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Unit – 6Application Layer 6L 6.1 Introduction To HTTP, FTP World Wide Web Telnet, Name System (DNS)HTTPHTTP stands for Hyper Text Transfer Protocol. It is a protocol used to access the data on the World Wide Web (www). The HTTP protocol can be used to transfer the data in the form of plain text, hypertext, audio, video, and so on. This protocol is known as HyperText Transfer Protocol because of its efficiency that allows us to use in a hypertext environment where there are rapid jumps from one document to another document. HTTP is similar to the FTP as it also transfers the files from one host to another host. But, HTTP is simpler than FTP as HTTP uses only one connection, i.e., no control connection to transfer the files. HTTP is used to carry the data in the form of MIME-like format. HTTP is similar to SMTP as the data is transferred between client and server. The HTTP differs from the SMTP in the way the messages are sent from the client to the server and from server to the client. SMTP messages are stored and forwarded while HTTP messages are delivered immediately. Features of HTTP:Connectionless protocol: HTTP is a connectionless protocol. HTTP client initiates a request and waits for a response from the server. When the server receives the request, the server processes the request and sends back the response to the HTTP client after which the client disconnects the connection. The connection between client and server exist only during the current request and response time only. Media independent: HTTP protocol is a media independent as data can be sent as long as both the client and server know how to handle the data content. It is required for both the client and server to specify the content type in MIME-type header. Stateless: HTTP is a stateless protocol as both the client and server know each other only during the current request. Due to this nature of the protocol, both the client and server do not retain the information between various requests of the web pages. FTPFTP stands for File transfer protocol. FTP is a standard internet protocol provided by TCP/IP used for transmitting the files from one host to another. It is mainly used for transferring the web page files from their creator to the computer that acts as a server for other computers on the internet. It is also used for downloading the files to computer from other servers. Objectives of FTPIt provides the sharing of files. It is used to encourage the use of remote computers. It transfers the data more reliably and efficiently. Why FTP?Although transferring files from one system to another is very simple and straightforward, but sometimes it can cause problems. For example, two systems may have different file conventions. Two systems may have different ways to represent text and data. Two systems may have different directory structures. FTP protocol overcomes these problems by establishing two connections between hosts. One connection is used for data transfer, and another connection is used for the control connection. Introduction to World Wide WebThe World Wide Web (WWW) is a collection of documents and other web resources which are identified by URLs, interlinked by hypertext links, and can be accessed and searched by browsers via the Internet. World Wide Web is also called the Web and it was invented by Tim Berners-Lee in 1989. Website is a collection of web pages belonging to a particular organization. The pages can be retrieved and viewed by using browser. 
Let us go through the scenario shown in above fig.The client wants to see some information that belongs to site 1. It sends a request through its browser to the server at site 2. The server at site 1 finds the document and sends it to the client. Client (Browser):Web browser is a program, which is used to communicate with web server on the Internet. Each browser consists of three parts: a controller, client protocol and interpreter. The controller receives input from input device and use the programs to access the documents. After accessing the document, the controller uses one of the interpreters to display the document on the screen. 
Server:A computer which is available for the network resources and provides service to the other computer on request is known as server. The web pages are stored at the server. Server accepts a TCP connection from a client browser. It gets the name of the file required. Server gets the stored file. Returns the file to the client and releases the top connection. Uniform Resource Locater (URL)The URL is a standard for specifying any kind of information on the Internet. The URL consists of four parts: protocol, host computer, port and path. The protocol is the client or server program which is used to retrieve the document or file. The protocol can be ftp or http. The host is the name of computer on which the information is located. The URL can optionally contain the port number and it is separated from the host name by a colon. Path is the pathname of the file where the file is stored.
Let us go through the scenario shown in above fig.
Server:
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