iSCSI: Canonical Targets

[Mark Bakke <mbakke@cisco.com>]

It was pointed out during the interim meeting that the
canonical target is defined somewhat ambiguously, and may
be a bit too flexible for interoperability purposes.  This
message is a stake in the ground for defining this a little
more tightly.

So here are the new canonical target rules.  Please let me
know if there are major problems with any of them.  I tried
to define them a-la-carte, to make it easier to pick out any
that might cause trouble.

1. Each iSCSI implementation MUST include a canonical target.

2. A canonical target MUST be accessible at the default, IANA-
   assigned TCP port on each IP address on which the iSCSI
   implementation is listening for iSCSI connections.

3. A canonical target MUST NOT be used for SCSI commands.

   A canonical target is an iSCSI construct only, and does not
   have a corresponding SCSI device.  This means it may not
   be used to access Logical Units.

   A session created to a canonical target is a discovery session
   only, and once in full feature phase, is used only for text
   commands and asynchronous messages.  (Do any other commands make
   sense)?

4. A device containing a single target MUST provide both the
   canonical target and the real target.  (This is implied by the
   above requirements).

   An initiator connecting to such a device using only its IP address
   would first connect to the canonical target, and use SendTargets
   to obtain the iSCSI name of the real target.  It would then create
   a separate session to the real target.  Essentially, this means
   there's nothing special about a single-target device. 

5. An iSCSI device MUST provide a unique iSCSI name for each of its
   targets.  Using the canonical target as a nameless iSCSI target
   is not supported.

6. We can further specify the order in which the SendTargets response
   fields are returned, to simplify things further, e.g. each target
   in the SendTargets response MUST return these fields in this order:

   - A TargetName= field
   - A TargetAlias= field (value left blank if there's no alias)
   - One or more TargetAddress= fields
   - Any vendor-specific fields (ignored by standard initiators)

The above rules will make the behavior of various target implementations
identical, regardless of the number of interfaces or targets they
support.  This will reduce the number of end-cases that initiator
implementations will have to handle.

If we agree on these, we can edit them into the iSCSI and NDT documents.


Additional questions to send input on:

1. Should a non-canonical target respond to a SendTargets command?

2. If so, should it respond only with addresses for its own target,
   or should it respond with other targets, as a canonical target
   might do?

3. If not, the initiator must connect to a canonical target to find
   the other addresses of a target to which it is already connected;
   is the information it has sufficient to do so?  (I think the answer
   is yes, given canonical requirement #2 above).


-- 
Mark A. Bakke
Cisco Systems
mbakke@cisco.com
763.398.1054