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

James M Snell <jasnell@gmail.com> Thu, 02 May 2013 17:16 UTC

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From: James M Snell <jasnell@gmail.com>
Date: Thu, 2 May 2013 10:14:17 -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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My apologies for missing that previous conversation. In my view,
allowing for the additional flag has only a minimal cost with
potentially significant benefit down the road. I see absolutely no
reason to rule it out. Forbidding it because it seems "less
complicated that way" is short sighted and, frankly, just rather
silly. However, I fully recognize that, currently, the HTTP/2 use case
does not require fully bidirectional pushed streams and that http/2
implementors may not want to have to do anything to deal with it. So
with that in mind, how about this...

Rather than make it a UNIDIRECTIONAL bit (on PUSH_PROMISE) that has to
be set in order for the stream to be viewed as being Unidirectional,
let's switch it and make it a BIDIRECTIONAL flag. If the flag is not
set (the default), then the stream is unidirectional and automatically
half-closed in the return direction. If the bit is set, the pushed
stream is bidirectional and would need to be half-closed by the
receiving side. Then, at the HTTP/2 semantics layer, we would simply
say that, for HTTP push streams, the bit always has to be switched
off. This would mean that any other subprotocol could come along later
and make use of the bit but http/2 only implementations would only be
required to contain minimal support. If an implementation that does
not support bidirectional pushed streams receives a PUSH_PROMISE with
the bidirectional flag set, it would respond with a RST_STREAM Cancel
or an empty data frame with the FINAL bit set. Simple, elegant and
flexible.

Also, this would address the original concern I had about push-streams
being left in the half-open state

- James

On Wed, May 1, 2013 at 1:15 PM, William Chan (陈智昌)
<willchan@chromium.org>; wrote:
> Well, it would have been great then if people supported me when I was the
> only one who stood up for this standpoint, but we went ahead and removed
> UNIDIRECTIONAL for being redundant anyways:
> http://lists.w3.org/Archives/Public/ietf-http-wg/2013JanMar/1055.html :P
>
> I'm with you, but we should fork this thread to have that discussion on what
> degree of flexibility we want in the layers below the application semantic
> layer. UNIDIRECTIONAL was removed for being redundant, conditioned on HTTP
> semantic usage. If we can reverse that decision, we can reopen these other
> discussions where simplifications were made due to assuming on HTTP semantic
> usage.
>
>
> On Wed, May 1, 2013 at 5:00 PM, James M Snell <jasnell@gmail.com>; wrote:
>>
>> 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 :)
>> >> >>>>>
>> >> >>>>>
>> >> >>>
>> >> >>
>> >> >
>> >
>> >
>
>