[ntp:questions] Re: Sufficient # servers to sync to
davids at webmaster.com
Sat Mar 19 21:04:05 UTC 2005
"Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message
news:IpidnYQnM441gKHfRVn-1w at comcast.com...
> Ummm.... When is the last time you saw two servers reporting exactly the
> same time? The difference is usually in the range 0.5 to 40 milliseconds
> but it's there. If they do happen to agree, is it random chance or do
> they both actually have the correct time?
I posted elsewhere with more detail. Servers respond with both a time
and an accuracy estimate. A server is working if the true UTC time is within
the time plus or minus the accuracy estimate. If you poll two servers, and
they are both working, their times plus their accuracy intervals plus the
round trip time *must* overlap, or else the servers are not working.
The assumption is that we have three servers, two working, two not.
Since two of the servers are working, by definition their time plus
uncertainty intervals must have some small amount of overlap in which the
true time lives. Since the third server is by assumption not working, the
true time must lay outside its interval or the interval must be absurdly
In this situation, it is trivial to isolate the incorrect server from
the two correct ones. However, one is not left with a very strong assurance
that the other two are in fact correct.
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