TCP or UDP ?

The NFS protocol is transport independent. It will run over a stream-oriented protocol like TCP or a datagram protocol like UDP. Historically, there are more UDP implementations of the NFS protocol because early implementations of TCP were notoriously slow. In recent years the performance of UDP and TCP implementations on local area networks is comparable and on wide area networks and the Internet there is a distinct preference for the congestion control and reliability of TCP. Additionally, it is much easier to control TCP connections at a firewall than UDP.

The NFS client will first attempt to use TCP to connect to any NFS server whether it be a local server or across the world on the Internet. Only if the server rejects the TCP connection attempt will the NFS client use UDP.