Hello @mlichvar, thanks for your reply. This is interesting. I’ve extracted delay and offset from my
chrony logs to understand this better.
The measurements log file has these, just as you said, and I’ve begun plotting delay and offset to some stratum 1 servers.
Plotting delay vs. offset seems to be the most useful approach for cutting through the noise and jitter, just like Dr. Mills on slide 12 of his seminar.
This screenshot with the Vee is from that presentation. I’m getting similar results.
But on the symmetry issue, I have to disagree, on offset at least. The 1/2 in the offset equation is a clue that symmetry is, indeed, assumed. The basic NTP time transfer approach assumes symmetric 2-way messaging and offset is estimated as the median of the two delta-t terms. This is mentioned in the ntp faq. The symmetry assumption is not terrible, especially with repeated attempts, but it’s part of the offset formulation.
On the delay, you are indeed correct. There is no symmetry assumed and the delay equation is a round trip delay, in total. There is no 1/2 term in the delay equation so highly asymmetric network links will report the correct round trip delay.
It may be interesting to know there is another time keeping community focused on Wireless Sensor Networks (WSNs). They use 1/2 in both offset and delay estimates. The NTP community, here, uses total round trip time and does not assume symmetry for total delay.