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.

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

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 …

HTML Tags Example HTML Tags Example document title body of document

HTML Tags formatting tags for line break headings tags … unordered lists list item 1 list item 2 embedding graphic images

Uniform Resource Locator (URL) format used to locate a remote item Protocol://computername:port/document_name – Where protocol is name of protocol (http, ftp, gopher) used to access the document. Eg.

Hypertext Link facilitates a user to navigate from one document to another via use of an anchor tags. ie. Prentice Hall textfile that can be ftped down –Note that the input side of anchor can include text or graphics images, allowing hypertext link to be words or a picture.

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. tag encountered), it must open a new connection to the same server to obtain a copy of the image.

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.

Browser Architecture (fig 32.5) (fig 32.5)(fig 32.5) contains a set of clients –http client –ftp client – 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).

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.