Electronic Mail

1
Electronic Mail
2
E-Mail

• Client Software and Mail Hosts
• Client PC has E-Mail client software that
  communicates with users 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

• Users Mail Host Stores Incoming Files in the
  Users Mailbox
• User later retrieves them
• User also sends outgoing mail

Receive Mail
Send Mail
Client PC
Mail Host With Users 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 servers 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
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
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
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