Browsing the World Wide Web

1
Browsing the World Wide Web

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Browsing Service

• Allows one to conveniently obtain and display
  information that is stored on a remote computer
• A browser (like Netscape, Explorer) can
• Obtain data (text, audio, video) from remote
  computers as a result of a request
• Display the retrieved information
• Store the retrieved information
• Print a copy of the data
• Follow a reference

3
Browsing vs. FTP

• Both browsing and FTP provide mechanisms for
  retrieving data from remote computers
• However, browsing
• Allow users to view data without necessarily
  requiring knowing the names of the individual
  files.
• Displays the content of documents to users.
• A document on one computer can point to documents
  stored on different computers? a user can jump
  from one computer to another without knowing or
  caring about which computers are being accessed.

4
Hypertext

• Information is stored as a set of documents
• Menus (pointers) are embedded in a document in
  order to make them easier to understand
• The combination of text and menus is called
  hypertext
• Note not all documents referred by a page reside
  on the same computer
• Mosaic---the first browser which displayed
  hypertext.

5
Hypermedia

• Multimedia computer--- a computer equipped with
  dedicated hardware that can display pictures or
  play video
• Hypermedia documents---documents that may include
  beside text information, other types of data
  including pictures, audio, video
• As for textual reference, the non-textual data
  can reside on different computers
• World Wide Web---an example of hypermedia
  browsing media.

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Hypermedia (cont.)

• Netscape, Explorer---browsers displaying
  hypermedia documents.
• The text corresponding to a link is displayed in
  a different color
• To select an item, one has to click it
• When an item is selected the browser
• Obtains the document
• Displays the document.

7
Browser Functionality

• A menu and a series of buttons allow a user to
• Print
• Save
• Navigate back and forth
• Find a word in a page
• View the source of a document
• Maintain a list of document bookmarks.

8
Document address---URL

• URL an acronym for Uniform Resource Locator
• An URL contains
• The protocol that a browser should use to
  retrieve the document (usually HTTP).
• The domain name (address) of the computer on
  which the server is running
• Location and name of the document
• Ex http//www.cs.albany.edu/

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URL (cont.)

• Not all elements in a URL are required
• If an element is not given, a default value is
  provided
• EX
• The default protocol is HTTP
• The default name of a file is index.html
• In a domain address, the default computer name is
  www, and the default type is com.
• The address cnn will be expanded to
  http//www.cnn.com/index.html

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URL (cont.)

• Note
• The vast majority of the URLs addresses call the
  server-computer www
• However, it is not necessary---but it makes the
  address (and server location) easier
• Moreover, a server may have a different name, and
  www is just an alias.
• The name and the alias(es) correspond to the same
  IP address.
• EX the server inbound.cs.albany.edu with IP
  address 128.6.25.4 has the alias
  www.cs.albany.edu.

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How a browser works

• Uses the client server paradigm
• The client is the browser program
• The server the remote HTTP server which serves
  requests
• A browser connects with one server at a time
• it retrieves the server address
• establishes a connection with it
• makes a request,
• receives and displays the data, and
• closes the connection.

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HTTP Hypertext Transfer Protocol

• It allows a client to request a specific item,
  which a server returns
• HTTP requests are sent as text encoded in ASCII
  
• Two main operations
• GET request a document from the server
• PUT (POST) sends data to the server

13
Browser Architecture

• Because a URL can specify a different service, a
  browser may handle several services? a browser
  consists of all software needed to support these
  services
• When a request is made, the interface (or
  controller) extracts the prefix, and uses it to
  choose the appropriate software

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Conclusion

• A browser integrates access to multiple Internet
  services into a single system.