[ntp:questions] Why does ntp keep changing my conf file?

Daniel Havey dhavey at yahoo.com
Fri Sep 17 23:50:03 UTC 2010

> On its own no.  With the help of ntpdate, yes.  I
> wouldn't actually expect to reach this level very often, but
> I'd certainly expect several milliseconds.  You will
> get these large steps because you are using individual
> measurements, which are very vulnerable to scheduling delays
> and network propagation delays, which ntpd, itself, will
> average out (it will effectively low pass filter the jitter
> out of the measurements).

Hehe ;^)  That was cool, with the help of ntpdate ;^)
Okay, I'll buy the scheduling delays argument, but latency?  On my control plane?  No way ;^)  That lan is soooo lightly loaded that any packet can get sent anywhere it wants at any time on Gigabit ethernet.  So lag, shmag, latency jitter does not scare me ;^)  I could see having a interrupt (perhaps for the nic on my lightning fast control network ;^) get thrown in the middle of a measurement.  This is a reasonable argument.  I am convinced.  

> If you seriously need very tight synchronisation, you
> should use ntpd or possibly chrony, with a pulse per second
> source parallel wired to all the machines.  In extreme
> cases, you may need to make sure the transmission lines from
> the PPS source are or equal length.

That might be a little extreme, we don't have a PPS source, or roof access for a GPS receiver.  Actually this whole discussion is becoming academic because of the "it's already good enough factor".  Fortunately, I am an academic so I enjoy the discussion ;^)

If we can get down to a millisecond then we are golden.  Wireless packets simply do not move that fast.  So I will do this.  I will switch my systems to ntpd, and I will also urge the emane community to switch.  Perhaps they will have other reasons for not wishing to use ntpd ;^)


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