The OSI Reference model has seven layers performing seven categories of functions necessary for computer communication.The TCP/IP protocol stack has only four layers.What functionality, if any is missing inthe TCP/IP protocol stack?
No functionality is missing. The TCP/IP stack incorporates the functions of the OSI Application, Presentation and Session layers in its Application layer. This is an example of the flexibility of the OSI architecture concept. Another example is the NetWare stack that does not have seven layers.