[Techtalk] NTP question

[Dominik Schramm]

John Stoneham <lyric.techtalk at lyrically.net> writes:

> [...]
> /etc/ntp.conf looks like this (it's based off the default one that came with
> the Fedora ntp install):
> [...]
> server  ntp-1.vt.edu
> server  ntp-2.vt.edu
> server  ntp.cmr.gov
> [...]
> 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?

Nothing in here tells the interval; the presence of the "server" lines
tells you that it actually does update from those servers. 
The (abbreviated) syntax of the "server" configuration line is:
server address [minpoll minpoll] [maxpoll maxpoll]

(see http://www.eecis.udel.edu/~mills/ntp/html/confopt.html for 
 details.)

The minpoll and maxpoll options specifiy exponents to the base 2 (time
is given in seconds) for the minimum and maximum intervals between
updates. Your configuration does not explicitly state update
intervals, therefore, the defaults are used:

"The maximum poll interval defaults to 10 (1,024 s) [...]
 The minimum poll interval defaults to 6 (64 s)" 
(ibid.)

Why this is configured this way, I don't know. I suppose it has 
something to do with NTP protocol details.

To make your server check with its peers on a more predictably
regular basis, you'd have to configure two low exponents, like
6 and 7, where the difference between the powers is still low:
2^7 - 2^6 = 128 - 64 = 64 (s). 
But to my knowledge there are no reasons why you should update 
so frequently, since AFAIK xntp learn the drift rate of your local 
system clock and corrects the time all by itself even when the 
servers are down or not checked all too frequently.

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

I find this page quite helpful for configuring xntp:
http://www.eecis.udel.edu/~mills/ntp/html/ntpd.html

and this one for general info on NTP:
http://www.eecis.udel.edu/~mills/ntp/html/index.html

Hope this helps,
dominik