Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams
William Chan (陈智昌) <willchan@chromium.org> Mon, 29 April 2013 21:33 UTC
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Date: Mon, 29 Apr 2013 18:32:28 -0300
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From: "William Chan (陈智昌)" <willchan@chromium.org>
To: James M Snell <jasnell@gmail.com>
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 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