[ntp:questions] Re: polling algorithm

David Schwartz davids at webmaster.com
Mon Sep 5 18:52:47 UTC 2005


"Terje Mathisen" <terje.mathisen at hda.hydro.com> wrote in message 
news:dfgphb$dkv$1 at osl016lin.hda.hydro.com...

> Brian (and David), thanks for finally making me really understand why
> NTP specifies four instead of three servers!
>
> David, your conclusion above is wrong, because there are two disjunct
> intervals where two servers agree that the correct time could be:
>
> A|B = 5:14 to 5:16
> B|C = 5:18 to 5:20
>
> With no other sources of correct time (i.e. a fourth server), there is
> no way to determine if the first or the second interval is correct. I.e.
> it is probable that B isn't a falseticker since it is the median choice,
> but there is no way to select a single time window as better than all
> others.

    You are correct, you would have to take 5:14 to 5:20. The clock accuracy 
can halve at each stratum, which is what it does in this case. (From +/-3 to 
+/-6.)

    DS





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