White Rabbit – sub-nanosecond synchronization for large distributed systems

there are several 10 gbps implementations made in Europe https://gitlab.com/ohwr/project/10G-wr-nic/-/blob/f_ltgt_spe...

In our lab tests phase lock jitter between WR client and master is about 10ps (picoseconds) over 50km fiber (with temperature change of the fiber, so WR actively compensates elongations), so relative clock of one system can be transmitted with about that accuracy to another.

P.S. There is WR workshop this week with some talks being publicly available on CERN's indico website.

It's totally possible to achieve synchronization better than light transmission time. For the purposes of synchronization, the speed of light delay, and any other delay are indistinguishable, and need not be distinguished.

Two-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.

The gravity well time dilation is about 3.5 nanoseconds per meter per year near the surface of the earth. (time changes rate with altitude in a gravity well)

Sub-nanosecond synchronization is getting into the relativity is measurable realm.

Yes, it uses phased locked loops and measures phase difference between the master clock and the local clock.

yes, it needs custom built hardware to work.

> Conventional wisdom is that distributed consensus is not possible at this kind of performance

I'm not sure why you would think that? If you can assume the fiber is the same in both directions you know the round trip time is exactly double the latency of the connection. Then you know to phase shift your start time by that much when you get a start signal and you're in sync.

Obviously it's not trivial in practice, but it's not a fundamentally insurmountable problem.

Probably even better to start at the collaboration website https://www.white-rabbit.tech/wr-technology/

Distributed systems spend most of their effort on one problem: agreeing on the order of events across machines. Without synchronized physical clocks you have two options. Logical clocks (Lamport, vector) give you causal order but not wall-clock truth, so you can’t answer “did A really happen before B” for events that don’t have a happens-before relationship. Or you run consensus, which gives total order but costs round trips. At geographic scale that’s tens of milliseconds per decision, and the floor is set by the speed of light.

Tight clock sync collapses this. If clock uncertainty ε is small and bounded, you can timestamp a write, wait ε, and trust the global order without talking to anyone. Spanner’s external consistency works because TrueTime’s ε was a few milliseconds, so commit-wait was tolerable. The latency cost of planet-scale serializability stops depending on how far apart your replicas are and starts depending on how good your clocks are.

That’s the real significance. Time sync converts a coordination problem (bounded by physics) into a local computation (bounded by clock quality). Spanner proved this is possible but required GPS receivers and atomic clocks in every datacenter, which kept the capability inside Google for years. White Rabbit-class sync pushes ε from milliseconds toward sub-nanoseconds over commodity Ethernet hardware, and it’s now in IEEE 1588 as a standard PTP profile. If sub-nanosecond sync becomes baseline network infrastructure, the long-held assumption that strong consistency has to be slow at geographic scale stops holding, and a meaningful chunk of what databases currently work around (HLCs, weak isolation defaults, application-level reconciliation) becomes unnecessary.

Used quite a bit by stock exchanges to ensure consumers and publishers have a reasonably aligned time.

it is useful e.g. to align the phase of signals being sent from different locations

Its on gitlab but even there I failed to find sources, documentation/presentations are there though