Computer Networks: An Introduction

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Session for Class XI and XII at Vidya Mandir, Mylapore, Chennai in Dec 2008

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Computer Networks: An Introduction

  1. 1. Computer Networks December 2008
  2. 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. 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. 4. Watch out I will deliberately lie once today You can guess where at the end of the class
  5. 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. 6. URL HTML XML HTTP SMTP CSS Javascript Hosting Hacking Privacy Piracy Security 1 ST TOPIC THE INTERNET
  7. 7. If you search on Google for “network”… http://www.google.com/search?hl=en&q=network&... scheme resource path query
  8. 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. 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. 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. 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. 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. 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. 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. 15. 2 ND TOPIC THE NETWORK
  16. 16. What is a network? Examples of devices you connect? What medium connects these devices? How to connect multiple devices? How to talk simultaneously?

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