Precision Time ProtocolSteven Kreuzer

steven@kreuzer.cx

NYCBUGJuly 2015

Why Clocks Drift• Accuracy is determined by the stability of the

oscillator

• No oscillator perfectly generates a specified frequency

• Stability describes how well the oscillator resists fluctuations such as variation in temperature, vibration, supply voltage

Reducing Clock Drift• Use a better oscillator

• TCXO, MCXO, and OCXO

• Atomic beam standard

• Use a time distribution protocol

• GPS, WWVB, IRIG-B

• Daytime, NTP, PTP

What is PTP• IEEE standard to synchronize time across an

ethernet network for systems requiring very high accuracies

• Fills a niche not well served by NTP and GPS

• GPS receivers are expensive or signals are inaccessible

• NTP can be accurate to about a millisecond under ideal conditions

Second

1.000000000000

Millisecond (ms)

0.001000000000

Microsecond (μs)

0.000001000000

Nanosecond (ns)

0.000000001000

Picosecond (ps)

0.000000000001

Synchronization Basics

• Master clock provides synchronization messages that slaves use to correct their local clock

• Sync message is transmitted to all slaves at a fixed interval of between 1 and 64 messages per second, configurable by the master clock

• Four timestamps are captured between the master and slave

Master Clock Slave ClockTim

e

Master Clock Slave ClockTim

eSyncT1

T2

Master Clock Slave ClockTim

eSync

Sync Followup

T1

T2

Master Clock Slave ClockTim

eSync

Sync Followup

Delay Request

T1

T2

T3

T4

Master Clock Slave ClockTim

eSync

Sync Followup

Delay Request

Delay Response

T1

T2

T3

T4

Master to Slave Offset

ms_diff = T2 - T1

Slave to Master Offset

sm_diff =T4 - T3

One Way Delay

owd =(ms_diff + sm_diff) /

2

Slave Clock Offset

offset = ms_diff - owd

-or-

((T2-T1) - (T4 -T3)) / 2

Implementation Methods

GrandmasterClock

SlaveClock

SlaveClock

SlaveClock

SlaveClock

SlaveClock

GPS

Ethernet Switch

Implementation Methods

GrandmasterClock

SlaveClock

SlaveClock

SlaveClock

SlaveClock

SlaveClock

GPS

Transparent Switch

Implementation Methods

GrandmasterClock

SlaveClock

GPS

Transparent Switch

Boundary Clock

SlaveClock

SlaveClock

Boundary Clock

SlaveClock

SlaveClock

Boundary Clock

SlaveClock

PTPd• Complete Implementation of the IEEE-1588

v2 specification for a standard clock

• Very well tested on FreeBSD and Linux

• Supports Mac OS X, NetBSD, OpenBSD and Solaris as well

• BSD License

PTPd Features• Can run as a grandmaster and successfully

service 1000 slaves running at 1 sync/sec and 1 delay/sec

• Can run as a slave on a client

• Uses PCAP/BPF timestamps for enhanced accuracy on FreeBSD

• Provide performance and quality metrics

PTPd Quality Measurement

• Provides a method to examine synchronization by using an external event that all slaves see

• Record the sequence id and arrival time of SYNC packets

• SYNC is multicast so all slaves should see it at the same time

• Post processing shows how well synchronized two slaves are

PTPd 2.3.1 Features• Improved sync and delay filtering with statistical

filters and an advanced outlier filter

• Extensive support for unicast transmission including unicast negotiation

• Improvements to leap second handling: support for leap second announcement in master state, leap seconds file support, support for leap second smearing

• Simple interface failover support

http://github.com/ptpd

pkg install net/ptpd2

Questions