Poul-Henning,
On 11/09/2014 09:44 PM, Poul-Henning Kamp wrote:
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In message <CABbxVHtommjwSWq1i=oH-u1S=G6P=xu8e0yzekadg-vchgk...@mail.gmail.com>
, Chris Albertson writes:
NTP does not pick the best clock. NTP finds
the subset of clocks that track each other.
NTP does indeed find the best clock from the subset of clocks
which pass its sanity check, and then it uses only that one.
There are several problems with that, and as we speak I'm developing
a new algorithm which at least so far, gives much superior performance.
You can read my musings about this here:
http://phk.freebsd.dk/time/20141107.html
My goal is to release the new NTP client before X-mas.
Just as you point out, there is no easy way around the fact that delays
may be asymmetric. When one analyzes the problem, the actual time-error
and delays in both directions cannot be solved with the two measurements
one do, three unknowns and two equations. The sum of the observations,
forming the Round-Trip-Time, is however the only value we can trust, and
worst case asymmetry will be that of the RTT, regardless of statistical
distribution. Using RTT as a worst-case estimator is thus enough if you
don't know better, and using RTT as a simple stability estimator isn't
all that bad for a simple system.
The NTP scatter-plot wedge is another way to present it, and finding the
"tip" of that wedge is really about finding the min-value of delay in
each direction prior to doing the two-way time-transfer equations.
Cheers,
Magnus
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