Chapter 29 World Wide Web

1
Chapter 29 World Wide Web Browsing

• World Wide Web (WWW) is a distributed hypermedia
  (hypertext graphics) on-line repository of
  information that users can access interactively
  using a client application program known as a
  browser.
• Users can navigate to other documents via
  pointers or selectable items known as hyperlinks.
• Since the computers used to store Web documents
  are administered independently, links among
  documents can become invalid.
• Homepage the main page of an organization or a
  person.

2
HyperText Markup Language (HTML)

• a standard (RFC 1866) that provides general
  display and formatting guidelines.
• Browsers have some freedom in the details of
  display.

3
HTML Format and Representation

• Each HTML document is a text file that contains
  tags along with other information.
• Tags provide structure for the document as well
  as formatting hints. Most tags occur in pairs
  lt/Tagnamegt lt/Tagnamegt

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HTML Tags Example

• ltHTMLgt
• ltHEADgt
• ltTITLEgt
• document title
• lt/TITLEgt
• lt/HEADgt
• ltBODYgt
• body of document
• lt/BODYgt
• lt/HTMLgt

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HTML Tags

• formatting tags
• ltBRgt for line break
• headings tags
• ltH1gt lt/H1gt
• ltH6gt lt/H6gt
• unordered lists
• ltULgt ltLIgt list item 1
• ltLIgt list item 2
• lt/ULgt
• embedding graphic images
• ltIMG SRCgraphicimage.gif ALIGNmiddlegt

6
Uniform Resource Locator (URL)

• format used to locate a remote item
• Protocol//computernameport/document_name
• Where protocol is name of protocol (http, ftp,
  gopher) used to access the document.
• Eg. http//www.calstatela.edu/faculty/egean/cs447
  grade02.htm

7
Hypertext Link

• facilitates a user to navigate from one document
  to another via use of an anchor tags . ie.
• ltA HREFhttp//www.prenhall.comgt Prentice Hall
  lt/Agt
• ltA REFftp//mercury.calstatela.edu/pub/README gt
  textfile that can be ftped down lt/Agt
• Note that the input side of anchor can include
  text or graphics images, allowing hypertext link
  to be words or a picture.

8
Web Browser/Server Interaction

• web browser is a client that contacts a server on
  the computer specified in the URL when requesting
  a document.
• The browser establishes a connection, sends a
  request, and receives the requested item or a
  messages that no such item exists.
• The connection is closed immediately after the
  document has been transferred.
• If the browser needs to display an image (eg.
  ltIMGgt tag encountered), it must open a new
  connection to the same server to obtain a copy of
  the image.

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Hypertext Transport Protocol (HTTP)

• RFC 2068
• application layer protocol which defines the
  exact format of request sent from a browser to a
  server as well as the format of replies that the
  server returns.
• The server waits for a browser to open a
  connection and request a specific page.
• The server then sends a copy of the requested
  item, closes the connection, and waits for the
  next connection.

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Browser Architecture (fig 32.5)

• contains a set of clients
• http client
• ftp client
• email client
• Contains a set of interpreters
• html interpreter
• JAVA interpreter
• Contains controller software
• handles inputs from keyboard mouse, and calls
  the other components (clients interpreter).

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Caching in Web Browsers

• browsers place a copy of each item it retrieves
  in a cache on local disk.
• When a user selects an item, the browser checks
  the disk cache before retrieving a new copy from
  remote site.
• Cache policy may be adjusted by user.