Re: [quicwg/base-drafts] PRIORITY frame on control stream referencing unopened request stream (#2502)

Kazuho Oku <notifications@github.com> Wed, 08 May 2019 23:36 UTC

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Subject: Re: [quicwg/base-drafts] PRIORITY frame on control stream referencing unopened request stream (#2502)
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Thank you for the clarifications.

> when something has implicitly_initialized set, what priority is given to it, or more relevantly, to its children? I'm arguing that solving these misprioritizations is more complex than suggested.

In my implementation, implicitly-initialized nodes are associated to the root with weight set to 16. I agree that we have the chance of misprioritization here, and that we do no specify how servers should handle that. I also think that what my implementation does now is not optimal - if we are to learn from the lesson of HTTP/2 that some servers respect only the weights but not the structure of the tree, the server should consider children of implicitly-initialized nodes as direct descendants of the root with their respective weights, until the PRIORITY frame for the implicitly-initialized node is received.

That said, I am not too worried about the status quo, due to the following reasons:
* We are encouraging (or we should encourage) web browsers requesting HTML and assets to use Firefox-style dependency tree. That means that chance of misprioritization only exists during the exchange of very first flights when the PRIORITY frames for the placeholders are sent.
* For video streaming, daisy-chaining of streams is preferable. But because we can expect several round-trips to be spent between each request, misprioritization would not be an issue in practice.

> I continue to believe the latter is probably out of both scope and time for this working group, unless HTTPbis directs us otherwise.

+1. I also agree that it would be beneficial to call out the strategy (or strategies) that the servers might employ, and if necessary the side-effects of those strategies that the clients should take into consideration.

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