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rfc:rfc1460

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:
  1. the RPOP facility is removed;
  1. the optional APOP facility is added (which is in interoperable,

operational use in at least three implementations);

  1. a typo was corrected with respect to the interaction of LAST

and RSET;

  1. section numbers were added; and,
  1. 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]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1460.txt · Last modified: 1993/06/04 21:13 (external edit)