# [ntp:questions] Re: Sufficient # servers to sync to

David L. Mills mills at udel.edu
Thu Mar 17 22:59:14 UTC 2005

```Brian,

I don't know how you got the correctness intervals. See
http://www.eecis.udel.edu/~mills/database/brief/arch/arch.pdf slide 9.

Brian Utterback wrote:
> Richard B. Gilbert wrote:
>
>> Take an extreme case:
>>
>> Server A says it's 11:53
>> Server B says it's 11:55
>> Server C says it's 23:52
>
>
> To add to what Brad said, what you really have is the following
> candidates for the time:
>
> 1. The interval 11:53-11:55
> 2. The interval 11:55-23:52
> 3. The interval 11:53-23:52
>
> Let's see... number 1 has two servers on it, number 2 has
> two servers, and number 3 has all three servers. So, with
> three servers voting for number 3, I guess it is the winner.
>
> This means that all three servers are allowed to proceed to
> the next stage in the selection, so server C may still get the nod.
>
> Just to clarify, the vote goes to the shortest interval that has
> at least n/2 servers on it. So, if we add one more server right
> around 11:5x, we see that this would then create a new interval
> with three servers, not allowing server C. The original long
> interval will have all four, but is trumped by the shorter one
> with three, since 3 is still more than n/2 = 4/2 = 2 servers on
> it.
>

```