1.1.Prior versions of HTTP

HTTP/1.1[HTTP11] is a TCP mapping which useswhitespace-delimited text fields to convey HTTP messages. While these exchangesare human-readable, using whitespace for message formatting leads to parsingcomplexity and motivates tolerance of variant behavior. Because eachconnection can transfer only a single HTTP request or response at a time in eachdirection, multiple parallel TCP connections are often used, reducing theability of the congestion controller to effectively manage traffic betweenendpoints.¶

HTTP/2[HTTP2] introduced a binary framing and multiplexing layer toimprove latency without modifying the transport layer. However, because theparallel nature of HTTP/2's multiplexing is not visible to TCP's loss recoverymechanisms, a lost or reordered packet causes all active transactions toexperience a stall regardless of whether that transaction was directly impactedby the lost packet.¶

1.2.Delegation to QUIC

The QUIC transport protocol incorporates stream multiplexing and per-stream flowcontrol, similar to that provided by the HTTP/2 framing layer. By providingreliability at the stream level and congestion control across the entireconnection, it has the capability to improve the performance of HTTP compared toa TCP mapping. QUIC also incorporates TLS 1.3[TLS13] at thetransport layer, offering comparable security to running TLS over TCP, withthe improved connection setup latency of TCP Fast Open[TFO].¶

This document defines a mapping of HTTP semantics over the QUIC transportprotocol, drawing heavily on the design of HTTP/2. While delegating streamlifetime and flow control issues to QUIC, a similar binary framing is used oneach stream. Some HTTP/2 features are subsumed by QUIC, while other features areimplemented atop QUIC.¶

QUIC is described in[QUIC-TRANSPORT]. For a full description of HTTP/2, see[HTTP2].¶

2.1.Document Organization

The following sections provide a detailed overview of the connection lifecycleand key concepts:¶

• Connection Setup and Management (Section 3) covers how an HTTP/3endpoint is discovered and a connection is established.¶
• HTTP Request Lifecycle (Section 4) describes how HTTPsemantics are expressed using frames.¶
• Connection Closure (Section 5) describes how connections areterminated, either gracefully or abruptly.¶

The details of the wire protocol and interactions with the transport aredescribed in subsequent sections:¶

• Stream Mapping and Usage (Section 6) describes the way QUIC streamsare used.¶
• HTTP Framing Layer (Section 7) describes the frames used onmost streams.¶
• Error Handling (Section 8) describes how error conditions are handled andexpressed, either on a particular stream or for the connection as a whole.¶

Additional resources are provided in the final sections:¶

• Extensions to HTTP/3 (Section 9) describes how new capabilities can beadded in future documents.¶
• A more detailed comparison between HTTP/2 and HTTP/3 can be found inAppendix A.¶

2.2.Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED","MAY", and "OPTIONAL" in this document are to be interpreted asdescribed in BCP 14[RFC2119][RFC8174] when, and only when, theyappear in all capitals, as shown here.¶

Field definitions are given in Augmented Backus-Naur Form (ABNF), as defined in[RFC5234].¶

This document uses the variable-length integer encoding from[QUIC-TRANSPORT].¶

The following terms are used:¶

abort:

An abrupt termination of a connection or stream, possibly due to an errorcondition.¶

client:

The endpoint that initiates an HTTP/3 connection. Clients send HTTP requestsand receive HTTP responses.¶

connection:

A transport-layer connection between two endpoints, using QUIC as thetransport protocol.¶

connection error:

An error that affects the entire HTTP/3 connection.¶

endpoint:

Either the client or server of the connection.¶

frame:

The smallest unit of communication on a stream in HTTP/3, consisting of aheader and a variable-length sequence of bytes structured according to theframe type.Protocol elements called "frames" exist in both this document and[QUIC-TRANSPORT]. Where frames from[QUIC-TRANSPORT] are referenced, theframe name will be prefaced with "QUIC." For example, "QUIC CONNECTION_CLOSEframes." References without this preface refer to frames defined inSection 7.2.¶

peer:

An endpoint. When discussing a particular endpoint, "peer" refers to theendpoint that is remote to the primary subject of discussion.¶

receiver:

An endpoint that is receiving frames.¶

sender:

An endpoint that is transmitting frames.¶

server:

The endpoint that accepts an HTTP/3 connection. Servers receive HTTP requestsand send HTTP responses.¶

stream:

A bidirectional or unidirectional bytestream provided by the QUIC transport.¶

stream error:

An error on the individual HTTP/3 stream.¶

The term "payload body" is defined in Section 6.3.3 of[SEMANTICS].¶

Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are definedin Section 2.2 of[SEMANTICS]. Intermediaries act as both client and serverat different times.¶

3.1.Draft Version Identification

• RFC Editor's Note: Please remove this section prior to publication of afinal version of this document.¶

HTTP/3 uses the token "h3" to identify itself in ALPN and Alt-Svc. Onlyimplementations of the final, published RFC can identify themselves as "h3".Until such an RFC exists, implementations MUST NOT identify themselves usingthis string.¶

Implementations of draft versions of the protocol MUST add the string "-" andthe corresponding draft number to the identifier. For example,draft-ietf-quic-http-01 is identified using the string "h3-01".¶

Draft versions MUST use the corresponding draft transport version as theirtransport. For example, the application protocol defined indraft-ietf-quic-http-25 uses the transport defined indraft-ietf-quic-transport-25.¶

Non-compatible experiments that are based on these draft versions MUST appendthe string "-" and an experiment name to the identifier. For example, anexperimental implementation based on draft-ietf-quic-http-09 which reserves anextra stream for unsolicited transmission of 1980s pop music might identifyitself as "h3-09-rickroll". Note that any label MUST conform to the "token"syntax defined in Section 4.4.1.1 of[SEMANTICS]. Experimenters areencouraged to coordinate their experiments on thequic@ietf.org mailing list.¶

3.2.Discovering an HTTP/3 Endpoint

HTTP relies on the notion of an authoritative response: a response that has beendetermined to be the most appropriate response for that request given the stateof the target resource at the time of response message origination by (or at thedirection of) the origin server identified within the target URI. Locating anauthoritative server for an HTTP URL is discussed in Section 5.4 of[SEMANTICS].¶

The "https" scheme associates authority with possession of a certificate thatthe client considers to be trustworthy for the host identified by the authoritycomponent of the URL. If a server presents a certificate and proof that itcontrols the corresponding private key, then a client will accept a securedconnection to that server as being authoritative for all origins with the"https" scheme and a host identified in the certificate.¶

A client MAY attempt access to a resource with an "https" URI by resolving thehost identifier to an IP address, establishing a QUIC connection to that addresson the indicated port, and sending an HTTP/3 request message targeting the URIto the server over that secured connection.¶

Connectivity problems (e.g., blocking UDP) can result in QUIC connectionestablishment failure; clients SHOULD attempt to use TCP-based versions of HTTPin this case.¶

Servers MAY serve HTTP/3 on any UDP port; an alternative service advertisementalways includes an explicit port, and URLs contain either an explicit port or adefault port associated with the scheme.¶

Alt-Svc: h3=":50781"

3.3.Connection Establishment

HTTP/3 relies on QUIC version 1 as the underlying transport. The use of otherQUIC transport versions with HTTP/3 MAY be defined by future specifications.¶

QUIC version 1 uses TLS version 1.3 or greater as its handshake protocol.HTTP/3 clients MUST support a mechanism to indicate the target host to theserver during the TLS handshake. If the server is identified by a DNS name,clients MUST send the Server Name Indication (SNI)[RFC6066] TLS extensionunless an alternative mechanism to indicate the target host is used.¶

QUIC connections are established as described in[QUIC-TRANSPORT]. Duringconnection establishment, HTTP/3 support is indicated by selecting the ALPNtoken "h3" in the TLS handshake. Support for other application-layer protocolsMAY be offered in the same handshake.¶

While connection-level options pertaining to the core QUIC protocol are set inthe initial crypto handshake, HTTP/3-specific settings are conveyed in theSETTINGS frame. After the QUIC connection is established, a SETTINGS frame(Section 7.2.4) MUST be sent by each endpoint as the initial frame of theirrespective HTTP control stream; seeSection 6.2.1.¶

3.4.Connection Reuse

HTTP/3 connections are persistent across multiple requests. For bestperformance, it is expected that clients will not close connections until it isdetermined that no further communication with a server is necessary (forexample, when a user navigates away from a particular web page) or until theserver closes the connection.¶

Once a connection exists to a server endpoint, this connection MAY be reused forrequests with multiple different URI authority components. In general, a serveris considered authoritative for all URIs with the "https" scheme for which thehostname in the URI is present in the authenticated certificate provided by theserver, either as the CN field of the certificate subject or as a dNSName in thesubjectAltName field of the certificate; see[RFC6125]. For a host that isan IP address, the client MUST verify that the address appears as an iPAddressin the subjectAltName field of the certificate. If the hostname or address isnot present in the certificate, the client MUST NOT consider the serverauthoritative for origins containing that hostname or address. See Section 5.4of[SEMANTICS] for more detail on authoritative access.¶

Clients SHOULD NOT open more than one HTTP/3 connection to a given host and portpair, where the host is derived from a URI, a selected alternative service[ALTSVC], or a configured proxy. A client MAY open multiple connections tothe same IP address and UDP port using different transport or TLS configurationsbut SHOULD avoid creating multiple connections with the same configuration.¶

Servers are encouraged to maintain open connections for as long as possible butare permitted to terminate idle connections if necessary. When either endpointchooses to close the HTTP/3 session, the terminating endpoint SHOULD first senda GOAWAY frame (Section 5.2) so that both endpoints can reliablydetermine whether previously sent frames have been processed and gracefullycomplete or terminate any necessary remaining tasks.¶

A server that does not wish clients to reuse connections for a particular origincan indicate that it is not authoritative for a request by sending a 421(Misdirected Request) status code in response to the request; see Section 9.1.2of[HTTP2].¶

4.1.HTTP Message Exchanges

A client sends an HTTP request on a client-initiated bidirectional QUIC stream.A client MUST send only a single request on a given stream. A server sends zeroor more interim HTTP responses on the same stream as the request, followed by asingle final HTTP response, as detailed below.¶

Pushed responses are sent on a server-initiated unidirectional QUIC stream; seeSection 6.2.2. A server sends zero or more interim HTTP responses, followedby a single final HTTP response, in the same manner as a standard response.Push is described in more detail inSection 4.4.¶

On a given stream, receipt of multiple requests or receipt of an additional HTTPresponse following a final HTTP response MUST be treated as malformed(Section 4.1.3).¶

An HTTP message (request or response) consists of:¶

1. the header field section (see Section 4 of[SEMANTICS]), sent as a singleHEADERS frame (seeSection 7.2.2),¶
2. optionally, the payload body, if present (see Section 6.3.3 of[SEMANTICS]), sent as a series of DATA frames (seeSection 7.2.1),¶
3. optionally, the trailer field section, if present (see Section 4.6 of[SEMANTICS]), sent as a single HEADERS frame.¶

Receipt of an invalid sequence of frames MUST be treated as a connection errorof type H3_FRAME_UNEXPECTED (Section 8). In particular, a DATA frame beforeany HEADERS frame, or a HEADERS or DATA frame after the trailing HEADERS frameis considered invalid.¶

A server MAY send one or more PUSH_PROMISE frames (seeSection 7.2.5)before, after, or interleaved with the frames of a response message. ThesePUSH_PROMISE frames are not part of the response; seeSection 4.4 for moredetails. These frames are not permitted in pushed responses; a pushed responsewhich includes PUSH_PROMISE frames MUST be treated as a connection error of typeH3_FRAME_UNEXPECTED.¶

Frames of unknown types (Section 9), including reserved frames(Section 7.2.8) MAY be sent on a request or push stream before, after, orinterleaved with other frames described in this section.¶

The HEADERS and PUSH_PROMISE frames might reference updates to the QPACK dynamictable. While these updates are not directly part of the message exchange, theymust be received and processed before the message can be consumed. SeeSection 4.1.1 for more details.¶

The "chunked" transfer encoding defined in Section 7.1 of[HTTP11] MUST NOTbe used.¶

A response MAY consist of multiple messages when and only when one or moreinformational responses (1xx; see Section 9.2 of[SEMANTICS]) precede a finalresponse to the same request. Interim responses do not contain a payload bodyor trailers.¶

An HTTP request/response exchange fully consumes a client-initiatedbidirectional QUIC stream. After sending a request, a client MUST close thestream for sending. Unless using the CONNECT method (seeSection 4.2), clientsMUST NOT make stream closure dependent on receiving a response to their request.After sending a final response, the server MUST close the stream for sending. Atthis point, the QUIC stream is fully closed.¶

When a stream is closed, this indicates the end of an HTTP message. Because somemessages are large or unbounded, endpoints SHOULD begin processing partial HTTPmessages once enough of the message has been received to make progress. If aclient stream terminates without enough of the HTTP message to provide acomplete response, the server SHOULD abort its response with the error codeH3_REQUEST_INCOMPLETE.¶

A server can send a complete response prior to the client sending an entirerequest if the response does not depend on any portion of the request that hasnot been sent and received. When the server does not need to receive theremainder of the request, it MAY abort reading the request stream, send acomplete response, and cleanly close the sending part of the stream. The errorcode H3_NO_ERROR SHOULD be used when requesting that the client stop sending onthe request stream. Clients MUST NOT discard complete responses as a result ofhaving their request terminated abruptly, though clients can always discardresponses at their discretion for other reasons. If the server sends a partialor complete response but does not abort reading, clients SHOULD continue sendingthe body of the request and close the stream normally.¶

4.2.The CONNECT Method

The CONNECT method requests that the recipient establish a tunnel to thedestination origin server identified by the request-target (Section 3.2 of[HTTP11]). It is primarily used with HTTP proxies to establish a TLSsession with an origin server for the purposes of interacting with "https"resources.¶

In HTTP/1.x, CONNECT is used to convert an entire HTTP connection into a tunnelto a remote host. In HTTP/2 and HTTP/3, the CONNECT method is used to establisha tunnel over a single stream.¶

A CONNECT request MUST be constructed as follows:¶

• The ":method" pseudo-header field is set to "CONNECT"¶
• The ":scheme" and ":path" pseudo-header fields are omitted¶
• The ":authority" pseudo-header field contains the host and port to connect to(equivalent to the authority-form of the request-target of CONNECT requests;see Section 3.2.3 of[HTTP11])¶

The request stream remains open at the end of the request to carry the data tobe transferred. A CONNECT request that does not conform to these restrictionsis malformed; seeSection 4.1.3.¶

A proxy that supports CONNECT establishes a TCP connection ([RFC0793]) to theserver identified in the ":authority" pseudo-header field. Once this connectionis successfully established, the proxy sends a HEADERS frame containing a 2xxseries status code to the client, as defined in Section 9.3 of[SEMANTICS].¶

All DATA frames on the stream correspond to data sent or received on the TCPconnection. Any DATA frame sent by the client is transmitted by the proxy to theTCP server; data received from the TCP server is packaged into DATA frames bythe proxy. Note that the size and number of TCP segments is not guaranteed tomap predictably to the size and number of HTTP DATA or QUIC STREAM frames.¶

Once the CONNECT method has completed, only DATA frames are permittedto be sent on the stream. Extension frames MAY be used if specificallypermitted by the definition of the extension. Receipt of any other frame typeMUST be treated as a connection error of type H3_FRAME_UNEXPECTED.¶

The TCP connection can be closed by either peer. When the client ends therequest stream (that is, the receive stream at the proxy enters the "Data Recvd"state), the proxy will set the FIN bit on its connection to the TCP server. Whenthe proxy receives a packet with the FIN bit set, it will terminate the sendstream that it sends to the client. TCP connections which remain half-closed ina single direction are not invalid, but are often handled poorly by servers, soclients SHOULD NOT close a stream for sending while they still expect to receivedata from the target of the CONNECT.¶

A TCP connection error is signaled by abruptly terminating the stream. A proxytreats any error in the TCP connection, which includes receiving a TCP segmentwith the RST bit set, as a stream error of type H3_CONNECT_ERROR(Section 8.1). Correspondingly, if a proxy detects an error with thestream or the QUIC connection, it MUST close the TCP connection. If theunderlying TCP implementation permits it, the proxy SHOULD send a TCP segmentwith the RST bit set.¶

4.3.HTTP Upgrade

HTTP/3 does not support the HTTP Upgrade mechanism (Section 9.9 of[HTTP11])or 101 (Switching Protocols) informational status code (Section 9.2.2 of[SEMANTICS]).¶

4.4.Server Push

Server push is an interaction mode which permits a server to push arequest-response exchange to a client in anticipation of the client making theindicated request. This trades off network usage against a potential latencygain. HTTP/3 server push is similar to what is described in HTTP/2[HTTP2],but uses different mechanisms.¶

Each server push is identified by a unique Push ID. This Push ID is used in oneor more PUSH_PROMISE frames (seeSection 7.2.5) that carry the requestfields, then included with the push stream which ultimately fulfills thosepromises. When the same Push ID is promised on multiple request streams, thedecompressed request field sections MUST contain the same fields in thesame order, and both the name and the value in each field MUST be exactmatches.¶

Server push is only enabled on a connection when a client sends a MAX_PUSH_IDframe; seeSection 7.2.7. A server cannot use server push until itreceives a MAX_PUSH_ID frame. A client sends additional MAX_PUSH_ID frames tocontrol the number of pushes that a server can promise. A server SHOULD use PushIDs sequentially, starting at 0. A client MUST treat receipt of a push streamwith a Push ID that is greater than the maximum Push ID as a connection error oftype H3_ID_ERROR.¶

The header section of the request message is carried by a PUSH_PROMISE frame(seeSection 7.2.5) on the request stream which generated the push.This allows the server push to be associated with a client request.¶

Not all requests can be pushed. A server MAY push requests which have thefollowing properties:¶

• cacheable; see Section 7.2.3 of[SEMANTICS]¶
• safe; see Section 7.2.1 of[SEMANTICS]¶
• does not include a request body or trailer section¶

The server MUST include a value in the ":authority" pseudo-header field forwhich the server is authoritative; seeSection 3.4.¶

Clients SHOULD send a CANCEL_PUSH frame upon receipt of a PUSH_PROMISE framecarrying a request which is not cacheable, is not known to be safe, thatindicates the presence of a request body, or for which it does not consider theserver authoritative.¶

Each pushed response is associated with one or more client requests. The pushis associated with the request stream on which the PUSH_PROMISE frame wasreceived. The same server push can be associated with additional clientrequests using a PUSH_PROMISE frame with the same Push ID on multiple requeststreams. These associations do not affect the operation of the protocol, butMAY be considered by user agents when deciding how to use pushed resources.¶

Ordering of a PUSH_PROMISE in relation to certain parts of the response isimportant. The server SHOULD send PUSH_PROMISE frames prior to sending HEADERSor DATA frames that reference the promised responses. This reduces the chancethat a client requests a resource that will be pushed by the server.¶

When a server later fulfills a promise, the server push response is conveyed ona push stream; seeSection 6.2.2. The push stream identifies the Push ID ofthe promise that it fulfills, then contains a response to the promised requestusing the same format described for responses inSection 4.1.¶

Due to reordering, push stream data can arrive before the correspondingPUSH_PROMISE frame. When a client receives a new push stream with anas-yet-unknown Push ID, both the associated client request and the pushedrequest header fields are unknown. The client can buffer the stream data inexpectation of the matching PUSH_PROMISE. The client can use stream flow control(see section 4.1 of[QUIC-TRANSPORT]) to limit the amount of data a server maycommit to the pushed stream.¶

If a promised server push is not needed by the client, the client SHOULD send aCANCEL_PUSH frame. If the push stream is already open or opens after sending theCANCEL_PUSH frame, the client can abort reading the stream with an error code ofH3_REQUEST_CANCELLED. This asks the server not to transfer additional data andindicates that it will be discarded upon receipt.¶

Pushed responses that are cacheable (see Section 3 of[CACHING]) can be stored by the client, if itimplements an HTTP cache. Pushed responses are considered successfullyvalidated on the origin server (e.g., if the "no-cache" cache response directiveis present (Section 5.2.2.3 of[CACHING])) at the time the pushed response isreceived.¶

Pushed responses that are not cacheable MUST NOT be stored by any HTTP cache.They MAY be made available to the application separately.¶

5.1.Idle Connections

Each QUIC endpoint declares an idle timeout during the handshake. If theconnection remains idle (no packets received) for longer than this duration, thepeer will assume that the connection has been closed. HTTP/3 implementationswill need to open a new connection for new requests if the existing connectionhas been idle for longer than the server's advertised idle timeout, and SHOULDdo so if approaching the idle timeout.¶

HTTP clients are expected to request that the transport keep connections openwhile there are responses outstanding for requests or server pushes, asdescribed in Section 10.2.2 of[QUIC-TRANSPORT]. If the client is notexpecting a response from the server, allowing an idle connection to time out ispreferred over expending effort maintaining a connection that might not beneeded. A gateway MAY maintain connections in anticipation of need rather thanincur the latency cost of connection establishment to servers. Servers SHOULDNOT actively keep connections open.¶

5.2.Connection Shutdown

Even when a connection is not idle, either endpoint can decide to stop using theconnection and initiate a graceful connection close. Endpoints initiate thegraceful shutdown of a connection by sending a GOAWAY frame (Section 7.2.6).The GOAWAY frame contains an identifier that indicates to the receiver the rangeof requests or pushes that were or might be processed in this connection. Theserver sends a client-initiated bidirectional Stream ID; the client sends a PushID. Requests or pushes with the indicated identifier or greater are rejected bythe sender of the GOAWAY. This identifier MAY be zero if no requests or pusheswere processed.¶

The information in the GOAWAY frame enables a client and server to agree onwhich requests or pushes were accepted prior to the connection shutdown. Uponsending a GOAWAY frame, the endpoint SHOULD explicitly cancel (seeSection 4.1.2 andSection 7.2.3) any requests or pushes thathave identifiers greater than or equal to that indicated, in order to clean uptransport state for the affected streams. The endpoint SHOULD continue to do soas more requests or pushes arrive.¶

Endpoints MUST NOT initiate new requests or promise new pushes on the connectionafter receipt of a GOAWAY frame from the peer. Clients MAY establish a newconnection to send additional requests.¶

Some requests or pushes might already be in transit:¶

• Upon receipt of a GOAWAY frame, if the client has already sent requests witha Stream ID greater than or equal to the identifier received in a GOAWAYframe, those requests will not be processed. Clients can safely retryunprocessed requests on a different connection.¶

  Requests on Stream IDs less than the Stream ID in a GOAWAY frame from theserver might have been processed; their status cannot be known until aresponse is received, the stream is reset individually, another GOAWAY isreceived, or the connection terminates.¶

  Servers MAY reject individual requests on streams below the indicated ID ifthese requests were not processed.¶

• If a server receives a GOAWAY frame after having promised pushes with a PushID greater than or equal to the identifier received in a GOAWAY frame, thosepushes will not be accepted.¶

Servers SHOULD send a GOAWAY frame when the closing of a connection is knownin advance, even if the advance notice is small, so that the remote peer canknow whether a request has been partially processed or not. For example, if anHTTP client sends a POST at the same time that a server closes a QUICconnection, the client cannot know if the server started to process that POSTrequest if the server does not send a GOAWAY frame to indicate what streams itmight have acted on.¶

A client that is unable to retry requests loses all requests that are in flightwhen the server closes the connection. An endpoint MAY send multiple GOAWAYframes indicating different identifiers, but the identifier in each frame MUSTNOT be greater than the identifier in any previous frame, since clients mightalready have retried unprocessed requests on another connection. Receiving aGOAWAY containing a larger identifier than previously received MUST be treatedas a connection error of type H3_ID_ERROR.¶

An endpoint that is attempting to gracefully shut down a connection can send aGOAWAY frame with a value set to the maximum possible value (2^62-4 for servers,2^62-1 for clients). This ensures that the peer stops creating new requests orpushes. After allowing time for any in-flight requests or pushes to arrive, theendpoint can send another GOAWAY frame indicating which requests or pushes itmight accept before the end of the connection. This ensures that a connectioncan be cleanly shut down without losing requests.¶

A client has more flexibility in the value it chooses for the Push ID in aGOAWAY that it sends. A value of 2^62 - 1 indicates that the server cancontinue fulfilling pushes which have already been promised, and the client cancontinue granting push credit as needed; seeSection 7.2.7. A smallervalue indicates the client will reject pushes with Push IDs greater than orequal to this value. Like the server, the client MAY send subsequent GOAWAYframes so long as the specified Push ID is strictly smaller than all previouslysent values.¶

Even when a GOAWAY indicates that a given request or push will not be processedor accepted upon receipt, the underlying transport resources still exist. Theendpoint that initiated these requests can cancel them to clean up transportstate.¶

Once all accepted requests and pushes have been processed, the endpoint canpermit the connection to become idle, or MAY initiate an immediate closure ofthe connection. An endpoint that completes a graceful shutdown SHOULD use theH3_NO_ERROR code when closing the connection.¶

If a client has consumed all available bidirectional stream IDs with requests,the server need not send a GOAWAY frame, since the client is unable to makefurther requests.¶

5.3.Immediate Application Closure

An HTTP/3 implementation can immediately close the QUIC connection at any time.This results in sending a QUIC CONNECTION_CLOSE frame to the peer indicatingthat the application layer has terminated the connection. The application errorcode in this frame indicates to the peer why the connection is being closed.SeeSection 8 for error codes which can be used when closing a connection inHTTP/3.¶

Before closing the connection, a GOAWAY frame MAY be sent to allow the client toretry some requests. Including the GOAWAY frame in the same packet as the QUICCONNECTION_CLOSE frame improves the chances of the frame being received byclients.¶

5.4.Transport Closure

For various reasons, the QUIC transport could indicate to the application layerthat the connection has terminated. This might be due to an explicit closureby the peer, a transport-level error, or a change in network topology whichinterrupts connectivity.¶

If a connection terminates without a GOAWAY frame, clients MUST assume that anyrequest which was sent, whether in whole or in part, might have been processed.¶

6.1.Bidirectional Streams

All client-initiated bidirectional streams are used for HTTP requests andresponses. A bidirectional stream ensures that the response can be readilycorrelated with the request. This means that the client's first request occurson QUIC stream 0, with subsequent requests on stream 4, 8, and so on. In orderto permit these streams to open, an HTTP/3 server SHOULD configure non-zerominimum values for the number of permitted streams and the initial stream flowcontrol window. So as to not unnecessarily limit parallelism, at least 100requests SHOULD be permitted at a time.¶

HTTP/3 does not use server-initiated bidirectional streams, though an extensioncould define a use for these streams. Clients MUST treat receipt of aserver-initiated bidirectional stream as a connection error of typeH3_STREAM_CREATION_ERROR unless such an extension has been negotiated.¶

6.2.Unidirectional Streams

Unidirectional streams, in either direction, are used for a range of purposes.The purpose is indicated by a stream type, which is sent as a variable-lengthinteger at the start of the stream. The format and structure of data thatfollows this integer is determined by the stream type.¶

Unidirectional Stream Header {  Stream Type (i),}

Some stream types are reserved (Section 6.2.3). Two stream types aredefined in this document: control streams (Section 6.2.1) and push streams(Section 6.2.2).[QPACK] defines two additional stream types. Other streamtypes can be defined by extensions to HTTP/3; seeSection 9 for moredetails.¶

The performance of HTTP/3 connections in the early phase of their lifetime issensitive to the creation and exchange of data on unidirectional streams.Endpoints that excessively restrict the number of streams or the flow controlwindow of these streams will increase the chance that the remote peer reachesthe limit early and becomes blocked. In particular, implementations shouldconsider that remote peers may wish to exercise reserved stream behavior(Section 6.2.3) with some of the unidirectional streams they are permittedto use. To avoid blocking, the transport parameters sent by both clients andservers MUST allow the peer to create at least one unidirectional stream for theHTTP control stream plus the number of unidirectional streams required bymandatory extensions (three being the minimum number required for the baseHTTP/3 protocol and QPACK), and SHOULD provide at least 1,024 bytes of flowcontrol credit to each stream.¶

Note that an endpoint is not required to grant additional credits to create moreunidirectional streams if its peer consumes all the initial credits beforecreating the critical unidirectional streams. Endpoints SHOULD create the HTTPcontrol stream as well as the unidirectional streams required by mandatoryextensions (such as the QPACK encoder and decoder streams) first, and thencreate additional streams as allowed by their peer.¶

If the stream header indicates a stream type which is not supported by therecipient, the remainder of the stream cannot be consumed as the semantics areunknown. Recipients of unknown stream types MAY abort reading of the stream withan error code of H3_STREAM_CREATION_ERROR, but MUST NOT consider such streamsto be a connection error of any kind.¶

Implementations MAY send stream types before knowing whether the peer supportsthem. However, stream types which could modify the state or semantics ofexisting protocol components, including QPACK or other extensions, MUST NOT besent until the peer is known to support them.¶

A sender can close or reset a unidirectional stream unless otherwise specified.A receiver MUST tolerate unidirectional streams being closed or reset prior tothe reception of the unidirectional stream header.¶

Push Stream Header {  Stream Type (i) = 0x01,  Push ID (i),}

7.1.Frame Layout

All frames have the following format:¶

HTTP/3 Frame Format {  Type (i),  Length (i),  Frame Payload (..),}

A frame includes the following fields:¶

Type:

A variable-length integer that identifies the frame type.¶

Length:

A variable-length integer that describes the length in bytes ofthe Frame Payload.¶

Frame Payload:

A payload, the semantics of which are determined by the Type field.¶

Each frame's payload MUST contain exactly the fields identified in itsdescription. A frame payload that contains additional bytes after theidentified fields or a frame payload that terminates before the end of theidentified fields MUST be treated as a connection error of typeH3_FRAME_ERROR.¶

When a stream terminates cleanly, if the last frame on the stream was truncated,this MUST be treated as a connection error (Section 8) of typeH3_FRAME_ERROR. Streams which terminate abruptly may be reset at any point ina frame.¶

7.2.Frame Definitions

DATA Frame {  Type (i) = 0x0,  Length (i),  Data (..),}

HEADERS Frame {  Type (i) = 0x1,  Length (i),  Encoded Field Section (..),}

CANCEL_PUSH Frame {  Type (i) = 0x3,  Length (i),  Push ID (..),}

Setting {  Identifier (i),  Value (i),}SETTINGS Frame {  Type (i) = 0x4,  Length (i),  Setting (..) ...,}

PUSH_PROMISE Frame {  Type (i) = 0x5,  Length (i),  Push ID (i),  Encoded Field Section (..),}

GOAWAY Frame {  Type (i) = 0x7,  Length (i),  Stream ID/Push ID (..),}

MAX_PUSH_ID Frame {  Type (i) = 0x1,  Length (i),  Push ID (..),}

8.1.HTTP/3 Error Codes

The following error codes are defined for use when abruptly terminating streams,aborting reading of streams, or immediately closing connections.¶

H3_NO_ERROR (0x100):

No error. This is used when the connection or stream needs to be closed, butthere is no error to signal.¶

H3_GENERAL_PROTOCOL_ERROR (0x101):

Peer violated protocol requirements in a way which doesn't match a morespecific error code, or endpoint declines to use the more specific error code.¶

H3_INTERNAL_ERROR (0x102):

An internal error has occurred in the HTTP stack.¶

H3_STREAM_CREATION_ERROR (0x103):

The endpoint detected that its peer created a stream that it will not accept.¶

H3_CLOSED_CRITICAL_STREAM (0x104):

A stream required by the connection was closed or reset.¶

H3_FRAME_UNEXPECTED (0x105):

A frame was received which was not permitted in the current state or on thecurrent stream.¶

H3_FRAME_ERROR (0x106):

A frame that fails to satisfy layout requirements or with an invalid sizewas received.¶

H3_EXCESSIVE_LOAD (0x107):

The endpoint detected that its peer is exhibiting a behavior that might begenerating excessive load.¶

H3_ID_ERROR (0x108):

A Stream ID or Push ID was used incorrectly, such as exceeding a limit,reducing a limit, or being reused.¶

H3_SETTINGS_ERROR (0x109):

An endpoint detected an error in the payload of a SETTINGS frame.¶

H3_MISSING_SETTINGS (0x10A):

No SETTINGS frame was received at the beginning of the control stream.¶

H3_REQUEST_REJECTED (0x10B):

A server rejected a request without performing any application processing.¶

H3_REQUEST_CANCELLED (0x10C):

The request or its response (including pushed response) is cancelled.¶

H3_REQUEST_INCOMPLETE (0x10D):

The client's stream terminated without containing a fully-formed request.¶

H3_CONNECT_ERROR (0x10F):

The connection established in response to a CONNECT request was reset orabnormally closed.¶

H3_VERSION_FALLBACK (0x110):

The requested operation cannot be served over HTTP/3. The peer shouldretry over HTTP/1.1.¶

Error codes of the format0x1f * N + 0x21 for non-negative integer values of Nare reserved to exercise the requirement that unknown error codes be treated asequivalent to H3_NO_ERROR (Section 9). Implementations SHOULD select anerror code from this space with some probability when they would have sentH3_NO_ERROR.¶

10.1.Server Authority

HTTP/3 relies on the HTTP definition of authority. The security considerationsof establishing authority are discussed in Section 11.1 of[SEMANTICS].¶

10.2.Cross-Protocol Attacks

The use of ALPN in the TLS and QUIC handshakes establishes the targetapplication protocol before application-layer bytes are processed. Because allQUIC packets are encrypted, it is difficult for an attacker to control theplaintext bytes of an HTTP/3 connection which could be used in a cross-protocolattack on a plaintext protocol.¶

10.3.Intermediary Encapsulation Attacks

The HTTP/3 field encoding allows the expression of names that are not validfield names in the syntax used by HTTP (Section 4.3 of[SEMANTICS]). Requestsor responses containing invalid field names MUST be treated as malformed(Section 4.1.3). An intermediary therefore cannot translate an HTTP/3 requestor response containing an invalid field name into an HTTP/1.1 message.¶

Similarly, HTTP/3 allows field values that are not valid. While most of thevalues that can be encoded will not alter field parsing, carriage return (CR,ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL, ASCII 0x0)might be exploited by an attacker if they are translated verbatim. Any requestor response that contains a character not permitted in a field value MUST betreated as malformed (Section 4.1.3). Valid characters are defined by the"field-content" ABNF rule in Section 4.4 of[SEMANTICS].¶

10.4.Cacheability of Pushed Responses

Pushed responses do not have an explicit request from the client; the request isprovided by the server in the PUSH_PROMISE frame.¶

Caching responses that are pushed is possible based on the guidance provided bythe origin server in the Cache-Control header field. However, this can causeissues if a single server hosts more than one tenant. For example, a servermight offer multiple users each a small portion of its URI space.¶

Where multiple tenants share space on the same server, that server MUST ensurethat tenants are not able to push representations of resources that they do nothave authority over. Failure to enforce this would allow a tenant to provide arepresentation that would be served out of cache, overriding the actualrepresentation that the authoritative tenant provides.¶

Pushed responses for which an origin server is not authoritative (seeSection 3.4) MUST NOT be used or cached.¶

10.5.Denial-of-Service Considerations

An HTTP/3 connection can demand a greater commitment of resources to operatethan an HTTP/1.1 or HTTP/2 connection. The use of field compression and flowcontrol depend on a commitment of resources for storing a greater amount ofstate. Settings for these features ensure that memory commitments for thesefeatures are strictly bounded.¶

The number of PUSH_PROMISE frames is constrained in a similar fashion. A clientthat accepts server push SHOULD limit the number of Push IDs it issues at atime.¶

Processing capacity cannot be guarded as effectively as state capacity.¶

The ability to send undefined protocol elements which the peer is required toignore can be abused to cause a peer to expend additional processing time. Thismight be done by setting multiple undefined SETTINGS parameters, unknown frametypes, or unknown stream types. Note, however, that some uses are entirelylegitimate, such as optional-to-understand extensions and padding to increaseresistance to traffic analysis.¶

Compression of field sections also offers some opportunities to waste processingresources; see Section 7 of[QPACK] for more details on potential abuses.¶

All these features - i.e., server push, unknown protocol elements, fieldcompression - have legitimate uses. These features become a burden only whenthey are used unnecessarily or to excess.¶

An endpoint that doesn't monitor this behavior exposes itself to a risk ofdenial-of-service attack. Implementations SHOULD track the use of thesefeatures and set limits on their use. An endpoint MAY treat activity that issuspicious as a connection error (Section 8) of type H3_EXCESSIVE_LOAD, butfalse positives will result in disrupting valid connections and requests.¶

10.6.Use of Compression

Compression can allow an attacker to recover secret data when it is compressedin the same context as data under attacker control. HTTP/3 enables compressionof fields (Section 4.1.1); the following concerns also apply to the useof HTTP compressed content-codings; see Section 6.1.2 of[SEMANTICS].¶

There are demonstrable attacks on compression that exploit the characteristicsof the web (e.g.,[BREACH]). The attacker induces multiple requestscontaining varying plaintext, observing the length of the resulting ciphertextin each, which reveals a shorter length when a guess about the secret iscorrect.¶

Implementations communicating on a secure channel MUST NOT compress content thatincludes both confidential and attacker-controlled data unless separatecompression dictionaries are used for each source of data. Compression MUST NOTbe used if the source of data cannot be reliably determined.¶

Further considerations regarding the compression of fields sections aredescribed in[QPACK].¶

10.7.Padding and Traffic Analysis

Padding can be used to obscure the exact size of frame content and is providedto mitigate specific attacks within HTTP, for example, attacks where compressedcontent includes both attacker-controlled plaintext and secret data (e.g.,[BREACH]).¶

Where HTTP/2 employs PADDING frames and Padding fields in other frames to make aconnection more resistant to traffic analysis, HTTP/3 can either rely ontransport-layer padding or employ the reserved frame and stream types discussedinSection 7.2.8 andSection 6.2.3. These methods of padding producedifferent results in terms of the granularity of padding, how padding isarranged in relation to the information that is being protected, whether paddingis applied in the case of packet loss, and how an implementation might controlpadding. Redundant padding could even be counterproductive.¶

To mitigate attacks that rely on compression, disabling or limiting compressionmight be preferable to padding as a countermeasure.¶

Use of padding can result in less protection than might seem immediatelyobvious. At best, padding only makes it more difficult for an attacker to inferlength information by increasing the number of frames an attacker has toobserve. Incorrectly implemented padding schemes can be easily defeated. Inparticular, randomized padding with a predictable distribution provides verylittle protection; similarly, padding payloads to a fixed size exposesinformation as payload sizes cross the fixed-sized boundary, which could bepossible if an attacker can control plaintext.¶

10.8.Frame Parsing

Several protocol elements contain nested length elements, typically in the formof frames with an explicit length containing variable-length integers. Thiscould pose a security risk to an incautious implementer. An implementation MUSTensure that the length of a frame exactly matches the length of the fields itcontains.¶

10.9.Early Data

The use of 0-RTT with HTTP/3 creates an exposure to replay attack. Theanti-replay mitigations in[HTTP-REPLAY] MUST be applied when usingHTTP/3 with 0-RTT.¶

10.10.Migration

Certain HTTP implementations use the client address for logging oraccess-control purposes. Since a QUIC client's address might change during aconnection (and future versions might support simultaneous use of multipleaddresses), such implementations will need to either actively retrieve theclient's current address or addresses when they are relevant or explicitlyaccept that the original address might change.¶

10.11.Privacy Considerations

Several characteristics of HTTP/3 provide an observer an opportunity tocorrelate actions of a single client or server over time. These include thevalue of settings, the timing of reactions to stimulus, and the handling of anyfeatures that are controlled by settings.¶

As far as these create observable differences in behavior, they could be used asa basis for fingerprinting a specific client.¶

HTTP/3's preference for using a single QUIC connection allows correlation of auser's activity on a site. Reusing connections for different origins allowsfor correlation of activity across those origins.¶

Several features of QUIC solicit immediate responses and can be used by anendpoint to measure latency to their peer; this might have privacy implicationsin certain scenarios.¶

11.1.Registration of HTTP/3 Identification String

This document creates a new registration for the identification ofHTTP/3 in the "Application Layer Protocol Negotiation (ALPN)Protocol IDs" registry established in[RFC7301].¶

The "h3" string identifies HTTP/3:¶

Protocol:

HTTP/3¶

Identification Sequence:

0x68 0x33 ("h3")¶

Specification:

This document¶

11.2.New Registries

New registries created in this document operate under the QUIC registrationpolicy documented in Section 22.1 of[QUIC-TRANSPORT]. These registries allinclude the common set of fields listed in Section 22.1.1 of[QUIC-TRANSPORT].¶

The initial allocations in these registries created in this document are allassigned permanent status and list as contact both the IESG (iesg@ietf.org) andthe HTTP working group (ietf-http-wg@w3.org).¶

A.1.Streams

HTTP/3 permits use of a larger number of streams (2^62-1) than HTTP/2. Theconsiderations about exhaustion of stream identifier space apply, though thespace is significantly larger such that it is likely that other limits in QUICare reached first, such as the limit on the connection flow control window.¶

In contrast to HTTP/2, stream concurrency in HTTP/3 is managed by QUIC. QUICconsiders a stream closed when all data has been received and sent data has beenacknowledged by the peer. HTTP/2 considers a stream closed when the framecontaining the END_STREAM bit has been committed to the transport. As a result,the stream for an equivalent exchange could remain "active" for a longer periodof time. HTTP/3 servers might choose to permit a larger number of concurrentclient-initiated bidirectional streams to achieve equivalent concurrency toHTTP/2, depending on the expected usage patterns.¶

Due to the presence of other unidirectional stream types, HTTP/3 does not relyexclusively on the number of concurrent unidirectional streams to control thenumber of concurrent in-flight pushes. Instead, HTTP/3 clients use theMAX_PUSH_ID frame to control the number of pushes received from an HTTP/3server.¶

A.2.HTTP Frame Types

Many framing concepts from HTTP/2 can be elided on QUIC, because the transportdeals with them. Because frames are already on a stream, they can omit thestream number. Because frames do not block multiplexing (QUIC's multiplexingoccurs below this layer), the support for variable-maximum-length packets can beremoved. Because stream termination is handled by QUIC, an END_STREAM flag isnot required. This permits the removal of the Flags field from the genericframe layout.¶

Frame payloads are largely drawn from[HTTP2]. However, QUIC includes manyfeatures (e.g., flow control) which are also present in HTTP/2. In these cases,the HTTP mapping does not re-implement them. As a result, several HTTP/2 frametypes are not required in HTTP/3. Where an HTTP/2-defined frame is no longerused, the frame ID has been reserved in order to maximize portability betweenHTTP/2 and HTTP/3 implementations. However, even equivalent frames between thetwo mappings are not identical.¶

Many of the differences arise from the fact that HTTP/2 provides an absoluteordering between frames across all streams, while QUIC provides this guaranteeon each stream only. As a result, if a frame type makes assumptions that framesfrom different streams will still be received in the order sent, HTTP/3 willbreak them.¶

Some examples of feature adaptations are described below, as well as generalguidance to extension frame implementors converting an HTTP/2 extension toHTTP/3.¶

A.3.HTTP/2 SETTINGS Parameters

An important difference from HTTP/2 is that settings are sent once, as the firstframe of the control stream, and thereafter cannot change. This eliminates manycorner cases around synchronization of changes.¶

Some transport-level options that HTTP/2 specifies via the SETTINGS frame aresuperseded by QUIC transport parameters in HTTP/3. The HTTP-level options thatare retained in HTTP/3 have the same value as in HTTP/2.¶

Below is a listing of how each HTTP/2 SETTINGS parameter is mapped:¶

SETTINGS_HEADER_TABLE_SIZE:

See[QPACK].¶

SETTINGS_ENABLE_PUSH:

This is removed in favor of the MAX_PUSH_ID which provides a more granularcontrol over server push.¶

SETTINGS_MAX_CONCURRENT_STREAMS:

QUIC controls the largest open Stream ID as part of its flow control logic.Specifying SETTINGS_MAX_CONCURRENT_STREAMS in the SETTINGS frame is an error.¶

SETTINGS_INITIAL_WINDOW_SIZE:

QUIC requires both stream and connection flow control window sizes to bespecified in the initial transport handshake. SpecifyingSETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame is an error.¶

SETTINGS_MAX_FRAME_SIZE:

This setting has no equivalent in HTTP/3. Specifying it in the SETTINGS frameis an error.¶

SETTINGS_MAX_FIELD_SECTION_SIZE:

SeeSection 7.2.4.1.¶

In HTTP/3, setting values are variable-length integers (6, 14, 30, or 62 bitslong) rather than fixed-length 32-bit fields as in HTTP/2. This will oftenproduce a shorter encoding, but can produce a longer encoding for settings whichuse the full 32-bit space. Settings ported from HTTP/2 might choose to redefinetheir value to limit it to 30 bits for more efficient encoding, or to make useof the 62-bit space if more than 30 bits are required.¶

Settings need to be defined separately for HTTP/2 and HTTP/3. The IDs ofsettings defined in[HTTP2] have been reserved for simplicity. Note thatthe settings identifier space in HTTP/3 is substantially larger (62 bits versus16 bits), so many HTTP/3 settings have no equivalent HTTP/2 code point. SeeSection 11.2.2.¶

As QUIC streams might arrive out-of-order, endpoints are advised to not wait forthe peers' settings to arrive before responding to other streams. SeeSection 7.2.4.2.¶

A.4.HTTP/2 Error Codes

QUIC has the same concepts of "stream" and "connection" errors that HTTP/2provides. However, the differences between HTTP/2 and HTTP/3 mean that errorcodes are not directly portable between versions.¶

The HTTP/2 error codes defined in Section 7 of[HTTP2] logically map tothe HTTP/3 error codes as follows:¶

NO_ERROR (0x0):

H3_NO_ERROR inSection 8.1.¶

PROTOCOL_ERROR (0x1):

This is mapped to H3_GENERAL_PROTOCOL_ERROR except in cases where morespecific error codes have been defined. This includes H3_FRAME_UNEXPECTEDand H3_CLOSED_CRITICAL_STREAM defined inSection 8.1.¶

INTERNAL_ERROR (0x2):

H3_INTERNAL_ERROR inSection 8.1.¶

FLOW_CONTROL_ERROR (0x3):

Not applicable, since QUIC handles flow control.¶

SETTINGS_TIMEOUT (0x4):

Not applicable, since no acknowledgement of SETTINGS is defined.¶

STREAM_CLOSED (0x5):

Not applicable, since QUIC handles stream management.¶

FRAME_SIZE_ERROR (0x6):

H3_FRAME_ERROR error code defined inSection 8.1.¶

REFUSED_STREAM (0x7):

H3_REQUEST_REJECTED (inSection 8.1) is used to indicate that arequest was not processed. Otherwise, not applicable because QUIC handlesstream management.¶

CANCEL (0x8):

H3_REQUEST_CANCELLED inSection 8.1.¶

COMPRESSION_ERROR (0x9):

Multiple error codes are defined in[QPACK].¶

CONNECT_ERROR (0xa):

H3_CONNECT_ERROR inSection 8.1.¶

ENHANCE_YOUR_CALM (0xb):

H3_EXCESSIVE_LOAD inSection 8.1.¶

INADEQUATE_SECURITY (0xc):

Not applicable, since QUIC is assumed to provide sufficient security on allconnections.¶

H3_1_1_REQUIRED (0xd):

H3_VERSION_FALLBACK inSection 8.1.¶

Error codes need to be defined for HTTP/2 and HTTP/3 separately. SeeSection 11.2.3.¶

B.1.Since draft-ietf-quic-http-28

• CANCEL_PUSH is recommended even when the stream is reset (#3698, #3700)¶
• Use H3_ID_ERROR when GOAWAY contains a larger identifier (#3631, #3634)¶

B.2.Since draft-ietf-quic-http-27

• Updated text to refer to latest HTTP revisions¶
• Use the HTTP definition of authority for establishing and coalescingconnections (#253, #2223, #3558)¶
• Define use of GOAWAY from both endpoints (#2632, #3129)¶
• Require either :authority or Host if the URI scheme has a mandatoryauthority component (#3408, #3475)¶

B.3.Since draft-ietf-quic-http-26

• No changes¶

B.4.Since draft-ietf-quic-http-25

• Require QUICv1 for HTTP/3 (#3117, #3323)¶
• Remove DUPLICATE_PUSH and allow duplicate PUSH_PROMISE (#3275, #3309)¶
• Clarify the definition of "malformed" (#3352, #3345)¶

B.5.Since draft-ietf-quic-http-24

• Removed H3_EARLY_RESPONSE error code; H3_NO_ERROR is recommended instead(#3130,#3208)¶
• Unknown error codes are equivalent to H3_NO_ERROR (#3276,#3331)¶
• Some error codes are reserved for greasing (#3325,#3360)¶

B.6.Since draft-ietf-quic-http-23

• Removedquic Alt-Svc parameter (#3061,#3118)¶
• Clients need not persist unknown settings for use in 0-RTT (#3110,#3113)¶
• Clarify error cases around CANCEL_PUSH (#2819,#3083)¶

B.7.Since draft-ietf-quic-http-22

• Removed priority signaling (#2922,#2924)¶

• Further changes to error codes (#2662,#2551):¶

  • Error codes renumbered¶
  • HTTP_MALFORMED_FRAME replaced by HTTP_FRAME_ERROR, HTTP_ID_ERROR, and others¶
• Clarify how unknown frame types interact with required frame sequence(#2867,#2858)¶
• Describe interactions with the transport in terms of defined interface terms(#2857,#2805)¶
• Require the use of thehttp-opportunistic resource (RFC 8164) when scheme ishttp (#2439,#2973)¶
• Settings identifiers cannot be duplicated (#2979)¶

• Changes to SETTINGS frames in 0-RTT (#2972,#2790,#2945):¶

  • Servers must send all settings with non-default values in their SETTINGSframe, even when resuming¶
  • If a client doesn't have settings associated with a 0-RTT ticket, it usesthe defaults¶
  • Servers can't accept early data if they cannot recover the settings theclient will have remembered¶
• Clarify that Upgrade and the 101 status code are prohibited (#2898,#2889)¶
• Clarify that frame types reserved for greasing can occur on any stream, butframe types reserved due to HTTP/2 correspondence are prohibited(#2997,#2692,#2693)¶
• Unknown error codes cannot be treated as errors (#2998,#2816)¶

B.8.Since draft-ietf-quic-http-21

No changes¶

B.9.Since draft-ietf-quic-http-20

• Prohibit closing the control stream (#2509, #2666)¶
• Change default priority to use an orphan node (#2502, #2690)¶
• Exclusive priorities are restored (#2754, #2781)¶
• Restrict use of frames when using CONNECT (#2229, #2702)¶
• Close and maybe reset streams if a connection error occurs for CONNECT (#2228,#2703)¶
• Encourage provision of sufficient unidirectional streams for QPACK (#2100,#2529, #2762)¶
• Allow extensions to use server-initiated bidirectional streams (#2711, #2773)¶
• Clarify use of maximum header list size setting (#2516, #2774)¶

• Extensive changes to error codes and conditions of their sending¶

  • Require connection errors for more error conditions (#2511, #2510)¶
  • Updated the error codes for illegal GOAWAY frames (#2714, #2707)¶
  • Specified error code for HEADERS on control stream (#2708)¶
  • Specified error code for servers receiving PUSH_PROMISE (#2709)¶
  • Specified error code for receiving DATA before HEADERS (#2715)¶
  • Describe malformed messages and their handling (#2410, #2764)¶
  • Remove HTTP_PUSH_ALREADY_IN_CACHE error (#2812, #2813)¶
  • Refactor Push ID related errors (#2818, #2820)¶
  • Rationalize HTTP/3 stream creation errors (#2821, #2822)¶

B.10.Since draft-ietf-quic-http-19

• SETTINGS_NUM_PLACEHOLDERS is 0x9 (#2443,#2530)¶
• Non-zero bits in the Empty field of the PRIORITY frame MAY be treated as anerror (#2501)¶

B.11.Since draft-ietf-quic-http-18

• Resetting streams following a GOAWAY is recommended, but not required(#2256,#2457)¶

• Use variable-length integers throughout (#2437,#2233,#2253,#2275)¶

  • Variable-length frame types, stream types, and settings identifiers¶
  • Renumbered stream type assignments¶
  • Modified associated reserved values¶
• Frame layout switched from Length-Type-Value to Type-Length-Value(#2395,#2235)¶
• Specified error code for servers receiving DUPLICATE_PUSH (#2497)¶
• Use connection error for invalid PRIORITY (#2507, #2508)¶

B.12.Since draft-ietf-quic-http-17

• HTTP_REQUEST_REJECTED is used to indicate a request can be retried (#2106,#2325)¶
• Changed error code for GOAWAY on the wrong stream (#2231, #2343)¶

B.13.Since draft-ietf-quic-http-16

• Rename "HTTP/QUIC" to "HTTP/3" (#1973)¶

• Changes to PRIORITY frame (#1865, #2075)¶

  • Permitted as first frame of request streams¶
  • Remove exclusive reprioritization¶
  • Changes to Prioritized Element Type bits¶
• Define DUPLICATE_PUSH frame to refer to another PUSH_PROMISE (#2072)¶
• Set defaults for settings, allow request before receiving SETTINGS (#1809,#1846, #2038)¶
• Clarify message processing rules for streams that aren't closed (#1972, #2003)¶
• Removed reservation of error code 0 and moved HTTP_NO_ERROR to this value(#1922)¶
• Removed prohibition of zero-length DATA frames (#2098)¶

B.14.Since draft-ietf-quic-http-15

Substantial editorial reorganization; no technical changes.¶

B.15.Since draft-ietf-quic-http-14

• Recommend sensible values for QUIC transport parameters (#1720,#1806)¶
• Define error for missing SETTINGS frame (#1697,#1808)¶
• Setting values are variable-length integers (#1556,#1807) and do not haveseparate maximum values (#1820)¶
• Expanded discussion of connection closure (#1599,#1717,#1712)¶
• HTTP_VERSION_FALLBACK falls back to HTTP/1.1 (#1677,#1685)¶

B.16.Since draft-ietf-quic-http-13

• Reserved some frame types for grease (#1333, #1446)¶
• Unknown unidirectional stream types are tolerated, not errors; some reservedfor grease (#1490, #1525)¶
• Require settings to be remembered for 0-RTT, prohibit reductions (#1541,#1641)¶
• Specify behavior for truncated requests (#1596, #1643)¶

B.17.Since draft-ietf-quic-http-12

• TLS SNI extension isn't mandatory if an alternative method is used (#1459,#1462, #1466)¶
• Removed flags from HTTP/3 frames (#1388, #1398)¶
• Reserved frame types and settings for use in preserving extensibility (#1333,#1446)¶
• Added general error code (#1391, #1397)¶
• Unidirectional streams carry a type byte and are extensible (#910,#1359)¶
• Priority mechanism now uses explicit placeholders to enable persistentstructure in the tree (#441,#1421,#1422)¶

B.18.Since draft-ietf-quic-http-11

• Moved QPACK table updates and acknowledgments to dedicated streams (#1121,#1122, #1238)¶

B.19.Since draft-ietf-quic-http-10

• Settings need to be remembered when attempting and accepting 0-RTT (#1157,#1207)¶

B.20.Since draft-ietf-quic-http-09

• Selected QCRAM for header compression (#228, #1117)¶
• The server_name TLS extension is now mandatory (#296, #495)¶
• Specified handling of unsupported versions in Alt-Svc (#1093, #1097)¶

B.21.Since draft-ietf-quic-http-08

• Clarified connection coalescing rules (#940, #1024)¶

B.22.Since draft-ietf-quic-http-07

• Changes for integer encodings in QUIC (#595,#905)¶
• Use unidirectional streams as appropriate (#515, #240, #281, #886)¶
• Improvement to the description of GOAWAY (#604, #898)¶
• Improve description of server push usage (#947, #950, #957)¶

B.23.Since draft-ietf-quic-http-06

• Track changes in QUIC error code usage (#485)¶

B.24.Since draft-ietf-quic-http-05

• Made push ID sequential, add MAX_PUSH_ID, remove SETTINGS_ENABLE_PUSH (#709)¶
• Guidance about keep-alive and QUIC PINGs (#729)¶
• Expanded text on GOAWAY and cancellation (#757)¶

B.25.Since draft-ietf-quic-http-04

• Cite RFC 5234 (#404)¶
• Return to a single stream per request (#245,#557)¶
• Use separate frame type and settings registries from HTTP/2 (#81)¶
• SETTINGS_ENABLE_PUSH instead of SETTINGS_DISABLE_PUSH (#477)¶
• Restored GOAWAY (#696)¶
• Identify server push using Push ID rather than a stream ID (#702,#281)¶
• DATA frames cannot be empty (#700)¶

B.26.Since draft-ietf-quic-http-03

None.¶

B.27.Since draft-ietf-quic-http-02

• Track changes in transport draft¶

B.28.Since draft-ietf-quic-http-01

• SETTINGS changes (#181):¶

  • SETTINGS can be sent only once at the start of a connection;no changes thereafter¶
  • SETTINGS_ACK removed¶
  • Settings can only occur in the SETTINGS frame a single time¶
  • Boolean format updated¶
• Alt-Svc parameter changed from "v" to "quic"; format updated (#229)¶
• Closing the connection control stream or any message control stream is afatal error (#176)¶
• HPACK Sequence counter can wrap (#173)¶
• 0-RTT guidance added¶
• Guide to differences from HTTP/2 and porting HTTP/2 extensions added(#127,#242)¶

B.29.Since draft-ietf-quic-http-00

• Changed "HTTP/2-over-QUIC" to "HTTP/QUIC" throughout (#11,#29)¶
• Changed from using HTTP/2 framing within Stream 3 to new framing format andtwo-stream-per-request model (#71,#72,#73)¶
• Adopted SETTINGS format from draft-bishop-httpbis-extended-settings-01¶
• Reworked SETTINGS_ACK to account for indeterminate inter-stream order (#75)¶
• Described CONNECT pseudo-method (#95)¶
• Updated ALPN token and Alt-Svc guidance (#13,#87)¶
• Application-layer-defined error codes (#19,#74)¶

B.30.Since draft-shade-quic-http2-mapping-00

• Adopted as base for draft-ietf-quic-http¶
• Updated authors/editors list¶