Code change:
streams, and those have stream IDs that are divisible by four.) A variable-length integer indicating the context ID of the <del> datagram (see datagram-contexts). </del> <ins> datagram (see datagram-contexts).  Whether or not this field is present depends on which registration capsules were exchanged on the associated stream: if a REGISTER_DATAGRAM_CONTEXT capsule (see register-capsule) has been sent or received on this stream, then the field is present; if a REGISTER_DATAGRAM_NO_CONTEXT capsule (see register-no-context-capsule) has been sent or received, then this field is absent; if neither has been sent or received, then it is not yet possible to parse this datagram and the receiver MUST either drop that datagram silently or buffer it temporarily while awaiting the registration capsule. </ins> The payload of the datagram, whose semantics are defined by individual applications.  Note that this field can be empty.

Related discussion:
text: This change allows clients to disable contexts for a given stream. This uses a new REGISTERDATAGRAMNO_CONTEXT capsule for negotiation. The important property here is that it requires capsule support even when contexts are not required. This is critical to the health of the ecosystem as it will ensure that intermediaries that support datagrams but not capsules cannot be deployed, as that would prevent future extensibility.
Editorial comments aside, I support the protocol feature of an optional DATAGRAM Context ID. Using a registration capsule, as proposed in this PR, is a very logical way of achieving that. I plan to implement this in my WIP code.
Looks like there's support for merging this. Some additional editorial work is needed, but we'll do that in followups.
This issue assumes that decide to go with the two-layer design described in . Given that design, some applications might not need the multiplexing provided by context IDs. We have multiple options here: Make context ID mandatory Applications that don't need it waste a byte per datagram Negotiate the presence of context IDs using an HTTP header Context IDs would still be mandatory to implement on servers because the client might send that header Have the method determine whether context IDs are present or not This would prevent extensibility on those methods Create a REGISTERNOCONTEXT message This assumes we use the Message design from We add a register message that means that this stream does not use context IDs This allows avoiding the byte overhead without sacrificing extensibility
Making the context ID mandatory is simple, although slightly wasteful. For the variations where it is optional, I prefer the one that uses a header. It could work with a variant of where we do Header + Message, and the Header determines whether or not you have contexts, and whether or not they can be dynamic.
What are the benefits of message over header to make it optional? To me it seems like header is sufficient, because the context IDs are end-to-end, and should not affect intermediaries, making the header a natural place to put it. I think having the context ID be the default behavior and the header is to opt-out is the right decision if we were to allow removing it.
This is a bit of a bike-shed, but why would it default to having context-IDs instead of not having them?