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Network Time Protocol (NTP)

The Network Time Protocol (NTP) keeps a computer's clock synchronised to the correct time. The UIS provides an NTP service that aims to provide millisecond-accurate time to systems within the University.

There are two ways to use the service: via NTP and SNTP - they are described below.

NTP Service

The NTP service is intended for use by clients which run a full implementation of the NTP protocol, including calculating network delays and jitter.

There are four NTP servers, resolvable by DNS hostname at  ntp0.cam.ac.ukntp1.cam.ac.ukntp2.cam.ac.uk and ntp3.cam.ac.uk.  The IP addresses these resolve to may change occasionally and must be re-resolved by DNS and updated periodically, at least once per year (the previous IP addresses should continue to work for some time, even after they cease to be advertised).

The individual hosts providing this service are not redundant and may be unavailable for short periods of time: redundancy is achieved through the client talking to multiple servers.  Clients must be configured with a minimum of three of the above hosts.

Note that the old ntpX.csx.cam.ac.uk names are still present the DNS and there are currently no plans to remove them.  You should, however, reconfigure clients to use the new names (directly in cam.ac.uk), when the opportunity arises, as they will likely be removed in the future (albeit with advance warning).

SNTP Service

Clients which do not implement the full NTP protocol and just retrieve the time from the first available server, without calculating delays and jitter, should use the SNTP (Simple Network Time Protocol) service.

The SNTP service is provided at a single host, resolvable by DNS hostname  sntp.cam.ac.uk.  As with the NTP service hostnames, the addresses this resolves to may change occasionally so must be re-resolved periodically, at least once per year.

This address is redundant and will fail over to another host, if the normal host providing it becomes unavailable, so clients should only need configuring with the single server.  Clients which do not support being configured with three or more servers should use this service instead of the full NTP service.

Last modified: 14th January 2021

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