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

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Date:	Tue, 10 Mar 2009 08:23:34 -0700
From:	John Heffner <johnwheffner@...il.com>
To:	Marian Ďurkovič <md@....sk>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

On Tue, Mar 10, 2009 at 4:46 AM, Marian Ďurkovič <md@....sk> wrote:
> On Tue, Mar 10, 2009 at 04:30:19AM -0700, David Miller wrote:
>> 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.
>
> Yes, but that's actual bandwidth between sender and receiver, not
> the hard BW limit of the receiver's NIC. My intention is just to introduce
> some safety belt preventing autotuning to increase the rx window
> into MB ranges when RTT is very low.

Nowhere in our proposal do you use NIC bandwidth.  What you proposed
can be done easily at the sender.

  -John
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