The Network Time Protocol (NTP) is a networking protocol used for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. It aims to synchronize participating computers to within a few milliseconds of Coordinated Universal Time (UTC). NTP has been in operation since before 1985, making it one of the oldest Internet protocols still in use.
NTP was designed by David L. Mills of the University of Delaware. It uses the intersection algorithm, a modified version of Marzullo’s algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency.
While NTP can usually maintain time to within tens of milliseconds over the public Internet, it can achieve better than one millisecond accuracy in local area networks under ideal conditions. The protocol typically operates using a client-server model, but can also be used in peer-to-peer relationships. Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port 123.
ntpd (NTP daemon)chronyd (Chrony daemon)ntpdate (utility for setting time once)