Re: iSCSI : Digest Error recovery causes data corruption

[Matt Wakeley <matt_wakeley@agilent.com>]

At first glance, the idea is nice.

However, options 1 and 2 require changes to the implementation and API of TCP
to provide the framing.  As you (should) know, we are not allowed to change
TCP.  Another problem is that your proposal to drop segments that have digest
errors violates the network layering.  TCP will deliver the PDUs to iSCSI. 
iSCSI is the layer that checks the CRC.  When TCP delivers the data to iSCSI,
it has already acknowledged the segment, so it can't be discarded and
retransmitted.  There is no way for iSCSI to tell TCP to accept or drop a
segment.

[all of the above assumes existing TCP definition/implementation.  Anything is
possible if one is allowed to change things]

-Matt

Pierre Labat wrote:
> 
> Hello,
> 
> I would invite you to revisit the Mike Krause mail about
> "iSCSI Data Integrity - Digests".
> 
> If Alternative 1 or 2 is adopted, when a digest error (CRC)
> occurs, the segment could be discarded, and TCP will
> retransmit. For software implementation with an hardware
> assist to calculate the CRC, (same kind of the one that exists
> now to calculate the checksum now), if a CRC error is detected
> it could be assimilated as a checksum error.
> Hence there is no need to overload the iSCSI protocol
> to deal with digest errors, because the iSCSI layer could not
> see them.
> 
> Why to do complicate when simple an efficient can be achieved?
> 
> Regards,
> 
> Pierre