[tgingold]

No, we don't know the distance between nodes (although we could deduce it). But using timestamps, we can know the round-trip time.

See https://www.ohwr.org/project/white-rabbit/uploads/2b9d42b664... (page 9 and later for the principle).

If you want all the details, see https://ohwr.org/project/white-rabbit/uploads/6a357829064b9e...

[Aune]

When synchronizing two nodes A and B, where there is a persistent difference in the travel times A->B and B->A, how do you achieve synchronization when knowing A->B->A or B->A->B?

[ooterness]

Delay symmetry is a critical assumption in any two-way time transfer process. White Rabbit goes to extreme lengths to maintain that property.

This includes mandating use of cables that share a single optical fiber, with specific wavelength pairs and fiber types so you can calibrate for unavoidable differences in propagation time.

More info on their wiki:

https://ohwr.org/projects/white-rabbit/wiki/SFP

[tgingold]

For a first approximation, you can assume A->B and B->A travel times are equal.

And because optical links are used, the asymmetry is mainly due to the wavelength difference which is known.

[rcxdude]

you can't. You can only assume that they are equal and attempt to make them as equal as possible. (the same issue arises when measuring the speed of light: it's actually not possible to distinguish if the speed of light is different in one direction to another, we only know accurately the average of each direction)

[ramdac]

What happens when the roundtrip time isn't consistent?

[nickez]

The roundtrip time is never consistent. Light travels with different speed in fiber depending on the temperature. This is why you calibrate every second.