Re: multiplexing -- don't do it

On Thu, Apr 5, 2012 at 9:43 AM, Peter Lepeska <bizzbyster@gmail.com> wrote:

> What's the main reason for the benefit? If it's just that SCTP is less of
> a good citizen in how it handles a lost packet and backs off slower than
> TCP then it's not something to get excited about. Have you done much
> analysis to explore the reason for SCTP being faster?
>

I believe it's due to unrealistic packet loss simulation.  The 1%/2% loss
is inaccurate (and in the wrong place for mobile networks, where this
happens below the transport) and the modeling of packet loss as random is
unrealistic (real internets don't randomly lose packets that much - its
much more correlated due to buffer overruns).  The net result is that SCTP
looks awesome with random loss, because it picks up packets that otherwise
would have been head-of-line blocked.  But, I don't believe this has
anything to do with the real world.

Sorry to condense over a year of SPDY research with packet loss into three
sentences :-)  I need to write all this stuff up.

Mike


>
> Peter
>
>
> On Thu, Apr 5, 2012 at 11:00 AM, Jon Leighton <leighton@cis.udel.edu>wrote:
>
>> **
>> On 04/04/2012 06:05 PM, Mike Belshe 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.
>>
>>
>> For anyone interested, here's a link to a summary of the work
>> http://www.eecis.udel.edu/~leighton/SPDY.html. Under the conditions
>> tested, SPDY over SCTP was clearly faster than SPDY over TCP. This is true
>> regardless of which mapping was used. Though the data is not provided in
>> the summary, using SCTP stream 0 as a control stream for SPDY headers was
>> only very slightly faster than sending headers on the same SCTP stream as
>> the data. There were some issues with the linux implementation of SCTP that
>> had to be worked around (most since fixed), but I can't comment on any
>> issues with SCTP on FreeBSD as we decided against trying to port Chrome and
>> the flip_in_mem_edsm_server to FreeBSD. The results were inconclusive with
>> regard to how best to map SPDY to SCTP. Regarding the benefit of using SCTP
>> to avoid HOL blocking, the results are pretty straightforward - SPDY
>> performed better over SCTP than TCP. Increasing the delay and loss
>> increases SCTP's advantage.
>>
>>
>>  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
>>>>
>>>
>