Lifting HTTP/3 Features into Extensions

To preface, this is really a separate discussion, linked to this one only by what is effectively whataboutism.  We don’t have consensus to do this complicating thing that optimizes for a common case, so why did we achieve consensus to do those other complicated things that optimize for uncommon cases?  But that consensus was achieved in HTTP/2, and my personal reading of the charter leads me to believe we’re prohibited from wholesale removal of H2 features in HTTP/3; we try to keep the same general spirit unless things are fundamentally incompatible with running over QUIC.

Moving everything post-DATA to a separate stream is a fairly dramatic change, and I’m having trouble seeing exactly how that would work.  And it suffers the same liability that it’s likely to be an under-exercised code path.

Priority and push were both controversial pieces of H2 for various reasons.  Unlike in HTTP/2, they are more decoupled from the stream state model in HTTP/3, and either or both could easily could become an extension.  I find the argument more persuasive for priority than for push, because one could very reasonably choose to experiment with several independent priority schemes – the pseudo-7540-style we have now, the three-bit SPDY-style priorities, a varint fixed priority, the weighted-group<https://lists.w3.org/Archives/Public/ietf-http-wg/2014JanMar/0396.html>, etc.

From: Ian Swett <ianswett@google.com>
Sent: Wednesday, December 12, 2018 9:27 AM
To: ianswett=40google.com@dmarc.ietf.org; Roberto Peon <fenix@fb.com>; Ryan Hamilton <rch@google.com>; Jana Iyengar <jri.ietf@gmail.com>; Mike Bishop <mbishop@evequefou.be>; HTTP Working Group <ietf-http-wg@w3.org>; Lucas Pardue <lucaspardue.24.7@gmail.com>; IETF QUIC WG <quic@ietf.org>; Martin Thomson <martin.thomson@gmail.com>; Kazuho Oku <kazuhooku@gmail.com>
Subject: Re: Final DATA frames

From a technical perspective, yes.  But from a goals/process perspective, I think they related.  We've spent a lot of time on very complex work in the QUIC WG, and I realize everyone has a different perspective, but I'm confused by the amount of pushback this got relative to all the work we have done to make PUSH and something approximating(since they're actually different) H2 priorities work on top of QUIC.

If I remember correctly, your main argument against this framing change was that the existing DATA frame with a length becomes largely useless and might be under-exercised.  That makes me feel like we've over-optimized for the complex special cases and made a design error or two at least one step prior to this proposal.

To make a specific and relevant suggestion, maybe DATA should never have a length and if we want features that most requests don't use(trailers/PUSH_PROMISE/etc) we should put them on a separate unidirectional stream?



On Wed, Dec 12, 2018 at 12:02 PM Dmitri Tikhonov <dtikhonov@litespeedtech.com<mailto:dtikhonov@litespeedtech.com>> wrote:
But you would agree that these are completely different discussions?

One is whether to modify HTTP/3 framing mechanism.  The other is
changing HTTP/3 feature set.

My point is that it is unjustified to to say that since we have
decided not to change the framing mechanism we should consider
dropping prioritization and push promises.

On Wed, Dec 12, 2018 at 11:38:09AM -0500, Ian Swett wrote:
> I don't want to derail this too much(probably too late for that), and no
> one has a PR anywhere close to landing, but I do think we should start
> thinking of these as potential extensions.  I disagree that they're
> HTTP/2's primary features, both due to lack of use and lack of measurable
> benefits years after standardization.  Push might actually benefit from
> being an extension, because then potential improvements(ie: cache-digest)
> could be integrated into the extension more quickly.
>
> On the other hand, QPACK is relatively complex but we have clearer
> demonstrations of it's benefit.
>
> On Wed, Dec 12, 2018 at 9:00 AM Dmitri Tikhonov <dtikhonov@litespeedtech.com<mailto:dtikhonov@litespeedtech.com>>
> wrote:
>
> > On Tue, Dec 11, 2018 at 09:02:01PM -0500, Ian Swett wrote:
> > > TLDR: If the focus is on shipping v1 and making sure we don't introduce
> > > premature optimizations into HTTP/3, I thInk we should seriously consider
> > > moving PUSH and the existing priorities to an extension.
> >
> > The EOS DATA frame and HTTP prioritization and push promises are not
> > equivalent.  One is simply fiddling with the way data is framed and
> > is not visible to the application.  The others are HTTP/2's primary
> > features.
> >
> >   - Dmitri.
> >