[ntp:questions] broadcast client

Unruh unruh-spam at physics.ubc.ca
Fri Sep 26 01:59:41 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>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
>> connected via 1 Gbps switch.  The network is lightly loaded and I
>> configured the clients as such
>> 
>> server ntp minpoll 4 maxpoll 4 iburst
>> 
>> Howerver, I notice that two clients have a relatively large offset
>> from the ntp server (greater than 100 micro-seconds according to "ntpq
>> -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
>> the clients and the server?  Should I then peer each client with the
>> ntp server and rely on broadcast messages for time synchronization?
>> 

>Remove the "minpoll 4 maxpoll 4" and let ntpd work as designed.  When 
>you force the polling interval to 16 seconds that way, you deprive ntpd 
>of the ability to accurately measure and correct small errors.

No it does a good job of correcting and measuring small phase errors. It is
the drift errors it has trouble with with such a small poll interval (bad
design, since it certainly has the information needed to get long term
drifts correct as well)  Ie, If
you expect the clock to be without a source for a length of time, then
having long poll intervals is good. If you do not want to pollute the net
with all your ntp packets, longer poll intervals are good. If you expect to
be constantly connected and do not give a damn about pollution because it
is your own system you are using as a source, then short intervals are
good. Why in the world does the OP care about the 16 clients polling every
16 sec? That is 16 bytes per second. That is trivial. If you have 16000000
clients polling, as some of the main time sources sometimes do then it
becomes a concern. 

Broadcast mode is far less accurate than polling-- no roundtrip
calculation, etc. 
If your offset is greater than 100usec and they are on a local lan, you hae
network problems. It is not a problem with ntp. Fix your network




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