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

Brian Utterback brian.utterback at oracle.com
Thu Dec 4 22:06:43 UTC 2014

On 12/4/2014 12:13 PM, Miroslav Lichvar wrote:
> 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:
> http://lists.ntp.org/pipermail/questions/2011-January/028313.html
>> 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.

I think we are in violent agreement. The point I am trying to make is 
that for a long time we recommended four servers then started saying 
that three was enough but four is better. I think that we should stick 
with recommending four, at least if you actually care about the time on 
your systems.

Brian Utterback

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