alt Hacker NewsAnother protocol that's not mentioned is PPS and its variants, such as WhiteRabbit.
A regular pulse is emitted from a specialized high-precision device, possibly over a specialized high-precision network.
Enables picosecond accuracy (or at least sub-nano).
As a user of WhiteRabbit, I can confirm a sub-10ps sync (two clocks phase lock) over 50km fiber connection for variable temperature of fiber (biggest problem of clock sync over fibers is temperature induced length change of the fiber itself, which needs to be measured and compensated).