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					Electronic Mail
                        E-Mail
• Client Software and Mail Hosts
  – Client PC has E-Mail client software that
    communicates with user’s mail host
  – Mail hosts deliver outgoing mail to other mail
    hosts



          PC with                                  PC with
        E-Mail Client    Mail Host   Mail Host   E-Mail Client
                         SMTP
• Simple Mail Transfer Protocol (SMTP)
  – Standard for mail host-mail host exchanges
  – E-Mail Client often sends messages to mail host
    via SMTP, but not always

                         SMTP               SMTP

           PC with
         E-Mail Client          Mail Host          Mail Host
        SMTP Operation
• For Each Message, the Sending Process
  – Makes a connection
  – Gives name of sender (From) and gets OK
  – Gives names of receivers individually and
    gets OK for each separately
  – Asks to send message, gets OK
  – Sends message, gets confirmation
  – Closes connection
  Receiving and Sending E-Mail
• User’s Mail Host Stores Incoming Files in
  the User’s Mailbox
  – User later retrieves them
  – User also sends outgoing mail

                      Receive Mail

                       Send Mail
          Client PC                 Mail Host
                               With User’s Mailbox
 File Server Program Access E-
              Mail
• Use proprietary ways to send messages, get
  messages, and in other ways interact with
  the mail host
  – Can be used only on LANs
  – Cannot be used over the Internet

                  PC with FSPA
                 E-Mail Program
                                       LAN
                  POP Clients
• POP (Post Office Protocol) is the most
  popular standard for mail downloading
  – Download messages all or selectively
  – Send outgoing messages via SMTP
  – Works via Internet

                        SMTP               SMTP

                         POP
             PC with
      Internet E-Mail Client   Mail Host          Mail Host
           POP Operation
• Several client-mail host interactions needed to
  download new mail
   – Log into mail host
   – Can ask how many new messages there are and how
     long they are
   – Can download all or download one at a time
   – If download one at a time, can decide based on
     length
   – Can delete messages on host after downloading
   – Close the session
                IMAP Clients
• IMAP (Internet Message Access Protocol)
  – But not as widely supported as POP
  – Send outgoing messages via SMTP
  – Works via the Internet

                       SMTP               SMTP

                       IMAP
            PC with
     Internet E-Mail Client   Mail Host          Mail Host
                IMAP Clients
• IMAP (Internet Message Access Protocol)
  – More sophisticated than POP
     • Can do more on mail server’s mailbox than
       download and delete messages; can fully manage
       the mailbox


                       SMTP               SMTP

                       IMAP
            PC with
     Internet E-Mail Client   Mail Host          Mail Host
Browser Clients (Web-Based E-Mail)
• Client is Browser
• Mail Host is a Webserver
   – Mail host sends HTML pages to client
   – User types messages and retrieval data in
     forms, sends back
• All communication is via HTTP
                  HTTP               SMTP


        PC with          Webserver          Mail Host
        Browser          Mail Host
            Telnet Clients
• Some mail hosts support Telnet
• Telnet client on PC emulates a simple
  terminal
  – No color or graphics
  – Monospaced Text
  – Sometimes only way to interact with a mail
    host
                      Telnet           SMTP


        PC with             Mail Host         Mail Host
      Telnet Client      Supporting Telnet
       Recap on Internet E-Mail
            Transmission
• Communication Between Mail Hosts
  – SMTP
• Communication From Client to its Mail Host
  – SMTP
  – Proprietary file server program access on LANs
  – HTTP
  – Telnet
       Recap on Internet E-Mail
            Transmission
• Communication to Client from its Mail Host
  to deliver messages
  – POP or IMAP
  – Proprietary file server program access on LANs
  – HTTP
  – Telnet
       Note on Internet E-Mail
            Transmission
• Client and Server can Communicate Over
  the Internet
  – Except for File Server Program Access
  – You can access your e-mail from anywhere
  – Must have the right client program



          Client PC                  Mail Host
                                 With User Mail Box
Message Structure Standards

• RFC 822
  – Text-only message bodies
• MIME
  – Multimedia message bodies and headers
  – Not widely used for bodies or headers
• HTML Bodies
  – Becoming common
  – Not well standardized; Limited interoperability
    between mail clients
               Attachments
• Send a message
  – Attach a file (word processing document,
    spreadsheet, graphic, etc.)
  – E-mail can be a file delivery mechanism
                Attachments
• Viruses
  – Attachments may contain viruses
     • Even messages without attachments may contain
       viruses today

  – Virus scanning before opening is critical
            Attachments
• Problem
  – Attached files use all 8 bits of each byte
     • Called binary data
  – On Internet, can only use the first seven bits
     • Called 7-bit ASCII
     • In Internet transmission, 7th bit may be truncated
       if send binary file

                 10101010                x1010101

                   Binary                Internet
                 Attachments
 • Internet Encoding
    – Files must be Internet encoded before
      transmission to travel over the Internet using
      only the first 7 bits in each byte
    – At the receiving end, files must be Internet
      decoded so that applications can read them
          Internet            Internet          Internet
          Encoding          Transmission        Decoding

10101010         x1010101            x1010101         10101010

 Binary              Internet        Internet          Binary
                     Attachments
 • Internet Encoding Example (There are
   Other Internet Encoding Standards)
     – Break file into groups of three data bytes (24
       bits)
     – Create group of four encoded bytes (32 bits)

   Data Bytes   11111111   00000000   11111111


Encoded Bytes
                     Attachments
 • Internet Encoding Example (There are
   Other Internet Encoding Standards)
     – Put six bits of each data byte in each outgoing
       byte
         • Leaves two bits free in each outgoing byte
                   6,2
   Data Bytes   11111111    00000000     11111111


Encoded Bytes   xx111111      xx11
                     Attachments
 • Internet Encoding Example (There are
   Other Internet Encoding Standards)
     – Put six bits of each data byte in each outgoing
       byte
         • Leaves two bits free in each outgoing byte
                               4,4
   Data Bytes   11111111    00000000


Encoded Bytes   xx111111    xx110000      xx0000
                     Attachments
 • Internet Encoding Example (There are
   Other Internet Encoding Standards)
     – Put six bits of each data byte in each outgoing
       byte
         • Leaves two bits free in each outgoing byte
                                            2,6
   Data Bytes   11111111    00000000     11111111


Encoded Bytes   xx111111    xx110000     xx000011       xx111111
                     Attachments
     • Internet Encoding Example (There are
       Other Internet Encoding Standards)
         – Lowest 31 ASCII codes are control codes
         – Add 32 (100000) to each outgoing byte so
           that it will not become a control code
         – 8th bit is still free, as required

Encoded Bytes   xx111111   xx110000   xx000011   xx111111

 Add 100000

Encoded Bytes   x1011111   x1010000   xx100011   x1011111
            Attachments
• Internet Encoding Standards
  – Communicating mail clients must use the
    same Internet encoding standard to encode
    and decode
  – UUENCODE is common in UNIX
  – MIME
     • Several versions of MIME exist
     • Basic MIME is almost universally supported by
       e-mail clients today
  – Binhex is commonly used on Macintoshes
            Attachments
• E-Mail users should negotiate before
  sending an attachment
  – Internet encoding standard they will use
  – Application file format they will use
     • If same application program and version, fine
     • If same application program and different
       versions, send in format of older version
     • If different application programs, send in a
       format and version the other can import
   E-Mail Standards Recap

• Transmission Standards
  – Sending messages (SMTP, etc.)
  – Receiving messages (POP, IMAP, etc.)
• Message Structure Standards
  – Message header and body (RFC 822, MIME,
    HTML)
  – Attachments: common Internet encoding
    standard
  – Attachments: common application file format

				
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posted:10/1/2012
language:English
pages:28