Ch2_Ed7_Network_Applications.ppt

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Chapter 2. Network Applications
Business Data Communications and
Networking Fitzgerald and Dennis,
7th Edition
Copyright © 2002 John Wiley & Sons, Inc.

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Chapter 2. Learning Objectives
• Understand host-based, client-based and
client-server architectures
• Understand how the Web works
• Understand how e-mail works
• Be aware of how FTP, Telnet and Instant
Messaging work

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Chapter 2. Outline
• Introduction
• Application Architectures
– 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 E-Mail Works, Inside an SMTP Packet
– Listserv, Attachments in MIME
• Other Applications
– Ftp, Telnet, Instant Messaging, Videoconferencing

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Application Architectures
• Application programs can be thought of as being
made up of four functions:
– Data storage
– Data access logic
– Application logic
– Presentation logic
• Application architectures are determined by how these
functions are split between clients and servers.
• Three basic application architectures are in use today:
– Host-based Architectures
– Client-based architectures
– Client-server architectures

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Host-based Architectures
• The first application architecture to be
developed was the host-based.
• The ‘server in host-based architectures, usually
a mainframe, performs all four functions.
• The client, usually a terminal or a PC running a
terminal emulation program captures key
strokes which are sent to the mainframe and
displays information according to the server’s
instructions (Figure 2-1)

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Figure 2-1 Host-Based Architectures

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Host-based Architectures
• There are two main problems with host-based
architectures:
– Since all processing is by the host, the host
becomes a bottleneck which can severely limit
network performance.
– Upgrades are typically expensive and “lumpy”,
meaning 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.

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Client-Based Architectures
• Client-based architectures became important in
the late 1980s with the widespread use of PCs,
local area networks and low-cost, PC-based
application programs such as spreadsheets and
word processors.
• In client-based architectures, application
programs on the clients are responsible for the
data access, application and presentation logic.
• The server is responsible for data storage only
(Figure 2-2).

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Figure 2-2 Client-Based Architectures

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Client-Based Architectures
• The main problem with Client-based
architectures is in the need for all of the data
travel back and forth between server and client.
• For example, if the client program is making a
database query, the entire database must travel
to the client before the query can be processed.
• The result can be poor network performance
because of the bottleneck created by constantly
sending large files back and forth over the
network.

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Client-Server Architectures
• In client-server architectures the application
program functions are divided up between
clients and servers.
• The client takes care of the presentation logic
• The server handles data storage and data access
logic.
• Application logic may reside on the client,
server or be split up between the two.
• Most networks today use a client-server
architecture (Figure 2-3 shows an example).

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Figure 2-3 Client-Server Architectures

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Client-Server Architectures
• Client-server architectures are more efficient since
they distribute processing between client and server.
• Another strength is that they allow hardware and
software from different servers to be used together.
• This is also a weakness, since it is sometimes difficult
to get software from different vendors to work
together smoothly.
• For this reason, a third category of software, called
Middleware was developed.

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Middleware
• Middleware provides an interface for allowing client
application programs to interact with server
application programs.
• Middleware has two main functions:
– Providing a standard way of translating between software
from different vendors.
– Manage message transfer between client and server, thus
insulating network changes (such as adding a new server)
from the clients.
• Two of the most important middleware standards are:
Distributed Computing Environment (DCE) and
Common Object Request Broker Architecture
(CORBA).

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Two-tier, Three-tier and N-tier
Architectures
• Figure 2-3 gives an example of a 2-tier client-server
architecture.
• In a three-tier architecture the application program
logic is split up between three computers. For
example (see Figure 2-4),
– the client handles the presentation logic
– an application server handles the application logic
– A database server handles the data storage and data access
logic.
• In an N-tier architecture more than three sets of
computers are used (see Figure 2-5).

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Figure 2-4 A Three-tier Architecture

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Figure 2-5 An N-tier Architecture

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Two-tier, Three-tier and N-tier
Architectures
• The primary advantage of N-tier architectures is in
distributing the application logic between several
servers makes load balancing possible, so that
demand for processing is more evenly distributed
between the servers.
• N-tiered architectures are also more easily scalable,
since only servers experiencing high demand, such as
the application server, need be upgraded.
• The primary disadvantage is that more distributed
processing means a more heavily loaded network.
• It is also more difficult to program and test an N-tier
architecture due to its increased complexity.

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Thin versus Fat Clients
• Thin and Fat (or thick) clients describe how much of
the application logic resides on the client.
– Little or no application logic resides on a thin client
– All or most of the application logic resides on a fat client
• Thin clients are becoming popular because they are
easier to manage, since only the server application
logic generally needs to be updated.
• The best example of a thin client is World Wide Web
architecture which uses a two-tier, thin client
architecture (Figure 2-6).

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Figure 2-6 Web Architecture

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Choosing An Architecture
• Often, the application architecture is given, so no
decision has to be made. When it is not, three
important criteria for choosing an architecture are:
– Cost of Infrastructure: mainframes are expensive, which
is why client-based and client-server architectures have
become popular.
– Cost of Development: because software is expensive to
develop, client-based and client-server architectures that use
off-the-shelf software tend to be much cheaper than
software solutions that require in-house development.
– Scalability refers to the ability to increase (or decrease) in
computing capacity as network demand changes. Client-
server architectures are the most scalable since both clients
and servers can be added to meet network needs.

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Host-Based Client-Based Client-Server
Cost of
Infrastructure
High Medium Low
Cost of
Development
Low Medium High
Scalability Low Medium High
Figure 2-7 Choosing an Architecture

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Origin of the World Wide Web
• Invented in 1989 by Tim Berners-Lee at the Centre
Européan pour Rechèrche Nucleaire (CERN) in
Geneva.
• Two central WWW ideas are hyperlinks and Uniform
Resource Locators (URLs).
• Marc Andressen, who later founded Netscape, wrote
the first web reader (browser) to gain widespread use
in 1993, called Mosaic at the Nat’l Center for
Supercomputing Applications (NCSA).

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How the Web works (Figure 2-8)
• Based on a Client-Server architecture
• The main Web communications protocol is Hypertext
Transfer Protocol.
• Web clients (Browsers) make HTTP requests to Web
Servers. Web Servers answer with HTTP responses
• Clicking on a hyperlink or typing a URL into a
browsers starts a request-response cycle.
• An HTTP Request-response cycle can have multiple
steps since many web pages often embedded files,
such as graphics, each requiring a separate response.

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Client computer
with
Web Browser
software
Server computer with
Web Server
software
HTTP Request
HTTP Response
Figure 2-8 How the Web Works

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HTTP Requests have 3 parts:
• Request line (required): starts with a command
and URL and ends with the http version number.
• Request header (optional): contains information
on the browser, date, and the referring page from
which the request originated. If the user types in
the URL by themselves, then the referring page is
blank.
• Request body (optional): contains information
sent to the server, such as from a form.

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Figure 2-9 Example of an HTTP
Request from a Web browser
GET http://www.kelley.indiana.edu/ardennis/home.htm HTTP/1.1
Date: Mon 06 Aug 2001 17:35:46 GMT
User-Agent: Mozilla/6.0
Referer: http://www.indiana.edu/~aisdept/faculty.htm Request Header
]- Request
Line
]- Date
]- Web browser (this is Netscape)
Command URL HTTP version
URL that contained the link to the requested URL

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HTTP Responses also have 3 parts:
• Response status (optional): http version number,
status code (e.g., 200 = ok, 404 = page not found)
and “reason” phrase (description of status code).
• Response header (optional): contains information on
the server, date, exact URL of the retreived page and
format used for the body of the response (e.g., html).
• Response body (required): contains the requested
Web page.

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Figure 2-10 Example of an HTTP
response from a Web server
HTTP/1.1 200 OK ]- Response Status
Date: Mon 06 Aug 2001 17:35:46 GMT ]- Date
Server: NCSA/1.3 ]- Web server
Location: http:// www.kelley.indiana.edu/adennis/home.htm ]- URL
Content-type: text/html]- Type of file
<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>
HTTP ver. Status code Reason
Response Header
Response
Body

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Hypertext Markup Language (HTML)
• Web pages are formatted using another protocol,
usually HTML, also developed at CERN.
• HTML documents include information on how to
format the file imbedded in the document called tags.
• HTML was initially developed for text files, not
multimedia files. This is limitation is why HTML
extensions have been created.
• And also why other markup language formats have
been created, such as Extensible Markup Language
(XML).

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Electronic Mail
• Still the most heavily used Internet
application because:
– It’s much, much faster than regular mail.
– It’s extremely inexpensive.
– It can substitute for other forms of
communication, such as telephone calls.
– E-mail users don’t have to play “telephone tag”,
since the respondent can answer at his/her
convenience, instead of time of call.

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E-mail standards
• Simple Mail Transfer Protocol (SMTP): part
of the TCP/IP protocol suite.
• SMTP is the main e-mail standard for mail
transfer agents in use today, but was written
to only handle text files.
• SMTP is usually implemented using a two-
tier client-server architecture.
• Two other competing standards for e-mail
are Common Messaging Calls (CMC) and
X.400.

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Two-Tier E-mail Architecture
• SMTP’s uses two types of programs:
• User agents, also called e-mail clients, are
programs running on client computers that send e-
mail to e-mail servers and download e-mail from
mailboxes on those servers.
– Examples: Eudora, Outlook.
• Mail transfer agents, also called mail server
software, are used by e-mail servers, send e-mail
through the Internet between e-mail servers and
maintains individual mailboxes.
– Post Office Protocol (POP) & Internet Mail Access
Protocol (IMAP) are the main protocols for user agent
to mail server communications.

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Sending An E-mail Message (Figure 2-11)
• When completed, an e-mail message is sent by the
sender’s e-mail client as an SMTP packet to the
local mail server.
• The mail server’s message transfer agent next
reads the packet’s destination address and sends it
over the Internet to the receiver’s mail server.
• The destination mail transfer agent then stores the
message in the receiver’s mail box.
• When the receiver next accesses e-mail, his or her
user agent contacts the local mail server which
then downloads the message to the receiver’s
client computer.

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Client computer with
e-mail client
software
(“user agent”)
e-mail server
software
(“message transfer
agent”)
SMTP packet
SMTP packet
LAN
e-mail client
software
(“user agent”)
e-mail server
software
(“message transfer
agent”)
SMTP packet
LAN
SMTP packet
IMAP or
SMTP packet
Internet
Figure 2-11 How SMTP works

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Three-Tier Client-Server Architecture
(Figure 2-12)
• Reading e-mail using a Web-based
interface, such as Hotmail, uses a three-tier
architecture. The three tiers are:
– The client’s web browser that sends HTTP
requests to the Web server.
– The Web server: 1) sends HTTP responses to
the Web client and 2) translates the client’s
HTTP requests into SMTP packets which are
then sent to the Mail server.
– The Mail server performs the same functions as
the mail server in the two-tier example.

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Web browser
Web server
software
HTTP request
SMTP packet
LAN
LAN
Internet
Server computer
with mail server
software
Figure 2-12 How
Web-based e-mail works
Web browser
Web server software
mail server
software
SMTP packet
HTTP request
HTTP response
SMTP packet
IMAP packet
HTTP response
IMAP or
SMTP packet

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Simple Mail Transfer Protocol
• SMTP E-mail messages have the following
structure:
– Header: lists source and destination addresses,
date, subject and other information about the e-
mail message.
– Body: the message itself
– Attachments: additional files that have been
included along with the e-mail message.

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Figure 2-13 Sample E-mail
Message using SMTP
From: “Alan Dennis;”<adennis@indiana.edu>
TO: “Pat Someone” <someone@somewhere.com>
Date: Mon 06 Aug 2001 19:03:02 GMT
Subject: Sample Note
Message-Id:
<4.1.20000623164823.009f5e80@IMAP.IU.EDU>
DATA:
This is an example of an e-mail message
Message
header
Message
body

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Attachments in MIME
• Since SMTP was developed for transferring text files,
special techniques needed to be developed to send
graphical information.
• Graphics capable mail transfer agent protocols include
MIME (Multipurpose Internet Mail Extension),
uuencode and binhex.
• Included as part of an e-mail client, MIME translates
graphical information into text allowing the graphic to
be sent as part of an SMTP message.
• The receiver’s e-mail client then translates the MIME
attachment from text back into graphical format.

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File Transfer Protocol (FTP) and Telnet
• FTP enables sending and receiving files
over the Internet.
• Telnet allows remote logins over the
Internet.
• Both were much more commonly used
during the pre-WWW days of the Internet.
• FTP still commonly used today for
uploading web pages.

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IM: Instant Messaging (Figure 2-14)
• IM is a client-server program that allows real-time
messages to be exchanged. Some types allow voice
and video packets to be sent. Examples include AOL
and ICQ.
• IM works as follows:
– Your IM client first 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.
– When you type some text, your IM client sends the text in a
packet to the IM server which relays it to your friend.
– 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.

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e-mail client
software
(“user agent”)
IM packet
LAN
Client computer
with IM client
software
IM packet
LAN
Internet
Figure 2-14
How Instant
Messaging works
LAN
Server computer
with IM server
software
IM packet
IM packet

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Videoconferencing
• Allows people to meet at the same time in
different locations, potentially saving
money and time by not having to move
people around.
• Conference calling does the same thing.
• Typically involves 2 meeting rooms only.
• Desktop videoconferencing is a related very
low cost application using small video
cameras and microphones over the Internet.

Ch2_Ed7_Network_Applications.ppt

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