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
