Code change:
However, QUIC DATAGRAM frames do not provide a means to demultiplex application contexts.  This document describes how to use QUIC DATAGRAM frames when the application protocol running over QUIC is <del> HTTP/3 H3.  It defines logical flows identified by a non-negative integer that are present at the start of the DATAGRAM frame payload. Flows are associated with HTTP messages using the Datagram-Flow-Id header field, allowing endpoints to match unreliable DATAGRAMS frames to the HTTP messages that they are related to. This design mimics the use of Stream Types in HTTP/3, which provide a demultiplexing identifier at the start of each unidirectional stream. </del> <ins> HTTP/3 H3.  It associates datagrams with client-initiated bidirectional streams and defines an optional additional demultiplexing layer. </ins> Discussion of this work is encouraged to happen on the MASQUE IETF mailing list (masque@ietf.org [2]) or on the GitHub repository which

Related discussion:
text: This PR aims to take the design discussions we had at the last interim, and propose a potential solution. This PR should in theory contain enough information to allow folks to implement this new design. That will allow us to perform interop testing and confirm whether the design is sound.
I'm confused by this pull request (probably because I was out of the loop for a while). My understanding, at least when I first read the proposal a month ago was that we would separate multiplexing streams and multiplexing contexts into different layers, so that methods that need context would get it (CONNECT-UDP), and features that don't (WebTransport/extended CONNECT) could use stream-associated datagrams directly.
NAME you're referring to which this PR isn't trying to address. That'll happen in a subsequent PR.
In , capsules, contexts and context IDs are explicitly defined as end-to-end and there are requirements for intermediaries to not modify them, and to not even parse them. The motivation for this was to ensure that we retain the ability to deploy extensions end-to-end without having to modify intermediaries. NAME mentioned that these requirements might be too strict, as he would like to have his intermediary be able to parse the context ID, for logging purposes.
Part of the comment from the PR is also about consistency of interface for generic HTTP libraries that can be used to build either endpoints or intermediaries. If the context ID is always present, it's overly prescriptive to say that an intermediary can't parse it, and such a requirement couldn't be enforced anyways. If context IDs are negotiated on/off, whether this is possible depends on the negotiation mechanism.
I think its fine for a component that has bytes passing through it to look at those bytes. The import thing to do is set expectations on what that means for the protocol behaviour we are defining. We don't expect intermediaries to do anything with a context ID; they should not enforce the protocol semantic rules (odd, even, reuse etc). H2 and H3 somewhat solve this problem by decoupling frame parsing from frame payload interpretation. E.g. if the form of a request or response is invalid, an intermediary shouldn't pass it on. DATAGRAM is different because it isn't defined as an actual HTTP/3 frame with frame headers and frame payload.