devans@timetools.co.uk.
Best regards
David Evans
http://www.timetools.co.uk
Overview of NTP Servers and the NTP Protocol
The Network Time Protocol, or NTP, is an internet protocol
developed to distribute accurate timing information to
network time clients. NTP is a client-server based protocol
widely used throughout the Internet for accurate time
synchronisation. This article describes some of the
features of NTP and attempts to simplify some of the
terminology used. NTP was originally developed 25 years ago
by Dr David Mills of the University of Delaware. The
protocol is one of the oldest Internet protocols still in
continuous use. Originally developed for the LINUX
operating systems, it has since been ported to the
Microsoft Windows operating systems.
NTP servers obtain accurate time from an external timing
reference, such as GPS or Radio time and frequency
transmissions. They provide network time clients with three
products: system clock offset, network delay and dispersion
relative to an external reference clock.
The NTP protocol is configured in a hierarchical manner;
primary servers, secondary servers and time clients.
Primary NTP servers are synchronised to an accurate
external clock and is attributed a stratum, or hierarchy,
level of one. Secondary NTP servers synchronise to primary
servers and provide a buffer between primary servers and
clients. Secondary servers are attributed a stratum of
greater than one. As the stratum increases, so the accuracy
of the server decreases.
NTP utilises the UDP (User Data-gram Protocol) protocol.
NTP protocol messages consist of a number of distinct
fields: Leap Indicator; Version Number; Mode; Stratum;
Poll; Precision; Root Delay; Root Dispersion; Reference
Identifier; Reference Timestamp; Originate Timestamp;
Receive Timestamp; Transmit Timestamp; Key Identifier and
Message Digest.
A NTP server can operate in three modes: unicast; anycast
and multicast. In unicast and anycast modes, the client
transmits a NTP request message to the server. The server
responds with a NTP time message that the client uses for
time synchronisation. In multicast mode, NTP time messages
are broadcast at periodic specified intervals.
The current release of NTP is version 4. The only
significant modification to the protocol between versions 3
and 4 is a slight header re-interpretation to accommodate
IPv6. All versions of NTP are backwards compatible and can
be freely interchanged.
SNTP, or Simple Network Time Protocol, is a simplification
of the full-blown Network Time Protocol. It can be used
where the complexities of the full-blown protocol are not
required. SNTP is often implemented on Microsoft Windows
platforms where many of the complex algorithms for
maintaining accurate time are unavailable.
SNTP can be implemented when the high synchronisation
performance of NTP is not required. The message format of
the SNTP protocol is almost identical to that of the NTP
protocol. However, the complex subroutines designed to
maintain a highly accurate synchronised time are removed or
simplified. Small computers with reduced processing power,
such as micro-controllers or monitoring equipment most
often use SNTP. The SNTP and NTP protocols are fully
interchangeable, a SNTP client can synchronise to a NTP
server without any issues.
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David Evans is a technical author in the field of computer
time synchronisation and telecommunications. If you would
like more information on NTP server systems, please visit:
http://www.timetools.co.uk/ntp-servers/ntp-server-s5000.htm
