Code change:
the fact that there are applications using HTTP/3 datagrams enabled on this endpoint. <del> 9. </del> <ins> 8. 8.1. This document will request IANA to register the following entry in the "HTTP/3 Frames" registry: </ins> <del> 9.1. </del> <ins> 8.2. </ins> This document will request IANA to register the following entry in the "HTTP/3 Settings" registry: <del> 9.2. </del> <ins> 8.3. </ins> <del> This document will request IANA to register the "Datagram-Flow-Id" header field in the "Permanent Message Header Field Names" registry maintained at <<https://www.iana.org/assignments/message-headers>>. </del> <ins> This document establishes a registry for HTTP/3 frame type codes. The "HTTP Capsule Types" registry governs a 62-bit space. Registrations in this registry MUST include the following fields: Type: A name or label for the capsule type. The value of the Capsule Type field (see capsule-frame) is a 62bit integer. An optional reference to a specification for the type.  This field MAY be empty. Registrations follow the "First Come First Served" policy (see Section 4.4 of IANA-POLICY) where two registrations MUST NOT have the same Type. </ins> <del> 9.3. </del> <ins> This registry initially contains the following entries: </ins> <del> This document will request IANA to create an "HTTP Datagram Flow Parameters" registry.  Registrations in this registry MUST include the following fields: </del> <ins> 8.4. </ins> <del> The key of a parameter that is associated with a datagram flow list member (see header).  Keys MUST be valid structured field parameter keys (see Section 3.1.2 of STRUCT-FIELD). </del> <ins> REGISTER_DATAGRAM_CONTEXT capsules carry key-value pairs, see register-capsule.  This document will request IANA to create an "HTTP Datagram Context Extension Keys" registry.  Registrations in this registry MUST include the following fields: </ins> <del> A brief description of the parameter semantics, which MAY be a summary if a specification reference is provided. </del> <ins> The key (see register-capsule).  Keys MUST be valid tokens as defined in Section 3.2.6 of RFC7230. </ins> <del> This field MUST be either Yes or No.  Yes indicates that this parameter is the name of a named element (see header).  No indicates that it is a parameter that is not a name. </del> <ins> A brief description of the key semantics, which MAY be a summary if a specification reference is provided. </ins> An optional reference to a specification for the parameter.  This field MAY be empty.

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.