WWW and Internet
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Transcript WWW and Internet
WWW and Internet
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The Internet
Creation of the Web
Languages for document description
Active web pages
Internet
• ARPANET – 1970s.
• Protocols
– TCP/IP: Transmission Control Program/Internet
Protocol - low level
– Telnet
– SMTP - Simple Mail Transport Protocol
– FTP - File Transfer Protocol
Creation of the Web
WWW provides a semantic description of the
information accessible through the Internet.
Server program: to provide the document
Client program: to read and display the
document.
Server and client communicate via
HTTP - HyperText Transport Protocol.
Basic Idea
WWW is based on the concept of a semantic
description language.
The power of the system is that the displayed
document contains pointers to other documents
called hypertext.
URL: Uniform Resource Locator
Client program:
uses URLs to locate documents and
display in the web browser
A database of addresses and names is
maintained by a Domain Name Server.
Other programs
Search engines – allow user to query site for
desired information
Web crawlers – crawler trace all links on a Web
page to find other Web pages
WEB NAVIGATION
Languages for document
description
SGML: Standard Generalized Markup Language
HTML – HyperText Markup Language
XML – Extensible Markup Language
SGML
SGML uses descriptive tags.
DTD –document type declaration, describes the
elements
What is described:
Document presentation (e.g. boldface,
italics)
Document structure (text split into divisions,
chapters, paragraphs, etc)
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Document history (author, revisions, etc)
HTML
HTML creates a virtual machine that web
browsers are programmed to execute.
Important elements-
• Sections – Separate parts of a document
• Presentation – How documents look.
• List – Lists of items
• URLs – links to other web pages
• Image URLs – displays pictures
XML
An instance of SGML.
Incorporates many features of HTML.
Uses DTD to describe the semantics of the
document.
Active web pages
• Forms and CGI scripts
• Java Applets
Forms and CGI scripts
• Forms – Method to pass information between
Web browsers and Web servers.
• Information is entered by the user and then
passed for processing to a program on the
server system
• CGI (Common Gateway Interface) scripts programs that process the information
HTML forms
<form method=“type”
action= “location of cgi script to
execute”>
text
</form>
Perl is often used as language for such scripts
Java Applets
Issue: Overloaded server
Solution: Do more processing on client side of
web
JAVA APPLETS
Java applets
• Run on the client machine
• Java programs compiled into a machine
independent code –
bytecode for JavaVirtual machine.
• Browsers on local systems have Java
Virtual machine interpreter
Java Applet Example
import java.awt.*; /* applet library */
public class hello extends java.applet.Applet
public void paint(Graphics x)
{x.drawString(“Hello World”, 100, 100);}
Displayed by:
<html><body>
<applet code = “hello.class” width=200 height=200>
</applet>
</body></html>