1. Business Data Communications & Networking Lecture 13 Distributed Applications

2. Business Data Communications & Networking Key Distributed Applications zElectronic MailElectronic Mail zElectronic Data Interchange (EDI)Electronic Data Interchange (EDI) zWeb-Based ApplicationsWeb-Based Applications

3. Business Data Communications & Networking Public E-Mail zPublic is provided by a vendor, generally via a dial-up network zMessages can be sent to any other registered user zGateway required to send messages to users outside the system ze.g. AOL, MCI Mail

4. Business Data Communications & Networking Private E-Mail zIntegrated with the user’s computer system zCan run on a central host, or as part of a LAN environment yHost examples: DEC All-In-One, IBM PROFS yLAN examples: CC:Mail, QuickMail zOwned and operated by an organization for internal messaging requirements

5. Business Data Communications & Networking Public v Private zFeatures and services can be quite similar zPrivate systems involve higher initial cost, low or no transaction costs zPublic systems involve little initial cost, ongoing transaction costs zPrivate e-mail offers better integration with installed systems zPublic systems offer wider range of delivery options

6. Business Data Communications & Networking Internet E-Mail zDoes not fit directly into either category zTransfer mechanism for exchanging mail among systems, rather than a mail system itself

7. Business Data Communications & Networking Single System E-Mail zOnly allows users of a shared system to exchange messages zEach user has unique identifier and mailbox zSending a message simply puts it into recipients’ box

8. Business Data Communications & Networking Multiple Systems E-Mail zDistributed system enables mail servers to connect over a network to exchange mail zFunctions split yUser agent handles preparation, submission, reading, filing, etc yTransfer agent receives mail from user, determines routing, communicates with remote systems zInterconnection requires standards

9. Business Data Communications & Networking Simple Mail Transfer Protocol (SMTP)SMTP zStandard for TCP/IP mail transfer, defined in RFC 821RFC 821 zConcerned addressing and delivery, not content, with two exceptions yCharacter set standardized as 7-bit ASCII yAdds log information to message that indicates message path

10. Business Data Communications & Networking Basic E-Mail Operation zUser creates message with user agent program yText includes RFC 822 header and body of message yList of destinations derived from header zMessages are queued and sent to SMTP sender program running on a host

11. Business Data Communications & Networking SMTP Mail Flow zSMTP server transmits messages to appropriate hosts via TCP yMultiple messages to same host can be sent on one connection yErrors handling necessary for faulty addresses and unreachable hosts zSMTP protocol attempts to provide error-free transmission, but does not provide end-to-end acknowledgement zSMTP receiver accepts messages, places it in mailbox or forwards

12. Business Data Communications & Networking SMTP Connection Setup zSender opens TCP connection to receiver zReceiver acknowledges connection with “220 Service Ready” or “421 Service Not Available” zIf connection is made, sender identifies itself with the “HELO” command zReceiver accepts identification with “250 OK”

13. Business Data Communications & Networking SMTP Mail Transfer zMAIL command identifies originator, provides reverse path for error reporting zRCPT commands identify recipient(s) for message yReceiver has several positive or negative responses to RCPT ySender will not send message until it is sure at least one copy can be delivered zDATA command transfers message

14. Business Data Communications & Networking Sample SMTP Exchange zS: MAILFROM: R: 250 OK zS: RCPT TO: R: 250 OK zS: RCPT TO: R: 550 No such user here  S: DATA R: 354 Start mail input; end with. S: Blah blah blah …. S: … etc. etc. etc. S:. R: 250 OK

15. Business Data Communications & Networking SMTP Connection Closing zSender sends a QUIT command to initiate TCP close operation zReceiver sends a reply to the QUIT command, then initiates its own close

16. Business Data Communications & Networking RFC 822 zDefines format for text messages via electronic mail zUsed by SMTP as accepted mail format zSpecifies both envelope and contents zIncludes a variety of headers that can be included in the message header lines

17. Business Data Communications & Networking Limitations of SMTP and RFC822 zCannot transmit executables or binary files without conversion into text through non- standard programs (e.g. UUENCODE) zCannot transmit diacritical marks zTransfers limited in size zGateways do not always map properly between EBCDIC and ASCII zCannot handle non-text data in X.400 messages zNot all SMTP implementations adhere completely to RFC821 (tabs, truncation, etc)

18. Business Data Communications & Networking MIME (Multipurpose Internet Mail Extensions) zIntended to resolve problems with SMTP and RFC822 zSpecifies five new header fields, providing info about body of message zDefines multiple content formats zDefines encodings to enable conversion of any type of content into transferable form

19. Business Data Communications & Networking MIME Header Fields zMIME-Version: Indicates compliance with RFCs 1521 and 1522 zContent-Type: Describes data in sufficient detail for receiver to pick method for representation zContent-Transfer-Encoding: Indicates type of transformation used to represent content zContent-ID: Used to uniquely identify MIME entities zContent-Description: Plain text description for use when object is not readable

20. Business Data Communications & Networking MIME Content Types zSeven major types: Text, Multipart, Message, Image, Video, Audio, Application zFourteen subtypes: See page 384 for details zText provides only plain subtype, but a richtext subtype is likely to be added zMultipart indicates separate parts, such as text and an attachment zMIME types are used by web servers, as well

21. Business Data Communications & Networking Electronic Data Interchange (EDI)EDI zDirect, computer-to-computer exchange of business data zReplaces use of paper documents zRequires two participants to agree on electronic format for the data yTwo parties within a company (depts) yCompanies and customers yMultiple companies

22. Business Data Communications & Networking Benefits of EDI zCost savings zSpeed zReduction of errors zSecurity zIntegration with office automation zJust-in-time delivery

23. Business Data Communications & Networking EDI Layered Architecture zSemantic Layer: describes the business application (e.g. procurement) zStandard Layer: X.12 from ANSI, EDIFCT from UN zTransport Layer: E-mail, Point to Point, WWW zPhysical Layer: Dial-up lines, Internet, WANs

24. Business Data Communications & Networking EDI v E-Mail zEDI yTypically no human involvement in processing the information; interface is software- to-software zE-Mail yData not necessarily structured for software processing. Human-to-software exchange is usually involved on at least one end.