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Chapter 23. Configuring PTP Using ptp4l
23.1. Introduction to PTP
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks in a network. When used in conjunction with hardware support,
PTP
is capable of sub-microsecond accuracy, which is far better than is normally obtainable with NTP
. PTP
support is divided between the kernel and user space. The kernel in Red Hat Enterprise Linux 6 now includes support for PTP
clocks, which are provided by network drivers. The actual implementation of the protocol is known as linuxptp, a PTPv2
implementation according to the IEEE standard 1588 for Linux.
The linuxptp package includes the ptp4l and phc2sys programs for clock synchronization. The ptp4l program implements the
PTP
boundary clock and ordinary clock. With hardware time stamping, it is used to synchronize the PTP
hardware clock to the master clock, and with software time stamping it synchronizes the system clock to the master clock. The phc2sys program is needed only with hardware time stamping, for synchronizing the system clock to the PTP
hardware clock on the network interface card (NIC).
23.1.1. Understanding PTP
The clocks synchronized by
PTP
are organized in a master-slave hierarchy. The slaves are synchronized to their masters which may be slaves to their own masters. The hierarchy is created and updated automatically by the best master clock (BMC) algorithm, which runs on every clock. When a clock has only one port, it can be master or slave, such a clock is called an ordinary clock (OC). A clock with multiple ports can be master on one port and slave on another, such a clock is called a boundary clock (BC). The top-level master is called the grandmaster clock, which can be synchronized by using a Global Positioning System (GPS) time source. By using a GPS-based time source, disparate networks can be synchronized with a high-degree of accuracy.
Figure 23.1. PTP grandmaster, boundary, and slave Clocks
23.1.2. Advantages of PTP
One of the main advantages that
PTP
has over the Network Time Protocol (NTP) is hardware support present in various network interface controllers (NIC) and network switches. This specialized hardware allows PTP
to account for delays in message transfer, and greatly improves the accuracy of time synchronization. While it is possible to use non-PTP enabled hardware components within the network, this will often cause an increase in jitter or introduce an asymmetry in the delay resulting in synchronization inaccuracies, which add up with multiple non-PTP aware components used in the communication path. To achieve the best possible accuracy, it is recommended that all networking components between PTP
clocks are PTP
hardware enabled. Time synchronization in larger networks where not all of the networking hardware supports PTP
might be better suited for NTP
.
With hardware
PTP
support, the NIC has its own on-board clock, which is used to time stamp the received and transmitted PTP
messages. It is this on-board clock that is synchronized to the PTP
master, and the computer's system clock is synchronized to the PTP
hardware clock on the NIC. With software PTP
support, the system clock is used to time stamp the PTP
messages and it is synchronized to the PTP
master directly. Hardware PTP
support provides better accuracy since the NIC can time stamp the PTP
packets at the exact moment they are sent and received while software PTP
support requires additional processing of the PTP
packets by the operating system.