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. It is designed particularly to resist the effects of variable latency by using a jitter buffer.

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

NTP uses the User Datagram Protocol (UDP) on port number 123.

NTP is not related to the simpler Daytime Protocol (RFC 867) or the Time Protocol (RFC 868).

The term also refers to the reference software implementation that is distributed by the NTP Public Services Project.

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Contents

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.

NTP provides Coordinated Universal Time (UTC). No information about time zones or daylight saving time is transmitted; this information is outside its scope and must be obtained separately. In isolated LANs, NTP could in principle be used to distribute a different time scale (e.g. local zone time), but this is uncommon.

As of June 2010, the current reference implementation is version 4 (NTPv4), which is a proposed standard as documented in RFC (RFC 5905).

A less complex implementation of NTP, using the same protocol but without requiring the storage of state over extended periods of time, is known as the Simple Network Time Protocol (SNTP). It is used in some embedded devices and in applications where high accuracy timing is not required (RFC 1361, RFC 1769, RFC 2030, RFC 4330 and RFC 5905).

NTP software implementations

Unix

For modern Unix systems, the NTP client is implemented as a daemon process that runs continuously in user space (ntpd). 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, Mac OS X and Solaris are implemented in this manner.

Microsoft Windows

All Microsoft Windows versions since Windows 2000 include the Windows Time Service,[1] 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.[2] Beginning with Windows Server 2003, the Microsoft documentation states that Windows Time Service implements the full NTPv3 protocol[3] 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."[4]

The reference implementation of NTP can be used on Microsoft Windows systems.[5]

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, semi-layered system of levels of clock sources. Each level of this hierarchy is termed a stratum and is 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.
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. NTP (depending on what version of NTP protocol in use) supports up to 256 strata.

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. The current NTP4 format has support for Era Number and Era Offset, that when used properly should aid fixing date rollover issues until NTP5 replaces NTP4.

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."[6] Indeed, 2−64 seconds is about 54 zeptoseconds, and 264 seconds is about 585 billion years.

Security concerns

Only a few security problems have been identified in the reference implementation of the NTP codebase in its 25+ year history.[7][8]

NTP itself has been undergoing revision and review over its entire history; no security vulnerabilities have ever been reported that have been traced to the NTP specification.[9]

The current codebase for the reference implementation has been undergoing security audits from several sources for several years now, and there are no known high-risk vulnerabilities in the current released software.[10]

There is a perceived hazard with the current NTP4 64-bit timestamp calculations spanning an era. It is possible to have a set of conditions arise in 2036 that might result in a time over-run or loop-around event.

See also

Other time synchronization protocols:

References

  1. ↑ Windows Time Service Technical Reference
  2. ↑ Windows Time Service page at NTP.org
  3. ↑ Windows Time Service Time Protocols
  4. ↑ Support boundary to configure the Windows Time service for high accuracy environments
  5. ↑ doc.ntp.org: NTP 4.x for Windows NT
  6. ↑ University of Delaware Digital Systems Seminar presentation by David Mills, 2006-04-26
  7. ↑ http://support.ntp.org/security
  8. ↑ results returned by a search on "Network Time Protocol" at http://cert.org]
  9. ↑ Look at RFC 778, RFC 891, RFC 956, RFC 958, RFC 1305, and the NTPv4 specification and notice the lack of revisions due to protocol security problems. Similarly, look at the results returned by a search on "Network Time Protocol" at http://cert.org]
  10. ↑ http://support.ntp.org/Main/CodeAudit

Further reading

External links