Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams
James M Snell <jasnell@gmail.com> Wed, 01 May 2013 20:01 UTC
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From: James M Snell <jasnell@gmail.com>
Date: Wed, 01 May 2013 13:00:31 -0700
Message-ID: <CABP7RbejssYWH+nEumVX__+4TnE1ec8e1YXeY8kqWF+AgszTrg@mail.gmail.com>
To: "William Chan (陈智昌)" <willchan@chromium.org>
Cc: Roberto Peon <grmocg@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>, Martin Thomson <martin.thomson@gmail.com>
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Subject: Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams
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On Wed, May 1, 2013 at 10:46 AM, William Chan (陈智昌) <willchan@chromium.org> wrote: > The only benefit to that is supporting non-HTTP/2 application layering > semantics, which is intended not to change from HTTP/1.X. So there's > currently no use to allow the server to initiate streams with the > client=>server direction open. > "currently no use" is not a very good reason to not allow for it. We (a) have the flag bits available and (b) can easily say that in the http/2 semantic layer, the unidirectional bit must always be set. That meets our current needs and leaves the door open for later. - James > I consider the current trend of our discussions to tend towards eliminating > complexity and targeting for HTTP/2 application layering semantics. I think > if we have another use case come up that would require supporting server > initiated bidirectional streams, I think at that point it'd be worthwhile to > revisit how we do this. > > I'd like to hear from others if they disagree with my assessment of how most > people feel so far. FWIW, I personally would like us to support server > initiated bidirectional streams. > > > On Wed, May 1, 2013 at 2:26 PM, James M Snell <jasnell@gmail.com> wrote: >> >> Why not just bring the UNIDIRECTIONAL flag back as a PUSH_PROMISE >> frame-specific flag? If a PUSH_PROMISE frame has the unidirectional >> flag set, the stream is automatically half-closed in the return >> direction. If the flag is unset, the promised stream remains half-open >> until the client half-closes or a rst_stream is sent. >> >> On Mon, Apr 29, 2013 at 2:44 PM, William Chan (陈智昌) >> <willchan@chromium.org> wrote: >> > Remember we originally *had* a flag for UNIDIRECTIONAL, which we removed >> > because it was redundant in the traditional HTTP use cases. >> > >> > >> > On Mon, Apr 29, 2013 at 6:39 PM, Roberto Peon <grmocg@gmail.com> wrote: >> >> >> >> At worst, we burn a flag which states it is half-closed or >> >> unidirectional, >> >> or provide some other information which identifies the IANA port number >> >> for >> >> the overlayed protocol or something. >> >> Anyway, *shrug*. >> >> -=R >> >> >> >> >> >> On Mon, Apr 29, 2013 at 2:32 PM, William Chan (陈智昌) >> >> <willchan@chromium.org> wrote: >> >>> >> >>> On Mon, Apr 29, 2013 at 6:17 PM, James M Snell <jasnell@gmail.com> >> >>> wrote: >> >>>> >> >>>> +1 on this. I like this approach. >> >>>> >> >>>> On Apr 29, 2013 2:15 PM, "Roberto Peon" <grmocg@gmail.com> wrote: >> >>>>> >> >>>>> I had thought to provide no explicit limit for PUSH_PROMISE, just as >> >>>>> there is no limit to the size of a webpage, or the number of links >> >>>>> upon it. >> >>>>> The memory requirements for PUSH are similar or the same (push >> >>>>> should >> >>>>> consume a single additional bit of overhead per url, when one >> >>>>> considers that >> >>>>> the URL should be parsed, enqueued, etc.). >> >>>>> If the browser isn't done efficiently, or, the server is for some >> >>>>> unknown reason being stupid and attempting to DoS the browser with >> >>>>> many >> >>>>> resources that it will never use, then the client sends RST_STREAM >> >>>>> for the >> >>>>> ones it doesn't want, and makes a request on its own. all tidy. >> >>> >> >>> >> >>> I don't feel too strongly here. I do feel like this is more of an edge >> >>> case, possibly important for forward proxies (or reverse proxies >> >>> speaking to >> >>> backends over a multiplexed channel like HTTP/2). It doesn't really >> >>> matter >> >>> for my browser, so unless servers chime in and say they'd prefer a >> >>> limit, >> >>> I'm fine with this. >> >>> >> >>>>> >> >>>>> As for PUSH'd streams, the easiest solution is likely to assume that >> >>>>> the stream starts out in a half-closed state. >> >>> >> >>> >> >>> I looked into our earlier email threads and indeed this is what we >> >>> agreed >> >>> on >> >>> (http://lists.w3.org/Archives/Public/ietf-http-wg/2013JanMar/1106.html). >> >>> I voiced some mild objection since if you view the HTTP/2 framing >> >>> layer as a >> >>> transport for another application protocol, then bidirectional server >> >>> initiated streams might be nice. But in absence of any such protocol, >> >>> this >> >>> is a nice simplification. >> >>> >> >>>>> >> >>>>> -=R >> >>>>> >> >>>>> >> >>>>> On Mon, Apr 29, 2013 at 12:33 PM, William Chan (陈智昌) >> >>>>> <willchan@chromium.org> wrote: >> >>>>>> >> >>>>>> On Mon, Apr 29, 2013 at 3:46 PM, James M Snell <jasnell@gmail.com> >> >>>>>> wrote: >> >>>>>>> >> >>>>>>> >> >>>>>>> On Apr 29, 2013 11:36 AM, "William Chan (陈智昌)" >> >>>>>>> <willchan@chromium.org> wrote: >> >>>>>>> > >> >>>>>>> [snip] >> >>>>>>> >> >>>>>>> > >> >>>>>>> > >> >>>>>>> > Oops, forgot about that. See, the issue with that is now we've >> >>>>>>> > made >> >>>>>>> > PUSH_PROMISE as potentially expensive as a HEADERS frame, since >> >>>>>>> > it does more >> >>>>>>> > than just simple stream id allocation. I guess it's not really a >> >>>>>>> > huge issue, >> >>>>>>> > since if it's used correctly (in the matter you described), then >> >>>>>>> > it >> >>>>>>> > shouldn't be too expensive. If clients attempt to abuse it, then >> >>>>>>> > servers >> >>>>>>> > should probably treat it in a similar manner as they treat >> >>>>>>> > people trying to >> >>>>>>> > abuse header compression in all other frames with the header >> >>>>>>> > block, and kill >> >>>>>>> > the connection accordingly. >> >>>>>>> > >> >>>>>>> >> >>>>>>> Not just "potentially" as expensive.. As soon as we get a push >> >>>>>>> promise we need to allocate state and hold onto it for an >> >>>>>>> indefinite period >> >>>>>>> of time. We do not yet know exactly when that compression context >> >>>>>>> can be let >> >>>>>>> go because it has not yet been bound to stream state. Do push >> >>>>>>> streams all >> >>>>>>> share the same compression state? Do those share the same >> >>>>>>> compression state >> >>>>>>> as the originating stream? The answers might be obvious but they >> >>>>>>> haven't yet >> >>>>>>> been written down. >> >>>>>> >> >>>>>> >> >>>>>> I guess I don't see per-stream state as being that expensive. >> >>>>>> Compression contexts are a fixed state on a per-connection basis, >> >>>>>> meaning >> >>>>>> that additional streams don't add to that state. The main cost, as >> >>>>>> I see it, >> >>>>>> is the decompressed headers. I said potentially since that >> >>>>>> basically only >> >>>>>> means the URL (unless there are other headers important for caching >> >>>>>> due to >> >>>>>> Vary), and additional headers can come in the HEADERS frame. Also, >> >>>>>> PUSH_PROMISE doesn't require allocating other state, like >> >>>>>> backend/DB >> >>>>>> connections, if you only want to be able to handle >> >>>>>> (#MAX_CONCURRENT_STREAMs) >> >>>>>> of those backend connections in parallel. >> >>>>>> >> >>>>>> If they're not specified, then we should specify it, but I've >> >>>>>> always >> >>>>>> understood the header compression contexts to be directional and >> >>>>>> apply to >> >>>>>> all frames sending headers in a direction. Therefore there should >> >>>>>> be two >> >>>>>> compression contexts in a connection, one for header blocks being >> >>>>>> sent and >> >>>>>> one for header blocks being received. If this is controversial, >> >>>>>> let's fork a >> >>>>>> thread and discuss it. >> >>>>>> >> >>>>>>> >> >>>>>>> >> >> >>>>>>> >> >> >>>>>>> >> > As far as the potential problem above, the root problem is >> >>>>>>> >> > that >> >>>>>>> >> > when you >> >>>>>>> >> > have limits you can have hangs. We see this all the time >> >>>>>>> >> > today >> >>>>>>> >> > with browsers >> >>>>>>> >> > (it's only reason people do domain sharding so they can >> >>>>>>> >> > bypass >> >>>>>>> >> > limits). I'm >> >>>>>>> >> > not sure I see the value of introducing the new proposed >> >>>>>>> >> > limits. >> >>>>>>> >> > They don't >> >>>>>>> >> > solve the hangs, and I don't think the granularity addresses >> >>>>>>> >> > any >> >>>>>>> >> > of the >> >>>>>>> >> > costs in a finer grained manner. I'd like to hear >> >>>>>>> >> > clarification >> >>>>>>> >> > on what >> >>>>>>> >> > costs the new proposed limits will address. >> >>>>>>> >> >> >>>>>>> >> I don't believe that the proposal improves the situation enough >> >>>>>>> >> (or at >> >>>>>>> >> all) to justify the additional complexity. That's something >> >>>>>>> >> that >> >>>>>>> >> you >> >>>>>>> >> need to assess for yourself. This proposal provides more >> >>>>>>> >> granular >> >>>>>>> >> control, but it doesn't address the core problem, which is that >> >>>>>>> >> you >> >>>>>>> >> and I can only observe each other actions after some delay, >> >>>>>>> >> which >> >>>>>>> >> means that we can't coordinate those actions perfectly. Nor >> >>>>>>> >> can >> >>>>>>> >> be >> >>>>>>> >> build a perfect model of the other upon which to observe and >> >>>>>>> >> act >> >>>>>>> >> upon. >> >>>>>>> >> The usual protocol issue. >> >>>>>>> > >> >>>>>>> > >> >>>>>>> > OK then. My proposal is to add a new limit for PUSH_PROMISE >> >>>>>>> > frames >> >>>>>>> > though, separately from the MAX_CONCURRENT_STREAMS limit, since >> >>>>>>> > PUSH_PROMISE >> >>>>>>> > exists as a promise to create a stream, explicitly so we don't >> >>>>>>> > have to count >> >>>>>>> > it toward the existing MAX_CONCURRENT_STREAMS limit (I searched >> >>>>>>> > the spec and >> >>>>>>> > this seems to be inadequately specced). Roberto and I discussed >> >>>>>>> > that before >> >>>>>>> > and may have written an email somewhere in spdy-dev@, but I >> >>>>>>> > don't think >> >>>>>>> > we've ever raised it here. >> >>>>>>> > >> >>>>>>> >> >>>>>>> Well, there is an issue tracking it in the github repo now, at >> >>>>>>> least. As currently defined in the spec, it definitely needs to >> >>>>>>> be >> >>>>>>> addressed. >> >>>>>> >> >>>>>> Great. You guys are way better than I am about tracking all known >> >>>>>> issues. I just have it mapped fuzzily in my head :) >> >>>>> >> >>>>> >> >>> >> >> >> > > >
- Design Issue: Max Concurrent Streams Limit and Un… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Poul-Henning Kamp
- RE: Design Issue: Max Concurrent Streams Limit an… RUELLAN Herve
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… Roberto Peon
- Re: Design Issue: Max Concurrent Streams Limit an… William Chan (陈智昌)
- Re: Design Issue: Max Concurrent Streams Limit an… James M Snell
- Re: Design Issue: Max Concurrent Streams Limit an… Martin Thomson