netdev - Re: TCP IPv4 strange retransmits

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Message-ID: <Pine.LNX.4.64.0803041507001.5423@kivilampi-30.cs.helsinki.fi>
Date:	Tue, 4 Mar 2008 15:36:42 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: TCP IPv4 strange retransmits

On Tue, 4 Mar 2008, Arnd Hannemann wrote:

> I'm observing some retransmits with kernel 2.6.24.2, which I don't 
> understand. For instance in this cutout[1] of a sequence diagram which 
> was captured[2] on the TCP sender, 4 retransmits are made.

They don't correspond to each other?

> According to netstat -st output[3][4] all those 4 retransmits were "fast 
> retransmit".
> But there are no three DUPACKs which I expected would be needed for fast 
> retransmit?

With FACK it's enough that you have fackets_out > tp->reordering 
(=dupThresh).

> Also interesting all retransmits happen _after_ those segments were
> already acked and sacked, internal queuing or latency issues?

I think your viewer is doing something wrong, sender.dump is not giving 
such information (or you draw that from wrong end?). Or it just draws
DSACK like that?

> It would be great if somebody could shed some light on this,
> why those segments are retransmitted.
> Dumps and xplots are available here[5].

...I quickly glanced over it and found no strange behavior in
the sender.dump.

-- 
 i.
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