Network Time Protocol

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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 port 123 as its transport layer. It is designed particularly to resist the effects of variable latency (jitter).

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 much simpler DAYTIME (RFC 867) and TIME (RFC 868) protocols.

[edit] Overview

NTP uses Marzullo's algorithm with the UTC time scale, including 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.

The NTP Unix daemon is a user-level process that runs continuously on a machine that supports NTP, and most of the protocol is implemented in this user process. To get the best performance from NTP, it is important to have the standard NTP clock phase-locked loop implemented in the operating system kernel, rather than using only the intervention of the external NTP daemon: all recent versions of the Linux and Solaris operating systems have this support.

The operational details of NTP are illustrated in RFC 778, RFC 891, RFC 956, RFC 958, 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.

Microsoft Windows 2000 and XP 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 SNTP. As of Windows Server 2003 Service Pack 1, the Windows Time Service implements the more robust NTPv3 protocol as specified in RFC 1305.

[edit] Clock strata

(Note that this is different from the notion of clock strata used in telecommunications systems.)

NTP uses a hierarchical system of "clock strata". The stratum levels define the distance from the reference clock and the associated accuracy.

Stratum 0

These are devices such as atomic (cesium, rubidium) clocks, GPS clocks or other radio clocks. Stratum-0 devices are 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 higher 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) that only 8 or 16 strata will be permitted.

[edit] Strata illustration

NTP stratum levels are indicated by the blue numerals. Green arrows indicate a direct connection, such as RS-232, while red arrows indicate a network connection.

[edit] 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 (0.233 nanoseconds).

The NTP timescale wraps around every 2³² seconds (136 years). NTP uses an epoch of January 1, 1900, so the first wrap will occur in 2036, reminiscent of the Year 2038 problem. This wraparound defect is specific to the 32 bit NTP timestamp in NTP3 that is held over into NTP4. NTP4 has a clean 64 bit mode that does not have this problem -- as well as a 128 bit mode that is in prototyping for NTP5.

Implementations should disambiguate NTP time using a knowledge of the approximate time from other sources. Since this only requires time accurate to a few decades, this is unlikely to ever be a problem in general use.

Even so, future versions of NTP will 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."^[1] Indeed, 2⁻⁶⁴ seconds is about 54 zeptoseconds, and 2⁶⁴ seconds is about 585 billion years.

[edit] References

1. ^ University of Delaware Digital Systems Seminar presentation by David Mills, April 26, 2006

[edit] Bibliography

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

[edit] See also

• Allan variance
• Clock synchronization
• ntpd and OpenNTPD
• Synchronization
• International Atomic Time
• Time server
• NTP server misuse and abuse
• NTP pool, a collection of worldwide computers that provide a highly accurate time via the Network Time Protocol
• DAYTIME protocol, older time synchronization protocol using TCP or UDP port 13
• TIME protocol, older time synchronization protocol using TCP or UDP port 37
• ICMP Timestamp and ICMP Timestamp Reply, older time synchronization protocol using ICMP

[edit] 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
• Stratum 2 NTP-Server (German website)
• [1]

[edit] Other time synchronization protocols

• Daytime protocol DAYTIME synchronization protocol via TCP or UDP port 13
• ICMP protocol Time synchronization with ICMP timestamps (basis for timed )
• HTTP Time Protocol (HTP) Time synchronization with HTTP headers
• Precision Time Protocol Time synchronization for wired networks with accuracy at the 10s of nanoseconds range
• Reference-Broadcast Synchronization (RBS) Time synchronization for wireless networks
• Time protocol Time synchronization protocol via TCP or UDP port 37