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

Brian Utterback brian.utterback at sun.removeme.com
Fri Mar 18 20:37:18 UTC 2005

```I made them up. I wasn't trying to be strictly correct, but rather
illustrative. I know, the intervals really extend +/- and you have to
include the mid-point of any and all included intervals, but that
wasn't my point. I was just trying to show in the simplest way
possible why three servers won't work.

David L. Mills wrote:
> 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.
>>

--
blu

If you put a submarine in a blender...
----------------------------------------------------------------------
Brian Utterback - OP/N1 RPE, Sun Microsystems, Inc.
Ph:877-259-7345, Em:brian.utterback-at-ess-you-enn-dot-kom

```