Delay-Request interval

Enhanced PTP User Guide (UG1602)

Document ID
UG1602
Release Date
2022-10-18
Revision
1.0 English

The interval between Delay_Request messages is determined by the master clock. A parameter of the Delay_Response message sent from the master is the logMessagePeriod. This is a power of two value that defines a send window period designed to ensure that:

  • Multiple slaves send at random intervals during the period.
  • The master clock is able to respond to Delay_Requests from multiple slaves without queuing these messages.
window = (2^logMessagePeriod)

So for a logMessagePeriod of 3:

window = (2^3) = 0-8 second window

sfptpd can override the logMessagePeriod value using the config file option ptp_delayreq_interval causing the sfptpd slave to send Delay_Request messages at a fixed interval. The ptp_delayreq_interval is also a power of two value.

Ideally a slave server should sent Delay-Request messages at the same rate as it receives Sync/Follow_up messages thus ensuring that the four timestamps: T1-T4 are current when calculating the one-way-delay and offset values. Refer to Message Sequence above.