15.3.6.5.1 Peer-to-Peer Transparent Clock Message Support
The IEEE 1588-2008 version supports Peer-to-Peer PTP (Pdelay) message in addition to SYNC, Delay Request, Follow-up, and Delay Response messages. Figure 15-12 shows the method to calculate the propagation delay in clocks supporting peer-to-peer path correction.
As shown in Figure 15-12, the propagation delay is calculated in the following way:
- Port-1 issues a Pdelay_Req message and generates a timestamp, t1, for the Pdelay_Req message.
- Port-2 receives the Pdelay_Req message and generates a timestamp, t2, for this message.
- Port-2 returns a Pdelay_Resp message and generates a timestamp, t3, for this message.
To minimize errors because of any frequency offset between the two ports, Port-2 returns the Pdelay_Resp message as quickly as possible after the receipt of the Pdelay_Req message. The Port-2 returns any one of the following:
- The difference between the timestamps t2 and t3 in the Pdelay_Resp message.
- The difference between the timestamps t2 and t3 in the Pdelay_Resp_Follow_Up message.
- The timestamps t2 and t3 in the Pdelay_Resp and Pdelay_Resp_Follow_Up messages respectively.
- Port-1 generates a timestamp, t4, on receiving the Pdelay_Resp message.
- Port-1 uses all four timestamps to compute the mean link delay.