Browsing the World Wide Web
Browsing Service


Allows one to conveniently obtain and display
information that is stored on a remote
computer
A browser (like Netscape, Explorer) can:





Spring 2002
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
Computer Networks Applications
Browsing vs. FTP


Both browsing and FTP provide mechanisms
for retrieving data from remote computers
However, browsing:



Spring 2002
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.
Computer Networks Applications
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.
Spring 2002
Computer Networks Applications
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.
Spring 2002
Computer Networks Applications
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:


Spring 2002
Obtains the document
Displays the document.
Computer Networks Applications
Browser Functionality

A menu and a series of buttons allow a
user to:






Spring 2002
Print;
Save;
Navigate back and forth;
Find a word in a page;
View the source of a document;
Maintain a list of document bookmarks.
Computer Networks Applications
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.rutgers.edu/almaMater/almamater.shtml
Spring 2002
Computer Networks Applications
URL (cont.)



Not all elements in a URL are required;
If an element is not given, the browser provides
a default value;
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 by the browser
to: “http://www.cnn.com/index.html”
Spring 2002
Computer Networks Applications
URL (cont.)

Note:





Spring 2002
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 “athos.rutgers.edu” with IP
address 128.6.25.4 has the alias
“www.cs.rutgers.edu”.
Computer Networks Applications
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:





Spring 2002
it retrieves the server address:
establishes a connection with it;
makes a request,
receives and displays the data, and
closes the connection.
Computer Networks Applications
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;
Spring 2002
Computer Networks Applications
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;
Spring 2002
Computer Networks Applications
Conclusion

A browser integrates access to multiple
Internet services into a single system.
Spring 2002
Computer Networks Applications