HP-UX Internet Services Administrator's Guide (August 2003)

Conﬁguring NTP

Getting Started with NTP

Chapter 4 63

The best primary server for the NTP client located in California is the

time server situated in New York because the ping command response

time is only 5 millisecond. The ping command response time for the time

server in Australia takes 500 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 .

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” on page 75.

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 tock.CS.UNLV 3 u 721 1024 377 2113.97 1004.94 824.

-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 22.6