netdev - Re: TCP: big bursts due to undos resulting from reordering

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Message-ID: <4CF8CACD.8060006@nets.rwth-aachen.de>
Date:	Fri, 3 Dec 2010 11:47:41 +0100
From:	Lennart Schulte <lennart.schulte@...s.rwth-aachen.de>
To:	John Heffner <johnwheffner@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: TCP: big bursts due to undos resulting from reordering

Hi John,
Thanks for your answer!

On 02.12.2010 18:55, John Heffner wrote:
> The problem that occurred, prior to this patch, was that cwnd was not
> able to effectively grow under persistent reordering.

Could you explain that deeper? What was the exact situation?
Do you have an example of when the smaller max_burst keep the cwnd down?
At the moment I can't think of a case.

Thanks,
Lennart

> On Thu, Dec 2, 2010 at 10:36 AM, Lennart Schulte
> <lennart.schulte@...s.rwth-aachen.de> wrote:
>> Hi John, hi Ilpo,
>>
>> at the moment I look on many TCP plots with reordering. When reordering
>> occurs there are some spurious retransmissions which are later undone by
>> e.g. DSACKs. This undo results in a very big burst of packets when
>> tp->reordering is high, since the function tcp_max_burst() returns
>> tp->reordering.
>>
>> This behavior was introduced because of a bug when using SACK instead of
>> Reno. The thread concerning this fix can be found at [1].
>>
>> Before the patch, which results from this thread, Linux has done a burst
>> of 3 packets and then slow started to the undone ssthresh value, which
>> is a much better way of handling an undo then it is after the patch.
>>
>> Also I patched a kernel to use the old max_burst value of 3 again to see
>> if it has any effect. Then I set up some virtual nodes and emulated a
>> network with netem as it was done in the thread.
>> The settings are:
>> - RTT 40ms
>> - no congestion, application sending rate 20 Mbps
>> - forward path: reordering rate 20%, reordering delay 20ms
>> - timestamps on
>>
>> Until now I have not found any evidence that the problem occurs (perhaps
>> because I don't get the settings right, since in the thread there is no
>> information concerning the settings for reordering and also the ones of
>> the sysctls).
>>
>> My problem is to understand why the patch was necessary and under what
>> circumstances SACK has a lower throughput so that it may be possible for
>> me to find another way of fixing this without introducing and old bug.
>> Since I can't figure it out on my own I hope to get some insights this
>> way :)
>>
>> Thanks,
>> Lennart Schulte
>>
>> [1] http://marc.info/?t=120728958000004&r=2&w=2
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