INTERNET-DRAFT S. Legg
draft-legg-xed-roadmap-03.txt eB2Bcom
Intended Category: Standards Track D. Prager
February 21, 2005
The XML Enabled Directory
Copyright (C) The Internet Society (2005). All Rights Reserved.
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Abstract
The XML Enabled Directory (XED) leverages existing Lightweight
Directory Access Protocol (LDAP) and X.500 directory technology to
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create a directory service that stores, manages and transmits
Extensible Markup Language (XML) format data, while maintaining
interoperability with LDAP clients and X.500 agents. This document
introduces the various XED specifications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions. . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Leveraging Existing Specifications . . . . . . . . . . . . . . 4
4. Incorporating XML Specifications . . . . . . . . . . . . . . . 4
5. User Defined Syntaxes. . . . . . . . . . . . . . . . . . . . . 5
6. Protocols. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References. . . . . . . . . . . . . . . . . . 7
10.2. Informative References. . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The XML Enabled Directory (XED) framework leverages existing
Lightweight Directory Access Protocol (LDAP) [LDAP] and X.500
directory [X.500] technology to create a directory service that
stores, manages and transmits Extensible Markup Language (XML) [XML]
format data, while maintaining interoperability with LDAP clients,
X.500 Directory User Agents (DUAs), and X.500 Directory System Agents
(DSAs). This document introduces the various XED specifications.
The main features of XED are:
- semantically equivalent XML renditions of existing directory
protocols,
- XML renditions of directory data,
- the ability to accept at run time, user defined attribute syntaxes
specified in a variety of XML schema languages,
- the ability to perform filter matching on the parts of XML format
attribute values,
- the flexibility for implementors to develop XED clients using only
their favoured XML schema language.
Section 3 describes how existing directory constructs are mapped into
XML.
Section 4 describes how separately developed XML schema
specifications can be incorporated into directory specifications.
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Section 5 discusses how user defined attribute syntaxes can be
imported into directory schema.
The XML renditions of existing directory protocols are described in
Section 6.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[BCP14].
3. Leveraging Existing Specifications
X.500 is defined in terms of Abstract Syntax Notation One (ASN.1)
[X.680]. LDAP is defined in part in ASN.1 and in part with ad-hoc
syntax definitions, though for the most part the ad-hoc definitions
have an underlying ASN.1 definition through association with X.500.
The XED framework enables data conforming to existing directory data
types in ASN.1 to have an XML representation by defining a set of
ASN.1 encoding rules, the Robust XML Encoding Rules (RXER) [RXER]
[RXEREI]. This representation can be accessed in LDAP through a
transfer encoding option [XFER]. Directory data are otherwise
accessed in their XML representation through one of the XED protocols
(Section 6).
4. Incorporating XML Specifications
The XED framework does not aim for a complete specification of the
directory in one schema language (e.g., by translating everything
that isn't ASN.1 into ASN.1, or by translating everything that isn't
XML Schema [XSD0] into XML Schema), but rather seeks to integrate
specifications in differing schema definition languages into a
cohesive whole.
The motivation for this approach is the observation that although
XML Schema [XSD0], RELAX NG [RNG] and ASN.1 are broadly similar, they
each have unique features that cannot be adequately expressed in the
other languages. Thus a guiding principle for XED is the assertion
that the best schema language in which to represent a data type is
the language of its original specification. Consequently, a need
arises for the means to reference definitions not only in different
documents, but specified in different schema languages.
The XED framework defines mechanisms [RXEREI] by which ASN.1
specifications may directly reference XML Schema definitions,
RELAX NG definitions and DTDs. This capability is a prerequisite for
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importing user defined non-ASN.1 data types into directory schemas.
The framework also defines how qualified names [XMLNS] are assigned
to ASN.1 definitions [ASD] so that XML Schema and RELAX NG documents
can reference ASN.1 definitions.
Multi-schema validators conforming to the XED framework will of
course understand and apply references across schema languages.
However, in order to accommodate non-ASN.1 aware validators, the XED
framework provides the means to translate ASN.1 into compatible
XML Schema [CXSD] or RELAX NG [CRNG]. Note that RXER is specifically
designed to facilitate the derivation of a compatible XML Schema or
RELAX NG specification from an original ASN.1 specification.
Where a specification in one schema language is translated into
another schema language the original specification is referred to as
the Primary Syntax Definition (PSD) and the translation is referred
to as a Derived Syntax Definition (DSD).
A DSD is compatible with the PSD in the sense that any XML document
that validates according to the PSD will also validate according to
the DSD, though the reverse is not necessarily true. So the DSD can
be said to be no more restrictive than the PSD, and may be less
restrictive than the PSD.
A single schema language validator can use a DSD to validate markup
conforming to a definition in a different schema language, though the
validation it performs will probably be less thorough than that
performed by a multi-schema validator.
XED servers MUST use the corresponding PSD when validating any part
of an XML document (whether the markup represents a protocol
operation or a data value to be stored) to ensure the integrity of
directory data.
XED clients SHOULD use the PSD to validate but may instead use a DSD,
though many XED clients will have no compelling need to perform
validation. That is, they will generate valid markup as a matter of
course, and will rely on XED servers to only return valid markup. In
such cases, DSDs serve as an aid to understanding rather than as a
basis for implementation.
5. User Defined Syntaxes
While the creation of user defined syntaxes for directory attributes
is discouraged by LDAP, it is a central capability of XED. The XED
framework defines additional schema operational attributes [SCHEMA]
to hold ASN.1 Schema documents, XML Schema documents, RELAX NG
documents and DTDs, whose definitions may be referenced in the syntax
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definitions of user defined directory attribute types.
The ability for users to import schema definitions in the form of
ASN.1 types has been notionally available in X.500 since the second
edition, however the capability has never previously been seriously
supported. In part this is because a full parser for the ASN.1
language is difficult to write. The XED framework overcomes this
obstacle to the runtime acquisition of ASN.1 definitions by defining
ASN.1 Schema [ASD], a semantically equivalent XML representation for
ASN.1 specifications. ASN.1 Schema completely avoids the inherent
ambiguities of the ASN.1 language. It is therefore much easier to
parse than ASN.1 specifications. Note that an ASN.1 Schema
translation of a PSD ASN.1 specification is not a DSD. Both the
original ASN.1 specification and the translation into ASN.1 Schema
are regarded as different representations of the same PSD.
ASN.1 Schema, in conjunction with RXER, makes ASN.1 a fully-fledged
schema language for XML.
ASN.1 Schema is the preferred form for representing ASN.1
specifications in XED, and is the primary medium through which
definitions in other schema languages are imported into directory
attribute syntax definitions [SCHEMA].
The XED framework extends directory matching rules [MATCH], and in
particular component matching rules [CMR], to apply to attribute
values with a user defined syntax specified in an XML schema language
other than ASN.1.
6. Protocols
The Internet Directly Mapped (IDM) protocol [X.519] is a recent
addition to X.500 whereby X.500 protocol operations can be exchanged
between directory agents using TCP/IP [TCP] with minimal
encapsulation.
Protocol operations in the IDM protocol are encoded according to the
Basic Encoding Rules (BER) [X.690] of ASN.1. The XED framework
introduces a new exclusively XML-based protocol, called the XML
Internet Directly Mapped (XIDM) protocol [XLDAP], which differs from
the IDM protocol only in that the protocol operations are encoded
using RXER [RXER] instead of BER.
Whilst the IDM protocol is amenable to a simple substitution of the
encoding rules to create a uniformly XML formatted protocol
operation, LDAP is not, due to discontinuities in the encoding, i.e.,
places where transfer syntax transitions occur (typically from BER to
LDAP-specific [SYNTAX] and back to BER). A straight application of
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RXER to an LDAP operation would inconveniently force directory
attribute values, among other things, to be represented as
hexadecimal strings.
However, within the XED framework the LDAP ASN.1 specification is
transformed to create a new specification without the
discontinuities. Essentially, the bland OCTET STRING [X.680]
containers for directory data items in LDAP are replaced by the open
types [X.681] and specific types used by X.500. The XML Lightweight
Directory Access Protocol (XLDAP) [XLDAP] is the result of applying
RXER to instances of the message types of the transformed
specification. Apart from the change in syntax, XLDAP is
semantically equivalent to LDAP.
Since the XED protocols are algorithmically generated from the LDAP
and X.500 specifications, all future extensions to LDAP and X.500
automatically acquire a XED protocol representation.
7. Security Considerations
Since XED is derived from LDAP and X.500 the security considerations
that apply to LDAP and X.500 apply equally to XED.
The XED protocols encode all attribute values using RXER, which does
not necessarily enable the exact BER encoding of an attribute value
to be recovered. Such recovery is needed for the verification of
digital signatures. The XED protocols MUST NOT be used by
applications requiring such recovery.
When interpreting security-sensitive fields, and in particular fields
used to grant or deny access, implementations MUST ensure that any
comparisons are done on the underlying abstract value, regardless of
the particular encoding used.
8. Acknowledgements
This document and the technology it describes are a product of a
joint research project between Adacel Technologies Limited and Deakin
University on leveraging existing directory technology to produce an
XML-based directory service.
9. IANA Considerations
This document has no actions for IANA.
10. References
10.1. Normative References
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[TCP] Postel, J., "TRANSMISSION CONTROL PROTOCOL", RFC 793,
September 1981.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress,
February 2005.
[RXER] Legg, S. and D. Prager, "Robust XML Encoding Rules for
Abstract Syntax Notation One (ASN.1)",
draft-legg-xed-rxer-xx.txt, a work in progress, June 2004.
[RXEREI] Legg, S., "Encoding Instructions for the Robust XML
Encoding Rules (RXER)", draft-legg-xed-rxer-ei-xx.txt, a
work in progress, February 2005.
[ASD] Legg, S. and D. Prager, "ASN.1 Schema: An XML
Representation for Abstract Syntax Notation One (ASN.1)
Specifications", draft-legg-xed-asd-xx.txt, a work in
progress, June 2004.
[CXSD] Legg, S. and D. Prager, "Translation of ASN.1
Specifications into XML Schema", draft-legg-xed-xsd-xx.txt,
a work in progress, to be published.
[CRNG] Legg, S. and D. Prager, "Translation of ASN.1
Specifications into RELAX NG", draft-legg-xed-rng-xx.txt, a
work in progress, to be published.
[XFER] Legg, S., "Lightweight Directory Access Protocol (LDAP):
Transfer Encoding Options",
draft-legg-ldap-transfer-xx.txt, a work in progress, June
2004.
[SCHEMA] Legg, S. and D. Prager, "The XML Enabled Directory: Schema
Operational Attributes", draft-legg-xed-schema-xx.txt, a
work in progress, June 2004.
[MATCH] Legg, S. and D. Prager, "The XML Enabled Directory:
Matching Rules", draft-legg-xed-matching-xx.txt, a work in
progress, February 2005.
[XLDAP] Legg, S. and D. Prager, "The XML Enabled Directory:
Protocols", draft-legg-xed-protocols-xx.txt, a work in
progress, May 2004.
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[X.500] ITU-T Recommendation X.500 (02/01) | ISO/IEC 9594-1:2001,
Information technology - Open Systems Interconnection - The
Directory: Overview of concepts, models and services
[X.519] ITU-T Recommendation X.519 (02/01) | ISO/IEC 9594-5:2001,
Information technology - Open Systems Interconnection - The
Directory: Protocol specifications
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and F.
Yergeau, "Extensible Markup Language (XML) 1.0 (Third
Edition)", W3C Recommendation,
http://www.w3.org/TR/2004/REC-xml-20040204, February 2004.
[XMLNS] Bray, T., Hollander, D. and A. Layman, "Namespaces in XML",
http://www.w3.org/TR/1999/REC-xml-names-19990114, January
1999.
10.2. Informative References
[CMR] Legg, S., "Lightweight Directory Access Protocol (LDAP) and
X.500 Component Matching Rules", RFC 3687, February 2004.
[SYNTAX] Legg, S. and K. Dally, "Lightweight Directory Access
Protocol (LDAP): Syntaxes and Matching Rules",
draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress,
February 2005.
[X.680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X.681] ITU-T Recommendation X.681 (07/02) | ISO/IEC 8824-2,
Information technology - Abstract Syntax Notation One
(ASN.1): Information object specification.
[X.690] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
[XSD0] Fallside, D., "XML Schema Part 0: Primer", W3C
Recommendation, http://www.w3.org/TR/2001/REC-
xmlschema-0-20010502, May 2001.
[RNG] Clark, J. and M. Makoto, "RELAX NG Tutorial", OASIS
Committee Specification, http://www.oasis-
open.org/committees/relax-ng/tutorial-20011203.html,
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December 2001.
Authors' Addresses
Dr. Steven Legg
eB2Bcom
Suite 3, Woodhouse Corporate Centre
935 Station Street
Box Hill North, Victoria 3129
AUSTRALIA
Phone: +61 3 9896 7830
Fax: +61 3 9896 7801
EMail: steven.legg@eb2bcom.com
Dr. Daniel Prager
EMail: dan@layabout.net
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Changes in Draft 01
The reference for the XED protocols has changed. Some of the text in
Section 6 has been moved to the protocols document.
References for XML Schema and RELAX NG have been added.
Changes in Draft 02
The Directory XML Encoding Rules (DXER) have been renamed to the
Robust XML Encoding Rules (RXER).
Changes in Draft 03
The document has been reformatted in line with current practice.
The document "Encoding Instructions for the Robust XML Encoding Rules
(RXER)" has been added to the roadmap. It obsoletes the document
"The XML Enabled Directory: Schema Language Integration".
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