Round-trip delay time measurement system, round-trip delay time measurement method, return method, communication device, program, and data structure

A communication device communicates with an external device. The communication device is provided with: a measurement packet transmission unit which transmits a measurement packet at a first transmission time point; a return packet reception unit which receives, at a second reception time point, a return packet that is transmitted as a reply to the measurement packet at a second transmission time point by the external device, that includes information relating to a processing time indicating a time from a first reception time point at which the measurement packet was received by the external device to the second transmission time point, and that has the same size as the measurement packet; and a delay time specification unit which specifies, as a round-trip delay time, a time obtained by subtracting the processing time from a time from the first transmission time point to the second reception time point.