sbr/mts.c: Delete mmdlm2; use same-valued mmdlm1 instead.

[nmh] / docs / historical / mh-6.8.5 / support / pop / pop.txt

Request For Comments:  draft












                      Post Office Protocol (revised)


                         Wed Jan 14 18:47:59 1987

                             Marshall T. Rose

                       Computer Science Laboratory
                  Northrop Research and Technical Center
                          Palos Verdes Peninsula

                         MRose@NRTC.NORTHROP.COM




    This memo suggests a simple method for workstations to dynamically
    access mail from a mailbox server.  This RFC specifies a proposed
    protocol for the ARPA Internet community, and requests discussion
    and suggestions for improvements.


                           Acknowledgements

    This memo is based on RFC918.  Although similar in form to the
    original POP proposed for the ARPA Internet community, the protocol
    discussed in this memo is similar in spirit to the ideas
    investigated by the MZnet project at the University of California,
    Irvine.

    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.
\f
Request For Comments:  draft                                     M. Rose
Post Office Protocol (revised)                                      UDel



                             Introduction

    On certain types of smaller nodes in the ARPA Internet it is often
    impractical to maintain a message transport system(MTS).  For
    example, a workstation may not have sufficient resources (cycles,
    disk space) in order to permit a SMTP server and associated local
    mail delivery system to be kept resident and continuously running.
    Similarly, it may be expensive (or impossible) to keep a personal
    computer interconnected to an IP-style network for long amounts of
    time (the node is lacking the resource known as "connectivity").

    Despite this, it is often very useful to be able to manage mail on
    these smaller nodes, and they often support a user agent(UA) to aid
    the tasks of mail handling.  To solve this problem, a node which
    can support an MTS entity offers a maildrop service to these less
    endowned nodes. The Post Office Protocol (POP) is intended to
    permit a workstation to dynamically access a maildrop on a server
    host in a useful fashion. Usually, this means that the POP is used
    to allow a workstation to retrieve mail that the server is holding
    for it.

    For the remainder of this memo, the term "client host" refers to a
    host making use of the POP service, while the term "server host"
    refers to a host which offers the POP service.



                          A Short Digression

    This memo does not specify how a client host enters mail into the
    transport system, although a method consistent with the philosophy
    of this memo is presented here:

         When the user agent on a client host wishes to enter a message
         into the transport system, it establishes an SMTP connection
         to its relay host (this relay host could be, but need not be,
         the POP server host for the client host).

    If this method is followed, then the client host appears to the MTS
    as a user agent, and should NOT be regarded as a "trusted" MTS
    entity in any sense whatsoever.  This concept, along with the role
    of the POP as a part of a split-UA model is discussed later in this
    memo.
\f
                              The Protocol

    Initially the server host starts the POP service by listening on
    TCP port 109.  When a client host wishes to make use of the service
    it establishes a TCP connection with the server host.  When the
    connection is established, the POP server sends a greeting.  The
    client and POP server then exchange commands and responses
    (respectively) until the connection is closed or aborted.

    Commands in the POP consist of a keyword possibly followed by an
    argument.  All commands are terminated by a CRLF pair.

    Responses in the POP consist of a success indicator and a keyword
    possibly followed by additional information.  All responses are
    terminated by a CRLF pair.  There are currently two success
    indicators: positive ("+OK") and negative ("-ERR").

    Responses to certain commands are multi-line.  In these cases,
    which are clearly indicated below, after sending the first line of
    the response and a CRLF, any additional lines are sent, each
    terminated by a CRLF pair.  When all lines of the response have
    been sent, a final line is sent, consisting of a termination octet
    (octal code 056, ".") and a CRLF pair. If any line of the
    multi-line response begins with the termination octet, the line is
    "bit-stuffed" by pre-pending the termination octet to that line of
    the response.  Hence a multi-line response is terminated with the
    five octets "CRLF.CRLF".  When examining a multi-line response, the
    client checks to see if the line begins with the termination
    octet. If so and if octets other than CRLF follow, the the first
    octet of the line (the termination octet) is stripped away.  If so
    and if CRLF immediately follows the termination character, then the
    response from the POP server is ended and the line containing
    ".CRLF" is not considered part of the multi-line response.

    A POP session progresses through a number of states during its
    lifetime.  Once the TCP connection has been opened and the POP
    server has sent the greeting, the session enters the AUTHORIZATION
    state.  In this state, the client must identify itself to the POP
    server.  Once the client has successfully done this, the server
    acquires resources associated with the client's maildrop, and the
    session enters the TRANSACTION state.  In this state, the client
    requests actions on the part of the POP server.  When the client
    has finished its transactions, the session enters the UPDATE state.
    In this state, the POP server releases any resources acquired
    during the TRANSACTION state and says goodbye.  The TCP connection
    is then closed.
\f
                        The AUTHORIZATION State

    Once the TCP connection has been opened by a POP client, the POP
    server issues a one line greeting.  This can be any string
    terminated by CRLF.  An example might be:

        S:    +OK dewey POP server ready (Comments to: PostMaster@UDel)

    Note that this greeting is a POP reply.  The POP server should always
    give a positive response as the greeting.

    The POP session is now in the AUTHORIZATION state.  The client must
    now issue the USER command.  If the POP server responds with a
    positive success indicator ("+OK"), then the client may issue
    either the PASS command to complete the authorization, or the QUIT
    command to terminate the POP session.  If the POP server responds
    with a negative success indicator ("-ERR") to the USER command,
    then the client may either issue a new USER command or may issue
    the QUIT command.

    When the client issues the PASS command, the POP server uses the
    argument pair from the USER and PASS commands to determine if the
    client should be given access to the appropriate maildrop.  If so,
    the POP server then acquires an exclusive-access lock on the
    maildrop.  If the lock is successfully acquired, the POP server
    parses the maildrop into individual messages (read note below),
    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 POP session now enters the TRANSACTION
    state.  If the lock can not be acquired or the client should is
    denied access to the appropriate maildrop or the maildrop can't be
    parsed for some reason, the POP server responds with a negative
    success indicator.  (If a lock was acquired but the POP server
    intends to respond with a negative success indicator, the POP server
    must release the lock prior to rejecting the command.)  At this
    point, the client may either issue a new USER command and start
    again, or the client may issue the QUIT command.  

              NOTE: Minimal implementations of the POP need only be
              able to break a maildrop into its component messages;
              they need NOT be able to parse individual messages.  More
              advanced implementations may wish to have this
              capability, for reasons discussed later.

    After the POP server has parsed the maildrop into individual
    messages, it assigns a message-id to each message, and notes the
    size of the message in octets.  The first message in the maildrop
    is assigned a message-id of "1", the second is assigned "2", and so
    on, so that the n'th message in a maildrop is assigned a message-id
    of "n".  In POP commands and responses, all message-id's and
    message sizes are expressed in base-10.

    It sets the "highest number accessed" to be that of the last
    message referenced by the RETR command.

    Here are summaries for the three POP command discussed thus far:

        USER name
            Arguments: a server specific user-id (required)
            Restrictions: may only be given in the AUTHORIZATION state
                after the POP greeting or after an unsuccessful USER
                or PASS command
            Possible Responses:
                +OK name is welcome here
                -ERR never heard of name
            Examples:
                C:    USER mrose
                S:    +OK mrose is a real hoopy frood
                  ...
                C:    USER frated
                S:    -ERR sorry, frated doesn't get his mail here

        PASS string
            Arguments: a server/user-id specific password (required)
            Restrictions: may only be given in the AUTHORIZATION state
                after a successful USER command
            Possible Responses:
                +OK maildrop locked and ready
                -ERR invalid password
                -ERR unable to lock maildrop
            Examples:
                C:    USER mrose
                S:    +OK mrose is a real hoopy frood
                C:    PASS secret
                S:    +OK mrose's maildrop has 2 messages (320 octets)
                  ...
                C:    USER mrose
                S:    +OK mrose is a real hoopy frood
                C:    PASS secret
                S:    -ERR unable to lock mrose's maildrop, file already locked

        QUIT
            Arguments: none
            Restrictions: none
            Possible Responses:
                +OK
            Examples:
                C:    QUIT
                S:    +OK dewey POP server signing off



                         The TRANSACTION State

    Once the client has successfully identified itself to the POP
    server and the POP server has locked and burst the appropriate
    maildrop, the POP session is now in the TRANSACTION state.  The
    client may now issue any of the following POP commands repeatedly.
    After each command, the POP server issues a response.  Eventually,
    the client issues the QUIT command and the POP session enters the
    UPDATE state.

    Here are the POP commands valid in the TRANSACTION state:

        STAT
            Arguments: none
            Restrictions: may only be given in the TRANSACTION state.
            Discussion:

              The POP server issues a positive response with a line
              containing information for the maildrop.  This line is
              called a "drop listing" for that maildrop.

              In order to simplify parsing, all POP servers are
              required to use a certain format for drop listings.  The
              first octets present must indicate the number of messages
              in the maildrop.  Following this is the size of the
              maildrop in octets. This memo makes no requirement on
              what follows the maildrop size.  Minimal implementations
              should just end that line of the response with a CRLF
              pair.  More advanced implementations may include other
              information.

                   NOTE: This memo STRONGLY discourages implementations
                   from supplying additional information in the drop
                   listing.  Other, optional, facilities are discussed
                   later on which permit the client to parse the
                   messages in the maildrop.

              Note that messages marked as deleted are not counted in
              either total.

            Possible Responses:
                +OK nn mm
            Examples:
                C:    STAT
                S:    +OK 2 320

        LIST [msg]
            Arguments: a message-id (optionally)  If a message-id is
                given, it may NOT refer to a message marked as deleted.
            Restrictions: may only be given in the TRANSACTION state.
            Discussion:

              If an argument was given and the POP server issues a
              positive response with a line containing information for
              that message.  This line is called a "scan listing"
              for that message.

              If no argument was given and the POP server issues a
              positive response, then the response given is multi-line.
              After the initial +OK, for each message in the maildrop,
              the POP server responds with a line containing information
              for that message.  This line is called a "scan listing"
              for that message.

              In order to simplify parsing, all POP servers are required
              to use a certain format for scan listings.  The first
              octets present must be the message-id of the message.
              Following the message-id is the size of the message in
              octets.  This memo makes no requirement on what follows
              the message size in the scan listing.  Minimal
              implementations should just end that line of the response
              with a CRLF pair.  More advanced implementations may
              include other information, as parsed from the message.

                   NOTE: This memo STRONGLY discourages implementations
                   from supplying additional information in the scan
                   listing.  Other, optional, facilities are discussed
                   later on which permit the client to parse the
                   messages in the maildrop.

              Note that messages marked as deleted are not listed.

            Possible Responses:
                +OK scan listing follows
                -ERR no such message
            Examples:
                C:    LIST
                S:    +OK 2 messages (320 octets)
                S:    1 120
                S:    2 200
                S:    .
                  ...
                C:    LIST 2
                S:    +OK 2 200
                  ...
                C:    LIST 3
                S:    -ERR no such message, only 2 messages in maildrop

        RETR msg
            Arguments: a message-id (required)  This message-id may NOT
                refer to a message marked as deleted.
            Restrictions: may only be given in the TRANSACTION state.
            Discussion:

              If the POP server issues a positive response, then the
              response given is multi-line.  After the initial +OK, the
              POP server sends the message corresponding to the given
              message-id, being careful to bit-stuff the termination
              character (as with all multi-line responses).

              If the number associated with this message is higher than
              the "highest number accessed" in the maildrop, the POP
              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 POP 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 POP server marks the message as deleted.  Any future
              reference to the message-id associated with the message
              in a POP command generates an error.  The POP server does
              not actually delete the message until the POP session
              enters the UPDATE state.

              If the number associated with this message is higher than
              the "highest number accessed" in the maildrop, the POP
              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 POP 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 POP 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 POP 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 POP
              server, they are unmarked.  The POP 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 POP 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 POP session enters the UPDATE state.  (Note that if the client
    issues the QUIT command from the AUTHORIZATION state, the POP
    session terminates but does NOT enter the UPDATE state).

        QUIT
            Arguments: none
            Restrictions: none
            Discussion:

              The POP server removes all messages marked as deleted
              from the maildrop.  It then releases the exclusive-access
              lock on the maildrop and replies as to the success of
              these operations.  The TCP connection is then closed.

            Possible Responses:
                +OK
            Examples:
                C:    QUIT
                S:    +OK dewey POP server signing off (maildrop empty)
                  ...
                C:    QUIT
                S:    +OK dewey POP server signing off (2 messages left)
                  ...
\f
                         Optional POP Commands

    The POP commands discussed above must be supported by all minimal
    implementations of POP servers.

    The optional POP commands described below permit a POP client
    greater freedom in message handling, while preserving a simple POP
    server implementation.

              NOTE: This memo STRONGLY encourages implementations to
              support these commands in lieu of developing augmented
              drop and scan listings.  In short, the philosophy of this
              memo is to put intelligence in the part of the POP client
              and not the POP server.

        TOP msg n
            Arguments: a message-id (required) and a number.  This
                message-id may NOT refer to a message marked as deleted.
            Restrictions: may only be given in the TRANSACTION state.
            Discussion:

              If the POP server issues a positive response, then the
              response given is multi-line.  After the initial +OK, the
              POP server sends the headers of the message, the blank
              line separating the headers from the body, and then the
              number of lines indicated message's body, being careful to
              bit-stuff the termination character (as with all
              multi-line responses).  

              Note that if the number of lines requested by the POP
              client is greater than than the number of lines in the
              body, then the POP server sends the entire message.

            Possible Responses:
                +OK top of message follows
                -ERR no such message
            Examples:
                C:    TOP 10
                S:    +OK
                S:    <the POP 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 (privileged)
                TCP port to connect to the server.
            Discussion:

              The RPOP command may be used instead of the PASS command
              to authenticate access to the maildrop.  In order for this
              command to be successful, the POP client must use a
              reserved TCP port (port < 1024) to connect to the server.
              The POP server uses the argument pair from the USER and
              RPOP commands to determine if the client should be given
              access to the appropriate maildrop.  Unlike the PASS
              command however, the POP server considers if the remote
              user specified by the RPOP command who resides on the POP
              client host is allowed to access the maildrop for the user
              specified by the USER command (e.g., on Berkeley UNIX, the
              .rhosts mechanism is used).  With the exception of this
              differing in authentication, this command is identical to
              the PASS command.  

            Possible Responses:
                +OK maildrop locked and ready
                -ERR permission denied
            Examples:
                C:    USER mrose
                S:    +OK mrose is a real hoopy frood
                C:    RPOP mrose
                S:    +OK mrose's maildrop has 2 messages (320 octets)
\f
                       POP Command/Reply Summary

    Minimal POP Commands:
        USER name               valid in the AUTHORIZATION state
        PASS string
        QUIT

        STAT                    valid in the TRANSACTION state
        LIST [msg]
        RETR msg
        DELE msg
        NOOP
        LAST
        RSET

        QUIT                    valid in the UPDATE state

    Optional POP Commands:
        RPOP user               valid in the AUTHORIZATION state

        TOP msg n               valid in the TRANSACTION state

    POP Replies:
        +OK
        -ERR

    Note that with the exception of the STAT command, the reply given
    by the POP server to any command is significant only to "+OK" and
    "-ERR".  Any text occurring after this reply may be ignored by the
    client.
\f
                          Example POP Session

    S: <wait for connection on TCP port 109>
        ...
    C: <open connection>
    S:    +OK dewey POP server ready (Comments to: PostMaster@UDel)
    C:    USER mrose
    S:    +OK mrose is a real hoopy frood
    C:    PASS secret
    S:    +OK mrose's maildrop has 2 messages (320 octets)
    C:    STAT
    S:    +OK 2 320
    C:    LIST
    S:    +OK 2 messages (320 octets)
    S:    1 120
    S:    2 200
    S:    .
    C:    RETR 1
    S:    +OK 120 octets
    S:    <the POP server sends message 1>
    S:    .
    C:    DELE 1
    S:    +OK message 1 deleted
    C:    RETR 2
    S:    +OK 200 octets
    S:    <the POP server sends message 2>
    S:    .
    C:    DELE 2
    S:    +OK message 2 deleted
    C:    QUIT
    S:    +OK dewey POP server signing off (maildrop empty)
    C:  <close connection>
    S:  <wait for next connection>



                            Message Format

    All messages transmitted during a POP session are assumed to
    conform to the standard for the format of ARPA Internet text
    messages [RFC822].

    It is important to note that the byte count for a message on the
    server host may differ from the octet count assigned to that
    message due to local conventions for designating end-of-line.
    Usually, during the AUTHORIZATION state of the POP session, the POP
    client can calculate the size of each message in octets when it
    parses the maildrop into messages.  For example, if the POP server
    host internally represents end-of-line as a single character, then
    the POP server simply counts each occurrence of this character in a
    message as two octets.  Note that lines in the message which start
    with the termination octet need not be counted twice, since the POP
    client will remove all bit-stuffed termination characters when it
    receives a multi-line response.



                    The POP and the Split-UA model

    The underlying paradigm in which the POP functions is that of a
    split-UA model.  The POP client host, being a remote PC based
    workstation, acts solely as a client to the message transport
    system.  It does not provide delivery/authentication services to
    others.  Hence, it is acting as a UA, on behalf of the person using
    the workstation.  Furthermore, the workstation uses SMTP to enter
    mail into the MTS. 

    In this sense we have two UA functions which interface to the
    message transport system: Posting (SMTP) and Retrieval (POP). The
    entity which supports this type of environment is called a split-UA
    (since the user agent is split between two hosts which must
    interoperate to provide these functions).  

              ASIDE: Others might term this a remote-UA instead.  There
              are arguments supporting the use of both terms.

    This memo has explicitly referenced TCP as the underlying transport
    agent for the POP.  This need not be the case.  In the MZnet
    split-UA, for example, personal micro-computer systems are used
    which do not have IP-style networking capability.  To connect to
    the POP server host, a PC establishes a terminal connection using
    some simple protocol (PhoneNet).  A program on the PC drives the
    connection, first establishing a login session as a normal user.
    The login shell for this pseudo-user is a program which drives the
    other half of the terminal protocol and communicates with one of
    two servers. Although MZnet can support several PCs, a single
    pseudo-user login is present on the server host.  The user-id and
    password for this pseudo-user login is known to all members of
    MZnet. Hence, the first action of the login shell, after starting
    the terminal protocol, is to demand a USER/PASS authorization pair
    from the PC.  This second level of authorization is used to
    ascertain who is interacting with the MTS. Although the server host
    is deemed to support a "trusted" MTS entity, PCs in MZnet are not.
    Naturally, the USER/PASS authorization pair for a PC is known only
    to the owner of the PC (in theory, at least).

    After successfully verifying the identity of the client, a modified
    SMTP server is started, and the PC posts mail with the server host.
    After the QUIT command is given to the SMTP server and it
    terminates, a modified POP server is started, and the PC retrieves
    mail from the server host. After the QUIT command is given to the
    POP server and it terminates, the login shell for the pseudo-user
    terminates the terminal protocol and logs the job out.  The PC then
    closes the terminal connection to the server host.

    The SMTP server used by MZnet is modified in the sense that it
    knows that it's talking to a user agent and not a "trusted" entity
    in the message transport system.  Hence, it does performs the
    validation activities normally performed by an entity in the MTS
    when it accepts a message from a UA.

    The POP server used by MZnet is modified in the sense that it does
    not require a USER/PASS combination before entering the TRANSACTION
    state.  The reason for this (of course) is that the PC has already
    identified itself during the second-level authorization step
    described above.

              NOTE: Truth in advertising laws require that the author
              of this memo state that MZnet has not actually been fully
              implemented.  The concepts presented and proven by the
              project led to the notion of the MZnet split-slot model.
              This notion has inspired the split-UA concept described
              in this memo, led to the author's interest in the POP,
              and heavily influenced the the description of the POP
              herein.

    In fact, some UAs present in the ARPA Internet already support the
    notion of posting directly to an SMTP server and retreiving mail
    directly from a POP server, even if the POP server and client
    resided on the same host!  

              ASIDE: this discussion raises an issue which this memo
              purposedly avoids: how does SMTP know that it's talking
              to a "trusted" MTS entity?
\f
                              References

    [MZnet]     E.A. Stefferud, J.N. Sweet, T.P. Domae.
                "MZnet: Mail Service for Personal Micro-Computer
                Systems",  Proceedings, IFIP 6.5 International
                Conference on Computer Message Systems, Nottingham, U.K.
                (May, 1984)

    [RFC821]    J.B. Postel.
                "Simple Mail Transfer Protocol", USC/Information Sciences
                Institute. (August, 1982)

    [RFC822]    D.H. Crocker.
                "Standard for the Format of ARPA Internet Text
                Messages", University of Delaware.  (August, 1982)

    [RFC918]    J.K. Reynolds.
                "Post Office Protocol", USC/Information Sciences Institute.
                (October, 1984)

    [RFC923]    J.K. Reynolds, J.B. Postel.
                "Assigned Numbers", USC/Information Sciences Institute.
                (October, 1984)