practice

Doi: 10.1145/2347736.2347750

Article development led by queue.acm.org

An introduction to PTP and its significance to NTP practitioners.

By RiCK RAtZEL AnD RoDnEy GREEnStREEt
toward
higher
Precision

it is difficUlt to overstate the importance of synchronized time to modern computer systems. Our lives today depend on the financial transactions, telecommunications, power generation and delivery, high-speed manufacturing, and discoveries in “big physics,” among many other things, that are driven by fast, powerful computing devices coordinated in time with each other.

Since the first complete specification of Network Time Protocol (NTP) version 1 and its accompanying algorithms appeared in RFC 1059 in 1988, NTP has played a large role in time synchronization by keeping the clocks of networked computer systems synchronized to within milliseconds of each other. NTP has been deployed to a vast number of systems over the years, yet it hardly bears the burden of clock synchronization alone. When users want to coordinate events in time between multiple systems, they typically have many options for accomplishing this, all with different trade-offs.

One of the emerging alternatives to NTP is PTP (Precision Time Protocol). PTP is defined by IEEE standard 1588, published in November 2002 and based on early prototypes built at Agilent Technologies between 1990 and 1998. A revision of PTP with additional features and improved performance was published in 2008; it is known as PTP version 2 or IEEE 1588-2008 (all references to PTP in this article refer to this later version).

PTP and NTP are similar in that both are packet-based and send timestamps over a network from a time reference device to one or more other devices. Additionally, both synchronize device clocks based on time offsets and network delays, and both support heterogeneous devices with varying clock time precision, resolution, and stability over varying amounts of physical separation. Each protocol has its unique strengths, and choosing one over the other often warrants an evaluation of a system’s environment, capabilities, and goals.

PTP is often chosen when the synchronization performance requirements of systems exceed the millisecond threshold of a typical NTP-based solution. When used with PTP-capable network hardware that has the ability to timestamp PTP packets precisely (something that is quickly becoming commonplace in industrial network interfaces), devices using PTP on a LAN (local-area network) can synchronize their clocks to within tens of nanoseconds of each other. Without hardware timestamping, referred to as a software-only configuration, PTP implementations can still achieve sub-millisecond precision.

NTP remains a popular synchronization technology, even as more PTP implementations have been made available to system designers on more platforms—both commercially and as freely available open source implementations. If PTP is available to a system designer and displays superior synchronization performance, why would NTP even be considered?