Chapter 16 distributed applications
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Chapter 16: Distributed Applications. Business Data Communications, 4e. Message Preparation Word processing Annotation Message Sending User directory Timed delivery Multiple addressing Message priority Status information Interface to other facilities. Message Receiving

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Chapter 16 distributed applications

Chapter 16:Distributed Applications

Business Data Communications, 4e

Electronic mail features

Message Preparation

Word processing


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

Business Data Communications, 4e

Single system e mail
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

Business Data Communications, 4e

Multiple systems e mail
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

Business Data Communications, 4e

Simple mail transfer protocol smtp
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

Business Data Communications, 4e

Basic e mail operation
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

Business Data Communications, 4e

Smtp mail flow
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

Business Data Communications, 4e

Smtp connection setup
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”

Business Data Communications, 4e

Smtp mail transfer
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

Business Data Communications, 4e

Sample smtp exchange
Sample SMTP Exchange

  • S: MAILFROM:<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

Business Data Communications, 4e

Smtp connection closing
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

Business Data Communications, 4e

Rfc 822
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

Business Data Communications, 4e

Limitations of smtp and rfc822
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)

Business Data Communications, 4e

Mime multipurpose internet mail extensions
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

Business Data Communications, 4e

Mime header fields
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

Business Data Communications, 4e

Mime content types
MIME Content Types

  • Seven major types: Text, Multipart, Message, Image, Video, Audio, Application

  • Fourteen subtypes: See page 384 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

Business Data Communications, 4e

Electronic data interchange edi
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

Business Data Communications, 4e

Benefits of edi
Benefits of EDI

  • Cost savings

  • Speed

  • Reduction of errors

  • Security

  • Integration with office automation

  • Just-in-time delivery

Business Data Communications, 4e

Edi v e mail


Typically no human involvement in processing the information; interface is software-to-software


Data not necessarily structured for software processing. Human-to-software exchange is usually involved on at least one end

EDI v E-Mail

Business Data Communications, 4e

Components of edi systems
Components of EDI Systems

  • Application

  • Translation Software

  • Communications Network

Business Data Communications, 4e

Edi internet integration
EDI/Internet Integration

  • RFC 1767, issued in 1995 defines a method for packaging EDI transactions in a MIME envelope.

  • 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.

Business Data Communications, 4e

Enterprise application integration eai
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

Business Data Communications, 4e

Eai illustrated
EAI Illustrated

Business Data Communications, 4e