About PTP in OKD cluster nodes

Precision Time Protocol (PTP) is used to synchronize clocks in a network. When used in conjunction with hardware support, PTP is capable of sub-microsecond accuracy, and is more accurate than Network Time Protocol (NTP).

You can configure linuxptp services and use PTP-capable hardware in OKD cluster nodes.

Use the OKD web console or OpenShift CLI (oc) to install PTP by deploying the PTP Operator. The PTP Operator creates and manages the linuxptp services and provides the following features:

• Discovery of the PTP-capable devices in the cluster.

• Management of the configuration of linuxptp services.

• Notification of PTP clock events that negatively affect the performance and reliability of your application with the PTP Operator cloud-event-proxy sidecar.

Elements of a PTP domain

PTP is used to synchronize multiple nodes connected in a network, with clocks for each node. The clocks synchronized by PTP are organized in a leader-follower hierarchy. The hierarchy is created and updated automatically by the best master clock (BMC) algorithm, which runs on every clock. Follower clocks are synchronized to leader clocks, and follower clocks can themselves be the source for other downstream clocks.

Figure 1. PTP nodes in the network

The three primary types of PTP clocks are described below.

Grandmaster clock

The grandmaster clock provides standard time information to other clocks across the network and ensures accurate and stable synchronisation. It writes time stamps and responds to time requests from other clocks. Grandmaster clocks synchronize to a Global Navigation Satellite System (GNSS) time source. The Grandmaster clock is the authoritative source of time in the network and is responsible for providing time synchronization to all other devices.

Boundary clock

The boundary clock has ports in two or more communication paths and can be a source and a destination to other destination clocks at the same time. The boundary clock works as a destination clock upstream. The destination clock receives the timing message, adjusts for delay, and then creates a new source time signal to pass down the network. The boundary clock produces a new timing packet that is still correctly synced with the source clock and can reduce the number of connected devices reporting directly to the source clock.

Ordinary clock

The ordinary clock has a single port connection that can play the role of source or destination clock, depending on its position in the network. The ordinary clock can read and write timestamps.

Advantages of PTP over NTP

One of the main advantages that PTP has over NTP is the hardware support present in various network interface controllers (NIC) and network switches. The specialized hardware allows PTP to account for delays in message transfer and improves the accuracy of time synchronization. To achieve the best possible accuracy, it is recommended that all networking components between PTP clocks are PTP hardware enabled.

Hardware-based PTP provides optimal accuracy, since the NIC can timestamp the PTP packets at the exact moment they are sent and received. Compare this to software-based PTP, which requires additional processing of the PTP packets by the operating system.

Using PTP with dual NIC hardware

OKD supports single and dual NIC hardware for precision PTP timing in the cluster.

For 5G telco networks that deliver mid-band spectrum coverage, each virtual distributed unit (vDU) requires connections to 6 radio units (RUs). To make these connections, each vDU host requires 2 NICs configured as boundary clocks.

Dual NIC hardware allows you to connect each NIC to the same upstream leader clock with separate ptp4l instances for each NIC feeding the downstream clocks.

Overview of linuxptp and gpsd in OKD nodes

OKD uses the PTP Operator with linuxptp and gpsd packages for high precision network synchronization. The linuxptp package provides tools and daemons for PTP timing in networks. Cluster hosts with Global Navigation Satellite System (GNSS) capable NICs use gpsd to interface with GNSS clock sources.

The linuxptp package includes the ts2phc, pmc, ptp4l, and phc2sys programs for system clock synchronization.

ts2phc

ts2phc synchronizes the PTP hardware clock (PHC) across PTP devices with a high degree of precision. ts2phc is used in grandmaster clock configurations. It receives the precision timing signal a high precision clock source such as Global Navigation Satellite System (GNSS). GNSS provides an accurate and reliable source of synchronized time for use in large distributed networks. GNSS clocks typically provide time information with a precision of a few nanoseconds.

The ts2phc system daemon sends timing information from the grandmaster clock to other PTP devices in the network by reading time information from the grandmaster clock and converting it to PHC format. PHC time is used by other devices in the network to synchronize their clocks with the grandmaster clock.

pmc

pmc implements a PTP management client (pmc) according to IEEE standard 1588.1588. pmc provides basic management access for the ptp4l system daemon. pmc reads from standard input and sends the output over the selected transport, printing any replies it receives.

ptp4l

ptp4l implements the PTP boundary clock and ordinary clock and runs as a system daemon. ptp4l does the following:

• Synchronizes the PHC to the source clock with hardware time stamping

• Synchronizes the system clock to the source clock with software time stamping

phc2sys

phc2sys synchronizes the system clock to the PHC on the network interface controller (NIC). The phc2sys system daemon continuously monitors the PHC for timing information. When it detects a timing error, the PHC corrects the system clock.

The gpsd package includes the ubxtool, gspipe, gpsd, programs for GNSS clock synchronization with the host clock.

ubxtool

ubxtool CLI allows you to communicate with a u-blox GPS system. The ubxtool CLI uses the u-blox binary protocol to communicate with the GPS.

gpspipe

gpspipe connects to gpsd output and pipes it to stdout.

gpsd

gpsd is a service daemon that monitors one or more GPS or AIS receivers connected to the host.

Overview of GNSS timing for PTP grandmaster clocks

OKD supports receiving precision PTP timing from Global Navigation Satellite System (GNSS) sources and grandmaster clocks (T-GM) in the cluster.

Figure 2. Overview of Synchronization with GNSS and T-GM

Global Navigation Satellite System (GNSS)

GNSS is a satellite-based system used to provide positioning, navigation, and timing information to receivers around the globe. In PTP, GNSS receivers are often used as a highly accurate and stable reference clock source. These receivers receive signals from multiple GNSS satellites, allowing them to calculate precise time information. The timing information obtained from GNSS is used as a reference by the PTP grandmaster clock.

By using GNSS as a reference, the grandmaster clock in the PTP network can provide highly accurate timestamps to other devices, enabling precise synchronization across the entire network.

Digital Phase-Locked Loop (DPLL)

DPLL provides clock synchronization between different PTP nodes in the network. DPLL compares the phase of the local system clock signal with the phase of the incoming synchronization signal, for example, PTP messages from the PTP grandmaster clock. The DPLL continuously adjusts the local clock frequency and phase to minimize the phase difference between the local clock and the reference clock.