Internet Engineering Course

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Internet Engineering Course

• Electronic Mail

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Electronic Mail

• Most heavily used application on any network
• Uses client-server architecture
• Electronic mail client accepts mail from user and
  delivers to server on destination computer
• Many variations and styles of delivery
• Simple Mail Transfer Protocol (SMTP)
• TCP/IP
• Delivery of simple text messages
• Multi-purpose Internet Mail Extension (MIME)
• Delivery of other types of data
• Voice, images, video clips

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SMTP Concept
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User Agents and Mail Transfer Agents
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Relay MTAs
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Mail Gateways
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The Entire E-mail System
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SMTP

• Simple Mail Transfer Protocol is standard
  application protocol for delivery of mail from
  source to destination
• Built on top of TCP provides reliable delivery
  of data
• SMTP does not normally use intermediate mail
  servers for sending mail
• SMTP uses persistent connections
• Other functions
• E-mail address lookup
• E-mail address verification
• Example protocol exchange

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Sample SMTP interaction
c\Telnet payk.ui.ac.ir 25
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(No Transcript)
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SMTP

• RFC 821
• Not concerned with format of messages or data
• Covered in RFC 822 (see later)
• SMTP uses info written on envelope of mail
• Message header
• Does not look at contents
• Message body
• Except
• Standardize message character set to 7 bit ASCII

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Basic Operation

• Mail created by user agent program (mail client)
• Message consists of
• Header containing recipients address and other
  info
• Body containing user data
• Messages queued and sent as input to SMTP sender
  program
• Typically a server process (daemon on UNIX)

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Mail Message Contents

• Each queued message has
• Message text
• RFC 822 header with message envelope and list of
  recipients
• Message body, composed by user
• A list of mail destinations
• Derived by user agent from header
• May be listed in header
• May require expansion of mailing lists
• May need replacement of mnemonic names with
  mailbox names
• If BCCs indicated, user agent needs to prepare
  correct message format

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SMTP Sender

• Takes message from queue
• Transmits to proper destination host
• Via SMTP transaction
• Over one or more TCP connections to port 25
• Host may have multiple senders active
• When delivery complete, sender deletes
  destination from list for that message
• When all destinations processed, message is
  deleted

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Optimization

• If message destined for multiple users on a given
  host, it is sent only once
• Delivery to users handled at destination host
• If multiple messages ready for given host, a
  single TCP connection can be used
• Saves overhead of setting up and dropping
  connection

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Possible Errors

• Host unreachable
• Host out of operation
• TCP connection fail during transfer
• Sender can re-queue mail
• Give up after a period
• Faulty destination address
• User error
• Target user changed address
• Redirect if possible
• Inform user if not

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SMTP Protocol - Reliability

• Used to transfer messages from sender to receiver
  over TCP connection
• Attempts to provide reliable service
• No guarantee to recover lost messages
• No end to end acknowledgement to originator
• Error indication delivery not guaranteed
• Generally considered reliable

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SMTP Receiver

• Accepts arriving message
• Places in user mailbox or copies to outgoing
  queue for forwarding
• Receiver must
• Verify local mail destinations
• Deal with errors
• Transmission
• Lack of disk space
• Sender responsible for message until receiver
  confirm complete transfer
• Indicates mail has arrived at host, not user

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SMTP Forwarding

• Mostly direct transfer from sender host to
  receiver host
• May go through intermediate machine via
  forwarding capability
• Sender can specify route
• Target user may have moved

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Conversation

• SMTP limited to conversation between sender and
  receiver
• Main function is to transfer messages
• Rest of mail handling beyond scope of SMTP
• May differ between systems

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SMTP Mail Flow
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SMTP System Overview

• Commands and responses between sender and
  receiver
• Initiative with sender
• Establishes TCP connection
• Sender sends commands to receiver
• e.g. HELOltSPgtltdomaingtltCRLFgt
• Each command generates exactly one reply
• e.g. 250 requested mail action ok completed

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SMTP Replies

• Leading digit indicates category
• Positive completion reply (2xx)
• Positive intermediate reply (3xx)
• Transient negative completion reply (4xx)
• Permanent negative completion reply (5xx)

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Operation Phases

• Connection setup
• Exchange of command-response pairs
• Connection termination

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Connection Setup

• Sender opens TCP connection with receiver
• Once connected, receiver identifies itself
• 220 ltdomaingt service ready
• Sender identifies itself
• HELO
• Receiver accepts senders identification
• 250 OK
• If mail service not available, step 2 above
  becomes
• 421 service not available

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Mail Transfer

• Sender may send one or more messages to receiver
• MAIL command identifies originator
• Gives reverse path to used for error reporting
• Receiver returns 250 OK or appropriate fail/error
  message
• One or more RCPT commands identifies recipients
  for the message
• Separate reply for each recipient
• DATA command transfers message text
• End of message indicated by line containing just
  period (.)

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Closing Connection

• Two steps
• Sender sends QUIT and waits for reply
• Then initiate TCP close operation
• Receiver initiates TCP close after sending reply
  to QUIT

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Electronic Mailboxes and Addresses

• E-mail users have an electronic mailbox into
  which mail is deposited
• Usually associated with computer account one
  user may have different electronic mailboxes
• User accesses mail with a mail reader program
• Electronic mailbox is identified by an e-mail
  address mailbox_at_computer

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E-mail message format

• Simple two-part format
• Header includes delivery information
• Body carries text of message
• Header and body is separated by blank line

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E-mail Headers

• Lines of text in format keyword information
• keyword identifies information information can
  appear in any order
• Essential information
• To list of recipients
• From sender
• Cc list of copy recipients
• Useful information
• Reply-to different address than From
• Received-by for debugging
• Frivolous information
• Favorite drink lemonade
• Phase-of-the-moon gibbous

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Data in E-mail

• Original Internet mail carried only 7-bit ASCII
  data
• Couldnt contain arbitrary binary values e.g.,
  executable program
• Techniques for encoding binary data allowed
  transport of binary data
• Uuencode three 8-bit binary values as four ASCII
  characters (6 bits each)
• Also carries file name and protection information
• Incurs 33 overhead

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Multipurpose Internet Mail Extension (MIME)

• Extension to RFC822
• SMTP can not transmit executables
• Uuencode and other schemes are available
• Not standardized
• Can not transmit text including international
  characters (e.g. â, å, ä, è, é, ê, ë)
• Need 8 bit ASCII
• Servers may reject mail over certain size
• Translation between ASCII and EBCDIC not standard
• Some SMTP implementations do not adhere to
  standard
• CRLF, truncate or wrap long lines, removal of
  white space, etc.

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MIME

• Multipart Internet Mail Extensions - Extends and
  automates encoding mechanisms
• Allows inclusion of separate components
  programs, pictures, audio clips in a single
  mail message
• Sending program identifies the components so
  receiving program can automatically extract and
  inform mail recipient
• Header includesMIME-Version 1.0Content-Type
  Multipart/Mixed BoundaryMime_separator
• Separator line gives information about specific
  encodingContent-Type text/plain

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MIME (cont.)

• MIME is extensible sender and receiver agree on
  encoding scheme
• MIME is compatible with existing mail systems
• Everything encoded as ASCII
• Headers and separators ignored by non-MIME mail
  systems
• MIME encapsulates binary data in ASCII mail
  envelope

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MIME
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Overview of MIME

• Five new message header fields
• MIME version
• Content type
• Content transfer encoding
• Content Id
• Content Description

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Content Types

• Text body
• Multipart
• Mixed, Parallel, Alternative, Digest
• Message
• RFC 822, Partial, External-body
• Image
• jpeg, gif
• Video
• mpeg
• Audio
• Basic
• Application
• Postscript
• octet stream

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MIME Transfer Encodings

• Reliable delivery across wide largest range of
  environments
• Content transfer encoding field
• Six values
• Three (7bit, 8bit, binary) no encoding done
• Provide info about nature of data
• Quoted-printable
• Data largely printable ASCII characters
• Non-printing characters represented by hex code
• Base64
• Maps arbitrary binary input onto printable output
• X-token
• Named nonstandard encoding

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Message format multimedia extensions

• MIME multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME
  content type

MIME version
method used to encode data
multimedia data type, subtype, parameter
declaration
encoded data
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Multipart Type
From alice_at_crepes.fr To bob_at_hamburger.edu
Subject Picture of yummy crepe. MIME-Version
1.0 Content-Type multipart/mixed
boundary98766789 --98766789 Content-Transfer-En
coding quoted-printable Content-Type
text/plain Dear Bob, Please find a picture of a
crepe. --98766789 Content-Transfer-Encoding
base64 Content-Type image/jpeg base64 encoded
data ..... .........................
......base64 encoded data --98766789--
Break into parts?
Compare with HTTP?
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Printable Encoding of Binary Data into Radix-64
Format
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Programs as mail recipients

• Can arrange for e-mailbox to be associated with a
  program rather than a users mail reader
• Incoming mail automatically processed as input to
  program
• Example mailing list subscription
  administration
• Can be used to implement client-server processing
• Client request in incoming mail message
• Server response in returned mail reply

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Mailing Lists and Forwarders

• E-mail addresses can be attached to programs as
  well as electronic mailboxes
• Mail exploder or mail forwarder resends copies of
  message to e-mail addresses in mailing list
• UNIX mail program sendmail provides mail aliases
• Mailing list processor, e.g. listserv, can also
  interpret subscription management commands

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Mail Gateways

• Mailing list processing may take significant
  resources in large organizations
• May be segregated to a dedicated server computer
  mail gateway
• Provides single mail destination point for all
  incoming mail
• Can use MX records in DNS to cause all mail to be
  delivered to gateway

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Mail gateways and forwarding

• Users within an organization may want to read
  mail on local or departmental computer
• Can arrange to have mail forwarded from mail
  gateway
• Message now makes multiple hops for delivery
• Hops may be recorded in header
• Forwarded mail may use proprietary (non-SMTP)
  mail system

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Mailbox Access

• Where should mailbox be located?
• Users want to access mail from most commonly used
  computer
• Cannot always use desktop computer as mail server
• Not always running
• Requires multitasking OS
• Requires local disk storage
• Can TELNET to remote computer with mail server
• Can use WEB based interfaces to access mailbox

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Mail Access Protocols

• Instead of TELNET, use protocol that accesses
  mail on remote computer directly
• Mail access protocol transfers mail from server
  to local PC
• 2 popular protocols
• POP3 (Post Office Protocol v3)
• IMAP (Internet Mail Access Protocol)
• Cant use SMTP because getting messages is a pull
  operation SMTP is a push protocol.

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Post Office Protocol (POP)

• TCP/IP suite includes Post Office Protocol (POP)
  for remote mailbox access
• Computer with mailboxes runs POP server
• User runs POP client on local computer
• POP client can access and retrieve messages from
  mailbox
• Requires authentication (password)
• Local computer uses SMTP for outgoing mail

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POP3 and SMTP
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POP3

• Stands for Post Office Protocol Version 3
• RFC in 1993
• POP3 is intended to permit a workstation to
  dynamically access a maildrop on a server host in
  a useful fashion. Usually, this means that the
  POP3 protocol is used to allow a workstation to
  retrieve mail that the server is holding for it.

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POP3

• Doesnt provide extensive mail operations on the
  server
• Normally mail is downloaded, then deleted
• Deletion doesnt really occur until QUIT
• Uses port 110

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POP3

• Three phases
• Authorization
• Transaction
• Update
• During a POP3 session, the server maintains state
  information
• POP3 downloads mail messages to the local machine
  not good for nomadic user

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POP3 protocol
S OK POP3 server ready C user bob S OK
C pass hungry S OK user successfully logged
on

• authorization phase
• client commands
• user declare username
• pass password
• server responses
• OK
• -ERR
• transaction phase, client
• list list message numbers
• retr retrieve message by number
• dele delete
• quit

C list S 1 498 S 2 912
S . C retr 1 S ltmessage 1
contentsgt S . C dele 1 C retr
2 S ltmessage 1 contentsgt S .
C dele 2 C quit S OK POP3 server
signing off
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Dialup Access and POP

• POP is useful for dialup connection
• Users computer not always connected
• Can download all mail at once and read off-line
• Can compose mail off-line and mail in one
  connection

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Minimal POP3 Commands

• USER name users name
• PASS string users password
• QUIT quits, and updates the mailbox
• STAT status
• LIST msg lists the mail messages (s only)
• RETR msg retrieves a mail message
• DELE msg deletes a message
• NOOP No operation
• RSET resets the mail to its original state

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IMAP

• Internet Mail Access Protocol
• Allows users to manipulate remote mailboxes
• Must maintain info about user (e.g., folder
  hierarchy) even after the user quits
• IMAP server is always in 1 of 4 states
• Nonauthenticated
• Authenticated
• Selected
• Logout

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POP3 (more) and IMAP

• More about POP3
• Previous example uses download and delete mode.
• Bob cannot re-read e-mail if he changes client
• Download-and-keep copies of messages on
  different clients
• POP3 is stateless across sessions

• IMAP
• Keep all messages in one place the server
• Allows user to organize messages in folders
• IMAP keeps user state across sessions
• names of folders and mappings between message IDs
  and folder name

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Security

• SMTP, POP3 and IMAP are plain text protocols
• Secured version available to protect the user
• SMTPS
• POP3s
• Need to be configured both on server and client

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Stunnel

• Name of an application/service
• Even if the mail server application does not
  implement secure version
• What does it do?
• gathers the encrypted message from client
• Decrypts the data
• Sends plain text data using a local connection to
  insecure port
• Server application considers it a normal client

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STunneling
Send decrypted data to port 25 of the server
Stunnel (on server) Listening to port 995
Encrypted data (Command/mail body)
Client
Server
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Web interface

• Nowadays there are many web interface user agents
• They allow accessing messages from any client
  with only a web browser
• However the interface is not as sophisticated as
  special mail agents
• But it is very useful if you need to check your
  emails from different computers

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Summary for Email

• Electronic mail allows quick, asynchronous
  communication across entire Internet
• Can attach e-mail addresses to programs for
  processing
• Mailing lists
• Other client-server applications
• Simple Mail Transfer Protocol (SMTP) is Internet
  standard for mail delivery
• Mail gateways
• Provide uniform user addressing outside
  organizations
• Translate from Internet mail (e.g. SMTP) to
  proprietary systems
• Post Office Protocol (POP) and Internet Mail
  Access Protocol (IMAP) allow remote access to
  electronic mailboxes.