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

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Date:	Wed, 27 Apr 2011 11:57:47 +0200
From:	Carsten Wolff <carsten@...ffcarsten.de>
To:	John Heffner <johnwheffner@...il.com>
Cc:	Dominik Kaspar <dokaspar.ietf@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Zimmermann Alexander <zimmermann@...s.rwth-aachen.de>,
	Lennart Schulte <Lennart.Schulte@...sys.rwth-aachen.de>,
	Arnd Hannemann <arnd@...dnet.de>
Subject: Re: Linux TCP's Robustness to Multipath Packet Reordering

Hi all,

On Tuesday 26 April 2011, John Heffner wrote:
> First, TCP is definitely not designed to work under such conditions.
> For example, assumptions behind RTO calculation and fast retransmit
> heuristics are violated.  However, in this particular case my first
> guess is that you are being limited by "cwnd moderation," which was
> the topic of recent discussion here.  Under persistent reordering,
> cwnd moderation can inhibit the ability of cwnd to grow.

it's not just cwnd moderation (of which I'm still in favor, even though I lost 
the argument by inactivity ;-)).

Anyway, there are a lot of things in reordering handling that can be improved. 
Our group (Alexander, Lennart, Arnd, myself and others) has worked on the 
problem for a long time now. This work resulted in an algorithm that is in 
large parts TCP-NCR (RFC4653), but also utilizes information gathered by 
reordering detection for determination of a good DupThresh, fixes a few 
problems in RFC4653 and improves on the reordering detection in Linux when the 
connection has no timestamps option. We implemented "pure" TCP-NCR and our own 
variant in Linux using a modular framework similar to the congestion control 
modules. A lot of measurements and evaluation have gone into the comparison of 
the three algorithms. We are now very close(TM) to a final patch, that is more 
suited for publication on this list and integrates our algorithm into tcp*.
[hc] without introducing the overhead of that modular framework.

Greetings,
Carsten
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