WWW and Internet 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) - 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 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: