Installing and Administering Internet Services

288 Chapter7
Configuring the Network Time Protocol (NTP)
Getting Started with NTP
Figure 7-1 Survey of Best Time Servers
Example 1: Locating the Best Primary Server
In Table 7-1, you can see that there are a number of servers the time
client can access. The primary time server is NAVOBS1.MIT.EDU. The
other time servers within reasonable physical and network distance are
cs.columbia.edu, 129.236.2.199, and c.epsydra.dec.c .
Choose three (or more) that are nearby geographically. If you are in
London, it would not be wise to choose time servers in Australia or
Brazil. Long distances over water usually mean a poor network
connection in terms of delay and path symmetry. Router hops also delay
the packets in unpredictable ways.
NTP Time Server in NY
86ms roundtrip PING time
NTP Time Server in Australia
500ms roundtrip PING time
NTP Time Server in CA
5ms roundtrip PING time
Your NTP
Time Client
Table 7-1 Available Time Servers
remote refid st t when poll reach delay offset disp
==============================================================================
clepsydra.dec.c usno.pa-x.dec 2 u 927 1024 355 108.49 -18.215 3.63
*NAVOBS1.MIT.EDU .USNO. 1 u 214 1024 377 38.48 -0.536 0.90
ticks.CS.UNLV.ED tock.CS.UNLV 3 u 721 1024 377 2113.97 1004.94 824.57
-cunixd-ether.cc 192.5.41.209 2 u 636 1024 377 47.99 3.090 9.75
+cs.columbia.edu haven.umd.edu 2 u 172 1024 377 3.39 12.573 1.14
+129.236.2.199 BITSY.MIT.E 2 u 423 1024 376 13.43 -14.707 22.60