HTTP Working Group R. Fielding, Ed.Internet-Draft AdobeObsoletes:7234 (if approved) M. Nottingham, Ed.Intended status: Standards Track FastlyExpires: September 10, 2019 J. Reschke, Ed. greenbytes March 9, 2019HTTP Cachingdraft-ietf-httpbis-cache-04Abstract The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. This document obsoletesRFC 7234.Editorial Note This note is to be removed before publishing as an RFC. Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at <https://lists.w3.org/Archives/Public/ietf-http-wg/>. Working Group information can be found at <https://httpwg.org/>; source code and issues list for this draft can be found at <https://github.com/httpwg/http-core>. The changes in this draft are summarized inAppendix C.5.Status of This Memo This Internet-Draft is submitted in full conformance with the provisions ofBCP 78 andBCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is athttps://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at anyFielding, et al. Expires September 10, 2019 [Page 1]
Internet-Draft HTTP Caching March 2019 time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 10, 2019.Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified 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 Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .41.1. Requirements Notation . . . . . . . . . . . . . . . . . .51.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . .51.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . .62. Overview of Cache Operation . . . . . . . . . . . . . . . . .63. Storing Responses in Caches . . . . . . . . . . . . . . . . .73.1. Storing Incomplete Responses . . . . . . . . . . . . . .83.2. Storing Responses to Authenticated Requests . . . . . . .93.3. Combining Partial Content . . . . . . . . . . . . . . . .94. Constructing Responses from Caches . . . . . . . . . . . . .94.1. Calculating Secondary Keys with Vary . . . . . . . . . .104.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . .114.2.1. Calculating Freshness Lifetime . . . . . . . . . . .134.2.2. Calculating Heuristic Freshness . . . . . . . . . . .13Fielding, et al. Expires September 10, 2019 [Page 2]
Internet-Draft HTTP Caching March 20194.2.3. Calculating Age . . . . . . . . . . . . . . . . . . .144.2.4. Serving Stale Responses . . . . . . . . . . . . . . .154.3. Validation . . . . . . . . . . . . . . . . . . . . . . .164.3.1. Sending a Validation Request . . . . . . . . . . . .164.3.2. Handling a Received Validation Request . . . . . . .174.3.3. Handling a Validation Response . . . . . . . . . . .184.3.4. Freshening Stored Responses upon Validation . . . . .184.3.5. Freshening Responses with HEAD . . . . . . . . . . .194.4. Invalidation . . . . . . . . . . . . . . . . . . . . . .205. Header Field Definitions . . . . . . . . . . . . . . . . . .205.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . .215.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . .215.2.1. Request Cache-Control Directives . . . . . . . . . .225.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . .225.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . .235.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . .235.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . .235.2.1.5. no-store . . . . . . . . . . . . . . . . . . . .245.2.1.6. no-transform . . . . . . . . . . . . . . . . . .245.2.1.7. only-if-cached . . . . . . . . . . . . . . . . .245.2.2. Response Cache-Control Directives . . . . . . . . . .245.2.2.1. must-revalidate . . . . . . . . . . . . . . . . .245.2.2.2. no-cache . . . . . . . . . . . . . . . . . . . .255.2.2.3. no-store . . . . . . . . . . . . . . . . . . . .265.2.2.4. no-transform . . . . . . . . . . . . . . . . . .265.2.2.5. public . . . . . . . . . . . . . . . . . . . . .265.2.2.6. private . . . . . . . . . . . . . . . . . . . . .265.2.2.7. proxy-revalidate . . . . . . . . . . . . . . . .275.2.2.8. max-age . . . . . . . . . . . . . . . . . . . . .275.2.2.9. s-maxage . . . . . . . . . . . . . . . . . . . .275.2.3. Cache Control Extensions . . . . . . . . . . . . . .285.2.4. Cache Directive Registry . . . . . . . . . . . . . .295.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . .295.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . .305.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . .306. Relationship to Applications . . . . . . . . . . . . . . . .307. Security Considerations . . . . . . . . . . . . . . . . . . .318. IANA Considerations . . . . . . . . . . . . . . . . . . . . .328.1. Header Field Registration . . . . . . . . . . . . . . . .328.2. Cache Directive Registration . . . . . . . . . . . . . .328.3. Warn Code Registry . . . . . . . . . . . . . . . . . . .329. References . . . . . . . . . . . . . . . . . . . . . . . . .329.1. Normative References . . . . . . . . . . . . . . . . . .329.2. Informative References . . . . . . . . . . . . . . . . .33Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . .35Appendix B. Changes fromRFC 7234 . . . . . . . . . . . . . . .35Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . .35C.1. BetweenRFC7234 and draft 00 . . . . . . . . . . . . . .35Fielding, et al. Expires September 10, 2019 [Page 3]
Internet-Draft HTTP Caching March 2019C.2. Sincedraft-ietf-httpbis-cache-00 . . . . . . . . . . . .36C.3. Sincedraft-ietf-httpbis-cache-01 . . . . . . . . . . . .36C.4. Sincedraft-ietf-httpbis-cache-02 . . . . . . . . . . . .36C.5. Sincedraft-ietf-httpbis-cache-03 . . . . . . . . . . . .37 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .39 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .391. Introduction The Hypertext Transfer Protocol (HTTP) is a stateless application- level request/response protocol that uses extensible semantics and self-descriptive messages for flexible interaction with network-based hypertext information systems. HTTP is defined by a series of documents that collectively form the HTTP/1.1 specification: o "HTTP Semantics" [Semantics] o "HTTP Caching" (this document) o "HTTP/1.1 Messaging" [Messaging] HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches. This document defines aspects of HTTP related to caching and reusing response messages. An HTTP cache is a local store of response messages and the subsystem that controls storage, retrieval, and deletion of messages in it. 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 by a server that is acting as a tunnel. A shared cache is a cache that stores responses to be reused by more than one user; shared caches are usually (but not always) deployed as a part of an intermediary. A private cache, in contrast, is dedicated to a single user; often, they are deployed as a component of a user agent. The goal of caching in HTTP is to significantly improve performance by reusing a prior response message to satisfy a current request. A stored response is considered "fresh", as defined inSection 4.2, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh response can therefore reduce both latency and network overhead each time it is reused. When a cached response is not fresh, it might still be reusable if it can be freshened byFielding, et al. Expires September 10, 2019 [Page 4]
Internet-Draft HTTP Caching March 2019 validation (Section 4.3) or if the origin is unavailable (Section 4.2.4). This document obsoletesRFC 7234, with the changes being summarized inAppendix B.1.1. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Conformance criteria and considerations regarding error handling are defined in Section 3 of [Semantics].1.2. 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 11 of [Semantics], 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. The following core rules are included by reference, as defined in[RFC5234], Appendix B.1: 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 [USASCII] character). The rules below are defined in [Semantics]: HTTP-date = <HTTP-date, see [Semantics], Section 10.1.1.1> OWS = <OWS, see [Semantics], Section 4.3> field-name = <field-name, see [Semantics], Section 4.2> quoted-string = <quoted-string, see [Semantics], Section 4.2.3> token = <token, see [Semantics], Section 4.2.3> uri-host = <host, see[RFC3986], Section 3.2.2> port = <port, see[RFC3986], Section 3.2.3> pseudonym = <pseudonym, see [Semantics], Section 5.5.1>Fielding, et al. Expires September 10, 2019 [Page 5]
Internet-Draft HTTP Caching March 20191.3. Delta Seconds The delta-seconds rule specifies a non-negative integer, representing time in seconds. delta-seconds = 1*DIGIT A recipient parsing a delta-seconds value and converting it to binary form ought to use an arithmetic type of at least 31 bits of non- negative integer range. If a cache receives a delta-seconds value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, the cache MUST consider the value to be either 2147483648 (2^31) or the greatest positive integer it can conveniently represent. Note: The value 2147483648 is here for historical reasons, effectively represents infinity (over 68 years), and does not need to be stored in binary form; an implementation could produce it as a canned string if any overflow occurs, even if the calculations are performed with an arithmetic type incapable of directly representing that number. What matters here is that an overflow be detected and not treated as a negative value in later calculations.2. Overview of Cache Operation Proper cache operation preserves the semantics of HTTP transfers ([Semantics]) while reducing the transfer of information already held in the cache. Although caching is an entirely OPTIONAL feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses. Each cache entry consists of a cache key and one or more HTTP responses corresponding to prior requests that used the same key. The most common form of cache entry is a successful result of a retrieval request: i.e., a 200 (OK) response to a GET request, which contains a representation of the resource identified by the request target (Section 7.3.1 of [Semantics]). However, it is also possible to cache permanent redirects, negative results (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial Content)), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key.Fielding, et al. Expires September 10, 2019 [Page 6]
Internet-Draft HTTP Caching March 2019 The primary cache key consists of the request method and target URI. However, since HTTP caches in common use today are typically limited to caching responses to GET, many caches simply decline other methods and use only the URI as the primary cache key. If a request target is subject to content negotiation, its cache entry might consist of multiple stored responses, each differentiated by a secondary key for the values of the original request's selecting header fields (Section 4.1). A cache is disconnected when it cannot contact the origin server or otherwise find a forward path for a given request. A disconnected cache can serve stale responses in some circumstances (Section 4.2.4).3. Storing Responses in Caches A cache MUST NOT store a response to any request, unless: o The request method is understood by the cache and defined as being cacheable, and o the response status code is final (see Section 9.3 of [Messaging]), and o the response status code is understood by the cache, and o the "no-store" cache directive (seeSection 5.2) does not appear in the response, and o the "private" response directive (seeSection 5.2.2.6) does not appear in the response, if the cache is shared, and o the Authorization header field (see Section 8.5.3 of [Semantics]) does not appear in the request, if the cache is shared, unless the response explicitly allows it (seeSection 3.2), and o the response either: * contains an Expires header field (seeSection 5.3), or * contains a max-age response directive (seeSection 5.2.2.8), or * contains a s-maxage response directive (seeSection 5.2.2.9) and the cache is shared, orFielding, et al. Expires September 10, 2019 [Page 7]
Internet-Draft HTTP Caching March 2019 * contains a Cache Control Extension (seeSection 5.2.3) that allows it to be cached, or * has a status code that is defined as cacheable by default (seeSection 4.2.2), or * contains a public response directive (seeSection 5.2.2.5). Note that any of the requirements listed above can be overridden by a cache-control extension; seeSection 5.2.3. In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements all specified caching-related behavior. Note that, in normal operation, some caches will not store a response that has neither a cache validator nor an explicit expiration time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.3.1. Storing Incomplete Responses A response message is considered complete when all of the octets indicated by the message framing ([Messaging]) are received prior to the connection being closed. If the request method is GET, the response status code is 200 (OK), and the entire response header section has been received, a cache MAY store an incomplete response message body if the cache entry is recorded as incomplete. Likewise, a 206 (Partial Content) response MAY be stored as if it were an incomplete 200 (OK) cache entry. However, a cache MUST NOT store incomplete or partial-content responses if it does not support the Range and Content-Range header fields or if it does not understand the range units used in those fields. A cache MAY complete a stored incomplete response by making a subsequent range request (Section 8.3 of [Semantics]) and combining the successful response with the stored entry, as defined inSection 3.3. A cache MUST NOT use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies a range that is wholly within the incomplete response. A cache MUST NOT send a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.Fielding, et al. Expires September 10, 2019 [Page 8]
Internet-Draft HTTP Caching March 20193.2. Storing Responses to Authenticated Requests A shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 8.5.3 of [Semantics]) to satisfy any subsequent request unless a response directive that allows such responses to be stored is present. In this specification, the following Cache-Control response directives (Section 5.2.2) have such an effect: must-revalidate, public, and s-maxage.3.3. Combining Partial Content A response might transfer only a partial representation if the connection closed prematurely or if the request used one or more Range specifiers (Section 8.3 of [Semantics]). After several such transfers, a cache might have received several ranges of the same representation. A cache MAY combine these ranges into a single stored response, and reuse that response to satisfy later requests, if they all share the same strong validator and the cache complies with the client requirements in Section 9.3.7.3 of [Semantics]. When combining the new response with one or more stored responses, a cache MUST use the header fields provided in the new response, aside from Content-Range, to replace all instances of the corresponding header fields in the stored response.4. Constructing Responses from Caches When presented with a request, a cache MUST NOT reuse a stored response, unless: o The presented effective request URI (Section 5.3 of [Semantics]) and that of the stored response match, and o the request method associated with the stored response allows it to be used for the presented request, and o selecting header fields nominated by the stored response (if any) match those presented (seeSection 4.1), and o the stored response does not contain the no-cache cache directive (Section 5.2.2.2), unless it is successfully validated (Section 4.3), and o the stored response is either:Fielding, et al. Expires September 10, 2019 [Page 9]
Internet-Draft HTTP Caching March 2019 * fresh (seeSection 4.2), or * allowed to be served stale (seeSection 4.2.4), or * successfully validated (seeSection 4.3). Note that any of the requirements listed above can be overridden by a cache-control extension; seeSection 5.2.3. When a stored response is used to satisfy a request without validation, a cache MUST generate an Age header field (Section 5.1), replacing any present in the response with a value equal to the stored response's current_age; seeSection 4.2.3. A cache MUST write through requests with methods that are unsafe (Section 7.2.1 of [Semantics]) to the origin server; i.e., a cache is not allowed to generate a reply to such a request before having forwarded the request and having received a corresponding response. Also, note that unsafe requests might invalidate already-stored responses; seeSection 4.4. When more than one suitable response is stored, a cache MUST use the most recent one (as determined by the Date header field). It can also forward the request with "Cache-Control: max-age=0" or "Cache- Control: no-cache" to disambiguate which response to use. A cache that does not have a clock available MUST NOT use stored responses without revalidating them upon every use.4.1. Calculating Secondary Keys with Vary When a cache receives a request that can be satisfied by a stored response that has a Vary header field (Section 10.1.4 of [Semantics]), it MUST NOT use that response unless all of the selecting header fields nominated by the Vary header field match in both the original request (i.e., that associated with the stored response), and the presented request. The selecting header fields from two requests are defined to match if and only if those in the first request can be transformed to those in the second request by applying any of the following: o adding or removing whitespace, where allowed in the header field's syntaxFielding, et al. Expires September 10, 2019 [Page 10]
Internet-Draft HTTP Caching March 2019 o combining multiple header fields with the same field name (see Section 4.2 of [Semantics]) o normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification (e.g., reordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive) If (after any normalization that might take place) a header field is absent from a request, it can only match another request if it is also absent there. A Vary header field-value of "*" always fails to match. The stored response with matching selecting header fields is known as the selected response. If multiple selected responses are available (potentially including responses without a Vary header field), the cache will need to choose one to use. When a selecting header field has a known mechanism for doing so (e.g., qvalues on Accept and similar request header fields), that mechanism MAY be used to select preferred responses; of the remainder, the most recent response (as determined by the Date header field) is used, as perSection 4. If no selected response is available, the cache cannot satisfy the presented request. Typically, it is forwarded to the origin server in a (possibly conditional; seeSection 4.3) request.4.2. Freshness A fresh response is one whose age has not yet exceeded its freshness lifetime. Conversely, a stale response is one where it has. A response's freshness lifetime is the length of time between its generation by the origin server and its expiration time. An explicit expiration time is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation, whereas a heuristic expiration time is assigned by a cache when no explicit expiration time is available. A response's age is the time that has passed since it was generated by, or successfully validated with, the origin server. When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server, thereby improving efficiency.Fielding, et al. Expires September 10, 2019 [Page 11]
Internet-Draft HTTP Caching March 2019 The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future, using either the Expires header field (Section 5.3) or the max-age response directive (Section 5.2.2.8). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation is not likely to change in a semantically significant way before the expiration time is reached. If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past to indicate that the response is already stale. Compliant caches will normally validate a stale cached response before reusing it for subsequent requests (seeSection 4.2.4). Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine an expiration time under certain circumstances (seeSection 4.2.2). The calculation to determine if a response is fresh is: response_is_fresh = (freshness_lifetime > current_age) freshness_lifetime is defined inSection 4.2.1; current_age is defined inSection 4.2.3. Clients can send the max-age or min-fresh request directives (Section 5.2.1) to constrain or relax freshness calculations for the corresponding response. However, caches are not required to honor them. When calculating freshness, to avoid common problems in date parsing: o Although all date formats are specified to be case-sensitive, a cache recipient SHOULD match day, week, and time-zone names case- insensitively. o If a cache recipient's internal implementation of time has less resolution than the value of an HTTP-date, the recipient MUST internally represent a parsed Expires date as the nearest time equal to or earlier than the received value. o A cache recipient MUST NOT allow local time zones to influence the calculation or comparison of an age or expiration time. o A cache recipient SHOULD consider a date with a zone abbreviation other than GMT or UTC to be invalid for calculating expiration.Fielding, et al. Expires September 10, 2019 [Page 12]
Internet-Draft HTTP Caching March 2019 Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload a resource. SeeSection 6 for an explanation of the difference between caches and history mechanisms.4.2.1. Calculating Freshness Lifetime A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of the following: o If the cache is shared and the s-maxage response directive (Section 5.2.2.9) is present, use its value, or o If the max-age response directive (Section 5.2.2.8) is present, use its value, or o If the Expires response header field (Section 5.3) is present, use its value minus the value of the Date response header field, or o Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; seeSection 4.2.2. Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server. When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives), the directive's value is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale.4.2.2. Calculating Heuristic Freshness Since origin servers do not always provide explicit expiration times, a cache MAY assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field values (such as the Last-Modified time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but does impose worst-case constraints on their results. A cache MUST NOT use heuristics to determine freshness when an explicit expiration time is present in the stored response. Because of the requirements inSection 3, this means that, effectively, heuristics can only be used on responses without explicit freshness whose status codes are defined as cacheable by default (see Section 9.1 of [Semantics]), and those responses without explicitFielding, et al. Expires September 10, 2019 [Page 13]
Internet-Draft HTTP Caching March 2019 freshness that have been marked as explicitly cacheable (e.g., with a "public" response directive). If the response has a Last-Modified header field (Section 10.2.2 of [Semantics]), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%. Note:Section 13.9 of [RFC2616] prohibited caches from calculating heuristic freshness for URIs with query components (i.e., those containing '?'). In practice, this has not been widely implemented. Therefore, origin servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to preclude caching.4.2.3. Calculating Age The Age header field is used to convey an estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the number of seconds since the response was generated or validated by the origin server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths. The following data is used for the age calculation: age_value The term "age_value" denotes the value of the Age header field (Section 5.1), in a form appropriate for arithmetic operation; or 0, if not available. date_value The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See Section 10.1.1.2 of [Semantics] for the definition of the Date header field, and for requirements regarding responses without it. now The term "now" means "the current value of the clock at the host performing the calculation". A host ought to use NTP ([RFC5905]) or some similar protocol to synchronize its clocks to Coordinated Universal Time. request_time The current value of the clock at the host at the time the request resulting in the stored response was made. response_time The current value of the clock at the host at the time the response was received. A response's age can be calculated in two entirely independent ways:Fielding, et al. Expires September 10, 2019 [Page 14]
Internet-Draft HTTP Caching March 2019 1. the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero. 2. the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1 or greater. A cache MUST interpret this value relative to the time the request was initiated, not the time that the response was received. apparent_age = max(0, response_time - date_value); response_delay = response_time - request_time; corrected_age_value = age_value + response_delay; These are combined as corrected_initial_age = max(apparent_age, corrected_age_value); unless the cache is confident in the value of the Age header field (e.g., because there are no HTTP/1.0 hops in the Via header field), in which case the corrected_age_value MAY be used as the corrected_initial_age. The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response was last validated by the origin server to the corrected_initial_age. resident_time = now - response_time; current_age = corrected_initial_age + resident_time;4.2.4. Serving Stale Responses A "stale" response is one that either has explicit expiry information or is allowed to have heuristic expiry calculated, but is not fresh according to the calculations inSection 4.2. A cache MUST NOT generate a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive; seeSection 5.2.2). A cache MUST NOT generate a stale response unless it is disconnected or doing so is explicitly permitted by the client or origin server (e.g., by the max-stale request directive inSection 5.2.1, by extension directives such as those defined in [RFC5861], or by configuration in accordance with an out-of-band contract).Fielding, et al. Expires September 10, 2019 [Page 15]
Internet-Draft HTTP Caching March 20194.3. Validation When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; seeSection 4.1), it can use the conditional request mechanism Section 8.2 of [Semantics] in the forwarded request to give the next inbound server an opportunity to select a valid stored response to use, updating the stored metadata in the process, or to replace the stored response(s) with a new response. This process is known as "validating" or "revalidating" the stored response.4.3.1. Sending a Validation Request When generating a conditional request for validation, a cache starts with either a request it is attempting to satisfy, or -- if it is initiating the request independently -- it synthesises a request using a stored response by copying the method, request-target, and request header fields used for identifying the secondary cache keySection 4.1. It then updates that request with one or more precondition header fields. These contain validator metadata sourced from stored response(s) that have the same cache key (both primary and secondary, as applicable). The precondition header fields are then compared by recipients to determine whether any stored response is equivalent to a current representation of the resource. One such validator is the timestamp given in a Last-Modified header field (Section 10.2.2 of [Semantics]), which can be used in an If- Modified-Since header field for response validation, or in an If- Unmodified-Since or If-Range header field for representation selection (i.e., the client is referring specifically to a previously obtained representation with that timestamp). Another validator is the entity-tag given in an ETag header field (Section 10.2.3 of [Semantics]). One or more entity-tags, indicating one or more stored responses, can be used in an If-None-Match header field for response validation, or in an If-Match or If-Range header field for representation selection (i.e., the client is referring specifically to one or more previously obtained representations with the listed entity-tags).Fielding, et al. Expires September 10, 2019 [Page 16]
Internet-Draft HTTP Caching March 20194.3.2. Handling a Received Validation Request Each client in the request chain may have its own cache, so it is common for a cache at an intermediary to receive conditional requests from other (outbound) caches. Likewise, some user agents make use of conditional requests to limit data transfers to recently modified representations or to complete the transfer of a partially retrieved representation. If a cache receives a request that can be satisfied by reusing one of its stored 200 (OK) or 206 (Partial Content) responses, the cache SHOULD evaluate any applicable conditional header field preconditions received in that request with respect to the corresponding validators contained within the selected response. A cache MUST NOT evaluate conditional header fields that are only applicable to an origin server, found in a request with semantics that cannot be satisfied with a cached response, or applied to a target resource for which it has no stored responses; such preconditions are likely intended for some other (inbound) server. The proper evaluation of conditional requests by a cache depends on the received precondition header fields and their precedence, as defined in Section 8.2.2 of [Semantics]. The If-Match and If- Unmodified-Since conditional header fields are not applicable to a cache. A request containing an If-None-Match header field (Section 8.2.4 of [Semantics]) indicates that the client wants to validate one or more of its own stored responses in comparison to whichever stored response is selected by the cache. If the field-value is "*", or if the field-value is a list of entity-tags and at least one of them matches the entity-tag of the selected stored response, a cache recipient SHOULD generate a 304 (Not Modified) response (using the metadata of the selected stored response) instead of sending that stored response. When a cache decides to revalidate its own stored responses for a request that contains an If-None-Match list of entity-tags, the cache MAY combine the received list with a list of entity-tags from its own stored set of responses (fresh or stale) and send the union of the two lists as a replacement If-None-Match header field value in the forwarded request. If a stored response contains only partial content, the cache MUST NOT include its entity-tag in the union unless the request is for a range that would be fully satisfied by that partial stored response. If the response to the forwarded request is 304 (Not Modified) and has an ETag header field value with an entity-tag that is not in the client's list, the cache MUST generate a 200 (OK) response for the client by reusing itsFielding, et al. Expires September 10, 2019 [Page 17]
Internet-Draft HTTP Caching March 2019 corresponding stored response, as updated by the 304 response metadata (Section 4.3.4). If an If-None-Match header field is not present, a request containing an If-Modified-Since header field (Section 8.2.5 of [Semantics]) indicates that the client wants to validate one or more of its own stored responses by modification date. A cache recipient SHOULD generate a 304 (Not Modified) response (using the metadata of the selected stored response) if one of the following cases is true: 1) the selected stored response has a Last-Modified field-value that is earlier than or equal to the conditional timestamp; 2) no Last- Modified field is present in the selected stored response, but it has a Date field-value that is earlier than or equal to the conditional timestamp; or, 3) neither Last-Modified nor Date is present in the selected stored response, but the cache recorded it as having been received at a time earlier than or equal to the conditional timestamp. A cache that implements partial responses to range requests, as defined in Section 8.3 of [Semantics], also needs to evaluate a received If-Range header field (Section 8.2.7 of [Semantics]) with respect to its selected stored response.4.3.3. Handling a Validation Response Cache handling of a response to a conditional request is dependent upon its status code: o A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; seeSection 4.3.4. o A full response (i.e., one with a payload body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the cache MUST use the full response to satisfy the request and MAY replace the stored response(s). o However, if a cache receives a 5xx (Server Error) response while attempting to validate a response, it can either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, the cache MAY send a previously stored response (seeSection 4.2.4).4.3.4. Freshening Stored Responses upon Validation When a cache receives a 304 (Not Modified) response and already has one or more stored 200 (OK) responses for the applicable cache key, the cache needs to identify which (if any) are to be updated by the new information provided, and then do so.Fielding, et al. Expires September 10, 2019 [Page 18]
Internet-Draft HTTP Caching March 2019 The stored response(s) to update are identified by using the first match (if any) of the following: o If the new response contains a strong validator (see Section 10.2.1 of [Semantics]), then that strong validator identifies the selected representation for update. All of the stored responses with the same strong validator are identified for update. If none of the stored responses contain the same strong validator, then the cache MUST NOT use the new response to update any stored responses. o If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then the most recent of those matching stored responses is identified for update. o If the new response does not include any form of validator (such as in the case where a client generates an If-Modified-Since request from a source other than the Last-Modified response header field), and there is only one stored response, and that stored response also lacks a validator, then that stored response is identified for update. For each stored response identified for update, the cache MUST use the header fields provided in the 304 (Not Modified) response to replace all instances of the corresponding header fields in the stored response.4.3.5. Freshening Responses with HEAD A response to the HEAD method is identical to what an equivalent request made with a GET would have been, except it lacks a body. This property of HEAD responses can be used to invalidate or update a cached GET response if the more efficient conditional GET request mechanism is not available (due to no validators being present in the stored response) or if transmission of the representation body is not desired even if it has changed. When a cache makes an inbound HEAD request for a given request target and receives a 200 (OK) response, the cache SHOULD update or invalidate each of its stored GET responses that could have been selected for that request (seeSection 4.1). For each of the stored responses that could have been selected, if the stored response and HEAD response have matching values for any received validator fields (ETag and Last-Modified) and, if the HEAD response has a Content-Length header field, the value of Content- Length matches that of the stored response, the cache SHOULD updateFielding, et al. Expires September 10, 2019 [Page 19]
Internet-Draft HTTP Caching March 2019 the stored response as described below; otherwise, the cache SHOULD consider the stored response to be stale. If a cache updates a stored response with the metadata provided in a HEAD response, the cache MUST use the header fields provided in the HEAD response to replace all instances of the corresponding header fields in the stored response and append new header fields to the stored response's header section unless otherwise restricted by the Cache-Control header field.4.4. Invalidation Because unsafe request methods (Section 7.2.1 of [Semantics]) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them to keep their contents up to date. A cache MUST invalidate the effective Request URI (Section 5.3 of [Semantics]) as well as the URI(s) in the Location and Content- Location response header fields (if present) when a non-error status code is received in response to an unsafe request method. However, a cache MUST NOT invalidate a URI from a Location or Content-Location response header field if the host part of that URI differs from the host part in the effective request URI (Section 5.3 of [Semantics]). This helps prevent denial-of-service attacks. A cache MUST invalidate the effective request URI (Section 5.3 of [Semantics]) when it receives a non-error response to a request with a method whose safety is unknown. Here, a "non-error response" is one with a 2xx (Successful) or 3xx (Redirection) status code. "Invalidate" means that the cache will either remove all stored responses related to the effective request URI or will mark these as "invalid" and in need of a mandatory validation before they can be sent in response to a subsequent request. Note that this does not guarantee that all appropriate responses are invalidated. For example, a state-changing request might invalidate responses in the caches it travels through, but relevant responses still might be stored in other caches that it has not.5. Header Field Definitions This section defines the syntax and semantics of HTTP header fields related to caching.Fielding, et al. Expires September 10, 2019 [Page 20]
Internet-Draft HTTP Caching March 2019 +-------------------+-----------+--------------+ | Header Field Name | Status | Reference | +-------------------+-----------+--------------+ | Age | standard |Section 5.1 | | Cache-Control | standard |Section 5.2 | | Expires | standard |Section 5.3 | | Pragma | standard |Section 5.4 | | Warning | obsoleted |Section 5.5 | +-------------------+-----------+--------------+5.1. Age The "Age" header field conveys the sender's estimate of the amount of time since the response was generated or successfully validated at the origin server. Age values are calculated as specified inSection 4.2.3. Age = delta-seconds The Age field-value is a non-negative integer, representing time in seconds (seeSection 1.3). The presence of an Age header field implies that the response was not generated or validated by the origin server for this request. However, lack of an Age header field does not imply the origin was contacted, since the response might have been received from an HTTP/1.0 cache that does not implement Age.5.2. Cache-Control The "Cache-Control" header field is used to specify directives for caches along the request/response chain. Such cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is present in the response, or to be repeated in it. SeeSection 5.2.3 for information about how Cache-Control directives defined elsewhere are handled. Note: Some HTTP/1.0 caches might not implement Cache-Control. A proxy, whether or not it implements a cache, MUST pass cache directives through in forwarded messages, regardless of their significance to that application, since the directives might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific cache.Fielding, et al. Expires September 10, 2019 [Page 21]
Internet-Draft HTTP Caching March 2019 Cache directives are identified by a token, to be compared case- insensitively, and have an optional argument, that can use both token and quoted-string syntax. For the directives defined below that define arguments, recipients ought to accept both forms, even if one is documented to be preferred. For any directive not defined by this specification, a recipient MUST accept both forms. Cache-Control = 1#cache-directive cache-directive = token [ "=" ( token / quoted-string ) ] For the cache directives defined below, no argument is defined (nor allowed) unless stated otherwise. +------------------------+-----------------------------------+ | Cache Directive | Reference | +------------------------+-----------------------------------+ | max-age |Section 5.2.1.1,Section 5.2.2.8 | | max-stale |Section 5.2.1.2 | | min-fresh |Section 5.2.1.3 | | must-revalidate |Section 5.2.2.1 | | no-cache |Section 5.2.1.4,Section 5.2.2.2 | | no-store |Section 5.2.1.5,Section 5.2.2.3 | | no-transform |Section 5.2.1.6,Section 5.2.2.4 | | only-if-cached |Section 5.2.1.7 | | private |Section 5.2.2.6 | | proxy-revalidate |Section 5.2.2.7 | | public |Section 5.2.2.5 | | s-maxage |Section 5.2.2.9 | | stale-if-error |[RFC5861], Section 4 | | stale-while-revalidate |[RFC5861], Section 3 | +------------------------+-----------------------------------+5.2.1. Request Cache-Control Directives This section defines cache request directives. They are advisory; caches MAY implement them, but are not required to.5.2.1.1. max-age Argument syntax: delta-seconds (seeSection 1.3) The "max-age" request directive indicates that the client prefers a response whose age is less than or equal to the specified number of seconds. Unless the max-stale request directive is also present, the client does not wish to receive a stale response.Fielding, et al. Expires September 10, 2019 [Page 22]
Internet-Draft HTTP Caching March 2019 This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the quoted-string form.5.2.1.2. max-stale Argument syntax: delta-seconds (seeSection 1.3) The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its freshness lifetime. If a value is present, then the client is willing to accept a response that has exceeded its freshness lifetime by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age. This directive uses the token form of the argument syntax: e.g., 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate the quoted-string form.5.2.1.3. min-fresh Argument syntax: delta-seconds (seeSection 1.3) The "min-fresh" request directive indicates that the client prefers a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number of seconds. This directive uses the token form of the argument syntax: e.g., 'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate the quoted-string form.5.2.1.4. no-cache The "no-cache" request directive indicates that the client prefers stored response not be used to satisfy the request without successful validation on the origin server.Fielding, et al. Expires September 10, 2019 [Page 23]
Internet-Draft HTTP Caching March 20195.2.1.5. no-store The "no-store" request directive indicates that a cache MUST NOT store any part of either this request or any response to it. This directive applies to both private and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best- effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping. Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply to the already stored response.5.2.1.6. no-transform The "no-transform" request directive indicates that the client is asking for intermediares (whether or not they implement a cache) to avoid transforming the payload, as defined in Section 5.5.2 of [Semantics].5.2.1.7. only-if-cached The "only-if-cached" request directive indicates that the client only wishes to obtain a stored response. Caches that honor this request directive SHOULD, upon receiving it, either respond using a stored response that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status code.5.2.2. Response Cache-Control Directives This section defines cache response directives. A cache MUST obey the requirements of the Cache-Control directives defined in this section.5.2.2.1. must-revalidate The "must-revalidate" response directive indicates that once it has become stale, the response MUST NOT be used to satisfy any other request without forwarding it for validation and receiving a successful response; seeSection 4.3.Fielding, et al. Expires September 10, 2019 [Page 24]
Internet-Draft HTTP Caching March 2019 The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances a cache MUST obey the must-revalidate directive; in particular, if a cache is disconnected, it MUST generate a 504 (Gateway Timeout) response. The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation could result in incorrect operation, such as a silently unexecuted financial transaction. The must-revalidate directive also has the effect of allowing a stored response to be used to satisfy a request with an Authorization header field; seeSection 3.2.5.2.2.2. no-cache Argument syntax: #field-name The "no-cache" response directive indicates that the response MUST NOT be used to satisfy any other request without forwarding it for validation and receiving a successful response; seeSection 4.3. This allows an origin server to prevent a cache from using it to satisfy a request without contacting it, even by caches that have been configured to send stale responses. If the no-cache response directive specifies one or more field-names, then a cache MAY use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, any header fields in the response that have the field-name(s) listed MUST NOT be sent in the response to a subsequent request without successful revalidation with the origin server. This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response. The field-names given are not limited to the set of header fields defined by this specification. Field names are case-insensitive. This directive uses the quoted-string form of the argument syntax. A sender SHOULD NOT generate the token form (even if quoting appears not to be needed for single-entry lists). Note: Although it has been back-ported to many implementations, some HTTP/1.0 caches will not recognize or obey this directive. Also, no- cache response directives with field-names are often handled byFielding, et al. Expires September 10, 2019 [Page 25]
Internet-Draft HTTP Caching March 2019 caches as if an unqualified no-cache directive was received; i.e., the special handling for the qualified form is not widely implemented.5.2.2.3. no-store The "no-store" response directive indicates that a cache MUST NOT store any part of either the immediate request or response, and MUST NOT use the response to satisfy any other request. This directive applies to both private and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best- effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.5.2.2.4. no-transform The "no-transform" response directive indicates that an intermediary (regardless of whether it implements a cache) MUST NOT transform the payload, as defined in Section 5.5.2 of [Semantics].5.2.2.5. public The "public" response directive indicates that any cache MAY store the response, even if the response would normally be non-cacheable or cacheable only within a private cache. (SeeSection 3.2 for additional details related to the use of public in response to a request containing Authorization, andSection 3 for details of how public affects responses that would normally not be stored, due to their status codes not being defined as cacheable by default; seeSection 4.2.2.)5.2.2.6. private Argument syntax: #field-name The "private" response directive indicates that the response message is intended for a single user and MUST NOT be stored by a shared cache. A private cache MAY store the response and reuse it for later requests, even if the response would normally be non-cacheable.Fielding, et al. Expires September 10, 2019 [Page 26]
Internet-Draft HTTP Caching March 2019 If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated with the listed response header fields. That is, a shared cache MUST NOT store the specified field-names(s), whereas it MAY store the remainder of the response message. The field-names given are not limited to the set of header fields defined by this specification. Field names are case-insensitive. This directive uses the quoted-string form of the argument syntax. A sender SHOULD NOT generate the token form (even if quoting appears not to be needed for single-entry lists). Note: This usage of the word "private" only controls where the response can be stored; it cannot ensure the privacy of the message content. Also, private response directives with field-names are often handled by caches as if an unqualified private directive was received; i.e., the special handling for the qualified form is not widely implemented.5.2.2.7. proxy-revalidate The "proxy-revalidate" response directive has the same meaning as the must-revalidate response directive, except that it does not apply to private caches.5.2.2.8. max-age Argument syntax: delta-seconds (seeSection 1.3) The "max-age" response directive indicates that the response is to be considered stale after its age is greater than the specified number of seconds. This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the quoted-string form.5.2.2.9. s-maxage Argument syntax: delta-seconds (seeSection 1.3) The "s-maxage" response directive indicates that, in shared caches, the maximum age specified by this directive overrides the maximum ageFielding, et al. Expires September 10, 2019 [Page 27]
Internet-Draft HTTP Caching March 2019 specified by either the max-age directive or the Expires header field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive. The must-revalidate directive also has the effect of allowing a stored response to be used to satisfy a request with an Authorization header field; seeSection 3.2. This directive uses the token form of the argument syntax: e.g., 's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the quoted-string form.5.2.3. Cache Control Extensions The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional value. A cache MUST ignore unrecognized cache directives. Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives. Both the new directive and the old directive are supplied, such that applications that do not understand the new directive will default to the behavior specified by the old directive, and those that understand the new directive will recognize it as modifying the requirements associated with the old directive. In this way, extensions to the existing cache-control directives can be made without breaking deployed caches. For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive: in addition to private caches, any cache that is shared only by members of the named community is allowed to cache the response. An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache(s) could do so by including Cache-Control: private, community="UCI" A cache that recognizes such a community cache-extension could broaden its behavior in accordance with that extension. A cache that does not recognize the community cache-extension would ignore it and adhere to the private directive. New extension directives ought to consider defining: o What it means for a directive to be specified multiple times,Fielding, et al. Expires September 10, 2019 [Page 28]
Internet-Draft HTTP Caching March 2019 o When the directive does not take an argument, what it means when an argument is present, o When the directive requires an argument, what it means when it is missing, o Whether the directive is specific to requests, responses, or able to be used in either.5.2.4. Cache Directive Registry The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" defines the namespace for the cache directives. It has been created and is now maintained at <https://www.iana.org/assignments/http-cache-directives>. A registration MUST include the following fields: o Cache Directive Name o Pointer to specification text Values to be added to this namespace require IETF Review (see[RFC8126], Section 4.8).5.3. Expires The "Expires" header field gives the date/time after which the response is considered stale. SeeSection 4.2 for further discussion of the freshness model. The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time. The Expires value is an HTTP-date timestamp, as defined in Section 10.1.1.1 of [Semantics]. Expires = HTTP-date For example Expires: Thu, 01 Dec 1994 16:00:00 GMT A cache recipient MUST interpret invalid date formats, especially the value "0", as representing a time in the past (i.e., "already expired").Fielding, et al. Expires September 10, 2019 [Page 29]
Internet-Draft HTTP Caching March 2019 If a response includes a Cache-Control field with the max-age directive (Section 5.2.2.8), a recipient MUST ignore the Expires field. Likewise, if a response includes the s-maxage directive (Section 5.2.2.9), a shared cache recipient MUST ignore the Expires field. In both these cases, the value in Expires is only intended for recipients that have not yet implemented the Cache-Control field. An origin server without a clock MUST NOT generate an Expires field unless its value represents a fixed time in the past (always expired) or its value has been associated with the resource by a system or user with a reliable clock. Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use of 32-bit integers for time values), and many caches will evict a response far sooner than that.5.4. Pragma The "Pragma" header field was defined for HTTP/1.0 caches, so that clients could specify a "no-cache" request (as Cache-Control was not defined until HTTP/1.1). However, support for Cache-Control is now widespread. As a result, this specification deprecates Pragma. Note: Because the meaning of "Pragma: no-cache" in responses was never specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in them.5.5. Warning The "Warning" header field was used to carry additional information about the status or transformation of a message that might not be reflected in the status code. This specification obsoletes it, as it is not widely generated or surfaced to users. The information it carried can be gleaned from examining other header fields, such as Age.6. Relationship to Applications Applications using HTTP often specify additional forms of caching. For example, Web browsers often have history mechanisms such as "Back" buttons that can be used to redisplay a representation retrieved earlier in a session.Fielding, et al. Expires September 10, 2019 [Page 30]
Internet-Draft HTTP Caching March 2019 Likewise, some Web browsers implement caching of images and other assets within a page view; they may or may not honor HTTP caching semantics. The requirements in this specification do not necessarily apply to how applications use data after it is retrieved from a HTTP cache. That is, a history mechanism can display a previous representation even if it has expired, and an application can use cached data in other ways beyond its freshness lifetime. This does not prohibit the application from taking HTTP caching into account; for example, a history mechanism might tell the user that a view is stale, or it might honor cache directives (e.g., Cache- Control: no-store).7. Security Considerations This section is meant to inform developers, information providers, and users of known security concerns specific to HTTP caching. More general security considerations are addressed in HTTP messaging [Messaging] and semantics [Semantics]. Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected as sensitive information. In particular, various attacks might be amplified by being stored in a shared cache; such "cache poisoning" attacks use the cache to distribute a malicious payload to many clients, and are especially effective when an attacker can use implementation flaws, elevated privileges, or other techniques to insert such a response into a cache. One common attack vector for cache poisoning is to exploit differences in message parsing on proxies and in user agents; see Section 6.3 of [Messaging] for the relevant requirements. Likewise, implementation flaws (as well as misunderstanding of cache operation) might lead to caching of sensitive information (e.g., authentication credentials) that is thought to be private, exposing it to unauthorized parties. Furthermore, the very use of a cache can bring about privacy concerns. For example, if two users share a cache, and the first one browses to a site, the second may be able to detect that the otherFielding, et al. Expires September 10, 2019 [Page 31]
Internet-Draft HTTP Caching March 2019 has been to that site, because the resources from it load more quickly, thanks to the cache. Note that the Set-Cookie response header field [RFC6265] does not inhibit caching; a cacheable response with a Set-Cookie header field can be (and often is) used to satisfy subsequent requests to caches. Servers who wish to control caching of these responses are encouraged to emit appropriate Cache-Control response header fields.8. IANA Considerations The change controller for the following registrations is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".8.1. Header Field Registration Please update the "Hypertext Transfer Protocol (HTTP) Header Field Registry" registry at <https://www.iana.org/assignments/http-headers> with the header field names listed in the two tables ofSection 5.8.2. Cache Directive Registration Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" at <https://www.iana.org/assignments/http-cache-directives> with the registration procedure ofSection 5.2.4 and the cache directive names summarized in the table ofSection 5.2.8.3. Warn Code Registry Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn Codes" registry at <https://www.iana.org/assignments/http-warn-codes> to the effect that Warning is obsoleted.9. References9.1. Normative References [Messaging] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1 Messaging",draft-ietf-httpbis-messaging-04 (work in progress), March 2019. [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>.Fielding, et al. Expires September 10, 2019 [Page 32]
Internet-Draft HTTP Caching March 2019 [RFC3986] 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>. [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>. [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",RFC 7405, DOI 10.17487/RFC7405, December 2014, <https://www.rfc-editor.org/info/rfc7405>. [Semantics] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics",draft-ietf-httpbis-semantics-04 (work in progress), March 2019. [USASCII] American National Standards Institute, "Coded Character Set -- 7-bit American Standard Code for Information Interchange", ANSI X3.4, 1986.9.2. Informative References [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>. [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale Content",RFC 5861, DOI 10.17487/RFC5861, April 2010, <https://www.rfc-editor.org/info/rfc5861>. [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>. [RFC6265] Barth, A., "HTTP State Management Mechanism",RFC 6265, DOI 10.17487/RFC6265, April 2011, <https://www.rfc-editor.org/info/rfc6265>. [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP): Caching",RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www.rfc-editor.org/info/rfc7234>.Fielding, et al. Expires September 10, 2019 [Page 33]
Internet-Draft HTTP Caching March 2019 [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>.Fielding, et al. Expires September 10, 2019 [Page 34]
Internet-Draft HTTP Caching March 2019Appendix A. Collected ABNF In the collected ABNF below, list rules are expanded as per Section 11 of [Semantics]. Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date HTTP-date = <HTTP-date, see [Semantics], Section 10.1.1.1> OWS = <OWS, see [Semantics], Section 4.3> cache-directive = token [ "=" ( token / quoted-string ) ] delta-seconds = 1*DIGIT field-name = <field-name, see [Semantics], Section 4.2> port = <port, see[RFC3986], Section 3.2.3> pseudonym = <pseudonym, see [Semantics], Section 5.5.1> quoted-string = <quoted-string, see [Semantics], Section 4.2.3> token = <token, see [Semantics], Section 4.2.3> uri-host = <host, see[RFC3986], Section 3.2.2>Appendix B. Changes fromRFC 7234 The Warning response header was obsoleted. Much of the information supported by Warning could be gleaned by examining the response), and the remaining warn-codes -- although potentially useful -- were entirely advisory, and in practice were not added by caches or intermediaries. (Section 5.5)Appendix C. Change Log This section is to be removed before publishing as an RFC.C.1. BetweenRFC7234 and draft 00 The changes were purely editorial:Fielding, et al. Expires September 10, 2019 [Page 35]
Internet-Draft HTTP Caching March 2019 o Change boilerplate and abstract to indicate the "draft" status, and update references to ancestor specifications. o Remove version "1.1" from document title, indicating that this specification applies to all HTTP versions. o Adjust historical notes. o Update links to sibling specifications. o Replace sections listing changes fromRFC 2616 by new empty sections referring to RFC 723x. o Remove acknowledgements specific to RFC 723x. o Move "Acknowledgements" to the very end and make them unnumbered.C.2. Sincedraft-ietf-httpbis-cache-00 The changes are purely editorial: o Moved all extensibility tips, registration procedures, and registry tables from the IANA considerations to normative sections, reducing the IANA considerations to just instructions that will be removed prior to publication as an RFC.C.3. Sincedraft-ietf-httpbis-cache-01 o CiteRFC 8126 instead ofRFC 5226 (<https://github.com/httpwg/http-core/issues/75>) o InSection 5.4, misleading statement about the relation between Pragma and Cache-Control (<https://github.com/httpwg/http-core/issues/92>, <https://www.rfc-editor.org/errata/eid4674>)C.4. Sincedraft-ietf-httpbis-cache-02 o InSection 3, explain that only final responses are cacheable (<https://github.com/httpwg/http-core/issues/29>) o InSection 5.2.2, clarify what responses various directives apply to (<https://github.com/httpwg/http-core/issues/52>) o InSection 4.3.1, clarify the source of validators in conditional requests (<https://github.com/httpwg/http-core/issues/110>) o ReviseSection 6 to apply to more than just History Lists (<https://github.com/httpwg/http-core/issues/126>)Fielding, et al. Expires September 10, 2019 [Page 36]
Internet-Draft HTTP Caching March 2019 o InSection 5.5, deprecated "Warning" header field (<https://github.com/httpwg/http-core/issues/139>) o InSection 3.2, remove a spurious note (<https://github.com/httpwg/http-core/issues/141>)C.5. Sincedraft-ietf-httpbis-cache-03 o InSection 2, define what a disconnected cache is (<https://github.com/httpwg/http-core/issues/5>) o InSection 4, clarify language around how to select a response when more than one matches (<https://github.com/httpwg/http-core/issues/23>) o inSection 4.2.4, mention stale-while-revalidate and stale-if- error (<https://github.com/httpwg/http-core/issues/122>) o Remove requirements around cache request directives (<https://github.com/httpwg/http-core/issues/129>) o Deprecate Pragma (<https://github.com/httpwg/http-core/issues/140>) o InSection 3.2 andSection 5.2.2, note effect of some directives on authenticated requests (<https://github.com/httpwg/http-core/issues/161>)Index A Age header field 21 age 11 C Cache-Control header field 21 cache 4 cache entry 6 cache key 6-7 E Expires header field 29 explicit expiration time 11 F fresh 11 freshness lifetime 11Fielding, et al. Expires September 10, 2019 [Page 37]
Internet-Draft HTTP Caching March 2019 G Grammar Age 21 ALPHA 5 Cache-Control 22 cache-directive 22 CR 5 CRLF 5 CTL 5 delta-seconds 6 DIGIT 5 DQUOTE 5 Expires 29 HEXDIG 5 HTAB 5 LF 5 OCTET 5 SP 5 VCHAR 5 H heuristic expiration time 11 M max-age (cache directive) 22, 27 max-stale (cache directive) 23 min-fresh (cache directive) 23 must-revalidate (cache directive) 24 N no-cache (cache directive) 23, 25 no-store (cache directive) 24, 26 no-transform (cache directive) 24, 26 O only-if-cached (cache directive) 24 P Pragma header field 30 private (cache directive) 26 private cache 4 proxy-revalidate (cache directive) 27 public (cache directive) 26 S s-maxage (cache directive) 27 shared cache 4 stale 11Fielding, et al. Expires September 10, 2019 [Page 38]
Internet-Draft HTTP Caching March 2019 strong validator 19 V validator 16 W Warning header field 30Acknowledgments See Appendix "Acknowledgments" of [Semantics].Authors' Addresses Roy T. Fielding (editor) Adobe 345 Park Ave San Jose, CA 95110 USA EMail: fielding@gbiv.com URI:https://roy.gbiv.com/ Mark Nottingham (editor) Fastly EMail: mnot@mnot.net URI:https://www.mnot.net/ Julian F. Reschke (editor) greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany EMail: julian.reschke@greenbytes.de URI:https://greenbytes.de/tech/webdav/Fielding, et al. Expires September 10, 2019 [Page 39]
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draft-ietf-httpbis-cache-04
This is an older version of an Internet-Draft that was ultimately published asRFC 9111.
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