Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams
William Chan (陈智昌) <willchan@chromium.org> Mon, 29 April 2013 21:45 UTC
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Date: Mon, 29 Apr 2013 18:44:21 -0300
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From: =?UTF-8?B?V2lsbGlhbSBDaGFuICjpmYjmmbrmmIwp?= <willchan@chromium.org>
To: Roberto Peon <grmocg@gmail.com>
Cc: James M Snell <jasnell@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>,
Martin Thomson <martin.thomson@gmail.com>
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X-Original-To: ietf-http-wg@w3.org
Subject: Re: Design Issue: Max Concurrent Streams Limit and Unidirectional
Streams
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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;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… 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… James M Snell
- 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