NTP was developed as an Internet Protocol to disseminate accurate timing information to client computers across the Internet. NTP, or Network Time Protocol, is utilised throughout the Internet to provide client machines with accurate time. This article attempts to simplify the terminology used in computer time synchronisation and describes some of the features of the Network Time Protocol. NTP is one of the oldest continuously used Internet protocols. It was developed over 25 years ago at the University of Delaware by Dr D Mills. NTP primarily runs on the LinuxUnix operating systems but has recently been ported to Microsoft Windows.



NTP servers obtain accurate time from an external timing reference, such as GPS or Radio time and frequency transmissions. A NTP Server provides networked clients primarily with three peices of information: 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.

The Network Time Protocol uses the TCPIP UDP protocol (User Data-gram Protocol). The protocol defines a number of distinct fields in the messages: 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 NTP server replies with a time stamp that the client can utilise for accurate timing. 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 new releases of NTP are backwards compatible with previous versions of the protocol.

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.