Message Sequence

Enhanced PTP User Guide (UG1602)

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The following diagram describes the PTP protocol message sequence which must occur for master and slave servers to synchronize.

Figure 1. PTP Message Sequence

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.

one_way_delay = ((T2-T1) + (T4-T3)) / 2
offset = ((T2-T1) - (T4-T3)) / 2