RNBD/RTRS were created with one aim: provide simple way to map a remote device on a client machine from a server machine. RNBD is nothing more but a block device which uses RTRS as a transport library to send I/O requests. RTRS in its turn manages RDMA (InfiniBand, RoCE, iWarp) connections between client and server.
RNBD documentation is here.
RTRS documentation is here.