Lecture#2 on Internet and World Wide Web

Internet Applications Electronic Mail ( ) Electronic Mail ( ) Domain mail server collects incoming mail and transmits outing mail Domain mail server collects incoming mail and transmits outing mail File Transfer Protocol (FTP) File Transfer Protocol (FTP) It is a standard protocol used to exchange and manipulate files over internet It is a standard protocol used to exchange and manipulate files over internet Its purpose is to remotely share files and software Its purpose is to remotely share files and software Voice Over IP (VoIP) Voice Over IP (VoIP) VoIP (voice over IP) is an IP telephony term for a set of facilities used to manage the delivery of voice information over the Internet. VoIP (voice over IP) is an IP telephony term for a set of facilities used to manage the delivery of voice information over the Internet.

Internet Applications Telnet Telnet A program that runs over your PC and connects it to a server on the network. A program that runs over your PC and connects it to a server on the network. You can enter commands using Telnet program and they will be executed as if you were directly entering them on the server command prompt. You can enter commands using Telnet program and they will be executed as if you were directly entering them on the server command prompt. This allows you to control the server and communicate with other servers on the network. This allows you to control the server and communicate with other servers on the network.

Internet Applications World Wide Web World Wide Web Web, is a way of accessing information over the medium of the Internet. It is an information-sharing model that is built on top of the Internet. Web, is a way of accessing information over the medium of the Internet. It is an information-sharing model that is built on top of the Internet. It uses HTTP(HyperText Transfer Protocol) for communication It uses HTTP(HyperText Transfer Protocol) for communication Web uses Web Browsers such as Internet Explorer. Firefox etc. to send data from one end to another end Web uses Web Browsers such as Internet Explorer. Firefox etc. to send data from one end to another end

World Wide Web (continue) Web page is a document that needs to be fetched Web page is a document that needs to be fetched Browser gets documents from Web server Browser gets documents from Web server Web Server is the place where web pages are stored/hosted Web Server is the place where web pages are stored/hosted Documents identified by Uniform Resource Locator Documents identified by Uniform Resource Locator

How the Web Works? like Internet communications, the Web relies on protocols to ensure that pages are accessible to any computer like Internet communications, the Web relies on protocols to ensure that pages are accessible to any computer HyperText Markup Language (HTML) defines the form of Web page content HyperText Markup Language (HTML) defines the form of Web page content HyperText Transfer Protocol (HTTP) defines how messages exchanged between browsers and servers are formatted HyperText Transfer Protocol (HTTP) defines how messages exchanged between browsers and servers are formatted the prefix in a URL specifies that the HTTP protocol is to be used in communicating with the server the prefix in a URL specifies that the HTTP protocol is to be used in communicating with the server the prefix is NOT used for local file access since no server communication is necessary the prefix is NOT used for local file access since no server communication is necessary

How Web Works? for efficiency reasons, browsers will sometimes cache pages/images for efficiency reasons, browsers will sometimes cache pages/images to avoid redundant downloads, the browser will store a copy of a page/image on the hard drive (along with a time stamp) to avoid redundant downloads, the browser will store a copy of a page/image on the hard drive (along with a time stamp) the next time the page/image is requested, it will first check the cache the next time the page/image is requested, it will first check the cache if a copy is found, it sends a conditional request to the server if a copy is found, it sends a conditional request to the server essentially: "send this page/image only if it has been changed since the timestamp" essentially: "send this page/image only if it has been changed since the timestamp" if the server copy has not changed, the server sends back a brief message and the browser simply uses the cached copy if the server copy has not changed, the server sends back a brief message and the browser simply uses the cached copy

A typical URL

Package-shipping example

Internet Software Layers Application: Constructs message with address Application: Constructs message with address Transport: Chops message into packets Transport: Chops message into packets Network: Handles routing through the Internet Network: Handles routing through the Internet Link: Handles actual transmission of packets Link: Handles actual transmission of packets

The Internet software layers

Following a message through the Internet

Security Attacks Attacks Malware (viruses, worms, Trojan horses, spyware, phishing software) Malware (viruses, worms, Trojan horses, spyware, phishing software) Denial of service Denial of service Spam Spam Protection Protection Firewalls Firewalls Spam filters Spam filters Proxy Servers Proxy Servers Antivirus software Antivirus software