[ntp:questions] NTP settings for machine with irregular, short connections to the Net

Rikishi 42 fsck_spam at telenet.be
Sat Sep 8 21:03:42 UTC 2007


On 2007-09-08, Steve Kostecke <kostecke at ntp.org> wrote:

>> The precision desired here is one of human scale, not milliseconds (or
>> worse).
> Please define "human scale".

Good enough(tm) for people who have never heard of NTP. :-)

The main concern is that that the laptop spends to much time adjusting,
correcting, etc... when it's only got a short time on the ISP. As mensionned
before, short time is a few (anywhere between 1 and 4) hours.
As the other machines of that LAN can't rely on the continuous presence of
the laptop, they too need to update quickly.


> A network connection is not the only way to acquire the time base used
> by NTP. Other sources include GPS/WWBV/CHU/MSF/DCF77/etc receivers,
> high quality external oscillators, and even a serial link (using the
> dumbclock driver).

Spending money is not really an option.
What is that latest option? A serial link? To what ?


> It is also possible, with a bit of work, to manually adjust the clock
> frequency. Depending on your requirements this may be Good Enough (tm).
It's precision is what it is. The main concern is to sync the clocks between
them from time to time, with as little fuss as possible. The type of
corrections currently active might cause the various machines to have
different time (compared to the second), because their time source is only
available for short times.


>> Imagine a laptop, connected to the net only once every few (2-3)
>> weeks. The connection lasts a few hours, maybe less. It gets it's time
>> updated from the NTP server from the ISP.
>
> Ntpd doesn't merely "update" the laptop's clock to the "correct time".
I know. And - sorry for the blasphemy - that's the main problem.


>> This laptop is 'brought back' to that isolated LAN, and subsequently
>> used as NTP reference to synchronise and update the time of the other
>> machines.
>
> Is the laptop actually moved and is it kept running when it is moved?

Yes, it's moved. That's why a laptop is used. The owner of laptop and the
isolated LAN is a friend who comes by every 23 weeks.
The laptop is not kept running dureing transport, that's not an option.
Besides, I've explained the poor precision on laptops. That's an accepted
given.

>> Obviously, the time of that laptop will have shifted in between
>> connections to the Net.
>
> That's because the laptop's clock, which is worse than the average wrist
> watch, is not being provided with a stable time base.
True. But as long as the other machines on that LAN sync to the same time as
the laptop, any time they detect it, that's fine.


>> What can be done to get it's time sync'ed as fast as possible,
> 1. You need to have a drift file that contains an accurate frequency.
Not an option for me, too fussy. Would know where to get the frequency. And
if I'm correct, that would cover what happens during boot/shutdown. 

> 2. You need to append 'iburst' to the server lines in the laptop's
> ntp.conf. 
We can try that, thanks !

> 3. When you connect the laptop to the Intenet:
>
> 	* stop ntpd
> 	* run 'ntpd -gq' or 'ntpdate' (deprecated)
> 	* start ntpd
>
> This should give you initial synchronization well within 1 minute.
Can be usefull in desperate situations, but we'd like to avoid doing it
manualy, for fear of skipping it when we're in a hurry.

>> and then become stable as quickly as possible?
>
> That depends on the factors which can affect the clock.
>
> If the laptop's ambient environment and operating state are largely the
> same regardless of which network (i.e. LAN or Internet) it is connected
> to AND the drift file contains an accurate frequency THEN you should
> have acceptable stability almost immediately.
Ambiant environment is equivalent. During transsport, the laptop might get
colder. But as long as the LAN is sync'ed rapidly and with minimal fuss,
then the occasional updates from the ISP will keep things within humanly
acceptable limits.

In other words, the clocks in the LAN may be off, but all by the same value.
And from time to time, the laptop 'brings home' a correction.



Many thanks to you and the others, allready. 
My friend was here yesterday, when I wrote the first post.
He's got that one, but the replies to it will only be fetched in about é
weeks, I guess.
So:
1. any more suggestions are welcome, we've got plently time (:-)
2. don't worry about absence of feedback, it'll be a while.


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