[ntp:questions] NTP absolute accuracy?

Unruh unruh-spam at physics.ubc.ca
Sun Nov 1 01:18:15 UTC 2009

"David J Taylor" <david-taylor at blueyonder.not-this-bit.nor-this.co.uk.invalid> writes:

>"Unruh" <> wrote in message news:kX%Gm.50022$Db2.12841 at edtnps83...
>> As I said, chrony does a linear regression on the last n offsets.



>I think we have seen these remarks about chrony before.

Some have, some have not. I was just putting out there what the difference between
ntp and chrony was, so people could make up their own minds. 

>What I was wanting was a yes/no answer (although numbers would be nice) to 
>the question:

>- is the 2-3 times improvement in the errors affected by the assumption of 
>equal round trip?

>Perhaps there is no definitive answer?

As I tried to emphasise, if the round trip is not symmetric, then neither ntp not
chrony can compensate for that lack of symmetry, and the absolute time will be
out. If occasionally it has an assymetric round trip, then ntp will probably
elimate it, and chrony may or may not, depending on how you set up chrony, but
typically chrony will be more sensitive to that asymmetry. 
Eb, say on exactly every 5th query, an extra .3ms is added to the return trip, and
lets say that the minimum round trip is .15ms plus or minus .1 ntp will always
throw out those fifth cases. but will have an error around .05 ms. chrony will
not, and without them would have an error of .02ms but with them would have an
error of about .05ms, just as would ntp. Ie, in this case the chrony advantage
would be more or less erased by those asymmetric round trips. 

There is no definative answer. 
It depends on the situation. 


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