[Webtransport] Head-of-Line-Blocking of WebTransport Flow Control Messages

[Marten Seemann <[e-mail]>]

The current proposal (
https://datatracker.ietf.org/doc/draft-thomson-webtrans-session-limit/) for
transmitting flow control messages serializes the capsules onto the
WebTransport control stream. Since the control stream is a QUIC stream,
this means that these messages suffer from HoL blocking in the case of
packet loss.
Depending on the usage pattern of the WebTransport application, this will
matter more or less.

Here's an alternative way to solve this problem: By allowing WebTransport
to send the flow control messages / capsules directly on the QUIC
connection, these messages can be transmitted independently from each
other. This can be achieved by introducing a new QUIC frame: the MESSAGE
frame. MESSAGEs are kind of similar to DATAGRAM frames, with the important
distinction that MESSAGE frames are 1. delivered reliably to the
application, and 2. retransmitted / updated in case of packet loss.

For more details, please refer to my (very early-stage) draft:
https://marten-seemann.github.io/draft-seemann-quic-reliable-message/draft-seemann-quic-reliable-message.html