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+Network Working Group                                            M. Rose
+Request for Comments: 1460                  Dover Beach Consulting, Inc.
+Obsoletes: 1225                                                June 1993
+
+
+                    Post Office Protocol - Version 3
+
+
+Status of this Memo
+
+   This RFC specifies an IAB standards track protocol for the Internet
+   community, and requests discussion and suggestions for improvements.
+   Please refer to the current edition of the "IAB Official Protocol
+   Standards" for the standardization state and status of this protocol.
+   Distribution of this memo is unlimited.
+
+Overview
+
+   This memo is a revision to RFC 1225, a Draft Standard.  It makes the
+   following changes from that document:
+
+        - the RPOP facility is removed;
+
+        - the optional APOP facility is added (which is in interoperable,
+          operational use in at least three implementations);
+
+        - a typo was corrected with respect to the interaction of LAST
+          and RSET;
+
+        - section numbers were added; and,
+
+        - an acknowledgements section was added.
+
+1. 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 [RFC821] 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
+
+
+
+Rose                                                            [Page 1]
+
+RFC 1460                          POP3                         June 1993
+
+
+   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.
+
+   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.
+
+2. 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.
+
+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 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
+
+
+
+Rose                                                            [Page 2]
+
+RFC 1460                          POP3                         June 1993
+
+
+   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
+   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.
+
+4. 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 POP3 server ready
+
+   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.
+
+
+
+
+Rose                                                            [Page 3]
+
+RFC 1460                          POP3                         June 1993
+
+
+   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
+   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
+
+
+
+Rose                                                            [Page 4]
+
+RFC 1460                          POP3                         June 1993
+
+
+                     ...
+                   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 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 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.
+
+
+
+Rose                                                            [Page 5]
+
+RFC 1460                          POP3                         June 1993
+
+
+               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
+                 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
+
+
+
+Rose                                                            [Page 6]
+
+RFC 1460                          POP3                         June 1993
+
+
+                 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
+                 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
+
+
+
+Rose                                                            [Page 7]
+
+RFC 1460                          POP3                         June 1993
+
+
+                 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.
+
+               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.
+
+
+
+Rose                                                            [Page 8]
+
+RFC 1460                          POP3                         June 1993
+
+
+               Discussion:
+
+                 The POP3 server does nothing, it merely replies with a
+                 positive response.
+
+               Possible Responses:
+                   +OK
+               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 0
+
+
+
+
+Rose                                                            [Page 9]
+
+RFC 1460                          POP3                         June 1993
+
+
+           RSET
+               Arguments: none
+               Restrictions: may only be given in the TRANSACTION
+                   state.
+               Discussion:
+
+                 If any messages have been marked as deleted by the POP3
+                 server, they are unmarked.  The POP3 server then
+                 replies with a positive response.  In addition, the
+                 "highest number accessed" is also reset to zero.
+
+               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.  (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)
+                     ...
+
+
+
+
+
+Rose                                                           [Page 10]
+
+RFC 1460                          POP3                         June 1993
+
+
+7. Optional POP3 Commands
+
+   The POP3 commands discussed above must be supported by all minimal
+   implementations of POP3 servers.
+
+   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
+
+
+
+
+Rose                                                           [Page 11]
+
+RFC 1460                          POP3                         June 1993
+
+
+           APOP name digest
+               Arguments: a server specific user-id and a digest string
+                    (both required).
+               Restrictions: may only be given in the AUTHORIZATION
+                   state after the POP3 greeting
+               Discussion:
+
+                 Normally, each POP3 session starts with a USER/PASS
+                 exchange.  This results in a server/user-id specific
+                 password being sent in the clear on the network.  For
+                 intermittent use of POP3, this may not introduce a
+                 sizable risk.  However, many POP3 client
+                 implementations connect to the POP3 server on a
+                 regular basis -- to check for new mail.  Further the
+                 interval of session initiation may be on the order of
+                 five minutes.  Hence, the risk of password capture is
+                 greatly enhanced.
+
+                 An alternate method of authentication is required
+                 which provides for both origin authentication and
+                 replay protection, but which does not involve sending
+                 a password in the clear over the network.  The APOP
+                 command provides this functionality.
+
+                 A POP3 server which implements the APOP command will
+                 include a timestamp in its banner greeting.  The
+                 syntax of the timestamp corresponds to the "msg-id"
+                 in [RFC822], and MUST be different each time the POP3
+                 server issues a banner greeting.  For example, on a
+                 UNIX implementation in which a separate UNIX process
+                 is used for each instance of a POP3 server, the
+                 syntax of the timestamp might be:
+
+                    <process-ID.clock@hostname>
+
+                 where "process-ID" is the decimal value of the
+                 process's PID, clock is the decimal value of the
+                 system clock, and hostname is the fully-qualified
+                 domain-name corresponding to the host where the POP3
+                 server is running.
+
+                 The POP3 client makes note of this timestamp, and
+                 then issues the APOP command.  The "name" parameter
+                 has identical semantics to the "name" parameter of
+                 the USER command. The "digest" parameter is
+                 calculated by applying the MD5 algorithm [RFC1321] to
+                 a string consisting of the timestamp (including
+                 angle-brackets) followed by a shared secret.  This
+
+
+
+Rose                                                           [Page 12]
+
+RFC 1460                          POP3                         June 1993
+
+
+                 shared secret is a string known only to the POP3
+                 client and server. Great care should be taken to
+                 prevent unauthorized disclosure of the secret, as
+                 knowledge of the secret will allow any entity to
+                 successfully masquerade as the named user.  The
+                 "digest" parameter itself is a 16-octet value which
+                 is sent in hexadecimal format, using lower-case ASCII
+                 characters.
+
+                 When the POP3 server receives the APOP command, it
+                 verifies the digest provided.  If the digest is
+                 correct, the POP3 server issues a positive response,
+                 and the POP3 session enters the TRANSACTION state.
+                 Otherwise, a negative response is issued and the POP3
+                 session remains in the AUTHORIZATION state.
+
+               Possible Responses:
+                   +OK maildrop locked and ready
+                   -ERR permission denied
+               Examples:
+                   S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
+                   C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
+                   S: +OK maildrop has 1 message (369 octets)
+
+                 In this example, the shared secret is the string "tanstaaf".
+                 Hence, the MD5 algorithm is applied to the string
+
+                    <1896.697170952@dbc.mtview.ca.us>tanstaaf
+
+                 which produces a digest value of
+
+                    c4c9334bac560ecc979e58001b3e22fb
+
+8. POP3 Command Summary
+
+       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 13]
+
+RFC 1460                          POP3                         June 1993
+
+
+           QUIT                    valid in the UPDATE state
+
+       Optional POP3 Commands:
+           APOP name digest        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.
+
+9. Example POP3 Session
+
+    S: <wait for connection on TCP port 110>
+        ...
+    C: <open connection>
+    S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
+    C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
+    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
+    S:    +OK dewey POP3 server signing off (maildrop empty)
+    C:  <close connection>
+    S:  <wait for next connection>
+
+
+
+
+Rose                                                           [Page 14]
+
+RFC 1460                          POP3                         June 1993
+
+
+10. 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.
+
+11. 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 [MZnet].  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 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
+
+
+
+Rose                                                           [Page 15]
+
+RFC 1460                          POP3                         June 1993
+
+
+   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 POP3 server, even if the POP3 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?
+
+
+
+
+Rose                                                           [Page 16]
+
+RFC 1460                          POP3                         June 1993
+
+
+12. References
+
+   [MZnet]   Stefferud, E., Sweet, J., 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", STD 10,
+             RFC 821, USC/Information Sciences Institute, August 1982.
+
+   [RFC822]  Crocker, D., "Standard for the Format of ARPA-Internet
+             Text Messages", STD 11, RFC 822, University of Delaware,
+             August 1982.
+
+   [RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", MIT
+             Laboratory for Computer Science, April 1992.
+
+13. Security Considerations
+
+   It is conjectured that use of the APOP command provides origin
+   identification and replay protection for a POP3 session.
+   Accordingly, a POP3 server which implements both the PASS and APOP
+   commands must not allow both methods of access for a given user; that
+   is, for a given "USER name" either the PASS or APOP command is
+   allowed, but not both.
+
+   Otherwise, security issues are not discussed in this memo.
+
+14. Acknowledgements
+
+   The POP family has a long and checkered history.  Although primarily
+   a minor revision to [RFC1225], POP3 is based on the ideas presented
+   in RFCs 918, 937, and 1081.
+
+   In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
+   provided significant comments on the APOP command.
+
+15. Author's Address
+
+   Marshall T. Rose
+   Dover Beach Consulting, Inc.
+   Mountain View, CA  94043-2186
+
+   Phone: +1 415 968 1052
+   Fax:   +1 415 968 2510
+
+   EMail: mrose@dbc.mtview.ca.us
+   X.500: rose, dbc, us
+
+
+
+Rose                                                           [Page 17]
+
\ No newline at end of file