1. Data Communications and Networking

2. Outline Application Architectures World Wide Web Electronic Mail
Host-Based, Client-Based, and Client-Server Architectures, Choosing Architectures
World Wide Web
How the Web Works, Inside an HTTP Request & HTTP Response
Electronic Mail
How Works, Inside an SMTP Packet
Listserv, Attachments in MIME
Other Applications
Ftp, Telnet, Instant Messaging, Videoconferencing

3. Application Layer - Introduction
Applications
(e.g., , web,
word processing)
Application Layer
Transport Layer
Network Layer
Functions of Applications
Data storage
Storing of data generated by programs (e.g., files, records)
Data access logic
Processing required to access stored data (e.g., SQL)
Application logic
Business logic
Presentation logic
Presentation of info to user and acceptance of user commands

4. Application Architectures
Determined by how functions of application programs are spread among clients and servers
Host-based Architectures
Server performs almost all functions
Client-based architectures
Client performs most functions
Client-server architectures
Functions shared between client and server

5. Host-Based Architectures
or a PC running a terminal emulation program
captures key strokes then sends them to the mainframe
displays information according to the server’s instructions

6. Problems with Host-based Arch.
Host becoming a bottleneck
All processing done by the host, which can severely limit network performance
Upgrades typically expensive and “lumpy”
Available upgrades require big jumps in processing and memory  $$$
Network demand grows more incrementally, so this often means a poor fit (too much or too little) between host performance and network demand.

7. Client-Based Architectures
Example: Using a word processing on a PC and storing data (file) on a server
Was popular in 1980s with the widespread use of PCs, LANs, and programs such as spreadsheets and word processors

8. Problems with Client-Based Arch.
Data MUST travel back and forth between server and client
Example: when the client program is making a database query, the ENTIRE database must travel to the client before the query can be processed
Result in poor network performance

9. Client-Server Architectures
(PC, mini, mainframe)
Used by most networks today
Client
(PC)
Example: Using a Web browser to get pages from
Presentation logic
Application logic
Data Access logic
Data Storage
Application logic
may reside on the client, server or be split up between the two

10. Client-Server Architectures
Advantages
More efficient because of distributed processing
Allow hardware and software from different vendors to be used together
Disadvantages
Difficulty in getting software from different vendors to work together smoothly
May require Middleware, a third category of software

11. client application programs server application programs
Middleware
client application programs
a standard way of translating between software from different vendors
Manages message transfers
Insulates network changes from the clients ((e.g., adding a new server)
Middleware
server application programs
Examples:
Distributed Computing Environment (DCE)
Common Object Request Broker Architecture (CORBA)
Open Database Connectivity (ODBC)

12. Multi-tier Architectures
Involve more than two computers in distributing application program logic
2-tier architecture (architectures discussed so far)
3-tier architecture
3 sets of computers involved
N-tier architecture
more than three sets of computers used

13. 3-tier Architecture

14. N-tier Architecture

15. Multi-tier Architectures
Advantages
Better load balancing:
More evenly distributed processing. (e.g., application logic distributed between several servers.)
More scalable:
Only servers experiencing high demand need be upgraded
Disadvantages
Heavily loaded network:
More distributed processing  more exchanges
Difficult to program and test due to increased complexity

16. Fat vs. Thin Clients
Depends on how much of the application logic resides on the client
Fat client: (a.k.a., thick client)
All or most of the application logic
Thin client:
Little or no application logic
Becoming popular because easier to manage, (only the server application logic generally needs to be updated)
The best example: World Wide Web architecture (uses a two-tier, thin client architecture)

17. Thin-Client Example: Web Architecture
Web Server
(PC, mini, mainframe)
Client
(PC)
Presentation logic
Application Logic
Data Access logic
Data Storage

18. Criteria for Choosing Architecture
Infrastructure Cost
Cost of servers, clients, and circuits
Mainframes: very expensive; terminals, PCs: very inexpensive
Development Cost
Mainly cost of software development
Software: expensive to develop; off-the-shelf software: inexpensive
Scalability
Ability to increase (or decrease) in computing capacity as network demand changes
Mainframes: not scalable; PCs: highly scalable

19. Choosing an Architecture
Host-Based
Client-Based
Client-Server
Cost of Infrastructure
High
Medium
Low
Cost of Development
Low
Medium
High
Scalability
Low
Medium
High

20. Applications World Wide Web E-mail File Transfer Videoconferencing
Instant Messaging

21. World Wide Web Two central ideas: Invention of WWW (1989)
Hypertext
A document containing links to other documents
Uniform Resource Locators (URLs)
A formal way of identifying links to other documents
Invention of WWW (1989)
By Tim Berners-Lee at CERN in Switzerland
First graphical browser, Mosaic, (1993)
By Marc Andressen at NCSA in USA; later founded Netscape
CERN - Centre Européan pour Rechèrche Nucleaire
NCSA - National Center for Supercomputing Applications

22. How the Web Works Main Web communications protocol:
HTTP - Hypertext Transfer Protocol
A request-response cycle: include multiple steps since web pages often contain embedded files, such as graphics, each requiring a separate response.
Clicking on a hyperlink or typing a URL into a browser starts a request-response cycle

23. HTTP Request Message Request line Request header Request body
(command, URL, HTTP version number)
required
(If the user types in the URL by themselves, then the referring page is blank.)
Request header
(information on the browser,
date, and the referring page )
optional
Request body
(information sent to the server,
e.g., from a form)
optional

24. Example of an HTTP Request
Command URL HTTP version
GET HTTP/1.1
Date: Mon 06 Aug :35:46 GMT
User-Agent: Mozilla/6.0
Referer:
Request
Line
Request Header
URL that contained the link to the requested URL
Web browser (code name for Netscape)
Note that this HTTP Request message has no “Body” part.
GMT – Greenwich Mean Time

25. HTTP Response Message Response status Response header Response body
(http version number, status code, reason)
optional
Response header
(information on the server, date,
URL of the page retrieved, format used )
optional
Response body
(requested web page)
required

26. Example of an HTTP Response
Response Status
HTTP/ OK
Date: Mon 06 Aug :35:46 GMT
Server: NCSA/1.3
Location:
Content-type: text/html
Response Header
<html>
<head>
<title>Allen R. Dennis</title>
</head>
<body>
<H2> Allen R. Dennis </H2>
<P>Welcome to the home page of Allen R. Dennis</P>
</body>
</html>
Response
Body
Another example of response status: HTTP/ page not found)

27. HTML - Hypertext Markup Language
A protocol used to format Web pages
Also developed at CERN (initially for text files)
TAGs embedded in HTML documents
include information on how to format the file
Extensions to HTML needed to format multimedia files
XML - Extensible Markup Language
A new markup language becoming popular

28. Electronic Mail Heavily used Internet application
Much, much faster than snail mail (regular mail)
Extremely inexpensive (compared to $3-$10 per paper mail cost)
Includes preparation, paper, postage, etc,
Can substitute for other forms of communication, such as telephone calls
Eliminates “telephone tag”
users can answer at his/her convenience, instead of time of call

29. E-mail Standards SMTP - Simple Mail Transfer Protocol
Main standard for
Originating user agent and the mail transfer agent
Between mail transfer agents
Originally written to handle only text files
Usually used in two-tier client-server architectures
Post Office Protocol (POP) and Internet Mail Access Protocol (IMAP)
Main protocols used between the receiver user agent and mail transfer agent
Main difference: with IMAP, messages can be left at the server after downloading them to the client
Other competing standards
Common Messaging Calls (CMC), X.400

30. Two-Tier E-mail Architecture
User agents (also called clients)
Run on client computers
Send to servers
Download from mailboxes on those servers
Examples: Eudora, Outlook, Netscape Messenger
Mail transfer agents (also called mail server)
Used by servers
Send between servers
Maintain individual mailboxes.

31. How SMTP Works LAN Internet LAN Server computer with
server software
SMTP packet
Client computer
(“message transfer agent”)
with client software
LAN
reads the packet’s destination address and sends it over the Internet to the receiver’s mail server.
SMTP packet
(“user agent”)
an message is sent as an SMTP packet to the local mail server
Internet
SMTP packet
Server computer with
IMAP or
POP packet
server software
Client computer
contacts the mail server which then downloads the message
LAN
stores the message in the receiver’s mail box
SMTP packet

32. Host Based e-mail Architectures
An old method used on UNIX based hosts
Similar to client-server architecture, except
Client PC replaced by a terminal (or emulator)
Sends all keystrokes to the server
Display characters received from the server
All software reside on server
Takes client keystrokes and understand user’s commands
Creates SMTP packets and sends them to next mail server

33. Three-Tier Client-Server Arch.
Best known example: Web based (e.g., Hotmail)
Server computer with server software
Client computer with Web Browser
Server computer with Web server software
sends HTTP requests to the Web server
sends HTTP responses to the Web client
translates the client’s HTTP requests into SMTP packets then send them to the Mail server
performs the same functions as the mail server in the two-tier example
No need for an user agent

34. Web-based e-mail LAN Internet LAN Server computer with
Web server software
HTTP request
Client computer with
LAN
SMTP packet
Web browser
Server computer with
server software
HTTP response
SMTP packet
Internet
SMTP packet
Server computer with
HTTP request
server software
Client computer with
Web browser
LAN
IMAP or
POP packet
Server computer with
HTTP response
IMAP packet
Web server software

35. SMTP Message Header Body Attachments
(source and destination addresses, date, subject,
and other information about the message)
Body
(message itself)
Attachments
(additional files included
along with the message)

36. Sample SMTP Message From: “Alan Dennis;”<adennis@indiana.edu>
Text in “ “ are ignored
From: “Alan
TO: “Pat Someone”
Date: Mon 06 Aug :03:02 GMT
Subject: Sample Note
Message-Id:
DATA:
This is an example of an message
Header
Body
Unique ID used to keep track of messages.
Note that this SMTP message has no attachments.

37. MIME Multipurpose Internet Mail Extension
A graphics capable mail transfer agent protocol (to send graphical information in addition to text)
SMPT designed for text transfer only
Included as part of an client
Translates graphical information into text allowing the graphic to be sent as part of an SMTP message (as a special attachment)
Receiver’s client then translates the MIME attachment from text back into graphical format
Other Graphics capable mail agent protocols
uuencode and binhex

38. Listserv Discussion Groups
Mailing lists of users who join to discuss some special topic (e.g., cooking, typing, networking)
Some permit any member to post messages, some are more restricted
Parts of listserv
Listserv Processor
Process commands ( subscriptions, etc,)
Listserv Mailer
Receive a message and resend it to everyone
To subscribe
Send an to Listserv processor (address of the processor is different than the address of mailer)

39. File Transfer Protocol (FTP)
Enables sending and receiving files over the Internet
Requires an application program on the client computer and a FTP server program on a server
Commonly used today for uploading web pages
Many packages available using FTP
WS-FTP (a graphical FTP software)
FTP sites
Closed sites
Requires account name and password
Anonymous sites
Account name: anonymous; pwd: your address

40. Telnet Allows one computer to log into other computers
Remote login enabling full control of the host
Requires an application program on the client computer and a Telnet server program on a server
Client program emulates a “dumb” terminal
Many packages available conforming Telnet
EWAN
Requires account name and password
Anonymous sites
Account name: anonymous; pwd: your address

41. Instant Messaging (IM)
A client-server program that allows real-time typed messages to be exchanged
Client needs an IM client software
Server needs an IM server package
Some types allow voice and video packets to be sent
Like a telephone
Examples include AOL and ICQ
Two step process:
Telling IM server that you are online
Chatting

42. How Instant Messaging Works
Client computer with
IM packet
When you type some text, your IM client sends the text in a packet to the IM server which relays it to your friend.
client software
LAN
IM packet
Server computer with
sends a request to the IM server telling it that you are online. If a friend connects, the IM server sends a packet to your IM client and vice versa
with IM server software
LAN
Internet
IM packet
Client computer with
If a chat session has more than two clients, multiple packets are sent by the IM server. IM servers can also relay information to other IM servers.
IM packet
IM client software
LAN

43. Videoconferencing
Provides real time transmission of video and audio signals among two or more locations
Allows people to meet at the same time in different locations
Saving money and time by not having to move people around (conference calling does the same thing)
Typically involves 2 special purpose rooms with cameras and displays
Desktop videoconferencing
Low cost application linking small video cameras and microphones together over the Internet
No need for special rooms
Example: Net Meeting sw on clients communicating through a videoconference server

45. Videoconferencing Standards
Proprietary early systems
Common standards in use today
H.320
Designed for room-to-room videoconferencing over high-speed phone lines
H.323
Family of standards designed for desktop videoconferencing and just simple audio conferencing over Internet
MPEG-2
Designed for faster connections such as LAN or privately owned WANs

46. Webcasting Special type of one-directional videoconferencing Process
Content is sent from the server to users
Process
Content created by developer
Downloaded as needed by the user
Played by a plug-in to a Web browser
No standards for webcasting yet
Defacto standards: products by RealNetworks

48. Implications for Management
Network must be used to provide a worry-free environment for applications
Network should not change the way an organization operates; application should!
Network should enable wide variety of applications
Dramatic increase in number and type of applications
Rapid growth in amount and type of traffic
Different implication on network design and management
Increased operating cost

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