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
<[email protected]> wrote:
On 2011-04-22, Roger <[email protected]> wrote:
On Thu, 21 Apr 2011 09:32:21 -0400, "Richard B. Gilbert"
<[email protected]> 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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