THE WORLD WIDE WEB &DOMAIN NAME SYSTEM

And HTTP (Hypertext Transfer Protocol)

World Wide Web (WWW)

What is the internet? It is a massive network of networks

Connects millions of computers globally

The information that travels over the internet does so via a variety of languages known as protocols

The World Wide Web (WWW) It is a way of accessing information over the

medium of internet

Client-Server model built over the internet for information sharing

The Web speaks HTTP (Hypertext Transfer Protocol)

It utilizes “browsers” to access “Web pages” that are linked to each other via “hyperlinks”

Hypertext Transfer Protocol (HTTP) The web’s application layer protocol

It is implemented in two programs: Client and Server

The client and server programs talk to each other by exchanging information HTTP messages

HTTP defines how the client requests web pages from web servers and how web servers transfer web pages to clients

Hypertext Transfer Protocol (HTTP) HTTP uses TCP as its underlying

transport protocol

Two versions: HTTP/1.0 – RFC 1945 HTTP/1.1 – RFC 2616

HTTP is connectionless and stateless

How does it work? (briefly)Client Server

1. Initiate TCP connection to server

2. Accept the connection, notifying the client

3. Send http request message to server

4. Form response message with requested object(s) & send to client

Close the TCP connection

Receive the object

Repeat Steps 1 to 4 for each object

NON-PERSISTENT CONNECTIONPERSISTENT CONNECTION

Repeat steps 3 and 4 for each object WithoutPipelining

WithPipelining

containing details of all needed objects

HTTP message format: request

Method URL Version

GET http://www.iiitb.ac.in/student/a.html HTTP/1.0

Header field name: ValueUser-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language: Eng

HTTP message format: request

Method URL Version

POST http://www.iiitb.ac.in/student/a.html HTTP/1.0

Header field name: ValueUser-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language: Eng

Entity BodyName=Amu&favorite+flavor=Mango

HTTP message format: response

Version Status Code Phrase

HTTP/1.1 200 OK

Header field name: ValueDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html

Entity Body(data data data data data data data . . .)

HTTP Status Codes The status code is a 3 digit integer The first digit identifies the general

category of response

1XX Information message of some kind2XX Success of some kind3XX Redirects the client to another URL4XX Indicates error on client’s part5XX Indicates error on server’s part

HTTP Status codes - examples

100 Continue

Only a part of the request has been received by the server, but as long as it has not been rejected, the client should continue with the request

200 OK The request succeeded

301 Moved Permanently

Requested object has been permanently moved. New URL specified in the Location: header of the response message

400 Bad Request Request could not be understood by the server

404 Not Found Requested object does not exist in server

505 HTTP Version Not Supported HTTP version not supported by the server

HTTP – Other important pieces User-Server Interaction

Authentication Cookies (Set-cookie: # and Cookie: #)

Web Caching (proxy server)

Conditional GET(e.g., If-modified-since: Mon, 23 Jul 2012 22:00:00)

Domain Name System (DNS)

Convenience People are comfortable with Names

BATMANI’m

Batman

I am the guy living in WAYNE

MANOR, GOTHAM

CITY

Convenience Machines, on the other hand, are

comfortable with addresses (IP addresses) Hey

there, what’s ya

name babe?

I’m 192.168.11.200 dear

Convenience The Internet, just like every other

network, is made up of millions and millions of computers (including Web Servers)

It is tough to remember the IP addresses of all the required web-servers in order to connect to them

This is where DNS comes to the rescue

Convenience DNS provides a map between IP

addresses and a real & easy-to-remember nameI forgot your

IP address.. How do I

connect with you?

*Chuckles*Sorry

mate !!! You can’t if you don’t remember

my address !!No fear

when I am here,

dear

DNS

How does DNS work? Say you enter

www.thedarkknightrises.com in your web browser

First the browser and the OS will find out whether they know the IP address of the site already

The OS, if it has no clue about the IP address, asks the Resolving Name Server (or the local name server) about the site and it’s IP

How does DNS work?

requesting hostsurf.eurecom.fr

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

1

23

45

6

root name server

The Root name Server may not know which authoritative name server to call (or forward the query to)

requesting hostsurf.eurecom.fr

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

It is hierarchical (easy to remember)

.

com

edu org

yahoo

www

web

mit

Other aspects of DNS & name-servers

Recursive and Iterated queries

Caching and updating records

DNS records; message formats:

http://www.tech-juice.org/2011/06/22/the-dns-protocol-explained/

Some useful videos – YouTube links DNS Explained:

http://www.youtube.com/watch?v=72snZctFFtA

How the internet works (in 5 minutes or less):http://www.youtube.com/watch?v=7_LPdttKXPc

World Wide Web (in plain English):

http://www.youtube.com/watch?v=1wUxWxYOdTE

WAKE UP.. IT’S OVER