netdev - RE: A Linux TCP SACK Question

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Date:	Fri, 4 Apr 2008 23:00:53 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Wenji Wu <wenji@...l.gov>
cc:	'John Heffner' <johnwheffner@...il.com>,
	Netdev <netdev@...r.kernel.org>
Subject: RE: A Linux TCP SACK Question

On Fri, 4 Apr 2008, Wenji Wu wrote:

> Every system runs Linux 2.6.24.

You should have reported kernel version right from the beginning. It may 
have a huge effect... ;-)

> When sack is on, the throughput is around 180Mbps
> When sack is off, the throughput is around 260Mbps

Not a surprise, once some reordering is detected, SACK TCP switches away 
from FACK to something that's not what you'd expect (in 2.6.24), you 
should try 2.6.25-rcs first in which the non-FACK is very close to 
RFC3517.

> I was thinking that if the reordered ACKs/SACKs cause confusion in the
> sender, and sender will unnecessarily reduce either the CWND or the
> TCP_REORDERING threshold. I might need to take a serious look at the 
> SACK implementation. 

I'd suggest that you don't waste too much effort for 2.6.24. ...Most of it 
is recoded/updated since then.

-- 
 i.
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