[ntp:questions] Re: Sufficient # servers to sync to
Richard B. Gilbert
rgilbert88 at comcast.net
Sat Mar 19 22:15:25 UTC 2005
David Schwartz wrote:
>"David L. Mills" <mills at udel.edu> wrote in message
>news:d1hqk9$861$1 at dewey.udel.edu...
>>It's not true for three servers. Assuming one is falseticker, the other
>>two cannot from a majority clique in the voting process. You need at least
>>three alleged truechimers for that. I hesitate to cite Leslie Lamport, the
>>source of the Byzantine Generals problem, on which this algorithm is
>>based, but the paper is among the citations at
> I hate to say this, and it's quite possible I'm wrong, but it seems to
>me that this is based on an erroneous understanding of the Byzantine
>Generals problem and its solution. If you have three servers, this is
>analogous to the Byzantine Generals problems with *four* entities, the three
>servers you have, and *you*. With four entities, each entity has three
>others to look at. If two are correct, they can outvote the third.
Aren't you assuming that the "*you*" already knows what time it is?
The objective of running NTP is to find out what time it is!
It is very useful to start up with a more or less correct estimate of
the correct time but it is by no means required. If you use four
servers, of which at least three are working properly, you will
eventually get to the correct time.
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