1. 1 WWW

2. 2 World Wide Web Major application protocol used on the Internet Simple interface Two concepts –Point –Click

3. 3 Web Components Browser Web server Hypermedia links Document representation Transfer protocol

4. 4 Browser Application program User’s interface to Web Becomes Web client to fetch information from Web server Displays information for user

5. 5 Web Server Running program Stores set of Web documents Responds to request from browser by sending copy of document

6. 6 Web Standards HyperText Markup Language (HTML) Uniform Resource Locator (URL) HyperText Transfer Protocol (HTTP)

7. 7 The General Form of an HTML Document

8. 8 HyperText Markup Language (HTML) ASCII representation Tags to give general layout guidelines Does not specify exact placement or format Allows document to contain –Text –Graphics –Links to other documents

9. 9 Uniform Resource Locator (URL) Specifies document on the Web Encodes –Protocol used to access document –Domain name of server –Protocol port number of server –Path to document

10. 10 General Form Of URL Only domain name required Defaults –Protocol is http –Port is 80 –Path is index.html

11. 11 Use Of Client-Server Paradigm Web server –Makes set of pages available –Uses port 80 Web client –Called a browser –Creates TCP connection to server –Sends requests for items Primary protocol known as HyperText Transfer Protocol (HTTP)

12. 12 Inside A Browser Main controller –Receives input from user –Invokes client and interpreter Client –One or more built into browser –Uses network to fetch items Interpreter –One or more built in –Displays items

13. 13 Illustration Of A Browser Browser contains many components

14. 14 Web Document Transfer/HTTP GET –Request a specific item from the server HEAD –Request status information from the server POST –Send data to the server –The server appends the data to a specified item PUT –Send data to the server –The server uses the data to replace a specified item –Seldom used

15. 15 Example HTTP Header Returned by a Server Status code –404: Requested item cannot be found –200: OK

16. 16 Alternative Protocol Example File transfer service Protocol is FTP Example URL ftp://ftp.cs.purdue.edu/pub/comer/netbook/clie nt.c Can be specified in anchor tag

17. 17 Caching In Browsers Cache for recently accessed –HTML pages –Images Item normally fetched from cache User can override HTTP can verify timestamp before fetching new copy

18. 18 Types Of Web Pages Static –Stored in file –Unchanging Dynamic –Formed by server –Created on demand –Output from a program –Use Common Gateway Interface (CGI) technology

19. 19 Types Of Web Pages – Con’t Active –Executed at client –Consist of a computer program –Can interact with user –Use Java technology