Internet-Draft WebTransport-H3 July 2022
Frindell, et al. Expires 7 January 2023 [Page]
Network Working Group
Intended Status:
Standards Track
A. Frindell
E. Kinnear
Apple Inc.
V. Vasiliev

WebTransport over HTTP/3


WebTransport [OVERVIEW] is a protocol framework that enables clients constrained by the Web security model to communicate with a remote server using a secure multiplexed transport. This document describes a WebTransport protocol that is based on HTTP/3 [HTTP3] and provides support for unidirectional streams, bidirectional streams and datagrams, all multiplexed within the same HTTP/3 connection.

Note to Readers

Discussion of this draft takes place on the WebTransport mailing list (, which is archived at <>.

The repository tracking the issues for this draft can be found at <>. The web API draft corresponding to this document can be found at <>.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 7 January 2023.

Table of Contents

1. Introduction

HTTP/3 [HTTP3] is a protocol defined on top of QUIC [RFC9000] 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 [OVERVIEW]. Using the mechanism described here, multiple WebTransport instances can be multiplexed simultaneously with regular HTTP traffic on the same HTTP/3 connection.

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This document follows terminology defined in Section 1.2 of [OVERVIEW]. 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.

2. Protocol Overview

WebTransport servers in general are identified by a pair of authority value and path value (defined in [RFC3986] Sections 3.2 and 3.3 correspondingly).

When an HTTP/3 connection is established, both the client and server have to send a SETTINGS_ENABLE_WEBTRANSPORT setting in order to indicate that they both support WebTransport over HTTP/3.

WebTransport sessions are initiated inside a given HTTP/3 connection by the client, who sends an extended CONNECT request [RFC8441]. If the server accepts the request, an 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 peers can exchange data using the following mechanisms:

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

3. Session Establishment

3.1. Establishing a Transport-Capable HTTP/3 Connection

In order to indicate support for WebTransport, both the client and the server MUST send a SETTINGS_ENABLE_WEBTRANSPORT value set to "1" in their SETTINGS frame. The SETTINGS_ENABLE_WEBTRANSPORT parameter value SHALL be either "0" or "1", with "0" being the default; an endpoint that receives a value other than "0" or "1" MUST close the connection with the H3_SETTINGS_ERROR error code.

The client MUST NOT send a WebTransport request until it has received the setting indicating WebTransport support from the server. Similarly, the server MUST NOT process any incoming WebTransport requests until the client settings have been received, as the client may be using a version of WebTransport extension that is different from the one used by the server.

3.2. Extended CONNECT in HTTP/3

[RFC8441] defines an extended CONNECT method in Section 4, enabled by the SETTINGS_ENABLE_CONNECT_PROTOCOL parameter. That parameter is only defined for HTTP/2. This document does not create a new multi-purpose parameter to indicate support for extended CONNECT in HTTP/3; instead, the SETTINGS_ENABLE_WEBTRANSPORT setting implies that an endpoint supports extended CONNECT.

3.3. Creating a New Session

As WebTransport sessions are established over HTTP/3, they are identified using the https URI scheme ([HTTP], Section 4.2.2).

In order to create a new WebTransport session, a client can send an HTTP CONNECT request. The :protocol pseudo-header field ([RFC8441]) MUST be set to webtransport. The :scheme field MUST be https. Both the :authority and the :path value MUST be set; those fields indicate the desired WebTransport server. An Origin header [RFC6454] MUST be provided within the request.

Upon receiving an extended CONNECT request with a :protocol field set to webtransport, 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 (Section 15.5.4, [HTTP]). If it does, it MAY accept the session by replying with a 2xx series status code, as defined in Section 15.3 of [HTTP]. The WebTransport server MUST verify the Origin header to ensure that the specified origin is allowed to access the server in question.

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. WebTransport over HTTP/3 does not support 0-RTT.

The webtransport HTTP Upgrade Token uses the Capsule Protocol as defined in [HTTP-DATAGRAM].

3.4. Limiting the Number of Simultaneous Sessions

From the flow control perspective, WebTransport sessions count against the stream flow control just like regular HTTP requests, since they are established via an HTTP CONNECT request. This document does not make any effort to introduce a separate flow control mechanism for sessions, nor to separate HTTP requests from WebTransport data streams. If the server needs to limit the rate of incoming requests, it has alternative mechanisms at its disposal:

  • HTTP_REQUEST_REJECTED error code defined in [HTTP3] 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 [RFC6585]. Unlike the previous method, this signal is directly propagated to the application.

4. WebTransport Features

WebTransport over HTTP/3 provides the following features described in [OVERVIEW]: unidirectional streams, bidirectional streams and datagrams, initiated by either endpoint.

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 [RFC9000].

If at any point a session ID is received that cannot a valid ID for a client-initiated bidirectional stream, the recepient MUST close the connection with an H3_ID_ERROR error code.

4.1. Unidirectional streams

Once established, both endpoints can open 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 (Figure 1).

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |                           0x54 (i)                          ...
  |                        Session ID (i)                       ...
  |                         Stream Body                         ...
Figure 1: Unidirectional WebTransport stream format

4.2. Bidirectional Streams

WebTransport endpoints can initiate bidirectional streams by opening an HTTP/3 bidirectional stream and sending an HTTP/3 frame with type WEBTRANSPORT_STREAM (type=0x41). The format of the frame SHALL be the frame type, followed by the session ID, encoded as a variable-length integer, followed by the user-specified stream data (Figure 2). The frame SHALL last until the end of the stream.

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |                           0x41 (i)                          ...
  |                        Session ID (i)                       ...
  |                         Stream Body                         ...
Figure 2: WEBTRANSPORT_STREAM frame format

HTTP/3 does not by itself define any semantics for server-initiated bidirectional streams. If WebTransport setting is negotiated by both endpoints, the syntax of the server-initiated bidirectional streams SHALL be the same as the syntax of client-initated bidirectional streams, that is, a sequence of HTTP/3 frames. The only frame defined by this document for use within server-initiated bidirectional streams is WEBTRANSPORT_STREAM.

TODO: move the paragraph above into a separate draft; define what happens with already existing HTTP/3 frames on server-initiated bidirectional streams.

4.3. 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 SHALL 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 8-bit integer, assuming values between 0x00 and 0xff. WebTransport implementations SHALL remap those error codes into an error range where 0x00 corresponds to 0x52e4a40fa8db, and 0xff corresponds to 0x52e4a40fa9e2. Note that there are code points inside that range of form "0x1f * N + 0x21" that are reserved by Section 8.1 of [HTTP3]; those have to be accounted for when mapping the error codes by skipping them (i.e. the two HTTP/3 error codepoints adjacent to a GREASE codepoint would map to two adjacent WebTransport application error codepoints). An example pseudocode can be seen in Figure 3.

    first = 0x52e4a40fa8db
    last = 0x52e4a40fa9e2

    def webtransport_code_to_http_code(n):
        return first + n + floor(n / 0x1e)

    def http_code_to_webtransport_code(h):
        assert(first <= h <= last)
        assert((h - 0x21) % 0x1f != 0)
        shifted = h - first
        return shifted - shifted // 0x1f
Figure 3: Pseudocode for converting between WebTransport application errors and HTTP/3 error codes; here, `//` is integer division

WebTransport data streams are associated with sessions through a header at the beginning of the stream; resetting a stream may result in that data being discarded. Because of that, WebTransport application error codes are best effort, as the WebTransport stack is not always capable of associating the reset code with a session. The only exception is the situation where there is only one session on a given HTTP/3 connection, and no intermediaries between the client and the server.

WebTransport implementations SHALL forward the error code for a stream associated with a known session to the application that owns that session; similarly, the intermediaries SHALL reset the streams with corresponding error code when receiving a reset from the peer. If a WebTransport implementation intentionally allows only one session over a given HTTP/3 connection, it SHALL forward the error codes within WebTransport application error code range to the application that owns the only session on that connection.

4.4. 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.

4.5. Buffering Incoming Streams and Datagrams

In WebTransport over HTTP/3, the client MAY send its SETTINGS frame, as well as 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 those can be associated with an established session. To avoid resource exhaustion, the 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 H3_WEBTRANSPORT_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.

4.6. Interaction with HTTP/3 GOAWAY frame

HTTP/3 defines a graceful shutdown mechanism (Section 5.2 of [HTTP3]) that allows a peer to send a GOAWAY frame indicating that it will no longer accept any new incoming requests or pushes. This mechanism applies to the CONNECT requests for new WebTransport sessions. A GOAWAY frame does not affect data streams for existing WebTransport sessions; those can continue to be opened even after the GOAWAY frame has been sent or received.

5. Session Termination

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

Upon learning that the session has been terminated, the endpoint MUST reset the send side and abort reading on the receive side of all of the streams associated with the session (see Section 2.4 of [RFC9000]) using the H3_WEBTRANSPORT_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 send an HTTP capsule [HTTP-DATAGRAM] of type CLOSE_WEBTRANSPORT_SESSION (0x2843). The format of the capsule SHALL be as follows:

  Length (i),
  Application Error Code (32),
  Application Error Message (..8192),

CLOSE_WEBTRANSPORT_SESSION has the following fields:

Application Error Code:

A 32-bit error code provided by the application closing the connection.

Application Error Message:

A UTF-8 encoded error message string provided by the application closing the connection. The message takes up the remainer of the capsule, and its length MUST NOT exceed 1024 bytes.

A CLOSE_WEBTRANSPORT_SESSION capsule MUST be followed by a FIN on the sender side. If any additional stream data is received on the CONNECT stream after CLOSE_WEBTRANSPORT_SESSION, the stream MUST be reset with code H3_MESSAGE_ERROR. The recipient MUST close the stream upon receiving a FIN. If the sender of CLOSE_WEBTRANSPORT_SESSION does not receive a FIN after some time, it SHOULD send STOP_SENDING on the CONNECT stream.

Cleanly terminating a CONNECT stream without a CLOSE_WEBTRANSPORT_SESSION capsule SHALL be semantically equivalent to terminating it with a CLOSE_WEBTRANSPORT_SESSION capsule that has an error code of 0 and an empty error string.

6. Negotiating the Draft Version

[[RFC editor: please remove this section before publication.]]

WebTransport over HTTP/3 uses two different mechanisms to negotiate versions for the different parts of the draft.

The hop-by-hop wire format aspects of the protocol are negotiated by changing the codepoint used for the SETTINGS_ENABLE_WEBTRANSPORT parameter. Because of that, any WebTransport endpoint MUST wait for the peer's SETTINGS frame before sending or processing any WebTransport traffic. When multiple versions are supported by both of the peers, the most recent version supported by both is selected.

The data exchanged over the CONNECT stream is transmitted across intermediaries, and thus cannot be versioned using a SETTINGS parameter. To indicate support for different versions of the protocol defined in this draft, the clients SHALL send a header for each version of the draft supported. The header corresponding to the version described in this draft is Sec-Webtransport-Http3-Draft02; its value SHALL be 1. The server SHALL reply with a Sec-Webtransport-Http3-Draft header indicating the selected version; its value SHALL be draft02 for the version described in this draft.

7. Security Considerations

WebTransport over HTTP/3 satisfies all of the security requirements imposed by [OVERVIEW] on WebTransport protocols, thus providing a secure framework for client-server communication in cases when the client 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, providing the server with the ability to deny access to Web-based clients 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 transports share both congestion control and flow control context, a single client aggressively using up those resources can cause other transports to stall. The user agent thus SHOULD implement a fairness scheme that ensures that each transport within connection gets a reasonable share of controlled resources; this applies both to sending data and to opening new streams.

8. IANA Considerations

8.1. Upgrade Token Registration

The following entry is added to the "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" registry established by Section 16.7 of [HTTP].

The "webtransport" label identifies HTTP/3 used as a protocol for WebTransport:




WebTransport over HTTP/3


This document and [I-D.ietf-webtrans-http2]

8.2. HTTP/3 SETTINGS Parameter Registration

The following entry is added to the "HTTP/3 Settings" registry established by [HTTP3]:

The SETTINGS_ENABLE_WEBTRANSPORT parameter indicates that the specified HTTP/3 connection is WebTransport-capable.

Setting Name:







This document

8.3. Frame Type Registration

The following entry is added to the "HTTP/3 Frame Type" registry established by [HTTP3]:

The WEBTRANSPORT_STREAM frame allows HTTP/3 client-initiated bidirectional streams to be used by WebTransport:



Frame Type:



This document

8.4. Stream Type Registration

The following entry is added to the "HTTP/3 Stream Type" registry established by [HTTP3]:

The "WebTransport stream" type allows unidirectional streams to be used by WebTransport:



Stream Type:

WebTransport stream


This document



8.5. HTTP/3 Error Code Registration

The following entry is added to the "HTTP/3 Error Code" registry established by [HTTP3]:






WebTransport data stream rejected due to lack of associated session.


This document.






WebTransport data stream aborted because the associated WebTransport session has been closed.


This document.

In addition, the following range of entries is registered:




0x52e4a40fa8db to 0x52e4a40fa9e2 inclusive, with the exception of 0x52e4a40fa8f9, 0x52e4a40fa918, 0x52e4a40fa937, 0x52e4a40fa956, 0x52e4a40fa975, 0x52e4a40fa994, 0x52e4a40fa9b3, and 0x52e4a40fa9d2.


WebTransport application error codes.


This document.

9. References

9.1. Normative References

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <>.
Schinazi, D. and L. Pardue, "HTTP Datagrams and the Capsule Protocol", Work in Progress, Internet-Draft, draft-ietf-masque-h3-datagram-11, , <>.
Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, , <>.
Vasiliev, V., "The WebTransport Protocol Framework", Work in Progress, Internet-Draft, draft-ietf-webtrans-overview-03, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <>.
Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, , <>.
Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
McManus, P., "Bootstrapping WebSockets with HTTP/2", RFC 8441, DOI 10.17487/RFC8441, , <>.
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, , <>.

9.2. Informative References

Frindell, A., Kinnear, E., Pauly, T., Thomson, M., Vasiliev, V., and G. Xie, "WebTransport using HTTP/2", Work in Progress, Internet-Draft, draft-ietf-webtrans-http2-03, , <>.

Authors' Addresses

Alan Frindell
Eric Kinnear
Apple Inc.
Victor Vasiliev