netdev - [PATCH] revert TCP retransmission backoff on ICMP destination unreachable

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Date:	Tue, 11 Aug 2009 13:27:42 +0200
From:	Damian Lukowski <damian@....rwth-aachen.de>
To:	netdev@...r.kernel.org
Subject: [PATCH] revert TCP retransmission backoff on ICMP destination
 unreachable

This patch is a proof-of-concept related to the draft "Make TCP more
Robust to Long Connectivity Disruptions"
(http://tools.ietf.org/html/draft-zimmermann-tcp-lcd-01).

Exponential backoff is TCP's standard behaviour during long connectivity
disruptions, which is a countermeasure against network congestion.
If congestion can be excluded as the reason for RTO retransmission loss,
backoff is not desirable, as it yields longer TCP recovery times, when
the communication path is repaired shortly after an unsuccessful
retransmission probe.
Here, an ICMP host/network unreachable message, whose payload fits to
TCPs SND.UNA, is taken as an indication that the RTO retransmission has
not been lost due to congestion, but because of a route failure
somewhere along the path. On receipt of such a message, the timeout is
halved (in order to revert to doubling after the retransmission).
With true congestion, a router won't trigger such a message and the
patched TCP will operate as standard TCP.

Comments are appreciated, especially regarding eventual side effects
with the zero window probing mechanism.
In Linux, the RTO retransmisson mechanism is coupled to the zero window
probing mechanism, but is not intended to be changed. I hope I have not
missed something, here.

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View attachment "TCP_ICMP_2.6.30.4.patch" of type "text/plain" (4323 bytes)