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Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams

From: Roberto Peon <grmocg@gmail.com>
Date: Mon, 29 Apr 2013 14:59:27 -0700
Message-ID: <CAP+FsNdB2wdaYPpSkosEsFKx=vDmMK7ZW7Sskjp+mnSVitOsDQ@mail.gmail.com>
To: William Chan (陈智昌) <willchan@chromium.org>
Cc: James M Snell <jasnell@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>, Martin Thomson <martin.thomson@gmail.com>

On Mon, Apr 29, 2013 at 2:58 PM, William Chan (陈智昌)

> OK :) Well, we can discuss that alternative if we want to revisit the
> earlier decision, but I'm happy to move forward with what we already agreed
> upon earlier.
> On Mon, Apr 29, 2013 at 6:56 PM, Roberto Peon <grmocg@gmail.com> wrote:
>> I do remember :)
>> Honestly I like the IANA port or NPN string thing or something similar,
>> because it gives us other information that we'd otherwise have to parse out
>> of the HEADERS thing, but, it is the lowest complexity alternative.
>> -=R
>> 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 :)
Received on Monday, 29 April 2013 21:59:54 UTC

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