





INTERNET-DRAFT                                                   S. Legg
draft-legg-ldap-transfer-06.txt                                  eB2Bcom
Intended status: Experimental                          21 September 2007


             Lightweight Directory Access Protocol (LDAP):
                       Transfer Encoding Options

Status of This Memo

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   Technical discussion of this document should take place on the LDAP
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   This Internet-Draft expires on 21 March 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Each attribute stored in a Lightweight Directory Access Protocol
   (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



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   distinguish it from other methods of encoding attribute values.  This
   document introduces a new category of attribute options, called
   transfer encoding options, that can be used to specify that the
   associated attribute values are encoded according to one of these
   other methods.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions .....................................................2
   3. Transfer Encoding Options .......................................3
   4. Defined Transfer Encoding Options ...............................4
   5. Attributes Returned in a Search .................................5
   6. Syntaxes Requiring Binary Transfer ..............................7
   7. Security Considerations .........................................7
   8. IANA Considerations .............................................7
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References ....................................10

1.  Introduction

   Each attribute stored in a Lightweight Directory Access Protocol
   (LDAP) directory [LDAP] has a defined syntax (i.e., data type) that
   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
   that 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



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   document are to be interpreted as described in BCP 14, RFC 2119
   [BCP14].

   This specification makes use of definitions from the XML Information
   Set (Infoset) [INFOSET].  In particular, information item property
   names follow the Infoset convention of being shown in square
   brackets, 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



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   subtype.  An attribute description containing a transfer encoding
   option references exactly the same attribute as the attribute
   description with the transfer encoding option removed.  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
   encoding is not supported for that ASN.1 type.

   The presence or absence of a transfer encoding option only affects
   the transfer of attribute and assertion values in the 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 that 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] [GSEREI].

   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] [RXEREI] as a Standalone RXER Encoding with the
   DirectoryString ASN.1 type amended by the inclusion of an RXER UNION
   encoding instruction as described in the specification for the XML-
   Enabled Directory (XED) [XED] protocols [XLDAP].

      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



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      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.
      A directory 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] "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 with the DirectoryString ASN.1 type amended
   as described in the specification for the XED protocols [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 (here
   called 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.

   Note that if the list of attributes in a search request is empty, or



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   contains the special attribute description string "*" [PROT], 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.  The most
   specific attribute description is determined by the following cases:

   (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 with 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.



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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
   for binary transfer.  For example, if the effective transfer encoding
   option is "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 those syntaxes 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 attributes of such syntaxes (e.g., into an LDIF file
   [LDIF]) 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



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      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
      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:




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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.

   [GSER]     Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
              Types", RFC 3641, October 2003.

   [LDAP]     Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510, June
              2006.

   [PROT]     Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511, June 2006.

   [MODELS]   Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Directory Information Models", RFC 4512, June
              2006.

   [SYNTAX]   Legg, S., Ed., "Lightweight Directory Access Protocol
              (LDAP): Syntaxes and Matching Rules", RFC 4517, June 2006.

   [BCP64]    Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
              Considerations for the Lightweight Directory Access
              Protocol (LDAP)", BCP 64, RFC 4520, June 2006.

   [BINARY]   Legg, S., "Lightweight Directory Access Protocol (LDAP):
              The Binary Encoding Option", RFC 4522, June 2006.

   [PKI]      Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP) Schema Definitions for X.509 Certificates", RFC
              4523, June 2006.

   [GSEREI]   Legg, S., "Encoding Instructions for the Generic String
              Encoding Rules (GSER)", RFC 4792, January 2007.

   [RXER]     Legg, S. and D. Prager, "Robust XML Encoding Rules (RXER)
              for Abstract Syntax Notation One (ASN.1)", RFC 4910, July
              2007.

   [RXEREI]   Legg, S., "Encoding Instructions for the Robust XML
              Encoding Rules (RXER)", RFC 4911, July 2007.

   [XLDAP]    Legg, S. and D. Prager, "The XML-Enabled Directory:
              Protocols", draft-legg-xed-protocols-05.txt, a work in
              progress, August 2007.



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   [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).

   [XML10]    Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", W3C Recommendation,
              http://www.w3.org/TR/2006/REC-xml-20060816, August 2006.

   [XML11]    Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
              Yergeau, F., and J. Cowan, "Extensible Markup Language
              (XML) 1.1 (Second Edition)", W3C Recommendation,
              http://www.w3.org/TR/2006/REC-xml11-20060816, August 2006.

   [INFOSET]  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

   [LDIF]     Good, G., "The LDAP Data Interchange Format (LDIF) -
              Technical Specification", RFC 2849, June 2000.

   [XED]      Legg, S. and D. Prager, "The XML-Enabled Directory",
              draft-legg-xed-roadmap-06.txt, a work in progress, August
              2007.

Author's Address

   Steven Legg
   eB2Bcom
   Suite 1, 85-87 Charles Street
   Kew, Victoria 3101
   AUSTRALIA

   Phone: +61 3 9851 8630
     Fax: +61 3 9851 8601
   Email: steven.legg@eb2bcom.com

Full Copyright Statement




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   Copyright (C) The IETF Trust (2007).

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Note to the RFC Editor: the remainder of this document is to be removed
before final publication.

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



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   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.

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.

Changes in Draft 05

   The references have been updated to the new LDAPv3 technical
   specification.

Changes in Draft 06

   This specification has been downgraded from an intended category of
   Proposed Standard to Experimental because the RXER specification on
   which it depends is in the Experimental category.

   XED's application of the RXER UNION encoding instruction to the
   DirectoryString ASN.1 type has been acknowledged.















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