[ntp:questions] Re: NTP, no external time source, peering "Undisciplined Local Clocks" ?

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 19 19:22:18 UTC 2005

Marco Molteni wrote:

>Say I have a network with no access to the internet, and I have no
>external time reference (no GPS or atomic or whatever clock available).
>I am using the NTP reference implementation ntpd on FreeBSD.
>The network is composed of 3 hosts: A, B and C.
>I know I can tell NTP to use the local clock as a time reference, what
>is called an "Undisciplined Local Clock" in the NTP documentation.
>Say I don't know if a clock is better than the other, so by configuring
>B and C to use A as server I risk to have a not so good time if A is 
>Can I configure A,B,C to peer with each other in a meshed fashion, so
>to have each clock influenced by all the others? From what I read I 
>should use the following configuration file on each host:
>========== /etc/ntp.conf ======
>peer hostA
>peer hostB
>peer hostC
>would it work? Is it the best way to obtain the "best" time assuming
>I don't know if a clock is better than another?
>questions mailing list
>questions at lists.ntp.isc.org
If you care about the CORRECT time, this configuration is an almost 
certain loser!  There is a White Paper available from Sun Microsystems 
that claims this configuration will synchronize all the systems to the 
same time.  See http://www.sun.com/blueprints.  The paper is entitled 
"Using NTP to Control and Synchronize System Clocks and can be 
downloaded in three parts.   It's by David Deeths and Glenn Brunette.  
You'll find a brief reference to the synchronization of an isolated 
network of peers on page 7 of part one.  I have not tried this 
configuration and cannot attest that it actually works.

To find the best clock, set all three to the correct time using your 
cellular phone if available or your wristwatch if not.   The cell phone 
display shows only hours and minutes but the rollover of the minute 
should be accurate to within a few microseconds.  If you have fast 
reflexes you should be able to set the time to within one second.   Let 
the three systems run for a few days and check the clocks.  The one 
closest to the correct time should have the best clock.  Typical 
computer clocks can gain or lose two to four minutes per month so don't 
expect too much.

If you want the correct time, invest in a GPS receiver.   I've heard of 
suitable receivers available in the $80-$90 US range.   I bought a 
Motorola receiver specifically designed for timing applications for $200 
US.  I keep one of my Sun Ultra 10 workstations synchronized to within 
ten microseconds or better with it.  This receiver is just a bare 
circuit board, a wall-wart, and an antenna.  If you wish, you can pay 
thousands of dollars for one in a fancy case with built-in NTP server 
but it's not necessary.

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