A conceptual row in the nlMatrixSDTable.
The hlMatrixControlIndex value in the index identifies the
hlMatrixControlEntry on whose behalf this entry was created.
The protocolDirLocalIndex value in the index identifies the
network-layer protocol of the nlMatrixSDSourceAddress and
nlMatrixSDDestAddress.
An example of the indexing of this table is
nlMatrixSDPkts.1.783495.18.4.128.2.6.6.4.128.2.6.7.
Note that some combinations of index values may result in an
index that exceeds 128 sub-identifiers in length, which exceeds
the maximum for the SNMP protocol. Implementations should take
care to avoid such combinations.