1. ITC242 – Introduction to Data Communications Internet Based Applications

2. 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.

3. 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

4. Question
Which layer of OSI we will study in this class?

5. Layering: The OSI Model
layer-to-layer communication
Application
Application 7 7
Presentation
Presentation 6 6
Session
Session 5 5
Peer-layer communication
Transport
Transport
Router
Router 4 4
Network
Network
Network
Network 3 3
Link
Link
Link
Link 2 2
Physical
Physical
Physical
Physical 1 1

6. 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

7. Public vs Private Email
Provided by a 3rd party vendor
Available over one or more public networks
Examples: MCI Mail, AOL
Private
Integrated with user’s 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

8. Electronic Mail SMTP SMTP SMTP Three major components: user agents
user mailbox
outgoing
message queue
user
agent
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
mail
server
user
agent
SMTP
mail
server
user
agent
SMTP
mail
server
SMTP
user
agent
user
agent
user
agent

9. Electronic Mail: mail servers
user
agent
Mail Servers
mailbox contains incoming messages for user
message queue of outgoing (to be sent) mail messages
SMTP protocol (see the next)
mail
server
user
agent
SMTP
mail
server
user
agent
SMTP
mail
server
SMTP
user
agent
user
agent
user
agent

10. SMTP [RFC 2821] messages must be in 7-bit ASCII
uses TCP to reliably transfer 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

11. Scenario: Alice sends message to Bob
1) Alice uses UA to compose message and “to”
2) Alice’s UA sends message to her mail server; message placed in message queue
3) Client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message
mail
server
mail
server 1
user
agent
user
agent 2 3 6 4 5

12. 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:
S: 250 Sender ok
C: RCPT TO:
S: 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

13. 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”

14. 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

15. 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

16. Single System

17. Single System
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

18. Multiple Systems E-Mail

19. 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

20. 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 don’t 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.

21. 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

22. 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

23. Mail access protocols SMTP SMTP access protocol
user
agent
user
agent
sender’s mail
server
receiver’s mail
server
SMTP: delivery/storage to receiver’s server
Mail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]
authorization (agent <-->server) 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.

24. 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

25. Mail message format SMTP: protocol for exchanging email msgs header
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

26. 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

27. 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)

28. 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)

29. 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

30. 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

31. Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056
additional lines in msg header declare MIME content type
From:
To:
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
encoded data

32. HTTP Overview Stateless protocol Flexible in format handling
TCP connection terminated as soon as transaction completes
Flexible in format handling

33. 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:
host name
path name

34. HTTP overview HTTP: hypertext transfer protocol
Web’s 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

35. 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

36. 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

37. Nonpersistent HTTP
(contains text,
references to 10
jpeg images)
Suppose user enters URL
1a. HTTP client initiates TCP connection to HTTP server (process) at on port 80
1b. HTTP server at host 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

38. 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

39. 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 = 2RTT+transmit time
time to
transmit
file
initiate TCP
connection
RTT
request
received
time

40. Persistent HTTP Nonpersistent HTTP issues: Persistent HTTP
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

41. HTTP request message two types of HTTP messages: request, response
ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Host:
User-agent: Mozilla/4.0
Connection: close
Accept-language:fr
(extra carriage return, line feed)
header
lines
Carriage return,
line feed
indicates end
of message

42. 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:

43. Method types HTTP/1.0 GET POST HEAD HTTP/1.1 GET, POST, HEAD PUT
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

44. HTTP response message status line (protocol status code status phrase)
HTTP/ OK
Connection close
Date: Thu, 06 Aug :00:15 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

45. HTTP response status codes
In first line in server->client 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

46. HTTP Operation

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

48. Intermediate System Examples

49. 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

50. 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

51. 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

52. SIP Component Illustration

53. Session Description Protocol
Media Streams
Addresses
Ports
Payload types
Start and stop times
Originator

54. 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

55. Topic 7 – Client Server and Intranet Computing
Learning Objectives
describe the features, characteristics and architecture of client/server applications; and
define intranets and extranets.

56. 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

57. Client-Server Environment

58. 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

59. 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

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

61. 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

62. The Intranet Web Web Content Web/Database Connectivity Electronic Mail
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

63. Web/Database Connectivity

64. Web/Database Connectivity
Advantages
Ease of administration
Deployment
Development speed
Flexible information presentation
Disadvantages
Limited functionality
Stateless operation makes tracking difficult

65. 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

66. 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

67. 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

68. Advantages of Extranets
Reduced costs
More marketable products
Increased productivity
Enhanced profits
Reduced inventories
Faster time to market

69. 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 company’s intranet data
Internet access to an external server that originates database queries to internal servers
Virtual private network

70. 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.