DATE:    April 19, 1993                     X3T9.2/93-065
TO:      X3T9.2 Committee (SCSI)
FROM:    Giles Frazier (IBM Austin)

SUBJECT: CLEARING AUTOCONTINGENT ALLEGIANCE (ACA) IN PARALLEL SCSI-3

In order to be able to use SCSI-3 targets with SCSI-2 initiator queue
handling microcode, a minor change to the behavior of a SCSI-3
target in the queueing model is needed. This change would remove
the following "lockup" condition which would occur when a SCSI-3
target had an error.

PROBLEM: When a SCSI-3 target returns a CHECK CONDITION, it
         freezes its queue for the task set until it receives
         a CLEAR ACA message from the initiatiator with the check
         condition. Also, during the ACA condition, the target will
         reject all commands without an ACA tag. SCSI-2 initiators,
         however, do not send these ACA messages nor tags.
         Therefore, they will lock up after any check condition and
         will require a reset to recover.

SOLUTION:
 This problem can be solved without affecting the SCSI-3 queueing
 model by simply allowing SCSI-3 targets to clear the ACA
 condition upon receipt of an untagged command from the
 initiator. SCSI-3 initiators will use ACA tagged commands to process
 the queue as specified in the queueing model. SCSI-2 initiators,
 however, will send an untagged REQUEST SENSE command which will clear
 the ACA condition. In summary, SCSI-3 targets will appear to be
 SCSI-3 devices to SCSI-3 initiators, but they will appear to be
 SCSI-2 devices to SCSI-2 initiators.

SPECIFICATION OF SOLUTION:
 Although this behavior is already documented as an exception
 for parallel SCSI in SAM rev. 7, (Please see SAM, section
 8.4.2.2, paragraph 2.), discussions at the March workgroup meeting
 indicated that the inclusion of this behavior needs a vote since it
 was not regarded as part of the original queueing model. It was
 further suggested that the behavior should be specified in SIP
 instead SAM since it only applies to parallel SCSI.

 In order to specify this behavior in SIP, SIP needs to be worded
 so that the receipt of an untagged command meets one of the 4
 conditions in paragraph 8.4.2.2 (SAM). The following wording in SIP
 accomplishes this by meeting condition c) in SAM 8.4.2.2.

        A target shall treat the receipt of any untagged command
        from an I_T_x nexus as a clear ACA task for the I_T_x nexus.

Therefore I am proposing that the above wording be included in
SCSI-3 SIP.
                                        Giles Frazier
                                        IBM AWS Austin
                                        Austin, Texas
                                        (512) 838-1802
                                        gfrazier@ausvm6.vnet.ibm.com