WebTransport over HTTP/3 Google
Transport WebTransport 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 Http3Transport, a WebTransport protocol that is based on HTTP/3 and provides support for unidirectional streams, bidirectional streams and datagrams, all multiplexed within the same HTTP/3 connection.
HTTP/3 is a protocol defined on top of QUIC that can multiplex HTTP requests over a QUIC connection. This document defines Http3Transport, a mechanism for multiplexing non-HTTP data with HTTP/3 in a manner that conforms with the WebTransport protocol framework . Using the mechanism described here, multiple Http3Transport instances can be multiplexed simultaneously with regular HTTP traffic on the same HTTP/3 connection.
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.
Http3Transport servers are identified by a pair of authority value and path value (defined in Sections 3.2 and 3.3 correspondingly). When an HTTP/3 connection is established, the client and server have to negotiate a specific set of QUIC transport parameters that indicate support for various Http3Transport features. Most notably, the http3_transport_support parameter signals Http3Transport support to the peer. Http3Transport sessions are initiated inside a given HTTP/3 connection by the client, who sends an extended CONNECT request . If the server accepts the request, an Http3Transport session is established. As a part of this process, the client proposes, and the server confirms, a session ID. A session ID (SID) is unique within a given HTTP/3 connection, and is used to associate all of the streams and datagrams with the specific session. After the session is established, the peers can exchange data using the following mechanisms: A client can create a bidirectional stream using a special indefinite-length HTTP/3 frame that transfers ownership of the stream to Http3Transport. A server can create a bidirectional stream, which is possible since HTTP/3 does not define any semantics for server-initiated bidirectional streams. Both client and server can create a unidirectional stream using a special stream type. A datagram can be sent using a QUIC DATAGRAM frame . An Http3Transport session is terminated when the CONNECT stream that created it is closed.
In order to allow multiple Http3Transport sessions to occur within the same HTTP/3 connection, Http3Transport assigns every session a unique ID, further referred to as session ID. A session ID is a 62-bit number that is unique within the scope of an HTTP/3 connection, and is never reused even after the session is closed. The client unilaterally picks the session ID. As the IDs are encoded using QUIC variable length integers, the client SHOULD start with zero and then sequentially increment the IDs. A session ID is considered to be used, and thus ineligible for new transports, as soon as the client sends a request proposing it. These reuse requirements guarantee that both HTTP/3 endpoints have a consistent view of the session ID space. The Session ID is a hop-by-hop property: if Http3Transport is proxied, the same session can have different IDs from the client’s and server’s perspective. Because of that, session IDs SHOULD NOT be exposed to the application.
In order to indicate support for Http3Transport, both the client and server MUST send an empty http3_transport_support transport parameter. Endpoints MUST NOT use any Http3Transport-related functionality unless the parameter has been negotiated. The negotiation is done through a QUIC transport parameter instead of an HTTP/3-level setting as it allows the server to customize the transport parameters it intends to send based on whether the client has indicated support for Http3Transport. If http3_transport_support is negotiated, support for the QUIC DATAGRAM extension MUST be negotiated. The initial_max_bidi_streams MUST be greater than zero, overriding the existing requirement in .
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 http3_transport_support transport parameter implies that an endpoint supports extended CONNECT.
As Http3Transport sessions are established over HTTP/3, they are identified using the https URI scheme . In order to create a new Http3Transport session, a client can send an HTTP CONNECT request. The :protocol pseudo-header field 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. The client MUST pick a new session ID as described in and send it encoded as a hexadecimal literal in :sessionid header. An Origin header 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 6.5.4, ). If it does, it MAY accept the session by replying with status code 200. Before accepting it, the HTTP/3 server MUST verify that the proposed session ID does not conflict with any currently open sessions, and it MAY verify that it was not used ever before on this connection. 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, an Http3Transport session is established when the client receives a 200 response. From the server’s perspective, a session is established once it sends a 200 response. Both endpoints MUST NOT open any streams or send any datagrams on a given session before that session is established. Http3Transport does not support 0-RTT.
From the flow control perspective, Http3Transport 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 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.
Http3Transport provides the following features described in : unidirectional streams, bidirectional streams and datagrams, initiated by either endpoint. Session IDs are used to demultiplex streams and datagrams belonging to different Http3Transport sessions. On the wire, session IDs are encoded using the QUIC variable length integer scheme described in .
Once established, both endpoints can open unidirectional streams. The HTTP/3 control 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 ().
Http3Transport clients 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 (). The frame SHALL last until the end of the stream.
Http3Transport servers can initiate bidirectional streams by opening a bidirectional stream within the HTTP/3 connection. Note that since HTTP/3 does not define any semantics for server-initiated bidirectional streams, this document is a normative reference for the semantics of such streams for all HTTP/3 connections in which the http3_transport_support option is negotiated. The format of those streams SHALL be the session ID, encoded as a variable-length integer, followed by the user-specified stream data ().
Datagrams can be sent using the DATAGRAM frame as defined in and . For all HTTP/3 connections in which the http3_transport_support option is negotiated, the Flow Identifier is set to the session ID. In other words, the format of datagrams SHALL be the session ID, followed by the user-specified payload ().
In QUIC, a datagram frame can span at most one packet. Because of that, the applications have to know the maximum size of the datagram they can send. However, when proxying the datagrams, the hop-by-hop MTUs can vary. TODO: Describe how the path MTU can be computed, specifically propagation across HTTP proxies.
An Http3Transport is terminated when either endpoint closes the stream associated with the CONNECT request that initiated the session. Upon learning about the session being terminated, the endpoint MUST stop sending new datagrams and reset all of the streams associated with the session.
Http3Transport supports most of the WebTransport features described in . Property Support Stream independence Always supported Partial reliability Always supported Pooling support Always supported Connection mobility Implementation-dependent
Http3Transport satisfies all of the security requirements imposed by on WebTransport protocols, thus providing a secure framework for client-server communication in cases when the client is potentially untrusted. Since HTTP/3 is QUIC-based, a lot of the analysis in applies here. Http3Transport requires explicit opt-in through the use of a QUIC transport parameter; 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/3 itself, Http3Transport 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.
The following entry is added to the “Hypertext Transfer Protocol (HTTP) Upgrade Token Registry” registry established by : The “webtransport” label identifies HTTP/3 used as a protocol for WebTransport: webtransport WebTransport over HTTP/3 This document
The following entry is added to the “QUIC Transport Parameter Registry” registry established by : The http3_transport_support parameter indicates that the specified HTTP/3 connection is Http3Transport-capable. 0x???? http3_transport_support This document
The following entry is added to the “HTTP/3 Frame Type” registry established by : The WEBTRANSPORT_STREAM frame allows HTTP/3 client-initiated bidirectional streams to be used by WebTransport: 0x54 WEBTRANSPORT_STREAM This document
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: 0x41 WebTransport stream This document Both