Add content for WebTransport

content/concepts/transport.md

@@ -218,6 +218,90 @@ two nodes using QUIC only takes a single RTT.
 Following the multiaddress format described earlier, a standard QUIC connection will
 look like: `/ip4/127.0.0.1/udp/65432/quic/`.
 
+## 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 exposes 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 
+(using the 
+[`serverCertificateHashes` option](https://www.w3.org/TR/webtransport/#dom-webtransportoptions-servercertificatehashes)), 
+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](#webtransport-in-libp2p) for how WebTransport addresses 
+achieve this).
+
+Therefore, WebTransport exhibits all the advantages of QUIC over TCP, that being 
+faster handshakes, no HoL blocking, and being future-proof.
+
+{{% notice "caution" %}}
+
+There is an experimental WebTransport transport in go-libp2p that is part 
+of the [v0.23 release](https://github.com/libp2p/go-libp2p/releases/tag/v0.23.0). 
+The implementation should be used experimentally and is not recommended for production 
+environments.
+
+js-libp2p also plans to release 
+[WebTransport support](https://github.com/libp2p/js-libp2p-webtransport) very soon.
+
+There are currently no concrete plans to support WebTransport beyond the Go and JS 
+implementations.
+
+{{% /notice %}}
+
+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](https://github.com/w3c/webtransport/issues/409).
+
+### WebTransport in libp2p
+
+WebTransport multiaddresses are composed of a QUIC multiaddr, followed 
+by `/webtransport` and a list of multihashes of the node certificates that the server uses.
+
+For instance, for multiaddress `/ip4/127.0.0.1/udp/123/quic/webtransport/certhash/<hash1>`, 
+a standard local QUIC connection is defined up until and including `/quic.` 
+Then, `/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 
+server advertising `/ip4/1.2.3.4/udp/1234/quic/webtransport/` would be 
+`https://1.2.3.4:1234/.well-known/libp2p-webtransport?type=noise` 
+(the ?type=noise refers to the authentication scheme using Noise).
+
 ## References
 
 [1] [What is HTTP/3 by Cloudspoint](https://cloudspoint.xyz/what-is-http3/)