HP Aruba JL253A Management And Configuration Manual page 607

One way delay
One way delay is defined as the time difference between the Initiator transmitting the frame and the Responder
receiving the frame. This requires the Initiator and the Responder to be time synchronized with the same clock
server. This is explained in the illustration below:
Round trip time
RTT is measured at the initiator on a per packet basis and is as illustrated below:
Chapter 17 IP Service Level Agreement 607