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

[Luben Tuikov <ltuikov@yahoo.com>]

--- "THALER,PAT (A-Roseville,ex1)" <pat_thaler@agilent.com>
wrote:
> Paul,
> 
> I agree that we should use the mathematical description
> of the CRC as the
> spec.

I've been trying to convince ppl this since I started
to work on iSCSI.

Pure math is much clearer, and simple than an
implementation. And it woudn't be more than a few
lines to describe the algorithm. (A treatment
of why it works, would of course be longer than that.)

Please note that the draft makes no mention of
the Ethernet CRC algo, and I assume there is a reason for
it. Else why not say: ``Ethernet CRC algo, with the
generator poly be ...''.

> Equivalent ways of obtaining the effect of
> inverting the first bits or
> checking the result (magic number) are implementation
> details.

I guess that would be true, but one thing needs mentioning:

======================================================
The start point of all those algoritms and circuits is a
message premultiplied by x^n (n=degG(x)), e.g. AUGMENTED,
and PREFIXED. And then division is done on that, I.e.
x^nM(x) + Prefix.
======================================================

Setting an initial value of the CRC register will result in
a constant apperaing here and there.

Now you want to object to this, and rightly so.
Why? Well after analysis, the above said can be so much
transformed that it doesn't look like that statement
anymore. E.g. Prefixing can be done away with, by adding a
constant value to the n MSb of the message; as well as
augmentation, by changing the div algo to include mult on
the fly.

As an exampe of this ``compilation'' is the circuits that
Vince has provided.

But we need a mathematical description, and by analogy,
reading source code makes understanding a lot easier than
reading compiled binary code.

> I'm not
> convinced that we need to show an implementation example
> in the annex.

Yes we should stick with the way it currently is in the
draft: mathematical steps, but make them clearer.

> Given
> the misleading information that Vince has found in some
> of the literature if
> we show an implementation example, we need to point out
> that the technique
> of initializing the register to 1's applies specifically
> to that
> implementation and may not work for other
> implementations.

It was I who first brought this up using pure analysis.
Maybe you should take a look at the iSCSI archives.

When I started work on iSCSI -- the CRC thead was quiet and
by word of mouth ppl were implementing Ethernet using the
CRC32/4 generator poly. But the description of the draft
had some inconsistencies and here we are.
 
> Regards,
> Pat

Regards,
Luben



=====
--

__________________________________________________
Do You Yahoo!?
Check out Yahoo! Shopping and Yahoo! Auctions for all of
your unique holiday gifts! Buy at http://shopping.yahoo.com
or bid at http://auctions.yahoo.com