esaProbeHistorySumOfSqOnewayP2RDelay is a 64-bit performance counter that reports the sum-of-squares of the one-way delay, in microseconds, from the probe to the reflector, for a completed interval that has already closed and been archived into the rolling history log. Because this figure isolates delay from the probe to the reflector only, it lets a technician tell whether a latency problem is one-sided rather than assuming the whole round trip is equally at fault. 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 one-way delay from the probe to the reflector during the incident window, long after that bin closed.