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Computer Networks: An Introduction
- 2. A few questions before we start… How many of you have… Access to a PC and the Internet (home or nearby) Chatted online Used a Bluetooth device Video-chatted Uploaded a photo or video Your own blog or website Written a web application Contributed to an open source project
- 3. What we will cover in What you need to learn in this session your syllabus Useful in Will help you Read up real life understand yourself
- 4. Watch out I will deliberately lie once today You can guess where at the end of the class
- 5. We will cover one or both these topics THE INTERNET THE NETWORK When you type a web site, how does it How did the information travel from appear the way it does on your browser? that machine into your machine?
- 6. URL HTML XML HTTP SMTP CSS Javascript Hosting Hacking Privacy Piracy Security 1 ST TOPIC THE INTERNET
- 7. If you search on Google for “network”… http://www.google.com/search?hl=en&q=network&... scheme resource path query
- 8. When you type a URL… http://www.google.com/search?hl=en&q=network&... scheme resource path query What to ask for? What method? Which machine? http: Use the HTTP protocol, and connect to port 80 ftp: Use the FTP protocol, and connect to port 21 mailto: Use the SMTP protocol, and connect to port 25 etc. Use DNS (Domain Name System) to find the address of www.google.com > nslookup www.google.com Non-authoritative answer: Name: www.google.com Address: 92.52.125.20
- 9. A HTTP request is sent to the resource GET /search?hl=en&q=network&meta= HTTP/1.1 Accept: image/gif, image/jpeg, */* Accept-Language: en-gb UA-CPU: x86 Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1) Host: www.google.com Proxy-Connection: Keep-Alive
- 10. The resource sends back a response HTTP/1.1 200 OK Cache-Control: private, max-age=0 Date: Sat, 06 Dec 2008 04:46:55 GMT Expires: -1 Content-Type: text/html; charset=UTF-8 Set-Cookie: SS=Q0=bmV0d29yaw; path=/search Server: gws Content-Length: 29240 <!DOCTYPE html> <html> <head> <title>network - Google Search</title> <style type=text/css> ...
- 11. This is what the resulting HTML looks like <!DOCTYPE html> <html> <head> <title>network - Google Search</title> <style type=text/css> body{background:#fff;... </style> <script type=text/javascript> ... </script> </head> <body id=gsr topmargin=3 marginheight=3> <div id=header> <div id=gbar> <nobr> <b class=gb1>Web</b> <a href=... class=gb1>Images</a> </nobr> .... </div> </div> </body> </html>
- 12. It’s easy to create a web page yourself • Type some text and save it as something.html Here is some simple text • Add tags around it Here is some <b>bold</b> <i>italics</i> text • Add links <a href=“http://www.google.com/”>Google</a> • Create headings <h1>Heading</h1> • Upload it. Search for “free web hosting” Freehostia.com, Awardspace.com • Rent your own server online Amazon EC2 (Rs 5 / hr)
- 13. The response need not be HTML • Text, images, video, audio as • Structured data transferred as XML / JSON • XML from Amazon’s ECommerce Service (ECS): <ItemSearchResponse xmlns=http://webservices.amazon.com/AWSECommerceService/2005-03-23> <Items> <Request> <IsValid>True</IsValid> <ItemSearchRequest><Author>J K Rowling</Author> <Condition>New</Condition> <DeliveryMethod>Ship</DeliveryMethod> ... </Items> </ItemSearchResponse> • HTML is a subset of XML with pre-defined tags
- 14. You don’t want to show everyone everything Authorization: Are you who you say you are? – Most online mail sites asks for login and password – Some will remember you, using cookies – Passwords are stolen. Hence, two-factor authentication • Something you have + and something you know • Examples: Credit Card and PIN, RSA SecureID and passcode – Biometric sensors are common on most new laptops Authentication: Are you allowed to do that? – Use of “Pro” features on Yahoo! Mail or Flickr restricted to Pro users – What you’re allowed to do is often stored in one server. Other machines ask this server, “Can this fellow do that?”
- 15. 2 ND TOPIC THE NETWORK
- 16. What is a network? Examples of devices you connect? What medium connects these devices? How to connect multiple devices? How to talk simultaneously?