[pkhuong]

You can use something like White Rabbit (https://en.wikipedia.org/wiki/White_Rabbit_Project) to keep clocks in sync. That still involves estimates, but a dedicated time sync network can do things like make sure all the cables are the same length.

[namibj]

Copper white rabbit is special, it uses the same wire in both directions (1000BASE-T phy with added carrier phase lock to and from outside clocks).