This is the first part of a multi-part series that will cover the basic concepts, terminology, behaviours, and applications of PTP.
PTP Goals
PTP was originally defined in the IEEE 1588-2002 standard, officially entitled “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” and published in 2002. In 2008, IEEE 1588-2008 was released as a revised standard; also known as PTP Version 2 [PTPv2], it improves accuracy, precision and robustness but is not backward compatible with the original 2002 version.
IEEE 1588 was designed to fill a niche that neither NTP nor GPS could fulfill by solving two problems:
- Local systems needing clock accuracy in the sub-microsecond range
- Systems needing synchronisation in areas that cannot afford the cost of a GPS receiver at each node, or that cannot receive GPS signals (consider an underground or windowless datacenter)
The origins of IEEE 1588 development come from test and measurement (instrumentation and data acquisition), industrial automation or power generation, and received strong support from the automation industry leading to the adoption of the IEEE 1588-2002 standard by the Industrial Electro-technical Commission [IEC] as IEEE/IEC 61588-2004.
Specific application domains and targets had driven first release of IEEE 1588 work. Applications (e.g. data acquisition, military instrumentation, industrial robots and high-speed printers) and their targets, had sufficient shared objective to define common criteria:
- Achievable sub-microsecond accuracy (if required).
- Operation over control localised areas, such as local area network [LAN], with no intent to cover Internet or wide area networks, but supporting a PTP system.
- Administration-free operation allowing “plug-and-play” deployment of IEEE 1588 clocks
- Designed for minimal resource requirement in devices supporting PTP (clock filtering or averaging might not be necessary).
Currently one of the main industries using PTP is the High-Frequency Trading [HFT], and financial sector, to ensure highly accurate timing and synchronisation for their market feeds and trading servers.
When reading the spec
Just to note, that as an IEEE spec, 1588 can be far more difficult to read than IETF/RFC documents. A key thing to note in this particular spec is that IEEE 1588 also specifies “the node, system, and communications properties necessary to support PTP” and proposes models of devices, called “clocks” that support the protocol and aim to help and enhance the transfer of packet-based timing references. In IEEE 1588 terms, a PTP system is a set of IEEE 1588 clocks with specialised functions to support the protocol, PTP.
For instance, specifications of an IEEE 1588 clock include some material, such as a port, that is not part of the protocol itself. However, the messages sent and received by each port of an IEEE 1588 clock and the resulting behaviour of that clock is part of the protocol. Nonetheless, the standard does not define any performance objective for transport network device or clock.
To simplify – a PTP clock is a device that runs the PTP protocol, and has one or more network interfaces
