[ntp:questions] Query about NTP accuracy

Brian Utterback brian.utterback at sun.com
Sat May 30 19:26:52 UTC 2009


David Woolley wrote:
> Brian Utterback wrote:
> 
>>
>> Also, remember that with NTP, it take three servers to outvote one bad 
>> one, not two. That is why best practice is to have four.
> 
> It only requires 2.  The argument about having four initially is about 
> having a clear majority even after rejecting one.

I am not sure what you are saying. Are you saying that 4 servers allow 
you to have a failed server and then outvote a falseticker. I don't 
think that is true. And it definitely takes three servers to outvote 
one falseticker in some cases. NTP deals in intervals, and it takes 
two servers to define one interval, and then one more to make that 
interval outvote all the others possible intervals. So, three servers 
will always outvote a single falseticker, but two will not always 
outvote a single falseticker.




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