Re: HTTP/2 and TCP CWND

"Eggert, Lars" <lars@netapp.com> Thu, 18 April 2013 17:38 UTC

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From: "Eggert, Lars" <lars@netapp.com>
To: Patrick McManus <mcmanus@ducksong.com>
CC: Wesley Eddy <wes@mti-systems.com>, Eliot Lear <lear@cisco.com>, "Gabriel Montenegro" <Gabriel.Montenegro@microsoft.com>, Roberto Peon <grmocg@gmail.com>, "Simpson, Robby (GE Energy Management)" <robby.simpson@ge.com>, Robert Collins <robertc@squid-cache.org>, Jitu Padhye <padhye@microsoft.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>, "Brian Raymor (MS OPEN TECH)" <Brian.Raymor@microsoft.com>, Rob Trace <Rob.Trace@microsoft.com>, Dave Thaler <dthaler@microsoft.com>, "Martin Thomson" <martin.thomson@skype.net>, Martin Stiemerling <martin.stiemerling@neclab.eu>
Thread-Topic: HTTP/2 and TCP CWND
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Subject: Re: HTTP/2 and TCP CWND
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We need to have this discussion over on the TCPM list, where the folks hang out that have looked at these issues for decades.

-- 
Sent from a mobile device; please excuse typos.

On Apr 17, 2013, at 11:12, "Patrick McManus" <mcmanus@ducksong.com> wrote:

> Hi Wes,
> 
> On Wed, Apr 17, 2013 at 12:48 PM, Wesley Eddy <wes@mti-systems.com> wrote:
>> 
>> It's definitely misinformation given the dynamic nature of the
>> CWND variable in TCP.  This is not a path property like MTU that
>> can be thought of as relatively static, and it can change on short
>> timescales with high granularity.
> 
> Granted, an old CWND measurement can be inaccurate. It's an informed guess based on path performance. I'm sure we agree the path plays a (non-definitive) role in this.
> 
> The alternative, IW, is an inaccurate guess too. Its an uninformed guess and I don't see why we should assume it would be more accurate.
> 
> We can't argue that IW10 is strictly more conservative because my data says it typically isn't.  (median SPDY CWND SETTING in firefox data is 30 x 1 session.. apples to apples that compares to at least 6 parallel HTTP/1 sessions of IW 10 each). Roberto suggested he's seen something similar. I'm not sure that more conservative is a better thing anyhow but I don't see how it applies in this case in any event.
> 
> -Patrick
>