Network Transmission Modes

Enhanced PTP User Guide (UG1602)

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1.1 English

The network transmission mode for an instance of the PTP sync module is set by the ptp_network_mode option in the configuration file. It can have the following values:

  • multicast.
  • hybrid.
  • hybrid-no-fallback.

Multicast Mode

The is the standard PTP mode whereby all PTP packets between master and slave are sent as multicast. The implication is that all slaves receive all Delay_Req, Delay_Resp message pairs between all other slaves and the PTP master clock thereby increasing the amount of traffic on the network. The traffic level generated by multicast transmission is usually not a problem on smaller networks employing only a few PTP slaves.

Multicast mode can be selected for sfptpd using the configuration file option ptp_network_mode multicast.

Hybrid Mode

PTP hybrid mode allows a PTP slave clock to use unicast transmission to send Delay_Req messages to the master clock which, in turn, will respond with a unicast Delay_Resp packet direct to the relevant slave. This reduces the level of PTP traffic on the network which can be a factor when scaling to larger networks employing many PTP slaves. Hybrid mode only requires the network to support multicast transmission from master to slave.

A sfptpd slave, using hybrid mode, will make three attempts to contact a master clock when sending a Delay_Req message using unicast transmission. If the master clock fails to respond to unicast transmissions the sfptpd slave will revert to multicast transmission. If hybrid mode communication is possible with the master clock, the slave will remain in hybrid mode until/if a new master clock is selected.

sfptpd will generate an error message if the PTP master fails to respond to the unicast Delay_Req message. Error messages will go to stderr or to syslog, depending on the logging configuration.

Hybrid mode is the default mode for sfptpd and can be enabled/disabled using the configuration file option ptp_network_mode. sfptpd does not support unicast negotiation.


Using this mode sfptpd will always stay in hybrid mode, sending unicast Delay-Request messages.