esaProbeHistorySumOfSqNegR2PJitter is a 64-bit performance counter that reports the sum-of-squares of the negative one-way jitter (packet-to-packet delay variation), in microseconds, from the reflector back to the probe, for a completed interval that has already closed and been archived into the rolling history log. Negative (a packet arriving earlier than the one before it) jitter from the reflector back to the probe is exactly the kind of packet-to-packet delay variation that causes stutter in voice and video traffic riding over the same service. For example, a technician investigating a resolved outage can combine this archived sum-of-squares value with the stored sum and count to reconstruct the variance of negative jitter from the reflector back to the probe during the incident window, long after that bin closed.