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

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Message-ID: <BANLkTinueBFkVmR-Hd6d1Yojt+gsAV9PYw@mail.gmail.com>
Date:	Sun, 19 Jun 2011 17:22:04 +0200
From:	Dominik Kaspar <dokaspar.ietf@...il.com>
To:	netdev@...r.kernel.org
Cc:	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>,
	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

Hello again,

I have another question to Linux TCP and packet reordering. What
exactly happens, when a packet is so much delayed (but not causing a
timeout), that it gets overtaken by a retransmitted version of itself?
It seems to me that this results in "SACK reneging", but I don't
really understand why...

The simplified situation goes this:
- Segment A gets sent and very much delayed (but not causing RTO)
- Segments B, C, D cause dupACKs
- Segment A_ret is retransmitted and ACKed (sent over new path)
- Some more segments E, F, ... are sent and ACKed
- Segment A (the delayed one) arrives at the receiver.
- Now what exactly happens next...?

I use default Linux TCP (with sack=1, dsack=1, fack=1, timestamps=1,
...) and the above described series of events is cause why
transparently forwarding IP packets over multiple paths with RTTs of
10 and 100 milliseconds.

I'd appreciate your help - best regards,
Dominik
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