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Stratum

NTP uses a hierarchical system of clock strata, and every NTP server is classified in a stratum, which is a synonym for parallel layers, the plural is strata. This indicates the distance of the server in an NTP network from an external time source, which may be an atomic clock, GPS or time signal receiver. A Stratum 1 server has an external source, a Stratum 2 computer uses a Stratum 1 server as the time reference, and so on. The highest stratum level is 16; a Stratum 16 server has not yet synchronized with other NTP servers. Generally only four strata are used.

NTP servers at the same level can synchronise with each other in case the connection to the higher stratum fails. They can also query the time from different higher servers.

For example, the National Physical Laboratory operates two publically accessible stratum 2 NTP servers. These are synchronised through two other servers that are each themselves synchronised to the NPL's atomic clocks, via their one pulse-per-second signals. These atomic clocks are the standard reference in the UK – the UK national time scale UTC(NPL). These two public servers are: ntp1.npl.co.uk (IP: 139.143.5.30) and ntp2.npl.co.uk (IP: 139.143.5.31). A user guide – PDF file – is available that describes how to access the system from an internet connected PC. Unlike the telephone time service mentioned earlier, which is only available from within the UK, this NTP service from the NPL is accessible worldwide – no premium phone connection is required.

The time inaccuracy increases from stratum to stratum due to the server's internal processes. The extent of the inaccuracy depends on the server, its configuration, and the packet delays on the internet, but even if the time source of a Stratum 2 server is off by, for example, 50 milliseconds, it is still sufficiently accurate for private use.

What time is it?

It would be excessive to install a full NTP server on your private or work computer. However, a Stratum 2 server provides sufficiently accurate time for your client program via SNTP (Simple NTP, RFC 4330) when booting up or when queried at certain intervals. These tools are available for almost all operating systems.

It is possible that the selected time server could possibly be shut down or be renamed at some point in the future; therefore it is a good idea to check the accessibility of the selected time sources from time to time with your program and, if necessary, search for alternatives. The client should also always be configured with the server name and not the IP address, because the IP can change but the name usually stays the same.

If you just need the correct time for your private PC or a DSL router, you can use the NTP pool, where volunteers register the addresses of publicly accessible NTP servers. Every time you query pool.ntp.org, the DNS server routes you to another address to distribute the traffic. A prefix before the domain name provides several different sources, such as 0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org; but that means you can be assigned to NTP servers all over the world. If you need the best possible synchronicity with your own region, you can add a continent or country code, such as 0/1/2.europe.pool.ntp.org or 0/1/2.uk.pool.ntp.org. Approximately 85 were active in the UK in May 2008; at the same time, Germany held the record in Europe with 240 – which is about one in seven of all the NTP pool servers in the world.

The PTB operates two Stratum 1 servers as the source of the official time in Germany. Anyone who wants to set up their own NTP server for a company intranet can use them once they have notified the PTB via email. There are similar servers operated by public institutions in other European countries, such as the Swiss Bundesamt für Metrologie or the Parisian Observatorium. Some internet service providers also operate NTP server that are publicly accessible. You need to enter at least three sources in the configuration of your own NTP server.

You need to obey each source's stipulated terms of use when selecting a Stratum 1 or Stratum 2 server. Many servers only support a single country or continent, while others only support clients in a specific network or service area. Access to the servers – the access policy – is also not always public. Usually all you need to do is send an email to the server's operator – notify. It is generally a good idea to select an NTP server that is close to you, because it decreases packet delays. A list of NTP servers in the UK is available from TimeTools, a manufacturer of NTP servers.

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