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UNIT – IVUNIT – IV

APPLICATION LAYER APPLICATION LAYER

WWW and HTTP – FTP – Email –

Telnet –SSH – DNS – SNMP

INTRODUCTIONINTRODUCTION

 (^) The application layer is the highest layer in the protocol suite.  (^) The application layer provides services to the user.  (^) Communication is provided using a logical connection, which means that the two application layers assume that there is an imaginary direct connection through which they can send and receive messages.  (^) The application layer is the only layer that provides services to the Internet user  (^) The application layer exchange messages with their peers on other machines  (^) Types of Application Protocols:  (^) Standard and Nonstandard Protocols05/04/25 (^) Karpagam Institute of Technology 2

Two very widely-used standardized application protocols:  SMTP : Simple Mail Transfer Protocol is used to exchange electronic mail.  HTTP : Hyper Text Transport Protocol is used to communicate between Web browsers and Web servers.

Nonstandard Application-Nonstandard Application- Layer Protocols Layer Protocols 

A programmer can create a nonstandard

application-layer program if they can

write two programs that provide service

to the user by interacting with the

transport layer.

Client-Server ParadigmClient-Server Paradigm

 (^) The traditional paradigm is called the client- server paradigm.  (^) It was the most popular Paradigm.  (^) In this paradigm, the service provider is an application program, called the server process; it runs continuously, waiting for another application program, called the client process, to make a connection through the Internet and ask for service.  (^) The server process must be running all the time; the client process is started when the client needs to receive service.  There are normally some server processes that can provide a specific type of service, but there are many clients that request service from any of these server processes.

Client-Server ParadigmClient-Server Paradigm

Peer-to-Peer(P2P)Peer-to-Peer(P2P)

Paradigm Paradigm

Mixed ParadigmMixed Paradigm



An application may choose to use a

mixture of the two paradigms by

combining the advantages of both.



For example, a light-load client-server

communication can be used to find the

address of the peer that can offer a

service.



When the address of the peer is found, the

actual service can be received from the

peer by using the peer-to-peer paradigm.

WWW (WORLD WIDE WEB)WWW (WORLD WIDE WEB)



This system allows document search and

retrieval from any part of the Internet.



The documents were having Hypertext as

the content



The units of information on the web can

be referred to as pages, documents or

resources.



A document can contain text, images,

sound and video, together called

Hypermedia.



Web is a vast collection of data,

information, software and protocols ,

spread across the world in web servers,

which are accessed by client machines by

browsers through the Internet.

COMPONENTS OF THE WEBCOMPONENTS OF THE WEB



Clients use browser application to send

URL’s via HTTP to servers requesting a Web

page.



Web pages constructed using HTML /XML

and consist of text, graphics, sounds plus

embedded files Servers (or caches) respond

with requested Web page.



Client’s browser renders Web page returned

by server



Web Page is written using Hyper Text

Markup Language (HTML)



Displays text, graphics and sound in

browser



The entire system runs over standard

networking protocols (TCP/IP, DNS)

WEB CLIENTS (BROWSERS)WEB CLIENTS (BROWSERS)

 (^) A browser is a software on the client on the web which initiates the communication with the server.  (^) Each browser usually consists of three parts:  (^) a controller,  (^) client protocols, and  (^) interpreters.  (^) The controller receives input from the keyboard or the mouse and uses the client programs to access the document.  (^) After the document has been accessed, the controller uses one of the interpreters to display the document on the screen.  (^) Examples are Internet Explorer, Mozilla FireFox, Netscape Navigator, Safari etc.

WEB SERVERSWEB SERVERS

 (^) All the communication between the web client and a web server use the standard protocol called as HTTP.  (^) Web server informs its operating system to accept incoming network connections using a specific port on the machine.  (^) The server also runs as a background process.  (^) A client (browser) opens a connection to the server, sends a request, receives information from server and closes the connection.

WEB SERVERSWEB SERVERS

 (^) Web server monitors a communications port on its host machine, accepts the http commands through it and performs specified operations.  (^) HTTP commands include a URL specifying the host machine.  (^) The URL received is translated into either a filename or a program name, accordingly the requested file or the output of the program execution is sent back to the browser.