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

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Message-ID: <BANLkTin6fsc=4GUY+1UKsLEbgzeybx7FHg@mail.gmail.com>
Date:	Mon, 20 Jun 2011 14:52:35 +0200
From:	Dominik Kaspar <dokaspar.ietf@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>,
	Netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Carsten Wolff <carsten@...ffcarsten.de>,
	John Heffner <johnwheffner@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Lennart Schulte <Lennart.Schulte@...sys.rwth-aachen.de>,
	Arnd Hannemann <arnd@...dnet.de>
Subject: Re: Linux TCP's Robustness to Multipath Packet Reordering

Hi Ilpo,

> Where did you get this idea of reneging?!?

I observed that my scenario of a retransmitted packet overtaking the
original somehow causes TCP to enter the "Loss" state although no RTO
was caused. And since the Loss state seems to be only entered due to
RTO timeout or SACK reneging, I got the idea that reneging must be
occurring.

> Reneging has nothing to do with DSACKs,
> instead it is only detected if the cumulative ACK stops to such
> boundary where the _next_ segment is SACKed (i.e., some reason
> the receiver "didn't bother" to cumulatively ACK for that too). ...
> That certainly does not happen (ever) for out of window DSACKs.

You are right. If I turn off DSACK, the same thing happens: TCP enters
the Loss state without timeouts occurring. Isn't that a sign of
reneging happening? What else can it be?

Dominik
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