Category: NTP Basics

NTP Server running a network (Part 2)

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Organising Strata

Stratum levels describe the distance between a device and the reference clock. For instance an atomic clock based in a physics laboratory or GPS satellite is a stratum 0 device. A stratum 1 device is a time server that receives time from a stratum 0 device so any dedicated NTP server is stratum 1. Devices that receive the time from the time server such as computers and routers are stratum 2 devices.

NTP can support up to 16 stratum levels and although there is a drop-off in accuracy the further away you go stratum levels are designed to allow huge networks to all receive a time from a single NTP server without causing network congestion or a blockage in the bandwidth.

When using a NTP server it is important to not overload the device with time requests so the network should be divided with a select number of machines taking requests from the NTP server (the NTP server manufacturer can recommend the number of requests it can handle). These stratum 2 devices can ten be used as time references for other devices (which become stratum 3 devices) on very large networks these can then be used as time references themselves.

NTP Server running a network (Part 1)

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NTP servers are a vital tool for any business that needs to communicate globally and securely. NTP servers distribute Coordinated Universal Time (UTC), the world’s global timescale based on the highly accurate time told by atomic clocks.

NTP (Network Time Protocol) is the protocol used to distribute the UTC time across a network it also ensures all time is accurate and stable. However, there are many pitfalls in setting up a NTP network, here are the most common:

Using the correct time source

Attaining the most suitable time source is fundamental in setting up a NTP network. The time source is going to be distributed amongst all machines and devices on a network so it is vital that it is not only accurate but also stable and secure.

Many system administrators cut corners with a time source. Some will decide to use an Internet based time source although these are not secure as the firewall will require an opening and also many internet sources are either wholly inaccurate or too far away to afford any useful precision.

There are two highly secure methods of receiving a UTC time source. The first is to utilise the GPS network which although doesn’t transmit UTC, GPS time is based on International atomic time and is therefore easy for NTP to convert. GPS time signals are also readily available all over the globe.

The second method is to use the long wave radio signals broadcast by some national physical laboratories. These signals, however, are not available in every country and they have a finite range and are susceptible to interference and local topography.

MSF Technical Information

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The MSF transmission from Anthorn (latitude 54° 55′ N, longitude 3° 15′ W) is the principal means of disseminating the UK national standards of time and frequency which are maintained by the National Physical Laboratory. The effective monopole radiated power is 15 kW and the antenna is substantially omnidirectional. The signal strength is greater than 10 mV/m at 100 km and greater than 100 μV/m at 1000 km from the transmitter. The signal is widely used in northern and western Europe. The carrier frequency is maintained at 60 kHz to within 2 parts in 1012.

Simple on-off carrier modulation is used, the rise and fall times of the carrier are determined by the combination of antenna and transmitter. The timing of these edges is governed by the seconds and minutes of Coordinated Universal Time (UTC), which is always within a second of Greenwich Mean Time (GMT). Every UTC second is marked by an ‘off’ preceded by at least 500 ms of carrier, and this second marker is transmitted with an accuracy better than ±1 ms.

The first second of the minute begins with a period of 500 ms with the carrier off, to serve as a minute marker. The other 59 (or, exceptionally, 60 or 58) seconds of the minute always begin with at least 100 ms ‘off’ and end with at least 700 ms of carrier. Seconds 01-16 carry information for the current minute about the difference (DUT1) between astronomical time and atomic time, and the remaining seconds convey the time and date code. The time and date code information is always given in terms of UK clock time and date, which is UTC in winter and UTC+1h when Summer Time is in effect, and it relates to the minute following that in which it is transmitted.

Dedicated MSF NTP Server devices are available that can connect directly to the MSF transmission.

Information Courtesy of NPL

Correcting Network Time

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Distributed networks rely completely on the correct time. Computers need timestamps to order events and when a collection of machines are working together it is imperative they run the same time.

Unfortunately modern PC’s are not designed to be perfect timekeepers. Their system clocks are simple electronic oscillators and are prone to drift. This is not normally a problem when the machines are working independently but when they are communicating across a network all sorts of problems can occur.

From emails arriving before they have been sent to entire system crashes, lack of synchronisation can causes untold problems across a network and it is for this reason that network time servers are used to ensure the entire network is synchronised together.

Network time servers come in two forms – The GPS time server and the radio referenced time server. GPS NTP servers use the time signal broadcast from GPS satellites. This is extremely accurate as it is generated by an atomic clock on board the GPS satellite. Radio referenced NTP servers use a long wave transmission broadcast by several national physics laboratories.

Both these methods are a good source of Coordinated Universal Time (UTC) the world’s global timescale. UTC is used by networks across the globe and synchronising to it allows computer networks to communicate confidently and partake of time sensitive transactions without error.

Some administrators use the Internet to receive a UTC time source. Whilst a dedicated network time server is not required to do this it does have security drawbacks in that a port is needed to be left open in the firewall for the computer to communicate with the NTP server, this can leave a system vulnerable and open to attack. Furthermore, Internet time sources are notoriously unreliable with many either too inaccurate or too far away to serve any useful purpose.

New Waterproof GPS Mushroom Antenna

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Galleon Systems’ new mushroom GPS antenna provide increased reliability in receiving GPS timing signals for NTP time servers.
The new Exactime 300 GPS Timing and Synchronization Receiver boasts waterproof protection, anti-UV, anti-acidity and anti-alkalinity properties to ensure reliable and continual communication with the GPS network.

The attractive white mushroom is smaller than conventional GPS antennas and sits just 77.5mm or 3.05-inch in height and is easily fitted and installed thanks to the inclusion of a full installation guide and CD manual.

Whilst an ideal unit for a GPS NTP time server this industry standard antenna is also ideal for all GPS receiving needs including: Marine Navigation, Control Vehicle Tracking and NTP synchronisation
The main features of the Exactime 300 mushroom antenna are:

• Built-in patch antenna • 12 parallel tracking channels • Fast TTFF (Time to first fix) and low power consumption • On-board, rechargeable battery sustained Real-Time Clock and control • parameters memory for fast satellite acquisition during power-up • Interference filter to major VHF channels of marine radar • WAAS compliant with EGNOS support • Perfect Static Drift for both of speed and course •  Magnetic Declination compensation • Is protected against reverse polarity voltage • Support RS-232 or RS-422 interface, Support 1 PPS output.

Utilising UTC

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To receive and distribute and authenticated UTC time source there are currently two types of NTP server, the GPS NTP server and the radio referenced NTP server. While both these systems distribute UTC in identical ways the way they receive the timing information differs.

A GPS NTP time server 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 radio 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 a suppressor is highly recommend being installed inline on the GPS cable.

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. This can provide difficulties in installation in larger buildings if the server is too far from the antenna.

An alternative solution is to use a radio referenced NTP time server. These rely on a number of national time and frequency radio transmissions that that broadcast UTC time. 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 the USA (WWVB) and in France, Germany and Japan.

A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. It should always be mounted horizontally at a right angle toward the transmission for optimum signal strength. Data is sent in pulses, 60 a second. These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

How a GPS Time Server Works

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A GPS time server is really a communication device. Its purpose is to receive a timing signal and then distribute it amongst all devices on a network. Time server s are often called different things from network time server, GPS time server, radio time server and NTP server.

Most time servers use the protocol NTP (Network Time Protocol). NTP is one of the Internet’s oldest protocols and is used by the majority of machines that use a time server. NTP is often installed, in a basic form, in most operating systems.

A GPS time server, as the names suggests, receives a timing signal from the GPS network. GPS satellites are really nothing more than orbiting clocks. Onboard each GPS satellite is an atomic clock. The ultra-precise time from this clock is what is transmitted from the satellite (along with the satellite’s position).

A satellite navigation system works by receiving the time signal from three or more satellites and by working out the position of the satellites and how long the signals took to arrive, it can triangulate a position.

A GPS time server needs even less information and only one satellite is required in order to receive a timing reference. A GPS time server’s antenna will receive a timing signal from one of the 33 orbiting satellites via line of sight, so the best place to fix the antenna is the roof.

Most dedicated GPS NTP time servers require a good 48 hours to locate and get a steady fix on a satellite but once they have it is rare for communication to be lost.

The time relayed by GPS satellites is known as GPS time and although it differs to the official global timescale UTC (Coordinated Universal Time) as they are both based on atomic time (TAI) GPS time is easily converted by NTP.

A GPS time server is often referred to as a stratum 1 NTP device, a stratum 2 device is a machine that receives the time from the GPS time server. Stratum 2 and stratum 3 devices can also be used as a time servers and in this way a single GPS time server can operate as a timing source for an unlimited amount of computers and devices as long as the hierarchy of NTP is followed.

Keeping Time with Network Time Protocol

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NTP (Network Time Protocol) is the most flexible, accurate and popular method of sending time over the Internet. It is perhaps the Internet’s oldest protocol having been around in one form or another since the mid 1980’s.

The main purpose of NTP is to ensure that all devices on a network are synchronised to the same time and to compensate for some network time delays. Across a LAN or WAN NTP manages to maintain an accuracy of a few milliseconds (Across the Internet, time transfer if far less accurate due to network traffic and distance).

NTP is by far the most widely used time synchronisation protocol (somewhere in the region of 95% of all time servers use NTP) and it owes much of its success to its continual updates and its flexibility. NTP will run on UNIX, LINUX, and Windows based operating systems (it is also free, another possible reason for its huge success).

NTP uses a single time source that it distributes among all devices on a network; it also checks each device for drift (the gaining or losing of time) and adjusts for each.  It is also hierarchical in that literally thousands of machines can be controlled using just one NTP server as each machine can in itself be used by neighbouring machines as a time server.

NTP is also highly secure (when using an external time reference not when using the Internet for a timing source) with an authentication protocol able to establish exactly where a timing source comes from.

For a network to be really effective most NTP time servers use an atomic clock as a basis for their time synchronisation. An international timescale based on the time told by atomic clocks has been developed for this very purpose. UTC (Coordinated Universal Time).

There are really two methods to receive a secure UTC atomic clock time signal to be utilised by NTP. The first being the time and frequency transmissions that several national physics laboratories broadcast on long wave around the world; the second (and by far the most readily available) is by using the timing information in the GPS satellite transmissions. These can be picked up anywhere on the globe and provide safe, secure and highly accurate timing information.

Importance of Preventing NTP Time Server Abuse

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NTP time server (Network Time Protocol) abuse is quite often unintentional and fortunately thanks to the NTP pool is less frequent than it was although incidents still happen.

NTP server abuse is any act that violates the access rules of a NTP time server or an act that damages it in any way. Public NTP servers are those servers that can be accessed from across the Internet by devices and routers to use as a timing source to synchronise a network to. Most public NTP time servers are non-profit and set up as acts of generosity, mostly by University’s or other technical centres.

For this reason access rules have to be set up as huge amounts of traffic can generate giant bandwidth bills and can lead to the NTP time server being turned off permanently. Access rules are used to prevent too much traffic from accessing stratum 1 servers, by convention stratum 1 servers should only be accessed by stratum 2 servers which in turn can pass the timing information on down the line.

However, the worst cases of NTP server abuse have been where thousands of devices have sent requests for time, where in the hierarchical nature of NTP only one is needed.

Whilst most acts of NTP abuse are intentional some of the worst abuses of NTP time servers have been committed (albeit unintentionally) by large companies. The first large firm discovered to have been guilty of NTP abuse was Netgear, who, in 2003 released four routers that were all hard coded to use the University of Wisconsin’s NTP server, the resulting DDS (Distributed Denial of Service) reached nearly 150 megabits a second.

Even now, five years on and despite the release of several patches to fix the problem and the University being compensated by Netgear the problem still continues as some people have never patched their routers.

Similar incidents have been committed by SMC and D-Link. D-Link in particular caused controversy as when the matter was drawn to their attention they decided to bring the lawyers in. Only after it was discovered that they violated nearly 50 NTP servers did they attempt resolve the problem (and only after scathing press coverage did they relent).

The easiest way to avoid such problems is to use a dedicated external stratum 1 time server. These devices are relatively inexpensive, simple to install and far more accurate and secure than online NTP servers. These devices receive the time from atomic clocks either from the GPS network (Global Positioning System) .

Common NTP Server Time Reference Problems

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The NTP server (Network Time Protocol) is one of the most used but least understood computer networking hardware items.

A NTP Server is just a time server that uses the protocol NTP. Other time protocols do exist but NTP is by far the most widely used. The terms ‘NTP server’, ‘time server’ and ‘network time server’ are interchangeable and often the terms ‘radio clock’ or ‘GPS time server’ are used but these simply describe the method which the time servers receive a time reference.

NTP servers receive a time source that they can then distribute amongst a network. NTP will check a devices system clock and advance or retreat the time depending on how much it has drifted. By regularly checking the system clock with the time server, NTP can ensure the device is synchronised.

The NTP server is a simple device to install and run. Most connect to a network via an Ethernet cable and the software included is easily configured. However, there are some common troubleshooting problems associated with NTP servers and in particular with receiving timing sources:

A dedicated NTP server will receive a time signal from various sources. The Internet is probably the most common sources of UTC time (Coordinated Universal Time), however, using the Internet as a timing source can be a cause for several time server problems.

Firstly Internet timing sources can’t be authenticated; authentication is NTP’s in-built security measure and ensures that a timing reference is coming from where it says it is. On a similar note to use an Internet timing source would mean that a gap would have to be created in the network firewall, this can obviously cause its own security issues.

Internet timing sources are also notoriously inaccurate. A survey by MIT (Massachusetts Institute of Technology) found less than a quarter of Internet timing sources were any where near accurate and often those that were, were too far away from clients to provide a reliable timing source.

The most common, secure and accurate method for receiving timing source is the GPS system (Global Positioning System). While a GPs signal can be received anywhere on the planet there are still common installation issues.

A GPS antenna has to have a good clear view of the sky; this is because the GPs satellite broadcast their signal by line of sight. He signal can not penetrate buildings and therefore the antenna has to be situated on the rood. Another common issue with a GPS time server is that they need to be left for at least 49 hours to ensure the GPS receiver gets a good satellite fix. Many users find that they are receiving an intermittent signal this is normally due to impatience and not letting the GPS system obtain a solid fix.

The other secure and reliable method for receiving a timing signal is the national radio transmissions. In the UK this is called MSF but similar systems exist in the US (WWVB), Germany (DCF) and several other countries. There are usually less problems faced when using the MSF/DCF/WWVB signal.

Although the radio signal can penetrate buildings it is susceptible to interference from topography and other electrical appliances.  Any issues with a MSF time server can normally be resolved by moving the server to another locale or often just angling the server so its ib-built antenna is perpendicular to the transmission.