SAM-2: Clarification of Protocol Services

Ralph Weber ralphoweber at
Tue Jun 1 08:11:51 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* Ralph Weber <ralphoweber at>

I believe that the notes below (from about page ix in the front
matter of the SAM-2 working draft) says about the same thing your
e-mail says but with less detail.

   The technical editor is considering a careful review of
   the working draft, with an eye toward overly abstract model
   abstractions. Examples are:

       a) Overly general layering terms and discussions; and
       b) Discussion of a new application client for each new
          request or task management function.

   The layering seems overly general and thus confusing. SCSI has
   two (or at most three) layers. The question of two or three layers
   depends on whether the service delivery port is a layer. The two
   "main" layers are the command and control layer (application client,
   device server, and task manager) and the service delivery subsystem.
   The description appears amenable to substantial simplifications.
   LLP and ULP could disappear. Generalized interfaces could be replaced
   with a small number of specific interfaces. Does T10 see value in this
   kind of simplification?

I will keep your e-mail to review before undertaking the changes described
above.  However, it could be several months before I start that work, as
I have a more immediate problem with multi-port devices.

If you feel that I do not properly understand your concerns, please contact

The one area where your e-mail raises a concern that I do not necessarily
share is in the following statement:

   ... the Application Client and the Device Server are members of this
   layer, however this is not made explicit.

To some degree, the Application Client and Device Server are peers.  However,
no rigorous effort has been made to formalize this relationship.  This is a
matter of history, as SCSI was developed without benefit of formal layered
models.  I am sympathetic to that history and reluctant to break long-held
concepts solely to create a rigorous formal model for SCSI.

Furthermore, it may not be possible to create a formal layer to house the
Application Client and Device Sever because SCSI intentionally defines the
latter more completely than the former.  At is core, SCSI is about what
the Target does, and thus about what the Device Sever does.  SCSI avoids
specifying Application Client behavior except where absolutely necessary.
This is a reasonable approach for SCSI, since the committee is dominated
by manufacturers of SCSI device hardware, not by system house software

For these reasons, I am not sanguine about making a formal layer to
house the Application Client and Device Server.  The looser, peer
relationship will always be there, however informal it may be.



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