rfc772.txt

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Network Working Group                                         S. Sluizer
Request for Comments: 772                                      J. Postel
                                                                     ISI
                                                          September 1980

                         MAIL TRANSFER PROTOCOL


PREFACE

   This is a first draft of this protocol and comments are very
   definitely requested.

INTRODUCTION

   The objective of Mail Transfer Protocol (MTP) is to transfer mail
   reliably and efficiently.

   This paper assumes knowledge of the following protocols described in
   the ARPA Internet Protocol Handbook.  The reader will note strong
   similarities to portions of the File Transfer Protocol; in part, this
   is due to the original ARPA Network implementation of computer mail
   as a feature of FTP.

      The ARPANET Host-to-Host Protocol [Network Control Protocol] (NCP)

      The Transmission Control Protocol (TCP)

      The TELNET Protocol (TELNET)

      The File Transfer Protocol (FTP)

DISCUSSION

   In this section, the terminology and the MTP model are discussed.
   The terms defined in this section are only those that have special
   significance in MTP.  Some of the terminology is very specific to the
   MTP model; some readers may wish to turn to the section on the MTP
   model while reviewing the terminology.

   TERMINOLOGY

      ASCII

         The ASCII character set as defined in the ARPA Internet
         Protocol Handbook.  In MTP, ASCII characters are defined to be
         the lower half of an eight-bit code set (i.e., the most
         significant bit is zero) and is called NVT-ASCII.






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September 1980                                                   RFC 772
Mail Transfer Protocol                                                  



      control connection

         The TCP full-duplex communication path or two NCP simplex
         communication paths between a sender-MTP and a receiver-MTP for
         the exchange of commands, replies, and mail text.  The control
         connection operates according to the TELNET Protocol.

      data mode

         The mail is transmitted over the control connection as a stream
         of octets.  (In FTP terminology this is called stream mode.)

      data structure

         The internal structure of mail is considered to be a continuous
         sequence of data octets.  (In FTP terminology this is called
         file-structure.)

      data representation

         The internal representation of all data (i.e., mail) is in
         NVT-ASCII.

      host

         A computer in the internetwork environment on which mailboxes
         reside.

      MTP commands

         A set of commands which comprise the control information
         flowing from the sender-MTP to the receiver-MTP.

      mail

         An ordered set of computer data of arbitrary length, which
         conforms to the standard set in RFC 733 (Standard for the
         Format of ARPA Network Text Messages).

      mailbox

         A character string (address) which identifies a user to whom
         mail is to be sent.  Mailbox normally consists of the host and
         user specifications.  The standard mailbox naming convention is
         defined to be "user@host".  Additionally, the "container" in
         which mail is stored.



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RFC 772                                                   September 1980
                                                  Mail Transfer Protocol



      NVT

         The Network Virtual Terminal as defined in the TELNET Protocol.

      octet

         Bytes in MTP are octets (8 bits).  This is not necessarily the
         same byte size in which data is stored in a host.

      reply

         A reply is an acknowledgment (positive or negative) sent from
         receiver to sender via the control connection in response to a
         MTP command.  The general form of a reply is a completion code
         (including error codes) followed by a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.

      receiver-MTP process

         A process which transfers mail in cooperation with a sender-MTP
         process.  It "listens" on its port/socket L for a connection
         from a sender-MTP and establishes a control connection using
         the TELNET Protocol.  It receives MTP commands from the
         sender-MTP, sends replies, and governs the transfer of mail.

      sender-MTP process

         A process which transfers mail in cooperation with a
         receiver-MTP process.  A local language may be used in the user
         interface command/reply dialogue.  The sender-MTP initiates the
         control connection from its port/socket U to the receiver-MTP
         process.  It initiates MTP commands, receives replies, and
         governs the transfer of mail.

      user

         A human being (or a process on behalf of a human being) wishing
         to obtain mail transfer service.  In addition, a recipient of
         computer mail.









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   THE MTP MODEL

      With the above definitions in mind, the following model (shown in
      Figure 1) may be diagrammed for an MTP service.

                  ------------                ------------
                  |          |                |          |    --------
                  |          |      MTP       |          |<-->| User |
                  | Receiver-|Commands/Replies|  Sender- |    --------
      --------    |   MTP    |<-------------->|    MTP   |    --------
      | Mail |<-->|          |      Mail      |          |<-->| Mail |
      |System|    |          |                |          |    |System|
      --------    ------------                ------------    --------
      

                  Receiver-MTP                 Sender-MTP

                           Model for MTP Use

                                Figure 1

      In the model described in Figure 1, the sender-MTP initiates the
      TCP/NCP control connection which follows the TELNET Protocol.  At
      the initiation of the user, standard MTP commands are generated by
      the sender-MTP and transmitted to the receiver-MTP via the control
      connection.  Standard replies are sent from the receiver-MTP to
      the sender-MTP over the control connection in response to the
      commands.  In addition, mail is sent over the control connection.

MAIL TRANSFER FUNCTIONS

   The control connection is used for the transfer of commands which
   describe the functions to be performed, the replies to commands, as
   well as the actual transfer of mail.  Mail is transferred only via
   the control connection.

   The communication channel from the sender-MTP to the receiver-MTP is
   established by a TCP/NCP control connection from the sender to a
   standard receiver port/socket.  The sender-MTP is responsible for
   sending MTP commands, interpreting the replies received, and sending
   the mail; the receiver-MTP interprets commands, sends replies, and
   receives the mail.







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RFC 772                                                   September 1980
                                                  Mail Transfer Protocol



   MAIL REPRESENTATION AND STORAGE

      Mail is transferred from a storage device in the sending host to a
      storage device in the receiving host.  It may be necessary to
      perform certain transformations on the mail because data storage
      representations in the two systems are different.  For example,
      NVT-ASCII has different data storage representations in different
      systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII
      characters, left-justified in a 36-bit word.  360's store
      NVT-ASCII as four 8-bit EBCDIC codes in a 32-bit word.  Multics
      stores NVT-ASCII as four 9-bit characters in a 36-bit word.

      For the sake of simplicity, all data must be represented in MTP as
      NVT-ASCII.  This means that characters must be converted into the
      standard NVT-ASCII representation when transmitting text,
      regardless of whether the sending and receiving hosts are
      dissimilar.  The sender converts the data from its internal
      character representation to the standard 8-bit NVT-ASCII
      representation (see the TELNET specification).  The receiver
      converts the data from the standard form to its own internal form.
      In accordance with this standard, the <CRLF> sequence should be
      used to denote the end of a line of text.

      The mail in MTP has no internal structure and is considered to be
      a continuous sequence of data octets.

   ERROR RECOVERY AND RESTART

      There is no provision for detecting bits lost or scrambled in data
      transfer; this level of error control is handled by the TCP/NCP.
      In addition, there is no restart procedure provided to protect
      senders from gross system failures (including failures of a host,
      an MTP-process, or the underlying network).

MTP COMMANDS

   COMMAND SEMANTICS

      The MTP commands define the mail transfer or the mail system
      function requested by the user.  The syntax of mailboxes must
      conform to receiver site conventions (with standard defaults
      applicable).  In response to an MTP transfer command, the mail
      shall always be transferred over the control connection.

      The Mail Transfer Protocol follows the specifications of the
      TELNET Protocol for all communications over the control



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      connection.  Although the language used for TELNET communication
      can be a negotiated option, the "TELNET language" and the
      corresponding "TELNET end of line code" are required to be
      NVT-ASCII and <CRLF> respectively.  No other specifications of the
      TELNET Protocol will be cited.

      MTP commands are NVT-ASCII strings terminated by <CRLF>.  The
      command codes themselves are alphabetic characters terminated by
      the character <SP> (space) if parameters follow and <CRLF>
      otherwise.

      The MTP commands are discussed below.  In the description of a few
      of the commands in this section the possible replies are given
      explicitly.  MTP replies are discussed in the next section.

         MAIL (MAIL)

            This command allows a sender-MTP to send mail over the
            control connection.  The argument field contains a sender
            and optional path sequence.  If the path sequence is
            present, it consists of an optional list of hosts and a
            destination mailbox.  When the list of hosts is present, it
            is source routing information and indicates that the mail
            must be forwarded to the first host on the list.  Following
            this command line the receiver treats all subsequent
            characters as mail text from the sender.  The mail text is
            terminated by the character sequence "CRLF.CRLF".

            As mail is forwarded along the path sequence, each
            forwarding host must remove itself from the list.  When mail
            reaches its ultimate destination (the path sequence has only
            a (possibly empty) destination mailbox), the receiver
            inserts it into the destination mailbox in accordance with
            its host mail conventions.  If the second argument field is
            blank (one or more spaces) or empty (<CRLF>), the mail is
            destined for a printer or other designated place for site
            general delivery mail.  The mail may be marked as sent from
            the sender as specified by the first argument field.

         MAIL RECIPIENT SCHEME QUESTION (MRSQ)

            This MTP command is used to select a scheme for the
            transmission of mail to several users at the same host.  The
            schemes are to list the recipients first, or to send the
            mail first.




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RFC 772                                                   September 1980
                                                  Mail Transfer Protocol



         MAIL RECIPIENT (MRCP)

            This command is used to identify the individual recipients
            of the mail in the transmission of mail for multiple users
            at one host.