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diff --git a/RFC/rfc1081.txt b/RFC/rfc1081.txt deleted file mode 100644 index c21d6e48..00000000 --- a/RFC/rfc1081.txt +++ /dev/null @@ -1,899 +0,0 @@ - - - - - - -Network Working Group M. Rose -Request for Comments: 1081 TWG - November 1988 - - Post Office Protocol - Version 3 - - -Status of this Memo - - This memo suggests a simple method for workstations to dynamically - access mail from a mailbox server. This RFC specifies a proposed - protocol for the Internet community, and requests discussion and - suggestions for improvements. Distribution of this memo is - unlimited. - - This memo is based on RFC 918 (since revised as RFC 937). Although - similar in form to the original Post Office Protocol (POP) proposed - for the 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. - - Further, substantial work was done on examining POP in a PC-based - environment. This work, which resulted in additional functionality - in this protocol, was performed by the ACIS Networking Systems Group - at Stanford University. The author gratefully acknowledges their - interest. - -Introduction - - On certain types of smaller nodes in the 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 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 is used - to allow a workstation to retrieve mail that the server is holding - for it. - - - - -Rose [Page 1] - -RFC 1081 POP3 November 1988 - - - For the remainder of this memo, the term "client host" refers to a - host making use of the POP3 service, while the term "server host" - refers to a host which offers the POP3 service. - -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 - POP3 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 - POP3 as a part of a split-UA model is discussed later in this memo. - - 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 keyword possibly followed by an - argument. All commands are terminated by a CRLF pair. - - Responses in the POP3 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 (decimal code - 046, ".") and 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. - 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 - - - -Rose [Page 2] - -RFC 1081 POP3 November 1988 - - - 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 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. When the client has - finished its transactions, the session enters the UPDATE state. In - this state, the POP3 server releases any resources acquired during - the TRANSACTION state and says goodbye. The TCP connection is then - closed. - -The AUTHORIZATION State - - Once the TCP connection has been opened by a POP3 client, the POP3 - server issues a one line greeting. This can be any string terminated - by CRLF. An example might be: - - S. +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU) - - Note that this greeting is a POP3 reply. The POP3 server should - always give a positive response as the greeting. - - The POP3 session is now in the AUTHORIZATION state. The client must - now issue the USER command. If the POP3 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 POP3 session. If the POP3 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 POP3 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 POP3 server then acquires an exclusive-access lock on the - maildrop. If the lock is successfully acquired, the POP3 server - parses the maildrop into individual messages (read note below), - determines the last message (if any) present in the maildrop that was - referenced by the RETR command, and responds with a positive success - indicator. The POP3 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 - - - -Rose [Page 3] - -RFC 1081 POP3 November 1988 - - - reason, the POP3 server responds with a negative success indicator. - (If a lock was acquired but the POP3 server intends to respond with a - negative success indicator, the POP3 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 POP3 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 POP3 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 POP3 commands and responses, all message-id's and message - sizes are expressed in base-10 (i.e., decimal). - - It sets the "highest number accessed" to be that of the last message - referenced by the RETR command. - - Here are summaries for the three POP3 commands discussed thus far: - - USER name - Arguments: a server specific user-id (required) - Restrictions: may only be given in the AUTHORIZATION - state after the POP3 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 - - - -Rose [Page 4] - -RFC 1081 POP3 November 1988 - - - -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 POP3 server signing off - - -The TRANSACTION State - - Once the client has successfully identified itself to the POP3 server - and the POP3 server has locked and burst the appropriate maildrop, - the POP3 session is now in the TRANSACTION state. The client may now - issue any of the following POP3 commands repeatedly. After each - command, the POP3 server issues a response. Eventually, the client - issues the QUIT command and the POP3 session enters the UPDATE state. - - 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. This line is - called a "drop listing" for that maildrop. - - In order to simplify parsing, all POP3 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 - - - -Rose [Page 5] - -RFC 1081 POP3 November 1988 - - - 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 POP3 server issues a - positive response with a line containing information - for that message. This line is called a "scan listing" - 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 called a "scan listing" for that message. - - In order to simplify parsing, all POP3 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 - - - -Rose [Page 6] - -RFC 1081 POP3 November 1988 - - - 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 POP3 server issues a positive response, then the - response given is multi-line. After the initial +OK, - the POP3 server sends the message corresponding to the - given message-id, being careful to byte-stuff the - termination character (as with all multi-line - responses). - - If the number associated with this message is higher - than the "highest number accessed" in the maildrop, the - POP3 server updates the "highest number accessed" to - the number associated with this message. - - - - - -Rose [Page 7] - -RFC 1081 POP3 November 1988 - - - Possible Responses: - +OK message follows - -ERR no such message - Examples: - C: RETR 1 - S: +OK 120 octets - S: <the POP3 server sends the entire message here> - 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 POP3 server marks the message as deleted. Any - future reference to the message-id associated with the - message in a POP3 command generates an error. The POP3 - server does not actually delete the message until the - POP3 session enters the UPDATE state. - - If the number associated with this message is higher - than the "highest number accessed" in the maildrop, - the POP3 server updates the "highest number accessed" - to the number associated with this message. - - 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 POP3 server does nothing, it merely replies with a - positive response. - - Possible Responses: - +OK - - - - - -Rose [Page 8] - -RFC 1081 POP3 November 1988 - - - Examples: - C: NOOP - S: +OK - - LAST - Arguments: none - Restrictions: may only be issued in the TRANSACTION state. - Discussion: - - The POP3 server issues a positive response with a line - containing the highest message number which accessed. - Zero is returned in case no message in the maildrop has - been accessed during previous transactions. A client - may thereafter infer that messages, if any, numbered - greater than the response to the LAST command are - messages not yet accessed by the client. - - Possible Response: - +OK nn - - Examples: - C: STAT - S: +OK 4 320 - C: LAST - S: +OK 1 - C: RETR 3 - S: +OK 120 octets - S: <the POP3 server sends the entire message - here> - S: . - C: LAST - S: +OK 3 - C: DELE 2 - S: +OK message 2 deleted - C: LAST - S: +OK 3 - C: RSET - S: +OK - C: LAST - S: +OK 1 - - RSET - Arguments: none - Restrictions: may only be given in the TRANSACTION - state. - Discussion: - - If any messages have been marked as deleted by the POP3 - - - -Rose [Page 9] - -RFC 1081 POP3 November 1988 - - - server, they are unmarked. The POP3 server then - replies with a positive response. In addition, the - "highest number accessed" is also reset to the value - determined at the beginning of the POP3 session. - - 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 POP3 session enters the UPDATE state. (Note that if the client - issues the QUIT command from the AUTHORIZATION state, the POP3 - session terminates but does NOT enter the UPDATE state.) - - QUIT - Arguments: none - Restrictions: none - Discussion: - - The POP3 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 POP3 server signing off (maildrop - empty) - ... - C: QUIT - S: +OK dewey POP3 server signing off (2 messages - left) - ... - - -Optional POP3 Commands - - The POP3 commands discussed above must be supported by all minimal - implementations of POP3 servers. - - - -Rose [Page 10] - -RFC 1081 POP3 November 1988 - - - The optional POP3 commands described below permit a POP3 client - greater freedom in message handling, while preserving a simple POP3 - 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 - POP3 client and not the POP3 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 POP3 server issues a positive response, then - the response given is multi-line. After the initial - +OK, the POP3 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 byte-stuff the termination character - (as with all multi-line responses). - - Note that if the number of lines requested by the POP3 - client is greater than than the number of lines in the - body, then the POP3 server sends the entire message. - - Possible Responses: - +OK top of message follows - -ERR no such message - Examples: - C: TOP 10 - S: +OK - S: <the POP3 server sends the headers of the - message, a blank line, and the first 10 lines - of the body of the message> - 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 only be given if the client used a reserved - - - -Rose [Page 11] - -RFC 1081 POP3 November 1988 - - - (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 POP3 - client must use a reserved TCP port (port < 1024) to - connect tothe server. The POP3 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 POP3 server considers if the remote user - specified by the RPOP command who resides on the POP3 - 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. - - Note that the use of this feature has allowed much wider - penetration into numerous hosts on local networks (and - sometimes remote networks) by those who gain illegal - access to computers by guessing passwords or otherwise - breaking into the system. - - 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) - - Minimal POP3 Commands: - USER name valid in the AUTHORIZATION state - PASS string - QUIT - - STAT valid in the TRANSACTION state - LIST [msg] - RETR msg - DELE msg - NOOP - LAST - RSET - - - - -Rose [Page 12] - -RFC 1081 POP3 November 1988 - - - QUIT valid in the UPDATE state - - Optional POP3 Commands: - RPOP user valid in the AUTHORIZATION state - - TOP msg n valid in the TRANSACTION state - - POP3 Replies: - +OK - -ERR - - Note that with the exception of the STAT command, the reply given - by the POP3 server to any command is significant only to "+OK" - and "-ERR". Any text occurring after this reply may be ignored - by the client. - -Example POP3 Session - - S: <wait for connection on TCP port 110> - ... - C: <open connection> - S: +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU) - 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: <the POP3 server sends message 1> - S: . - C: DELE 1 - S: +OK message 1 deleted - C: RETR 2 - S: +OK 200 octets - S: <the POP3 server sends message 2> - S: . - C: DELE 2 - S: +OK message 2 deleted - C: QUIT - - - - - -Rose [Page 13] - -RFC 1081 POP3 November 1988 - - - S: +OK dewey POP3 server signing off (maildrop empty) - C: <close connection> - S: <wait for next connection> - -Message Format - - All messages transmitted during a POP3 session are assumed to conform - to the standard for the format of 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 POP3 session, the POP3 client - can calculate the size of each message in octets when it parses the - maildrop into messages. For example, if the POP3 server host - internally represents end-of-line as a single character, then the - POP3 server simply counts each occurrence of this character in a - message as two octets. Note that lines in the message which start - with the termination octet need not be counted twice, since the POP3 - client will remove all byte-stuffed termination characters when it - receives a multi-line response. - -The POP and the Split-UA model - - The underlying paradigm in which the POP3 functions is that of a - split-UA model. The POP3 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 (POP3). 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 POP3. 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 POP3 - 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 - - - -Rose [Page 14] - -RFC 1081 POP3 November 1988 - - - 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 POP3 server is started, and the PC retrieves mail from the - server host. After the QUIT command is given to the POP3 server and - it terminates, the login shell for the pseudo-user 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 POP3 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 POP3 herein. - - In fact, some UAs present in the Internet already support the notion - of posting directly to an SMTP server and retrieving 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 - - - -Rose [Page 15] - -RFC 1081 POP3 November 1988 - - - purposedly avoids: how does SMTP know that it's talking - to a "trusted" MTS entity? - -References - - [MZnet] Stefferud, E., J. Sweet, and T. 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] Postel, J., "Simple Mail Transfer Protocol", - USC/Information Sciences Institute, August 1982. - - [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet - Text Messages", University of Delaware, August 1982. - - [RFC937] Butler, M., J. Postel, D. Chase, J. Goldberger, and J. - Reynolds, "Post Office Protocol - Version 2", RFC 937, - USC/Information Sciences Institute, February 1985. - - [RFC1010] Reynolds, J., and J. Postel, "Assigned Numbers", RFC - 1010, USC/Information Sciences Institute, May 1987. - -Author's Address: - - - Marshall Rose - The Wollongong Group - 1129 San Antonio Rd. - Palo Alto, California 94303 - - Phone: (415) 962-7100 - - Email: MRose@TWG.COM - - - - - - - - - - - - - - - - - -Rose [Page 16] -
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