The Network Time Protocol (NTP) defines a set of procedures for synchronizing clocks on hosts connected to a network with access to the Internet. NTP goes beyond simple routines that occasionally query a time server and adjust a local clock to a time value received from the server. NTP has the ability to continually keep a server or workstation's time in sync with Coordinated Universal Time (UTC).
NTP is a tiered time distribution system with redundancy capability. NTP measures delays within the network and within the algorithms on the machine on which it is running. Using these tools and techniques, it is able to synchronize clocks to within milliseconds of each other when connected on a Local Area Network and within hundreds of milliseconds of each other when connected to a Wide Area Network. The tiered nature of the NTP time distribution tree enables a user to choose the accuracy needed by selecting a level (stratum) within the tree for machine placement. A time server placed higher in the tree (lower stratum number), provides a higher likelihood of agreement with the UTC time standard.
Some of the hosts act as time servers, that is, they provide what they believe is the correct time to other hosts. Other hosts act as clients, that is, they find out what time it is by querying a time server. Some hosts act as both clients and time servers, because these hosts are links in a chain over which the correct time is forwarded from one host to the next. As part of this chain, a host acts first as a client to get the correct time from another host that is a time server. It then turns around and functions as a time server when other hosts, acting as clients, send requests to it for the correct time.
Should the time server from which NTP is synchronizing fail, NTP retains the ability to maintain an accurate clock.
The network time daemon, in.xntpd (see xntpd(1Mtcp)) is the program running on each of the hosts in the network that attempts to establish the correct time using NTP. It does this by using the best available source of time to synchronize the host clock with the time at zero longitude (as Coordinated Universal Time or UTC).