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HISTORIC

Network Working Group                                          M. Butler
Request for Comments: 937                                      J. Postel
                                                                D. Chase
                                                           J. Goldberger
                                                          J. K. Reynolds
Obsoletes: RFC 918                                                   ISI
                                                           February 1985


                    POST OFFICE PROTOCOL - VERSION 2


Status of this Memo

   This RFC 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 improvement.  This memo is a revision of RFC 918.
   Distribution of this memo is unlimited.

Introduction

   The intent of the Post Office Protocol Version 2 (POP2) is to allow a
   user's workstation to access mail from a mailbox server.  It is
   expected that mail will be posted from the workstation to the mailbox
   server via the Simple Mail Transfer Protocol (SMTP).  For further
   information see RFC-821 [1] and RFC-822 [2].

   This protocol assumes a reliable data stream such as provided by TCP
   or any similar protocol.  When TCP is used, the POP2 server listens
   on port 109 [4].

System Model and Philosophy

   While we view the workstation as an Internet host in the sense that
   it implements IP, we do not expect the workstation to contain the
   user's mailbox.  We expect the mailbox to be on a server machine.

   We believe it is important for the mailbox to be on an "always up"
   machine and that a workstation may be frequently powered down, or
   otherwise unavailable as an SMTP server.

   POP2 is designed for an environment of workstations and servers on a
   low-delay, high-throughput, local networks (such as Ethernets).  POP2
   may be useful in other environments as well, but if the environment
   is substantially different, a different division of labor between the
   client and server may be appropriate, and a different protocol
   required.

   Suppose the user's real name is John Smith, the user's machine is
   called FIDO, and that the mailbox server is called DOG-HOUSE.  Then



Butler, et. al.                                                 [Page 1]

RFC 937                                                    February 1985
Post Office Protocol


   we expect the user's mail to be addressed to JSmith@DOG-HOUSE.ARPA
   (not JSmith@FIDO.ARPA).

   That is, the destination of the mail is the mailbox on the server
   machine.  The POP2 protocol and the workstation are merely a
   mechanism for viewing the messages in the mailbox.

   The user is not tied to any particular workstation for accessing his
   mail.  The workstation does not appear as any part of the mailbox
   address.

   This is a very simple protocol.  This is not a user interface.  We
   expect that there is a program in the workstation that is friendly to
   the user.  This protocol is not "user friendly".  One basic rule of
   this protocol is "if anything goes wrong close the connection".
   Another basic rule is to have few options.

   POP2 does not parse messages in any way.  It does not analyze message
   headers (Date:, From:, To:, Cc:, or Subject:).  POP2 simply transmits
   whole messages from a mailbox server to a client workstation.

The Protocol

   The POP2 protocol is a sequence of commands and replies.  The design
   draws from many previous protocols of the ARPA-Internet community.

      The server must be listening for a connection.  When a connection
      is opened the server sends a greeting message and waits for
      commands.  When commands are received the server acts on them and
      responds with replies.

      The client opens a connection, waits for the greeting, then sends
      the HELO command with the user name and password arguments to
      establish authorization to access mailboxes.  The server returns
      the number of messages in the default mailbox.

      The client may read the default mailbox associated with the user
      name or may select another mailbox by using the FOLD command.  The
      server returns the number of messages in the mailbox selected.

      The client begins a message reading transaction with a READ
      command.  The read command may optionally indicate which message
      number to read, the default is the current message (incremented
      when a message is read and set to one when a new folder is
      selected).  The server returns the number of characters in the
      message.




Butler, et. al.                                                 [Page 2]

RFC 937                                                    February 1985
Post Office Protocol


      The client asks for the content of the message to be sent with the
      RETR command.  The server sends the message data.

      When all the data has been received the client sends an
      acknowledgment command.  This is one of ACKS, ACKD, and NACK.

         ACKS means "I've received the message successfully and please
         keep it in the mailbox".

         ACKD means "I've received the message successfully and please
         delete it from the mailbox".

         NACK means "I did not receive the message and please keep it in
         the mailbox".

      In the case of ACKS or ACKD the server increments the current
      message indicator.  In the case of NACK the current message
      indicator stays the same.

      In all cases the server returns the number of characters in the
      (now) current message.

      The client terminates the session with the QUIT command.  The
      server returns an ok.


























Butler, et. al.                                                 [Page 3]

RFC 937                                                    February 1985
Post Office Protocol


   The Normal Scenario

           Client                    Server
           ------                    ------
                                Wait for Connection
      Open Connection  -->
                           <--  + POP2 Server Ready
                                Wait for Command
      HELO Fred Secret -->
                           <--  #13 messages for you
                                Wait for Command
      READ 13          -->
                           <--  =537 characters in that message
                                Wait for Command
      RETR             -->
                           <--  (send the message data)
                                Wait for Command
      ACKS             -->
                           <--  =0 no more messages
                                Wait for Command
      QUIT             -->
                           <--  + OK
      Close connection --> <--  Close connection
                                Wait for Connection (go back to start)

Conventions

   Arguments

      These arguments have system specific definitions.

         user - A login account name.

         password - The password for the login account.

         mailbox - A mailbox name (also called a mail folder).














Butler, et. al.                                                 [Page 4]

RFC 937                                                    February 1985
Post Office Protocol


   Default Mailboxes

      TOPS-20

         MAIL.TXT.1 - from login directory

      UNIX

         both
            /usr/spool/mail/user
         and
            /usr/user/Mail/inbox/*

            where "user" is the user value supplied in the HELO command.

   End of Line

      End of Line is Carriage Return (CR) followed by Line Feed (LF).
      This sequence is indicated by "CRLF" in this document.  This end
      of line convention must be used for commands and replies.

   Message Length

      The reply to the READ command or an acknowledgment command (ACKS,
      ACKD, NACK) is the length (a character count) of the next message
      to be transmitted.  This includes all the characters in the data
      transmitted.  CRLF counts as two characters.  A length of zero
      means the message does not exist or is empty.  A request to
      transmit a message of zero length will result in the server
      closing the connection.  The message is transmitted in the
      standard internet format described in RFC-822 [2] and NVT-ASCII.
      This may be different from the storage format and may make
      computing the message length from the stored message non-trivial.

   Message Numbers

      The reply to the HELO and FOLD commands is a count of the number
      of messages in a the selected mailbox.  The READ command has a
      message number as an optional argument.  These numbers are
      decimal, start at one, and computed with respect to the current
      mailbox.  That is, the first message in a mailbox is message
      number 1.

   Numbers

      All numbers in this memo and protocol are decimal.




Butler, et. al.                                                 [Page 5]

RFC 937                                                    February 1985
Post Office Protocol


   Quoting

      In a few cases, there may be a need to have a special character in
      an argument (user, password, or mailbox) that is not allowed by
      the syntax.  For example, a space in a password. To allow for
      this, a quoting convention is defined.  Unfortunately, such
      quoting conventions "use up" another otherwise uninteresting
      character.  In this protocol the back slash "\" is used as the
      quote character.  To include a space in an argument the two
      character sequence "back-slash, space" is transmitted.  To include
      a back-slash in an argument the two character sequence
      "back-slash, back-slash" is transmitted.  This quoting convention
      is used in the command arguments only, it is not used in the mail
      data transmitted in response to a RETR command.

   Reply Strings

      The first character is required to be as specified (i.e.,
      "+", "-", "=", "#").  The optional strings that follow can be
      whatever the implementer thinks is appropriate.

Definitions of Commands and Replies

   Summary of Commands and Replies

      Commands                          Replies
      --------                          -------
      HELO user password                + OK
      FOLD mailbox                      - Error
      READ [n]                          #xxx
      RETR                              =yyy
      ACKS
      ACKD
      NACK
      QUIT















Butler, et. al.                                                 [Page 6]

RFC 937                                                    February 1985
Post Office Protocol


   Commands

      HELO user password

         The Hello command identifies the user to the server and carries
         the password authenticating this user.  This information is
         used by the server to control access to the mailboxes.  The
         Hello command is the "HELO" keyword, followed by the user
         argument, followed by the password argument, followed by CRLF.

            Possible responses:

               "#nnn"

                  where nnn is the number of messages in the default
                  mailbox,"

               "- error report" and Close the connection.

      FOLD mailbox

         The Folder command selects another mailbox or mail folder.  The
         server must check that the user is permitted read access to
         this mailbox.  If the mailbox is empty or does not exist, the
         number of messages reported is zero.  The Folder command is the
         "FOLD" keyword, followed by the mailbox argument, followed by
         CRLF.

            Possible responses:

               "#nnn"

                  where nnn is the number of messages in this mailbox.

      READ [nnn]

         The Read command begins a message reading transaction.  If the
         Read command is given without an argument the current message
         is  implied (the current message indicator is incremented  by
         the ACKS or ACKD commands).  If an argument is used with the
         Read command it is the message number to be read, and this
         command sets the current message indicator to that value.  The
         server returns the count of characters in the message to be
         transmitted.  If there is no message to be read, the count of
         zero is returned.  If the message was previously deleted with
         the ACKD command, the count of zero is returned.  The Read
         command is followed by the RETR command, the READ command, the
         FOLD command, or the QUIT command.  Do not attempt to RETR a


Butler, et. al.                                                 [Page 7]

RFC 937                                                    February 1985
Post Office Protocol


         message of zero characters.  The Read command is the "READ"
         keyword, optionally followed by the message number argument,
         followed by CRLF.

            Possible responses:

               "=ccc"

                  where ccc is the number of characters in this message.

      RETR

         The Retrieve command confirms that the client is ready to
         receive the mail data.  It must be followed by an
         acknowledgment command.  The server will close the connection
         if asked to transmit a message of zero characters (i.e.,
         transmit a non-existent message).  The message is transmitted
         according to the Internet mail format standard RFC-822 [2] in
         NVT-ASCII.  The Retrieve command is the "RETR" keyword,
         followed by CRLF.

            Possible responses:

               the message data

               Close the connection

      ACKS

         The Acknowledge and Save command confirms that the client has
         received and accepted the message.  The ACKS command ends the
         message reading transaction.  The message is kept in the
         mailbox.  The current message indicator is incremented.  The
         server returns the count of characters in the now current
         message to be transmitted.  If there is no message to be read
         or the message is marked deleted, the count of zero is
         returned.  The Acknowledge and Save command is the "ACKS"
         keyword, followed by CRLF.

            Possible responses:

               "=ccc"

                  where ccc is the number of characters in the next
                  message.





Butler, et. al.                                                 [Page 8]

RFC 937                                                    February 1985
Post Office Protocol


      ACKD

         The Acknowledge and Delete command confirms that the client has
         received and accepted the message.  The ACKD command ends the
         message reading transaction.  If the user is authorized to have
         write access to the mailbox, the message is deleted from the
         mailbox.  Actually, the message is only marked for deletion.
         The actual change is made  when the mailbox is released at the
         end of the session or when the client selects another mailbox
         with the FOLD command.  The messages are not renumbered until
         the mailbox is released.  If the user does not have write
         access to the mailbox no change is made to the mailbox.  The
         response is the same whether or not the message was actually
         deleted.  The current message indicator is incremented.  The
         server returns the count of characters in the now current
         message to be transmitted.  If there is no message to be read
         or the message is marked deleted, the count of zero is
         returned.  The Acknowledge and Delete command is the "ACKD"
         keyword, followed by CRLF.

            Possible responses:

               "=ccc"

                  where ccc is the number of characters in the next
                  message.

      NACK

         The Negative Acknowledge command reports that the client did
         not receive the message.  The NACK command ends the message
         reading transaction.  The message is kept in the mailbox.  The
         current message indicator remains the same.  The server returns
         the count of characters in the current message.  Since the
         count to be returned is for the message just transmitted it the
         message must exist and not be marked deleted, and the count
         must be positive (non-zero).  The Negative Acknowledge command
         is the "NACK" keyword, followed by CRLF.

            Possible responses:

               "=ccc"

                  where ccc is the number of characters in this message.






Butler, et. al.                                                 [Page 9]

RFC 937                                                    February 1985
Post Office Protocol


      QUIT

         The Quit command indicates the client is done with the session.
         The server sends an OK response and then closes the connection.
         The Quit command is the "QUIT" keyword, followed by CRLF.

            Possible responses:

               "+ OK" and Close the connection

   Replies

      Greeting

         The greeting is sent by the server as soon as the connection is
         established.  The greeting is a plus sign, followed by the
         protocol name ("POP2"), followed by the server host name,
         optionally followed by text, and ending with a CRLF.

      +

         The success or plus sign response indicates successful
         completion of the operation specified in the command.  The
         success response is a plus sign, optionally followed by text,
         and ending with a CRLF.

      -

         The failure or minus sign response indicates the failure of the
         operation specified in the command.  The failure response is a
         minus sign, optionally followed by text, and ending with a
         CRLF.

      =

         The length or equal sign response tells the length in
         characters of the message referenced by the command.  The
         length response is a equal sign, followed by a number,
         optionally followed by text, and ending with a CRLF.

      #

         The count or number sign response tells the number of messages
         in a folder or mailbox referenced by the command.  The count
         response is a number sign, followed by a number, optionally
         followed by text, and ending with a CRLF.




Butler, et. al.                                                [Page 10]

RFC 937                                                    February 1985
Post Office Protocol


   Timeouts

      In any protocol of this type there have to be timeouts.  Neither
      side wants to get stuck waiting forever for the other side
      (particularly is the other side has gone crazy or crashed).

      The client expects a reply to a command fairly quickly and so
      should have a short timeout for this.  This timeout is called T1.

         For some servers, it may take some processing to compute the
         number of messages in a mailbox, or the length of a message, or
         to reformat a stored message for transmission, so this time out
         has to allow for such processing time.  Also care must be taken
         not to timeout waiting for the completion of a RETR reply while
         a long message is in fact being transfered.

      The server expects the session to progress with some but not
      excessive delay between commands and so should have a long timeout
      waiting for the next command.  This time out is T2.

         One model of use of this protocol is that any number of
         different types of clients can be built with different ways of
         interacting with the human user and the server, but still
         expecting the client to open the connection to the server,
         present a sequence of commands, and close the connection,
         without waiting for intervention by the human user.  With such
         client implementations, it is reasonable for the server to have
         a fairly small value for timeout T2.

         On the other hand, one could easily have the client be very
         human user directed with the user making decisions between
         commands.  This would cause arbitrary delays between client
         commands to the server, and require the value of timeout T2 to
         be quite large.

Implementation Discussion

   Comments on a Server on TOPS-20

      On TOPS-20, a mailbox is a single file.  New messages are appended
      to the file.  There is a separator line between messages.

      The tricky part of implementing a POP2 server on TOPS-20 is to
      provide for deleting messages.  This only has to be done for the
      mailboxes (files) for which the user has write access.  The
      problem is to avoid both (1) preventing other users from accessing
      or updating the mailbox for long periods, and (2) accidentally
      deleting a message the user has not seen.


Butler, et. al.                                                [Page 11]

RFC 937                                                    February 1985
Post Office Protocol


      One suggestion is as follows:

         When a mailbox is first selected, if the user has write access,
         rename the mailbox file to some temporary name.  Thus new
         messages will be placed in a new instance of the mailbox file.
         Conduct all POP2 operation on the temporary mailbox file
         (including deleting messages).  When the POP2 session is over
         or another mailbox is selected, prepend any messages left
         undeleted in the temporary file to the new instance of the
         mailbox file.

   Sizes

      The maximum length of a command line is 512 characters (including
      the command word and the CRLF).

      The maximum length of a reply line is 512 characters (including
      the success indicator (+, -, =, #) and the CRLF).

      The maximum length of a text line is 1000 characters (including
      CRLF).

   ISI has developed a POP2 server for TOPS-20 and for Berkeley 4.2
   Unix, and a POP2 client for an IBM-PC and for Berkeley 4.2 Unix.

Extensions Not Supported

   POP2 does not examine the internal data of messages.  In particular,
   the server does not parse message headers.

   The server doesn't have any state information (i.e., it doesn't know
   from one session to the next what has happened).  For example, the
   server doesn't know which messages were received since the last time
   the user used POP2, so it can't send just the "new" messages.
















Butler, et. al.                                                [Page 12]

RFC 937                                                    February 1985
Post Office Protocol


Examples

   Example 1:

           Client                    Server
           ------                    ------
                                Wait for connection
      Open connection  -->
                           <--  + POP2 USC-ISIF.ARPA Server
      HELO POSTEL SECRET -->
                           <--  #2 messages in your mailbox
      READ             -->
                           <--  =537 characters in message 1
      RETR             -->
                           <--  [data of message 1]
      ACKD             -->
                           <--  =234 characters in message 2
      RETR             -->
                           <--  [data of message 2]
      ACKD             -->
                           <--  =0 no more messages
      QUIT             -->
                           <--  + OK, bye, bye
      Close connection --> <--  Close connection
                                Go back to start

























Butler, et. al.                                                [Page 13]

RFC 937                                                    February 1985
Post Office Protocol


   Example 2:

           Client                    Server
           ------                    ------
                                Wait for connection
      Open connection  -->
                           <--  + POP2 ISI-VAXA.ARPA server here
      HELO smith secret -->
                           <--  #35 messages
      FOLD /usr/spool/mail/smith -->
                           <--  #27 messages
      READ  27         -->
                           <--  =10123 characters in that message
      RETR             -->
                           <--  [data of message 27]
      ACKS             -->
                           <--  =0 no more messages
      QUIT             -->
                           <--  + bye, call again sometime.
      Close connection --> <--  Close connection
                                Go back to start

   Example 3:

           Client                    Server
           ------                    ------
                                Wait for connection
      Open connection  -->
                           <--  + POP2 ISI-VAXA.ARPA server here
      HELO Jones secret -->
                           <--  #0 messages
      READ             -->
                           <--  Close connection
      Close connection -->
                                Go back to start















Butler, et. al.                                                [Page 14]

RFC 937                                                    February 1985
Post Office Protocol


Formal Syntax

   <digit>    = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

   <letter>   = A | B | C | ... | Z
                a | b | c | ... | z

   <punct>    = ! | " | # | $ | % | & | ' | ( | ) | * |
                + | , | - | / | : | < | = | > | ? | @ |
                [ | ] | ^ | _ | ` | { | | | } | ~

   <quote>    = \

   <any>      = any one of the 128 ASCII codes

   <CR>       = carriage return, code 10

   <LF>       = line feed, code 13

   <SP>       = space, code 32

   <CRLF>     = <CR> <LF>

   <print>    = <letter> | <digit> | <punct> | <quote> <any>

   <char>     = <print> | <SP>

   <word>     = <print> | <print> <word>

   <string>   = <char> | <char> <string>

   <ld>       = <letter> | <digit>

   <ldh>      = <letter> | <digit> | -

   <ldhs>     = <ldh> | <ldh> <ldhs>

   <name>     = <letter> [ [ <ldhs> ] <ld> ]

   <host>     =  <name> | <name> . <host>

   <user>     = <word>

   <password> = <word>

   <mailbox>  = <string>

   <number>   = <digit> | <digit> <number>


Butler, et. al.                                                [Page 15]

RFC 937                                                    February 1985
Post Office Protocol


   <helo>     = HELO <SP> <user> <SP> <password> <CRLF>

   <fold>     = FOLD <SP> <mailbox> <CRLF>

   <read>     = READ [<SP> <number>] <CRLF>

   <retr>     = RETR <CRLF>

   <acks>     = ACKS <CRLF>

   <ackd>     = ACKD <CRLF>

   <nack>     = NACK <CRLF>

   <quit>     = QUIT <CRLF>

   <ok>       = + [<SP> <string>] <CRLF>

   <err>      = - [<SP> <string>] <CRLF>

   <count>    = # <number> [<SP> <string>] <CRLF>

   <greet>    = + <SP> POP2 <SP> <host> [<SP> <string>] <CRLF>

   <length>   = = <number> [<SP> <string>] <CRLF>

   <command>  = <helo> | <fold> | <read> | <retr> |
                <acks> | <ackd> | <nack> | <quit>

   <reply>    = <ok> | <err> | <count> | <length> | <greet>




















Butler, et. al.                                                [Page 16]

RFC 937                                                    February 1985
Post Office Protocol


Client State Diagram


                          |                    ^  + BYE
                          |  Open              |  -----
                          |           Greet    |  Close
                          V           -----    |
                      +-------+       QUIT    +-------+
                      | CALL  |-------------->| EXIT  |
                      +-------+               +-------+
                          |                       ^
                          |  Greet                |
                          |  -----                |
                          |  HELO                 |
              +---->+     |                       |
        #NNN  ^     |     |        #NNN           |
        ----  |     V     V        ----           |
        FOLD  |    +-------+       QUIT           |
              +<---| NMBR  |--------------------->+
                   +-------+                      ^
                    ^     |                       |
                    |     |  #NNN                 |
                    |     |  ----                 |
              =CCC  |     |  READ                 |
              ----  |     |                       |
              FOLD  |     |        =CCC           |
                    |     V        ----           |
        =CCC  +--->+-------+       QUIT           |
        ----  ^    | SIZE  |--------------------->+
        READ  +<---+-------+
                    ^     |
                    |     |  =CCC
              data  |     |  ----
              ----  |     |  RETR
              ack   |     |
                    |     V
                   +-------+
                   | XFER  |
                   +-------+











Butler, et. al.                                                [Page 17]

RFC 937                                                    February 1985
Post Office Protocol


Server State Diagram


                       +<----------------------+  Close
                       |                       |  -----
               Listen  |                       |  Close
                       V                       |
                   +-------+                  +-------+
                   | LSTN  |                  | DONE  |
                   +-------+                  +-------+
                       |                          ^
                       |  Open                    |
                       |  -----                   |
                       |  Greet                   |
                       |                          |
                       |           QUIT           |
                       V           -----          |
                   +-------+       + BYE          |
                   | AUTH  |--------------------->+
                   +-------+                      ^
                       |                          |
                       |  HELO                    |
                       |  ----                    |
                       |  #NNN                    |
                       |                          |
                       |           QUIT           |
                       V           -----          |
        FOLD  +--->+-------+       + BYE          |
        ----  ^    | MBOX  |--------------------->+
        #NNN  +<---+-------+                      ^
                    ^     |                       |
                    |     |  READ                 |
              FOLD  |     |  ----                 |
              ----  |     |  =CCC                 |
              #NNN  |     |        QUIT           |
                    |     V        -----          |
        READ  +--->+-------+       + BYE          |
        ----  ^    | ITEM  |--------------------->+
        =CCC  +<---+-------+
                    ^     |
                    |     |  RETR
              ack   |     |  ----
              ----  |     |  data
              =CCC  |     |
                    |     V
                   +-------+
                   | NEXT  |
                   +-------+


Butler, et. al.                                                [Page 18]

RFC 937                                                    February 1985
Post Office Protocol


Combined Flow Diagram


   +----+
   |CALL|<------------------------------------------------------------+
   |LSTN|                                                             ^
   +----+                                                             |
    | Greet                                                           |
    |                                                                 |
    |  +----------------------------------------------------->+       |
    |  ^ QUIT                                                 |       |
    V  |                                                      V       |
   +----+        +----+                                      +----+   |
   |CALL| HELO   |NMBR|                                      |EXIT|   |
   |AUTH|------->|AUTH|                                      |AUTH|   |
   +----+        +----+                                      +----+   |
                  | #NNN                                   + Bye |    |
                  |                                              |    |
                  |  +------------------------------------>+     |    |
                  |  ^ QUIT                                |     |    |
                  V  |                                     V     |    |
            +--->+----+        +----+                     +----+ |    |
       FOLD ^    |NMBR| READ   |SIZE|                     |EXIT| |    |
       ---- |    |MBOX|------->|MBOX|                     |MBOX| |    |
       #NNN +<---+----+        +----+                     +----+ |    |
                     ^           | =CCC                 + Bye |  |    |
                     |           |                            |  |    |
                FOLD +<--------+ | +------------------->+     |  |    |
                ----           ^ | ^ QUIT               |     |  |    |
                #NNN           | V |                    V     |  |    |
                         +--->+-----+        +----+    +----+ |  |    |
                    READ ^    |SIZE | RETR   |XFER|    |EXIT| |  |    |
                    ---- |    | ITEM|------->|ITEM|    |ITEM| |  |    |
                    =CCC +<---+-----+        +----+    +----+ |  |    |
                                 ^             | data      |  |  |    |
                                 |             |           |  |  |    |
                            =CCC |             V     + Bye |  |  |    |
                               +----+        +----+        |  |  |    |
                               |SIZE|    Ack |XFER|        |  |  |    |
                               |NEXT|<-------|NEXT|        |  |  |    |
                               +----+        +----+        |  |  |    |
                                                           |  |  |    |
                                                           |  |  |    |
                                                           V  V  V    |
                                                          +-------+   |
                                                          | EXIT  |-->+
                                                          | DONE  |
                                                          +-------+


Butler, et. al.                                                [Page 19]

RFC 937                                                    February 1985
Post Office Protocol


Client Decision Table


          |            STATE                 |
   -------+----------------------------------|
   INPUT  | CALL | NMBR | SIZE | XFER | EXIT |
   -------+----------------------------------|
   Greet  |  2   |  1   |  1   |  1   |  6   |
   -------+----------------------------------|
   #NNN   |  1   |  3   |  1   |  1   |  6   |
   -------+----------------------------------|
   =CCC   |  1   |  1   |  4   |  1   |  6   |
   -------+----------------------------------|
   data   |  1   |  1   |  1   |  5   |  6   |
   -------+----------------------------------|
   + Bye  |  1   |  1   |  1   |  1   |  6   |
   -------+----------------------------------|
   Close  |  1   |  1   |  1   |  1   |  6   |
   -------+----------------------------------|
   other  |  1   |  1   |  1   |  1   |  6   |
   -------+----------------------------------|
   Timeout|  1   |  1   |  1   |  1   |  6   |
   -------+----------------------------------|



























Butler, et. al.                                                [Page 20]

RFC 937                                                    February 1985
Post Office Protocol


   Actions:

      1.  This is garbage.  Send "QUIT", and go to EXIT state.

      2.  (a) If the greeting is right then send "HELO"
              and go to NMBR state,
          (b) Else send "QUIT" and go to EXIT state.

      3.  (a) If user wants this folder and NNN > 0
              then send "READ" and go to SIZE state,
          (b) If user wants a this folder and NNN = 0
              then send "QUIT" and go to EXIT state,
          (c) If user wants a different folder
              then send "FOLD" and go to NMBR state.

      4.  (a) If user wants this message and CCC > 0
              then send "RETR" and go to XFER state,
          (b) If user wants a this message and CCC = 0
              then send "QUIT" and go to EXIT state,
          (c) If user wants a different message
              then send "READ" and go to SIZE state.

      5.  (a) If user wants this message kept
              then send "ACKS" and go to SIZE state,
          (b) If user wants a this message deleted
              then send "ACKD" and go to SIZE state,
          (c) If user wants a this message again
              then send "NACK" and go to SIZE state.

      6.  Close the connection.




















Butler, et. al.                                                [Page 21]

RFC 937                                                    February 1985
Post Office Protocol


Server Decision Table


          |              STATE
   -------+-----------------------------------------
   INPUT  | LSTN | AUTH | MBOX | ITEM | NEXT | DONE |
   -------+-----------------------------------------|
   Open   |  2   |  1   |  1   |  1   |  1   |  1   |
   -------+-----------------------------------------|
   HELO   |  1   |  3   |  1   |  1   |  1   |  1   |
   -------+-----------------------------------------|
   FOLD   |  1   |  1   |  5   |  5   |  1   |  1   |
   -------+-----------------------------------------|
   READ   |  1   |  1   |  6   |  6   |  1   |  1   |
   -------+-----------------------------------------|
   RETR   |  1   |  1   |  1   |  7   |  1   |  1   |
   -------+-----------------------------------------|
   ACKS   |  1   |  1   |  1   |  1   |  8   |  1   |
   -------+-----------------------------------------|
   ACKD   |  1   |  1   |  1   |  1   |  8   |  1   |
   -------+-----------------------------------------|
   NACK   |  1   |  1   |  1   |  1   |  8   |  1   |
   -------+-----------------------------------------|
   QUIT   |  1   |  4   |  4   |  4   |  1   |  1   |
   -------+-----------------------------------------|
   Close  |  1   |  1   |  1   |  1   |  1   |  9   |
   -------+-----------------------------------------|
   other  |  1   |  1   |  1   |  1   |  1   |  1   |
   -------+-----------------------------------------|
   Timeout|      |  1   |  1   |  1   |  1   |  1   |
   -------+-----------------------------------------|



















Butler, et. al.                                                [Page 22]

RFC 937                                                    February 1985
Post Office Protocol


   Actions:

      1.  This is garbage.  Send "- error", and Close the connection.

      2.  Send the greeting. Go to AUTH state.

      3.  (a) If authorized user then send "#NNN" and go tp MBOX state,
          (b) Else send "- error" and Close the connection.

      4.  Send "+ Bye" and go to DONE state.

      5.  Send "+NNN" and go to MBOX state.

      6.  Send "=CCC" and go to ITEM state.

      7.  If message exists then send the data and got to NEXT state,
          Else Close the connection.

      8.  Do what ACKS/ACKD/NACK require and go to ITEM state.

      9.  Close the connection.





























Butler, et. al.                                                [Page 23]

RFC 937                                                    February 1985
Post Office Protocol


Acknowledgment

   We would like to acknowledge the helpful comments that we received on
   the first version of POP described in RFC 918, and the draft of POP2
   distributed to interested parties.

References

   [1]  Postel, J., "Simple Mail Transfer Protocol", RFC 821,
   USC/Information Sciences Institute, August 1982.

   [2]  Crocker, D., "Standard for the Format of ARPA-Internet Text
   Messages", RFC 822, University of Delaware, August 1982.

   [3]  Reynolds, J.K., "Post Office Protocol", RFC 918, USC/Information
   Sciences Institute, October 1984.

   [4]  Reynolds, J.K., and J. Postel, "Assigned Numbers", RFC 923,
   USC/Information Sciences Institute, October 1984.































Butler, et. al.                                                [Page 24]


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