esaProbeHistorySumOnewayR2PDelay is a 64-bit performance counter that reports the running total (sum) of the one-way delay, in microseconds, from the reflector back to the probe, for a completed interval that has already closed and been archived into the rolling history log. Because this figure isolates delay from the reflector back to the probe 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, re-deriving the average one-way delay from the reflector back to the probe from this archived running total and the matching archived packet count lets an auditor validate the device's own math well after the fact.