The Network Time Protocol (NTP) is a
networking protocol for
clock synchronization between computer systems over
packet-switched
In telecommunications, packet switching is a method of grouping data into '' packets'' that are transmitted over a digital network. Packets are made of a header and a payload. Data in the header is used by networking hardware to direct the pac ...
, variable-
latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed by
David L. Mills of the
University of Delaware.
NTP is intended to
synchronize
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
all participating computers to within a few
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds.
A unit of 10 milliseconds may be called ...
s of
Coordinated Universal Time
Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently ...
(UTC).
It uses the
intersection algorithm, a modified version of
Marzullo's algorithm Marzullo's algorithm, invented by Keith Marzullo for his Ph.D. dissertation in 1984, is an agreement algorithm used to select sources for estimating accurate time from a number of noisy time sources. A refined version of it, renamed the "intersect ...
, to select accurate
time servers and is designed to mitigate the effects of variable
network latency. NTP can usually maintain time to within tens of milliseconds over the public
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
, and can achieve better than one millisecond accuracy in
local area networks under ideal conditions. Asymmetric
routes
Route or routes may refer to:
* Route (gridiron football), a path run by a wide receiver
* route (command), a program used to configure the routing table
* Route, County Antrim, an area in Northern Ireland
* ''The Route'', a 2013 Ugandan film
* Ro ...
and
network congestion can cause errors of 100 ms or more.
The protocol is usually described in terms of a
client–server model, but can as easily be used in
peer-to-peer relationships where both peers consider the other to be a potential time source.
Implementations send and receive
timestamps using the
User Datagram Protocol (UDP) on
port number 123.
They can also use
broadcasting
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began ...
or
multicasting
In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused with ...
, where clients passively listen to time updates after an initial round-trip calibrating exchange.
NTP supplies a warning of any impending
leap second adjustment, but no information about local
time zones or
daylight saving time
Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typical ...
is transmitted.
The current protocol is version 4 (NTPv4), which is
backward compatible with version 3.
Network Time Security (NTS) is a secure version of NTP with
TLS
TLS may refer to:
Computing
* Transport Layer Security, a cryptographic protocol for secure computer network communication
* Thread level speculation, an optimisation on multiprocessor CPUs
* Thread-local storage, a mechanism for allocating vari ...
and
AEAD.
History
In 1979, network
time synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
technology was used in what was possibly the first public demonstration of
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
services running over a trans-Atlantic satellite network, at the
National Computer Conference in New York. The technology was later described in the 1981 Internet Engineering Note (IEN) 173
and a public protocol was developed from it that was documented in . The technology was first deployed in a local area network as part of the Hello routing protocol and implemented in the
Fuzzball router Fuzzball routers were the first modern routers on the Internet. They were DEC PDP-11 computers (usually LSI-11 personal workstations) loaded with the Fuzzball software written by David L. Mills (of the University of Delaware). The name "Fuzzbal ...
, an experimental operating system used in network prototyping, where it ran for many years.
Other related network tools were available both then and now. They include the
Daytime
Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemis ...
and
Time protocols for recording the time of events, as well as the
ICMP Timestamp
The Internet Control Message Protocol (ICMP) is a supporting Communications protocol, protocol in the Internet protocol suite. It is used by network devices, including Router (computing), routers, to send error messages and operational informatio ...
messages and IP Timestamp option (). More complete synchronization systems, although lacking NTP's data analysis and clock disciplining algorithms, include the
Unix daemon ''
timed'', which uses an election algorithm to appoint a server for all the clients;
and the Digital Time Synchronization Service (DTSS), which uses a hierarchy of servers similar to the NTP stratum model.
In 1985, NTP version 0 (NTPv0) was implemented in both Fuzzball and Unix, and the NTP packet header and
round-trip delay and offset calculations, which have persisted into NTPv4, were documented in . Despite the relatively slow computers and networks available at the time, accuracy of better than 100
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds.
A unit of 10 milliseconds may be called ...
s was usually obtained on Atlantic spanning links, with accuracy of tens of milliseconds on
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in ...
networks.
In 1988, a much more complete specification of the NTPv1 protocol, with associated algorithms, was published in . It drew on the experimental results and clock filter algorithm documented in and was the first version to describe the
client–server and
peer-to-peer modes. In 1991, the NTPv1 architecture, protocol and algorithms were brought to the attention of a wider engineering community with the publication of an article by
David L. Mills in the
IEEE Transactions on Communications.
In 1989, was published defining NTPv2 by means of a
state machine
A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
, with
pseudocode
In computer science, pseudocode is a plain language description of the steps in an algorithm or another system. Pseudocode often uses structural conventions of a normal programming language, but is intended for human reading rather than machine re ...
to describe its operation. It introduced a management protocol and
cryptographic authentication scheme which have both survived into NTPv4, along with the bulk of the algorithm. However the design of NTPv2 was criticized for lacking
formal correctness by the DTSS community, and the clock selection procedure was modified to incorporate
Marzullo's algorithm Marzullo's algorithm, invented by Keith Marzullo for his Ph.D. dissertation in 1984, is an agreement algorithm used to select sources for estimating accurate time from a number of noisy time sources. A refined version of it, renamed the "intersect ...
for NTPv3 onwards.
In 1992, defined NTPv3. The RFC included an analysis of all sources of error, from the
reference clock down to the final client, which enabled the calculation of a
metric that helps choose the best server where several candidates appear to disagree. Broadcast mode was introduced.
In subsequent years, as new features were added and algorithm improvements were made, it became apparent that a new protocol version was required.
In 2010, was published containing a proposed specification for NTPv4. The protocol has significantly progressed since then, and , an updated RFC has yet to be published.
Following the retirement of Mills from the
University of Delaware, the reference implementation is currently maintained as an
open source
Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...
project led by Harlan Stenn.
Clock strata
NTP uses a hierarchical, semi-layered system of time sources. Each level of this hierarchy is termed a ''stratum'' and is assigned a number starting with zero for the reference clock at the top. A server synchronized to a stratum ''n'' server runs at stratum ''n'' + 1. The number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy. Stratum is not always an indication of quality or reliability; it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources. A brief description of strata 0, 1, 2 and 3 is provided below.
; Stratum 0
: These are high-precision timekeeping devices such as
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betw ...
s,
GNSS (including
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
) or other
radio clock
A radio clock or radio-controlled clock (RCC), and often (incorrectly) referred to as an atomic clock is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter connected to a time st ...
s. They generate a very accurate
pulse per second signal that triggers an
interrupt and timestamp on a connected computer. Stratum 0 devices are also known as reference clocks. NTP servers cannot advertise themselves as stratum 0. A stratum field set to 0 in NTP packet indicates an unspecified stratum.
[RFC 5905, p. 21]
; Stratum 1
: These are computers whose
system time is synchronized to within a few microseconds of their attached stratum 0 devices. Stratum 1 servers may peer with other stratum 1 servers for
sanity check
A sanity check or sanity test is a basic test to quickly evaluate whether a claim or the result of a calculation can possibly be true. It is a simple check to see if the produced material is rational (that the material's creator was thinking ration ...
and backup.
They are also referred to as primary time servers.
; Stratum 2
: These are computers that are synchronized over a network to stratum 1 servers. Often a stratum 2 computer queries several stratum 1 servers. Stratum 2 computers may also peer with other stratum 2 computers to provide more stable and robust time for all devices in the peer group.
; Stratum 3
: These are computers that are synchronized to stratum 2 servers. They employ the same algorithms for peering and data sampling as stratum 2, and can themselves act as servers for stratum 4 computers, and so on.
The upper limit for stratum is 15; stratum 16 is used to indicate that a device is unsynchronized. The NTP algorithms on each computer interact to construct a
Bellman-Ford shortest-path
spanning tree
In the mathematical field of graph theory, a spanning tree ''T'' of an undirected graph ''G'' is a subgraph that is a tree which includes all of the vertices of ''G''. In general, a graph may have several spanning trees, but a graph that is not ...
, to minimize the accumulated round-trip delay to the stratum 1 servers for all the clients.
In addition to stratum, the protocol is able to identify the synchronization source for each server in terms of a reference identifier (refid).
Timestamps
The 64-bit
binary fixed-point timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part for fractional second, giving a time scale that
rolls over every 2
32 seconds (136 years) and a theoretical resolution of 2
−32 seconds (233 picoseconds). NTP uses an
epoch of January 1, 1900. Therefore, the first rollover occurs on February 7, 2036.
NTPv4 introduces a 128-bit date format: 64 bits for the second and 64 bits for the fractional-second. The most-significant 32-bits of this format is the ''Era Number'' which resolves rollover ambiguity in most cases.
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."
[ University of Delaware Digital Systems Seminar presentation by David Mills, 2006-04-26]
Clock synchronization algorithm
A typical NTP client regularly
polls one or more NTP servers. The client must compute its time offset and
round-trip delay. Time offset ''θ'' is positive or negative (client time > server time) difference in absolute time between the two clocks. It is defined by
and the round-trip delay ''δ'' by
where
*''t''
0 is the client's timestamp of the request packet transmission,
*''t''
1 is the server's timestamp of the request packet reception,
*''t''
2 is the server's timestamp of the response packet transmission and
*''t''
3 is the client's timestamp of the response packet reception.
To derive the expression for the offset, note that for the request packet,
and for the response packet,
Solving for ''θ'' yields the definition of the time offset.
The values for ''θ'' and ''δ'' are passed through filters and subjected to statistical analysis.
Outliers are discarded and an estimate of time offset is derived from the best three remaining candidates. The clock frequency is then adjusted to reduce the offset gradually, creating a
feedback loop.
Accurate synchronization is achieved when both the incoming and outgoing routes between the client and the server have symmetrical nominal delay. If the routes do not have a common nominal delay, a
systematic bias exists of half the difference between the forward and backward travel times.
Software implementations
Reference implementation
The NTP
reference implementation, along with the protocol, has been continuously developed for over 20 years. Backwards compatibility has been maintained as new features have been added. It contains several sensitive algorithms, especially to discipline the clock, that can misbehave when synchronized to servers that use different algorithms. The software has been
ported to almost every computing platform, including personal computers. It runs as a
daemon called
ntpd under Unix or as a
service
Service may refer to:
Activities
* Administrative service, a required part of the workload of university faculty
* Civil service, the body of employees of a government
* Community service, volunteer service for the benefit of a community or a pu ...
under Windows. Reference clocks are supported and their offsets are filtered and analysed in the same way as remote servers, although they are usually polled more frequently.
This implementation was audited in 2017, finding numerous potential security issues.
SNTP
Simple Network Time Protocol (SNTP) is a less complex implementation of NTP, using the same protocol but without requiring the storage of
state over extended periods of time.
It is used in some
embedded systems and in applications where full NTP capability is not required.
Windows Time
All
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
versions since
Windows 2000 include the Windows Time service (W32Time),
which has the ability to synchronize the computer clock to an NTP server.
W32Time was originally implemented for the purpose of the
Kerberos version 5 authentication protocol, which required time to be within 5 minutes of the correct value to prevent
replay attacks. The version in Windows 2000 and Windows XP only implements SNTP, and violates several aspects of the NTP version 3 standard.
Beginning with
Windows Server 2003 and
Windows Vista, W32Time became compatible with a significant subset of NTPv3.
Microsoft states that W32Time cannot reliably maintain time synchronization with one second accuracy.
If higher accuracy is desired, Microsoft recommends using a newer version of Windows or different NTP implementation.
Beginning with
Windows 10 version 1607 and
Windows Server 2016, W32Time can be configured to reach time accuracy of 1 s, 50 ms or 1 ms under certain specified operating conditions.
OpenNTPD
In 2004, Henning Brauer presented
OpenNTPD, an NTP implementation with a focus on security and encompassing a privilege separated design. Whilst it is aimed more closely at the simpler generic needs of
OpenBSD
OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project em ...
users, it also includes some protocol security improvements while still being compatible with existing NTP servers. A portable version is available in Linux package repositories.
Ntimed
Ntimed was started by
Poul-Henning Kamp in 2014 and abandoned in 2015.
The implementation was sponsored by the
Linux Foundation
The Linux Foundation (LF) is a non-profit technology consortium founded in 2000 as a merger between Open Source Development Labs and the Free Standards Group to standardize Linux, support its growth, and promote its commercial adoption. Additi ...
.
NTPsec
NTPsec is a
fork of the reference implementation that has been systematically
security-hardened. The fork point was in June 2015 and was in response to a series of compromises in 2014. The first production release shipped in October 2017.
Between removal of unsafe features, removal of support for obsolete hardware, and removal of support for obsolete Unix variants, NTPsec has been able to pare away 75% of the original codebase, making the remainder easier to
audit
An audit is an "independent examination of financial information of any entity, whether profit oriented or not, irrespective of its size or legal form when such an examination is conducted with a view to express an opinion thereon.” Auditing ...
.
A 2017 audit of the code showed eight security issues, including two that were not present in the original reference implementation, but NTPsec did not suffer from eight other issues that remained in the reference implementation.
chrony
chrony comes by default in
Red Hat
Red Hat, Inc. is an American software company that provides open source software products to enterprises. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina, with other offices worldwide.
Red Hat has become ass ...
distributions
and is available in the
Ubuntu repositories.
is aimed at ordinary computers, which are unstable, go into sleep mode or have intermittent connection to the Internet.
is also designed for virtual machines, a much more unstable environment. It is characterized by low resource consumption (cost) and supports
Precision Time Protocol hardware for greater timestamp precision.
It has two main components: , a daemon that is executed when the computer starts, and , a command line interface to the user for its configuration. It has been evaluated as very safe and with just a few incidents,
its advantage is the versatility of its code, written from scratch to avoid unnecessary complexity.
Support for Network Time Security (NTS) was added on version 4.0. is available under
GNU General Public License version 2, was created by
Richard Curnow
Richard is a male given name. It originates, via Old French, from Old Frankish and is a compound of the words descending from Proto-Germanic ''*rīk-'' 'ruler, leader, king' and ''*hardu-'' 'strong, brave, hardy', and it therefore means 'stron ...
in 1997 and is currently maintained by
Miroslav Lichvar Miroslav may refer to:
* Miroslav (given name), a Slavic masculine given name
* ''Young America'' (clipper) or ''Miroslav'', an Austrian clipper ship in the Transatlantic case oil trade
* Miroslav (Znojmo District), a town in the Czech Republic
S ...
.
Leap seconds
On the day of a
leap second event, ntpd receives notification from either a
configuration file, an attached reference clock, or a remote server. Although the NTP clock is actually halted during the event, because of the requirement that time must appear to be
strictly increasing
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order ...
, any
processes
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
that query the system time cause it to increase by a tiny amount, preserving the order of events. If a negative leap second should ever become necessary, it would be deleted with the sequence 23:59:58, 00:00:00, skipping 23:59:59.
An alternative implementation, called leap smearing, consists in introducing the leap second incrementally during a period of 24 hours, from noon to noon in UTC time. This implementation is used by Google (both internally and on their public NTP servers) and by Amazon AWS.
Security concerns
Only a few other security problems have been identified in the reference implementation of the NTP codebase, but those that appeared in 2009 were cause for significant concern.
The protocol has been undergoing revision and review throughout its history. The codebase for the reference implementation has undergone security audits from several sources for several years.
A
stack buffer overflow exploit was discovered and patched in 2014.
Apple
An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ances ...
was concerned enough about this vulnerability that it used its auto-update capability for the first time.
Some implementation errors are basic, such as a missing return statement in a routine, that can lead to unlimited access to systems that are running some versions of NTP in the root daemon. Systems that do not use the root daemon, such as those derived from
Berkeley Software Distribution
The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...
(BSD), are not subject to this flaw.
A 2017 security audit of three NTP implementations, conducted on behalf of the Linux Foundation's Core Infrastructure Initiative, suggested that both NTP
NTP SecurityNotice Page
''
NVD NIST Product Search NTP
' and NTPsec
NVD NIST Product Search NTPsec
'' were more problematic than Chrony
NVD NIST Product Search Chrony
'' from a security standpoint.
NTP servers can be susceptible to
man-in-the-middle attacks unless packets are cryptographically signed for authentication.
The computational overhead involved can make this impractical on busy servers, particularly during
denial of service attacks.
NTP message
spoofing from a man-in-the-middle attack can be used to alter clocks on client computers and allow a number of attacks based on bypassing of cryptographic key expiration.
Some of the services affected by fake NTP messages identified are
TLS
TLS may refer to:
Computing
* Transport Layer Security, a cryptographic protocol for secure computer network communication
* Thread level speculation, an optimisation on multiprocessor CPUs
* Thread-local storage, a mechanism for allocating vari ...
,
DNSSEC
The Domain Name System Security Extensions (DNSSEC) are a suite of extension specifications by the Internet Engineering Task Force (IETF) for securing data exchanged in the Domain Name System (DNS) in Internet Protocol (IP) networks. The protoco ...
, various caching schemes (such as DNS cache),
Border Gateway Protocol (BGP), Bitcoin and a number of persistent login schemes.
NTP has been used in
distributed denial of service attacks.
A small query is sent to an NTP server with the return
IP address spoofed to be the target address. Similar to the
DNS amplification attack, the server responds with a much larger reply that allows an attacker to substantially increase the amount of data being sent to the target. To avoid participating in an attack, NTP server software can be upgraded or servers can be configured to ignore external queries.
To improve NTP security, a secure version called Network Time Security (NTS) was developed and currently supported by several time servers.
See also
*
Allan variance
*
Clock network
A clock network or clock system is a set of synchronized clocks designed to always show exactly the same time by communicating with each other. Clock networks usually consist of a central master clock kept in sync with an official time source, ...
*
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 ato ...
*
IRIG timecode
*
NITZ
*
NTP pool
*
Ntpdate
Notes
References
Further reading
*
*
External links
*
Official Stratum One Time Servers listIETF NTP working groupMicrosft Windows accurate time guidean
moreTime and NTP paperNTP Survey 2005Current NIST leap seconds file compatible with ntpd* {{citation , author=David L. Mills , url=https://www.eecis.udel.edu/~mills/database/papers/history.pdf , title=A Brief History of NTP Time: Confessions of an Internet Timekeeper , access-date=2021-02-07
Application layer protocols
Internet Standards
Network time-related software