[Techtalk] NTP question

[John Stoneham]

So, I set up NTP as set forth in Carla's article "Keeping Accurate Time on
Linux" <http://networking.earthweb.com/netsysm/article.php/10954_3302411_1>.
An excellent article, by the way!

I've got /etc/localtime symlinked to /usr/share/zoneinfo/US/Eastern, I've got
server lines in ntp.conf, I ntpdated with each server and they all respond,
and ntpd is running. No problems here.

/etc/ntp.conf looks like this (it's based off the default one that came with
the Fedora ntp install):

restrict default ignore
restrict 127.0.0.1 
restrict 172.16.0.0 mask 255.255.255.0
server  ntp-1.vt.edu
server  ntp-2.vt.edu
server  ntp.cmr.gov
driftfile /var/lib/ntp/drift
broadcastdelay	0.008
logfile /var/log/ntp.log
keys		/etc/ntp/keys

So I ought to be able to access my NTP server over loopback and 172.16.0.*,
which is correct, and it should update from those servers. What I don't
understand is how it knows how often to contact those servers and update the
local time? What in here tells it to keep itself updated from those servers
and then serve that time out to other machines that contact it?

I think there's a few things about NTP I don't understand here...

Thanks,

- John