Re: [tcpm] draft-eggert-tcpm-historicize-00

<L.Wood@surrey.ac.uk> Mon, 28 June 2010 08:35 UTC

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From: <L.Wood@surrey.ac.uk>
To: <touch@ISI.EDU>
Date: Mon, 28 Jun 2010 09:35:37 +0100
Thread-Topic: [tcpm] draft-eggert-tcpm-historicize-00
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Cc: tcpm@ietf.org, ananth@cisco.com, Anumita.Biswas@netapp.com, L.Wood@surrey.ac.uk
Subject: Re: [tcpm] draft-eggert-tcpm-historicize-00
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On 26 Jun 2010, at 20:25, Joe Touch wrote:
> The only way a packet is accepted erroneously at a receiver, but would cause an
> problem, would be:
> 	link check OK
> 	TCP (1s compl) checksum OK
> 	new checksum FAIL
> 
> I.e., the only reason a new TCP checksum would be useful is in this case. If
> either of the link or existing TCP checks succeed, there's no new problem.

An example of that scenario is an echo server:
http://en.wikipedia.org/wiki/Echo_protocol
http://www.faqs.org/rfcs/rfc862.html
http://www.faqs.org/rfcs/rfc2075.html

which swaps source and destination addresses. These are covered by the pseudo-header
check; the 1's complement checksum is oblivious to the byte swapping, and the
echoed packet is still considered valid, even though it has been altered.
A stronger TCP checksum would detect this modification, and have to be recomputed.
much as payload checksums are recomputed through NAT boxes.


Lloyd Wood
L.Wood@surrey.ac.uk
http://sat-net.com/L.Wood