Electronic Mail Networking Concepts

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Electronic MailNetworking Concepts

• D.A. Clements

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Just a thought
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Electronic Mail

• Most successful and ubiquitous Internet
  application
• Used to transmit messages
• From one person to another or
• From one person to many people
• Initially exchanged only plain text messages
• Today messages routinely include
• Pictures
• Advanced text markup (like different fonts and
  colors)
• File attachments (like Word or Excel files)
• Most follow open Internet standards
• Some proprietary systems incorporate additional
  features like calendaring, address books,
  security features and virus protection

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Client/Server Architecture

• Central technology of both LANs and the Internet
  today
• Describes the relationship between two network
  computer programs
• Client a local program/computer makes a service
  request
• Server remote program/computer fulfills the
  request
• Both sides can perform relevant data processing
• eMail Server performs the Post Office functions
• eMail Client performs creation, formatting,
  display funtions
• Many different email clients and servers

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Email Delivery Scenario
Stored here until UserConnects
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The Email Client

• Software the user uses to send and receive email
• Available for all computer operating systems
• Unix / Linux
• Windows
• Mac
• Can take the following forms
• Dumb-terminal/character applications pine, elm
• GUI applications Outlook, Netscape
• Web based Pine, Outlook Express, gmail, AOL,
  Hotmail, etc.
• No matter what they look like, they all have to
  do the same thing to send mail on the Internet
• They all must talk SMTP

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Email Client Features

• Common features
• Alias and alias-lists, (address books)
• Folders (longer-term storage)
• Forwarding, carbon copies (cc/bcc), reply
• File inclusion, attachments
• Signature file...signature-file
• Evolving
• Security
• PGP, S/MIME
• Authentication
• Digital certificates user authentication

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The Outlook Express Client
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Outlook Email Window
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Email Servers

• Sendmail for UNIX systems
• Microsoft Exchange Server
• Eudora Internet Mail Server (EIMS) for Macintosh
• MDaemon
• Merak Mail Server
• MailEnable
• SurgeMail
• Many more

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Email Addresses

• Internet standard
• userid_at_host.domain
• Such as daclem_at_u.washington.edu
• Some variations are possible like
• Mark.Palmer_at_xyz.com
• 12345.65362_at_xyz.com
• Where xyz.com is the domain name of eMail server
• DNS resolves name exactly like for web sites
• Early commercial competing alternatives
• MCIMail
• SprintMail
• CompuServe
• Each had proprietary address formats
• Couldnt readily exchange information between
  systems

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Delivering the Message

• Once sent an email message is usually placed in
  an outgoing mail queue folder on the local
  computer
• A server process on that box checks the folder
  periodically
• If it finds an outgoing message
• It initiates an SMTP (Simple Mail Transfer
  Protocol) connection to the destination mail
  server
• If the destination server is up and accepting
  messages
• The message will be delivered immediately
• If the destination is not accepting messages
• Message will be placed back in the outgoing mail
  folder
• Queued-up for another attempt (usually 10-15
  minutes)
• If the message cant be delivered after specific
  period of time (typically 3 days)
• It is returned to the sender as undeliverable

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SMTP

• Simple Mail Transport Protocol
• Originally defined by RFC 821 in 1982
• Modified by RFC 2821 in 2001
• A host to host mail transport and delivery
  protocol
• Connect and listen on port 25 (OSI Layer 4)
• SMTP servers can be
• The source
• The final destination
• Or they can relay mail for others
• Many servers limit this service to computers
  connected to the network
• Prevents spammers from using the server to
  distribute their mail
• UW mail system does this
• Has nothing to do with how you read the mail once
  it arrives
• POP and IMAP protocols for PC/Mac-based clients
  retrieve mail

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So far we are here
Stored here until UserConnects
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Checking/Reading Mail

• Sue previously configured her email client to
  talk to a POP or IMAP server
• She connects to Internet and runs her email
  client
• The email client talks to her server using the
  POP or IMAP protocol and either
• Downloads the messages to Sues computer (POP),
  or
• Downloads the list of messages stored on the
  server to Sues client (IMAP)
• Sue selects a message and is able to read it
  either from
• Her local computers hard drive (POP), or
• The mail server (IMAP)

Stored here until UserConnects
POP3 or IMAP
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POP

• Post Office Protocol
• Version 3 mostly widely used today POP3
• Defined by RFC 1939 (which obsoletes RFC1725)
• Runs over port 110 (OSI Layer 4)
• Simple protocol
• Essentially a store, copy, delete protocol
• Mail is stored on the server until the client
  connects
• It is downloaded to the client by copying
• It is then usually deleted from the server
• A simple text-based protocol
• Widely used and supported
• Many ISPs suggest you use a POP client for mail
• Good for them because the mail is removed from
  their server
• Many email clients support it including
• Outlook and Outlook Express
• Eudora the King of POP
• Netscape Mail

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POP Weaknesses Strength

• Not convenient for users that use multiple
  machines during the day. Why?
• Where is the email you read yesterday?
• Cant selectively retrieve messages or parts of
  messages
• What if there is a big attachment and you have a
  SLOW connection? Say sitting in a airport
• Since mail is downloaded to your machine
• If you use only one computer
• You can read and compose messages offline

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IMAP

• Internet Message Access Protocol
• Defined by RFC 2060 and several other additions
• Most work done at UW by Mark Crispin and others
  in CAC
• Most IMAP servers run on port 143
• Folders and Messages can be stored on the server
  or in local folders on your machine
• Can selectively copy messages from server to the
  local client
• Much better than POP for people using multiple
  machines
• IMAP4 has a mechanism to work off-line
• Then resynchronize the changes when you reconnect
• MUCH more complex protocol to implement
• Result not as many clients as POP

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So why not
Why do we need SMTP servers to deliver mail and
other protocols for retrieving and displaying
messages? Why not just connect directly? What
address would we use? Most hosts use dynamic IP
addresses (DHCP) What if the recipients address
changes before email arrives? Most PCs do not
have IP names that are registered in DNS What if
the destination host is powered off? What if we
turn off our machine, how will we receive new
mail? How do we effectively queue messages for
later delivery since either host machine can be
powered off? What if the sending / receiving
hosts are 10-12 time zones apart?
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Non-Standard Email Systems

• SMTP, POP, and IMAP are great open standards
• But they address just a portion of what people
  want to do with email
• Address book is not part of the protocols
• What if you want (need) to recall a sent
  message?
• What if you want to do workflow through your
  email system?
• What if you want to use shared calendaring and
  scheduling in email?
• These protocols have no mechanisms for that type
  of functionality.

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Proprietary Servers/Clients

• Many products today add these missing features
• Most notable are
• Microsoft Exchange (they claim 100 million
  licenses sold to businesses)
• Lotus Notes
• Novell Groupwise
• Most offer scheduling/calendaring and some do
  workflow
• Unfortunately, no standard has emerged for
  calendaring
• You are often limited to exchanging data with
  other clients using the same product
• A new standard called iCalendar may be emerging
• While considered proprietary they
• All use SMTP for sending/receiving mail
• Most can act as a POP or IMAP server
• If you use a standard client , you lose the
  added functionality
• Example Using Unix Pine configured as an IMAP
  client to the iSchool Exchange server
• You can see all your folders and messages, but
  cant do any scheduling

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Proprietary Email System

• Microsoft Exchange Server
• IMAP
• Added value
• Work offline modeif no Internet connection
  available
• Disconnected modeif server or Internet
  connection goes down
• Synchs files between client and server
• Downloads mail to client
• Uploads mail when connection is restored

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Proprietary Email System
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Web Client Outlook Web Access (OWA)
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End papers
Your mailbox is your window to your heart! Make
sure you leave that window open! Takayuki
Ikkaku, Arisa Hosaka, and Toshihiro Kawabata,
Animal Crossing Wild World, 2005