ftp.isi.edu/in-notes/rfc1939.txt
Similarly, it may be expensive (or impossible) to keep a personal computer interconnected to an IP-style network for long amounts of time (the node is lacking the resource known as "connectivity"). Despite this, it is often very useful to be able to manage mail on these smaller nodes, and they often support a user agent (UA) to aid the tasks of mail handling. To solve this problem, a node which can support an MTS entity offers a maildrop service to these less endowed nodes. The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually, this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it. POP3 is not intended to provide extensive manipulation operations of mail on the server;
RFC 1939 POP3 May 1996 3 Basic Operation Initially, the server host starts the POP3 service by listening on TCP port 110. When a client host wishes to make use of the service, it establishes a TCP connection with the server host. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses (respectively) until the connection is closed or aborted. Commands in the POP3 consist of a case-insensitive keyword, possibly followed by one or more arguments. Keywords and arguments consist of printable ASCII characters. Keywords and arguments are each separated by a single SPACE character. Responses in the POP3 consist of a status indicator and a keyword possibly followed by additional information. Responses may be up to 512 characters long, including the terminating CRLF. There are currently two status indicators: positive ("+OK") and negative ("-ERR"). In these cases, which are clearly indicated below, after sending the first line of the response and a CRLF, any additional lines are sent, each terminated by a CRLF pair. If any line of the multi-line response begins with the termination octet, the line is "byte-stuffed" by pre-pending the termination octet to that line of the response. When examining a multi-line response, the client checks to see if the line begins with the termination octet. If so and if octets other than CRLF follow, the first octet of the line (the termination octet) is stripped away. CRLF" is not considered part of the multi-line response. A POP3 session progresses through a number of states during its lifetime. Once the TCP connection has been opened and the POP3 server has sent the greeting, the session enters the AUTHORIZATION state. In this state, the client must identify itself to the POP3 server. Once the client has successfully done this, the server acquires resources associated with the client's maildrop, and the session enters the TRANSACTION state. In this state, the client requests actions on the part of the POP3 server.
RFC 1939 POP3 May 1996 issued the QUIT command, the session enters the UPDATE state. In this state, the POP3 server releases any resources acquired during the TRANSACTION state and says goodbye. A server MUST respond to an unrecognized, unimplemented, or syntactically invalid command by responding with a negative status indicator. A server MUST respond to a command issued when the session is in an incorrect state by responding with a negative status indicator. There is no general method for a client to distinguish between a server which does not implement an optional command and a server which is unwilling or unable to process the command. The receipt of any command from the client during that interval should suffice to reset the autologout timer. When the timer expires, the session does NOT enter the UPDATE state--the server should close the TCP connection without removing any messages or sending any response to the client. An example might be: S: +OK POP3 server ready The POP3 session is now in the AUTHORIZATION state. The client must now identify and authenticate itself to the POP3 server. Two possible mechanisms for doing this are described in this document, the USER and PASS command combination and the APOP command.
RFC 1939 POP3 May 1996 maildrop, or the maildrop cannot be parsed), the POP3 server responds with a negative status indicator. If the server does not close the connection, the client may either issue a new authentication command and start again, or the client may issue the QUIT command. After the POP3 server has opened the maildrop, it assigns a message- number to each message, and notes the size of each message in octets. The first message in the maildrop is assigned a message-number of "1", the second is assigned "2", and so on, so that the nth message in a maildrop is assigned a message-number of "n". Here is the summary for the QUIT command when used in the AUTHORIZATION state: QUIT Arguments: none Restrictions: none Possible Responses: +OK Examples: C: QUIT S: +OK dewey POP3 server signing off 5 The TRANSACTION State Once the client has successfully identified itself to the POP3 server and the POP3 server has locked and opened the appropriate maildrop, the POP3 session is now in the TRANSACTION state. The client may now issue any of the following POP3 commands repeatedly. Eventually, the client issues the QUIT command and the POP3 session enters the UPDATE state.
RFC 1939 POP3 May 1996 Here are the POP3 commands valid in the TRANSACTION state: STAT Arguments: none Restrictions: may only be given in the TRANSACTION state Discussion: The POP3 server issues a positive response with a line containing information for the maildrop. In order to simplify parsing, all POP3 servers are required to use a certain format for drop listings. The positive response consists of "+OK" followed by a single space, the number of messages in the maildrop, a single space, and the size of the maildrop in octets. This memo makes no requirement on what follows the maildrop size. Minimal implementations should just end that line of the response with a CRLF pair. More advanced implementations may include other information. NOTE: This memo STRONGLY discourages implementations from supplying additional information in the drop listing. Other, optional, facilities are discussed later on which permit the client to parse the messages in the maildrop. Note that messages marked as deleted are not counted in either total.
RFC 1939 POP3 May 1996 Restrictions: may only be given in the TRANSACTION state Discussion: If an argument was given and the POP3 server issues a positive response with a line containing information for that message. If no argument was given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP3 server responds with a line containing information for that message. This line is also called a "scan listing" for that message. If there are no messages in the maildrop, then the POP3 server responds with no scan listings--it issues a positive response followed by a line containing a termination octet and a CRLF pair. In order to simplify parsing, all POP3 servers are required to use a certain format for scan listings. A scan listing consists of the message-number of the message, followed by a single space and the exact size of the message in octets. Methods for calculating the exact size of the message are described in the "Message Format" section below. This memo makes no requirement on what follows the message size in the scan listing. Minimal implementations should just end that line of the response with a CRLF pair. More advanced implementations may include other information, as parsed from the message. NOTE: This memo STRONGLY discourages implementations from supplying additional information in the scan listing. Other, optional, facilities are discussed later on which permit the client to parse the messages in the maildrop.
Possible Responses: +OK Examples: C: RSET S: +OK maildrop has 2 messages (320 octets) 6 The UPDATE State When the client issues the QUIT command from the TRANSACTION state, the POP3 session enters the UPDATE state. QUIT Arguments: none Restrictions: none Discussion: The POP3 ser...
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