Inband priority exchange for stream/path scheduler

[markusa]

Problem

Scheduling and re-ordering mechanism are usually out of scope of a multi-path standard and is up to implementers and/or separate IETF work. However, there might be the wish to align between sender and receiver the priority and usage of paths. Multi-path protocols like MPTCP and MP-DCCP, see this in scope and define therefore a MP_PRIO option. So far, MP-QUIC does not define anything in this direction. Beyond path scheduling MP-QUIC also offers stream multiplexing, where it can be useful to have a prioritization indication for stream schedulers at both ends. Per https://datatracker.ietf.org/doc/html/rfc9000#section-2.3, stream prioritization is so far not not part of QUIC v1.

Request

Define a prioritization exchange for paths and streams within the MP-QUIC draft.

Benefit

This facilitates scenarios where both sides should apply the same path/stream prioritization (e.g. trigger a handover or respect cost) or the information about the sender prioritization is useful for e.g. re-ordering mechanisms on receiver side. While this can be applied in end-to-end scenarios it will be in particular useful for Hybrid Access and 3GPP ATSSS.

Btw. that was the original question I intended to ask in #81 , but probably misleading :grinning:

[mirjak]

This might be a bit related to issue #22. I believe we already decided that a more advanced priority scheme will not be part of the base draft, however, you can make a proposal for an additional extension to provide such a scheme in a separate draft.

[markusa]

I see that there was a consensus at least from the few people who engaged in the #22 discussion. In respect to the 3GPP ATSSS Rel. 18 development, where MP-QUIC was adopted (beside MP-DCCP) for the Study Phase, it's probably not useful to have too much defined outside the basic MP-QUIC draft?!