Protocols are assigned to each layer of the layered model to perform each layers task. Here, we would define the functions of each protocol as based on the DoD four layered model.

APPLICATION LAYER

Several protocols make up the Application layer of the TCP/IP suite. This layer of the TCP/IP protocol suite corresponds roughly to the Application, Presentation, and Session layers of the OSI Reference Model. The protocols implemented at the Application layer are:

Telnet This is a terminal emulation service that provides remote login over the network. FTP File Transfer Protocol is used for interactive file transfer. SMTP The Simple Mail Transfer Protocol delivers electronic mail on a system.
DNS Domain Name Service maps host names to IP addresses. RIP Routing Information Protocol advertises routes to different network numbers on the internetwork. NFS The network File System allows directories on a host computer t be shared by other machine on the network.

Each application normally requires two separate programs: a client program and a server program (often referred to as daemon). The daemon program runs in the background on the host server and may start when the system starts. More often daemon programs are required by the system. A user on the other hand, executes the client program to gain access to the server. Some Application layer protocols use different Transport layer protocols; for example, Telnet and SMTP rely on TCP, whereas RIP and DNS rely on UDP.