World Wide Web Components

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World Wide Web Components

• Browsers and Servers

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WWW is example of Client/Server Computing

• Server computers are located all around the world
  and respond to requests (messages) from Computers
  running Browsers.
• Browsers understand HTML, (and now Javascript,
  Java etc.)

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History

• Originally developed as a protocol that would
  withstand wide network outages (Arpanet)
• Now it has become the standard protocol for
  almost all data communications
• Consists of two parts, IP ad TCP

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TCP/IP Language of the Internet

• TCP/IP is the basic protocol used by all Internet
  applications (including the WWW)
• TCP/IP connects all of the internet clients and
  servers together

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TCPTransmission Control Protocol

• Handles communications between 2 processes
  (programs) anywhere on internet
• Guarantees correct data is received by receiver
• Uses IP for delivery and routing of packets

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IP - Internet Protocol

• Handle datagrams
• Defines Internet addressing scheme
• routes datagrams to remote hosts
• Performs fragmentation and reassembly of datagrams

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IP Addressing

• Every host has an address in the form of
  www.zzz.yyy.zzz (the IP address)
• Each subcomponent refers to a particular
  breakdown of all of the machines on the internet.
• 128.122.197.133 sales.stern.nyu.edu
• 128 edu domain
• 122 nyu network
• 197 subnet at stern
• 133 address of sales computer

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Name Resolution

• The TCP/IP protocols include the concept of name
  lookup, where names like sales.stern.nyu.edu
  are translated into their numeric addresses
• (128.122.197.133)
• Hence one has to specify the locations of the
  name servers you wish to use.

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Gateways

• Gateways (also called IP routers) are the
  computers used to connect your network to the
  internet

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Sockets and ports

• TCP/IP uses ports to connect services between
  machines
• The destination machine has particular ports
  offering different types of service
• 23 telnet , 80 WWW etc.
• Client machine uses random port
• Combination of destination port and originating
  port socket (i.e a destination port that is in
  use)

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Standard TCP/IP Services

• telnet
• rlogin
• ftp
• NFS - Network File Service
• Talk
• Ping
• Finger
• Sendmail

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Other TCP/IP Services

• Gopher
• Menu based interface to internet
• World Wide Web
• Hyper media interface to internet
• Connects clients and servers using HTTP
• Hyper Text Transfer Protocol
• Subset of the internet
• CUSEEME, NETMEETING
• Interactive video

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TCP/IP Problems

• Current standard has several major problems
• Address space limitations
• xxx.yyy.www.zzz limits number of nodes
• Out of space in another year or so
• No Quality of Service guarantees
• Difficult to stream audio/video etc
• Need client and server to be able to negotiate
  quality of service, especially for streamed
  multimedia

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Summary

• TCP/IP is the language of the internet
• Many other services are based on the basic TCP/IP
  support
• All of these services are client/server
• Like the WWW
• WWW uses HTTP (Hyper Text Transfer Protocol)
• New Version of TC/IP IPv6 coming

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Server Browser Interaction (simple) (requests
sent using TCP/IP)
http request
B
Server
HTML
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Sample HTTP Request

• Browser sends http request to web server
• GET INDEX.HTML or
• GET userid/multim
• WEB SERVER sends back HTML document

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Sample HTML File

• lthtmlgt
• ltHeadgt
• ltTitlegt MY Homepagelt/Title)
• lt/Headgt
• ltbodygt
• .
• lt/bodygt
• lt/htmlgt

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Non-HTML files

• Server sends a header in front of each file
  identifying the file type (HTML,GIF,JPEG etc.)
• Most Browsers understand HTML, GIF and TEXT
• Browsers can be configured to call external
  programs to handle new types of files

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Helper Apps

• These programs are called HELPER applications and
  dramatically extend the capabilities of the
  browser, since they can be developed
  independently of the client software
• Examples - Quicktime viewers, sound players, VRML
  viewers etc.
• To see the currently configured viewers go to
  options on the Netscape title bar

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Forms and CGI Programming

• HTML provides a simple to use FORM capability,
  which allows a wide variety of input forms to be
  easily generated.
• Forms include
• Text input - One line of text
• Textarea - Multiple lines of text
• Check boxes (on/off
• Radio boxes (1 of N)
• Etc.

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Forms Processing Logic

• Output of Form is formatted and sent to Server,
  along with the name of a program to process the
  output of the form.
• Server takes information from form, and passes it
  on to a Common Gateway Interface Program (CGI)
• Output of CGI program is sent back to Client
  browser as an HTML (or other) file.

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CGI programming extends power of WWW

• CGI programs can do an almost unlimited set of
  activities ...
• Look up info in a database and send it to
  Browser.
• Take input from user and add to a file.
• Take input and send to a standard business
  application
• CGI program can be in any language (including C
  Shell language)

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CGI Programming
http forms
B
http server
HTML
input
output
CGI Program
(Note, all processing is on server)
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PERL, TCL etc.

• These are specialized languages for developing
  internet applications that make it very easy to
  write CGI programs
• Techies might want to use them for a final
  project.....

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New Developments - Browsers

• Plugins- ability to hook code directly to
  Netscape browser instead of having a helper app
• Means that Netscape can invoke the code directly,
  allowing live audio and vide and VRML apps
• JAVA and JAVAscript
• Code that is included in HTML document or sent
  with it to be executed at the client machine (as
  opposed to CGI)

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Java and JavaScript

• Both are programs which are downloaded and
  executed by the browser on the client
• JavaScript -
• Interpreted language like Visual Basic
• Source code imbedded in html document
• Java
• Compiled program
• Object code interpreted by Browser

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Client Pull and Server Push

• Usual model is that user pulls info from server
• Can also use server Push, where server keeps a
  connection open to the client and continually
  sends new updated info Useful for animation of
  pages (but takes a lot of bandwidth)

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Security problems

• Plugins can do anything, how do we control
• Java and Javascript run on client so have to be
  restricted in what they can do.

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Conclusion

• World-Wide-Web model is much more powerful than
  it appears on the surface
• Easily integrated with existing applications
• Easy to add new functionality