RE: iSCSI: 12-97 Bit Rule

[pat_thaler@agilent.com]

Thank you Julian, I am very happy with this text.

 

Regards,

Pat

-----Original Message-----
From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
Sent: Thursday, June 13, 2002 7:20 AM
To: pat_thaler@agilent.com
Cc: ips@ece.cmu.edu; owner-ips@ece.cmu.edu
Subject: Re: iSCSI: 12-97 Bit Rule
Importance: High

I took out completely the bit rule. 
I reformulated the CRC text as: 

The CRC MUST be calculated by a method that produces the same results as 
the following process: 

 - The PDU bits are 
considered as the coefficients of a polyno-mial M(x) of degree n-1; bit 7 of the 
lowest numbered byte is considered the most significant bit (x^n-1), followed by 
bit 6 of the lowest numbered byte and through bit 0 of the high-est numbered 
byte (x^0). 

- The most 
significant 32 bits are complemented. 

- The polynomial is multiplied by x^32 then divided by G(x). The 
generator polynomial produces a remainder R(x) of degree <= 31. 

- The coefficients of R(x) are 
considered a 32 bit sequence. 

- 
The bit sequence is complemented and the result is the CRC. 

- the CRC bits are mapped into the digest word - the 
x^31 coef-ficient in bit 7 of the lowest numbered byte of the digest continuing 
to through the byte up to the x^24 coefficient in bit 0 of the lowest numbered 
byte, continuing with the x^23 coefficient in bit 7 of next byte through x^0 in 
bit 0 of the highest numbered byte. 

- Computing the CRC over any segment (data or header) extended to include 
the CRC built using the generator 0x11edc6f41 will get always the value 
0x1c2d19ed as its final CRC. This value is given here in its polynomial form - 
i.e. not mapped as the digest word 

I 
hope you will find it less confusing 

Julo

pat_thaler@agilent.com 06/12/2002 08:41 PM Please respond to pat_thaler

To:        Julian Satran/Haifa/IBM@IBMIL         cc:        ips@ece.cmu.edu, owner-ips@ece.cmu.edu         Subject:        iSCSI: 12-97 Bit Rule

Julian, 
  
The bit rule added to 12-97 
(1.3.4 page 31) is inconsistant with the clauses above it: Word Rule, 
 Half-Word Rule and Byte Rule. 
  
It says "when the sequence of 
bits has a positional significance (e.g., a modulo 2 polynomial) then bit 7 of 
the lowest numbered byte is considered the most significant bit (2**n), followed 
by ...." 
  
When bits represent a number, they have positional 
significance so that sentence applies. However, the other rules apply the 
opposite significance to the bits within each byte. 
  
Also, 
note that bits represent polynomials at other times than the CRC such as for the 
purpose of authentication or encryption algorithms and those algorithms do not 
necessarily use the bit significance of the Bit Rule. The bit rule only applies 
to bit significance for CRC calculation. 
  
Bit Rule should be CRC Bit Rule 
and the associated text should make it clear that it only applies to the CRC 
calculation. Since it only applies there, it would be kinder to the reader to 
put the text in 11.1 rather than making the reader of 11.1 look nearly 200 pages 
back in the spec. 
  
Regards, 
Pat