Chapter 16: Distributed Applications

1. Chapter 16:Distributed Applications Business Data Communications, 4e

2. 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 Electronic Mail Features Page 442

3. Single System E-Mail • 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 • e.g. RITVAX, AOL

4. 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

5. Multiple Systems E-Mail

6. Simple Mail Transfer Protocol (SMTP) • Standard for TCP/IP mail transfer, defined in RFC 821 • Concerned addressing and delivery, not content, with two exceptions • Character set standardized as 7-bit ASCII • Adds log information to message that indicates message path

7. 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

8. 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

10. 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”

11. 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

12. Sample SMTP Exchange • S: HELO pc-ycc R: 250 euler.im.ncnu.edu.tw Hello …, pleased to meet you • S: MAIL FROM:<Smith@Alpha.ARPA>R: 250 OK • S: RCPT TO:<Jones@Beta.ARPA>R: 250 OK • S: RCPT TO:<Green@Beta.ARPA>R: 550 No such user here • S: DATAR: 354 Start mail input; end with <CRLF>.<CRLF> S: Blah blah blah….S:…etc. etc. etc.S: <CRLF>.<CRLF>R: 250 OK • S: QUIT

13. 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

14. 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

15. 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)

16. 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

17. 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

18. MIME Content Types • Seven major types: Text, Multipart, Message, Image, Video, Audio, Application • Fourteen subtypes: See page 452 for details • Text provides only plain subtype, but a richtext subtype is likely to be added • Multipart indicates separate parts, such as text and an attachment • MIME types are used by web servers, as well

19. MIME Content Types

20. multipart/mixed ... From: "Yen-Cheng Chen" <ycchen@ncnu.edu.tw> To: <ycchen@im.ncnu.edu.tw> Subject: test mail! Date: Wed, 22 May 2002 14:27:07 +0800 MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_NextPart_000_0047_01C2019E.69F32C40“ Status: This is a multi-part message in MIME format. ------=_NextPart_000_0047_01C2019E.69F32C40 Content-Type: text/plain; charset="big5" Content-Transfer-Encoding: 7bit Start of Body! Yen-Cheng ------=_NextPart_000_0047_01C2019E.69F32C40 Content-Type: text/plain; name="a.txt" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="a.txt“ This is the first and only one line of the file. ------=_NextPart_000_0047_01C2019E.69F32C40--

21. POP3Post Office Protocol - Version 3 • RFC 1939 / Std 53 • To retrieve emails from server (POP3 server) • Three states • Authorization State • Transaction State • Update State

22. Authorization State USER username PASS password APOP name digest Transaction State STAT LIST [msgNo] RETR msgNo DELE msgNo NOOP RSET TOP msgNo line UIDL [msgNo] Update State QUIT POP3 Commands

23. S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us> C: USER mrose S: +OK User name accepted, password please C: PASS pwd999 S: +OK Mailbox open, 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: QUIT S: +OK dewey POP3 server signing off (maildrop empty)

24. Electronic Data Interchange (EDI) • Direct, computer-to-computer exchange of business data • Replaces use of paper documents • Requires two participants to agree on electronic format for the data • Two departments within a company • Companies and customers • Multiple companies

25. Benefits of EDI • Cost savings • Speed • Reduction of errors • Security • Integration with office automation • Just-in-time delivery

26. EDI Typically no human involvement in processing the information; interface is software-to-software E-Mail Data not necessarily structured for software processing. Human-to-software exchange is usually involved on at least one end EDI v E-Mail

27. Components of EDI Systems • Application • Translation Software • Communications Network

28. EDI/Internet Integration • RFC 1767, issued in 1995 defines a method for packaging EDI transactions in a MIME envelope. • Application/EDIFACT • Application/EDI-X12 • Application/EDI-consent • Additional requirements have since emerged: • Security issues such as EDI transaction integrity, privacy and non-repudiation • Support for exchanges by point-to-point, FTP, and SMTP protocols. • An IETF working group is currently addressing these unresolved issues.

29. Enterprise Application Integration (EAI) • linking applications, whether purchased or developed in-house, so they can better support a business process. • Critical for implementation of Internet-based business strategies

30. EAI Illustrated