Slave Clock Offset by 37 Seconds

Enhanced PTP User Guide (UG1602)

Document ID
UG1602
Release Date
2022-10-18
Revision
1.0 English
Figure 1. Flowchart for Slave Clock Offset by 37 Seconds

PTP Master clocks use the atomic timescale (TAI). Servers in business networks use the UTC timescale (UTC).

The difference between the two timescales is the UTC offset (currently 37 seconds). This increments by one second whenever a leap second occurs.

UTC time = (TAI time - UTC offset)
Note: When sfptpd is used as a master clock it uses UTC time. Ensure the ptp_utc_offset option in the sfptpd master config file is set to 0:

If the sfptpd slave clocks are observed to be ~37 seconds offset from the master clock, you should collect a small tcpdump pcap file and examine the UTC offset values in the received Announce messages.

A pcap file allows you to examine the fields of the received PTP Announce messages:

flags: 0x023c
        0... .... .... .... = PTP_SECURITY: False
        .0.. .... .... .... = PTP profile Specific 2: False
        ..0. .... .... .... = PTP profile Specific 1: False
        .... .0.. .... .... = PTP_UNICAST: False
        .... ..1. .... .... = PTP_TWO_STEP: True
        .... ...0 .... .... = PTP_ALTERNATE_MASTER: False
        .... .... ..1. .... = FREQUENCY_TRACEABLE: True
        .... .... ...1 .... = TIME_TRACEABLE: True
        .... .... .... 1... = PTP_TIMESCALE: True
        .... .... .... .1.. = PTP_UTC_REASONABLE: True
        .... .... .... ..0. = PTP_LI_59: False
        .... .... .... ...0 = PTP_LI_61: False
      ...
    originCurrentUTCOffset: 37