esaProbeHistoryR2PLostPkts is a 64-bit performance counter counting test packets sent from the reflector back to the probe that never came back before the configured timeout, for a completed interval that has already closed and been archived into the rolling history log; for Y.1731 probes these are actual lost data frames rather than timed-out test packets. Because loss is broken out by direction, a technician can tell which leg of the circuit is actually dropping traffic instead of only knowing that loss happened somewhere on the round trip. For example, reviewing a week of archived bins and finding loss consistently on this one direction only, night after night, points a technician at an asymmetric-routing problem rather than a symmetric link fault.