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Applicazioni
Simple Terminal-Host System
TelephoneNetwork
ModemModem
HostDumbTerminal
DumbTerminal
Direct Wire Connection
All Processing isDone on the Host
Slow Response TimeMonochrome Text
Little Graphics
Client/Server Computing
1.ProgramStored onFile Server
File Server
Client PC
2.Program
Downloaded toClient PC
When Called
3.Program
Executed onClient PC!
Only useful for small programs (e-mail, word processing, etc.)Because clients do not get very large
File Server Program Access
Client/Server Computing
Server
Client PC
Highly scalable: Use larger server as number of clients increases
Client/Server Processing with Request-Response Cycle
Client Program Server Program
Request Message
Response Message
Does Light I/O andPost-DownloadProcessing
Does HeavyDatabase
and Other HeavyProcessing
Web-Enabled Application (E-Mail)
E-Mail Server
Client PC
Client is Browser Server Program
HTTP Request Message
HTTP Response Message
Almost all client PCsnow have browsers.
No need to install new software.
Importance of EMail Universal service on the Internet
Attachments make email a general file delivery mechanism
Viruses, worms, and spam, and other abuses
E-Mail Standards
SMTPTo Send
SMTPTo Send
SendingE-MailClient
ReceivingE-MailClient
Sender’s MailServer
Receiver’s MailServer
Simple Mail Transfer Protocol (SMTP) to transmit mail in real time to a user’s mail server or between mail servers
Sender-initiated
E-Mail Standards
Sending E-MailClient
ReceivingE-MailClient
Sender’s MailServer
Receiver’s MailServer
POP orIMAP
To Receive
POP or IMAP to download mail to receiver when the receiver is next capable of downloading mail.
Receiver-initiated
EMail Standards Downloading mail to client
Post Office Protocol (POP): Simple and widely used
Internet Message Application Program (IMAP): More powerful, can manage messages on the receiver’s mail host, less widely used
E-Mail Standards
SendingE-MailClient
ReceivingE-MailClient
Sender’s MailServer
Receiver’s MailServer
MessageRFC 822 or 2822HTML bodyUNICODE
Message Body Format Standard
EMail Standards
Message body standards Receiver must understand sender’s message RFC 822 and RFC 2822 for alltext bodies HTML bodies with fancy text and graphics UNICODE for nonEnglish languages
Viruses, Worms, and Trojan Horses
Widespread Problems Often delivered by email attachments
Use of antivirus software is almost universal but not fully effective
Viruses, Worms, and Trojan Horses Where to Do Scanning?
On client PCs But users often turn off their software,
Fail to download virus definitions regularly
Or let their contracts lapse
On the corporate mail server or application firewall Users cannot turn off
Viruses, Worms, and Trojan Horses Where to Do Scanning?
At an antivirus outsourcing company Before mail ever reaches the company
High level of expertise
Defense in Depth Do it in at least two places
Spam Unsolicited commercial email Why filter?
Potential sexual harassment suits
Time consumed by users deleting them
Time consumed by staff to delete them
Bandwidth and storage consumed
Legitimate messages lost because overlooked
Spam Separating SPAM from legitimate email is
difficult Many spam messages get through to users
Some legitimate messages are deleted
Some firms merely mark messages as probable spam
HTML and HTTP
Figure 11-7: HTML and HTTP
HTTP Response
HTTP Request
HTML DocumentClient PCWebserver
BrowserWebserverApplication
Downloading a Complex Webpage with Two Graphics Files
Client PC
BrowserWebserverApplication
Webserver
Webpage Consists of Three FilesRendered as a Single Page On-Screen
AsDisplayed
2Graphics
Files
HTML Document
Figure 11-8: Downloading a Complex Webpage with
Two Graphics FilesHTML Document
Client PCBrowser Webserver
Application
Webserver
Download Requires 3HTTP Request-Response Cycles;
Downloads HTML Page FirstIt has Tags to Identify Other Files
AsDisplayed
2Graphics
Files
HTML Document
Downloading a Complex Webpage with Two Graphics Files
Client PC
Browser WebserverApplication
Webserver
Based on Tags in HTML Document,Browser Requests Downloads of
Remaining Graphics or Other Files
Browser Renders Combined Webpage on Screen
AsDisplayed
2Graphics
Files
Downloading a Complex Webpage with Two Graphics Files
Quiz: Your browser downloads a webpage with three graphics images, a sound sequence, and a Java applet. How many files will your browser have to download?
Examples of HTTP Request and Response Messages
HTTP Request Message GET /panko/home.htm HTTP/1.1[CRLF] Host: voyager.cba.hawaii.edu[CRLF]
Examples of HTTP Request and Response Messages
HTTP Response Message HTTP/1.1 200 OK[CRLF] Date: Tuesday, 20JAN2004 18:32:15
GMT[CRLF] Server: name of server software[CRLF] MIMEversion: 1.0[CRLF] Contenttype: text/plain[CRLF] [CRLF] File to be downloaded
E-Commerce
Electronic Commerce Functions
CustomerPC
Browser WebserverApplication
E-CommerceServer
ExternalBank
InternalWarehouseDatabase
Electronic Commerce Functions
Webserver Functionality, Plus… ECommerce functionality
Online catalog Shopping cart Checkout, including payment Customer resource management Links to External Systems
Credit card number checking Bank settlement
Electronic Commerce Functions
ECommerce functionality Links to internal systems
Accounting Pricing Warehousing (Product Availability) Shipment Etc.
