esaProbeStatsSumOfSqOnewayR2PDelay is a 64-bit performance counter that reports the sum-of-squares 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, combining this live sum-of-squares value with the bin's packet count and delay sum lets the technician compute the variance of one-way delay from the reflector back to the probe to judge, right now, whether the circuit is consistently mediocre or wildly erratic.