RE: CRC vs CHKSUM presentation slides

[vince_cavanna@agilent.com]

To save Julian from having to write a memo I would like to add that the
ethernet polynomial has a certain weakness for 3 bit errors that the others
don't. The ethernet polynomial is only guaranteed to catch all combination
sof 3 bit errors when the message length is up to 12144 bits. For larger
message lengths some 3 bit errors may get through. 
Vince


|-----Original Message-----
|From: CAVANNA,VICENTE V (A-Roseville,ex1) 
|Sent: Thursday, March 22, 2001 3:42 PM
|To: 'Mark Bakke'; 'Jim Williams'; ips@ece.cmu.edu
|Cc: CAVANNA,VICENTE V (A-Roseville,ex1)
|Subject: RE: CRC vs CHKSUM presentation slides
|
|
|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.
||
|