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Chapter 27
HTTP
and
WWW
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HTTP
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Hypertext Transfer Protocol (HTTP) is used
mainly to access data on the World Wide
Web.
It can jump from one document to another
Functions like FTP and SMTP
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Transfers files and uses services of TCP; Uses TCP
port 80
Transfer data between client and server
HTTP information is read and interpreted by HTTP
server and HTTP client
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Figure 27.1
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HTTP itself is a stateless protocol
Client initializes the transaction by sending a
request message.
Server replies by sending a response.
Two types of HTTP messages
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HTTP Transaction
Request
Response
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Figure 27.2
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Request line
 Request type
 Uniform Resource Locator (URL): address of the web page
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Request Message
Method: Protocol used to retrieve the document.
Host computer: Name of the computer where the information
is located
Port: [Optional] Port number of server
Path: Path name of the file where the information is located.
Version: HTTP 1.1 OR 1.0 OR 0.9
Headers
Body
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Figure 27.3
Request line
URL
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Methods
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Request method is the actual command or request that a client
issues to the server
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GET: Client wants to retrieve a document from server
HEAD: client wants information about a document and not the
document itself.
POST: Client provides information to the server.
PUT: Client provides a document to the server.
PATCH: similar to PUT but only with differences that should be
implemented in existing file.
COPY: Copies a file to another location. Source is in request line and
destination is in entity header.
MOVE: Moves a file to another location.
DELETE: Removes a document from server.
LINK: Creates a link or links from a document to another location.
UNLINK: Deletes links created by LINK method.
OPTION: Used by client to ask the server about available options.
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Figure 27.5
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Status Line
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HTTP Version
Status code: Status code field as in FTP & SMTP; three digits
Status Phrase: Explains the status code in text form.
Header
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Response Message
Exchange additional information between client and server
Header name, colon, space, header value.
Body
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Figure 27.6
Status Line
Header Format
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Figure 27.8
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General header: Info about message.
Request header: specifies client’s configuration and client’s
preferred document format.
Response header: Specifies server’s configuration and special
information about the request.
Entity header:
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Header Categories
Gives info about the body of document.
Mostly in response message but in request messages of POST &
PUT methods
Request message has Only general, request and entity headers.
Response message has general, response and entity headers.
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Example 1
This example retrieves a document. We use the GET method to
retrieve an image with the path /usr/bin/image1. The request line
shows the method (GET), the URL, and the HTTP version (1.1).
The header has two lines that show that the client can accept
images in GIF and JPEG format. The request does not have a body.
The response message contains the status line and four lines of
header. The header lines define the date, server, MIME version, and
length of the document. The body of the document follows the
header (see Fig. 27.9, next slide).
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Figure 27.9
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Example 1
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Example 2
This example retrieves information about a document. We use the
HEAD method to retrieve information about an HTML document
(see the next section). The request line shows the method (HEAD),
URL, and HTTP version (1.1). The header is one line showing that
the client can accept the document in any format (wild card). The
request does not have a body. The response message contains the
status line and five lines of header. The header lines define the date,
server, MIME version, type of document, and length of the
document (see Fig. 27.10, next slide). Note that the response
message does not contain a body.
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Figure 27.10
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Example 2
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Features of HTTP 1.1
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Persistent connection
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HTTP 1.1 default option
Server leaves the connection open for more requests after sending a
response.
Server can close the connection at the request of a client or if a timeout has
been reached.
 Usually length of data is sent along with each response but when the
length is not known, server informs the client that the length is not
known and closes the connection after sending data so the client knows
that the end of data has been reached.
Nonpersistent connection
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HTTP 1.0
One TCP connection is made for each request/response.
 Client opens a TCP connection and sends a request
 Server sends the response and closes the connection
 Client reads the data until it encounters an end-of-file marker; the client
then closes the connection.
 For N different images in different files, the connection must be opened
and closed N times; impose high overhead on server.
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Proxy Server
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HTTP support Proxy server.
Proxy server is a computer that keeps copies of
responses to recent requests.
If proxy server is present, HTTP client sends a
request to proxy server and the proxy server checks
its cache.
If the response is not stored in cache, the proxy
server sends the request to corresponding server.
Incoming responses are sent to proxy server and
stored for further requests from other clients.
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Reduces load on original sever, decreases traffic, and
improves latency.
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Figure 27.11
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World wide web (WWW)
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Distributed services
Repository of information spread all over the world.
Unique combination of flexibility, portability and user
friendliness.
WWW today is a distributed client-server service, in which a
client using a browser can access a service using a server.
However, the service provided is distributed over many
locations called websites.
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Figure 27.12
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Hypertext
Linking of documents is done using pointers
Hypertext documents only contain text, hypermedia
documents can contain pictures, graphics, and sound
Unit of hypertext or hypermedia available on web is
called a page. The main page for an organization or
an individual is called homepage.
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Figure 27.13
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Browser has three parts
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Browser architecture
Controller: receives input from keyboard or mouse and uses
the client programs to access the document.
Client programs
Interpreters: After the document has been accessed, the
controller use one of the interpreters to display the
document on the screen; HTML or Java.
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Figure 27.14
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Static documents
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Categories of Web documents
Fixed-content documents that are created and stored in the
server.
Client can get only the copy of the document.
The contents in the server can be changed, but the user
cannot change it.
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Figure 27.15
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Static Document
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Figure 27.16
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HTML(Hypertext Markup Language)
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Boldface Tags
Language for creating web pages.
Tags are instructions to the browser.
HTML allows us to embed formatting instructions in the file
itself.
HTML lets us use only ASCII characters for both the main
text and formatting instructions.
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Figure 27.18
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Beginning and Ending Tags
Structure of a web page
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Head
st part of a web page
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 Contains the title of the page and other parameters that the
browser will use.
Body
 Actual contents of a page are in the body, which includes text and
tags. Tags define the appearance of the document.
Tags
 Marks that are embedded into the text.
 Enclosed in two signs (< and >) and usually comes in pairs.
 Beginning tag starts with the name of the tag, and the ending tag
starts with a slash followed by the name of the tag.
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Table 27.1 Common tags
Beginning
Tag
Ending
Tag
Meaning
Skeletal Tags
<HTML>
</HTML>
Defines an HTML document
<HEAD>
</HEAD>
Defines the head of the document
<BODY>
</BODY>
Defines the body of the document
Title and Header Tags
<TITLE>
</TITLE>
Defines the title of the document
<Hn>
</Hn>
Defines the title of the document
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Table 27.1 Common tags (continued)
Beginning
Tag
Ending
Tag
Meaning
Text Formatting Tags
<B>
</B>
Boldface
<I>
</I>
Italic
<U>
</U>
Underlined
<SUB>
</SUB>
Subscript
<SUP>
</SUP>
Superscript
Data Flow Tag
<CENTER>
</CENTER>
<BR>
</BR>
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Centered
Line break
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Table 27.1 Common tags (continued)
Beginning
Tag
Ending
Tag
Meaning
List Tags
<OL>
</OL>
Ordered list
<UL>
</UL>
Unordered list
<LI>
</LI>
An item in a list
Image Tag
<IMG>
Defines an image
Hyperlink Tag
<A>
</A>
Defines an address (hyperlink)
Executable Contents
<APPLET>
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</APPLET>
The document is an applet
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Example 3
This example shows how tags are used to let the browser format the
appearance of the text.
<HTML>
<HEAD>
<TITLE> First Sample Document </TITLE>
</HEAD>
<BODY>
<CENTER>
<H1><B> ATTENTION </B></H1>
</CENTER>
You can get a copy of this document by:
<UL>
<LI> Writing to the publisher
<LI> Ordering online
<LI> Ordering through a bookstore
</UL>
</BODY>
</HTML>
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Example 4
This example shows how tags are used to import an image and
insert it into the text.
<HTML>
<HEAD>
<TITLE> Second Sample Document </TITLE>
</HEAD>
<BODY>
This is the picture of a book:
<IMG SRC="Pictures/book1.gif" ALIGN=MIDDLE>
</BODY>
</HTML>
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Example 5
This example shows how tags are used to make a hyperlink to
another document.
<HTML>
<HEAD>
<TITLE> Third Sample Document </TITLE>
</HEAD>
<BODY>
This is a wonderful product that can save you money and time.
To get information about the producer, click on
<A HREF="http://www.phony.producer">
Producer </A>
</BODY>
</HTML>
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Figure 27.19
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Dynamic Document
Dynamic documents do not exist in a predefined format.
Dynamic document is created by a Web server whenever a
browser requests the document.
When a request arrives, the Web server runs an application
program that creates the dynamic document. The server returns
the output of the program as a response to the browser that
requested the document.
As fresh document is created for each request, the contents of a
dynamic document can vary from one request to another.
Example is getting date and time from the server.
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Steps involved in handling dynamic documents.
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Server examines the URL to find if it defines a dynamic document.
URL defines a dynamic document, the server executes the
program.
Sends the output of the program to the client (browser).
Common Gateway Interface (CGI)
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Technology that creates and handles dynamic documents.
CGI is a set of standards that defines how a dynamic document
should be written, how input data should be supplied to the
program, and how the output result should be used.
Can use C, C++, Perl, …
Use of common in CGI indicates that the standard defines a set of
rules that are common to any language or platform.
Gateway here means that a CGI program is a gateway that can be
used to access other resources such as databases and graphics
packages.
Interface means that there is a set of predefined terms, variables,
calls, and so on that can be used in any CGI program.
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Example 6
Example 6 is a CGI program written in Bourne shell script. The
program accesses the UNIX utility (date) that returns the date and
the time. Note that the program output is in plain text.
#!/bin/sh
# The head of the program
echo Content_type: text/plain
echo
# The body of the program
now='date'
echo $now
exit 0
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Example 7
Example 7 is similar to Example 6 except that program output is in
HTML.
#!/bin/sh
# The head of the program
echo Content_type: text/html
echo
# The body of the program
echo <HTML>
echo <HEAD><TITLE> Date and Time </TITLE></HEAD>
echo <BODY>
now='date'
echo <CENTER><B> $now </B></CENTER>
echo </BODY>
echo </HTML>
exit 0
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Example 8
Example 8 is similar to Example 7 except that the program is
written in Perl.
#!/bin/perl
# The head of the program
print "Content_type: text/html\n";
print "\n";
# The body of the program
print "<HTML>\n";
print "<HEAD><TITLE> Date and Time </TITLE></HEAD>\n";
print "<BODY>\n";
$now = 'date';
print "<CENTER><B> $now </B></CENTER>\n";
print "</BODY>\n";
print "</HTML>\n";
exit 0
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Figure 27.20
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Active document
For active documents, we need a program to be run at the
client side.
For example, to run animations.
When a browser requests an active document, the server sends
a copy of the document in the form of byte code. The document
is then run at the client (browser) site; the client can store this
document in its own storage area also.
Active document is stored in binary code in the server.
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Creation, compilation and execution
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At server site, programmer writes a program, in source code, and
stores it in a file.
Compile the code into byte code. Path name of the file is the one
used by a URL to refer to the file. In this file, each program
command (statement) is in binary form, and each identifier
(variable, constants, function names, and so on) is referred to by a
binary offset address.
Client (browser) requests a copy of the binary code, which is
probably transported in compressed form from the server to the
client (browser).
Client (browser) uses its own software to change the binary code
into executable code. The software links all the library modules and
makes it ready for execution.
Client (browser) runs the program and creates the result that can
include animation or interaction with the user.
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Java
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Java is combination of a high-level programming language, a runtime environment, and a class library that allows a programmer to
write an active document (an applet) and a browser to run it.
Java can also be a stand-alone program without using a browser.
Java is an object-oriented language like C++ without operator
overloading or multiple inheritance.
Java is platform-independent and does not use pointer arithmetic.
Java is an object-oriented language, a programmer defines a set of
objects and a set of operations (methods) to operate on those
objects.
Java is a typed language which means that the programmer must
declare the type of any piece of data before using it.
Java is also a concurrent language, which means the programmer can
use multiple threads to create concurrency.
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Classes and Objects
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Inheritance
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Inheritance defines a hierarchy of objects, in which one
object can inherit data and methods from other objects.
In Java, we can define a class as the base class that
contains data and methods common to many classes.
Inherited classes can inherit these data and methods and
can also have their own data and methods.
Packages
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Object is an instance of a class that uses methods
(procedures or functions) to manipulate encapsulated data.
Java has a rich library of classes, which allows the
programmer to create and use different objects in an applet.
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Figure 27.21
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Skeleton of an applet
Applet is an active document written in Java.
It is actually the definition of a publicly inherited
class, which inherits from the applet class defined in
the java.applet library.
Programmer can define private data and public and
private methods in this definition.
Client process (browser) creates an instance of this
applet. The browser then uses the public methods
defined in the applet to invoke private methods or to
access data.
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Figure 27.23
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Creation and compilation
Use an editor to create a java source file.
Name of the file is the same as the name of the
publicly inherited class with the “java” extension.
Java compiler creates the bytecode for the file, with
the “class” extension.
Create an applet which can be run by a browser.
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Figure 27.24
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HTML document carrying an Applet
To use an applet, an HTML document is created and
the name of the applet is inserted between the
<APPLET> tags.
The tag also defines the size of the window used for
the applet.
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Example 9
In this example, we first import two packages, java.awt and java.applet. They
contain the declarations and definitions of classes and methods that we need. Our
example uses only one publicly inherited class called First. We define only one
public method, paint. The browser can access the instance of First through the
public method paint. The paint method, however, calls another method called
drawString, which is defined in java.awt.*.
import java.applet.*;
import java.awt.*;
public class First extends Applet
{
public void paint (Graphics g)
{
g.drawString ("Hello World", 100, 100);
}
}
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Example 10
In this example, we modify the program in Example 9 to draw
a line. Instead of method drawString, we use another method
called drawLine. This method needs four parameters: the x and
y coordinates at the beginning of the line and the x and y
coordinates at the end of the line. We use 0, 0 for the beginning
and 80, 90 for the end.
import java.applet.*;
import java.awt.*;
public class Second extends Applet
{
public void paint (Graphics g)
{
g.drawLine (0, 0, 80, 90);
}
}
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