23.2.3. Starting ptp4l

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The ptp4l program tries to use hardware time stamping by default. To use ptp4l with hardware time stamping capable drivers and NICs, you must provide the network interface to use with the -i option. Enter the following command as root:
~]# ptp4l -i eth3 -m
Where eth3 is the interface you want to configure. Below is example output from ptp4l when the PTP clock on the NIC is synchronized to a master:
~]# ptp4l -i eth3 -m
selected eth3 as PTP clock
port 1: new foreign master 00a069.fffe.0b552d-1
selected best master clock 00a069.fffe.0b552d
master offset -23947 s0 freq +0 path delay       11350
master offset -28867 s0 freq +0 path delay       11236
master offset -32801 s0 freq +0 path delay       10841
master offset -37203 s1 freq +0 path delay       10583
master offset  -7275 s2 freq -30575 path delay   10583
master offset  -4552 s2 freq -30035 path delay   10385
The master offset value is the measured offset from the master in nanoseconds. The s0, s1, s2 strings indicate the different clock servo states: s0 is unlocked, s1 is clock step and s2 is locked. Once the servo is in the locked state (s2), the clock will not be stepped (only slowly adjusted) unless the pi_offset_const option is set to a positive value in the configuration file (described in the ptp4l(8) man page). The freq value is the frequency adjustment of the clock in parts per billion (ppb). The path delay value is the estimated delay of the synchronization messages sent from the master in nanoseconds. Port 0 is a Unix domain socket used for local PTP management. Port 1 is the eth3 interface (based on the example above.) INITIALIZING, LISTENING, UNCALIBRATED and SLAVE are some of possible port states which change on the INITIALIZE, RS_SLAVE, MASTER_CLOCK_SELECTED events. In the last state change message, the port state changed from UNCALIBRATED to SLAVE indicating successful synchronization with a PTP master clock.
The ptp4l program can also be started as a service by running:
~]# service ptp4l start
When running as a service, options are specified in the /etc/sysconfig/ptp4l file. More information on the different ptp4l options and the configuration file settings can be found in the ptp4l(8) man page.
By default, messages are sent to /var/log/messages. However, specifying the -m option enables logging to standard output which can be useful for debugging purposes.
To enable software time stamping, the -S option needs to be used as follows:
~]# ptp4l -i eth3 -m -S Selecting a Delay Measurement Mechanism

There are two different delay measurement mechanisms and they can be selected by means of an option added to the ptp4l command as follows:
The -P selects the peer-to-peer (P2P) delay measurement mechanism.
The P2P mechanism is preferred as it reacts to changes in the network topology faster, and may be more accurate in measuring the delay, than other mechanisms. The P2P mechanism can only be used in topologies where each port exchanges PTP messages with at most one other P2P port. It must be supported and used by all hardware, including transparent clocks, on the communication path.
The -E selects the end-to-end (E2E) delay measurement mechanism. This is the default.
The E2E mechanism is also referred to as the delay request-response mechanism.
The -A enables automatic selection of the delay measurement mechanism.
The automatic option starts ptp4l in E2E mode. It will change to P2P mode if a peer delay request is received.


All clocks on a single PTP communication path must use the same mechanism to measure the delay. A warning will be printed when a peer delay request is received on a port using the E2E mechanism. A warning will be printed when a E2E delay request is received on a port using the P2P mechanism.
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