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

Roberto Peon <grmocg@gmail.com> Wed, 01 May 2013 19:57 UTC

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Date: Wed, 1 May 2013 12:55:44 -0700
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From: Roberto Peon <grmocg@gmail.com>
To: =?UTF-8?B?V2lsbGlhbSBDaGFuICjpmYjmmbrmmIwp?= <willchan@chromium.org>
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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Subject: Re: Design Issue: Max Concurrent Streams Limit and Unidirectional Streams
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I still want to be able to support the WS API over HTTP/2. It would be
tragic to have N+1 connections instead of 1 when 1 works better anyway...

-=R


On Wed, May 1, 2013 at 10:46 AM, William Chan (陈智昌)
<willchan@chromium.org>wrote:

> The only benefit to that is supporting non-HTTP/2 application layering
> semantics, which is intended not to change from HTTP/1.X. So there's
> currently no use to allow the server to initiate streams with the
> client=>server direction open.
>
> I consider the current trend of our discussions to tend towards
> eliminating complexity and targeting for HTTP/2 application layering
> semantics. I think if we have another use case come up that would require
> supporting server initiated bidirectional streams, I think at that point
> it'd be worthwhile to revisit how we do this.
>
> I'd like to hear from others if they disagree with my assessment of how
> most people feel so far. FWIW, I personally would like us to support server
> initiated bidirectional streams.
>
>
> On Wed, May 1, 2013 at 2:26 PM, James M Snell <jasnell@gmail.com>; wrote:
>
>> 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 :)
>> >>>>>
>> >>>>>
>> >>>
>> >>
>> >
>>
>
>