From TFA: "Some commercial PTP implementations use packet delay variation (PDV) filters [27], and compensate for known latencies in the receive and transmit pat

From TFA: "Some commercial PTP implementations use packet delay variation (PDV) filters [27], and compensate for known latencies in the receive and transmit paths." This is phrased vaguely. All gPTP implementations are required to compensate for link delay (not necessarily rx/tx asymmetry).