Editing Network Time Protocol (section)

{{Short description|Networking protocol for clock synchronization}}
{{Use dmy dates|date=January 2025}}
{{Distinguish|Daytime Protocol|Time Protocol|NNTP}}

{{Infobox protocol
| name        = Network Time Protocol
| image       = Network_Time_Protocol_servers_and_clients.svg
| caption     = 
| standard    = {{IETF RFC|5905}}
| developer   = [[David L. Mills]], Harlan Stenn, Network Time Foundation <!--organization(s) involved in development-->
| introdate   = {{Start date and age|1985}} <!--{{Start date|YYYY|MM|DD}}-->
| industry    = <!--industries used (such as PC/Chemical/Multimedia)-->
| connector   = <!--connector(s) usable with protocol-->
| hardware    = <!--examples of compatible hardware-->
| range       = <!--{{convert|X|mi|abbr=on}}-->
| newer       = <!--superseded by which protocol-->
| website     = <!--URL to specs; OD wrap in template -->
}}
{{IPstack}}

The '''Network Time Protocol''' ('''NTP''') is a [[networking protocol]] for [[clock synchronization]] between computer systems over [[packet-switched]], variable-[[Network latency|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]] participating computers to within a few [[millisecond]]s of [[Coordinated Universal Time]] (UTC).<ref name="Mills2010" />{{rp|3}} It uses the [[intersection algorithm]], a modified version of [[Marzullo's algorithm]], to select accurate [[time server]]s 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 network]]s under ideal conditions. Asymmetric [[Routing|routes]] and [[network congestion]] can cause errors of 100&nbsp;ms or more.<ref name="summary">{{cite web |url=http://www.eecis.udel.edu/~mills/exec.html |title=Executive Summary: Computer Network Time Synchronization |access-date=2011-11-21 |archive-url=https://web.archive.org/web/20111102204926/http://www.eecis.udel.edu/~mills/exec.html |archive-date=2011-11-02 |url-status=live}}</ref><ref name="faq">{{cite web |url=http://www.ntp.org/ntpfaq/NTP-s-algo.htm#Q-ACCURATE-CLOCK |title=NTP FAQ |publisher=The NTP Project |access-date=2011-08-27 |archive-url=https://web.archive.org/web/20110906014530/http://www.ntp.org/ntpfaq/NTP-s-algo.htm#Q-ACCURATE-CLOCK |archive-date=2011-09-06 |url-status=live}}</ref>

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.<ref name="Mills2010" />{{Rp|20}} Implementations send and receive [[timestamp]]s using the [[User Datagram Protocol]] (UDP) on [[port number]] 123.<ref name="qNvGW">{{cite web |url= https://www.iana.org/assignments/port-numbers |title= Port Numbers |publisher= The Internet Assigned Numbers Authority (IANA) |access-date= 2011-01-19 |archive-url= https://web.archive.org/web/20010604223215/https://www.iana.org/assignments/port-numbers |archive-date= 2001-06-04 |url-status= live}}</ref>{{Ref RFC|5905|rp=16}} They can also use [[Broadcasting (networking)|broadcasting]] or [[multicasting]], where clients passively listen to time updates after an initial round-trip calibrating exchange.<ref name="faq" /> NTP supplies a warning of any impending [[leap second]] adjustment, but no information about local [[time zone]]s or [[daylight saving time]] is transmitted.<ref name="summary" /><ref name="faq" />

The current protocol is version 4 (NTPv4),{{Ref RFC|5905}} which is [[backward compatible]] with version 3.{{Ref RFC|1305}}