The following diagram describes the PTP protocol message sequence which must occur for master and slave servers to synchronize.
The Sync message is multicast to all slaves at a fixed interval of between 1 and 64 messages per second, configurable by the master clock. On most PTP networks a sync interval of between 1-4 sync messages per second is sufficient to ensure accurate synchronization and increasing the sync interval does not always result in greater accuracy of synchronization. When using 1-step synchronization, the Sync message contains the time the message was transmitted (T1). The slave generates a hardware timestamp (T2) when the message is received.
The Follow_up message is sent immediately following every Sync by master clocks using 2-step synchronization. The Follow_up message contains the actual time the preceding Sync message was sent. A master clock using 1-step synchronization does not transmit the Follow_up message.
When the slave has received the Follow_up message (or just Sync message in the case of 1-step synchronization) it will generate a Delay_Request message. When this message is sent the slave generates and retains a hardware timestamp (T3).
The master will record the time the Delay_Request is received (T4) and this timestamp is then relayed back to the slave in the Delay_Response message.
Using the timestamp information derived from the message sequence, the slave is able to calculate the one-way-delay between slave and master clocks and the time offset from the master clock.