esaProbeStatsSumRoundTripDelay is a 64-bit performance counter that reports the running total (sum) 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, the technician can divide this live running total by the packet count reported in the same still-open bin to independently recompute the average round-trip delay before the bin rolls over.