HTTP Priority - proposal to support headers and frames

Hello WG,

If you recall, in March we published draft-ietf-http-priority-00, which
fixed a few issues that we gave an overview on the mailing list [1].

One of the remaining issues is concerned about the use of headers vs
frames. Tracking the range of different threads, the answer seems to having
an ability to use either.

PR 1167 is a proposal that tweaks the PRIORITY_UPDATE frame in a few ways.
The most interesting tweak is to make it more explicit that the frame can
be used to signal the initial priority of a request [2], this frame can
only be sent on stream 0 / control stream. This reflects my understanding
of how Chrome is using the PRIORITY_UPDATE frame today.

This proposal avoids sending a new priority frame on the request stream.
Doing that adds some complication for H2. Further, H3 would have to deal
with the possibility of out-of-order delivery anyway, so having another
frame does not help it.

We welcome comments on the issue, the PR or in this thread.

Cheers
Lucas

[1] - https://lists.w3.org/Archives/Public/ietf-http-wg/2020JanMar/0171.html
[2] - https://github.com/httpwg/http-extensions/pull/1167