For packet-oriented interfaces, the number of packets
received via the interface that were discarded because
of an unknown or unsupported protocol. For
character-oriented or fixed-length interfaces that
support protocol multiplexing, the number of
transmission units received via the interface that were
discarded because of an unknown or unsupported protocol.
For any interface that does not support protocol
multiplexing, this counter is not present.
Discontinuities in the value of this counter can occur
at re-initialization of the management system, and at
other times as indicated by the value of
'last-clear'.Reference : RFC 2863: The Interfaces Group MIB - ifInUnknownProtos