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

Roger invalid at invalid.invalid
Fri Apr 22 15:48:54 UTC 2011

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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