INTERNET-DRAFT S. Legg
draft-legg-ldap-transfer-03.txt Adacel Technologies
Intended Category: Standards Track 16 June 2004
Updates: RFC 2252bis
Lightweight Directory Access Protocol (LDAP):
Transfer Encoding Options
Copyright (C) The Internet Society (2004). All Rights Reserved.
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Abstract
Each attribute stored in a Lightweight Directory Access Protocol
(LDAP) directory has a defined syntax (i.e., data type). A syntax
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definition specifies how attribute values conforming to the syntax
are normally represented when transferred in LDAP operations. This
representation is referred to as the LDAP-specific encoding to
distinguish it from other methods of encoding attribute values. This
document introduces a new category of attribute options, called
transfer encoding options, which can be used to specify that the
associated attribute values are encoded according to one of these
other methods.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions. . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Transfer Encoding Options. . . . . . . . . . . . . . . . . . . 4
4. Defined Transfer Encoding Options. . . . . . . . . . . . . . . 5
5. Attributes Returned in a Search. . . . . . . . . . . . . . . . 6
6. Syntaxes Requiring Binary Transfer . . . . . . . . . . . . . . 7
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Each attribute stored in a Lightweight Directory Access Protocol
(LDAP) directory [ROADMAP] has a defined syntax (i.e., data type)
which constrains the structure and format of its values.
The description of each syntax [SYNTAX] specifies how attribute or
assertion values [MODELS] conforming to the syntax are normally
represented when transferred in LDAP operations [PROT]. This
representation is referred to as the LDAP-specific encoding to
distinguish it from other methods of encoding attribute values.
This document introduces a new category of attribute options
[MODELS], called transfer encoding options, that allow attribute and
assertion values to be transferred using an alternative method of
encoding. This document defines several transfer encoding options
which can be used in an attribute description [MODELS] in an LDAP
operation to specify that the associated attribute values or
assertion value are, or are requested to be, encoded according to
specific Abstract Syntax Notation One (ASN.1) [X680] encoding rules,
instead of the usual LDAP-specific encoding. One option in
particular allows Extensible Markup Language (XML) [XML] encodings.
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
[KEYWORD].
This specification makes use of definitions from the XML Information
Set (Infoset) [ISET]. In particular, information item property names
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are presented per the Infoset, e.g., [local name].
3. Transfer Encoding Options
Transfer encoding options enable attribute and assertion values to be
transferred using an alternative method of encoding to the default
LDAP-specific encoding. In fact, some attribute and assertion
syntaxes do not have a defined LDAP-specific encoding in which case
the only way values of those syntaxes can be transferred is by using
an alternative encoding.
The binary option [BINARY] is not formally regarded as a transfer
encoding option, though it has much in common with transfer encoding
options. The requirements governing the use of transfer encoding
options do not apply to the binary option. The requirements
governing the use of the binary option are described elsewhere
[BINARY].
In terms of the protocol [PROT], a transfer encoding option specifies
that the contents octets of an associated AttributeValue or
AssertionValue OCTET STRING are a complete encoding of the relevant
value according to the encoding method specified by the option.
Where a transfer encoding option is present in an attribute
description the associated attribute values or assertion value MUST
be encoded according to the encoding method corresponding to the
option. Note that it is possible for a syntax to be defined such
that its LDAP-specific encoding is exactly the same as its encoding
according to some transfer encoding option (e.g., the LDAP-specific
encoding might be defined to be the same as the BER encoding).
Transfer encoding options are mutually exclusive. An attribute
description SHALL NOT contain more than one transfer encoding option,
and SHALL NOT contain both a transfer encoding option and the binary
option.
Transfer encoding options are not tagging options [MODELS] so the
presence of a transfer encoding option does not specify an attribute
subtype. An attribute description containing a transfer encoding
option references exactly the same attribute as the same attribute
description without the transfer encoding option. The
supertype/subtype relationships of attributes with tagging options
are not altered in any way by the presence or absence of transfer
encoding options.
An attribute description SHALL be treated as unrecognized if it
contains a transfer encoding option and the syntax of the attribute
does not have an associated ASN.1 type [SYNTAX], or if the nominated
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encoding is not supported for that type.
The presence or absence of a transfer encoding option only affects
the transfer of attribute and assertion values in protocol; servers
store any particular attribute value in a single format of their
choosing.
4. Defined Transfer Encoding Options
The attribute option string "transfer-ber" specifies that the
associated attribute values or assertion value are (to be) encoded
according to the Basic Encoding Rules [X690] of ASN.1. This option
is similar to the binary option [BINARY], however servers are more
restricted in when they can use "transfer-ber" which leads to more
predictability in the results returned to clients who request
"transfer-ber".
The attribute option string "transfer-der" specifies that the
associated attribute values or assertion value are (to be) encoded
according to the Distinguished Encoding Rules [X690] of ASN.1.
The attribute option string "transfer-gser" specifies that the
associated attribute values or assertion value are (to be) encoded
according to the Generic String Encoding Rules (GSER) [RFC3641].
The attribute option string "transfer-rxer" specifies that the
associated attribute values or assertion value are (to be) encoded as
an XML document [XML]. The [local name] of the [document element] of
the document information item representing the associated value SHALL
be the identifier of the NamedType [X680] in the LDAP ASN.1
specification [PROT] defining the associated attribute value or
assertion value, or "item" if the associated value isn't in a
NamedType. This means:
- for an AttributeValue the identifier is "value" in every case,
- for an AssertionValue in an AttributeValueAssertion the identifier
is "assertionValue",
- for an AssertionValue in a SubstringFilter the identifier is
"initial", "any" or "final", as appropriate,
- for an AssertionValue in a MatchingRuleAssertion the identifier is
"matchValue".
The [namespace name] of the [document element] SHALL have no value.
The content of the [document element] is the Robust XML Encoding
Rules (RXER) [RXER] encoding of the associated attribute or assertion
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value. Note that the enclosing element for the RXER encoding has the
form it would take in an XLDAP operation [XLDAP].
Note that, like all attribute options, the strings representing
transfer encoding options are case insensitive.
All future registrations of option strings for transfer encoding
options should use the "transfer-" prefix so that LDAP clients and
servers can recognize that an option is a transfer encoding option
even though the particular encoding rules may be unrecognized.
5. Attributes Returned in a Search
An LDAP search request [PROT] contains a list of the attributes (the
requested attributes list) to be returned from each entry matching
the search filter. An attribute description in the requested
attributes list also implicitly requests all subtypes of the
attribute type in the attribute description, whether through
attribute subtyping or attribute tagging option subtyping [MODELS].
The requested attributes list MAY contain attribute descriptions with
a transfer encoding option, but MUST NOT contain two attribute
descriptions with the same attribute type and the same tagging
options (even if only one of them has a transfer encoding option). A
transfer encoding option in an attribute description in the requested
attributes list implicitly applies to the subtypes of the attribute
type in the attribute description.
If the list of attributes in a search request is empty, or contains
the special attribute description string "*", then all user
attributes are requested to be returned.
In general, it is possible for a particular attribute to be
explicitly requested by an attribute description and/or implicitly
requested by the attribute descriptions of one or more of its
supertypes. In such cases the effective transfer encoding being
requested for a particular attribute is determined by the transfer
encoding option (or lack thereof) in the most specific attribute
description in the requested attributes list that applies to the
attribute.
An applicable attribute description with an actual attribute type is
more specific than the special attribute description string "*".
If the attribute type of one applicable attribute description is a
direct or indirect subtype of the attribute type in another
applicable attribute description then the former attribute
description is more specific than the latter attribute description.
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If two applicable attribute descriptions have the same attribute type
and the tagging options of one attribute description are a superset
of the tagging options of the other attribute description then the
former attribute description is more specific than the latter
attribute description.
Attributes MUST either be returned in the effective transfer encoding
requested, be returned with no attribute values, or be omitted from
the results. An attribute SHALL NOT be returned using an encoding
other than the effective transfer encoding requested.
Regardless of the encoding chosen, a particular attribute value is
returned at most once.
6. Syntaxes Requiring Binary Transfer
Certain syntaxes are required to be transferred in the BER encoded
form. These syntaxes are said to have a binary transfer requirement.
The Certificate, Certificate List, Certificate Pair and Supported
Algorithm syntaxes [PKI] are examples of syntaxes with a binary
transfer requirement. These syntaxes also have an additional
requirement that the exact BER encoding must be preserved. Note that
this is a property of the syntaxes themselves, and not a property of
the binary option or any of the transfer encoding options.
Transfer encoding options SHALL take precedence over the requirement
for binary transfer. For example, if the effective transfer encoding
requested is say "transfer-gser", then attribute values of a syntax
with a binary transfer requirement will be GSER encoded. In the
absence of a transfer encoding option the normal rules on binary
transfer and the use of the binary option SHALL apply.
7. Security Considerations
There is a requirement on some attribute syntaxes that the exact BER
encoding of values of that syntax be preserved. In general, a
transformation from the BER encoding into some other encoding (e.g.,
GSER) and back into the BER encoding will not necessarily reproduce
exactly the octets of the original BER encoding.
Applications needing the original BER encoding to be preserved, e.g.,
for the verification of digital signatures, MUST NOT request
attributes with such a requirement using a transfer encoding option.
These attributes MUST be requested explicitly or implicitly with the
binary option, or implicitly without the binary option (or any
transfer encoding option).
When interpreting security-sensitive fields, and in particular fields
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used to grant or deny access, implementations MUST ensure that any
matching rule comparisons are done on the underlying abstract value,
regardless of the particular encoding used.
8. IANA Considerations
The Internet Assigned Numbers Authority (IANA) is requested to update
the LDAP attribute description option registry [BCP64] as indicated
by the following templates:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-ber
Family of Options: NO
Person & email address to contact for further information:
Steven Legg <[email protected]>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-der
Family of Options: NO
Person & email address to contact for further information:
Steven Legg <[email protected]>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-gser
Family of Options: NO
Person & email address to contact for further information:
Steven Legg <[email protected]>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-rxer
Family of Options: NO
Person & email address to contact for further information:
Steven Legg <[email protected]>
Specification: RFC XXXX
Author/Change Controller: IESG
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Comments:
9. References
9.1. Normative References
[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[BCP64] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
Considerations for the Lightweight Directory Access
Protcol (LDAP)", BCP 64, RFC 3383, September 2002.
[ROADMAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress,
June 2004.
[MODELS] Zeilenga, K., "LDAP: Directory Information Models",
draft-ietf-ldapbis-models-xx.txt, a work in progress, June
2004.
[PROT] Sermersheim, J., "LDAP: The Protocol",
draft-ietf-ldapbis-protocol-xx.txt, a work in progress,
May 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,
May 2004.
[RFC3641] Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
Types", RFC 3641, October 2003.
[BINARY] Legg, S., "Lightweight Directory Access Protocol (LDAP):
The Binary Encoding Option",
draft-legg-ldap-binary-xx.txt, a work in progress, June
2004.
[RXER] Legg, S. and D. Prager, "Robust XML Encoding Rules for
ASN.1 Types", draft-legg-xed-rxer-00.txt, a work in
progress, June 2004.
[X680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1,
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Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
[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.
[ISET] Cowan, J. and R. Tobin, "XML Information Set",
http://www.w3.org/TR/2001/REC-xml-infoset-20011024,
October 2001.
9.2. Informative References
[XLDAP] Legg, S. and D. Prager, "The XML Enabled Directory:
Protocols", draft-legg-xed-protocols-xx.txt, a work in
progress, May 2004.
Author's Address
Steven Legg
Adacel Technologies Ltd.
250 Bay Street
Brighton, Victoria 3186
AUSTRALIA
Phone: +61 3 8530 7710
Fax: +61 3 8530 7888
Email: [email protected]
Full Copyright Statement
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Changes in Draft 01
A transfer encoding option for RXER has been added.
Changes in Draft 02
The local name of the root element of the XML document representing
an attribute value encoded according to the transfer-rxer encoding
option has been changed from "item" to "value" to align with
revisions to the LDAP protocol specification [PROT].
The Directory XML Encoding Rules (DXER) have been renamed to the
Robust XML Encoding Rules (RXER).
Changes in Draft 03
The special attribute description strings that consist of the
asterisk character followed by a transfer encoding option, e.g.,
"*;transfer-ber", "*;transfer-gser", have been removed from this
specification. An LDAP control will be defined in a separate
document to provide equivalent functionality.
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