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Transport QUIC The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic. Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-http.

Introduction HTTP semantics are used for a broad range of services on the Internet. These semantics have commonly been used with two different TCP mappings, HTTP/1.1 and HTTP/2. HTTP/3 supports the same semantics over a new transport protocol, QUIC.

Prior versions of HTTP HTTP/1.1 is a TCP mapping which uses whitespace-delimited text fields to convey HTTP messages. While these exchanges are human-readable, using whitespace for message formatting leads to parsing difficulties and workarounds to be tolerant of variant behavior. Because each connection can transfer only a single HTTP request or response at a time in each direction, multiple parallel TCP connections are often used, reducing the ability of the congestion controller to accurately manage traffic between endpoints. HTTP/2 introduced a binary framing and multiplexing layer to improve latency without modifying the transport layer. However, because the parallel nature of HTTP/2's multiplexing is not visible to TCP's loss recovery mechanisms, a lost or reordered packet causes all active transactions to experience a stall regardless of whether that transaction was impacted by the lost packet.

Delegation to QUIC The QUIC transport protocol incorporates stream multiplexing and per-stream flow control, similar to that provided by the HTTP/2 framing layer. By providing reliability at the stream level and congestion control across the entire connection, it has the capability to improve the performance of HTTP compared to a TCP mapping. QUIC also incorporates TLS 1.3 at the transport layer, offering comparable security to running TLS over TCP, with the improved connection setup latency of TCP Fast Open . This document defines a mapping of HTTP semantics over the QUIC transport protocol, drawing heavily on the design of HTTP/2. While delegating stream lifetime and flow control issues to QUIC, a similar binary framing is used on each stream. Some HTTP/2 features are subsumed by QUIC, while other features are implemented atop QUIC. QUIC is described in . For a full description of HTTP/2, see .

HTTP/3 Protocol Overview HTTP/3 provides a transport for HTTP semantics using the QUIC transport protocol and an internal framing layer similar to HTTP/2. Once a client knows that an HTTP/3 server exists at a certain endpoint, it opens a QUIC connection. QUIC provides protocol negotiation, stream-based multiplexing, and flow control. An HTTP/3 endpoint can be discovered using HTTP Alternative Services; this process is described in greater detail in . Within each stream, the basic unit of HTTP/3 communication is a frame (). Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses (). Multiplexing of requests is performed using the QUIC stream abstraction, described in Section 2 of . Each request-response pair consumes a single QUIC stream. Streams are independent of each other, so one stream that is blocked or suffers packet loss does not prevent progress on other streams. Server push is an interaction mode introduced in HTTP/2 which permits a server to push a request-response exchange to a client in anticipation of the client making the indicated request. This trades off network usage against a potential latency gain. Several HTTP/3 frames are used to manage server push, such as PUSH_PROMISE, MAX_PUSH_ID, and CANCEL_PUSH. As in HTTP/2, request and response headers are compressed for transmission. Because HPACK relies on in-order transmission of compressed header blocks (a guarantee not provided by QUIC), HTTP/3 replaces HPACK with QPACK . QPACK uses separate unidirectional streams to modify and track header table state, while header blocks refer to the state of the table without modifying it.

Document Organization The following sections provide a detailed overview of the connection lifecycle and key concepts:

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

The details of the wire protocol and interactions with the transport are described in subsequent sections:

• Stream Mapping and Usage () describes the way QUIC streams are used.
• HTTP Framing Layer () describes the frames used on most streams.
• Error Handling () describes how error conditions are handled and expressed, either on a particular stream or for the connection as a whole.

Additional resources are provided in the final sections:

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

Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Field definitions are given in Augmented Backus-Naur Form (ABNF), as defined in . This document uses the variable-length integer encoding from . The following terms are used:

abort:

An abrupt termination of a connection or stream, possibly due to an error condition.

client:

The endpoint that initiates an HTTP/3 connection. Clients send HTTP requests and receive HTTP responses.

connection:

A transport-layer connection between two endpoints, using QUIC as the transport 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 a header and a variable-length sequence of bytes structured according to the frame type. Protocol elements called "frames" exist in both this document and . Where frames from are referenced, the frame name will be prefaced with "QUIC." For example, "QUIC CONNECTION_CLOSE frames." References without this preface refer to frames defined in .

peer:

An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint 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 requests and 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 3.3 of . Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in Section 2.3 of . Intermediaries act as both client and server at different times.

Connection Setup and Management

Draft Version Identification

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

HTTP/3 uses the token "h3" to identify itself in ALPN and Alt-Svc. Only implementations of the final, published RFC can identify themselves as "h3". Until such an RFC exists, implementations MUST NOT identify themselves using this string. Implementations of draft versions of the protocol MUST add the string "-" and the 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 their transport. For example, the application protocol defined in draft-ietf-quic-http-25 uses the transport defined in draft-ietf-quic-transport-25. Non-compatible experiments that are based on these draft versions MUST append the string "-" and an experiment name to the identifier. For example, an experimental implementation based on draft-ietf-quic-http-09 which reserves an extra stream for unsolicited transmission of 1980s pop music might identify itself as "h3-09-rickroll". Note that any label MUST conform to the "token" syntax defined in Section 3.2.6 of . Experimenters are encouraged to coordinate their experiments on the quic@ietf.org mailing list.

Discovering an HTTP/3 Endpoint An HTTP origin advertises the availability of an equivalent HTTP/3 endpoint via the Alt-Svc HTTP response header field or the HTTP/2 ALTSVC frame (), using the ALPN token defined in . For example, an origin could indicate in an HTTP response that HTTP/3 was available on UDP port 50781 at the same hostname by including the following header field: On receipt of an Alt-Svc record indicating HTTP/3 support, a client MAY attempt to establish a QUIC connection to the indicated host and port and, if successful, send HTTP requests using the mapping described in this document. Connectivity problems (e.g. firewall blocking UDP) can result in QUIC connection establishment failure, in which case the client SHOULD continue using the existing connection or try another alternative endpoint offered by the origin. Servers MAY serve HTTP/3 on any UDP port, since an alternative always includes an explicit port.

Connection Establishment HTTP/3 relies on QUIC version 1 as the underlying transport. The use of other QUIC 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 the server during the TLS handshake. If the server is identified by a DNS name, clients MUST send the Server Name Indication (SNI) TLS extension unless an alternative mechanism to indicate the target host is used. QUIC connections are established as described in . During connection establishment, HTTP/3 support is indicated by selecting the ALPN token "h3" in the TLS handshake. Support for other application-layer protocols MAY be offered in the same handshake. While connection-level options pertaining to the core QUIC protocol are set in the initial crypto handshake, HTTP/3-specific settings are conveyed in the SETTINGS frame. After the QUIC connection is established, a SETTINGS frame () MUST be sent by each endpoint as the initial frame of their respective HTTP control stream (see ).

Connection Reuse Once a connection exists to a server endpoint, this connection MAY be reused for requests with multiple different URI authority components. The client MAY send any requests for which the client considers the server authoritative. An authoritative HTTP/3 endpoint is typically discovered because the client has received an Alt-Svc record from the request's origin which nominates the endpoint as a valid HTTP Alternative Service for that origin. As required by , clients MUST check that the nominated server can present a valid certificate for the origin before considering it authoritative. Clients MUST NOT assume that an HTTP/3 endpoint is authoritative for other origins without an explicit signal. Prior to making requests for an origin whose scheme is not "https," the client MUST ensure the server is willing to serve that scheme. If the client intends to make requests for an origin whose scheme is "http", this means that it MUST obtain a valid http-opportunistic response for the origin as described in prior to making any such requests. Other schemes might define other mechanisms. A server that does not wish clients to reuse connections for a particular origin can indicate that it is not authoritative for a request by sending a 421 (Misdirected Request) status code in response to the request (see Section 9.1.2 of ). The considerations discussed in Section 9.1 of also apply to the management of HTTP/3 connections.

HTTP Request Lifecycle

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 zero or more non-final HTTP responses on the same stream as the request, followed by a single final HTTP response, as detailed below. Pushed responses are sent on a server-initiated unidirectional QUIC stream (see ). A server sends zero or more non-final HTTP responses, followed by a single final HTTP response, in the same manner as a standard response. Push is described in more detail in . On a given stream, receipt of multiple requests or receipt of an additional HTTP response following a final HTTP response MUST be treated as malformed (). An HTTP message (request or response) consists of:

1. the message header (see Section 3.2 of ), sent as a single HEADERS frame (see ),
2. optionally, the payload body, if present (see Section 3.3 of ), sent as a series of DATA frames (see ),
3. optionally, trailing headers, if present (see Section 4.1.2 of ), sent as a single HEADERS frame.

Receipt of DATA and HEADERS frames in any other sequence MUST be treated as a connection error of type H3_FRAME_UNEXPECTED (). A server MAY send one or more PUSH_PROMISE frames (see ) before, after, or interleaved with the frames of a response message. These PUSH_PROMISE frames are not part of the response; see for more details. These frames are not permitted in pushed responses; a pushed response which includes PUSH_PROMISE frames MUST be treated as a connection error of type H3_FRAME_UNEXPECTED. Frames of unknown types (), including reserved frames () MAY be sent on a request or push stream before, after, or interleaved with other frames described in this section. The HEADERS and PUSH_PROMISE frames might reference updates to the QPACK dynamic table. While these updates are not directly part of the message exchange, they must be received and processed before the message can be consumed. See for more details. The "chunked" transfer encoding defined in Section 4.1 of MUST NOT be used. A response MAY consist of multiple messages when and only when one or more informational responses (1xx; see Section 6.2 of ) precede a final response to the same request. Non-final responses do not contain a payload body or trailers. If an endpoint receives an invalid sequence of frames on either a request or a push stream, it MUST respond with a connection error of type H3_FRAME_UNEXPECTED (). In particular, a DATA frame before any HEADERS frame, or a HEADERS or DATA frame after the trailing HEADERS frame is considered invalid. An HTTP request/response exchange fully consumes a client-initiated bidirectional QUIC stream. After sending a request, a client MUST close the stream for sending. Unless using the CONNECT method (see ), clients MUST 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. At this point, the QUIC stream is fully closed. When a stream is closed, this indicates the end of an HTTP message. Because some messages are large or unbounded, endpoints SHOULD begin processing partial HTTP messages once enough of the message has been received to make progress. If a client stream terminates without enough of the HTTP message to provide a complete response, the server SHOULD abort its response with the error code H3_REQUEST_INCOMPLETE. A server can send a complete response prior to the client sending an entire request if the response does not depend on any portion of the request that has not been sent and received. When the server does not need to receive the remainder of the request, it MAY abort reading the request stream, send a complete response, and cleanly close the sending part of the stream. The error code H3_NO_ERROR SHOULD be used when requesting that the client stop sending on the request stream. Clients MUST NOT discard complete responses as a result of having their request terminated abruptly, though clients can always discard responses at their discretion for other reasons. If the server sends a partial or complete response but does not abort reading, clients SHOULD continue sending the body of the request and close the stream normally.

Header Formatting and Compression HTTP message headers carry information as a series of key-value pairs, called header fields. For a listing of registered HTTP header fields, see the "Message Header Field" registry maintained at https://www.iana.org/assignments/message-headers. Just as in previous versions of HTTP, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. Properties of HTTP header field names and values are discussed in more detail in Section 3.2 of , though the wire rendering in HTTP/3 differs. As in HTTP/2, header field names MUST be converted to lowercase prior to their encoding. A request or response containing uppercase header field names MUST be treated as malformed (). Like HTTP/2, HTTP/3 does not use the Connection header field to indicate connection-specific header fields; in this protocol, connection-specific metadata is conveyed by other means. An endpoint MUST NOT generate an HTTP/3 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed (). The only exception to this is the TE header field, which MAY be present in an HTTP/3 request; when it is, it MUST NOT contain any value other than "trailers". This means that an intermediary transforming an HTTP/1.x message to HTTP/3 will need to remove any header fields nominated by the Connection header field, along with the Connection header field itself. Such intermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding, and Upgrade, even if they are not nominated by the Connection header field.

Pseudo-Header Fields As in HTTP/2, HTTP/3 uses special pseudo-header fields beginning with the ':' character (ASCII 0x3a) to convey the target URI, the method of the request, and the status code for the response. Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields other than those defined in this document, except as negotiated via an extension; see . Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined for requests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear in requests. Pseudo-header fields MUST NOT appear in trailers. Endpoints MUST treat a request or response that contains undefined or invalid pseudo-header fields as malformed (). All pseudo-header fields MUST appear in the header block before regular header fields. Any request or response that contains a pseudo-header field that appears in a header block after a regular header field MUST be treated as malformed (). The following pseudo-header fields are defined for requests:

":method":

Contains the HTTP method (, Section 4)

":scheme":

Contains the scheme portion of the target URI (, Section 3.1)

":scheme" is not restricted to "http" and "https" schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services.

":authority":

Contains the authority portion of the target URI (Section 3.2 of ). The authority MUST NOT include the deprecated "userinfo" subcomponent for "http" or "https" schemed URIs.

To ensure that the HTTP/1.1 request line can be reproduced accurately, this pseudo-header field MUST be omitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see Section 5.3 of ). Clients that generate HTTP/3 requests directly SHOULD use the ":authority" pseudo-header field instead of the Host header field. An intermediary that converts an HTTP/3 request to HTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the ":authority" pseudo-header field.

":path":

Contains the path and query parts of the target URI (the "path-absolute" production and optionally a '?' character followed by the "query" production (see Sections 3.3 and 3.4 of ). A request in asterisk form includes the value '*' for the ":path" pseudo-header field.

This pseudo-header field MUST NOT be empty for "http" or "https" URIs; "http" or "https" URIs that do not contain a path component MUST include a value of '/'. The exception to this rule is an OPTIONS request for an "http" or "https" URI that does not include a path component; these MUST include a ":path" pseudo-header field with a value of '*' (see Section 5.3.4 of ).

All HTTP/3 requests MUST include exactly one value for the ":method", ":scheme", and ":path" pseudo-header fields, unless it is a CONNECT request (). An HTTP request that omits mandatory pseudo-header fields or contains invalid values for those fields is malformed (). HTTP/3 does not define a way to carry the version identifier that is included in the HTTP/1.1 request line. For responses, a single ":status" pseudo-header field is defined that carries the HTTP status code field (see Section 6 of ). This pseudo-header field MUST be included in all responses; otherwise, the response is malformed (). HTTP/3 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

Header Compression HTTP/3 uses QPACK header compression as described in , a variation of HPACK which allows the flexibility to avoid header-compression-induced head-of-line blocking. See that document for additional details. To allow for better compression efficiency, the cookie header field MAY be split into separate header fields, each with one or more cookie-pairs, before compression. If a decompressed header list contains multiple cookie header fields, these MUST be concatenated before being passed into a non-HTTP/2, non-HTTP/3 context, as described in Section 8.1.2.5 of .

Header Size Constraints An HTTP/3 implementation MAY impose a limit on the maximum size of the message header it will accept on an individual HTTP message. A server that receives a larger header field list than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code . A client can discard responses that it cannot process. The size of a header field list is calculated based on the uncompressed size of header fields, including the length of the name and value in bytes plus an overhead of 32 bytes for each header field. If an implementation wishes to advise its peer of this limit, it can be conveyed as a number of bytes in the SETTINGS_MAX_HEADER_LIST_SIZE parameter. An implementation which has received this parameter SHOULD NOT send an HTTP message header which exceeds the indicated size, as the peer will likely refuse to process it. However, because this limit is applied at each hop, messages below this limit are not guaranteed to be accepted.

Request Cancellation and Rejection Clients can cancel requests by resetting and aborting the request stream with an error code of H3_REQUEST_CANCELLED (). When the client aborts reading a response, it indicates that this response is no longer of interest. Implementations SHOULD cancel requests by abruptly terminating any directions of a stream that are still open. When the server rejects a request without performing any application processing, it SHOULD abort its response stream with the error code H3_REQUEST_REJECTED. In this context, "processed" means that some data from the stream was passed to some higher layer of software that might have taken some action as a result. The client can treat requests rejected by the server as though they had never been sent at all, thereby allowing them to be retried later on a new connection. Servers MUST NOT use the H3_REQUEST_REJECTED error code for requests which were partially or fully processed. When a server abandons a response after partial processing, it SHOULD abort its response stream with the error code H3_REQUEST_CANCELLED. When a client resets a request with the error code H3_REQUEST_CANCELLED, a server MAY abruptly terminate the response using the error code H3_REQUEST_REJECTED if no processing was performed. Clients MUST NOT use the H3_REQUEST_REJECTED error code, except when a server has requested closure of the request stream with this error code. If a stream is cancelled after receiving a complete response, the client MAY ignore the cancellation and use the response. However, if a stream is cancelled after receiving a partial response, the response SHOULD NOT be used. Automatically retrying such requests is not possible, unless this is otherwise permitted (e.g., idempotent actions like GET, PUT, or DELETE).

Malformed Requests and Responses A malformed request or response is one that is an otherwise valid sequence of frames but is invalid due to:

• the presence of prohibited header fields or pseudo-header fields,
• the absence of mandatory pseudo-header fields,
• invalid values for pseudo-header fields,
• pseudo-header fields after header fields,
• an invalid sequence of HTTP messages, or
• the inclusion of uppercase header field names.

A request or response that includes a payload body can include a content-length header field. A request or response is also malformed if the value of a content-length header field does not equal the sum of the DATA frame payload lengths that form the body. A response that is defined to have no payload, as described in Section 3.3.2 of can have a non-zero content-length header field, even though no content is included in DATA frames. Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response. Malformed requests or responses that are detected MUST be treated as a stream error () of type H3_GENERAL_PROTOCOL_ERROR. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict because being permissive can expose implementations to these vulnerabilities.

The CONNECT Method The pseudo-method CONNECT (Section 4.3.6 of ) is primarily used with HTTP proxies to establish a TLS session 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 tunnel to a remote host. In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes. A CONNECT request in HTTP/3 functions in the same manner as in HTTP/2. The request MUST be formatted as described in Section 8.3 of . A CONNECT request that does not conform to these restrictions is malformed (see ). The request stream MUST NOT be closed at the end of the request. A proxy that supports CONNECT establishes a TCP connection () to the server identified in the ":authority" pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in Section 4.3.6 of . All DATA frames on the stream correspond to data sent or received on the TCP connection. Any DATA frame sent by the client is transmitted by the proxy to the TCP server; data received from the TCP server is packaged into DATA frames by the proxy. Note that the size and number of TCP segments is not guaranteed to map predictably to the size and number of HTTP DATA or QUIC STREAM frames. Once the CONNECT method has completed, only DATA frames are permitted to be sent on the stream. Extension frames MAY be used if specifically permitted by the definition of the extension. Receipt of any other frame type MUST be treated as a connection error of type H3_FRAME_UNEXPECTED. The TCP connection can be closed by either peer. When the client ends the request 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. When the proxy receives a packet with the FIN bit set, it will terminate the send stream that it sends to the client. TCP connections which remain half-closed in a single direction are not invalid, but are often handled poorly by servers, so clients SHOULD NOT close a stream for sending while they still expect to receive data from the target of the CONNECT. A TCP connection error is signaled by abruptly terminating the stream. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error of type H3_CONNECT_ERROR (). Correspondingly, if a proxy detects an error with the stream or the QUIC connection, it MUST close the TCP connection. If the underlying TCP implementation permits it, the proxy SHOULD send a TCP segment with the RST bit set.

HTTP Upgrade HTTP/3 does not support the HTTP Upgrade mechanism (Section 6.7 of ) or 101 (Switching Protocols) informational status code (Section 6.2.2 of ).

Server Push Server push is an interaction mode introduced in HTTP/2 which permits a server to push a request-response exchange to a client in anticipation of the client making the indicated request. This trades off network usage against a potential latency gain. HTTP/3 server push is similar to what is described in HTTP/2 , but uses different mechanisms. Each server push is identified by a unique Push ID. This Push ID is used in one or more PUSH_PROMISE frames (see ) that carry the request headers, then included with the push stream which ultimately fulfills those promises. When the same Push ID is promised on multiple request streams, the decompressed request header sets MUST contain the same fields in the same order, and both the name and the value in each field MUST be exact matches. Server push is only enabled on a connection when a client sends a MAX_PUSH_ID frame (see ). A server cannot use server push until it receives a MAX_PUSH_ID frame. A client sends additional MAX_PUSH_ID frames to control the number of pushes that a server can promise. A server SHOULD use Push IDs sequentially, starting at 0. A client MUST treat receipt of a push stream with a Push ID that is greater than the maximum Push ID as a connection error of type H3_ID_ERROR. The header of the request message is carried by a PUSH_PROMISE frame (see ) on the request stream which generated the push. Promised requests MUST conform to the requirements in Section 8.2 of . Each pushed response is associated with one or more client requests. The push is associated with the request stream on which the PUSH_PROMISE frame was received. The same server push can be associated with additional client requests using a PUSH_PROMISE frame with the same Push ID on multiple request streams. These associations do not affect the operation of the protocol, but MAY 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 is important. The server SHOULD send PUSH_PROMISE frames prior to sending HEADERS or DATA frames that reference the promised responses. This reduces the chance that 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 on a push stream (see ). The push stream identifies the Push ID of the promise that it fulfills, then contains a response to the promised request using the same format described for responses in . Due to reordering, push stream data can arrive before the corresponding PUSH_PROMISE frame. When a client receives a new push stream with an as-yet-unknown Push ID, both the associated client request and the pushed request headers are unknown. The client can buffer the stream data in expectation of the matching PUSH_PROMISE. The client can use stream flow control (see section 4.1 of ) to limit the amount of data a server may commit to the pushed stream. If a promised server push is not needed by the client, the client SHOULD send a CANCEL_PUSH frame. If the push stream is already open or opens after sending the CANCEL_PUSH frame, the client can abort reading the stream with an error code of H3_REQUEST_CANCELLED. This asks the server not to transfer additional data and indicates that it will be discarded upon receipt.

Connection Closure Once established, an HTTP/3 connection can be used for many requests and responses over time until the connection is closed. Connection closure can happen in any of several different ways.

Idle Connections Each QUIC endpoint declares an idle timeout during the handshake. If the connection remains idle (no packets received) for longer than this duration, the peer will assume that the connection has been closed. HTTP/3 implementations will need to open a new connection for new requests if the existing connection has been idle for longer than the server's advertised idle timeout, and SHOULD do so if approaching the idle timeout. HTTP clients are expected to request that the transport keep connections open while there are responses outstanding for requests or server pushes, as described in Section 19.2 of . If the client is not expecting a response from the server, allowing an idle connection to time out is preferred over expending effort maintaining a connection that might not be needed. A gateway MAY maintain connections in anticipation of need rather than incur the latency cost of connection establishment to servers. Servers SHOULD NOT actively keep connections open.

Connection Shutdown Even when a connection is not idle, either endpoint can decide to stop using the connection and let the connection close gracefully. Since clients drive request generation, clients perform a connection shutdown by not sending additional requests on the connection; responses and pushed responses associated to previous requests will continue to completion. Servers perform the same function by communicating with clients. Servers initiate the shutdown of a connection by sending a GOAWAY frame (). The GOAWAY frame indicates that client-initiated requests on lower stream IDs were or might be processed in this connection, while requests on the indicated stream ID and greater were rejected. This enables client and server to agree on which requests were accepted prior to the connection shutdown. This identifier MAY be zero if no requests were processed. Servers SHOULD NOT permit additional QUIC streams after sending a GOAWAY frame. Clients MUST NOT send new requests on the connection after receiving GOAWAY; a new connection MAY be established to send additional requests. Some requests might already be in transit. If the client has already sent requests on streams with a Stream ID greater than or equal to that indicated in the GOAWAY frame, those requests will not be processed and MAY be retried by the client on a different connection. The client MAY cancel these requests. It is RECOMMENDED that the server explicitly reject such requests (see ) in order to clean up transport state for the affected streams. Requests on Stream IDs less than the Stream ID in the GOAWAY frame might have been processed; their status cannot be known until a response is received, the stream is reset individually, or the connection terminates. Servers MAY reject individual requests on streams below the indicated ID if these requests were not processed. Servers SHOULD send a GOAWAY frame when the closing of a connection is known in advance, even if the advance notice is small, so that the remote peer can know whether a request has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a QUIC connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on. A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. A server MAY send multiple GOAWAY frames indicating different stream IDs, but MUST NOT increase the value they send in the last Stream ID, since clients might already have retried unprocessed requests on another connection. A server that is attempting to gracefully shut down a connection can send an initial GOAWAY frame with the last Stream ID set to the maximum possible value for a client-initiated, bidirectional stream (i.e. 2^62-4 in case of QUIC version 1). This GOAWAY frame signals to the client that shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight requests to reach the server, the server can send another GOAWAY frame indicating which requests it will accept before the end of the connection. This ensures that a connection can be cleanly shut down without causing requests to fail. Once all accepted requests have been processed, the server can permit the connection to become idle, or MAY initiate an immediate closure of the connection. An endpoint that completes a graceful shutdown SHOULD use the H3_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 make further requests.

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; the error code in this frame indicates to the peer why the connection is being closed. See for error codes which can be used when closing a connection. Before closing the connection, a GOAWAY MAY be sent to allow the client to retry some requests. Including the GOAWAY frame in the same packet as the QUIC CONNECTION_CLOSE frame improves the chances of the frame being received by clients.

Transport Closure For various reasons, the QUIC transport could indicate to the application layer that the connection has terminated. This might be due to an explicit closure by the peer, a transport-level error, or a change in network topology which interrupts connectivity. If a connection terminates without a GOAWAY frame, clients MUST assume that any request which was sent, whether in whole or in part, might have been processed.

Stream Mapping and Usage A QUIC stream provides reliable in-order delivery of bytes, but makes no guarantees about order of delivery with regard to bytes on other streams. On the wire, data is framed into QUIC STREAM frames, but this framing is invisible to the HTTP framing layer. The transport layer buffers and orders received QUIC STREAM frames, exposing the data contained within as a reliable byte stream to the application. Although QUIC permits out-of-order delivery within a stream, HTTP/3 does not make use of this feature. QUIC streams can be either unidirectional, carrying data only from initiator to receiver, or bidirectional. Streams can be initiated by either the client or the server. For more detail on QUIC streams, see Section 2 of . When HTTP headers and data are sent over QUIC, the QUIC layer handles most of the stream management. HTTP does not need to do any separate multiplexing when using QUIC - data sent over a QUIC stream always maps to a particular HTTP transaction or connection context.

Bidirectional Streams All client-initiated bidirectional streams are used for HTTP requests and responses. A bidirectional stream ensures that the response can be readily correlated with the request. This means that the client's first request occurs on QUIC stream 0, with subsequent requests on stream 4, 8, and so on. In order to permit these streams to open, an HTTP/3 server SHOULD configure non-zero minimum values for the number of permitted streams and the initial stream flow control window. So as to not unnecessarily limit parallelism, at least 100 requests SHOULD be permitted at a time. HTTP/3 does not use server-initiated bidirectional streams, though an extension could define a use for these streams. Clients MUST treat receipt of a server-initiated bidirectional stream as a connection error of type H3_STREAM_CREATION_ERROR unless such an extension has been negotiated.

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-length integer at the start of the stream. The format and structure of data that follows this integer is determined by the stream type.

Unidirectional Stream Header

Some stream types are reserved (). Two stream types are defined in this document: control streams () and push streams (). defines two additional stream types. Other stream types can be defined by extensions to HTTP/3; see for more details. The performance of HTTP/3 connections in the early phase of their lifetime is sensitive to the creation and exchange of data on unidirectional streams. Endpoints that excessively restrict the number of streams or the flow control window of these streams will increase the chance that the remote peer reaches the limit early and becomes blocked. In particular, implementations should consider that remote peers may wish to exercise reserved stream behavior () with some of the unidirectional streams they are permitted to use. To avoid blocking, the transport parameters sent by both clients and servers MUST allow the peer to create at least one unidirectional stream for the HTTP control stream plus the number of unidirectional streams required by mandatory extensions (three being the minimum number required for the base HTTP/3 protocol and QPACK), and SHOULD provide at least 1,024 bytes of flow control credit to each stream. Note that an endpoint is not required to grant additional credits to create more unidirectional streams if its peer consumes all the initial credits before creating the critical unidirectional streams. Endpoints SHOULD create the HTTP control stream as well as the unidirectional streams required by mandatory extensions (such as the QPACK encoder and decoder streams) first, and then create additional streams as allowed by their peer. If the stream header indicates a stream type which is not supported by the recipient, the remainder of the stream cannot be consumed as the semantics are unknown. Recipients of unknown stream types MAY abort reading of the stream with an error code of H3_STREAM_CREATION_ERROR, but MUST NOT consider such streams to be a connection error of any kind. Implementations MAY send stream types before knowing whether the peer supports them. However, stream types which could modify the state or semantics of existing protocol components, including QPACK or other extensions, MUST NOT be sent 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 to the reception of the unidirectional stream header.

Control Streams A control stream is indicated by a stream type of 0x00. Data on this stream consists of HTTP/3 frames, as defined in . Each side MUST initiate a single control stream at the beginning of the connection and send its SETTINGS frame as the first frame on this stream. If the first frame of the control stream is any other frame type, this MUST be treated as a connection error of type H3_MISSING_SETTINGS. Only one control stream per peer is permitted; receipt of a second stream which claims to be a control stream MUST be treated as a connection error of type H3_STREAM_CREATION_ERROR. The sender MUST NOT close the control stream, and the receiver MUST NOT request that the sender close the control stream. If either control stream is closed at any point, this MUST be treated as a connection error of type H3_CLOSED_CRITICAL_STREAM. A pair of unidirectional streams is used rather than a single bidirectional stream. This allows either peer to send data as soon as it is able. Depending on whether 0-RTT is enabled on the connection, either client or server might be able to send stream data first after the cryptographic handshake completes.

Push Streams Server push is an optional feature introduced in HTTP/2 that allows a server to initiate a response before a request has been made. See for more details. A push stream is indicated by a stream type of 0x01, followed by the Push ID of the promise that it fulfills, encoded as a variable-length integer. The remaining data on this stream consists of HTTP/3 frames, as defined in , and fulfills a promised server push by zero or more non-final HTTP responses followed by a single final HTTP response, as defined in . Server push and Push IDs are described in . Only servers can push; if a server receives a client-initiated push stream, this MUST be treated as a connection error of type H3_STREAM_CREATION_ERROR.

Push Stream Header

Each Push ID MUST only be used once in a push stream header. If a push stream header includes a Push ID that was used in another push stream header, the client MUST treat this as a connection error of type H3_ID_ERROR.

Reserved Stream Types Stream types of the format 0x1f * N + 0x21 for integer values of N are reserved to exercise the requirement that unknown types be ignored. These streams have no semantics, and can be sent when application-layer padding is desired. They MAY also be sent on connections where no data is currently being transferred. Endpoints MUST NOT consider these streams to have any meaning upon receipt. The payload and length of the stream are selected in any manner the implementation chooses. Implementations MAY terminate these streams cleanly, or MAY abruptly terminate them. When terminating abruptly, the error code H3_NO_ERROR or a reserved error code () SHOULD be used.

HTTP Framing Layer HTTP frames are carried on QUIC streams, as described in . HTTP/3 defines three stream types: control stream, request stream, and push stream. This section describes HTTP/3 frame formats and the streams types on which they are permitted; see for an overview. A comparison between HTTP/2 and HTTP/3 frames is provided in . HTTP/3 Frames and Stream Type Overview

Frame	Control Stream	Request Stream	Push Stream	Section
DATA	No	Yes	Yes
HEADERS	No	Yes	Yes
CANCEL_PUSH	Yes	No	No
SETTINGS	Yes (1)	No	No
PUSH_PROMISE	No	Yes	No
GOAWAY	Yes	No	No
MAX_PUSH_ID	Yes	No	No
Reserved	Yes	Yes	Yes

Certain frames can only occur as the first frame of a particular stream type; these are indicated in with a (1). Specific guidance is provided in the relevant section. Note that, unlike QUIC frames, HTTP/3 frames can span multiple packets.

Frame Layout All frames have the following format:

HTTP/3 Frame Format

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 of the 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 its description. A frame payload that contains additional bytes after the identified fields or a frame payload that terminates before the end of the identified fields MUST be treated as a connection error of type H3_FRAME_ERROR. When a stream terminates cleanly, if the last frame on the stream was truncated, this MUST be treated as a connection error () of type H3_FRAME_ERROR. Streams which terminate abruptly may be reset at any point in a frame.

Frame Definitions

DATA DATA frames (type=0x0) convey arbitrary, variable-length sequences of bytes associated with an HTTP request or response payload. DATA frames MUST be associated with an HTTP request or response. If a DATA frame is received on a control stream, the recipient MUST respond with a connection error () of type H3_FRAME_UNEXPECTED.

DATA Frame Payload

HEADERS The HEADERS frame (type=0x1) is used to carry a header block, compressed using QPACK. See for more details.

HEADERS Frame Payload

HEADERS frames can only be sent on request / push streams. If a HEADERS frame is received on a control stream, the recipient MUST respond with a connection error () of type H3_FRAME_UNEXPECTED.

CANCEL_PUSH The CANCEL_PUSH frame (type=0x3) is used to request cancellation of a server push prior to the push stream being received. The CANCEL_PUSH frame identifies a server push by Push ID (see ), encoded as a variable-length integer. When a client sends CANCEL_PUSH, it is indicating that it does not wish to receive the promised resource. The server SHOULD abort sending the resource, but the mechanism to do so depends on the state of the corresponding push stream. If the server has not yet created a push stream, it does not create one. If the push stream is open, the server SHOULD abruptly terminate that stream. If the push stream has already ended, the server MAY still abruptly terminate the stream or MAY take no action. When a server sends CANCEL_PUSH, it is indicating that it will not be fulfilling a promise and has not created a push stream. The client should not expect the corresponding promise to be fulfilled. Sending CANCEL_PUSH has no direct effect on the state of existing push streams. A server SHOULD NOT send a CANCEL_PUSH when it has already created a corresponding push stream, and a client SHOULD NOT send a CANCEL_PUSH when it has already received a corresponding push stream. A CANCEL_PUSH frame is sent on the control stream. Receiving a CANCEL_PUSH frame on a stream other than the control stream MUST be treated as a connection error of type H3_FRAME_UNEXPECTED.

CANCEL_PUSH Frame Payload

The CANCEL_PUSH frame carries a Push ID encoded as a variable-length integer. The Push ID identifies the server push that is being cancelled (see ). If a CANCEL_PUSH frame is received which references a Push ID greater than currently allowed on the connection, this MUST be treated as a connection error of type H3_ID_ERROR. If the client receives a CANCEL_PUSH frame, that frame might identify a Push ID that has not yet been mentioned by a PUSH_PROMISE frame due to reordering. If a server receives a CANCEL_PUSH frame for a Push ID that has not yet been mentioned by a PUSH_PROMISE frame, this MUST be treated as a connection error of type H3_ID_ERROR.

SETTINGS The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. Individually, a SETTINGS parameter can also be referred to as a "setting"; the identifier and value of each setting parameter can be referred to as a "setting identifier" and a "setting value". SETTINGS frames always apply to a connection, never a single stream. A SETTINGS frame MUST be sent as the first frame of each control stream (see ) by each peer, and MUST NOT be sent subsequently. If an endpoint receives a second SETTINGS frame on the control stream, the endpoint MUST respond with a connection error of type H3_FRAME_UNEXPECTED. SETTINGS frames MUST NOT be sent on any stream other than the control stream. If an endpoint receives a SETTINGS frame on a different stream, the endpoint MUST respond with a connection error of type H3_FRAME_UNEXPECTED. SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which can be used by the receiving peer. However, a negotiation can be implied by the use of SETTINGS - each peer uses SETTINGS to advertise a set of supported values. The definition of the setting would describe how each peer combines the two sets to conclude which choice will be used. SETTINGS does not provide a mechanism to identify when the choice takes effect. Different values for the same parameter can be advertised by each peer. For example, a client might be willing to consume a very large response header, while servers are more cautious about request size. The same setting identifier MUST NOT occur more than once in the SETTINGS frame. A receiver MAY treat the presence of duplicate setting identifiers as a connection error of type H3_SETTINGS_ERROR. The payload of a SETTINGS frame consists of zero or more parameters. Each parameter consists of a setting identifier and a value, both encoded as QUIC variable-length integers.

SETTINGS Parameter Format

An implementation MUST ignore the contents for any SETTINGS identifier it does not understand.

Defined SETTINGS Parameters The following settings are defined in HTTP/3:

SETTINGS_MAX_HEADER_LIST_SIZE (0x6):

The default value is unlimited. See for usage.

Setting identifiers of the format 0x1f * N + 0x21 for integer values of N are reserved to exercise the requirement that unknown identifiers be ignored. Such settings have no defined meaning. Endpoints SHOULD include at least one such setting in their SETTINGS frame. Endpoints MUST NOT consider such settings to have any meaning upon receipt. Because the setting has no defined meaning, the value of the setting can be any value the implementation selects. Additional settings can be defined by extensions to HTTP/3; see for more details.

Initialization An HTTP implementation MUST NOT send frames or requests which would be invalid based on its current understanding of the peer's settings. All settings begin at an initial value. Each endpoint SHOULD use these initial values to send messages before the peer's SETTINGS frame has arrived, as packets carrying the settings can be lost or delayed. When the SETTINGS frame arrives, any settings are changed to their new values. This removes the need to wait for the SETTINGS frame before sending messages. Endpoints MUST NOT require any data to be received from the peer prior to sending the SETTINGS frame; settings MUST be sent as soon as the transport is ready to send data. For servers, the initial value of each client setting is the default value. For clients using a 1-RTT QUIC connection, the initial value of each server setting is the default value. 1-RTT keys will always become available prior to SETTINGS arriving, even if the server sends SETTINGS immediately. Clients SHOULD NOT wait indefinitely for SETTINGS to arrive before sending requests, but SHOULD process received datagrams in order to increase the likelihood of processing SETTINGS before sending the first request. When a 0-RTT QUIC connection is being used, the initial value of each server setting is the value used in the previous session. Clients SHOULD store the settings the server provided in the connection where resumption information was provided, but MAY opt not to store settings in certain cases (e.g., if the session ticket is received before the SETTINGS frame). A client MUST comply with stored settings - or default values, if no values are stored - when attempting 0-RTT. Once a server has provided new settings, clients MUST comply with those values. A server can remember the settings that it advertised, or store an integrity-protected copy of the values in the ticket and recover the information when accepting 0-RTT data. A server uses the HTTP/3 settings values in determining whether to accept 0-RTT data. If the server cannot determine that the settings remembered by a client are compatible with its current settings, it MUST NOT accept 0-RTT data. Remembered settings are compatible if a client complying with those settings would not violate the server's current settings. A server MAY accept 0-RTT and subsequently provide different settings in its SETTINGS frame. If 0-RTT data is accepted by the server, its SETTINGS frame MUST NOT reduce any limits or alter any values that might be violated by the client with its 0-RTT data. The server MUST include all settings which differ from their default values. If a server accepts 0-RTT but then sends settings that are not compatible with the previously specified settings, this MUST be treated as a connection error of type H3_SETTINGS_ERROR. If a server accepts 0-RTT but then sends a SETTINGS frame that omits a setting value that the client understands (apart from reserved setting identifiers) that was previously specified to have a non-default value, this MUST be treated as a connection error of type H3_SETTINGS_ERROR.

PUSH_PROMISE The PUSH_PROMISE frame (type=0x5) is used to carry a promised request header set from server to client on a request stream, as in HTTP/2.

PUSH_PROMISE Frame Payload

The payload consists of:

Push ID:

A variable-length integer that identifies the server push operation. A Push ID is used in push stream headers (), CANCEL_PUSH frames ().

Header Block:

QPACK-compressed request header fields for the promised response. See for more details.

A server MUST NOT use a Push ID that is larger than the client has provided in a MAX_PUSH_ID frame (). A client MUST treat receipt of a PUSH_PROMISE frame that contains a larger Push ID than the client has advertised as a connection error of H3_ID_ERROR. A server MAY use the same Push ID in multiple PUSH_PROMISE frames. If so, the decompressed request header sets MUST contain the same fields in the same order, and both the name and and value in each field MUST be exact matches. Clients SHOULD compare the request header sets for resources promised multiple times. If a client receives a Push ID that has already been promised and detects a mismatch, it MUST respond with a connection error of type H3_GENERAL_PROTOCOL_ERROR. If the decompressed header sets match exactly, the client SHOULD associate the pushed content with each stream on which a PUSH_PROMISE was received. Allowing duplicate references to the same Push ID is primarily to reduce duplication caused by concurrent requests. A server SHOULD avoid reusing a Push ID over a long period. Clients are likely to consume server push responses and not retain them for reuse over time. Clients that see a PUSH_PROMISE that uses a Push ID that they have already consumed and discarded are forced to ignore the PUSH_PROMISE. If a PUSH_PROMISE frame is received on the control stream, the client MUST respond with a connection error () of type H3_FRAME_UNEXPECTED. A client MUST NOT send a PUSH_PROMISE frame. A server MUST treat the receipt of a PUSH_PROMISE frame as a connection error of type H3_FRAME_UNEXPECTED. See for a description of the overall server push mechanism.

GOAWAY The GOAWAY frame (type=0x7) is used to initiate graceful shutdown of a connection by a server. GOAWAY allows a server to stop accepting new requests while still finishing processing of previously received requests. This enables administrative actions, like server maintenance. GOAWAY by itself does not close a connection.

GOAWAY Frame Payload

The GOAWAY frame is always sent on the control stream. It carries a QUIC Stream ID for a client-initiated bidirectional stream encoded as a variable-length integer. A client MUST treat receipt of a GOAWAY frame containing a Stream ID of any other type as a connection error of type H3_ID_ERROR. Clients do not need to send GOAWAY to initiate a graceful shutdown; they simply stop making new requests. A server MUST treat receipt of a GOAWAY frame on any stream as a connection error () of type H3_FRAME_UNEXPECTED. The GOAWAY frame applies to the connection, not a specific stream. A client MUST treat a GOAWAY frame on a stream other than the control stream as a connection error () of type H3_FRAME_UNEXPECTED. See for more information on the use of the GOAWAY frame.

MAX_PUSH_ID The MAX_PUSH_ID frame (type=0xD) is used by clients to control the number of server pushes that the server can initiate. This sets the maximum value for a Push ID that the server can use in PUSH_PROMISE and CANCEL_PUSH frames. Consequently, this also limits the number of push streams that the server can initiate in addition to the limit maintained by the QUIC transport. The MAX_PUSH_ID frame is always sent on the control stream. Receipt of a MAX_PUSH_ID frame on any other stream MUST be treated as a connection error of type H3_FRAME_UNEXPECTED. A server MUST NOT send a MAX_PUSH_ID frame. A client MUST treat the receipt of a MAX_PUSH_ID frame as a connection error of type H3_FRAME_UNEXPECTED. The maximum Push ID is unset when a connection is created, meaning that a server cannot push until it receives a MAX_PUSH_ID frame. A client that wishes to manage the number of promised server pushes can increase the maximum Push ID by sending MAX_PUSH_ID frames as the server fulfills or cancels server pushes.

MAX_PUSH_ID Frame Payload

The MAX_PUSH_ID frame carries a single variable-length integer that identifies the maximum value for a Push ID that the server can use (see ). A MAX_PUSH_ID frame cannot reduce the maximum Push ID; receipt of a MAX_PUSH_ID that contains a smaller value than previously received MUST be treated as a connection error of type H3_ID_ERROR.

Reserved Frame Types Frame types of the format 0x1f * N + 0x21 for integer values of N are reserved to exercise the requirement that unknown types be ignored (). These frames have no semantics, and can be sent on any open stream when application-layer padding is desired. They MAY also be sent on connections where no data is currently being transferred. Endpoints MUST NOT consider these frames to have any meaning upon receipt. The payload and length of the frames are selected in any manner the implementation chooses. Frame types which were used in HTTP/2 where there is no corresponding HTTP/3 frame have also been reserved (). These frame types MUST NOT be sent, and receipt MAY be treated as an error of type H3_FRAME_UNEXPECTED.

Error Handling QUIC allows the application to abruptly terminate (reset) individual streams or the entire connection when an error is encountered. These are referred to as "stream errors" or "connection errors" and are described in more detail in . An endpoint MAY choose to treat a stream error as a connection error under certain circumstances. Implementations need to consider the impact on outstanding requests before making this choice. Because new error codes can be defined without negotiation (see ), use of an error code in an unexpected context or receipt of an unknown error code MUST be treated as equivalent to H3_NO_ERROR. However, closing a stream can have other effects regardless of the error code (see ). This section describes HTTP/3-specific error codes which can be used to express the cause of a connection or stream error.

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, but there is no error to signal.

H3_GENERAL_PROTOCOL_ERROR (0x101):

Peer violated protocol requirements in a way which doesn't match a more specific 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 the current stream.

H3_FRAME_ERROR (0x106):

A frame that fails to satisfy layout requirements or with an invalid size was received.

H3_EXCESSIVE_LOAD (0x107):

The endpoint detected that its peer is exhibiting a behavior that might be generating 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 or abnormally closed.

H3_VERSION_FALLBACK (0x110):

The requested operation cannot be served over HTTP/3. The peer should retry over HTTP/1.1.

Error codes of the format 0x1f * N + 0x21 for integer values of N are reserved to exercise the requirement that unknown error codes be treated as equivalent to H3_NO_ERROR (). Implementations SHOULD select an error code from this space with some probability when they would have sent H3_NO_ERROR.

Extensions to HTTP/3 HTTP/3 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/3 connection. This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or header fields. Extensions are permitted to use new frame types (), new settings (), new error codes (), or new unidirectional stream types (). Registries are established for managing these extension points: frame types (), settings (), error codes (), and stream types (). Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames and unidirectional streams that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation. However, where a known frame type is required to be in a specific location, such as the SETTINGS frame as the first frame of the control stream (see ), an unknown frame type does not satisfy that requirement and SHOULD be treated as an error. Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. Coordinating when such a revised layout comes into effect could prove complex. As such, allocating new identifiers for new definitions of existing protocol elements is likely to be more effective. This document doesn't mandate a specific method for negotiating the use of an extension but notes that a setting () could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the default value MUST be defined in such a fashion that the extension is disabled if the setting is omitted.

Security Considerations The security considerations of HTTP/3 should be comparable to those of HTTP/2 with TLS; the considerations from Section 10 of apply in addition to those listed here. When HTTP Alternative Services is used for discovery for HTTP/3 endpoints, the security considerations of also apply.

Traffic Analysis Where HTTP/2 employs PADDING frames and Padding fields in other frames to make a connection more resistant to traffic analysis, HTTP/3 can either rely on transport-layer padding or employ the reserved frame and stream types discussed in and . These methods of padding produce different results in terms of the granularity of padding, how padding is arranged in relation to the information that is being protected, whether padding is applied in the case of packet loss, and how an implementation might control padding.

Frame Parsing Several protocol elements contain nested length elements, typically in the form of frames with an explicit length containing variable-length integers. This could pose a security risk to an incautious implementer. An implementation MUST ensure that the length of a frame exactly matches the length of the fields it contains.

Early Data The use of 0-RTT with HTTP/3 creates an exposure to replay attack. The anti-replay mitigations in MUST be applied when using HTTP/3 with 0-RTT.

Migration Certain HTTP implementations use the client address for logging or access-control purposes. Since a QUIC client's address might change during a connection (and future versions might support simultaneous use of multiple addresses), such implementations will need to either actively retrieve the client's current address or addresses when they are relevant or explicitly accept that the original address might change.

IANA Considerations This document registers a new ALPN protocol ID () and creates new registries that manage the assignment of codepoints in HTTP/3.

Registration of HTTP/3 Identification String This document creates a new registration for the identification of HTTP/3 in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in . The "h3" string identifies HTTP/3:

Protocol:

HTTP/3

Identification Sequence:

0x68 0x33 ("h3")

Specification:

This document

New Registries New registries created in this document operate under the QUIC registration policy documented in Section 22.1 of . These registries all include the common set of fields listed in Section 22.1.1 of . The initial allocations in these registries created in this document are all assigned permanent status and list as contact both the IESG (iesg@ietf.org) and the HTTP working group (ietf-http-wg@w3.org).

Frame Types This document establishes a registry for HTTP/3 frame type codes. The "HTTP/3 Frame Type" registry governs a 62-bit space. This registry follows the QUIC registry policy; see . Permanent registrations in this registry are assigned using the Specification Required policy , except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of . While this registry is separate from the "HTTP/2 Frame Type" registry defined in , it is preferable that the assignments parallel each other where the code spaces overlap. If an entry is present in only one registry, every effort SHOULD be made to avoid assigning the corresponding value to an unrelated operation. In addition to common fields as described in , permanent registrations in this registry MUST include the following field:

Frame Type:

A name or label for the frame type.

Specifications of frame types MUST include a description of the frame layout and its semantics, including any parts of the frame that are conditionally present. The entries in are registered by this document. Initial HTTP/3 Frame Types

Frame Type	Value	Specification
DATA	0x0
HEADERS	0x1
Reserved	0x2	N/A
CANCEL_PUSH	0x3
SETTINGS	0x4
PUSH_PROMISE	0x5
Reserved	0x6	N/A
GOAWAY	0x7
Reserved	0x8	N/A
Reserved	0x9	N/A
MAX_PUSH_ID	0xD

Additionally, each code of the format 0x1f * N + 0x21 for integer values of N (that is, 0x21, 0x40, ..., through 0x3FFFFFFFFFFFFFFE) MUST NOT be assigned by IANA.

Settings Parameters This document establishes a registry for HTTP/3 settings. The "HTTP/3 Settings" registry governs a 62-bit space. This registry follows the QUIC registry policy; see . Permanent registrations in this registry are assigned using the Specification Required policy , except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of . While this registry is separate from the "HTTP/2 Settings" registry defined in , it is preferable that the assignments parallel each other. If an entry is present in only one registry, every effort SHOULD be made to avoid assigning the corresponding value to an unrelated operation. In addition to common fields as described in , permanent registrations in this registry MUST include the following fields:

Setting Name:

A symbolic name for the setting. Specifying a setting name is optional.

Default:

The value of the setting unless otherwise indicated. A default SHOULD be the most restrictive possible value.

The entries in are registered by this document. Initial HTTP/3 Settings

Setting Name	Value	Specification	Default
Reserved	0x2	N/A	N/A
Reserved	0x3	N/A	N/A
Reserved	0x4	N/A	N/A
Reserved	0x5	N/A	N/A
MAX_HEADER_LIST_SIZE	0x6		Unlimited

Additionally, each code of the format 0x1f * N + 0x21 for integer values of N (that is, 0x21, 0x40, ..., through 0x3FFFFFFFFFFFFFFE) MUST NOT be assigned by IANA.

Error Codes This document establishes a registry for HTTP/3 error codes. The "HTTP/3 Error Code" registry manages a 62-bit space. This registry follows the QUIC registry policy; see . Permanent registrations in this registry are assigned using the Specification Required policy , except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of . Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated. In addition to common fields as described in , permanent registrations in this registry MUST include the following fields:

Name:

A name for the error code. Specifying an error code name is optional.

Description:

A brief description of the error code semantics.

The entries in the are registered by this document. Initial HTTP/3 Error Codes

Name	Value	Description	Specification
H3_NO_ERROR	0x0100	No error
H3_GENERAL_PROTOCOL_ERROR	0x0101	General protocol error
H3_INTERNAL_ERROR	0x0102	Internal error
H3_STREAM_CREATION_ERROR	0x0103	Stream creation error
H3_CLOSED_CRITICAL_STREAM	0x0104	Critical stream was closed
H3_FRAME_UNEXPECTED	0x0105	Frame not permitted in the current state
H3_FRAME_ERROR	0x0106	Frame violated layout or size rules
H3_EXCESSIVE_LOAD	0x0107	Peer generating excessive load
H3_ID_ERROR	0x0108	An identifier was used incorrectly
H3_SETTINGS_ERROR	0x0109	SETTINGS frame contained invalid values
H3_MISSING_SETTINGS	0x010A	No SETTINGS frame received
H3_REQUEST_REJECTED	0x010B	Request not processed
H3_REQUEST_CANCELLED	0x010C	Data no longer needed
H3_REQUEST_INCOMPLETE	0x010D	Stream terminated early
H3_CONNECT_ERROR	0x010F	TCP reset or error on CONNECT request
H3_VERSION_FALLBACK	0x0110	Retry over HTTP/1.1

Additionally, each code of the format 0x1f * N + 0x21 for integer values of N (that is, 0x21, 0x40, ..., through 0x3FFFFFFFFFFFFFFE) MUST NOT be assigned by IANA.

Stream Types This document establishes a registry for HTTP/3 unidirectional stream types. The "HTTP/3 Stream Type" registry governs a 62-bit space. This registry follows the QUIC registry policy; see . Permanent registrations in this registry are assigned using the Specification Required policy , except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of . In addition to common fields as described in , permanent registrations in this registry MUST include the following fields:

Stream Type:

A name or label for the stream type.

Sender:

Which endpoint on a connection may initiate a stream of this type. Values are "Client", "Server", or "Both".

Specifications for permanent registrations MUST include a description of the stream type, including the layout semantics of the stream contents. The entries in the following table are registered by this document.

Stream Type	Value	Specification	Sender
Control Stream	0x00		Both
Push Stream	0x01		Server

Additionally, each code of the format 0x1f * N + 0x21 for integer values of N (that is, 0x21, 0x40, ..., through 0x3FFFFFFFFFFFFFFE) MUST NOT be assigned by IANA.

References Normative References Fastly Mozilla Google, Inc Akamai Technologies Facebook This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients. This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. This document specifies "Alternative Services" for HTTP, which allow an origin's resources to be authoritatively available at a separate network location, possibly accessed with a different protocol configuration. This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] This document specifies "Alternative Services" for HTTP, which allow an origin's resources to be authoritatively available at a separate network location, possibly accessed with a different protocol configuration. This document describes how "http" URIs can be accessed using Transport Layer Security (TLS) and HTTP/2 to mitigate pervasive monitoring attacks. This mechanism not a replacement for "https" URIs; it is vulnerable to active attacks. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK] Using TLS early data creates an exposure to the possibility of a replay attack. This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Informative References This document describes an experimental TCP mechanism called TCP Fast Open (TFO). TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, and saves up to one full round-trip time (RTT) compared to the standard TCP, which requires a three-way handshake (3WHS) to complete before data can be exchanged. However, TFO deviates from the standard TCP semantics, since the data in the SYN could be replayed to an application in some rare circumstances. Applications should not use TFO unless they can tolerate this issue, as detailed in the Applicability section. This specification defines HPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/2. This document specifies additional HyperText Transfer Protocol (HTTP) status codes for a variety of common situations. [STANDARDS-TRACK] This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection.

Considerations for Transitioning from HTTP/2 HTTP/3 is strongly informed by HTTP/2, and bears many similarities. This section describes the approach taken to design HTTP/3, points out important differences from HTTP/2, and describes how to map HTTP/2 extensions into HTTP/3. HTTP/3 begins from the premise that similarity to HTTP/2 is preferable, but not a hard requirement. HTTP/3 departs from HTTP/2 where QUIC differs from TCP, either to take advantage of QUIC features (like streams) or to accommodate important shortcomings (such as a lack of total ordering). These differences make HTTP/3 similar to HTTP/2 in key aspects, such as the relationship of requests and responses to streams. However, the details of the HTTP/3 design are substantially different than HTTP/2. These departures are noted in this section.

Streams HTTP/3 permits use of a larger number of streams (2^62-1) than HTTP/2. The considerations about exhaustion of stream identifier space apply, though the space is significantly larger such that it is likely that other limits in QUIC are 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. QUIC considers a stream closed when all data has been received and sent data has been acknowledged by the peer. HTTP/2 considers a stream closed when the frame containing 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 period of time. HTTP/3 servers might choose to permit a larger number of concurrent client-initiated bidirectional streams to achieve equivalent concurrency to HTTP/2, depending on the expected usage patterns. Due to the presence of other unidirectional stream types, HTTP/3 does not rely exclusively on the number of concurrent unidirectional streams to control the number of concurrent in-flight pushes. Instead, HTTP/3 clients use the MAX_PUSH_ID frame to control the number of pushes received from an HTTP/3 server.

HTTP Frame Types Many framing concepts from HTTP/2 can be elided on QUIC, because the transport deals with them. Because frames are already on a stream, they can omit the stream number. Because frames do not block multiplexing (QUIC's multiplexing occurs below this layer), the support for variable-maximum-length packets can be removed. Because stream termination is handled by QUIC, an END_STREAM flag is not required. This permits the removal of the Flags field from the generic frame layout. Frame payloads are largely drawn from . However, QUIC includes many features (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 frame types are not required in HTTP/3. Where an HTTP/2-defined frame is no longer used, the frame ID has been reserved in order to maximize portability between HTTP/2 and HTTP/3 implementations. However, even equivalent frames between the two mappings are not identical. Many of the differences arise from the fact that HTTP/2 provides an absolute ordering between frames across all streams, while QUIC provides this guarantee on each stream only. As a result, if a frame type makes assumptions that frames from different streams will still be received in the order sent, HTTP/3 will break them. Some examples of feature adaptations are described below, as well as general guidance to extension frame implementors converting an HTTP/2 extension to HTTP/3.

Prioritization Differences HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in HEADERS frames. HTTP/3 does not provide a means of signaling priority. Note that while there is no explicit signaling for priority, this does not mean that prioritization is not important for achieving good performance.

Header Compression Differences HPACK was designed with the assumption of in-order delivery. A sequence of encoded header blocks must arrive (and be decoded) at an endpoint in the same order in which they were encoded. This ensures that the dynamic state at the two endpoints remains in sync. Because this total ordering is not provided by QUIC, HTTP/3 uses a modified version of HPACK, called QPACK. QPACK uses a single unidirectional stream to make all modifications to the dynamic table, ensuring a total order of updates. All frames which contain encoded headers merely reference the table state at a given time without modifying it. provides additional details.

Guidance for New Frame Type Definitions Frame type definitions in HTTP/3 often use the QUIC variable-length integer encoding. In particular, Stream IDs use this encoding, which allows for a larger range of possible values than the encoding used in HTTP/2. Some frames in HTTP/3 use an identifier rather than a Stream ID (e.g., Push IDs). Redefinition of the encoding of extension frame types might be necessary if the encoding includes a Stream ID. Because the Flags field is not present in generic HTTP/3 frames, those frames which depend on the presence of flags need to allocate space for flags as part of their frame payload. Other than this issue, frame type HTTP/2 extensions are typically portable to QUIC simply by replacing Stream 0 in HTTP/2 with a control stream in HTTP/3. HTTP/3 extensions will not assume ordering, but would not be harmed by ordering, and would be portable to HTTP/2 in the same manner.

Mapping Between HTTP/2 and HTTP/3 Frame Types

DATA (0x0):

Padding is not defined in HTTP/3 frames. See .

HEADERS (0x1):

The PRIORITY region of HEADERS is not defined in HTTP/3 frames. Padding is not defined in HTTP/3 frames. See .

PRIORITY (0x2):

As described in , HTTP/3 does not provide a means of signaling priority.

RST_STREAM (0x3):

RST_STREAM frames do not exist, since QUIC provides stream lifecycle management. The same code point is used for the CANCEL_PUSH frame ().

SETTINGS (0x4):

SETTINGS frames are sent only at the beginning of the connection. See and .

PUSH_PROMISE (0x5):

The PUSH_PROMISE does not reference a stream; instead the push stream references the PUSH_PROMISE frame using a Push ID. See .

PING (0x6):

PING frames do not exist, since QUIC provides equivalent functionality.

GOAWAY (0x7):

GOAWAY is sent only from server to client and does not contain an error code. See .

WINDOW_UPDATE (0x8):

WINDOW_UPDATE frames do not exist, since QUIC provides flow control.

CONTINUATION (0x9):

CONTINUATION frames do not exist; instead, larger HEADERS/PUSH_PROMISE frames than HTTP/2 are permitted.

Frame types defined by extensions to HTTP/2 need to be separately registered for HTTP/3 if still applicable. The IDs of frames defined in have been reserved for simplicity. Note that the frame type space in HTTP/3 is substantially larger (62 bits versus 8 bits), so many HTTP/3 frame types have no equivalent HTTP/2 code points. See .

HTTP/2 SETTINGS Parameters An important difference from HTTP/2 is that settings are sent once, as the first frame of the control stream, and thereafter cannot change. This eliminates many corner cases around synchronization of changes. Some transport-level options that HTTP/2 specifies via the SETTINGS frame are superseded by QUIC transport parameters in HTTP/3. The HTTP-level options that are 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 .

SETTINGS_ENABLE_PUSH:

This is removed in favor of the MAX_PUSH_ID which provides a more granular control 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 be specified in the initial transport handshake. Specifying SETTINGS_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 frame is an error.

SETTINGS_MAX_HEADER_LIST_SIZE:

See .

In HTTP/3, setting values are variable-length integers (6, 14, 30, or 62 bits long) rather than fixed-length 32-bit fields as in HTTP/2. This will often produce a shorter encoding, but can produce a longer encoding for settings which use the full 32-bit space. Settings ported from HTTP/2 might choose to redefine their value to limit it to 30 bits for more efficient encoding, or to make use of 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 of settings defined in have been reserved for simplicity. Note that the settings identifier space in HTTP/3 is substantially larger (62 bits versus 16 bits), so many HTTP/3 settings have no equivalent HTTP/2 code point. See . As QUIC streams might arrive out-of-order, endpoints are advised to not wait for the peers' settings to arrive before responding to other streams. See .

HTTP/2 Error Codes QUIC has the same concepts of "stream" and "connection" errors that HTTP/2 provides. However, there is no direct portability of HTTP/2 error codes to HTTP/3 error codes; the values are shifted in order to prevent accidental or implicit conversion. The HTTP/2 error codes defined in Section 7 of logically map to the HTTP/3 error codes as follows:

NO_ERROR (0x0):

H3_NO_ERROR in .

PROTOCOL_ERROR (0x1):

This is mapped to H3_GENERAL_PROTOCOL_ERROR except in cases where more specific error codes have been defined. This includes H3_FRAME_UNEXPECTED and H3_CLOSED_CRITICAL_STREAM defined in .

INTERNAL_ERROR (0x2):

H3_INTERNAL_ERROR in .

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 in .

REFUSED_STREAM (0x7):

H3_REQUEST_REJECTED (in ) is used to indicate that a request was not processed. Otherwise, not applicable because QUIC handles stream management.

CANCEL (0x8):

H3_REQUEST_CANCELLED in .

COMPRESSION_ERROR (0x9):

Multiple error codes are defined in .

CONNECT_ERROR (0xa):

H3_CONNECT_ERROR in .

ENHANCE_YOUR_CALM (0xb):

H3_EXCESSIVE_LOAD in .

INADEQUATE_SECURITY (0xc):

Not applicable, since QUIC is assumed to provide sufficient security on all connections.

H3_1_1_REQUIRED (0xd):

H3_VERSION_FALLBACK in .

Error codes need to be defined for HTTP/2 and HTTP/3 separately. See .

Change Log

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

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)

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)

Since draft-ietf-quic-http-23

• Removed quic 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)

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 the http-opportunistic resource (RFC 8164) when scheme is http (#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 SETTINGS frame, even when resuming
  • If a client doesn't have settings associated with a 0-RTT ticket, it uses the defaults
  • Servers can't accept early data if they cannot recover the settings the client 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, but frame types reserved due to HTTP/2 correspondence are prohibited (#2997,#2692,#2693)
• Unknown error codes cannot be treated as errors (#2998,#2816)

Since draft-ietf-quic-http-21 No changes

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)

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 an error (#2501)

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)

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)

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)

Since draft-ietf-quic-http-15 Substantial editorial reorganization; no technical changes.

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 have separate maximum values (#1820)
• Expanded discussion of connection closure (#1599,#1717,#1712)
• HTTP_VERSION_FALLBACK falls back to HTTP/1.1 (#1677,#1685)

Since draft-ietf-quic-http-13

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

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 persistent structure in the tree (#441,#1421,#1422)

Since draft-ietf-quic-http-11

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

Since draft-ietf-quic-http-10

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

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)

Since draft-ietf-quic-http-08

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

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)

Since draft-ietf-quic-http-06

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

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)

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)

Since draft-ietf-quic-http-03 None.

Since draft-ietf-quic-http-02

• Track changes in transport draft

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 a fatal 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)

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 and two-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)

Since draft-shade-quic-http2-mapping-00

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

Acknowledgements The original authors of this specification were Robbie Shade and Mike Warres. A substantial portion of Mike's contribution was supported by Microsoft during his employment there.