Network Time Protocol

Network Time Protocol
International standard	RFC 5905
Developed by	David L. Mills, Harlan Stenn, Network Time Foundation
Introduced	1985

The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed by David L. Mills of the University of Delaware.

NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC).^[1]^: 3 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. NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.^[2]^[3]

The protocol is usually described in terms of a client–server model, but can as easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source.^[1]^: 20 Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123.^[4]^[5]^: 16 They can also use broadcasting or multicasting, where clients passively listen to time updates after an initial round-trip calibrating exchange.^[3] NTP supplies a warning of any impending leap second adjustment, but no information about local time zones or daylight saving time is transmitted.^[2]^[3]

The current protocol is version 4 (NTPv4),^[5] which is backward compatible with version 3.^[6]

^ ^a ^b Cite error: The named reference Mills2010 was invoked but never defined (see the help page).
^ ^a ^b "Executive Summary: Computer Network Time Synchronization". Archived from the original on 2011-11-02. Retrieved 2011-11-21.
^ ^a ^b ^c "NTP FAQ". The NTP Project. Archived from the original on 2011-09-06. Retrieved 2011-08-27.
^ "Port Numbers". The Internet Assigned Numbers Authority (IANA). Archived from the original on 2001-06-04. Retrieved 2011-01-19.
^ ^a ^b D. Mills; J. Burbank; W. Kasch (August 2010). J. Martin (ed.). Network Time Protocol Version 4: Protocol and Algorithms Specification. Internet Engineering Task Force (IETF). doi:10.17487/RFC5905. ISSN 2070-1721. RFC 5905. Proposed Standard. Obsoletes RFC 1305, 4330. Updated by RFC 7822, 8573 and 9109.
^ David L. Mills (March 1992). Network Time Protocol (Version 3) - Specification, Implementation and Analysis. Network Working Group. doi:10.17487/RFC1305. RFC 1305. Obsolete. Obsoleted by RFC 5905. Obsoletes RFC 958, 1059 and 1119.