35.1 Introduction to NTP | HP Certified: HP-UX System Administration

The Network Time Protocol (NTP) package comes bundled with HP-UX. The xntpd daemon is responsible for synchronization of time. The daemon uses a configuration file, /etc/ntp.conf . The synchronization process depends on a reference time source. After the initial synchronization process, NTP keeps track of any drift in the local system clock compared with the reference time and continuously synchronizes itself. Depending on the accuracy of the local clock, the frequency of consulting the reference time source decreases with the passage of time.

NTP Time Sources

There are usually three types of time sources used by xntpd . These are listed in the order of their reliability.

The clock from a global positioning system (GPS) satellite or a radio broadcast. These are the most reliable sources and you need to attach additional hardware to your system to get GPS or a radio broadcast. An example of a radio broadcast is the National Institute of Standards and Technology (NIST) radio station WWVB that broadcasts time signals at 60 KHz. This radio station does not broadcast voice announcements.
Some NTP servers are available on the Internet. If your system is connected to the Internet, you may use time from one or more of these servers as reference time. A list of NTP servers is available at http://www.ntp.org. The NTP protocols make corrections in the time synchronization information received from any of these servers by making adjustments caused by data propagation delays on the Internet.
If your local network is not connected to the Internet, you may use one of the machines on your LAN as your timeserver and consider its clock as standard. This is the least accurate method.

Some Definitions

Before moving further in this chapter, let's go through some basic terminology used with NTP.

SERVER

Any host that provides time to other systems is a server. The server itself gets time from one of the reference sources mentioned earlier.

CLIENT

A client is a system that gets its time information from a server. A client may direct a polling client where it has information about a timeserver in its configuration file and polls that server. Some servers may also broadcast time information on the network, and a client may be configured to listen to these broadcasts and synchronize its time accordingly . This type of client is called a broadcast client. A client may also act as a server for some other clients .

PEER

Two or more systems may act as peers if they use time sources of equal reliability or stratum level. If the primary source of time for a server becomes unavailable, a server may synchronize its time with its peer servers. Peer servers also synchronize their time even in the case of availability of the reference time source. If there is a time difference among peer servers, NTP employs a special algorithm to reconcile peer server time.

STRATUM LEVEL

A stratum level is an indication of the accuracy of a reference time source. Stratum levels are numbered from 1 to 15. A reference at stratum level 1 is the most accurate time source, while a source at stratum level 15 is the least accurate time source. A server at stratum level 1 receives its time from an external accurate time source such as a GPS radio signal. If your network is connected to the Internet and you are getting time from a server at stratum level 1, you will assign a stratum level greater than 1 to your local time server.

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