[ntpwg] Testing NTP performance

[Harlan Stenn]

Dave,

Please help me out here.

> In your question below the measured offset is the maximum liklihood 
> estimate of the client offset relative to the server, while the jitter 
> represents the estimated error of the offset estimate itself. The 
> synchronization distance represents the maximum error due all causes 
> with the assumption that the maximum inherent clock frequency error is 
> bounded. The assumed parameters for phase and frequency error estimate 
> are represented by the limbs of the Allan deviation specific to each 
> installation. There's not much more than can be said abount the 
> probabilistics.

So if somebody wants the answer to ask the question "Does ntpd think the
time on the machine is OK and sync'd?" the first thing we all agree on
is that's not the best question to ask.  But we should be able to
determine:

Q: Does ntpd think it is sync'd?  To something "useful"?
A: Look at the (system) status word in the response to a mode-6 control
   message, and make sure:
   - LI is not 11
   - the value of "clock source" is something tolerable

Q: What time does ntpd think it is?
A: do "the dance of the 4 timestamps".

Q: How close can we expect the previous answer to be to the "correct"
   time?
A: Check out lots of error-related variables and pick the ones you think
   you want.  This includes root dispersion, jitter, and synchronization
   distance.  Remember that the answer is a probability.  (OK, we also
   need to be able to report that probability.)

Q: How is the previous answer affected by "slew-only" mode?
A: (Harlan does not currently know the answer to this one.)

Q: Are any of the above questions superfluous?  If so, please explain.

Q: Should we be asking any additional or different questions?

This general topic is a FAQ, and there is a need to have it covered in
sufficient detail.  Dave, while it may not be productive for *you* to
answer this (either once or rarely), I believe it is critical that we
can point people at a place to get an understanding of the issues that
is more than "read the spec and Dave's book and then study the problem".

Of course, the next thing that will happen is that people will tell us:

- We want to meet the following "performance" target at a given
  statistical level.
- Based on the spec, we can see the target above is reachable.
- We are monitoring the values (X), and we are seeing (Y) which seems to
  be out of spec.
- - how do we figure out what the problem is?
- - how do we fix it?

-- 
Harlan Stenn <stenn at ntp.org>
http://ntpforum.isc.org  - be a member!