RE: iSCSI CRC: A CRC-checking example

["Douglas Otis" <dotis@sanlight.net>]

Vince,

Thanks for the explanation and here is how I understand your comments.  Even
with the CRC unlikely to find zero as a stable state with the all ones
initialization, in a rare event where a CRC value becomes zero over a
trailing list of zeros, then an inverted CRC store will create an all ones
as an ending value.  This is to better ensure detection of the end of the
block as there are encoding changes that help the underlying decoding
process.  It would seem that there is still the requirement to know the end
of the frame regardless of the CRC value but there may be an advantage to
this encoding change.  Again, thanks.

Doug


> In answer to Doug's question about the reason for using the COMPLEMENT of
> the remainder of the division as the CRC ....
>
> One reason I recall, is that this method permits the receiver to detect
> extra trailing zeroes in the protected packet. If the remainder were sent
> directly as the CRC, in the absence of errors the remainder of a similar
> computation at the receiver would be expected to be zero. This means that
> errors consisting of extra trailing zeroes would not be caught since zero
> input bits would cause no changes in the CRC circuit once the CRC register
> is in the so called "inaccessible state" consisting of all zeroes.