Considered from the perspective of the TCP/IP protocol suite as a whole, the Network File System (NFS) is a single protocol that resides at the application layer of the TCP/IP (DOD) model. This TCP/IP layer encompasses the session, presentation and application layers of the OSI Reference Model. As I have said before in this Guide, I don't see much value in trying to differentiate between layers 5 through 7 most of the time. In some cases, however, these layers can be helpful in understanding the architecture of a protocol, and that's the case with NFS.

The operation of NFS is defined in the form of three main components that can be viewed as logically residing at each of the three OSI model layers corresponding to the TCP/IP application layer (see Figure 253). These components are:

• Remote Procedure Call (RPC): RPC is a generic session layer service used to implement client/server internetworking functionality. It extends the notion of a program calling a local procedure on a particular host computer, to the calling of a procedure on a remote device across a network.
  
  
• External Data Representation (XDR): XDR is a descriptive language that allows data types to be defined in a consistent manner. XDR conceptually resides at the presentation layer; its universal representations allow data to be exchanged using NFS between computers that may use very different internal methods of storing data.
  
  
• NFS Procedures and Operations: The actual functionality of NFS is implemented in the form of procedures and operations that conceptually function at layer seven of the OSI model. These procedures specify particular tasks to be carried out on files over the network, using XDR to represent data and RPC to carry the commands across an internetwork.

These three key “subprotocols” if you will, comprise the bulk of the NFS protocol. Each is described in more detail in a separate topic of this section on NFS.