An index that uniquely identifies an entry in
the portTopN table among those in the same report.
This index is between 1 and N, where N is the
portTopNGrantedSize of ports with the highest
portTopNRateBase. Increasing values of
portTopNIndex shall be assigned to entries with
decreasing values of portTopNRateBase until index
N is assigned to the entry with the lowest value
of portTopNRateBase or there are no more
portTopNEntries.