Re: BITS

["C. M. Heard" <heard@pobox.com>]

On Tue, 28 Oct 2003, David T. Perkins wrote:
> Sure, one could "remove bit definitions", but it provides no
> real value. Likewise, in compliance specifications, but what
> does it mean, and what are you saying about interoperability
> when you "remove bits in a compliance specification". What value
> do you achieve?


Here is an example.  In the SONET-MIB (RFC 3592) there exists the
following object definition:

   sonetMediumLoopbackConfig OBJECT-TYPE
       SYNTAX      BITS {
                     sonetNoLoop(0),
                     sonetFacilityLoop(1),
                     sonetTerminalLoop(2),
                     sonetOtherLoop(3) }
       MAX-ACCESS  read-write
       STATUS      current
       DESCRIPTION
          "The current loopback state of the SONET/SDH interface.  The
           values mean:

             sonetNoLoop
                Not in the loopback state. A device that is not
                capable of performing a loopback on this interface
                shall always return this value.

             sonetFacilityLoop
                The received signal at this interface is looped back
                out through the corresponding transmitter in the return
                direction.

             sonetTerminalLoop
                The signal that is about to be transmitted is connected
                to the associated incoming receiver.

             sonetOtherLoop
                Loopbacks that are not defined here."

       ::= { sonetMediumEntry 8 }

The ETHER-WIS compliance statement has the following OBJECT clause:

           OBJECT       sonetMediumLoopbackConfig
           SYNTAX       BITS {
               sonetNoLoop(0),
               sonetFacilityLoop(1)
               }
           MIN-ACCESS   read-only
           DESCRIPTION
               "Write access is not required, nor is support for values
               other than sonetNoLoop(0) and sonetFacilityLoop(1)."

What does this achieve?  It says that an implementation is not required
to allow bit positions other than the ones listed to be set, even if
write access is alowed.  Seems useful to me.

//cmh