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

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Date:	Tue, 21 Jun 2011 14:46:11 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Carsten Wolff <carsten@...ffcarsten.de>
cc:	Alexander Zimmermann <zimmermann@...s.rwth-aachen.de>,
	Dominik Kaspar <dokaspar.ietf@...il.com>,
	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 Tue, 21 Jun 2011, Carsten Wolff wrote:

> On Tuesday 21 June 2011, Ilpo Järvinen wrote:
> > On Wed, 27 Apr 2011, Alexander Zimmermann wrote:
> > > 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).
> 
> the NewReno detection is clever, but there's no exact information it could 
> utilize for a good metric, because it detects the event too late, when the 
> information is already gone. In my experiments it always under-estimated the 
> reordering extent, though. I also remmember thinking that the metric of the 
> Eifel-detection has an off-by-one bug.

That might be true for most of the cases but IIRC I figured out a
a scenario where it miscalculates RTT worth of extra into the reordering 
(but I never really confirmed that in real tests or so, just figured it 
a bit).

-- 
 i.