An index object used to disambiguate conceptual rows in
the table, since the ipv6UdpLocalAddress/ipv6UdpLocalPort
pair may not be unique.
This object identifies the local interface that is
associated with ipv6UdpLocalAddress for this UDP listener.
If such a local interface cannot be determined, this object
should take on the value 0. (A possible example of this
would be if the value of ipv6UdpLocalAddress is ::0.)
The interface identified by a particular non-0 value of
this index is the same interface as identified by the same
value of ipv6IfIndex.
The value of this object must remain constant during
the life of this UDP endpoint.
This object is obsoleted by the zone identifier in
an InetAddressIPv6z address in
UDP-MIB::udpEndpointLocalAddress.