Before building your network application, you also need a basic understanding of how the programs, running in multiple end systems, communicate with each other. In the jargon of operating systems, it is not actually programs but processes that communicate. A process can be thought of as a program that is running within an end system. When processes are running on the same end system, they can communicate with each other with inter process communication, using rules that are governed by the end system’s operating system. We are not particularly interested in how processes in the same host communicate, but instead in how processes run fling on different hosts (with potentially different operating systems) communicate.

Processes on two different end systems communicate with each other by exchanging messages across the, computer network. A sending process creates and sends messages into the network; a receiving process receives these messages and possibly responds by sending messages back.

Client and Server Processes

A network application consists of pain of processes that send messages to each other over a network. For example, in the Web application a client browser process exchanges messages with a Web server process. In a P2P file-sharing system, a file is transferred from a process in one peer to a process in another peer. For each pair of communicating processes, we typically label one of the two processes as the client and

the other process as the server. With the Web, a browser is a client process and a Web server is a server process. With P2P file sharing, the peer that is downloading the file is labeled as the client, and the peer that is uploading the file is labeled as the server, You may have observed that in some applications, such as in P2P file sharing, a process can be both a client and a server Indeed, a process in a P2P file-sharing system can both upload and download files. Nevertheless, in the context of any given communication session between a pair of processes we can still label one process the client and the other process as the server. We define the client and server processes as follows:

In the context of a communication session between a pair of processes the process that initiates the communication (that is, initially contacts the other process at the beginning of the session) is labeled as the client. The process that waits to be contacted to begin the session is the server.

In the Web, a browser process initializes contact with a Web server process; hence the browser process is the client and the Web server process is the server. In P2P file sharing, when Peer A asks Peer B to send a specific file, Peer A is the client and Peer B is the server in the context of this specific communication session. When there’s no confusion, we’ll sometimes also use the terminology "client side and server side of an application." we’ll step through simple code for both the client and server sides of network applications.