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

Joe Touch <touch@isi.edu> Sat, 26 June 2010 18:10 UTC

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Date: Sat, 26 Jun 2010 11:09:19 -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:
> Ethernet uses CRC-32, TCP's existing 16-bit checksum
> isn't a CRC. So I'm a bit puzzled by the question.
> 
> The link CRC and TCP checksum can disagree. See

*can*. The paper below did mathematical analyses, but didn't observe them either
in a lab or in the wild. I.e., that work didn't see places where it *did*. Thus
my question.

I'm not asking whether this is important to address, just what the current
motivation is. The work below suggests using application-layer checksums. That
seems prudent for large transfers, but I don't see a motivation for needing this
fixed at the TCP layer...

Joe

> Stone, J., Greenwald, M., Hughes, J., and C. Partridge,
> "Performance of checksums and CRCs over real data", IEEE
> Transactions on Networks vol. 6 issue 5, pp. 529-543,
> October 1998.
> 
> Stone, J. and C. Partridge, "When the CRC and TCP Checksum
> Disagree", Proceedings of ACM SIGCOMM , September 2000.
> 
> 
> On 25 Jun 2010, at 19:36, Joe Touch wrote:
>>> Our expirience has shown, that TCP often runs over links with much
>>> higher error rates, than intended for the technology (ie. Design goal of
>>> BER <= 1E-12 over Ethernet often has an actual BER >> 1E-12). If such
>>> error-prone links are running at a high speed and with large segment
>>> sizes (jumbo / giant frames), there exists the real problem that within
>>> a short timeframe (weeks to months), TCP will deliver corrupted data, as
>>> CRC-16 was not able to catch the error.
>>
>> Just to confirm, is this a measured problem or a mathematical one? I.e., have
>> you seen this actually occur (i.e., TCP with invalid CRC-16 but valid checksum)
>> in the wild, in a lab, or have you never seen it but consider it important anyway?
>