Network Working Group
`Request for Comments: 918
`
`J. K. Reynolds
`ISI
`October 1984
`
`POST OFFICE PROTOCOL
`
`Status of this Memo
`
` This RFC suggests a simple method for workstations to dynamically
` access mail from a mailbox server. This RFC specifies a proposed
` protocol for the ARPA-Internet community, and requests discussion and
` suggestions for improvement.
`
`Introduction
`
` The intent of the Post Office Protocol (POP) is to allow a user’s
` workstation to access mail from a mailbox server. It is expected
` that mail will be posted from the workstation to the mailbox server
` via the Simple Mail Transfer Protocol (SMTP). For further
` information see RFC-821 [1] and RFC-822 [2].
`
` The status of this protocol is experimental, and this protocol is
` dependent upon TCP.
`
`The Protocol
`
` The protocol is for the server to be listening for a connection.
` When a connection is opened the server sends a greeting message and
` waits for commands. When commands are received the server acts on
` them and responds with replies.
`
` The client opens a connection, waits for the greeting, then sends the
` USER and then the PASS commands to establish authorization to access
` mailboxes. The client begins a mail reading transaction with either
` an RDEL (to read and delete all messages from a mailbox) or a RETR
` (to simply read all messages from a mailbox). The server opens and
` locks the mailbox, and responds with the number of characters in the
` mailbox. Then the client asks for the data to be sent by issuing the
` RCEV command. The server responds by sending the mail data. When
` all the data has been received the client sends the RCVD command. If
` the transaction started with the RDEL command the server now deletes
` the mail data from the mailbox. In any case, the server closes and
` unlocks the mailbox. The client terminates the session with the QUIT
` command.
`
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`Post Office Protocol RFC 918
`
` The Normal Scenario
`
` Client Server
` ------ ------
` Wait for Connection
` Open Connection -->
` <-- +OK
` Wait for Command
` USER Fred -->
` <-- +OK
` Wait for Command
` PASS password -->
` <-- +OK
` Wait for Command
` RDEL mailbox --> (open and lock mailbox)
` <-- #xxx
` Wait for Command
` RCEV -->
` <-- (send a copy of mail)
` Wait for Command
` RCVD --> (deletes mail from mailbox, unlock
` and close mailbox)
` <-- +OK
` Wait for Command
` QUIT -->
` <-- +OK
` Close connection --> <-- Close connection
` Wait for Connection (go back to start)
`
`Definitions of Commands and Replies
`
` Summary of Commands and Replies
`
` Commands Replies
` -------- -------
` USER name +OK
` PASS password -Error
` RETR mailbox #xxx
` RDEL mailbox
` RCEV
` RCVD
` QUIT
` NOOP
` RSET
`
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`Post Office Protocol RFC 918
`
` Commands
`
` USER name
`
` This command identifies the user to the server. It must be
` followed by the PASS command.
`
` Possible responses: "+OK" or "-ERR"
`
` PASS password
`
` The PASS command carries the password authenticating this user.
` Together the USER name and PASS password are used by the server
` to control access to the mailboxes.
`
` Possible responses: "+OK" or "-ERR"
`
` RETR mailbox
`
` This command begins a mail reading transaction. The RETR
` command is used to read all the messages in a mailbox without
` deleting them. It must be followed by the RCEV command.
`
` Possible responses: "#xxx" or "-ERR"
`
` RDEL mailbox
`
` This command begins a mail reading transaction. The RDEL
` command is used to read all the messages in a mailbox and
` delete them. It must be followed by the RCEV command.
`
` Possible responses: "#xxx" or "-ERR"
`
` RCEV
`
` This command confirms that the client is ready to receive the
` mail data. It must be followed by the RCVD command.
`
` Possible responses: "+OK" or (connection aborted)
`
` RCVD
`
` This command confirms that the client has received and accepted
` the mail. The RCVD command ends the mail reading transaction.
` In the case of the RDEL transaction, it is possible that the
` mail is not necessarily deleted. This is indicated by an error
` reply.
`
` Possible responses: "+OK" or "-ERR"
`
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`Post Office Protocol RFC 918
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` QUIT
`
` This command indicates the client is done with the session.
` The server sends an OK response and then closes the connection.
`
` Possible responses: "+OK" then Close
`
` NOOP
`
` This is the no operation command. It causes no action on the
` part of the server except an OK response.
`
` Possible response: "+OK"
`
` RSET
`
` This command causes the server to abort the current transaction
` and return to waiting for a command (one of RDEL, RETR, QUIT,
` NOOP, or RSET). When aborting a transaction the server must
` take care to properly close and unlock the mailbox.
`
` Possible response: "+OK"
`
`Acknowledgment
`
` I would like to acknowledge the contributions of Jon Postel, Joel
` Goldberger, Dale Chase, and Michael Butler in the development of the
` Post Office Protocol.
`
`References
`
` [1] Postel, J., "Simple Mail Transfer Protocol", RFC-821,
` USC/Information Sciences Institute, August 1982.
`
` [2] Crocker, D., "Standard for the Format of ARPA-Internet Text
` Messages", RFC-822, University of Delaware, August 1982.
`
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`Post Office Protocol RFC 918
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`Diagram
` +---+ +---+ +---+ +---+ +---+
` Open | | +OK | | | | +OK | | | | +OK
` ----->| S |--+-->| C |------>| S |------>| C |------>| S |---->+
` | | ^ | | USER | | | | PASS | | |
` +---+ | +---+ +---+ +---+ +---+ |
` | | -ERR | -ERR |
` | V V |
` +<-----------+<----------------+<----------------------+ |
` | |
` V +---+ +---+ |
` +----->| | QUIT | | +OK |
` | C |------>| S |----->Close |
` +----->| | | | |
` ^ +---+ +---+ |
` | V
` +<-------------------------------------------------------------+
` | ^
` | +---+ +---+ +---+ +---+ +---+ +---+ |
` | | |RETR | | #xxx | |RECV | |DATA | |RCVD | |+OK |
` +->| C |---->| S |----->| C |---->| S |---->| C |---->| S |--->+
` | | | or | | | | | | | | | | ^
` | +---+ RDEL+---+ +---+ +---+ +---+ +---+ |
` | | -ERR | | | | -ERR |
` | V | V | | |
` +<-------------+ | Abort | | |
` | V V V |
` | +<---------------+-------------------+ +----->+
` | V
` | +---+ +---+
` | | | RSET | | +OK
` +----->| C |------>| S |----->+
` | | | | | |
` | +---+ +---+ |
` | V
` +<----------------------------+
` | ^
` | +---+ +---+ |
` | | | NOOP | | +OK |
` +----->| C |------>| S |----->+
` | | | |
` +---+ +---+
`
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