[ntp:questions] Re: ntpdc vs iburst

Richard B. Gilbert rgilbert88 at comcast.net
Fri Nov 19 01:33:33 UTC 2004

Nagy Bela wrote:

>  Hello all,
>  As I mentioned it in my earlier posts, I would like to synchronize
>my (only one) PC's clock. What makes it tricky is that the PC is connected
>to the internet only for 5-25 minutes one-three times a day 
>(every second week).
>  Now I have found out the following solutions:
>A) The ntpd is started from ip-up (when the computer has finished the ppp
>negotiation and already connected). Plus several servers
>with iburst in ntp.conf. And ip-down stops ntpd. It works, I tested it.

I believe this is the best of the three options you have outlined.  
Starting with iburst gets the maximum benefit from the brief uptime.  
Ntpd will fill the clock filter pipeline in the first 30 seconds and 
will poll the server every 64 seconds thereafter.  After the first 30 
seconds, ntpd has enough information to begin correcting the clock.  
Ntpd not only corrects the time but also the frequency of the clock 
oscillator.  This means that the clock will stay closer to the correct 
time than it would if you simply set it to the correct time and did 
nothing else.  Of course the oscillator frequency will also drift  but I 
think it is best to start with the correct clock frequency.

>B) The ntpd starts up with the computer with only one server:
>The ip-up adds several server with ntpdc's addserver command.
>And the ip-down removes all of them except for the local clock.
>The only one drawback of this option is that this way ntpd
>sets the clock a bit slowly.
I don't see that using the local clock as a server gains you anything at 
all!  As I understand it, you are not serving time to other machines 
from your PC.

>C) A simple ntpdate from ip-up.
This option simply sets your clock to approximately the correct time.  
It does nothing to adjust the frequency of the clock oscillator; the 
clock will continue to drift at its native rate.  You could be gaining 
or losing two milliseconds per minute or worse.

>My questions (may seem to be trivial, but I am not an expert 
>in ntpd; perhaps these informations might be useful for others too):
>1) Are there any other options?
>2) What is the 'best' and why?
>3) Is there any disadvantage of A) against B)?
>4) Could I combine the iburst solution with the local clock solution?
>That is, is there a way to specify iburst in ntpdc?
>If not, please regard this email as a feature wish.
>Thanks for your help in advance.

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