[ntp:questions] Isolated Network Drift Problem

Unruh unruh-spam at physics.ubc.ca
Wed Nov 26 00:40:00 UTC 2008

cwebster at ec.rr.com (Cal Webster) writes:

>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?

Ah, I thought you had it on your clients. 

In your situation I might well strongly advice chrony instead of ntpd as
the time source. chrony has the option of using wristwatch time as a time
source, and will discipline the clock against that. It will also as as an
ntp server. 

>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.

>> What is a KVM?

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

Ah, OK, I can imagine the setup now. 

My strong suggestions would be to a) use chrony for your servers and set it
with your wristwatch (or your KVM) b) get a GPS PPS receiver. Both should
work. You can use eitehr chrony or ntpd on the clients. (chrony disciplines
the clocks better and converges MUCH faster than does ntpd. chrony will not
do any kind of refclocks, except wristwatch time-- ie manual entry of the

>Thanks for the feedback Bill! :-)


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