The World Wide Web

1. The World Wide Web

2. Web Growth

3. Web Server Statistics • Apache is the most popular web server today (freely available) • Microsoft IIS is gaining ground

4. The World Wide Web: A Brief History

5. Vannevar Bush 1945

6. MEMory EXtender system • Problem: • Bush was concerned about “new knowledge not reaching the people who would benefit from it”

7. MEMory EXtender system • Store publications, correspondence, and personal work on microfilm • Items retrieved rapidly using index codes • Can annotate text with margin notes, comments • Can construct a trail through the material and save it • Acts as an external memory

8. MEMory EXtender system limitations • Basic unit of content is an image page • No links to/from sub-text • No digital content • No keyword search, only TOC/index codes • No networking • No rapid info sharing

9. Results • The MEMory EXtender system was the inspiration for the creators of hypertext and the web

10. McLuhan is known for coining the expressions the medium is the message and the global village, and for predicting the World Wide Web almost thirty years before it was invented Marshall McLuhan 1964 – Hypertext Coined

11. Sir Timothy John "Tim" Berners-Lee is an English computer scientist known as the inventor of the World Wide Web. He made a proposal for an information management system in March 1989, and on 25 December 1990, with the help of Robert Cailliau and a young student at CERN, he implemented the first successful communication between a Hypertext Transfer Protocol (HTTP) client and server via the Internet. By the mid 1990s, the World Wide Web had replaced Mark P. McCahill's Gopher protocol as the dominant Internet protocol. Berners-Lee is the director of the World Wide Web Consortium (W3C), which oversees the Web's continued development. He is also the founder of the World Wide Web Foundation, and is a senior researcher and holder of the Founders Chair at the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL). He is a director of the Web Science Research Initiative (WSRI),and a member of the advisory board of the MIT Center for Collective Intelligence. Tim Berners-Lee invents Global Hypertext WWW 1991

12. World First Browser and Web page.

13. Then…

14. What has happened through the years? Appearance of JavaScript (JS) in Late 1995. Standardization of HTML in Jan 1997. Introduction of Cascading Style Sheet (CSS) in 1998. Document Object Model (DOM) was standardized in End of 1998. Begin to be working in 2001. AJAX (Widely used by Google in 2004).

15. Web Architecture

16. The World Wide Web: HTTP Protocol

17. HTTP vs HTML • HTML: hypertext markup language • Definitions of tags that are added to Web documents to control their appearance • HTTP: hypertext transfer protocol • The rules governing the conversation between a Web client and a Web server Both were invented at the same time by the same person

18. What is a protocol? • In diplomatic circles, a protocol is the set of rules governing a conversation between people • We have seen that the client and server carry on a machine-to-machine conversation • A network protocol is the set of rules governing a conversation between a client and a server • There are many protocols, HTTP is just one

19. I would like to open a connection GET <file location> Display response Close connection OK Send page or error message OK An HTTP conversation Client Server HTTP is the set of rules governing the format and content of the conversation between a Web client and server

20. An HTTP example The message requesting a Web page must begin with the work “GET” and be followed by a space and the location of a file on the server, like this: GET /fac/lpress/shortbio.htm The protocol spells out the exact message format, so any Web client can retrieve pages from any Web server.

21. Network protocols • The details are only important to developers. • The rules are defined by the inventor of the protocol – may be a group or a single person. • The rules must be precise and complete so programmers can write programs that work with other programs. • The rules are often published as an RFC along with running client and server programs. • The HTTP protocol used for Web applications was invented by Tim Berners Lee. RFC = request for comments

22. Tim Berners-Lee Tim Berners-Lee was knighted by Queen Elizabeth for his invention of the World Wide Web. He is shown here, along with the first picture posted on the Web and a screen shot from an early version of his Web browser.

23. HTTP is an application layer protocol • The Web client and the Web server are application programs • Application layer programs do useful work like retrieving Web pages, sending and receiving email or transferring files • Lower layers take care of the communication details • The client and server send messages and data without knowing anything about the communication network

24. The application layer is boss – the top layer • Your boss says: Send this package to Miami -- I don't care if you use Federal Express, UPS, or any other means. Also, let me know when it arrives or if it cannot be delivered for some reason. • The application program says:Send this request to the server -- I don't care how you do it or whether it goes over phone lines, radio, or anything else about the details. Just send the message, and let me know when it arrives or if it cannot be delivered for some reason. There are five TCP/IP layers, the application layer and four lower layers.

25. Many application layer protocols are used on the Internet, HTTP is only one

26. The TCP/IP protocol layers The application program is king – it gets work done using the lower level layers for communication between the client and server. Get useful work done – retrieve Web pages, copy files, send and receive email, etc. Make client-server connections and optionally control transmission speed, check for errors, etc. Route packets between networks Route data packets within the local area network Specify what medium connects two nodes, how binary ones and zeros are differentiated, etc,

27. HTTP Operations • GET: retrieves URL (most widely used) • HEAD: retrieves only response header • POST: posts data to server • PUT: puts page on server • DELETE: deletes page from server

28. Simple HTTP Request and Reply Request: GET http://www.server.com/page.html HTTP/1.0 Response: HTTP-Version: HTTP/1.0 200 OK Content-Length: 3012 Content-Type: text/html <body>

29. HTTP 1.0 • Client opens a separate TCP connection for each requested object • Object is served and connection is closed • Advantages • maximum concurrency • Limitations • TCP connection setup/tear-down overhead • TCP slow start overhead

30. HTTP 1.0 Client Server connect() SYN SYN, ACK write() ACK, GET www.cs.uiuc.edu Retrieve Data From Disk close() connect() SYN SYN, ACK write() ACK, GET image.gif Retrieve Image From Disk close()

31. HTTP 1.1 Client Server connect() SYN SYN, ACK write() ACK, GET www.cs.uiuc.edu Retrieve Data From Disk write() GET image.gif Retrieve Image From Disk close()

32. Server Side Close() Client Server connect() SYN Set timeout SYN, ACK Reset timeout write() ACK, GET www.cs.uiuc.edu Retrieve Data From Disk write() GET image.gif Retrieve Image From Disk Timeout! close()

33. Dynamic Content • Web pages can be created as requests arrive • Advantages • Personalization (e.g., my.yahoo.com), • interaction with client input • interaction with back-end applications • Disadvantages • Performance penalty • Generating dynamic content (CGI, ASP, PHP, ColdFusion, JavaScript, Flash, …)

34. CGI Scripts • CGI scripts are URLs with a .cgi extension • The script is a program (e.g., C, JAVA, …) • When the URL is requested, server invokes the named script, passing to it client info • Script outputs HTML page to standard output (redirected to server) • Server sends page to client

35. CGI Execution fork() Server CGI Script Send page Request Response

36. Active Server Pages (ASPs) • Active server pages are HTML documents with extensions for embedded program execution • When request arrives, server fetches and parses the HTML document • Server executes embedded executable code and plugs output into page • Expanded page is sent to client