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

Joe Touch <touch@isi.edu> Sat, 26 June 2010 19:26 UTC

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Date: Sat, 26 Jun 2010 12:25:41 -0700
From: Joe Touch <touch@isi.edu>
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Subject: Re: [tcpm] draft-eggert-tcpm-historicize-00
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L.Wood@surrey.ac.uk wrote:
>> The do not seek, nor did they find in the wild, a single packet where the CRC
>> was valid and the TCP checksum was valid, but the packet had changed in
>> contents.
> 
> Before going any further - did you really mean to write that?

Yes. FWIW, the CRC I'm referring to above is the link CRC (CRC32 for Ethernet).

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.

Please see draft-ietf-tcpm-anumita-tcp-stronger-checksum-00, e.g., in the
abstract, or Sec 1. In fact, the case they're considering is where real link
errors occur that are not detectable - not TCP/IP implementation errors or
router memory errors, as were the only ones seen in the wild in the Sigcomm paper.

Joe