[ntp:questions] ntpq -p command query

Charles Swiger cswiger at mac.com
Wed Jul 31 14:48:39 UTC 2013

On Jul 31, 2013, at 6:01 AM, Charles Elliott <elliott.ch at verizon.net> wrote:
> If he is using broadcast mode, what earthly difference does it make how
> often he broadcasts?

If you have a small # of machines per collision domain, then broadcasting
frequently probably won't matter.  If you've got thousands or tens of thousands
of machines to deal with, and need to pay attention to the MAC table sizes of
your various switches and routers, then excessive broadcast traffic is a much
more significant concern.

> It broadcasts the time to my home network every 16 seconds. It is also
> connected to 9 nearby stratum 2 time servers with a minpoll of 4 and a maxpoll
> of 5 (=32 seconds).  Unless this list is it, no one has ever criticized me for
> querying a time server too frequently.

See http://tools.ietf.org/html/rfc4330, section 10, "Best Practices".

Polling time more often than once a minute is widely regarded as abusive.
Unless the server operators of those machines agreed to allow this, you should stop.

> It is completely unrealistic to query a time server every 1024 seconds (poll = 10)
> and expect accurate time thru a WAN whose delay varies between about 40 ms and about 200 ms
> continually and unpredictably.

In point of fact, longer poll intervals give a longer baseline for comparison and would
mitigate the unpredicable latency you describe better than short poll intervals.

> Where the normal delay is closer to 40 ms, one reading thru a delay of 180 ms can
> throw the time off for hours, given that offset is significantly and negatively
> correlated with delay.

The output from 'ntpdate -d' would likely help explain what you meant by this.


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