[time-nuts] NTP stratum 0

Hal Murray hmurray at suespammers.org
Sat Jan 14 16:05:40 EST 2006

> If you are within about a millisecond, then no one will notice if the
> time exchange is over the internet...

How about almost no one.  This is the time-nuts mailing list after all.

It's pretty easy to see a 1 ms offset if you have a good local clock (a place 
to stand) and reasonable network connectivity to the clock you want to check.

Setup ntpd to use the remote system as a server.  Turn on logging.  Wait a while, say a day.  Maybe set minpoll to be nice and/or maxpoll to get more data.

Pull the info for that system from peerstats.  Make two graphs.

First, just plot the offset and RTT as a function of time.  That should give you the big picture.  The normal case is the RTT is flat bottom with spikes going up.  Sometimes you see shifts as the network routing changes.  The spikes are packets that hit queuing in the network.

Next, plot RTT as X and offset as Y.  That should give an arrow pointing left.  The blob at the point is the case where your request didn't encounter any network queuing.  The points along the lines leading to the point are samples with network queuing delays - top line is delays in one direction, bottom line is the other direction.  Points in the middle hit delays in both directions.

The size of the blob determines how accurately you can measure the time offset of the remote system.  (or systematic errors like ADSL or asymmetric routing)

If the fuzzy blob is 1 ms in dia, a 10 ms offset is trivial to see but you have to look closer to see a 1 ms offset.
[My system is messed up.  A good reminder to go fix it.]

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