Time Synchronisation

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.

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 network time 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.

Computer networks rely on timekeeping for nearly all their applications, from sending an email to saving data, a timestamp is necessary for computer to keep track. All routers and switches need to run at the same rate, out of sync devices can lead to data being lost and even entire connections.

All PC’s and networking devices use clocks to maintain an internal system time. These clocks, called Real Time Clock chips (RTC) provide time and date information. The chips are battery backed so that even during power outages, they can maintain time.

However, 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. However, these internal clocks are prone to drift and although for many application this can be quite adequate, often machines need to work together on a network and if the computers drift at different rates the computers will become out of sync with each other and problems can arise particularly with time sensitive transactions.

For some transactions it is necessary for computers to be perfectly synchronised, even a few seconds difference between machines can have serious effects, such as finding an airline ticket you had booked had been sold moments later to another customer or you could draw your savings out of a cash machine and when your account is empty you could quickly going to another machine and withdraw it all again.

Time servers are like other computer servers in the sense they are usually located on a network. A time server gathers timing information, usually from an external hardware source and then synchronises the network to that time.

Most time servers use NTP (Network Time Protocol) which 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 utilises an external timing reference and then synchronises all devices on the network to that time.

Often time servers are synchronised to a UTC (Coordinated Universal time) source which is the global standard time scale and allows computers all over the world to synchronised to exactly the same time. This has obvious importance in industries where exact timing is crucial such as the stock exchange or airline industry.

There are various sources that a time server can use as a timing reference. The Internet is an obvious source, however, internet timing references from the Internet such as nist.gov and windows.time can not be authenticated, leaving the time server and therefore the network vulnerable to security threats.

Just as your computer is only as good as the software it is running, a time server is only as useful as the timing source it uses. Time servers are similar to other servers in that they are located on a network but the primary function of a time server is to supply timing information rather than storing or routing data.

For computers a time stamp is the only point of a reference a computer can use to establish a sequence of events. Timestamps are used in everything from sending an email to debugging a system and ensuring your network is secure.

Nearly all time servers rely on NTP (Network Time Protocol) to synchronise devices once a timing source is selected Whilst it is not the only protocol designed for synchronisation it is by far the most widely used, primarily because it has been under constant development since its inception nearly three decades ago.

A NTP server works by receiving a time reference from an authoritative source, normally a UTC time source. UTC (Coordinated Universal Time) is an International timescale based on the time told by atomic clocks.

Unfortunately many administrators opt to use an Internet timing source to run their time server with. However, several studies of Internet timing references discovered that nearly two thirds were inaccurate by over several seconds (almost an eternity in the world of NTP) and those that weren’t were often too far away from a client to provide reliable accuracy.

Probably the most accurate and widely used method of retrieving a reliable time reference is to use a dedicated GPS Time Server, many national physics laboratories also broadcast the time told by their atomic clocks via long wave radio. These signals can be received by a time server if it is equipped with a radio receiver although not every country broadcasts one and the transmissions are susceptible to interference and local topography.

Once this secure UTC time reference is received the time server can then distribute the time across the network. The timestamp itself is represented as a single ascending number. The number is the amount of seconds from a prime epoch (start number) normally 01 January 1900. The time server checks the system clocks on its clients and advances or retracts the time until all machines on the network are synchronised.

As important as telling the time is for the smooth running of our daily lives, it is even more essential for computer networks. Everything from sending and receiving email to debugging a network is dependent on a timestamp. If timestamps on machines differ then all sorts of complications can arise from email arriving before it was sent to systems being left vulnerable to attacks and even fraud.

Computer networks use Network Time Protocol (NTP) to synchronise their networks to an authoritative time source. The ideal sources are those that relay UTC (Coordinated Universal Time). UTC was developed after the invention of atomic clocks and is a global timescale that allows computer networks and devices all over the world to synchronise to the exact same time.

There are different ways to receive UTC most commonly is from an Internet source. However, Internet timing sources vary in accuracy and are affected by distance and bandwidth problems. Also, Microsoft and others suggest they should not be used as they can’t be authenticated. This is a security measure used by NTP to ensure that a servers is what it is supposed to be.

Fortunately a dedicated time server can receive a UTC source from either a specialist radio broadcast (available in certain countries only – including France, Germany, UK, USA and Japan) or the GPS network (available anywhere on Earth).

Not only can a dedicated time server provide accuracy to within a few milliseconds of UTC it can also be authenticated by NTP ensuring security.

Dedicated time servers are relatively inexpensive and simple to install and can allow networks of hundreds and even thousands to be synchronised to UTC.