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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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 :)
>>>
>>
>>