RE: iSCSI: 12-97 Bit Rule

["CAVANNA,VICENTE V (A-Roseville,ex1)" <vince_cavanna@agilent.com>]

I believe Bill is correct. The receiver, unlike the transmitter, should not complement the remainder if he expects to get 0x1c2d19ed.
I am afraid I may be reponsible for this mistake during an email exchange I and others had with Julian recently. Julian and those others must have trusted me a little too much. Sorry Julian and others.
Vince

|-----Original Message-----
|From: Bill Studenmund [mailto:wrstuden@wasabisystems.com]
|Sent: Thursday, June 13, 2002 2:36 PM
|To: Julian Satran
|Cc: pat_thaler@agilent.com; ips@ece.cmu.edu; owner-ips@ece.cmu.edu
|Subject: Re: iSCSI: 12-97 Bit Rule
|
|
|On Thu, 13 Jun 2002, Julian Satran wrote:
|
|One minor question.
|
|> 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.
|
|Call the above step 5.
|
|> - 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
|
|About the use of "final CRC" with respect to 0x1c2d19ed. Step 
|5 says the
|"CRC" is after the complementation, but my experiments indicate that
|0x1c2d19ed is the uncomplimented result, and that the 
|complimented result
|would be 0xe3d2e612. Thus 0xe3d2e612 would be the "CRC."
|
|> I hope you will find it less confusing
|
|I'm not sure what to do. The thoughts which come to my mind 
|take up space.
|The simplest would be to mention what coefficents one uses in 
|well-known
|crc packages. A more complex one would be to include source 
|code, as the
|sctp crc draft did.
|
|Take care,
|
|Bill
|