RE: New iFCP draft

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

Elliott,

While correct about the need for CRC on Ethernet, IP is more than just
Ethernet.  The Ethernet-Fibre-Channel-FDDI CRC added is not to improve error
detection for Ethernet.  While moving through DSLAMs, Switches, Routers, and
the like, errors happen without the aid of CRC to catch this event as the
CRC is unfortunately stripped and regenerated as the world is not entirely
Ethernet-Fibre-Channel-FDDI.  This helps explain the Digest added to iSCSI.
iFCP could have added a separate CRC to protect their header.  FCIP does not
add any additional information yet and so they encapsulate the CRC found on
Fibre-Channel.  This is one area where we just can't get along.

Doug

> With regards to the new iFCP draft
>
> 5.3        Trailing iFCP CRC
>
>     This original Fibre Channel CRC shall not be encapsulated into the
>     iFCP message.  Rather, iFCP MAY use a 32-bit field containing a CRC
>     calculated over the data contained in the frame from the iFCP
>     AUGMENTED DATA to the iFCP Payload (inclusive).  This includes the
>     Fibre Channel header and payload.  The iFCP CRC follows the iFCP
>     payload, and the CRC algorithm is the same used for the Frame Check
>     Sequence (FCS) of IEEE 802.3 Ethernet frames.  A bit in the iFCP
>     FLAGS field in the iFCP header indicates the presence or absence of
>     this 32-bit trailing CRC.
>
> I would suggest "Rather, iFCP MUST use a 32-bit field containing
> a CRC ..."
> unless ethernet provides an equivalent CRC elsewhere.  The reason
> is that I
> believe gigabit ethernet uses the same physical layer as fibre channel.
> This physical layer is specified for fibre channel to have a data bit loss
> rate of 1 in 10^12 or less, and is not "perfect".  I think the CRC is
> designed to throw out these imperfect frames instead of resulting in data
> corruption.  Without the CRC, physical layer imperfections within
> fibre-channel specificaiton may lead to data corruption.
>
> Also, I am not intending to show preference for iSCSI or iFCP with this
> email.  I just was skimming over the standard and thought this feedback
> might be helpful.
>
> Stephen Elliott
>
>