computer networks: an introduction
DESCRIPTION
Session for Class XI and XII at Vidya Mandir, Mylapore, Chennai in Dec 2008TRANSCRIPT
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Computer Networks
December 2008
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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
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this sessionWhat we will cover in
Will help you understand
Read up yourself
Useful in real life
your syllabusWhat you need to learn in
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Watch outI will deliberately lie once today
You can guess where at the end of the class
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We will cover one or both these topics
When you type a web site, how does it appear the way it does on your browser?
How did the information travel fromthat machine into your machine?
THE INTERNET THE NETWORK
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ST TOPICTHE INTERNET1
HTMLHTTP
CSS
URL
HostingPrivacy
Security
XMLSMTPJavascript
HackingPiracy
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If you search on Google for “network”…
http://www.google.com/search?hl=en&q=network&...
scheme resource path query
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When you type a URL…
http://www.google.com/search?hl=en&q=network&...
scheme resource path query
Which machine?What method? What to ask for?
http: Use the HTTP protocol, and connect to port 80ftp: Use the FTP protocol, and connect to port 21mailto: Use the SMTP protocol, and connect to port 25etc.
Use DNS (Domain Name System) to find the address of www.google.com> nslookup www.google.comNon-authoritative answer:Name: www.google.comAddress: 92.52.125.20
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A HTTP request is sent to the resourceGET /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
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The resource sends back a responseHTTP/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">
...
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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>
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It’s easy to create a web page yourself
• Type some text and save it as something.htmlHere is some simple text
• Add tags around itHere 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 onlineAmazon EC2 (Rs 5 / hr)
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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
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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?”
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ND TOPICTHE NETWORK2
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What is a network?
Examples of devices you connect?
What medium connects these devices?
How to connect multiple devices?
How to talk simultaneously?