PTP support on SONiC

Introduction

Precision Time Protocol (PTP)

  • Standardized protocol, IEEE1588

  • Synchronize the clocks in local computing systems and devices

  • Microsecond to sub-microsecond accuracy and precision

  • Administration free

    • Capability to autonomously decide time server(master)

    • called Best Master Clock Algorithm (BMCA)

How PTP works

Time offset between master and slave clocks is calculated based on timestamps at packet sending and receiving.

path delay = ((t4-t1) - (t3-t2))/2

offset from master clock = (t2 - t1) - path delay

Achieving high precision requires hardware timestamping

  • Hardware assisted timestamp at PHY or MAC layer

  • Get time from PTP Hardware Clock (PHC) on NIC

PTP on Linux

  • PTP protocol is implemented on user-land

  • Kernel feature for PTP

    • Socket option SO_TIMESTAMPING for packet timestamping

      • Hardware timestamping

    • PHC subsystem

      • Allow to access PHC via clock_gettime/settime/adjtime system calls

LinuxPTP

  • ptp4l

    • Implementation of PTP (Ordinary Clock, Boundary Clock)

  • phc2sys

    • Synchronize two clocks (typically PHC and system clock)

ptp4l adjust PHC, phc2sys adjusts system clock.

ptp4l servo

ptp4l has PI (proportional-integral) controller servo

  • A kind of feedback loop

  • Determine frequency set to PHC

LinuxPTP on SONiC

Requirements:

Reference