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 Mail Host PC with E-Mail Client

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 PC with E-Mail Client Mail Host SMTP

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 Client PC Mail Host With User’s Mailbox Receive Mail Send Mail

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 LAN PC with FSPA E-Mail Program

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 PC with Internet E-Mail Client Mail Host SMTP POP

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 PC with Internet E-Mail Client Mail Host SMTP IMAP

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 PC with Internet E-Mail Client Mail Host SMTP IMAP

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 PC with Browser Webserver Mail Host HTTP SMTP

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 PC with Telnet Client Mail Host Supporting Telnet Mail Host Telnet SMTP

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. 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

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. 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, 7 th bit may be truncated if send binary file 10101010 Binary x1010101 Internet

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

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 11111111 00000000 11111111 xx111111 xx11 Data Bytes Encoded Bytes 6,2

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 11111111 00000000 xx111111 xx110000 xx0000 Data Bytes Encoded Bytes 4,4

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 11111111 00000000 11111111 xx111111 xx110000 xx000011 xx111111 Data Bytes Encoded Bytes 2,6

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 –8 th bit is still free, as required xx111111 xx110000 xx000011 xx111111 Encoded Bytes x1011111 x1010000 xx100011 x1011111 Encoded Bytes Add 100000

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