





INTERNET-DRAFT                              Editor: Kurt D. Zeilenga
Intended Category: Standard Track                   OpenLDAP Foundation
Expires: 20 May 2002                                20 November 2001



             Named Subordinate References in LDAP Directories
                  <draft-zeilenga-ldap-namedref-05.txt>


Status of this Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  This document is intended to be, after appropriate review and
  revision, submitted to the RFC Editor as a Standard Track document.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDAP Extension Working Group
  mailing list <ietf-ldapext@netscape.com>.  Please send editorial
  comments directly to the author <Kurt@OpenLDAP.org>.

  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups.  Note that other
  groups may also distribute working documents as Internet-Drafts.
  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as ``work in progress.''

  The list of current Internet-Drafts can be accessed at
  <http://www.ietf.org/ietf/1id-abstracts.txt>. The list of
  Internet-Draft Shadow Directories can be accessed at
  <http://www.ietf.org/shadow.html>.

  Copyright 2001, The Internet Society.  All Rights Reserved.

  Please see the Copyright section near the end of this document for
  more information.


Abstract

  This document details schema and protocol elements for representing
  and managing named subordinate references in LDAP directories.


Conventions



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

  The key words "SHALL", "SHALL NOT", "MUST", "MUST NOT", "SHOULD",
  "SHOULD NOT", "MAY" and "MAY NOT" 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
  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.



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      ( 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 directory
  entries whose structural object class is (or is derived from)
  referral.

  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].

  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 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.



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  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).

  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.




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  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].

  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



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  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.


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



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  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.

  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
          }





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  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.

  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].




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  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.


  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.



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  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.


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
          }



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          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:

          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.



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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.


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.  Author's Address

  Kurt D. Zeilenga
  OpenLDAP Foundation
  <Kurt@OpenLDAP.org>


9. Normative References

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

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

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

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

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

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




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

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


10. 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,
            1993.


Copyright 2001, The Internet Society.  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published and
  distributed, in whole or in part, without restriction of any kind,
  provided that the above copyright notice and this paragraph are
  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
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  The limited permissions granted above are perpetual and will not be
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