Post Office Protocol

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Post Office Protocol IMAP
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Using POP

• Send or receive email from remote server (mail
  drop)
• delete or retain (option)
• any user that wishes to receive email on his own
  machine, but does not have a permanent Internet
  connection.
• Mail drop services
• SMTP MTA for local users and POP users
• MDA (local users)
• MRA (POP)

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How to Retrieve email

• POP3 client makes connection to POP server using
  TCP port 110.
• Concurrent server
• Two styles
• individual retrieval
• mailhost of LAN

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Comparing POP and IMAP

• Where mail is ultimately stored.
• Criteria
• network bandwidth
• security
• speed
• roaming

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POP States

• Authorization
• Transaction
• Update

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The Authorization State

• Many forms are available but there is no way for
  POP server to advertise which authentication
  mechanism it supports.
• 2 standards and 1 proposed standard
• username/password
• APOP
• RFC 1734 (AUTH)

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Username/password APOP

• Authenticate the user but does not provide
  transmission security.
• Servers must decide when to return errors.
• After user name or password
• APOP
• server sends timestamp to client as a part of
  banner string, the client uses this information
  to encode password using MD5 algorithm.
• pid.clock_at_hostname.domain

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AUTH authentication

• Provides authentication negotiation method.
• Client proposes and server accepts or rejects.
• Server does not advertise which authentication
  mechanisms it supports.
• Provides transmission encryption negotiation.

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The Transaction State

• STAT
• server sends a drop listing of the mailbox with
  the no. of messages and the size of the mailbox
  in octets.
• LIST
• LIST n or LIST
• RETR, NOOP, DELE, REST
• TOP 3 1 displays first line of message 3
• QUIT

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IMAP

• A successor to POP
• remote mailbox operation.
• Accessed from multiple client machines, which is
  good for mobile users.
• A protocol for remote editing
• mailbox may be created, manipulated, and
  destroyed remotely.
• More complex than POP

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Using IMAP

• Send or receive email from remote server (mail
  drop)
• copy locally and delete or retain (option) like a
  POP or no copy at a local machine
• any user that wishes to receive email on his own
  machine, but does not have a permanent Internet
  connection.
• Connecting newsgroup service
• Mail drop services
• SMTP MTA for local users and IMAP users
• MDA
• MRA

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IMAP States

• Nonauthenticated initial connection
• Authenticated when a user log into the server,
  providing an appropriate type of authentication
  credentials.
• Selected by selecting a mailbox
• Logout

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Commands Valid in any State

• Capability to check the servers capability
• Client C678 CAPABILITY
• Server CAPABILITY IMAP4rev1 AUTH-KERBEROS_V4
• Server C678 OK CAPABILITY completed
• NOOP
• to reset inactivity timeouts on the server.
• LOGOUT
• to inform the server that the client wishes to
  close the network connection.

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Nonauthenticated State

• To enter authenticate state, use AUTHENTICATE OR
  LOGIN Command.
• Login
• basic mechanism
• username/password
• Authenticate
• name the required mechanism
• use capability command to check available
  service.
• To support an unlimited number of authentication
  mechanism and allows the client to choose the
  most secure one at the time of request.

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System Flags

• Seen
• Answered
• Flagged for urgent/special attention.
• Deleted
• Draft has not completed composition.
• Recent the current session is the first session
  to have been presented with this message.

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Authenticated State

• SELECT to select mailbox
• the client may retrieve a list of available
  mailboxes with the LIST command.
• Example
• Client FR56 SELECT Conference
• Server 55 Exists
• Server 5 Recent
• Server OK Unseen 51 Message 51 is the first
  unseen
• Server OK UIDvalidity 21475678 UIDs valid
• Server FLAGS (\Answered \Flagged \Deleted
  \Seen \Draft)
• Server OK permanentflags (\) Limited
• Server FR56 OK READ-WRITE SELECT completed

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Authenticated State

• Examine open mailbox for read only purpose
• Example
• Client FR56 EXAMIME Conference
• Server 55 Exists
• Server 5 Recent
• Server OK Unseen 51 Message 51 is the first
  unseen
• Server OK UIDvalidity 21475678 UIDs valid
• Server FLAGS (\Answered \Flagged \Deleted
  \Seen \Draft)
• Server OK permanentflags () No permanent
  flags permitted
• Server FR56 OK READ-Only EXAMINE completed

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Authenticated State Other commands

• CREATE mailbox
• DELETE mailbox
• RENAME mailbox
• (UN)SUBSCRIBE make active
• LIST
• LSUB
• STATUS takes a mailbox name and a status code
• APPEND takes a mailbox name and a message.

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Selected State

• The client may operate on mail messages
  themselves.
• Access to message contents, search messages or
  parts of messages for strings, change the values
  of certain message parts and copy messages.