User manual

Page 12-1
12 Network Time Protocol
Introduction
The Network Time Protocol (NTP) is used to synchronize the time of a computer client or
server to another server or reference time source, such as a radio or satellite receiver. It
provides client time accuracies within a millisecond on LANs, and up to a few tens of millisec-
onds on WANs relative to a primary server synchronized to Coordinated Universal Time (UTC)
(via a Global Positioning Service receiver, for example). Typical NTP configurations utilize
multiple redundant servers and diverse network paths in order to achieve high accuracy and
reliability. Some configurations include cryptographic authentication to prevent accidental or
malicious protocol attacks.
It is important for networks to maintain accurate time synchronization between network
nodes. The standard timescale used by most nations of the world is based on a combination
of Universal Coordinated Time (
UTC) (representing the Earth's rotation about its axis) and the
Gregorian Calendar (representing the Earth's rotation about the Sun). The
UTC timescale is
disciplined with respect to International Atomic Time (TAI) by inserting leap seconds at inter-
vals of about 18 months. UTC time is disseminated by various means, including radio and
satellite navigation systems, telephone modems, and portable clocks.
Special purpose receivers are available for many time-dissemination services, including the
Global Position System (
GPS) and other services operated by various national governments.
For reasons of cost and convenience, it is not possible to equip every computer with one of
these receivers. However, it is possible to equip some computers with these clocks, which
then act as primary time servers to synchronize a much larger number of secondary servers
and clients connected by a common network. In order to do this, a distributed network clock
synchronization protocol is required which can read a server clock, transmit the reading to
one or more clients, and adjust each client clock as required. Protocols that do this include
the Network Time Protocol (NTP).