Re: iSCSI: 12-97 Bit Rule

["Julian Satran" <Julian_Satran@il.ibm.com>]

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