HP-UX Internet Services Administrator's Guide (May 2010)

milliseconds. Therefore, selecting the time server situated in Australia is not
recommended because it may cause network delays.
Example 1: Locating the Best Primary Server
Table 4-1 shows the 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 clepsydra.dec.c .
Table 4-1 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 to ck.CS.UNLV 3 u 721 1024 377 2113.97 1004.94 824.57
-cunixd-ether.cc 192.5.4 1.209 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
Choose three (or more) time servers that are geographically close to the NTP client. If
you are located in London, it is not preferable to choose time servers in Australia or
Brazil. Long distances over water usually pose a poor network connection due to delay
and path symmetry. Router hops also delay the packets in unpredictable ways.
Before using a time server, you must evaluate these potential time servers (and the
network paths) to decide if they are physically close to the NTP client (consider ping
time, delay and variation) and configured properly. You may also have to send
notification to the time server before using them (consider the ettiquitte of the listings
at UDelaware). Do not point more than three machines to a single public time server.
Use a small group of machines (at stratum-2 or stratum-3) as the main time server for
remaining systems. For more information about stratum levels, see “Stratum Levels
and Time Server Hierarchy” (page 58).
The public stratum-2 servers provides time service for all clients. Also, their access
policies are less restrictive than the stratum-1 servers. The errors displayed while
connecting your machine with the public time server (or ISP) denotes the quality of
the network service.
This makes the distinction between stratum-1 and stratum-2 almost meaningless for
most purposes.
Unlike other applications (such as FTP, DNS, NFS, Sendmail) which can tolerate huge
delays in packet, NTP is sensitive to the network service quality. Therefore, in addition
to the quality of the time servers, you must ensure that the network is fast and is not
overloaded. A minor delay in the network can be harmful for your time service. Delays
shows up immediately in the dispersion figures.
If you can afford time delays in the time service upto milliseconds, then pay special
attention to the network service quality. If you can afford time delays in time service
upto microseconds, you must abandon the network time servers and choose a radio
clock for each NTP client.
50 Configuring NTP