World Wide Web(WWW)

Seminar
on
WWW(World Wide Web)
Presented By
Mr. Pratik R. Tambekar
Roll No.:-19
M.Tech II Sem(CSE)
Department of Computer Science & Engineering
YCCE, Nagpur

The World Wide Web (WWW) can be viewed as a huge
distributed system with millions of clients and servers for
accessing linked documents.
Servers maintain collections of documents while clients
provide users an easy-to-use interface for presenting and
accessing those documents.
A document is fetched from a server, transferred to a
client, and presented on the screen. To a user there is
conceptually no difference between a document stored
locally or in another part of the world.
2
INTRODUCTION

CONT…..
Now, Web has become more than just a simple
document based system.
With the emergence of Web services, it is becoming a
system of distributed services rather than just documents
offered to any user or machine.
What can we get from WWW?
Read news, listen to music and watch video;
Buy or sell goods such as books, airline tickets;
Make reservations on hotel room, rental car, restaurant, etc.;
Pay bills and transfer money from one bank account to another;
…
3

TRADITIONAL WEB-BASED SYSTEMS
4
Many Web-based systems are still organized as
simple client-server architectures.

The core of a Web site: a process that has access
to a local file system storing documents.
How to refer to a document?
URL (Uniform Resource Locator)?
Example:
http://www.cse.unl.edu/~ylu/csce855/notes/web-system.
ppt
A client interacts with Web servers through a special
application known as browser.
What’s the key function of a browser?
Responsible for displaying documents.
5

Document Model
 A Web document does not only contain text, but it can
include all kinds of dynamic features such as audio, video,
animations, etc.
In many cases special helper applications (interpreters)
are needed, and they are integrated into the browser.
 E.g., Windows Media Player and QuickTime Player for playing
streaming content
The variety of document types forces browser to be
extensible. As a result, plug-ins are required to follow a
standard interfaces so that they can be easily integrated
with the browsers. 6

CONT…..
 User data comes from an HTML form, specifying the
program and parameters.
Server-side scripting technologies are used to generate
dynamic content:
Microsoft: Active Server Pages (ASP.NET)
Sun: Java Server Pages (JSP)
Netscape: JavaScript
Free Software Foundation: PHP
7

8
Document Model (1)
<HTML> <!- Start of HTML document -->
<BODY> <!- Start of the main body -->
<H1>Hello World/H1> <!- Basic text to be displayed -->
<P> <!- Start of a new paragraph -->
<SCRIPT type = "text/javascript"> <!- identify scripting language -->
document.writeln ("<H1>Hello World</H1>; // Write a line of text
</SCRIPT> <!- End of scripting section -->
</P> <!- End of paragraph section -->
</BODY> <!- End of main body -->
</HTML> <!- End of HTML section -->
• A simple Web page embedding a script written in JavaScript.

When a web page is loaded, the browser creates a
Document Object Model of the page.
The HTML DOM model is constructed as a tree of
Objects:
9

Document Model (2)
10
(1) <!ELEMENT article (title, author+,journal)>
(2) <!ELEMENT title (#PCDATA)>
(3) <!ELEMENT author (name, affiliation?)>
(4) <!ELEMENT name (#PCDATA)>
(5) <!ELEMENT affiliation (#PCDATA)>
(6) <!ELEMENT journal (jname, volume, number?, month? pages, year)>
(7) <!ELEMENT jname (#PCDATA)>
(8) <!ELEMENT volume (#PCDATA)>
(9) <!ELEMENT number (#PCDATA)>
(10) <!ELEMENT month (#PCDATA)>
(11) <!ELEMENT pages (#PCDATA)>
(12) <!ELEMENT year (#PCDATA)>
• An XML definition for referring to a journal article.

11
(1) <?xml = version "1.0">
(2) <!DOCTYPE article SYSTEM "article.dtd">
(3) <article>
(4) <title> Prudent Engineering Practice for Cryptographic Protocols</title>
(5) <author><name>M. Abadi</name></author>
(6) <author><name>R. Needham</name></author>
(7) <journal>
(8) <jname>IEEE Transactions on Software Engineering</jname>
(9) <volume>22</volume>
(10) <number>12</number>
(11) <month>January</month>
(12) <pages>6 – 15</pages>
(13) <year>1996</year>
(14) </journal>
(15) </article>
• An XML document using the XML definitions from previous slide

Document Types
12
Type Subtype Description
Text Plain Unformatted text
HTML Text including HTML markup commands
XML Text including XML markup commands
Image GIF Still image in GIF format
JPEG Still image in JPEG format
Audio Basic Audio, 8-bit PCM sampled at 8000 Hz
Tone A specific audible tone
Video MPEG Movie in MPEG format
Pointer Representation of a pointer device for presentations
Application Octet-stream An uninterrupted byte sequence
Postscript A printable document in Postscript
PDF A printable document in PDF
Multipart Mixed Independent parts in the specified order
Parallel Parts must be viewed simultaneously
• Six top-level MIME types and some common subtypes.

Architectural Overview
13
• The principle of using server-side CGI programs.

14
CONT………..
(1) <HTML>
(2) <BODY>
(3) <P>The current content of <pre>/data/file.txt</PRE>is:</P>
(4) <P>
(5) <SERVER type = "text/javascript");
(6) clientFile = new File("/data/file.txt");
(7) if(clientFile.open("r")){
(8) while (!clientFile.eof())
(9) document.writeln(clientFile.readln());
(10) clientFile.close();
(11) }
(12) </SERVER>
(13) </P>
(14) <P>Thank you for visiting this site.</P>
(15) </BODY>
(16) </HTML>
• An HTML document containing a JavaScript to be executed by the server

CONT………..
• Architectural details of a client and server in the Web.

HTTP
All communication between clients and servers is based
on HTTP. Servers listen on port 80.
HTTP is a simple protocol; a client sends a request to a
server and waits for a response.
HTTP is stateless; it does not have any concept of open
connection and does not require a server to maintain
information on its clients. (Can use HTTP cookies to store
session information.)
HTTP is based on TCP; whenever a client issues a request
to a server, it first sets up a TCP connection and sends
the message on that connection. The same connection is
used for receiving the response.
One of the problems with the first versions of HTTP was
its inefficient use of TCP connections.
HTTP 1.0 vs. HTTP 1.1
16

HTTP CONNECTIONS
 A Web document is constructed from a collection of
different files from the same server.
In HTTP version 1.0 and older, each request to a server
required setting up a separate connection. When server
had responded, the connection was broken down. These
connections are referred as nonpersistent.
In HTTP version 1.1, several requests and their responses
can be issued without the need for a separate
connection. These connections are referred as
persistent.
Furthermore, a client can issue several requests in a row
without waiting for the response to the first request
which is referred as pipelining.
17

CONT……….
(a) Using non-persistent connections. (b) Using persistent connections. 18

HTTP Methods
19
Operation Description
Head Request to return the header of a document
Get Request to return a document to the client
Put Request to store a document
Post Provide data that is to be added to a document (collection)
Delete Request to delete a document
• Operations supported by HTTP.

HTTP Messages
• HTTP request message 20

• HTTP response message. 21
CONT……….

22
Header Source Contents
CONT……….
Accept Client The type of documents the client can handle
Accept-Charset Client The character sets are acceptable for the client
Accept-Encoding Client The document encodings the client can handle
Accept-Language Client The natural language the client can handle
Authorization Client A list of the client's credentials
WWW-Authenticate Server Security challenge the client should respond to
Date Both Date and time the message was sent
ETag Server The tags associated with the returned document
Expires Server The time how long the response remains valid
From Client The client's e-mail address
Host Client The TCP address of the document's server
If-Match Client The tags the document should have
If-None-Match Client The tags the document should not have
If-Modified-Since Client Tells the server to return a document only if it has been
modified since the specified time
If-Unmodified-Since Client Tells the server to return a document only if it has not been
modified since the specified time
Last-Modified Server The time the returned document was last modified
Location Server A document reference to which the client should redirect its
request
Referer Client Refers to client's most recently requested document
Upgrade Both The application protocol the sender wants to switch to
Warning Both Information about the status of the data in the message
• Some
HTTP
message
headers.

Processes
A plug-in is a small program that can be dynamically
loaded into a browser for handling a specific document
type.
When a browser encounters a document type for which
it needs a plug-in, it loads the plug-in locally and creates
an instance
 The plug-in is removed from the browser when it isno
longer needed.
23
1.Clients:

CONT……….
• Using a plug-in in a Web browser. 24

CONT……..
25
• Using a Web proxy when the browser does not speak FTP.

Important: The majority of Web servers is a
configured Apache server, which breaks down
each HTTP request handling into eight phases. This
approach allows flexible configuration of servers.
26
2.Servers:
• General organization of the Apache Web server.

CONT…….
In order to invoke the appropriate handler at the right time,
processing HTTP requests is broken down into sevral phases.
A module can register a handler for a specific phase.
Whenever a phase is reached, the core module inspects which
handlers have been registered for that phase and invoke one
of them.
27
1. Resolving document reference to local file name
2. Client authentication
3. Client access control
4. Request access control
5. MIME type determination of the response
6. General phase for handling leftovers
7. Transmission of the response
8. Logging data on the processing of the request

28
Server Clusters
• Essence: To improve performance and availability,
WWW servers are often clustered in a way that is
transparent to clients:
• The principle of using a cluster of workstations to implement a Web service.

CONT…….
• Problem: The front end may easily get overloaded,
so that special measures need to be taken.
• Transport-layer switching: Front end simply
passes the TCP request to one of the servers,
taking some performance metric into account.
• Content-aware distribution: Front end reads the
content of the HTTP request and then selects the
best server.
• A crucial aspect of this organization is the the
design of the front end as it can easily become a
serious performance bottleneck 29

30
CONT…….
(a) The principle of TCP handoff.

CONT……
(b) A scalable content-aware cluster of Web servers. 31

1.URL(Uniform Resource Locators):
Uniform Resource Locator tells how and
where to access a resource
32
Naming
• Often-used structures for URLs.
a) Using only a DNS name.
b) Combining a DNS name with a port number.
c) combining an IP address with a port number.

33
CONT…….
Name Used for Example
http HTTP http://www.cs.vu.nl:80/globe
ftp FTP ftp://ftp.cs.vu.nl/pup/minx/README
file Local file file:/edu/book/work/chp/11/11
data Inline data data:text/plain;charset=iso-8859-7,%e1%e2%e3
telnet Remote login telnet://flits.cs.vu.nl
tel Telephone tel:+31201234567
modem Modem modem:+31201234567;type=v32
Examples of URLs.

2.URN(Uniform Resource Names):
URNs are location-independent references to
documents.
34
The general structure of a URN
• A typical example of a URN is the one used for
identifying books by means of their ISBN such as
urn:isbn:0-13-349945-6

Synchronization: WebDAV
• Problem: There is a growing need for collaborative
auditing of Web documents, but bare-bones HTTP can’t
help here.
• Solution: Web Distributed Authoring and Versioning.
• Supports exclusive and shared write locks, which
operate on entire documents
• A lock is passed by means of a lock token; the server
registers the client(s) holding the lock
• Clients modify the document locally and post it back
to the server along with the lock token
• Note: There is no specific support for crashed clients
holding a lock.
35

Caching and Replication
Web Proxy Caching:
• Basic idea: Sites install a separate proxy server that
handles all outgoing requests. Proxies subsequently
cache incoming documents. Cache-consistency
protocols:
• Always verify validity by contacting server
• Age-based consistency:
Texpire = α·(Tcached – Tlast_modified) + Tcached
• Cooperative caching, by which you first check your
neighbors on a cache miss: 36

CONT………
37
The principle of cooperative caching

• A nontransparent form of replication that is widely
deployed is to make an entire copy of a web site
available at a different server. This approach is also
called Mirroring.
• Content Delivery Network: CDNs act as Web hosting
services to replicate documents across the Internet
providing their customers guarantees on high availability
and performance (example: Akamai).
38
Server Replication:

CONT…..
The principle working of the Akami CDN. 39

• Transport Layer Security: Modern version of the
the Secure Socket Layer (SSL), which “sits”
between transport layer and application protocols.
Relatively simple protocol that can support mutual
authentication using certificates:
40
Security
The position of TLS in the Internet protocol stack.

Cont……
41 TLS with mutual authentication.

CONT……
1.first, The client informs the server of the cryptographic
algorithms it can handle, as well as any compression methods
it supports.
2.In the second phase, authentication takes place. The server is
always required to authenticate itself, for which reason it
passes the client a certificate containing its public key signed
by a certification authority CA.
3.If server requires that the client be authenticated, the client
will have to send a certificate to the server.
4.The client generates a random number that will be used by
both sides for constructing a session key, and sends this
number to the server, encrypted with the server’s public key
5.If client authenticated is required, the client signs the number
with its private key.
42

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