Network Time Protocol |
The Network Time Protocol (NTP) is a protocol for clock synchronization of computer systems over packet-switched, variable-latency data networks. NTP uses User Datagram Protocol port 123 as its transport layer. It is designed particularly to resist the effects of variable latency.
NTP uses Marzullo s algorithm with the UTC time scale, including support for features such as leap seconds. NTPv4 can usually maintain time to within 10 milliseconds (1/100 s) over the public Internet, and can achieve accuracies of 200 microseconds (1/5000 s) or better in local area networks under ideal conditions.
NTP is one of the oldest internet protocol suites still in use (since before 1985). NTP was originally designed by David L. Mills of the University of Delaware, who still maintains it, along with a team of volunteers.
The NTP operating systems have this support.
NTP uses a hierarchical system of clock strata , where stratum 1 systems are synchronised to an accurate external clock such as a GPS clock or other radio clock. NTP stratum 2 systems derive their time from one or more stratum 1 systems, and so on. (Note that this is different from the notion of clock stratum used in telecommunications systems.)
The 64-bit timestamps used by NTP consist of a 32-bit seconds part and a 32-bit fractional second part, giving NTP a time scale of 232 seconds (136 years), with a theoretical resolution of 2−32 seconds (0.233 nanoseconds). Although the NTP timescale wraps round every 232 seconds, implementations should disambiguate NTP time using a knowledge of the approximate time from other sources. Since this only requires time accurate to a few decades, this is not a problem in general use.
The operational details of NTP are illustrated in RFC 778, RFC 891, RFC 956, RFC 958, and RFC 1305. (NTP should not be confused with the daytime protocol (RFC 867) or time protocol (RFC 868) protocols.) The current version is NTP version 4; however, as of 2005, only NTP up to version 3 has been documented in Request for Commentss. The IETF NTP Working Group has formed to standardise the work of the NTP community since RFC 1305 et al.
A less complex form of NTP that does not require storing information about previous communications is known as the Simple Network Time Protocol or SNTP. It has gained a following in embedded devices and in applications where extremely high accuracy is not required. See RFC 1361, RFC 1769, and RFC 2030.
Besides the reference software implementation of NTP, which can be found on the [http://www.ntp.org NTP homepage] and for which ports to the most popular of any size.
=See also=
= External links =
General NTP links:
NTP-related software:
Manufacturers of NTP-based timeservers:
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