Description: An index. When the measurement is for a round-trip measurement, then this table entry is on the same probe as the corresponding sspmSourceEntry, and the value of this attribute should correspond to the value of sspmSourceInstance. Management applications configuring sinks for one-way measurements could define some scheme whereby the sspmSinkInstance is unique across all probes. Note that the unique key to this entry is also constructed with sspmSinkType, sspmSinkSourceAddressType, and sspmSinkSourceAddress. To make the implementation simpler, those other attributes are not included in the index but uniqueness is still needed to receive all the packets.