esaProbeStatsSumOfSqRoundTripDelay is a 64-bit performance counter that reports the sum-of-squares of the round-trip delay of the test packets, in microseconds, with the reflector's own processing time excluded, for the interval that is still being actively collected (the live bin). Round-trip delay is the core proof-of-performance number for an Etherjack SLA test because it reflects everything affecting the circuit in both directions combined, so a technician watches it to confirm the customer's contracted latency bound is being met. 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 round-trip delay to judge, right now, whether the circuit is consistently mediocre or wildly erratic.