    Network Working Group                                    V. Cerf  (ARPA)
    Request for Comments: 771                                J. Postel (ISI)
                                                              September 1980

                              MAIL TRANSITION PLAN

    PREFACE

       This is a draft memo and comments are requested.

    INTRODUCTION

       The principal aim of the mail service transition plan is to provide
       orderly support for computer mail service during the period of
       transition from the old ARPANET protocols to the new Internet
       protocols.

       This plan covers only the transition from the current text computer
       mail in the ARPANET environment to text computer mail in an Internet
       environment.  This plan does not address a second transition from
       text only mail to multimedia mail [10,11].

       The goal is to provide equivalent or better service in the new
       Internet environment as was available in the ARPANET environment.
       During the interim period, when both protocol environments are in
       use, the goal is to minimize the impact on users and existing
       software, yet to permit the maximum mail exchange connectivity.

       It is assumed that the user is familiar with both the ARPANET and
       Internet protocol environments [1-8].  The Internet protocols are
       designed to be used in a diverse collection of networks including the
       ARPANET, Packet Radio nets, Satellite nets, and local nets (e.g.,
       Ethernets, Ring nets); while the ARPANET protocol are, of course,
       limited to the ARPANET.

       The Internet protocol environment specifies TCP as the host-to-host
       transport protocol.  The ARPANET protocol environment specifies NCP
       as the host-to-host transport protocol.  Both TCP and NCP provide
       connection type process-to-process communication.  The problem in the
       transition is to bridge these two different interprocess
       communication systems.

       The objective of this plan is to specify the means by which the
       ARPANET computer mail services may be extended into the Internet
       system without disruptive changes for the users during the
       transition.

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    September 1980                                                   RFC 771
    Mail Transition Plan

    MODEL OF MAIL SERVICE

       The model of the computer mail system taken here separates the mail
       composition and reading functions from the mail transport functions.
       In the following, the discussion will be hoplessly TOPS20-oriented.
       We appologize to users of other systems, but  we feel it is better to
       discuss examples we know than to attempt to be abstract.

       In the ARPANET mail service, composition and reading is done with
       user programs such as HERMES, MSG, MM, etc., while mail transmission
       is done by system programs such as MAILER (sending) and FTPSRV
       (receiving).

       One element of the ARPANET mail service is the assumption that every
       source of mail can have a direct interprocess communication
       connection (via the NCPs) to every destination for mail.  (There are
       some cases where special handling and forwarding of mail violates
       this assumption.)

       Mailbox names are of the form "MAILBOX@HOST", and it is assumed that
       MAILBOX is a destination mailbox on that host.

       The messages are actually transmitted according to the provisions of
       the File Transfer Protocol.  Mail may be transimitted via either the
       control connection (MAIL command), or via a data connection (MLFL
       command).  In either case, the argument specifies only the mailbox
       since the destination host is assumed to be the host receiving the
       transmission.

          For example:  messages sent from Postel at USC-ISIF to Cerf at
          USC-ISIA would arrive at ISIA with the argument "Cerf" but no
          indication of the host.

    COMPOUND AND ALTERNATE NAMES

       Mailboxes are of the form "mailbox@host" where mailbox is usually a
       name like "Postel" and host is a host identifier like "USC-ISIF".  In
       some cases it will be useful to allow the host to be a compound name
       such as:

          USC-ISIA
          ARPANET-ISIA
          SATNET-NDRE
          PPSN-RSRE
          HOST1.SRINET
          LSCNET/MAILROOM

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    RFC 771                                                   September 1980
                                                        Mail Transition Plan

       or even the name of an organization:

          BBN
          ARPA
          MIT
          SRI

       The only restriction is that "@" not appear in either the "mailbox"
       or the "host" strings in the destination address.

       To actually send the message the mailer program must convert the host
       string into the physical address to which to transmit the message.
       This name-to-address conversion is typically done by looking the name
       up in a table and finding the physical address in another field of
       that table entry.  This means that all the compound and organization
       names (and any other alternate names or synonyms) must also be in the
       host table.

    HIDDEN HOSTS

       Sometimes the mailbox part of the destination address is a compound
       name and is used to mark a set of mailboxes which are not really on
       the host at all, but rather on another host which is connected to
       this host in a non-standard way.

       It is important to users of computer mail that replies to messages
       may be easily composed with automatic assistance from the mail
       processing programs.  To preserve this capability it is important
       that a host understand the mailbox part of every address in every
       message it sends if the host part of the address is itself.

       That is, for every message, in every header field, in every address
       "m@h", host h must understand all values of m.  Thus when a host
       prepares a message it should check all the addresses that appear in
       the header and for any address whose host part is this host the
       mailbox part should be verified.

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    September 1980                                                   RFC 771
    Mail Transition Plan

    THE TRANSITION PLAN

       The basic ground rules for the transition are:

          1.  ARPANET mailbox names must continue to work correctly.

          2.  No changes should be required to mail editor software which
          parses message headers to compose replies and the like.
          Specifically,  non-ARPANET mailbox designators must be
          accommodated without change to the parsing and checking mechanisms
          of mail processing programs.

          3.  Automatic forwarding of messages between NCP and TCP
          environments without user (or operator) intervention.

       For the communication of messages between NCP and TCP hosts a mail
       relay service will be provided on a few hosts that implement both TCP
       and NCP.  These will be "well known" in the same sense that sockets
       or ports for contacting Telnet or FTP servers are well known.

       To make use of these relay servers changes will be made to the mailer
       programs.  The mailer program will be responsible for determining if
       the destination address of the message is directly reachable via the
       interprocess communication system it has available (TCP or NCP or
       both), or if the mail must be relayed.  If the mail must be relayed,
       the mailer must choose a relay server and transmit the message to it.

       The basis for the decision the mailer must make is an expanded host
       name table.  There will be a table which translates host names to
       physical addresses.  The physical addresses in this table will be the
       32-bit Internet addresses. (This makes sense for even NCP-only hosts,
       since after 1 January 1981 even they must use 96-bit leader format
       which requires 24-bit ARPANET physical addresses).  Each entry in
       this table will also have some flag bits.

       The flag bits will include information to indicate if the host in
       this entry is (1) a  NCP host with "old tables", (2) a NCP host with
       "new tables", (3) a TCP host, or (4) some other kind of host.  All
       TCP hosts are assumed to have "new tables".  "Old tables" are those
       without these flag bits, while "new tables" do have these flags.

       A separate table may be useful to list the addresses of the hosts
       with relay servers.

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    RFC 771                                                   September 1980
                                                        Mail Transition Plan

       When a message is sent to a relay server, the control information (in
       the argument of the mail transfer command) must be augmented to
       include the destination host identifier.

       The relay server may accept messages to be relayed without knowing
       that destination mailbox is actually reachable.  This means that it
       may later discover that the destination mailbox does not exist (or
       some other condition prevents mail delivery).  To be able to report
       the error to the originating user, the mailbox (mailbox@host) of the
       originating user must be included in the argument of the mail
       transfer command.  If the argument does not contain the address of
       the originating user no error response is attempted.  The error
       report, which is itself a message, does not carry an originator
       address in the command argument to avoid the possibility of a endless
       chain of error reports (however, an originator address does appear
       the header).

       Since the originating host will act as if the mail was successfully
       delivered when it is accepted by the relay server, it deletes any
       back up copies of the message it was keeping in case of errors.  For
       this reason, the relay server must include the complete message in
       any error report it sends to the originator.  The relay server should
       parse the addresses in the argument before accepting a message.  If
       it does not understand how deliver locally, or both relay and reply
       (if the originating address is present) to the message, it should not
       accept it.

       There are enough differences in the transmission procedure that the
       relay server will use a distinct mail transfer protocol, separate
       from the file transfer protocol.

    MAIL TRANSFER PROTOCOL

       The mail trasfer protocol to be used by the relay server and all TCP
       hosts is documented in reference [9].

    CONNECTIVITY

       There are nine cases of mail exchange, the three by three matrix of
       (1) old-table NCP hosts, (2) new-table NCP hosts, (3) TCP hosts.
       There are also two transfer mechanisms:  file transfer and mail
       transfer.  The diagonal is easy, each type of host can exchange mail
       with other hosts of its type.  The other cases are more subtle.

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    September 1980                                                   RFC 771
    Mail Transition Plan

       An old-table NCP host is assumed to have a table with 32-bit physical
       addresses, but no flag bits.  It has NCP and file transfer.  It does
       not have the separate mail transfer protocol.

       An new-table NCP host is assumed to have a table with 32-bit physical
       addresses, and the flag bits.  It has NCP and file transfer.  It also
       has the new separate mail transfer.

       An TCP host is assumed to have a table with 32-bit physical
       addresses, and the flag bits.  It has the new separate mail transfer.
       It probably has a file transfer, but does not use it for mail.

       1. Old-table NCP to Old-table NCP

          This transfer is direct and uses the old mechanisms -- NCP and
          file transfer.

       2. New-table NCP to Old-table NCP

          This transfer is direct and uses the old mechanisms -- NCP and
          file transfer.

       3. TCP to Old-table NCP

          This transfer must use a relay server.  The first transfer (from
          the TCP host to the relay server) is via TCP and the mail transfer
          protocol.  The second transfer (from the relay server to the
          old-table NCP) is via NCP and file transfer protocol.

       4. Old-table NCP to New-table NCP

          This transfer is direct and uses the old mechanisms -- NCP and
          file transfer.

       5. New-table NCP to New-table NCP

          This transfer is done with the NCP and the mail transfer protocol,
          that is, using the old interprocess communication system and the
          new mail transmission scheme.

       6. TCP to New-table NCP

          This transfer must use a relay server.  The first transfer (from
          the TCP host to the relay server) is via TCP and the mail transfer
          protocol.  The second transfer (from the relay server to the
          new-table NCP) is via NCP and mail transfer protocol.

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    RFC 771                                                   September 1980
                                                        Mail Transition Plan

       7. Old-table NCP to TCP

          This transfer must use a special relay server.  The first transfer
          (from the old-table NCP to the relay server) is via NCP and the
          file transfer protocol.  The second transfer (from the relay
          server to the TCP host) is via TCP and mail transfer protocol.
          This relay server must be special because the messages coming from
          the old-table NCP host will not have the destination host
          information in the command argument.  This relay server must have
          a list of registered TCP user mailboxes and their associated TCP
          host identifiers.  Since such a registry could be potentially
          large and frequently changing (and will grow as more TCP hosts
          come into existence) it will be necessary to limit the mailboxes
          on the registry.

       8. New-table NCP to TCP

          This transfer must use a relay server.  The first transfer (from
          the new-table NCP to the relay server) is via NCP and the mail
          transfer protocol.  The second transfer (from the relay server to
          the TCP host) is via TCP and mail transfer protocol.

       9. TCP to TCP

          This transfer is direct and uses the new mechanisms -- TCP and the
          mail transfer protocol.

       In general, whenever possible the new procedures are to be used.

    MULTIPLE RECIPIENTS

       A substantial portion of the mail sent is addressed to multiple
       recipients.  It would substantially cut the transmission and
       processing costs if such multiple recipient mail were transfered
       using the multiple recipient technique available for use in both the
       old file transfer protocol [12] and new mail transfer protocol [9].

       The relay servers will attempt to use a multiple recipient commands
       whenever applicable on transmitting messages, and will accept such
       commands when revceiving messages.

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    September 1980                                                   RFC 771
    Mail Transition Plan

    COMPOSITION AND READING PROGRAMS

       The impact on the mail composition and reading programs is minimal.
       If these programs use a table to recognize, complete, or verify host
       identifiers, then they must be modified to use the new table.

       To assist the user in replying to messages it will be important that
       all addresses in the header fields (TO:, CC:, etc.) be complete with
       both the mailbox and host parts.  In some cases this has not
       previously been necessary since the addresses without host parts
       could be assumed to be local to the originating host, and the sending
       host was recorded by the receiving host.  When the messages were sent
       directly the originating host was the sending host, but when messages
       are relayed the originating host will not be the host sending the
       mail to the destination host.

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    RFC 771                                                   September 1980
                                                        Mail Transition Plan

    REFERENCES

       [1]     Cerf, V., "The Catenet Model for Internetworking," IEN 48,
               DARPA/IPTO, July 1978.

       [2]     Postel, J., "Internet Protocol," RFC 760, USC/Information
               Sciences Institute, NTIS ADA079730, January 1980.

       [3]     Postel, J., "Transmission Control Protocol," RFC 761,
               USC/Information Sciences Institute, NTIS ADA082609,
               January 1980.

       [4]     Postel, J., "Telnet Protocol Specification," RFC 764,
               USC/Information Sciences Institute, June 1980.

       [4]     Postel, J., "File Transfer Protocol," RFC 765,
               USC/Information Sciences Institute, June 1980.

       [5]     Postel, J., "Assigned Numbers," USC/Information Sciences
               Institute, RFC 762, January 1980.

       [6]     Postel, J., "Internet Protocol Handbook," USC/Information
               Sciences Institute, RFC 766, July 1980.

       [7]     Feinler, E. and, J. Postel, "ARPANET Protocol Handbook,"
               NIC 7104, Network Information Center, SRI International,
               January 1978.

       [8]     Crocker, D., J. Vittal, K. Pogran, and, D. Henderson,
               "Standards for the Format of ARPA Network Text Messages,"
               RFC 733 7104, Network Information Center, SRI International,
               November 1977.

       [9]     Sluizer, S. and, J. Postel, "Mail Transfer Protocol,"
               USC/Information Sciences Institute, RFC rrr, September 1980.

       [10]    Postel, J., "Internet Message Protocol," USC/Information
               Sciences Institute, RFC 759, August 1980.

       [11]    Postel, J., "A Structured Format for Transmission of
               Multi-Media Documents," USC/Information Sciences Institute,
               RFC 767, August 1980.

       [12]    Harrenstien, K., "FTP Extension: XRSQ/XRCP,"
               SRI International, RFC 743, December 1977.

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