Accurate time on a network is essential for all businesses and institutions. Without an accurately synchronised system a computer network can be vulnerable to all sorts of problems, from malicious hackers and other security threats to fraud and data loss.

Network Time Protocol is the key to keeping accurate time it is a software algorithm that has been constantly developed for over two decades. NTP takes a single time source that is received by the NTP server and distributes it across a network ensuring all machines in that network are running to exact same time.

Whilst NTP can maintain synchronisation of a network to within a few milliseconds it is only as good as the time source it receives. A dedicated NTP server will use a time signal from an external source and so keep the network secure as the firewall will not have to be disturbed.

The two preferred methods for most users of NTP servers is the GPS network (Global Positioning System) or specialist time and frequency transmissions put out be several national physics labs such as the UK’s NPL.

These time signals are UTC (Coordinated Universal Time) which is the world’s civil timescale. A NTP server receiving time source from either a frequency transmission or the GPS network can realistically provide accuracy to within a few milliseconds of UTC

In selecting a timing source to synchronise a computer network to using a NTP server (Network Time Protocol) it is important that the time source is accurate, secure and a source of UTC (Coordinated Universal Time). UTC is a global timescale used by computer networks, business and commerce across the globe.

Whilst UTC is freely available across the Internet it is neither accurate nor secure (being as it is external to your firewall).  Also Internet time sources cannot be authenticated which is NTP’s own method of ensuring a time source is what it says it is. There are two secure, accurate and reliable methods for receiving UTC via a NTP server and both come with their own advantages and drawbacks.

The first method is to use the GPS network (Global Positioning System).The main advantage of using the signals transmitted from a GPS satellite’s onboard atomic clock is that a signal is available anywhere on the planet. However it does come with a downside. As the signals are all line-of-sight it means that the GPS antenna needs to be placed on a roof to ensure connectivity with a satellite.

An alternative to the GPS signal but equally as accurate and reliable is to make use of the long wave radio transmissions broadcast by several national physics laboratories. These signals, such as the UK’s MSF, Germany’s DCF-77 and the United States’ WWVB transmissions, can often be picked up inside buildings making them ideal for a solution if a rooftop is unavailable for a GPS antenna. It must be noted that not every country broadcasts such a signal and whilst most transmissions can be picked up in neighbouring countries the signals are vulnerable to interference and local geography.

NTP.org – The home for the NTP Project who provides support and additional development resources for the Official Reference Implementation of NTP.

NTP Project support pages

THE NTP pool – list of public servers

NPL – The National Physical Laboratory in the UK who control the MSF radio signal.

The University of Delaware and David Mills’ information page, Professor Mills is the original inventor and developer of NTP

David Mills’ list of Public NTP Time Servers a list of public NTP servers

National Institute of Standards and Technology (NIST) who operate the USA’s WWVB radio signal

Europe’s largest supplier of NTP server related products.

Galleon UK – NTP server products for the UK

ntp-time-server.com – one of the largest time and frequency suppliers in the United States

NTP – Wikipedia article on NTP

NTP server checker – free tool to ensure time server accuracy

Time synchronisation in the modern age is highly precise. Modern atomic clocks can keep time so accurately that in 100 million years these timekeeping devices will not lose even a second.

Bit is this sort of accuracy necessary in the modern world? How important can a second possibly be, after all, a second has always been seen as one of the smallest units of time.

However, when you consider modern technology such as the satellite navigation then a second suddenly becomes a huge gulf in time. Modern satellite navigation devices work by calculating distance by using the time form the atomic clocks on GPS (Global Positioning System) satellites.

However, when you consider that the speed of light is close to 300, 000 km a second then you can understand that if a GPS clock is a second out then your navigation could be inaccurate by hundreds of thousands of kilometres.

The same is true for modern computer networks. Computers can process thousands of transactions a second so when it comes to global network communication a second can be a huge amount of time.

That is why modern NTP server’s, responsible for synchronising networks offer precision to the millisecond, ensuring that network across the globe are within a hare’s breath of each other.

For users of MSF time server or other NTP related devices:

Notice of Interruption MSF 60 kHz Time and Frequency Signal – NPL
The MSF 60 kHz time and frequency signal broadcast from Anthorn Radio Station will be shut down over the period:

11 December 2008 – from 10:00 UTC to 14:00 UTC

The interruption to the transmission is required to allow scheduled maintenance work to be carried out in safety.

If you require any additional information, please contact NPL

Or alternatively please see the NPL time website.

Galleon’s free NTP Server Checker allows you to check the following items
* IP address – the time server you are checking.
* NTP Version
* Reference timestamp (the prime epoch ) used by NTP to work out the time from this set point
* Round trip delay (the time it takes request to arrive and come back in milliseconds)
* Local clock offset – time difference between host and client
* Leap indicator (if there is to be a leap second that day – normally only on 31 December)
* Mode 3 – indicates a client request
* Stratum level – which stratum level the NTP server is (a stratum 1 server receives the time from an atomic clock source a stratum 2 server receives the time from a stratum 1 server)
* Poll Interval (Will be 1 as only 1 requests is made by the SNTP client)
* Precision – how accurate in milliseconds
* Root Delay – This is a signed fixed-point number indicating the total roundtrip delay to the primary reference source at the root
* Root dispersion (in milliseconds)- The root dispersion is the maximum (worst case) difference between the local system clock and the root of the NTP tree (stratum 1 clock)
* Ref ID – the host name
* Originate time stamp (time before synchronisation request)
* Receive timestamp – the time the host got request
* Transmit timestamp – the time the host sends back to us
* Is response valid – synchronised or not

Please download from Galleon Systems

NTP - Network Time Protocol

SNTP – Simple Network Time Protocol

GPS – Global Positioning System

UTC – Coordinated Universal Time

MSF – Radio Time Signal for United Kingdom

WWVB – Radio Time Signal for American

DCF – Radio Time Signal for Germany

LAN – Local Area Network

UDP – User Datagram Protocol

TCP – Transmission Control Protocol

IP – Internet Protocol

TDF – Radio Time Signal for France

CHU – Radio Time Signal for Canada

JJY – Radio Time Signal for Japan

HBG – Radio Time Signal for Switzerland

USB – Universal Serial Bus

RTC – Real Time Clock

AM – Amplitude Modulation

APM – Automatic Power Management

DES – Data Encryption Standard

ESD – Electrostatic Discharge

FM – Frequency Modulation

IETF – Internet Engineering Task Force

IRIG – Inter-Range Instrumentation Group

MD5 – Message Digest

PPM – Part Per Million

PPS – Pulse Per Second

RFC – Request For Comments

SA – Selective Availability

TAI – International Atomic Time

SI – International System of Units

A public NTP Server is a time server on the Internet that, as the name suggests, members of the public can use as a timing source. The best location on the Internet to find a list of public NTP servers is the home of NTP – www.ntp.org

There are two lists of public NTP servers on ntp.org, one for primary servers and one for secondary servers. Primary servers have up to several hundred clients each. However, many primary servers are ‘closed access’ meaning that only agreed clients can access them. This is because if there is too much traffic attempting to receive a timing source from a primary source then it will clog the network making the server useless.

Primary servers are known as a stratum 1 server in that they get their timing source direct from an atomic clock often using the GPS or national time and frequency transmissions. Secondary NTP servers tend to be stratum 2 time servers, that is a time server that receives its timing source from a stratum 1 server.

Most users that require a public NTP server will find that most primary servers are closed access and that they will have to use a secondary NTP server. When using a public NTP server it is important that access policies are adhered to as many institutions require on these servers for timing information.

NTP (Network Time Protocol) is the most prevalent time synchronisation software available. On of the reasons NTP is so successful is the way it organises its clients into a hierarchy.

The hierarchy of NTP is divided into stratum with each strata representing the distance from the original reference clock.  For instance an atomic clock that generates a UTC (coordinated universal time) signal is referred to as a stratum 0 device.

A NTP server that receives a stratum  1 time signal is referred to as a stratum 1 device and a device that receives a time source from a NTP server is a stratum 2 device. NTP can support up to 16 strata although the further away from the reference clock you get (stratum 0) the less accurate the device will be.

However, by arranging the network into stratum and allowing stratum 2 devices to pass on the time to a stratum 3 device (and so on) it reduced the demand on the NTP server and the network. By using a stratum based network, realistically thousands of machines can be synchronised to just one NTP server.

Network Time Protocol (NTP) is an Internet based protocol designed to distribute and synchronise time across a network.

NTP is in fact one of the oldest Internet protocols having been developed in the late 1980’s at Delaware University when the Internet was still in its infancy. It was devised by Professor David Mills and his team when they realised the need for accurate time synchronisation if computers were needed to communicate with each other.

A NTP server is a dedicated device that receives a single timing source and then distributes it amongst all network devices. A NTP server will receive the timing information through a number of ways but normally it is a UTC source (coordinated universal time) a global timescale based on the time as told by atomic clocks.

NTP handles the time in a different way to how humans perceive and deal with it. While we may split a time into seconds, minutes, hours, days, months and years; NTP regards time  as a single number which is the number of seconds since the ‘prime epoch’.

The prime epoch is a date set for when NTP began counting seconds. For NTP the prime Epoch is 00.01 on 1 January 1900 so that means on 1 January 2008 the time according to NTP will be 3405888000, which is the number of seconds since 1900.

Time synchronisation is vital for the modern computer network particularly when computer networks across the globe need to communicate with each other.

A lack of synchronisation would make impossible many online activities such as Internet auctions, seat reservation and trading in stocks and shares. It can also leave a system open to security threats and even fraud.

The NTP server (Network Time Protocol) can provide the most secure and accurate method of synchronising a network.  Many NTP servers are rack-mountable devices that can connect to a network and distributes time information between all devices on that system.

They work by using a single time reference, most commonly a source of UTC (Coordinated Universal Time), which NTP then checks all the system clocks to ensure all devices are keeping the same time. When it finds a computer or device that is drifting it advances or retreats the system clock until it matches UTC.

A NTP server will receive a timing source from either across the Internet (although not very secure or accurate), a specialist long-wave radio transmission or from the GPS network (global positioning system).

By utilising dedicated NTP server, not only can all devices on a network be synchronised together but also by using UTC the network will be synchronised with millions of computer networks all over the world.

Using a long wave time and frequency transmission is perhaps the simplest and most efficient way of receiving an accurate and secure UTC timing reference (coordinated universal time). Dedicated NTP servers are available that receive a time code this way and distribute the timing information to a network. Often these time servers are referred to as radio clocks, although this title is a little misleading.

The long wave transmissions are usually broadcast at 60 khz but are not available everywhere. Only certain countries have these broadcasts and most come from their country of origin’s national physics laboratory.

In the UK the signal is known as MSF as is broadcast by the NPL (National Physical Laboratory) in Cumbria. The USA signal, WWVB, is broadcast Near Fort Collins in Colorado while the signal in Germany is known as DCF and is broadcast near to Frankfurt. Other nations such as Switzerland, Japan and Finland also have their own signals.

These transmissions are not however, available everywhere. While in many neighbouring countries it is possible to receive one of these transmissions, the long-wave signal is finite in range and susceptible to interference from topography and other electrical devices

However, where they are available, these time and frequency signals make an ideal source for a NTP server to synchronise a network too making them a logical choice for securing UTC time.

The NTP server is now an essential part of the modern computer network. Without a dedicated NTP server administrators are forced to rely on unsecure and inaccurate Internet sources to synchronise their network clocks too.

The potential risks involved in this, namely leaving a hole open in the network firewall and the lack of the NTP security measure: authentication, means that networks that use an Internet based timing source are risking their system to attacks from malicious user and hackers.

It should also be noted that a survey of Internet based timing sources found less than a third were accurate to UTC time and those that were could still be too far away from client to make any useful synchronisation.

There are two types of dedicated NTP server, the GPS NTP server and the radio referenced NTP server. The difference between the two is based solely on the method they receive their UTC time source from. A GPS NTP server will use the signals broadcast from the GPS satellites above the Earth’s atmosphere. These signals are very reliable and can be picked up anywhere in the Worlds as long as the GPS antenna has a clear view of the sky.

The alternative is to use a dedicated NTP server that can receive a signal from the national time and frequency transmissions broadcast by several national physics laboratories. While not available in every country and quite vulnerable to interference these long-wave time signals are still an accurate and secure method of receiving UTC time. They are also ideally suited for network administrators who, for reasons of logistics can’t place a GPS antenna on the roof.

A NTP server is really just a time server that utilises Network Time Protocol (NTP). Whilst other time protocols do exist, NTP is by far the most commonly used and is utilised in over ninety percent of time servers.

NTP server and time server are therefore interchangeable terms but describe the same thing: a device used to receive and distribute a timing signal.

The timing signal utilised by most NTP servers is a UTC time source. UTC (Coordinated Universal Time) is a global time scale based on the time told by atomic clocks. By utilising UTC a NTP server can in affect, synchronise a network to the same time as millions of other computer networks from around the world. This has made possible many online global transactions that just simply wouldn’t be possible without UTC.

The timing signal is received by the NTP server (or time server) via a number of ways; the Internet, national time and frequency transmission (long wave) or the GPS (global positioning system) network. Once received the time server (NTP server) checks the authenticity of this signal (except from Internet sources where authentication is not possible), evaluates its accuracy then distributes it amongst the network.

To prevent a possible overload of time requests to the time server, machines that receive a time signal from the NTP server, can themselves be used as a time reference and the machines that receive a time signal from those can again be used as a reference. This hierarchy is called stratum levels. A NTP server is a stratum 1 device, a machine that receives a signal directly from the time server is a stratum 2 device and if a machine receives a signal from that it becomes stratum 3.

The NTP server is an essential network tool. Whilst other protocols do exist, NTP is by far the standard time synchronisation protocol and is utilised in the majority of time servers.

A NTP server is reliant on a single time source it is this time reference that it uses to distribute amongst the network and synchronise to. This timing reference tends to be a UTC time source (coordinated universal time) which is a global time source based on the time told by atomic clocks.

There are only two viable options for receiving a UTC timing source. Although the Internet can be used, the signal can’t be authenticated this is a security measure used by NTP to ensure the reference is what it says it is. Also by using an Internet time source a hole must be left open in the network firewall to allow for communication to the server, this has its own security risks.

The only two secure methods for receiving a UTC time signal is to either use the GPS network or national time and frequency transmissions that are broadcast by several countries’ national physics laboratories.

In selecting a timing source for a NTP server, location is the key consideration. The national time and frequency transmissions are not available in every country. Whilst the USA, UK, Germany, France, Japan and Finland have a signal there are many countries that do not. Furthermore being a long wave radio transmission it can easily be blocked by local topography, although the radio aerial can pick op a signal indoors which is something a GPS NTP server can’t do.

GPS antennas have to be situated on a roof. This can have logistical problems if the server room is in the basement of a high storey building but on the plus inside the GPS signal can be received literally anywhere in the world.

Network Time Protocol (NTP) is one of the Internet’s oldest protocols still in use. Developed by Dr David Mills from the University of Delaware, it has been in constant use and continually updated since 1985. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local or Wider Area Networks (LANs/WANS).

In a modern global economy time synchronisation is essential for carrying out time sensitive transactions such as booking an airline ticket to bidding on an Internet auction site. If clocks were not synchronised to the same time you may find your airline seat sold after you had bought it and Ebay’s administrators would not be able to discover whose bid was the latest.

NTP is a multi-tiered system, each tier being called a stratum. Servers at each tier communicate with each other (peer) and provide time to lower strata. Servers at the top stratum, stratum 1 connect to an atomic clock either over the Internet or by a radio or GPS receiver while a stratum 2 server will connect to a stratum 1.

NTP uses an algorithm (Marzullo’s algorithm) to synchronise time on a network using time scales like UTC (Coordinated Universal Time or Temps Universel Coordonné) and can support such features as leap seconds – added to compensate for the slowing of the Earth’s rotation.

NTP (version 4 being the latest) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second) and can perform even better over LANs with accuracies of 200 microseconds (1/5000th of a second) under ideal conditions.

NTP time servers work within the TCP/IP suite and rely on UDP (User Datagram Protocol). A less complex form of NTP called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP, is used in some devices and applications where high accuracy timing is not as important and is also included as standard in Windows software (although more recent versions of Microsoft Windows have the full NTP installed and the source code is free and readily available on the Internet).

The NTP program (known as a daemon on UNIX and a service on Windows) runs in the background and refuses to believe the time it is told until several exchanges have taken place, each passing a set of tests. If the replies from a server satisfy these ‘protocol specifications’, the server is accepted. It usually takes about five good samples (five minutes) until a NTP server is accepted as a source for synchronisation.

Synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source such as an atomic clock, although these are extremely expensive and are generally only to be found in large-scale physics laboratories, however NTP can use either the Global Positioning system (GPS) network or specialist radio transmission to receive UTC time from these clocks.

A simplified version of NTP called Simple Network Time Protocol (SNTP) exists that does not require the storing of information about previous communications as required by NTP. It is used in some devices and applications where high accuracy timing is not as important and is installed on older versions of Microsoft Windows. Windows since 2000 has included the Windows Time Service (w32time.exe) which uses SNTP to synchronise the computer clock. NTP is also available on UNIX and LINUX (download via NTP.org).

Computer networking is one of the most difficult aspects of information and communications technology (ICT). The logistics of connecting terminals, routers, printers and all the other devices can leave many administrators with a constant headache.
One of the most important aspects that often gets overlooked and can have disastrous consequences is that of time synchronization.

It is imperative that all devices on a network are telling the same time as timestamps, the format a computer relays time to each other, are the only form of reference a computer can use to establish a sequence of events. If different machines on a network are telling different times then unforeseen consequences such as emails arriving before they have technically been sent and other anomalies will make the administrator’s headache even worse.

What’s more a computer network that is not synchronized is open to security threats and even fraud. Fortunately the NTP time server has been around for many years and can ease the headache of time synchronization .

NTP (Network Time Protocol) is one of the oldest protocols used by computer networks. Developed nearly three decades ago NTP is a protocol that checks the time on all devices on network and adds or subtracts enough time to ensure they are all synchronized.

NTP requires a time reference to synchronise the network’s clocks to. Whilst NTP can synchronize a network to any time an authoritative time source is obviously the best solution. UTC (Coordinated Universal Time) is a globally used timescale based on the time told by atomic clocks. As atomic clocks lose less than a second of time in over a thousand years, UTC is by far the best timing source to synchronize a network to. Not only will your network be perfectly synchronized together but also your network will be synchronized to the same time as millions of computer networks all from around the world.

A NTP server can receive a UTC time reference from several sources. The Internet is the most obvious source, however Internet timing sources are notoriously inaccurate and those that are not can be relatively useless if the distance is too far away. Also having placed your NTP server securely behind your firewall it does seem pointless to have to keep a hole open in it to allow the NTP server to poll the timing reference from across the web and leave the entire network vulnerable, particularly as NTP authentication (NTP’s own security measure) is not possible over the Internet.

There are two far more secure and accurate methods of receiving a UTC timing reference. The first is to utilise the national time and frequency transmissions that several countries broadcast from their national physics laboratories. These are usually broadcast via long wave which has an advantage of being able to be picked up inside a server room although many countries do not have such a signal.

However, many NTP servers can utilize the timing signal broadcast by the onboard atomic clocks of the GPS (Global Positioning System) satellites.  This signal is available everywhere but a GPS antenna is required that can get a clear view of the sky.

By utilizing a UTC timing source either through the GPS network of radio transmission a computer network can be synchronized to within a few milliseconds of UTC time.

Many network administrators boast that there networks are perfectly synchronised because they have an atomic clock as an NTP server. In actual fact as atomic clocks cost several millions of pounds and are quite vast in size it is doubtful the average server room contains such a timepiece.

What in fact they are referring to is that they have an NTP server that receives a timing source from an atomic clock. However, just because atomic clocks are the most accurate chronometers in the world, accurate to a few nanoseconds (billionth of a second) it doesn’t necessarily mean that a network using one as a timing source is receiving the same sort of accuracy

Atomic clocks work on the principle that certain atoms (in most atomic clocks the caesium -133 atom) oscillates at an exact frequency at certain energy levels. In the case of the caesium atom it resonates at exactly 9,192,631,770 every second.  Because of this exact resonance, atomic clocks lose less than a second in millions of years. In fact, the resonance of the caesium atom is so precise that the International System of Units has defined the second as exactly that number of oscillations of the caesium atom.

NTP servers can receive the time from an atomic clock through several sources. Obviously the Internet contains thousands of timing servers, some of which are hooked up to an atomic clock, others however, can be over ten seconds out of sync.

Furthermore, using an Internet timing source can leave a system open to abuse as the timing references cannot be authenticated. Also, the distance from a host, client and server can make dramatic differences in the accuracy.

The most accurate and effective way of receiving a timing source from an atomic clock is to use the national time and frequency broadcast that several country’s national physics laboratories transmit. Alternatively the American GPS (Global Positioning System) transmits the time from its own satellite’s atomic clocks. both methods can provide perfect synchronisation and accuracy to within a few milliseconds.

All computers are prone to drift and as accurate timing is essential for many time critical applications, Network Time Protocol has been developed to keep computers synchronized

NTP is installed on most versions of Windows (although a stripped down version called SNTP is in older versions) and Linux but regardless is open source an free to download from ntp.org.

To synchronise a network it is preferable to use a dedicated NTP server that receives a timing source from an atomic clock either from specialist national radio transmissions or the US GPS system, although Internet time references are available but some are more reliable than others (and none can be authenticated leaving a system open to attack).

NTP is hierarchical, it is arranged into stratum. Stratum 0 is a timing source (such as an atomic clock) while stratum 1 is a server connected to a stratum 0 server and a stratum 2 is a computer (or device) attached to a stratum 1 server.

There is an understanding that if using a public Internet based time server, stratum 0 servers are not used by most applications as too many requests would disable them. Instead NTP should be configured to receive a timing reference from several stratum 1 and stratum 2 servers (it is good housekeeping to use more than one as it is possible one server could go down).

The most accurate and secure way of synchronising a network is to use a dedicated NTP server. These can receive a timing reference from either the GPS network (as each GPS satellite contains an atomic clock and broadcasts the signal) or a specialist national radio transmission. Both of these signals come from a stratum 0 source and both provide accuracy to within a few milliseconds.

Network Time Protocol (NTP) has been around for nearly 25 years. It is one of the Internet’s oldest protocols and is still widely used and under constant development.

NTP was developed and designed in 1985 by Professor David Mills from the University of Delaware in the United States. Its success owes much to the fact that it was one of the first ever protocols on the Internet and was first used when the World Wide Web was in its infancy.

The importance of NTP on modern computer networks cannot be stressed highly enough, without NTP and NTP servers many of the applications and processes that we conduct over the Internet and now take for granted

Internet auction sites, email and global trading all require accurate time synchronisation. Just imagine booking an airline ticket only to discover your seat was resold after you had purchased it because the buyer had a slower clock on their computer?

Confusion and problems such as these would be commonplace without NTP and NTP servers, just think of the hysteria around the millennium bug!

NTP servers allow not only computers on a particular network to be perfectly synchronised but as most NTP servers are set to receive time from a UTC time source, computers around the entire globe can be synchronised together. UTC or Coordinated Universal Time is a global time scale based on the time told by atomic clocks.

NTP servers can receive a timing reference from the Internet, although this is fairly inaccurate, or from dedicated time and frequency radio signals or the GPS network.

Currently an NTP server receiving an authoritative timing source can provide accuracy over the Internet to within a few hundred nanoseconds (a nanosecond is 1 second every billion years.)

An NTP server (network time protocol) is a device to ensure all machines on a computer network are running the exact same time. Without an NTP server time would be inconsistent between devices which could problems for the network, seconds could be lost here or gained there which could cause major confusion as well as leaving your network vulnerable.

Time, in the form of timestamps, provides the only frame of reference between all devices on a network and the way an NTP server works is pretty straightforward. The timestamp relayed to the server is in the form of an ever increasing number that started from a set point in time, this is known as the prime epoch and for most systems this started on 1 January, 1900.

The NTP server checks the time stamp from an authoritative source, normally a UTC source (Coordinated Universal Time, a global timescale based on the time told by atomic clocks), from either the Internet, a radio transmission or via the GPS network.

The NTP server uses the timestamp to calculate if the network clocks are drifting and adds or subtracts a second to match the reference clock. The NTP server will do this at set intervals, normally every fifteen minutes to ensure perfect accuracy.

NTP is accurate to within 1/100th of a second (10 milliseconds) over the public Internet and can perform even better over LANs and WANS with accuracies of 1/5000th of a second (200 microseconds) not unheard of.

To ensure further accuracy the NTP service (or daemon on Linux) which runs in the background and does not believe the time it is told until after several exchanges and each one has passed a protocol specification (a test), the server is then considered. It usually takes about five good samples) until a NTP server is accepted as a timing source.

Time is essential for modern computer networks, as the majority of applications and processes conducted by a PC are  reliant on a timestamp, from sending an email, debugging a server to preventing fraud, therefore, accurate time and synchronisation is vital.

NTP (Network Time Protocol) is an Internet protocol designed for the synchronisation of computer networks. NTP servers are used by millions of system administrators to ensure their networks are keeping accurate time.

However, just as a computer is only as a good as the software it is running, a NTP server is only as good as the timing source it receives.

Despite the hundreds and probably thousands of possible timing references on the Internet, administrators should be aware of some of the possible pitfalls in selecting a timing reference.

A survey conducted by the Massachusetts Institute of Technology (MIT) discovered that not only were half of Internet timing sources inaccurate by over a ten seconds (a lifetime if we are attempting millisecond accuracy) but many were too far away to be provide any useful accuracy.

If an Internet timing source is to be used then not only should the accuracy of the server be checked but also the closest host should be selected to ensure the best accuracy.

Another consideration in using an Internet timing source is to be aware that they cannot be authenticated which means that your system could be vulnerable to malicious attacks and it is recommended by Microsoft and Novell that an external hardware source should be used.

The most secure and accurate method of receiving a timing source is to use a dedicated NTP server that can receive either a national time and frequency transmission (such as WWVB in the US or MSF in the UK). Alternatively a timing source can be received from the GPS network (Global Positioning System), both methods are authenticated and can provide millisecond accuracy.