some numbering clarifications

[julian_satran@il.ibm.com]

Dear colleagues,

To clarify some issues related to numbering in iSCSI here is an excerpt
from the text supposed to go into the next draft.

I hope it helps.

Regards,
Julo

1.1.1     Ordering and iSCSI numbering

   The NOP message PDUs are not associated with a task, are meant for
   immediate delivery, and their only purpose is synchronizing the ordering
   registers of the target and initiator.


1.1.1.1   Command numbering

   iSCSI supports ordered command delivery within a session.  All commands
   (initiator-to-target) and responses (target-to-initiator) are numbered.
   Any SCSI activity is related to a task (SAM-2). The task is identified
   by the Initiator Task Tag for the life of the task.  Commands in transit
   from the initiator SCSI layer to the target SCSI layer are numbered by
   iSCSI and the number is carried by the iSCSI PDU as CmdRN
   (Command-Reference-Number).
   The numbering is session-wide.  All iSCSI PDUs that have a task
   association carry this number. CmdRNs are allocated by the initiator
   iSCSI within a 32 bit unsigned counter (modulo 2**32).  The value 0 is
   reserved and used to mean immediate delivery. Comparisons and arithmetic
   on CmdRN SHOULD use Serial Number Arithmetic as defined in [RFC1982]
   where SERIAL_BITS = 32.
   The target may choose to deliver some task management commands for
   immediate delivery.  The means by which the SCSI layer may request
   immediate delivery for a command or by which iSCSI will decide by itself
   to mark a PDU for immediate delivery are outside the scope of this
   document.
   CmdRNs are significant only during command delivery to the target. Once
   the device serving part of the target SCSI has received a command, CmdRN
   ceases to be significant.  During command delivery to the target, the
   allocated numbers are unique session wide.  The initiator and target are
   assumed to have three registers that define the allocation mechanism -
   CmdRN - the current command reference number advanced by 1 on each
   command shipped; ExpCmdRN - the next expected command by the target -
   acknowledges all commands up to it; MaxCmdRN - the maximum number to be
   shipped - MaxCmdRN - ExpCmdRN defines the queuing capacity of the
   receiving iSCSI layer.
   The target SHOULD NOT transmit a MaxCmdRN that is more than 2**31 - 1
   above the last ExpCmdRN.  CmdRN can take any value from ExpCmdRN to
   MaxCmdRN except 0. The target will reject any command outside this range
   or duplicates within the range not flagged with the retry bit (the X bit
   in the opcode).  The target and initiator registers MUST uphold causal
   ordering.

   iSCSI initiators MUST implement the command/request numbering scheme
   only if they support more than one connection.

   iSCSI targets MAY use the command/request numbering scheme for ordered
   delivery when they support multiple connections. However, they MUST
   provide ExpCmdRN and MaxCmdRN values that will enable the initiator to
   make progress.

1.1.1.2   Response/Status numbering

   Responses in transit from the target to the initiator are numbered.  The
   StatRN (Status Reference Number) is used for this purpose.  To enable
   command recovery the target MAY maintain enough state to enable data and
   status recovery after a connection failure.
   A target can discard all the state information maintained for recovery
   after the status delivery is acknowledged through ExpStatRN.
   A large difference between StatRN and ExpStatRN indicates a failed
   connection.

   Initiators and Targets MUST support the response-numbering scheme
   regardless of the support for command recovery.

1.1.1.3   Data PDU numbering

   Incoming Data PDUs MAY be numbered by a target to enable fast recovery
   of long running READ commands.
   Data PDUs are numbered with DataRN.  NOP command PDUs carrying the same
   Initiator Tag as the Data PDUs are used to acknowledge the incoming Data
   PDUs.  Support for Data PDU acknowledgement and the
   maximum number of unacknowledged data PDUs are negotiated at login.
   In a PDU carrying both data and status the number refers to the Status
   and the last set of data blocks is implicitly acknowledged when Status
   is acknowledged.