[ntp:questions] Number of Stratum 1 & Stratum 2 Peers

Miroslav Lichvar mlichvar at redhat.com
Thu Dec 4 17:13:39 UTC 2014

On Thu, Dec 04, 2014 at 10:46:17AM -0500, brian utterback wrote:
> I remain unconvinced. I believe that it takes three correct servers to
> outvote a single falseticker, meaning that if you want to be safe
> against one of your servers becoming a falseticker and still being
> accepted as the system server by a client, the client needs at least
> four servers.

Four (or any larger number) of servers still doesn't guarantee the
source selection algorithm will mark one bad source as a falseticker.
There was a very similar discussion about this few years ago,
including an example:


> Now imagine that the falseticker has a similar overlap with T1, but on
> the interval T1off-T1disp to T1off. That interval does not include the
> real time, so F is indeed a falseticker. So, we have a completely
> symmetric situation, with T1 and F "voting" for an interval that does
> not include the real time and T1 and T2 "voting" for an interval that
> does include the real time. By what mechanism are we to presume that the
> client will choose the interval that includes the real time?

The intersection interval determined in the source selection algorithm
will be equal to the interval of T1 and all three servers will pass as
truechimers. Adding a third good server may not be enough to change
the result.

Miroslav Lichvar

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