Installing and Administering Internet Services

Chapter 7 291
Configuring the Network Time Protocol (NTP)
Getting Started with NTP
server also has several good stratum-1 and stratum-2 servers which it
can fall back on if the GPS receiver stops working for any reason.
Notice the line for hpsdlo.sdd.hp.com which has delay, offset, and
dispersion measures that are markedly worse than any of the other
sources. The time server hpsdlo is good enough, but the network in
between has some problems, mainly evidenced by the large dispersion
figure. There is nothing that NTP can do to reduce the dispersion. NTP
is simply reporting to you what it sees out on the network. You must
complain to your network service provider if the dispersion numbers are
too high.
In summary, ntp-cup.external.hp.com is a well-configured time server
that is only 5 milliseconds away from my location (in California) on the
network. It would be a good choice for a public time server for my
location. Whether it is good for you depends on the "ping" round-trip
times at your location.
Example 2: Evaluating Time Servers in Eastern United States
Look at the time server located on the east coast of North America. Here
are the details:
ntp.ctr.columbia.edu (128.59.64.60)
Location: Columbia University Center for Telecommunications
Research; NYC
Synchronization: NTP secondary (stratum 2), Sun/Unix
Service Area: Sprintlink/NYSERnet
Access Policy: open access, authenticated NTP (DES/MD5)
available
Contact: Seth Robertson ([e-mail])
Note: IP addresses are subject to change; please use DNS
/usr/sbin/ping ntp.ctr.columbia.edu 64 5
PING 128.59.64.60: 64 byte packets
64 bytes from 128.59.64.60: icmp_seq=0. time=83. ms
64 bytes from 128.59.64.60: icmp_seq=1. time=86. ms
64 bytes from 128.59.64.60: icmp_seq=2. time=85. ms
64 bytes from 128.59.64.60: icmp_seq=3. time=86. ms
64 bytes from 128.59.64.60: icmp_seq=4. time=83. ms
----128.59.64.60 PING Statistics----
5 packets transmitted, 5 packets received, 0% packet loss
round-trip (ms) min/avg/max = 83/84/86
These ping round-trip times are significantly greater than the west coast
example; the target is 5000 kilometers (3000 miles) further away.