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How a NTP Server Works
By FeaturesMan | June 2, 2008
This article explains what an NTP server is and how it works.
Network
Time Protocol (NTP)
is one of the Internet’s oldest protocols still in use. Invented by Dr David
Mills from the University of Delaware it has been utilized since 1985. NTP is
designed to synchronize the clocks on computers and networks across the
Internet or Local Area Networks (LANs).
NTP
(currently version 4) is actually three
things in one; a software program that runs in the background of Windows or UNIX;
a protocol that exchanges time values between servers and clients; and a suite
of algorithms that process the time values to advance or retreat the system
clock.
NTP
uses an algorithm (Marzullo’s algorithm) to synchronise time on a network using a time reference. Although networks can
be synchronized with internal clocks or Internet based timing references, it is
highly recommended by Microsoft and others that an external timing reference
should be used to guarantee authentication. An absolute timing reference should
use UTC (Coordinated Universal Time or Temps
Universel Coordonné) which supports such features as leap seconds - added to
compensate for the slowing of the Earth’s rotation.
NTP works within the TCP/IP suite and relies on UDP, a less complex
form of NTP exists called Simple Network Time Protocol (SNTP) that does not
require the storing of information about previous communications, needed by
NTP. It is used in some devices and applications where high accuracy timing is
not as important, it is also included in most Windows operating systems but
more recent versions have the full NTP already installed, which is also free to
download via the Internet.
Synchronisation
with NTP is relatively simple, it synchronises time with reference to a reliable clock source such
as an atomic clock, although these are extremely expensive and are generally
only to be found in large-scale physics laboratories, however NTP can use
either the Global Positioning system (GPS) network or specialist radio transmission
to receive UTC time from these clocks.
NTP uses timestamps to represent the current time of the day
each timestamp is ephemeral, in other words it is always greater than the
previous timestamp as time never runs backwards. NTP analyses the timestamp values
including the frequency of errors and the stability. A NTP server
will maintain an estimate of the quality of its reference
clocks and of itself.
The distance from the reference clock is known as the
stratum levels and they exist to prevent cycles in the NTP. Stratum 0 are
devices such as reference clocks connected directly to a computer. Stratum 1
are computers attached to stratum 0 devices, while Stratum 2 are computers that
send NTP requests to Stratum 1 servers. NTP can support up to 256 strata.
NTP timestamps are in two formats but they relay the seconds
from a set point in time (known as the prime epoch, set at 00:00 1 January
1900) The NTP algorithm then uses this timestamp to determine the amount to
advance or retreat the system or network clock.
The NTP program (known as a daemon on UNIX and a service on
Windows) runs in the system background. NTP refuses to believe the time it is
told until several packet exchanges have taken place, each passing a set of
tests. Only if the replies from a server satisfy the test, known as protocol
specifications, the server is considered. It usually takes about five minutes
(five good samples) until a NTP server is accepted as a
synchronization source.
A typical GPS time server can provide timing information to
within a few nanoseconds of UTC as long as there is an antenna situated with a
good view of the sky.
There are also a number of national time and frequency radio
transmissions that can be used to synchronise a NTP server. In Britain the
signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria
which serves as the , the radio signal has a finite range and is vulnerable to interference.
About the author
Richard N Williams is a technical author and a
specialist in the telecommunications and network time synchronisation industry
helping to develop dedicated time server products. Please visit us for
more information about a GPS time server or other NTP products.
Tags: absolute timing, atomic clock, clock source, coordinated universal time, earths rotation, internal clocks, leap seconds, local area networks, most windows operating systems, network time protocol, ntp server, physics laboratories, sntp, system clock, tcp ip, time reference, time values, timing reference, university of delaware, windows operating systems
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