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RFC 1138


Mapping between X.400(1988) / ISO 10021 and RFC 822

Part 2 of 3, p. 29 to 60
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prevText      Top       Page 29 
Chapter 4 -- Addressing

   Addressing is probably the trickiest problem of an X.400 <-> RFC 822
   gateway.  Therefore it is given a separate chapter.  This chapter, as
   a side effect, also defines a textual representation of an X.400 O/R

   Initially, we consider an address in the (human) mail user sense of
   "what is typed at the mailsystem to reference a mail user".  A basic
   RFC 822 address is defined by the EBNF EBNF.822-address:

                822-address     = [ route ] addr-spec

   In an 822-MTS protocol, the originator and each recipient should be
   considered to be defined by such a construct.  In an RFC 822 header,
   the EBNF.822-address is encapsulated in the 822.address syntax rule,
   and there may also be associated comments.  None of this extra
   information has any semantics, other than to the end user.

   The basic X.400 O/R Address, used by the MTS for routing, is defined
   by MTS.ORAddress.  In IPMS, the MTS.ORAddress is encapsulated within

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   It can be seen that RFC 822 822.address must be mapped with
   IPMS.ORDescriptor, and that RFC 822 EBNF.822-address must be mapped
   with MTS.ORAddress.

4.1.  A textual representation of MTS.ORAddress

   MTS.ORAddress is structured as a set of attribute value pairs.  It is
   clearly necessary to be able to encode this in ASCII for gatewaying
   purposes.  All aspects should be encoded, in order to guarantee
   return of error messages, and to optimise third party replies.

4.2.  Basic Representation

   An O/R Address has a number of structured and unstructured
   attributes.  For each unstructured attribute, a key and an encoding
   is specified.  For structured attributes, the X.400 attribute is
   mapped onto one or more attribute value pairs.  For domain defined
   attributes, each element of the sequence will be mapped onto a triple
   (key and two values), with each value having the same encoding.  The
   attributes are as follows, with 1984 attributes given in the first
   part of the table.  For each attribute, a reference is given,
   consisting of the relevant sections in X.402 / ISO 10021-2, and the
   extension identifier for 88 only attributes:

Attribute (Component)               Key            Enc     Ref       Id

84/88 Attributes

MTS.CountryName                     C              P       18.3.3
MTS.AdministrationDomainName        ADMD           P       18.3.1
MTS.PrivateDomainName               PRMD           P       18.3.21
MTS.NetworkAddress                  X121           N       18.3.7
MTS.TerminalIdentifier              T-ID           N       18.3.23
MTS.OrganizationName                O              P/T     18.3.9
MTS.OrganizationalUnitNames.value   OU             P/T     18.3.10
MTS.NumericUserIdentifier           UA-ID          N       18.3.8
MTS.PersonalName                    PN             P/T     18.3.12
MTS.PersonalName.surname            S              P/T     18.3.12
MTS.PersonalName.given-name         G              P/T     18.3.12
MTS.PersonalName.initials           I              P/T     18.3.12
   .generation-qualifier            GQ             P/T     18.3.12
MTS.DomainDefinedAttribute.value    DD             P/T     18.1

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88 Attributes

MTS.CommonName                      CN             P/T     18.3.2    1
MTS.TeletexCommonName               CN             P/T     18.3.2    2
MTS.TeletexOrganizationName         O              P/T     18.3.9    3
MTS.TeletexPersonalName             PN             P/T     18.3.12   4
MTS.TeletexPersonalName.surname     S              P/T     18.3.12   4
MTS.TeletexPersonalName.given-name  G              P/T     18.3.12   4
MTS.TeletexPersonalName.initials    I              P/T     18.3.12   4
   .generation-qualifier            GQ             P/T     18.3.12   4
   .value                           OU             P/T     18.3.10   5
   .value                           DD             P/T     18.1      6
MTS.PDSName                         PD-SYSTEM      P       18.3.11   7
MTS.PhysicalDeliveryCountryName     PD-C           P       18.3.13   8
MTS.PostalCode                      POSTCODE       P       18.3.19   9
MTS.PhysicalDeliveryOfficeName      PD-OFFICE      P/T     18.3.14  10
MTS.PhysicalDeliveryOfficeNumber    PD-OFFICE-NUM  P/T     18.3.15  11
MTS.ExtensionORAddressComponents    PD-EXT-D       P/T     18.3.4   12
MTS.PhysicalDeliveryPersonName      PD-PN          P/T     18.3.17  13
MTS.PhysicalDelivery                PD-O           P/T     18.3.16  14
   AddressComponents                PD-EXT-LOC     P/T     18.3.5   15
MTS.UnformattedPostalAddress        PD-ADDRESS     P/T     18.3.25  16
MTS.StreetAddress                   STREET         P/T     18.3.22  17
MTS.PostOfficeBoxAddress            PO-BOX         P/T     18.3.18  18
MTS.PosteRestanteAddress            POSTE-RESTANTE P/T     18.3.20  19
MTS.UniquePostalName                PD-UNIQUE      P/T     18.3.26  20
MTS.LocalPostalAttributes           PD-LOCAL       P/T     18.3.6   21
   .e163-4-address.number           NET-NUM        N       18.3.7   22
   .e163-4-address.sub-address      NET-SUB        N       18.3.7   22
  .psap-address                     NET-PSAP       X       18.3.7   22
MTS.TerminalType                    NET-TTYPE      I       18.3.24  23

   The following keys identify different EBNF encodings, which are
   associated with the ASCII representation of MTS.ORAddress.

                 Key         Encoding

                 P     printablestring
                 N     numericstring
                 T     teletex-string

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                 P/T   teletex-and-or-ps
                 I     labelled-integer
                 X     presentation-address

   The BNF for presentation-address is taken from the specification "A
   String Encoding of Presentation Address" [Kille89a].

   In most cases, the EBNF encoding maps directly to the ASN.1 encoding
   of the attribute.  There are a few exceptions. In cases where an
   attribute can be encoded as either a PrintableString or NumericString
   (Country, ADMD, PRMD), either form should be mapped into the BNF.
   When generating ASN.1, the NumericString encoding should be used if
   the string contains only digits.

   There are a number of cases where the P/T (teletex-and-or-ps)
   representation is used.  Where the key maps to a single attribute,
   this choice is reflected in the encoding of the attribute (attributes
   10-21).  For most of the 1984 attributes and common name, there is a
   printablestring and a teletex variant.   This pair of attributes is
   mapped onto the single component here.  This will give a clean
   mapping for the common cases where only one form of the name is used.

4.2.1.  Encoding of Personal Name

   Handling of Personal Name and Teletex Personal Name based purely on
   the EBNF.standard-type syntax defined above is likely to be clumsy.
   It seems desirable to utilise the "human" conventions for encoding
   these components.  A syntax is defined, which is designed to provide
   a clean encoding for the common cases of O/R address specification

      1.   There is no generational qualifier

      2.   Initials contain only letters

      3.   Given Name does not contain full stop ("."), and is at least
           two characters long.

      4.   If Surname contains full stop, then it may not be in the
           first two characters, and either initials or given name is

   The following EBNF is defined:

                encoded-pn      = [ given "." ] *( initial "." ) surname

                given           = 2*<ps-char not including ".">

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                initial         = ALPHA

                surname         = printablestring

   This can be used to map from any string containing only printable
   string characters to an O/R address personal name. Parse the string
   according to the EBNF.  The given name and surname are assigned
   directly.  All EBNF.initial tokens are concatenated without
   intervening full stops to generate the initials.

   For an O/R address which follows the above restrictions, a string can
   be derived in the natural manner.  In this case, the mapping will be

        For example:

                GivenName       = "Marshall"
                Surname         = "Rose"

                Maps with  "Marshall.Rose"

                Initials        = "MT"
                Surname         = "Rose"

                Maps with  "M.T.Rose"

                GivenName       = "Marshall"
                Initials        = "MT"
                Surname         = "Rose"

                Maps with  "Marshall.M.T.Rose"

   Note that X.400 suggest that Initials is used to encode ALL initials.
   Therefore, the proposed encoding is "natural" when either GivenName
   or Initials, but not both, are present.  The case where both are
   present can be encoded, but this appears to be contrived!

4.2.2.  Standard Encoding of MTS.ORAddress

   Given this structure, we can specify a BNF representation of an O/R

                std-or-address  = 1*( "/" attribute "=" value ) "/"
                attribute       = standard-type
                                / "RFC-822"
                                / registered-dd-type
                                / dd-key "." std-printablestring
                standard-type   = key-string

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                                = key-string
                dd-key          = key-string

                value           = std-printablestring

                                = *( std-char / std-pair )
                std-char        = <"{", "}", "*", and any ps-char
                                                except "/" and "=">
                std-pair        = "$" ps-char

   The standard-type is any key defined in the table in Section 4.2,
   except PN, and DD.  The value, after quote removal, should be
   interpreted according to the defined encoding.

   If the standard-type is PN, the value is interpreted according to
   EBNF.encoded-pn, and the components of MTS.PersonalName and/or
   MTS.TeletexPersonalName derived accordingly.

   If dd-key is the recognised Domain Defined string (DD), then the type
   and value should be interpreted according to the syntax implied from
   the encoding, and aligned to either the teletex or printable string
   form.  Key and value should have the same encoding.

   If value is "RFC-822", then the (printable string) Domain Defined
   Type of "RFC-822" is assumed.  This is an optimised encoding of the
   domain defined type defined by this specification.

   The matching of all keywords should be done in a case- independent

   If the value is registered-dd-type, the value is registered with the
   IANA and will be listed in the Assigned Numbers RFC, then the value
   should be interpreted accordingly.  This restriction maximises the
   syntax checking which can be done at a gateway.

4.3.  EBNF.822-address <-> MTS.ORAddress

   Ideally, the mapping specified would be entirely symmetrical and
   global, to enable addresses to be referred to transparently in the
   remote system, with the choice of gateway being left to the Message
   Transfer Service.  There are two fundamental reasons why this is not

      1.   The syntaxes are sufficiently different to make this

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      2.   In the general case, there would not be the necessary
           administrative co-operation between the X.400 and RFC 822
           worlds, which would be needed for this to work.

   Therefore, an asymmetrical mapping is defined, which can be
   symmetrical where there is appropriate administrative control.

4.3.1.  X.400 encoded in RFC 822

   The std-or-address syntax is  used to encode O/R Address information
   in the 822.local-part of EBNF.822-address.  Further O/R Address
   information may be associated with the 822.domain component.  This
   cannot be used in the general case, basically due to character set
   problems, and lack of order in X.400 O/R Addresses.  The only way to
   encode the full PrintableString character set in a domain is by use
   of the 822.domain-ref syntax (i.e., 822.atom).  This is likely to
   cause problems on many systems.  The effective character set of
   domains is in practice reduced from the RFC 822 set, by restrictions
   imposed by domain conventions and policy.

   A generic 822.address consists of a 822.local-part and a sequence of (e.g., <@domain1,@domain2:user@domain3>).  All except the
   822.domain associated with the 822.local-part (domain3 in this case)
   should be considered to specify routing within the RFC 822 world, and
   will not be interpreted by the gateway (although they may have
   identified the gateway from within the RFC 822 world).

      This form of source routing is now discouraged in the Internet
      (Host Requirements, page 58 [Braden89a]).

   The 822.domain associated with the 822.local-part may also identify
   the gateway from within the RFC 822 world.  This final 822.domain may
   be used to determine some number of O/R Address attributes.  The
   following O/R Address attributes are considered as a hierarchy, and
   may be specified by the domain.  They are (in order of hierarchy):

      Country, ADMD, PRMD, Organisation, Organisational Unit

      There may be multiple Organisational Units.

      Associations may be defined between domain specifications, and
      some set of attributes.  This association proceeds hierarchically.
      For example, if a domain implies ADMD, it also implies country.
      Subdomains under this are associated according to the O/R Address
      hierarchy.  For example:

      => "AC.UK" might be associated with
      C="GB", ADMD="GOLD 400", PRMD="UK.AC"

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      then domain "R-D.Salford.AC.UK" maps with
      C="GB", ADMD="GOLD 400", PRMD="UK.AC", O="Salford", OU="R-D"

      There are three basic reasons why a domain/attribute mapping might
      be maintained, as opposed to using simply subdomains:

      1.   As a shorthand to avoid redundant X.400 information.  In
           particular, there will often be only one ADMD per country,
           and so it does not need to be given explicitly.

      2.   To deal with cases where attribute values do not fit the

              domain-syntax   = alphanum [ *alphanumhyphen alphanum ]
              alphanum        = <ALPHA or DIGIT>
              alphanumhyphen  = <ALPHA or DIGIT or HYPHEN>

           Although RFC 822 allows for a more general syntax, this
           restricted syntax is chosen as it is the one chosen by the
           various domain service administrations.

      3.   To deal with missing elements in the hierarchy.  A domain
           may be associated with an omitted attribute in conjunction
           with several present ones.  When performing the algorithmic
           insertion of components lower in the hierarchy, the omitted
           value should be skipped.  For example, if "HNE.EGM" is
           associated with "C=TC", "ADMD=ECQ", "PRMD=HNE", and omitted
           organisation, then "ZI.HNE.EGM" is mapped with "C=TC",
           "ADMD=ECQ", "PRMD=HNE", "OU=ZI". It should be noted that
           attributes may have null values, and that this is treated
           separately from omitted attributes (whilst it would be bad
           practice to treat these two cases differently, they must be
           allowed for).

   This set of mappings need only be known by the gateways relaying
   between the RFC 822 world, and the O/R Address space associated with
   the mapping in question.  However, it is desirable (for the optimal
   mapping of third party addresses) for all gateways to know these
   mappings.  A format for the exchange of this information is defined
   in Appendix F.

   The remaining attributes are encoded on the LHS, using the EBNF.std-
   or-address syntax.  For example:


   encodes the MTS.ORAddress consisting of:

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                MTS.CountryName                       = "TC"
                MTS.AdministrationDomainName          = "BTT"
                MTS.OrganizationName                  = "Widget"
                MTS.OrganizationalUnitNames.value     = "Marketing"
                MTS.PersonalName.surname              = "Linnimouth"
                MTS.PersonalName.initials             = "J"
                MTS.PersonalName.generation-qualifier = "5"

   The first three attributes are determined by the domain Widget.COM.
   Then, the first element of OrganizationalUnitNames is determined
   systematically, and the remaining attributes are encoded on the LHS.
   In an extreme case, all of the attributes will be on the LHS.  As the
   domain cannot be null, the RHS will simply be a domain indicating the

   The RHS (domain) encoding is designed to deal cleanly with common
   addresses, and so the amount of information on the RHS should be
   maximised.  In particular, it covers the Mnemonic O/R Address using a
   1984 compatible encoding.  This is seen as the dominant form of O/R
   Address.  Use of other forms of O/R Address, and teletex encoded
   attributes will require an LHS encoding.

   There is a further mechanism to simplify the encoding of common
   cases, where the only attributes to be encoded on the LHS is a (non-
   Teletex) Personal Name attributes which comply with the restrictions
   of 4.2.1.  To achieve this, the 822.local-part shall be encoded as
   EBNF.encoded-pn.  In the previous example, if the GenerationQualifier
   was not present, the encoding J.Linnimouth@Marketing.Widget.COM would

   From the standpoint of the RFC 822 Message Transfer System, the
   domain specification is simply used to route the message in the
   standard manner.  The standard domain mechanisms are are used to
   select appropriate gateways for the corresponding O/R Address space.
   In most cases, this will be done by registering the higher levels,
   and assuming that the gateway can handle the lower levels.

4.3.2.  RFC 822 encoded in X.400

   In some cases, the encoding defined above may be reversed, to give a
   "natural" encoding of genuine RFC 822 addresses.  This depends
   largely on the allocation of appropriate management domains.

   The general case is mapped by use of domain defined attributes.  A
   Domain defined type "RFC-822" is defined.  The associated attribute
   value is an ASCII string encoded according to Section 3.3.3 of this
   specification.  The interpretation of the ASCII string depends on the
   context of the gateway.

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      1.   In the context of RFC 822, and RFC 1034
           [Crocker82a, Mockapetris87a], the string can be used

      2.   In the context of the JNT Mail protocol, and the NRS
           [Kille84a, Larmouth83a], the string should be interpreted
           according to Mailgroup Note 15 [Kille84b].

      3.   In the context of UUCP based systems, the string should be
           interpreted as defined in [Horton86a].

   Other O/R Address attributes will be used to identify a context in
   which the O/R Address will be interpreted.  This might be a
   Management Domain, or some part of a Management Domain which
   identifies a gateway MTA.  For example:

                C               = "GB"
                ADMD            = "GOLD 400"
                PRMD            = "UK.AC"
                O               = "UCL"
                OU              = "CS"
                "RFC-822"      =  "Jimmy(a)WIDGET-LABS.CO.UK"


                C               = "TC"
                ADMD            = "Wizz.mail"
                PRMD            = "42"
                "rfc-822"       = "Postel(a)"

   Note in each case the PrintableString encoding of "@" as "(a)".  In
   the second example, the "RFC-822" domain defined attribute is
   interpreted everywhere within the (Private) Management Domain.  In
   the first example, further attributes are needed within the
   Management Domain to identify a gateway.  Thus, this scheme can be
   used with varying levels of Management Domain co-operation.

4.3.3.  Component Ordering

   In most cases, ordering of O/R Address components is not significant
   for the mappings specified.  However, Organisational Units (printable
   string and teletex forms) and Domain Defined Attributes are specified
   as SEQUENCE in MTS.ORAddress, and so their order may be significant.
   This specification needs to take account of this:

      1.   To allow consistent mapping into the domain hierarchy

      2.   To ensure preservation of order over multiple mappings.

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   There are three places where an order must be specified:

      1.   The text encoding (std-or-address) of MTS.ORAddress as used
           in the local-part of an RFC 822 address.  An order is needed
           for those components which may have multiple values
           (Organisational Unit, and Domain Defined Attributes). When
           generating an 822.std-or-address, components of a given type
           shall be in hierarchical order with the most significant
           component on the RHS.  If there is an Organisation
           Attribute, it shall be to the right of any Organisational
           Unit attributes.  These requirements are for the following

           -    Alignment to the hierarchy of other components in RFC
                822 addresses (thus, Organisational Units will appear
                in the same order, whether encoded on the RHS or LHS).
                Note the differences of JNT Mail as described in
                Appendix B.

           -    Backwards compatibility with RFC 987/1026.

           -    To ensure that gateways generate consistent addresses.
                This is both to help end users, and to generate
                identical message ids.

           Further, it is recommended that all other attributes are
           generated according to this ordering, so that all attributes
           so encoded follow a consistent hierarchy.

           There will be some cases where an X.400 O/R address of this
           encoding will be generated by an end user from external
           information.  The ordering of attributes may be inverted or
           mixed.  For this reason, the following heuristics may be

           -    If there is an Organisation attribute to the left of
                any Org Unit attribute, assume that the hierarchy is

           -    If an inversion of the Org Unit hierarchy generates a
                valid address, when the preferred order does not,
                assume that the hierarchy is inverted.

      2.   For the Organisational Units (OU) in MTS.ORAddress, the
           first OU in the SEQUENCE is the most significant, as
           specified in X.400.

      3.   For the Domain Defined Attributes in MTS.ORAddress, the

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           First Domain Defined Attribute in the SEQUENCE is the most

           Note that although this ordering is mandatory for this
           mapping, there are NO implications on ordering significance
           within X.400, where this is a Management Domain issue.

4.3.4.  RFC 822 -> X.400

   There are two basic cases:

      1.   X.400 addresses encoded in RFC 822.  This will also include
           RFC 822 addresses which are given reversible encodings.

      2.   "Genuine" RFC 822 addresses.

   The mapping should proceed as follows, by first assuming case 1).


   1.   If the 822-address is not of the form:

                local-part "@" domain

        Go to stage II.

   NOTE:It may be appropriate to reduce a source route address
        to this form by removal of all bar the last domain.  In
        terms of the design intentions of RFC 822, this would
        be an incorrect action.  However, in most real cases,
        it will do the "right" thing and provide a better
        service to the end user.  This is a reflection on the
        excessive and inappropriate use of source routing in
        RFC 822 based systems.  Either approach, or the
        intermediate approach of stripping only domain
        references which reference the local gateway are
        conformant to this specification.

   2.   Attempt to parse EBNF.domain as:

                *( domain-syntax "." ) known-domain

        Where EBNF.known-domain is the longest possible match in a
        list of supported mappings (see Appendix F).  If this fails,
        and the EBNF.domain does not explicitly identify the local
        gateway, go to stage II.  If it succeeds, allocate the
        attributes associated with EBNF.known-domain, and
        systematically allocate the attributes implied by each

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        EBNF.domain-syntax component.  If the domain explicitly
        identifies the gateway, allocate no attributes.

   3.   If the local-part contains any characters not in
        PrintableString, go to stage II.

   4.   If the 822.local-part uses the 822.quoted-string encoding,
        remove this quoting.  Parse the (unquoted) 822.local-part
        according to the EBNF EBNF.std-or-address.  If this parse
        fails, parse the local-part according to the EBNF
        EBNF.encoded-pn.  The result is a set of type/value pairs.
        If the values generated conflict with those derived in step
        2 (e.g., a duplicated country attribute), the domain should
        be assumed to be an RFC 987 gateway.  In this case, take
        only the LHS derived attributes.  Otherwise add LHS and RHS
        derived attributes together.

   5.   Associate the EBNF.attribute-value syntax (determined from
        the identified type) with each value, and check that it
        conforms.  If not, go to stage II.

   6.   Ensure that the set of attributes conforms both to the
        MTS.ORAddress specification and to the restrictions on this
        set given in X.400.  If not go to stage II.

   7.   Build the O/R Address from this information.


   This will only be reached if the RFC 822 EBNF.822-address is not
   a valid X.400 encoding.  If the address is an 822-MTS recipient
   address, it must be rejected, as there is a need to interpret
   such an address in X.400.  For the 822-MTS return address, and
   any addresses in the RFC 822 header, they should now be encoded
   as RFC 822 addresses in an X.400 O/R Name:

   1.   Convert the EBNF.822-address to PrintableString, as
        specified in Chapter 3.

   2.   The "RFC-822" domain defined attribute should be generated
        from this string.

   3.   Build the rest of the O/R Address in the local Management
        Domain agreed manner, so that the O/R Address will receive a
        correct global interpretation.

   Note that the domain defined attribute value has a maximum length

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   of MTS.ub-domain-defined-attribute-value-length (128).  If this
   is exceeded by a mapping at the MTS level, then the gateway
   should reject the message in question.  If this occurs at the
   IPMS level, then the action should depend on the policy being
   taken, which is discussed in Section 5.1.3.

4.3.5.  X.400 -> RFC 822

   There are two basic cases:

   1.   RFC 822 addresses encoded in X.400.

   2.   "Genuine" X.400 addresses.  This may include symmetrically
        encoded RFC 822 addresses.

   When a MTS Recipient O/R Address is interpreted, gatewaying will be
   selected if there a single "RFC-822" domain defined attribute
   present.  In this case, use mapping A.  For other O/R Addresses

   1.   Contain the special attribute.


   2.   Identifies the local gateway or any other known gateway with
        the other attributes.

   Use mapping A.  In other cases, use mapping B.

        A pragmatic approach would be to assume that any O/R
        Address with the special domain defined attribute identifies
        an RFC 822 address.  This will usually work correctly, but is
        in principle not correct.

   Mapping A

   1.   Map the domain defined attribute value to ASCII, as defined
        in Chapter 3.

   Mapping B

   This will be used for X.400 addresses which do not use the explicit
   RFC 822 encoding.

   1.   For all string encoded attributes, remove any leading or
        trailing spaces, and replace adjacent spaces with a single

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   2.   Noting the hierarchy specified in 4.3.1, determine the
        maximum set of attributes which have an associated domain
        specification.  If no match is found, allocate the domain as
        the domain specification of the local gateway, and go to
        step 4.

   3.   Following the 4.3.1 hierarchy and noting any omitted
        components implied by the mapping tables (see Appendix F),
        if each successive component exists, and conforms to the
        syntax EBNF.domain-syntax (as defined in 4.3.1), allocate
        the next subdomain.  At least one attribute of the X.400
        address should not be mapped onto subdomain, as
        822.local-part cannot be null.

   4.   If the remaining components are personal-name components,
        conforming to the restrictions of 4.2.1, then EBNF.encoded-
        pn should be derived to form 822.local-part.  In other cases
        the remaining components should simply be encoded as a
        822.local-part using the EBNF.std-or-address syntax.  If
        necessary, the 822.quoted-string encoding should be used.

        If the derived 822.local-part can only be encoded by use of
        822.quoted-string, then use of the mapping defined
        in [Kille89b] may be appropriate.  Use of this mapping is

4.4.  Repeated Mappings

   The mappings defined are symmetrical and reversible across a single
   gateway.  The symmetry is particularly useful in cases of (mail
   exploder type) distribution list expansion.  For example, an X.400
   user sends to a list on an RFC 822 system which he belongs to.  The
   received message will have the originator and any 3rd party X.400 O/R
   Addresses in correct format (rather than doubly encoded).  In cases
   (X.400 or RFC 822) where there is common agreement on gateway
   identification, then this will apply to multiple gateways.

   When a message traverses multiple gateways, the mapping will always
   be reversible, in that a reply can be generated which will correctly
   reverse the path.  In many cases, the mapping will also be
   symmetrical, which will appear clean to the end user.  For example,
   if countries "AB" and "XY" have RFC 822 networks, but are
   interconnected by X.400, the following may happen:  The originator


Top       Page 44 
   This is routed to a gateway, which generates:

                C               = "XY"
                ADMD            = "PTT"
                PRMD            = "Griddle MHS Providers"
                Organisation    = "Widget Corporation"
                Surname         = "Soap"
                Given Name      = "Joe"

   This is then routed to another gateway where the mapping is reversed
   to give:


   Here, use of the gateway is transparent.

   Mappings will only be symmetrical where mapping tables are defined.
   In other cases, the reversibility is more important, due to the (far
   too frequent) cases where RFC 822 and X.400 services are partitioned.

   The syntax may be used to source route.  THIS IS STRONGLY
   DISCOURAGED.  For example:

      X.400 -> RFC 822  -> X.400

      C             = "UK"
      ADMD          = "Gold 400"
      PRMD          = "UK.AC"
      "RFC-822"     = "/PN=Duval/DD.Title=Manager/(a)Inria.ATLAS.FR"

   This will be sent to an arbitrary UK Academic Community gateway by
   X.400.  Then it will be sent by JNT Mail to another gateway
   determined by the domain Inria.ATLAS.FR (FR.ATLAS.Inria).  This will
   then derive the X.400 O/R Address:

      C             = "FR"
      ADMD          = "ATLAS"
      PRMD          = "Inria"
      PN.S          = "Duval"
      "Title"       = "Manager"

   RFC 822 -> X.400 -> RFC 822


   This will be sent to by RFC 822, then to the AC

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   PRMD by X.400, and then to by RFC 822.

4.5.  Directory Names

   Directory Names are an optional part of O/R Name, along with O/R
   Address.  The RFC 822 addresses are mapped onto the O/R Address
   component.  As there is no functional mapping for the Directory Name
   on the RFC 822 side, a textual mapping should be used.  There is no
   requirement for reversibility in terms of the goals of this
   specification.  There may be some loss of functionality in terms of
   third party recipients where only a directory name is given, but this
   seems preferable to the significant extra complexity of adding a full
   mapping for Directory Names.

4.6.  MTS Mappings

   The basic mappings at the MTS level are:

      1) 822-MTS originator ->
         MTS.OtherMessageDeliveryFields.originator-name ->
                    822-MTS originator

      2) 822-MTS recipient ->
      MTS.OtherMessageDeliveryFields.this-recipient-name ->
                    822-MTS recipient

   822-MTS recipients and return addresses are encoded as EBNF.822-

   The MTS Originator is always encoded as MTS.OriginatorName, which
   maps onto MTS.ORAddressAndOptionalDirectoryName, which in turn maps
   onto MTS.ORName.

4.6.1.  RFC 822 -> X.400

   From the 822-MTS Originator, use the basic ORAddress mapping, to
   generate MTS.PerMessageSubmissionFields.originator-name (MTS.ORName),
   without a DirectoryName.

   For recipients, the following settings should be made for each
   component of MTS.PerRecipientMessageSubmissionFields.

             This should be derived from the 822-MTS recipient by the
             basic ORAddress mapping.

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             This should be set according to content return policy, as
             discussed in Section 5.2.

             This optional component should be omitted, as this service
             is not needed.

             The default value (no extensions) should be used.

4.6.2.  X.400 -> RFC 822

   The basic functionality is to generate the 822-MTS originator and
   recipients.  There is information present on the X.400 side, which
   cannot be mapped into analogous 822-MTS services.  For this reason,
   new RFC 822 fields are added for the MTS Originator and Recipients.
   The information discarded at the 822-MTS level should be present in
   these fields.  There may also be the need to generate a delivery
   report.  822-MTS Mappings

   Use the basic ORAddress mapping, to generate the 822-MTS originator
   (return address) from MTS.OtherMessageDeliveryFields.originator-name
   (MTS.ORName).  If is present, it should be

   The 822-MTS recipient is conceptually generated from
   MTS.OtherMessageDeliveryFields.this-recipient-name.  This is done by
   taking MTS.OtherMessageDeliveryFields.this-recipient-name, and
   generating an 822-MTS recipient according to the basic ORAddress
   mapping, discarding if present.  However,
   if this model was followed exactly, there would be no possibility to
   have multiple 822-MTS recipients on a single message.  This is
   unacceptable, and so layering is violated.  The mapping needs to use
   the MTA level information, and map each value of
   MTA.PerRecipientMessageTransferFields.recipient-name, where the
   responsibility bit is set, onto an 822-MTS recipient.  Generation of RFC 822 Headers

   Not all per-recipient information can be passed at the 822-MTS level.
   For this reason, two new RFC 822 headers are created, in order to
   carry this information to the RFC 822 recipient.  These fields are
   "X400-Originator:"  and "X400-Recipients:".

   The "X400-Originator:" field should be set to the same value as the

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   822-MTS originator.  In addition, if
   MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName) contains then this Directory Name should be
   represented in an 822.comment.

   Recipient names, taken from each value of
   MTS.OtherMessageDeliveryFields.this-recipient-name and
   MTS.OtherMessageDeliveryFields.other-recipient-names should  be made
   available to the RFC 822 user by use of the "X400-Recipients:" field.
   By taking the recipients at the MTS level, disclosure of recipients
   will be dealt with correctly.  If any is
   present, it should be represented in an 822.comment.  If
   MTS.OtherMessageDeliveryFields.orignally-intended-recipient-name is
   present, then it should be represented in an associated 822.comment,
   starting with the string "Originally Intended Recipient".

   In addition, the following per-recipient services from
   MTS.OtherMessageDeliveryFields.extensions should be represented in
   comments if they are used.  None of these services can be provided on
   RFC 822 networks, and so in general these will be informative strings
   associated with other MTS recipients. In some cases, string values
   are defined.  For the remainder, the string value may be chosen by
   the implementor.   If the parameter has a default value, then no
   comment should be inserted.


             "(Physical Forwarding Prohibited)".

             "(Physical Forwarding Address Requested)".





             "(Physical Delivery Report Requested)".

             "(Proof of Delivery Requested)".

Top       Page 48  Delivery Report Generation

   If MTA.PerRecipientMessageTransferFields.per-recipient-indicators
   requires a positive delivery notification, this should be
   generated by the gateway.  Supplementary Information should be
   set to indicate that the report is gateway generated.

4.6.3.  Message IDs (MTS)

   A mapping from 822.msg-id to MTS.MTSIdentifier is defined.  The
   reverse mapping is not needed, as MTS.MTSIdentifier is always
   mapped onto new RFC 822 fields.  The value of
   MTS.MTSIdentifier.local-part will facilitate correlation of
   gateway errors.

   To map from 822.msg-id, apply the standard mapping to
   822.msg-id, in order to generate an MTS.ORAddress.  The Country,
   ADMD, and PRMD components of this should be used to generate
   MTS.MTSIdentifier.local-identifier should be set to the
   822.msg-id, including the braces "<" and ">".  If this string is
   longer than MTS.ub-local-id-length (32), then it should be
   truncated to this length.

   The reverse mapping is not used in this specification.  It
   would be applicable where MTS.MTSIdentifier.local-identifier is
   of syntax 822.msg-id, and it algorithmically identifies

4.7.  IPMS Mappings

   All RFC 822 addresses are assumed to use the 822.mailbox syntax.
   This should include all 822.comments associated with the lexical
   tokens of the 822.mailbox.  In the IPMS O/R Names are encoded as
   MTS.ORName.  This is used within the  IPMS.ORDescriptor,
   IPMS.RecipientSpecifier, and IPMS.IPMIdentifier.  An asymmetrical
   mapping is defined between these components.

4.7.1.  RFC 822 -> X.400

   To derive IPMS.ORDescriptor from an RFC 822 address.

      1.   Take the address, and extract an EBNF.822-address.  This can
           be derived trivially from either the 822.addr-spec or
           822.route-addr syntax.  This is mapped to MTS.ORName as
           described above, and used as IMPS.ORDescriptor.formal-name.

      2.   A string should be built consisting of (if present):

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           - The 822.phrase component if the 822.address is an
             822.phrase 822.route-addr construct.

           - Any 822.comments, in order, retaining the parentheses.

      This string should then be encoded into T.61 us a human
      oriented mapping (as described in Chapter 3).  If the string
      is not null, it should be assigned to

      3.   IPMS.ORDescriptor.telephone-number should be omitted.

   If IPMS.ORDescriptor is being used in IPMS.RecipientSpecifier,
   IPMS.RecipientSpecifier.reply-request and
   IPMS.RecipientSpecifier.notification-requests should be set to
   default values (none and false).

   If the construct is present, any included 822.mailbox
   should be encoded as above to generate a separate IPMS.ORDescriptor.
   The should be mapped to T.61, and a IPMS.ORDescriptor with
   only an free-form-name component built from it.

4.7.2.  X.400 -> RFC 822

   Mapping from IPMS.ORDescriptor to RFC 822 address.  In the basic
   case, where IPMS.ORDescriptor.formal-name is present, proceed as

      1.   Encode IPMS.ORDescriptor.formal-name (MTS.ORName) as

      2a.  If is present, convert it
           to ASCII (Chapter 3), and use this as the 822.phrase
           component of 822.mailbox using the 822.phrase 822.route-addr

      2b.  If is absent.  If
           EBNF.822-address is parsed as 822.addr-spec use this as the
           encoding of 822.mailbox.  If EBNF.822-address is parsed as
           822.route 822.addr-spec, then a 822.phrase taken from
           822.local-part should be added.

      3.   If IPMS.ORDescriptor.telephone-number is present, this
           should be placed in an 822.comment, with the string "Tel ".
           The normal international form of number should be used.  For

                  (Tel +44-1-387-7050)

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      4.   If is present,
           then a text representation should be placed in a trailing

      5.   If has any non-
           default values, then an 822.comment "(Receipt Notification
           Requested)", and/or "(Non Receipt Notification Requested)",
           and/or "(IPM Return Requested)" should be appended to the
           address.  The effort of correlating P1 and P2 information is
           too great to justify the gateway sending Receipt

      6.   If IPMS.RecipientSpecifier.reply-request is True, an
           822.comment "(Reply requested)" should be appended to the

   If IPMS.ORDescriptor.formal-name is absent,
   form-name should be converted to ASCII, and used as 822.phrase within
   the RFC 822 syntax.  For example:

      Free Form Name ":" ";"

   Steps 3-6 should then be followed.

4.7.3.  IP Message IDs

   There is a need to map both ways between 822.msg-id and
   IPMS.IPMIdentifier.  This allows for X.400 Receipt Notifications,
   Replies, and Cross References to reference an RFC 822 Message ID,
   which is preferable to a gateway generated ID.  A reversible and
   symmetrical mapping is defined.  This allows for good things to
   happen when messages pass multiple times across the X.400/RFC 822

   An important issue with messages identifiers is mapping to the exact
   form, as many systems use these ids as uninterpreted keys.  The use
   of table driven mappings is not always symmetrical, particularly in
   the light of alternative domain names, and alternative management
   domains.  For this reason, a purely algorithmic mapping is used.  A
   mapping which is simpler than that for addresses can be used for two

        -    There is no major requirement to make message IDs "natural"

        -    There is no issue about being able to reply to message IDs.
             (For addresses, creating a return path which works is more
             important than being symmetrical).

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   The mapping works by defining a way in which message IDs generated on
   one side of the gateway can be represented on the other side in a
   systematic manner.  The mapping is defined so that the possibility of
   clashes is is low enough to be treated as impossible.  822.msg-id represented in X.400

   IPMS.IPMIdentifier.user is omitted.  The IPMS.IPMIdentifier.user-
   relative-identifier is set to a printable string encoding of the
   822.msg-id with the angle braces ("<" and ">") removed.  IPMS.IPMIdentifier represented in RFC 822

   The 822.domain of 822.msg-id is set to the value "MHS".  The
   822.local-part of 822.msg-id is built as:

                [ printablestring ] "*"  [ std-or-address ]

   with EBNF.printablestring being the IPMS.IPMIdentifier.user-
   relative-identifier, and std-or-address being an encoding of the
   IPMS.IPMIdentifier.user.  If necessary, the 822.quoted-string
   encoding is used.  For example:

        <"147*/S=Dietrich/O=Siemens/ADMD=DBP/C=DE/"@MHS>  822.msg-id -> IPMS.IPMIdentifier

   If the 822.local-part can be parsed as:

                [ printablestring ] "*"  [ std-or-address ]

   and the 822.domain is "MHS", then this ID was X.400 generated.  If
   EBNF.printablestring is present, the value is assigned to
   IPMS.IPMIdentifier.user-relative-identifier.  If EBNF.std-or-address
   is present, the O/R Address components derived from it are used to
   set IPMS.IPMIdentifier.user.

   Otherwise, this is an RFC 822 generated ID.  In this case, set
   IPMS.IPMIdentifier.user-relative-identifier to a printable string
   encoding of the 822.msg-id without the angle braces.  IPMS.IPMIdentifier -> 822.msg-id

   If IPMS.IPMIdentifier.user is absent, and IPMS.IPMIdentifier.user-
   relative-identifier mapped to ASCII and angle braces added parses as
   822.msg-id, then this is an RFC 822 generated ID.

   Otherwise, the ID is X.400 generated.  Use the

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   IPMS.IPMIdentifier.user to generate an EBNF.std-or-address form
   string.  Build the 822.local-part of the 822.msg-id with the syntax:

                [ printablestring ] "*"  [ std-or-address ]

   The printablestring is taken from IPMS.IPMIdentifier.user-relative-
   identifier.  Use 822.quoted-string if necessary.  The 822.msg-id is
   generated with this 822.local-part, and "MHS" as the 822.domain.  Phrase form

   In "Reply-To:" and "References:", the encoding 822.phrase may be used
   as an alternative to 822.msg-id.  To map from 822.phrase to
   IPMS.IPMIdentifier, assign IPMS.IPMIdentifier.user-relative-
   identifier to the phrase.  When mapping from IPMS.IPMIdentifier for
   "Reply-To:" and "References:", if IPMS.IPMIdentifier.user is absent
   and IPMS.IPMIdentifier.user-relative-identifier does not parse as
   822.msg-id, generate an 822.phrase rather than adding the domain MHS.  RFC 987 backwards compatibility

   The mapping proposed here is different to that used in RFC 987, as
   the RFC 987 mapping lead to changed message IDs in many cases.
   Fixing the problems is preferable to retaining backwards
   compatibility.  An implementation of this standard is encouraged to
   recognise message IDs generated by RFC 987.

Chapter 5 -- Detailed Mappings

   This chapter gives detailed mappings for the functions outlined in
   Chapters 1 and 2.  It makes extensive use of the notations and
   mappings defined in Chapters 3 and 4.

5.1.  RFC 822 -> X.400

5.1.1.  Basic Approach

   A single IP Message is generated.  The RFC 822 headers are used to
   generate the IPMS.Heading.  The IP Message will have one IA5
   IPMS.BodyPart containing the RFC 822 message body.

   Some RFC 822 fields cannot be mapped onto a standard IPM Heading
   field, and so an extended field is defined in Section 5.1.2.  This is
   then used for fields which cannot be mapped onto existing services.

   The message is submitted to the MTS, and the services required can be
   defined by specifying MTS.MessageSubmissionEnvelope.  A few
   parameters of the MTA Abstract service are also specified, which are

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   not in principle available to the MTS User.  Use of these services
   allows RFC 822 MTA level parameters to be carried in the analogous
   X.400 service elements.  The advantages of this mapping far outweigh
   the layering violation.

5.1.2.  X.400 Extension Field

   An IPMS Extension is defined:

                rfc-822-field HEADING-EXTENSION
                        VALUE RFC822Field
                        ::= id-rfc-822-field

                RFC822Field ::= IA5String

   The Object Identifier id-rfc-822-field is defined in Appendix D.

   To encode any RFC 822 Header using this extension, the RFC822Field
   should be set to the  822.field omitting the trailing CRLF (e.g.,
   "Fruit-Of-The-Day: Kiwi Fruit"). Structured fields should be
   unfolded.  There should be no space before the ":".  The reverse
   mapping builds the RFC 822 field in a straightforward manner.

5.1.3.  Generating the IPM

   The IPM (IPMS Service Request) is generated according to the rules of
   this section.  The IPMS.IPM.body usually consists of one
   IPMS.BodyPart of type IPMS.IA5TextbodyPart with
   IPMS.IA5TextBodyPart.parameters.repertoire set to the default (ia5)
   which contains the body of the RFC 822 message.  The exception is
   where there is a "Comments:" field in the RFC 822 header.

   If no specific 1988 features are used, the IPM generated should be
   encoded as content type 2.  Otherwise, it should be encoded as
   content type 22.  The latter will always be the case if extension
   heading fields are generated.

   When generating the IPM, the issue of upper bounds must be
   considered.  At the MTS and MTA level, this specification is strict
   about enforcing upper bounds.  Three options are available at the IPM
   level.  Use of any of these options conforms to this standard.

      1.   Ignore upper bounds, and generate messages in the natural
           manner.  This assumes that if any truncation is done, it
           will happen at the recipient UA.  This will maximise
           transfer of information, but may break some recipient UAs.

      2.   Reject any inbound message which would cause a message

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           violating constraints to be generated.  This will be robust,
           but may prevent useful communication.

      3.   Truncate fields to the upper bounds specified in X.400.
           This will prevent problems with UAs which enforce upper
           bounds, but will sometimes discard useful information.

   These choices have different advantages and disadvantages, and the
   choice will depend on the exact application of the gateway.

   The rest of this section concerns IPMS.IPM.heading (IPMS.Heading).
   The only mandatory component of IPMS.Heading is the
   IPMS.Heading.this-IPM (IPMS.IPMIdentifier).  A default should be
   generated by the gateway.  With the exception of "Received:", the
   values of multiple fields should be merged (e.g., If there are two
   "To:" fields, then the mailboxes of both should be used).
   Information should be generated from the standard RFC 822 Headers as

             Ignore (Handled at MTS level)

             Ignore (Handled at MTA level)

             Mapped to IPMS.Heading.this-IPM.  For these, and all other
             fields containing 822.msg-id the mappings of Chapter 4 are
             used for each 822.msg-id.

             If Sender: is present, this is mapped to
             IPMS.Heading.authorizing-users.  If not, it is mapped to
             IPMS.Heading.originator.  For this, and other components
             containing addresses, the mappings of Chapter 4 are used
             for each address.

             Mapped to IPMS.Heading.originator.

             Mapped to IPMS.Heading.reply-recipients.

        To:  Mapped to IPMS.Heading.primary-recipients

        Cc:  Mapped to IPMS.Heading.copy-recipients.

        Bcc: Mapped to IPMS.Heading.blind-copy-recipients.

Top       Page 55 
             If there is one value, it is mapped to
             IPMS.Heading.replied-to-IPM, using the 822.phrase or
             822.msg-id mapping as appropriate.  If there are several
             values, they are mapped to IPMS.Heading.related-IPMs, along
             with any values from a "References:" field.

             Mapped to IPMS.Heading.related-IPMs.

             Mapped onto a heading extension.

             Mapped to IPMS.Heading.subject.  The field-body uses the
             human oriented mapping referenced in Chapter 3 from ASCII to

             Generate an IPMS.BodyPart of type IPMS.IA5TextbodyPart with
             IPMS.IA5TextBodyPart.parameters.repertoire set to the
             default (ia5), containing the value of the fields, preceded
             by the string "Comments: ".  This body part should precede
             the other one.

             Mapped onto a heading extension.

             Mapped onto a heading extension.

             Note that it would be possible to use a ForwardedIPMessage
             for these fields, but the semantics are (arguably) slightly
             different, and it is probably not worth the effort.

        Other Fields
             In particular X-* fields, and "illegal" fields in common
             usage (e.g., "Fruit-of-the-day:") are mapped onto a heading
             extension, unless covered by another section or appendix of
             this specification.  The same treatment should be applied to
             RFC 822 fields where the content of the field does not
             conform to RFC 822 (e.g., a Date: field with unparsable

5.1.4.  Mappings to the MTS Abstract Service

   The MTS.MessageSubmissionEnvelope comprises
   MTS.PerMessageSubmissionFields, and

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   MTS.PerRecipientMessageSubmissionFields.  The mandatory parameters
   should be defaulted as follows.

             This is always generated from 822-MTS, as defined in
             Chapter 4.

             Set to the value implied by the encoding of the IPM (2 or

             These will always be supplied from 822-MTS, as defined in
             Chapter 4.

   Optional components should be left out, and default components
   defaulted, with two exceptions.  For
   MTS.PerMessageSubmissionFields.per-message-indicators, the following
   settings should be made:

   -    Alternate recipient should be allowed, as it seems desirable
        to maximise the opportunity for (reliable) delivery.

   -    Content return request should be set according to the issues
        discussed in Section 5.2.

   should be made a set of one element

   The MTS.PerMessageSubmissionFields.content-correlator should be
   encoded as IA5String, and contain the Subject:, Message-ID:, Date:,
   and To: fields (if present).  This should include the strings
   "Subject:", "Date:", "To:", "Message-ID:", and appropriate folding.
   This should be truncated to MTS.ub-content-correlator-length (512)
   characters.  In addition, if there is a "Subject:" field, the
   MTS.PerMessageSubmissionFields.content-identifier, should be set to a
   printable string representation of the contents of it, truncated to
   MTS.ub-content-id-length (16).  Both are used, due to the much larger
   upper bound of the content correlator, and that the content id is
   available in X.400(1984).

5.1.5.  Mappings to the MTA Abstract Service

   There is a need to map directly onto some aspects of the MTA Abstract
   service, for the following reasons:

   -    So the the MTS Message Identifier can be generated from the

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        RFC 822 Message-ID:.

   -    So that the submission date can be generated from the

   -    To prevent loss of trace information.

   -    To prevent RFC 822/X.400 looping caused by distribution
        lists or redirects.

   The following mappings are defined.

        If this is present, the
        MTA.PerMessageTransferFields.message-identifier should be
        generated from it, using the mappings described in
        Chapter 4.

        This is used to set the first component of
        (MTA.TraceInformationElement).  The 822-MTS originator
        should be mapped into an MTS.ORAddress, and used to derive  The
        optional components of
        MTA.TraceInformationElement.domain-supplied-information are
        omitted, and the mandatory components are set as follows:

             This is set to the date derived from Date:

             Set to relayed.

        The first element of
        should be generated in an analogous manner, although this
        may later be dropped (see the procedures for "Received:").

        All RFC 822 trace is used to derive
        MTA.PerMessageTransferFields.trace-information and
        Processing of Received: lines should follow processing of
        Date:, and should be done from the the bottom to the top of
        the RFC 822 header (i.e., in chronological order).  If other
        trace elements are processed (Via:, X400-Received:), care
        should be taken to keep the relative ordering correct.  The

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        initial element of
        MTA.PerMessageTransferFields.trace-information will be
        generated already (from Date:).

        Consider the Received: field in question.  If the "by"  part
        of the received is present, use it to derive an
        MTS.GlobalDomainIdentifier.  If this is different from the
        one in the last element of
        create a new MTA.TraceInformationElement, and optionally
        This removal should be done in cases where the message is
        being transferred to another MD where there is no bilateral
        agreement to preserve internal trace beyond the local MD.
        The trace creation is as for internal trace described below,
        except that no MTA field is needed.

        Then add a new element (MTA.InternalTraceInformationElement)
        to MTA.PerMessageTransferFields.internal-trace-information,
        creating this if needed.  This shall be done, even if
        inter-MD trace is created.  The
        should be set to the value derived.  The
        (MTA.MTASuppliedInformation) should be set as follows:

             Derived from the date of the Received: line

             Set to relayed

        The MTA.InternalTraceInformationElement.mta-name should be
        taken from the "by" component of the "Received:" field,
        truncated to MTS.ub-mta-name-length (32).  For example:

           Received: from by
              vs6.Cs.Ucl.AC.UK via Janet with NIFTP  id aa03794;
              28 Mar 89 16:38 GMT

   Generates the string:


   Note that before transferring the message to some ADMDs, additional
   trace stripping may be required, as the implied path through multiple

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   MDs would violate ADMD policy.

   Two extended fields must be mapped, in order to prevent looping.
   "DL-Expansion-History:" is mapped to
   "Redirection-History:" is mapped to

5.1.6.  Mapping New Fields

   This specification defines a number of new fields for Reports,
   Notifications and IP Messages in Section 5.3.  As this specification
   only aims to preserve existing services, a gateway conforming to this
   specification does not need to map these fields to X.400, with the
   exception of "DL-Expansion-History" and "Redirection-History"
   described in the previous section.  However, it is usually desirable
   and beneficial to do so, particularly to facilitate support of a
   message traversing multiple gateways.  These mappings may be onto
   MTA, MTS, or IPMS services.

5.2.  Return of Contents

   It is not clear how widely supported the X.400 return of contents
   service will be.  Experience with X.400(1984) suggests that support
   of this service may not be universal.  As this service is expected in
   the RFC 822 world, two approaches are specified.  The choice will
   depend on the use of X.400 return of contents withing the X.400
   community being serviced by the gateway.

   In environments where return of contents is widely supported, content
   return can be requested as a service.  The content return service can
   then be passed back to the end (RFC 822) user in a straightforward

   In environments where return of contents is not widely supported, a
   gateway must make special provision to handle return of contents.
   For every message passing from RFC 822 -> X.400, content return
   request will not be requested, and report request always will be.
   When the delivery report comes back, the gateway can note that the
   message has been delivered to the recipient(s) in question.  If a
   non-delivery report is received, a meaningful report (containing some
   or all of the original message) can be sent to the 822-MTS
   originator.  If no report is received for a recipient, a (timeout)
   failure notice should be sent to the 822-MTS originator.  The gateway
   may retransmit the X.400 message if it wishes.  When this approach is
   taken, routing must be set up so that error reports are returned
   through the same MTA.  This approach may be difficult to use in
   conjunction with some routing strategies.

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5.3.  X.400 -> RFC 822

5.3.1.  Basic Approach

   A single RFC 822 message is generated from the incoming IP Message,
   Report, or IP Notification.   All IPMS.BodyParts are mapped onto a
   single RFC 822 body.  Other services are mapped onto RFC 822 header
   fields.  Where there is no appropriate existing field, new fields are
   defined for IPMS, MTS and MTA services.

   The gateway mechanisms will correspond to MTS Delivery.  As with
   submission, there are aspects where the MTA (transfer) services are
   also used. In particular, there is an optimisation to allow for
   multiple 822-MTS recipients.

5.3.2.  RFC 822 Settings

   An RFC 822 Service requires to have a number of mandatory fields in
   the RFC 822 Header.  Some 822-MTS services mandate specification of
   an 822-MTS Originator.  Even in cases where this is optional, it is
   usually desirable to specify a value.  The following defaults are
   defined, which should be used if the mappings specified do not derive
   a value:

   822-MTS Originator
        If this is not generated by the mapping (e.g., for a
        Delivery Report), a value pointing at a gateway
        administrator should be assigned.

        A value will always be generated

   From:If this is not generated by the mapping, it should be
        assigned equal to the 822-MTS Originator.  If this is
        gateway generated, an appropriate 822.phrase should be

   At least one recipient field
        If no recipient fields are generated, a field "To: list:;",
        should be added.

   This will ensure minimal RFC 822 compliance.  When generating RFC
   822 headers, folding should be used in an appropriate manner.

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