[ntp:questions] Isolated Network Drift Problem

Cal Webster cwebster at ec.rr.com
Tue Nov 25 18:44:48 UTC 2008

On Tue, 2008-11-25 at 17:12 +0000, Unruh wrote:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
> >A more adequate test would be to set up a server with a GPS receiver and 
> >compare the offset of all the local clocks under test every thirty 
> >minutes over a period of several days.  But, if you are going to do 
> >that, you might as well make it permanent and use it for your master clock!
> Or connect your system to the net for a while allowing only ntp packets in
> or out and use one of the time servers on the net. Since this is good to
> 1ms or so, the tests you could do with your wristwatch in 10 days can now
> be done in a couple of hours. Plus ntp will tell you what the rate
> fluctuations are. 

Unfortunately, connection to the Internet is not an option I'm allowed
to consider.

> >Consider that the Garmin GPS18LVC has a pulse per second output and 
> >costs less than $100 US.  If you can site an antenna with a good view of 
> >the sky, you can have a stratum 1 server of your very own and have the 
> >time accurate to within a millisecond or less.  Note that while the GPS 
> >is accurate to 50ns or better the process of getting the time into your 
> >computer may introduce several hundred microseconds of uncertainty.
> The Garmin 18LVC only claims an accuracy of the PPS of about 1micro second.
> However getting the time in can be done to the usec level. My system which
> has a Garmin 18LVC running a parallel port interrupt has fluctuations of
> about 2usec rms. And tests I ran in which I sent a signal out on the
> parallel port output line to the interrupt input, timing when I sent it out
> and when the interrupt was triggered showed about a 1-2us difference. 

This might be a future option but will take time to get it installed and
configured on a local time server.

> >> I've changed my server ntp.conf files so that one machine (jato) is
> >> designated as a server with its undisciplined clock set at stratum 5.
> >> The other three are peers to each other, each pointing to the one
> >> stratum 5 server. The peers still have the undisciplined clock
> >> configured but at stratum 8. I guess I'll see how this goes.
> I see absolutely no advantage to have the local clock as a possible source
> for the clients. It does nothing for you.

None of our client machines use the local clock. The servers configured
like this are the other 3 "peer" NTP servers. Don't they need some type
of time reference if the master server becomes unreachable?

I'd like to setup Orphan mode as a backup but I could not determine
where to add the command "tos orphan stratum" described at
[http://www.cis.udel.edu/~mills/ntp/html/assoc.html#orphan]. It only
says to put it in the "configuration file". There is no mention of the
"tos" command in ntp.conf(5).

When I had previously set up all 4 servers as peers, using local clocks,
they would all select the same one. When I took that one off-line they
selected another. Originally I assumed this was Orphan mode but now I'm
pretty sure it's not. I would be grateful if you could clarify this.

> >> Before restarting the ntp daemons I zeroed out the drift files then set
> >> the system times to the exact second showing on
> >> "http://wwp.greenwichmeantime.com/time-zone/usa/eastern-time/". I then
> If you can get onto the net why in the world are you not using ntp from a
> server on the net? 

Long story... The short version is that I have Internet access via a KVM
to a connected machine. The R&D network cannot be permitted to connect.
I have to play with the hand I'm dealt.

> >> set the hardware clocks to match system time with "hwclock --systohc".
> harware clocks tend to have worse drift behaviour than does the CPU clock,
> and can only be read on the second boundary. And recent Linux kernels have
> almost completely broken the harware clock as a time source ( the harware
> clock roll over interrrupt is simulated by routine which gives only about a
> 10ms accuracy.)

Well, since the ntpd (start) and the halt init scripts set the hardware
clock from system time and the sysinit script sets the system time from
the hardware clock, this seemed like a logical choice. Besides, this is
only done shortly after setting system time based on the Internet time
source. If not this method, then what would you suggest?

> >> Although the network has no Internet connection, I have a KVM to an
> What is a KVM?

Keyboard Video Mouse switch (connected via KVM cables to the Internet
connected machine and my workstation).

Thanks for the feedback Bill! :-)


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