1 / 28

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.

norm
Download Presentation

Electronic Mail

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related