The Precision Time Protocol (PTP) is a time synchronization protocol for nodes distributed across a network. Its hardware timestamp feature provides greater accuracy than the Network Time Protocol (NTP).
A PTP system can consist of a combination of PTP and non-PTP devices. These PTP devices include ordinary clocks, boundary clocks, and transparent clocks. Non-PTP devices include ordinary network switches, routers, and other infrastructure devices.
PTP is a distributed protocol that specifies how real-time PTP clocks in the system synchronize with each other. These clocks are organized into a master-member synchronization hierarchy with the grandmaster clock, the clock at the top of the hierarchy, determining the reference time for the entire system. Synchronization is achieved by exchanging PTP timing messages, with the members using the timing information to adjust their clocks to the time of their master in the hierarchy. PTP operates within a logical scope called a PTP domain. Figure 6-7 shows an example of a PTP domain.
Figure 6-7 PTP Domain
The following clocks are common PTP devices:
Ordinary clock: Communicates with the network based on a single physical port, similar to an end host. An ordinary clock can function as a grandmaster clock.
Boundary clock: Typically has several physical ports, with each port behaving like a port of an ordinary clock. However, each port shares the local clock, and the clock data sets are common to all ports. Each port decides its individual state, either master (synchronizing other ports connected to it) or member (synchronizing to a downstream port), based on the best clock available to it through all of the other ports on the boundary clock. Messages related to synchronization and establishing the master-member hierarchy terminate in the protocol engine of a boundary clock and are not forwarded.
Transparent clock: Forwards all PTP messages like an ordinary switch or router but measures the residence time of a packet in the switch (the time that the packet takes to traverse the transparent clock) and in some cases the link delay of the ingress port for the packet. The ports have no state because the transparent clock does not need to synchronize to the grandmaster clock.
End-to-end transparent clock: Measures the residence time of a PTP message and accumulates the times in the correction field of the PTP message or an associated follow-up message.
Peer-to-peer transparent clock: Measures the residence time of a PTP message and computes the link delay between each port and a similarly equipped port on another node that shares the link. For a packet, this incoming link delay is added to the residence time in the correction field of the PTP message or an associated follow-up message.
The PTP process consists of two phases: establishing the master-member hierarchy and synchronizing the clocks.
Within a PTP domain, each port of an ordinary or boundary clock follows this process to determine its state:
Examines the contents of all received announce messages (issued by ports in the master state).
Compares the data sets of the foreign master (in the announce message) and the local clock for priority, clock class, accuracy, and so on.
