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

unruh unruh at wormhole.physics.ubc.ca
Fri Apr 22 15:19:31 UTC 2011

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. 

> doesn't 7 allow for the failure of 4? Assuming that this
> wasn't a mistake in the dim and distant past which keeps
> on being repeated what am I missing?

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