Time Synchronisation

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).

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

NTP (version 4) 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 works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

Time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source. This source could be relative (a computer’s internal clock or the time on a wrist-watch) or absolute (A UTC – Universal Coordinated Time – clock source that is accurate as is humanely possible).

Atomic clocks are the most absolute time-keeping devices. They work on the principle that the atom, caesium-133, has an exact number of cycles of radiation every second (9,192,631,770). This has proved so accurate the International System of Units (SI) has now defined the second as the duration of 9,192,631,770 cycles of radiation of the caesium-133 atom.

However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP can synchronise networks to an atomic clock by using either the Global Positioning System (GPS) or a specialist radio transmission.

The most widely used is the GPS system which consists of a number of satellites providing accurate positioning and location information. Each GPS satellite can only do this by utilising an atomic clock which in turn can be can be used as a timing reference.

A typical GPS receiver can provide timing information to within a few nanoseconds of UTC as long as there is an antenna situated with a good view of the sky.

There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

The distance from the reference clock is known as the stratum levels and they exist to prevent cycles in the NTP. Stratum 0, are devices such as atomic clocks connected directly to a computer. Stratum 1, are computers attached to stratum 0 devices, while Stratum 2 are computers that send NTP requests to Stratum 1 servers. NTP can support up to 256 strata.

All Microsoft Windows versions since 2000 include the Windows Time Service (w32time.exe) which has the ability to synchronise the computer clock to an NTP server (or an SNTP server – a simplified version of NTP) Many LINUX and UNIX based operating systems also have a version of NTP but the source code is free to download (current version 4.2.4) at the NTP website (NTP.org).

It is strongly recommended by Microsoft and others, that external based timing should be used rather than Internet based, as these can’t be authenticated. Specialist NTP time servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS signal.

Summary: This article gives a step-by-step guide in configuring LINUX to act as an authoritative time server using NTP (Network Time Protocol).

Computer time synchronisation is highly important in modern computer networks, precision and time synchronization is critical in many applications, particularly time sensitive transactions. Just imagine buying an airline seat only to be told at the airport that the ticket was sold twice because it was purchased afterwards on a computer that had a slower clock!

Modern computers do have internal clocks called Real Time Clock chips (RTC) that provide time and date information. These chips are battery backed so that even during power outages, they can maintain time but personal computers are not designed to be perfect clocks. Their design has been optimized for mass production and low-cost rather than maintaining accurate time.

For many applications, this is can be quite adequate, although, quite often machines need time to be synchronised with other PC’s on a network and when computers are out of sync with each other problems can arise such as sharing network files or in some environments even fraud!

Network Time Protocol (NTP) is an Internet protocol used for the transfer of accurate time, providing time information along so that a precise time can be obtained. As NTP was originally written for LINUX many LINUX based operating systems already have a version of NTP installed. However the source code is free to download from the NTP website (NTP.org) the most recent version being v 4.2.4.

NTP (version 4) 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 works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

The NTP background program is configured with the file ‘NTP.conf’. this may contain a list of public NTP server references that can be used to synchronise time. NTP time servers are specified using the ’server’ command, any characters after the ‘#’ symbol are comments:

Example
server time-a.nist.gov # Public NTP server: Maryland
When configured, NTP can be controlled using the commands ‘ntpd start’ ‘ntpd stop’ ‘ ntpq –p’ (displays status)

NTP can also authenticate timing resources Note: It is strongly recommends that you configure a time server with a hardware source rather than from the internet where there is no authentication. Authentication codes are specified in the ‘NTP.keys’ file.

Specialist NTP servers are available that can receive transmissions from either GPS or national time reference broadcasts. They are relatively cheap and the signal is authenticated providing a secure time reference.

Authentication for NTP has been developed to prevent malicious tampering with system synchronisation just as firewalls have been developed to protect networks from attack but as with any system of security it only works if it is utilised.

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

NTP (version 4) 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 works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

Time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source. This source could be relative (a computer’s internal clock or the time on a wrist-watch) or absolute (A UTC – Universal Coordinated Time – clock source that is accurate as is humanely possible).

Atomic clocks are the most absolute time-keeping devices; however, they are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP can synchronise networks to an atomic clock by using either the Global Positioning system (GPS) network, a specialist radio transmission or over the Internet. However, it must be noted that Microsoft strongly recommends that an external based timing should be used rather than Internet based, as these can’t be authenticated.

GPS is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.  Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at 1575 MHz, Low-cost GPS antennas and receivers are now widely available.

The signal transmitted by the satellite can pass through windows but can be blocked by buildings so the ideal location for a GPS antenna is on a rooftop with a good view of the sky. The more satellites it can receive from the better the signal. However, roof-mounted antennas can be prone to lighting strikes or other voltage surges, so installation of a suppressor inline on the GPS cable is highly recommend.

The cable between the GPS antenna and receiver is also critical. The maximum distance that a cable can run is normally only 20-30 metres but a high quality coax cable combined with a GPS amplifier placed in-line to boost the gain of the antenna can allow in excess of 100 metre cable runs.

A GPS receiver then decodes the GPS signal sent from the antenna to a computer readable protocol which can be utilised by most time servers and operating systems including, Windows, LINUX and UNIX.

The GPS receiver also outputs a precise pulse every second that GPS Network Time Protocol (NTP) servers and computer time servers may utilise to provide ultra-precise timing. The pulse-per-second timing on most receivers is accurate to within 0.001 of a second of UTC.

GPS is ideal in providing NTP time servers or stand-alone computers with a highly accurate external reference for synchronisation. Even with relatively low cost equipment, accuracy of hundred nanoseconds (a nanosecond = a billionth of a second) can be reasonably achieved using GPS as an external reference.

An accurate time source is needed for many computer applications. Every Personal Computer comprises of an internal Clock, it is beneficial to check the date and time settings on your PC daily. For critical application, you should synchronise the time base with a highly accurate external time source.

Personal Computers are not designed to be perfect clocks. Their design has been optimised for mass production and low-cost rather than maintaining accurate time. Where time is crucial to the application, there are a number of precise external references available that enable computers to maintain accurate system time. This article looks at the various sources of time references to show how they can be utilised to maintain synchronised time on your computer.

Working to a synchronised time base is essential in computer networks. Without any external reference, individual computers will start to drift, anything from a few seconds to a few minutes each day. Clearly such a situation would not be acceptable when processing transactions or performing time critical tasks.

On the Internet this problem has been solved by introducing the Network Time Protocol (NTP). The NTP protocol supports the distribution of accurate time from a highly precise time server to network time clients. Most modern operating systems have the ability to synchronise time with an NTP server. Generally all that is required is the IP address or domain name of Stratum 1 or Stratum 2 NTP servers.

LINUX and UNIX operating systems can download the full NTP implementation from the NTP website at www.ntp.org NTP is freely available, open source software, available under the GNU public license.

Mirosoft Windows XP/2000/2003 and Vista system software use a standart SNTP client for Simple Network Time Protocol. This is based on a sub-set of the Network Time Protocol, using a simplified NTP algorithm with many of the more complex high precision routines removed.

The Windows operating systems provide facilities for an IP address or domain name of an Internet or Intranet NTP server to be entered in the time properties tab. The SNTP client will then contact the NTP server periodically in order to update and synchronise system time.

Alternative methods will be required for standalone computers and systems which do not have access to the Internet. These can be provided with a local access to national radio time references which are transmitted free-to-air.

All that is required is a small RS232 serial or USB radio receiver, and the PC can obtain continuous accurate time. The Computer Time synchronises to the received time and frequency radio source.

Radio broadcasts are identified by their ‘call sign’ The UK time transmitter call sign, MSF, is located at Anthorn, Cumbria. Similar arrangements exist in Noth America – call sign WWVB from Colarado. Germany is covered by DCF broadcast from Mineflingen, near Frankfurt. National broadcasts are also available in France, Switzerland, Japan and Canada.

The only shortcoming with national radio time and frequency solutions is that they have a finite transmission range. In general they are limited to geographic boundaries also. Such issues do not apply to the Global Position System (GPS) a satellite based universal navigation system.

Each GPS satellite carries a highly accurate synchronised atomic clock. This enables GPS to provide precise timing information anywhere on the face of the planet. All that is required to receive the transmission is a low cost-cost GPS receiver and antenna with a clear view of the sky. PC connections are similar to the radio transmissions configuration, using a serial or USB port, enabling accurate timing information to be available continuously.

This article is about the Importance of network time synchronisation and how significant it is to have an accurate network time server resource in the business.

Every Computer has a clock that sustains the internal system time. PC clocks have low- cost crystal oscillators that are notoriously prone to drift. Each Computer can drift at different rates.

The time difference can become bigger and bigger over a period of time. This can lead to huge problems within the company. This article conveys the value of network time synchronization and having an accurate network time server source in an organization.

Computer Time: Computer system time is maintained by low cost real time clock and crystal oscillator components. Crystal oscillators are built with very low tolerance, each oscillates at a slightly different rate. The temperature variation make the crystal change its frequency. This is one of the reason why computers are weak in keeping up an accurate time. A Companies Computers need to be synchronized otherwise each computer will drift at a slightly different quantity.

Each computer need a network time that is correctly synchronized otherwise it can cause huge problems, especially in transaction-processing surroundings. Unsynchronized PCs can lead to fraud and other problems that can be caused in an environment where synchronized time is needed. Problems can arise with shared network files, the reason is because the unsynchronized computers are using the same resource.

Network Time Servers: Network Time servers gain precise time from an external time reference and offer (presents) an accurate time source to network time clients. Time server use the Network Time Protocol (NTP), which is a UDP based protection over TCP/IP. There is a number of options that can be used to get an accurate time. GPS (Global Positioning Systems) offers a highly accurate time; all that is needed is a clear view to the sky. Another alternative would be radio frequency. The main countries that provide radio frequency are England (MSF); Germany (DCF-77) and the US (WWVB). Other countries are France (TDF); Canada (CHU); Japan (JJY) and Switzerland (HBG) and it is possible to obtain time from mobile phone network.

Internet Time Servers: There are a lot of Internet based NTP time server resources offered that can be used for network time synchronization. The Internet Time Server can cause Problem for the Company’s Computers. The time server is located outside the firewall, so a port must be left open to access the time server. This does not just lead to Security implications, but also to problems with the time server accuracy, reliability and provision.

Local Dedicated Network Time Server: A local devoted network time server overcome many problems related to Internet based references. The time server is located inside the security firewall and does not need any external access. Also, a local time server can improve accuracy due to reliable network paths between clients and server.

To resume, the network time synchronization is an important factor in Computer network installations. Unlike the Internet based network time servers, which are known for they security problems, local dedicated network time servers overcome the security problems by offering secure, accurate and reliable network time synchronization resolution.