[ntp:questions] Using two NTP Server: Bad?

David L. Mills mills at udel.edu
Fri Apr 22 17:35:39 UTC 2011


Roger,

See the "How NTP Works" page in the current online documentation at 
ntp.org. Pay particular attention to the description of the selection 
algorithm.

Dave

Roger wrote:

>On Fri, 22 Apr 2011 15:19:31 GMT, unruh
><unruh at wormhole.physics.ubc.ca> wrote:
>
>  
>
>>On 2011-04-22, Roger <invalid at invalid.invalid> wrote:
>>    
>>
>>>On Thu, 21 Apr 2011 09:32:21 -0400, "Richard B. Gilbert"
>>><rgilbert88 at comcast.net> wrote:
>>>
>>>      
>>>
>>>>One server: if it fails you have nothing!
>>>>Two servers: If the two differ, which one do you believe?
>>>>Three servers: degenerates too easily to the two server case.
>>>>Four servers: Allows the failure of one server.
>>>>Five servers: Allows the failure of two.
>>>>Seven servers: Allows the failure of three.
>>>>        
>>>>
>>>I've seen these number quoted before and I don't understand
>>>the last one. Why doesn't 6 allow for the failure of 3? Why
>>>      
>>>
>>Because 3-3 is a tie and the system cannot decide which is best. Ie by
>>failure, read "bad timekeepers". If 3 fail-- ie stop responding to 
>>packets, 6 is pleanty. 4 would be enough. But if they fail by delivering
>>the wrong time, and all three deliver the same wrong time (say because
>>all three are in Chicago and all three used a cell phone system to set
>>the time and .... ) then you have a tie. 
>>It starts to get a bit absurd, I know. 
>>    
>>
>
>Thank you, and David and Dave.
>
>I hadn't thought about a 3-3 tie. I hadn't even considered that
>that might happen. But if that is possible then so is a 2-2 tie
>with 4 servers. Ho hum, nothing is perfect in this life.
>  
>




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