RFC1838 - Use of the X.500 Directory to support mapping between X.400 and RFC822

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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 RFC822 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 RFC1327 [2].

1. X.400/RFC822 Mappings

RFC1327 defines an algorithm for maintaining a global mapping
between X.400 and RFC822 addresses directory [2]. RFC1327 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.

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/RFC822 Mapping, O=Internet

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

X.400 to RFC822 This table gives the equivalence mapping from X.400
to RFC822. 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 RFC822,
OU=X.400/RFC822 Mapping, O=Internet

RFC822 to X.400 There is a domain tree under this. This table holds
the equivalence mapping from RFC822 to X.400, and the gateway
mapping defined in RFC1327. An example entry is:

DomainComponent=ISODE, DomainComponent=COM,
CN=RFC822 to X.400,
OU=X.400/RFC822 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.

2. Omitted Components

In RFC1327, 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 RFC822
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 RFC1327 mappings

-----------------------------------------------------------------------
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 RFC822

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 RFC822,
OU=X.400/RFC822 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 RFC1327 mapping shall make the
appropriate entry.

Functionally, mapping takes place exactly according to RFC1327. 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 RFC1327 key in the in
the X.400 to RFC822 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.

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 RFC822 to X.400

There is an analogous structure for mappings in the reverse
direction. The domain hierarchy is represented in the DIT according
to RFC1279. The domain:

AC.UK

Is represented in the DIT as:

DomainComponent=AC, DomainComponent=UK, CN=RFC822 to X.400,
OU=X.400/RFC822 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 RFC1327 [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 RFC1327. The longest match is found by the following algorithm.

1. Derive a directory name from the domain part of the RFC822
address.

2. Look up this name in the RFC822 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

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", RFC1279,
Department of Computer Science, University College London,
November 1991.

[2] Kille, S., "Mapping between X.400(1988)/ISO 10021 and RFC822",
RFC1327, Department of Computer Science, University College
London, May 1992.

[3] Kille, S., "MHS Use of the X.500 Directory to Support MHS
Routing", RFC1801, ISODE Consortium, June 1995.

[4] Kille, S., "Representing Tables and Subtrees in the X.500
Directory", RFC1837, ISODE Consortium, August 1995.

[5] Kille, S., "Representing the O/R Address Hierarchy in the X.500
Directory Information Tree", RFC1836, ISODE Consortium, August
1995.

6. Security Considerations

Security issues are not discussed in this memo.

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

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

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