netdev - Re: TCP rx window autotuning harmful at LAN context

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Message-Id: <20090310.043019.132650585.davem@davemloft.net>
Date:	Tue, 10 Mar 2009 04:30:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	md@....sk
Cc:	johnwheffner@...il.com, netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

From: Marian Ďurkovič <md@....sk>
Date: Tue, 10 Mar 2009 11:49:56 +0100

> Sender does not have the relevant info to implement this - it might be
> connected by 10 GE to the highspeed backbone.

Yes, the sender does indeed have this information, and using it is
exactly what congestion control algorithms such as VEGAS try to do.

They look at both round trip times and bandwith as they increase the
send congestion window.  And if round trips increase without a
corresponding increase in bandwidth, they stop increasing.  This is
because in such a situation we can infer that we're just consuming
more queue space at some intermediate router/switch rather than using
more of the available bandwidth.