3. State and Flow Diagram
Once the connection between client and server is established, an IMAP4rev1 connection is in one of four states. The initial state is identified in the server greeting. Most commands are only valid in certain states. It is a protocol error for the client to attempt a command while the connection is in an inappropriate state, and the server will respond with a BAD or NO (depending upon server implementation) command completion result.
3.1. Not Authenticated State
In the not authenticated state, the client MUST supply authentication credentials before most commands will be permitted. This state is entered when a connection starts unless the connection has been pre-authenticated.
3.2. Authenticated State
In the authenticated state, the client is authenticated and MUST select a mailbox to access before commands that affect messages will be permitted. This state is entered when a pre-authenticated connection starts, when acceptable authentication credentials have been provided, after an error in selecting a mailbox, or after a successful CLOSE command.
3.3. Selected State
In a selected state, a mailbox has been selected to access. This state is entered when a mailbox has been successfully selected.
RFC 3501 IMAPv4 March 2003
3.4. Logout State
In the logout state, the connection is being terminated. This state can be entered as a result of a client request (via the LOGOUT command) or by unilateral action on the part of either the client or server.
If the client requests the logout state, the server MUST send an untagged BYE response and a tagged OK response to the LOGOUT command before the server closes the connection; and the client MUST read the tagged OK response to the LOGOUT command before the client closes the connection.
A server MUST NOT unilaterally close the connection without sending an untagged BYE response that contains the reason for having done so. A client SHOULD NOT unilaterally close the connection, and instead SHOULD issue a LOGOUT command. If the server detects that the client has unilaterally closed the connection, the server MAY omit the untagged BYE response and simply close its connection.
RFC 3501 IMAPv4 March 2003
+----------------------+ |connection established| +----------------------+ || / +--------------------------------------+ | server greeting | +--------------------------------------+ || (1) || (2) || (3) / || || +-----------------+ || || |Not Authenticated| || || +-----------------+ || || || (7) || (4) || || || / / || || +----------------+ || || | Authenticated |<=++ || || +----------------+ || || || || (7) || (5) || (6) || || || / || || || || +--------+ || || || || |Selected|==++ || || || +--------+ || || || || (7) || / / / / +--------------------------------------+ | Logout | +--------------------------------------+ || / +-------------------------------+ |both sides close the connection| +-------------------------------+
(1) connection without pre-authentication (OK greeting) (2) pre-authenticated connection (PREAUTH greeting) (3) rejected connection (BYE greeting) (4) successful LOGIN or AUTHENTICATE command (5) successful SELECT or EXAMINE command (6) CLOSE command, or failed SELECT or EXAMINE command (7) LOGOUT command, server shutdown, or connection closed
RFC 3501 IMAPv4 March 2003
- Data Formats
IMAP4rev1 uses textual commands and responses. Data in IMAP4rev1 can be in one of several forms: atom, number, string, parenthesized list, or NIL. Note that a particular data item may take more than one form; for example, a data item defined as using "astring" syntax may be either an atom or a string.