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Re: multiplexing -- don't do it

From: Peter Lepeska <bizzbyster@gmail.com>
Date: Thu, 5 Apr 2012 10:05:39 -0400
Message-ID: <CANmPAYGV5AGDUfPjX+Qs8UZRE+7AVoLCapuYCabBZ5gtyDyBuA@mail.gmail.com>
To: William Chan (陈智昌) <willchan@chromium.org>
Cc: Mike Belshe <mike@belshe.com>, Patrick McManus <pmcmanus@mozilla.com>, Poul-Henning Kamp <phk@phk.freebsd.dk>, Willy Tarreau <w@1wt.eu>, "Roy T. Fielding" <fielding@gbiv.com>, "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
The idea is just a partial solution to the HOL blocking problem that at
least decreases the likelihood that HOL issue will block high priority
objects. I don't yet know the SPDY protocol well enough but the idea is
that you would have two SPDY streams running over two TCP connections.
Anything that the browser deems as blocking page load, or for another
reason high priority, will go over one SPDY stream and everything else will
go over the other.

I agree that a UDP-based transport that supports out of order packets would
completely solve HOL blocking for SPDY. My misunderstanding was that SCTP
was such a transport but Mike set me straight. In any case, there are no
obvious candidates for this so I agree we can't really design for it.

Still I think we should support inter-object dependencies only if there's a
real benefit, which I believe there is in the upstream but not so sure in
the downstream, as compared to simply going binary.


On Thu, Apr 5, 2012 at 4:25 AM, William Chan (陈智昌) <willchan@chromium.org>wrote:

> On Thu, Apr 5, 2012 at 9:48 AM, Peter L <bizzbyster@gmail.com> wrote:
>> Thanks for the explanation on SCTP -- I have only a high level
>> understanding and based on what you said I can't even claim that.
>> In general, the in-flight HOL blocking problem is really serious for long
>> fat networks: some mobile, intercontinental, satellite to name a few. So
>> I'd like for HTTP 2.0 to at least hold the promise that it will be
>> addressed. For instance, I proposed two SPDY streams, one high and one low
>> priority. This would at least prevent low pri drops from blocking high pri
>> objects. Do you have other better ideas?
> Can you explain this proposal further? It doesn't make sense to me as
> stated (What do you mean by two SPDY streams? Are these actual SPDY streams
> from the spec?). If HTTP/2.0 is layered on TCP, then it fundamentally will
> have TCP-level HOL blocking. At the TCP level, there's no concept of
> different priority objects, so any packet drop will delay other packets
> behind it. SPDY did not attempt to address this problem, since there were
> already so many problems higher in the stack. The real solution is to fix
> the transport (new UDP-based protocol anyone?), but that's beyond the scope
> of this work.
>> Peter
>> On Apr 4, 2012, at 6:05 PM, Mike Belshe <mike@belshe.com> wrote:
>> On Wed, Apr 4, 2012 at 1:58 PM, Peter Lepeska <bizzbyster@gmail.com>wrote:
>>> As long as SPDY is sent over TCP, it also suffers from HOL problems,
>>> just not as bad as pipelining.
>>> I think SPDY (or whatever the HTTP 2.0 muxing protocol is) should framed
>>> in such a way that if running over a protocol like SCTP, that solves the
>>> HOL problems, we should be able to take advantage of it. Due to gzip
>>> compression of headers, even if the transport allowed me to grab messages
>>> out of order, I'd still have to wait for all packets prior in order to
>>> decode the HTTP headers.
>> I think people have confusion about layering on top of transport
>> protocols.  Any time you have an app protocol that wants to take advantage
>> of new transport features you *MUST* change the definition of how the app
>> protocol is bound to the lower level transport.  This absolutely applies to
>> HTTP and TCP/SCTP (not talking about SPDY yet).  For example, RFC2616 does
>> *not* specify how to use HTTP over SCTP, and a whole I-D exists for that:
>> http://tools.ietf.org/html/draft-natarajan-http-over-sctp-01  This I-D
>> defines one possible binding between HTTP and SCTP, but others could exist
>> too.
>> Why is this necessary?  Well, TCP has a single, bidirectional stream.
>>  SCTP doesn't have bi-directional streams at all, and instead only has
>> multiple, uni-directional streams.  So, if you want to leverage the new
>> feature (streams) you need to define how you're going to bind onto it.
>>  It's trivial, but doable.
>> If we had SCTP, we wouldn't do MUX and SPDY.  It's redundant.  That's not
>> to say that HTTP/2.0 is worse with multiplexing, its just to say that we
>> wouldn't need to consider it if the transport already had it.  But SCTP is
>> not viable today or even within the next decade.
>> Google sponsored a project at the Univ of Delaware where they
>> investigated SPDY over SCTP already.  A couple of bindings for SPDY over
>> SCTP were considered:  use stream 0 for a control stream, and send the
>> headers over that stream.  This has the nice property that it binds the
>> stateful compression to a single, in-order stream.  Also, as you can
>> imagine, it introduces a small amount of HoL there.  Another solution was
>> to use N SCTP streams, with M mux'd SPDY streams on top of it.  Other such
>> bindings could be considered.  Overall, SCTP was too immature to really
>> benchmark.  The FreeBSD and Linux implementations of SCTP have many
>> problems already.  The UoD guys might want to comment, I found the whole
>> thing non-conclusive.
>> My recommendation:
>> Designing HTTP/2.0 for SCTP is a mistake and should NOT be a requirement.
>>  SCTP is not a viable transport over the Internet today, and will not be in
>> the foreseeable future.  When it is available, an appropriate binding for
>> HTTP/2.0 can be determined, trivially, and we can worry about it then.
>>  This is the same approach that was taken for HTTP with
>> http://tools.ietf.org/html/draft-natarajan-http-over-sctp-01.
>> Mike
>>> Peter
>>> On Wed, Apr 4, 2012 at 9:07 AM, Patrick McManus <pmcmanus@mozilla.com>wrote:
>>>> On Wed, 2012-04-04 at 07:02 +0000, Poul-Henning Kamp wrote:
>>>> > In message <20120404054903.GA13883@1wt.eu>, Willy Tarreau writes:
>>>> >
>>>> > >> I'm starting to get data back, but not in a state that I'd reliably
>>>> > >> release. That said, there are very clear indicators of
>>>> intermediaries
>>>> > >> causing problems, especially when the pipeline depth exceeds 3
>>>> requests.
>>>> >
>>>> > I always thought that the problem in HTTP/1.x is that you can never
>>>> > quite be sure if there is an un-warranted entity comming after at GET,
>>>> its not uncommon to have the consumer RST the whole TCP session when
>>>> asked to recv too far beyond the current request it is processing. For
>>>> some devices "too far" appears to be defined as "any new packet". I
>>>> presume some variation of this is where Will's data point comes from.
>>>> (Often 3 uncompressed requests fit in 1 packet).
>>>> That class of bug sounds absurd, but its really a pretty common pattern.
>>>> As an example: hosts that fail TLS False Start (for which I understand
>>>> second hand that Chrome needs to keep a black-list), react badly because
>>>> there is TCP data queued when they are in a state that the expect their
>>>> peer to be quiet. Same pattern.
>>>> The lesson to me is that you want to define a tight set of functionality
>>>> that is reasonably testable up front - and that's what you can depend
>>>> widely on later. Using anything beyond that demands excessive levels of
>>>> pain, complexity, and cleverness.
>>>> (and all this pipelining talk as if it were equivalent to spdy mux is
>>>> kind of silly. Pipelining's intrinsic HOL problems are at least as bad
>>>> of an issue as the interop bugs.)
>>>> -Patrick
Received on Thursday, 5 April 2012 14:06:14 UTC

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