Update WebTransport authentication process.

Transport ID is created during publishing and subscribing, it would
be too late for authentication. A token for WebTransport connection
is issued during joining so client can create a WebTransport connection
just after joining.

Author: jianjunz

doc/design/quic-transport-payload-format.md

@@ -25,7 +25,20 @@ Each package has a 128 bit header for session ID.
 
 It may increase about 2% network cost.
 
+## Signaling Session
+
+After creating a WebTransport, a stream with session 0 should be created for authentication and signaling. Every signaling message is followed by a 32 bit length integer that indicates the body's length.
+
+```
+    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
+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+   |                      Message length                           |
+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+   |                          Message                            ...
+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
 
 ## Authentication
 
-After creating a WebTransport, a stream with session 0 should be created for authentication. If signaling is transmitted over WebTransport, authentication follows the regular process defined by [Client-Portal Protocol](https://github.com/open-webrtc-toolkit/owt-server/blob/master/doc/Client-Portal%20Protocol.md). Otherwise, client sends a 128 bit length transport ID to the server.
+If signaling messages are transmitted over WebTransport, authentication follows the regular process defined by [Client-Portal Protocol](https://github.com/open-webrtc-toolkit/owt-server/blob/master/doc/Client-Portal%20Protocol.md). Otherwise, client sends a token for WebTransport as a signaling message. WebTransport token is issued during joining a conference.