[ntp:questions] broadcast client

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Sep 26 07:07:06 UTC 2008


rochertov at gmail.com wrote:
> Hello,
>   I have a question regarding broadcast mode.  I have 6 machines
> synchronizing with the same ntp server.  That server uses uses a local
> ntp displined clock (we are looking into a GPS one).  The machines are

"local ntp discipline" is a contradiction in terms.  I think you mean a 
completely undisciplined local clock configured to be the NTP reference 
clock.

> connected via 1 Gbps switch.  The network is lightly loaded and I
> configured the clients as such
> 
> server ntp minpoll 4 maxpoll 4 iburst

Dave Mills, please note, yet another non-believer in the NTP algorithms.

Rochertov, please note that this will increase your vulnerability to 
short term network propagation delay variations, although it may help if 
you are subject to temperature variations.  Note that NTP doesn't 
correct the time on each poll but rather updates the frequency, and rate 
of change of frequency.
> 
> Howerver, I notice that two clients have a relatively large offset
> from the ntp server (greater than 100 micro-seconds according to "ntpq

Firstly, see other posts over the last month about the fact that offset 
is not the same as error.  I would say that, for a suitable OS and 
hardware, with negligible work load, 100 microseconds was a reasonable 
90 percentile figure.  You should not expect to achieve it 100% of the time.

What OS are you using?

How do the offsets vary with time?  An offset with a constant sign would 
be very weird and would indicate a serious problem.  To the extent that 
the time discipline algorithm is good,

> -p").  I considered setting the server into broadcast mode and
> enabling broadcastclient on the clients to avoid 6 machines polling
> the same server every 16 seconds.  I have two questions regarding
> this.  What is the syntax for setting the frequency of broadcasts on
> the server?  Also, how can I check the approximate time offset between

I believe you can measure the "offset" the same way as with a named 
server, although it will be an even worse measure of error, as it will 
also include the effects of changes in network loading between the time 
that the round trip time was calibrated and the time of the current 
measurement.

For error, you can also use the same methods, probably using OS kernel 
modifications to output exact phase information using a PCI or ISA 
parallel port, or modem control line, and then using specialist hardware 
to compare the timing between two systems.  I'd suggest outputting the 
clock interrupt time this way, and outputting the difference between 
tick time and a round number multiple of the nominal tick time via a 
software channel.

 > the clients and the server?  Should I then peer each client with the
 > ntp server and rely on broadcast messages for time synchronization?

Peering and broadcasting are different.




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