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

David Woolley david at ex.djwhome.demon.invalid
Sun Nov 1 11:12:02 UTC 2009


berra.84 wrote:

> 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]

This is (more) correct. I'm not sure if it fully accounts for some of 
the details of the intersection algorithm.  One would need to double 
check whether it was valid during startup transients and in the light of 
the averaging of multiple sources that is actually used to estimate the 
time.

> 
> 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.

This is an oversimplification that is quite often valid in cases where 
the error is significant, as delay tends to dominate dispersion and most 
of the delay, and in particular, nearly all its asymmetry, is associated 
with the "retail" hop.

Another factor, I think is that the RFC attempts to be a mathematical 
treatise, and the use of lots of Greek letters, rather than spelling out 
what they mean, turns people off, so very few users can actually read 
and understand the RFC.

Please note that RFC 1305 is de facto obsolete, and current versions of 
NTP actually correspond with a new, draft, RFC.

Also note that this is not a frequently asked question, although maybe 
it should be.




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