WebTransport over HTTP/3

Introduction HTTP/3 is a protocol defined on top of QUIC that can multiplex HTTP requests over a QUIC connection. This document defines a mechanism for multiplexing non-HTTP data with HTTP/3 in a manner that conforms with the WebTransport protocol requirements and semantics . Using the mechanism described here, multiple WebTransport instances, or sessions, can be multiplexed simultaneously with regular HTTP traffic on the same HTTP/3 connection.

Terminology The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document follows terminology defined in Section 1.2 of . Note that this document distinguishes between a WebTransport server and an HTTP/3 server. An HTTP/3 server is the server that terminates HTTP/3 connections; a WebTransport server is an application that accepts WebTransport sessions, which can be accessed via an HTTP/3 server. An application client is user or developer-provided code, often untrusted, that utilizes the interface offered by the WebTransport client to communicate with an application server. The application server uses the interface offered by the WebTransport server to accept incoming WebTransport sessions.

Overview

QUIC, WebTransport, and HTTP/3 QUIC version 1 is a secure transport protocol with flow control and congestion control. QUIC supports application data exchange via streams; reliable and ordered byte streams that can be multiplexed. Stream independence can mitigate head-of-line blocking. While QUIC provides streams as a transport service, it is unopinionated about their usage. The applicability of streams is described by section 4 of . HTTP is an application-layer protocol, defined by "HTTP Semantics" . HTTP/3 is the application mapping for QUIC, defined in . It describes how QUIC streams are used to carry control data or HTTP request and response message sequences in the form of frames and describes details of stream and connection lifecycle management. HTTP/3 offers two features in addition to HTTP Semantics: QPACK header compression and Server Push . WebTransport session establishment involves interacting at the HTTP layer with a resource. For Web user agents and other WebTransport clients, this interaction is important for security reasons, especially to ensure that the resource is willing to use WebTransport. Although WebTransport requires HTTP for its handshake, when HTTP/3 is in use, HTTP is not used for anything else related to an established session. Instead, QUIC streams begin with a sequence of header bytes that links them to the established session. The remainder of the stream is the body, which carries the payload supplied by the application using WebTransport. This process is similar to WebSockets over HTTP/1.1 , where access to the underlying byte stream is enabled after both sides have completed an initial handshake. The layering of QUIC, HTTP/3, and WebTransport is shown in . Once a WebTransport session is established, applications have nearly direct access to QUIC.

WebTransport Layering

Minimizing Implementation Complexity WebTransport has minimal interaction with HTTP and HTTP/3. Clients and servers can constrain their use of features to only those required to complete a WebTransport handshake:

Generating/parsing the request method, host, path, protocol, optional Origin header field, and perhaps some extra header fields.
Generating/parsing the response status code and possibly some extra header fields.

A WebTransport endpoint, whether a client or a server, can likely perform several of its HTTP-level requirements using bytestring comparisons. While HTTP/3 encodes HTTP messages using QPACK, this complexity can be minimized. When receiving, a WebTransport endpoint can disable dynamic decompression entirely but must always support static decompression and Huffman decoding. When sending, endpoints can opt to never use dynamic compression, static compression, or Huffman encoding.

Capsule-Based WebTransport over HTTP/3 WebTransport over HTTP/3 as defined in this document provides the best performance by using native QUIC streams and datagrams. Endpoints SHOULD always use this protocol when using WebTransport over an HTTP/3 connection. However, it is also possible to use WebTransport over a single HTTP/3 stream using the capsule-based protocol defined in . The two protocols are distinguished by their upgrade tokens: this document uses the "webtransport-h3" token (), while uses the "webtransport" token. The capsule-based protocol can be useful for intermediaries that proxy WebTransport sessions between HTTP/2 and HTTP/3 connections, as it avoids the need to translate between the two wire formats. It can also be useful in deployment environments such as data centers where existing routing infrastructure supports forwarding streams but does not support the HTTP/3 extensions required by this document. Endpoints that use the capsule-based protocol over HTTP/3 lose the benefits of stream independence, as all WebTransport streams within a session share a single QUIC stream and are subject to head-of-line blocking. Datagrams sent using the capsule-based protocol are also retransmitted by QUIC, and therefore do not provide unreliable delivery.

Protocol Overview WebTransport servers in general are identified by a pair of authority value and path value (defined in Sections 3.2 and 3.3 respectively). When an HTTP/3 connection is established, the server sends a SETTINGS_WT_ENABLED setting to indicate support for WebTransport over HTTP/3. This process also negotiates the use of additional HTTP/3 extensions to enable both endpoints to open WebTransport streams. WebTransport sessions are initiated inside a given HTTP/3 connection by the client, who sends an extended CONNECT request . If the server accepts the request, a WebTransport session is established. The resulting stream will be further referred to as a CONNECT stream, and its stream ID is used to uniquely identify a given WebTransport session within the connection. The ID of the CONNECT stream that established a given WebTransport session will be further referred to as a Session ID. After the session is established, the endpoints can exchange data using the following mechanisms:

A client can create a bidirectional stream and transfer its ownership to WebTransport by providing a special signal in the first bytes.
A server can create a bidirectional stream and transfer its ownership to WebTransport by providing a special signal in the first bytes.
Both client and server can create a unidirectional stream using a special stream type.
Both client and server can send datagrams using HTTP Datagrams .

A WebTransport session is terminated when the CONNECT stream that created it is closed.

Session Establishment

Establishing a WebTransport-Capable HTTP/3 Connection A WebTransport-Capable HTTP/3 connection requires the server to signal support for WebTransport over HTTP/3 using a setting. Clients also signal support by using the "webtransport-h3" upgrade token in extended CONNECT requests when establishing sessions (see ). This document defines a SETTINGS_WT_ENABLED setting that WebTransport servers use to indicate their support for WebTransport. The default value for the SETTINGS_WT_ENABLED setting is "0", meaning that the server does not support WebTransport. Clients MUST NOT attempt to establish WebTransport sessions until they have received the setting indicating WebTransport support from the server. WebTransport over HTTP/3 uses extended CONNECT in HTTP/3 as described in , which defines the SETTINGS_ENABLE_CONNECT_PROTOCOL setting. WebTransport over HTTP/3 requires support for HTTP/3 datagrams and the Capsule Protocol, and both the client and the server indicate support for HTTP/3 datagrams by sending a SETTINGS_H3_DATAGRAM setting value set to 1 in their SETTINGS frame (see ). WebTransport over HTTP/3 also requires support for QUIC datagrams. To indicate support, both the client and the server send a max_datagram_frame_size transport parameter with a value greater than 0 (see ). WebTransport over HTTP/3 relies on the RESET_STREAM_AT frame defined in . To indicate support, both the client and the server enable the extension by sending an empty reset_stream_at transport parameter as described in . In summary, servers supporting WebTransport over HTTP/3 send:

A SETTINGS_WT_ENABLED setting with a value greater than "0"
A SETTINGS_ENABLE_CONNECT_PROTOCOL setting with a value of "1"
A SETTINGS_H3_DATAGRAM setting with a value of 1
A max_datagram_frame_size transport parameter with a value greater than 0
An empty reset_stream_at transport parameter

Clients supporting WebTransport over HTTP/3 send:

A SETTINGS_H3_DATAGRAM setting with a value of 1
A max_datagram_frame_size transport parameter with a value greater than 0
An empty reset_stream_at transport parameter

[[RFC editor: please remove the following paragraph before publication.]] For draft versions of WebTransport only, clients MUST also send SETTINGS_WT_ENABLED with the draft-specific codepoint to allow the server to identify the client's supported version; see . Servers should note that CONNECT requests to establish new WebTransport sessions, in addition to other messages, can arrive before the client's SETTINGS are received (see ). If the server receives SETTINGS that do not have correct values for every required setting, or transport parameters that do not have correct values for every required transport parameter, the server MUST treat all established and newly incoming WebTransport sessions as malformed, as described in . A client MUST NOT establish WebTransport sessions if the server's SETTINGS do not have correct values for every required setting or if the server's transport parameters do not have correct values for every required transport parameter. If a client does not wish to use the connection for purposes other than WebTransport when the requirements for WebTransport are not met, the client MAY close the HTTP/3 connection with a WT_REQUIREMENTS_NOT_MET error code to aid in debugging. [[RFC editor: please remove the following paragraph before publication.]] For draft versions of WebTransport only, the server MUST NOT process any incoming WebTransport requests until the client's SETTINGS have been received; see .

Creating a New Session As WebTransport sessions are established over HTTP/3, they are identified using the https URI scheme (). In order to create a new WebTransport session, a WebTransport client sends an HTTP extended CONNECT request. In this request:

The :protocol pseudo-header field () MUST be set to webtransport-h3.
The :scheme field MUST be https.
Both the :authority and the :path value MUST be set; these fields identify the desired WebTransport server resource.
If the WebTransport session is coming from a browser client, an Origin header MUST be provided within the request. Otherwise, the header is OPTIONAL.

Upon receiving an extended CONNECT request with a :protocol field set to webtransport-h3, the HTTP/3 server can check if it has a WebTransport server associated with the specified :authority and :path values. If it does not, it SHOULD reply with status code 404 (). When the request contains the Origin header, the WebTransport server MUST verify the Origin header to ensure that the specified origin is allowed to access the server in question. If the verification fails, the WebTransport server SHOULD reply with status code 403 (). If all checks pass, the WebTransport server MAY accept the session by replying with a 2xx series status code, as defined in . From the client's perspective, a WebTransport session is established when the client receives a 2xx response. From the server's perspective, a session is established once it sends a 2xx response. The server may reply with a 3xx response, indicating a redirection (). The WebTransport client MUST NOT automatically follow such redirects, as it potentially could have already sent data for the WebTransport session in question; it MAY notify the application client about the redirect. Clients cannot initiate WebTransport in 0-RTT packets, as the CONNECT method is not considered safe (see ). However, WebTransport-related SETTINGS parameters may be retained from the previous session as described in . If the server accepts 0-RTT, the server MUST NOT reduce the limit of maximum open WebTransport sessions, or other initial flow control values, from the values negotiated during the previous session; such change would be deemed incompatible, and MUST result in a H3_SETTINGS_ERROR connection error. The "webtransport-h3" HTTP Upgrade Token uses the Capsule Protocol as defined in . The Capsule Protocol is negotiated when the server sends a 2xx response. The capsule-protocol header field is not required by WebTransport and can safely be ignored by WebTransport endpoints.

Application Protocol Negotiation WebTransport over HTTP/3 offers a protocol negotiation mechanism, similar to the TLS Application-Layer Protocol Negotiation (ALPN) extension ; the intent is to simplify porting existing protocols that use QUIC and rely on this functionality. The client MAY include a WT-Available-Protocols header field in the CONNECT request. The WT-Available-Protocols field enumerates the possible protocols in preference order, with the most preferred protocol listed first. If the server receives such a header, it MAY include a WT-Protocol field in a successful (2xx) response. If it does, the server MUST include a single choice from the client's list in that field. Servers MAY reject the request if the client did not include a suitable protocol. Both WT-Available-Protocols and WT-Protocol are Structured Fields . WT-Available-Protocols is a List. WT-Protocol is defined as an Item. In both cases, the only valid value type is a String. Any value type other than String MUST be treated as an error that causes the entire field to be ignored as recommended in , allowing application protocol negotiation to remain optional. No semantics are defined for parameters on either field; parameters MUST be ignored. A server that requires application protocol negotiation MAY reject the session if the WT-Available-Protocols header field is absent or if it is malformed and therefore ignored. A client that requires application protocol negotiation MUST close the WebTransport session with a WT_ALPN_ERROR error code if the server does not include a WT-Protocol header field, or if it is malformed and therefore ignored, in a successful response. If the client sends a WT-Available-Protocols header field and the server responds with a WT-Protocol header field, the value in the WT-Protocol response header field MUST be one of the values listed in WT-Available-Protocols of the request. If the client receives a WT-Protocol value that was not included in its WT-Available-Protocols list, the client MUST close the WebTransport session with a WT_ALPN_ERROR error code. The semantics of individual values used in WT-Available-Protocols and WT-Protocol are determined by the WebTransport resource in question and are not required to be registered in IANA's "ALPN Protocol IDs" registry.

Prioritization WebTransport sessions are initiated using extended CONNECT. While describes how extensible priorities can be applied to data sent on a CONNECT stream, WebTransport extends the types of data that are exchanged in relation to the request and response, which requires additional considerations. WebTransport CONNECT requests and responses MAY contain the Priority header field (); clients MAY reprioritize by sending PRIORITY_UPDATE frames (). In extension to , it is RECOMMENDED that clients and servers apply the scheduling guidance in both and for all data that they send in the enclosing WebTransport session, including Capsules, WebTransport streams and datagrams. WebTransport does not provide any priority signaling mechanism for streams and datagrams within a WebTransport session; such mechanisms can be defined by application protocols using WebTransport. It is RECOMMENDED that such mechanisms only affect scheduling within a session and not scheduling of other data on the same HTTP/3 connection. The client/server priority merging guidance in also applies to WebTransport sessions. For example, a client that receives a response Priority header field could alter its view of a WebTransport session priority and alter the scheduling of outgoing data as a result. Endpoints that prioritize WebTransport sessions need to consider how they interact with other sessions or requests on the same HTTP/3 connection.

WebTransport Features WebTransport over HTTP/3 provides the following features described in : unidirectional streams, bidirectional streams, and datagrams, all of which can be initiated by either endpoint. Protocols designed for use with WebTransport over HTTP/3 are constrained to these features. The Capsule Protocol is an implementation detail of WebTransport over HTTP/3 and is not a WebTransport feature. Session IDs are used to demultiplex streams and datagrams belonging to different WebTransport sessions. On the wire, session IDs are encoded using the QUIC variable length integer scheme described in . The client MAY optimistically open unidirectional and bidirectional streams, as well as send datagrams, on a session for which it has sent the CONNECT request, even if it has not yet received the server's response to the request. On the server side, opening streams and sending datagrams is possible as soon as the CONNECT request has been received. Session IDs are derived from the stream ID of the CONNECT stream that established the session and therefore MUST always correspond to a client-initiated bidirectional stream, as defined in . If an endpoint receives a session ID on a unidirectional stream, bidirectional stream, or datagram that does not correspond to a client-initiated bidirectional stream ID, the endpoint MUST close the connection with an H3_ID_ERROR error code. Session IDs that correspond to closed sessions are not considered invalid for the purposes of this check; endpoints handle data for closed sessions as described in .

Transport Properties The WebTransport framework defines a set of optional transport properties that clients can use to determine the presence of features which might allow additional optimizations beyond the common set of properties available via all WebTransport protocols. Below are details about support in WebTransport over HTTP/3 for the properties defined by the WebTransport framework.

Unreliable Delivery:: WebTransport over HTTP/3 supports unreliable delivery. Resetting a stream results in lost stream data no longer being retransmitted. WebTransport over HTTP/3 also supports datagrams, which are not retransmitted.
Pooling:: WebTransport over HTTP/3 provides optional support for pooling. Endpoints that do not support pooling can reply to CONNECT requests with a header indicating a rate limit policy with a quota of "1" ().

Unidirectional streams WebTransport endpoints can initiate unidirectional streams. The HTTP/3 unidirectional stream type SHALL be 0x54. The body of the stream SHALL be the stream type, followed by the session ID, encoded as a variable-length integer, followed by the user-specified stream data ().

Unidirectional WebTransport stream format

Bidirectional Streams All client-initiated bidirectional streams are reserved by HTTP/3 as request streams, which are a sequence of HTTP/3 frames with a variety of rules (see Sections and of ). WebTransport extends HTTP/3 to allow clients to declare and to use alternative request stream rules. Once a client receives settings indicating WebTransport support (), it MUST send a special signal value, encoded as a variable-length integer, as the first bytes of each bidirectional WebTransport stream it initiates to indicate how the remaining bytes on the stream are used. WebTransport extends HTTP/3 by defining rules for all server-initiated bidirectional streams. Once a server receives an incoming CONNECT request establishing a WebTransport session (), it can open a bidirectional stream for use with that session and MUST send a special signal value, encoded as a variable-length integer, as the first bytes of the stream in order to indicate how the remaining bytes on the stream are used. Clients and servers use the signal value 0x41 to open a bidirectional WebTransport stream. Following this is the associated session ID, encoded as a variable-length integer; the rest of the stream is the application payload of the WebTransport stream ().

Bidirectional WebTransport stream format This document reserves the special signal value 0x41 as a WT_STREAM frame type. While it is registered as an HTTP/3 frame type to avoid collisions, WT_STREAM lacks length and is not a proper HTTP/3 frame; it is an extension of HTTP/3 frame syntax that MUST be supported by any peer negotiating WebTransport. Endpoints that implement this extension are also subject to additional frame handling requirements. Endpoints MUST NOT send WT_STREAM as a frame type on HTTP/3 streams other than the very first bytes of a request stream. Receiving this frame type in any other circumstances MUST be treated as a connection error of type H3_FRAME_ERROR.

Resetting Data Streams A WebTransport endpoint may send a RESET_STREAM or a STOP_SENDING frame for a WebTransport data stream. Those signals are propagated by the WebTransport implementation to the application. A WebTransport application MUST provide an error code for those operations. Since WebTransport shares the error code space with HTTP/3, WebTransport application errors for streams are limited to an unsigned 32-bit integer, assuming values between 0x00000000 and 0xffffffff. WebTransport implementations MUST remap those error codes into the error range reserved for WT_APPLICATION_ERROR, where 0x00000000 corresponds to 0x52e4a40fa8db, and 0xffffffff corresponds to 0x52e5ac983162. Note that there are codepoints inside that range of form "0x1f * N + 0x21" that are reserved by ; those have to be skipped when mapping the error codes (i.e., the two HTTP/3 error codepoints adjacent to a reserved codepoint would map to two adjacent WebTransport application error codepoints). An example pseudocode can be seen in .

Pseudocode for converting between WebTransport application errors and HTTP/3 error codes WebTransport data streams are associated with sessions through a header at the beginning of the stream; resetting a stream might result in that data being discarded when using a RESET_STREAM frame. To prevent this, WebTransport implementations MUST use the RESET_STREAM_AT frame with a Reliable Size set to at least the size of the WebTransport header when resetting a WebTransport data stream. This ensures reliable delivery of the ID field associating the data stream with a WebTransport session. WebTransport endpoints MUST forward the error code for a stream associated with a known session to the application that owns that session; similarly, intermediaries MUST reset such streams with a corresponding error code when receiving a reset from their peer. If a RESET_STREAM or STOP_SENDING frame is received with an error code outside the range reserved for WT_APPLICATION_ERROR, the stream is still considered reset, but the error code is not mapped to a WebTransport application error code. The WebTransport implementation SHOULD deliver this to the application as a stream reset with no application error code.

Datagrams Datagrams can be sent using HTTP Datagrams. The WebTransport datagram payload is sent unmodified in the "HTTP Datagram Payload" field of an HTTP Datagram (). Note that the payload field directly follows the Quarter Stream ID field, which is at the start of the QUIC DATAGRAM frame payload and refers to the CONNECT stream that established the WebTransport session.

Buffering Incoming Streams and Datagrams In WebTransport over HTTP/3, the client MUST wait for receipt of the server's SETTINGS frame before establishing any WebTransport sessions by sending CONNECT requests using the WebTransport upgrade token (see ). This ensures that the client will always know what versions of WebTransport can be used on a given HTTP/3 connection. Clients can, however, send a SETTINGS frame, multiple WebTransport CONNECT requests, WebTransport data streams, and WebTransport datagrams all within a single flight. As those can arrive out of order, a WebTransport server could be put into a situation where it receives a stream or a datagram without a corresponding session. Similarly, a client may receive a server-initiated stream or a datagram before receiving the CONNECT response headers from the server. To handle this case, WebTransport endpoints SHOULD buffer streams and datagrams until they can be associated with an established session. To avoid resource exhaustion, endpoints MUST limit the number of buffered streams and datagrams. When the number of buffered streams is exceeded, a stream SHALL be closed by sending a RESET_STREAM and/or STOP_SENDING with the WT_BUFFERED_STREAM_REJECTED error code. When the number of buffered datagrams is exceeded, a datagram SHALL be dropped. It is up to an implementation to choose what stream or datagram to discard.

Interaction with the HTTP/3 GOAWAY frame HTTP/3 defines a graceful shutdown mechanism () that allows a peer to send a GOAWAY frame indicating that it will no longer accept any new incoming requests or pushes. A client receiving GOAWAY cannot initiate CONNECT requests for new WebTransport sessions on that HTTP/3 connection; it must open a new HTTP/3 connection to initiate new WebTransport sessions with the same peer. An HTTP/3 GOAWAY frame is also a signal to applications to initiate shutdown for all WebTransport sessions. To shut down a single WebTransport session, either endpoint can send a WT_DRAIN_SESSION (0x78ae) capsule.

WT_DRAIN_SESSION Capsule Format After sending or receiving either a WT_DRAIN_SESSION capsule or a HTTP/3 GOAWAY frame, an endpoint MAY continue using the session and MAY open new WebTransport streams. The signal is intended for the application using WebTransport, which is expected to attempt to gracefully terminate the session as soon as possible. The WT_DRAIN_SESSION capsule is useful when an end-to-end WebTransport session passes through an intermediary. For example, when the backend shuts down, it sends a GOAWAY to the intermediary. The intermediary can convert this signal to a WT_DRAIN_SESSION capsule on the client-facing session, without impacting other requests or sessions carried on that connection.

Use of Keying Material Exporters WebTransport over HTTP/3 supports the use of TLS keying material exporters . Since the underlying QUIC connection may be shared by multiple WebTransport sessions, WebTransport defines a mechanism for deriving a TLS exporter that separates keying material for different sessions. If the application requests an exporter for a given WebTransport session with a specified label and context, the resulting exporter SHALL be a TLS exporter as defined in with the label set to "EXPORTER-WebTransport" and the context set to the serialization of the "WebTransport Exporter Context" struct as defined below.

WebTransport Exporter Context struct A TLS exporter API might permit the context field to be omitted. In this case, as with TLS 1.3, the WebTransport Application-Supplied Exporter Context becomes zero-length if omitted.

Flow Control Flow control governs the amount of resources that can be consumed or data that can be sent. When using WebTransport over HTTP/3, endpoints can limit the number of sessions that a peer can create on a single HTTP/3 connection and the number of streams that a peer can create within a session. Endpoints can also limit the amount of data that can be consumed by each session and by each stream within a session. WebTransport over HTTP/3 provides a connection-level limit that governs the number of sessions that can be created on an HTTP/3 connection (see ). It also provides session-level limits that govern the number of streams that can be created in a session and limit the amount of data that can be exchanged across all streams in each session (see ). The underlying QUIC connection provides connection and stream level flow control. The QUIC connection data limit defines the total amount of data that can be sent across all WebTransport sessions and other non-WebTransport streams. A QUIC stream's data limit controls the amount of data that can be sent on that stream, WebTransport or otherwise (see ).

Negotiating the Use of Flow Control A WebTransport endpoint that allows a WebTransport session to share an underlying transport connection with other WebTransport sessions MUST enable flow control. This prevents an application from consuming excessive resources on a single session and starving traffic for other sessions (see ). Flow control is enabled when both endpoints declare their intent to use flow control by taking any of the following actions:

Sending SETTINGS_WT_INITIAL_MAX_STREAMS_UNI with any value other than "0".
Sending SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI with any value other than "0".
Sending SETTINGS_WT_INITIAL_MAX_DATA with any value other than "0".

If both endpoints take at least one of these actions, flow control is enabled, and the limits described in the entirety of apply. Flow control can be enabled regardless of the number of WebTransport sessions a server supports. If flow control is not enabled, clients MUST NOT attempt to establish more than one simultaneous WebTransport session. A server that receives more than one session on an underlying transport connection when flow control is not enabled MUST reset the excessive CONNECT streams with a H3_REQUEST_REJECTED status (see ). Also, if flow control is not enabled, an endpoint MUST ignore receipt of any flow control capsules (see ), since the peer might not have received SETTINGS at the time they were sent or packets might have been reordered.

Limiting the Number of Simultaneous Sessions Servers SHOULD limit the rate of incoming WebTransport sessions on HTTP/3 connections to prevent excessive consumption of resources. To do so, they have multiple mechanisms available:

The H3_REQUEST_REJECTED error code defined in indicates to the receiving HTTP/3 stack that the request was not processed in any way.
HTTP status code 429 indicates that the request was rejected due to rate limiting . Unlike the previous method, this signal is directly propagated to the application.

WebTransport servers can use rate limit header fields in responses to CONNECT requests to signal quota policies and service limits to WebTransport clients (see ). This provides hints to clients about how many sessions they can reasonably expect to be able to open. An endpoint that does not support pooling and flow control MUST NOT accept more than one incoming WebTransport session at a time.

Limiting the Number of Streams Within a Session The WT_MAX_STREAMS capsule () establishes a limit on the number of streams within a WebTransport session. Like the QUIC MAX_STREAMS frame (), this capsule has two types that provide separate limits for unidirectional and bidirectional streams that a peer initiates. Note that the CONNECT stream for the session is not included in either the bidirectional or the unidirectional stream limits; the number of CONNECT streams a client can open is limited by QUIC flow control's stream limits and any rate limit that a WebTransport server enforces. The session-level stream limit applies in addition to the QUIC MAX_STREAMS frame, which provides a connection-level stream limit. New streams can only be created within the session if both the stream- and the connection-level limit permit, see for details on how QUIC stream limits are applied. Unlike the QUIC MAX_STREAMS frame, there is no simple relationship between the value in this frame and stream IDs in QUIC STREAM frames. This especially applies if there are other users of streams on the connection. The WT_STREAMS_BLOCKED capsule () can be sent to indicate that an endpoint was unable to create a stream due to the session-level stream limit. Note that enforcing this limit requires reliable resets for stream headers so that both endpoints can agree on the number of streams that are open.

Data Limits The WT_MAX_DATA capsule () establishes a limit on the amount of data that can be sent within a WebTransport session. This limit counts all data that is sent on streams of the corresponding type, excluding the stream header (see and ). The stream header is excluded from this limit so that this limit does not prevent the sending of information that is essential in linking new streams to a specific WebTransport session. For streams that were reset, implementing WT_MAX_DATA requires that the QUIC stack provide the WebTransport implementation with information about the final size of streams; see . This guarantees that both endpoints agree on how much WebTransport session flow control credit was consumed by the sender on that stream. The WT_DATA_BLOCKED capsule () can be sent to indicate that an endpoint was unable to send data due to a limit set by the WT_MAX_DATA capsule. Because WebTransport over HTTP/3 uses a native QUIC stream for each WebTransport stream, per-stream data limits are provided by QUIC natively (see ). The WT_MAX_STREAM_DATA and WT_STREAM_DATA_BLOCKED capsules (Sections and of ) are not used and so are prohibited. Endpoints MUST treat receipt of a WT_MAX_STREAM_DATA or a WT_STREAM_DATA_BLOCKED capsule as a session error.

Flow Control SETTINGS Initial flow control limits can be exchanged via HTTP/3 SETTINGS () by providing non-zero values for

WT_MAX_STREAMS via SETTINGS_WT_INITIAL_MAX_STREAMS_UNI and SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI
WT_MAX_DATA via SETTINGS_WT_INITIAL_MAX_DATA

SETTINGS_WT_INITIAL_MAX_STREAMS_UNI The SETTINGS_WT_INITIAL_MAX_STREAMS_UNI setting indicates the initial value for the unidirectional max stream limit, otherwise communicated by the WT_MAX_STREAMS capsule (see ). The default value for the SETTINGS_WT_INITIAL_MAX_STREAMS_UNI setting is "0", indicating that the endpoint needs to send WT_MAX_STREAMS capsules on each individual WebTransport session before its peer is allowed to create any unidirectional streams within that session. Note that this limit applies to all WebTransport sessions that use the HTTP/3 connection on which this SETTING is sent.

SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI The SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI setting indicates the initial value for the bidirectional max stream limit, otherwise communicated by the WT_MAX_STREAMS capsule (see ). The default value for the SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI setting is "0", indicating that the endpoint needs to send WT_MAX_STREAMS capsules on each individual WebTransport session before its peer is allowed to create any bidirectional streams within that session. Note that this limit applies to all WebTransport sessions that use the HTTP/3 connection on which this SETTING is sent.

SETTINGS_WT_INITIAL_MAX_DATA The SETTINGS_WT_INITIAL_MAX_DATA setting indicates the initial value for the session data limit, otherwise communicated by the WT_MAX_DATA capsule (see ). The default value for the SETTINGS_WT_INITIAL_MAX_DATA setting is "0", indicating that the endpoint needs to send a WT_MAX_DATA capsule within each session before its peer is allowed to send any stream data within that session. Note that this limit applies to all WebTransport sessions that use the HTTP/3 connection on which this SETTING is sent.

Flow Control Capsules WebTransport over HTTP/3 uses several capsules for flow control, and all of these capsules define special intermediary handling as described in . These capsules, referred to as the "flow control capsules", are WT_MAX_DATA, WT_MAX_STREAMS, WT_DATA_BLOCKED, and WT_STREAMS_BLOCKED. An endpoint MUST NOT wait for a WT_DATA_BLOCKED or WT_STREAMS_BLOCKED capsule before sending a WT_MAX_DATA or WT_MAX_STREAMS capsule; doing so could result in the sender being blocked for at least an entire round trip. Endpoints SHOULD send WT_MAX_DATA and WT_MAX_STREAMS capsules as they consume data or close streams (similar to the mechanism used in QUIC, see ).

Flow Control and Intermediaries Because flow control in WebTransport is hop-by-hop and does not provide an end-to-end signal, intermediaries MUST consume flow control signals and express their own flow control limits to the next hop. The intermediary can send these signals via HTTP/3 flow control messages, HTTP/2 flow control messages, or as WebTransport flow control capsules, where appropriate. Intermediaries are responsible for storing any data for which they advertise flow control credit if that data cannot be immediately forwarded to the next hop. In practice, an intermediary that translates flow control signals between similar WebTransport protocols, such as between two HTTP/3 connections, can often simply reexpress the same limits received on one connection directly on the other connection. An intermediary that does not want to be responsible for storing data that cannot be immediately sent on its translated connection can ensure that it does not advertise a higher flow control limit on one connection than the corresponding limit on the translated connection.

WT_MAX_STREAMS Capsule An HTTP capsule called WT_MAX_STREAMS is introduced to inform the peer of the cumulative number of streams of a given type it is permitted to open. A WT_MAX_STREAMS capsule with a type of 0x190B4D3F applies to bidirectional streams, and a WT_MAX_STREAMS capsule with a type of 0x190B4D40 applies to unidirectional streams. Note that, because Maximum Streams is a cumulative value representing the total allowed number of streams, including previously closed streams, endpoints repeatedly send new WT_MAX_STREAMS capsules with increasing Maximum Streams values as streams are opened.

WT_MAX_STREAMS Capsule Format WT_MAX_STREAMS capsules contain the following field:

Maximum Streams:: A count of the cumulative number of streams of the corresponding type that can be opened over the lifetime of the session. This value cannot exceed 2⁶⁰, as it is not possible to encode stream IDs larger than 2⁶²-1. Receipt of a capsule with a Maximum Streams value larger than this limit MUST be treated as an HTTP/3 error of type H3_DATAGRAM_ERROR.

An endpoint MUST NOT open more streams than permitted by the current stream limit set by its peer. For instance, a server that receives a unidirectional stream limit of 3 is permitted to open streams 3, 7, and 11, but not stream 15. Note that this limit includes streams that have been closed as well as those that are open. If an endpoint receives an incoming stream for a session that would exceed the advertised Maximum Streams value, it MUST close the WebTransport session with a WT_FLOW_CONTROL_ERROR error code. Unlike in QUIC, where MAX_STREAMS frames can be delivered in any order, WT_MAX_STREAMS capsules are sent on the WebTransport session's connect stream and are delivered in order. If an endpoint receives a WT_MAX_STREAMS capsule with a Maximum Streams value less than a previously received value, it MUST close the WebTransport session with a WT_FLOW_CONTROL_ERROR error code. The WT_MAX_STREAMS capsule defines special intermediary handling, as described in . Intermediaries MUST consume WT_MAX_STREAMS capsules for flow control purposes and MUST generate and send appropriate flow control signals for their limits. Initial values for these limits MAY be communicated by sending non-zero values for SETTINGS_WT_INITIAL_MAX_STREAMS_UNI and SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI.

WT_STREAMS_BLOCKED Capsule A sender SHOULD send a WT_STREAMS_BLOCKED capsule (type=0x190B4D43 or 0x190B4D44) when it wishes to open a stream but is unable to do so due to the maximum stream limit set by its peer. A WT_STREAMS_BLOCKED capsule of type 0x190B4D43 is used to indicate reaching the bidirectional stream limit, and a WT_STREAMS_BLOCKED capsule of type 0x190B4D44 is used to indicate reaching the unidirectional stream limit. A WT_STREAMS_BLOCKED capsule does not open the stream, but informs the peer that a new stream was needed and the stream limit prevented the creation of the stream. A sender is not required to send WT_STREAMS_BLOCKED capsules, however WT_STREAMS_BLOCKED capsules can be used as input to tuning of flow control algorithms and for debugging purposes.

WT_STREAMS_BLOCKED Capsule Format WT_STREAMS_BLOCKED capsules contain the following field:

Maximum Streams:: A variable-length integer indicating the maximum number of streams allowed at the time the capsule was sent. This value cannot exceed 2⁶⁰, as it is not possible to encode stream IDs larger than 2⁶²-1.

The WT_STREAMS_BLOCKED capsule defines special intermediary handling, as described in . Intermediaries MUST consume WT_STREAMS_BLOCKED capsules for flow control purposes and MUST generate and send appropriate flow control signals for their limits.

WT_MAX_DATA Capsule An HTTP capsule called WT_MAX_DATA (type=0x190B4D3D) is introduced to inform the peer of the maximum amount of data that can be sent on the WebTransport session as a whole. This limit counts all data that is sent on streams of the corresponding type, excluding the stream header (see and ). For streams that were reset, implementing WT_MAX_DATA requires that the QUIC stack provide the WebTransport implementation with information about the final size of streams (see ).

WT_MAX_DATA Capsule Format WT_MAX_DATA capsules contain the following field:

Maximum Data:: A variable-length integer indicating the maximum amount of data that can be sent on the entire session, in units of bytes.

The sum of the lengths of Stream Body data sent on all streams associated with this session MUST NOT exceed the Maximum Data value advertised by a receiver. Note that capsules sent on the CONNECT stream, and the Signal Value, Stream Type, and Session ID fields, are not included in this limit. If an endpoint receives Stream Body data in excess of this limit, it MUST close the WebTransport session with a WT_FLOW_CONTROL_ERROR error code. Unlike in QUIC, where MAX_DATA frames can be delivered in any order, WT_MAX_DATA capsules are sent on the WebTransport session's connect stream and are delivered in order. If an endpoint receives a WT_MAX_DATA capsule with a Maximum Data value less than a previously received value, it MUST close the WebTransport session with a WT_FLOW_CONTROL_ERROR error code. The WT_MAX_DATA capsule defines special intermediary handling, as described in . Intermediaries MUST consume WT_MAX_DATA capsules for flow control purposes and MUST generate and send appropriate flow control signals for their limits (see ). The initial value for this limit MAY be communicated by sending a non-zero value for SETTINGS_WT_INITIAL_MAX_DATA.

WT_DATA_BLOCKED Capsule A sender SHOULD send a WT_DATA_BLOCKED capsule (type=0x190B4D41) when it wishes to send data but is unable to do so due to WebTransport session-level flow control. A sender is not required to send WT_DATA_BLOCKED capsules, however WT_DATA_BLOCKED capsules can be used as input to tuning of flow control algorithms and for debugging purposes.

WT_DATA_BLOCKED Capsule Format WT_DATA_BLOCKED capsules contain the following field:

Maximum Data:: A variable-length integer indicating the session-level limit at which blocking occurred.

The WT_DATA_BLOCKED capsule defines special intermediary handling, as described in . Intermediaries MUST consume WT_DATA_BLOCKED capsules for flow control purposes and MUST generate and send appropriate flow control signals for their limits (see ).

Session Termination A WebTransport session over HTTP/3 is considered terminated when either of the following conditions is met:

the CONNECT stream is closed, either cleanly or abruptly, on either side; or
a WT_CLOSE_SESSION capsule is either sent or received.

Upon learning that the session has been terminated, the endpoint MUST reset the send side and abort reading on the receive side of all unidirectional and bidirectional streams associated with the session (see ) using the WT_SESSION_GONE error code; it MUST NOT send any new datagrams or open any new streams. To terminate a session with a detailed error message, an application MAY provide such a message for the WebTransport endpoint to send in an HTTP capsule of type WT_CLOSE_SESSION (0x2843). The format of the capsule SHALL be as follows:

WT_CLOSE_SESSION Capsule Format WT_CLOSE_SESSION has the following fields:

Application Error Code:: A 32-bit error code provided by the application closing the session.
Application Error Message:: A UTF-8 encoded error message string provided by the application closing the session. The message takes up the remainder of the capsule, and its length MUST NOT exceed 1024 bytes.

Note that the Application Error Code field does not mirror the Error Code field in QUIC's CONNECTION_CLOSE frame () because WebTransport application errors use a subset of the HTTP/3 Error Code space and need to fit within those bounds, see . An endpoint that sends a WT_CLOSE_SESSION capsule MUST immediately send a FIN on the CONNECT Stream. The endpoint MAY also send a STOP_SENDING with error code WT_SESSION_GONE to indicate it is no longer reading from the CONNECT stream. The recipient MUST either close or reset the stream in response. If any additional stream data is received on the CONNECT stream after receiving a WT_CLOSE_SESSION capsule, the stream MUST be reset with code H3_MESSAGE_ERROR. Cleanly terminating a CONNECT stream without a WT_CLOSE_SESSION capsule SHALL be semantically equivalent to terminating it with a WT_CLOSE_SESSION capsule that has an error code of 0 and an empty error string. In some scenarios, an endpoint might want to send a WT_CLOSE_SESSION with detailed close information and then immediately close the underlying QUIC connection. If the endpoint were to do both of those simultaneously, the peer could potentially receive the CONNECTION_CLOSE before receiving the WT_CLOSE_SESSION, thus never receiving the application error data contained in the latter. To avoid this, the endpoint SHOULD wait until all CONNECT streams have been closed by the peer before sending the CONNECTION_CLOSE; this gives WT_CLOSE_SESSION properties similar to that of the QUIC CONNECTION_CLOSE mechanism as a best-effort mechanism of delivering application close metadata.

Considerations for Future Versions Future versions of WebTransport that change the syntax of the CONNECT requests used to establish WebTransport sessions will need to modify the upgrade token used to identify WebTransport, allowing servers to offer multiple versions simultaneously (see ). Servers that support future incompatible versions of WebTransport signal that support by changing the codepoint used for the SETTINGS_WT_ENABLED setting (see ). Clients can select the associated upgrade token, if applicable, to use when establishing a new session, ensuring that servers will always know the syntax in use for every incoming request. Changes to future stream formats require changes to the Unidirectional Stream type (see ) and Bidirectional Stream signal value (see ) to allow recipients of incoming frames to determine the WebTransport version, and corresponding wire format, used for the session associated with that stream.

Negotiating the Draft Version [[RFC editor: please remove this section before publication.]] The wire format aspects of the protocol are negotiated by changing the codepoint used for the SETTINGS_WT_ENABLED setting. Each draft version defines a distinct codepoint for SETTINGS_WT_ENABLED. Both the client and the server MUST send SETTINGS_WT_ENABLED with the codepoint corresponding to their supported draft version. An endpoint that supports multiple draft versions sends a SETTINGS_WT_ENABLED value for each supported version, as each version uses a different setting identifier. The highest version supported by both endpoints is selected. Because data streams can arrive at the server before the CONNECT request that establishes the associated session, and the wire format of the stream header depends on the negotiated version, the server needs to know the client's version before processing any incoming WebTransport streams. For this reason, the server MUST NOT process any incoming WebTransport requests until the client's SETTINGS have been received.

Security Considerations WebTransport over HTTP/3 satisfies all of the security requirements imposed by on WebTransport protocols, thus providing a secure framework for client-server communication in cases when the application is potentially untrusted. WebTransport over HTTP/3 requires explicit opt-in through the use of an HTTP/3 setting; this avoids potential protocol confusion attacks by ensuring the HTTP/3 server explicitly supports it. It also requires the use of the Origin header for browser traffic, providing the server with the ability to deny access to Web-based applications that do not originate from a trusted origin. Just like HTTP traffic going over HTTP/3, WebTransport pools traffic to different origins within a single connection. Different origins imply different trust domains, meaning that the implementations have to treat each transport as potentially hostile towards others on the same connection. One potential attack is a resource exhaustion attack: since all of the WebTransport sessions share both congestion control and flow control context, a single application aggressively using up those resources can cause other sessions to stall. A WebTransport endpoint MUST implement flow control mechanisms if it allows a WebTransport session to share the transport connection with other WebTransport sessions. WebTransport endpoints SHOULD implement a fairness scheme that ensures that each session that shares a transport connection gets a reasonable share of controlled resources; this applies both to sending data and to opening new streams. An application could attempt to exhaust resources by opening too many WebTransport sessions at once. In cases when the application is untrusted, a WebTransport client SHOULD limit the number of outgoing sessions it will open. Note that the security considerations of HTTP/3 apply to WebTransport over HTTP/3. In particular, the denial-of-service considerations in are relevant. WebTransport extends HTTP/3 with additional features that have legitimate uses but can become a burden when they are used unnecessarily or to excess. WebTransport introduces new interaction modes that permit either endpoint to send streams and datagrams to its peer. This is particularly novel for clients, which previously had limited exposure to unsolicited server-initiated traffic beyond server push (see ). An endpoint that does not monitor use of these features exposes itself to a risk of denial-of-service attack. Implementations SHOULD track the use of WebTransport features, such as the number of incoming streams and datagrams, and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error of type H3_EXCESSIVE_LOAD.

IANA Considerations This document registers an upgrade token (), HTTP/3 settings (), an HTTP/3 stream type (), an HTTP/3 error code (), and an HTTP header field ().

Upgrade Token Registration The following entry is added to the "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" registry established by Section 16.7 of . The "webtransport-h3" label identifies HTTP/3 used as a protocol for WebTransport:

Value:: webtransport-h3
Description:: WebTransport over HTTP/3
Reference:: This document

HTTP/3 SETTINGS Parameter Registration The following entry is added to the "HTTP/3 Settings" registry established by :

Setting Name:: SETTINGS_WT_ENABLED
Value:: 0x2c7cf000
Default:: 0
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Setting Name:: SETTINGS_WT_INITIAL_MAX_STREAMS_UNI
Value:: 0x2b64
Default:: 0
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Setting Name:: SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI
Value:: 0x2b65
Default:: 0
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Setting Name:: SETTINGS_WT_INITIAL_MAX_DATA
Value:: 0x2b61
Default:: 0
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org

Frame Type Registration The following entry is added to the "HTTP/3 Frame Type" registry established by : The WT_STREAM frame is reserved for the purpose of avoiding collision with WebTransport HTTP/3 extensions:

Value:: 0x41
Frame Type:: WT_STREAM
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org

Stream Type Registration The following entry is added to the "HTTP/3 Stream Type" registry established by : The "WebTransport stream" type allows unidirectional streams to be used by WebTransport:

Value:: 0x54
Stream Type:: WebTransport stream
Reference:: This document
Sender:: Both
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org

HTTP/3 Error Code Registration The following entries are added to the "HTTP/3 Error Code" registry established by :

Name:: WT_BUFFERED_STREAM_REJECTED
Value:: 0x3994bd84
Description:: WebTransport data stream rejected due to lack of associated session.
Reference:: This document.
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Name:: WT_SESSION_GONE
Value:: 0x170d7b68
Description:: WebTransport data stream aborted because the associated WebTransport session has been closed. Also used to indicate that the endpoint is no longer reading from the CONNECT stream.
Specification:: This document.
Name:: WT_FLOW_CONTROL_ERROR
Value:: 0x045d4487
Description:: WebTransport session aborted because a flow control error was encountered.
Reference:: This document.
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Name:: WT_ALPN_ERROR
Value:: 0x0817b3dd
Description:: WebTransport session aborted because application protocol negotiation failed.
Reference:: This document.
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Name:: WT_REQUIREMENTS_NOT_MET
Value:: 0x212c0d48
Description:: HTTP/3 connection closed because the features required for WebTransport are not supported. Either the client or server is missing required SETTINGS or transport parameters needed for WebTransport.
Reference:: This document.
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org

In addition, the following range of entries is registered:

Name:: WT_APPLICATION_ERROR
Value:: 0x52e4a40fa8db to 0x52e5ac983162 inclusive, with the exception of the codepoints of form 0x1f * N + 0x21.
Description:: WebTransport application error codes.
Reference:: This document.
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org

Capsule Types The following entries are added to the "HTTP Capsule Types" registry established by : The WT_CLOSE_SESSION capsule.

Value:: 0x2843
Capsule Type:: WT_CLOSE_SESSION
Status:: permanent
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

The WT_DRAIN_SESSION capsule.

Value:: 0x78ae
Capsule Type:: WT_DRAIN_SESSION
Status:: provisional (when this document is approved this will become permanent)
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

The WT_MAX_STREAMS capsule:

Value:: 0x190B4D3F..0x190B4D40
Capsule Type:: WT_MAX_STREAMS
Status:: permanent
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

The WT_STREAMS_BLOCKED capsule:

Value:: 0x190B4D43..0x190B4D44
Capsule Type:: WT_STREAMS_BLOCKED
Status:: permanent
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

The WT_MAX_DATA capsule:

Value:: 0x190B4D3D
Capsule Type:: WT_MAX_DATA
Status:: permanent
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

The WT_DATA_BLOCKED capsule:

Value:: 0x190B4D41
Capsule Type:: WT_DATA_BLOCKED
Status:: permanent
Reference:: This document
Change Controller:: IETF
Contact:: WebTransport Working Group webtransport@ietf.org
Notes:: None

Protocol Negotiation HTTP Header Fields The following HTTP header fields are used for negotiating a protocol (). These are added to the "HTTP Field Name" registry established in : The WT-Available-Protocols field:

Field Name:: WT-Available-Protocols
Status:: permanent
Structured Type:: List
Reference:
Comments:: None

The WT-Protocol field:

Field Name:: WT-Protocol
Status:: permanent
Structured Type:: Item
Reference:
Comments:: None