1 internet services. 2 telnet protocol 3 telnet allows a person sitting on one computer to work on...

1

Internet Services

2

TELNET Protocol

3

TELNET

• Allows a person sitting on one computer to work on another computer.

• Starts a remote session on another machine.• Requires a person to supply login name and

password to gain entry.• Command:

telnet <domain_name>

telnet <ip_address> A B

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TELNET (contd.)

• Why required?– For using software only available on the remote

host.– For using devices (like printer) connected to the

remote host.

• Typical scenario– Many users do a telnet to a remote server, and

work there.– Server is a bigger and faster computer.

5

TELNET (contd.)

• By default connection is established over port 23.

• Any other port number can also be specified. telnet cse.iitkgp.ernet.in 25

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A Sample TELNET Session

SUNNY>> telnet cseTrying 144.16.192.57...Connected to cse.Escape character is '^]'. Linux 1.2.8 (cse) (ttyp0)  cse login: isgPassword: Last login: Wed Jun 11 12:17:55 from 144.16.202.210Linux 1.2.8. (POSIX).You have new mail. cse:~$

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FTP Protocol

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File Transfer Protocol (FTP)

• Allows a person to transfer files between two machines.

• Requires a person to supply login name and password to gain entry.

• Command: ftp <domain_name>

ftp <ip_address>

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Anonymous FTP

• A special form of FTP which has become very popular.– Does not require a person to know login name

and password.– In place of login name, type the word

anonymous.– In place of password, type the email address.

• Huge amount of resources are available in anonymous FTP sites.

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FTP Process Model

Control

DataTransfer

Control

INTERNET

Port 21

DataTransfer

SERVERCLIENT

Port 22

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How FTP Works?

• Two connections established:– A control connection over port 21, that

remains all through a session.– A temporary port number, used for every file

being transferred.• New connection established for every file

transfer.

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Typical FTP Commands

• put, mput– Send one (or more) file(s).

• get, mget– Receive one (or more) file(s).

• hash– Indicates progress of file transfer.

• prompt– Prompts user before actual transfer of each file.

• bin, asc– Selects binary or ascii mode of transfer.

• exit– Terminates connection with ftp server.

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Sample FTP Sessions

SUNNY>> ftp cseindigoConnected to cseindigo.220 cseindigo FTP server ready.Name (cseindigo:isg): isg331 Password required for isg.Password:230 User isg logged in.Remote system type is UNIX.Using binary mode to transfer files.ftp>

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Sample FTP Sessions (contd.)

ftp> dir200 PORT command successful.150 Opening ASCII mode data connection for '/bin/ls'.total 97drwxr-xr-x 2 isg faculty 512 Oct 8 1995 PROJECTS-rw-r--r-- 1 isg faculty 7693 Mar 31 1995 lan-cse.fig-rw-r--r-- 1 isg faculty 6578 Apr 26 1995 lan-cse.fig-rw-r--r-- 1 isg faculty 6721 Jul 12 1995 newlan.fig226 Transfer complete.

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Sample FTP Sessions (contd.)

ftp> get lan-cse.fig200 PORT command successful.150 Opening BINARY mode data connection for 'lan-cse.fig' (6721 bytes).226 Transfer complete.6721 bytes received in 0.0389 secs (1.7e+02 Kbytes/sec) ftp> quit221 Goodbye. SUNNY>>

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Electronic Mail

Indian Institute of Technology Kharagpur

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Electronic Mail

• Most widely used application on the Internet.

• For sending mails:– Simple Mail Transfer Protocol (SMTP)– Multi-purpose Internet Mail Extension (MIME)

• For receiving mails:– Post office protocol version 3 (POP3)– Internet mail access protocol (IMAP)

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Simple Mail Transfer Protocol

• Based on RFC 821.• Transmits simple text messages only.

– 7-bit ASCII format.

• Uses information written on envelope of mail.– Message header.– Contains recipient address and other information.

• Does not look at contents.– Message body.

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Message Header

Message Body

MailMessage

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Basic Operation

• Mail is created by user agent program (mail client).– mailx, pine, Outlook, etc.

• Messages queued and sent as input to SMTP sender program.– Typically a server process.– Daemon on UNIX.

• sendmail or qmail• Listens on port number 25

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SMTP Mail Flow

UserAgent

SMTPServer

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

User Mailboxes

SMTPServer

SMTPServer

Port 25

Port 25

Port 25

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Mail Message Contents

• Each queued message has:– Message text

• RFC 822 header with message envelope and list of recipients.

• Message body, composed by user.– A list of mail destinations

• Derived by user agent / SMTP server from header.

• May require expansion of mailing lists.

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SMTP Sender

• Takes message from queue.• Transmits to proper destination host.

– Via SMTP transaction.– Over one or more TCP connections to port 25.

• When all destinations have been processed, message is deleted.

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Optimization

• If message is sent to multiple users on a given host, it is sent only once.– Delivery to users handled at destination host.

• If multiple messages are ready for given host, a single TCP connection can be used.– Saves overhead of setting up and dropping

connection.

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Possible Errors

• Host unreachable• Host out of operation• TCP connection fail during transfer• Faulty destination address

– User error– Target user address has changed– Redirect if possible– Inform user if not

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SMTP Protocol - Reliability

• Used to transfer messages from sender to receiver over TCP connection.– Uses port number 25.

• Attempts to provide reliable service.• No guarantee to recover lost messages.• No end-to-end ACK to sender.• Error indication report not guaranteed.

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SMTP Receiver

• Accepts arriving message.• Places in user mailbox or copies to

outgoing queue for forwarding.• Receiver must:

– Verify local mail destinations.– Deal with errors

• Transmission• Lack of disk space

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SMTP Forwarding

• Mostly direct transfer from sender host to receiver host.

• May go through intermediate mail servers via forwarding capability.– Sender can specify route.

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SMTP Mail Flow

UserAgent

SMTPServer

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

Header

MailBody

User Mailboxes

SMTPServer

SMTPServer

Port 25

Port 25

Port 25

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SMTP System Overview

• Commands and responses exchanged between sender and receiver.

• Initiative with sender.– Establishes TCP connection.

• Sender sends commands to receiver.– e.g. HELO <domain><CRLF>

• Each command generates exactly one reply.– e.g. 250 requested mail action ok; completed.

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SMTP Replies

• Starts with 3-digit code.• Leading digit indicates category.

– 2xx -- Positive completion reply– 3xx -- Positive intermediate reply– 4xx -- Transient negative completion reply – 5xx -- Permanent negative completion reply

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Operation Phases

a) Connection setup

b) Exchange of command-response pairs

c) Connection termination

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a) Connection Setup

• Sender opens TCP connection with receiver.• Once connected, receiver identifies itself.

220 <domain> service ready• Sender identifies itself.

HELO• Receiver accepts sender’s identification.

250 OK• If mail service not available, the response to the

second step above becomes: 421 service not available

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b) Mail Transfer Commands

• The MAIL FROM command identifies originator.– Gives reverse path to be used for error

reporting.– Receiver returns 250 OK or appropriate failure /

error message.

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• One or more RCPT TO commands identify recipients for the message.– Separate reply for each recipient.

• The DATA command transfers message text.– End of message indicated by a line containing

just period (.)

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c) Closing Connection

• Two steps:– Sender sends QUIT and waits for reply.– Then initiate TCP close operation.

• Receiver initiates TCP close after sending reply to QUIT.

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An Example SMTP Session

• How to connect to an SMTP server? telnet servername 25

– A TCP connection gets established over port number 25.

– The telnet client and the mail server can now start a dialogue.

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An Example SMTP Session

S: 220 hotmail.com Simple Mail Transfer Service ReadyC: HELO yahoo.comS: 250 hotmail.com

C: MAIL FROM: <isg@yahoo.com> S: 250 OK

C: RCPT TO: <myfriend@hotmail.com> S: 250 OK

C: RCPT TO: <somebody@rediffmail.com> S: 250 OK

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An Example SMTP Session

C: DATA

S: 354 Start mail input; end with (.)

C: … actual contents of the message …

C: ………

C: ………

C: .S: 250 OK

C: QUIT

S: 221 hotmail.com Service closing transmission channel

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Mail Access Protocols

• Two mail access protocols are widely used:1. Post Office Protocol, version 3 (POP3)

2. Internet Mail Access Protocol version 4 (IMAP4).

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What do they do?

User Mailboxes

on SMTP server

Mail Client

POP3 / IMAP

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POP3

• The client POP3 software is installed on the recipient machine, and the server POP3 software installed on mail server.– The client (user agent) opens a connection

with the server on TCP port number 110.– Sends user name and password.– Can access the mails, one by one.

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POP3 (contd.)

– Two modes:• Delete mode – mails deleted as they are read• Keep mode – mails remain in the mailbox

– POP3 has commands for:• Log in• Log out• Fetch messages• Delete messages

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IMAP4

• Provides the following extra features:– A user can check the email header before

downloading.– A user can search the contents of the email for

a specific string prior to downloading.– A user can create, delete, or rename mailboxes

on the mail server.– A user can create a hierarchy of mailboxes in a

folder for email storage.

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Multipurpose Internet Mail Extension (MIME)

• SMTP cannot transmit non-text messages.– Solutions (like uuencode) exists on some

systems, but are not standardized.

• Cannot transmit text that includes international characters (e.g. â, å, ä, è, é, ê, ë).– Need 8 bit ASCII.

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• Servers may reject mail over certain size.• Some SMTP implementations do not

adhere to standard.– CRLF, truncate or wrap long lines, removal of

white space, etc.

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Overview of MIME

• Five new message header fields:– MIME-version– Content-type– Content-transfer-encoding– Content-Id– Content-description

• A number of content types and transfer encoding formats have been defined.

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Content Types

• Text body• Multipart

– Mixed, Parallel, Alternative

• Message– RFC 822, Partial,

External-body• Image

– jpeg, gif

• Video– mpeg

• Audio– Basic

• Application– Postscript– octet stream

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MIME Transfer Encodings

• Specifies how the mail body is wrapped for transmission.

• Content transfer encoding field can have six possible values.– 7bit, 8bit, binary:

• No encoding done for these three.

• Provide information about nature of data.

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– Quoted-printable• Data mostly printable ASCII characters.• Non-printing characters represented by hex code.

– Base64• Maps arbitrary binary input onto printable output.

– X-token• Named nonstandard encoding.

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Base64 Encoding

6 bits 6 bits 6 bits6 bits

8 bits8 bits 8 bits 8 bits

R64 R64 R64 R64

• Expands the message by 33%.

• Uses the symbols A..Z, a..z, 0..9, +, /

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MIME Header ExampleFrom: Indranil Sengupta <isg@iitkgp.ac.in>To: Jaswinder Ahuja <jassi@cadence.com>Subject: Simple MessageMIME-Version: 1.0Content-type: multipart/mixed; boundary="simple boundary"

This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note.--simple boundary

This is implicitly typed plain text. It does NOT end with a linebreak.-- simple boundaryContent-type: text/plain; charset=us-ascii

This is explicitly typed plain ASCII text. It DOES end with a linebreak.

--simple boundary--This is the epilogue. It is also to be ignored.

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Another MIME ExampleFrom: Indranil Sengupta <isg@iitkgp.ac.in>To: Jaswinder Ahuja <jassi@cadence.com>Subject: Formatted text mailMIME-Version: 1.0Content-type: multipart/alternative; boundary=boun42

--boun42

Content-type: text/plain; charset=us-ascii

... plain text version of message goes here ...

--boun42Content-type: text/enriched

... RFC1896 text/enriched version of the same message goes here

...--boun42--

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World Wide Web

Indian Institute of Technology Kharagpur

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World Wide Web (WWW)

• Latest revolution in the internet scenario.• Allows multimedia documents to be

shared between machines.– Containing text, image, audio, video,

animation.

• Basically a huge collection of inter-linked documents.– Billions of documents.– Inter-linked in any possible way.– Resembles a cob-web.

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WWW (contd.)

• Where do the documents reside?– On web servers.– Also called Hyper Text Transfer Protocol

(HTTP) servers.

• They are typically written in– Hyper Text Markup Language (HTML).

• Documents get formatted/displayed using– Web browsers

• Internet Explorer• Netscape• Mosaic• Konquerer

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What is HTTP?

• Hyper Text Transfer Protocol– A protocol using which web clients (browsers)

interact with web servers.

• It is a stateless protocol.– Fresh connection for every item to be

downloaded.

• Transfers hypertext across the Internet.– A text with links to other text documents.– Resembles a cob-web, and hence the name

World Wide Web (WWW).

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HTTP Protocol

• Web clients (browsers) and web servers communicate via HTTP protocol.

• Basic steps:– Client opens socket connection to the HTTP

server.• Typically over port 80.

– Client sends HTTP requests to server.– Server sends back response.– Server closes connection.

• HTTP is a stateless protocol.

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Illustration

Web Servers

WebClient

http request

http response

http request

http response

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HTTP Request Format

• A client request to a server consists of:– Request method– Path portion of the HTTP URL– Version number of the HTTP protocol– Optional request header information– Blank line– POST or PUT data if present.

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HTTP Request Methods

• GET– Most common HTTP method.– Returns the contents of the specified

document.– Places any parameters in request header.– Can also be used to submit forms:

• The form data is URL-encoded and appended to the GET command URL.

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Illustration of GET

– A very simple HTTP connection to a server. telnet www.facweb.iitkgp.ac.in http

– Client sends request for a file: GET /test.html HTTP/1.0

– The server sends back the response: HTTP/1.1 200 OK Date: Sun, 22 May 2005 09:51:42 GMT Server: Apache/1.3.33 (Win32) Last-Modified: Sun, 22 May 2005 09:51:10 GMT Accept-Ranges: bytes Content-Length: 119 Connection: close

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Illustration of GET (contd.)

Content-Type: text/html

<html> <head> <title> A test page </title> </head>

<body>

This is the body of the test page.

</body>

</html>

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HTTP Request Methods (contd.)

• HEAD– Returns only the header information of the

specified document.– Used by clients to determine the file size,

modification date, server version, etc.

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Illustration of HEAD

• Client sendsHEAD /index.html HTTP/1.0

• Server responds back with:HTTP/1.1 200 OK

Date: Sun, 22 May 2005 10:08:37 GMT

Server: Apache/1.3.33 (Win32)

Last-Modified: Thu, 03 May 2001 11:30:38 GMT

Accept-Ranges: bytes

Content-Length: 1494

Connection: close

Content-Type: text/html

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HTTP Request Methods (contd.)

• POST– Used to send data to the server to be processed in

some way, as in a CGI script.– Basic difference from GET:

• A block of data is sent along with the request. Extra headers like Content-Type and Content-Length are used for this purpose.

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• The requested object is not a resource to retrieve. Rather, it is a script that can handle the data being sent.

• The server response is not a static file; but is generated dynamically as the program output.

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Illustration of POST

– A typical form submission, using POST is illustrated below:

POST /cgi-bin/myscript.cgi HTTP/1.0

From: isg@hotmail.com

User-Agent: HTTPTool/1.0

Content-Type: application/x-www-form-urlencoded

Content-Length: 32

Roll=1234&Sex=M&Age=20

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HTTP Request Methods (contd.)

• PUT– Replaces the contents of the specified

document with data supplied along with the command.

– Not used widely.

• DELETE:– Deletes the specified document from the

server.– Not used widely.

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HTTP Request Headers

• After a HTTP request line, a client can send any number of header fields.– Usually optional – used to convey some

information.– Some commonly used fields:

• Accept: MIME types client accepts, in order of preference.

• Connection: connection options, close or Keep-Alive.

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• Content-Length: number of bytes of data to follow.

• Content-Type: MIME type and subtype of the data that follows.

• Pragma: “no-cache” option directs the server/proxy to return a fresh document even though a cached copy may exist.

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HTTP Request Data

• To be given if the request type is either PUT or POST.– Send the data immediately after the HTTP

request header, and a blank line.

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HTTP Response

• An initial response line.– Also called the status line.– Consists of three parts separated by spaces

• The HTTP version• A 3-digit response status code• An English phrase describing the status code.

HTTP/1.0 200 OK

HTTP/1.0 404 Not Found

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HTTP Response (contd.)• Header information, followed by a blank

line, and then the data.

HTTP/1.1 200 OKDate: Sun, 22 May 2005 09:51:42 GMTServer: Apache/1.3.33 (Win32)Last-Modified: Sun, 22 May 2005 09:51:10 GMTContent-Length: 119Connection: closeContent-Type: text/html

<html> <head> <title> A test page </title> </head><body> This is the body of the test page.</body> </html>

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3-digit Status Code

• 1xx– Indicates informational messages only.

• 2xx– Indicates successful transaction.

• 3xx– Redirects the client to another URL.

• 4xx– Indicates client error, such as unauthorized

request.

• 5xx– Indicates internal server error.

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Common Status Codes

• 200 OK• 301 Moved Permanently• 302 Moved Temporarily• 401 Unauthorized• 403 Forbidden• 404 Not Found• 500 Internal Server Error

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HTTP Response Headers

• Common response headers include:– Content-Length

• Size of the data in bytes.

– Content-Type• MIME type and subtype of data being sent.

– Date• Current date.

– Expires• Date at which document expires.

– Last-Modified– Set-Cookie

• Name/value pair to be stored as cookie.

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HTTP Response Data

• A blank line follows the response header, and the data follows next.– No upper limit on data size.

• HTTP/1.0– Server typically closes connection after

completing a transaction.

• HTTP/1.1– Server keeps the connection open by default,

across transactions.

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HTTP version 1.1

• Current standard and widely used.– Became IETF draft standard in 2001.

• Improvements over HTTP 1.0:– Requires host identification.

• Allows multi-homed servers.• More than one domain living on same server.

GET /index.html HTTP/1.1Host: www.facweb.iitkgp.ac.in<blank line>

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HTTP version 1.1 (contd.)

– Default support for persistent connections.• Multiple transactions over a single connection.

– Support for content negotiation.• Decides on the best among the available representations.• Server-driven or browser-driven.

– Browsers can request part of document.• Specify the bytes using Range header.• Browser can ask for more than one range.• Continue interrupted downloads.

Range: bytes=1200-3500

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HTTP version 1.1 (contd.)

– Efficient caching support• A document caching model that allows both

the server and the client to control the level of cachability and update conditions and requirements.

• HTTP 1.1 requires several extra things from both clients and servers.– Mandatory to know these if one is trying to

write a HTTP client or server.

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HTTP 1.1 Client Requirements

• The clients must do the following:– Include the Host: header with each request.– Either support persistent connections, or include

the Connection: close header with each request.

– Handle the 100 Continue response.– Accept responses with chunked data.

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HTTP 1.1 Server Requirements

• The servers must do the following:– Require the Host: header from HTTP 1.1 clients.– Accepts absolute URL’s in a request.– Accept requests with chunked data.– Include the Date: header in each response.– Support at least the GET and HEAD methods.– Support HTTP 1.0 requests.– Either support persistent connections, or include

the Connection: close header with each request.

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How a Web Server Works?

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Requirements of a Web Server

• Simple requirements:– Able to accept HTTP requests, and respond to

them.– Support for GET and HEAD, possibly also

POST.– Able to handle server-side scripts.

• Executables residing on the server.• They get executed when specified.• Their output sent back to the client;

typically as a HTML page.

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Directory Structure

• When a web server is installed:– A http root directory gets created.

• For example, “/home/httpd”– There is a directory under the http root that

acts as the http home directory.• For example, “/home/httpd/docs”

– There is a directory under the http root under which all the CGI and other scripts are to be stored.

• For example, “/home/cgi-bin”• Server-side exec permission provided

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Directory Structure (contd.)

HTTP Root

cgi-binHTTP Home Other

WebFolders

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Default Web Page

• There is a default web page that gets returned by the server if no explicit document path is specified.– index.htm or index.html.– Any other name can also be specified through

server script configuration.

• Can be accessed as:GET www.xyz.com HTTP/1.0

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How are Scripts Handled?

• Server-side scripts– A file that is to be executed by the server, and

the output sent back to the client.

• How does the server know?– Two ways:

• GET command with a “?”.• POST command.

90

GET Command with a “?”

• Consider an example HTTP command:GET /cgi-bin/xyz.pl ? roll=1234 & sex=M

• What happens?– Server identifies the “?” following the GET.– Identifies xyz.pl as a program to be executed.– Allows the xyz.com program to read the values

present in the string following the “?”.• How, to be discussed later

– The output generated by the xyz.com program is sent back to the client.

91

POST Command

• Works similar to get.• Differences:

– The name-value pairs are present as data following the header lines and a blank line.

– Not limited by the maximum size of a string (as in GET).

– The executable program can read the data values.

• How, to be discussed later.

92

POST Command :: Example

POST /cgi-bin/myscript.cgi HTTP/1.0

From: isg@hotmail.com

User-Agent: HTTPTool/1.0

Content-Type: application/x-www-form-urlencoded

Content-Length: 32

Roll = 1234 & Sex = M & Age = 20

93

Points to Note

• The executable program that runs on the server can be written in any language.– Shell script (C shell, bourne shell, etc.)– Perl– ASP– PHP– C, Java (requires servlet support)

• Necessary support for executing must be there in the server.

• For example, ASP can run under IIS but not under Apache.