The Network Time Protocol (NTP)

NTP (Network Time Protocol) was originally developed as a solution to synchronising time critical process on the Internet. Dr David Mills, of the University of Delaware, developed the protocol over twenty-five years ago. It is one of the oldest protocols still in continuous operation. The protocol is the standard means of synchronising network time clients across the Internet. This article provides a technical overview of NTP and NTP servers and some of the terminology involved in computer time synchronisation.

NTP was originally developed to solve the requirement of synchronisation of critical time processes across the Internet. The NTP protocol is primarily intended for operation on LINUX servers; however, it has recently been ported to other operating systems such as Microsoft Windows. NTP is most widely used on LINUX as many of the algorithms needed to provide precise time are embedded into its kernel.

NTP is based upon the User Data-gram Protocol (UDP) which is in-turn reliant on the TCP/IP protocol. NTP messages are communicated using UDP port 23, which is reserved solely for the use of NTP traffic. The protocol basically consists of a number of fields, which specify: clock-offset, round-trip delay and dispersion relative to a precise time source. The information stored in each NTP packet allow a network time client to accurately synchronise time with a NTP server.

NTP is a structured protocol that operates in a hierarchical manner. At the top of the tree, a primary time reference is known as a stratum 1 time server. Servers that synchronise to a stratum 1 server are known as stratum 2 servers and so on down each level of the hierarchy. As the stratum increases, so generally precision decreases.

Over a number of years NTP has been enhanced to operate with a plethora of precision hardware clock devices, or reference clocks. NTP reference clocks are available for GPS hardware and also many of the National Radio Time and Frequency standards such as MSF, DCF-77 and WWVB. A number of third-party timing hardware manufacturers have installed precision crystals into their reference clocks to provide an accurate backup timing reference,.

A spin-off of the NTP protocol is SNTP or Simple Network Time Protocol, which is basically as the name implies, a simplified version of NTP. SNTP is generally used in small low-powered computing devices such as micro-controllers. It allows low-powered devices the ability to synchronise time to NTP servers over a network.

To summarise, NTP is a long-standing and widely used protocol for synchronising time between time critical processes. NTP has a linear heirarchical structure that can provide a timing resource for large numbers of clients. For applications that require critical timing, NTP provides a de-facto standard solution.

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Dave W Evans provides technical authoring services to the time and frequency industries. Dave has published a number of articles and white-papers detailing the operation of NTP. Click here, if you would like more information on NTP server solutions.

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