The most important application of RPC is the NFS file sharing protocol. UDP or TCP transport. Standardized data representation. RPC provides support for authenticating the calling program on one machine to the target subroutine on the other.
Authentication can operate in several different modes.
During NFS operations, authentication usually takes the form of relaying UNIX user and group IDs to the file server for permission checking. Both TCP and UDP rely on well-known port numbers to perform service rendezvous. Connecting to TCP and UDP services simply requires connecting to the right port number. RPC introduces another step in this process, to divorce services from being tied to a given port number. Other RPC services, running on any port number, can register themselves using an RPC call to port 111.