Hypertext Transfer Protocol Version 3 (HTTP/3)

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.¶

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 [RFC7413].¶

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

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

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

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

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

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

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

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 [QUIC-TRANSPORT]. Where frames from [QUIC-TRANSPORT] 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 Section 7.2.

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 [RFC7230].¶

Finally, the terms “gateway”, “intermediary”, “proxy”, and “tunnel” are defined in Section 2.3 of [RFC7230]. Intermediaries act as both client and server at different times.¶

• 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”.¶

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 [RFC7230]. Experimenters are encouraged to coordinate their experiments on the quic@ietf.org mailing list.¶

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 ([ALTSVC]), using the ALPN token defined in Section 3.3.¶

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.¶

HTTP/3 relies on QUIC as the underlying transport. The QUIC version being used MUST use TLS version 1.3 or greater as its handshake protocol. HTTP/3 clients MUST indicate the target domain name during the TLS handshake. This may be done using the Server Name Indication (SNI) [RFC6066] extension to TLS or using some other mechanism.¶

QUIC connections are established as described in [QUIC-TRANSPORT]. 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 (Section 7.2.4) MUST be sent by each endpoint as the initial frame of their respective HTTP control stream (see Section 6.2.1).¶

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 [RFC7838], 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 [RFC8164] 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 [HTTP2]).¶

The considerations discussed in Section 9.1 of [HTTP2] also apply to the management of HTTP/3 connections.¶

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.¶

1. the message header (see Section 3.2 of [RFC7230]), sent as a single HEADERS frame (see Section 7.2.2),
2. optionally, the payload body, if present (see Section 3.3 of [RFC7230]), sent as a series of DATA frames (see Section 7.2.1),
3. optionally, trailing headers, if present (see Section 4.1.2 of [RFC7230]), sent as a single HEADERS frame.

A server MAY send one or more PUSH_PROMISE (see Section 7.2.5) or DUPLICATE_PUSH (see Section 7.2.8) frames before, after, or interleaved with the frames of a response message. These PUSH_PROMISE and DUPLICATE_PUSH frames are not part of the response; see Section 4.4 for more details.¶

Frames of unknown types (Section 9), including reserved frames (Section 7.2.9) 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 Section 4.1.1 for more details.¶

The “chunked” transfer encoding defined in Section 4.1 of [RFC7230] 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 [RFC7231]) 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 (Section 8). 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 bidirectional QUIC stream. After sending a request, a client MUST close the stream for sending. Unless using the CONNECT method (see Section 4.2), 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.¶

The pseudo-method CONNECT (Section 4.3.6 of [RFC7231]) 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 [HTTP2]. A CONNECT request that does not conform to these restrictions is malformed (see Section 4.1.3). The request stream MUST NOT be closed at the end of the request.¶

A proxy that supports CONNECT establishes a TCP connection ([RFC0793]) 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 [RFC7231].¶

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 (Section 8.1). 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/3 does not support the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]) or 101 (Switching Protocols) informational status code (Section 6.2.2 of [RFC7231]).¶

Server push is an interaction mode introduced in HTTP/2 [HTTP2] 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 [HTTP2], but uses different mechanisms.¶

Each server push is identified by a unique Push ID. This Push ID is used in a single PUSH_PROMISE frame (see Section 7.2.5) which carries the request headers, possibly included in one or more DUPLICATE_PUSH frames (see Section 7.2.8), then included with the push stream which ultimately fulfills those promises.¶

Server push is only enabled on a connection when a client sends a MAX_PUSH_ID frame (see Section 7.2.7). 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 Section 7.2.5) on the request stream which generated the push. Promised requests MUST conform to the requirements in Section 8.2 of [HTTP2].¶

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 DUPLICATE_PUSH frame (see Section 7.2.8). These associations do not affect the operation of the protocol, but MAY be used by user agents when deciding how to use pushed resources.¶

Ordering of a PUSH_PROMISE or DUPLICATE_PUSH in relation to certain parts of the response is important. The server SHOULD send PUSH_PROMISE or DUPLICATE_PUSH 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 Section 6.2.2). 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 Section 4.1.¶

Due to reordering, DUPLICATE_PUSH frames or push stream data can arrive before the corresponding PUSH_PROMISE frame. When a client receives a DUPLICATE_PUSH frame for an as-yet-unknown Push ID, the request headers of the push are not immediately available. The client can either delay generating new requests for content referenced following the DUPLICATE_PUSH frame until the request headers become available, or can initiate requests for discovered resources and cancel the requests if the requested resource is already being pushed. 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 [QUIC-TRANSPORT]) 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.¶

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 [QUIC-TRANSPORT]. 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.¶

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 (Section 7.2.6). 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 Section 4.1.2) 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.¶

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 Section 8 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.¶

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.¶

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, 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.¶

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

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 (Section 8) of type H3_FRAME_ERROR. Streams which terminate abruptly may be reset at any point in a frame.¶

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 (Section 9). Implementations SHOULD select an error code from this space with some probability when they would have sent H3_NO_ERROR.¶

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 Section 7.2.9 and Section 6.2.3. These methods of padding produce different results in terms of the granularity of padding, the effect of packet loss and recovery, and how an implementation might control padding.¶

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.¶

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

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.¶

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

Protocol:

HTTP/3

Identification Sequence:

0x68 0x33 (“h3”)

Specification:

This document

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

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

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.¶

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 [HTTP2]. 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.¶

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.¶

SETTINGS_HEADER_TABLE_SIZE:

See [QPACK].

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 Section 7.2.4.1.

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 [HTTP2] 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 Section 11.2.2.¶

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 Section 7.2.4.2.¶

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.¶

NO_ERROR (0x0):

H3_NO_ERROR in Section 8.1.

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 Section 8.1.

INTERNAL_ERROR (0x2):

H3_INTERNAL_ERROR in Section 8.1.

FLOW_CONTROL_ERROR (0x3):

Not applicable, since QUIC handles flow control.

SETTINGS_TIMEOUT (0x4):

Not applicable, since no acknowledgement of SETTINGS is defined.

STREAM_CLOSED (0x5):

Not applicable, since QUIC handles stream management.

FRAME_SIZE_ERROR (0x6):

H3_FRAME_ERROR error code defined in Section 8.1.

REFUSED_STREAM (0x7):

H3_REQUEST_REJECTED (in Section 8.1) is used to indicate that a request was not processed. Otherwise, not applicable because QUIC handles stream management.

CANCEL (0x8):

H3_REQUEST_CANCELLED in Section 8.1.

COMPRESSION_ERROR (0x9):

Multiple error codes are defined in [QPACK].

CONNECT_ERROR (0xa):

H3_CONNECT_ERROR in Section 8.1.

ENHANCE_YOUR_CALM (0xb):

H3_EXCESSIVE_LOAD in Section 8.1.

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 Section 8.1.

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

• 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)

• 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)

• 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)

No changes¶

• 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)

• 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)

• 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)

• 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)

• 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)

Substantial editorial reorganization; no technical changes.¶

• 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)

• 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)

• 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)

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

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

• 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)

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

• 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)

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

• 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)

• 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)

None.¶

• Track changes in transport draft

• 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)

• 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)

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