses.12.1.  Proactive Negotiation   When content negotiation preferences are sent by the user agent in a   request to encourage an algorithm located at the server to select the   preferred representation, it is called "proactive negotiation"   (a.k.a., "server-driven negotiation").  Selection is based on the   available representations for a response (the dimensions over which   it might vary, such as language, content coding, etc.) compared to   various information supplied in the request, including both the   explicit negotiation header fields below and implicit   characteristics, such as the client's network address or parts of the   User-Agent field.   Proactive negotiation is advantageous when the algorithm for   selecting from among the available representations is difficult to   describe to a user agent, or when the server desires to send its   "best guess" to the user agent along with the first response (when   that "best guess" is good enough for the user, this avoids the round-   trip delay of a subsequent request).  In order to improve the   server's guess, a user agent MAY send request header fields that   describe its preferences.   Proactive negotiation has serious disadvantages:   *  It is impossible for the server to accurately determine what might      be "best" for any given user, since that would require complete      knowledge of both the capabilities of the user agent and the      intended use for the response (e.g., does the user want to view it      on screen or print it on paper?);   *  Having the user agent describe its capabilities in every request      can be both very inefficient (given that only a small percentage      of responses have multiple representations) and a potential risk      to the user's privacy;   *  It complicates the implementation of an origin server and the      algorithms for generating responses to a request; and,   *  It limits the reusability of responses for shared caching.   A user agent cannot rely on proactive negotiation preferences being   consistently honored, since the origin server might not implement   proactive negotiation for the requested resource or might decide that   sending a response that doesn't conform to the user agent's   preferences is better than sending a 406 (Not Acceptable) response.   A Vary header field (Section 12.5.5) is often sent in a response   subject to proactive negotiation to indicate what parts of the   request information were used in the selection algorithm.   The request header fields Accept, Accept-Charset, Accept-Encoding,   and Accept-Language are defined below for a user agent to engage in   proactive negotiation of the response content.  The preferences sent   in these fields apply to any content in the response, including   representations of the target resource, representations of error or   processing status, and potentially even the miscellaneous text   strings that might appear within the protocol.12.2.  Reactive Negotiation   With "reactive negotiation" (a.k.a., "agent-driven negotiation"),   selection of content (regardless of the status code) is performed by   the user agent after receiving an initial response.  The mechanism   for reactive negotiation might be as simple as a list of references   to alternative representations.   If the user agent is not satisfied by the initial response content,   it can perform a GET request on one or more of the alternative   resources to obtain a different representation.  Selection of such   alternatives might be performed automatically (by the user agent) or   manually (e.g., by the user selecting from a hypertext menu).   A server might choose not to send an initial representation, other   than the list of alternatives, and thereby indicate that reactive   negotiation by the user agent is preferred.  For example, the   alternatives listed in responses with the 300 (Multiple Choices) and   406 (Not Acceptable) status codes include information about available   representations so that the user or user agent can react by making a   selection.   Reactive negotiation is advantageous when the response would vary   over commonly used dimensions (such as type, language, or encoding),   when the origin server is unable to determine a user agent's   capabilities from examining the request, and generally when public   caches are used to distribute server load and reduce network usage.   Reactive negotiation suffers from the disadvantages of transmitting a   list of alternatives to the user agent, which degrades user-perceived   latency if transmitted in the header section, and needing a second   request to obtain an alternate representation.  Furthermore, this   specification does not define a mechanism for supporting automatic   selection, though it does not prevent such a mechanism from being   developed.12.3.  Request Content Negotiation   When content negotiation preferences are sent in a server's response,   the listed preferences are called "request content negotiation"   because they intend to influence selection of an appropriate content   for subsequent requests to that resource.  For example, the Accept   (Section 12.5.1) and Accept-Encoding (Section 12.5.3) header fields   can be sent in a response to indicate preferred media types and   content codings for subsequent requests to that resource.   Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch"   response header field, which allows discovery of which content types   are accepted in PATCH requests.12.4.  Content Negotiation Field Features12.4.1.  Absence   For each of the content negotiation fields, a request that does not   contain the field implies that the sender has no preference on that   dimension of negotiation.   If a content negotiation header field is present in a request and   none of the available representations for the response can be   considered acceptable according to it, the origin server can either   honor the header field by sending a 406 (Not Acceptable) response or   disregard the header field by treating the response as if it is not   subject to content negotiation for that request header field.  This   does not imply, however, that the client will be able to use the   representation.      |  *Note:* A user agent sending these header fields makes it      |  easier for a server to identify an individual by virtue of the      |  user agent's request characteristics (Section 17.13).12.4.2.  Quality Values   The content negotiation fields defined by this specification use a   common parameter, named "q" (case-insensitive), to assign a relative   "weight" to the preference for that associated kind of content.  This   weight is referred to as a "quality value" (or "qvalue") because the   same parameter name is often used within server configurations to   assign a weight to the relative quality of the various   representations that can be selected for a resource.   The weight is normalized to a real number in the range 0 through 1,   where 0.001 is the least preferred and 1 is the most preferred; a   value of 0 means "not acceptable".  If no "q" parameter is present,   the default weight is 1.     weight = OWS ";" OWS "q=" qvalue     qvalue = ( "0" [ "." 0*3DIGIT ] )            / ( "1" [ "." 0*3("0") ] )   A sender of qvalue MUST NOT generate more than three digits after the   decimal point.  User configuration of these values ought to be   limited in the same fashion.12.4.3.  Wildcard Values   Most of these header fields, where indicated, define a wildcard value   ("*") to select unspecified values.  If no wildcard is present,   values that are not explicitly mentioned in the field are considered   unacceptable.  Within Vary, the wildcard value means that the   variance is unlimited.      |  *Note:* In practice, using wildcards in content negotiation has      |  limited practical value because it is seldom useful to say, for      |  example, "I prefer image/* more or less than (some other      |  specific value)".  By sending Accept: */*;q=0, clients can      |  explicitly request a 406 (Not Acceptable) response if a more      |  preferred format is not available, but they still need to be      |  able to handle a different response since the server is allowed      |  to ignore their preference.12.5.  Content Negotiation Fields12.5.1.  Accept   The "Accept" header field can be used by user agents to specify their   preferences regarding response media types.  For example, Accept   header fields can be used to indicate that the request is   specifically limited to a small set of desired types, as in the case   of a request for an in-line image.   When sent by a server in a response, Accept provides information   about which content types are preferred in the content of a   subsequent request to the same resource.     Accept = #( media-range [ weight ] )     media-range    = ( "*/*"                        / ( type "/" "*" )                        / ( type "/" subtype )                      ) parameters   The asterisk "*" character is used to group media types into ranges,   with "*/*" indicating all media types and "type/*" indicating all   subtypes of that type.  The media-range can include media type   parameters that are applicable to that range.   Each media-range might be followed by optional applicable media type   parameters (e.g., charset), followed by an optional "q" parameter for   indicating a relative weight (Section 12.4.2).   Previous specifications allowed additional extension parameters to   appear after the weight parameter.  The accept extension grammar   (accept-params, accept-ext) has been removed because it had a   complicated definition, was not being used in practice, and is more   easily deployed through new header fields.  Senders using weights   SHOULD send "q" last (after all media-range parameters).  Recipients   SHOULD process any parameter named "q" as weight, regardless of   parameter ordering.      |  *Note:* Use of the "q" parameter name to control content      |  negotiation would interfere with any media type parameter      |  having the same name.  Hence, the media type registry disallows      |  parameters named "q".   The example   Accept: audio/*; q=0.2, audio/basic   is interpreted as "I prefer audio/basic, but send me any audio type   if it is the best available after an 80% markdown in quality".   A more elaborate example is   Accept: text/plain; q=0.5, text/html,          text/x-dvi; q=0.8, text/x-c   Verbally, this would be interpreted as "text/html and text/x-c are   the equally preferred media types, but if they do not exist, then   send the text/x-dvi representation, and if that does not exist, send   the text/plain representation".   Media ranges can be overridden by more specific media ranges or   specific media types.  If more than one media range applies to a   given type, the most specific reference has precedence.  For example,   Accept: text/*, text/plain, text/plain;format=flowed, */*   have the following precedence:   1.  text/plain;format=flowed   2.  text/plain   3.  text/*   4.  */*   The media type quality factor associated with a given type is   determined by finding the media range with the highest precedence   that matches the type.  For example,   Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,          text/plain;format=fixed;q=0.4, */*;q=0.5   would cause the following values to be associated:   +==========================+===============+   | Media Type               | Quality Value |   +==========================+===============+   | text/plain;format=flowed | 1             |   +--------------------------+---------------+   | text/plain               | 0.7           |   +--------------------------+---------------+   | text/html                | 0.3           |   +--------------------------+---------------+   | image/jpeg               | 0.5           |   +--------------------------+---------------+   | text/plain;format=fixed  | 0.4           |   +--------------------------+---------------+   | text/html;level=3        | 0.7           |   +--------------------------+---------------+                     Table 5      |  *Note:* A user agent might be provided with a default set of      |  quality values for certain media ranges.  However, unless the      |  user agent is a closed system that cannot interact with other      |  rendering agents, this default set ought to be configurable by      |  the user.12.5.2.  Accept-Charset   The "Accept-Charset" header field can be sent by a user agent to   indicate its preferences for charsets in textual response content.   For example, this field allows user agents capable of understanding   more comprehensive or special-purpose charsets to signal that   capability to an origin server that is capable of representing   information in those charsets.     Accept-Charset = #( ( token / "*" ) [ weight ] )   Charset names are defined in Section 8.3.2.  A user agent MAY   associate a quality value with each charset to indicate the user's   relative preference for that charset, as defined in Section 12.4.2.   An example is   Accept-Charset: iso-8859-5, unicode-1-1;q=0.8   The special value "*", if present in the Accept-Charset header field,   matches every charset that is not mentioned elsewhere in the field.      |  *Note:* Accept-Charset is deprecated because UTF-8 has become      |  nearly ubiquitous and sending a detailed list of user-preferred      |  charsets wastes bandwidth, increases latency, and makes passive      |  fingerprinting far too easy (Section 17.13).  Most general-      |  purpose user agents do not send Accept-Charset unless      |  specifically configured to do so.12.5.3.  Accept-Encoding   The "Accept-Encoding" header field can be used to indicate   preferences regarding the use of content codings (Section 8.4.1).   When sent by a user agent in a request, Accept-Encoding indicates the   content codings acceptable in a response.   When sent by a server in a response, Accept-Encoding provides   information about which content codings are preferred in the content   of a subsequent request to the same resource.   An "identity" token is used as a synonym for "no encoding" in order   to communicate when no encoding is preferred.     Accept-Encoding  = #( codings [ weight ] )     codings          = content-coding / "identity" / "*"   Each codings value MAY be given an associated quality value (weight)   representing the preference for that encoding, as defined in   Section 12.4.2.  The asterisk "*" symbol in an Accept-Encoding field   matches any available content coding not explicitly listed in the   field.   Examples:   Accept-Encoding: compress, gzip   Accept-Encoding:   Accept-Encoding: *   Accept-Encoding: compress;q=0.5, gzip;q=1.0   Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0   A server tests whether a content coding for a given representation is   acceptable using these rules:   1.  If no Accept-Encoding header field is in the request, any content       coding is considered acceptable by the user agent.   2.  If the representation has no content coding, then it is       acceptable by default unless specifically excluded by the Accept-       Encoding header field stating either "identity;q=0" or "*;q=0"       without a more specific entry for "identity".   3.  If the representation's content coding is one of the content       codings listed in the Accept-Encoding field value, then it is       acceptable unless it is accompanied by a qvalue of 0.  (As       defined in Section 12.4.2, a qvalue of 0 means "not acceptable".)   A representation could be encoded with multiple content codings.   However, most content codings are alternative ways to accomplish the   same purpose (e.g., data compression).  When selecting between   multiple content codings that have the same purpose, the acceptable   content coding with the highest non-zero qvalue is preferred.   An Accept-Encoding header field with a field value that is empty   implies that the user agent does not want any content coding in   response.  If a non-empty Accept-Encoding header field is present in   a request and none of the available representations for the response   have a content coding that is listed as acceptable, the origin server   SHOULD send a response without any content coding unless the identity   coding is indicated as unacceptable.   When the Accept-Encoding header field is present in a response, it   indicates what content codings the resource was willing to accept in   the associated request.  The field value is evaluated the same way as   in a request.   Note that this information is specific to the associated request; the   set of supported encodings might be different for other resources on   the same server and could change over time or depend on other aspects   of the request (such as the request method).   Servers that fail a request due to an unsupported content coding   ought to respond with a 415 (Unsupported Media Type) status and   include an Accept-Encoding header field in that response, allowing   clients to distinguish between issues related to content codings and   media types.  In order to avoid confusion with issues related to   media types, servers that fail a request with a 415 status for   reasons unrelated to content codings MUST NOT include the Accept-   Encoding header field.   The most common use of Accept-Encoding is in responses with a 415   (Unsupported Media Type) status code, in response to optimistic use   of a content coding by clients.  However, the header field can also   be used to indicate to clients that content codings are supported in   order to optimize future interactions.  For example, a resource might   include it in a 2xx (Successful) response when the request content   was big enough to justify use of a compression coding but the client   failed do so.12.5.4.  Accept-Language   The "Accept-Language" header field can be used by user agents to   indicate the set of natural languages that are preferred in the   response.  Language tags are defined in Section 8.5.1.     Accept-Language = #( language-range [ weight ] )     language-range  =               <language-range, see [RFC4647], Section 2.1>   Each language-range can be given an associated quality value   representing an estimate of the user's preference for the languages   specified by that range, as defined in Section 12.4.2.  For example,   Accept-Language: da, en-gb;q=0.8, en;q=0.7   would mean: "I prefer Danish, but will accept British English and   other types of English".   Note that some recipients treat the order in which language tags are   listed as an indication of descending priority, particularly for tags   that are assigned equal quality values (no value is the same as q=1).   However, this behavior cannot be relied upon.  For consistency and to   maximize interoperability, many user agents assign each language tag   a unique quality value while also listing them in order of decreasing   quality.  Additional discussion of language priority lists can be   found in Section 2.3 of [RFC4647].   For matching, Section 3 of [RFC4647] defines several matching   schemes.  Implementations can offer the most appropriate matching   scheme for their requirements.  The "Basic Filtering" scheme   ([RFC4647], Section 3.3.1) is identical to the matching scheme that   was previously defined for HTTP in Section 14.4 of [RFC2616].   It might be contrary to the privacy expectations of the user to send   an Accept-Language header field with the complete linguistic   preferences of the user in every request (Section 17.13).   Since intelligibility is highly dependent on the individual user,   user agents need to allow user control over the linguistic preference   (either through configuration of the user agent itself or by   defaulting to a user controllable system setting).  A user agent that   does not provide such control to the user MUST NOT send an Accept-   Language header field.      |  *Note:* User agents ought to provide guidance to users when      |  setting a preference, since users are rarely familiar with the      |  details of language matching as described above.  For example,      |  users might assume that on selecting "en-gb", they will be      |  served any kind of English document if British English is not      |  available.  A user agent might suggest, in such a case, to add      |  "en" to the list for better matching behavior.12.5.5.  Vary   The "Vary" header field in a response describes what parts of a   request message, aside from the method and target URI, might have   influenced the origin server's process for selecting the content of   this response.     Vary = #( "*" / field-name )   A Vary field value is either the wildcard member "*" or a list of   request field names, known as the selecting header fields, that might   have had a role in selecting the representation for this response.   Potential selecting header fields are not limited to fields defined   by this specification.   A list containing the member "*" signals that other aspects of the   request might have played a role in selecting the response   representation, possibly including aspects outside the message syntax   (e.g., the client's network address).  A recipient will not be able   to determine whether this response is appropriate for a later request   without forwarding the request to the origin server.  A proxy MUST   NOT generate "*" in a Vary field value.   For example, a response that contains   Vary: accept-encoding, accept-language   indicates that the origin server might have used the request's   Accept-Encoding and Accept-Language header fields (or lack thereof)   as determining factors while choosing the content for this response.   A Vary field containing a list of field names has two purposes:   1.  To inform cache recipients that they MUST NOT use this response       to satisfy a later request unless the later request has the same       values for the listed header fields as the original request       (Section 4.1 of [CACHING]) or reuse of the response has been       validated by the origin server.  In other words, Vary expands the       cache key required to match a new request to the stored cache       entry.   2.  To inform user agent recipients that this response was subject to       content negotiation (Section 12) and a different representation       might be sent in a subsequent request if other values are       provided in the listed header fields (proactive negotiation).   An origin server SHOULD generate a Vary header field on a cacheable   response when it wishes that response to be selectively reused for   subsequent requests.  Generally, that is the case when the response   content has been tailored to better fit the preferences expressed by   those selecting header fields, such as when an origin server has   selected the response's language based on the request's   Accept-Language header field.   Vary might be elided when an origin server considers variance in   content selection to be less significant than Vary's performance   impact on caching, particularly when reuse is already limited by   cache response directives (Section 5.2 of [CACHING]).   There is no need to send the Authorization field name in Vary because   reuse of that response for a different user is prohibited by the   field definition (Section 11.6.2).  Likewise, if the response content   has been selected or influenced by network region, but the origin   server wants the cached response to be reused even if recipients move   from one region to another, then there is no need for the origin   server to indicate such variance in Vary.13.  Conditional Requests   A conditional request is an HTTP request with one or more request   header fields that indicate a precondition to be tested before   applying the request method to the target resource.  Section 13.2   defines when to evaluate preconditions and their order of precedence   when more than one precondition is present.   Conditional GET requests are the most efficient mechanism for HTTP   cache updates [CACHING].  Conditionals can also be applied to state-   changing methods, such as PUT and DELETE, to prevent the "lost   update" problem: one client accidentally overwriting the work of   another client that has been acting in parallel.13.1.  Preconditions   Preconditions are usually defined with respect to a state of the   target resource as a whole (its current value set) or the state as   observed in a previously obtained representation (one value in that   set).  If a resource has multiple current representations, each with   its own observable state, a precondition will assume that the mapping   of each request to a selected representation (Section 3.2) is   consistent over time.  Regardless, if the mapping is inconsistent or   the server is unable to select an appropriate representation, then no   harm will result when the precondition evaluates to false.   Each precondition defined below consists of a comparison between a   set of validators obtained from prior representations of the target   resource to the current state of validators for the selected   representation (Section 8.8).  Hence, these preconditions evaluate   whether the state of the target resource has changed since a given   state known by the client.  The effect of such an evaluation depends   on the method semantics and choice of conditional, as defined in   Section 13.2.   Other preconditions, defined by other specifications as extension   fields, might place conditions on all recipients, on the state of the   target resource in general, or on a group of resources.  For   instance, the "If" header field in WebDAV can make a request   conditional on various aspects of multiple resources, such as locks,   if the recipient understands and implements that field ([WEBDAV],   Section 10.4).   Extensibility of preconditions is only possible when the precondition   can be safely ignored if unknown (like If-Modified-Since), when   deployment can be assumed for a given use case, or when   implementation is signaled by some other property of the target   resource.  This encourages a focus on mutually agreed deployment of   common standards.13.1.1.  If-Match   The "If-Match" header field makes the request method conditional on   the recipient origin server either having at least one current   representation of the target resource, when the field value is "*",   or having a current representation of the target resource that has an   entity tag matching a member of the list of entity tags provided in   the field value.   An origin server MUST use the strong comparison function when   comparing entity tags for If-Match (Section 8.8.3.2), since the   client intends this precondition to prevent the method from being   applied if there have been any changes to the representation data.     If-Match = "*" / #entity-tag   Examples:   If-Match: "xyzzy"   If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"   If-Match: *   If-Match is most often used with state-changing methods (e.g., POST,   PUT, DELETE) to prevent accidental overwrites when multiple user   agents might be acting in parallel on the same resource (i.e., to   prevent the "lost update" problem).  In general, it can be used with   any method that involves the selection or modification of a   representation to abort the request if the selected representation's   current entity tag is not a member within the If-Match field value.   When an origin server receives a request that selects a   representation and that request includes an If-Match header field,   the origin server MUST evaluate the If-Match condition per   Section 13.2 prior to performing the method.   To evaluate a received If-Match header field:   1.  If the field value is "*", the condition is true if the origin       server has a current representation for the target resource.   2.  If the field value is a list of entity tags, the condition is       true if any of the listed tags match the entity tag of the       selected representation.   3.  Otherwise, the condition is false.   An origin server that evaluates an If-Match condition MUST NOT   perform the requested method if the condition evaluates to false.   Instead, the origin server MAY indicate that the conditional request   failed by responding with a 412 (Precondition Failed) status code.   Alternatively, if the request is a state-changing operation that   appears to have already been applied to the selected representation,   the origin server MAY respond with a 2xx (Successful) status code   (i.e., the change requested by the user agent has already succeeded,   but the user agent might not be aware of it, perhaps because the   prior response was lost or an equivalent change was made by some   other user agent).   Allowing an origin server to send a success response when a change   request appears to have already been applied is more efficient for   many authoring use cases, but comes with some risk if multiple user   agents are making change requests that are very similar but not   cooperative.  For example, multiple user agents writing to a common   resource as a semaphore (e.g., a nonatomic increment) are likely to   collide and potentially lose important state transitions.  For those   kinds of resources, an origin server is better off being stringent in   sending 412 for every failed precondition on an unsafe method.  In   other cases, excluding the ETag field from a success response might   encourage the user agent to perform a GET as its next request to   eliminate confusion about the resource's current state.   A client MAY send an If-Match header field in a GET request to   indicate that it would prefer a 412 (Precondition Failed) response if   the selected representation does not match.  However, this is only   useful in range requests (Section 14) for completing a previously   received partial representation when there is no desire for a new   representation.  If-Range (Section 13.1.5) is better suited for range   requests when the client prefers to receive a new representation.   A cache or intermediary MAY ignore If-Match because its   interoperability features are only necessary for an origin server.   Note that an If-Match header field with a list value containing "*"   and other values (including other instances of "*") is syntactically   invalid (therefore not allowed to be generated) and furthermore is   unlikely to be interoperable.13.1.2.  If-None-Match   The "If-None-Match" header field makes the request method conditional   on a recipient cache or origin server either not having any current   representation of the target resource, when the field value is "*",   or having a selected representation with an entity tag that does not   match any of those listed in the field value.   A recipient MUST use the weak comparison function when comparing   entity tags for If-None-Match (Section 8.8.3.2), since weak entity   tags can be used for cache validation even if there have been changes   to the representation data.     If-None-Match = "*" / #entity-tag   Examples:   If-None-Match: "xyzzy"   If-None-Match: W/"xyzzy"   If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"   If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"   If-None-Match: *   If-None-Match is primarily used in conditional GET requests to enable   efficient updates of cached information with a minimum amount of   transaction overhead.  When a client desires to update one or more   stored responses that have entity tags, the client SHOULD generate an   If-None-Match header field containing a list of those entity tags   when making a GET request; this allows recipient servers to send a   304 (Not Modified) response to indicate when one of those stored   responses matches the selected representation.   If-None-Match can also be used with a value of "*" to prevent an   unsafe request method (e.g., PUT) from inadvertently modifying an   existing representation of the target resource when the client   believes that the resource does not have a current representation   (Section 9.2.1).  This is a variation on the "lost update" problem   that might arise if more than one client attempts to create an   initial representation for the target resource.   When an origin server receives a request that selects a   representation and that request includes an If-None-Match header   field, the origin server MUST evaluate the If-None-Match condition   per Section 13.2 prior to performing the method.   To evaluate a received If-None-Match header field:   1.  If the field value is "*", the condition is false if the origin       server has a current representation for the target resource.   2.  If the field value is a list of entity tags, the condition is       false if one of the listed tags matches the entity tag of the       selected representation.   3.  Otherwise, the condition is true.   An origin server that evaluates an If-None-Match condition MUST NOT   perform the requested method if the condition evaluates to false;   instead, the origin server MUST respond with either a) the 304 (Not   Modified) status code if the request method is GET or HEAD or b) the   412 (Precondition Failed) status code for all other request methods.   Requirements on cache handling of a received If-None-Match header   field are defined in Section 4.3.2 of [CACHING].   Note that an If-None-Match header field with a list value containing   "*" and other values (including other instances of "*") is   syntactically invalid (therefore not allowed to be generated) and   furthermore is unlikely to be interoperable.13.1.3.  If-Modified-Since   The "If-Modified-Since" header field makes a GET or HEAD request   method conditional on the selected representation's modification date   being more recent than the date provided in the field value.   Transfer of the selected representation's data is avoided if that   data has not changed.     If-Modified-Since = HTTP-date   An example of the field is:   If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT   A recipient MUST ignore If-Modified-Since if the request contains an   If-None-Match header field; the condition in If-None-Match is   considered to be a more accurate replacement for the condition in If-   Modified-Since, and the two are only combined for the sake of   interoperating with older intermediaries that might not implement   If-None-Match.   A recipient MUST ignore the If-Modified-Since header field if the   received field value is not a valid HTTP-date, the field value has   more than one member, or if the request method is neither GET nor   HEAD.   A recipient MUST ignore the If-Modified-Since header field if the   resource does not have a modification date available.   A recipient MUST interpret an If-Modified-Since field value's   timestamp in terms of the origin server's clock.   If-Modified-Since is typically used for two distinct purposes: 1) to   allow efficient updates of a cached representation that does not have   an entity tag and 2) to limit the scope of a web traversal to   resources that have recently changed.   When used for cache updates, a cache will typically use the value of   the cached message's Last-Modified header field to generate the field   value of If-Modified-Since.  This behavior is most interoperable for   cases where clocks are poorly synchronized or when the server has   chosen to only honor exact timestamp matches (due to a problem with   Last-Modified dates that appear to go "back in time" when the origin   server's clock is corrected or a representation is restored from an   archived backup).  However, caches occasionally generate the field   value based on other data, such as the Date header field of the   cached message or the clock time at which the message was received,   particularly when the cached message does not contain a Last-Modified   header field.   When used for limiting the scope of retrieval to a recent time   window, a user agent will generate an If-Modified-Since field value   based on either its own clock or a Date header field received from   the server in a prior response.  Origin servers that choose an exact   timestamp match based on the selected representation's Last-Modified   header field will not be able to help the user agent limit its data   transfers to only those changed during the specified window.   When an origin server receives a request that selects a   representation and that request includes an If-Modified-Since header   field without an If-None-Match header field, the origin server SHOULD   evaluate the If-Modified-Since condition per Section 13.2 prior to   performing the method.   To evaluate a received If-Modified-Since header field:   1.  If the selected representation's last modification date is       earlier or equal to the date provided in the field value, the       condition is false.   2.  Otherwise, the condition is true.   An origin server that evaluates an If-Modified-Since condition SHOULD   NOT perform the requested method if the condition evaluates to false;   instead, the origin server SHOULD generate a 304 (Not Modified)   response, including only those metadata that are useful for   identifying or updating a previously cached response.   Requirements on cache handling of a received If-Modified-Since header   field are defined in Section 4.3.2 of [CACHING].13.1.4.  If-Unmodified-Since   The "If-Unmodified-Since" header field makes the request method   conditional on the selected representation's last modification date   being earlier than or equal to the date provided in the field value.   This field accomplishes the same purpose as If-Match for cases where   the user agent does not have an entity tag for the representation.     If-Unmodified-Since = HTTP-date   An example of the field is:   If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT   A recipient MUST ignore If-Unmodified-Since if the request contains   an If-Match header field; the condition in If-Match is considered to   be a more accurate replacement for the condition in If-Unmodified-   Since, and the two are only combined for the sake of interoperating   with older intermediaries that might not implement If-Match.   A recipient MUST ignore the If-Unmodified-Since header field if the   received field value is not a valid HTTP-date (including when the   field value appears to be a list of dates).   A recipient MUST ignore the If-Unmodified-Since header field if the   resource does not have a modification date available.   A recipient MUST interpret an If-Unmodified-Since field value's   timestamp in terms of the origin server's clock.   If-Unmodified-Since is most often used with state-changing methods   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when   multiple user agents might be acting in parallel on a resource that   does not supply entity tags with its representations (i.e., to   prevent the "lost update" problem).  In general, it can be used with   any method that involves the selection or modification of a   representation to abort the request if the selected representation's   last modification date has changed since the date provided in the If-   Unmodified-Since field value.   When an origin server receives a request that selects a   representation and that request includes an If-Unmodified-Since   header field without an If-Match header field, the origin server MUST   evaluate the If-Unmodified-Since condition per Section 13.2 prior to   performing the method.   To evaluate a received If-Unmodified-Since header field:   1.  If the selected representation's last modification date is       earlier than or equal to the date provided in the field value,       the condition is true.   2.  Otherwise, the condition is false.   An origin server that evaluates an If-Unmodified-Since condition MUST   NOT perform the requested method if the condition evaluates to false.   Instead, the origin server MAY indicate that the conditional request   failed by responding with a 412 (Precondition Failed) status code.   Alternatively, if the request is a state-changing operation that   appears to have already been applied to the selected representation,   the origin server MAY respond with a 2xx (Successful) status code   (i.e., the change requested by the user agent has already succeeded,   but the user agent might not be aware of it, perhaps because the   prior response was lost or an equivalent change was made by some   other user agent).   Allowing an origin server to send a success response when a change   request appears to have already been applied is more efficient for   many authoring use cases, but comes with some risk if multiple user   agents are making change requests that are very similar but not   cooperative.  In those cases, an origin server is better off being   stringent in sending 412 for every failed precondition on an unsafe   method.   A client MAY send an If-Unmodified-Since header field in a GET   request to indicate that it would prefer a 412 (Precondition Failed)   response if the selected representation has been modified.  However,   this is only useful in range requests (Section 14) for completing a   previously received partial representation when there is no desire   for a new representation.  If-Range (Section 13.1.5) is better suited   for range requests when the client prefers to receive a new   representation.   A cache or intermediary MAY ignore If-Unmodified-Since because its   interoperability features are only necessary for an origin server.13.1.5.  If-Range   The "If-Range" header field provides a special conditional request   mechanism that is similar to the If-Match and If-Unmodified-Since   header fields but that instructs the recipient to ignore the Range   header field if the validator doesn't match, resulting in transfer of   the new selected representation instead of a 412 (Precondition   Failed) response.   If a client has a partial copy of a representation and wishes to have   an up-to-date copy of the entire representation, it could use the   Range header field with a conditional GET (using either or both of   If-Unmodified-Since and If-Match.)  However, if the precondition   fails because the representation has been modified, the client would   then have to make a second request to obtain the entire current   representation.   The "If-Range" header field allows a client to "short-circuit" the   second request.  Informally, its meaning is as follows: if the   representation is unchanged, send me the part(s) that I am requesting   in Range; otherwise, send me the entire representation.     If-Range = entity-tag / HTTP-date   A valid entity-tag can be distinguished from a valid HTTP-date by   examining the first three characters for a DQUOTE.   A client MUST NOT generate an If-Range header field in a request that   does not contain a Range header field.  A server MUST ignore an If-   Range header field received in a request that does not contain a   Range header field.  An origin server MUST ignore an If-Range header   field received in a request for a target resource that does not   support Range requests.   A client MUST NOT generate an If-Range header field containing an   entity tag that is marked as weak.  A client MUST NOT generate an If-   Range header field containing an HTTP-date unless the client has no   entity tag for the corresponding representation and the date is a   strong validator in the sense defined by Section 8.8.2.2.   A server that receives an If-Range header field on a Range request   MUST evaluate the condition per Section 13.2 prior to performing the   method.   To evaluate a received If-Range header field containing an HTTP-date:   1.  If the HTTP-date validator provided is not a strong validator in       the sense defined by Section 8.8.2.2, the condition is false.   2.  If the HTTP-date validator provided exactly matches the       Last-Modified field value for the selected representation, the       condition is true.   3.  Otherwise, the condition is false.   To evaluate a received If-Range header field containing an   entity-tag:   1.  If the entity-tag validator provided exactly matches the ETag       field value for the selected representation using the strong       comparison function (Section 8.8.3.2), the condition is true.   2.  Otherwise, the condition is false.   A recipient of an If-Range header field MUST ignore the Range header   field if the If-Range condition evaluates to false.  Otherwise, the   recipient SHOULD process the Range header field as requested.   Note that the If-Range comparison is by exact match, including when   the validator is an HTTP-date, and so it differs from the "earlier   than or equal to" comparison used when evaluating an   If-Unmodified-Since conditional.13.2.  Evaluation of Preconditions13.2.1.  When to Evaluate   Except when excluded below, a recipient cache or origin server MUST   evaluate received request preconditions after it has successfully   performed its normal request checks and just before it would process   the request content (if any) or perform the action associated with   the request method.  A server MUST ignore all received preconditions   if its response to the same request without those conditions, prior   to processing the request content, would have been a status code   other than a 2xx (Successful) or 412 (Precondition Failed).  In other   words, redirects and failures that can be detected before significant   processing occurs take precedence over the evaluation of   preconditions.   A server that is not the origin server for the target resource and   cannot act as a cache for requests on the target resource MUST NOT   evaluate the conditional request header fields defined by this   specification, and it MUST forward them if the request is forwarded,   since the generating client intends that they be evaluated by a   server that can provide a current representation.  Likewise, a server   MUST ignore the conditional request header fields defined by this   specification when received with a request method that does not   involve the selection or modification of a selected representation,   such as CONNECT, OPTIONS, or TRACE.   Note that protocol extensions can modify the conditions under which   preconditions are evaluated or the consequences of their evaluation.   For example, the immutable cache directive (defined by [RFC8246])   instructs caches to forgo forwarding conditional requests when they   hold a fresh response.   Although conditional request header fields are defined as being   usable with the HEAD method (to keep HEAD's semantics consistent with   those of GET), there is no point in sending a conditional HEAD   because a successful response is around the same size as a 304 (Not   Modified) response and more useful than a 412 (Precondition Failed)   response.13.2.2.  Precedence of Preconditions   When more than one conditional request header field is present in a   request, the order in which the fields are evaluated becomes   important.  In practice, the fields defined in this document are   consistently implemented in a single, logical order, since "lost   update" preconditions have more strict requirements than cache   validation, a validated cache is more efficient than a partial   response, and entity tags are presumed to be more accurate than date   validators.   A recipient cache or origin server MUST evaluate the request   preconditions defined by this specification in the following order:   1.  When recipient is the origin server and If-Match is present,       evaluate the If-Match precondition:       *  if true, continue to step 3       *  if false, respond 412 (Precondition Failed) unless it can be          determined that the state-changing request has already          succeeded (see Section 13.1.1)   2.  When recipient is the origin server, If-Match is not present, and       If-Unmodified-Since is present, evaluate the If-Unmodified-Since       precondition:       *  if true, continue to step 3       *  if false, respond 412 (Precondition Failed) unless it can be          determined that the state-changing request has already          succeeded (see Section 13.1.4)   3.  When If-None-Match is present, evaluate the If-None-Match       precondition:       *  if true, continue to step 5       *  if false for GET/HEAD, respond 304 (Not Modified)       *  if false for other methods, respond 412 (Precondition Failed)   4.  When the method is GET or HEAD, If-None-Match is not present, and       If-Modified-Since is present, evaluate the If-Modified-Since       precondition:       *  if true, continue to step 5       *  if false, respond 304 (Not Modified)   5.  When the method is GET and both Range and If-Range are present,       evaluate the If-Range precondition:       *  if true and the Range is applicable to the selected          representation, respond 206 (Partial Content)       *  otherwise, ignore the Range header field and respond 200 (OK)   6.  Otherwise,       *  perform the requested method and respond according to its          success or failure.   Any extension to HTTP that defines additional conditional request   header fields ought to define the order for evaluating such fields in   relation to those defined in this document and other conditionals   that might be found in practice.14.  Range Requests   Clients often encounter interrupted data transfers as a result of   canceled requests or dropped connections.  When a client has stored a   partial representation, it is desirable to request the remainder of   that representation in a subsequent request rather than transfer the   entire representation.  Likewise, devices with limited local storage   might benefit from being able to request only a subset of a larger   representation, such as a single page of a very large document, or   the dimensions of an embedded image.   Range requests are an OPTIONAL feature of HTTP, designed so that   recipients not implementing this feature (or not supporting it for   the target resource) can respond as if it is a normal GET request   without impacting interoperability.  Partial responses are indicated   by a distinct status code to not be mistaken for full responses by   caches that might not implement the feature.14.1.  Range Units   Representation data can be partitioned into subranges when there are   addressable structural units inherent to that data's content coding   or media type.  For example, octet (a.k.a. byte) boundaries are a   structural unit common to all representation data, allowing   partitions of the data to be identified as a range of bytes at some   offset from the start or end of that data.   This general notion of a "range unit" is used in the Accept-Ranges   (Section 14.3) response header field to advertise support for range   requests, the Range (Section 14.2) request header field to delineate   the parts of a representation that are requested, and the   Content-Range (Section 14.4) header field to describe which part of a   representation is being transferred.     range-unit       = token   All range unit names are case-insensitive and ought to be registered   within the "HTTP Range Unit Registry", as defined in Section 16.5.1.   Range units are intended to be extensible, as described in   Section 16.5.14.1.1.  Range Specifiers   Ranges are expressed in terms of a range unit paired with a set of   range specifiers.  The range unit name determines what kinds of   range-spec are applicable to its own specifiers.  Hence, the   following grammar is generic: each range unit is expected to specify   requirements on when int-range, suffix-range, and other-range are   allowed.   A range request can specify a single range or a set of ranges within   a single representation.     ranges-specifier = range-unit "=" range-set     range-set        = 1#range-spec     range-spec       = int-range                      / suffix-range                      / other-range   An int-range is a range expressed as two non-negative integers or as   one non-negative integer through to the end of the representation   data.  The range unit specifies what the integers mean (e.g., they   might indicate unit offsets from the beginning, inclusive numbered   parts, etc.).     int-range     = first-pos "-" [ last-pos ]     first-pos     = 1*DIGIT     last-pos      = 1*DIGIT   An int-range is invalid if the last-pos value is present and less   than the first-pos.   A suffix-range is a range expressed as a suffix of the representation   data with the provided non-negative integer maximum length (in range   units).  In other words, the last N units of the representation data.     suffix-range  = "-" suffix-length     suffix-length = 1*DIGIT   To provide for extensibility, the other-range rule is a mostly   unconstrained grammar that allows application-specific or future   range units to define additional range specifiers.     other-range   = 1*( %x21-2B / %x2D-7E )                   ; 1*(VCHAR excluding comma)   A ranges-specifier is invalid if it contains any range-spec that is   invalid or undefined for the indicated range-unit.   A valid ranges-specifier is "satisfiable" if it contains at least one   range-spec that is satisfiable, as defined by the indicated   range-unit.  Otherwise, the ranges-specifier is "unsatisfiable".14.1.2.  Byte Ranges   The "bytes" range unit is used to express subranges of a   representation data's octet sequence.  Each byte range is expressed   as an integer range at some offset, relative to either the beginning   (int-range) or end (suffix-range) of the representation data.  Byte   ranges do not use the other-range specifier.   The first-pos value in a bytes int-range gives the offset of the   first byte in a range.  The last-pos value gives the offset of the   last byte in the range; that is, the byte positions specified are   inclusive.  Byte offsets start at zero.   If the representation data has a content coding applied, each byte   range is calculated with respect to the encoded sequence of bytes,   not the sequence of underlying bytes that would be obtained after   decoding.   Examples of bytes range specifiers:   *  The first 500 bytes (byte offsets 0-499, inclusive):           bytes=0-499   *  The second 500 bytes (byte offsets 500-999, inclusive):           bytes=500-999   A client can limit the number of bytes requested without knowing the   size of the selected representation.  If the last-pos value is   absent, or if the value is greater than or equal to the current   length of the representation data, the byte range is interpreted as   the remainder of the representation (i.e., the server replaces the   value of last-pos with a value that is one less than the current   length of the selected representation).   A client can refer to the last N bytes (N > 0) of the selected   representation using a suffix-range.  If the selected representation   is shorter than the specified suffix-length, the entire   representation is used.   Additional examples, assuming a representation of length 10000:   *  The final 500 bytes (byte offsets 9500-9999, inclusive):           bytes=-500      Or:           bytes=9500-   *  The first and last bytes only (bytes 0 and 9999):           bytes=0-0,-1   *  The first, middle, and last 1000 bytes:           bytes= 0-999, 4500-5499, -1000   *  Other valid (but not canonical) specifications of the second 500      bytes (byte offsets 500-999, inclusive):           bytes=500-600,601-999           bytes=500-700,601-999   For a GET request, a valid bytes range-spec is satisfiable if it is   either:   *  an int-range with a first-pos that is less than the current length      of the selected representation or   *  a suffix-range with a non-zero suffix-length.   When a selected representation has zero length, the only satisfiable   form of range-spec in a GET request is a suffix-range with a non-zero   suffix-length.   In the byte-range syntax, first-pos, last-pos, and suffix-length are   expressed as decimal number of octets.  Since there is no predefined   limit to the length of content, recipients MUST anticipate   potentially large decimal numerals and prevent parsing errors due to   integer conversion overflows.14.2.  Range   The "Range" header field on a GET request modifies the method   semantics to request transfer of only one or more subranges of the   selected representation data (Section 8.1), rather than the entire   selected representation.     Range = ranges-specifier   A server MAY ignore the Range header field.  However, origin servers   and intermediate caches ought to support byte ranges when possible,   since they support efficient recovery from partially failed transfers   and partial retrieval of large representations.   A server MUST ignore a Range header field received with a request   method that is unrecognized or for which range handling is not   defined.  For this specification, GET is the only method for which   range handling is defined.   An origin server MUST ignore a Range header field that contains a   range unit it does not understand.  A proxy MAY discard a Range   header field that contains a range unit it does not understand.   A server that supports range requests MAY ignore or reject a Range   header field that contains an invalid ranges-specifier   (Section 14.1.1), a ranges-specifier with more than two overlapping   ranges, or a set of many small ranges that are not listed in   ascending order, since these are indications of either a broken   client or a deliberate denial-of-service attack (Section 17.15).  A   client SHOULD NOT request multiple ranges that are inherently less   efficient to process and transfer than a single range that   encompasses the same data.   A server that supports range requests MAY ignore a Range header field   when the selected representation has no content (i.e., the selected   representation's data is of zero length).   A client that is requesting multiple ranges SHOULD list those ranges   in ascending order (the order in which they would typically be   received in a complete representation) unless there is a specific   need to request a later part earlier.  For example, a user agent   processing a large representation with an internal catalog of parts   might need to request later parts first, particularly if the   representation consists of pages stored in reverse order and the user   agent wishes to transfer one page at a time.   The Range header field is evaluated after evaluating the precondition   header fields defined in Section 13.1, and only if the result in   absence of the Range header field would be a 200 (OK) response.  In   other words, Range is ignored when a conditional GET would result in   a 304 (Not Modified) response.   The If-Range header field (Section 13.1.5) can be used as a   precondition to applying the Range header field.   If all of the preconditions are true, the server supports the Range   header field for the target resource, the received Range field-value   contains a valid ranges-specifier with a range-unit supported for   that target resource, and that ranges-specifier is satisfiable with   respect to the selected representation, the server SHOULD send a 206   (Partial Content) response with content containing one or more   partial representations that correspond to the satisfiable   range-spec(s) requested.   The above does not imply that a server will send all requested   ranges.  In some cases, it may only be possible (or efficient) to   send a portion of the requested ranges first, while expecting the   client to re-request the remaining portions later if they are still   desired (see Section 15.3.7).   If all of the preconditions are true, the server supports the Range   header field for the target resource, the received Range field-value   contains a valid ranges-specifier, and either the range-unit is not   supported for that target resource or the ranges-specifier is   unsatisfiable with respect to the selected representation, the server   SHOULD send a 416 (Range Not Satisfiable) response.14.3.  Accept-Ranges   The "Accept-Ranges" field in a response indicates whether an upstream   server supports range requests for the target resource.     Accept-Ranges     = acceptable-ranges     acceptable-ranges = 1#range-unit   For example, a server that supports byte-range requests   (Section 14.1.2) can send the field   Accept-Ranges: bytes   to indicate that it supports byte range requests for that target   resource, thereby encouraging its use by the client for future   partial requests on the same request path.  Range units are defined   in Section 14.1.   A client MAY generate range requests regardless of having received an   Accept-Ranges field.  The information only provides advice for the   sake of improving performance and reducing unnecessary network   transfers.   Conversely, a client MUST NOT assume that receiving an Accept-Ranges   field means that future range requests will return partial responses.   The content might change, the server might only support range   requests at certain times or under certain conditions, or a different   intermediary might process the next request.   A server that does not support any kind of range request for the   target resource MAY send   Accept-Ranges: none   to advise the client not to attempt a range request on the same   request path.  The range unit "none" is reserved for this purpose.   The Accept-Ranges field MAY be sent in a trailer section, but is   preferred to be sent as a header field because the information is   particularly useful for restarting large information transfers that   have failed in mid-content (before the trailer section is received).14.4.  Content-Range   The "Content-Range" header field is sent in a single part 206   (Partial Content) response to indicate the partial range of the   selected representation enclosed as the message content, sent in each   part of a multipart 206 response to indicate the range enclosed   within each body part (Section 14.6), and sent in 416 (Range Not   Satisfiable) responses to provide information about the selected   representation.     Content-Range       = range-unit SP                           ( range-resp / unsatisfied-range )     range-resp          = incl-range "/" ( complete-length / "*" )     incl-range          = first-pos "-" last-pos     unsatisfied-range   = "*/" complete-length     complete-length     = 1*DIGIT   If a 206 (Partial Content) response contains a Content-Range header   field with a range unit (Section 14.1) that the recipient does not   understand, the recipient MUST NOT attempt to recombine it with a   stored representation.  A proxy that receives such a message SHOULD   forward it downstream.   Content-Range might also be sent as a request modifier to request a   partial PUT, as described in Section 14.5, based on private   agreements between client and origin server.  A server MUST ignore a   Content-Range header field received in a request with a method for   which Content-Range support is not defined.   For byte ranges, a sender SHOULD indicate the complete length of the   representation from which the range has been extracted, unless the   complete length is unknown or difficult to determine.  An asterisk   character ("*") in place of the complete-length indicates that the   representation length was unknown when the header field was   generated.   The following example illustrates when the complete length of the   selected representation is known by the sender to be 1234 bytes:   Content-Range: bytes 42-1233/1234   and this second example illustrates when the complete length is   unknown:   Content-Range: bytes 42-1233/*   A Content-Range field value is invalid if it contains a range-resp   that has a last-pos value less than its first-pos value, or a   complete-length value less than or equal to its last-pos value.  The   recipient of an invalid Content-Range MUST NOT attempt to recombine   the received content with a stored representation.   A server generating a 416 (Range Not Satisfiable) response to a byte-   range request SHOULD send a Content-Range header field with an   unsatisfied-range value, as in the following example:   Content-Range: bytes */1234   The complete-length in a 416 response indicates the current length of   the selected representation.   The Content-Range header field has no meaning for status codes that   do not explicitly describe its semantic.  For this specification,   only the 206 (Partial Content) and 416 (Range Not Satisfiable) status   codes describe a meaning for Content-Range.   The following are examples of Content-Range values in which the   selected representation contains a total of 1234 bytes:   *  The first 500 bytes:      Content-Range: bytes 0-499/1234   *  The second 500 bytes:      Content-Range: bytes 500-999/1234   *  All except for the first 500 bytes:      Content-Range: bytes 500-1233/1234   *  The last 500 bytes:      Content-Range: bytes 734-1233/123414.5.  Partial PUT   Some origin servers support PUT of a partial representation when the   user agent sends a Content-Range header field (Section 14.4) in the   request, though such support is inconsistent and depends on private   agreements with user agents.  In general, it requests that the state   of the target resource be partly replaced with the enclosed content   at an offset and length indicated by the Content-Range value, where   the offset is relative to the current selected representation.   An origin server SHOULD respond with a 400 (Bad Request) status code   if it receives Content-Range on a PUT for a target resource that does   not support partial PUT requests.   Partial PUT is not backwards compatible with the original definition   of PUT.  It may result in the content being written as a complete   replacement for the current representation.   Partial resource updates are also possible by targeting a separately   identified resource with state that overlaps or extends a portion of   the larger resource, or by using a different method that has been   specifically defined for partial updates (for example, the PATCH   method defined in [RFC5789]).14.6.  Media Type multipart/byteranges   When a 206 (Partial Content) response message includes the content of   multiple ranges, they are transmitted as body parts in a multipart   message body ([RFC2046], Section 5.1) with the media type of   "multipart/byteranges".   The "multipart/byteranges" media type includes one or more body   parts, each with its own Content-Type and Content-Range fields.  The   required boundary parameter specifies the boundary string used to   separate each body part.   Implementation Notes:   1.  Additional CRLFs might precede the first boundary string in the       body.   2.  Although [RFC2046] permits the boundary string to be quoted, some       existing implementations handle a quoted boundary string       incorrectly.   3.  A number of clients and servers were coded to an early draft of       the byteranges specification that used a media type of       "multipart/x-byteranges", which is almost (but not quite)       compatible with this type.   Despite the name, the "multipart/byteranges" media type is not   limited to byte ranges.  The following example uses an "exampleunit"   range unit:   HTTP/1.1 206 Partial Content   Date: Tue, 14 Nov 1995 06:25:24 GMT   Last-Modified: Tue, 14 July 04:58:08 GMT   Content-Length: 2331785   Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES   --THIS_STRING_SEPARATES   Content-Type: video/example   Content-Range: exampleunit 1.2-4.3/25   ...the first range...   --THIS_STRING_SEPARATES   Content-Type: video/example   Content-Range: exampleunit 11.2-14.3/25   ...the second range   --THIS_STRING_SEPARATES--   The following information serves as the registration form for the   "multipart/byteranges" media type.   Type name:  multipart   Subtype name:  byteranges   Required parameters:  boundary   Optional parameters:  N/A   Encoding considerations:  only "7bit", "8bit", or "binary" are      permitted   Security considerations:  see Section 17   Interoperability considerations:  N/A   Published specification:  RFC 9110 (see Section 14.6)   Applications that use this media type:  HTTP components supporting      multiple ranges in a single request   Fragment identifier considerations:  N/A   Additional information:  Deprecated alias names for this type:  N/A                            Magic number(s):  N/A                            File extension(s):  N/A                            Macintosh file type code(s):  N/A   Person and email address to contact for further information:  See Aut      hors' Addresses section.   Intended usage:  COMMON   Restrictions on usage:  N/A   Author:  See Authors' Addresses section.   Change controller:  IESG15.  Status Codes   The status code of a response is a three-digit integer code that   describes the result of the request and the semantics of the   response, including whether the request was successful and what   content is enclosed (if any).  All valid status codes are within the   range of 100 to 599, inclusive.   The first digit of the status code defines the class of response.   The last two digits do not have any categorization role.  There are   five values for the first digit:   *  1xx (Informational): The request was received, continuing process   *  2xx (Successful): The request was successfully received,      understood, and accepted   *  3xx (Redirection): Further action needs to be taken in order to      complete the request   *  4xx (Client Error): The request contains bad syntax or cannot be      fulfilled   *  5xx (Server Error): The server failed to fulfill an apparently      valid request   HTTP status codes are extensible.  A client is not required to   understand the meaning of all registered status codes, though such   understanding is obviously desirable.  However, a client MUST   understand the class of any status code, as indicated by the first   digit, and treat an unrecognized status code as being equivalent to   the x00 status code of that class.   For example, if a client receives an unrecognized status code of 471,   it can see from the first digit that there was something wrong with   its request and treat the response as if it had received a 400 (Bad   Request) status code.  The response message will usually contain a   representation that explains the status.   Values outside the range 100..599 are invalid.  Implementations often   use three-digit integer values outside of that range (i.e., 600..999)   for internal communication of non-HTTP status (e.g., library errors).   A client that receives a response with an invalid status code SHOULD   process the response as if it had a 5xx (Server Error) status code.   A single request can have multiple associated responses: zero or more   "interim" (non-final) responses with status codes in the   "informational" (1xx) range, followed by exactly one "final" response   with a status code in one of the other ranges.15.1.  Overview of Status Codes   The status codes listed below are defined in this specification.  The   reason phrases listed here are only recommendations -- they can be   replaced by local equivalents or left out altogether without   affecting the protocol.   Responses with status codes that are defined as heuristically   cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,   414, and 501 in this specification) can be reused by a cache with   heuristic expiration unless otherwise indicated by the method   definition or explicit cache controls [CACHING]; all other status   codes are not heuristically cacheable.   Additional status codes, outside the scope of this specification,   have been specified for use in HTTP.  All such status codes ought to   be registered within the "Hypertext Transfer Protocol (HTTP) Status   Code Registry", as described in Section 16.2.15.2.  Informational 1xx   The 1xx (Informational) class of status code indicates an interim   response for communicating connection status or request progress   prior to completing the requested action and sending a final   response.  Since HTTP/1.0 did not define any 1xx status codes, a   server MUST NOT send a 1xx response to an HTTP/1.0 client.   A 1xx response is terminated by the end of the header section; it   cannot contain content or trailers.   A client MUST be able to parse one or more 1xx responses received   prior to a final response, even if the client does not expect one.  A   user agent MAY ignore unexpected 1xx responses.   A proxy MUST forward 1xx responses unless the proxy itself requested   the generation of the 1xx response.  For example, if a proxy adds an   "Expect: 100-continue" header field when it forwards a request, then   it need not forward the corresponding 100 (Continue) response(s).15.2.1.  100 Continue   The 100 (Continue) status code indicates that the initial part of a   request has been received and has not yet been rejected by the   server.  The server intends to send a final response after the   request has been fully received and acted upon.   When the request contains an Expect header field that includes a   100-continue expectation, the 100 response indicates that the server   wishes to receive the request content, as described in   Section 10.1.1.  The client ought to continue sending the request and   discard the 100 response.   If the request did not contain an Expect header field containing the   100-continue expectation, the client can simply discard this interim   response.15.2.2.  101 Switching Protocols   The 101 (Switching Protocols) status code indicates that the server   understands and is willing to comply with the client's request, via   the Upgrade header field (Section 7.8), for a change in the   application protocol being used on this connection.  The server MUST   generate an Upgrade header field in the response that indicates which   protocol(s) will be in effect after this response.   It is assumed that the server will only agree to switch protocols   when it is advantageous to do so.  For example, switching to a newer   version of HTTP might be advantageous over older versions, and   switching to a real-time, synchronous protocol might be advantageous   when delivering resources that use such features.15.3.  Successful 2xx   The 2xx (Successful) class of status code indicates that the client's   request was successfully received, understood, and accepted.15.3.1.  200 OK   The 200 (OK) status code indicates that the request has succeeded.   The content sent in a 200 response depends on the request method.   For the methods defined by this specification, the intended meaning   of the content can be summarized as:   +================+============================================+   | Request Method | Response content is a representation of:   |   +================+============================================+   | GET            | the target resource                        |   +----------------+--------------------------------------------+   | HEAD           | the target resource, like GET, but without |   |                | transferring the representation data       |   +----------------+--------------------------------------------+   | POST           | the status of, or results obtained from,   |   |                | the action                                 |   +----------------+--------------------------------------------+   | PUT, DELETE    | the status of the action                   |   +----------------+--------------------------------------------+   | OPTIONS        | communication options for the target       |   |                | resource                                   |   +----------------+--------------------------------------------+   | TRACE          | the request message as received by the     |   |                | server returning the trace                 |   +----------------+--------------------------------------------+                               Table 6   Aside from responses to CONNECT, a 200 response is expected to   contain message content unless the message framing explicitly   indicates that the content has zero length.  If some aspect of the   request indicates a preference for no content upon success, the   origin server ought to send a 204 (No Content) response instead.  For   CONNECT, there is no content because the successful result is a   tunnel, which begins immediately after the 200 response header   section.   A 200 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).   In 200 responses to GET or HEAD, an origin server SHOULD send any   available validator fields (Section 8.8) for the selected   representation, with both a strong entity tag and a Last-Modified   date being preferred.   In 200 responses to state-changing methods, any validator fields   (Section 8.8) sent in the response convey the current validators for   the new representation formed as a result of successfully applying   the request semantics.  Note that the PUT method (Section 9.3.4) has   additional requirements that might preclude sending such validators.15.3.2.  201 Created   The 201 (Created) status code indicates that the request has been   fulfilled and has resulted in one or more new resources being   created.  The primary resource created by the request is identified   by either a Location header field in the response or, if no Location   header field is received, by the target URI.   The 201 response content typically describes and links to the   resource(s) created.  Any validator fields (Section 8.8) sent in the   response convey the current validators for a new representation   created by the request.  Note that the PUT method (Section 9.3.4) has   additional requirements that might preclude sending such validators.15.3.3.  202 Accepted   The 202 (Accepted) status code indicates that the request has been   accepted for processing, but the processing has not been completed.   The request might or might not eventually be acted upon, as it might   be disallowed when processing actually takes place.  There is no   facility in HTTP for re-sending a status code from an asynchronous   operation.   The 202 response is intentionally noncommittal.  Its purpose is to   allow a server to accept a request for some other process (perhaps a   batch-oriented process that is only run once per day) without   requiring that the user agent's connection to the server persist   until the process is completed.  The representation sent with this   response ought to describe the request's current status and point to   (or embed) a status monitor that can provide the user with an   estimate of when the request will be fulfilled.15.3.4.  203 Non-Authoritative Information   The 203 (Non-Authoritative Information) status code indicates that   the request was successful but the enclosed content has been modified   from that of the origin server's 200 (OK) response by a transforming   proxy (Section 7.7).  This status code allows the proxy to notify   recipients when a transformation has been applied, since that   knowledge might impact later decisions regarding the content.  For   example, future cache validation requests for the content might only   be applicable along the same request path (through the same proxies).   A 203 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.3.5.  204 No Content   The 204 (No Content) status code indicates that the server has   successfully fulfilled the request and that there is no additional   content to send in the response content.  Metadata in the response   header fields refer to the target resource and its selected   representation after the requested action was applied.   For example, if a 204 status code is received in response to a PUT   request and the response contains an ETag field, then the PUT was   successful and the ETag field value contains the entity tag for the   new representation of that target resource.   The 204 response allows a server to indicate that the action has been   successfully applied to the target resource, while implying that the   user agent does not need to traverse away from its current "document   view" (if any).  The server assumes that the user agent will provide   some indication of the success to its user, in accord with its own   interface, and apply any new or updated metadata in the response to   its active representation.   For example, a 204 status code is commonly used with document editing   interfaces corresponding to a "save" action, such that the document   being saved remains available to the user for editing.  It is also   frequently used with interfaces that expect automated data transfers   to be prevalent, such as within distributed version control systems.   A 204 response is terminated by the end of the header section; it   cannot contain content or trailers.   A 204 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.3.6.  205 Reset Content   The 205 (Reset Content) status code indicates that the server has   fulfilled the request and desires that the user agent reset the   "document view", which caused the request to be sent, to its original   state as received from the origin server.   This response is intended to support a common data entry use case   where the user receives content that supports data entry (a form,   notepad, canvas, etc.), enters or manipulates data in that space,   causes the entered data to be submitted in a request, and then the   data entry mechanism is reset for the next entry so that the user can   easily initiate another input action.   Since the 205 status code implies that no additional content will be   provided, a server MUST NOT generate content in a 205 response.15.3.7.  206 Partial Content   The 206 (Partial Content) status code indicates that the server is   successfully fulfilling a range request for the target resource by   transferring one or more parts of the selected representation.   A server that supports range requests (Section 14) will usually   attempt to satisfy all of the requested ranges, since sending less   data will likely result in another client request for the remainder.   However, a server might want to send only a subset of the data   requested for reasons of its own, such as temporary unavailability,   cache efficiency, load balancing, etc.  Since a 206 response is self-   descriptive, the client can still understand a response that only   partially satisfies its range request.   A client MUST inspect a 206 response's Content-Type and Content-Range   field(s) to determine what parts are enclosed and whether additional   requests are needed.   A server that generates a 206 response MUST generate the following   header fields, in addition to those required in the subsections   below, if the field would have been sent in a 200 (OK) response to   the same request: Date, Cache-Control, ETag, Expires,   Content-Location, and Vary.   A Content-Length header field present in a 206 response indicates the   number of octets in the content of this message, which is usually not   the complete length of the selected representation.  Each   Content-Range header field includes information about the selected   representation's complete length.   A sender that generates a 206 response to a request with an If-Range   header field SHOULD NOT generate other representation header fields   beyond those required because the client already has a prior response   containing those header fields.  Otherwise, a sender MUST generate   all of the representation header fields that would have been sent in   a 200 (OK) response to the same request.   A 206 response is heuristically cacheable; i.e., unless otherwise   indicated by explicit cache controls (see Section 4.2.2 of   [CACHING]).15.3.7.1.  Single Part   If a single part is being transferred, the server generating the 206   response MUST generate a Content-Range header field, describing what   range of the selected representation is enclosed, and a content   consisting of the range.  For example:   HTTP/1.1 206 Partial Content   Date: Wed, 15 Nov 1995 06:25:24 GMT   Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT   Content-Range: bytes 21010-47021/47022   Content-Length: 26012   Content-Type: image/gif   ... 26012 bytes of partial image data ...15.3.7.2.  Multiple Parts   If multiple parts are being transferred, the server generating the   206 response MUST generate "multipart/byteranges" content, as defined   in Section 14.6, and a Content-Type header field containing the   "multipart/byteranges" media type and its required boundary   parameter.  To avoid confusion with single-part responses, a server   MUST NOT generate a Content-Range header field in the HTTP header   section of a multiple part response (this field will be sent in each   part instead).   Within the header area of each body part in the multipart content,   the server MUST generate a Content-Range header field corresponding   to the range being enclosed in that body part.  If the selected   representation would have had a Content-Type header field in a 200   (OK) response, the server SHOULD generate that same Content-Type   header field in the header area of each body part.  For example:   HTTP/1.1 206 Partial Content   Date: Wed, 15 Nov 1995 06:25:24 GMT   Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT   Content-Length: 1741   Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES   --THIS_STRING_SEPARATES   Content-Type: application/pdf   Content-Range: bytes 500-999/8000   ...the first range...   --THIS_STRING_SEPARATES   Content-Type: application/pdf   Content-Range: bytes 7000-7999/8000   ...the second range   --THIS_STRING_SEPARATES--   When multiple ranges are requested, a server MAY coalesce any of the   ranges that overlap, or that are separated by a gap that is smaller   than the overhead of sending multiple parts, regardless of the order   in which the corresponding range-spec appeared in the received Range   header field.  Since the typical overhead between each part of a   "multipart/byteranges" is around 80 bytes, depending on the selected   representation's media type and the chosen boundary parameter length,   it can be less efficient to transfer many small disjoint parts than   it is to transfer the entire selected representation.   A server MUST NOT generate a multipart response to a request for a   single range, since a client that does not request multiple parts   might not support multipart responses.  However, a server MAY   generate a "multipart/byteranges" response with only a single body   part if multiple ranges were requested and only one range was found   to be satisfiable or only one range remained after coalescing.  A   client that cannot process a "multipart/byteranges" response MUST NOT   generate a request that asks for multiple ranges.   A server that generates a multipart response SHOULD send the parts in   the same order that the corresponding range-spec appeared in the   received Range header field, excluding those ranges that were deemed   unsatisfiable or that were coalesced into other ranges.  A client   that receives a multipart response MUST inspect the Content-Range   header field present in each body part in order to determine which   range is contained in that body part; a client cannot rely on   receiving the same ranges that it requested, nor the same order that   it requested.15.3.7.3.  Combining Parts   A response might transfer only a subrange of a representation if the   connection closed prematurely or if the request used one or more   Range specifications.  After several such transfers, a client might   have received several ranges of the same representation.  These   ranges can only be safely combined if they all have in common the   same strong validator (Section 8.8.1).   A client that has received multiple partial responses to GET requests   on a target resource MAY combine those responses into a larger   continuous range if they share the same strong validator.   If the most recent response is an incomplete 200 (OK) response, then   the header fields of that response are used for any combined response   and replace those of the matching stored responses.   If the most recent response is a 206 (Partial Content) response and   at least one of the matching stored responses is a 200 (OK), then the   combined response header fields consist of the most recent 200   response's header fields.  If all of the matching stored responses   are 206 responses, then the stored response with the most recent   header fields is used as the source of header fields for the combined   response, except that the client MUST use other header fields   provided in the new response, aside from Content-Range, to replace   all instances of the corresponding header fields in the stored   response.   The combined response content consists of the union of partial   content ranges within the new response and all of the matching stored   responses.  If the union consists of the entire range of the   representation, then the client MUST process the combined response as   if it were a complete 200 (OK) response, including a Content-Length   header field that reflects the complete length.  Otherwise, the   client MUST process the set of continuous ranges as one of the   following: an incomplete 200 (OK) response if the combined response   is a prefix of the representation, a single 206 (Partial Content)   response containing "multipart/byteranges" content, or multiple 206   (Partial Content) responses, each with one continuous range that is   indicated by a Content-Range header field.15.4.  Redirection 3xx   The 3xx (Redirection) class of status code indicates that further   action needs to be taken by the user agent in order to fulfill the   request.  There are several types of redirects:   1.  Redirects that indicate this resource might be available at a       different URI, as provided by the Location header field, as in       the status codes 301 (Moved Permanently), 302 (Found), 307       (Temporary Redirect), and 308 (Permanent Redirect).   2.  Redirection that offers a choice among matching resources capable       of representing this resource, as in the 300 (Multiple Choices)       status code.   3.  Redirection to a different resource, identified by the Location       header field, that can represent an indirect response to the       request, as in the 303 (See Other) status code.   4.  Redirection to a previously stored result, as in the 304 (Not       Modified) status code.      |  *Note:* In HTTP/1.0, the status codes 301 (Moved Permanently)      |  and 302 (Found) were originally defined as method-preserving      |  ([HTTP/1.0], Section 9.3) to match their implementation at      |  CERN; 303 (See Other) was defined for a redirection that      |  changed its method to GET.  However, early user agents split on      |  whether to redirect POST requests as POST (according to then-      |  current specification) or as GET (the safer alternative when      |  redirected to a different site).  Prevailing practice      |  eventually converged on changing the method to GET.  307      |  (Temporary Redirect) and 308 (Permanent Redirect) [RFC7538]      |  were later added to unambiguously indicate method-preserving      |  redirects, and status codes 301 and 302 have been adjusted to      |  allow a POST request to be redirected as GET.   If a Location header field (Section 10.2.2) is provided, the user   agent MAY automatically redirect its request to the URI referenced by   the Location field value, even if the specific status code is not   understood.  Automatic redirection needs to be done with care for   methods not known to be safe, as defined in Section 9.2.1, since the   user might not wish to redirect an unsafe request.   When automatically following a redirected request, the user agent   SHOULD resend the original request message with the following   modifications:   1.  Replace the target URI with the URI referenced by the redirection       response's Location header field value after resolving it       relative to the original request's target URI.   2.  Remove header fields that were automatically generated by the       implementation, replacing them with updated values as appropriate       to the new request.  This includes:       1.  Connection-specific header fields (see Section 7.6.1),       2.  Header fields specific to the client's proxy configuration,           including (but not limited to) Proxy-Authorization,       3.  Origin-specific header fields (if any), including (but not           limited to) Host,       4.  Validating header fields that were added by the           implementation's cache (e.g., If-None-Match,           If-Modified-Since), and       5.  Resource-specific header fields, including (but not limited           to) Referer, Origin, Authorization, and Cookie.   3.  Consider removing header fields that were not automatically       generated by the implementation (i.e., those present in the       request because they were added by the calling context) where       there are security implications; this includes but is not limited       to Authorization and Cookie.   4.  Change the request method according to the redirecting status       code's semantics, if applicable.   5.  If the request method has been changed to GET or HEAD, remove       content-specific header fields, including (but not limited to)       Content-Encoding, Content-Language, Content-Location,       Content-Type, Content-Length, Digest, Last-Modified.   A client SHOULD detect and intervene in cyclical redirections (i.e.,   "infinite" redirection loops).      |  *Note:* An earlier version of this specification recommended a      |  maximum of five redirections ([RFC2068], Section 10.3).      |  Content developers need to be aware that some clients might      |  implement such a fixed limitation.15.4.1.  300 Multiple Choices   The 300 (Multiple Choices) status code indicates that the target   resource has more than one representation, each with its own more   specific identifier, and information about the alternatives is being   provided so that the user (or user agent) can select a preferred   representation by redirecting its request to one or more of those   identifiers.  In other words, the server desires that the user agent   engage in reactive negotiation to select the most appropriate   representation(s) for its needs (Section 12).   If the server has a preferred choice, the server SHOULD generate a   Location header field containing a preferred choice's URI reference.   The user agent MAY use the Location field value for automatic   redirection.   For request methods other than HEAD, the server SHOULD generate   content in the 300 response containing a list of representation   metadata and URI reference(s) from which the user or user agent can   choose the one most preferred.  The user agent MAY make a selection   from that list automatically if it understands the provided media   type.  A specific format for automatic selection is not defined by   this specification because HTTP tries to remain orthogonal to the   definition of its content.  In practice, the representation is   provided in some easily parsed format believed to be acceptable to   the user agent, as determined by shared design or content   negotiation, or in some commonly accepted hypertext format.   A 300 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).      |  *Note:* The original proposal for the 300 status code defined      |  the URI header field as providing a list of alternative      |  representations, such that it would be usable for 200, 300, and      |  406 responses and be transferred in responses to the HEAD      |  method.  However, lack of deployment and disagreement over      |  syntax led to both URI and Alternates (a subsequent proposal)      |  being dropped from this specification.  It is possible to      |  communicate the list as a Link header field value [RFC8288]      |  whose members have a relationship of "alternate", though      |  deployment is a chicken-and-egg problem.15.4.2.  301 Moved Permanently   The 301 (Moved Permanently) status code indicates that the target   resource has been assigned a new permanent URI and any future   references to this resource ought to use one of the enclosed URIs.   The server is suggesting that a user agent with link-editing   capability can permanently replace references to the target URI with   one of the new references sent by the server.  However, this   suggestion is usually ignored unless the user agent is actively   editing references (e.g., engaged in authoring content), the   connection is secured, and the origin server is a trusted authority   for the content being edited.   The server SHOULD generate a Location header field in the response   containing a preferred URI reference for the new permanent URI.  The   user agent MAY use the Location field value for automatic   redirection.  The server's response content usually contains a short   hypertext note with a hyperlink to the new URI(s).      |  *Note:* For historical reasons, a user agent MAY change the      |  request method from POST to GET for the subsequent request.  If      |  this behavior is undesired, the 308 (Permanent Redirect) status      |  code can be used instead.   A 301 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.4.3.  302 Found   The 302 (Found) status code indicates that the target resource   resides temporarily under a different URI.  Since the redirection   might be altered on occasion, the client ought to continue to use the   target URI for future requests.   The server SHOULD generate a Location header field in the response   containing a URI reference for the different URI.  The user agent MAY   use the Location field value for automatic redirection.  The server's   response content usually contains a short hypertext note with a   hyperlink to the different URI(s).      |  *Note:* For historical reasons, a user agent MAY change the      |  request method from POST to GET for the subsequent request.  If      |  this behavior is undesired, the 307 (Temporary Redirect) status      |  code can be used instead.15.4.4.  303 See Other   The 303 (See Other) status code indicates that the server is   redirecting the user agent to a different resource, as indicated by a   URI in the Location header field, which is intended to provide an   indirect response to the original request.  A user agent can perform   a retrieval request targeting that URI (a GET or HEAD request if   using HTTP), which might also be redirected, and present the eventual   result as an answer to the original request.  Note that the new URI   in the Location header field is not considered equivalent to the   target URI.   This status code is applicable to any HTTP method.  It is primarily   used to allow the output of a POST action to redirect the user agent   to a different resource, since doing so provides the information   corresponding to the POST response as a resource that can be   separately identified, bookmarked, and cached.   A 303 response to a GET request indicates that the origin server does   not have a representation of the target resource that can be   transferred by the server over HTTP.  However, the Location field   value refers to a resource that is descriptive of the target   resource, such that making a retrieval request on that other resource   might result in a representation that is useful to recipients without   implying that it represents the original target resource.  Note that   answers to the questions of what can be represented, what   representations are adequate, and what might be a useful description   are outside the scope of HTTP.   Except for responses to a HEAD request, the representation of a 303   response ought to contain a short hypertext note with a hyperlink to   the same URI reference provided in the Location header field.15.4.5.  304 Not Modified   The 304 (Not Modified) status code indicates that a conditional GET   or HEAD request has been received and would have resulted in a 200   (OK) response if it were not for the fact that the condition   evaluated to false.  In other words, there is no need for the server   to transfer a representation of the target resource because the   request indicates that the client, which made the request   conditional, already has a valid representation; the server is   therefore redirecting the client to make use of that stored   representation as if it were the content of a 200 (OK) response.   The server generating a 304 response MUST generate any of the   following header fields that would have been sent in a 200 (OK)   response to the same request:   *  Content-Location, Date, ETag, and Vary   *  Cache-Control and Expires (see [CACHING])   Since the goal of a 304 response is to minimize information transfer   when the recipient already has one or more cached representations, a   sender SHOULD NOT generate representation metadata other than the   above listed fields unless said metadata exists for the purpose of   guiding cache updates (e.g., Last-Modified might be useful if the   response does not have an ETag field).   Requirements on a cache that receives a 304 response are defined in   Section 4.3.4 of [CACHING].  If the conditional request originated   with an outbound client, such as a user agent with its own cache   sending a conditional GET to a shared proxy, then the proxy SHOULD   forward the 304 response to that client.   A 304 response is terminated by the end of the header section; it   cannot contain content or trailers.15.4.6.  305 Use Proxy   The 305 (Use Proxy) status code was defined in a previous version of   this specification and is now deprecated (Appendix B of [RFC7231]).15.4.7.  306 (Unused)   The 306 status code was defined in a previous version of this   specification, is no longer used, and the code is reserved.15.4.8.  307 Temporary Redirect   The 307 (Temporary Redirect) status code indicates that the target   resource resides temporarily under a different URI and the user agent   MUST NOT change the request method if it performs an automatic   redirection to that URI.  Since the redirection can change over time,   the client ought to continue using the original target URI for future   requests.   The server SHOULD generate a Location header field in the response   containing a URI reference for the different URI.  The user agent MAY   use the Location field value for automatic redirection.  The server's   response content usually contains a short hypertext note with a   hyperlink to the different URI(s).15.4.9.  308 Permanent Redirect   The 308 (Permanent Redirect) status code indicates that the target   resource has been assigned a new permanent URI and any future   references to this resource ought to use one of the enclosed URIs.   The server is suggesting that a user agent with link-editing   capability can permanently replace references to the target URI with   one of the new references sent by the server.  However, this   suggestion is usually ignored unless the user agent is actively   editing references (e.g., engaged in authoring content), the   connection is secured, and the origin server is a trusted authority   for the content being edited.   The server SHOULD generate a Location header field in the response   containing a preferred URI reference for the new permanent URI.  The   user agent MAY use the Location field value for automatic   redirection.  The server's response content usually contains a short   hypertext note with a hyperlink to the new URI(s).   A 308 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).      |  *Note:* This status code is much younger (June 2014) than its      |  sibling codes and thus might not be recognized everywhere.  See      |  Section 4 of [RFC7538] for deployment considerations.15.5.  Client Error 4xx   The 4xx (Client Error) class of status code indicates that the client   seems to have erred.  Except when responding to a HEAD request, the   server SHOULD send a representation containing an explanation of the   error situation, and whether it is a temporary or permanent   condition.  These status codes are applicable to any request method.   User agents SHOULD display any included representation to the user.15.5.1.  400 Bad Request   The 400 (Bad Request) status code indicates that the server cannot or   will not process the request due to something that is perceived to be   a client error (e.g., malformed request syntax, invalid request   message framing, or deceptive request routing).15.5.2.  401 Unauthorized   The 401 (Unauthorized) status code indicates that the request has not   been applied because it lacks valid authentication credentials for   the target resource.  The server generating a 401 response MUST send   a WWW-Authenticate header field (Section 11.6.1) containing at least   one challenge applicable to the target resource.   If the request included authentication credentials, then the 401   response indicates that authorization has been refused for those   credentials.  The user agent MAY repeat the request with a new or   replaced Authorization header field (Section 11.6.2).  If the 401   response contains the same challenge as the prior response, and the   user agent has already attempted authentication at least once, then   the user agent SHOULD present the enclosed representation to the   user, since it usually contains relevant diagnostic information.15.5.3.  402 Payment Required   The 402 (Payment Required) status code is reserved for future use.15.5.4.  403 Forbidden   The 403 (Forbidden) status code indicates that the server understood   the request but refuses to fulfill it.  A server that wishes to make   public why the request has been forbidden can describe that reason in   the response content (if any).   If authentication credentials were provided in the request, the   server considers them insufficient to grant access.  The client   SHOULD NOT automatically repeat the request with the same   credentials.  The client MAY repeat the request with new or different   credentials.  However, a request might be forbidden for reasons   unrelated to the credentials.   An origin server that wishes to "hide" the current existence of a   forbidden target resource MAY instead respond with a status code of   404 (Not Found).15.5.5.  404 Not Found   The 404 (Not Found) status code indicates that the origin server did   not find a current representation for the target resource or is not   willing to disclose that one exists.  A 404 status code does not   indicate whether this lack of representation is temporary or   permanent; the 410 (Gone) status code is preferred over 404 if the   origin server knows, presumably through some configurable means, that   the condition is likely to be permanent.   A 404 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.5.6.  405 Method Not Allowed   The 405 (Method Not Allowed) status code indicates that the method   received in the request-line is known by the origin server but not   supported by the target resource.  The origin server MUST generate an   Allow header field in a 405 response containing a list of the target   resource's currently supported methods.   A 405 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.5.7.  406 Not Acceptable   The 406 (Not Acceptable) status code indicates that the target   resource does not have a current representation that would be   acceptable to the user agent, according to the proactive negotiation   header fields received in the request (Section 12.1), and the server   is unwilling to supply a default representation.   The server SHOULD generate content containing a list of available   representation characteristics and corresponding resource identifiers   from which the user or user agent can choose the one most   appropriate.  A user agent MAY automatically select the most   appropriate choice from that list.  However, this specification does   not define any standard for such automatic selection, as described in   Section 15.4.1.15.5.8.  407 Proxy Authentication Required   The 407 (Proxy Authentication Required) status code is similar to 401   (Unauthorized), but it indicates that the client needs to   authenticate itself in order to use a proxy for this request.  The   proxy MUST send a Proxy-Authenticate header field (Section 11.7.1)   containing a challenge applicable to that proxy for the request.  The   client MAY repeat the request with a new or replaced   Proxy-Authorization header field (Section 11.7.2).15.5.9.  408 Request Timeout   The 408 (Request Timeout) status code indicates that the server did   not receive a complete request message within the time that it was   prepared to wait.   If the client has an outstanding request in transit, it MAY repeat   that request.  If the current connection is not usable (e.g., as it   would be in HTTP/1.1 because request delimitation is lost), a new   connection will be used.15.5.10.  409 Conflict   The 409 (Conflict) status code indicates that the request could not   be completed due to a conflict with the current state of the target   resource.  This code is used in situations where the user might be   able to resolve the conflict and resubmit the request.  The server   SHOULD generate content that includes enough information for a user   to recognize the source of the conflict.   Conflicts are most likely to occur in response to a PUT request.  For   example, if versioning were being used and the representation being   PUT included changes to a resource that conflict with those made by   an earlier (third-party) request, the origin server might use a 409   response to indicate that it can't complete the request.  In this   case, the response representation would likely contain information   useful for merging the differences based on the revision history.15.5.11.  410 Gone   The 410 (Gone) status code indicates that access to the target   resource is no longer available at the origin server and that this   condition is likely to be permanent.  If the origin server does not   know, or has no facility to determine, whether or not the condition   is permanent, the status code 404 (Not Found) ought to be used   instead.   The 410 response is primarily intended to assist the task of web   maintenance by notifying the recipient that the resource is   intentionally unavailable and that the server owners desire that   remote links to that resource be removed.  Such an event is common   for limited-time, promotional services and for resources belonging to   individuals no longer associated with the origin server's site.  It   is not necessary to mark all permanently unavailable resources as   "gone" or to keep the mark for any length of time -- that is left to   the discretion of the server owner.   A 410 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.5.12.  411 Length Required   The 411 (Length Required) status code indicates that the server   refuses to accept the request without a defined Content-Length   (Section 8.6).  The client MAY repeat the request if it adds a valid   Content-Length header field containing the length of the request   content.15.5.13.  412 Precondition Failed   The 412 (Precondition Failed) status code indicates that one or more   conditions given in the request header fields evaluated to false when   tested on the server (Section 13).  This response status code allows   the client to place preconditions on the current resource state (its   current representations and metadata) and, thus, prevent the request   method from being applied if the target resource is in an unexpected   state.15.5.14.  413 Content Too Large   The 413 (Content Too Large) status code indicates that the server is   refusing to process a request because the request content is larger   than the server is willing or able to process.  The server MAY   terminate the request, if the protocol version in use allows it;   otherwise, the server MAY close the connection.   If the condition is temporary, the server SHOULD generate a   Retry-After header field to indicate that it is temporary and after   what time the client MAY try again.15.5.15.  414 URI Too Long   The 414 (URI Too Long) status code indicates that the server is   refusing to service the request because the target URI is longer than   the server is willing to interpret.  This rare condition is only   likely to occur when a client has improperly converted a POST request   to a GET request with long query information, when the client has   descended into an infinite loop of redirection (e.g., a redirected   URI prefix that points to a suffix of itself) or when the server is   under attack by a client attempting to exploit potential security   holes.   A 414 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.5.16.  415 Unsupported Media Type   The 415 (Unsupported Media Type) status code indicates that the   origin server is refusing to service the request because the content   is in a format not supported by this method on the target resource.   The format problem might be due to the request's indicated   Content-Type or Content-Encoding, or as a result of inspecting the   data directly.   If the problem was caused by an unsupported content coding, the   Accept-Encoding response header field (Section 12.5.3) ought to be   used to indicate which (if any) content codings would have been   accepted in the request.   On the other hand, if the cause was an unsupported media type, the   Accept response header field (Section 12.5.1) can be used to indicate   which media types would have been accepted in the request.15.5.17.  416 Range Not Satisfiable   The 416 (Range Not Satisfiable) status code indicates that the set of   ranges in the request's Range header field (Section 14.2) has been   rejected either because none of the requested ranges are satisfiable   or because the client has requested an excessive number of small or   overlapping ranges (a potential denial of service attack).   Each range unit defines what is required for its own range sets to be   satisfiable.  For example, Section 14.1.2 defines what makes a bytes   range set satisfiable.   A server that generates a 416 response to a byte-range request SHOULD   generate a Content-Range header field specifying the current length   of the selected representation (Section 14.4).   For example:   HTTP/1.1 416 Range Not Satisfiable   Date: Fri, 20 Jan 2012 15:41:54 GMT   Content-Range: bytes */47022      |  *Note:* Because servers are free to ignore Range, many      |  implementations will respond with the entire selected      |  representation in a 200 (OK) response.  That is partly because      |  most clients are prepared to receive a 200 (OK) to complete the      |  task (albeit less efficiently) and partly because clients might      |  not stop making an invalid range request until they have      |  received a complete representation.  Thus, clients cannot      |  depend on receiving a 416 (Range Not Satisfiable) response even      |  when it is most appropriate.15.5.18.  417 Expectation Failed   The 417 (Expectation Failed) status code indicates that the   expectation given in the request's Expect header field   (Section 10.1.1) could not be met by at least one of the inbound   servers.15.5.19.  418 (Unused)   [RFC2324] was an April 1 RFC that lampooned the various ways HTTP was   abused; one such abuse was the definition of an application-specific   418 status code, which has been deployed as a joke often enough for   the code to be unusable for any future use.   Therefore, the 418 status code is reserved in the IANA HTTP Status   Code Registry.  This indicates that the status code cannot be   assigned to other applications currently.  If future circumstances   require its use (e.g., exhaustion of 4NN status codes), it can be re-   assigned to another use.15.5.20.  421 Misdirected Request   The 421 (Misdirected Request) status code indicates that the request   was directed at a server that is unable or unwilling to produce an   authoritative response for the target URI.  An origin server (or   gateway acting on behalf of the origin server) sends 421 to reject a   target URI that does not match an origin for which the server has   been configured (Section 4.3.1) or does not match the connection   context over which the request was received (Section 7.4).   A client that receives a 421 (Misdirected Request) response MAY retry   the request, whether or not the request method is idempotent, over a   different connection, such as a fresh connection specific to the   target resource's origin, or via an alternative service [ALTSVC].   A proxy MUST NOT generate a 421 response.15.5.21.  422 Unprocessable Content   The 422 (Unprocessable Content) status code indicates that the server   understands the content type of the request content (hence a 415   (Unsupported Media Type) status code is inappropriate), and the   syntax of the request content is correct, but it was unable to   process the contained instructions.  For example, this status code   can be sent if an XML request content contains well-formed (i.e.,   syntactically correct), but semantically erroneous XML instructions.15.5.22.  426 Upgrade Required   The 426 (Upgrade Required) status code indicates that the server   refuses to perform the request using the current protocol but might   be willing to do so after the client upgrades to a different   protocol.  The server MUST send an Upgrade header field in a 426   response to indicate the required protocol(s) (Section 7.8).   Example:   HTTP/1.1 426 Upgrade Required   Upgrade: HTTP/3.0   Connection: Upgrade   Content-Length: 53   Content-Type: text/plain   This service requires use of the HTTP/3.0 protocol.15.6.  Server Error 5xx   The 5xx (Server Error) class of status code indicates that the server   is aware that it has erred or is incapable of performing the   requested method.  Except when responding to a HEAD request, the   server SHOULD send a representation containing an explanation of the   error situation, and whether it is a temporary or permanent   condition.  A user agent SHOULD display any included representation   to the user.  These status codes are applicable to any request   method.15.6.1.  500 Internal Server Error   The 500 (Internal Server Error) status code indicates that the server   encountered an unexpected condition that prevented it from fulfilling   the request.15.6.2.  501 Not Implemented   The 501 (Not Implemented) status code indicates that the server does   not support the functionality required to fulfill the request.  This   is the appropriate response when the server does not recognize the   request method and is not capable of supporting it for any resource.   A 501 response is heuristically cacheable; i.e., unless otherwise   indicated by the method definition or explicit cache controls (see   Section 4.2.2 of [CACHING]).15.6.3.  502 Bad Gateway   The 502 (Bad Gateway) status code indicates that the server, while   acting as a gateway or proxy, received an invalid response from an   inbound server it accessed while attempting to fulfill the request.15.6.4.  503 Service Unavailable   The 503 (Service Unavailable) status code indicates that the server   is currently unable to handle the request due to a temporary overload   or scheduled maintenance, which will likely be alleviated after some   delay.  The server MAY send a Retry-After header field   (Section 10.2.3) to suggest an appropriate amount of time for the   client to wait before retrying the request.      |  *Note:* The existence of the 503 status code does not imply      |  that a server has to use it when becoming overloaded.  Some      |  servers might simply refuse the connection.15.6.5.  504 Gateway Timeout   The 504 (Gateway Timeout) status code indicates that the server,   while acting as a gateway or proxy, did not receive a timely response   from an upstream server it needed to access in order to complete the   request.15.6.6.  505 HTTP Version Not Supported   The 505 (HTTP Version Not Supported) status code indicates that the   server does not support, or refuses to support, the major version of   HTTP that was used in the request message.  The server is indicating   that it is unable or unwilling to complete the request using the same   major version as the client, as described in Section 2.5, other than   with this error message.  The server SHOULD generate a representation   for the 505 response that describes why that version is not supported   and what other protocols are supported by that server.16.  Extending HTTP   HTTP defines a number of generic extension points that can be used to   introduce capabilities to the protocol without introducing a new   version, including methods, status codes, field names, and further   extensibility points within defined fields, such as authentication   schemes and cache directives (see Cache-Control extensions in   Section 5.2.3 of [CACHING]).  Because the semantics of HTTP are not   versioned, these extension points are persistent; the version of the   protocol in use does not affect their semantics.   Version-independent extensions are discouraged from depending on or   interacting with the specific version of the protocol in use.  When   this is unavoidable, careful consideration needs to be given to how   the extension can interoperate across versions.   Additionally, specific versions of HTTP might have their own   extensibility points, such as transfer codings in HTTP/1.1   (Section 6.1 of [HTTP/1.1]) and HTTP/2 SETTINGS or frame types   ([HTTP/2]).  These extension points are specific to the version of   the protocol they occur within.   Version-specific extensions cannot override or modify the semantics   of a version-independent mechanism or extension point (like a method   or header field) without explicitly being allowed by that protocol   element.  For example, the CONNECT method (Section 9.3.6) allows   this.   These guidelines assure that the protocol operates correctly and   predictably, even when parts of the path implement different versions   of HTTP.16.1.  Method Extensibility16.1.1.  Method Registry   The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained   by IANA at <https://www.iana.org/assignments/http-methods>, registers   method names.   HTTP method registrations MUST include the following fields:   *  Method Name (see Section 9)   *  Safe ("yes" or "no", see Section 9.2.1)   *  Idempotent ("yes" or "no", see Section 9.2.2)   *  Pointer to specification text   Values to be added to this namespace require IETF Review (see   [RFC8126], Section 4.8).16.1.2.  Considerations for New Methods   Standardized methods are generic; that is, they are potentially   applicable to any resource, not just one particular media type, kind   of resource, or application.  As such, it is preferred that new   methods be registered in a document that isn't specific to a single   application or data format, since orthogonal technologies deserve   orthogonal specification.   Since message parsing (Section 6) needs to be independent of method   semantics (aside from responses to HEAD), definitions of new methods   cannot change the parsing algorithm or prohibit the presence of   content on either the request or the response message.  Definitions   of new methods can specify that only a zero-length content is allowed   by requiring a Content-Length header field with a value of "0".   Likewise, new methods cannot use the special host:port and asterisk   forms of request target that are allowed for CONNECT and OPTIONS,   respectively (Section 7.1).  A full URI in absolute form is needed   for the target URI, which means either the request target needs to be   sent in absolute form or the target URI will be reconstructed from   the request context in the same way it is for other methods.   A new method definition needs to indicate whether it is safe   (Section 9.2.1), idempotent (Section 9.2.2), cacheable   (Section 9.2.3), what semantics are to be associated with the request   content (if any), and what refinements the method makes to header   field or status code semantics.  If the new method is cacheable, its   definition ought to describe how, and under what conditions, a cache   can store a response and use it to satisfy a subsequent request.  The   new method ought to describe whether it can be made conditional   (Section 13.1) and, if so, how a server responds when the condition   is false.  Likewise, if the new method might have some use for   partial response semantics (Section 14.2), it ought to document this,   too.      |  *Note:* Avoid defining a method name that starts with "M-",      |  since that prefix might be misinterpreted as having the      |  semantics assigned to it by [RFC2774].16.2.  Status Code Extensibility16.2.1.  Status Code Registry   The "Hypertext Transfer Protocol (HTTP) Status Code Registry",   maintained by IANA at <https://www.iana.org/assignments/http-status-   codes>, registers status code numbers.   A registration MUST include the following fields:   *  Status Code (3 digits)   *  Short Description   *  Pointer to specification text   Values to be added to the HTTP status code namespace require IETF   Review (see [RFC8126], Section 4.8).16.2.2.  Considerations for New Status Codes   When it is necessary to express semantics for a response that are not   defined by current status codes, a new status code can be registered.   Status codes are generic; they are potentially applicable to any   resource, not just one particular media type, kind of resource, or   application of HTTP.  As such, it is preferred that new status codes   be registered in a document that isn't specific to a single   application.   New status codes are required to fall under one of the categories   defined in Section 15.  To allow existing parsers to process the   response message, new status codes cannot disallow content, although   they can mandate a zero-length content.   Proposals for new status codes that are not yet widely deployed ought   to avoid allocating a specific number for the code until there is   clear consensus that it will be registered; instead, early drafts can   use a notation such as "4NN", or "3N0" .. "3N9", to indicate the   class of the proposed status code(s) without consuming a number   prematurely.   The definition of a new status code ought to explain the request   conditions that would cause a response containing that status code   (e.g., combinations of request header fields and/or method(s)) along   with any dependencies on response header fields (e.g., what fields   are required, what fields can modify the semantics, and what field   semantics are further refined when used with the new status code).   By default, a status code applies only to the request corresponding   to the response it occurs within.  If a status code applies to a   larger scope of applicability -- for example, all requests to the   resource in question or all requests to a server -- this must be   explicitly specified.  When doing so, it should be noted that not all   clients can be expected to consistently apply a larger scope because   they might not understand the new status code.   The definition of a new final status code ought to specify whether or   not it is heuristically cacheable.  Note that any response with a   final status code can be cached if the response has explicit   freshness information.  A status code defined as heuristically   cacheable is allowed to be cached without explicit freshness   information.  Likewise, the definition of a status code can place   constraints upon cache behavior if the must-understand cache   directive is used.  See [CACHING] for more information.   Finally, the definition of a new status code ought to indicate   whether the content has any implied association with an identified   resource (Section 6.4.2).16.3.  Field Extensibility   HTTP's most widely used extensibility point is the definition of new   header and trailer fields.   New fields can be defined such that, when they are understood by a   recipient, they override or enhance the interpretation of previously   defined fields, define preconditions on request evaluation, or refine   the meaning of responses.   However, defining a field doesn't guarantee its deployment or   recognition by recipients.  Most fields are designed with the   expectation that a recipient can safely ignore (but forward   downstream) any field not recognized.  In other cases, the sender's   ability to understand a given field might be indicated by its prior   communication, perhaps in the protocol version or fields that it sent   in prior messages, or its use of a specific media type.  Likewise,   direct inspection of support might be possible through an OPTIONS   request or by interacting with a defined well-known URI [RFC8615] if   such inspection is defined along with the field being introduced.16.3.1.  Field Name Registry   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines   the namespace for HTTP field names.   Any party can request registration of an HTTP field.  See   Section 16.3.2 for considerations to take into account when creating   a new HTTP field.   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is   located at <https://www.iana.org/assignments/http-fields/>.   Registration requests can be made by following the instructions   located there or by sending an email to the "ietf-http-wg@w3.org"   mailing list.   Field names are registered on the advice of a designated expert   (appointed by the IESG or their delegate).  Fields with the status   'permanent' are Specification Required ([RFC8126], Section 4.6).   Registration requests consist of the following information:   Field name:      The requested field name.  It MUST conform to the field-name      syntax defined in Section 5.1, and it SHOULD be restricted to just      letters, digits, and hyphen ('-') characters, with the first      character being a letter.   Status:      "permanent", "provisional", "deprecated", or "obsoleted".   Specification document(s):      Reference to the document that specifies the field, preferably      including a URI that can be used to retrieve a copy of the      document.  Optional but encouraged for provisional registrations.      An indication of the relevant section(s) can also be included, but      is not required.   And optionally:   Comments:  Additional information, such as about reserved entries.   The expert(s) can define additional fields to be collected in the   registry, in consultation with the community.   Standards-defined names have a status of "permanent".  Other names   can also be registered as permanent if the expert(s) finds that they   are in use, in consultation with the community.  Other names should   be registered as "provisional".   Provisional entries can be removed by the expert(s) if -- in   consultation with the community -- the expert(s) find that they are   not in use.  The expert(s) can change a provisional entry's status to   permanent at any time.   Note that names can be registered by third parties (including the   expert(s)) if the expert(s) determines that an unregistered name is   widely deployed and not likely to be registered in a timely manner   otherwise.16.3.2.  Considerations for New Fields   HTTP header and trailer fields are a widely used extension point for   the protocol.  While they can be used in an ad hoc fashion, fields   that are intended for wider use need to be carefully documented to   ensure interoperability.   In particular, authors of specifications defining new fields are   advised to consider and, where appropriate, document the following   aspects:   *  Under what conditions the field can be used; e.g., only in      responses or requests, in all messages, only on responses to a      particular request method, etc.   *  Whether the field semantics are further refined by their context,      such as their use with certain request methods or status codes.   *  The scope of applicability for the information conveyed.  By      default, fields apply only to the message they are associated      with, but some response fields are designed to apply to all      representations of a resource, the resource itself, or an even      broader scope.  Specifications that expand the scope of a response      field will need to carefully consider issues such as content      negotiation, the time period of applicability, and (in some cases)      multi-tenant server deployments.   *  Under what conditions intermediaries are allowed to insert,      delete, or modify the field's value.   *  If the field is allowable in trailers; by default, it will not be      (see Section 6.5.1).   *  Whether it is appropriate or even required to list the field name      in the Connection header field (i.e., if the field is to be hop-      by-hop; see Section 7.6.1).   *  Whether the field introduces any additional security      considerations, such as disclosure of privacy-related data.   Request header fields have additional considerations that need to be   documented if the default behavior is not appropriate:   *  If it is appropriate to list the field name in a Vary response      header field (e.g., when the request header field is used by an      origin server's content selection algorithm; see Section 12.5.5).   *  If the field is intended to be stored when received in a PUT      request (see Section 9.3.4).   *  If the field ought to be removed when automatically redirecting a      request due to security concerns (see Section 15.4).16.3.2.1.  Considerations for New Field Names   Authors of specifications defining new fields are advised to choose a   short but descriptive field name.  Short names avoid needless data   transmission; descriptive names avoid confusion and "squatting" on   names that might have broader uses.   To that end, limited-use fields (such as a header confined to a   single application or use case) are encouraged to use a name that   includes that use (or an abbreviation) as a prefix; for example, if   the Foo Application needs a Description field, it might use "Foo-   Desc"; "Description" is too generic, and "Foo-Description" is   needlessly long.   While the field-name syntax is defined to allow any token character,   in practice some implementations place limits on the characters they   accept in field-names.  To be interoperable, new field names SHOULD   constrain themselves to alphanumeric characters, "-", and ".", and   SHOULD begin with a letter.  For example, the underscore ("_")   character can be problematic when passed through non-HTTP gateway   interfaces (see Section 17.10).   Field names ought not be prefixed with "X-"; see [BCP178] for further   information.   Other prefixes are sometimes used in HTTP field names; for example,   "Accept-" is used in many content negotiation headers, and "Content-"   is used as explained in Section 6.4.  These prefixes are only an aid   to recognizing the purpose of a field and do not trigger automatic   processing.16.3.2.2.  Considerations for New Field Values   A major task in the definition of a new HTTP field is the   specification of the field value syntax: what senders should   generate, and how recipients should infer semantics from what is   received.   Authors are encouraged (but not required) to use either the ABNF   rules in this specification or those in [RFC8941] to define the   syntax of new field values.   Authors are advised to carefully consider how the combination of   multiple field lines will impact them (see Section 5.3).  Because   senders might erroneously send multiple values, and both   intermediaries and HTTP libraries can perform combination   automatically, this applies to all field values -- even when only a   single value is anticipated.   Therefore, authors are advised to delimit or encode values that   contain commas (e.g., with the quoted-string rule of Section 5.6.4,   the String data type of [RFC8941], or a field-specific encoding).   This ensures that commas within field data are not confused with the   commas that delimit a list value.   For example, the Content-Type field value only allows commas inside   quoted strings, which can be reliably parsed even when multiple   values are present.  The Location field value provides a counter-   example that should not be emulated: because URIs can include commas,   it is not possible to reliably distinguish between a single value   that includes a comma from two values.   Authors of fields with a singleton value (see Section 5.5) are   additionally advised to document how to treat messages where the   multiple members are present (a sensible default would be to ignore   the field, but this might not always be the right choice).16.4.  Authentication Scheme Extensibility16.4.1.  Authentication Scheme Registry   The "Hypertext Transfer Protocol (HTTP) Authentication Scheme   Registry" defines the namespace for the authentication schemes in   challenges and credentials.  It is maintained at   <https://www.iana.org/assignments/http-authschemes>.   Registrations MUST include the following fields:   *  Authentication Scheme Name   *  Pointer to specification text   *  Notes (optional)   Values to be added to this namespace require IETF Review (see   [RFC8126], Section 4.8).16.4.2.  Considerations for New Authentication Schemes   There are certain aspects of the HTTP Authentication framework that   put constraints on how new authentication schemes can work:   *  HTTP authentication is presumed to be stateless: all of the      information necessary to authenticate a request MUST be provided      in the request, rather than be dependent on the server remembering      prior requests.  Authentication based on, or bound to, the      underlying connection is outside the scope of this specification      and inherently flawed unless steps are taken to ensure that the      connection cannot be used by any party other than the      authenticated user (see Section 3.3).   *  The authentication parameter "realm" is reserved for defining      protection spaces as described in Section 11.5.  New schemes MUST      NOT use it in a way incompatible with that definition.   *  The "token68" notation was introduced for compatibility with      existing authentication schemes and can only be used once per      challenge or credential.  Thus, new schemes ought to use the auth-      param syntax instead, because otherwise future extensions will be      impossible.   *  The parsing of challenges and credentials is defined by this      specification and cannot be modified by new authentication      schemes.  When the auth-param syntax is used, all parameters ought      to support both token and quoted-string syntax, and syntactical      constraints ought to be defined on the field value after parsing      (i.e., quoted-string processing).  This is necessary so that      recipients can use a generic parser that applies to all      authentication schemes.      *Note:* The fact that the value syntax for the "realm" parameter      is restricted to quoted-string was a bad design choice not to be      repeated for new parameters.   *  Definitions of new schemes ought to define the treatment of      unknown extension parameters.  In general, a "must-ignore" rule is      preferable to a "must-understand" rule, because otherwise it will      be hard to introduce new parameters in the presence of legacy      recipients.  Furthermore, it's good to describe the policy for      defining new parameters (such as "update the specification" or      "use this registry").   *  Authentication schemes need to document whether they are usable in      origin-server authentication (i.e., using WWW-Authenticate), and/      or proxy authentication (i.e., using Proxy-Authenticate).   *  The credentials carried in an Authorization header field are      specific to the user agent and, therefore, have the same effect on      HTTP caches as the "private" cache response directive      (Section 5.2.2.7 of [CACHING]), within the scope of the request in      which they appear.      Therefore, new authentication schemes that choose not to carry      credentials in the Authorization header field (e.g., using a newly      defined header field) will need to explicitly disallow caching, by      mandating the use of cache response directives (e.g., "private").   *  Schemes using Authentication-Info, Proxy-Authentication-Info, or      any other authentication related response header field need to      consider and document the related security considerations (see      Section 17.16.4).16.5.  Range Unit Extensibility16.5.1.  Range Unit Registry   The "HTTP Range Unit Registry" defines the namespace for the range   unit names and refers to their corresponding specifications.  It is   maintained at <https://www.iana.org/assignments/http-parameters>.   Registration of an HTTP Range Unit MUST include the following fields:   *  Name   *  Description   *  Pointer to specification text   Values to be added to this namespace require IETF Review (see   [RFC8126], Section 4.8).16.5.2.  Considerations for New Range Units   Other range units, such as format-specific boundaries like pages,   sections, records, rows, or time, are potentially usable in HTTP for   application-specific purposes, but are not commonly used in practice.   Implementors of alternative range units ought to consider how they   would work with content codings and general-purpose intermediaries.16.6.  Content Coding Extensibility16.6.1.  Content Coding Registry   The "HTTP Content Coding Registry", maintained by IANA at   <https://www.iana.org/assignments/http-parameters/>, registers   content-coding names.   Content coding registrations MUST include the following fields:   *  Name   *  Description   *  Pointer to specification text   Names of content codings MUST NOT overlap with names of transfer   codings (per the "HTTP Transfer Coding Registry" located at   <https://www.iana.org/assignments/http-parameters/>) unless the   encoding transformation is identical (as is the case for the   compression codings defined in Section 8.4.1).   Values to be added to this namespace require IETF Review (see   Section 4.8 of [RFC8126]) and MUST conform to the purpose of content   coding defined in Section 8.4.1.16.6.2.  Considerations for New Content Codings   New content codings ought to be self-descriptive whenever possible,   with optional parameters discoverable within the coding format   itself, rather than rely on external metadata that might be lost   during transit.16.7.  Upgrade Token Registry   The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"   defines the namespace for protocol-name tokens used to identify   protocols in the Upgrade header field.  The registry is maintained at   <https://www.iana.org/assignments/http-upgrade-tokens>.   Each registered protocol name is associated with contact information   and an optional set of specifications that details how the connection   will be processed after it has been upgraded.   Registrations happen on a "First Come First Served" basis (see   Section 4.4 of [RFC8126]) and are subject to the following rules:   1.  A protocol-name token, once registered, stays registered forever.   2.  A protocol-name token is case-insensitive and registered with the       preferred case to be generated by senders.   3.  The registration MUST name a responsible party for the       registration.   4.  The registration MUST name a point of contact.   5.  The registration MAY name a set of specifications associated with       that token.  Such specifications need not be publicly available.   6.  The registration SHOULD name a set of expected "protocol-version"       tokens associated with that token at the time of registration.   7.  The responsible party MAY change the registration at any time.       The IANA will keep a record of all such changes, and make them       available upon request.   8.  The IESG MAY reassign responsibility for a protocol token.  This       will normally only be used in the case when a responsible party       cannot be contacted.17.  Security Considerations   This section is meant to inform developers, information providers,   and users of known security concerns relevant to HTTP semantics and   its use for transferring information over the Internet.   Considerations related to caching are discussed in Section 7 of   [CACHING], and considerations related to HTTP/1.1 message syntax and   parsing are discussed in Section 11 of [HTTP/1.1].   The list of considerations below is not exhaustive.  Most security   concerns related to HTTP semantics are about securing server-side   applications (code behind the HTTP interface), securing user agent   processing of content received via HTTP, or secure use of the   Internet in general, rather than security of the protocol.  The   security considerations for URIs, which are fundamental to HTTP   operation, are discussed in Section 7 of [URI].  Various   organizations maintain topical information and links to current   research on Web application security (e.g., [OWASP]).17.1.  Establishing Authority   HTTP relies on the notion of an "authoritative response": a response   that has been determined by (or at the direction of) the origin   server identified within the target URI to be the most appropriate   response for that request given the state of the target resource at   the time of response message origination.   When a registered name is used in the authority component, the "http"   URI scheme (Section 4.2.1) relies on the user's local name resolution   service to determine where it can find authoritative responses.  This   means that any attack on a user's network host table, cached names,   or name resolution libraries becomes an avenue for attack on   establishing authority for "http" URIs.  Likewise, the user's choice   of server for Domain Name Service (DNS), and the hierarchy of servers   from which it obtains resolution results, could impact the   authenticity of address mappings; DNS Security Extensions (DNSSEC,   [RFC4033]) are one way to improve authenticity, as are the various   mechanisms for making DNS requests over more secure transfer   protocols.   Furthermore, after an IP address is obtained, establishing authority   for an "http" URI is vulnerable to attacks on Internet Protocol   routing.   The "https" scheme (Section 4.2.2) is intended to prevent (or at   least reveal) many of these potential attacks on establishing   authority, provided that the negotiated connection is secured and the   client properly verifies that the communicating server's identity   matches the target URI's authority component (Section 4.3.4).   Correctly implementing such verification can be difficult (see   [Georgiev]).   Authority for a given origin server can be delegated through protocol   extensions; for example, [ALTSVC].  Likewise, the set of servers for   which a connection is considered authoritative can be changed with a   protocol extension like [RFC8336].   Providing a response from a non-authoritative source, such as a   shared proxy cache, is often useful to improve performance and   availability, but only to the extent that the source can be trusted   or the distrusted response can be safely used.   Unfortunately, communicating authority to users can be difficult.   For example, "phishing" is an attack on the user's perception of   authority, where that perception can be misled by presenting similar   branding in hypertext, possibly aided by userinfo obfuscating the   authority component (see Section 4.2.1).  User agents can reduce the   impact of phishing attacks by enabling users to easily inspect a   target URI prior to making an action, by prominently distinguishing   (or rejecting) userinfo when present, and by not sending stored   credentials and cookies when the referring document is from an   unknown or untrusted source.17.2.  Risks of Intermediaries   HTTP intermediaries are inherently situated for on-path attacks.   Compromise of the systems on which the intermediaries run can result   in serious security and privacy problems.  Intermediaries might have   access to security-related information, personal information about   individual users and organizations, and proprietary information   belonging to users and content providers.  A compromised   intermediary, or an intermediary implemented or configured without   regard to security and privacy considerations, might be used in the   commission of a wide range of potential attacks.   Intermediaries that contain a shared cache are especially vulnerable   to cache poisoning attacks, as described in Section 7 of [CACHING].   Implementers need to consider the privacy and security implications   of their design and coding decisions, and of the configuration   options they provide to operators (especially the default   configuration).   Intermediaries are no more trustworthy than the people and policies   under which they operate; HTTP cannot solve this problem.17.3.  Attacks Based on File and Path Names   Origin servers frequently make use of their local file system to   manage the mapping from target URI to resource representations.  Most   file systems are not designed to protect against malicious file or   path names.  Therefore, an origin server needs to avoid accessing   names that have a special significance to the system when mapping the   target resource to files, folders, or directories.   For example, UNIX, Microsoft Windows, and other operating systems use   ".." as a path component to indicate a directory level above the   current one, and they use specially named paths or file names to send   data to system devices.  Similar naming conventions might exist   within other types of storage systems.  Likewise, local storage   systems have an annoying tendency to prefer user-friendliness over   security when handling invalid or unexpected characters,   recomposition of decomposed characters, and case-normalization of   case-insensitive names.   Attacks based on such special names tend to focus on either denial-   of-service (e.g., telling the server to read from a COM port) or   disclosure of configuration and source files that are not meant to be   served.17.4.  Attacks Based on Command, Code, or Query Injection   Origin servers often use parameters within the URI as a means of   identifying system services, selecting database entries, or choosing   a data source.  However, data received in a request cannot be   trusted.  An attacker could construct any of the request data   elements (method, target URI, header fields, or content) to contain   data that might be misinterpreted as a command, code, or query when   passed through a command invocation, language interpreter, or   database interface.   For example, SQL injection is a common attack wherein additional   query language is inserted within some part of the target URI or   header fields (e.g., Host, Referer, etc.).  If the received data is   used directly within a SELECT statement, the query language might be   interpreted as a database command instead of a simple string value.   This type of implementation vulnerability is extremely common, in   spite of being easy to prevent.   In general, resource implementations ought to avoid use of request   data in contexts that are processed or interpreted as instructions.   Parameters ought to be compared to fixed strings and acted upon as a   result of that comparison, rather than passed through an interface   that is not prepared for untrusted data.  Received data that isn't   based on fixed parameters ought to be carefully filtered or encoded   to avoid being misinterpreted.   Similar considerations apply to request data when it is stored and   later processed, such as within log files, monitoring tools, or when   included within a data format that allows embedded scripts.17.5.  Attacks via Protocol Element Length   Because HTTP uses mostly textual, character-delimited fields, parsers   are often vulnerable to attacks based on sending very long (or very   slow) streams of data, particularly where an implementation is   expecting a protocol element with no predefined length (Section 2.3).   To promote interoperability, specific recommendations are made for   minimum size limits on fields (Section 5.4).  These are minimum   recommendations, chosen to be supportable even by implementations   with limited resources; it is expected that most implementations will   choose substantially higher limits.   A server can reject a message that has a target URI that is too long   (Section 15.5.15) or request content that is too large   (Section 15.5.14).  Additional status codes related to capacity   limits have been defined by extensions to HTTP [RFC6585].   Recipients ought to carefully limit the extent to which they process   other protocol elements, including (but not limited to) request   methods, response status phrases, field names, numeric values, and   chunk lengths.  Failure to limit such processing can result in   arbitrary code execution due to buffer or arithmetic overflows, and   increased vulnerability to denial-of-service attacks.17.6.  Attacks Using Shared-Dictionary Compression   Some attacks on encrypted protocols use the differences in size   created by dynamic compression to reveal confidential information;   for example, [BREACH].  These attacks rely on creating a redundancy   between attacker-controlled content and the confidential information,   such that a dynamic compression algorithm using the same dictionary   for both content will compress more efficiently when the attacker-   controlled content matches parts of the confidential content.   HTTP messages can be compressed in a number of ways, including using   TLS compression, content codings, transfer codings, and other   extension or version-specific mechanisms.   The most effective mitigation for this risk is to disable compression   on sensitive data, or to strictly separate sensitive data from   attacker-controlled data so that they cannot share the same   compression dictionary.  With careful design, a compression scheme   can be designed in a way that is not considered exploitable in   limited use cases, such as HPACK ([HPACK]).17.7.  Disclosure of Personal Information   Clients are often privy to large amounts of personal information,   including both information provided by the user to interact with   resources (e.g., the user's name, location, mail address, passwords,   encryption keys, etc.) and information about the user's browsing   activity over time (e.g., history, bookmarks, etc.).  Implementations   need to prevent unintentional disclosure of personal information.17.8.  Privacy of Server Log Information   A server is in the position to save personal data about a user's   requests over time, which might identify their reading patterns or   subjects of interest.  In particular, log information gathered at an   intermediary often contains a history of user agent interaction,   across a multitude of sites, that can be traced to individual users.   HTTP log information is confidential in nature; its handling is often   constrained by laws and regulations.  Log information needs to be   securely stored and appropriate guidelines followed for its analysis.   Anonymization of personal information within individual entries   helps, but it is generally not sufficient to prevent real log traces   from being re-identified based on correlation with other access   characteristics.  As such, access traces that are keyed to a specific   client are unsafe to publish even if the key is pseudonymous.   To minimize the risk of theft or accidental publication, log   information ought to be purged of personally identifiable   information, including user identifiers, IP addresses, and user-   provided query parameters, as soon as that information is no longer   necessary to support operational needs for security, auditing, or   fraud control.17.9.  Disclosure of Sensitive Information in URIs   URIs are intended to be shared, not secured, even when they identify   secure resources.  URIs are often shown on displays, added to   templates when a page is printed, and stored in a variety of   unprotected bookmark lists.  Many servers, proxies, and user agents   log or display the target URI in places where it might be visible to   third parties.  It is therefore unwise to include information within   a URI that is sensitive, personally identifiable, or a risk to   disclose.   When an application uses client-side mechanisms to construct a target   URI out of user-provided information, such as the query fields of a   form using GET, potentially sensitive data might be provided that   would not be appropriate for disclosure within a URI.  POST is often   preferred in such cases because it usually doesn't construct a URI;   instead, POST of a form transmits the potentially sensitive data in   the request content.  However, this hinders caching and uses an   unsafe method for what would otherwise be a safe request.   Alternative workarounds include transforming the user-provided data   prior to constructing the URI or filtering the data to only include   common values that are not sensitive.  Likewise, redirecting the   result of a query to a different (server-generated) URI can remove   potentially sensitive data from later links and provide a cacheable   response for later reuse.   Since the Referer header field tells a target site about the context   that resulted in a request, it has the potential to reveal   information about the user's immediate browsing history and any   personal information that might be found in the referring resource's   URI.  Limitations on the Referer header field are described in   Section 10.1.3 to address some of its security considerations.17.10.  Application Handling of Field Names   Servers often use non-HTTP gateway interfaces and frameworks to   process a received request and produce content for the response.  For   historical reasons, such interfaces often pass received field names   as external variable names, using a name mapping suitable for   environment variables.   For example, the Common Gateway Interface (CGI) mapping of protocol-   specific meta-variables, defined by Section 4.1.18 of [RFC3875], is   applied to received header fields that do not correspond to one of   CGI's standard variables; the mapping consists of prepending "HTTP_"   to each name and changing all instances of hyphen ("-") to underscore   ("_").  This same mapping has been inherited by many other   application frameworks in order to simplify moving applications from   one platform to the next.   In CGI, a received Content-Length field would be passed as the meta-   variable "CONTENT_LENGTH" with a string value matching the received   field's value.  In contrast, a received "Content_Length" header field   would be passed as the protocol-specific meta-variable   "HTTP_CONTENT_LENGTH", which might lead to some confusion if an   application mistakenly reads the protocol-specific meta-variable   instead of the default one.  (This historical practice is why   Section 16.3.2.1 discourages the creation of new field names that   contain an underscore.)   Unfortunately, mapping field names to different interface names can   lead to security vulnerabilities if the mapping is incomplete or   ambiguous.  For example, if an attacker were to send a field named   "Transfer_Encoding", a naive interface might map that to the same   variable name as the "Transfer-Encoding" field, resulting in a   potential request smuggling vulnerability (Section 11.2 of   [HTTP/1.1]).   To mitigate the associated risks, implementations that perform such   mappings are advised to make the mapping unambiguous and complete for   the full range of potential octets received as a name (including   those that are discouraged or forbidden by the HTTP grammar).  For   example, a field with an unusual name character might result in the   request being blocked, the specific field being removed, or the name   being passed with a different prefix to distinguish it from other   fields.17.11.  Disclosure of Fragment after Redirects   Although fragment identifiers used within URI references are not sent   in requests, implementers ought to be aware that they will be visible   to the user agent and any extensions or scripts running as a result   of the response.  In particular, when a redirect occurs and the   original request's fragment identifier is inherited by the new   reference in Location (Section 10.2.2), this might have the effect of   disclosing one site's fragment to another site.  If the first site   uses personal information in fragments, it ought to ensure that   redirects to other sites include a (possibly empty) fragment   component in order to block that inheritance.17.12.  Disclosure of Product Information   The User-Agent (Section 10.1.5), Via (Section 7.6.3), and Server   (Section 10.2.4) header fields often reveal information about the   respective sender's software systems.  In theory, this can make it   easier for an attacker to exploit known security holes; in practice,   attackers tend to try all potential holes regardless of the apparent   software versions being used.   Proxies that serve as a portal through a network firewall ought to   take special precautions regarding the transfer of header information   that might identify hosts behind the firewall.  The Via header field   allows intermediaries to replace sensitive machine names with   pseudonyms.17.13.  Browser Fingerprinting   Browser fingerprinting is a set of techniques for identifying a   specific user agent over time through its unique set of   characteristics.  These characteristics might include information   related to how it uses the underlying transport protocol, feature   capabilities, and scripting environment, though of particular   interest here is the set of unique characteristics that might be   communicated via HTTP.  Fingerprinting is considered a privacy   concern because it enables tracking of a user agent's behavior over   time ([Bujlow]) without the corresponding controls that the user   might have over other forms of data collection (e.g., cookies).  Many   general-purpose user agents (i.e., Web browsers) have taken steps to   reduce their fingerprints.   There are a number of request header fields that might reveal   information to servers that is sufficiently unique to enable   fingerprinting.  The From header field is the most obvious, though it   is expected that From will only be sent when self-identification is   desired by the user.  Likewise, Cookie header fields are deliberately   designed to enable re-identification, so fingerprinting concerns only   apply to situations where cookies are disabled or restricted by the   user agent's configuration.   The User-Agent header field might contain enough information to   uniquely identify a specific device, usually when combined with other   characteristics, particularly if the user agent sends excessive   details about the user's system or extensions.  However, the source   of unique information that is least expected by users is proactive   negotiation (Section 12.1), including the Accept, Accept-Charset,   Accept-Encoding, and Accept-Language header fields.   In addition to the fingerprinting concern, detailed use of the   Accept-Language header field can reveal information the user might   consider to be of a private nature.  For example, understanding a   given language set might be strongly correlated to membership in a   particular ethnic group.  An approach that limits such loss of   privacy would be for a user agent to omit the sending of Accept-   Language except for sites that have been explicitly permitted,   perhaps via interaction after detecting a Vary header field that   indicates language negotiation might be useful.   In environments where proxies are used to enhance privacy, user   agents ought to be conservative in sending proactive negotiation   header fields.  General-purpose user agents that provide a high   degree of header field configurability ought to inform users about   the loss of privacy that might result if too much detail is provided.   As an extreme privacy measure, proxies could filter the proactive   negotiation header fields in relayed requests.17.14.  Validator Retention   The validators defined by this specification are not intended to   ensure the validity of a representation, guard against malicious   changes, or detect on-path attacks.  At best, they enable more   efficient cache updates and optimistic concurrent writes when all   participants are behaving nicely.  At worst, the conditions will fail   and the client will receive a response that is no more harmful than   an HTTP exchange without conditional requests.   An entity tag can be abused in ways that create privacy risks.  For   example, a site might deliberately construct a semantically invalid   entity tag that is unique to the user or user agent, send it in a   cacheable response with a long freshness time, and then read that   entity tag in later conditional requests as a means of re-identifying   that user or user agent.  Such an identifying tag would become a   persistent identifier for as long as the user agent retained the   original cache entry.  User agents that cache representations ought   to ensure that the cache is cleared or replaced whenever the user   performs privacy-maintaining actions, such as clearing stored cookies   or changing to a private browsing mode.17.15.  Denial-of-Service Attacks Using Range   Unconstrained multiple range requests are susceptible to denial-of-   service attacks because the effort required to request many   overlapping ranges of the same data is tiny compared to the time,   memory, and bandwidth consumed by attempting to serve the requested   data in many parts.  Servers ought to ignore, coalesce, or reject   egregious range requests, such as requests for more than two   overlapping ranges or for many small ranges in a single set,   particularly when the ranges are requested out of order for no   apparent reason.  Multipart range requests are not designed to   support random access.17.16.  Authentication Considerations   Everything about the topic of HTTP authentication is a security   consideration, so the list of considerations below is not exhaustive.   Furthermore, it is limited to security considerations regarding the   authentication framework, in general, rather than discussing all of   the potential considerations for specific authentication schemes   (which ought to be documented in the specifications that define those   schemes).  Various organizations maintain topical information and   links to current research on Web application security (e.g.,   [OWASP]), including common pitfalls for implementing and using the   authentication schemes found in practice.17.16.1.  Confidentiality of Credentials   The HTTP authentication framework does not define a single mechanism   for maintaining the confidentiality of credentials; instead, each   authentication scheme defines how the credentials are encoded prior   to transmission.  While this provides flexibility for the development   of future authentication schemes, it is inadequate for the protection   of existing schemes that provide no confidentiality on their own, or   that do not sufficiently protect against replay attacks.   Furthermore, if the server expects credentials that are specific to   each individual user, the exchange of those credentials will have the   effect of identifying that user even if the content within   credentials remains confidential.   HTTP depends on the security properties of the underlying transport-   or session-level connection to provide confidential transmission of   fields.  Services that depend on individual user authentication   require a secured connection prior to exchanging credentials   (Section 4.2.2).17.16.2.  Credentials and Idle Clients   Existing HTTP clients and user agents typically retain authentication   information indefinitely.  HTTP does not provide a mechanism for the   origin server to direct clients to discard these cached credentials,   since the protocol has no awareness of how credentials are obtained   or managed by the user agent.  The mechanisms for expiring or   revoking credentials can be specified as part of an authentication   scheme definition.   Circumstances under which credential caching can interfere with the   application's security model include but are not limited to:   *  Clients that have been idle for an extended period, following      which the server might wish to cause the client to re-prompt the      user for credentials.   *  Applications that include a session termination indication (such      as a "logout" or "commit" button on a page) after which the server      side of the application "knows" that there is no further reason      for the client to retain the credentials.   User agents that cache credentials are encouraged to provide a   readily accessible mechanism for discarding cached credentials under   user control.17.16.3.  Protection Spaces   Authentication schemes that solely rely on the "realm" mechanism for   establishing a protection space will expose credentials to all   resources on an origin server.  Clients that have successfully made   authenticated requests with a resource can use the same   authentication credentials for other resources on the same origin   server.  This makes it possible for a different resource to harvest   authentication credentials for other resources.   This is of particular concern when an origin server hosts resources   for multiple parties under the same origin (Section 11.5).  Possible   mitigation strategies include restricting direct access to   authentication credentials (i.e., not making the content of the   Authorization request header field available), and separating   protection spaces by using a different host name (or port number) for   each party.17.16.4.  Additional Response Fields   Adding information to responses that are sent over an unencrypted   channel can affect security and privacy.  The presence of the   Authentication-Info and Proxy-Authentication-Info header fields alone   indicates that HTTP authentication is in use.  Additional information   could be exposed by the contents of the authentication-scheme   specific parameters; this will have to be considered in the   definitions of these schemes.18.  IANA Considerations   The change controller for the following registrations is: "IETF   (iesg@ietf.org) - Internet Engineering Task Force".18.1.  URI Scheme Registration   IANA has updated the "Uniform Resource Identifier (URI) Schemes"   registry [BCP35] at <https://www.iana.org/assignments/uri-schemes/>   with the permanent schemes listed in Table 2 in Section 4.2.18.2.  Method Registration   IANA has updated the "Hypertext Transfer Protocol (HTTP) Method   Registry" at <https://www.iana.org/assignments/http-methods> with the   registration procedure of Section 16.1.1 and the method names   summarized in the following table.                 +=========+======+============+=========+                 | Method  | Safe | Idempotent | Section |                 +=========+======+============+=========+                 | CONNECT | no   | no         | 9.3.6   |                 +---------+------+------------+---------+                 | DELETE  | no   | yes        | 9.3.5   |                 +---------+------+------------+---------+                 | GET     | yes  | yes        | 9.3.1   |                 +---------+------+------------+---------+                 | HEAD    | yes  | yes        | 9.3.2   |                 +---------+------+------------+---------+                 | OPTIONS | yes  | yes        | 9.3.7   |                 +---------+------+------------+---------+                 | POST    | no   | no         | 9.3.3   |                 +---------+------+------------+---------+                 | PUT     | no   | yes        | 9.3.4   |                 +---------+------+------------+---------+                 | TRACE   | yes  | yes        | 9.3.8   |                 +---------+------+------------+---------+                 | *       | no   | no         | 18.2    |                 +---------+------+------------+---------+                                  Table 7   The method name "*" is reserved because using "*" as a method name   would conflict with its usage as a wildcard in some fields (e.g.,   "Access-Control-Request-Method").18.3.  Status Code Registration   IANA has updated the "Hypertext Transfer Protocol (HTTP) Status Code   Registry" at <https://www.iana.org/assignments/http-status-codes>   with the registration procedure of Section 16.2.1 and the status code   values summarized in the following table.            +=======+===============================+=========+            | Value | Description                   | Section |            +=======+===============================+=========+            | 100   | Continue                      | 15.2.1  |            +-------+-------------------------------+---------+            | 101   | Switching Protocols           | 15.2.2  |            +-------+-------------------------------+---------+            | 200   | OK                            | 15.3.1  |            +-------+-------------------------------+---------+            | 201   | Created                       | 15.3.2  |            +-------+-------------------------------+---------+            | 202   | Accepted                      | 15.3.3  |            +-------+-------------------------------+---------+            | 203   | Non-Authoritative Information | 15.3.4  |            +-------+-------------------------------+---------+            | 204   | No Content                    | 15.3.5  |            +-------+-------------------------------+---------+            | 205   | Reset Content                 | 15.3.6  |            +-------+-------------------------------+---------+            | 206   | Partial Content               | 15.3.7  |            +-------+-------------------------------+---------+            | 300   | Multiple Choices              | 15.4.1  |            +-------+-------------------------------+---------+            | 301   | Moved Permanently             | 15.4.2  |            +-------+-------------------------------+---------+            | 302   | Found                         | 15.4.3  |            +-------+-------------------------------+---------+            | 303   | See Other                     | 15.4.4  |            +-------+-------------------------------+---------+            | 304   | Not Modified                  | 15.4.5  |            +-------+-------------------------------+---------+            | 305   | Use Proxy                     | 15.4.6  |            +-------+-------------------------------+---------+            | 306   | (Unused)                      | 15.4.7  |            +-------+-------------------------------+---------+            | 307   | Temporary Redirect            | 15.4.8  |            +-------+-------------------------------+---------+            | 308   | Permanent Redirect            | 15.4.9  |            +-------+-------------------------------+---------+            | 400   | Bad Request                   | 15.5.1  |            +-------+-------------------------------+---------+            | 401   | Unauthorized                  | 15.5.2  |            +-------+-------------------------------+---------+            | 402   | Payment Required              | 15.5.3  |            +-------+-------------------------------+---------+            | 403   | Forbidden                     | 15.5.4  |            +-------+-------------------------------+---------+            | 404   | Not Found                     | 15.5.5  |            +-------+-------------------------------+---------+            | 405   | Method Not Allowed            | 15.5.6  |            +-------+-------------------------------+---------+            | 406   | Not Acceptable                | 15.5.7  |            +-------+-------------------------------+---------+            | 407   | Proxy Authentication Required | 15.5.8  |            +-------+-------------------------------+---------+            | 408   | Request Timeout               | 15.5.9  |            +-------+-------------------------------+---------+            | 409   | Conflict                      | 15.5.10 |            +-------+-------------------------------+---------+            | 410   | Gone                          | 15.5.11 |            +-------+-------------------------------+---------+            | 411   | Length Required               | 15.5.12 |            +-------+-------------------------------+---------+            | 412   | Precondition Failed           | 15.5.13 |            +-------+-------------------------------+---------+            | 413   | Content Too Large             | 15.5.14 |            +-------+-------------------------------+---------+            | 414   | URI Too Long                  | 15.5.15 |            +-------+-------------------------------+---------+            | 415   | Unsupported Media Type        | 15.5.16 |            +-------+-------------------------------+---------+            | 416   | Range Not Satisfiable         | 15.5.17 |            +-------+-------------------------------+---------+            | 417   | Expectation Failed            | 15.5.18 |            +-------+-------------------------------+---------+            | 418   | (Unused)                      | 15.5.19 |            +-------+-------------------------------+---------+            | 421   | Misdirected Request           | 15.5.20 |            +-------+-------------------------------+---------+            | 422   | Unprocessable Content         | 15.5.21 |            +-------+-------------------------------+---------+            | 426   | Upgrade Required              | 15.5.22 |            +-------+-------------------------------+---------+            | 500   | Internal Server Error         | 15.6.1  |            +-------+-------------------------------+---------+            | 501   | Not Implemented               | 15.6.2  |            +-------+-------------------------------+---------+            | 502   | Bad Gateway                   | 15.6.3  |            +-------+-------------------------------+---------+            | 503   | Service Unavailable           | 15.6.4  |            +-------+-------------------------------+---------+            | 504   | Gateway Timeout               | 15.6.5  |            +-------+-------------------------------+---------+            | 505   | HTTP Version Not Supported    | 15.6.6  |            +-------+-------------------------------+---------+                                  Table 818.4.  Field Name Registration   This specification updates the HTTP-related aspects of the existing   registration procedures for message header fields defined in   [RFC3864].  It replaces the old procedures as they relate to HTTP by   defining a new registration procedure and moving HTTP field   definitions into a separate registry.   IANA has created a new registry titled "Hypertext Transfer Protocol   (HTTP) Field Name Registry" as outlined in Section 16.3.1.   IANA has moved all entries in the "Permanent Message Header Field   Names" and "Provisional Message Header Field Names" registries (see   <https://www.iana.org/assignments/message-headers/>) with the   protocol 'http' to this registry and has applied the following   changes:   1.  The 'Applicable Protocol' field has been omitted.   2.  Entries that had a status of 'standard', 'experimental',       'reserved', or 'informational' have been made to have a status of       'permanent'.   3.  Provisional entries without a status have been made to have a       status of 'provisional'.   4.  Permanent entries without a status (after confirmation that the       registration document did not define one) have been made to have       a status of 'provisional'.  The expert(s) can choose to update       the entries' status if there is evidence that another is more       appropriate.   IANA has annotated the "Permanent Message Header Field Names" and   "Provisional Message Header Field Names" registries with the   following note to indicate that HTTP field name registrations have   moved:      |  *Note*      |        |  HTTP field name registrations have been moved to      |  [https://www.iana.org/assignments/http-fields] per [RFC9110].   IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name   Registry" with the field names listed in the following table.   +===========================+============+=========+============+   | Field Name                | Status     | Section | Comments   |   +===========================+============+=========+============+   | Accept                    | permanent  | 12.5.1  |            |   +---------------------------+------------+---------+------------+   | Accept-Charset            | deprecated | 12.5.2  |            |   +---------------------------+------------+---------+------------+   | Accept-Encoding           | permanent  | 12.5.3  |            |   +---------------------------+------------+---------+------------+   | Accept-Language           | permanent  | 12.5.4  |            |   +---------------------------+------------+---------+------------+   | Accept-Ranges             | permanent  | 14.3    |            |   +---------------------------+------------+---------+------------+   | Allow                     | permanent  | 10.2.1  |            |   +---------------------------+------------+---------+------------+   | Authentication-Info       | permanent  | 11.6.3  |            |   +---------------------------+------------+---------+------------+   | Authorization             | permanent  | 11.6.2  |            |   +---------------------------+------------+---------+------------+   | Connection                | permanent  | 7.6.1   |            |   +---------------------------+------------+---------+------------+   | Content-Encoding          | permanent  | 8.4     |            |   +---------------------------+------------+---------+------------+   | Content-Language          | permanent  | 8.5     |            |   +---------------------------+------------+---------+------------+   | Content-Length            | permanent  | 8.6     |            |   +---------------------------+------------+---------+------------+   | Content-Location          | permanent  | 8.7     |            |   +---------------------------+------------+---------+------------+   | Content-Range             | permanent  | 14.4    |            |   +---------------------------+------------+---------+------------+   | Content-Type              | permanent  | 8.3     |            |   +---------------------------+------------+---------+------------+   | Date                      | permanent  | 6.6.1   |            |   +---------------------------+------------+---------+------------+   | ETag                      | permanent  | 8.8.3   |            |   +---------------------------+------------+---------+------------+   | Expect                    | permanent  | 10.1.1  |            |   +---------------------------+------------+---------+------------+   | From                      | permanent  | 10.1.2  |            |   +---------------------------+------------+---------+------------+   | Host                      | permanent  | 7.2     |            |   +---------------------------+------------+---------+------------+   | If-Match                  | permanent  | 13.1.1  |            |   +---------------------------+------------+---------+------------+   | If-Modified-Since         | permanent  | 13.1.3  |            |   +---------------------------+------------+---------+------------+   | If-None-Match             | permanent  | 13.1.2  |            |   +---------------------------+------------+---------+------------+   | If-Range                  | permanent  | 13.1.5  |            |   +---------------------------+------------+---------+------------+   | If-Unmodified-Since       | permanent  | 13.1.4  |            |   +---------------------------+------------+---------+------------+   | Last-Modified             | permanent  | 8.8.2   |            |   +---------------------------+------------+---------+------------+   | Location                  | permanent  | 10.2.2  |            |   +---------------------------+------------+---------+------------+   | Max-Forwards              | permanent  | 7.6.2   |            |   +---------------------------+------------+---------+------------+   | Proxy-Authenticate        | permanent  | 11.7.1  |            |   +---------------------------+------------+---------+------------+   | Proxy-Authentication-Info | permanent  | 11.7.3  |            |   +---------------------------+------------+---------+------------+   | Proxy-Authorization       | permanent  | 11.7.2  |            |   +---------------------------+------------+---------+------------+   | Range                     | permanent  | 14.2    |            |   +---------------------------+------------+---------+------------+   | Referer                   | permanent  | 10.1.3  |            |   +---------------------------+------------+---------+------------+   | Retry-After               | permanent  | 10.2.3  |            |   +---------------------------+------------+---------+------------+   | Server                    | permanent  | 10.2.4  |            |   +---------------------------+------------+---------+------------+   | TE                        | permanent  | 10.1.4  |            |   +---------------------------+------------+---------+------------+   | Trailer                   | permanent  | 6.6.2   |            |   +---------------------------+------------+---------+------------+   | Upgrade                   | permanent  | 7.8     |            |   +---------------------------+------------+---------+------------+   | User-Agent                | permanent  | 10.1.5  |            |   +---------------------------+------------+---------+------------+   | Vary                      | permanent  | 12.5.5  |            |   +---------------------------+------------+---------+------------+   | Via                       | permanent  | 7.6.3   |            |   +---------------------------+------------+---------+------------+   | WWW-Authenticate          | permanent  | 11.6.1  |            |   +---------------------------+------------+---------+------------+   | *                         | permanent  | 12.5.5  | (reserved) |   +---------------------------+------------+---------+------------+                                Table 9   The field name "*" is reserved because using that name as an HTTP   header field might conflict with its special semantics in the Vary   header field (Section 12.5.5).   IANA has updated the "Content-MD5" entry in the new registry to have   a status of 'obsoleted' with references to Section 14.15 of [RFC2616]   (for the definition of the header field) and Appendix B of [RFC7231]   (which removed the field definition from the updated specification).18.5.  Authentication Scheme Registration   IANA has updated the "Hypertext Transfer Protocol (HTTP)   Authentication Scheme Registry" at <https://www.iana.org/assignments/   http-authschemes> with the registration procedure of Section 16.4.1.   No authentication schemes are defined in this document.18.6.  Content Coding Registration   IANA has updated the "HTTP Content Coding Registry" at   <https://www.iana.org/assignments/http-parameters/> with the   registration procedure of Section 16.6.1 and the content coding names   summarized in the table below.   +============+===========================================+=========+   | Name       | Description                               | Section |   +============+===========================================+=========+   | compress   | UNIX "compress" data format [Welch]       | 8.4.1.1 |   +------------+-------------------------------------------+---------+   | deflate    | "deflate" compressed data ([RFC1951])     | 8.4.1.2 |   |            | inside the "zlib" data format ([RFC1950]) |         |   +------------+-------------------------------------------+---------+   | gzip       | GZIP file format [RFC1952]                | 8.4.1.3 |   +------------+-------------------------------------------+---------+   | identity   | Reserved                                  | 12.5.3  |   +------------+-------------------------------------------+---------+   | x-compress | Deprecated (alias for compress)           | 8.4.1.1 |   +------------+-------------------------------------------+---------+   | x-gzip     | Deprecated (alias for gzip)               | 8.4.1.3 |   +------------+-------------------------------------------+---------+                                 Table 1018.7.  Range Unit Registration   IANA has updated the "HTTP Range Unit Registry" at   <https://www.iana.org/assignments/http-parameters/> with the   registration procedure of Section 16.5.1 and the range unit names   summarized in the table below.   +=================+==================================+=========+   | Range Unit Name | Description                      | Section |   +=================+==================================+=========+   | bytes           | a range of octets                | 14.1.2  |   +-----------------+----------------------------------+---------+   | none            | reserved as keyword to indicate  | 14.3    |   |                 | range requests are not supported |         |   +-----------------+----------------------------------+---------+                               Table 1118.8.  Media Type Registration   IANA has updated the "Media Types" registry at   <https://www.iana.org/assignments/media-types> with the registration   information in Section 14.6 for the media type "multipart/   byteranges".   IANA has updated the registry note about "q" parameters with a link   to Section 12.5.1 of this document.18.9.  Port Registration   IANA has updated the "Service Name and Transport Protocol Port Number   Registry" at <https://www.iana.org/assignments/service-names-port-   numbers/> for the services on ports 80 and 443 that use UDP or TCP   to:   1.  use this document as "Reference", and   2.  when currently unspecified, set "Assignee" to "IESG" and       "Contact" to "IETF_Chair".18.10.  Upgrade Token Registration   IANA has updated the "Hypertext Transfer Protocol (HTTP) Upgrade   Token Registry" at <https://www.iana.org/assignments/http-upgrade-   tokens> with the registration procedure described in Section 16.7 and   the upgrade token names summarized in the following table.   +======+===================+=========================+=========+   | Name | Description       | Expected Version Tokens | Section |   +======+===================+=========================+=========+   | HTTP | Hypertext         | any DIGIT.DIGIT (e.g.,  | 2.5     |   |      | Transfer Protocol | "2.0")                  |         |   +------+-------------------+-------------------------+---------+                               Table 1219.  References19.1.  Normative References   [CACHING]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "HTTP Caching", STD 98, RFC 9111,              DOI 10.17487/RFC9111, June 2022,              <https://www.rfc-editor.org/info/rfc9111>.   [RFC1950]  Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format              Specification version 3.3", RFC 1950,              DOI 10.17487/RFC1950, May 1996,              <https://www.rfc-editor.org/info/rfc1950>.   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,              <https://www.rfc-editor.org/info/rfc1951>.   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",              RFC 1952, DOI 10.17487/RFC1952, May 1996,              <https://www.rfc-editor.org/info/rfc1952>.   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail              Extensions (MIME) Part Two: Media Types", RFC 2046,              DOI 10.17487/RFC2046, November 1996,              <https://www.rfc-editor.org/info/rfc2046>.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC4647]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language              Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September              2006, <https://www.rfc-editor.org/info/rfc4647>.   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,              <https://www.rfc-editor.org/info/rfc4648>.   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68, RFC 5234,              DOI 10.17487/RFC5234, January 2008,              <https://www.rfc-editor.org/info/rfc5234>.   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,              Housley, R., and W. Polk, "Internet X.509 Public Key              Infrastructure Certificate and Certificate Revocation List              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,              <https://www.rfc-editor.org/info/rfc5280>.   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,              DOI 10.17487/RFC5322, October 2008,              <https://www.rfc-editor.org/info/rfc5322>.   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,              September 2009, <https://www.rfc-editor.org/info/rfc5646>.   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and              Verification of Domain-Based Application Service Identity              within Internet Public Key Infrastructure Using X.509              (PKIX) Certificates in the Context of Transport Layer              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March              2011, <https://www.rfc-editor.org/info/rfc6125>.   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in              Internationalization in the IETF", BCP 166, RFC 6365,              DOI 10.17487/RFC6365, September 2011,              <https://www.rfc-editor.org/info/rfc6365>.   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",              RFC 7405, DOI 10.17487/RFC7405, December 2014,              <https://www.rfc-editor.org/info/rfc7405>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,              RFC 793, DOI 10.17487/RFC0793, September 1981,              <https://www.rfc-editor.org/info/rfc793>.   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,              <https://www.rfc-editor.org/info/rfc8446>.   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,              RFC 3986, DOI 10.17487/RFC3986, January 2005,              <https://www.rfc-editor.org/info/rfc3986>.   [USASCII]  American National Standards Institute, "Coded Character              Set -- 7-bit American Standard Code for Information              Interchange", ANSI X3.4, 1986.   [Welch]    Welch, T., "A Technique for High-Performance Data              Compression", IEEE Computer 17(6),              DOI 10.1109/MC.1984.1659158, June 1984,              <https://ieeexplore.ieee.org/document/1659158/>.19.2.  Informative References   [ALTSVC]   Nottingham, M., McManus, P., and J. Reschke, "HTTP              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,              April 2016, <https://www.rfc-editor.org/info/rfc7838>.   [BCP13]    Freed, N. and J. Klensin, "Multipurpose Internet Mail              Extensions (MIME) Part Four: Registration Procedures",              BCP 13, RFC 4289, December 2005.              Freed, N., Klensin, J., and T. Hansen, "Media Type              Specifications and Registration Procedures", BCP 13,              RFC 6838, January 2013.              <https://www.rfc-editor.org/info/bcp13>   [BCP178]   Saint-Andre, P., Crocker, D., and M. Nottingham,              "Deprecating the "X-" Prefix and Similar Constructs in              Application Protocols", BCP 178, RFC 6648, June 2012.              <https://www.rfc-editor.org/info/bcp178>   [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines              and Registration Procedures for URI Schemes", BCP 35,              RFC 7595, June 2015.              <https://www.rfc-editor.org/info/bcp35>   [BREACH]   Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the              CRIME Attack", July 2013,              <http://breachattack.com/resources/              BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.   [Bujlow]   Bujlow, T., Carela-Español, V., Solé-Pareta, J., and P.              Barlet-Ros, "A Survey on Web Tracking: Mechanisms,              Implications, and Defenses", In Proceedings of the IEEE              105(8), DOI 10.1109/JPROC.2016.2637878, August 2017,              <https://doi.org/10.1109/JPROC.2016.2637878>.   [COOKIE]   Barth, A., "HTTP State Management Mechanism", RFC 6265,              DOI 10.17487/RFC6265, April 2011,              <https://www.rfc-editor.org/info/rfc6265>.   [Err1912]  RFC Errata, Erratum ID 1912, RFC 2978,              <https://www.rfc-editor.org/errata/eid1912>.   [Err5433]  RFC Errata, Erratum ID 5433, RFC 2978,              <https://www.rfc-editor.org/errata/eid5433>.   [Georgiev] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,              D., and V. Shmatikov, "The Most Dangerous Code in the              World: Validating SSL Certificates in Non-Browser              Software", In Proceedings of the 2012 ACM Conference on              Computer and Communications Security (CCS '12), pp. 38-49,              DOI 10.1145/2382196.2382204, October 2012,              <https://doi.org/10.1145/2382196.2382204>.   [HPACK]    Peon, R. and H. Ruellan, "HPACK: Header Compression for              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,              <https://www.rfc-editor.org/info/rfc7541>.   [HTTP/1.0] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext              Transfer Protocol -- HTTP/1.0", RFC 1945,              DOI 10.17487/RFC1945, May 1996,              <https://www.rfc-editor.org/info/rfc1945>.   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,              June 2022, <https://www.rfc-editor.org/info/rfc9112>.   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,              DOI 10.17487/RFC9113, June 2022,              <https://www.rfc-editor.org/info/rfc9113>.   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,              June 2022, <https://www.rfc-editor.org/info/rfc9114>.   [ISO-8859-1]              International Organization for Standardization,              "Information technology -- 8-bit single-byte coded graphic              character sets -- Part 1: Latin alphabet No. 1", ISO/              IEC 8859-1:1998, 1998.   [Kri2001]  Kristol, D., "HTTP Cookies: Standards, Privacy, and              Politics", ACM Transactions on Internet Technology 1(2),              November 2001, <http://arxiv.org/abs/cs.SE/0105018>.   [OWASP]    The Open Web Application Security Project,              <https://www.owasp.org/>.   [REST]     Fielding, R.T., "Architectural Styles and the Design of              Network-based Software Architectures", Doctoral              Dissertation, University of California, Irvine, September              2000, <https://roy.gbiv.com/pubs/dissertation/top.htm>.   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",              RFC 1919, DOI 10.17487/RFC1919, March 1996,              <https://www.rfc-editor.org/info/rfc1919>.   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)              Part Three: Message Header Extensions for Non-ASCII Text",              RFC 2047, DOI 10.17487/RFC2047, November 1996,              <https://www.rfc-editor.org/info/rfc2047>.   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",              RFC 2068, DOI 10.17487/RFC2068, January 1997,              <https://www.rfc-editor.org/info/rfc2068>.   [RFC2145]  Mogul, J. C., Fielding, R., Gettys, J., and H. Frystyk,              "Use and Interpretation of HTTP Version Numbers",              RFC 2145, DOI 10.17487/RFC2145, May 1997,              <https://www.rfc-editor.org/info/rfc2145>.   [RFC2295]  Holtman, K. and A. Mutz, "Transparent Content Negotiation              in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,              <https://www.rfc-editor.org/info/rfc2295>.   [RFC2324]  Masinter, L., "Hyper Text Coffee Pot Control Protocol              (HTCPCP/1.0)", RFC 2324, DOI 10.17487/RFC2324, 1 April              1998, <https://www.rfc-editor.org/info/rfc2324>.   [RFC2557]  Palme, J., Hopmann, A., and N. Shelness, "MIME              Encapsulation of Aggregate Documents, such as HTML              (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,              <https://www.rfc-editor.org/info/rfc2557>.   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext              Transfer Protocol -- HTTP/1.1", RFC 2616,              DOI 10.17487/RFC2616, June 1999,              <https://www.rfc-editor.org/info/rfc2616>.   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,              Leach, P., Luotonen, A., and L. Stewart, "HTTP              Authentication: Basic and Digest Access Authentication",              RFC 2617, DOI 10.17487/RFC2617, June 1999,              <https://www.rfc-editor.org/info/rfc2617>.   [RFC2774]  Nielsen, H., Leach, P., and S. Lawrence, "An HTTP              Extension Framework", RFC 2774, DOI 10.17487/RFC2774,              February 2000, <https://www.rfc-editor.org/info/rfc2774>.   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,              DOI 10.17487/RFC2818, May 2000,              <https://www.rfc-editor.org/info/rfc2818>.   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,              October 2000, <https://www.rfc-editor.org/info/rfc2978>.   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web              Replication and Caching Taxonomy", RFC 3040,              DOI 10.17487/RFC3040, January 2001,              <https://www.rfc-editor.org/info/rfc3040>.   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration              Procedures for Message Header Fields", BCP 90, RFC 3864,              DOI 10.17487/RFC3864, September 2004,              <https://www.rfc-editor.org/info/rfc3864>.   [RFC3875]  Robinson, D. and K. Coar, "The Common Gateway Interface              (CGI) Version 1.1", RFC 3875, DOI 10.17487/RFC3875,              October 2004, <https://www.rfc-editor.org/info/rfc3875>.   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "DNS Security Introduction and Requirements",              RFC 4033, DOI 10.17487/RFC4033, March 2005,              <https://www.rfc-editor.org/info/rfc4033>.   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based              Kerberos and NTLM HTTP Authentication in Microsoft              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,              <https://www.rfc-editor.org/info/rfc4559>.   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",              RFC 5789, DOI 10.17487/RFC5789, March 2010,              <https://www.rfc-editor.org/info/rfc5789>.   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,              "Network Time Protocol Version 4: Protocol and Algorithms              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,              <https://www.rfc-editor.org/info/rfc5905>.   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,              DOI 10.17487/RFC6454, December 2011,              <https://www.rfc-editor.org/info/rfc6454>.   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,              <https://www.rfc-editor.org/info/rfc6585>.   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Message Syntax and Routing",              RFC 7230, DOI 10.17487/RFC7230, June 2014,              <https://www.rfc-editor.org/info/rfc7230>.   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,              DOI 10.17487/RFC7231, June 2014,              <https://www.rfc-editor.org/info/rfc7231>.   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,              DOI 10.17487/RFC7232, June 2014,              <https://www.rfc-editor.org/info/rfc7232>.   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",              RFC 7233, DOI 10.17487/RFC7233, June 2014,              <https://www.rfc-editor.org/info/rfc7233>.   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",              RFC 7234, DOI 10.17487/RFC7234, June 2014,              <https://www.rfc-editor.org/info/rfc7234>.   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Authentication", RFC 7235,              DOI 10.17487/RFC7235, June 2014,              <https://www.rfc-editor.org/info/rfc7235>.   [RFC7538]  Reschke, J., "The Hypertext Transfer Protocol Status Code              308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,              April 2015, <https://www.rfc-editor.org/info/rfc7538>.   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,              DOI 10.17487/RFC7540, May 2015,              <https://www.rfc-editor.org/info/rfc7540>.   [RFC7578]  Masinter, L., "Returning Values from Forms: multipart/              form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,              <https://www.rfc-editor.org/info/rfc7578>.   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-              Authentication-Info Response Header Fields", RFC 7615,              DOI 10.17487/RFC7615, September 2015,              <https://www.rfc-editor.org/info/rfc7615>.   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP              Digest Access Authentication", RFC 7616,              DOI 10.17487/RFC7616, September 2015,              <https://www.rfc-editor.org/info/rfc7616>.   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",              RFC 7617, DOI 10.17487/RFC7617, September 2015,              <https://www.rfc-editor.org/info/rfc7617>.   [RFC7694]  Reschke, J., "Hypertext Transfer Protocol (HTTP) Client-              Initiated Content-Encoding", RFC 7694,              DOI 10.17487/RFC7694, November 2015,              <https://www.rfc-editor.org/info/rfc7694>.   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for              Writing an IANA Considerations Section in RFCs", BCP 26,              RFC 8126, DOI 10.17487/RFC8126, June 2017,              <https://www.rfc-editor.org/info/rfc8126>.   [RFC8187]  Reschke, J., "Indicating Character Encoding and Language              for HTTP Header Field Parameters", RFC 8187,              DOI 10.17487/RFC8187, September 2017,              <https://www.rfc-editor.org/info/rfc8187>.   [RFC8246]  McManus, P., "HTTP Immutable Responses", RFC 8246,              DOI 10.17487/RFC8246, September 2017,              <https://www.rfc-editor.org/info/rfc8246>.   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,              DOI 10.17487/RFC8288, October 2017,              <https://www.rfc-editor.org/info/rfc8288>.   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",              RFC 8336, DOI 10.17487/RFC8336, March 2018,              <https://www.rfc-editor.org/info/rfc8336>.   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,              <https://www.rfc-editor.org/info/rfc8615>.   [RFC8941]  Nottingham, M. and P-H. Kamp, "Structured Field Values for              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,              <https://www.rfc-editor.org/info/rfc8941>.   [Sniffing] WHATWG, "MIME Sniffing",              <https://mimesniff.spec.whatwg.org>.   [WEBDAV]   Dusseault, L., Ed., "HTTP Extensions for Web Distributed              Authoring and Versioning (WebDAV)", RFC 4918,              DOI 10.17487/RFC4918, June 2007,              <https://www.rfc-editor.org/info/rfc4918>.Appendix A.  Collected ABNF   In the collected ABNF below, list rules are expanded per   Section 5.6.1.   Accept = [ ( media-range [ weight ] ) *( OWS "," OWS ( media-range [    weight ] ) ) ]   Accept-Charset = [ ( ( token / "*" ) [ weight ] ) *( OWS "," OWS ( (    token / "*" ) [ weight ] ) ) ]   Accept-Encoding = [ ( codings [ weight ] ) *( OWS "," OWS ( codings [    weight ] ) ) ]   Accept-Language = [ ( language-range [ weight ] ) *( OWS "," OWS (    language-range [ weight ] ) ) ]   Accept-Ranges = acceptable-ranges   Allow = [ method *( OWS "," OWS method ) ]   Authentication-Info = [ auth-param *( OWS "," OWS auth-param ) ]   Authorization = credentials   BWS = OWS   Connection = [ connection-option *( OWS "," OWS connection-option )    ]   Content-Encoding = [ content-coding *( OWS "," OWS content-coding )    ]   Content-Language = [ language-tag *( OWS "," OWS language-tag ) ]   Content-Length = 1*DIGIT   Content-Location = absolute-URI / partial-URI   Content-Range = range-unit SP ( range-resp / unsatisfied-range )   Content-Type = media-type   Date = HTTP-date   ETag = entity-tag   Expect = [ expectation *( OWS "," OWS expectation ) ]   From = mailbox   GMT = %x47.4D.54 ; GMT   HTTP-date = IMF-fixdate / obs-date   Host = uri-host [ ":" port ]   IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT   If-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]   If-Modified-Since = HTTP-date   If-None-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]   If-Range = entity-tag / HTTP-date   If-Unmodified-Since = HTTP-date   Last-Modified = HTTP-date   Location = URI-reference   Max-Forwards = 1*DIGIT   OWS = *( SP / HTAB )   Proxy-Authenticate = [ challenge *( OWS "," OWS challenge ) ]   Proxy-Authentication-Info = [ auth-param *( OWS "," OWS auth-param )    ]   Proxy-Authorization = credentials   RWS = 1*( SP / HTAB )   Range = ranges-specifier   Referer = absolute-URI / partial-URI   Retry-After = HTTP-date / delay-seconds   Server = product *( RWS ( product / comment ) )   TE = [ t-codings *( OWS "," OWS t-codings ) ]   Trailer = [ field-name *( OWS "," OWS field-name ) ]   URI-reference = <URI-reference, see [URI], Section 4.1>   Upgrade = [ protocol *( OWS "," OWS protocol ) ]   User-Agent = product *( RWS ( product / comment ) )   Vary = [ ( "*" / field-name ) *( OWS "," OWS ( "*" / field-name ) )    ]   Via = [ ( received-protocol RWS received-by [ RWS comment ] ) *( OWS    "," OWS ( received-protocol RWS received-by [ RWS comment ] ) ) ]   WWW-Authenticate = [ challenge *( OWS "," OWS challenge ) ]   absolute-URI = <absolute-URI, see [URI], Section 4.3>   absolute-path = 1*( "/" segment )   acceptable-ranges = range-unit *( OWS "," OWS range-unit )   asctime-date = day-name SP date3 SP time-of-day SP year   auth-param = token BWS "=" BWS ( token / quoted-string )   auth-scheme = token   authority = <authority, see [URI], Section 3.2>   challenge = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","    OWS auth-param ) ] ) ]   codings = content-coding / "identity" / "*"   comment = "(" *( ctext / quoted-pair / comment ) ")"   complete-length = 1*DIGIT   connection-option = token   content-coding = token   credentials = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","    OWS auth-param ) ] ) ]   ctext = HTAB / SP / %x21-27 ; '!'-'''    / %x2A-5B ; '*'-'['    / %x5D-7E ; ']'-'~'    / obs-text   date1 = day SP month SP year   date2 = day "-" month "-" 2DIGIT   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )   day = 2DIGIT   day-name = %x4D.6F.6E ; Mon    / %x54.75.65 ; Tue    / %x57.65.64 ; Wed    / %x54.68.75 ; Thu    / %x46.72.69 ; Fri    / %x53.61.74 ; Sat    / %x53.75.6E ; Sun   day-name-l = %x4D.6F.6E.64.61.79 ; Monday    / %x54.75.65.73.64.61.79 ; Tuesday    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday    / %x54.68.75.72.73.64.61.79 ; Thursday    / %x46.72.69.64.61.79 ; Friday    / %x53.61.74.75.72.64.61.79 ; Saturday    / %x53.75.6E.64.61.79 ; Sunday   delay-seconds = 1*DIGIT   entity-tag = [ weak ] opaque-tag   etagc = "!" / %x23-7E ; '#'-'~'    / obs-text   expectation = token [ "=" ( token / quoted-string ) parameters ]   field-content = field-vchar [ 1*( SP / HTAB / field-vchar )    field-vchar ]   field-name = token   field-value = *field-content   field-vchar = VCHAR / obs-text   first-pos = 1*DIGIT   hour = 2DIGIT   http-URI = "http://" authority path-abempty [ "?" query ]   https-URI = "https://" authority path-abempty [ "?" query ]   incl-range = first-pos "-" last-pos   int-range = first-pos "-" [ last-pos ]   language-range = <language-range, see [RFC4647], Section 2.1>   language-tag = <Language-Tag, see [RFC5646], Section 2.1>   last-pos = 1*DIGIT   mailbox = <mailbox, see [RFC5322], Section 3.4>   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) )    parameters   media-type = type "/" subtype parameters   method = token   minute = 2DIGIT   month = %x4A.61.6E ; Jan    / %x46.65.62 ; Feb    / %x4D.61.72 ; Mar    / %x41.70.72 ; Apr    / %x4D.61.79 ; May    / %x4A.75.6E ; Jun    / %x4A.75.6C ; Jul    / %x41.75.67 ; Aug    / %x53.65.70 ; Sep    / %x4F.63.74 ; Oct    / %x4E.6F.76 ; Nov    / %x44.65.63 ; Dec   obs-date = rfc850-date / asctime-date   obs-text = %x80-FF   opaque-tag = DQUOTE *etagc DQUOTE   other-range = 1*( %x21-2B ; '!'-'+'    / %x2D-7E ; '-'-'~'    )   parameter = parameter-name "=" parameter-value   parameter-name = token   parameter-value = ( token / quoted-string )   parameters = *( OWS ";" OWS [ parameter ] )   partial-URI = relative-part [ "?" query ]   path-abempty = <path-abempty, see [URI], Section 3.3>   port = <port, see [URI], Section 3.2.3>   product = token [ "/" product-version ]   product-version = token   protocol = protocol-name [ "/" protocol-version ]   protocol-name = token   protocol-version = token   pseudonym = token   qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['    / %x5D-7E ; ']'-'~'    / obs-text   query = <query, see [URI], Section 3.4>   quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )   quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )   range-resp = incl-range "/" ( complete-length / "*" )   range-set = range-spec *( OWS "," OWS range-spec )   range-spec = int-range / suffix-range / other-range   range-unit = token   ranges-specifier = range-unit "=" range-set   received-by = pseudonym [ ":" port ]   received-protocol = [ protocol-name "/" ] protocol-version   relative-part = <relative-part, see [URI], Section 4.2>   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT   second = 2DIGIT   segment = <segment, see [URI], Section 3.3>   subtype = token   suffix-length = 1*DIGIT   suffix-range = "-" suffix-length   t-codings = "trailers" / ( transfer-coding [ weight ] )   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA   time-of-day = hour ":" minute ":" second   token = 1*tchar   token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )    *"="   transfer-coding = token *( OWS ";" OWS transfer-parameter )   transfer-parameter = token BWS "=" BWS ( token / quoted-string )   type = token   unsatisfied-range = "*/" complete-length   uri-host = <host, see [URI], Section 3.2.2>   weak = %x57.2F ; W/   weight = OWS ";" OWS "q=" qvalue   year = 4DIGITAppendix B.  Changes from Previous RFCsB.1.  Changes from RFC 2818   None.B.2.  Changes from RFC 7230   The sections introducing HTTP's design goals, history, architecture,   conformance criteria, protocol versioning, URIs, message routing, and   header fields have been moved here.   The requirement on semantic conformance has been replaced with   permission to ignore or work around implementation-specific failures.   (Section 2.2)   The description of an origin and authoritative access to origin   servers has been extended for both "http" and "https" URIs to account   for alternative services and secured connections that are not   necessarily based on TCP.  (Sections 4.2.1, 4.2.2, 4.3.1, and 7.3.3)   Explicit requirements have been added to check the target URI   scheme's semantics and reject requests that don't meet any associated   requirements.  (Section 7.4)   Parameters in media type, media range, and expectation can be empty   via one or more trailing semicolons.  (Section 5.6.6)   "Field value" now refers to the value after multiple field lines are   combined with commas -- by far the most common use.  To refer to a   single header line's value, use "field line value".  (Section 6.3)   Trailer field semantics now transcend the specifics of chunked   transfer coding.  The use of trailer fields has been further limited   to allow generation as a trailer field only when the sender knows the   field defines that usage and to allow merging into the header section   only if the recipient knows the corresponding field definition   permits and defines how to merge.  In all other cases,   implementations are encouraged either to store the trailer fields   separately or to discard them instead of merging.  (Section 6.5.1)   The priority of the absolute form of the request URI over the Host   header field by origin servers has been made explicit to align with   proxy handling.  (Section 7.2)   The grammar definition for the Via field's "received-by" was expanded   in RFC 7230 due to changes in the URI grammar for host [URI] that are   not desirable for Via. For simplicity, we have removed uri-host from   the received-by production because it can be encompassed by the   existing grammar for pseudonym.  In particular, this change removed   comma from the allowed set of characters for a host name in received-   by.  (Section 7.6.3)B.3.  Changes from RFC 7231   Minimum URI lengths to be supported by implementations are now   recommended.  (Section 4.1)   The following have been clarified: CR and NUL in field values are to   be rejected or mapped to SP, and leading and trailing whitespace   needs to be stripped from field values before they are consumed.   (Section 5.5)   Parameters in media type, media range, and expectation can be empty   via one or more trailing semicolons.  (Section 5.6.6)   An abstract data type for HTTP messages has been introduced to define   the components of a message and their semantics as an abstraction   across multiple HTTP versions, rather than in terms of the specific   syntax form of HTTP/1.1 in [HTTP/1.1], and reflect the contents after   the message is parsed.  This makes it easier to distinguish between   requirements on the content (what is conveyed) versus requirements on   the messaging syntax (how it is conveyed) and avoids baking   limitations of early protocol versions into the future of HTTP.   (Section 6)   The terms "payload" and "payload body" have been replaced with   "content", to better align with its usage elsewhere (e.g., in field   names) and to avoid confusion with frame payloads in HTTP/2 and   HTTP/3.  (Section 6.4)   The term "effective request URI" has been replaced with "target URI".   (Section 7.1)   Restrictions on client retries have been loosened to reflect   implementation behavior.  (Section 9.2.2)   The fact that request bodies on GET, HEAD, and DELETE are not   interoperable has been clarified.  (Sections 9.3.1, 9.3.2, and 9.3.5)   The use of the Content-Range header field (Section 14.4) as a request   modifier on PUT is allowed.  (Section 9.3.4)   A superfluous requirement about setting Content-Length has been   removed from the description of the OPTIONS method.  (Section 9.3.7)   The normative requirement to use the "message/http" media type in   TRACE responses has been removed.  (Section 9.3.8)   List-based grammar for Expect has been restored for compatibility   with RFC 2616.  (Section 10.1.1)   Accept and Accept-Encoding are allowed in response messages; the   latter was introduced by [RFC7694].  (Section 12.3)   "Accept Parameters" (accept-params and accept-ext ABNF production)   have been removed from the definition of the Accept field.   (Section 12.5.1)   The Accept-Charset field is now deprecated.  (Section 12.5.2)   The semantics of "*" in the Vary header field when other values are   present was clarified.  (Section 12.5.5)   Range units are compared in a case-insensitive fashion.   (Section 14.1)   The use of the Accept-Ranges field is not restricted to origin   servers.  (Section 14.3)   The process of creating a redirected request has been clarified.   (Section 15.4)   Status code 308 (previously defined in [RFC7538]) has been added so   that it's defined closer to status codes 301, 302, and 307.   (Section 15.4.9)   Status code 421 (previously defined in Section 9.1.2 of [RFC7540])   has been added because of its general applicability. 421 is no longer   defined as heuristically cacheable since the response is specific to   the connection (not the target resource).  (Section 15.5.20)   Status code 422 (previously defined in Section 11.2 of [WEBDAV]) has   been added because of its general applicability.  (Section 15.5.21)B.4.  Changes from RFC 7232   Previous revisions of HTTP imposed an arbitrary 60-second limit on   the determination of whether Last-Modified was a strong validator to   guard against the possibility that the Date and Last-Modified values   are generated from different clocks or at somewhat different times   during the preparation of the response.  This specification has   relaxed that to allow reasonable discretion.  (Section 8.8.2.2)   An edge-case requirement on If-Match and If-Unmodified-Since has been   removed that required a validator not to be sent in a 2xx response if   validation fails because the change request has already been applied.   (Sections 13.1.1 and 13.1.4)   The fact that If-Unmodified-Since does not apply to a resource   without a concept of modification time has been clarified.   (Section 13.1.4)   Preconditions can now be evaluated before the request content is   processed rather than waiting until the response would otherwise be   successful.  (Section 13.2)B.5.  Changes from RFC 7233   Refactored the range-unit and ranges-specifier grammars to simplify   and reduce artificial distinctions between bytes and other   (extension) range units, removing the overlapping grammar of other-   range-unit by defining range units generically as a token and placing   extensions within the scope of a range-spec (other-range).  This   disambiguates the role of list syntax (commas) in all range sets,   including extension range units, for indicating a range-set of more   than one range.  Moving the extension grammar into range specifiers   also allows protocol specific to byte ranges to be specified   separately.   It is now possible to define Range handling on extension methods.   (Section 14.2)   Described use of the Content-Range header field (Section 14.4) as a   request modifier to perform a partial PUT.  (Section 14.5)B.6.  Changes from RFC 7235   None.B.7.  Changes from RFC 7538   None.B.8.  Changes from RFC 7615   None.B.9.  Changes from RFC 7694   This specification includes the extension defined in [RFC7694] but   leaves out examples and deployment considerations.Acknowledgements   Aside from the current editors, the following individuals deserve   special recognition for their contributions to early aspects of HTTP   and its core specifications: Marc Andreessen, Tim Berners-Lee, Robert   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jim Gettys,   Jean-François Groff, Phillip M. Hallam-Baker, Koen Holtman, Jeffery   L. Hostetler, Shel Kaphan, Dave Kristol, Yves Lafon, Scott   D. Lawrence, Paul J. Leach, Håkon W. Lie, Ari Luotonen, Larry   Masinter, Rob McCool, Jeffrey C. Mogul, Lou Montulli, David Morris,   Henrik Frystyk Nielsen, Dave Raggett, Eric Rescorla, Tony Sanders,   Lawrence C. Stewart, Marc VanHeyningen, and Steve Zilles.   This document builds on the many contributions that went into past   specifications of HTTP, including [HTTP/1.0], [RFC2068], [RFC2145],   [RFC2616], [RFC2617], [RFC2818], [RFC7230], [RFC7231], [RFC7232],   [RFC7233], [RFC7234], and [RFC7235].  The acknowledgements within   those documents still apply.   Since 2014, the following contributors have helped improve this   specification by reporting bugs, asking smart questions, drafting or   reviewing text, and evaluating issues:   Alan Egerton, Alex Rousskov, Amichai Rothman, Amos Jeffries, Anders   Kaseorg, Andreas Gebhardt, Anne van Kesteren, Armin Abfalterer, Aron   Duby, Asanka Herath, Asbjørn Ulsberg, Asta Olofsson, Attila Gulyas,   Austin Wright, Barry Pollard, Ben Burkert, Benjamin Kaduk, Björn   Höhrmann, Brad Fitzpatrick, Chris Pacejo, Colin Bendell, Cory   Benfield, Cory Nelson, Daisuke Miyakawa, Dale Worley, Daniel   Stenberg, Danil Suits, David Benjamin, David Matson, David Schinazi,   Дилян Палаузов (Dilyan Palauzov), Eric Anderson, Eric Rescorla, Éric   Vyncke, Erik Kline, Erwin Pe, Etan Kissling, Evert Pot, Evgeny   Vrublevsky, Florian Best, Francesca Palombini, Igor Lubashev, James   Callahan, James Peach, Jeffrey Yasskin, Kalin Gyokov, Kannan Goundan,   奥 一穂 (Kazuho Oku), Ken Murchison, Krzysztof Maczyński, Lars Eggert,   Lucas Pardue, Martin Duke, Martin Dürst, Martin Thomson, Martynas   Jusevičius, Matt Menke, Matthias Pigulla, Mattias Grenfeldt, Michael   Osipov, Mike Bishop, Mike Pennisi, Mike Taylor, Mike West, Mohit   Sethi, Murray Kucherawy, Nathaniel J. Smith, Nicholas Hurley, Nikita   Prokhorov, Patrick McManus, Piotr Sikora, Poul-Henning Kamp, Rick van   Rein, Robert Wilton, Roberto Polli, Roman Danyliw, Samuel Williams,   Semyon Kholodnov, Simon Pieters, Simon Schüppel, Stefan Eissing,   Taylor Hunt, Todd Greer, Tommy Pauly, Vasiliy Faronov, Vladimir   Lashchev, Wenbo Zhu, William A. Rowe Jr., Willy Tarreau, Xingwei Liu,   Yishuai Li, and Zaheduzzaman Sarker.Index   1 2 3 4 5 A B C D E F G H I L M N O P R S T U V W X      1         100 Continue (status code)  *_Section 15.2.1_*         100-continue (expect value)  *_Section 10.1.1_*         101 Switching Protocols (status code)  *_Section 15.2.2_*         1xx Informational (status code class)  *_Section 15.2_*      2         200 OK (status code)  *_Section 15.3.1_*         201 Created (status code)  *_Section 15.3.2_*         202 Accepted (status code)  *_Section 15.3.3_*         203 Non-Authoritative Information (status code)  *_Section 15.3            .4_*         204 No Content (status code)  *_Section 15.3.5_*         205 Reset Content (status code)  *_Section 15.3.6_*         206 Partial Content (status code)  *_Section 15.3.7_*         2xx Successful (status code class)  *_Section 15.3_*      3         300 Multiple Choices (status code)  *_Section 15.4.1_*         301 Moved Permanently (status code)  *_Section 15.4.2_*         302 Found (status code)  *_Section 15.4.3_*         303 See Other (status code)  *_Section 15.4.4_*         304 Not Modified (status code)  *_Section 15.4.5_*         305 Use Proxy (status code)  *_Section 15.4.6_*         306 (Unused) (status code)  *_Section 15.4.7_*         307 Temporary Redirect (status code)  *_Section 15.4.8_*         308 Permanent Redirect (status code)  *_Section 15.4.9_*         3xx Redirection (status code class)  *_Section 15.4_*      4         400 Bad Request (status code)  *_Section 15.5.1_*         401 Unauthorized (status code)  *_Section 15.5.2_*         402 Payment Required (status code)  *_Section 15.5.3_*         403 Forbidden (status code)  *_Section 15.5.4_*         404 Not Found (status code)  *_Section 15.5.5_*         405 Method Not Allowed (status code)  *_Section 15.5.6_*         406 Not Acceptable (status code)  *_Section 15.5.7_*         407 Proxy Authentication Required (status code)  *_Section 15.5            .8_*         408 Request Timeout (status code)  *_Section 15.5.9_*         409 Conflict (status code)  *_Section 15.5.10_*         410 Gone (status code)  *_Section 15.5.11_*         411 Length Required (status code)  *_Section 15.5.12_*         412 Precondition Failed (status code)  *_Section 15.5.13_*         413 Content Too Large (status code)  *_Section 15.5.14_*         414 URI Too Long (status code)  *_Section 15.5.15_*         415 Unsupported Media Type (status code)  *_Section 15.5.16_*         416 Range Not Satisfiable (status code)  *_Section 15.5.17_*         417 Expectation Failed (status code)  *_Section 15.5.18_*         418 (Unused) (status code)  *_Section 15.5.19_*         421 Misdirected Request (status code)  *_Section 15.5.20_*         422 Unprocessable Content (status code)  *_Section 15.5.21_*         426 Upgrade Required (status code)  *_Section 15.5.22_*         4xx Client Error (status code class)  *_Section 15.5_*      5         500 Internal Server Error (status code)  *_Section 15.6.1_*         501 Not Implemented (status code)  *_Section 15.6.2_*         502 Bad Gateway (status code)  *_Section 15.6.3_*         503 Service Unavailable (status code)  *_Section 15.6.4_*         504 Gateway Timeout (status code)  *_Section 15.6.5_*         505 HTTP Version Not Supported (status code)  *_Section 15.6.6_            *         5xx Server Error (status code class)  *_Section 15.6_*      A         accelerator  *_Section 3.7, Paragraph 6_*         Accept header field  *_Section 12.5.1_*         Accept-Charset header field  *_Section 12.5.2_*         Accept-Encoding header field  *_Section 12.5.3_*         Accept-Language header field  *_Section 12.5.4_*         Accept-Ranges header field  *_Section 14.3_*         Allow header field  *_Section 10.2.1_*         Authentication-Info header field  *_Section 11.6.3_*         authoritative response  *_Section 17.1_*         Authorization header field  *_Section 11.6.2_*      B         browser  *_Section 3.5_*      C         cache  *_Section 3.8_*         cacheable  *_Section 3.8, Paragraph 4_*         client  *_Section 3.3_*         clock  *_Section 5.6.7_*         complete  *_Section 6.1_*         compress (Coding Format)  Section 8.4.1.1         compress (content coding)  *_Section 8.4.1_*         conditional request  *_Section 13_*         CONNECT method  *_Section 9.3.6_*         connection  *_Section 3.3_*         Connection header field  *_Section 7.6.1_*         content  Section 6.4         content coding  *_Section 8.4.1_*         content negotiation  Section 1.3, Paragraph 4         Content-Encoding header field  *_Section 8.4_*         Content-Language header field  *_Section 8.5_*         Content-Length header field  *_Section 8.6_*         Content-Location header field  *_Section 8.7_*         Content-MD5 header field  *_Section 18.4, Paragraph 10_*         Content-Range header field  *_Section 14.4_*; Section 14.5         Content-Type header field  *_Section 8.3_*         control data  *_Section 6.2_*      D         Date header field  *_Section 6.6.1_*         deflate (Coding Format)  Section 8.4.1.2         deflate (content coding)  *_Section 8.4.1_*         DELETE method  *_Section 9.3.5_*         Delimiters  Section 5.6.2, Paragraph 3         downstream  *_Section 3.7, Paragraph 4_*      E         effective request URI  *_Section 7.1, Paragraph 8.1_*         ETag field  *_Section 8.8.3_*         Expect header field  *_Section 10.1.1_*      F         field  *_Section 5_*; Section 6.3         field line  Section 5.2, Paragraph 1         field line value  Section 5.2, Paragraph 1         field name  Section 5.2, Paragraph 1         field value  Section 5.2, Paragraph 2         Fields            *  *_Section 18.4, Paragraph 9_*            Accept  *_Section 12.5.1_*            Accept-Charset  *_Section 12.5.2_*            Accept-Encoding  *_Section 12.5.3_*            Accept-Language  *_Section 12.5.4_*            Accept-Ranges  *_Section 14.3_*            Allow  *_Section 10.2.1_*            Authentication-Info  *_Section 11.6.3_*            Authorization  *_Section 11.6.2_*            Connection  *_Section 7.6.1_*            Content-Encoding  *_Section 8.4_*            Content-Language  *_Section 8.5_*            Content-Length  *_Section 8.6_*            Content-Location  *_Section 8.7_*            Content-MD5  *_Section 18.4, Paragraph 10_*            Content-Range  *_Section 14.4_*; Section 14.5            Content-Type  *_Section 8.3_*            Date  *_Section 6.6.1_*            ETag  *_Section 8.8.3_*            Expect  *_Section 10.1.1_*            From  *_Section 10.1.2_*            Host  *_Section 7.2_*            If-Match  *_Section 13.1.1_*            If-Modified-Since  *_Section 13.1.3_*            If-None-Match  *_Section 13.1.2_*            If-Range  *_Section 13.1.5_*            If-Unmodified-Since  *_Section 13.1.4_*            Last-Modified  *_Section 8.8.2_*            Location  *_Section 10.2.2_*            Max-Forwards  *_Section 7.6.2_*            Proxy-Authenticate  *_Section 11.7.1_*            Proxy-Authentication-Info  *_Section 11.7.3_*            Proxy-Authorization  *_Section 11.7.2_*            Range  *_Section 14.2_*            Referer  *_Section 10.1.3_*            Retry-After  *_Section 10.2.3_*            Server  *_Section 10.2.4_*            TE  *_Section 10.1.4_*            Trailer  *_Section 6.6.2_*            Upgrade  *_Section 7.8_*            User-Agent  *_Section 10.1.5_*            Vary  *_Section 12.5.5_*            Via  *_Section 7.6.3_*            WWW-Authenticate  *_Section 11.6.1_*         Fragment Identifiers  Section 4.2.5         From header field  *_Section 10.1.2_*      G         gateway  *_Section 3.7, Paragraph 6_*         GET method  *_Section 9.3.1_*         Grammar            ALPHA  *_Section 2.1_*            Accept  *_Section 12.5.1_*            Accept-Charset  *_Section 12.5.2_*            Accept-Encoding  *_Section 12.5.3_*            Accept-Language  *_Section 12.5.4_*            Accept-Ranges  *_Section 14.3_*            Allow  *_Section 10.2.1_*            Authentication-Info  *_Section 11.6.3_*            Authorization  *_Section 11.6.2_*            BWS  *_Section 5.6.3_*            CR  *_Section 2.1_*            CRLF  *_Section 2.1_*            CTL  *_Section 2.1_*            Connection  *_Section 7.6.1_*            Content-Encoding  *_Section 8.4_*            Content-Language  *_Section 8.5_*            Content-Length  *_Section 8.6_*            Content-Location  *_Section 8.7_*            Content-Range  *_Section 14.4_*            Content-Type  *_Section 8.3_*            DIGIT  *_Section 2.1_*            DQUOTE  *_Section 2.1_*            Date  *_Section 6.6.1_*            ETag  *_Section 8.8.3_*            Expect  *_Section 10.1.1_*            From  *_Section 10.1.2_*            GMT  *_Section 5.6.7_*            HEXDIG  *_Section 2.1_*            HTAB  *_Section 2.1_*            HTTP-date  *_Section 5.6.7_*            Host  *_Section 7.2_*            IMF-fixdate  *_Section 5.6.7_*            If-Match  *_Section 13.1.1_*            If-Modified-Since  *_Section 13.1.3_*            If-None-Match  *_Section 13.1.2_*            If-Range  *_Section 13.1.5_*            If-Unmodified-Since  *_Section 13.1.4_*            LF  *_Section 2.1_*            Last-Modified  *_Section 8.8.2_*            Location  *_Section 10.2.2_*            Max-Forwards  *_Section 7.6.2_*            OCTET  *_Section 2.1_*            OWS  *_Section 5.6.3_*            Proxy-Authenticate  *_Section 11.7.1_*            Proxy-Authentication-Info  *_Section 11.7.3_*            Proxy-Authorization  *_Section 11.7.2_*            RWS  *_Section 5.6.3_*            Range  *_Section 14.2_*            Referer  *_Section 10.1.3_*            Retry-After  *_Section 10.2.3_*            SP  *_Section 2.1_*            Server  *_Section 10.2.4_*            TE  *_Section 10.1.4_*            Trailer  *_Section 6.6.2_*            URI-reference  *_Section 4.1_*            Upgrade  *_Section 7.8_*            User-Agent  *_Section 10.1.5_*            VCHAR  *_Section 2.1_*            Vary  *_Section 12.5.5_*            Via  *_Section 7.6.3_*            WWW-Authenticate  *_Section 11.6.1_*            absolute-URI  *_Section 4.1_*            absolute-path  *_Section 4.1_*            acceptable-ranges  *_Section 14.3_*            asctime-date  *_Section 5.6.7_*            auth-param  *_Section 11.2_*            auth-scheme  *_Section 11.1_*            authority  *_Section 4.1_*            challenge  *_Section 11.3_*            codings  *_Section 12.5.3_*            comment  *_Section 5.6.5_*            complete-length  *_Section 14.4_*            connection-option  *_Section 7.6.1_*            content-coding  *_Section 8.4.1_*            credentials  *_Section 11.4_*            ctext  *_Section 5.6.5_*            date1  *_Section 5.6.7_*            day  *_Section 5.6.7_*            day-name  *_Section 5.6.7_*            day-name-l  *_Section 5.6.7_*            delay-seconds  *_Section 10.2.3_*            entity-tag  *_Section 8.8.3_*            etagc  *_Section 8.8.3_*            field-content  *_Section 5.5_*            field-name  *_Section 5.1_*; Section 6.6.2            field-value  *_Section 5.5_*            field-vchar  *_Section 5.5_*            first-pos  *_Section 14.1.1_*; Section 14.4            hour  *_Section 5.6.7_*            http-URI  *_Section 4.2.1_*            https-URI  *_Section 4.2.2_*            incl-range  *_Section 14.4_*            int-range  *_Section 14.1.1_*            language-range  *_Section 12.5.4_*            language-tag  *_Section 8.5.1_*            last-pos  *_Section 14.1.1_*; Section 14.4            media-range  *_Section 12.5.1_*            media-type  *_Section 8.3.1_*            method  *_Section 9.1_*            minute  *_Section 5.6.7_*            month  *_Section 5.6.7_*            obs-date  *_Section 5.6.7_*            obs-text  *_Section 5.5_*            opaque-tag  *_Section 8.8.3_*            other-range  *_Section 14.1.1_*            parameter  *_Section 5.6.6_*            parameter-name  *_Section 5.6.6_*            parameter-value  *_Section 5.6.6_*            parameters  *_Section 5.6.6_*            partial-URI  *_Section 4.1_*            port  *_Section 4.1_*            product  *_Section 10.1.5_*            product-version  *_Section 10.1.5_*            protocol-name  *_Section 7.6.3_*            protocol-version  *_Section 7.6.3_*            pseudonym  *_Section 7.6.3_*            qdtext  *_Section 5.6.4_*            query  *_Section 4.1_*            quoted-pair  *_Section 5.6.4_*            quoted-string  *_Section 5.6.4_*            qvalue  *_Section 12.4.2_*            range-resp  *_Section 14.4_*            range-set  *_Section 14.1.1_*            range-spec  *_Section 14.1.1_*            range-unit  *_Section 14.1_*            ranges-specifier  *_Section 14.1.1_*            received-by  *_Section 7.6.3_*            received-protocol  *_Section 7.6.3_*            rfc850-date  *_Section 5.6.7_*            second  *_Section 5.6.7_*            segment  *_Section 4.1_*            subtype  *_Section 8.3.1_*            suffix-length  *_Section 14.1.1_*            suffix-range  *_Section 14.1.1_*            t-codings  *_Section 10.1.4_*            tchar  *_Section 5.6.2_*            time-of-day  *_Section 5.6.7_*            token  *_Section 5.6.2_*            token68  *_Section 11.2_*            transfer-coding  *_Section 10.1.4_*            transfer-parameter  *_Section 10.1.4_*            type  *_Section 8.3.1_*            unsatisfied-range  *_Section 14.4_*            uri-host  *_Section 4.1_*            weak  *_Section 8.8.3_*            weight  *_Section 12.4.2_*            year  *_Section 5.6.7_*         gzip (Coding Format)  Section 8.4.1.3         gzip (content coding)  *_Section 8.4.1_*      H         HEAD method  *_Section 9.3.2_*         Header Fields            Accept  *_Section 12.5.1_*            Accept-Charset  *_Section 12.5.2_*            Accept-Encoding  *_Section 12.5.3_*            Accept-Language  *_Section 12.5.4_*            Accept-Ranges  *_Section 14.3_*            Allow  *_Section 10.2.1_*            Authentication-Info  *_Section 11.6.3_*            Authorization  *_Section 11.6.2_*            Connection  *_Section 7.6.1_*            Content-Encoding  *_Section 8.4_*            Content-Language  *_Section 8.5_*            Content-Length  *_Section 8.6_*            Content-Location  *_Section 8.7_*            Content-MD5  *_Section 18.4, Paragraph 10_*            Content-Range  *_Section 14.4_*; Section 14.5            Content-Type  *_Section 8.3_*            Date  *_Section 6.6.1_*            ETag  *_Section 8.8.3_*            Expect  *_Section 10.1.1_*            From  *_Section 10.1.2_*            Host  *_Section 7.2_*            If-Match  *_Section 13.1.1_*            If-Modified-Since  *_Section 13.1.3_*            If-None-Match  *_Section 13.1.2_*            If-Range  *_Section 13.1.5_*            If-Unmodified-Since  *_Section 13.1.4_*            Last-Modified  *_Section 8.8.2_*            Location  *_Section 10.2.2_*            Max-Forwards  *_Section 7.6.2_*            Proxy-Authenticate  *_Section 11.7.1_*            Proxy-Authentication-Info  *_Section 11.7.3_*            Proxy-Authorization  *_Section 11.7.2_*            Range  *_Section 14.2_*            Referer  *_Section 10.1.3_*            Retry-After  *_Section 10.2.3_*            Server  *_Section 10.2.4_*            TE  *_Section 10.1.4_*            Trailer  *_Section 6.6.2_*            Upgrade  *_Section 7.8_*            User-Agent  *_Section 10.1.5_*            Vary  *_Section 12.5.5_*            Via  *_Section 7.6.3_*            WWW-Authenticate  *_Section 11.6.1_*         header section  *_Section 6.3_*         Host header field  *_Section 7.2_*         http URI scheme  *_Section 4.2.1_*         https URI scheme  *_Section 4.2.2_*      I         idempotent  *_Section 9.2.2_*         If-Match header field  *_Section 13.1.1_*         If-Modified-Since header field  *_Section 13.1.3_*         If-None-Match header field  *_Section 13.1.2_*         If-Range header field  *_Section 13.1.5_*         If-Unmodified-Since header field  *_Section 13.1.4_*         inbound  *_Section 3.7, Paragraph 4_*         incomplete  *_Section 6.1_*         interception proxy  *_Section 3.7, Paragraph 10_*         intermediary  *_Section 3.7_*      L         Last-Modified header field  *_Section 8.8.2_*         list-based field  Section 5.5, Paragraph 7         Location header field  *_Section 10.2.2_*      M         Max-Forwards header field  *_Section 7.6.2_*         Media Type            multipart/byteranges  *_Section 14.6_*            multipart/x-byteranges  Section 14.6, Paragraph 4, Item 3         message  Section 3.4; *_Section 6_*         message abstraction  *_Section 6_*         messages  *_Section 3.4_*         metadata  *_Section 8.8_*         Method            *  *_Section 18.2, Paragraph 3_*            CONNECT  *_Section 9.3.6_*            DELETE  *_Section 9.3.5_*            GET  *_Section 9.3.1_*            HEAD  *_Section 9.3.2_*            OPTIONS  *_Section 9.3.7_*            POST  *_Section 9.3.3_*            PUT  *_Section 9.3.4_*            TRACE  *_Section 9.3.8_*         multipart/byteranges Media Type  *_Section 14.6_*         multipart/x-byteranges Media Type  Section 14.6, Paragraph 4,            Item 3      N         non-transforming proxy  *_Section 7.7_*      O         OPTIONS method  *_Section 9.3.7_*         origin  *_Section 4.3.1_*; Section 11.5         origin server  *_Section 3.6_*         outbound  *_Section 3.7, Paragraph 4_*      P         phishing  *_Section 17.1_*         POST method  *_Section 9.3.3_*         Protection Space  Section 11.5         proxy  *_Section 3.7, Paragraph 5_*         Proxy-Authenticate header field  *_Section 11.7.1_*         Proxy-Authentication-Info header field  *_Section 11.7.3_*         Proxy-Authorization header field  *_Section 11.7.2_*         PUT method  *_Section 9.3.4_*      R         Range header field  *_Section 14.2_*         Realm  Section 11.5         recipient  *_Section 3.4_*         Referer header field  *_Section 10.1.3_*         representation  *_Section 3.2_*         request  *_Section 3.4_*         request target  *_Section 7.1_*         resource  *_Section 3.1_*; Section 4         response  *_Section 3.4_*         Retry-After header field  *_Section 10.2.3_*         reverse proxy  *_Section 3.7, Paragraph 6_*      S         safe  *_Section 9.2.1_*         satisfiable range  *_Section 14.1.1_*         secured  *_Section 4.2.2_*         selected representation  *_Section 3.2, Paragraph 4_*;            Section 8.8; Section 13.1         self-descriptive  *_Section 6_*         sender  *_Section 3.4_*         server  *_Section 3.3_*         Server header field  *_Section 10.2.4_*         singleton field  Section 5.5, Paragraph 6         spider  *_Section 3.5_*         Status Code  Section 15         Status Codes            Final  Section 15, Paragraph 7            Informational  Section 15, Paragraph 7            Interim  Section 15, Paragraph 7         Status Codes Classes            1xx Informational  *_Section 15.2_*            2xx Successful  *_Section 15.3_*            3xx Redirection  *_Section 15.4_*            4xx Client Error  *_Section 15.5_*            5xx Server Error  *_Section 15.6_*      T         target resource  *_Section 7.1_*         target URI  *_Section 7.1_*         TE header field  *_Section 10.1.4_*         TRACE method  *_Section 9.3.8_*         Trailer Fields  *_Section 6.5_*            ETag  *_Section 8.8.3_*         Trailer header field  *_Section 6.6.2_*         trailer section  *_Section 6.5_*         trailers  *_Section 6.5_*         transforming proxy  *_Section 7.7_*         transparent proxy  *_Section 3.7, Paragraph 10_*         tunnel  *_Section 3.7, Paragraph 8_*      U         unsatisfiable range  *_Section 14.1.1_*         Upgrade header field  *_Section 7.8_*         upstream  *_Section 3.7, Paragraph 4_*         URI  *_Section 4_*            origin  *_Section 4.3.1_*         URI reference  *_Section 4.1_*         URI scheme            http  *_Section 4.2.1_*            https  *_Section 4.2.2_*         user agent  *_Section 3.5_*         User-Agent header field  *_Section 10.1.5_*      V         validator  *_Section 8.8_*            strong  *_Section 8.8.1_*            weak  *_Section 8.8.1_*         Vary header field  *_Section 12.5.5_*         Via header field  *_Section 7.6.3_*      W         WWW-Authenticate header field  *_Section 11.6.1_*      X         x-compress (content coding)  *_Section 8.4.1_*         x-gzip (content coding)  *_Section 8.4.1_*Authors' Addresses   Roy T. Fielding (editor)   Adobe   345 Park Ave   San Jose, CA 95110   United States of America   Email: fielding@gbiv.com   URI:   https://roy.gbiv.com/   Mark Nottingham (editor)   Fastly   Prahran   Australia   Email: mnot@mnot.net   URI:   https://www.mnot.net/   Julian Reschke (editor)   greenbytes GmbH   Hafenweg 16   48155 Münster   Germany   Email: julian.reschke@greenbytes.de   URI:   https://greenbytes.de/tech/webdav/