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Network Working Group J. Houttuin Request for Comments: 1506 RARE Secretariat RARE Technical Report: 6 August 1993

      A Tutorial on Gatewaying between X.400 and Internet Mail

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard.  Distribution of this memo is


 There are many ways in which X.400 and Internet (STD 11, RFC 822)
 mail systems can be interconnected. Addresses and service elements
 can be mapped onto each other in different ways. From the early
 available gateway implementations, one was not necessarily better
 than another, but the sole fact that each handled the mappings in a
 different way led to major interworking problems, especially when a
 message (or address) crossed more than one gateway. The need for one
 global standard on how to implement X.400 - Internet mail gatewaying
 was satisfied by the Internet Request For Comments 1327, titled
 "Mapping between X.400(1988)/ISO 10021 and RFC 822."
 This tutorial was produced especially to help new gateway managers
 find their way into the complicated subject of mail gatewaying
 according to RFC 1327. The need for such a tutorial can be
 illustrated by quoting the following discouraging paragraph from RFC
 1327, chapter 1: "Warning: the remainder of this specification is
 technically detailed. It will not make sense, except in the context
 of RFC 822 and X.400 (1988). Do not attempt to read this document
 unless you are familiar with these specifications."
 The introduction of this tutorial is general enough to be read not
 only by gateway managers, but also by e-mail managers who are new to
 gatewaying or to one of the two e-mail worlds in general. Parts of
 this introduction can be skipped as needed.
 For novice end-users, even this tutorial will be difficult to read.
 They are encouraged to use the COSINE MHS pocket user guide [14]
 To a certain extent, this document can also be used as a reference
 guide to X.400 <-> RFC 822 gatewaying. Wherever there is a lack of
 detail in the tutorial, it will at least point to the corresponding
 chapters in other documents. As such, it shields the RFC 1327 novice

RARE Working Group on Mail and Messaging (WG-MSG) [Page 1] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 from too much detail.


 This tutorial is heavily based on other documents, such as [2], [6],
 [7], [8], and [11], from which large parts of text were reproduced
 (slightly edited) by kind permission from the authors.
 The author would like to thank the following persons for their
 thorough reviews: Peter Cowen (Nexor), Urs Eppenberger (SWITCH), Erik
 Huizer (SURFnet), Steve Kille (ISODE Consortium), Paul Klarenberg
 (NetConsult), Felix Kugler (SWITCH), Sabine Luethi.


 This document is not everywhere exact and/or complete in describing
 the involved standards. Irrelevant details are left out and some
 concepts are simplified for the ease of understanding. For reference
 purposes, always use the original documents.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 2] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

Table of Contents

     1. An overview of relevant standards ........................   4
       1.1. What is X.400 ? ......................................   5
       1.2. What is an RFC ? .....................................   8
       1.3. What is RFC 822 ? ....................................   9
       1.4. What is RFC 1327 ? ...................................  11
     2. Service Elements .........................................  12
     3. Address mapping ..........................................  14
       3.1. X.400 addresses ......................................  15
         3.1.1. Standard Attributes ..............................  15
         3.1.2. Domain Defined Attributes ........................  17
         3.1.3. X.400 address notation ...........................  17
       3.2. RFC 822 addresses ....................................  19
       3.3. RFC 1327 address mapping .............................  20
         3.3.1. Default mapping ..................................  20
  X.400 -> RFC 822 .............................  20
  RFC 822 -> X.400 .............................  22
         3.3.2. Exception mapping ................................  23
  PersonalName and localpart mapping ...........  25
  X.400 domain and domainpart mapping ..........  26
    X.400 -> RFC 822 .........................  27
    RFC 822 -> X.400 .........................  28
       3.4. Table co-ordination ..................................  31
       3.5. Local additions ......................................  31
       3.6. Product specific formats .............................  32
       3.7. Guidelines for mapping rule definition ...............  34
     4. Conclusion ...............................................  35
     Appendix A. References ......................................  36
     Appendix B. Index  (Only available in the Postscript version)  37
     Appendix C. Abbreviations ...................................  37
     Appendix D. How to access the MHS Co-ordination Server ......  38
     Security Considerations .....................................  39
     Author's Address ............................................  39

RARE Working Group on Mail and Messaging (WG-MSG) [Page 3] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

1. An overview of relevant standards

 This chapter describes the history, status, future, and contents of
 the involved standards.
 There is a major difference between mail systems used in the USA and
 Europe. Mail systems originated mainly in the USA, where their
 explosive growth started as early as in the seventies. Different
 company-specific mail systems were developed simultaneously, which,
 of course, led to a high degree of incompatibility. The Advanced
 Research Projects Agency (ARPA), which had to use machines of many
 different manufacturers, triggered the development of the Internet
 and the TCP/IP protocol suite, which was later accepted as a standard
 by the US Department of Defense (DoD). The Internet mail format is
 defined in STD 11, RFC 822 and the protocol used for exchanging mail
 is known as the simple mail transfer protocol (SMTP) [1]. Together
 with UUCP and the BITNET protocol NJE, SMTP has become one of the
 main de facto mail standards in the US.
 Unfortunately, all these protocols were incompatible, which explains
 the need to come to an acceptable global mail standard.  CCITT and
 ISO began working on a norm and their work converged in what is now
 known as the X.400 Series Recommendations. One of the objectives was
 to define a superset of the existing systems, allowing for easier
 integration later on. Some typical positive features of X.400 are the
 store-and-forward mechanism, the hierarchical address space and the
 possibility of combining different types of body parts into one
 message body.
 In Europe, the mail system boom came later. Since there was not much
 equipment in place yet, it made sense to use X.400 as much as
 possible right from the beginning. A strong X.400 lobby existed,
 especially in West-Germany (DFN). In the R&D world, mostly EAN was
 used because it was the only affordable X.400 product at that time
 (Source-code licenses were free for academic institutions).
 At the moment, the two worlds of X.400 and SMTP are moving closer
 together. For instance, the United States Department of Defense, one
 of the early forces behind the Internet, has decided that future DoD
 networking should be based on ISO standards, implying a migration
 from SMTP to X.400. As an important example of harmonisation in the
 other direction, X.400 users in Europe have a need to communicate
 with the Internet. Due to the large traffic volume between the two
 nets it is not enough interconnecting them with a single
 international gateway.  The load on such a gateway would be too
 heavy. Direct access using local gateways is more feasible.
 Although the expected success of X.400 has been a bit disappointing

RARE Working Group on Mail and Messaging (WG-MSG) [Page 4] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 (mainly because no good products were available), many still see the
 future of e-mail systems in the context of this standard.
 And regardless if in the long run X.400 will or will not take over
 the world of e-mail systems, SMTP cannot be neglected over the next
 ten years. Especially the simple installation procedures and the high
 degree of connectivity will contribute to a growing number of RFC 822
 installations in Europe and world-wide in the near future.

1.1. What is X.400 ?

 In October 1984, the Plenary Assembly of the CCITT accepted a
 standard to facilitate international message exchange between
 subscribers to computer based store-and-forward message services.
 This standard is known as the CCITT X.400 series recommendations
 ([16], from now on called X.400(84)) and happens to be the first
 CCITT recommendation for a network application. It should be noted
 that X.400(84) is based on work done in the IFIP Working Group 6.5,
 and that ISO at the same time was proceeding towards a compatible
 document. However, the standardisation efforts of CCITT and ISO did
 not converge in time (not until the 1988 version), to allow the
 publication of a common text.
 X.400(84) triggered the development of software implementing (parts
 of) the standard in the laboratories of almost all major computer
 vendors and many software houses. Similarly, public carriers in many
 countries started to plan X.400(84) based message systems that would
 be offered to the users as value added services. Early
 implementations appeared shortly after first drafts of the standard
 were published and a considerable number of commercial systems are
 available nowadays.
 X.400(84) describes a functional model for a Message Handling System
 (MHS) and associates services and protocols. The model illustrated in
 Figure 1.1. defines the components of a distributed messaging system.
 Users in the MHS environment are provided with the capability of
 sending and receiving messages. Users in the context of an MHS may be
 humans or application processes. The User Agent (UA) is a process
 that makes the services of the MTS available to the user. A UA may be
 implemented as a computer program that provides utilities to create,
 send, receive and perhaps archive messages. Each UA, and thus each
 user, is identified by a name (each user has its own UA).

RARE Working Group on Mail and Messaging (WG-MSG) [Page 5] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

  1. —————————————————————-

| user user Message Handling Environment|

  |                 |            |                                |
  |     ----------------------------------------------------------|
  |     |           |            |    Message Handling System    ||
  |     |         ----          ----                             ||
  |     |         |UA|          |UA|                             ||
  |     |         ----          ----                             ||
  |     |           |             |                              ||
  |     |       -------------------------------------------------||
  |     |       |   |             |   Message Transfer System   |||
  |     | ----  |  -----         -----                          |||
  |user-|-|UA|--|--|MTA|         |MTA|                          |||
  |     | ----  |  -----         -----                          |||
  |     |       |    \             /                            |||
  |     |       |     \           /                             |||
  |     |       |      \         /                              |||
  |     |       |       \       /                               |||
  |     |       |        \     /                                |||
  |     | ----  |         -----                                 |||
  |user-|-|UA|--|---------|MTA|                                 |||
  |     | ----  |         -----                                 |||
  |     |       -------------------------------------------------||
  |     ----------------------------------------------------------|
                  Fig. 1.1. X.400 functional model
 The Message Transfer system (MTS) transfers messages from an
 originating UA to a recipient UA. As implied by the Figure 1.1, data
 sent from UA to UA may be stored temporarily in several intermediate
 Message Transfer Agents (MTA), i.e., a store-and- forward mechanism
 is being used. An MTA forwards received messages to a next MTA or to
 the recipient UA.
 X.400(84) divides layer 7 of the OSI Reference Model into 2
 sublayers, the User Agent Layer (UAL) and the Message Transfer Layer
 (MTL) as shown in the Figure 1.2.
 The MTL is involved in the transport of messages from UA to UA, using
 one or several MTAs as intermediaries. By consequence, routing issues
 are entirely dealt with in the MTL. The MTL in fact corresponds to
 the postal service that forwards letters consisting of an envelope
 and a content. Two protocols, P1 and P3, are used between the MTL
 entities (MTA Entity (MTAE), and Submission and Delivery Entity
 (SDE)) to reliably transport messages. The UAL embodies  peer UA
 Entities (UAE), which interpret the content of a message and offer
 specific services to the application process.  Depending on the
 application to be supported on top of the MTL, one of several end-

RARE Working Group on Mail and Messaging (WG-MSG) [Page 6] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 to-end protocols (Pc) is used between UAEs. For electronic mail,
 X.400(84) defines the protocol P2 as part of the InterPersonal
 Messaging Service (IPMS). Conceivably other UAL protocols may be
 defined, e.g., a protocol to support the exchange of electronic
 business documents.
  1. ————————————————————-
    1. —- —–

UA layer |UAE|←—- P2, Pc ———–>|UAE|

  1. —- —–
  2. ————————————————————-
  3. —– —— —–

MTA layer |MTAE|←- P1 –>|MTAE|←- P3–>|SDE|

  1. —– —— —–
  2. ————————————————————-

xxxE = xxx Entity ; SDE = Submission & Delivery Entity

  1. ————————————————————-

Fig. 1.2. X.400 Protocols

 The structure of an InterPersonal Message (IPM) can be visualised as
 in Figure 1.3. (Note that the envelope is not a part of the IPM; it
 is generated by the MTL).
  Message                                                 IP-message
  -                     ----------      --- ----------    -
  |  message-           |envelope|     /    | PDI    |    |
  |  content   IPM      ----------    /     ----------    |
  |  -         -        ----------   /      ----------    |
  |  |         |  IPM-  |heading |  /       |heading |    |
  |  |         |  body  ---------- /        ----------    |
  |  |         |  -     ----------/         ----------    |
  |  |         |  |     |bodypart|          |bodypart|    |
  |  |         |  |     ----------\         ----------    |
  |  |         |  |     ---------- \        ----------    |
  |  |         |  |     |bodypart|  \       |bodypart|    |
  |  |         |  |     ----------   \      ----------    |
  |  |         |  |          .        \                   |
  |  |         |  |          .         \                  |
  |  |         |  |     ----------      \   ----------    |
  |  |         |  |     |bodypart|       \  |bodypart|    |
  -  -         -  -     ----------        - ----------    -
                                    (PDI = Previous Delivery Info.)
                  Fig. 1.3. X.400 message structure
 An IPM heading contains information that is specific for an
 interpersonal message like 'originator', 'subject', etc. Each
 bodypart can contain one information type, text, voice or as a

RARE Working Group on Mail and Messaging (WG-MSG) [Page 7] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 special case, a forwarded message. A forwarded message consists of
 the original message together with Previous Delivery Information
 (PDI), which is drawn from the original delivery envelope.
 Early experience with X.400(84) showed that the standard had various
 shortcomings. Therefore CCITT, in parallel with ISO, corrected and
 extended the specification during its 1984 to 1988 study period and
 produced a revised standard [17], which was accepted at the 1988
 CCITT Plenary Meeting [10].  Amongst others, X.400(88) differs from
 X.400(84) in that it defines a Message Store (MS), which can be seen
 as a kind of database for messages. An MS enables the end-user to run
 a UA locally, e.g., on a PC, whilst the messages are stored in the
 MS, which is co-located with the MTA. The MTA can thus always deliver
 incoming messages to the MS instead of to the UA. The MS can even
 automatically file incoming messages according to certain criteria.
 Other enhancements in the 88 version concern security and
 distribution lists.

1.2. What is an RFC ?

 The Internet, a loosely-organised international collaboration of
 autonomous, interconnected networks, supports host-to-host
 communication through voluntary adherence to open protocols and
 procedures defined by Internet Standards. There are also many
 isolated internets, i.e., sets of interconnected networks, that are
 not connected to the Internet but use the Internet Standards. The
 architecture and technical specifications of the Internet are the
 result of numerous research and development activities conducted over
 a period of two decades, performed by the network R&D community, by
 service and equipment vendors, and by government agencies around the
 In general, an Internet Standard is a specification that is stable
 and well-understood, is technically competent, has multiple,
 independent, and interoperable implementations with operational
 experience, enjoys significant public support, and is recognisably
 useful in some or all parts of the Internet.
 The principal set of Internet Standards is commonly known as the
 "TCP/IP protocol suite". As the Internet evolves, new protocols and
 services, in particular those for Open Systems Interconnection (OSI),
 have been and will be deployed in traditional TCP/IP environments,
 leading to an Internet that supports multiple protocol suites.
 The following organisations are involved in setting Internet
 Internet standardisation is an organised activity of the Internet

RARE Working Group on Mail and Messaging (WG-MSG) [Page 8] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 Society (ISOC). The ISOC is a professional society that is concerned
 with the growth and evolution of the world-wide Internet, with the
 way in which the Internet is and can be used, and with the social,
 political, and technical issues that arise as a result.
 The Internet Engineering Task Force (IETF) is the primary body
 developing new Internet Standard specifications. The IETF is composed
 of many Working Groups, which are organised into areas, each of which
 is co-ordinated by one or more Area Directors.
 The Internet Engineering Steering Group (IESG) is responsible for
 technical management of IETF activities and the approval of Internet
 standards specifications, using well-defined rules. The IESG is
 composed of the IETF Area Directors, some at-large members, and the
 chairperson of the IESG/IETF.
 The Internet Architecture Board (IAB) has been chartered by the
 Internet Society Board of Trustees to provide quality control and
 process appeals for the standards process, as well as external
 technical liaison, organizational oversight, and long-term
 architectural planning and research.
 Any individual or group (e.g., an IETF or RARE working group) can
 submit a document as a so-called Internet Draft. After the document
 is proven stable, the IESG may turn the Internet-Draft into a
 "Requests For Comments" (RFC). RFCs cover a wide range of topics,
 from early discussion of new research concepts to status memos about
 the Internet. All Internet Standards (STDs) are published as RFCs,
 but not all RFCs specify standards. Another sub-series of the RFCs
 are the RARE Technical Reports (RTRs).
 As an example, this tutorial also started out as an Internet-Draft.
 After almost one year of discussions and revisions it was approved by
 the IESG as an Informational RFC.
 Once a document is assigned an RFC number and published, that RFC is
 never revised or re-issued with the same number. Instead, a revision
 will lead to the document being re-issued with a higher number
 indicating that an older one is obsoleted.

1.3. What is RFC 822 ?

 STD 11, RFC 822 defines a standard for the format of Internet text
 messages. Messages consist of lines of text. No special provisions
 are made for encoding drawings, facsimile, speech, or structured
 text. No significant consideration has been given to questions of
 data compression or to transmission and storage efficiency, and the
 standard tends to be free with the number of bits consumed. For

RARE Working Group on Mail and Messaging (WG-MSG) [Page 9] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 example, field names are specified as free text, rather than special
 terse codes.
 A general "memo" framework is used. That is, a message consists of
 some information in a rigid format (the 'headers'), followed by the
 main part of the message (the 'body'), with a format that is not
 specified in STD 11, RFC 822. It does define the syntax of several
 fields of the headers section; some of these fields must be included
 in all messages.
 STD 11, RFC 822 is used in conjunction with a number of different
 message transfer protocol environments (822-MTSs).
  1. SMTP Networks: On the Internet and other TCP/IP networks,

STD 11, RFC 822 is used in conjunction with two other

        standards: STD 10, RFC 821, also known as Simple Mail
        Transfer Protocol (SMTP) [1], and RFCs 1034 and 1035
        which specify the Domain Name System [3].
  1. UUCP Networks: UUCP is the UNIX to UNIX CoPy protocol, which

is usually used over dialup telephone networks to provide a

        simple message transfer mechanism.
  1. BITNET: Some parts of Bitnet and related networks use STD

11, RFC 822 related protocols, with EBCDIC encoding.

  1. JNT Mail Networks: A number of X.25 networks, particularly

those associated with the UK Academic Community, use the JNT

        (Joint Network Team) Mail Protocol, also known as Greybook.
 STD 11, RFC 822 is based on the assumption that there is an
 underlying service, which in RFC 1327 is called the 822-MTS service.
 The 822-MTS service provides three basic functions:
      1. Identification of a list of recipients.
      2. Identification of an error return address.
      3. Transfer of an RFC 822 message.
 It is possible to achieve 2) within the RFC 822 header.  Some 822-
 MTS protocols, in particular SMTP, can provide additional
 functionality, but as these are neither mandatory in SMTP, nor
 available in other 822-MTS protocols, they are not considered here.
 Details of aspects specific to two 822-MTS protocols are given in
 Appendices B and C of RFC 1327. An RFC 822 message consists of a
 header, and content which is uninterpreted ASCII text. The header is
 divided into fields, which are the protocol elements. Most of these
 fields are analogous to P2 heading fields, although some are
 analogous to MTS Service Elements.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 10] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

1.4. What is RFC 1327 ?

 There is a large community using STD 11, RFC 822 based protocols for
 mail services, who will wish to communicate with users of the
 InterPersonal Messaging Service (IPMS) provided by X.400 systems, and
 the other way around. This will also be a requirement in cases where
 RFC 822 communities intend to make a transition to use X.400 (or the
 other way around, which also happens), as conversion will be needed
 to ensure a smooth service transition.
 The basic function of a mail gateway can be described as follows:
 receive a mail from one mail world, translate it into the formats of
 the other mail world and send it out again using the routing rules
 and protocols of that other world.
 Especially if a message crosses more than one gateway, it is
 important that all gateways have the same understanding of how things
 should be mapped. A simple example of what could go wrong otherwise
 is the following: A sends a message to B through a gateway and B's
 reply to A is being routed through another gateway.
 If the two gateways don't use the same mappings, it can be expected
 that the From and To addresses in the original mail and in the answer
 don't match, which is, to say the least, very confusing for the end-
 users (consider what happens if automated processes communicate via
 mail). More serious things can happen to addresses if a message
 crosses more than one gateway on its way from the originator to the
 recipient. As a real-life example, consider receiving a message from:
    Mary Plork <MMP_+a_ARG_+lMary_Plork+r%MHS+d_A0CD8A2B01F54FDC->
 This is not what you would call user-friendly addressing.... RFC 1327
 describes a set of mappings that will enable a more transparent
 interworking between systems operating X.400 (both 84 and 88) and
 systems using RFC 822, or protocols derived from STD 11, RFC 822.
 RFC 1327 describes all mappings in term of X.400(88). It defines how
 these mappings should be applied to X.400(84) systems in its Appendix
 Some words about the history of RFC 1327: It started out in June
 1986, when RFC 987 defined for X.400(84) what RFC 1327 defines for
 X.400(84 and 88). RFC 1026 specified a number of additions and
 corrections to RFC 987. In December 1989, RFC 1138, which had a very
 short lifetime, was the first one to deal with X.400(88). It was
 obsoleted by RFC 1148 in March 1990. Finally, in May 1992, RFC 1327
 obsoleted all of its ancestors.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 11] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

2. Service Elements

 Both RFC 822 and X.400 messages consist of certain service elements
 (such as 'originator' and 'subject'). As long as a message stays
 within its own world, the behaviour of such service elements is well
 defined. An important goal for a gateway is to maintain the highest
 possible service level when a message crosses the boundary between
 the two mail worlds.
 When a user originates a message, a number of services are available.
 RFC 1327 describes, for each service elements, to what extent it is
 supported for a recipient accessed through a gateway.  There are
 three levels of support:
  1. Supported: Some of the mappings are quite straight-forward,

such as '822.Subject:' ↔ 'IPMS.Subject'.

  1. Not supported: There may be a complete mismatch: certain

service elements exist only in one of the two worlds (e.g.,

        interpersonal notifications).
  1. Partially supported: When similar service elements exist in

both worlds, but with slightly different interpretations,

        some tricks may be needed to provide the service over the
        gateway border.
 Apart from mapping between the service elements, a gateway must also
 map the types and values assigned to these service elements.  Again,
 this may in certain cases be very simple, e.g., 'IA5 -> ASCII'. The
 most complicated example is mapping address spaces. The problem is
 that address spaces are not something static that can be defined
 within RFC 1327. Address spaces change continuously, and they are
 defined by certain addressing authorities, which are not always
 parallel in the RFC 822 and the X.400 world. A valid mapping between
 two addresses assumes however that there is 'administrative
 equivalence' between the two domains in which the addresses exist
 (see also [13]).
 The following basic mappings are defined in RFC 1327. When going from
 RFC 822 to X.400, an RFC 822 message and the associated 822- MTS
 information is always mapped into an IPM (MTA, MTS, and IPMS
 Services). Going from X.400 to RFC 822, an RFC 822 message and the
 associated 822-MTS information may be derived from:
  1. A Report (MTA, and MTS Services)
  1. An InterPersonal Notification (IPN) (MTA, MTS, and IPMS


RARE Working Group on Mail and Messaging (WG-MSG) [Page 12] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

  1. An InterPersonal Message (IPM) (MTA, MTS, and IPMS services)
 Probes (MTA Service) have no equivalent in STD 10, RFC 821 or STD 11,
 RFC 822 and are thus handled by the gateway. The gateway's Probe
 confirmation should be interpreted as if the gateway were the final
 MTA to which the Probe was sent. Optionally, if the gateway uses RFC
 821 as an 822-MTS, it may use the results of the 'VRFY' command to
 test whether it would be able to deliver (or forward) mail to the
 mailbox under probe.
 MTS Messages containing Content Types other than those defined by the
 IPMS are not mapped by the gateway, and should be rejected at the
 Some basic examples of mappings between service elements are listed
  Service elements:
       RFC 822         X.400
       Reply-To:       IPMS.Heading.reply-recipients
       Subject:        IPMS.Heading.subject
       In-Reply-To:    IPMS.Heading.replied-to-ipm
       References:     IPMS.Heading.related-IPMs
       To:             IPMS.Heading.primary-recipients
       Cc:             IPMS.Heading.copy-recipients
  Service element types:
       RFC 822         X.400
       ASCII           PrintableString
       Boolean         Boolean
  Service element values:
       RFC 822         X.400
       oh_dear         oh(u)dear
       False           00000000
 There are some mappings between service elements that are rather
 tricky and important enough to mention in this tutorial. These are
 the mappings of origination-related headers and some envelope fields:

RARE Working Group on Mail and Messaging (WG-MSG) [Page 13] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

  RFC 822 -> X.400:
  1. If Sender: is present, Sender: is mapped to

IPMS.Heading.originator, and From: is mapped to

        IPMS.Heading.authorizing-users. If not, From: is mapped to
  X.400 -> RFC 822
  1. If IPMS.Heading.authorizing-users is present,

IPMS.Heading.originator is mapped to Sender:, and

        IPMS.Heading.authorizing-users is mapped to From: . If not,
        IPMS.Heading.originator is mapped to From:.
  Envelope attributes
  1. RFC 1327 doesn't define how to map the MTS.OriginatorName and

the MTS.RecipientName (often referred to as the P1.originator

        and P1.recipient), since this depends on which underlying 822-
        MTS is used. In the very common case that RFC 821 (SMTP) is
        used for this purpose, the mapping is normally as follows:
          MTS.Originator-name <->   MAIL FROM:
          MTS.Recipient-name  <->   RCPT TO:
 For more details, refer to RFC 1327, chapters 2.2 and 2.3.

3. Address mapping

 As address mapping is often considered the most complicated part of
 mapping between service element values, this subject is given a
 separate chapter in this tutorial.
 Both RFC 822 and X.400 have their own specific address formats. RFC
 822 addresses are text strings (e.g., ""), whereas X.400
 addresses are binary encoded sets of attributes with values. Such
 binary addresses can be made readable for a human user by a number of
 notations; for instance:
      O=a bank
 The rest of this chapter deals with addressing issues and mappings
 between the two address forms in more detail.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 14] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

3.1. X.400 addresses

 As already stated above, an X.400 address is modelled as a set of
 attributes. Some of these attributes are mandatory, others are
 optional. Each attribute has a type and a value, e.g., the Surname
 attribute has type IA5text, and an instance of this attribute could
 have the value 'Kille'. Attributes are divided into Standard
 Attributes (SAs) and Domain Defined Attributes (DDAs).
 X.400 defines four basic forms of addresses ([17], 18.5), of which
 the 'Mnemonic O/R Address' is the form that is most used, and is the
 only form that is dealt with in this tutorial. This is roughly the
 same address format as what in the 84 version was known as 'O/R
 names: form 1, variant 1' ([16] 3.3.2).

3.1.1. Standard Attributes

 Standard Attributes (SAs) are attributes that all X.400 installations
 are supposed to 'understand' (i.e., use for routing), for example:
 'country name', 'given name' or 'organizational unit'.  The most
 commonly used SAs in X.400(84) are:
      surName (S)
      givenName (G)
      initials (I*) (Zero or more)
      generationQualifier (GQ)
      OrganizationalUnits (OU1 OU2 OU3 OU4)
      OrganizationName (O)
      PrivateDomainName (PRMD)
      AdministrationDomainName (ADMD)
      CountryName (C)
 The combination of S, G, I* and GQ is often referred to as the
 PersonalName (PN).
 Although there is no hierarchy (of addressing authorities) defined by
 the standards, the following hierarchy is considered natural:
      PersonalName < OU4 < OU3 < OU2 < OU1 < O < P < A < C
 In addition to the SAs listed above, X.400(88) defines some extra
 attributes, the most important of which is
      Common Name (CN)
 CN can be used instead of or even together with PN. The problem in
 X.400(84) was that PN (S G I* GQ) was well suited to represent
 persons, but not roles and abstract objects, such as distribution

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 lists. Even though postmaster clearly is a role, not someone's real
 surname, it is quite usual in X.400(84) to address a postmaster with
 S=postmaster. In X.400(88), the same postmaster would be addressed
 with CN=postmaster .
 The attributes C and ADMD are mandatory (i.e., they must be present),
 and may not be empty. At least one of the attributes PRMD, O, OU, PN
 and CN must be present.
 PRMD and ADMD are often felt to be routing attributes that don't
 really belong in addresses. As an example of how such address
 attributes can be used for the purpose of routing, consider two
 special values for ADMD:
  1. ADMD=0; (zero) should be interpreted as 'the PRMD in this

address is not connected to any ADMD'

  1. ADMD= ; (single SPACE) should be interpreted as 'the PRMD in

this address is reachable via any ADMD in this country'. It

        is expected that ISO will express this 'any' value by means
        of a missing ADMD attribute in future versions of MOTIS.
        This representation can uniquely identify the meaning 'any',
        as a missing or empty ADMD field as such is not allowed.
 Addresses are defined in X.400 using the Abstract Syntax Notation One
 (ASN.1). X.409 defines how definitions in ASN.1 should be encoded
 into binary format. Note that the meaning, and thus the ASN.1
 encoding, of a missing attribute is not the same as that of an empty
 attribute. In addressing, this difference is often represented as
  1. PRMD=; means that this attribute is present in the address,

but its value is empty. Since this is not very useful, it's

        hardly ever used. The only examples the author knows of
        were caused by mail managers who should have had this
        tutorial before they started defining their addresses :-)
  1. PRMD=@; means that this attribute is not present in the

address. {NB. This is only necessary if an address notation

        (see 3.1.3) requires that every single attribute in the
        hierarchy is somehow listed. Otherwise, a missing attribute
        can of course be represented by simply not mentioning it.
        This means that this syntax is mostly used in mapping rules,
        not by end users.}
 Addresses that only contain SAs are often referred to as Standard
 Attribute Addresses (SAAs).

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3.1.2. Domain Defined Attributes

 Domain Defined Attributes (DDAs) can be used in addition to Standard
 Attributes. An instance of a DDA consists of a type and a value. DDAs
 are meant to have a meaning only within a certain context (originally
 this was supposed to be the context of a certain management domain,
 hence the name DDA), such as a company context.
 As an example, a company might want to define a DDA for describing
 internal telephone numbers: DDA type=phone value=9571.
 A bit tricky is the use of DDAs to encode service element types or
 values that are only available on one side of a service gateway.  The
 most important examples of such usage are defined in:
     RFC 1327 (e.g., DDA type=RFC-822 value=u(u)ser(a)
     RFC 1328 (e.g., DDA type=CommonName value=mhs-discussion-list)
 Addresses that contain both SAs and DDAs are often referred to as DDA

3.1.3. X.400 address notation

 X.400 only prescribes the binary encoding of addresses, it doesn't
 standardise how such addresses should be written on paper or what
 they should look like in a user interface on a computer screen.
 There exist a number of recommendations for X.400 address
 representation though.
  1. JTC proposed an annex to CCITT Rec. F.401 and ISO/IEC 10021-2,

called 'Representation of O/R addresses for human usage'. According

  to this proposal, an X.400 address would look as follows:
  G=jo; S=plork; O=a bank; OU1=owe; OU2=you; P=fhbo; A=ade; C=zz
    Note that in this format, the order of O and the OUs is exactly
    the opposite of what one would expect intuitively (the attribute
    hierarchy is increasing from left to right, except for the O and
    OUs, where it's right to left. The reasoning behind this is that
    this sequence is following the example of a postal address). This
    proposal has been added (as a recommendation) to the 1992 version
    of the standards.
  1. Following what was originally used in the DFN-EAN software, most

EAN versions today use an address representation similar to the JTC

  proposal, with a few differences:

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  1. natural ordering for O and OUs
  2. no numbering of OUs.
  3. allows writing ADMD and PRMD instead of A and P
  The address in the example above could, in EAN, be represented as:
  G=jo; S=plork; OU=you; OU=owe; O=a bank; PRMD=fhbo; ADMD=ade; C=zz
  This DFN-EAN format is still often referred to as _the_ 'readable
  1. The RARE Working Group on Mail and Messaging, WG-MSG, has made a

recommendation that is very similar to the DFN-EAN format, but with

  the hierarchy reversed. Further, ADMD and PRMD are used instead of
  A and P. This results in the address above being represented as:
  C=zz; ADMD=ade; PRMD=fhbo; O=a bank; OU=owe; OU=you; S=plork; G=jo
  This format is recognised by most versions of the EAN software. In
  the R&D community, this is one of the most popular address
  representations for business cards, letter heads, etc. It is also
  the format that will be used for the examples in this tutorial.
  (NB. The syntax used here for describing DDAs is as follows:
  DD.'type'='value', e.g.,
  1. RFC 1327 defines a slash separated address representation:
  /G=jo/S=plork/OU=you/OU=owe/O=a bank/P=fhbo/A=ade/C=zz/
  Not only is this format used by the PP software, it is also
  widespread for business cards and letter heads in the R&D
  1. RFC 1327 finally defines yet another format for X.400 _domains_

(not for human users):

  OU$you.OU$owe.O$a bank.P$fhbo.A$ade.C$zz
  The main advantage of this format is that it is better machine-
  parseble than the others, which also immediately implies its main
  disadvantage: it is barely readable for humans. Every attribute
  within the hierarchy should be listed, thus a missing attribute
  must be represented by the '@' sign
  (e.g., $a bank.P$@.A$ade.C$zz).
  1. Paul-Andre Pays (INRIA) has proposed a format that combines the

readability of the JTC format with the parsebility of the RFC 1327

  domain format. Although a number of operational tools within the GO-

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  MHS community are already based on (variants of) this proposal, its
  future is still uncertain.

3.2. RFC 822 addresses

 An RFC 822 address is an ASCII string of the following form:
  "domainpart" is sub-divided into
  domainpart = sdom(n).sdom(n-1)....sdom(2).sdom(1).dom
  "sdom" stands for "subdomain", "dom" stands for "top-level-domain".
  "localpart" ;is normally a login name, and thus typically is a
  surname or an abbreviation for this. It can also designate a local
  distribution list.
  The hierarchy (of addressing authorities) in an RFC 822 address is
  as follows:
      localpart < sdom(n) < sdom(n-1) <...< dom
  Some virtual real-life examples:
  In the above examples, 'nl', 'dk', and 'edu' are valid,
  registered, top level domains. Note that some networks that have
  their own addressing schemes are also reachable by way of 'RFC
  822-like' addressing. Consider the following addresses:
      oops!user          (a UUCP address)
      V13ENZACC@CZKETH5A (a BITNET address)
  These addresses can be expressed in RFC 822 format:
 Note that the domains '.uucp' and '.bitnet' have no registered
 Internet routing.  Such addresses must always be routed to a gateway
 (how this is done is outside the scope of this tutorial).
 As for mapping such addresses to X.400, there is no direct mapping

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 defined between X.400 on the one hand and UUCP and BITNET on the
 other, so they are normally mapped to RFC 822 style first, and then
 to X.400 if needed.

3.3. RFC 1327 address mapping

 Despite the difference in address formats, the address spaces defined
 by RFC 822 and X.400 are quite similar. The most important parallels
  1. both address spaces are hierarchical
  2. top level domains and country codes are often the same
  3. localparts and surnames are often the same
 This similarity can of course be exploited in address mapping
 algorithms. This is also done in RFC 1327 (NB only in the exception
 mapping algorithm. See chapter 3.3.2).
 Note that the actual mapping algorithm is much more complicated than
 shown below. For details, see RFC 1327, chapter 4.

3.3.1. Default mapping

 The default RFC 1327 address mapping can be visualised as a function
 with input and output parameters:
        address information of the gateway performing the mapping
      RFC 822 address <--->| address mapping | <---> X.400 address
 I.e., to map an address from X.400 to RFC 822 or vice versa, the only
 extra input needed is the address information of the local gateway. X.400 → RFC 822

 There are two kinds of default address mapping from X.400 to RFC 822:
 one to map a real X.400 address to RFC 822, and another to decode an
 RFC 822 address that was mapped to X.400 (i.e., to reverse the
 default RFC 822 -> X.400 mapping).
 To map a real X.400 address to RFC 822, the slash separated notation
 of the X.400 address (see chapter 3.1.) is mapped to 'localpart', and
 the local RFC 822 domain of the gateway that performs the mapping is
 used as the domain part. As an example, the gateway ''
 would perform the following mappings:

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      C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork; ->
      C=zz; ADMD=ade; PRMD=fhbo; O=a bank; S=plork->
      "/C=zz/ADMD=ade/PRMD=fhbo/O=a bank/S=plork/"
 The quotes in the second example are mandatory if the X.400 address
 contains spaces, otherwise the syntax rules for the RFC 822 localpart
 would be violated.
 This default mapping algorithm is generally referred to as 'left-
 hand-side encoding'.
 To reverse the default RFC 822 -> X.400 mapping (see chapter if the X.400 address contains a DDA of the type RFC-822,
 the SAs can be discarded, and the value of this DDA is the desired
 RFC 822 address (NB. Some characters in the DDA value must be decoded
 first. See chapter For example, the gateway
      DD.RFC-822=bush(a); C=nl; ADMD=tlec; PRMD=GW
      -> RFC 822 → X.400

 There are also two kinds of default address mapping from RFC 822 to
 X.400: one to map a real RFC 822 address to X.400, and another to
 decode an X.400 address that was mapped to RFC 822 (i.e., to reverse
 the default X.400 -> RFC 822 mapping).
 To map a real RFC 822 address to X.400, the RFC 822 address is
 encoded in a DDA of type RFC-822 , and the SAs of the local gateway
 performing the mapping are added to form the complete X.400 address.
 This mapping is generally referred to as 'DDA mapping'. As an
 example, the gateway 'C=nl; ADMD=tlec; PRMD=GW' would perform the
 following mapping:  ->
      DD.RFC-822=bush(a); C=nl; ADMD=tlec; PRMD=GW
 As for the encoding/decoding of RFC 822 addresses in DDAs, it is
 noted that RFC 822 addresses may contain characters (@ ! % etc.) that
 cannot directly be represented in a DDA. DDAs are of the restricted
 character set type 'PrintableString', which is a subset of IA5
 (=ASCII). Characters not in this set need a special encoding. Some
 examples (For details, refer to RFC 1327, chapter 3.4.):

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      100%name@address   -> DD.RFC-822;=100(p)name(a)address
      u_ser!name@address -> DD.RFC-822;=u(u)ser(b)name(a)address
 To decode an X.400 address that was mapped to RFC 822: if the RFC 822
 address has a slash separated representation of a complete X.400
 mnemonic O/R address in its localpart, that address is the result of
 the mapping. As an example, the gateway '' would perform
 the following mapping:
      C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork; G=mary

3.3.2. Exception mapping according to mapping tables

 Chapter 3.3.1. showed that it is theoretically possible to use RFC
 1327 with default mapping only. Although this provides a very simple,
 straightforward way to map addresses, there are some very good
 reasons not to use RFC 1327 this way:
  1. RFC 822 users are used to writing simple addresses of the

form 'localpart@domainpart'. They often consider X.400

        addresses, and thus also the left-hand-side encoded
        equivalents, as unnecessarily long and complicated. They
        would rather be able to address an X.400 user as if she had a
        'normal' RFC 822 address. For example, take the mapping
          C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork;     ->
        from chapter RFC 822 users would find it much more
        'natural' if this address could be expressed in RFC 822 as:

  1. X.400 users are used to using X.400 addresses with SAs only.

They often consider DDA addresses as complicated, especially

        if they have to encode the special characters, @ % ! etc,
        manually. They would rather be able to address an RFC 822
        user as if he had a 'normal' X.400 address. For example, take
        the mapping

          C=nl; ADMD= ; PRMD=tlec; O=gateway
        from chapter X.400 users would find it much more

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        'natural' if this address could be expressed in X.400 as:
          C=us; ADMD=dole; S=bush
  1. Many organisations are using both RFC 822 and X.400

internally, and still want all their users to have a simple,

        unique address in both mail worlds. Note that in the default
        mapping, the mapped form of an address completely depends on
        which gateway  performed the mapping. This also results in a
        complication of a more technical nature:
  1. The tricky 'third party problem'. This problem need not

necessarily be understood to read the rest of this chapter.

        If it looks too complicated, please feel free to skip it
        until you are more familiar with the basics.
        The third party problem is a routing problem caused by
        mapping. As an example for DDA mappings (the example holds
        just as well for left-hand-side encoding), consider the
        following situation (see Fig. 3.1.): RFC 822 user X in
        country A sends a message to two recipients: RFC 822 user Y,
        and X.400 user Z, both in country B:
          From: X@A
          To:   Y@B ,
        Since the gateway in country A maps all addresses in the
        message, Z will see both X's and Y's address as DDA-encoded
        RFC 822 addresses, with the SAs of the gateway in country A:
          From: DD.RFC-822=X(a)A; C=A;....;O=GW
          To:   DD.RFC-822=Y(a)B; C=A;....;O=GW ,

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          |       ------------         ---------
          |       |X: RFC 822|<------->|gateway|
          |       ------------         ---------
          | A           |                  ^
          \             |                  |
                        |                  |
          /             |                  |
          | B           |                  v
          |             |              -----------
          |             |              |Z: X.400 |
          |             |              -----------
          |             |                  .
          |             |                  .
          |             |                  .
          |             |                  .
          |             |                  .
          |             v                  v
          |        ------------         ---------
          |        |Y: RFC 822|<........|gateway|
          |        ------------         ---------
                  Fig. 3.1 The third party problem
       Now if Z wants to 'group reply' to both X and Y, his reply to Y
       will be routed over the gateway in country A, even though Y is
       located in the same country:
                   From: C=B;...;S=Z
                   To:   DD.RFC-822=Y(a)B; C=A;....;O=GW ,
                         DD.RFC-822=X(a)A; C=A;....;O=GW
       The best way to travel for a message from Z to Y would of
       course have been over the gateway in country B:
                   From: C=B;...;S=Z
                   To:   DD.RFC-822=Y(a)B; C=B;....;O=GW ,
                         DD.RFC-822=X(a)A; C=A;....;O=GW
       The third party problem is caused by the fact that routing
       information is mapped into addresses.
       Ideally, the third party problem shouldn't exist. After all,
       address mapping affects addresses, and an address is not a
       route.... The reality is different however. For instance, very

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       few X.400 products are capable to route messages on the
       contents of a DDA (actually, only RFC 1327 gateways will be
       able to interpret this type of DDA, and who says that the reply
       will pass a local gateway on its route back?).  Similar
       limitations hold for the other direction: an RFC 822 based
       mailer is not even allowed (see [5]) to make routing decisions
       of the content of a left-hand-side encoded X.400 address if the
       domain part is not its own.  So in practice, addressing and
       (thus also mapping) will very well affect routing.
 To make mapping between addresses more user friendly, and to avoid
 the problems shown above, RFC 1327 allows for overruling the default
 left-hand-side encoding and DDA mapping algorithms. This is done by
 specifying associations (mapping rules) between certain domainparts
 and X.400 domains. An X.400 domain (for our purposes; CCITT has a
 narrower definition...) consists of the domain-related SAs of a
 Mnemonic O/R address (i.e., all SAs except PN and CN). The idea is to
 use the similarities between both address spaces, and directly map
 similar address parts onto each other. If, for the domain in the
 address to be mapped, an explicit mapping rule can be found, the
 mapping is performed between:
      localpart     <->   PersonalName
      domainpart    <->   X.400 domain
 The address information of the gateway is only used as an input
 parameter if no mapping rule can be found, i.e., if the address
 mapping must fall back to its default algorithm.
 The complete mapping function can thus be visualised as follows:
        address information of the gateway performing the mapping
      RFC 822 address <--->| address mapping | <---> X.400 address
                  domain associations (mapping rules) PersonalName and localpart mapping

 Since the mapping between these address parts is independent of the
 mapping rules that are used, and because it follows a simple, two-
 way algorithmic approach, this subject is discussed in a separate
 sub-chapter first.

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 The X.400 PersonalName consists of givenName, initials, and surName.
 RFC 1327 assumes that generationQualifier is not used.
 To map a localpart to an X.400 PN, the localpart is scanned for dots,
 which are considered delimiters between the components of PN, and
 also between single initials. In order not to put too much detail in
 this tutorial, only a few examples are shown here. For the detailed
 algorithm, see RFC 1327, chapter 4.2.1.
      Marshall.Rose             <->   G=Marshall;S=Rose
      M.T.Rose                  <->   I=MT;S=Rose
      Marshall.M.T.Rose         <->   G=Marshall;I=MT;S=Rose
 To map an X.400 PN to an RFC 822 localpart, take the non-empty PN
 attributes, put them into their hierarchical order (G I* S), and
 connect them with periods.
 Some exceptions are caused by the fact that left-hand-side encoding
 can also be mixed with exception mapping. This is shown in more
 detail in the following sub-chapters. X.400 domain and domainpart mapping

 A mapping rule associates two domains: an X.400 domain and an RFC 822
 domain. The X.400 domain is written in the RFC 1327 domain notation
 (See 3.1.3.), so that both domains have the same hierarchical order.
 The domains are written on one line, separated by a '#' sign. For
 A mapping rule must at least contain a top level domain and a country
 code. If an address must be mapped, a mapping rule with the longest
 domain match is sought. The associated domain in the mapping rule is
 used as the domain of the mapped address. The remaining domains are
 mapped one by one following the natural hierarchy. Concrete examples
 are shown in the following subchapters. X.400 → RFC 822

 As an example, assume the following mapping rule is defined:

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 Then the address C=nl; ADMD=ade; PRMD=tlec; O=you; OU=owe; S=plork
         S      OU  O  PRMD  ADMD  Country
         |      |   |  |     |     |
         plork owe you tlec  ade   nl
 would be mapped as follows. The Surname 'plork' is mapped to the
 localpart 'plork', see chapter The domain
            |  sdom3
            |    | sdom2
            |    |   |  sdom1
            |    |   |   |  top-level-domain
            |    |   |   |   |
 The remaining SAs (O and one OU) are mapped one by one following the
 natural hierarchy: O is mapped to sdom2, OU is mapped to sdom3:
            | sdom3
            |  | sdom2
            |  |   |  sdom1
            |  |   |   |  top-level-domain
            |  |   |   |    |
 Thus the mapped address is:
 The table containing the listing of all such mapping rules, which is
 distributed to all gateways world-wide, is normally referred to as
 'mapping table 1'. Other commonly used filenames (also depending on
 which software your are using) are:
         'mapping 1'
         'table 1'
 As already announced, there is an exceptional case were localpart and
 PN are not directly mapped onto each other: sometimes it is necessary
 to use the localpart for other purposes. If the X.400 address
 contains attributes that would not allow for the simple mapping:

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         localpart     <->   PersonalName
         domainpart    <->   X.400 domain
 (e.g., spaces are not allowed in an RFC 822 domain, GQ and CN cannot
 be directly mapped into localpart, DDAs of another type than RFC-
 822), such attributes, together with the PN, are left-hand-side
 encoded. The domainpart must still be mapped according to the mapping
 rule as far as possible. This probably needs some examples:
         C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=you; S=plork; GQ=jr
         C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=spc ctr; OU=u; S=plork
         "/S=plork/OU=u/OU=spc ctr/"
 Note that in the second example, 'O=owe' is still mapped to a
 subdomain following the natural hierarchy. The problems start with
 the space in 'OU=spc ctr'. RFC 822 → X.400

 As an example, assume the following mapping rule is defined:$tlec.ADMD$ade.C$nl#
 Then the address '' :
            |  sdom3
            |    | sdom2
            |    |   |  sdom1
            |    |   |   |  top-level-domain
            |    |   |   |   |
 would be mapped as follows.
 The localpart 'plork' is mapped to 'S=plork', see chapter
 The domain '' is mapped according to the mapping rule:
         S     OU  OU  O  PRMD  ADMD  Country
         |                |     |    |
         plork            tlec  ade  nl
 The remaining domains ( are mapped one by one following the

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 natural hierarchy: sdom2 is mapped to O, sdom3 is mapped to OU:
         S     OU  OU  O  PRMD  ADMD  Country
         |         |   |  |     |     |
         plork     |   |  tlec  ade   nl
                   owe you
 Thus the mapped address is (in a readable notation):
         C=nl; ADMD=ade; PRMD=tlec; O=you; OU=owe; S=plork
 Had there been any left-hand-side encoded SAs in the localpart that
 didn't represent a complete mnemonic O/R address, the localpart would
 be mapped to those SAs. E.g.,
         "/S=plork/GQ=jr/OU=u/OU=spc ctr/"
         C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=space ctr;
         OU=u; S=plork; GQ=jr
 This is necessary to reverse the special use of localpart to left-
 hand-side encode certain attributes. See
 You might ask yourself by now why such rules are needed at all. Why
 don't we just use map1 in the other direction? The problem is that a
 symmetric mapping function (a bijection) would indeed be ideal, but
 it's not feasible. Asymmetric mappings exist for a number of reasons:
  1. To make sure that uucp addresses etc. get routed over local


  1. Preferring certain address forms, while still not forbidding

others to use another form. Examples of such reasons are:

  1. Phasing out old address forms.
  1. If an RFC 822 address is mapped to ADMD= ; it means that

the X.400 mail can be routed over any ADMD in that

               country. One single ADMD may of course send out an
               address containing: ADMD=ade; . It must also be possible
               to map such an address back.
 So we do need mapping rules from RFC 822 to X.400 too. The table
 containing the listing of all such mapping rules, which is
 distributed to all gateways world-wide, is normally referred to as on
 which software your are using) are:

RARE Working Group on Mail and Messaging (WG-MSG) [Page 29] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

         'mapping 2'
         'table 2'
 If the RFC 822 localpart and/or domainpart contain characters that
 would not immediately fit in the value of a PN attribute (! % _), the
 mapping algorithm falls back to DDA mapping. In this case, the SAs
 that will be used are still determined by mapping the domainpart
 according to the mapping rule. In our case:
         C=nl; ADMD=ade; PRMD=tlec; O=work
 If no map2 rule can be found, a third table of rules is scanned: the
 gateway table. This table has the same syntax as mapping table 2, but
 its semantics are different. First of all, a domain that only has an
 entry in the gateway table is always mapped into an RFC 822 DDA. For
 a domain that is purely RFC 822 based, but whose mail may be relayed
 over an X.400 network, the gateway table associates with such a
 domain the SAs of the gateway to which the X.400 message should be
 routed. That gateway will then be responsible for gatewaying the
 message back into the RFC 822 world. E.g., if we have the gateway
 table entry:
 (and we assume that no overruling map2 rule for the top level domain
 'gov' exists), this would force all gateways to perform the following
         C=us; ADMD=Internet; PRMD=gateway
 This is very similar to the default DDA mapping, except the SAs are
 those of a gateway that has declared to be responsible for a certain
 RFC 822 domain, not those of the local gateway. And thus, this
 mechanism helps avoid the third party problem discussed in chapter
 The table containing the listing of all such gateway rules, which is
 distributed to all gateways world-wide, is normally referred to as
 the 'gateway table'. Other commonly used filenames (also depending on

RARE Working Group on Mail and Messaging (WG-MSG) [Page 30] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 which software your are using) are:
         'rfc1148gate' {From the predecessor of RFC 1327, RFC 1148}
         'gate table'
 Only when no rule at all (map2 or gateway rule) is defined for a
 domain, the algorithm falls back to the default DDA mapping as
 described in

3.4. Table co-ordination

 As already stated, the use of mapping tables will only function
 smoothly if all gateways in the world use the same tables. On the
 global level, the collection and distribution of RFC 1327 address
 mapping tables is co-ordinated by the MHS Co-ordination Service:
        SWITCH Head Office
        MHS Co-ordination Service
        Limmatquai 138
        CH-8001 Zurich, Europe
        Tel. +41 1 268 1550
        Fax. +41 1 268 1568
        RFC 822:
        X.400:   C=ch;ADMD=arcom;PRMD=switch;O=switch;S=project-team;
 The procedures for collection and distribution of mapping rules can
 be found on the MHS Co-ordination Server, in the directory
 "/procedures".  Appendix D describes how this server can be accessed.
 If you want to define mapping rules for your own local domain, you
 can find the right contact person in your country or network (the
 gateway manager) on the same server, in the directory "/mhs-

3.5. Local additions

 Since certain networks want to define rules that should only be used
 within their networks, such rules should not be distributed world-
 wide. Consider two networks that both want to reach the old top-
 level-domain 'arpa' over their local gateway. They would both like to
 use a mapping 2 rule for this purpose:
         TLec in NL:     arpa#PRMD$gateway.ADMD$tlec.C$nl#
         SWITCH in CH:   arpa#PRMD$gateway.ADMD$switch.C$ch#

RARE Working Group on Mail and Messaging (WG-MSG) [Page 31] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 (You may have noticed correctly that they should have defined such
 rules in the gateway table, but for the sake of the example, we
 assume they defined it in mapping table 2. This was the way things
 were done in the days of RFC 987, and many networks are still doing
 it this way these days.)
 Since a mapping table cannot contain two mapping rules with the same
 domain on the left hand side, such 'local mappings' are not
 distributed globally. There exists a RARE draft proposal [13] which
 defines a mechanism for allowing and automatically dealing with
 conflicting mapping rules, but this mechanism has not been
 implemented as to date. After having received the global mapping
 tables from the MHS Co-ordination Service, many networks add 'local'
 rules to map2 and the gateway table before installing them on their
 gateways. Note that the reverse mapping 2 rules for such local
 mappings _are_ globally unique, and can thus be distributed world-
 wide. This is even necessary, because addresses that were mapped with
 a local mapping rule may leak out to other networks (here comes the
 third party problem again...). Such other networks should at least be
 given the possibility to map the addresses back. So the global
 mapping table 1 would in this case contain the two rules:
 Note that if such rules would have been defined as local gate table
 entries instead of map2 entries, there would have been no need to
 distribute the reverse mappings world-wide (the reverse mapping of a
 DDA encoded RFC 822 address is simply done by stripping the SAs, see

3.6. Product specific formats

 Not all software uses the RFC 1327 format of the mapping tables
 internally. Almost all formats allow comments on a line starting with
 a # sign. Some examples of different formats:

RARE Working Group on Mail and Messaging (WG-MSG) [Page 32] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

  RFC 1327
      # This is pure RFC 1327 format
      # table 1: X.400 -> RFC 822
      # etc.
      # table 2: RFC 822 -> X.400
      # etc.
      # This is EAN format
      # It uses the readable format for X.400 domains and TABs
      # to make a 'readable mapping table format'.
      # table 1: X.400 -> RFC 822
      P=tlec; A=ade; C=nl;       #
      # etc.
      # table 2: RFC 822 -> X.400
      #                   # A=arcom; C=ch;
      # etc.
      # This is PP format
      # table 1: X.400 -> RFC 822
      # etc.
      # table 2: RFC 822 -> X.400
      # etc.
 Most R&D networks have tools to automatically generate these formats
 from the original RFC 1327 tables;, some even distribute the tables
 within their networks in several formats. If you need mapping tables
 in a specific format, please contact your national or R&D network's
 gateway manager. See chapter 3.4.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 33] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

3.7. Guidelines for mapping rule definition

 Beware that defining mapping rules without knowing what you are doing
 can be disastrous not only for your network, but also for others. You
 should be rather safe if you follow at least these rules:
  1. First of all, read this tutorial;.
  1. Avoid local mappings; prefer gate table entries. (See chapter


  1. Make sure any domain you map to can also be mapped back;.
  1. Aim for symmetry.
  1. Don't define a gateway table entry if the same domain already

has a map2 entry. Such a rule would be redundant.

  1. Map to "ADMD=0;" if you will not be connected to any ADMD for

the time being.

  1. Only map to "ADMD= ;" if you are indeed reachable through

_any_ ADMD in your country.

  1. Mind the difference between "PRMD=;" and "PRMD=@;" and make

sure which one you need. (Try to avoid empty or unused

           attributes in the O/R address hierarchy from the beginning!)
  1. Don't define mappings for domains over which you have no

naming authority.

  1. Before defining a mapping rule, make sure you have the

permission from the naming authority of the domain you want

           to map to. Normally, this should be the same organisation as
           the mapping authority of the domain in the left hand side of
           the mapping rule. This principle is called 'administrative
  1. Avoid redundant mappings. E.g., if all domains under ''

are in your control, don't define:

           but rather have only one mapping rule:

RARE Working Group on Mail and Messaging (WG-MSG) [Page 34] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

  1. Before introducing a new mapped version of a domain, make

sure the world can route to that mapped domain;.

           E.g., If you are operating a PRMD: C=zz; ADMD=ade; PRMD=ergo;
           and you want to define the mapping rules:
             map1: PRMD$ergo.ADMD$ade.C$zz#ergo.zz#
             map2: ergo.zz#PRMD$ergo.ADMD$ade.C$zz#
           Make sure that ergo.zz (or at least all of its subdomains) is
           DNS routeable (register an MX or A record) and will be routed
           to a gateway that agreed to route the messages from the
           Internet to you over X.400.
           In the other direction, if you are operating the Internet
           domain, and you want to define a mapping for
           that domain:
             map2:$cs.PRMD$woodstock.ADMD$ .C$us#
             map1: O$cs.PRMD$woodstock.ADMD$ .C$
           Make sure that C=us; ADMD= ; PRMD=woodstock; O=cs; (or at
           least all of its subdomains) is routeable in the X.400 world,
           and will be routed to a gateway that agreed to route the
           messages from X.400 to your RFC 822 domain over SMTP. Within
           the GO-MHS community, this would be done by registering a
           line in a so-called domain document, which will state to
           which mail relay this domain should be routed.
           Co-ordinate any such actions with your national or MHS'
           gateway manager. See chapter 3.4.

4. Conclusion

 Mail gatewaying remains a complicated subject. If after reading this
 tutorial, you feel you understand the basics, try solving some real-
 life problems. This is indeed a very rewarding area to work in: even
 after having worked with it for many years, you can make amazing
 discoveries every other week........

RARE Working Group on Mail and Messaging (WG-MSG) [Page 35] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

Appendix A. References

 [1]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
      USC/Information Sciences Institute, August 1982.
 [2]  Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages", STD 11, RFC 822, University of Delaware, August 1982.
 [3]  Mockapetris, P., "Domain Names - Concepts and Facilities", and
      "Domain Names - Implementation and Specification", STD 13, RFCs
      1034 and 1035, USC/Information Sciences Institute, November
 [4]  Kille, S., "Mapping Between X.400 and RFC 822", RFC 987, UK
      Academic Community Report (MG.19), UCL, June 1986.
 [5]  Braden, R., Editor, "Requirements for Internet Hosts --
      Application and Support", STD 3, RFC 1123, USC/Information
      Sciences Institute, October 1989.
 [6]  Postel, J., Editor, "Internet Official Protocol Standards", STD
      1, RFC 1500, USC/Information Sciences Institute, August 1993.
 [7]  Chapin, L., Chair, "The Internet Standards Process", RFC 1310,
      Internet Activities Board, March 1992.
 [8]  Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC
      822", RFC 1327 / RARE RTR 2, University College London, May
 [9]  Kille, S., "X.400 1988 to 1984 downgrading", RFC 1328 / RARE RTR
      3, University College London, May 1992.
 [10] Plattner, B., and H. Lubich, "Electronic Mail Systems and
      Protocols Overview and Case Study", Proceedings of the IFIP WG
      6.5 International working conference on message handling systems
      and distributed applications; Costa Mesa 1988; North-Holland,
 [11] Houttuin, J., "@route:100%name@address, a practical guide to MHS
      configuration", Top-Level EC, 1993, (not yet published).
 [12] Alvestrand, H., "Frequently asked questions on X.400", regularly
      posted on USEnet in newsgroup comp.protocols.iso.x400.
 [13] Houttuin, J., Hansen, K., and S. Aumont, "RFC 1327 Address
      Mapping Authorities", RARE WG-MSG Working Draft, Work in
      Progress, May 1993.

RARE Working Group on Mail and Messaging (WG-MSG) [Page 36] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

 [14] "COSINE MHS Pocket User Guide", COSINE MHS Project Team 1992.
      Also available in several languages from the MHS Co-ordination
      Server:/user-guides. See Appendix D.
 [15] Grimm, R., and S. Haug, "A Minimum Profile for RFC 987", GMD,
      November 1987; RARE MHS Project Team; July 1990. Also available
      from the MHS Co-ordination Server:/procedures/min-rfc987-
      profile. See Appendix D.
 [16] CCITT Recommendations X.400 - X.430. Data Communication
      Networks: Message Handling Systems.  CCITT Red Book, Vol. VIII -
      Fasc. VIII.7, Malaga-Torremolinos 1984.
 [17] CCITT Recommendations X.400 - X.420. Data Communication
      Networks: Message Handling Systems.  CCITT Blue Book, Vol. VIII
      - Fasc. VIII.7, Melbourne 1988.

Appendix B. Index

 <<Only available in the Postscript version>>

Appendix C. Abbreviations

    ADMD     Administration Management Domain
    ARPA     Advanced Research Projects Agency
    ASCII    American Standard Code for Information Exchange
    ASN.1    Abstract Syntax Notation One
    BCD      Binary-Coded Decimal
    BITNET   Because It's Time NETwork
    CCITT    Comite Consultatif International de Telegraphique et
    COSINE   Co-operation for OSI networking in Europe
    DFN      Deutsches Forschungsnetz
    DL       Distribution List
    DNS      Domain Name System
    DoD      Department of Defense
    EBCDIC   Extended BCD Interchange Code
    IAB      Internet Architecture Board
    IEC      International Electrotechnical Commission
    IESG     Internet Engineering Steering Group
    IETF     Internet Engineering Task Force
    IP       Internet Protocol
    IPM      Inter-Personal Message
    IPMS     Inter-Personal Messaging Service
    IPN      Inter-Personal Notification
    ISO      International Organisation for Standardisation
    ISOC     Internet Society

RARE Working Group on Mail and Messaging (WG-MSG) [Page 37] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

    ISODE    ISO Development Environment
    JNT      Joint Network Team (UK)
    JTC      Joint Technical Committee (ISO/IEC)
    MHS      Message Handling System
    MOTIS    Message-Oriented Text Interchange Systems
    MTA      Message Transfer Agent
    MTL      Message Transfer Layer
    MTS      Message Transfer System
    MX       Mail eXchanger
    OSI      Open Systems Interconnection
    OU(s)    Organizational Unit(s)
    PP       Mail gatewaying software (not an abbreviation)
    PRMD     Private Management Domain
    RARE     Reseaux Associes pour la Recherche Europeenne
    RFC      Request for comments
    RTC      RARE Technical Committee
    RTR      RARE Technical Report
    SMTP     simple mail transfer protocol
    STD      Internet Standard
    TCP      Transmission Control Protocol
    UUCP     Unix to Unix CoPy

Appendix D. How to access the MHS Co-ordination Server

 Here is an at-a-glance sheet on the access possibilities of the MHS
 Co-ordination server:
        X.400:  S=mhs-server; OU1=nic; O=switch; P=switch; A=arcom;
        help                       # you receive this document
        index ['directory']        # you receive a directory listing
        send 'directory''filename' # you receive the specified file
      address:  Internet:
      account:  cosine
      password: 'your email address'

RARE Working Group on Mail and Messaging (WG-MSG) [Page 38] RFC 1506 X.400-Internet Mail Gatewaying Tutorial August 1993

      address:   Internet:
      address:   PSPDN:    +22847971014540
      address:   EMPB/IXI: 20432840100540
      account:   info
      directory: e-mail/COSINE-MHS/
      address:  Internet:
      address:  PSPDN   : +22847971014540
      address:  EMPB/IXI: 20432840100540
      address:  ISO CLNS: NSAP=39756f11112222223333aa0004000ae100,
      account:  ANON
      address:  Internet:

Security Considerations

 Security issues are not discussed in this memo.

Author's Address

 Jeroen Houttuin
 RARE Secretariat
 Singel 466-468
 NL-1017 AW Amsterdam
 Tel. +31 20 6391131
 Fax. +31 20 6393289
 RFC 822:
 X.400:   C=nl;ADMD=400net;PRMD=surf;O=rare;S=houttuin

RARE Working Group on Mail and Messaging (WG-MSG) [Page 39]

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