In addition to NTP, there's also IEEE 1588. All of these "clock synch over packet switched network" mechanisms are pretty similar, the differences are mainly in the timestamp filtering and processing. NTP details the algorithms, IEEE 1588 leaves them open to implementation and RADClock has its own algorithms (details unknown at least to me).
This story is also a bit dupeish, as RADClock has been recently featured here. Thus I'll copypaste the relevant parts from my previous reply:
(disclaimer: just finished my Master's thesis on a related subject) About the 1588 in general: its main selling point is the ability to do hardware timestamping (when using hardware with support!) of the two-way timing messages between master and slave. This eliminates the very significant timing jitter that happens in the software stack before the messages are timestamped. For reference, commercially available master-slave implementations using IEEE 1588 achieve synchronisation within tens of nanoseconds within LAN, and microseconds to tens of microseconds within WAN, depending on network conditions.
So overall I think that while RADclock might be ok as an alternative between NTP and IEEE 1588, it doesn't really bring anything new to the table. Some of the stuff in the Rideaux/Veitch paper has also been used with IEEE 1588 for quite some time, for instance the filtering for fast timing packets is a necessity for accurate synchronisation with IEEE 1588.