Installing and Administering Internet Services

210 Chapter 7
Configuring NTP
Overview
change due to server or network path availability, the stratum level of
your server can also change. The maximum stratum level that a server
can assume is 15.
Figure 7-1 depicts the organization of time servers into strata. The
arrows show the direction of time synchronization.
Figure 7-1 Hierarchy of NTP Time Servers
A time server processes a client’s request for time and immediately
returns a message to the client. From the returned message, the client
determines the server’s time, compares it to its local time, and then
adjusts its local clock. Instead of adjusting the local clock all at once
(which could cause the clock to be set backward), xntpd adjusts the clock
to its new value at small, constant increments over a period of time.
Note that a client of a time server can be another time server. Or a client
can be a workstation that is simply synchronizing its local system clock
to a time server but is not providing time to any other system.
NOTE /usr/sbin/ntpdate is a program that can be used to set the date and
time on a system by polling specified NTP servers. If precise timing is not
absolutely essential on your local host, ntpdate is an alternative to
running xntpd that consumes less memory than running the daemon.
The program must be run as root; typically, the ntpdate command is
included in the startup script or a cron script. See the ntpdate man
page for more information.
Stratum 1
Server
Stratum 2
Server
Stratum 2
Server
Stratum 3
Server
Stratum 3
Server
Stratum 3
Server
external
clock