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Introduction to NTP

NTP (Network Time Protocol) provides accurate and syncronised time across the Internet. This introductory article will try to show you how to use NTP to control and synchronize your system clock.

NTP is organised in a hierarchical client-server model. In the top of this hierarchy there are a small number of machines known as reference clocks. A reference clock is known as stratum 0 and is typically a cesium clock or a Global Positioning System (GPS) that receives time from satellites. Attached to these machines there are the so-called stratum 1 servers (that is, stratum 0 clients), which are the top level time servers available to the Internet, that is, they are the best NTP servers available.

Following this hierarchy, the next level in the structure are the stratum 2 servers which in turn are the clients for stratum 1 servers. The lowest level of the hierarchy is made up by stratum 16 servers. Generally speaking, every server syncronized with a stratum n server is termed as being at stratum n+1 level. So, there are a few stratum 1 servers which are referenced by stratum 2 servers, wich in turn are refenced by stratum 3 servers, which are referenced by stratum 4 and so on.

NTP servers operating in the same stratum may be associated with others in a peer to peer basis, so they may decide who has the higher quality of time and then can synchronise to the most accurate.

In addition to the client-server model and the peer to peer model, a server may broadcast time to a broadcast or multicast IP addresses and clients may be configured to synchronise to these broadcast time signals.

Basic features of NTP

  • NTP needs some reference clock that defines the true time to operate. All clocks are set towards that true time. (It will not just make all systems agree on some time, but will make them agree upon the true time as defined by some standard.)
  • NTP uses UTC as reference time
  • NTP is a fault-tolerant protocol that will automatically select the best of several available time sources to synchronize to. Multiple candidates can be combined to minimize the accumulated error. Temporarily or permanently insane time sources will be detected and avoided.
  • NTP is highly scalable: A synchronization network may consist of several reference clocks. Each node of such a network can exchange time information either bidirectional or unidirectional. Propagating time from one node to another forms a hierarchical graph with reference clocks at the top.
  • Having available several time sources, NTP can select the best candidates to build its estimate of the current time. The protocol is highly accurate, using a resolution of less than a nanosecond (about 2^-32 seconds). (The popular protocol used by rdate and defined in [RFC 868] only uses a resolution of one second).
  • Even when a network connection is temporarily unavailable, NTP can use measurements from the past to estimate current time and error.

UTC (Universal Time Coordinated)

UTC (Universal Time Coordinated, Temps Universel Coordonné) is an official standard for the current time. UTC evolved from the former GMT (Greenwich Mean Time) that once was used to set the clocks on ships before they left for a long journey. Later GMT had been adopted as the world's standard time. One of the reasons that GMT had been replaced as official standard time was the fact that it was based on the mean solar time. Newer methods of time measurement showed that the mean solar time varied a lot by itself.The following list will explain the main components of UTC:

  • Universal means that the time can be used everywhere in the world, meaning that it is independent from time zones (i.e. it's not local time). To convert UTC to local time, one would have to add or subtract the local time zone.
  • Coordinated means that several institutions contribute their estimate of the current time, and UTC is built by combining these estimates.

Online ActiveSocket/NTP samples

Click here for online NTP samples for various development and scripting platforms, including VBScript, Visual Basic, Visual C++, ASP, ASP .NET, Visual Basic .NET and Visual C# .NET.