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Network Working Group                                    V. Cerf  (ARPA)Request for Comments: 771                                J. Postel (ISI)                                                          September 1980MAIL TRANSITION PLANPREFACE   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.                                   1

September 1980RFC 771Mail Transition PlanMODEL 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                                   2

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.                                   3

September 1980RFC 771Mail Transition PlanTHE 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.                                   4

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.                                   5

September 1980RFC 771Mail 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.                                   6

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.                                   7

September 1980RFC 771Mail Transition PlanCOMPOSITION 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.                                   8

RFC 771                                                   September 1980                                                    Mail Transition PlanREFERENCES   [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.                                   9