Introduction

• Internet History
• Basic Internet Concepts
• World Wide Web
• HTTP
• Web Applications

Internet

• A global network of networks through which computers communicate by sending information in packets
• Envisioned by its inventors to serve as
  • a world-wide broadcasting medium
  • a mechanism for information dissemination
  • a medium for global collaboration
• Came to public attention only 25 years after its inception
  • appearance of web browsers
  • commercial use and "dot-com" revolution

History of the Internet

Growth of the Internet

• The number of hosts connected by the Internet follows an exponential growth pattern (network effect)

  

Hosts

• A host is a computing device connected to the Internet (PC, Internet appliance, mobile phone)
• Sends and receives packets
• Every host constantly monitors the network for packets
• Collision detection with retry

Internet Protocol (IP)

• The protocol (a set of conventions) that governs how packets are sent across the Internet
• Defines a format for assigning addresses to hosts
• A mechanism for transmitting packets between source and destination

IP Addresses

• Host addresses are 4 bytes long
• Assigned by Internet Assigned Numbers Authority (IANA)
• Usually presented in dotted decimal notation

  134.117.5.8

Domain Names

• The dotted decimal notation is not very memorable
  • On top, IP address often change for any number of reasons, and should not be hardcoded
• Symbolic names provide an alternative way of naming hosts

  www.scs.carleton.ca

• Given a domain name, how do we resolve it to its IP address?
  • Domain Name Server (DNS)

Domain Name Server (DNS)

• A DNS is a federated database
• Each domain has a unique name, and each of the subdomains in a domain

  

Domains and Zones

• Domain name space divided into zones
• Zones start at a domain and extend down to leaf domains or to domains where other zones start
• Each zone has two or more authoritative DNS servers

  

Ports

• Every host has multiple ports (ako mailboxes) on which it can receive packets
• Ports identify the source and destination applications
• Certain ports (0-1024) are reserved for standard services

Ports for Standard Services

• 20: File Transfer Protocol (FTP)
• 22: Secure Shell (SSH)
• 23: Telnet
• 25: Simple Mail Transfer Protocol (SMTP)
• 80: Hypertext Transfer Protocol (HTTP)

History of the World Wide Web

• Vannevar Bush envisions the Memex in 1945
• Ted Nelson introduces the term "hypertext" in 1965
• Tim Berners-Lee invents HTML in 1989
• Netscape releases the first commercial browser in 1994
• W3C begins work on XML in 1997

Web System

• A web system is a hypertext document system with three fundamental components
  • client browser
  • network
  • web server

  

Resources

• Anything accessible via the web is referred to as a resource
  • an HTML document
  • an image
• Static resources don't change from request to request, such as an HTML document or an image
• Dynamic resources may vary with each request, depending on a number of conditions
  • a changing data source
  • user identity
  • input from the user passed with the request
  • time of day

Uniform Resource Locators (URLs)

• Provide a standard way to locate resources on the Web
• Web equivalent for specifying a file name within a file system
• An URL identifies the protocol, host, optional port number, and document path.
  • Common protocols include http, https, ftp, and file
  • Host identifies the machine running the web server
  • Host and port number are separated by a colon (80 is used as default for http URLs)
  • Path is the location of the resource requested

  

HTTP

• HyperText Transfer Protocol
• Implements a request-response model
• Connectionless
  • As soon as a request is processed, the connection between client and server is terminated
• Defines format for requests and responses
• Response includes MIME type

Sample HTTP Request

• At a minimum, a request includes
  • type of request
  • resource requested
  • name of server
• The following request is of type GET
• It asks the server for the resource /index.html

  GET /~weiss/courses/205/index.html HTTP/1.0
  Host: www.scs.carleton.ca

Sample HTTP Response

• This is a sample response the client browser would receive

  HTTP/1.0 200 OK
  Date: Sat, 06 Jan 2001 10:05:30 GMT
  Server: Apache/1.2.13 (Linux)
  Last-Modified: Wed, 03 Jan 2001 20:14:26 GMT
  ETag: "0-9db-3a538822"
  Accept-Ranges: bytes
  Content-Length: 2523
  Connection: close
  Content-Type: text/html
  
  <html>
  <head>
  &lt;title&gt;95.205 C/D - Internet Applications&lt;/title&gt;
  ...

MIME

• Multipurpose Internet Mail Extensions
• A specification for formatting non-text messages (graphics, audio, and video files) so they can be sent over the Internet
• Originally developed for formatting e-mail messages
• Many predefined MIME types
  • text/html for HTML documents
  • image/gif for image/gif for graphics
  • application/pdf for PDF files
• Browsers can specify helper applications (plug-ins or external applications) to handle unsupported MIME types

Web Applications

• Web applications are online applications that users can access via standard web browsers
• Distinct from web sites that only serve static content
• Web applications operate in background when you
  • fill out an online registration form (writing your registration information to a database)
  • make an online purchse (validating your credit card, logging the transaction)
  • request a stock chart for a given time range (dynamically generating the image)
• Use enabling technologies such as CGI, PHP, ASP, or Java servlets to create dynamic content

How Does a Web Application Work?

• When the web server gets a CGI request, it executes the application in a separate process
• The server passes the parameters received with the request (if any) to the application and collects its output
• This output is then returned to the client just as if it had been fetched from a static document

  

Time Server

• The following web application is implemented in Perl
• It computes the local time on the web server and returns the result to the browser

  #!/usr/bin/perl
  
  use strict;
  
  my $time = localtime;
  
  print "Content-Type: text/html\n\n";
  print "The current time on this server is: $time";

• It produces the following output:

  The current time on this server is: Sat Jan 6 21:58:25 2001