About PTP

Use PTP to synchronise system clocks on a LAN more accurately than than NTP. If network drivers support either hardware or software time stamping, a PTP clock can use the time stamps in PTP messages to resolve propagation delays across a network. With software time stamping, PTP synchronises systems to within a few tens of microseconds. With hardware time stamping, PTP can synchronise systems to within a few tenths of a microsecond. If you require high-precision time synchronization of systems, use hardware time stamping.

A typical PTP configuration on an enterprise local area network consists of:

One or more grandmaster clock systems.

A grandmaster clock is typically implemented as specialized hardware that can use high-accuracy GPS signals or lower-accuracy code division several access (CDMA) signals, radio clock signals, or NTP as a time reference source. If several grandmaster clocks are available, the best master clock (BMC) algorithm selects the grandmaster clock based on the settings of their priority1, clockClass, clockAccuracy, offsetScaledLogVariance, and priority2 parameters and their unique identifier, in that order.
Several boundary clock systems.

Each boundary clock is backed up to a grandmaster clock on one subnetwork and relays PTP messages to one or more added subnetworks. A boundary clock is usually implemented as a function of a network switch.
Several secondary clock systems.

Each secondary clock on a subnetwork is backed up to a boundary clock, which acts as the master clock for that secondary clock.

For a basic configuration, set up a single grandmaster clock and several secondary clocks on the same network segment and thus eliminates any need for an intermediate layer of boundary clocks.

Grandmaster and secondary clock systems that use only one network interface for PTP are termed ordinary clocks.

Boundary clocks require at least two network interfaces for PTP: one interface acts a secondary to a grandmaster clock or a higher-level boundary clock; the other interfaces act as masters to secondary clocks or lower-level boundary clocks.

Synchronization of boundary and secondary clock systems is achieved by sending time stamps in PTP messages. By default, PTP messages are sent in UDPv4 datagrams. You can also configure PTP to use UDPv6 datagrams or Ethernet frames as its transport mechanism.

To use PTP on a system, the driver for at least one of the system's network interfaces must support either software or hardware time stamping. To find out whether the driver for a network interface supports time stamping, use the ethtool command:

sudo ethtool -T en1

Time stamping parameters for en1:
Capabilities:
	hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
	software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
	hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
	software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
	software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
	hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
...

The output in the example shows that the en1 interface supports both hardware and software time stamping capabilities.

With software time stamping, ptp4l synchronises the system clock to an external grandmaster clock.

If hardware time stamping is available, ptp4l can synchronise the PTP hardware clock to an external grandmaster clock. In this case, you use the phc2sys daemon to synchronise the system clock with the PTP hardware clock.

Configuring the PTP Service

To configure the PTP service on a system:

Install the linuxptp package.
```
sudo dnf install linuxptp
```
Edit /etc/sysconfig/ptp4l and define the start-up options for the ptp4l daemon.

Grandmaster clocks and secondary clocks require that you define only one interface.

For example, to use hardware time stamping with interface en1 on a secondary clock:
```
OPTIONS="-f /etc/ptp4l.conf -i en1 -s"
```
To use software time stamping instead of hardware time stamping, specify the -S option:
```
OPTIONS="-f /etc/ptp4l.conf -i en1 -S -s"
```
Note:

The -s option specifies that the clock operates only as a secondary (slaveOnly mode). Don't specify this option for a grandmaster clock or a boundary clock.

For a grandmaster clock, omit the -s option, for example:
```
OPTIONS="-f /etc/ptp4l.conf -i en1"
```
A boundary clock requires that you define at least two interfaces, for example:
```
OPTIONS="-f /etc/ptp4l.conf -i en1 -i en2"
```
You might need to edit the /etc/ptp4l.conf file to customize ptp4l further, for example:
- For a grandmaster clock, set the value of the priority1 parameter to a value between 0 and 127, where lesser values have greater priority when the BMC algorithm selects the grandmaster clock. For a configuration that has a single grandmaster clock, a value of 127 is suggested.
- If you set the value of summary_interval to an integer value N instead of 0, ptp4l writes summary clock statistics to /var/log/messages every 2^N seconds instead of every second (2⁰ = 1). For example, a value of 10 would correspond to an interval of 2¹⁰ or 1024 seconds.
- The logging_level parameter controls the amount of logging information that ptp4l records. The default value of logging_level is 6, which corresponds to LOG_INFO. To turn off logging, set the value of logging_level to 0. Alternatively, specify the -q option to ptp4l.
See the ptp4l(8) manual page.
Configure the system firewall to accept access by PTP event and general messages to UDP ports 319 and 320 in the appropriate zone, for example:
```
sudo firewall-cmd --zone=zone --add-port=319/udp --add-port=320/udp
```
```
sudo firewall-cmd --permanent --zone=zone --add-port=319/udp --add-port=320/udp
```
Start the ptp4l service and configure it to start following a system reboot.
```
sudo systemctl start ptp4l
```
```
sudo systemctl enable ptp4l
```
To configure phc2sys on a clock system that uses hardware time stamping:
1. Edit the /etc/sysconfig/phc2sys file and define the start-up options for the phc2sys daemon.
  
  On a boundary clock or secondary clock, synchronise the system clock with the PTP hardware clock that's associated with the secondary network interface, for example:
```
OPTIONS="-c CLOCK_REALTIME -s en1 -w"
```
  Note:
  
  The secondary network interface on a boundary clock is the one that it uses to communicate with the grandmaster clock.
  
  The -w option specifies that phc2sys waits until ptp4l has synchronised the PTP hardware clock before synchronising the system clock.
  
  On a grandmaster clock, which derives its system time from a reference time source such as GPS, CDMA, NTP, or a radio time signal, synchronise the network interface's PTP hardware clock from the system clock, for example:
```
OPTIONS="-c en1 -s CLOCK_REALTIME -w"
```
  See the phc2sys(8) manual page.
2. Start the phc2sys service and configure it to start following a system reboot.
```
sudo systemctl start phc2sys
```
```
sudo systemctl enable phc2sys
```

You can use the pmc command to query the status of ptp4l operation. The following example shows the results of running pmc on a slave clock system that's directly connected to the grandmaster clock system without any intermediate boundary clocks:

sudo pmc -u -b 0 'GET TIME_STATUS_NP'

sending: GET TIME_STATUS_NP
	080027.fffe.7f327b-0 seq 0 RESPONSE MANAGEMENT TIME_STATUS_NP 
		master_offset              -98434
		ingress_time               1412169090025854874
		cumulativeScaledRateOffset +1.000000000
		scaledLastGmPhaseChange    0
		gmTimeBaseIndicator        0
		lastGmPhaseChange          0x0000'0000000000000000.0000
		gmPresent                  true
		gmIdentity                 080027.fffe.d9e453

sudo pmc -u -b 0 'GET CURRENT_DATA_SET'

sending: GET CURRENT_DATA_SET
	080027.fffe.7f327b-0 seq 0 RESPONSE MANAGEMENT CURRENT_DATA_SET 
		stepsRemoved     1
		offsetFromMaster  42787.0
		meanPathDelay    289207.0

This output examples include the following useful information:

gmIdentity: The unique identifier of the grandmaster clock, which is based on the MAC address of its network interface.
gmPresent: Whether an external grandmaster clock is available. This value is displayed as false on the grandmaster clock itself.
meanPathDelay: An estimate of how many nanoseconds by which synchronization messages are delayed.
offsetFromMaster: The most recent measurement of the time difference in nanoseconds relative to the grandmaster clock.
stepsRemoved: The number of network steps between this system and the grandmaster clock.

For more information, see the phc2sys(8), pmc(8), and ptp4l(8) manual pages, and IEEE 1588.

Using PTP as a Time Source for NTP

To make the PTP-adjusted system time on an NTP server available to NTP clients, include the following entries in the /etc/chrony.conf file on the NTP server:

server    127.127.1.0
fudge     127.127.1.0 stratum 0

These entries define the local system clock as the time reference.

Note:

Don't configure any added server lines in the file.