Installing and Administering Internet Services

Chapter 7 289
Configuring the Network Time Protocol (NTP)
Getting Started with NTP
You will need to evaluate these potential time servers (and the network
paths) to decide if they are close enough (ping time, delay and variation)
and well configured before you use them. Some time servers may also
require notification before you use them, so pay attention to the
ettiquitte of the listings at UDelaware. Do not point more than three of
your machines at any one public time server. Use that small group of
your machines (at stratum-2 or stratum-3) as the main time servers for
the rest of your organization. For more information about stratum levels,
see the section “Stratum Levels and Time Server Hierarchy” on page
298.
The public stratum-2 servers can provide good timeservice for almost
anybody. Also, their access policies are less restrictive than the
stratum-1 servers. The quality of the network service between your
machine and the public time server (or your ISP) dominates the errors
you will see. This makes the distinction between stratum-1 and
stratum-2 almost meaningless for most purposes.
Dispersion is a measurement of time server quality plus network quality.
In reality, the network quality swamps everything else. If your network
is slow or overloaded, then dispersion will be high no matter how good
the time servers themselves are. NTP may be your first experience with
an application that is actually sensitive to network service quality.
Other applications (FTP, DNS, NFS, sendmail) can tolerate huge delays
in packet delivery because their data is not time-critical.
But NTP is different. Delays are deadly for your time service. Delays
immediately show up in the dispersion figures. If you care about
milliseconds, you must vigorously pursue your dispersion measurements
and pay attention to network service quality. If you care about
microseconds, you must abandon the network time servers and purchase
a radio clock for each NTP client.
You can evaluate different public time servers from the stratum-2 list.
First is a machine that HP is providing in Silicon Valley for public use in
North America. This machine was recently upgraded from stratum-2 to
stratum-1 with a new GPS receiver, but the lists at UDelaware might not
have been updated yet.