stantiations of a PTP stack, each operating in a different domain. If the network is well designed, and each domain takes different paths, then this results in a very ro- bust and resilient distribution of time. The slave ports can cross-validate the different values of tim- ing they are receiving in different domains and discard any rogue results. So, the default version of BCMA is supported within a domain on a clock, but the clock may be tracking multiple grandmas- ters if they are in different domains. Message rates for this profile are a default of 1 per second with a range from 128 per second to one every 128 seconds. Only the Announce message is fixed at 1 per second. Negotiation of message rates is not allowed. 7.5.1 White Rabbit White Rabbit (WR) is a project that originated within the European Organization for Nuclear Re- search (CERN) facility and has now spread out to other research institutions. The initial require- ment was to support scientific experiments in particle physics, but it has since been applied to many applications requiring highly accurate time. The project aimed at creating a low-latency network based on Ethernet that could provide high-accuracy timing distribution. The goal was to achieve synchronization between thousands of nodes, spread across tens of kilometers to sub-nanosecond accuracy. WR makes use of the following technology: Ethernet-based transport (over fiber, which is typically a single BiDi fiber to reduce any asymmetry) PTP for phase/time transfer SyncE for frequency synchronization Methods to reduce link asymmetry and increase clock accuracy Several techniques were used to increase the accuracy: Use SyncE to deliver frequency synchronization in combination with PTP (also available in other implementations, such as the G.8275.1 telecom profile). Increase the granularity and accuracy of the timestamping (which was commonly 8 ns in previ- ous implementations, limiting the possible accuracy). Measure and calibrate any asymmetry in the hardware implementation of time distrib