What is WebTransport?
While browsers perform HTTP request using HTTP/3, so far they don’t offer an API to allow applications to gain access to the underlying QUIC stream. WebTransport is a new specification that uses QUIC to offer an alternative to WebSocket. Conceptually, it can be considered WebSocket over QUIC. It allows browsers to establish a stream-multiplexed and bidirectional connection to servers, and use streams to send and receive application data.
While WebSocket provides a single bidirectional, full-duplex communication between a browser and a server over a TCP connection, WebTransport allows the endpoints to use multiple streams in parallel.
When connecting to a WebSocket server, browsers require the server to present a
TLS certificate signed by a trusted CA (certificate authority). Few nodes have such
a certificate, which is the reason that WebSocket never saw widespread adoption in the
libp2p network. libp2p WebTransport offers a browser API that includes a way to
accept the server’s certificate by checking the (SHA-256) hash of the certificate
even if the certificate is “just” a self-signed certificate. This allows us to connect
any browser node to any server node, as long as the browser knows the certificate hash in
advance (see WebTransport in libp2p for how WebTransport addresses
Therefore, WebTransport exhibits all the advantages of QUIC over TCP, that being faster handshakes, no HoL blocking, and being future-proof.
For network stacks like libp2p, WebTransport is a pluggable protocol that fits well with a modular network design.
For a standard WebSocket connection, the roundtrips required are as follows:
- 1 RTT for TCP handshake
- 1 RTT for TLS 1.3 handshake
- 1 RTT for WebSocket upgrade
- 1 RTT for multistream security negotiation (Noise or TLS 1.3)
- 1 RTT for security handshake (Noise or TLS 1.3)
- 1 RTT for multistream muxer negotiation (mplex or yamux)
In total, 6 RTTs.
WebTransport running over QUIC only requires 3 RTTs, as:
- 1 RTT for QUIC handshake
- 1 RTT for WebTransport handshake
- 1 RTT for libp2p handshake; one for multistream and one for authentication (with a Noise handshake)
In principle, the WebTransport protocol would even allow running the WebTransport handshake and the Noise handshake in parallel. However, this is currently not possible since the browser API doesn’t allow that yet.
WebTransport in libp2p
WebTransport multiaddresses are composed of a QUIC multiaddr, followed
/webtransport and a list of multihashes of the node certificates that the server uses.
For instance, for multiaddress
a standard local QUIC connection is defined up until and including
/webtransport/ runs over QUIC. The self-signed certificate hash that the
server will use to verify the connection.
The WebTransport CONNECT request is sent to an HTTPS endpoint. libp2p WebTransport server use
/.well-known/libp2p-webtransport. For instance, the WebTransport URL of a WebTransport
/ip4/192.0.2.0/udp/1234/quic/webtransport/ would be
(the ?type=noise refers to the authentication scheme using Noise).