Re: iSCSI : On the subject of R2T and Task Tags.

[Matt Wakeley <matt_wakeley@agilent.com>]

The task tags in iSCSI should work the same as they do in FCP:

The initiator supplies a initiator task tag per task that it uses to associate
any received messages from the target to that task. [a "undefined" initiator
task tag should have a value of 0xffffffff since a value of zero is a valid
number for a table look up ].

The target supplies a target task tag per task that it sends to the initiator
where a response from the initiator is expected.  The target uses the target
task tag to associate any solicited received messages from the initiator
(other than the initial command) to that task.

For SCSI READ commands, there is no need to specifiy a target task tag.
Typically in FCP today, the targets to NOT assign target task tags (RX_IDs in
FC terms) for READs.

For SCSI WRITE commands, the target MAY assign a target task tag to the I/O
that it uses when sending R2Ts to the initiator.  The initiator must then echo
that target task tag in replies it sends to the target.  I strongly object to
the words in section 2.16 "All outstanding R2T should have different Target
Transfer Tags".  This specification should not dictate to the target how it
chooses or uses the Target Task Tags.

-Matt Wakeley
Agilent Technologies