Hypertext Transfer Protocol Version 3 (HTTP/3)

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].¶

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 4.2.¶

• 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 2.3.¶

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 4.2.5) MUST be sent by each endpoint as the initial frame of their respective HTTP control stream (see Section 3.2.1).¶

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.¶

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 [RFC7540]).¶

The considerations discussed in Section 9.1 of [RFC7540] also apply to the management of HTTP/3 connections.¶

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 client SHOULD send non-zero values for the QUIC transport parameters initial_max_stream_data_bidi_local. An HTTP/3 server SHOULD send non-zero values for the QUIC transport parameters initial_max_stream_data_bidi_remote and initial_max_bidi_streams. It is recommended that initial_max_bidi_streams be no smaller than 100, so as to not unnecessarily limit parallelism.¶

These streams carry frames related to the request/response (see Section 5.1). When a stream terminates cleanly, if the last frame on the stream was truncated, this MUST be treated as a connection error (see HTTP_MALFORMED_FRAME in Section 8.1). Streams which terminate abruptly may be reset at any point in the frame.¶

HTTP/3 does not use server-initiated bidirectional streams; clients MUST omit or specify a value of zero for the QUIC transport parameter initial_max_bidi_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.¶

Some stream types are reserved (Section 3.2.3). Two stream types are defined in this document: control streams (Section 3.2.1) and push streams (Section 3.2.2). Other stream types can be defined by extensions to HTTP/3; see Section 7 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 set low values for the QUIC transport parameters initial_max_uni_streams and initial_max_stream_data_uni will increase the chance that the remote peer reaches the limit early and becomes blocked. In particular, the value chosen for initial_max_uni_streams should consider that remote peers may wish to exercise reserved stream behaviour (Section 3.2.3). To reduce the likelihood of blocking, both clients and servers SHOULD send a value of three or greater for the QUIC transport parameter initial_max_uni_streams, and a value of 1,024 or greater for the QUIC transport parameter initial_max_stream_data_uni.¶

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 trigger a QUIC STOP_SENDING frame with an error code of HTTP_UNKNOWN_STREAM_TYPE, but MUST NOT consider such streams to be an 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 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 HTTP_MALFORMED_FRAME.¶

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 one or more HTTP responses on the same stream as the request, as detailed below.¶

1. the message header (see [RFC7230], Section 3.2), sent as a single HEADERS frame (see Section 4.2.2),
2. the payload body (see [RFC7230], Section 3.3), sent as a series of DATA frames (see Section 4.2.1),
3. optionally, one HEADERS frame containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

A server MAY interleave one or more PUSH_PROMISE frames (see Section 4.2.6) with the frames of a response message. These PUSH_PROMISE frames are not part of the response; see Section 5.4 for more details.¶

The “chunked” transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used.¶

Trailing header fields are carried in an additional HEADERS frame following the body. Senders MUST send only one HEADERS frame in the trailers section; receivers MUST discard any subsequent HEADERS frames.¶

A response MAY consist of multiple messages when and only when one or more informational responses (1xx, see [RFC7231], Section 6.2) precede a final response to the same request. Non-final responses do not contain a payload body or trailers.¶

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 5.2), 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 HTTP_INCOMPLETE_REQUEST.¶

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 this is true, a server MAY request that the client abort transmission of a request without error by triggering a QUIC STOP_SENDING frame with error code HTTP_EARLY_RESPONSE, sending a complete response, and cleanly closing its 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.¶

The pseudo-method CONNECT ([RFC7231], Section 4.3.6) 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 [RFC7540], Section 8.3. A CONNECT request that does not conform to these restrictions is malformed. 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 [RFC7231], Section 4.3.6.¶

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.¶

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 with QUIC RESET_STREAM frame. 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 HTTP_CONNECT_ERROR (Section 8.1). Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the QUIC connection.¶

• Requests, identified by the ID of the request stream
• Pushes, identified by the Push ID of the promised resource (Section 4.2.6)
• Placeholders, identified by a Placeholder ID

Taken together, the dependencies across all prioritized elements in a connection form a dependency tree. An element can depend on another element or on the root of the tree. A reference to an element which is no longer in the tree is treated as a reference to the root of the tree. The structure of the dependency tree changes as PRIORITY frames modify the dependency links between prioritized elements.¶

Due to reordering between streams, an element can also be prioritized which is not yet in the tree. Such elements are added to the tree with the requested priority.¶

When a prioritized element is first created, it has a default initial weight of 16 and a default dependency. Requests and placeholders are dependent on the root of the priority tree; pushes are dependent on the client request on which the PUSH_PROMISE frame was sent.¶

Requests may override the default initial values by including a PRIORTIY frame (see Section 4.2.3) at the beginning of the stream. These priorities can be updated by sending a PRIORITY frame on the control stream.¶

HTTP/3 server push is similar to what is described in HTTP/2 [RFC7540], 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 4.2.6) which carries the request headers, possibly included in one or more DUPLICATE_PUSH frames (see Section 4.2.9), 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 4.2.8). 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 HTTP_LIMIT_EXCEEDED.¶

The header of the request message is carried by a PUSH_PROMISE frame (see Section 4.2.6) on the request stream which generated the push. This allows the server push to be associated with a client request. Ordering of a PUSH_PROMISE in relation to certain parts of the response is important (see Section 8.2.1 of [RFC7540]). Promised requests MUST conform to the requirements in Section 8.2 of [RFC7540].¶

The same server push can be associated with additional client requests using a DUPLICATE_PUSH frame (see Section 4.2.9). Ordering of a DUPLICATE_PUSH in relation to certain parts of the response is similarly important.¶

When a server later fulfills a promise, the server push response is conveyed on a push stream (see Section 3.2.2). 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 5.1.¶

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, a QUIC STOP_SENDING frame with an appropriate error code can also be used (e.g., HTTP_PUSH_REFUSED, HTTP_PUSH_ALREADY_IN_CACHE; see Section 8). 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 4.2.7). 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 lower than the stream limit identified by a QUIC MAX_STREAM_ID frame, and MAY be zero if no requests were processed. Servers SHOULD NOT increase the QUIC MAX_STREAM_ID limit 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 5.1.2) 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 SHOULD send an initial GOAWAY frame with the last Stream ID set to the current value of QUIC’s MAX_STREAM_ID and SHOULD NOT increase the MAX_STREAM_ID thereafter. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight requests (at least one round-trip time), the server MAY send another GOAWAY frame with an updated last Stream ID. This ensures that a connection can be cleanly shut down without losing requests.¶

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 HTTP_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.¶

The following error codes are defined for use in QUIC RESET_STREAM frames, STOP_SENDING frames, and CONNECTION_CLOSE frames when using HTTP/3.¶

HTTP_NO_ERROR (0x00):

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

HTTP_WRONG_SETTING_DIRECTION (0x01):

A client-only setting was sent by a server, or a server-only setting by a client.

HTTP_PUSH_REFUSED (0x02):

The server has attempted to push content which the client will not accept on this connection.

HTTP_INTERNAL_ERROR (0x03):

An internal error has occurred in the HTTP stack.

HTTP_PUSH_ALREADY_IN_CACHE (0x04):

The server has attempted to push content which the client has cached.

HTTP_REQUEST_CANCELLED (0x05):

The request or its response is cancelled.

HTTP_INCOMPLETE_REQUEST (0x06):

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

HTTP_CONNECT_ERROR (0x07):

The connection established in response to a CONNECT request was reset or abnormally closed.

HTTP_EXCESSIVE_LOAD (0x08):

The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load.

HTTP_VERSION_FALLBACK (0x09):

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

HTTP_WRONG_STREAM (0x0A):

A frame was received on a stream where it is not permitted.

HTTP_LIMIT_EXCEEDED (0x0B):

A Stream ID, Push ID, or Placeholder ID greater than the current maximum for that identifier was referenced.

HTTP_DUPLICATE_PUSH (0x0C):

A Push ID was referenced in two different stream headers.

HTTP_UNKNOWN_STREAM_TYPE (0x0D):

A unidirectional stream header contained an unknown stream type.

HTTP_WRONG_STREAM_COUNT (0x0E):

A unidirectional stream type was used more times than is permitted by that type.

HTTP_CLOSED_CRITICAL_STREAM (0x0F):

A stream required by the connection was closed or reset.

HTTP_WRONG_STREAM_DIRECTION (0x0010):

A unidirectional stream type was used by a peer which is not permitted to do so.

HTTP_EARLY_RESPONSE (0x0011):

The remainder of the client’s request is not needed to produce a response. For use in STOP_SENDING only.

HTTP_MISSING_SETTINGS (0x0012):

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

HTTP_UNEXPECTED_FRAME (0x0013):

A frame was received which was not permitted in the current state.

HTTP_REQUEST_REJECTED (0x0014):

A server rejected a request without performing any application processing.

HTTP_GENERAL_PROTOCOL_ERROR (0x00FF):

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.

HTTP_MALFORMED_FRAME (0x01XX):

An error in a specific frame type. If the frame type is 0xfe or less, the type is included as the last byte of the error code. For example, an error in a MAX_PUSH_ID frame would be indicated with the code (0x10D). The last byte 0xff is used to indicate any frame type greater than 0xfe.

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

This document creates a new registration for version-negotiation hints in the “Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter” registry established in [RFC7838].¶

Parameter:

“quic”

Specification:

This document, Section 2.2.1

This document establishes a registry for HTTP/3 frame type codes. The “HTTP/3 Frame Type” registry governs a 62-bit space. This space is split into three spaces that are governed by different policies. Values between 0x00 and 0x3f (in hexadecimal) are assigned via the Standards Action or IESG Review policies [RFC8126]. Values from 0x40 to 0x3fff operate on the Specification Required policy [RFC8126]. All other values are assigned to Private Use [RFC8126].¶

While this registry is separate from the “HTTP/2 Frame Type” registry defined in [RFC7540], 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.¶

Frame Type:

A name or label for the frame type.

Code:

The 62-bit code assigned to the frame type.

Specification:

A reference to a specification that includes a description of the frame layout and its semantics, including any parts of the frame that are conditionally present.

The entries in the following table are registered by this document.¶

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.¶

This document establishes a registry for HTTP/3 settings. The “HTTP/3 Settings” registry governs a 62-bit space. This space is split into three spaces that are governed by different policies. Values between 0x00 and 0x3f (in hexadecimal) are assigned via the Standards Action or IESG Review policies [RFC8126]. Values from 0x40 to 0x3fff operate on the Specification Required policy [RFC8126]. All other values are assigned to Private Use [RFC8126]. The designated experts are the same as those for the “HTTP/2 Settings” registry defined in [RFC7540].¶

While this registry is separate from the “HTTP/2 Settings” registry defined in [RFC7540], 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.¶

Name:

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

Code:

The 62-bit code assigned to the setting.

Specification:

An optional reference to a specification that describes the use of the setting.

The entries in the following table are registered by this document.¶

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.¶

This document establishes a registry for HTTP/3 error codes. The “HTTP/3 Error Code” registry manages a 16-bit space. The “HTTP/3 Error Code” registry operates under the “Expert Review” policy [RFC8126].¶

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.¶

Name:

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

Code:

The 16-bit error code value.

Description:

A brief description of the error code semantics, longer if no detailed specification is provided.

Specification:

An optional reference for a specification that defines the error code.

The entries in the following table are registered by this document.¶

This document establishes a registry for HTTP/3 unidirectional stream types. The “HTTP/3 Stream Type” registry governs a 62-bit space. This space is split into three spaces that are governed by different policies. Values between 0x00 and 0x3f (in hexadecimal) are assigned via the Standards Action or IESG Review policies [RFC8126]. Values from 0x40 to 0x3fff operate on the Specification Required policy [RFC8126]. All other values are assigned to Private Use [RFC8126].¶

Stream Type:

A name or label for the stream type.

Code:

The 62-bit code assigned to the stream type.

Specification:

A reference to a specification that includes a description of the stream type, including the layout semantics of its payload.

Sender:

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

The entries in the following table are registered by this document.¶

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.¶

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.¶

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 [RFC7540]. 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.¶

For example, implicit in the HTTP/2 prioritization scheme is the notion of in-order delivery of priority changes (i.e., dependency tree mutations): since operations on the dependency tree such as reparenting a subtree are not commutative, both sender and receiver must apply them in the same order to ensure that both sides have a consistent view of the stream dependency tree. HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in HEADERS frames. To achieve in-order delivery of priority changes in HTTP/3, PRIORITY frames are sent as the first frame on a request stream or on the control stream and exclusive prioritization has been removed. HTTP/3 permits the prioritisation of requests, pushes and placeholders that each exist in separate identifier spaces. The HTTP/3 PRIORITY frame replaces the stream dependency field with fields that can identify the element of interest and its dependency.¶

Likewise, 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. As a result, HTTP/3 uses a modified version of HPACK, described in [QPACK].¶

Frame type definitions in HTTP/3 often use the QUIC variable-length integer encoding. In particular, Stream IDs use this encoding, which allow 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 in PRIORITY frames). 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.¶

DATA (0x0):

Padding is not defined in HTTP/3 frames. See Section 4.2.1.

HEADERS (0x1):

The PRIORITY region of HEADERS is not defined in HTTP/3 frames. A separate PRIORITY frame is used in all cases. Padding is not defined in HTTP/3 frames. See Section 4.2.2.

PRIORITY (0x2):

As described above, the PRIORITY frame references a variety of identifiers. It is sent as the first frame on a request streams or on the control stream. See Section 4.2.3.

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 (Section 4.2.4).

SETTINGS (0x4):

SETTINGS frames are sent only at the beginning of the connection. See Section 4.2.5 and Appendix A.3.

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 Section 4.2.6.

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 Section 4.2.7.

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 [RFC7540] 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 Section 10.3.¶

An important difference from HTTP/2 is that settings are sent once, at the beginning of the connection, 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 4.2.5.1.

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 the format of their settings to avoid using the 62-bit encoding.¶

Settings need to be defined separately for HTTP/2 and HTTP/3. The IDs of settings defined in [RFC7540] 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 10.4.¶

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.¶

NO_ERROR (0x0):

HTTP_NO_ERROR in Section 8.1.

PROTOCOL_ERROR (0x1):

No single mapping. See new HTTP_MALFORMED_FRAME error codes defined in Section 8.1.

INTERNAL_ERROR (0x2):

HTTP_INTERNAL_ERROR in Section 8.1.

FLOW_CONTROL_ERROR (0x3):

Not applicable, since QUIC handles flow control. Would provoke a QUIC_FLOW_CONTROL_RECEIVED_TOO_MUCH_DATA from the QUIC layer.

SETTINGS_TIMEOUT (0x4):

Not applicable, since no acknowledgement of SETTINGS is defined.

STREAM_CLOSED (0x5):

Not applicable, since QUIC handles stream management. Would provoke a QUIC_STREAM_DATA_AFTER_TERMINATION from the QUIC layer.

FRAME_SIZE_ERROR (0x6):

HTTP_MALFORMED_FRAME error codes defined in Section 8.1.

REFUSED_STREAM (0x7):

HTTP_REQUEST_REJECTED (in Section 8.1) is used to indicate that a request was not processed. Otherwise, not applicable because QUIC handles stream management. A STREAM_ID_ERROR at the QUIC layer is used for streams that are improperly opened.

CANCEL (0x8):

HTTP_REQUEST_CANCELLED in Section 8.1.

COMPRESSION_ERROR (0x9):

Multiple error codes are defined in [QPACK].

CONNECT_ERROR (0xa):

HTTP_CONNECT_ERROR in Section 8.1.

ENHANCE_YOUR_CALM (0xb):

HTTP_EXCESSIVE_LOAD in Section 8.1.

INADEQUATE_SECURITY (0xc):

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

HTTP_1_1_REQUIRED (0xd):

HTTP_VERSION_FALLBACK in Section 8.1.

Error codes need to be defined for HTTP/2 and HTTP/3 separately. See Section 10.5.¶

• 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