What Is the Application Layer?
The Application layer is the top layer in TCP/IP's protocol suite. In the Application layer, you'll find network applications and services that communicate with lower layers through the TCP and UDP ports discussed in Hour 6, "The Transport Layer." You might ask why the Application layer is considered part of the stack at all, as the TCP and UDP ports form such a well-defined interface to the network. But it is important to remember that, in a layered architecture such as TCP/IP, every layer is an interface to the network. The Application layer must be as aware of TCP and UDP ports as the Transport layer is and must channel data accordingly.
TCP/IP's Application layer is really an assortment of network-aware software components sending information to and receiving information from the TCP and UDP ports. These Application layer components are not really parallel in the sense of being logically similar or equivalent. Some of the components at the Application layer are simple utilities that collect information about the network configuration. Other Application layer components might be a user interface system (such as the X Window interface) or an Application Program Interface (API) that supports a desktop operating environment. Some Application layer components provide services for the network, such as file and print services or name resolution services. (You'll learn more about name resolution in Hour 11, "Name Resolution.") This hour shows you some of the kinds of services and applications that are usually found in the Application layer. The actual implementation of these components hinges on details of programming and software design.
But first this hour begins with a quick comparison of TCP/IP's Application layer with the corresponding layers defined through TCP/IP's counterpart, the OSI model.