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Electronic Mail

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 E-Mail Client. PC with E-Mail Client. Mail Host. Mail Host. SMTP.

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Electronic Mail

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  1. Electronic Mail

  2. 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 E-Mail Client PC with E-Mail Client Mail Host Mail Host

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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 Browser Webserver Mail Host Mail Host

  12. 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 Telnet Client Mail Host Supporting Telnet Mail Host

  13. 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

  14. 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

  15. Client PC Mail Host With User Mail Box 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

  16. 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

  17. Attachments • Send a message • Attach a file (word processing document, spreadsheet, graphic, etc.) • E-mail can be a file delivery mechanism

  18. Attachments • Viruses • Attachments may contain viruses • Even messages without attachments may contain viruses today • Virus scanning before opening is critical

  19. 10101010 x1010101 Internet Binary 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

  20. 10101010 x1010101 x1010101 Internet Internet Binary 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 Encoding Internet Transmission Internet Decoding 10101010 Binary

  21. 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) 11111111 00000000 11111111 Data Bytes Encoded Bytes

  22. 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 11111111 00000000 11111111 Data Bytes xx111111 xx11 Encoded Bytes

  23. 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 11111111 00000000 Data Bytes xx111111 xx110000 xx0000 Encoded Bytes

  24. 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 11111111 00000000 11111111 Data Bytes xx111111 xx110000 xx000011 xx111111 Encoded Bytes

  25. 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 xx111111 xx110000 xx000011 xx111111 Encoded Bytes Add 100000 x1011111 x1010000 xx100011 x1011111 Encoded Bytes

  26. 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

  27. 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

  28. 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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