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rfc:rfc937

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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