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

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Message-id: <B35A6FDF-714A-4226-AFF4-D12ECCD4A152@comsys.rwth-aachen.de>
Date:	Thu, 28 Apr 2011 08:11:15 +0200
From:	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Dominik Kaspar <dokaspar.ietf@...il.com>,
	Carsten Wolff <carsten@...ffcarsten.de>,
	John Heffner <johnwheffner@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>, 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

Am 27.04.2011 um 23:41 schrieb Yuchung Cheng:

> AFAIK, FACK is disabled throughout the life of the connection after
> sender detects reordering degree > 3.

Right.

> 
> But Alex said the reordering has some bugs. I suspect these bugs may
> affect FACK/sack auto-tuning.

No. It affects reordering detection only. 

> Maybe Alex could describe the reordering
> bugs?
> 

Yes, I can. With DSACK, you have two cases. DSACK below and above SEG.ACK.
DSACK below SEG.ACK is the come case. However, Linux doesn't calculate a
reordering extent in this case. We will fix this. In the other case, DSACK
above SEG.ACK, Linux quantifies the reordering as the distance between the
received DSACK and snd_fack. However, this is only correct if the reordering
delay is greater the RTT. We will fix that too.

As a result, in the first case, we waste the opportunity to calculate an
extent (loosing performance). In the second case we overestimate the
reordering.

Alex

//
// Dipl.-Inform. Alexander Zimmermann
// Department of Computer Science, Informatik 4
// RWTH Aachen University
// Ahornstr. 55, 52056 Aachen, Germany
// phone: (49-241) 80-21422, fax: (49-241) 80-22222
// email: zimmermann@...rwth-aachen.de
// web: http://www.umic-mesh.net
//

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