Hacker News Reader

The roundtrip time is measured and compensated. Even NTP does this. Knowing the distance is not necessary for time synchronization.

gPTP estimates the link delay and the peer clock ratio, see for example this random link I just found for you: https://blog.meinbergglobal.com/2024/03/27/what-is-gptp/

Hmm one would expect heat expansion to change the length of fiber over tens of kilometers. Does it also affect light speed in the fiber? I think consumer fiber is not buried very deep on average, but maybe for these use cases you use something hefty anyway.

It doesn't matter if the length changes provided:

* you measure the round trip time often enough

* the shift affects light in both directions equally

delay is easy

jitter kills

... why would cm precision be often not possible?

Because you're using fibres laid years ago. Often pulse velocity isn't known to better than one part in 1000.