[e2e] Re: [Tsvwg] Really End-to-end or CRC vs everything else?

Douglas Otis dotis at sanlight.net
Thu Jun 7 17:41:36 PDT 2001


The stuck bit tests for the more elaborate two Fletcher-16 sums indicated
that errors were undetected 1.3% of the time using disk files as test cases
if a stuck memory bus bit affected only half of the packet.  This
pathological failure indicates a low level of assurance that data has not
been corrupted.  The same test using CRC never failed.  This exceeds the
number of bits assured by CRC, but the random nature of CRC still provides
n:2^32 protection.  I did not bother with a splice test as the headers
within SCTP should provide this level of protection unless these segments
are out of order within the packet.  I had no information on the nature of
this type of error.


>    Thu, 07 Jun 2001 09:02:54 -0700
>    "Matt Wakeley" <matt_wakeley at agilent.com>
>    Jonathan Stone wrote:
>    > But *why* is crc32 thought to be better than a 32-bit mod-2^32
>    > checksum or a fletcher checksum with two 16-bit halves?
>    > A citation would be wonderful.
>    try draft-sheinwald-iSCSI-CRC-00.txt  It has lots of citations in it.
> While it may (probably) be true that CRC32 performs better than Fletcher
> and/or 32bit ones complement sum on real data, the draft-sheinwald-...txt
> says "For real data ... checksums behave substantially worse than
> CRCs" and
> cites our paper in TON '98 [Stone98].  The results there,
> however, compared
> a *32bit* CRC against a *16bit* ones-complement sum and *16bit* Fletcher
> (both one and twos complement).
> Also, the measurements in our paper were based on a very specific error
> model: packet-splices.  Care must be exercised when generalizing those
> measurements.
> I am not quibbling with the results quoted in the draft for the
> burst-error
> model over uniformly distributed data: the advantage of CRC32 there over
> Fletcher (in terms of error-rate and size of protected block) is
> compelling.  And, given the factor of 10^4 advantage, CRC32 is probably
> *still* the way to go.  However, as pointed out by our TON paper, this
> should not make us complacent.  In the real world it is possible for
> pathological distributions of data to be common, and combined with a
> particular error model the resulting protection offered by *any* checksum,
> CRC, or what-have-you may be *much* weaker than the theoretical model.
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