iSCSI: CmdSN during the Login phase

["Scott M. Ferris" <sferris@acm.org>]

  At what point during the Login Phase of a leading connection (null
TSID) does a session exist?  

  Section 2.10.7 describing CmdSN for the Login PDU indicates that
when TSID is null, CmdSN indicates the starting Command Sequence
number for this session.

  For interoperability, it is important to define precisely when a
session exists, so that all initiators and targets agree on when
CmdSNs are significant, and do not reject or ignore PDUs due to
differing assumptions of how the CmdSN numbering should be done during
the Login Phase.

Some of the possible choices for session start are:

1) a session starts before the initiator sends anything, and the Login
   PDU is the first PDU of a session.

2) a session starts when the initiator receives a (possibly partial)
   Login Response with an accept status class and a non-zero TSID, and
   the next PDU sent by the initiator is the first PDU of the session,
   which uses the same CmdSN as the Login PDU.

3) a session starts when the initiator receives a Final Login
   Response, and the next PDU sent by the initiator is the first PDU
   of the session, which uses the same CmdSN as the Login PDU.

  After searching through draft 6, I was unable to find anything that
clearly indicated which if any of the above possibilities was correct,
or, if more than one is possible, how the initiator and target are
supposed to determine what CmdSN numbering scheme the other side is
using for the Login Phase.

  The UNH draft-6 initiator appears to use choice #1.  The Cisco
draft-6 initiator and target use choice #2.  This can cause the UNH
initiator to hang when trying to login to the Cisco target, since the
initiator may send a Text PDU with CmdSN 2, while the target is
waiting for a PDU with CmdSN 1.

  Section 1.2.2.2 states that "Status numbering starts after
Login. During login, there is always only one outstanding command per
connection and status numbering is not strictly needed but may be used
as a sanity check."

  A similar argument could be made that CmdSN is not needed during the
Login Phase, suggesting choice #3 may be the simplest. 

  I also think section 1.2.2.2 is problematic, as it appears to
contradict itself by saying that status numbering is not used during
login, and then follows up saying status numbering may be used during
login.  

-- 
Scott M. Ferris,
sferris@acm.org