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

Conﬁguring NTP

Getting Started with NTP

Chapter 470

When the time server in Silicon Valley is conﬁgured to use

sirius.ctr.columbia.edu and gpo.adelaide.edu as time sources, the

output from ntpq -p looks like this (about 10 minutes after daemon

startup):

The time server in Australia has a delay of 470 milliseconds, which is

very similar to the ping round-trip times seen earlier. This leads to an

offset value of 54 milliseconds, which is signiﬁcantly worse than any of

the other time sources. It is interesting to note that the offest is much

less than the delay, which means that the round-trip is almost

symmetric. NTP must assume the outbound and inbound travel times

are equal, and the offset value gives an idea how unequal they might be.

This is considerably better than 470/2 which would be the offset if NTP

did not make this assumption. Also interesting is the very low dispersion

value, which means that the round-trip time does not vary a lot as more

packets are exchanged. Less than 1 millisecond is an excellent

dispersion value for a trip of 15,000 kilometers. The time server in

Australia is working out better than expected at this distance, but it is

still noticeably poorer than the other choices that are in North America.

The time server at Columbia is better than the time server in Australia,

due to the closer distance, but still noticeably worse than all of the other

time sources.

You must choose a minimum of one time server, and it is a good idea to

choose three or more for redundancy. Then put lines like this at the end

of your /etc/ntp.conf ﬁle:

Table 4-5 Output from ntpq for Conﬁguring Silicon Valley Time Server

remote refid st t when poll reach delay offset disp

=========================================================================

*REFCLK(29,1) .GPS. 0 l 25 32 377 0.00 0.413 0.03

+bigben.cac.wash .USNO. 1 u 56 64 377 39.54 -0.466 1.68

clepsydra.dec.c usno.pa-x. 2 u 122 512 377 6.32 -0.250 0.92

-clock.isc.org .GOES. 1 u 149 512 357 5.98 -3.045 0.46

hpsdlo.sdd.hp.c wwvb.col.h 2 u 25 32 126 56.29 -8.078 8.50

+tick.ucla.edu .USNO. 1 u 13 64 177 19.29 -0.265 0.26

+usno.pa-x.dec.c .USNO. 1 u 56 64 277 6.82 0.034 0.20

gpo.adelaide.ed tictoc.tip 2 u 15 16 377 470.52 54.789 0.90

sirius.ctr.colu NAVOBS1.MI 2 u 3 16 377 83.37 -8.372 1.24