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9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
9 TCP/IP Internet Message Access Protocol (IMAP/IMAP4)
IMAP General Operation, Client/Server Communication and Session States
(Page 2 of 3)
The session between an IMAP4 client
and server is described in the IMAP standards using a finite state machine
(FSM), a descriptive tool that I explain in the
TCP operational overview. Again, this
is similar to how
the operation of POP3 is explained, except
that IMAP4 is a bit more complex. Its FSM defines four states instead
of three, and where a POP3 session is linear (only going through each
state once) in IMAP4 the session is not. However, the state flow is
still fairly straight-forward, mostly following a logical sequence from
one state to the next. The IMAP FSM is illustrated in Figure 309.
Figure 309: Internet Message Access Protocol (IMAP) Finite State Machine
The IMAP finite state machine is slightly more complex than that of POP (Figure 306) but still rather straight-forward. Once the TCP connection is made between client and server, the Not Authenticated state is entered; after successful authorization, the session moves to the Authenticated state. The session may move between Authenticated and Selected several times, as different mailboxes are selected for use and then closed when no longer needed. From any state the session may be terminated, entering the Logout state.
The following are the IMAP
states, in the usual sequence in which they occur for a session:
- Not Authenticated State:
The session normally begins in this state after a TCP connection is
established, unless the special IMAP preauthentication feature
has been used (well get to this feature shortly). The client at
this point cannot really do much aside from providing authentication
information so it can move to the next state.
- Authenticated State:
The client has completed authentication, either through an authentication
process in the prior state or through preauthentication. The client
is now allowed to perform operations on whole mailboxes. The client
must select a mailbox before individual message operations are permitted.
- Selected State: After
a mailbox has been chosen, the client is allowed to access and manipulate
individual messages within the mailbox. When the client is done with
the current mailbox it can close it and return to the Authenticated
state to select a new one to work with, or can log out to end the session.
- Logout State: The client
may issue a Logout command from any of the other states to request
that the IMAP session be ended. The session may also enter this state
if the session inactivity timer expires. The server sends a response
and the connection is terminated.
Of the four IMAP states, only the
first three are interactive, meaning states where commands
are actively issued by the client and responses provided by the server.
Some IMAP commands can be used while the session is in any state, while
others are state-specific. The
next topic describes IMAP commands and
responses in general terms.
Key Concept: IMAP is a client/server application, and an IMAP session begins with the client making a TCP connection to the server. The session then normally starts in the Not Authenticated state and remains there until successful authentication. In the Authenticated state, the client may perform operations on whole mailboxes, but a mailbox must be selected to transition to the Selected state where individual messages can be manipulated. The client can work with many mailboxes by selecting each one in turn; it then logs out from the server.
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Version 3.0 - Version Date: September 20, 2005
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