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:

Trap PTP packet/events
- opencomputeproject/SAI#841
NETDEV device support SO_TIMESTAMP option
- https://www.kernel.org/doc/Documentation/networking/timestamping.txt
- Section 3.1
Develop PHC driver
- https://www.kernel.org/doc/Documentation/ptp/ptp.txt

Reference