<pre>RFC 754                                                        J. Postel
                                                                     ISI
                                                            6 April 1979



                   <span class="h1">Out-of-Net Host Addresses for Mail</span>

There is now interest in sustantially extending the scope of the
computer mail system used in the ARPANET to allow communication of
voice, fax, graphics, as well as text information between users in
different networks as wells as within the ARPANET.

The discussion of a transition from the current ARPANET sndmsg
environment and mechanisms to a more general internet environment and
richer mechanisms must consider techniques for continued activity during
the transition.  In addition, there is a current need for a mechanism to
support the interaction of the several already existing NSW-like message
environments with the ARPANET message environment.

This memo discusses some possible alternatives for computer mail
addressing for hosts outside the ARPANET in the short term.  This memo
is hopelessly Tenex oriented in its descriptions and examples.

It helps to keep a few goals in mind while considering the alternative
solutions:

Goals:

   1) Minimum Change to Existing Software.

   2) Maximum User Acceptance.

   3) Maximum Compatibility with the future Internet Message
   Environment.

   4) Minimum Special Transition Software.

These goals are to some degree incompatible, so the evaluation should be
expected to involve a trade off.

At this point, it would be good to have a model of the current situation
and mechanisms of the ARPANET message environment.  It is assumed the
reader understands it well enough to dispense with a long description of
how a message gets from A to B.  The important thing is to note the
types of players in the picture.  There are:

   message composition (or sending) programs (e.g., Hermes, SNDMSG), in
   general there are several message composition programs for each type
   of operating system or host in the network,




Postel                                                          [page 1]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



   mailers,

   mail servers (i.e., FTP servers) that receive the mail coming into at
   host and deposit it in mailboxes,

   message processing (or reading) programs (e.g., Hermes, MSG, RD), in
   general there are several message processing programs for each type
   of operating system or host in the network,  and note that the more
   developed mail are both reading and sending programs.

Messages are transmitted as a character string to an address which is
specified "outside" the message.  The destination host ("YYY") is
specified to the sending (or user) FTP as the argument of the "open
connection" command, and the destination user ("XXX") is specified to
the receiving (or server) FTP as the argument of the "MAIL" (or "MLFL")
command.  In Tenex, when mail is queued this outside information is
saved in the file name ("[---].XXX@YYY").

The proposed solutions are briefly characterized.

Proposed Solutions:

   This first pass at describing the solutions is rather brief and
   intended to set the scene for a subsequent discussion based on
   examples.

   A) SINGLE MAILBOX

      This solution suggests that all mail for another network be routed
      to a single mailbox on a forwarding host on the ARPANET.  The FTP
      server would naturally put all the mail for this mailbox into a
      single file to be examined by a routing deamon process.  The
      routing deamon process would use information in new header lines
      to determine the actual destination.

      Format:

         Outside:  [---].NSW-MAIL@FWDR

         Inside:   To:       NSW-MAIL@FWDR
                   From:     Sam@ISIB
                   NSW-User: Joe








Postel                                                          [page 2]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



   B) GLOBAL NAMES INSIDE

      This proposal suggests that all mail for users in another network
      be sent to a single mailbox on a forwarding host.  The FTP server
      would naturally put all the mail for this mailbox into a single
      file to be examined by a routing deamon process.  The routing
      deamon process would use information in existing header lines to
      determine the actual destination.

      Format:

         Outside: [---].NSW-MAIL@FWDR

         Inside:  To:   Joe@NSW
                  From: Sam@ISIB

   C) GLOBAL NAMES OUTSIDE

      This proposal suggests that mail for users in another network be
      sent to distinct per user mailbox names on a forwarding host.  The
      FTP server would somehow put all the mail for these mailboxes into
      a single file to be examined by a routing deamon process.  The
      routing deamon process would use information in existing header
      lines to determine the actual destination.

      Format:

         Outside: [---].Joe@FWDR or [---].Joe@NSW

         Inside:  To:   Joe@NSW
                  From: Sam@ISIB

   D) STRUCTURED NAMES

      This proposal suggests that mail for users in another network be
      sent to distinct per user mailbox names on a forwarding host,
      however, these mailbox names would have a common "network" part
      and a unique "user" part.  By recognizing the common part the FTP
      server would put the mail and the mailbox name into a single file
      to be examined by a routing deamon process.  The routing deamon
      process would use mailbox name information to determine the actual
      destination.








Postel                                                          [page 3]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



      Format:

         Outside:  [---].NSW-Joe@FWDR

         Inside:  To:   NSW-Joe@FWDR
                  From: Sam@ISIB

Before further examination of the advantages and disadvantages of these
proposals, it would be well to have some more detailed criteria in mind
to help expose the degree to which the goals are met.

Criteria:

   1) What changes are needed?

   2) How many instances of the change need to be implemented?

   3) What information does the routing deamon use?

   4) How does the "answer" command work?

   5) How is the name space used?

   It is particularly instructive to work through examples with a
   mixture of mailbox destinations in the ARPANET and other networks in
   each of the "To:" and "CC:" fields and to see what happens when one
   wants to send an answer to all, just the "To:", or just the "CC:", or
   just the "From:" or "Sender:" mailboxes.

Solutions Reconsidered:

   It is easier to talk about these things in terms of examples.  In the
   following "NSW" is an example of a network name.  "FWDR" is a host
   name, or nickname for the forwarding host.  Also note that for all of
   these solutions it is assumed that host tables can have alternate or
   nicknames for hosts, e.g., FWDR could map to 86 while ISI also maps
   to 86, although this is not essential.

   In addition, all these solutions provide a single forwarding point
   from the ARPANET into the destination net.

   All forwarded messages are handled by a routing deamon which lives in
   the FWDR host.

   Also note that the information shown as the "outside" information is
   the Tenex representation.  The key thing is the mailbox argument
   value that is passed to the FTP server is the one in the string



Postel                                                          [page 4]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



   "[---].XXX@YYY", not anything from the header.  Only the string "XXX"
   is passed to the FTP server.

   A) SINGLE MAILBOX

      Example:

         Outside:  [---].NSW-MAIL@FWDR

         Inside:   To:       NSW-MAIL@FWDR,Bill@ISIA
                   CC:       Jeff@ISIB
                   From:     Joe@ISIB
                   NSW-User-To: SAM,Fred
                   NSW-User-CC: Bob,Mike

         or

         Outside:  [---].NSW-MAIL@FWDR

         Inside:   To:       NSW-MAIL@FWDR,Bill@ISIA
                   CC:       Jeff@ISIB
                   From:     NSW-MAIL@FWDR
                   NSW-User-To: SAM,Fred
                   NSW-User-CC: Bob,Mike
                   NSW-User-From: Paul

      Every mail composition program has to change to make it easy for
      users to put the "NSW-User:" line in the header.  Every mail
      reading program has to change to notice and make use of this line.
      In an "answer" command the mail processing program has to know to
      copy this line into the answer message.  The deamon has to examine
      the inside message header to find the "NSW-User:" line and forward
      the message to the users listed there.  If there is a message that
      has both NSW and ARPANET mailboxes in both the "To:" and "CC:"
      lines, then it seems there must be both a "NSW-Users-To:" and a
      "NSW-Users-CC:" lines if it is to be possible to send an answer to
      just the users in the "To:" lines.  If there is another network,
      e.g. PRNET, then another set of header lines must be introduced,
      e.g. PRNET-USER-To: etc., that is up to four new lines per network
      (To, CC, From, Sender).

      This solution has the advantage of saving some transmissions:
      when several of the destination mailboxes are in NSW, the sending
      program sends just one copy to the FWDR and routing deamon, the
      routing deamon sends copies to all NSW users it finds.  If this is
      not done, the deamon would have difficulty avoiding sending
      multiple copies to each destination user.



Postel                                                          [page 5]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



      A problem arises about acknowledgements of mail receipt.  First
      the normal ARPANET message delivery mechanisms will say the mail
      is delivered when the FTP server puts the mail in the file for the
      routing deamon to examine.  Second if the routing deamon discovers
      an message is to be forwarded to a nonexistent user, care must be
      used to notify the original sender unambiguously.

      Changes:

         all composition programs

   B) GLOBAL NAMES INSIDE

      Example:

         Outside:  [---].NSW-MAIL@FWDR

         Inside:   To:       Joe@NSW, Bill@ISIA, Fred@NSW
                   CC:       Mike@NSW, Paul@NSW, John@ISIB
                   From:     Sam@ISIB

      Every mail composition program has to know that NSW is a very
      special host name, for which it uses a different mailbox argument
      and sends to the FWDR host.  The FTP server naturally puts all the
      NSW mail into a single mailbox file which the routing deamon
      examines.  The "answer" command works fine.  The routing deamon
      has to look at the inside header to determine where to forward the
      messages.  It has to check the "To:" and "CC:" lines.

      The sending programs must also send just one copy to the FWDR and
      routing deamon, the routing deamon will send copies to all NSW
      users it finds.  If this is not done, the deamon would have
      difficulty avoiding sending multiple copies to each destination
      user.  This is an advantage in terms of number of transmissions.

      A problem arises about acknowledgements of mail receipt.  First
      the normal ARPANET message delivery mechanisms will say the mail
      is delivered when the FTP server puts the mail in the file for the
      routing deamon to examine.  Second if the routing deamon discovers
      an message is to be forwarded to a nonexistent user, care must be
      used to notify the original sender unambiguously.

      Changes:

         all sending programs





Postel                                                          [page 6]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



   C) GLOBAL NAMES OUTSIDE

      Example:

         Outside:  [---].Joe@NSW

         Inside:   To:       Joe@NSW, Bill@ISIA, Fred@NSW
                   CC:       Mike@NSW, Paul@NSW, John@ISIB
                   From:     Sam@ISIB

      No changes to mail composition or processing programs are needed.
      The FTP server has to put all the NSW users mail into a single
      mailbox file which the routing deamon examines.  The cheapest way
      to do this is to put all the names of the NSW users in the ARPANET
      user forwarding file with the same destination ARPANET mailbox.
      This means the local users of the FWDR host and the users in the
      destination networks share the name space for user names.  The
      routing deamon has to look at the inside header to determine where
      to forward the messages.  It has to check the "To:" and "CC:"
      lines.

      This appears to be the solution with the minimum change to
      existing software.  The "answer" command works fine.

      There is a problem with the name space, for example, if ISIA
      serves as FWDR host, then Fred@ISI and Fred@NSW cannot co-exist.
      Further, there is the database update problem.  Every time a new
      user is added to NSW or any of the hosts in any of the nets that
      the FWDR host serves the forwarding file at the FWDR host has to
      be updated.  The names added have to be unique so all user names
      assigned in NSW and all the hosts on all the networks served by
      the same FWDR host have to be oked by the "forwarding file data
      base administrator" before they can actually be used.  Also note
      that Fred@NSW and Fred@PRNET cannot be routed through the same
      FWDR host.

      This doesn't work too well, if the sending programs are not
      changed they will send one copy of this message for each NSW user
      and all these copies will end up in the file to be examined by the
      routing deamon.  If the FTP server code is not changed the outside
      information will be lost and the routing deamon will have no idea
      which NSW user this copy is for.  To do the job right with the
      information available the routing deamon would have to keep a
      substantial record about each message it handled checking to see
      if it received for, and send a copy to, each intended destination
      user.




Postel                                                          [page 7]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



      A problem arises about acknowledgements of mail receipt.  First
      the normal ARPANET message delivery mechanisms will say the mail
      is delivered when the FTP server puts the mail in the file for the
      routing deamon to examine.  Second if the routing deamon discovers
      an message is to be forwarded to a nonexistent user, care must be
      used to notify the original sender unambiguously.

      Changes:

         ARPANET user forwarding file at FWDR host

   D) STRUCTURED NAMES

      Example:

         Outside:  [---].NSW-Joe@NSW

         Inside:   To:       NSW-Joe@NSW, Bill@ISIA, NSW-Fred@NSW
                   CC:       NSW-Mike@NSW, NSW-Paul@NSW, John@ISIB
                   From:     Sam@ISIB

      No changes to mail composition or processing programs are needed.
      The FTP server has to put all the NSW-x users mail into a single
      file which the routing deamon examines.  The FTP server can do
      this on the recognition of the "NSW-" prefix without knowing all
      the legal individual users.  In addition the FTP server puts the
      mailbox argument into the file with the message.  This is
      necessary to avoid the loss of the "outside" information.  The
      routing deamon can then look at the mailbox argument to determine
      where to forward the messages.  It need not look at the inside of
      the message at all.  The "answer" command works fine.

      A problem arises about acknowledgements of mail receipt.  First
      the normal ARPANET message delivery mechanisms will say the mail
      is delivered when the FTP server puts the mail in the file for the
      routing deamon to examine.  However, if the routing deamon
      discovers an message is to be forwarded to a nonexistent user, the
      deamon can easily tell the original sender the exact destination
      user that is unreachable.

      Changes:

         FTP server at FWDR host







Postel                                                          [page 8]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



Summary:

                                 A         B        C        D
                               Single    Global   Global   Structured
                               Mailbox   Names    Names    Names
                                         Inside   Outside

   Criteria:

      1) What changes?         Composer  Composer None      FTP server

      2) How many?             100       100      0         1

      3) Routing information?  New       Old      Old       Old
                               Inside    Inside   Inside    Outside

      4) "Answer" command?     Changes   Same     Same      Same

      5) ARPANET name space    1 per     1 per    1 per     1 per
         use?                  FWDR      FWDR     user      user

   Goals:

      1) Software Change       Bad       Bad      Good      Good

      2) User Acceptance       Bad       Good     Good      Poor

      3) Future Compatibility  Bad       Poor     Poor      Fair

      4) Transition Software   Fair      Good     Bad       Good

   Conclusions:

      Solution D is recommended.

      Only solution D is based on the use of strictly "outside"
      information.  Please note that the existing ARPANET message
      DELIVERY system is based strictly on the use of "outside"
      information only.  Also note that the problems that keep coming up
      in ARPANET message processing &amp; composition programs have to do
      with the different possibilities for syntax (and semanitcs) of the
      "inside" information.  This is a major advantage of solution D.








Postel                                                          [page 9]</pre>
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<a href="./rfc754">RFC 754</a>                                                     6 April 1979
Out-of-Net Host Addresses for Mail



      Please note that the syntax NET-USER@FWDR in the examples is not
      the only form that could be used.  Any of the following (or even
      others) would be fine:

         Net-User@FWDR       User-Net@FWDR
         Net/User@FWDR       User/Net@FWDR
         Net.User@FWDR       User.Net@FWDR
         Net.and.User@FWDR   User.on.Net@FWDR










































Postel                                                         [page 10]
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