Request For Comments: draft Post Office Protocol (revised) Sun Oct 28 19:40:26 1984 Last Revision Wed Oct 23 20:54:26 1985 Marshall T. Rose Department of Computer and Information Sciences University of Delaware Newark, DE 19716 MRose@UDel This memo 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 improvements. Acknowledgements This memo is based on RFC918. Although similar in form to the original POP proposed for the ARPA Internet community, the protocol discussed in this memo is similar in spirit to the ideas investigated by the MZnet project at the University of California, Irvine. Request For Comments: draft M. Rose Post Office Protocol (revised) UDel Introduction On certain types of smaller nodes in the ARPA Internet it is often impractical to maintain a message transport system(MTS). For example, a workstation may not have sufficient resources (cycles, disk space) in order to permit a SMTP server and associated local mail delivery system to be kept resident and continuously running. 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 endowned nodes. The Post Office Protocol (POP) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually, this means that the POP is used to allow a workstation to retrieve mail that the server is holding for it. For the remainder of this memo, the term "client host" refers to a host making use of the POP service, while the term "server host" refers to a host which offers the POP service. The status of this protocol is experimental. A Short Digression This memo does not specify how a client host enters mail into the transport system, although a method consistent with the philosophy of this memo is presented here: When the user agent on a client host wishes to enter a message into the transport system, it establishes an SMTP connection to its relay host (this relay host could be, but need not be, the POP server host for the client host). If this method is followed, then the client host appears to the MTS as a user agent, and should NOT be regarded as a "trusted" MTS entity in any sense whatsoever. This concept, along with the role of the POP as a part of a split-UA model is discussed later in this memo. Page 1 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel The Protocol Initially the server host starts the POP service by listening on TCP port 109. 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 POP server sends a greeting. The client and POP server then exchange commands and responses (respectively) until the connection is closed or aborted. Commands in the POP consist of a keyword possibly followed by an argument. All commands are terminated by a CRLF pair. Responses in the POP consist of a success indicator and a keyword possibly followed by additional information. All responses are terminated by a CRLF pair. There are currently two success indicators: positive ("+OK") and negative ("-ERR"). Responses to certain commands are multi-line. 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. When all lines of the response have been sent, a final line is sent, consisting of a termination octet (octal code 056, ".") and a CRLF pair. If any line of the multi-line response begins with the termination octet, the line is "bit-stuffed" by pre-pending the termination octet to that line of the response. Hence a multi-line response is terminated with the five octets "CRLF.CRLF". 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 the first octet of the line (the termination octet) is stripped away. If so and if CRLF immediately follows the termination character, then the response from the POP server is ended and the line containing ".CRLF" is not considered part of the multi-line response. A POP session progresses through a number of states during its lifetime. Once the TCP connection has been opened and the POP server has sent the greeting, the session enters the AUTHORIZATION state. In this state, the client must identify itself to the POP 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 POP server. When the client has finished its transactions, the session enters the UPDATE state. In this state, the POP server releases any resources acquired during the TRANSACTION state and says goodbye. The TCP connection is then closed. Page 2 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel The AUTHORIZATION State Once the TCP connection has been opened by a POP client, the POP server issues a one line greeting. This can be any string terminated by CRLF. An example might be: S: +OK dewey POP server ready (Comments to: PostMaster@UDel) Note that this greeting is a POP reply. The POP server should always give a positive response as the greeting. The POP session is now in the AUTHORIZATION state. The client must now issue the USER command. If the POP server responds with a positive success indicator ("+OK"), then the client may issue either the PASS command to complete the authorization, or the QUIT command to terminate the POP session. If the POP server responds with a negative success indicator ("-ERR") to the USER command, then the client may either issue a new USER command or may issue the QUIT command. When the client issues the PASS command, the POP server uses the argument pair from the USER and PASS commands to determine if the client should be given access to the appropriate maildrop. If so, the POP server then acquires an exclusive-access lock on the maildrop. If the lock is successfully acquired, the POP server parses the maildrop into individual messages (read note below) and responds with a positive success indicator. The POP session now enters the TRANSACTION state. If the lock can not be acquired or the client should is denied access to the appropriate maildrop or the maildrop can't be parsed for some reason, the POP server responds with a negative success indicator. (If a lock was acquired but the POP server intends to respond with a negative success indicator, the POP server must release the lock prior to rejecting the command.) At this point, the client may either issue a new USER command and start again, or the client may issue the QUIT command. NOTE: Minimal implementations of the POP need only be able to break a maildrop into its component messages; they need NOT be able to parse individual messages. More advanced implementations may wish to have this capability, for reasons discussed later. After the POP server has parsed the maildrop into individual messages, it assigns a message-id to each message, and notes the size of the message in octets. The first message in the maildrop is assigned a message-id of "1", the second is assigned "2", and so on, so that the n'th message in a maildrop is assigned a message-id of "n". In POP commands and responses, all message-id's and message sizes are expressed in base-10. Page 3 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel Here are summaries for the three POP command discussed thus far: USER name Arguments: a server specific user-id (required) Restrictions: may only be given in the AUTHORIZATION state after the POP greeting or after an unsuccessful USER or PASS command Possible Responses: +OK name is welcome here -ERR never heard of name Examples: C: USER mrose S: +OK mrose is a real hoopy frood ... C: USER frated S: -ERR sorry, frated doesn't get his mail here PASS string Arguments: a server/user-id specific password (required) Restrictions: may only be given in the AUTHORIZATION state after a successful USER command Possible Responses: +OK maildrop locked and ready -ERR invalid password -ERR unable to lock maildrop Examples: C: USER mrose S: +OK mrose is a real hoopy frood C: PASS secret S: +OK mrose's maildrop has 2 messages (320 octets) ... C: USER mrose S: +OK mrose is a real hoopy frood C: PASS secret S: -ERR unable to lock mrose's maildrop, file already locked QUIT Arguments: none Restrictions: none Possible Responses: +OK Examples: C: QUIT S: +OK dewey POP server signing off The TRANSACTION State Once the client has successfully identified itself to the POP server and the POP server has locked and burst the appropriate Page 4 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel maildrop, the POP session is now in the TRANSACTION state. The client may now issue any of the following POP commands repeatedly. After each command, the POP server issues a response. Eventually, the client issues the QUIT command and the POP session enters the UPDATE state. Here are the POP commands valid in the TRANSACTION state: STAT Arguments: none Restrictions: may only be given in the TRANSACTION state. Discussion: The POP server issues a positive response with a line containing information for the maildrop. This line is called a "drop listing" for that maildrop. In order to simplify parsing, all POP servers are required to use a certain format for drop listings. The first octets present must indicate the number of messages in the maildrop. Following this is 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. Possible Responses: +OK nn mm Examples: C: STAT S: +OK 2 320 LIST [msg] Arguments: a message-id (optionally) If a message-id is given, it may NOT refer to a message marked as deleted. Restrictions: may only be given in the TRANSACTION state. Discussion: If an argument was given and the POP server issues a positive response with a line containing information for that message. This line is called a "scan listing" Page 5 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel for that message. If no argument was given and the POP server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP server responds with a line containing information for that message. This line is called a "scan listing" for that message. In order to simplify parsing, all POP servers are required to use a certain format for scan listings. The first octets present must be the message-id of the message. Following the message-id is the size of the message in octets. 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. Note that messages marked as deleted are not listed. Possible Responses: +OK scan listing follows -ERR no such message Examples: C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . ... C: LIST 2 S: +OK 2 200 ... C: LIST 3 S: -ERR no such message, only 2 messages in maildrop RETR msg Arguments: a message-id (required) This message-id may NOT refer to a message marked as deleted. Restrictions: may only be given in the TRANSACTION state. Discussion: If the POP server issues a positive response, then the response given is multi-line. After the initial +OK, the Page 6 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel POP server sends the message corresponding to the given message-id, being careful to bit-stuff the termination character (as with all multi-line responses). Possible Responses: +OK message follows -ERR no such message Examples: C: RETR 1 S: +OK 120 octets S: S: . DELE msg Arguments: a message-id (required) This message-id may NOT refer to a message marked as deleted. Restrictions: may only be given in the TRANSACTION state. Discussion: The POP server marks the message as deleted. Any future reference to the message-id associated with the message in a POP command generates an error. The POP server does not actually delete the message until the POP session enters the UPDATE state. Possible Responses: +OK message deleted -ERR no such message Examples: C: DELE 1 S: +OK message 1 deleted ... C: DELE 2 S: -ERR message 2 already deleted NOOP Arguments: none Restrictions: may only be given in the TRANSACTION state. Discussion: The POP server does nothing, it merely replies with a positive response. Possible Responses: +OK Examples: C: NOOP S: +OK RSET Arguments: none Page 7 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel Restrictions: may only be given in the TRANSACTION state. Discussion: If any messages have been marked as deleted by the POP server, they are unmarked. The POP server then replies with a positive response. Possible Responses: +OK Examples: C: RSET S: +OK maildrop has 2 messages (320 octets) The UPDATE State When the client issues the QUIT command from the TRANSACTION state the POP session enters the UPDATE state. (Note that if the client issues the QUIT command from the AUTHORIZATION state, the POP session terminates but does NOT enter the UPDATE state). QUIT Arguments: none Restrictions: none Discussion: The POP server removes all messages marked as deleted from the maildrop. It then releases the exclusive-access lock on the maildrop and replies as to the success of these operations. The TCP connection is then closed. Possible Responses: +OK Examples: C: QUIT S: +OK dewey POP server signing off (maildrop empty) ... C: QUIT S: +OK dewey POP server signing off (2 messages left) ... Page 8 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel Optional POP Commands The POP commands discussed above must be supported by all minimal implementations of POP servers. The optional POP commands described below permit a POP client greater freedom in message handling, while preserving a simple POP server implementation. NOTE: This memo STRONGLY encourages implementations to support these commands in lieu of developing augmented drop and scan listings. In short, the philosophy of this memo is to put intelligence in the part of the POP client and not the POP server. TOP msg n Arguments: a message-id (required) and a number. This message-id may NOT refer to a message marked as deleted. Restrictions: may only be given in the TRANSACTION state. Discussion: If the POP server issues a positive response, then the response given is multi-line. After the initial +OK, the POP server sends the headers of the message, the blank line separating the headers from the body, and then the number of lines indicated message's body, being careful to bit-stuff the termination character (as with all multi-line responses). Note that if the number of lines requested by the POP client is greater than than the number of lines in the body, then the POP server sends the entire message. Possible Responses: +OK top of message follows -ERR no such message Examples: C: TOP 10 S: +OK S: S: . ... C: TOP 100 S: -ERR no such message RPOP user Arguments: a client specific user-id (required) Restrictions: may only be given in the AUTHORIZATION state after a successful USER command; in addition, may Page 9 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel only be given if the client used a reserved (privileged) TCP port to connect to the server. Discussion: The RPOP command may be used instead of the PASS command to authenticate access to the maildrop. In order for this command to be successful, the POP client must use a reserved TCP port (port < 1024) to connect to the server. The POP server uses the argument pair from the USER and RPOP commands to determine if the client should be given access to the appropriate maildrop. Unlike the PASS command however, the POP server considers if the remote user specified by the RPOP command who resides on the POP client host is allowed to access the maildrop for the user specified by the USER command (e.g., on Berkeley UNIX, the .rhosts mechanism is used). With the exception of this differing in authentication, this command is identical to the PASS command. Possible Responses: +OK maildrop locked and ready -ERR permission denied Examples: C: USER mrose S: +OK mrose is a real hoopy frood C: RPOP mrose S: +OK mrose's maildrop has 2 messages (320 octets) Page 10 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel POP Command/Reply Summary Minimal POP Commands: USER name valid in the AUTHORIZATION state PASS string QUIT STAT valid in the TRANSACTION state LIST [msg] RETR msg DELE msg NOOP RSET QUIT valid in the UPDATE state Optional POP Commands: RPOP user valid in the AUTHORIZATION state TOP msg n valid in the TRANSACTION state POP Replies: +OK -ERR Note that with the exception of the STAT command, the reply given by the POP server to any command is significant only to "+OK" and "-ERR". Any text occurring after this reply may be ignored by the client. Page 11 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel Example POP Session S: ... C: S: +OK dewey POP server ready (Comments to: PostMaster@UDel) C: USER mrose S: +OK mrose is a real hoopy frood C: PASS secret S: +OK mrose's maildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +OK dewey POP server signing off (maildrop empty) C: S: Message Format All messages transmitted during a POP session are assumed to conform to the standard for the format of ARPA Internet text messages [RFC822]. It is important to note that the byte count for a message on the server host may differ from the octet count assigned to that message due to local conventions for designating end-of-line. Usually, during the AUTHORIZATION state of the POP session, the POP client can calculate the size of each message in octets when it parses the maildrop into messages. For example, if the POP server host internally represents end-of-line as a single character, then the POP server simply counts each occurrence of this character in a message as two octets. Note that lines in the message which start Page 12 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel with the termination octet need not be counted twice, since the POP client will remove all bit-stuffed termination characters when it receives a multi-line response. The POP and the Split-UA model The underlying paradigm in which the POP functions is that of a split-UA model. The POP client host, being a remote PC based workstation, acts solely as a client to the message transport system. It does not provide delivery/authentication services to others. Hence, it is acting as a UA, on behalf of the person using the workstation. Furthermore, the workstation uses SMTP to enter mail into the MTS. In this sense we have two UA functions which interface to the message transport system: Posting (SMTP) and Retrieval (POP). The entity which supports this type of environment is called a split-UA (since the user agent is split between two hosts which must interoperate to provide these functions). ASIDE: Others might term this a remote-UA instead. There are arguments supporting the use of both terms. This memo has explicitly referenced TCP as the underlying transport agent for the POP. This need not be the case. In the MZnet split-UA, for example, personal micro-computer systems are used which do not have IP-style networking capability. To connect to the POP server host, a PC establishes a terminal connection using some simple protocol (PhoneNet). A program on the PC drives the connection, first establishing a login session as a normal user. The login shell for this pseudo-user is a program which drives the other half of the terminal protocol and communicates with one of two servers. Although MZnet can support several PCs, a single pseudo-user login is present on the server host. The user-id and password for this pseudo-user login is known to all members of MZnet. Hence, the first action of the login shell, after starting the terminal protocol, is to demand a USER/PASS authorization pair from the PC. This second level of authorization is used to ascertain who is interacting with the MTS. Although the server host is deemed to support a "trusted" MTS entity, PCs in MZnet are not. Naturally, the USER/PASS authorization pair for a PC is known only to the owner of the PC (in theory, at least). After successfully verifying the identity of the client, a modified SMTP server is started, and the PC posts mail with the server host. After the QUIT command is given to the SMTP server and it terminates, a modified POP server is started, and the PC retrieves mail from the server host. After the QUIT command is given to the POP server and it terminates, the login shell for the pseudo-user Page 13 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel terminates the terminal protocol and logs the job out. The PC then closes the terminal connection to the server host. The SMTP server used by MZnet is modified in the sense that it knows that it's talking to a user agent and not a "trusted" entity in the message transport system. Hence, it does performs the validation activities normally performed by an entity in the MTS when it accepts a message from a UA. The POP server used by MZnet is modified in the sense that it does not require a USER/PASS combination before entering the TRANSACTION state. The reason for this (of course) is that the PC has already identified itself during the second-level authorization step described above. NOTE: Truth in advertising laws require that the author of this memo state that MZnet has not actually been fully implemented. The concepts presented and proven by the project led to the notion of the MZnet split-slot model. This notion has inspired the split-UA concept described in this memo, led to the author's interest in the POP, and heavily influenced the the description of the POP herein. In fact, some UAs present in the ARPA Internet already support the notion of posting directly to an SMTP server and retreiving mail directly from a POP server, even if the POP server and client resided on the same host! ASIDE: this discussion raises an issue which this memo purposedly avoids: how does SMTP know that it's talking to a "trusted" MTS entity? Page 14 Request For Comments: draft M. Rose Post Office Protocol (revised) UDel References [MZnet] E.A. Stefferud, J.N. Sweet, T.P. Domae. "MZnet: Mail Service for Personal Micro-Computer Systems", Proceedings, IFIP 6.5 International Conference on Computer Message Systems, Nottingham, U.K. (May, 1984) [RFC821] J.B. Postel. "Simple Mail Transfer Protocol", USC/Information Sciences Institute. (August, 1982) [RFC822] D.H. Crocker. "Standard for the Format of ARPA Internet Text Messages", University of Delaware. (August, 1982) [RFC918] J.K. Reynolds. "Post Office Protocol", USC/Information Sciences Institute. (October, 1984) [RFC923] J.K. Reynolds, J.B. Postel. "Assigned Numbers", USC/Information Sciences Institute. (October, 1984) Page 15