[ntp:questions] Re: NTP Pool rules vs. four-server advice

David Schwartz davids at webmaster.com
Tue Dec 20 01:20:39 UTC 2005

"Brian Utterback" <brian.utterback at sun.removeme.com> wrote in message 
news:do6n6k$rol$1 at news1brm.Central.Sun.COM...

> Three
> servers can protect from certain classes of falseticker situations
> involving one falseticker, but some combinations may not be prevented.

    This is really not true. If we define falseticker as "a clock that is so 
far off that we are screwed if we trust it", the only possibilities are:

    1) There are no falsetickers, in which case, it doesn't matter which 
clock we follow.

    2) There is one falseticker and two accurate clocks. In which case, 
using either of the two clocks in closest agreement is fine. You cannot put 
the falseticker close enough to the other two clocks to confuse us without 
making it no longer a falseticker.

    3) There is more than one falseticker, in which case we know we can't 
keep accurate time.

    The mathematical argument for why you need four clocks considers *you* 
to be one of the clocks. If there's you, and three configured references, 
then you do have four clocks.

    You do not need four servers to protect against one inaccurate or 
unreachable clock.


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