[ntp:questions] Frequently asked question about error bounds?

Unruh unruh-spam at physics.ubc.ca
Sun Nov 1 17:36:39 UTC 2009


"berra.84" <berra.84 at spray.se> writes:

>What is the correct error bound between client and server? In the RFC
>documentation I read one thing and in many theads another. I'm
>confused so please help me. Which is correct?

>1) In the documentation for RFC1305 p. 102 it can be read that the
>true  offset between
>client and server must lie somewhere in the correctness  interval,
>defined by
>I=[theta - delta/2 - epsilon, theta + delta/2 + epsilon]

>2) And in threads and websites I usuallly see the statement that the
>error is bounded with half the roundtrip. And it sure look like that
>when scatter diagram(wedge plots) with offset as a function rtt are
>viewed.
That is the delta term above. 
But it is clear that if the clock you are comparing to is wildly off, then the
roundtrip time does not encompas the error. Lets say that the server gets its time
from a GPS NMEA which has a best accuracy of about 1ms ( and often worse) but the
roundtrip time is 200usec. Then the accuracy will be a few ms, not 200usec. 
That is epsilon.


....

>If the 1) alternative is right, all the other would be much more
>consistently.

It is right.

>Thanks in advance




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