esaProbeStatsSumOnewayR2PDelay 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 the interval that is still being actively collected (the live bin). 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, the technician can divide this live running total by the packet count reported in the same still-open bin to independently recompute the average one-way delay from the reflector back to the probe before the bin rolls over.