[ntp:questions] ntpdate.c unsafe buffer write

David L. Mills mills at udel.edu
Thu Feb 14 15:05:49 UTC 2008


You are correct about the frequency, but the statistic of interest here 
is the phase or offset from one server to another. Accumulated over many 
poll intervals, the phase does a random walk. In your terms, the 
integral of a Gaussian distribution is a random walk.


Maarten Wiltink wrote:
> "David L. Mills" <mills at udel.edu> wrote in message
> news:fp0fpa$p5c$1 at scrotar.nss.udel.edu...
>>>>>Is there also a random backoff after an increase of the polling
>>>>No. However, there is a small dither of a few percent at all poll
>>>>intervals to resist self-synchronization.
>>The natural behavior of a bunch of oscillators near the same frequency
>>is to become one giant phase-locked oscillator. Adding a bit of random
>>fuzz at each poll turns each oscillator into a mini random-walk which
>>breaks up that tendency. The fuzz is not a lot, like 10 percent.
> Do you mean the dither alluded to above is cumulative?
> I was never much good with statistics and remember only that the
> expectation of the offset after N steps in a random walk is sqrt(N)
> times the average step size. Not a clue what the distribution might
> be. Intuitively, I would be aiming for uniform, and randomly adding
> half a polling interval delay when doubling it seemed to me like it
> would do that.
> Groetjes,
> Maarten Wiltink

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