RE: iSCSI: 12-97 Bit Rule

[vince_cavanna@agilent.com]

Hi Luben,

When Bill pointed out that the text of the spec required the receiver to
complement the remainder in order to obtain the magic number 0x1c2d19ed I
re-read the text and came to the same interpretation as Bill. With that
interpretation I do believe there is a problem in the text. I had previously
read the text and not seen a problem. 

The reason I thought it was my fault is that in recent private
correspondence between you, Pat and Julian I noticed that Pat had it correct
in one of her memos and I corrected her (erroneously) and nobody objected
and Julian changed the text and I, for one, felt comfortable with it. I know
that you also had it right (from my perspective) in your description but
what we are commenting on is what is in the text.

As Bill pointed out, the text defines, in its description of the CRC
proceesing at the transmitter, that the "CRC computation" includes bit
complementing.

Later, in the description of the receiver computation at the receiver, the
text refers to "CRC computation" once more. The reader has no clue that "CRC
computation" at the receiver should not include the complement operation if
he is to obtain the value 0x1c2d19ed. Thus I agree that hte text is
misleading. 

We need to make it explicit that the receiver follows a different algorithm
(no complementing) and expects 0x1c2d19ed or that it implements the same
algorithm and expects 0xe3d2e612.

Vince



|-----Original Message-----
|From: Luben Tuikov [mailto:luben@splentec.com]
|Sent: Thursday, June 13, 2002 5:43 PM
|To: CAVANNA,VICENTE V (A-Roseville,ex1)
|Cc: 'Bill Studenmund'; Julian Satran; THALER,PAT (A-Roseville,ex1);
|ips@ece.cmu.edu
|Subject: Re: iSCSI: 12-97 Bit Rule
|
|
|"CAVANNA,VICENTE V (A-Roseville,ex1)" wrote:
|> 
|> 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, you had it right. The text was mentioning that it was 
|the CRC that
|was the magic constant, and since the CRC is complemented in 
|the course of
|computation, one more was needed to get ``back'' to the remainder.
|
|Yes, Bill, your empirical conclusion is correct.
|
|The magic constant _is_ the pure remainder in polynomial
|form (0x1c2d19ed).
|
|The CRC is the complemented (and byte mirrored) remainder.
|
|That is: you have 2 algoritms, one to compute the pure
|remainder, and another to complement it (and maybe byte-mirror)
|and inject it at the end of the message to be sent.
|
|The sender does both algorithms (compute and inject),
|and the receiver does just the first (compute) and compares
|the result with the magic value.
|
|This has already been mentioned in a more formal (and
|maybe confusing :-)) manner in an email from me at the
|beginning of this thread (take a look at it, the one
|with bit-sequences).
|
|-- 
|Luben
|