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Re: HTTP/2 and TCP CWND

Patrick McManus <mcmanus@ducksong.com> Wed, 17 April 2013 18:13 UTC

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Date: Wed, 17 Apr 2013 14:12:15 -0400
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From: Patrick McManus <mcmanus@ducksong.com>
To: Wesley Eddy <wes@mti-systems.com>
Cc: 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>, "Eggert, Lars" <lars@netapp.com>, Martin Stiemerling <martin.stiemerling@neclab.eu>
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Subject: Re: HTTP/2 and TCP CWND
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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

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