Description: The value of this object identifies a particular
conceptual row associated with the session identified by
casnSessionId.  The conceptual row that this object points
to represents a port that is used to transport a session.

If the port transporting the session cannot be determined,
the value of this object will be zeroDotZero.

For example, suppose a session is established using an ATM
PVC.  If the ifIndex of the ATM interface is 7, and the 
VPI/VCI values of the PVC are 1, 100 respectively, then
the value of this object might be as follows:

       casnNasPort.15 = atmVclAdminStatus.7.1.100
                   ^                      ^ ^  ^
                   |                      | |  |
   casnSessionId --+                      | |  |
         ifIndex -------------------------+ |  |
       atmVclVpi ---------------------------+  |
       atmVclVci ------------------------------+

where atmVclAdminStatus is the first accessible object
of the atmVclTable of the ATM-MIB.