INTERNET-DRAFT S. Legg
draft-legg-ldap-transfer-04.txt eB2Bcom
Intended Category: Standards Track 24 November 2005
Updates: [SYNTAX]
Lightweight Directory Access Protocol (LDAP):
Transfer Encoding Options
Copyright (C) The Internet Society (2005).
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Abstract
Each attribute stored in a Lightweight Directory Access Protocol
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(LDAP) directory has a defined syntax (i.e., data type). A syntax
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. . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Each attribute stored in a Lightweight Directory Access Protocol
(LDAP) directory [LDAP] 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) [X.680] encoding rules,
instead of the usual LDAP-specific encoding. One option in
particular allows Extensible Markup Language (XML) [XML10][XML11]
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
[BCP14].
This specification makes use of definitions from the XML Information
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Set (Infoset) [ISET]. In particular, information item property names
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. In the absence of a transfer encoding option the usual
encoding for LDAP applies, i.e., the LDAP-specific encoding, or a
Basic Encoding Rules (BER) [X.690] encoding if the binary option is
present. 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 Generic String
Encoding Rules (GSER) [GSER] 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
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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
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 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, or are requested
to be, encoded according to BER [X.690]. 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, or are requested
to be, encoded according to the Distinguished Encoding Rules (DER)
[X.690].
The attribute option string "transfer-gser" specifies that the
associated attribute values or assertion value are, or are requested
to be, encoded according to GSER [GSER].
The attribute option string "transfer-rxer" specifies that the
associated attribute values or assertion value are, or are requested
to be, encoded according to the Robust XML Encoding Rules (RXER)
[RXER] as a Standalone RXER Encoding.
ASIDE: The Standalone RXER Encoding of an attribute or assertion
value is an XML document [XML10][XML11] where the root element
(i.e., the [document element] of the document information item)
has the [local name] "value" and no value for the [namespace name]
or [prefix]. The RXER encoding of the attribute or assertion
value, an ASN.1 abstract value, is not the root element as such,
but rather the content of that root element. When the abstract
value appears in an XML Lightweight Directory Access Protocol
(XLDAP) [XLDAP] operation, the [local name] of the encapsulating
element is determined from the surrounding context. An attribute
value in an XLDAP operation is also encapsulated in an element
information item with the [local name] "value", however this is
coincidental. An assertion value in an AttributeValueAssertion is
encapsulated in an element information item with the [local name]
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"assertionValue". An assertion value in a SubstringFilter is
encapsulated in an element information item with the [local name]
"initial", "any" or "final", as appropriate. An assertion value
in a MatchingRuleAssertion is encapsulated in an element
information item with the [local name] "matchValue".
The attribute option string "transfer-crxer" specifies that the
associated attribute values or assertion value are, or are requested
to be, encoded according to Canonical RXER (CRXER) [RXER] as a
Standalone CRXER Encoding.
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 and/or the special attribute description string "*". In
such cases the effective transfer encoding option for a particular
requested attribute is determined by the transfer encoding option (or
absence thereof) in the most specific attribute description in the
requested attributes list that applies to the attribute:
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(1) An applicable attribute description with an actual attribute type
is more specific than the special attribute description string
"*".
(2) 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.
(3) 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.
If an attribute is requested with a particular effective transfer
encoding option then the attribute SHALL NOT be returned with a
different transfer encoding option, or no transfer encoding option,
in its attribute description. Recall from Section 3 that when a
transfer encoding option is present in an attribute description the
associated attribute values must be encoded according to the encoding
method corresponding to the option. If for any reason an attribute
value cannot be encoded according to the requested transfer encoding
then the value MUST be omitted from the result.
If an attribute is requested with no effective transfer encoding
option then the attribute SHALL NOT be returned with a transfer
encoding option in its attribute description. This does not preclude
the attribute being returned with the binary option in its attribute
description.
Regardless of the encoding chosen, a particular attribute value is
returned at most once.
6. Syntaxes Requiring Binary Transfer
Certain syntaxes are defined without an LDAP-specific encoding and
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
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for binary transfer. For example, if the effective transfer encoding
option is say, "transfer-gser", then attribute values of a syntax
with a binary transfer requirement will be GSER encoded instead (if
they appear at all). 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 [PKI] that the
exact BER encoding of values of that syntax must 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, e.g., for the
verification of digital signatures, MUST NOT request attributes of
such syntaxes using an explicit or implicit transfer encoding option.
Directory servers MUST NOT use a transfer encoding option when
exporting (e.g., in an LDIF file [LDIF]) attributes of such syntaxes
if those attributes might later be imported into the same or another
directory server.
When interpreting security-sensitive fields, and in particular fields
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 <steven.legg@eb2bcom.com>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-der
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Family of Options: NO
Person & email address to contact for further information:
Steven Legg <steven.legg@eb2bcom.com>
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 <steven.legg@eb2bcom.com>
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 <steven.legg@eb2bcom.com>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
Subject: Request for
LDAP Attribute Description Option Registration
Option Name: transfer-crxer
Family of Options: NO
Person & email address to contact for further information:
Steven Legg <steven.legg@eb2bcom.com>
Specification: RFC XXXX
Author/Change Controller: IESG
Comments:
9. References
9.1. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDIF] Good, G., "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.
[BCP64] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
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Considerations for the Lightweight Directory Access
Protocol (LDAP)", BCP 64, RFC 3383, September 2002.
[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress,
September 2005.
[MODELS] Zeilenga, K., "LDAP: Directory Information Models",
draft-ietf-ldapbis-models-xx.txt, a work in progress,
February 2005.
[PROT] Sermersheim, J., "LDAP: The Protocol",
draft-ietf-ldapbis-protocol-xx.txt, a work in progress,
October 2005.
[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,
June 2005.
[GSER] 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
2005.
[PKI] Zeilenga, Kurt D., "Lightweight Directory Access Protocol
(LDAP) schema definitions for X.509 Certificates",
draft-zeilenga-ldap-x509-xx.txt, a work in progress, July
2005.
[RXER] Legg, S. and D. Prager, "Robust XML Encoding Rules (RXER)
for Abstract Syntax Notation One (ASN.1)",
draft-legg-xed-rxer-xx.txt, a work in progress, October
2005.
[X.680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X.690] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1,
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
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[XML10] 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.
[XML11] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
Yergeau, F., and J. Cowan, "Extensible Markup Language
(XML) 1.1", W3C Recommendation,
http://www.w3.org/TR/2004/REC-xml11-20040204, February
2004.
[ISET] Cowan, J. and R. Tobin, "XML Information Set (Second
Edition)", W3C Recommendation,
http://www.w3.org/TR/2004/REC-xml-infoset-20040204,
February 2004.
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, November 2005.
Author's Address
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
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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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Changes in Draft 04
The [local name] of the root element for RXER encoded assertion
values has been changed to "value" in all cases, for the sake of
simplicity.
A transfer encoding option for Canonical RXER has been defined.
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