RE: iSCSI CRC: A CRC-checking example

["Douglas Otis" <dotis@sanlight.net>]

Vince,

Thanks again as you have made this issue clear.  It would be beneficial to
maintain the "Ethernet" techniques even if only to ensure an accurate length
of packet detection.

Doug

> Doug,
>
> If the transmit side CRC generator used for the frame check sequence the
> remainder of the polynomial division directly (without complementing) the
> receiver-side CRC checker would be expected to end up with zeroes after
> processing an error-free frame. Unfortunately the receiver side
> CRC checker
> would also end up with zeroes after processing a frame whose only "errors"
> are extra trailing zeroes. Thus this type of error would not be
> detected by
> the CRC checker.
>
> Note that the expected final state of the receiver-side CRC checker is
> expected to be zero if hte remainder is not complemented even though both
> transmit and receiver sides initialize their CRC register to 1s. It is the
> operation of complementing the remainder that causes the receiver
> to have a
> non-zero (but constant) ending state after processing an error-free frame.
>
> If I understand you correctly, you have described another type of
> error that
> would be undetected were the remainder not complemented. Whenever the
> receiver CRC register is in its zero state (perhaps partially through a
> packet) the receiver checker would be blind to extra zeroes
> inserted in the
> packet at that point. I agree.
>
> Again, the improved protection resulting from initializing the
> CRC to 1s and
> complementing the remainder is not important for iSCSI because the packet
> length can be checked independently but I suppose it is advantageous to do
> things the ethernet way.
>
>
> Vince