Re: CRC vs CHKSUM presentation slides

[Michael Krause <krause@cup.hp.com>]

At 04:29 PM 3/22/2001 -0500, Jim Williams wrote:
>From: "Mark Bakke" <mbakke@cisco.com>
>To: "CAVANNA,VICENTE V (A-Roseville,ex1)" <vince_cavanna@agilent.com>
>Cc: <ips@ece.cmu.edu>; <ipsan@rtl.rose.agilent.com>
>Sent: Thursday, March 22, 2001 4:00 PM
>Subject: Re: CRC vs CHKSUM presentation slides
>
>
> > Vicente-
> >
> > I just took another look through your slides after seeing the
> > presentation on Monday.  They were very well-done.  I have
> > one question, though.  If the CCITT-CRC32 is considered "good
> > enough", then would the Ethernet CRC32 also be good enough?  The
> > reason I ask is that every hardware vendor involved in building
> > iSCSI stuff already has implementations of the Ethernet CRC,
> > which is used for both Ethernet and Fibre Channel.
> >
> > The Ethernet poly has more terms than CCITT, and perhaps is
> > not as good as CRC-32C (any thoughts?), but everyone has hardware
> > and software for this, with proven interoperability (bit and
> > byte order, etc).  Performance-wise, it will be there for
> > 10Gb Ethernet, so it should be fast enough.
> >
> > So if the Ethernet poly is deemed good enough (even if it's not
> > the best), and fast enough (even if it's not the fastest), why
> > not use it?  I think we would stand a much better chance of
> > achieving interoperability in a short time.
> >
> > Please let me know what you think of this; I realize that a few
> > of my questions were speculative.
>
>Since the iSCSI messages will often be encapsulated in ethernet
>packets, there is some value to using a different CRC.  Link
>errors are double protected with two different CRCs.  If
>ethernet and iSCSI use the same polynomial, there is little
>additional coverage against link errors.  This point may not
>be decisive, but all other things being equal or almost
>equal, it is worth considering.

Ethernet link (layer 2) protection only applies within a subnet; iSCSI CRC 
protection is end-to-end and is layer 5.  The amount of bytes within a 
given packet covered by each is different as well.   The arguments for 
using two different polynomials may be found in the selection proof used 
for GSN (HIPPI6400) which used two 16-bit CRCs to provide overall data 
protection.  It would be interesting to see a mathematical analysis showing 
the data protection provided by two 32-bit CRCs using the same polynomial 
by across two over-lapping but not identical data sets versus two different 
CRCs but I doubt that there is that much improvement and there is clearly 
benefit from being able to use the same cell for both purposes.

Mike


Mike