| Re: [chrony-users] Poor synchronization to local NTP server |
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On Tue, 16 Mar 2021, Hal Murray wrote:
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Maybe we should add geographic coordinates to the NTP protocol
so a client can sanity check the round trip times.
Uh, the round trip time is easily measureable. It is one of the things the ntp
Right. And NTP assumes the routing delays are symmetric. But it doesn't know
anything about the delays so it has to add 1/2 the round trip time to the
possible error budget.
If you knew the server was on the other side of the country, you could make a
rough estimate of the minimal round trip time and remove that from the error
budget.
Unfortunately the round trip time is largely determined by all of the
switches/routers/... on the way there an back ( which may well be different
routes). It is not simply the straight line distance. While it is true that
the one way trip cannot be less than the distance divided by c, that is not
terriby helpful. So to my uni, 6km away speed of light roundtrip is 40
microseconds, the ping time is 3ms, 100 times as long.
To a computer in Toronto (4000km away) the speed of light round trip is about
26ms, and the ping time is 60ms, about 2.5 times as long. Now one could
perhaps argue that the max error in that one way tip is that excess time
(34ms), which would be better than the 60 ms one might naively assign to it,
but even that reduction by 40% is really irrelevant. None of those estimates
are that accurate anyway.
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