RE: iSCSI: PDU formats

["Robert D. Russell" <rdr@mars.iol.unh.edu>]

Julian:

I would like to request 3 small changes in the format of some of
the PDUs.  One of the design features that you have employed
very successfully to date is to have a given field, such as the
"Initiator Task Tag" for example, always appear in the same position
(bytes 16-19) in any PDU in which it appears.  This makes it easy
to understand, to implement, and to debug.  However, a few small
inconsistencies in the application of this design principle have
crept in with draft 7, and I would like to propose that we fix them.

1.  In draft 6 the SCSI Response PDU had one Status/Response field
	in byte 3 -- in draft 7-90 it now has a Status field in byte 2
	and a Response field in byte 3.  In draft 6 the Data-in PDU also
	had a Status field in byte 3, and in draft 7-90 it is still in
	byte 3, with byte 2 unused (reserved).  Would you please either:

	a.	Reorder the bytes in the SCSI Response PDU so that the Status
		field will be in byte 3 (so it is consistent with the Data-in
		PDU) and the Response field will be in byte 2; or

	b.	Move the status field in the Data-in PDU from byte 3 to byte 2
		(so it remains consistent with the SCSI Response PDU).

	I would prefer alternative a. because it would leave the Data-in
	PDU unchanged for drafts 6, 7 and 8, and the SCSI Response PDU
	has to change in any case.  However, obviously either solution
	would work.

2.	In draft 7-90, a field called "Response" appears in 3 PDUs:

	a.	In byte 36 of the Task Management Response PDU.
	b.	In byte 36 of the Logout Response PDU.
	c.	In byte 2 (if my request 1a above is taken) in the
		SCSI Response PDU.  This is clearly inconsistent with a and b.

	Since bytes 2 and 3 are currently unused (reserved) in both
	the Task Management Response PDU and the Logout Response PDU,
	the simplest solution would be to move the "Response" field in
	those 2 PDUs to byte 2 in order to be consistent with the SCSI
	Response PDU.  To keep the design clean, the new "Qualifier" field
	in the Task Management Response PDU should probably also be
	moved to byte 3.

3.	In draft 7-90 the Login and Login Response PDUs have been modified
	with the introduction of the T, C, and CNxSG fields in byte 37.
	However, in the Login Response PDU these fields overlay the
	Status-Detail field, which is also in byte 37.  Although the way
	to interpret this field is uniquely determined by the context,
	it is context dependent and I believe that this will lead to a lot
	of needless errors in coding, and that it also makes debugging more
	difficult, because the use of this byte changes during the login phase
	exchanges.  This means that you can't always look at it the same way.
	To avoid this overlay, would you please move the new fields
	(T, C, and CNxSG) to one of the currently unused bytes.  Many bytes
	(2, 10-11, 20-23, 38-39, 40-47) are currently unused in both of these
	PDUs, so there would appear to be no urgent need to overlay the new
	fields on top of an existing field in order to save space.

Thanks,

Bob Russell
InterOperability Lab
University of New Hampshire
rdr@iol.unh.edu
603-862-3774