The TCP/IP Application Layer and OSI
As was mentioned in Hour 2, "How TCP/IP Works," TCP/IP does not officially conform to the seven-layer OSI networking model. The OSI model, however, has been very influential in the development of networking systems, and the recent trend toward multiprotocol networking has increased reliance on OSI terminology and concepts. The Application layer can draw from a vast range of operating and networking environments, and in many of those environments the OSI model is an important tool for defining and describing network systems. A look at the OSI model will help you understand the processes that take place at the TCP/IP Application layer.
The TCP/IP Application layer corresponds with the OSI Application, Presentation, and Session layers (see Figure 7.1). The extra subdivisions of the OSI model (three layers instead of one) provide some additional organization of features that TCP/IP theorists have traditionally grouped into the heading of Application-level (sometimes called Process/Application-level) services.
Descriptions of the OSI layers corresponding to TCP/IP's Application layer are as follows:
All of these services are not necessary for all applications and implementations. In the TCP/IP model, implementations are not required to follow the layering of these OSI subdivisions, but overall, the duties defined for OSI's Application, Presentation, and Session layers fall within the range of the TCP/IP Application layer's responsibility.