UDP and TCP Negotiation

During initiation, the transport protocol is also negotiated. By default, the first connection-oriented transport supported on both the client and the server is selected. If this does not succeed, then the first available connectionless transport protocol is used. The transport protocols supported on a system are listed in /etc/netconfig. TCP is the connection-oriented transport protocol supported by the release. UDP is the connectionless transport protocol.

When both the NFS protocol version and the transport protocol are determined by negotiation, the NFS protocol version is given precedence over the transport protocol. The NFS version 3 protocol using UDP is given higher precedence than the NFS version 2 protocol using TCP. You can manually select both the NFS protocol version and the transport protocol with the mount command (see the mount_nfs(1M) man page). Under most conditions, it is better to allow the negotiation to select the best options.