Re: iSCSI: Nailing down CRC-32C

[bj nima <bj_nima@excite.com>]

Assumptions:

1) The iSCSI DATA PDU Header and Data Integrity checks uses CRC-32
2) Support of Integrity check is mandatory for both header and Data 
   digest
3) An iSCSI PDU may span multiple TCP segments. 
4) Any of these segments may arrive out of order at the receiver.
5) To regenerate the CRC for comparison with the value present in the 
PDU received, the receiver needs to keep in a temporary buffers all 
segments, waiting for the missing one, as CRC must be computed using 
the whole re-assembled PDU

Question: Does CRC-32 support partial computation and compensation 
for wrong initial values later (I guess NO!), to enable partial CRC 
calc on avail parts of PDU?


Possible answer: No

Possible Conclusion: The RCV side is further complicated b/c of this 
CRC

Possible solutions:

1) Use CRC that allows partial calc
2) Limit iSCSI PDU to TCP MSS if data CRC is used (added o/h)...
3) Leave as it, though it has cost , performance implication and hurts
scaling to 10G..


Nima



-----Original Message-----
From: julian_satran@il.ibm.com [mailto:julian_satran@il.ibm.com]
Sent: Saturday, May 05, 2001 9:21 AM
To: ips@ece.cmu.edu
Subject: Re: iSCSI: Nailing down CRC-32C




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 <mbakke@cisco.com> on 05-05-2001 00:13:09

Please respond to Mark Bakke <mbakke@cisco.com>

To:   IPS <ips@ece.cmu.edu>
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
mbakke@cisco.com
763.398.1054










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