Network Time Protocol

The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. NTP uses UDP on port 123 as its transport layer. It is designed particularly to resist the effects of variable latency by using a jitter buffer. NTP also refers to a reference software implementation that is distributed by the NTP Public Services Project.

NTP is one of the oldest Internet protocols still in use (since before 1985). NTP was originally designed by Dave Mills of the University of Delaware, who still maintains it, along with a team of volunteers.

NTP is not related to the simpler DAYTIME (RFC 867) and TIME (RFC 868) protocols.

Overview

NTP uses Marzullo's algorithm, and includes support for features such as leap seconds. NTPv4 can usually maintain time to within 10 milliseconds (1/100 s) over the public Internet, and can achieve accuracies of 200 microseconds (1/5000 s) or better in local area networks under ideal conditions.

In the Internet, NTP synchronizes computer system clocks to UTC; in isolated LANs, NTP is also commonly used to synchronize to UTC, but in principle it could be used to distribute a different time scale, for example local zone time.

The operational details of NTP are illustrated in RFC 778, RFC 891, RFC 956, RFC 958 (obsolete by 1305), and RFC 1305. The current reference implementation is version 4 (NTPv4); however, as of 2005, only versions up to 3 (1992) have been documented in RFCs. The IETF NTP Working Group has formed to standardize the work of the NTP community since RFC 1305 et al.

A less complex form of NTP that does not require storing information about previous communications is known as the Simple Network Time Protocol or SNTP. It is used in some embedded devices and in applications where high accuracy timing is not required. See RFC 1361, RFC 1769, RFC 2030, and RFC 4330.

Note that NTP provides just the UTC time, and no information about time zones or daylight saving time. This information is outside its scope and must be obtained separately (most systems allow it to be set manually).

NTP software implementations

Unix

For modern Unix systems, the NTP client is implemented as a daemon process that runs continuously in user space. Because of sensitivity to timing, however, it is important to have the standard NTP clock phase-locked loop implemented in kernel space. All recent versions of Linux, BSD, and Solaris are implemented in this manner.

Microsoft Windows

All Microsoft Windows versions since 2000 include the Windows Time Service, which has the ability to sync the computer clock to an NTP server. However, the version in Windows 2000 only implements Simple NTP, and violates several aspects of the NTP version 3 standard. Beginning with Windows Server 2003, the Microsoft documentation states that Windows Time Service implements the full NTPv3 protocol as specified in RFC 1305.

However, the Windows Time Service cannot maintain the system time more accurately than about a 1-2 second range. Microsoft "[does] not guarantee and [does] not support the accuracy of the W32Time service between nodes on a network. The W32Time service is not a full-featured NTP solution that meets time-sensitive application needs."

The reference implementation of NTP can be used on Microsoft Windows systems. It is available free of charge from a commercial manufacturer of GPS and radio reference clocks in a form that can be installed using the Microsoft Installer.

Clock strata

Yellow arrows indicate a direct connection; red arrows indicate a network connection.

The U.S. Naval Observatory Alternate Master Clock at Schriever AFB (Colorado) is a Stratum-0 source for NTP

NTP uses a hierarchical, layered system of levels of clock sources, each level of this hierarchy is termed a stratum and assigned a layer number starting with 0 (zero) at the top. The stratum level defines its distance from the reference clock and exists to prevent cyclical dependencies in the hierarchy. It is important to note that the stratum is not an indication of quality or reliability, it is quite common to find "stratum 3" time sources that are higher quality than other "stratum 2" time sources. This definition of "stratum" is also different from the notion of clock strata used in telecommunication systems.

Stratum 0

These are devices such as atomic (caesium, rubidium) clocks, GPS clocks or other radio clocks. Stratum-0 devices are traditionally not attached to the network; instead they are locally connected to computers (e.g., via an RS-232 connection using a Pulse per second signal).

Stratum 1

These are computers attached to Stratum 0 devices. Normally they act as servers for timing requests from Stratum 2 servers via NTP. These computers are also referred to as time servers. Many Stratum 1 servers (for NTP v3 and earlier versions) may not actually be operating with Stratum 1 precision. As the NTP protocol is developed, it will become less and less possible for misleading Stratum 1 servers to run — instead the protocol would automatically bump the server Stratum level down accordingly.

Stratum 2

These are computers that send NTP requests to Stratum 1 servers. Normally a Stratum 2 computer will reference a number of Stratum 1 servers and use the NTP algorithm to gather the best data sample, dropping any Stratum 1 servers that seem obviously wrong. Stratum 2 computers will peer with other Stratum 2 computers to provide more stable and robust time for all devices in the peer group. Stratum 2 computers normally act as servers for Stratum 3 NTP requests.

Stratum 3

These computers employ exactly the same NTP functions of peering and data sampling as Stratum 2, and can themselves act as servers for lower strata, potentially up to 16 levels. NTP (depending on what version of NTP protocol in use) supports up to 256 strata.

It is hoped that in NTP 5, a protocol still in development, only 8 strata will be permitted. As most NTP clients call on Stratum 2 servers, it is expected that no users will be disadvantaged by the loss of granularity.

NTP timestamps

The 64-bit timestamps used by NTP consist of a 32-bit seconds part and a 32-bit fractional second part, giving NTP a time scale of 2³² seconds (136 years) and a theoretical resolution of 2⁻³² seconds (233 picoseconds).

The NTP timescale wraps around every 2³² seconds (136 years). NTP uses an epoch of January 1, 1900, so the first rollover will occur in 2036, well before the familiar UNIX Year 2038 problem.

Implementations should disambiguate NTP time using a knowledge of the approximate time from other sources. Since NTP only works with the differences between timestamps and never their absolute values, the wraparound is invisible as long as the timestamps are within 68 years of each other. This means that the rollover will be invisible for most running systems, since they will have the correct time to within a very small tolerance. However, systems that are starting up need to know the date within no more than 68 years. Given the large allowed error, it is not expected that this is too onerous a requirement. One suggested method is to set the clock to no earlier than the system build date. Many systems use a battery powered hardware clock to avoid this problem.

Even so, future versions of NTP may extend the time representation to 128 bits: 64 bits for the second and 64 bits for the fractional-second.

According to Mills, "The 64 bit value for the fraction is enough to resolve the amount of time it takes a photon to pass an electron at the speed of light. The 64 bit second value is enough to provide unambiguous time representation until the universe goes dim." Indeed, 2⁻⁶⁴ seconds is about 54 zeptoseconds, and 2⁶⁴ seconds is about 585 billion years.

See also

• Allan variance
• Clock synchronization
• International Atomic Time
• ntpd, OpenNTPD and Ntpdate
• NTP pool, a collection of worldwide computers that provide a highly accurate time via the Network Time Protocol
• NTP server misuse and abuse
• Synchronization
• Time server

Other time synchronization protocols:

• DAYTIME protocol, older time synchronization protocol using TCP or UDP port 13
• ICMP Timestamp and ICMP Timestamp Reply, older time synchronization protocol using ICMP
• Precision Time Protocol
• TIME protocol, older time synchronization protocol using TCP or UDP port 37

Further reading

• Mills, David L. Computer Network Time Synchronization: The Network Time Protocol. Taylor & Francis / CRC Press. ISBN 0849358051.

External links

• NTP Public Services Project This is the home for the NTP (Network Time Protocol) Public Services Project which provides public support for the NTP Project and the IETF NTP Working Group. Also a Wiki with lists of NTP servers.
• IETF NTP working group
• David Mills' NTP project page
• NTP version 4 Release Notes
• Time and NTP paper
• NTP Survey 2005
• Pool of public NTP Time Servers
• For a comprehensive list of NTP Servers