Encapsulating TCP over UDP allows the support of peers which implement TCP in userland without requiring special permissions by encapsulating the TCP segments in UDP.
Support for this is already available in [[https://www.wireshark.org | Wireshark ]], [[ https://github.com/freebsd-net/packetdrill | packetdrill ]], and [[ https://github.com/uperf/uperf | uperf ]].
It is planned to merge this to the main tree, after FreeBSD 13.0 has been released.