Subject: Re: iSCSI: Nailing down CRC-32C

From: [email-masked]
Date: Sat, 5 May 2001 19:21:18 +0300

The CRC part of the appendix (for 07) reads:

   The following table lists cyclic integrity checksums that can be
   negotiated for the digests and MUST be implemented by every iSCSI
   initiator and target. Note that these digest options have only error
   detection significance.

   +---------------------------------------------+
   | Name          | Description                 |
   +---------------------------------------------+
   | crc-32C       | 32 bit CRC      | 11EDC6F41 |
   +---------------------------------------------+
   | none          | no digest                   |
   +---------------------------------------------+

   The generator polynomials for those digests are given in hex-notation,
   for example 3a stands for 0011 1010 - the polynomial x**5+X**4+x**3+x+1.

   The generator polynomial selected is evaluated in [Castagnioli93].
   When using the CRC the CRC register must be initialized to all 1s
   (0xFFFFFFFF) and the CRC bits must be complemented before transmission.
   Padding bytes, when presents in a segment covered by a CRC, have to be
   set to 0 and are included in the CRC.

   Regards,
   Julo

Mark Bakke <[email-masked]> on 05-05-2001 00:13:09

Please respond to Mark Bakke <[email-masked]>

To:   IPS <[email-masked]>
cc:
Subject:  iSCSI: Nailing down CRC-32C

At the interim meeting, it was stated that iSCSI has selected
the CRC-32C polynomial as its required iSCSI-level header and
data CRC.  Now that we have it, it's time to move on and make
sure we can implement it.

In the interest of interoperability, we need to not only specify
the polynomial, but also the initial values, bit and byte
ordering, etc.

"A Painless Guide to CRC Error Detection Algorithms"
(http://www.ross.net/crc/crcpaper.html) specifies a
method to unambiguously characterize these parameters
(sections 15 and 16).  Has anyone taken a shot at defining
these yet?  Otherwise, here is what it might look like:

Name   : "CRC-32C"
Width  : 32
Poly   : 1EDC6F41   (note that the leading "1" is implied)
Init   : FFFFFFFF
RefIn  : True
RefOut : True
XorOut : FFFFFFFF
Check  : ?

I haven't attempted to create check data based on these yet.  As
soon as the other parameters are nailed down, we need to do this.

Anyway, I am not a CRC expert, and can't make any statement about
the above values being the "best" way to do this, but instead just
copied them (except the polynomial itself) from the Ethernet CRC,
since that is likely the easiest for everyone implementing hardware
to deal with.

If someone else is already doing this, let me know; I just wanted
to start this thread so we can get closure.

Regards,

Mark

--
Mark A. Bakke
Cisco Systems
[email-masked]
763.398.1054

---

From: Mark Bakke <[email-masked]>
Date: Mon, 07 May 2001 15:00:28 -0500

Julian-

That's great; it covers nearly everything.  The only thing left
is the byte order; are they taken in the same order Ethernet uses?

If so, I can generate a few test patterns that our implementations
can all check against.

Thanks,

--
Mark

-- 
Mark A. Bakke
Cisco Systems
[email-masked]
763.398.1054