RE: iSCSI CRC: A CRC-checking example

[Robert Snively <rsnively@Brocade.COM>]

Pat,

I agree with you.

Bob

> -----Original Message-----
> From: THALER,PAT (A-Roseville,ex1) [mailto:pat_thaler@agilent.com]
> Sent: Friday, August 24, 2001 10:36 AM
> To: joeg@alacritech.com; Robert Snively; Black_David@emc.com;
> sanjay_goyal@ivivity.com; ips@ece.cmu.edu
> Subject: RE: iSCSI CRC: A CRC-checking example
> 
> 
> Joe and Bob,
> 
> I doubt that any implementation today will be doing CRC 
> computation in a bit
> serial circuit. The computation is generally done either byte 
> parallel or,
> increasingly, multi-byte parallel. The order in which the 
> bits of a byte are
> assigned to polynomial coefficients makes no difference to 
> byte parallel CRC
> implementation complexity. The only time complexity would be 
> effected by bit
> order is when one was doing a serial shift register 
> implementation of the
> CRC generator as some of the earliest Ethernet 
> implementations did. The
> technical reasons which applied to Ethernet in chosing the 
> CRC bit checking
> order, burst protection and serial implementation complexity, 
> don't apply to
> iSCSI.
> 
> The only reason for choosing one bit order over another for iSCSI CRC
> checking is the limits of human comprehension. Which bit order is most
> likely to be gotten right by the humans implementing the standard?
> 
>  Ethernet 
>           + many people are already accustomed to its bit order
>           - most significant byte first, least significant 
> bit first can
> cause confusion
>  Most significant everything first 
>           + more internally consistent than Ethernet
>           - different from Ethernet
> 
> The most important thing is that we specify whichever one we chose as
> clearly as possible.
> 
> Regards,
> Pat 
>              
> 
> -----Original Message-----
> From: Joe Gervais [mailto:joeg@alacritech.com]
> Sent: Thursday, August 23, 2001 7:21 PM
> To: Robert Snively; Black_David@emc.com; sanjay_goyal@ivivity.com;
> ips@ece.cmu.edu
> Subject: RE: iSCSI CRC: A CRC-checking example
> 
> 
> Bob,
> 
> If the data was going on token ring or some other media, 
> sure, who cares
> about an Ethernet approach, but the data is generally being sent and
> received over Ethernet, and there is advantage at least in some
> implementations to be in Ethernet bit/byte order.
> 
> Additionally, as David said earlier today, if it isn't broke, 
> let's not fix
> it. Can we get consensus and move on?
> 
> Joe
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Robert Snively
> 
> ... snip ...
> So I would have expected that we would avoid any choice that
> makes that less convenient.  The Ethernet approach to those
> bytes would strike me as being the second-least convenient
> approach.  Clearly, throwing hardware at this could correct
> for that inconvenience, but it seems to me that any
> optimizations we make should focus on what we want to do, not
> what Ethernet did.
> 
> Bob
> 
>