Installing and Administering Internet Services

Chapter 7 209

Conﬁguring NTP

Overview

Many internetwork services and applications depend upon the system

clocks of the networked systems being synchronized with each other. For

example, network management systems need to be able to determine the

order in which events in an internetwork occur. Without clock

synchronization, the timestamps of networked or distributed ﬁle systems

(such as NFS, AFS, or DFS) may not reﬂect the actual time at which a

ﬁle was created. Software distribution programs like rdist depend upon

reliable timestamps to update software. NTP is a way to help

synchronize time in an internetwork so that these types of services or

applications operate properly.

An NTP synchronization subnet is a network of timekeeping systems,

called time servers. These time servers are a subset of the systems on a

network or an internetwork. Each time server synchronizes to Universal

Coordinated Time (also known by the acronym UTC). Each server

measures the time difference between its local system clock and the

system clocks of its neighbors in the subnet.

NTP Time Server Hierarchy

Time servers are organized into levels, or strata. Stratum-1 servers are

directly connected to an external time source. The external time source

can be a device such as a radio clock, which decodes UTC timecodes that

are broadcast from radio services in the United States, Canada, and in

some European countries.

NTP assumes that servers that are not stratum-1 servers have several

possible sources to which they can synchronize their time. NTP then

chooses a server to synchronize to, based on factors such as which server

is at the lowest-numbered stratum, and which is the closest in terms of

network delay. If the chosen synchronization source becomes unavailable

(due to failure of the server or in the network path), NTP automatically

selects a different source from the available servers.

The stratum level of your local NTP server is always one more than the

stratum level of the time server to which your server is synchronizing.

Thus, if your server is synchronizing to a stratum-1 server, your server is

a stratum-2 server. If your server is synchronizing to a stratum-2 server,

then it is a stratum-3 server. Because the synchronization source can