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

Richard B. Gilbert rgilbert88 at comcast.net
Sat Mar 19 12:40:07 UTC 2005


David Schwartz wrote:

>"Brad Knowles" <brad at stop.mail-abuse.org> wrote in message 
>news:mailman.29.1111192970.588.questions at lists.ntp.isc.org...
>
>  
>
>>At 3:16 PM -0800 2005-03-18, David Schwartz wrote:
>>    
>>
>
>  
>
>>>     The assumption is that you have three servers, two of which are 
>>>working
>>> and one of which is not. If two of them are working, then their 
>>>intervals
>>> *must* overlap.
>>>      
>>>
>
>  
>
>>They can't overlap, since it takes two machines to *define* an interval. 
>>If you have three machines, you may have as many as two different 
>>intervals.  If one of those machines is insane, then you're back to the 
>>well-known two-machine problem.
>>    
>>
>
>    A machine reports a time of X and it reports its own accuracy as Y and 
>the round trip time is Z. Why does this one machine not define an interval 
>X+Y+Z to X-Y-Z?
>
>  
>
>>>                  Obviously if you have three servers and no two of them 
>>>are
>>> reliable, you are screwed.
>>>      
>>>
>>You're pretty much screwed if you have just three servers and any one of 
>>them is unreliable.
>>    
>>
>
>    I don't see why. Common sense say that if you have three servers and 
>they report:
>
>10:11
>10:11
>12:03
>
>    The time is probably 10:11.
>
>    DS
>
>
>  
>
Ummm....   When is the last time you saw two servers reporting exactly 
the same time?   The difference is usually in the range 0.5 to 40 
milliseconds but it's there.  If they do happen to agree, is it random 
chance or do they both actually have the correct time?




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