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Network Working Group
Request for Comments: 3876
Category: Standards Track
D. Chadwick
University of Salford
S. Mullan
Sun Microsystems
September 2004

Returning Matched Values with the

Lightweight Directory Access Protocol version 3 (LDAPv3)

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2004).

Abstract

This document describes a control for the Lightweight Directory Access Protocol version 3 that is used to return a subset of attribute values from an entry. Specifically, only those values that match a "values return" filter. Without support for this control, a client must retrieve all of an attribute's values and search for specific values locally.

1. Introduction

When reading an attribute from an entry using the Lightweight Directory Access Protocol version 3 (LDAPv3) [2], it is normally only possible to read either the attribute type, or the attribute type and all its values. It is not possible to selectively read just a few of the attribute values. If an attribute holds many values, for example, the userCertificate attribute, or the subschema publishing operational attributes objectClasses and attributeTypes [3], then it may be desirable for the user to be able to selectively retrieve a subset of the values, specifically, those attribute values that match some user defined selection criteria. Without the control specified in this document, a client must read all of the attribute's values and filter out the unwanted values, necessitating the client to implement the matching rules. It also requires the client to potentially read and process many irrelevant values, which can be inefficient if the values are large or complex, or there are many values stored per attribute.

This document specifies an LDAPv3 control to enable a user to return only those values that matched (i.e., returned TRUE to) one or more elements of a newly defined "values return" filter. This control can be especially useful when used in conjunction with extensible matching rules that match on one or more components of complex binary attribute values. 

   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 [4].

2. The valuesReturnFilter Control

The valuesReturnFilter control is either critical or non-critical as determined by the user. It only has meaning for the Search operation, and SHOULD only be added to the Search operation by the client. If the server supports the control and it is present on a Search operation, the server MUST obey the control, regardless of the value of the criticality flag.

If the control is marked as critical, and either the server does not support the control or the control is applied to an operation other than Search, the server MUST return an unavailableCriticalExtension error. If the control is not marked as critical, and either the server does not support the control or the control is applied to an operation other than Search, then the server MUST ignore the control.

The object identifier for this control is 1.2.826.0.1.3344810.2.3.

The controlValue is an OCTET STRING, whose value is the BER encoding [6], as per Section 5.1 of RFC 2251 [2], of a value of the ASN.1 [5] type ValuesReturnFilter.

ValuesReturnFilter ::= SEQUENCE OF SimpleFilterItem

SimpleFilterItem ::= CHOICE { 

                   equalityMatch   [3] AttributeValueAssertion,
                   substrings      [4] SubstringFilter,
                   greaterOrEqual  [5] AttributeValueAssertion,
                   lessOrEqual     [6] AttributeValueAssertion,
                   present         [7] AttributeDescription,
                   approxMatch     [8] AttributeValueAssertion,
                   extensibleMatch [9] SimpleMatchingAssertion }
   
            SimpleMatchingAssertion ::= SEQUENCE {
                   matchingRule    [1] MatchingRuleId OPTIONAL,
                   type            [2] AttributeDescription OPTIONAL,
   --- at least one of the above must be present
                   matchValue      [3] AssertionValue}

All the above data types have their standard meanings as defined in [2].

If the server supports this control, the server MUST make use of the control as follows:

  1. The Search Filter is first executed in order to determine which entries satisfy the Search criteria (these are the filtered entries). The control has no impact on this step.
  1. If the typesOnly parameter of the Search Request is TRUE, the control has no effect and the Search Request is processed as if the control had not been specified.
  1. If the attributes parameter of the Search Request consists of a list containing only the attribute with OID "1.1" (specifying that no attributes are to be returned), the control has no effect and the Search Request is processed as if the control had not been specified.
  1. For each attribute listed in the attributes parameter of the Search Request, the server MUST apply the control as follows to each entry in the set of filtered entries:

i) Every attribute value that evaluates TRUE against one or more

elements of the ValuesReturnFilter is placed in the corresponding SearchResultEntry.

ii) Every attribute value that evaluates FALSE or undefined

against all elements of the ValuesReturnFilter is not placed in the corresponding SearchResultEntry. An attribute that has no values selected is returned with an empty set of values.

Note. If the AttributeDescriptionList (see [2]) is empty or comprises "*", then the control MUST be applied against every user attribute. If the AttributeDescriptionList contains a "+", then the control MUST be applied against every operational attribute.

3. Relationship to X.500

The control is a superset of the matchedValuesOnly (MVO) boolean of the X.500 Directory Access Protocol (DAP) [8] Search argument, as amended in the latest version [9]. Close examination of the matchedValuesOnly boolean by the LDAP Extensions (LDAPEXT) Working Group revealed ambiguities and complexities in the MVO boolean that could not easily be resolved. For example, it was not clear if the MVO boolean governed only those attribute values that contributed to the overall truth of the filter, or all of the attribute values, even if the filter item containing the attribute was evaluated as false. For this reason the LDAPEXT group decided to replace the MVO boolean with a simple filter that removes any uncertainty as to whether an attribute value has been selected or not.

4. Relationship to other LDAP Controls

The purpose of this control is to select zero, one, or more attribute values from each requested attribute in a filtered entry, and to discard the remainder. Once the attribute values have been discarded by this control, they MUST NOT be re-instated into the Search results by other controls.

This control acts independently of other LDAP controls such as server side sorting [13] and duplicate entries [10]. However, there might be interactions between this control and other controls so that a different set of Search Result Entries are returned, or the entries are returned in a different order, depending upon the sequencing of this control and other controls in the LDAP request. For example, with server side sorting, if sorting is done first, and value return filtering second, the set of Search Results may appear to be in the wrong order since the value filtering may remove the attribute values upon which the ordering was done. (The sorting document specifies that entries without any sort key attribute values should be treated as coming after all other attribute values.) Similarly with duplicate entries, if duplication is performed before value filtering, the set of Search Result Entries may contain identical duplicate entries, each with an empty set of attribute values, because the value filtering removed the attribute values that were used to duplicate the results.

For these reasons, the ValuesReturnFilter control in a SearchRequest SHOULD precede other controls that affect the number and ordering of SearchResultEntrys.

5. Examples

All entries are provided in an LDAP Data Interchange Format (LDIF)[11].

The string representation of the valuesReturnFilter in the examples below uses the following ABNF [15] notation:

valuesReturnFilter = "(" 1*simpleFilterItem ")" simpleFilterItem = "(" item ")"

where item is as defined below (adapted from RFC2254 [14]). 

      item         = simple / present / substring / extensible
      simple       = attr filtertype value
      filtertype   = equal / approx / greater / less
      equal        = "="
      approx       = "~="
      greater      = ">="
      less         = "<="
      extensible   = attr [":" matchingrule] ":=" value
                     / ":" matchingrule ":=" value
      present      = attr "=*"
      substring    = attr "=" [initial] any [final]
      initial      = value
      any          = "*" *(value "*")
      final        = value
      attr         = AttributeDescription from Section 4.1.5 of [1]
      matchingrule = MatchingRuleId from Section 4.1.9 of [1]
      value        = AttributeValue from Section 4.1.6 of [1]
  1. The first example shows how the control can be set to return all attribute values from one attribute type (e.g., telephoneNumber) and a subset of values from another attribute type (e.g., mail).

The entries below represent organizationalPerson object classes located somewhere beneath the distinguished name dc=ac,dc=uk. 

   dn: cn=Sean Mullan,ou=people,dc=sun,dc=ac,dc=uk
   cn: Sean Mullan
   sn: Mullan
   objectClass: organizationalPerson
   objectClass: person
   objectClass: inetOrgPerson
   mail: sean.mullan@hotmail.com
   mail: mullan@east.sun.com
   telephoneNumber: + 781 442 0926
   telephoneNumber: 555-9999
   dn: cn=David Chadwick,ou=isi,o=salford,dc=ac,dc=uk
   cn: David Chadwick
   sn: Chadwick
   objectClass: organizationalPerson
   objectClass: person
   objectClass: inetOrgPerson
   mail: d.w.chadwick@salford.ac.uk

An LDAP search operation is specified with a baseObject set to the DN of the search base (i.e., dc=ac,dc=uk), a subtree scope, a filter set to (sn=mullan), and the list of attributes to be returned set to "mail,telephoneNumber" or "*". In addition, a ValuesReturnFilter control is set to ((mail=*hotmail.com)(telephoneNumber=*)).

The search results returned by the server would consist of the following entry: 

   dn: cn=Sean Mullan,ou=people,dc=sun,dc=ac,dc=uk
   mail: sean.mullan@hotmail.com
   telephoneNumber: + 781 442 0926
   telephoneNumber: 555-9999

Note that the control has no effect on the values returned for the "telephoneNumber" attribute (all of the values are returned), since the control specified that all values should be returned.

  1. The second example shows how one might retrieve a single attribute type subschema definition for the "gunk" attribute with OID 1.2.3.4.5 from the subschema subentry.

Assume the subschema subentry is held below the root entry with DN cn=subschema subentry,o=myorg and this holds an attributeTypes operational attribute holding the descriptions of the 35 attributes known to this server (each description is held as a single attribute value of the attributeTypes attribute). 

   dn: cn=subschema subentry,o=myorg
   cn: subschema subentry
   objectClass: subschema
   attributeTypes: ( 2.5.4.3 NAME 'cn' SUP name )
   attributeTypes: ( 2.5.4.6 NAME 'c' SUP name SINGLE-VALUE )
   attributeTypes: ( 2.5.4.0 NAME 'objectClass' EQUALITY obj
    ectIdentifierMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 )
   attributeTypes: ( 2.5.18.2 NAME 'modifyTimestamp' EQUALITY gen
    eralizedTimeMatch ORDERING generalizedTimeOrderingMatch SYN
    TAX 1.3.6.1.4.1.1466.115.121.1.24 SINGLE-VALUE NO-USER-
    MODIFICATION USAGE directoryOperation )
   attributeTypes: ( 2.5.21.6 NAME 'objectClasses' EQUALITY obj
    ectIdentifierFirstComponentMatch SYNTAX 1.3.
    6.1.4.1.1466.115.121.1.37 USAGE directoryOperation )
   attributeTypes: ( 1.2.3.4.5 NAME 'gunk' EQUALITY caseIgnoreMat
    ch SUBSTR caseIgnoreSubstringsMatch SYNTAX 1.3.
    6.1.4.1.1466.115.121.1.44{64} )
   attributeTypes: ( 2.5.21.5 NAME 'attributeTypes' EQUALITY obj
    ectIdentifierFirstComponentMatch SYNTAX 1.3.
    6.1.4.1.1466.115.121.1.3 USAGE directoryOperation )

plus another 28 - you get the idea.

The user creates an LDAP search operation with a baseObject set to cn=subschema subentry,o=myorg, a scope of base, a filter set to (objectClass=subschema), the list of attributes to be returned set to "attributeTypes", and the ValuesReturnFilter set to ((attributeTypes=1.2.3.4.5))

The search result returned by the server would consist of the following entry: 

   dn: cn=subschema subentry,o=myorg
   attributeTypes: ( 1.2.3.4.5 NAME 'gunk' EQUALITY caseIgnoreMat
    ch SUBSTR caseIgnoreSubstringsMatch SYNTAX 1.3.
    6.1.4.1.1466.115.121.1.44{64} )
  1. The final example shows how the control can be used to match on a userCertificate attribute value. Note that this example requires the LDAP server to support the certificateExactMatch matching rule defined in [12] as the EQUALITY matching rule for userCertificate.

The entry below represents a pkiUser object class stored in the directory.

dn: cn=David Chadwick,ou=people,o=University of Salford,c=gb cn: David Chadwick
objectClass: person
objectClass: organizationalPerson
objectClass: pkiUser
objectClass: inetOrgPerson
sn: Chadwick
mail: d.w.chadwick@salford.ac.uk
userCertificate;binary: {binary representation of a certificate with a serial number of 2468 issued by o=truetrust ltd,c=gb} userCertificate;binary: {binary representation of certificate with a serial number of 1357 issued by o=truetrust ltd,c=gb} userCertificate;binary: {binary representation of certificate with a serial number of 1234 issued by dc=certsRus,dc=com}

An LDAP search operation is specified with a baseObject set to o=University of Salford,c=gb, a subtree scope, a filter set to (sn=chadwick), and the list of attributes to be returned set to "userCertificate;binary". In addition, a ValuesReturnFilter control is set to ((userCertificate=1357$o=truetrust ltd,c=gb)).

The search result returned by the server would consist of the following entry:

dn: cn=David Chadwick,ou=people,o=University of Salford,c=gb userCertificate;binary: {binary representation of certificate with a serial number of 1357 issued by o=truetrust ltd,c=gb}

6. Security Considerations

This document does not primarily discuss security issues.

Note however that attribute values MUST only be returned if the access controls applied by the LDAP server allow them to be returned, and in this respect the effect of the ValuesReturnFilter control is of no consequence.

Note that the ValuesReturnFilter control may have a positive effect on the deployment of public key infrastructures. Certain PKI operations, like searching for specific certificates, become more scalable, and more practical when combined with X.509 certificate matching rules at the server, since the control avoids the downloading of potentially large numbers of irrelevant certificates which would have to be processed and filtered locally (which in some cases is very difficult to perform).

7. IANA Considerations

The Matched Values control as an LDAP Protocol Mechanism [7] has been registered as follows:

Subject:

Request for LDAP Protocol Mechanism Registration 

      Object Identifier: 1.2.826.0.1.3344810.2.3
      Description: Matched Values Control
      Person & email address to contact for further information:
        David Chadwick <d.w.chadwick@salford.ac.uk>
      Usage: Control
      Specification: RFC3876
      Author/Change Controller: IESG
      Comments: none

This document uses the OID 1.2.826.0.1.3344810.2.3 to identify the matchedValues control described here. This OID was assigned by TrueTrust Ltd, under its BSI assigned English/Welsh Registered Company number [16].

8. Acknowledgements

The authors would like to thank members of the LDAPExt list for their constructive comments on earlier versions of this document, and in particular to Harald Alvestrand who first suggested having an attribute return filter and Bruce Greenblatt who first proposed a syntax for this control.

9. References

9.1. Normative References

   [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.
   
   [2]  Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
        Protocol (w3)", RFC 2251, December 1997.
   
   [3]  Wahl, M., Coulbeck, A., Howes, T., and S. Kille, "Lightweight
        Directory Access Protocol (v3): Attribute Syntax Definitions",
        RFC 2252, December 1997.
   
   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.
   
   [5]  ITU-T Recommendation X.680 (1997) | ISO/IEC 8824-1:1998,
        Information Technology - Abstract Syntax Notation One (ASN.1):
        Specification of Basic Notation
   
   [6]  ITU-T Recommendation X.690 (1997) | ISO/IEC 8825-1,2,3:1998
        Information technology - ASN.1 encoding rules: Specification of
        Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and
        Distinguished Encoding Rules (DER)
   
   [7]  Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
        Considerations for the Lightweight Directory Access Protocol
        (LDAP)", BCP 64, RFC 3383, September 2002.

9.2. Informative References

   [8]  ITU-T Rec. X.511, "The Directory: Abstract Service Definition",
        1993.
   
   [9]  ISO/IEC 9594 / ITU-T Rec X.511 (2001) The Directory: Abstract
        Service Definition.
   
   [10] Sermersheim, J., "LDAP Control for a Duplicate Entry
        Representation of Search Results", Work in Progress, October
        2000.
   
   [11] Good, G., "The LDAP Data Interchange Format (LDIF) - Technical
        Specification", RFC 2849, June 2000.
   
   [12] Chadwick, D. and S.Legg, "Internet X.509 Public Key
        Infrastructure - Additional LDAP Schema for PKIs", Work in
        Progress, June 2002
   
   [13] Howes, T., Wahl, M., and A. Anantha, "LDAP Control Extension for
        Server Side Sorting of Search Results", RFC 2891, August 2000.
   
   [14] Howes, T., "The String Representation of LDAP Search Filters",
        RFC 2254, December 1997.
   
   [15] Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

[16] BRITISH STANDARD BS 7453 Part 1. Procedures for UK Registration

for Open System Standards Part 1: Procedures for the UK Name Registration Authority.

10. Authors' Addresses

David Chadwick
IS Institute
University of Salford
Salford M5 4WT
England 

   Phone: +44 161 295 5351
   EMail: d.w.chadwick@salford.ac.uk

Sean Mullan
Sun Microsystems
One Network Drive
Burlington, MA 01803
USA 

   EMail: sean.mullan@sun.com

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