Re: iSCSI: [Fwd: Crc-32c example in iSCSI spec]

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

Here are some clarifications about how the CRC is to be calculated (Network
byte order etc.),
the assumptions on data flow, the value of the CRC register for a good
segment and a corrected set of examples:

The Appendix A part will read:

   The following table lists cyclic integrity checksums that can be
   negotiated for the digests and MUST be implemented by every iSCSI
   initiator and target. Note that these digest options have only error
   detection significance.

   +---------------------------------------------+
   | Name          | Description     | Generator |
   +---------------------------------------------+
   | crc-32C       | 32 bit CRC      | 11EDC6F41 |
   +---------------------------------------------+
   | none          | no digest                   |
   +---------------------------------------------+

   The generator polynomial for this digest is given in hex-notation, for
   example 3b stands for 0011 1011 - the polynomial x**5+X**4+x**3+x+1.

   The generator polynomial selected is evaluated in [Castagnioli93].
   When using the CRC the CRC register must be initialized to all 1s
   (0xFFFFFFFF) and the CRC bits must be complemented before transmission.
   Padding bytes, when present, in a segment covered by a CRC, should be
   set to 0 and are included in the CRC. The CRC should be calculated as
   follows:

      - data are assumed to be  in the numbering order that appears in the
      draft - start with byte 0 bit 0 continue byte 1 bit 0 etc. (Big
      Endian on bytes / Little Endian on bits)
      - the CRC register is initialized with all 1s (equivalent to
      complementing the first 32 bits of the message)
      - the n PDU bits are considered coefficients of a polynomial M(x) of
      order n-1, with bit 0 of byte 0 being x^(n-1)
      - the polynomial is multiplied by x^32 and divided by G(x)- the
      generator polynomial - producing 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
      - after the last bit of the original segment the CRC bits are
      transmitted with x^31 first followed by x^30 etc. ( whenever examples
      are given the value to be specified in examples follows the same
      rules of representation as the rest of this document)
      - a receiver of a "good" segment (data or header) built using the
      generator 0x11EDC6F41 will end-up having in the CRC register the
      value 0x1c2d19ed (this a register value and not a word as outlined in
      this draft)


CRC Examples will read:

01   CRC Examples

   N.B. all Values are Hexadecimal

   32 bytes of zeroes:

     Byte:        0  1  2  3

        0:       00 00 00 00
      ...
       28:       00 00 00 00

      CRC:       aa 36 91 8a

   32 bytes of ones:

     Byte:        0  1  2  3

        0:       ff ff ff ff
      ...
       28:       ff ff ff ff

      CRC:       43 ab a8 62

   32 bytes of incrementing 00..1f:

     Byte:        0  1  2  3

        0:       00 01 02 03
      ...
       28:       1c 1d 1e 1f

      CRC:       4e 79 dd 46

   32 bytes of decrementing ff..e0:

     Byte:        0  1  2  3

        0:       ff fe fd fc
      ...
       28:       e3 e2 e1 e0

      CRC:       5c db 3f 11