[time-nuts] Divide by five ->Ensemble

Bob Camp kb8tq at n1k.org
Sun Nov 9 18:18:26 EST 2014


Here’s what I’m saying:

The NTP algorithm as it is written and as it is implemented results in an output clock that does *not* improve when a number of very good clocks are being used and the output is compared to the input. In that case and that case alone, the system simply does what one of the clocks is doing. That’s the case that matters when you are looking at a bunch of similar Rb’s.


> On Nov 9, 2014, at 6:06 PM, Paul <tic-toc at bodosom.net> wrote:
> On Sun, Nov 9, 2014 at 5:22 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> What is going on is that people are confusing the estimation process that
>> is used by the selection process (which does look at a lot of stuff) and
>> how that is described. They are then making the leap to the locking
>> process, which is something else altogether.
> Have you read the referenced documentation?  It quite clearly describes
> five major components:
>   - Clock Filter Algorithm
>   <http://www.eecis.udel.edu/%7Emills/ntp/html/filter.html>
>   - Clock Select Algorithm
>   <http://www.eecis.udel.edu/%7Emills/ntp/html/select.html>
>   - Clock Cluster Algorithm
>   <http://www.eecis.udel.edu/%7Emills/ntp/html/cluster.html>
>   - Mitigation Rules and the prefer Keyword [viz. Clock Combining
>   Algorithm]
>   <http://www.eecis.udel.edu/%7Emills/ntp/html/prefer.html>
>   - Clock Discipline Algorithm
>   <http://www.eecis.udel.edu/%7Emills/ntp/html/discipline.html>
> Admittedly saying "Mitigation Rules and the prefer Keyword" rather than
> saying Combining Algorithm could lead one astray but there is a step
> between Cluster and Discipline.  As noted earlier this bit: "designate one
> of them as the *system peer*"
> does not mean one clocks offset is used to discipline the client system
> clock.
>  It's easy to see that this is what's going on by looking at the
>> performance of an NTP implementation.
> I'm not sure what you mean here but "root distance" degradation dictates
> that no Stratum N+1 clock can be "better" than its upstream Stratum N
> servers.
>> In certain cases, with equally good  / very good clocks, the selection
>> process falls apart and the output actually degrades compared to the best
>> clock in the group due to "best clock" selection changes.
> Do you mean this?
> "In practice, with fast LANs and modern computers, the correctness interval
> can be quite small, especially when the candidates are multiple reference
> clocks. In such cases the intersection interval might be empty, due to
> insignificant differences in the reference clock offsets."
> or clock-hopping?
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