[ntp:questions] Magice Server Numbers: 4,5,7,9

unruh unruh at wormhole.physics.ubc.ca
Fri Sep 16 06:24:40 UTC 2011

On 2011-09-16, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> On 9/15/2011 10:34 PM, Doug Calvert wrote:
>> Hello,
>>   From time to time people mention that the magic number of servers is
>> 4,5,7 and nine. However I can not find a reference or explanation for
>> the magical properties. Can anyone explain the background/basis in
>> general? And specifically how is five or seven preferable to six?
> Okay!  It looks as if you read something I posted here.
> 1.  A man with two clocks can never be certain what time it is!
> 2.  Three clocks are the minimum necessary to get reasonably good and 
> reliable time.
> 3.  The weakness of three clocks is that the failure of any one clock
>      leaves you with case 1.

And the failure or 2 leaves you with 1. And the failure of 3 leaves you
with nothing. 
> 4.  Four clocks is the minimum robust configuration.

And two clocks going wrong in the same way leaves you with case 1.

robust in what sense. As you say below, 4 is not immune to failure of 2.

> 5.  Five clocks allows for two failed clocks.

But the loss of three leaves you with case 1

> 6.  Seven clocks allow for the failure of three.
> Etc, etc. . . .

The only answer is to have at least 10000000000001 clocks although that
is also not foolproof:-) 

You decide what safety you want, and get enough backup to get that


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