ICSA 411: Data Communication and Computer Networks Lawley

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ICSA 411 Week 8

• The Internet Distributed Applications

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Distributed Applications

• Primary examples
• Electronic Mail
• Electronic Data Interchange (EDI)
• Web-Based Applications
• Client-Server/Groupware Environments
• Increasingly implemented using Internet
  technology (TCP/IP)
• Internets, Intranets, and Extranets

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Client-Server Basics

• Clients are usually single-user PCs or
  workstations, with GUIs
• Servers enable many clients to share access to
  the same data
• Network provides communication services between
  clients and server (many to one)

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Client-Server Characteristics

• Strong emphasis on bringing user-friendly,
  familiar apps and interface to the user
• Applications are distributed, but data is
  centralized
• Encourages the use of open and modular systems
• Networking is fundamental to the process (the
  network is the computer)

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Advantages of Client-Server Computing

• Distributed systems means redundancy and
  robustness
• Distributed processing provides power without
  monopolization of resources
• Modularity allows choices in hardware and
  software
• Upgrading and growth are relatively easy

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Disadvantages of Client-Server Computing

• Maintenance problems are more difficult with
  different vendors and interconnected systems
• Difficult to find support tools for applications
  developed in-house
• Requires different skills sets from software
  developers (Windows, Mac, RDBMS rather than
  COBOL, C

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Client-Server Architecture

• Protocols at the communication and application
  layers must be compatible
• Hardware, operating systems, and software
  interface layers can operate independently of
  each other

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Classes of Client-Server Applications

• Host-based processing
• traditional mainframe/central host environment
  workstation role is limited to emulation
• Server-based processing
• Client primarily provides interface

• Client-based processing
• All application processing on client
• Cooperative processing
• Processing distributed optimally

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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
• Uses proprietary software, usually in a mainframe
  environment
• 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
• e.g. RITVAX, AOL

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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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DNS Address System

• Distributed host address lookup system, necessary
  because of proliferation of hosts on the Internet
• All second-level domains (e.g. rit.edu,
  yahoo.com) are registered with InterNIC server,
  and specify an authoritative DNS server for hosts
  within that domain
• When a local system needs the IP address
  (physical location) of an Internet host, it
  queries its local DNS system first, then up the
  line, until the InterNIC provides the
  authoritative source for the info (if needed)

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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 delivered
• DATA command transfers message

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Sample SMTP Exchange

• S MAILFROMR 250 OK
• S RCPT TOR 250 OK
• S RCPT TOR 550 No such user
  here
• S DATAR 354 Start mail input end with
  .S Blah blah blah.Setc. etc.
  etc.S .R 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 departments within a company
• 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 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