Subject: RE: iSCSI: CRC description

From: "Julian Satran" <[email-masked]>
Date: Wed, 12 Jun 2002 21:41:17 +0300

The thing about this text that is going on and on is very frustrating.
First Vince insisted on emphasis.
Now you find it confusing.

I gave it as a quiz to a undergraduate class and no one made a mistake.

How much effort do you think I will spend on it? And who are the people 
that are confused?

Julo

[email-masked]
06/12/2002 09:24 PM
Please respond to pat_thaler

        To:     Julian Satran/Haifa/IBM@IBMIL
        cc:     [email-masked], [email-masked]
        Subject:        RE: iSCSI: CRC description

Julian,
 
The changes to the text on CRC are a step down rather than up in clarity.
 
In particular: 
 " - In the bit stream mentioned above the CRC bits are appended   to the message bits with x^31 first followed by x^30 etc. In   the examples provided in Appendix B.4 - CRC Examples, the   value is outlined as a word sent or received and therefore   the CRC bits are mapped into the CRC word according to Section   1.3.4 Bit Rule - i.e., the x^31 coefficient in bit 7 of   the first byte of the digest continuing to through the byte the x^24 
coefficient in bit 0 of the first byte, continuing with the x^23 
coefficient in bit 7 of second byte through x^0 in bit 7 of the last byte. 
 
This statement will be very confusing to readers. The there is no abstract 
bit stream in which the CRC bits follow each other. Trying to specify the 
CRC with respect a bit stream will just lead to confusion. The old text 
with the changes I had suggested would provide more clarity.
Also, there is no MUST on the CRC computation method. Using a compatible 
computation method is necessary to produce interoperability so there 
should be a MUST. I suggest the following  which should allow for 
compatible variations in implementation:
Replace, "The CRC should be calculated as follows:" with "The CRC MUST be 
calculated using a method that produces the same result as the following 
process:"
Regards,
Pat

---

From: Bill Studenmund <[email-masked]>
Date: Wed, 12 Jun 2002 13:48:01 -0700 (PDT)

> How much effort do you think I will spend on it? And who are the people

I'm not sure how much effort you need to spend on this. I'll admit I was
one of the people confused, but that was more about CRC methods and the
bit reflection we have happening than anything else.

The only thing I might suggest would be to include parameters appropriate
for one of the CRC libraries. For instance including settings for Ross
Williams's CRC discussion could be quite useful, and remove much
ambiguity.

If there still is confusion and size becomes a concern, perhaps we should
generate an ID covering the CRC process. Something like
draft-ietf-tsvwg-sctpcsum-07.txt but for iSCSI.

> coefficient in bit 7 of second byte through x^0 in bit 7 of the last byte.

Shouldn't that be "x^0 in bit 0 of the last byte"?

> calculated using a method that produces the same result as the following

Take care,

Bill