ter FIGURE 8-15 PTP master selection process. PTP, Precision Time Protocol. Follow-Up Master Slave Delay Request Delay Response Timestamp captured as message passes this point t1 timestamp is sent to slave either in sync message itself or in separate follow-up message FIGURE 8-16 PTP synchronization message flow. PTP, Precision Time Protocol. Chapter 8 Synchronization in open radio access networks 283 As can be seen in Fig. 8-16, the Sync and Delay Request messages are event messages- they get timestamped at each end-whereas the Delay Response and Follow-Up messages are general messages and are not time-critical. The flow of messages is such: The master sends a Sync message to the slaves and captures the exact instant this was sent (the tl timestamp). This timestamp is either included in the Sync message as it is sent, or sent in a related Follow-Up message if the master's hardware cannot handle retime modifying of a message. Either way is equivalent in performance and all slaves are required to support both methods. The time at which the slave receives the Sync message is captured as t2 using the slave's local clock. The time at which the slave sends the Delay Request message is captured as t3, again using the slave's local clock. Finally, the time at which the master receives the Delay Request message is captured against actual time as t4. The Delay Response message does not involve any timestamping but instead is used to pass the final t4 timestamp back to the slave, since it cannot piggyback on another message as tl does with the Sync message. Once all these stages have been completed, the slave has all four timestamps t1-t4 ready for processing. The information in these timestamps can be used by the slave to achieve both syntonization and synchronization to the master. Syntonization is achieved by comparing the difference between tl timestamps across many messages with the difference between the t2 timestamps. This is shown in Fig. In Fig. 8-17, it can be seen that the master sends the Sync messages with a c