Browsing the World Wide Web

Spring 2002Computer Networks Applications 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

Spring 2002Computer Networks Applications 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.

Spring 2002Computer 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 2002Computer 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 2002Computer 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: Obtains the document Displays the document.

Spring 2002Computer Networks Applications 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.

Spring 2002Computer 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:

Spring 2002Computer 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: “

Spring 2002Computer Networks Applications 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 “athos.rutgers.edu” with IP address has the alias “

Spring 2002Computer 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: it retrieves the server address: establishes a connection with it; makes a request, receives and displays the data, and closes the connection.

Spring 2002Computer 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 2002Computer 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 2002Computer Networks Applications Conclusion A browser integrates access to multiple Internet services into a single system.