This article describes the Network Time Protocol and provides an overview of how NTP servers utilise NTP to provide an accurate source of time to network time clients.
A Brief History
The NTP protocol is one of the oldest Internet protocols still in use today. It has been in continuous use for more than 25 years. The protocol was originally designed to synchronise computers and time critical processes across the Internet. Initially developed for the Linux operating system, the protocol has recently been partly ported to the Microsoft Windows operating system. NTP is installed by default in many Linux Unix and Free BSD distributions. Most NTP server systems utilise the Linux operating system due to its specialised kernel timing algorithms.
About the Protocol
NTP is a client-server based protocol. The NTP protocol provides clients with 3 fundamental products: clock offset, network round-trip delay and reference clock dispersion. The clock offset specifies the difference between the local system time and an external reference clock. The round-trip delay specifies the time latencies measured during the transfer of packets around the network. Time dispersion specifies the maximum error associated with timing information received from the external reference clock.
The NTP protocol has a hierarchical structure. A stratum 1 time server is the primary reference server and sits at the top of the heirachy. The primary reference server it is followed by secondary reference servers and clients. A radio or GPS external time reference is generally used to synchronise a stratum 1 NTP Server. At the highest level a primary reference NTP server has a stratum of one. Each lower-level NTP server and client is attributed a stratum one greater than the previous level. As the stratum increases, accuracy decreases due to inconsistencies in network path timing.
NTP utilises the User Data-gram Protocol (UDP), part of the TCP/IP protocol suite. Each NTP message consists of a number of 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. The fields are used to identify messages and provide time stamp and round-trip delay information.
The current release of NTP is version 4. All versions of the protocol are backwards compatible with previous versions. The only significant modification to the protocol between versions 3 and 4 is a slight header re-interpretation to accommodate IPv6.
NTP Server Modes of Operation
A stratum 1 NTP server has three modes of operation: unicast; anycast and multicast. The client initiates the unicast and anycast modes, the server responds with a NTP time message that the client uses for synchronisation. The multicast mode, is a broadcast mode, NTP time messages are broadcast throughout the sub-domain at specified periodic intervals.
Accurate External Reference Clocks
The standard NTP distribution can operate with a number of external time references, or ?reference clocks'. The Global Positioning System (GPS) is a popular choice since it provides a global precise time reference. A number of national time and frequency radio broadcasts are also available, such as WWVB (US), DCF-77 (Germany) and MSF (UK).
The Simple Network Time Protocol (SNTP)
SNTP is a simplified version of the full-blown Network Time Protocol. SNTP is generally used where the complexities and high accuracy of the NTP protocol is not required. The SNTP protocol is often used by micro-controllers and SBC computers. The SNTP and NTP protocols are fully interchangeable. Any SNTP client can synchronise with a full-blown NTP server.
The Network Time Protocol (NTP) is utilised by NTP Server systems to distribute accurate time information to network time clients. The NTP protocol is widely used throughout the Internet to provide synchronisation of computers and processes. This article discusses how NTP server systems utilise the Network Time Protocol to provide networks with an accurate reference of time.
NTP Overview
NTP has been in use as an Internet protocol for over 25 years. It is the longest running continuously operating Internet protocol. The protocol was born through the need to provide synchronisation of time critical processes across the Internet. NTP primarily runs on LINUX and UNIX platforms including Free-BSD but has also been, in part, ported to Windows operating systems. Dedicated NTP server systems generally utilise the LINUX operating system.
The NTP Protocol
NTP is designed to provide network time clients with three products: system clock offset, round-trip delay and dispersion relative to a specified reference clock. Clock offset is the time difference between the local clock and reference clock. Round-trip delay measures the amount of time the protocol takes to receive a response from the server. Dispersion is the maximum error of the local clock relative to the specified reference.
NTP Stratum
NTP operates in a hierarchical manner, the primary reference followed by secondary references and clients. At the top of the hierarchy, the primary reference is usually synchronised to an external time source such as a radio or GPS clock. The primary reference is attributed a stratum of one. Each level down in the hierarchy is attributed a stratum one greater than the preceding level. As the stratum increases, the accuracy of the reference degrades slightly due to inconsistencies in network path timing. Secondary references have a stratum of between two and fifteen.
NTP Messages
NTP utilises the UDP (User Data-gram Protocol) protocol. The NTP message consists of a number of 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.
The leap indicator warns of an impending leap second addition or deletion. The version number indicates the NTP version in use. Mode specified the NTP mode of the current message. Stratum is an eight-bit value indicating the hierarchical level of the reference clock. Poll interval specifies the maximum interval between messages. Precision specifies the accuracy of the local clock. Root delay indicates the round-trip delay time to the reference. Root dispersion indicates the nominal error relative to the primary reference. Reference identifier is a 4-character ASCII string identifying the reference source, such as GPS, DCF or MSF. Reference timestamp specifies the time at which the reference clock was last corrected. The Originate Timestamp specifies the time the NTP request message departed the client for the NTP server. Receive timestamp specifies the time the NTP request message was received by the NTP server. Transmit timestamp specifies the time the NTP response message was transmitted by the server to the client. The key identifier field is utilised when authentication is required and provides the message authentication code.
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.
NTP Versions
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.
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