NST

1
NST

• Electronic Mail and MIME

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Email

• Has been around for over two decades
• The first email system simply consisted of File
  Transfer Protocols
• The first line of each message consisted of
  recipient's address

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limitations of the earlier design

• Sending a message to a group was inconvenient
• Messages had no internal structure
• The sender couldnt tell if the message arrived

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limitations of the earlier design

• The message interface was poorly integrated
• The user had to first edit a file,
• then leave the editor and invoke the FTP program

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limitations of the earlier design

• It was not possible to create and send messages
  containing a mixture of
• Text
• Voice
• Images, etc.

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ARPANET email Proposal

• ARPANET published the following email proposals
• RFC 821 Transmission Protocol
• RFC 822 Message Format
• These then became Internet de facto standards.

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X.400

• Was also drafted by CCITT aimed to be the the
  flagship application for OSI.
• This system was so poorly designed and so complex
  to implement.
• So RFC 821 and RFC 822 are actually the working
  de facto standards

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Email architecture and services

• Email systems consist of two subsystems
• User Agents
• Message Transfer Agents

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User Agents

• Allows people to read and send email
• Are local programs that provide a command line
  based, menu based or a GUI-based method to
  interact with the email system.

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Message Transfer Agents

• Move messages from source to the destination
• Implemented as system daemons that
• run in the background and
• move email through the system.

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Email Model
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E-mail transfer

• Store and forward
• each MTA stores the message before forward it on
  to its destination
• when forwarding the message a new TCP connection
  is setup for each MTA to MTA hop
• E-mail addressing
• for most users follows the Internet standard e.g.
  myEmail_at_hotmail.com but others exist (e.g. X.400)

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Email basic functions

• Composition
• Creates messages and replies
• Transfer
• Moving messages from sender to recipient and
  requires
• establishing a connection
• Outputting the message and,
• Releasing the connection

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Email basic functions

• Reporting function
• tells the originator
• What happened to the message?
• Was it delivered?
• Was it rejected?
• Was it lost?
• displays incoming message

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Email basic functions

• Disposition function implements
• What the recipient does with the message
• Delete before reading the message
• Delete after reading the message
• Save the message
• Retrieve and reread saved messages
• Forward the message

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Other features

• Mail boxes
• Storing the incoming email
• Commands to create or destroy mail box, inspect
  its contents
• Insert/ delete messages from the mail box,.
• Registered email - acknowledge receipt of email
• Carbon copies
• High priority email
• Secret (encrypted) email
• ..

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Modern email

• The key idea is to distinguish between the
  envelope and its contents
• Envelop encapsulates the message. It contains the
  information for transporting the message
• Destination address
• Priority
• Security level, etc

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Modern email

• The message transport agents use the envelope
  for routing, just as the post office does.
• The message content inside the envelope has two
  parts
• The header contains control information for user
  agents
• The body is entirely used for human recipient

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Email user agent

• User agent is a program that accepts a variety of
  commands for composing, receiving , and replying
  to messages as well as manipulating mail boxes.
• It can use a fancy mouse driven, GUI-menu or
  simply a command-line based menu

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User Agents- Sending email

• User provides the message using
• a text editor(can be built in to the user agent),
• The destination address in a format that the
  user agent can deal with
• e.g myMailbox_at_staffs.ac.uk
• Mailing lists

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User Agents- Reading email

• When a user agent is started up it looks at the
  users mail box for incoming mail
• Then it may announce the number of messages and
• waits for a command

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User Agents Message Formats

• Now we have some idea about the user interface,
  lets consider two message formats
• The ASCII email using RFC 822,
• The multimedia extensions to RFC 822

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RFC 822 Message format

• A message consists of
• A primitive envelope
• A number of header fields
• A blank line
• A message body

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RFC 822 Header Fields

• Header fields related to message transport
• Header Meaning
• To Email address of recipient
• Cc Email address of secondary recipient
• Bcc Email address for blind carbon copies
• From Person who created the message
• Sender Email address of the actual sender
• Received Line added by each transfer agent along
  the route
• Return-Path can be used to identify a path back
  to the sender

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RFC 822 Message Body

• Provides users a mechanism to include their
  messages
• These messages may terminate with signatures,
  disclaimer, etc.

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MIME Multipurpose Internet Mail Extensions

• Early implementations of email, using RFC 822
  only specified the headers.
• The content was left entirely up to the users.
• This is not an adequate method for the current
  use of the Internet.
• Problems include languages with accents,
  non-Latin alphabet and not containing text at all
  (audio and video..)

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Solution MIME (RFC 1341 RFC 1521)

• MIME continues to use RFC 822 format.
• But adds structure to the message body and
• defines encoding rules for non-ASCII messages

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MIME (new) Message Headers

• RFC 822 headers added by MIME
• Header Meaning
• MIME-Version Identifies the MIME version
• Content-Description Human-readable string
  telling what is in the message
• Content-Id Unique identifier
• Content-Transfer-Encoding How the body is
  wrapped for transmission
• Content-Type Nature of the message

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Content-Transfer-Encoding header

• Uses a number of schemes including
• ASCII Text using 7-bit or 8-bit encoding but
  lines should not exceed 1000 characters.
• Arbitrary binary files using all 8 bits and may
  exceed 1000 characters line limit.
• E.g. executable programs. But there is no
  guarantee for the correct arrival of binary
  messages.

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Encoding binary messages correctly with base64 -
MIME body

• Each block of 24 bits (3 bytes) broken up into 4
  6 bit units.
• A -gt0 B -gt 1 Z -gt 25
• a -gt 26 b -gt 27 z-gt 51
• 0 -gt 52 1-gt53 9 -gt 61
• -gt 62 / -gt 63

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MIME Example

• BAMCAQ
• The files contains the following bytes in hex 04
  03 02 01
• Code Decimal value Binary value
• B 1 000001
• A 0 000000
• M 12 001100
• C 2 000010
• A 0 000000
• Q 16 010000

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MIME Example

• File contents 0000010000000011000000110000000100
  00
• Split into bytes 00000100 00000011 00000010
  00000001
• Note You lose the last group of 4 zeroes.

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Base64 encoding

• If the last group of bits only contains a byte
  then the message is finished with , if it only
  contains 2 bytes then the message is finished
  with .
• Carriage return and line feeds are ignored.
• ..So binary data should be sent encoded in
  base64!

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Content-Type Header

• Seven original types are defined in RFC 1521,
  each of which has one or more subtypes
• E.g. Content-Type video/mpeg

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RFC 1521 MIME Types

• Text(Plain/Richtext)
• Image(Gif/Jpeg)
• Audio (Basic)
• Video(Mpeg)
• Application(Octet-stream/Postscript)
• Message(RFC822/Partial/External-body)
• Multipart(Mixed/Alternative/Parallel/Digest)

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Simple Mail Transfer ProtocolSMTP (port 25)

• The standard protocol for electronic mail
  transfer over the Internet. SMTP header contains
  forward and reverse paths.
• To route the packet to the destination the
  forward path is defined (SMTP command TO
  ltforward_pathgt

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SMTP in practise

• SMTP commands always initiate sending mail (there
  is not a specific command for receive)
• SMTP can be UA to MTA or MTA to MTA

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SMTP by telnet

• Telnet stmail.staffs.ac.uk 25
• 220 cr10m.staffs.ac.uk ESMTP Sendmail 8.9.1/8.9.1
• HELO staffs.ac.uk
• 250 cr10m.staffs.ac.uk Hello 192.168.9.7,
  pleased to meet you
• MAIL FROMltastudent_at_stmail.staffs.ac.ukgt
• 250 ltastudent_at_stmail.staffs.ac.ukgt... Sender ok
• RCPT TOltPs1_at_staffs.ac.ukgt
• 250 ltPS1_at_staffs.ac.ukgt... Recipient ok

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SMTP by telnet

• DATA
• 354 Enter mail, end with "." on a line by itself
• HELLO HOW ARE YOU ?
• .
• 250 PAA01415 Message accepted for delivery
• QUIT
• 220 cr10m.staffs.ac.uk closing connection

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SMTP summary

• Mail sending protocol defined by RFC821
• Maximum message size 64K
• ESMTP allows longer messages (10M on student
  server) is defined by RFC 1870
• A server which supports ESMTP responds to the
  command EHLO

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Post Office Protocol (POP3)port110

• SMTP has commands to transmit e-mail but not to
  receive
• There is an assumption that all machines that are
  due to receive e-mail will be connected to the
  Internet all the time
• POP allows users to connect to a mail server and
  download messages
• POP version 3 is the latest version

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POP3 example via Telnet

• telnet mail.staffs.ac.uk 110
• Trying 193.60.4.62...
• Connected to mail.staffs.ac.uk.
• Escape character is ''.
• OK mail.staffs.ac.uk Cyrus POP3 v1.5.14 server
  ready
• user ps1
• OK Name is a valid mailbox
• pass givePassword
• OK Maildrop locked and ready
• list
• OK scan listing follows
• 1 592
• .

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POP3 via telnet

• RETR 1
• OK Message follows
• Return-Path ltps1_at_staffs.ac.ukgt
• Received from staffs.ac.uk (bspps.soc.staffs.ac.u
  k 193.61.121.226)
• by mail.staffs.ac.uk (8.9.1/8.9.1) with
  ESMTP id RAA23481
• for ltp.saeidi_at_staffs.ac.ukgt Thu, 28 Nov
  2002 172704 GMT
• Message-ID lt3DE651EE.4C9C7E55_at_staffs.ac.ukgt
• Date Thu, 28 Nov 2002 172710 0000
• From pirooz saeidi ltps1_at_staffs.ac.ukgt
• X-Mailer Mozilla 4.73 en (WinNT U)
• X-Accept-Language en
• MIME-Version 1.0
• To p.saeidi_at_staffs.ac.uk
• Subject Test Pop3
• Content-Type text/plain charsetus-ascii
• Content-Transfer-Encoding 7bit
• Hello
• Testing Pop3

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IMAP(Internet Message Access Protocol)

• Provides the same service as POP3 but with a
  number of important improvements
• Allows proper secure authentication mechanisms,
  this is very important POP3 is very insecure
• Allows multiple mailboxes to be managed at the
  same time
• Allows multiple mail commands to be executed
  concurrently