ICSA 411: Week 8

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ICSA 411 Week 8

• Distributed Applications

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Key Distributed Applications

• Electronic Mail
• Electronic Data Interchange (EDI)
• Web-Based Applications

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Public E-Mail

• Public is provided by a vendor, generally via a
  dial-up network
• Messages can be sent to any other registered user
• Gateway required to send messages to users
  outside the system
• e.g. AOL, MCI Mail

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Private E-Mail

• Integrated with the users computer system
• Can run on a central host, or as part of a LAN
  environment
• Host examples DEC All-In-One, IBM PROFS
• LAN examples CCMail, QuickMail
• Owned and operated by an organization for
  internal messaging requirements

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Public v Private

• Features and services can be quite similar
• Private systems involve higher initial cost, low
  or no transaction costs
• Public systems involve little initial cost,
  ongoing transaction costs
• Private e-mail offers better integration with
  installed systems
• Public systems offer wider range of delivery
  options

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Internet E-Mail

• Does not fit directly into either category
• Transfer mechanism for exchanging mail among
  systems, rather than a mail system itself

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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

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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

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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

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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

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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

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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

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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 delievered
• DATA command transfers message

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Sample SMTP Exchange

• S MAILFROMltSmith_at_Alpha.ARPAgtR 250 OK
• S RCPT TOltJones_at_Beta.ARPAgtR 250 OK
• S RCPT TOltGreen_at_Beta.ARPAgtR 550 No such user
  here
• S DATAR 354 Start mail input end with
  ltCRLFgt.ltCRLFgtS Blah blah blah.Setc. etc.
  etc.S ltCRLFgt.ltCRLFgtR 250 OK

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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

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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

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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)

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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

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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

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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

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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 parties within a company (depts)
• Companies and customers
• Multiple companies

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Benefits of EDI

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

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EDI Layered Architecture

• Semantic Layer describes the business
  application (e.g. procurement)
• Standard Layer X.12 from ANSI, EDIFCT from UN
• Transport Layer E-mail, Point to Point, WWW
• Physical Layer Dial-up lines, Internet, WANs

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EDI v E-Mail

• 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.