1. Internet Applications (Cont’d) Basic Internet Applications – World Wide Web (WWW) Browser Architecture Static Documents Dynamic Documents Active Documents HTTP 1

2. WWW: How A Browser Works WWW project was initiated by CERN (European Laboratory for Particle Physics) to create a system to handle resources necessary for scientific research. Web browsing uses client-server paradigm Browser: – Makes TCP connection – Sends request for page – Reads page If a page contains several images, Each different item - e.g., tag (image) - requires separate TCP connection HyperText Transport Protocol (HTTP) specifies commands and client-server interaction 2 Fig. 22.1 (text book)

3. How The World Wide Web Works Use client-server interaction The client: – browser program uses the Internet to contact a remote server The server: – returns a copy of the requested page with additional information A URL (Uniform Resource Locator) tells a browser which server to contact – Each URL uniquely identifies a page of information by giving the name of a remote computer and a specific page of information available from the server URL specifies – an access protocol – the domain name of the remote computer on which the server runs – any specific item 3 Fig. 22.7 (text book)

4. Inside a Browser Program Examples: Netscape Communicator, Microsoft IE A Web Browser Architecture usually consists of 3 parts: – controller: receives and interpret input from the keyboard or the mouse calls other components to perform operations specified by users (e.g., uses a client program to access a document) – client programs: uses different protocols HTTP client fetches HTML documents from WWW server optional clients: FTP client and E-mail client FTP client - access file transfer service E-mail client - send & receive e-mails – interpreters: HTML interpreter displays HTML-formatted documents 4

5. Inside a Browser Program 5

6. Web Documents Documents in WWW can be grouped into 3 categories: – static documents: fixed-content, created and stored in the a server. – dynamic documents: created by a Web server whenever a browser requests the document. contents can vary from one request to another. – active documents: contain program to be run at the client side, e.g. animation stored in the form of binary code. transported to the client in a binary form. 6

7. Static Documents 7 Fig. 24.1 (text book) (1) A user enters a URL, causing a browser to contact a Web server and request the item (2) the server extracts the specified item from its local disk and sends a copy to the browser.

8. Dynamic Documents 8 Figure 24.3 (text book) (1) A browser requests a URL that corresponds to a CGI program, causing the server to start the program (2) the CGI program computes a response, which the server returns to the browser.

9. Dynamic Documents Dynamic documents: – e.g. documents generated using CGI A server follows these steps: – it examines the URL to find if it defines a dynamic document, e.g. with extension.cgi – if it is dynamic, then it executes the program. – it sends the output of the program to the client. 9

10. Active Documents 10 Figure 24.4 (text book) (1) A browser requests a URL that corresponds to an active document, (2) Server sent a copy of active program to client (browser) (3) the browser runs a copy of the active document, which then controls the screen.

11. Active Documents Active documents: – e.g. documents containing Javascript. Its execution is as follows: – the binary code of the program is created and stored at the server side. – upon client request, the binary code is transported to the client. – the client uses its own software to change the binary code into executable code. – the client runs the program. 11

12. Comparing Static, Dynamic & Active Documents Static Document Advantages: – Simplicity: straightforward formatting specifications – can be created by non-technical person – Reliability: after it has been created and tested thoroughly, a static document remains valid indefinitely – Performance: browser can (1) display static documents more rapidly and (2) place a copy on cache to speed up future requests Disadvantage: – Inflexibility: must be revised whenever information changes; not useful for reporting information that changes frequently 12

13. Comparing Static, Dynamic & Active Documents Dynamic Document Advantage: – Able to report current information Disadvantage: – Increase cost: more expensive to develop than static document – Inability to update information continuously: Do not deliver a continuous stream of images 13

14. Comparing Static, Dynamic & Active Documents Active Document Advantages: – ability to update information continuously – Ability to change the display quickly enough to show an animated image Disadvantage: – Additional cost (more programming skills needed, requires more sophisticated browser software and more powerful computer) 14

15. HTTP HTTP (Hypertext Transfer Protocol) – protocol used to access data on the WWW. – uses one TCP connection on well-known port 80. – two types of http messages: request, response – transfer data in the form of plain text, hypertext, audio, video, and so on. – data transferred look like SMTP messages (see Lecture note 2). 15

16. HTTP HTTP requests use the concept of resource locator (URL). In more detail, URL defines four things: method, host computer, port and path – Method is the protocol used to retrieve the document, e.g. FTP, & HTTP. An access protocol (see slide 3) – Host is the computer where the information is located. Domain name of the computer (see slide 3) – Port: port number of the server (optional) – Path is the pathname of the file where the information is located. Specify a specific item to request (see slide 3) 16

17. HTTP: Request and Response 17

18. HTTP: Request and Response 18 Status Line Request Line

19. HTTP Example: Retrieving a File 19

20. HTTP Example: Retrieving a File Request line – request type (method): GET – path: /usr/bin/image1 – HTTP version: 1.1 client can accept images in GIF and JPEG formats status line: – HTTP version: 1.1 – status code (a 3 digit number): 200 – status phrase: OK (200 OK indicates request is successful) Header lines defines: – date: 04-Jan-99 – server name: challenger – MIME version: 1.0 - (MIME (Multipurpose Internet Mail Extensions) is an SMTP extension to allow non-ASCII data to be sent through SMTP.) – length of document: 2048 bytes 20