Chirag's Blog: Principle of Application Layer | Introduction of Application Layer

Principle of Application Layer

Applications layer provides the interface between the applications, use communicate and underlying network. When communicating processes are running on the same system, they communicate with each other using inter-process communication.

Figure: Inter-Process Communication

Processes running on two different end systems communicate with each other by message passing between computer networks.

Figure: Message Passing

Application Layer Protocols

World wide web, electronic mail system and domain name system are the traditional application of the application layer network. Hypertext Transport Protocol (HTTP) is an application protocol. HTTP is used to retrieve Web pages from remote servers. Widely used application protocol are SMTP and HTTP. Simple mail transfer protocol is used to exchange e-mail. HTTP is used to communicate between Web browser and Web server.

Client and Server-Side Application

Web browser is client-side application of HTTP. Web server is server-side application of HTTP. In Electronic mail system, sending mail is the client side of SMTP and the receiving mail is the server-side SMTP.

Processes Communicating Across Network

Two processes communicate with each other by sending and receiving messages. Socket is used for process communication. Socket is an interface between application layer and transport layer with in a machine. Socket is also referred as the application programmer’s interface (API) between the application and the network. Multiple sockets might exist in each host. A port number identifies each such socket in each host.

Figure: Process Communicating across network

Client Process: Process that initiates communication.

Server Process: Process that waits to be contacted.

Application protocols used by both the source and destination device during a communication session. The protocols implemented on the source and destination host must match. The client process begins the exchange by requesting data from the server. Server responds by sending one or more streams of data to the client.

Addressing Processes

A sender uses the address of the destination host to specify where the message should send. In host-to-host delivery, receiver identifies in the network through the IP Address. When data is received at the destination, the port number examined to determine which application or process is the correct destination for the data. Each device on a network must be uniquely defined. In internet applications, the destination host is identified by its IP address.