The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Network File and Resource Sharing Protocols and the TCP/IP Network File System (NFS)            9  TCP/IP Network File System (NFS)

NFS Data Storage and Data Types, and the External Data Representation (XDR) Standard 1 2 3 NFS Server Procedures and Operations

RPC Operation and Transport Protocol Usage

When a client wants to perform some type of action on a file on a particular machine, it uses RPC to make a call to the NFS server on that machine. The server accepts the request and performs the action required, then returns a result code and possibly data back to the client, depending on the request. The result code indicates if the action was successful. If it was, the client can assume that whatever it asked to be done was completed. For example, in the case of writing data, the client can assume the data has been successfully written to long-term storage.

NFS can operate over any transport mechanism that has a valid RPC implementation at the session layer. Of course in TCP/IP we have two transport protocols, UDP and TCP. It's interesting to see that NFS has seen an evolution of sorts in its use of transport protocol. The NFSv2 standard says that it operates “normally” using UDP, and this is still a common way that NFS information is carried. NFSv3 says that either UDP or TCP may be used, but NFSv4 specifies TCP to carry data. The nominal registered port number for use by NFS is 2049, but in fact other port numbers are sometimes used for NFS, through the use of RPC's “port mapper” capability.

NFS Data Storage and Data Types, and the External Data Representation (XDR) Standard 1 2 3 NFS Server Procedures and Operations