[ntp:questions] Re: Time sync over bad modem line

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 27 16:12:36 UTC 2004

Nagy Bela wrote:

>Hello everyone!
>I guess this question has been asked several times but I could not
>find the answer. (Excuse me, if it is too simple, but I could not
>figure out any solution.)
>  The problem is the following: my computer rarely connects to the
>internet via a dial-up connection. Sometimes this connection is
>terrible, sometimes it is good (not bad). I would like NTP to
>synchronize the system clock whenever it's connected.
>  Practically, a solution would be the following: I specify some NTP
>servers; actually, several servers, since it happens that I could not
>access a remote server directly (though it is working -- this is the
>fault of the connection). NTP should try to synchronize
>with them, say, every fifth minute. And, of course, log when it
>successfully connected and synchronized the local clock.
>  How should I configure this? (regular ntpdate as a cron job would 
>not be bad, but as far as I understand 	ntpd would be better for this)
>Thanks for any help in advance.
NTP cannot really synchronize your clock with an intermittent connection 
of poor quality.  What you can do is SET your clock whenever you connect 
to the internet;  ntpdate is good for this.  One or two phone calls a 
day could keep your clock within a few seconds of the correct time.

In order to work as designed, ntpd must query one or more (four are 
good, five or more better) servers at intervals of, at most, 1024 
seconds.  The intervals normally start at 64 seconds and increase to 
128, 256, 512, and 1024 seconds as the clock synchronizes.  ntpd expects 
a connection "on demand"; the delay involved in making a phone call each 
time would introduce a ten or fifteen second error!

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