NTP Server Operation
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.
NTP Reference Clocks
Primary NTP servers can synchronise with a number of external reference time sources. The GPS (Global Positioning System) is often used to provide an accurate timing reference. There is also a number of national time and frequency radio broadcasts available. The WWVB radio time broadcast is transmitted from Colorado and covers most of the US. The DCF-77 radio time signal is broadcast from Frankfurt, Germany, and covers much of central and Western Europe. The MSF radio time signal is broadcast from Rugby, UK, and covers the British Isles and much of Western Europe. Other local time and frequency radio broadcasts include: TDF, France, and CHU, Canada.
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.
SNTP Simple Network Time Protocol
SNTP is a simplification of the Network Time Protocol, where the complexities of the full-blown protocol are not required. 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.
More Information: NTP RFC’s
The original Network Time Protocol was introduced in RFC 958 (Request for Comments: 958). The full NTP version 3 protocol and synchronisation algorithms are described in RFC 1305. The SNTP version 4 protocols are described in RFC 2030.