1. Basics of Web Technologies

2. Learning Outcomes
The evolution of the Internet, Internet standards organizations, and the difference between the Internet, intranets, and extranets.
The beginning of the World Wide Web, ethical use of information on the Web, Web Accessibility, and future Internet trends.
The Client/Server Model, Internet Protocols, Networks, URLs and Domain Names.

3. The Internet Interconnected network of computer networks.
Internet is composed of collection of networks connecting millions of computers together.
Internet consists of hardware and software infrastructure that enables computers to communicate and exchange information.

4. The Evolution of the Internet
ARPAnet
Advanced Research Project Agency
1969 – four computers connected
NSFnet
National Science Foundation
Use of the Internet was originally limited to government, research and academic use
1991 Commercial ban lifted

5. Intranet & Extranets Intranet
A private network contained within an organization or business used to share information and resources among coworkers.
Extranet
A private network that securely shares part of an organization’s information or operations with external partners

6. The World Wide Web
The World Wide Web (WWW) is a system of interlinked hypertext documents accessed via the Internet.
With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them by using hyperlinks.

7. WWW: The World Wide Web
The World Wide Web (WWW) was developed by Tim Berners-Lee and other research scientists at CERN, the European center for nuclear research, in the late 1980s and early 1990s.
WWW is a client-server model and uses TCP connections to transfer information or web pages from server to client.
WWW uses a Hypertext model. Hypertext allows interactive accesses to a collection of documents.
Documents can hold
Text (hypertext), Graphics, Sound, Animations, Video
Documents are linked together
Non-distributed – all documents stored locally (e.g on CD-Rom).
Distributed – documents stored at remote servers on the Internet.

8. WWW - Hyperlinks (or links)
Each document contains links (pointers) to other documents.
The link represented by "active area" on screen
Graphic - button
Text - highlighted
By selecting a particular link, the client fetches the referenced document from a server for display.
Links may become invalid.
Link is simply a text name for a remote document.
Remote document may be moved to a new location while name in link remains in place.

9. WWW – Document Representation
Each WWW document is called a page.
Initial page for individual or organization is called a home page.
Page can contain many different types of information; page must specify:
Content – The actual information
Type of content – The type of information, e.g. text, pictures etc
Links to other documents
Rather than having a fixed representation for every browser, pages are formatted with a mark up language.
This allows browser to format page to fit display.
Standard is called HyperText Markup Language (HTML).

10. Web Documents – Static Documents
Fixed-content documents that are created and stored in a server.
The client can get only a copy of the document.
The content of the file is determined when the file is created, not when it is used.
When a client accesses the document, a copy of the document is sent. The user can then use a browsing program to display the document.

11. Web Documents – Dynamic Documents
Created by a Web server whenever a browser requests the document.
When a request arrives, the Web server runs an application program or a script that creates a dynamic document.
The server returns the output of the program or script as a response to the browser that requested the document.

12. Basic Internet Protocols
TCP/IP is fundamental to the Internet
, web browsing, file downloads, accessing database are built on top of TCP and IP protocols
TCP is the Transmission Control Protocol
TCP extends IP to provide added functionality
However, only IP is fundamental to the definition of Internet
IP address:
Each device connected to the Internet has a unique numeric IP address.
32 bit number (sequence of 4 decimal numbers separated by dots, for example:
Other protocols: UDP, FTP, SMTP etc.

13. UDP- User Datagram Protocol
An alternative protocol to TCP
It builds on IP
Does not provide two-way connection
Does not provide guaranteed delivery unlike TCP
Faster than TCP for simple tasks
Used for tasks like downloading video, short message etc.

14. FTP: File Transfer Protocol
People required a protocol to reliably transfer files between any two computers connected to the Internet.
FTP is a client/server program
An FTP client program enables the user to interact with an ftp server in order to access files on the ftp server computer.
FTP provides similar services to those available on most filesystems: list directories, create new files, download files, delete files.
FTP uses TCP connections and the default server port for FTP is 21.

15. Web Standards and the W3C Consortium
W3C – World Wide Web Consortium
Develops recommendations and prototype technologies related to the Web
Produces specifications, called Recommendations, in an effort to standardize web technologies

16. Web Accessibility WAI – Web Accessibility Initiative
Develops recommendations for
Web content developers,
Web authoring tool developers,
Developers of web browsers, and
Developers of other user agents to facilitate use of the web by those with special needs.
WCAG
Web Content Accessibility Guidelines

17. Web Architecture Resources are identified by URIs.
URIs identify any resource (e.g., multimedia, databases and publications).
Resources have different representations (e.g., HTML and PDF).

18. Uniform Resource Locator (URL)
A client that wants to access a Web page needs the address to that page.
The Uniform Resource Locator (URL) is a standard for specifying any kind of information on the Internet.
The URL defines four things: protocol, host computer, port and path.
:// : /
Protocol
Host
Port
Path

19. Domain Name What is “Domain Name”?
It is a unique name for your website. To get a domain name you have to register the name from domain name registration companies like eHost.com.
Example:
What is “Domain Name System”?
It is a hierarchical decentralized naming system for computers, services, or any resource connected to the Internet or a private network.
It associates various information with domain names , including IP addresses.

20. Top-Level Domain Name (TLD)
A top-level domain (TLD) identifies the right-most label of the domain name.
Current generic TLDs:
.com, .org, .net, .mil, .gov, .edu, .int, .aero, .asia, .cat, .jobs, .name, .biz, .mobi, .museum, .info, .coop, .post, .pro, .tel, .travel.

21. WWW – Identifying a web page
A web page is identified by:
The protocol used to access the web page.
The host is the computer on which the web page is stored.
The TCP port that the server is listening on to allow a client to access the web page.
The path is the directory pathname of web page on server (the file where the information is located).
Specific syntax for Uniform Resource Locator (URL): protocol://computer_name:port/document_name
Protocol can be http, UDP, SMTP, FTP, mailto.

22. WWW – Identifying a web page
Computer name can be DNS name or IP address.
TCP port is optional (http uses port 80 as its default port).
document_name is path on server to web page (file).
E.g.
Protocol is http
Computer name or DNS name is
Port number is the default port for http, i.e. port 80.
Document name is /Recreation/Sports/Soccer/index.html

23. Checkpoint 1.1
1. Describe the difference between the Internet and an intranet.
2. Describe the difference between the Internet and the Web.

24. The Web Experience A user clicks on a link in a browser
The browser communicates with a web server using HTTP.
The server sends an HTML document back.
The browser displays the document.
The user clicks on another link and activates another URL.
Web client (browser)
Request
Reponse
Web server

25. WWW – Client Server Model
The browser is the client, WWW (or web) server is the server.
Browser:
The browser makes TCP connection to the web server.
The browser sends request for the particular web page that it wishes to display.
The browser reads the contents of the web page from the TCP connection and displays it in the browsers window.
The browser closes the TCP connection used to transfer the web page.
Each separate item in a web page (e.g., pictures, audio) require a separate TCP connection.
HyperText Transport Protocol (HTTP) specifies commands that the client (browser) issues to the server (web server) and the responses that the server sends back to the client.

26. Figure 1-1: Web client/server architecture
WWW – Client Server Model
Figure 1-1: Web client/server architecture

27. Web Server Basics
Web servers are continually connected to the Internet
All communications between browsers and servers use Hypertext Transfer Protocol (HTTP).
Web servers provide responses to browser requests, either existing documents or dynamically built documents.
When a client is connected. The server reads the HTTP request, performs some lookup function and sends HTTP response and the requested data.

28. Serving a Page
User of client machine types in a URL

29. Serving a Page Server name is translated to an IP address via DNS
client
server
(Netscape)
(Apache)
/index.html

30. Serving a Page
Client connects to server using IP address and port number

31. Serving a Page
Client determines path and file to request

32. Serving a Page
Client sends HTTP request to server

33. Serving a Page
Server determines which file to send

34. Serving a Page
Server sends response code and the document

35. Serving a Page
Connection is broken

36. The basic client/server request/response sequence

37. A dynamic client/server request/response sequence

38. HTTP Protocol The Hypertext Transfer Protocol (HTTP) is:
A protocol used mainly to access data on the World Wide Web.
Designed for document transfer
Specifies how a server and client communicate
Most HTTP messages are sent using TCP
Stateless
no persistent client/server connection

39. WWW – HTTP Protocol
When a user types in the broswer creates a HTTP GET Request message and sends it over a TCP connection to the web server.
In the above case, the HTTP GET Request message would be
GET /Recreation/Sports/Soccer/index.html HTTP/1.0
User-Agent: InternetExplorer/5.0
Accept: text/html, text/plain, image/gif, audio/au
“\r\n”

40. Message body (Optional Data)
WWW – HTTP Request messages
HTTP Request messages are sent from client to server.
A blank line
Request Line
Message body (Optional Data)
HTTP Headers
Additional information
such as brower being
used, media types accepted
Delimiter
Carriage return
Line feed
User data e.g. contents of completed form
Type of Request
(e.g. GET)

41. WWW – HTTP Request messages – Request line
Request type
URL
HTTP version
Space
Space
Request type: This field is used in request messages. In version 1.1 of HTTP, several request types as defined. The request type is categorized into methods as follows:
Get – Used to request a web page from a web server
Post – Used to send data (e.g. results of registration form) to a web server
Head – Requests information about a document but not the document itself. Used by search engines to test the validity of hyperlinks
Put / Delete – Not typically implemented by browsers.

42. Message body (Optional Data)
WWW – HTTP Response messages
HTTP Response messages are sent from server to client.
A blank line
Status Line
Message body (Optional Data)
HTTP Headers
Success/Failure
Indication
Number between
200 and 599
Type of content returned
e.g. text/html or image/gif
Requested Data e.g. web page
Delimiter

43. WWW – HTTP response messages – Status line
HTTP version
Status code
Status phrase
Space
Space
The Status Line gives information about the success of the previous HTTP Request
Status code: this field is used in the response message. It consists of three digits, as follows:
The codes in the 200 range indicate a successful request (200 – 299).
The codes in the 300 range redirect the client to another URL; meaning that the document has been moved (300 – 399).
The codes in the 400 range indicate an error at the client side– Bad Request, Unauthorised, Not found (400 – 499)
The codes in the 500 range indicate an error at the server side– Internal Error, Service Overloaded (500 – 599).
Status phrase: this field explains the status code in text form (e.g., OK for 200 and Bad request for 400)

44. Example: HTTP request and response messages
Request message
Server
Client
POST /cgi-bin/doc.pl HTTP/1.1
Accept: */*
Accept: image/gif
Accept: image/jpeg
Content-length: 50
(Input information)
Response message
HTTP/ OK
Date: Mon, 07-Jan-02 13:15:14 GMT
Server: Challenger
MIME-version: 1.0
Content-length: 2000
(Body of the document)

45. MIME MIME Multipurpose Internet Mail Extensions
Standards for encoding different media types in a message
Originally developed for ing files and messages in different languages

46. WWW – Caching Web pages
Downloading HTML documents from servers can be slow due to a number of conditions:
Parts of the Internet can be congested
Dialup connection is typically very slow, 33Kbps or 56Kbps
Web server can have a lot of clients connecting to it at the same time, causing it to be overloaded.
If a user returns to previous HTML document, then this could require downloading the document from the server again.
A browser can hold copies of recently visited pages. This avoids having to download pages again.
An organization can use a HTTP proxy that caches documents for multiple users. Thus improving the speed at which pages can be displayed on each users computer.

47. WWW – Browser ArchitectureD i s p l a y r v e
Input from
keyboard and
mouse
Controller
html
interpreter
Output sent to
display …
optional
plugins
HTTP
client
Other
client …
Network Interface
Communication with
remote server

48. WWW – Browser Architecture
Browser has more components than a server:
Display driver for painting screen.
HTML interpreter for formatting HTML documents.
Plugins to display different content (e.g., Shockwave or Real Audio content)
HTTP client to fetch HTML documents from WWW server.
Other clients for other protocols (e.g., ftp, mail)
Controller also must accept input from the computer user through the mouse or keyboard.