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

James M Snell <jasnell@gmail.com> Wed, 01 May 2013 20:01 UTC

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From: James M Snell <jasnell@gmail.com>
Date: Wed, 1 May 2013 13:00:31 -0700
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To: =?UTF-8?B?V2lsbGlhbSBDaGFuICjpmYjmmbrmmIwp?= <willchan@chromium.org>
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 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.
>

"currently no use" is not a very good reason to not allow for it. We
(a) have the flag bits available and (b) can easily say that in the
http/2 semantic layer, the unidirectional bit must always be set. That
meets our current needs and leaves the door open for later.

- James

> 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 :)
>> >>>>>
>> >>>>>
>> >>>
>> >>
>> >
>
>