RE: CRC vs CHKSUM presentation slides

[vince_cavanna@agilent.com]

Mark and Jim,
I think any of the 32 bit CRC polynomials that have been proposed are more
than good enough. My main reason for recommending hte CCITT-CRC32 is that it
is half the cost of the others in terms of gate count - and unless you are
doing a serial divider, which would be too slow, the gate count is very
significant. Also, the leverage may not be as high as you think unless we
are willing to use the datapath width of existing implementations. For 10
gig we will probably need to have larger than normal datapath widths. If you
change the datapath width to handle, say, 64 bits at a time you change the
implementation. I would otherwise have no problem with using the ethernet
polynomial. I will try to explain later why I am not concerned about using
the same polynomial and potentially giving up some cross-checking (I am not
quite sure yet).
Vince

|-----Original Message-----
|From: Jim Williams [mailto:jim.williams@emulex.com]
|Sent: Thursday, March 22, 2001 1:30 PM
|To: ips@ece.cmu.edu
|Subject: Re: CRC vs CHKSUM presentation slides
|
|
|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.
|