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Network Working Group                                        K. Zeilenga
Request for Comments: 3296                           OpenLDAP Foundation
Category: Standards Track                                      July 2002

Named Subordinate References in
        Lightweight Directory Access Protocol (LDAP) Directories

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.

Abstract

This document details schema and protocol elements for representing
   and managing named subordinate references in Lightweight Directory
   Access Protocol (LDAP) Directories.

Conventions

Schema definitions are provided using LDAPv3 description formats
   [RFC2252].  Definitions provided here are formatted (line wrapped)
   for readability.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" used in
   this document are to be interpreted as described in BCP 14 [RFC2119].

1.  Background and Intended Usage

The broadening of interest in LDAP (Lightweight Directory Access
   Protocol) [RFC2251] directories beyond their use as front ends to
   X.500 [X.500] directories has created a need to represent knowledge
   information in a more general way.  Knowledge information is
   information about one or more servers maintained in another server,
   used to link servers and services together.

This document details schema and protocol elements for representing
   and manipulating named subordinate references in LDAP directories.  A
   referral object is used to hold subordinate reference information in

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the directory.  These referral objects hold one or more URIs
   [RFC2396] contained in values of the ref attribute type and are used
   to generate protocol referrals and continuations.

A control, ManageDsaIT, is defined to allow manipulation of referral
   and other special objects as normal objects.  As the name of control
   implies, it is intended to be analogous to the ManageDsaIT service
   option described in X.511(97) [X.511].

Other forms of knowledge information are not detailed by this
   document.  These forms may be described in subsequent documents.

This document details subordinate referral processing requirements
   for servers.  This document does not describe protocol syntax and
   semantics.  This is detailed in RFC 2251 [RFC2251].

This document does not detail use of subordinate knowledge references
   to support replicated environments nor distributed operations (e.g.,
   chaining of operations from one server to other servers).

2.  Schema

2.1.  The referral Object Class

A referral object is a directory entry whose structural object class
   is (or is derived from) the referral object class.

( 2.16.840.1.113730.3.2.6
          NAME 'referral'
          DESC 'named subordinate reference object'
          STRUCTURAL
          MUST ref )

The referral object class is a structural object class used to
   represent a subordinate reference in the directory.  The referral
   object class SHOULD be used in conjunction with the extensibleObject
   object class to support the naming attributes used in the entry's
   Distinguished Name (DN) [RFC2253].

Referral objects are normally instantiated at DSEs immediately
   subordinate to object entries within a naming context held by the
   DSA.  Referral objects are analogous to X.500 subordinate knowledge
   (subr) DSEs [X.501].

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In the presence of a ManageDsaIT control, referral objects are
   treated as normal entries as described in section 3.  Note that the
   ref attribute is operational and will only be returned in a search
   entry response when requested.

In the absence of a ManageDsaIT control, the content of referral
   objects are used to construct referrals and search references as
   described in Section 4 and, as such, the referral entries are not
   themselves visible to clients.

2.2  The ref Attribute Type

( 2.16.840.1.113730.3.1.34
          NAME 'ref'
          DESC 'named reference - a labeledURI'
          EQUALITY caseExactMatch
          SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
          USAGE distributedOperation )

The ref attribute type has directoryString syntax and is case
   sensitive.  The ref attribute is multi-valued.  Values placed in the
   attribute MUST conform to the specification given for the labeledURI
   attribute [RFC2079].  The labeledURI specification defines a format
   that is a URI, optionally followed by whitespace and a label.  This
   document does not make use of the label portion of the syntax.
   Future documents MAY enable new functionality by imposing additional
   structure on the label portion of the syntax as it appears in the ref
   attribute.

If the URI contained in a ref attribute value refers to a LDAP
   [RFC2251] server, it MUST be in the form of a LDAP URL [RFC2255].
   The LDAP URL SHOULD NOT contain an explicit scope specifier, filter,
   attribute description list, or any extensions.  The LDAP URL SHOULD
   contain a non-empty DN.  The handling of LDAP URLs with absent or
   empty DN parts or with explicit scope specifier is not defined by
   this specification.

Other URI schemes MAY be used so long as all operations returning
   referrals based upon the value could be performed.  This document
   does not detail use of non-LDAP URIs.  This is left to future
   specifications.

The referential integrity of the URI SHOULD NOT be validated by the
   server holding or returning the URI (whether as a value of the
   attribute or as part of a referral result or search reference
   response).

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When returning a referral result or search continuation, the server
   MUST NOT return the separator or label portions of the attribute
   values as part of the reference.  When the attribute contains
   multiple values, the URI part of each value is used to construct the
   referral result or search continuation.

The ref attribute values SHOULD NOT be used as a relative name-
   component of an entry's DN [RFC2253].

This document uses the ref attribute in conjunction with the referral
   object class to represent subordinate references.  The ref attribute
   may be used for other purposes as defined by other documents.

3.  The ManageDsaIT Control

The client may provide the ManageDsaIT control with an operation to
   indicate that the operation is intended to manage objects within the
   DSA (server) Information Tree.  The control causes Directory-specific
   entries (DSEs), regardless of type, to be treated as normal entries
   allowing clients to interrogate and update these entries using LDAP
   operations.

A client MAY specify the following control when issuing an add,
   compare, delete, modify, modifyDN, search request or an extended
   operation for which the control is defined.

The control type is 2.16.840.1.113730.3.4.2.  The control criticality
   may be TRUE or, if FALSE, absent.  The control value is absent.

When the control is present in the request, the server SHALL NOT
   generate a referral or continuation reference based upon information
   held in referral objects and instead SHALL treat the referral object
   as a normal entry.  The server, however, is still free to return
   referrals for other reasons.  When not present, referral objects
   SHALL be handled as described above.

The control MAY cause other objects to be treated as normal entries
   as defined by subsequent documents.

4.  Named Subordinate References

A named subordinate reference is constructed by instantiating a
   referral object in the referencing server with ref attribute values
   which point to the corresponding subtree maintained in the referenced
   server.  In general, the name of the referral object is the same as
   the referenced object and this referenced object is a context prefix
   [X.501].

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That is, if server A holds "DC=example,DC=net" and server B holds
   "DC=sub,DC=example,DC=net", server A may contain a referral object
   named "DC=sub,DC=example,DC=net" which contains a ref attribute with
   value of "ldap://B/DC=sub,DC=example,DC=net".

dn: DC=sub,DC=example,DC=net
      dc: sub
      ref: ldap://B/DC=sub,DC=example,DC=net
      objectClass: referral
      objectClass: extensibleObject

Typically the DN of the referral object and the DN of the object in
   the referenced server are the same.

If the ref attribute has multiple values, all the DNs contained
   within the LDAP URLs SHOULD be equivalent.  Administrators SHOULD
   avoid configuring naming loops using referrals.

Named references MUST be treated as normal entries if the request
   includes the ManageDsaIT control as described in section 3.

5.  Scenarios

The following sections contain specifications of how referral objects
   should be used in different scenarios followed by examples that
   illustrate that usage.  The scenarios described here consist of
   referral object handling when finding target of a non-search
   operation, when finding the base of a search operation, and when
   generating search references.  Lastly, other operation processing
   considerations are presented.

It is to be noted that, in this document, a search operation is
   conceptually divided into two distinct, sequential phases: (1)
   finding the base object where the search is to begin, and (2)
   performing the search itself.  The first phase is similar to, but not
   the same as, finding the target of a non-search operation.

It should also be noted that the ref attribute may have multiple
   values and, where these sections refer to a single ref attribute
   value, multiple ref attribute values may be substituted and SHOULD be
   processed and returned (in any order) as a group in a referral or
   search reference in the same way as described for a single ref
   attribute value.

Search references returned for a given request may be returned in any
   order.

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5.1.  Example Configuration

For example, suppose the contacted server (hosta) holds the entry
   "O=MNN,C=WW" and the entry "CN=Manager,O=MNN,C=WW" and the following
   referral objects:

dn: OU=People,O=MNN,C=WW
      ou: People
      ref: ldap://hostb/OU=People,O=MNN,C=US
      ref: ldap://hostc/OU=People,O=MNN,C=US
      objectClass: referral
      objectClass: extensibleObject

dn: OU=Roles,O=MNN,C=WW
      ou: Roles
      ref: ldap://hostd/OU=Roles,O=MNN,C=WW
      objectClass: referral
      objectClass: extensibleObject

The first referral object provides the server with the knowledge that
   subtree "OU=People,O=MNN,C=WW" is held by hostb and hostc (e.g., one
   is the master and the other a shadow).  The second referral object
   provides the server with the knowledge that the subtree
   "OU=Roles,O=MNN,C=WW" is held by hostd.

Also, in the context of this document, the "nearest naming context"
   means the deepest context which the object is within.  That is, if
   the object is within multiple naming contexts, the nearest naming
   context is the one which is subordinate to all other naming contexts
   the object is within.

5.2.  Target Object Considerations

This section details referral handling for add, compare, delete,
   modify, and modify DN operations.  If the client requests any of
   these operations, there are four cases that the server must handle
   with respect to the target object.

The DN part MUST be modified such that it refers to the appropriate
   target in the referenced server (as detailed below).  Even where the
   DN to be returned is the same as the target DN, the DN part SHOULD
   NOT be trimmed.

In cases where the URI to be returned is a LDAP URL, the server
   SHOULD trim any present scope, filter, or attribute list from the URI
   before returning it.  Critical extensions MUST NOT be trimmed or
   modified.

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Case 1: The target object is not held by the server and is not within
      or subordinate to any naming context nor subordinate to any
      referral object held by the server.

The server SHOULD process the request normally as appropriate for
      a non-existent base which is not within any naming context of the
      server (generally return noSuchObject or a referral based upon
      superior knowledge reference information).  This document does not
      detail management or processing of superior knowledge reference
      information.

Case 2: The target object is held by the server and is a referral
      object.

The server SHOULD return the URI value contained in the ref
      attribute of the referral object appropriately modified as
      described above.

Example: If the client issues a modify request for the target object
      of "OU=People,O=MNN,c=WW", the server will return:

ModifyResponse (referral) {
             ldap://hostb/OU=People,O=MNN,C=WW
             ldap://hostc/OU=People,O=MNN,C=WW
         }

Case 3: The target object is not held by the server, but the nearest
      naming context contains no referral object which the target object
      is subordinate to.

If the nearest naming context contains no referral object which
      the target is subordinate to, the server SHOULD process the
      request as appropriate for a nonexistent target (generally return
      noSuchObject).

Case 4: The target object is not held by the server, but the nearest
      naming context contains a referral object which the target object
      is subordinate to.

If a client requests an operation for which the target object is
      not held by the server and the nearest naming context contains a
      referral object which the target object is subordinate to, the
      server SHOULD return a referral response constructed from the URI
      portion of the ref value of the referral object.

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Example: If the client issues an add request where the target object
      has a DN of "CN=Manager,OU=Roles,O=MNN,C=WW", the server will
      return:

AddResponse (referral) {
             ldap://hostd/CN=Manager,OU=Roles,O=MNN,C=WW"
         }

Note that the DN part of the LDAP URL is modified such that it
      refers to the appropriate entry in the referenced server.

5.3.  Base Object Considerations

This section details referral handling for base object processing
   within search operations.  Like target object considerations for
   non-search operations, there are the four cases.

In cases where the URI to be returned is a LDAP URL, the server MUST
   provide an explicit scope specifier from the LDAP URL prior to
   returning it.  In addition, the DN part MUST be modified such that it
   refers to the appropriate target in the referenced server (as
   detailed below).

If aliasing dereferencing was necessary in finding the referral
   object, the DN part of the URI MUST be replaced with the base DN as
   modified by the alias dereferencing such that the return URL refers
   to the new target object per [RFC2251, 4.1.11].

Critical extensions MUST NOT be trimmed nor modified.

Case 1: The base object is not held by the server and is not within
      nor subordinate to any naming context held by the server.

The server SHOULD process the request normally as appropriate for
      a non-existent base which not within any naming context of the
      server (generally return a superior referral or noSuchObject).
      This document does not detail management or processing of superior
      knowledge references.

Case 2: The base object is held by the server and is a referral
      object.

The server SHOULD return the URI value contained in the ref
      attribute of the referral object appropriately modified as
      described above.

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Example: If the client issues a subtree search in which the base
      object is "OU=Roles,O=MNN,C=WW", the server will return

SearchResultDone (referral) {
             ldap://hostd/OU=Roles,O=MNN,C=WW??sub
         }

If the client were to issue a base or oneLevel search instead of
      subtree, the returned LDAP URL would explicitly specify "base" or
      "one", respectively, instead of "sub".

Case 3: The base object is not held by the server, but the nearest
      naming context contains no referral object which the base object
      is subordinate to.

If the nearest naming context contains no referral object which
      the base is subordinate to, the request SHOULD be processed
      normally as appropriate for a nonexistent base (generally return
      noSuchObject).

Case 4: The base object is not held by the server, but the nearest
      naming context contains a referral object which the base object is
      subordinate to.

If a client requests an operation for which the target object is
      not held by the server and the nearest naming context contains a
      referral object which the target object is subordinate to, the
      server SHOULD return a referral response which is constructed from
      the URI portion of the ref value of the referral object.

Example: If the client issues a base search request for
      "CN=Manager,OU=Roles,O=MNN,C=WW", the server will return

SearchResultDone (referral) {
             ldap://hostd/CN=Manager,OU=Roles,O=MNN,C=WW??base"
         }

If the client were to issue a subtree or oneLevel search instead
      of subtree, the returned LDAP URL would explicitly specify "sub"
      or "one", respectively, instead of "base".

Note that the DN part of the LDAP URL is modified such that it
      refers to the appropriate entry in the referenced server.

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5.4.  Search Continuation Considerations

For search operations, once the base object has been found and
   determined not to be a referral object, the search may progress.  Any
   entry matching the filter and scope of the search which is not a
   referral object is returned to the client normally as described in
   [RFC2251].

For each referral object within the requested scope, regardless of
   the search filter, the server SHOULD return a SearchResultReference
   which is constructed from the URI component of values of the ref
   attribute.  If the URI component is not a LDAP URL, it should be
   returned as is.  If the LDAP URL's DN part is absent or empty, the DN
   part must be modified to contain the DN of the referral object.  If
   the URI component is a LDAP URL, the URI SHOULD be modified to add an
   explicit scope specifier.

Subtree Example:

If a client requests a subtree search of "O=MNN,C=WW", then in
      addition to any entries within scope which match the filter, hosta
      will also return two search references as the two referral objects
      are within scope.  One possible response might be:

SearchEntry for O=MNN,C=WW
          SearchResultReference {
              ldap://hostb/OU=People,O=MNN,C=WW??sub
              ldap://hostc/OU=People,O=MNN,C=WW??sub
          }
          SearchEntry for CN=Manager,O=MNN,C=WW
          SearchResultReference {
              ldap://hostd/OU=Roles,O=MNN,C=WW??sub
          }
          SearchResultDone (success)

One Level Example:

If a client requests a one level search of "O=MNN,C=WW" then, in
      addition to any entries one level below the "O=MNN,C=WW" entry
      matching the filter, the server will also return two search
      references as the two referral objects are within scope.  One
      possible sequence is shown:

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SearchResultReference {
              ldap://hostb/OU=People,O=MNN,C=WW??base
              ldap://hostc/OU=People,O=MNN,C=WW??base
          }
          SearchEntry for CN=Manager,O=MNN,C=WW
          SearchResultReference {
              ldap://hostd/OU=Roles,O=MNN,C=WW??base
          }
          SearchResultDone (success)

Note: Unlike the examples in Section 4.5.3.1 of RFC 2251, the LDAP
      URLs returned with the SearchResultReference messages contain, as
      required by this specification, an explicit scope specifier.

5.6.  Other Considerations

This section details processing considerations for other operations.

5.6.1 Bind

Servers SHOULD NOT return referral result code if the bind name (or
   authentication identity or authorization identity) is (or is
   subordinate to) a referral object but MAY use the knowledge
   information to process the bind request (such as in support a future
   distributed operation specification).  Where the server makes no use
   of the knowledge information, the server processes the request
   normally as appropriate for a non-existent authentication or
   authorization identity (e.g., return invalidCredentials).

5.6.2 Modify DN

If the newSuperior is a referral object or is subordinate to a
   referral object, the server SHOULD return affectsMultipleDSAs.  If
   the newRDN already exists but is a referral object, the server SHOULD
   return affectsMultipleDSAs instead of entryAlreadyExists.

6.  Security Considerations

This document defines mechanisms that can be used to tie LDAP (and
   other) servers together.  The information used to tie services
   together should be protected from unauthorized modification.  If the
   server topology information is not public information, it should be
   protected from unauthorized disclosure as well.

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7.  Acknowledgments

This document borrows heavily from previous work by IETF LDAPext
   Working Group.  In particular, this document is based upon "Named
   Referral in LDAP Directories" (an expired Internet Draft) by
   Christopher Lukas, Tim Howes, Michael Roszkowski, Mark C. Smith, and
   Mark Wahl.

8. Normative References

[RFC2079] Smith, M., "Definition of an X.500 Attribute Type and an
             Object Class to Hold Uniform Resource Identifiers (URIs)",
             RFC 2079, January 1997.

[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
             Access Protocol (v3)", RFC 2251, December 1997.

[RFC2252] Wahl, M., Coulbeck, A., Howes, T. and S. Kille,
             "Lightweight Directory Access Protocol (v3): Attribute
             Syntax Definitions", RFC 2252, December 1997.

[RFC2253] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory
             Access Protocol (v3): UTF-8 String Representation of
             Distinguished Names", RFC 2253, December 1997.

[RFC2255] Howes, T. and M. Smith, "The LDAP URL Format", RFC 2255,
             December, 1997.

[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
             Resource Identifiers (URI): Generic Syntax", RFC 2396,
             August 1998.

[X.501]   ITU-T, "The Directory: Models", X.501, 1993.

9. Informative References

[X.500]   ITU-T, "The Directory: Overview of Concepts, Models, and
             Services", X.500, 1993.

[X.511]   ITU-T, "The Directory: Abstract Service Definition", X.500,
             1997.

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10.  Author's Address

Kurt D. Zeilenga
   OpenLDAP Foundation

EMail: Kurt@OpenLDAP.org

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11.  Full Copyright Statement

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   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
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   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
   Internet Society.

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