Re: The TCP and UDP checksum algorithm may soon need updating

Craig Partridge <craig@tereschau.net> Tue, 09 June 2020 19:32 UTC

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From: Craig Partridge <craig@tereschau.net>
Date: Tue, 9 Jun 2020 13:31:58 -0600
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Subject: Re: The TCP and UDP checksum algorithm may soon need updating
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Following up on notes from various folks, most notably John K. and
Christian H., about checksums and encryption-based integrity protection, I
wanted to make a couple of clarifying points.

* If you are not worried about an adversary who seeks to alter your data in
transit, then integrity protection is a lousy method for ensuring the
received data is correct.  The reality is that a checksum of width X bits
can potentially catch 10x as many errors as an integrity check of X bits
and will likely require 1/X computational power to compute.  The
requirement to make an integrity check proof against an adversary makes it
less good. [Happy to explain more -- just trying to keep this note short].

* Some folks have assumed the TCP checksum, at 16 bits, misses an error
about 1 in 2^16 times.  That's not correct.  There are classes of errors
that TCP catches 100% of the time (e.g. single-bit errors).  There are also
classes of errors that the TCP checksum misses about 1 in 2^10 times.  This
latter is because the TCP checksum is not terribly good -- a better
designed checksum would miss only about 1 in 2^16.

Thanks!

Craig

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