esaProbeHistorySumRoundTripDelay 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microseconds, with the reflector's own processing time excluded, for a completed interval that has already closed and been archived into the rolling history log. 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, re-deriving the average round-trip delay from this archived running total and the matching archived packet count lets an auditor validate the device's own math well after the fact.