Lecture#2 on Lecture#2 on Internet and Internet and

World Wide WebWorld Wide Web

Internet ApplicationsInternet Applications Electronic Mail (email)Electronic Mail (email)

Domain mail server collects incoming mail Domain mail server collects incoming mail and transmits outing mail and transmits outing mail

File Transfer Protocol (FTP)File Transfer Protocol (FTP) It is a standard protocol used to exchange It is a standard protocol used to exchange

and manipulate files over internetand manipulate files over internet Its purpose is to remotely share files and Its purpose is to remotely share files and

softwaresoftware Voice Over IP (VoIP)Voice Over IP (VoIP)

VoIP (voice over IP) is an IP telephony term VoIP (voice over IP) is an IP telephony term for a set of facilities used to manage the for a set of facilities used to manage the delivery of voice information over the delivery of voice information over the Internet.Internet.

Internet ApplicationsInternet Applications

TelnetTelnet A program that runs over your PC and A program that runs over your PC and

connects it to a server on the network. connects it to a server on the network. You can enter commands using Telnet You can enter commands using Telnet

program and they will be executed as if you program and they will be executed as if you were directly entering them on the server were directly entering them on the server command prompt.command prompt.

This allows you to control the server and This allows you to control the server and communicate with other servers on the communicate with other servers on the network.network.

Internet ApplicationsInternet Applications

World Wide WebWorld Wide Web Web, is a way of accessing information Web, is a way of accessing information

over the medium of the Internet. It is an over the medium of the Internet. It is an information-sharing model that is built on information-sharing model that is built on top of the Internet.top of the Internet.

It uses HTTP(HyperText Transfer It uses HTTP(HyperText Transfer Protocol) for communicationProtocol) for communication

Web uses Web Browsers such as Internet Web uses Web Browsers such as Internet Explorer. Firefox etc. to send data from Explorer. Firefox etc. to send data from one end to another end one end to another end

World Wide Web World Wide Web (continue)(continue)

Web page is a document that needs to Web page is a document that needs to be fetchedbe fetched

Browser gets documents from Web Browser gets documents from Web serverserver

Web Server is the place where web Web Server is the place where web pages are stored/hostedpages are stored/hosted

Documents identified by Uniform Documents identified by Uniform Resource LocatorResource Locator

How the Web Works?How the Web Works?

like Internet communications, the Web relies like Internet communications, the Web relies on protocols to ensure that pages are on protocols to ensure that pages are accessible to any computeraccessible to any computer HyperText Markup Language (HTML) defines the HyperText Markup Language (HTML) defines the

form of Web page contentform of Web page content HyperText Transfer Protocol (HTTP) defines how HyperText Transfer Protocol (HTTP) defines how

messages exchanged between browsers and messages exchanged between browsers and servers are formattedservers are formatted

the prefix the prefix http://http:// in a URL specifies that the HTTP in a URL specifies that the HTTP protocol is to be used in communicating with the serverprotocol is to be used in communicating with the server

the prefix is NOT used for local file access since no server the prefix is NOT used for local file access since no server communication is necessarycommunication is necessary

How Web Works?How Web Works? for efficiency reasons, browsers will sometimes for efficiency reasons, browsers will sometimes cachecache

pages/imagespages/images

to avoid redundant downloads, the to avoid redundant downloads, the browser will store a copy of a browser will store a copy of a page/image on the hard drive (along page/image on the hard drive (along with a time stamp)with a time stamp)

the next time the page/image is the next time the page/image is requested, it will first check the cacherequested, it will first check the cache

if a copy is found, it sends a if a copy is found, it sends a conditionalconditional request to the server request to the server essentially: "send this page/image only if it has been changed essentially: "send this page/image only if it has been changed

since the timestamp"since the timestamp" if the server copy has not changed, the server sends back a if the server copy has not changed, the server sends back a

brief message and the browser simply uses the cached copybrief message and the browser simply uses the cached copy

A typical URLA typical URL

Package-shipping Package-shipping exampleexample

Internet Software LayersInternet Software Layers

Application:Application: Constructs message Constructs message with addresswith address

Transport:Transport: Chops message into Chops message into packetspackets

Network:Network: Handles routing through Handles routing through the Internetthe Internet

Link:Link: Handles actual transmission of Handles actual transmission of packetspackets

The Internet The Internet software layerssoftware layers

Following a message Following a message through the Internetthrough the Internet

SecuritySecurity AttacksAttacks

Malware (viruses, worms, Trojan horses, Malware (viruses, worms, Trojan horses, spyware, phishing software)spyware, phishing software)

Denial of serviceDenial of service SpamSpam

ProtectionProtection FirewallsFirewalls Spam filtersSpam filters Proxy ServersProxy Servers Antivirus softwareAntivirus software