This managed object returns the next available DD ID that
MAY be used for DD creation for this iSNS Server instance.
The user should read and use the value of cIsnsNextIdxDd
before performing a cIsnsDdTable row creation. The
cIsnsNextIdxDd value returned MAY be used as the index
value when creating a new row in the cIsnsDdTable. Use
of this managed object reduces the chance for an index
conflict.