ITC242

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ITC242 Introduction to Data Communications
Internet Based Applications
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Last Week

• TCP/IP and OSI
• Describe the definition of a protocol
• Discuss the need and benefits of using protocol
  architectures
• Describe the TCP/IP protocol architecture
• Describe the OSI reference model.

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Topic 6 Internet Based Applications

• Learning Objectives
• describe electronic mail and the basic function
  of SMTP
• explain the need for MIME
• explain the role of HTTP in the operation of the
  Web
• describe the functions of proxies, gateways and
  tunnels in HTTP
• recognise the role of SIP

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Question

• Which layer of OSI we will study in this class?

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Layering The OSI Model
layer-to-layer communication
Application
Application
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Presentation
Presentation
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Session
Session
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Peer-layer communication
Transport
Transport
Router
Router
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4
Network
Network
Network
Network
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3
Link
Link
Link
Link
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Physical
Physical
Physical
Physical
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Electronic Mail Features

• Message Preparation
• Word processing
• Annotation
• Message Sending
• User directory
• Timed delivery
• Multiple addressing
• Message priority
• Status information
• Interface to other facilities

• Message Receiving
• Mailbox scanning
• Message selection
• Message notification
• Message reply
• Message rerouting

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Public vs Private Email

• Public
• Provided by a 3rd party vendor
• Available over one or more public networks
• Examples MCI Mail, AOL
• Private
• Integrated with users computer equipment
• Often part of an integrated tool, eg PROFS
• Used for internal messaging
• Internet mail does not fit into either of these
  categories it is a transfer mechanism rather
  than a contained system

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Electronic Mail

• Three major components
• user agents
• mail servers
• simple mail transfer protocol SMTP
• User Agent
• a.k.a. mail reader
• composing, editing, reading mail messages
• e.g., Eudora, Outlook, elm, Mozilla Thunderbird
• outgoing, incoming messages stored on server

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Electronic Mail mail servers

• Mail Servers
• mailbox contains incoming messages for user
• message queue of outgoing (to be sent) mail
  messages
• SMTP protocol (see the next)

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SMTP RFC 2821

• uses TCP to reliably transfer email message from
  client to server, port 25
• direct transfer sending server ( client) to
  receiving server( server)
• three phases of transfer
• handshaking (greeting)
• transfer of messages
• closure
• command/response interaction
• commands ASCII text
• response status code and phrase
• messages must be in 7-bit ASCII

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Scenario Alice sends message to Bob

• 1) Alice uses UA to compose message and to
  bob_at_someschool.edu
• 2) Alices UA sends message to her mail server
  message placed in message queue
• 3) Client side of SMTP opens TCP connection with
  Bobs mail server

• 4) SMTP client sends Alices message over the TCP
  connection
• 5) Bobs mail server places the message in Bobs
  mailbox
• 6) Bob invokes his user agent to read message

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Sample SMTP interaction
SMIT Client C SMIT server S
S 220 hamburger.edu C HELO crepes.fr
S 250 Hello crepes.fr, pleased to meet
you C MAIL FROM ltalice_at_crepes.frgt
S 250 alice_at_crepes.fr... Sender ok C RCPT
TO ltbob_at_hamburger.edugt S 250
bob_at_hamburger.edu ... Recipient ok C DATA
S 354 Enter mail, end with "." on a line
by itself C Do you like ketchup?
C How about pickles? C . S
250 Message accepted for delivery C QUIT
S 221 hamburger.edu closing connection
Sending message
Status code 220 Service Ready 250 OK 421
Service Not Available
Commands HELO(HELLO), MAIL FROM, RCPT TO, DATA,
QUIT
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SMTP Connection Setup

• Sender opens TCP connection to receiver
• Receiver acknowledges connection with 220
  Service Ready or 421 Service Not Available
• If connection is made, sender identifies itself
  with the HELO command
• Receiver accepts identification with 250 OK

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SMTP Mail Transfer

• MAIL command identifies originator, provides
  reverse path for error reporting
• RCPT commands identify recipient(s) for message
• Receiver has several positive or negative
  responses to RCPT
• Sender will not send message until it is sure at
  least one copy can be delivered
• DATA command transfers message

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SMTP Connection Closing

• Sender sends a QUIT command to initiate TCP close
  operation
• Receiver sends a reply to the QUIT command, then
  initiates its own close

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Single System E-Mail
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Single System E-Mail

• Only allows users of a shared system to exchange
  messages
• Each user has unique identifier and mailbox
• Sending a message simply puts it into recipients
  box
• Example AOL

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Multiple Systems E-Mail
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Multiple Systems E-Mail

• Distributed system enables mail servers to
  connect over a network to exchange mail
• Functions split
• User agent handles preparation, submission,
  reading, filing, etc
• Transfer agent receives mail from user,
  determines routing, communicates with remote
  systems
• Interconnection requires standards

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Request For Comments

• Form the basis of the Internet's technical
  documentation.
• Conceptually, the Internet as a layered series of
  protocols, each is documented by one or more
  RFCs.
• RFCs dont change. Updates documented by new
  RFCs.
• Not all RFCs document protocols Some for
  discussion, informational purposes, April Fools'
  RFCs distinguished by their date.
• There are more than 2000 RFCs in existence,
  dating back to ARPANET in the 1970s.
• Most RFCs are available as large text files, with
  graphics made out of typewriter characters.

http//www.freesoft.org/CIE/RFC/
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Basic E-Mail Operation

• User creates message with user agent program
• Text includes RFC 822 header and body of message
• List of destinations derived from header
• Messages are queued and sent to SMTP sender
  program running on a host

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SMTP Mail Flow

• SMTP server transmits messages to appropriate
  hosts via TCP
• Multiple messages to same host can be sent on one
  connection
• Errors handling necessary for faulty addresses
  and unreachable hosts
• SMTP protocol attempts to provide error-free
  transmission, but does not provide end-to-end
  acknowledgement
• SMTP receiver accepts messages, places it in
  mailbox or forwards

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Mail access protocols
SMTP
access protocol
receivers mail server

• SMTP delivery/storage to receivers server
• Mail access protocol retrieval from server
• POP Post Office Protocol RFC 1939
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
• more features (more complex)
• manipulation of stored msgs on server
• HTTP gmail, Hotmail, Yahoo! Mail, etc.

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RFC 822

• Defines format for text messages via electronic
  mail
• Used by SMTP as accepted mail format
• Specifies both envelope and contents
• Includes a variety of headers that can be
  included in the message header lines

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Mail message format

• SMTP protocol for exchanging email msgs
• RFC 822 standard for text message format
• header lines, e.g.,
• To
• From
• Subject
• different from SMTP commands!
• body
• the message, ASCII characters only

header
blank line
body
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SMTP final words

• SMTP uses persistent connections
• SMTP requires message (header body) to be in
  7-bit ASCII
• SMTP server uses CRLF.CRLF to determine end of
  message

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Question

• Why we are able to send emails with attached
  image files?
• RFC 822 was defined for sending ordinary ASCII
  text, but not sufficiently rich for multimedia
  messages, or for carrying non-ASCII text formats(
  for example, characters used by language other
  than English)

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Limitations of SMTP and RFC822

• Cannot transmit executables or binary files
  without conversion into text through non-standard
  programs (e.g. UUENCODE)
• Cannot transmit diacritical marks
• Transfers limited in size
• Gateways do not always map properly between
  EBCDIC and ASCII
• Cannot handle non-text data in X.400 messages
• Not all SMTP implementations adhere completely to
  RFC821 (tabs, truncation, etc)

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MIME (Multipurpose Internet Mail Extensions)

• Intended to resolve problems with SMTP and RFC822
• Specifies five new header fields, providing info
  about body of message
• Defines multiple content formats
• Defines encodings to enable conversion of any
  type of content into transferable form

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MIME Header Fields

• MIME-Version Indicates compliance with RFCs 1521
  and 1522
• Content-Type Describes data in sufficient detail
  for receiver to pick method for representation
• Content-Transfer-Encoding Indicates type of
  transformation used to represent content
• Content-ID Used to uniquely identify MIME
  entities
• Content-Description Plain text description for
  use when object is not readable

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Message format multimedia extensions

• MIME multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME
  content type

MIME version
method used to encode data
multimedia data type, subtype, parameter
declaration
encoded data
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HTTP Overview

• Stateless protocol
• TCP connection terminated as soon as transaction
  completes
• Flexible in format handling

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Web and HTTP

• First some jargon
• Web page consists of objects
• Object can be HTML file, JPEG image, Java applet,
  audio file,
• Web page consists of base HTML-file which
  includes several referenced objects
• Each object is addressable by a URL
• Example URL

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HTTP overview

• HTTP hypertext transfer protocol
• Webs application layer protocol
• client/server model
• client browser that requests, receives,
  displays Web objects
• server Web server sends objects in response to
  requests
• HTTP 1.0 RFC 1945
• HTTP 1.1 RFC 2068

HTTP request
PC running Explorer
HTTP response
HTTP request
Server running Apache Web server
HTTP response
Mac running Navigator
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HTTP overview (continued)

• HTTP is stateless
• server maintains no information about past client
  requests

• Uses TCP
• client initiates TCP connection (creates socket)
  to server, port 80
• server accepts TCP connection from client
• HTTP messages (application-layer protocol
  messages) exchanged between browser (HTTP client)
  and Web server (HTTP server)
• TCP connection closed

• Protocols that maintain state are complex!
• past history (state) must be maintained
• if server/client crashes, their views of state
  may be inconsistent, must be reconciled

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HTTP connections

• Nonpersistent HTTP
• At most one object is sent over a TCP connection.
• HTTP/1.0 uses nonpersistent HTTP

• Persistent HTTP
• Multiple objects can be sent over single TCP
  connection between client and server.
• HTTP/1.1 uses persistent connections in default
  mode

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Nonpersistent HTTP
(contains text, references to 10 jpeg images)

• Suppose user enters URL www.someSchool.edu/someDep
  artment/home.index

• 1a. HTTP client initiates TCP connection to HTTP
  server (process) at www.someSchool.edu on port 80

1b. HTTP server at host www.someSchool.edu
waiting for TCP connection at port 80. accepts
connection, notifying client
2. HTTP client sends HTTP request message
(containing URL) into TCP connection socket.
Message indicates that client wants object
someDepartment/home.index
3. HTTP server receives request message, forms
response message containing requested object, and
sends message into its socket
time
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Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.

• 5. HTTP client receives response message
  containing html file, displays html. Parsing
  html file, finds 10 referenced jpeg objects

time
6. Steps 1-5 repeated for each of 10 jpeg objects
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Non-Persistent HTTP Response time

• Definition of RTT time to send a small packet to
  travel from client to server and back.
• Response time
• one RTT ( round-trip time) to initiate TCP
  connection
• one RTT for HTTP request and first few bytes of
  HTTP response to return
• file transmission time
• total 2RTTtransmit time

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Persistent HTTP

• Nonpersistent HTTP issues
• requires 2 RTTs per object
• OS overhead for each TCP connection
• browsers often open parallel TCP connections to
  fetch referenced objects
• Persistent HTTP
• server leaves connection open after sending
  response
• subsequent HTTP messages between same
  client/server sent over open connection

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HTTP request message

• two types of HTTP messages request, response
• HTTP request message
• ASCII (human-readable format)

request line (GET, POST, HEAD commands)
GET /somedir/page.html HTTP/1.1 Host
www.someschool.edu User-agent
Mozilla/4.0 Connection close Accept-languagefr
(extra carriage return, line feed)
header lines
Carriage return, line feed indicates end of
message
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Uploading form input

• Post method
• Web page often includes form input
• Input is uploaded to server in entity body

• URL method
• Uses GET method
• Input is uploaded in URL field of request line

www.somesite.com/animalsearch?monkeysbanana
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Method types

• HTTP/1.0
• GET
• POST
• HEAD
• asks server to leave requested object out of
  response

• HTTP/1.1
• GET, POST, HEAD
• PUT
• uploads file in entity body to path specified in
  URL field
• DELETE
• deletes file specified in the URL field

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HTTP response message
status line (protocol status code status phrase)
HTTP/1.1 200 OK Connection close Date Thu, 06
Aug 1998 120015 GMT Server Apache/1.3.0
(Unix) Last-Modified Mon, 22 Jun 1998 ...
Content-Length 6821 Content-Type text/html
data data data data data ...
header lines
data, e.g., requested HTML file
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HTTP response status codes
In first line in server-gtclient response
message. A few sample codes

• 200 OK
• request succeeded, requested object later in this
  message
• 301 Moved Permanently
• requested object moved, new location specified
  later in this message (Location)
• 400 Bad Request
• request message not understood by server
• 404 Not Found
• requested document not found on this server
• 505 HTTP Version Not Supported

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HTTP Operation
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HTTP Intermediate Systems

• Proxy
• Forwarding agent
• Security intermediary
• Different versions of HTTP
• Gateway
• Security intermediary
• Non-HTTP server
• Tunnel
• Relay point between two TCP connections
• Cache
• Facility storing previous requests and responses

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Intermediate System Examples
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SMTP and HTTP

• HTTP pull
• SMTP push
• both have ASCII command/response interaction,
  status codes
• HTTP each object encapsulated in its own
  response msg
• SMTP multiple objects sent in multipart msg

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Session Initiation Protocol (SIP)

• Defined in RFC 3261
• Manages real-time sessions over IP data network
• Intended to enable Internet telephony/VoIP
• Based on HTTP-like request/response transaction
  model
• Five facets
• User location
• User availability
• User capabilities
• Session setup
• Session management

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SIP Components and Protocols

• Client/server elements
• Client sends/receives SIP messages
• Includes user agents, proxies
• Network elements
• User agent (client/UAC, server/UAS)
• Redirect server
• Proxy server
• Registrar
• Location service

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SIP Component Illustration
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Session Description Protocol

• Media Streams
• Addresses
• Ports
• Payload types
• Start and stop times
• Originator

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Summary

• SMTP - transmits messages to appropriate hosts
  via TCP, attempts to provide error-free
  transmission.
• MIME - Intended to resolve problems with SMTP,
  provides info about body of message, defines
  multiple content formats, and encodings
• HTTP - Stateless protocol, flexible format
  handling, Proxy, Gateway, Tunnel, Cache
• SIP - Manages real-time sessions over IP, enable
  Internet telephony/VoIP, HTTP-like
  request/response transaction model

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Topic 7 Client Server and Intranet Computing

• Learning Objectives
• describe the features, characteristics and
  architecture of client/server applications and
• define intranets and extranets.

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What is Client/Server?

• Client
• Server
• Network
• How is client/server different from other
  distributed computing?
• Heavy reliance on user-friendly applications
• Emphasis on centralizing databases and management
  functions
• Commitment to openness/modularity
• Networking fundamental to operation

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Client-Server Environment
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Why is Client-Server Different?

• Emphasis on user-friendly client applications
• Focus on access to centralized databases
• Commitment to open and modular applications
• Networking is fundamental to the organization

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Client/Server Applications

• Emphasis on GUI for users
• Database Example
• Database on server, applications for access on
  client, glue (like SQL) enables requests)
• Application logic can be client-only, or split
  between client and server

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Intranets

• Implementation of internet-based client/server
  technology within an organization, rather than
  for global connectivity
• Immensely successful in corporate computing
  contexts

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Advantages of Intranets

• Rapid prototyping
• Scales effectively
• Little training required
• Can be implemented on variety of systems
• Open architecture allows interaction across
  platforms

• Supports a range of distributed servers
• Allows integration of legacy systems on client
  and server side
• Supports a range of media types
• Inexpensive to implement

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The Intranet Web

• Web Content
• The web can be used to effectively distribute
  content in a way that requires no new training
  for end-users
• Web/Database Connectivity
• Multiple tools exist to serve as middleware
  between web servers and data sources
• Electronic Mail
• Network News

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Web/Database Connectivity
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Web/Database Connectivity

• Advantages
• Ease of administration
• Deployment
• Development speed
• Flexible information presentation

• Disadvantages
• Limited functionality
• Stateless operation makes tracking difficult

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Intranet Disadvantages

• Long development cycles
• Difficulty in partitioning applications, and
  modifying based on user feedback
• Effort in distributing upgrades to clients
• Difficult in scaling servers to respond to
  increased load
• Continuous requirement for more powerful desktop
  machines

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Other Intranet Technologies

• Electronic Mail
• Closed internal mail systems (delivery
  verification, etc)
• Internal mailing lists
• Network news (USENET)
• Can be adopted for internal intranet uses

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The Extranet Web

• Extends the intranet concept to provide
  information and services to selected outside
  populations, such as customers and suppliers
• Enables the sharing of information between
  companies
• A TCP/IP enabled form of EDI

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Advantages of Extranets

• Reduced costs
• More marketable products
• Increased productivity
• Enhanced profits
• Reduced inventories
• Faster time to market

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Methods for Converting Intranets to Extranets

• Long-distance dial-up access
• Internet access to intranet with security
• Internet access to an external server that
  duplicates some of a companys intranet data
• Internet access to an external server that
  originates database queries to internal servers
• Virtual private network

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Summary

• Client/server - user-friendly client
  applications, centralized databases, open and
  modular applications, the network is fundamental
• Intranet - internet-based client/server
  technology within an organization, immensely
  successful
• Extranets Extend intranet concept to outside
  community, e.g customers and suppliers, enables
  sharing of information between companies, TCP/IP
  enabled form of EDI.