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IETF RFC 918
Post Office Protocol
Last modified on Tuesday, September 22nd, 1992
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Network Working Group J. K. Reynolds
Request for Comments: 918 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.
Reynolds PAGE 1 
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
Reynolds PAGE 2
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
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
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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Post Office Protocol
RFC TOTAL SIZE: 9876 bytes
PUBLICATION DATE: Tuesday, September 22nd, 1992
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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