What is NTP ?
The Network Time Protocol (NTP) is a protocol for distributing the Coordinated Universal Time (UTC) by means of synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. NTP uses UDP port 123 as its transport layer. It is designed particularly to resist the effects of variable latency by using a jitter buffer.
Network Time Server
There are hundreds of internet based timing sources that allow computers running NTP to synchronise to a UTC time - however, there are several drawbacks in relying on the Internet for a timing reference:
Security is paramount for most computer networks and NTP, one of the Internet's oldest protocols, is equipped with its own security measures in the form of authentication. Authentication verifies that each timestamp has come from the intended time reference by analysing a set of agreed encryption keys that are sent along with the timing information.
Unfortunately, internet time sources can not be authenticated and Microsoft, Novell and others "strongly recommend" that internet sources are not used for a timing reference as it could leave a network vulnerable and open to malicious hackers, viruses or even a DDoS attack (Distributed Denial of Service - where a server is inundated with traffic rendering it useless).
Also a survey by MIT (Massachusetts Institute of Technology) of over 900 internet time references, discovered nearly half were offset by over ten seconds (one by a staggering six years - but there were fortunately not many peers) and less that a third where described as being at all "useful".
The report also discovered that many internet time reference hosts were too far away from their peers to allow any accurate time synchronisation to take place.
Fortunately a dedicated NTP network time server can receive other sources of UTC time which do offer complete security, authentication and are far more accurate and reliable.
The first is the national time and frequency broadcasts transmitted by several countries. In the US the signal is referred to as WWVB and is broadcast by NIST (National Institute for Standards and Technology) in Colorado. In the UK the National Physical Laboratory (NPL) broadcasts the MSF signal from Cumbria and similar systems are broadcast in Germany (DCF-77), Japan (JJY) and France (TDF).
Another, equally accurate and secure method is to use the timing signals broadcast by the American GPS (Global Positioning System).
The GPS is currently the world's only Global Navigation Satellite System, although Europe's Galileo and the Russian GLONASS system are expected to be up and running over the next five years.
It is a consequence of needing accurate timing information to be able to pin-point positioning that GPS satellites all contain an atomic clocks and the signal that is broadcast can be received and used by a network time server.
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