Re: iSCSI error recovery

[Matt Wakeley <matt_wakeley@agilent.com>]

Black_David@emc.com wrote:

> - Given some of the CRC discussion, I think the conclusion
>         in Orlando to have separate header and data CRCs is
>         NOT the rough consensus of the WG.  We need to go
>         back to a requirements discussion on this rather than
>         debating exactly where to put the CRCs.  Would those
>         envisioning middleboxes/gateways/etc. that would
>         benefit from this sort of CRC separation please post
>         short use cases/descriptions indicating the basic
>         functionality of the box and which fields it needs
>         to change (let's do this with reference to the header
>         layout in -03 rather than subsequent changes)?  As
>         part of the use case/description, please explain
>         how/why Fibre Channel's single CRC covering both
>         the frame header and data causes problems/difficulties.

David,

The use of a separate header/data CRC is required in order to perform
efficient data steering.  If the iSCSI header and data are both protected by
the same CRC (which for efficiencies in transmission would be after the data),
then an iSCSI adapter would have to buffer up an entire iSCSI PDU before it
could begin DMAing the PDU payload to host memory (because it could not
guarantee that the information in the iSCSI header was correct to begin DMAing
it earlier).  Since Julian insists that PDUs could be in lengths requiring 32
bit length fields, this is a lot of unnecessary storage.

-Matt Wakeley
Agilent Technologies