netdev - Re: Linux TCP's Robustness to Multipath Packet Reordering

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Date:	Tue, 21 Jun 2011 14:25:20 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Alexander Zimmermann <zimmermann@...s.rwth-aachen.de>
cc:	Dominik Kaspar <dokaspar.ietf@...il.com>,
	Carsten Wolff <carsten@...ffcarsten.de>,
	John Heffner <johnwheffner@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Netdev <netdev@...r.kernel.org>,
	Lennart Schulte <Lennart.Schulte@...sys.rwth-aachen.de>,
	Arnd Hannemann <arnd@...dnet.de>
Subject: Re: Linux TCP's Robustness to Multipath Packet Reordering

On Wed, 27 Apr 2011, Alexander Zimmermann wrote:

> Hi,
> 
> Am 27.04.2011 um 18:22 schrieb Dominik Kaspar:
> 
> > Hi Carsten,
> > 
> > Thanks for your feedback. I made some new tests with the same setup of
> > packet-based forwarding over two emulated paths (600 KB/s, 10 ms) +
> > (400 KB/s, 100 ms). In the first experiments, which showed a step-wise
> > adaptation to reordering, SACK, DSACK, and Timestamps were all
> > enabled. In the experiments, I individually disabled these three
> > mechanisms and saw the following:
> > 
> > - Disabling timestamps causes TCP to never adjust to reordering at all.
> 
> Reordering detection with DSACK is broken in Linux. We will fix that in
> a couple of weeks...
> 
> > - Disabling SACK allows TCP to adapt very rapidly ("perfect" aggregation!).
> 
> If you disable SACK, you will use the NewReno detection

Which probably has some reordering over-estimate bugs on its own... 
(but I've forgotten details of my suspicion long time ago so please don't 
ask for the them).

-- 
 i.
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