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

James M Snell <jasnell@gmail.com> Wed, 01 May 2013 17:28 UTC

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From: James M Snell <jasnell@gmail.com>
Date: Wed, 1 May 2013 10:26:26 -0700
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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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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 :)
>>>>>
>>>>>
>>>
>>
>