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Network Working Group                                           S. Kille
Request for Comments: 1838                              ISODE Consortium
Category: Experimental                                       August 1995


      Use of the X.500 Directory to support mapping between X.400
                         and RFC 822 Addresses

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This document defines how to use directory to support the mapping
   between X.400 O/R Addresses and mailboxes defined in RFC 1327 [2].

1.  X.400/RFC 822 Mappings

   RFC 1327 defines an algorithm for maintaining a global mapping
   between X.400 and RFC 822 addresses directory [2].  RFC 1327 also
   defines a table based mechanism for maintaining this mapping.  There
   is substantial benefit to maintaining this mapping within the
   directory.  In particular, this will lead to an approach for managing
   the mapping which is both distributed and scalable.

   Mechanisms for representing O/R Address and Domain hierarchies within
   the DIT are defined in [1, 5].  These techniques are used to define
   two independent subtrees in the DIT, which contain the mapping
   information.  The benefits of this approach are:

   1.  The mapping information is kept in a clearly defined area which
       can be widely replicated in an efficient manner.  The tree is
       constrained to hold only information needed to support the
       mapping.  This is important as gateways need good access to the
       entire mapping.

   2.  It facilitates migration from the currently deployed table-based
       approach.

   3.  It handles the issues of "missing components" in a natural
       manner.






Kille                         Experimental                      [Page 1]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


          An alternative approach which is not taken is to locate the
          information in the routing subtrees.  The benefits of this
          would be:

        o  It is the "natural" location, and will also help to
           ensure correct administrative authority for a mapping
           definition.

        o  The tree will usually be accessed for routing, and so it
           will be efficient for addresses which are being routed.

          This is not done, as the benefits of the approach proposed
          are greater.

   There are three mappings, which are represented by two subtrees
   located under:

   OU=X.400/RFC 822 Mapping,  O=Internet

   These subtree roots are of object class subtree, and use the
   mechanism for representing subtrees defined in [4].

   X.400 to RFC 822 This table gives the equivalence mapping from X.400
       to RFC 822.  There is an O/R Address tree under this.  An example
       entry is:

       PRMD=UK.AC, ADMD=Gold 400, C=GB, CN=X.400 to RFC 822,
       OU=X.400/RFC 822 Mapping,  O=Internet


   RFC 822 to X.400 There is a domain tree under this.  This table holds
       the equivalence mapping from RFC 822 to X.400, and the gateway
       mapping defined in RFC 1327.  An example entry is:

       DomainComponent=ISODE, DomainComponent=COM,
       CN=RFC 822 to X.400,
       OU=X.400/RFC 822 Mapping,  O=Internet

   The values of the table mapping are defined by use of two new object
   classes, as specified in Figure 1.  The objects give pointers to the
   mapped components.










Kille                         Experimental                      [Page 2]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


2.  Omitted Components

   In RFC 1327, it is possible to have omitted components in O/R
   Addresses on either side of the mapping.  A mechanism to represent
   such omitted components is defined in Figure 2.

   The attribute at-or-address-component-type is set to the X.500
   attribute type associated with the omitted component (e.g., at-prmd-
   name).  This mechanism is for use only within the X.400 to RFC 822
   subtree and for the at-associated-or-address attribute.

-----------------------------------------------------------------------
rFC822ToX400Mapping OBJECT-CLASS ::= {
    SUBCLASS OF {domain-component}
    MAY CONTAIN {
        associatedORAddress|
        associatedX400Gateway}
    ID oc-rfc822-to-x400-mapping}

x400ToRFC822Mapping OBJECT-CLASS ::= {
    SUBCLASS OF {top}
    MAY CONTAIN {                                                   10
        associatedDomain}
    ID oc-x400-to-rfc822-mapping}


associatedORAddress ATTRIBUTE ::= {
    SUBTYPE OF distinguishedName
    SINGLE VALUE
    ID at-associated-or-address}

                                                                    20
associatedX400Gateway ATTRIBUTE ::= {
    SUBTYPE OF mhs-or-addresses
    MULTI VALUE
    ID at-associated-x400-gateway}

associatedDomain ATTRIBUTE ::= {
    SUBTYPE OF name
    WITH SYNTAX caseIgnoreIA5String
    SINGLE VALUE
    ID at-associated-domain}                                        30


             Figure 1:  ObjectClasses for RFC 1327 mappings






Kille                         Experimental                      [Page 3]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


-----------------------------------------------------------------------
omittedORAddressComponent OBJECT-CLASS ::=
        SUBCLASS OF {top}
        MUST Contain {
                oRAddressComponentType
        }
        ID oc-omitted-or-address-component}


oRAddressComponentType ATTRIBUTE ::= {
        SUBTYPE OF  objectIdentifier                                10
        SINGLE VALUE
        ID at-or-address-component-type}

                Figure 2:  Omitted O/R Address Component

3.  Mapping from X.400 to RFC 822

   As an example, consider the mapping from the O/R Address:

   P=UK.AC; A=Gold 400; C=GB

   This would be keyed by the directory entry:

   PRMD=UK.AC, ADMD=Gold 400, C=GB, CN=X.400 to RFC 822,
   OU=X.400/RFC 822 Mapping,  O=Internet

   and return the mapping from the associatedDomain attribute, which
   gives the domain which this O/R address maps to.  This attribute is
   used to define authoritative mappings, which are placed in the open
   community tree.  The manager of an RFC 1327 mapping shall make the
   appropriate entry.

   Functionally, mapping takes place exactly according to RFC 1327.  The
   longest match is found by the following algorithm.

   1.  Take the O/R Address, and derive a directory name.  This will be
       the O/R Address as far as the lowest OU.

   2.  Look up the entire name derived from the RFC 1327 key in the in
       the X.400 to RFC 822 subtree.  This lookup will either succeed,
       or it will fail and indicate the longest possible match, which
       can then be looked up.

   3.  Check for an associatedDomain attribute in the matched entry.






Kille                         Experimental                      [Page 4]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


   The mapping can always be achieved with two lookups.

   Because of the availability of aliases, some of the table mappings
   may be simplified.  In addition, the directory can support mapping
   from addresses using the numeric country codes.

4.  Mapping from RFC 822 to X.400

   There is an analogous structure for mappings in the reverse
   direction.  The domain hierarchy is represented in the DIT according
   to RFC 1279.  The domain:

   AC.UK

   Is represented in the DIT as:

   DomainComponent=AC, DomainComponent=UK,  CN=RFC 822 to X.400,
   OU=X.400/RFC 822 Mapping,  O=Internet

   This has associated with it the attribute associatedORAddress encoded
   as a distinguished name with a value:

   PRMD=UK.AC, ADMD=Gold 400, C=GB

   The "table 3" mapping defined in RFC 1327 [2] is provided by the
   associatedX400Gateway attribute.  This value may identify multiple
   possible associated gateways.  This information is looked up at the
   same time as mapped O/R addresses.  In effect, this provides a
   fallback mapping, which is found if there is no equivalence mapping.
   Because of the nature of the mapping a domain will map to either a
   gateway or a domain, but not both.  Thus, there shall never be both
   an associatedX400Gateway and associatedORAddress attribute present in
   the same entry.  Functionally, mapping takes place exactly according
   to RFC 1327.  The longest match is found by the following algorithm.

   1.  Derive a directory name from the domain part of the RFC 822
       address.

   2.  Look up this name in the RFC 822 to X.400 subtree to find the
       mapped value (either associatedORAddress or
       associatedX400Gateway.).  If the lookup fails, the error will
       indicate the longest match, which can then be looked up.

   If associatedORAddress is found, this will define the mapped O/R
   Address.  The mapping can always be achieved with two lookups.  If an
   associatedX400Gateway is present, the address in question will be
   encoded as a domain defined attribute, relative to the O/R Address
   defined by this attribute.  If multiple associatedX400Gateway



Kille                         Experimental                      [Page 5]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


   attributes are found, the MTA may select the one it chooses to use.

   Because of the availability of aliases, some of the table mappings
   may be simplified.  In addition, the directory can support mapping
   from addresses using the numeric country codes.

5.  Acknowledgements

   Acknowledgements for work on this document are given in [3].

References

   [1] Kille, S. "X.500 and Domains", RFC 1279,
       Department of Computer Science, University College London,
       November 1991.

   [2] Kille, S., "Mapping between X.400(1988)/ISO 10021 and RFC 822",
       RFC 1327, Department of Computer Science, University College
       London, May 1992.

   [3] Kille, S., "MHS Use of the X.500 Directory to Support MHS
       Routing", RFC 1801, ISODE Consortium, June 1995.

   [4] Kille, S., "Representing Tables and Subtrees in the X.500
       Directory", RFC 1837, ISODE Consortium, August 1995.

   [5] Kille, S., "Representing the O/R Address Hierarchy in the X.500
       Directory Information Tree", RFC 1836, ISODE Consortium, August
       1995.

6.  Security Considerations

   Security issues are not discussed in this memo.


















Kille                         Experimental                      [Page 6]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


7.  Author's Address

   Steve Kille
   ISODE Consortium
   The Dome
   The Square
   Richmond
   TW9 1DT
   England

   Phone:  +44-81-332-9091
   Internet EMail:  S.Kille@ISODE.COM

   X.400:  I=S; S=Kille; O=ISODE Consortium; P=ISODE;
   A=Mailnet; C=FI;

   UFN: S. Kille, ISODE Consortium, GB


































Kille                         Experimental                      [Page 7]

RFC 1838             RFC 822/X.400 Mapping by X.500          August 1995


A  Object Identifier Assignment

-----------------------------------------------------------------------
mhs-ds OBJECT IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
          private(4) enterprises(1) isode-consortium (453) mhs-ds (7)}

mapping OBJECT IDENTIFIER ::= {mhs-ds 4}

oc OBJECT IDENTIFIER ::= {mapping 1}
at OBJECT IDENTIFIER ::= {mapping 2}


oc-rfc822-to-x400-mapping OBJECT IDENTIFIER ::= {oc 1}              10
oc-x400-to-rfc822-mapping OBJECT IDENTIFIER ::= {oc 2}
oc-omitted-or-address-component OBJECT IDENTIFIER ::= {oc 3}

at-associated-or-address OBJECT IDENTIFIER ::= {at 6}
at-associated-x400-gateway OBJECT IDENTIFIER ::= {at 3}
at-associated-domain OBJECT IDENTIFIER ::= {at 4}
at-or-address-component-type OBJECT IDENTIFIER ::= {at 7}

Figure 3:  Object Identifier Assignment





























Kille                         Experimental                      [Page 8]