RE: effect of initializing CRC reg to 1's depends on implementati on? iSCSI

[Luben Tuikov <ltuikov@yahoo.com>]

--- "CAVANNA,VICENTE V (A-Roseville,ex1)"
<vince_cavanna@agilent.com> wrote:
>
> When Paul, in his 12/14/01 memo "re:effect of
> initializing CRC reg to 1's
> depends on implementation? iSCSI", says to multiply the
> message by x^32 he
> means that is what the implied circuit does. He does not
> mean to multiply
> the message by x^32 and _then_ to perform the CRC
> computation with a circuit
> that multiplies by x^32 and divides by the CRC
> polynomial. Note also that
> Paul said to complement the first 32 bits of the PDU and
> he did not say that
> this is equivalent to initializing the CRC to ones like
> the iSCSI spec
> currently says.
>
> Anyone who attempts to implement Paul's process using a
> multiply-divide
> circuit  will obtain the results in the examples in hte
> iSCSI spec. If that
> person then, instead of complementing the first 32 bits
> of the PDU,
> initializes the circuit to 1s, he will also obtain the
> results in the iSCSI
> spec.
> 
> On the other hand, if that person implements paul's
> process using a
> divide-only circuit as Luben I and others have tried to
> do, the circuit will
> not yield the results in the examples and furthermore for
> that circuit
> initializing the circuit to ones is not the same as
> complementing the first
> 32 bits of the PDU.
> 
> So it is important that we either specify the circuit or
> we specify the
> process more rigorously. Again, specifying the circuit
> means specifying its
> response to an input _and_ its response to an initial
> state. 

All true.

But let's not really aim at ``circuit''.

A simple explanation of how to compute the
CRC, bit at a time will suffice.

From it an engineer can make a circuit,
and a computer scientist can make a program.

It would also be nice to make a document
describing all the math, derivations, etc.
cleanly and clearly and make a reference
to it in the draft. So if anyone is interested
they can follow the reference, if not, an implementaion
is clearly defined.

> I repeat below two ways to specify the implementation
> unambiguously:
> 
> One way to specify the circuit is to show the reference
> implementation (e.g.

Circuit?

> the serial implementation of the multiply-divide circuit
> that I posted) and
> to say that any circuit that performs the same function,
> ie. has the exact
> same response to initial state and to the input is also
> acceptable.
> 
> Another way is to describe the response of the implied
> circuit (the

Circuit again?

> I also claim that the description in the ethernet spec is
> also ambiguous but
> got away with it because of the implementation that they
> referenced (a
> multiply-divide implementation) and which removed the
> ambiguity - whether
> folk realized it or not.

Absolutely AGREE!

-l




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