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Understanding the Network Time Protocol
NTP is an acronym for Network Time Protocol. It is a computer protocol developed to disseminate precise timing information to computers on a network. NTP is a client-server based protocol used widely throughout the Internet and local networks for computer time synchronisation. This article discusses some of the lesser-known features of the protocol and attempts to describe its use in jargon free terminology.
NTP was originally developed in the mid 80's by Dr David Mills of the University of Delaware. He saw an urgent requirement to provide a standard means of synchronising time across the Internet. NTP was born which provided an accurate, simple and secure means of maintaining synchronisation of network infrastructure.
The NTP application was originally implemented on the LINUX operating system. However, recently it has been ported to other operating systems, notably Microsoft Windows. The application can be freely downloaded and used under a public licence from the NTP website. No charge or subscription fee needs to be paid for utilising NTP.
Precise timing information is obtained from an external timing reference, such as GPS or LF time and frequency transmissions. The information is used to accurately synchronise system time on a NTP time server. The server can then provide network time clients with three products: system clock offset, network delay and dispersion relative to an external reference clock. A client can use the information to accurately synchronise its system time to the server.
NTP is configured in a hierarchical manner; primary time servers, secondary servers and time clients. Primary servers synchronise to an accurate external hardware clock and are attributed the highest stratum, or hierarchy, level of one. Secondary servers synchronise to the level above and provide a buffer between primary servers and clients, they take some of load off primary servers. Secondary servers are attributed a stratum of greater than one. As the server stratum increases, so the accuracy of the time reference decreases.
NTP uses the UDP (User Data-gram Protocol) protocol, which is part of the TCP/IP protocol suite on which the Internet is based. Protocol messages consist of a number of distinct fields: Leap Indicator; Version Number; Mode; Stratum; Poll; Precision; Root Delay; Root Dispersion; Reference Identifier; Reference Timestamp; Originate Timestamp; Receive Timestamp; Transmit Timestamp; Key Identifier and Message Digest.
Any NTP server can operate in one of three modes: unicast, anycast and multicast. In unicast mode, the client transmits a request for time message to the server. The server responds with a time message that the client can utilise for time synchronisation. In anycast and multicast mode, NTP time messages are broadcast at periodic specified intervals.
The current release of NTP is version 4. The only significant modification to the protocol between versions 3 and 4 is a slight header re-interpretation to accommodate IPv6. All versions of NTP are backwards compatible and can be freely interchanged.
SNTP, or Simple Network Time Protocol, is a simplified version of NTP. It is ideally utilised where the synchronisation complexities of the full-blown protocol are not necessary. It is often implemented on platforms that have low processing power such as microcontrollers and PLC's where many of the complex algorithms for maintaining accurate time are unavailable. SNTP can also be implemented when the high synchronisation performance of NTP is not required. The message format of the protocols are almost identical. However, the complex subroutines designed to maintain a highly accurate synchronised time are simplified or even removed. Still, the SNTP and NTP protocols are fully interchangeable, a SNTP client can synchronise to a NTP server without any issues.
About the Author TimeTools has a number of articles and white papers relating to computer network time synchronisation. Please visit our website if you would like more information on Time Server solutions.
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