1. The Web
The World Wide Web
What does it do?

2. The Web Looking at …. The World Wide Web HTTP URLs HTML
World Wide Web consortium (W3C)
Search Engines

3. World Wide Web (WWW)
The World Wide Web began as a networked information project led by Tim Berners-Lee, now Director of the W3C consortium.
In essence, it is an application of the Internet.
It has a body of software, and a set of protocols and conventions.
Using Hypertext and Multimedia techniques, the web makes it easy to roam, browse and contribute to the Internet.

4. World Wide Web (WWW) Server Client Internet
Client sends a request to the Server
Server authenticates Client and checks rights
Server retrieves resource, makes a copy and sends response
Client retrieves response

5. World Wide Web (WWW)
Client (Internet Browser) and Server (Web Server) communicate using Requests and Responses
A protocol is required (as we’ve seen)
what is best to use ?
HTTP (HyperText Transport Protocol) is what is used
Let’s have a look at HTTP ….

6. HyperText Transfer Protocol (HTTP)
HTTP is a protocol used in which Requests and Responses are sent between a Client and a Server
HTTP communication usually takes place over TCP/IP. It presumes reliable transport. It may be used over other network protocols (outside scope of course).
Standard (and default port) for HTTP is 80 (other ports may also be used).

7. HTTP HTTP is used to transmit resources (not just files)
a resource is piece of info. Identified by a URL (Uniform Resource Locator) - we’ll see this later …
resources include: files, query results, output of CGI script, document in any language, etc….
During communication process, HTTP is a Statefull protocol (Server maintains a connection with the Client)
After delivering the response, the server closes the connection (making HTTP become a Stateless protocol)

8. HTTP message format
The format of the request and response messages are similar
English oriented
Both messages consist of
initial line
zero or more header lines
a blank line (CRLF)
optional message body (e.g. a file, query data, docs..)
Initial line  different for request and response

9. HTTP - Operation Request Get resource Type of browser Name of host etc
First line is request-line
GET: Get a file from the server
HEAD: Like GET but no content returned
POST: Invokes a program on a form
PUT: Store some resource
GET path/index.html HTTP/1.1 User-Agent: Lynx/2.4 Connection: Keep-Alive Host: Accept: text/html

10. HTTP - Operation Response 200 = Status code All’s well
Type of server etc.
HTTP/ OK Date: Thu, 22 July :40:55 GMT Server: Apache (Unix) PHP/ Last-Modified: Mon, 19 July :03:22 GMT Content-Type: text/html Content-Length:

11. HTTP - response status codes
1XX: Provide information to the client
2XX: Correct response has occurred.
3XX: Browser must carry out some further action in order for the request to be successful. For example, the code indicates that the resource that was requested has been permanently moved to another location.
4XX: Something has gone wrong; for example, the most frequent status code that is returned is 404 which indicates that the resource that has been requested cannot be found.
5XX: Server has experienced a problem. For example, the status code 503 indicates that the service requested has not been able to be carried out.

12. HTTP Proxies
HTTP proxies act as intermediaries between clients and servers. It receives requests from clients and forwards them to the intended servers (and vice-versa).
Proxies are commonly used in Firewalls and LAN caches found mostly in company setups.
When a client uses a proxy, all requests are sent to the proxy and not the intended URL. Requests to a proxy differ in 1 way to those that do not involve proxies

13. GET http://www.rte.ie/aertel/sport.html HTTP/1.1
HTTP Proxies
Request difference:
the first line of the request uses a complete URL of the resource being requested instead of just the path
e.g.
GET HTTP/1.1
this way the proxy knows which server to forward the request to
Note: Proxies add a level of security and caching that may increase efficiency in an organisation

14. Uniform Resource Locator (URL)
URLs are the method by which documents or data are addressed in the World Wide Web
URLs contain:
type of service the resource is served by (HTTP, FTP,..)
Host address and port number of the service (if omitted, browser assumes the default port number, e.g HTTP port 80)
the location of the resource in the directory structure of the server
name of the resource (data, document, etc..)

15. URL structure host path & resource service port
service
port
the structure of a URL can differ to the above

16. URL structure
URLs can include a query-string that is passed to the specific server.
Indicated by including a question-mark at the end of the URL followed by the query-string
e.g
the server will process the query using the query-string and return the result in the response back to the client

17. HyperText Markup Language (HTML)
HTML is a language for the Web
HTML instructions, along with the text to which the instructions apply, are called HTML elements
HTML instructions themselves are called tags
e.g. <element_name>
<B> everything is Bold </B>
Your browser looks at the tags, interpret them and perform whatever instruction they mean to the “media” between them - (text, pictures, audio, ..)

18. HyperText Markup Language (HTML)
<B>Note: </B> this is just an example. <I>Happy?</I>
<BR>Try this:<A HREF=“ now</A>
Note: this is just an example. Happy?
Try this: Search now
Browser output

19. HyperText Markup Language (HTML)
Note: this is just an example. Happy?
Try this: Search now
Browser output
HyperText in Browser

20. World Wide Web consortium (W3C)
“Founded in 1994 to lead the World Wide Web to its full potential by developing common protocols that promote its evolution and ensure its operability”
- W3C.org
Over 500 members world-wide
Members contribute to growth of the Web
Share technology, research, write papers, etc..
To date, has developed more than 40 technical specs for the Web’s infrastructure

21. World Wide Web consortium (W3C)
Role
Vision: Promote and develop its vision of the future of the Web. To identify the technical requirements that must be satisfied if the web is to be a truly universal information space.
Design: Designs web technologies to realise its vision, taking into account existing technologies as well as those of the future
Standardisation: Produces specifications (“Recommendations) that describe the building blocks of the web.

22. World Wide Web consortium (W3C)
Technologies standardised: “Recommendations”
HyperText Markup Language (HTML), Extensible HyperText Markup Language (XHTML), Cascading Style Sheets (CSS), Extensible Markup Language (XML) and many others …
Homepage:

23. Search Engines
Web sites that search the Internet selecting information based on keywords and categories
Store information in very large databases (100’s of Gigabytes)
Returns lists of sites as hyperlinks

24. Search Engines Meta-search engines Search tips
Do not maintain databases
Aggregate results from multiple search engines
e.g. Microsoft’s MSN (
Search tips
Use quotes around specific words and sentences that you search for i.e. “ “
Use a “+” to concatenate more than one word/sentence you are searching for

25. Summary We have looked at The Web HTTP URLs HTML
Operation, Format, Proxies
URLs
HTML
World Wide Web Consortium (W3C)
Search Engines