Re: Linux TCP's Robustness to Multipath Packet Reordering

Carsten Wolff
2011-04-27T09:58:00Z

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