





INTERNET-DRAFT                              Editor: Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                               15 February 2004
Obsoletes: RFC 2251, RFC 2252, RFC 2256



                    LDAP: Directory Information Models
                    <draft-ietf-ldapbis-models-10.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 published as a Standard Track RFC.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDAP Revision Working Group
  mailing list <ietf-ldapbis@openldap.org>.  Please send editorial
  comments directly to the editor <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 (C) The Internet Society (2004).  All Rights Reserved.

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


Abstract

  The Lightweight Directory Access Protocol (LDAP) is an Internet
  protocol for accessing distributed directory services which act in
  accordance with X.500 data and service models.  This document
  describes the X.500 Directory Information Models, as used in LDAP.



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Table of Contents

  Status of this Memo                                             1
  Abstract
  Table of Contents                                               2
  1.       Introduction                                           3
  1.1.     Relationship to Other LDAP Specifications
  1.2.     Relationship to X.501                                  4
  1.3.     Conventions
  1.4.     Common ABNF Productions
  2.       Model of Directory User Information                    6
  2.1.     The Directory Information Tree                         7
  2.2.     Naming of Entries
  2.3.     Structure of an Entry                                  8
  2.4.     Object Classes                                         9
  2.5.     Attribute Descriptions                                11
  2.6.     Alias Entries                                         15
  3.       Directory Administrative and Operational Information  17
  3.1.     Subtrees
  3.2.     Subentries
  3.3.     The 'objectClass' attribute
  3.4.     Operational attributes                                18
  4.       Directory Schema                                      21
  4.1.     Schema Definitions                                    22
  4.2.     Subschema Subentries                                  31
  4.3.     'extensibleObject'                                    35
  4.4.     Subschema Discovery
  5.       DSA (Server) Informational Model
  5.1.     Server-specific Data Requirements                     36
  6.       Other Considerations                                  39
  6.1.     Preservation of User Information
  6.2.     Short Names                                           40
  6.3.     Cache and Shadowing
  7.       Implementation Guidelines
  7.1.     Server Guidelines
  7.2.     Client Guidelines                                     41
  8.       Security Considerations
  9.       IANA Considerations                                   42
  10.      Acknowledgments                                       43
  11.      Editor's Address
  12.      References
  12.1.    Normative References
  12.2.    Informative References                                45
  Appendix A.  Changes
  Intellectual Property Rights                                   49
  Full Copyright





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1. Introduction

  This document discusses the X.500 Directory Information Models
  [X.501], as used by the Lightweight Directory Access Protocol (LDAP)
  [Roadmap].

  The Directory is "a collection of open systems cooperating to provide
  directory services" [X.500].  The information held in the Directory is
  collectively known as the Directory Information Base (DIB).  A
  Directory user, which may be a human or other entity, accesses the
  Directory through a client (or Directory User Agent (DUA)).  The
  client, on behalf of the directory user, interacts with one or more
  servers (or Directory System Agents (DSA)).  A server holds a fragment
  of the DIB.

  The DIB contains two classes of information:

      1) user information (e.g., information provided and administrated
         by users).  Section 2 describes the Model of User Information.

      2) administrative and operational information (e.g., information
         used to administer and/or operate the directory).  Section 3
         describes the model of Directory Administrative and Operational
         Information.

  These two models, referred to as the generic Directory Information
  Models, describe how information is represented in the Directory.
  These generic models provide a framework for other information models.
  Section 4 discusses the subschema information model and subschema
  discovery.  Section 5 discusses the DSA (Server) Informational Model.

  Other X.500 information models, such as access control distribution
  knowledge, and replication knowledge information models, may be
  adapted for use in LDAP.  Specification of how these models apply to
  LDAP is left to future documents.


1.1. Relationship to Other LDAP Specifications

  This document is a integral part of the LDAP technical specification
  [Roadmap] which obsoletes the previously defined LDAP technical
  specification, RFC 3377, in its entirety.

  This document obsoletes RFC 2251 sections 3.2 and 3.4, as well as
  portions of sections 4 and 6.  Appendix A.1 summaries changes to these
  sections.  The remainder of RFC 2251 is obsoleted by the [Protocol],
  [AuthMeth], and [Roadmap] documents.




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  This document obsoletes RFC 2252 sections 4, 5 and 7.  Appendix A.2
  summaries changes to these sections.  The remainder of RFC 2252 is
  obsoleted by [Syntaxes].

  This document obsoletes RFC 2256 sections 5.1, 5.2, 7.1 and 7.2.
  Appendix A.3 summarizes changes to these sections.  The remainder of
  RFC 2256 is obsoleted by [Schema] and [Syntaxes].


1.2. Relationship to X.501

  This document includes material, with and without adaptation, from
  [X.501].  The material in this document takes precedence over that in
  [X.501].


1.3. Conventions

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

  Schema definitions are provided using LDAP description formats (as
  defined in Section 4.1).  Definitions provided here are formatted
  (line wrapped) for readability.  Matching rules and LDAP syntaxes
  referenced in these definitions are specified in [Syntaxes].


1.4. Common ABNF Productions

  A number of syntaxes in this document are described using Augmented
  Backus-Naur Form (ABNF) [RFC2234].  These syntaxes (as well as a
  number of syntaxes defined in other documents) rely on the following
  common productions:

      keystring = leadkeychar *keychar
      leadkeychar = ALPHA
      keychar = ALPHA / DIGIT / HYPHEN
      number  = DIGIT / ( LDIGIT 1*DIGIT )

      ALPHA   = UALPHA / %x61-7A    ; "A"-"Z" / "a"-"z"
      UALPHA  = %x41-5A             ; "A"-"Z"
      DIGIT   = %x30 / LDIGIT       ; "0"-"9"
      LDIGIT  = %x31-39             ; "1"-"9"
      HEX     = DIGIT / %x41-46 / %x61-66 ; "0"-"9" / "A"-"F" / "a"-"f"

      SP      = 1*SPACE  ; one or more " "
      WSP     = 0*SPACE  ; zero or more " "



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      NULL    = %x00 ; null (0)
      SPACE   = %x20 ; space (" ")
      DQUOTE  = %x22 ; quote (""")
      SHARP   = %x23 ; octothorpe (or sharp sign) ("#")
      DOLLAR  = %x24 ; dollar sign ("$")
      SQUOTE  = %x27 ; single quote ("'")
      LPAREN  = %x28 ; left paren ("(")
      RPAREN  = %x29 ; right paren (")")
      PLUS    = %x2B ; plus sign ("+")
      COMMA   = %x2C ; comma (",")
      HYPHEN  = %x2D ; hyphen ("-")
      DOT     = %x2E ; period (".")
      SEMI    = %x3B ; semicolon (";")
      LANGLE  = %x3C ; left angle bracket ("<")
      EQUALS  = %x3D ; equals sign ("=")
      RANGLE  = %x3E ; right angle bracket (">")
      ESC     = %x5C ; backslash ("\")
      USCORE  = %x5F ; underscore ("_")
      LCURLY  = %x7B ; left curly brace "{"
      RCURLY  = %x7D ; right curly brace "}"

      ; Any UTF-8 [UTF-8] encoded UCS [ISO10646] character
      UTF8    = UTF1 / UTFMB
      UTFMB   = UTF2 / UTF3 / UTF4
      UTF0    = %x80-BF
      UTF1    = %x00-7F
      UTF2    = %xC2-DF UTF0
      UTF3    = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
                %xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
      UTF4    = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
                %xF4 %x80-8F 2(UTF0)

      OCTET   = %x00-FF ; Any octet

  Object identifiers (OIDs) [X.680] are represented in LDAP using a dot-
  decimal format conforming to the ABNF:

      numericoid = number 1*( DOT number )

  Short names, also known as descriptors, are used as more readable
  aliases for object identifiers.  Short names are case insensitive and
  conform to the ABNF:

      descr = keystring

  Where either an object identifier or a short name may be specified,
  the following production is used:




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      oid = descr / numericoid

  While the <descr> form is generally preferred when the usage is
  restricted to short names referring to object identifiers which
  identify like kinds of objects (e.g., attribute type descriptions,
  matching rule descriptions, object class descriptions), the
  <numericoid> form should be used when the object identifiers may
  identify multiple kinds of objects or when an unambiguous short name
  (descriptor) is not available.

  Implementations SHOULD treat short names (descriptors) used in an
  unambiguous manner (as discussed above) as unrecognized.

  Short Names (descriptors) are discussed further in Section 6.2.


2. Model of Directory User Information

  As [X.501] states:

      The purpose of the Directory is to hold, and provide access to,
      information about objects of interest (objects) in some 'world'.
      An object can be anything which is identifiable (can be named).

      An object class is an identified family of objects, or conceivable
      objects, which share certain characteristics. Every object belongs
      to at least one class.  An object class may be a subclass of other
      object classes, in which case the members of the former class, the
      subclass, are also considered to be members of the latter classes,
      the superclasses.  There may be subclasses of subclasses, etc., to
      an arbitrary depth.

  A directory entry, a named collection of information, is the basic
  unit of information held in the Directory.  There are multiple kinds
  of directory entries.

  An object entry represents a particular object.  An alias entry
  provides alternative naming.  A subentry holds administrative and/or
  operational information.

  The set of entries representing the DIB are organized hierarchically
  in a tree structure known as the Directory Information Tree (DIT).

  Section 2.1 describes the Directory Information Tree
  Section 2.2 discusses naming of entries.
  Section 2.3 discusses the structure of entries.
  Section 2.4 discusses object classes.
  Section 2.5 discusses attribute descriptions.



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  Section 2.6 discusses alias entries.


2.1. The Directory Information Tree

  As noted above, the DIB is composed of a set of entries organized
  hierarchically in a tree structure known as the Directory Information
  Tree (DIT).  Specifically, a tree where vertices are the entries.

  The arcs between vertices define relations between entries.  If an arc
  exists from X to Y, then the entry at X is the immediate superior of Y
  and Y is the immediate subordinate of X.  An entry's superiors are the
  entry's immediate superior and its superiors.  An entry's subordinates
  are all of its immediate subordinates and their subordinates.

  Similarly, the superior/subordinate relationship between object
  entries can be used to derive a relation between the objects they
  represent.  DIT structure rules can be used to govern relationships
  between objects.

  Note: An entry's immediate superior is also known as the entry's
        parent and an entry's immediate subordinate is also known as the
        entry's child.  Entries which have the same parent are known as
        siblings.


2.2. Naming of Entries

2.2.1.  Relative Distinguished Names

  Each entry is named relative to its immediate superior.  This relative
  name, known as its Relative Distinguished Name (RDN) [X.501], is
  composed of an unordered set of one or more attribute value assertions
  (AVA) consisting of an attribute description with zero options and an
  attribute value.  These AVAs are chosen from the attributes of the
  entry.

  An entry's relative distinguished name must be unique among all
  immediate subordinates of the entry's immediate superior (i.e., all
  siblings).

  The following are examples of string representations of RDNs [LDAPDN]:

      UID=12345
      OU=Engineering
      CN=Kurt Zeilenga+L=Redwood Shores

  The last is an example of a multi-valued RDN.  That is, an RDN



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  composed of multiple AVAs.


2.2.2. Distinguished Names

  An entry's fully qualified name, known as its Distinguished Name (DN)
  [X.501], is the concatenation of its RDN and its immediate superior's
  DN.  A Distinguished Name unambiguously refers to an entry in the
  tree.  The following are examples of string representations of DNs
  [LDAPDN]:

      UID=nobody@example.com,DC=example,DC=com
      CN=John Smith,OU=Sales,O=ACME Limited,L=Moab,ST=Utah,C=US


2.2.3. Alias Names

  An alias, or alias name, is "an name for an object, provided by the
  use of alias entries" [X.501].  Alias entries are described in Section
  2.6.


2.3. Structure of an Entry

  An entry consists of a set of attributes which hold information about
  the object which the entry represents.  Some attributes represent user
  information and are called user attributes.  Other attributes
  represent operational and/or administrative information and are called
  operational attributes.

  An attribute is an attribute description (a type and zero or more
  options) with one or more associated values.  An attribute is often
  referred to by its attribute description.  For example, the
  'givenName' attribute is the attribute which consists of the attribute
  description 'givenName' (the 'givenName' attribute type [Schema] and
  zero options) and one or more associated values.

  The attribute type governs whether the attribute can have multiple
  values, the syntax and matching rules used to construct and compare
  values of that attribute, and other functions.  Options indicate
  subtypes and other functions.  No two values of an attribute may be
  equivalent.

  Two values are considered equivalent if they would match according to
  the equality matching rule of the attribute type.  If the attribute
  type is defined with no equality matching rule, two values are
  equivalent if and only if they are identical.




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  For example, a 'givenName' attribute can have can have more than one
  value, they must be Directory Strings, and they are case insensitive.
  A 'givenName' attribute cannot hold both "John" and "JOHN" as these
  are equivalent values per the equality matching rule of the attribute
  type.


2.4. Object Classes

  An object class is "an identified family of objects (or conceivable
  objects) which share certain characteristics" [X.501].

  As defined in [X.501]:

      Object classes are used in the Directory for a number of purposes:

        - describing and categorising objects and the entries that
          correspond to these objects;

        - where appropriate, controlling the operation of the Directory;

        - regulating, in conjunction with DIT structure rule
          specifications, the position of entries in the DIT;

        - regulating, in conjunction with DIT content rule
          specifications, the attributes that are contained in entries;

        - identifying classes of entry that are to be associated with a
          particular policy by the appropriate administrative authority.

      An object class (a subclass) may be derived from an object class
      (its direct superclass) which is itself derived from an even more
      generic object class.  For structural object classes, this process
      stops at the most generic object class, 'top' (defined in Section
      2.4.1).  An ordered set of superclasses up to the most superior
      object class of an object class is its superclass chain.

      An object class may be derived from two or more direct
      superclasses (superclasses not part of the same superclass chain).
      This feature of subclassing is termed multiple inheritance.

  Each object class identifies the set of attributes required to be
  present in entries belonging to the class and the set of attributes
  allowed to be present in entries belonging to the class.  As an entry
  of a class must meet the requirements of each class it belongs to, it
  can be said that an object class inherits the sets of allowed and
  required attributes from its superclasses.  A subclass can identify an
  attribute allowed by its superclass as being required.  If an



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  attribute is a member of both sets, it is required to be present.

  Each object class is defined to be one of three kinds of object
  classes: Abstract, Structural, or Auxiliary.

  Each object class is identified by an object identifier (OID) and,
  optionally, one or more short names (descriptors).


2.4.1. Abstract Object Classes

  An abstract object class, as the name implies, provides a base of
  characteristics from which other object classes can be defined to
  inherit from.  An entry cannot belong to an abstract object class
  unless it belongs to a structural or auxiliary class which inherits
  from that abstract class.

  Abstract object classes can not derive from structural nor auxiliary
  object classes.

  All structural object classes derive (directly or indirectly) from the
  'top' abstract object class.  Auxiliary object classes do not
  necessarily derive from 'top'.

  The following is the object class definition (see Section 4.1.1) for
  the 'top' object class:

      ( 2.5.6.0 NAME 'top' ABSTRACT MUST objectClass )

  All entries belong to the 'top' abstract object class.


2.4.2. Structural Object Classes

  As stated in [X.501]:

      An object class defined for use in the structural specification of
      the DIT is termed a structural object class.  Structural object
      classes are used in the definition of the structure of the names
      of the objects for compliant entries.

      An object or alias entry is characterised by precisely one
      structural object class superclass chain which has a single
      structural object class as the most subordinate object class.
      This structural object class is referred to as the structural
      object class of the entry.

      Structural object classes are related to associated entries:



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        - an entry conforming to a structural object class shall
          represent the real-world object constrained by the object
          class;

        - DIT structure rules only refer to structural object classes;
          the structural object class of an entry is used to specify the
          position of the entry in the DIT;

        - the structural object class of an entry is used, along with an
          associated DIT content rule, to control the content of an
          entry.

        The structural object class of an entry shall not be changed.

  Each structural object class is a (direct or indirect) subclass of the
  'top' abstract object class.

  Structural object classes cannot subclass auxiliary object classes.

  Each entry is said to belong to its structural object class as well as
  all classes in its structural object class's superclass chain.


2.4.3. Auxiliary Object Classes

  Auxiliary object classes are used augment the characteristics of
  entries.  They are commonly used to augment the sets of attributes
  required and allowed to be present in an entry.  They can be used to
  describe entries or classes of entries.

  Auxiliary object classes cannot subclass structural object classes.

  An entry can belong to any subset of the set of auxiliary object
  classes allowed by the DIT content rule associated with the structural
  object class of the entry.  If no DIT content rule is associated with
  the structural object class of the entry, the entry cannot belong to
  any auxiliary object class.

  The set of auxiliary object classes which an entry belongs to can
  change over time.


2.5. Attribute Descriptions

  An attribute description is composed of an attribute type (see Section
  2.5.1) and a set of zero or more attribute options (see Section
  2.5.2).




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  An attribute description is represented by the ABNF:

      attributedescription = attributetype options
      attributetype = oid
      options = *( SEMI option )
      option = 1*keychar

  where <attributetype> identifies the attribute type and each <option>
  identifies an attribute option.  Both <attributetype> and <option>
  productions are case insensitive.  The order in which <option>s appear
  is irrelevant.  That is, any two <attributedescription>s which consist
  of the same <attributetype> and same set of <option>s are equivalent.

  Examples of valid attribute descriptions:

      2.5.4.0
      cn;lang-de;lang-en
      owner

  An attribute description with an unrecognized attribute type is to be
  treated as unrecognized.  Servers SHALL treat an attribute description
  with an unrecognized attribute option as unrecognized.  Clients MAY
  treat an unrecognized attribute option as a tagging option (see
  Section 2.5.2.1).

  All attributes of an entry must have distinct attribute descriptions.


2.5.1. Attribute Types

  An attribute type governs whether the attribute can have multiple
  values, the syntax and matching rules used to construct and compare
  values of that attribute, and other functions.

  The attribute type indicates whether the attribute is a user attribute
  or an operational attribute.  If operational, the attribute type
  indicates the operational usage and whether the attribute is
  modifiable by users or not.  Operational attributes are discussed in
  Section 3.4.

  An attribute type (a subtype) may derive from a more generic attribute
  type (a direct supertype).  The following restrictions apply to
  subtyping:
    - a subtype must have the same usage as its direct supertype,
    - a subtype's syntax must be the same, or a refine of, its
      supertype's syntax, and
    - a subtype must be collective [RFC3671] if its supertype is
      collective.



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  An attribute description consisting of a subtype and no options is
  said to be the direct description subtype of the attribute description
  consisting of the subtype's direct supertype and no options.

  Each attribute type is identified by an object identifier (OID) and,
  optionally, one or more short names (descriptors).


2.5.2. Attribute Options

  There are multiple kinds of attribute description options.  The LDAP
  technical specification details one kind: tagging options.

  Not all options can be associated with attributes held in the
  directory.  Tagging options can be.

  Not all options can be used in conjunction with all attribute types.
  In such cases, the attribute description is to be treated as
  unrecognized.

  An attribute description that contains mutually exclusive options
  shall be treated as unrecognized.  That is, "cn;x-bar;x-foo", where
  "x-foo" and "x-bar" are mutually exclusive, is to be treated as
  unrecognized.

  Other kinds of options may be specified in future documents.  These
  documents must detail how new kinds of options they define relate to
  tagging options.  In particular, these documents must detail whether
  or not new kinds of options can be associated with attributes held in
  the directory, how new kinds of options affect transfer of attribute
  values, and how new kinds of options are treated in attribute
  description hierarchies.

  Options are represented as short case insensitive textual strings
  conforming to the <option> production defined in Section 2.5 of this
  document.

  Procedures for registering options are detailed in BCP 64 [BCP64bis].


2.5.2.1. Tagging Options

  Attributes held in the directory can have attribute descriptions with
  any number of tagging options.  Tagging options are never mutually
  exclusive.

  An attribute description with N tagging options is a direct
  (description) subtype of all attribute descriptions of the same



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  attribute type and all but one of the N options.  If the attribute
  type has a supertype, then the attribute description is also a direct
  (description) subtype of the attribute description of the supertype
  and the N tagging options.  That is, 'cn;lang-de;lang-en' is a direct
  (description) subtype of 'cn;lang-de', 'cn;lang-en', and
  'name;lang-de;lang-en' ('cn' is a subtype of 'name', both are defined
  in [Schema]).


2.5.3. Attribute Description Hierarchies

  An attribute description can be the direct subtype of zero or more
  other attribute descriptions as indicated by attribute type subtyping
  (as described in Section 2.5.1) or attribute tagging option subtyping
  (as described in Section 2.5.2.1).  These subtyping relationships are
  used to form hierarchies of attribute descriptions and attributes.

  As adapted from [X.501]:

      Attribute hierarchies allow access to the DIB with varying degrees
      of granularity.  This is achieved by allowing the value components
      of attributes to be accessed by using either their specific
      attribute description (a direct reference to the attribute) or by
      a more generic attribute description (an indirect reference).

      Semantically related attributes may be placed in a hierarchical
      relationship, the more specialized being placed subordinate to the
      more generalized.  Searching for, or retrieving attributes and
      their values is made easier by quoting the more generalized
      attribute description; a filter item so specified is evaluated for
      the more specialized descriptions as well as for the quoted
      description.

      Where subordinate specialized descriptions are selected to be
      returned as part of a search result these descriptions shall be
      returned if available.  Where the more general descriptions are
      selected to be returned as part of a search result both the
      general and the specialized descriptions shall be returned, if
      available.  An attribute value shall always be returned as a value
      of its own attribute description.

      All of the attribute descriptions in an attribute hierarchy are
      treated as distinct and unrelated descriptions for user
      modification of entry content.

      An attribute value stored in a object or alias entry is of
      precisely one attribute description.  The description is indicated
      when the value is originally added to the entry.



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  For the purpose of subschema administration of the entry, a
  specification that an attribute is required is fulfilled if the entry
  contains a value of an attribute description belonging to an attribute
  hierarchy where the attribute type of that description is the same as
  the required attribute's type.  That is, a "MUST name" specification
  is fulfilled by 'name' or 'name;x-tag-option', but is not fulfilled by
  'CN' nor by 'CN;x-tag-option' (even though 'CN' is a subtype of
  'name').  Likewise, an entry may contain a value of an attribute
  description belonging to an attribute hierarchy where the attribute
  type of that description is either explicitly included in the
  definition of an object class to which the entry belongs or allowed by
  the DIT content rule applicable to that entry.  That is, 'name' and
  'name;x-tag-option' are allowed by "MAY name" (or by "MUST name"), but
  'CN' and 'CN;x-tag-option' are not allowed by "MAY name" (nor by "MUST
  name").

  For the purposes of other policy administration, unless stated
  otherwise in the specification of the particular administrative model,
  all of the attribute descriptions in an attribute hierarchy are
  treated as distinct and unrelated descriptions.


2.5.4. Attribute Values

  Attribute values conform to the defined syntax of the attribute.

  When an attribute is used for naming of the entry, one and only one
  value of the attribute is selected to appear in the Relative
  Distinguished Name.  This value is known as a distinguished value.

  Only attributes whose descriptions have no options can be used for
  naming.


2.6. Alias Entries

  As adapted from [X.501]:

      An alias, or an alias name, for an object is a an alternative name
      for an object or object entry which is provided by the use of
      alias entries.

      Each alias entry contains, within the 'aliasedObjectName'
      attribute (known as the 'aliasedEntryName' attribute in X.500]), a
      name of some object.  The distinguished name of the alias entry is
      thus also a name for this object.

          NOTE - The name within the 'aliasedObjectName' is said to be



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                 pointed to by the alias.  It does not have to be the
                 distinguished name of any entry.

      The conversion of an alias name to an object name is termed
      (alias) dereferencing and comprises the systematic replacement of
      alias names, where found within a purported name, by the value of
      the corresponding 'aliasedObjectName' attribute.  The process may
      require the examination of more than one alias entry.

      Any particular entry in the DIT may have zero or more alias names.
      It therefore follows that several alias entries may point to the
      same entry.  An alias entry may point to an entry that is not a
      leaf entry and may point to another alias entry.

      An alias entry shall have no subordinates, so that an alias entry
      is always a leaf entry.

      Every alias entry shall belong to the 'alias' object class.

  An entry with the 'alias' object class must also belong to an object
  class (or classes), or be governed by a DIT content rule, which allows
  suitable naming attributes to be present.

  Example:
      dn: cn=bar,dc=example,dc=com
      objectClass: top
      objectClass: alias
      objectClass: extensibleObject
      cn: bar
      aliasedObjectName: cn=foo,dc=example,dc=com


2.6.1. 'alias' object class

  Alias entries belong to the 'alias' object class.

     ( 2.5.6.1 NAME 'alias'
       SUP top STRUCTURAL
       MUST aliasedObjectName )


2.6.2. 'aliasedObjectName' attribute type

  The 'aliasedObjectName' attribute holds the name of the entry an alias
  points to.  The 'aliasedObjectName' attribute is known as the
  'aliasedEntryName' attribute in X.500.

      ( 2.5.4.1 NAME 'aliasedObjectName'



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        EQUALITY distinguishedNameMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
        SINGLE-VALUE )

  The 'distinguishedNameMatch' matching rule and the DistinguishedName
  (1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [Syntaxes].


3. Directory Administrative and Operational Information

  This section discusses select aspects of the X.500 Directory
  Administrative and Operational Information model [X.501].  LDAP
  implementations MAY support other aspects of this model.


3.1. Subtrees

  As defined in [X.501]:

      A subtree is a collection of object and alias entries situated at
      the vertices of a tree.  Subtrees do not contain subentries.  The
      prefix sub, in subtree, emphasizes that the base (or root) vertex
      of this tree is usually subordinate to the root of the DIT.

      A subtree begins at some vertex and extends to some identifiable
      lower boundary, possibly extending to leaves.  A subtree is always
      defined within a context which implicitly bounds the subtree.  For
      example, the vertex and lower boundaries of a subtree defining a
      replicated area are bounded by a naming context.


3.2. Subentries

  A subentry is a "special sort of entry, known by the Directory, used
  to hold information associated with a subtree or subtree refinement"
  [X.501].  Subentries are used in Directory to hold for administrative
  and operational purposes as defined in [X.501].  Their use in LDAP is
  detailed in [RFC3672].

  The term "(sub)entry" in this specification indicates that servers
  implementing X.500(93) models are, in accordance with X.500(93) as
  described in [RFC3672], to use a subentry and that other servers are
  to use an object entry belonging to the appropriate auxiliary class
  normally used with the subentry (e.g., 'subschema' for subschema
  subentries) to mimic the subentry.  This object entry's RDN SHALL be
  formed from a value of the 'cn' (commonName) attribute [Schema] (as
  all subentries are named with 'cn').




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3.3. The 'objectClass' attribute

  Each entry in the DIT has an 'objectClass' attribute.

      ( 2.5.4.0 NAME 'objectClass'
        EQUALITY objectIdentifierMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 )

  The 'objectIdentifierMatch' matching rule and the OBJECT IDENTIFIER
  (1.3.6.1.4.1.1466.115.121.1.38) syntax are defined in [Syntaxes].

  The 'objectClass' attribute specifies the object classes of an entry,
  which (among other things) is used in conjunction with the controlling
  schema to determine the permitted attributes of an entry.  Values of
  this attribute can be modified by clients, but the 'objectClass'
  attribute cannot be removed.

  Servers which follow X.500(93) models SHALL restrict modifications of
  this attribute to prevent the basic structural class of the entry from
  being changed.  That is, one cannot change a 'person' into a
  'country'.

  When creating an entry or adding an 'objectClass' value to an entry,
  all superclasses of the named classes SHALL be implicitly added as
  well if not already present.  That is, if the auxiliary class 'x-a' is
  a subclass of the class 'x-b', adding 'x-a' to 'objectClass' causes
  'x-b' to be implicitly added (if is not already present).

  Servers SHALL restrict modifications of this attribute to prevent
  superclasses of remaining 'objectClass' values from being deleted.
  That is, if the auxiliary class 'x-a' is a subclass of the auxiliary
  class 'x-b' and the 'objectClass' attribute contains 'x-a' and 'x-b',
  an attempt to delete only 'x-b' from the 'objectClass' attribute is an
  error.


3.4. Operational attributes

  Some attributes, termed operational attributes, are used or maintained
  by servers for administrative and operational purposes.  As stated in
  [X.501]: "There are three varieties of operational attributes:
  Directory operational attributes, DSA-shared operational attributes,
  and DSA-specific operational attributes."

  A directory operational attribute is used to represent operational
  and/or administrative information in the Directory Information Model.
  This includes operational attributes maintained by the server (e.g.
  'createTimestamp') as well as operational attributes which hold values



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  administrated by the user (e.g. 'ditContentRules').

  A DSA-shared operational attribute is used to represent information of
  the DSA Information Model which is shared between DSAs.

  A DSA-specific operational attribute is used to represent information
  of the DSA Information Model which is specific to the DSA (though, in
  some cases, may be derived from information shared between DSAs)
  (e.g., 'namingContexts').

  The DSA Information Model operational attributes are detailed in
  [X.501].

  Operational attributes are not normally visible.  They are not
  returned in search results unless explicitly requested by name.

  Not all operational attributes are user modifiable.

  Entries may contain, among others, the following operational
  attributes:

    - creatorsName: the Distinguished Name of the user who added this
      entry to the directory,

    - createTimestamp: the time this entry was added to the directory,

    - modifiersName: the Distinguished Name of the user who last
      modified this entry, and

    - modifyTimestamp: the time this entry was last modified.

  Servers SHOULD maintain the 'creatorsName', 'createTimestamp',
  'modifiersName', and 'modifyTimestamp' attributes for all entries of
  the DIT.


3.4.1. 'creatorsName'

  This attribute appears in entries which were added using the protocol
  (e.g., using the Add operation).  The value is the distinguished name
  of the creator.

      ( 2.5.18.3 NAME 'creatorsName'
        EQUALITY distinguishedNameMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )




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  The 'distinguishedNameMatch' matching rule and the DistinguishedName
  (1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [Syntaxes].


3.4.2. 'createTimestamp'

  This attribute appears in entries which were added using the protocol
  (e.g., using the Add operation).  The value is the time the entry was
  added.

      ( 2.5.18.1 NAME 'createTimestamp'
        EQUALITY generalizedTimeMatch
        ORDERING generalizedTimeOrderingMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.24
        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )

  The 'generalizedTimeMatch' and 'generalizedTimeOrderingMatch' matching
  rules and the GeneralizedTime (1.3.6.1.4.1.1466.115.121.1.24) syntax
  are defined in [Syntaxes].


3.4.3. 'modifiersName'

  This attribute appears in entries which have been modified using the
  protocol (e.g., using Modify operation).  The value is the
  distinguished name of the last modifier.

      ( 2.5.18.4 NAME 'modifiersName'
        EQUALITY distinguishedNameMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )

  The 'distinguishedNameMatch' matching rule and the DistinguishedName
  (1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [Syntaxes].


3.4.4. 'modifyTimestamp'

  This attribute appears in entries which have been modified using the
  protocol (e.g., using the Modify operation).  The value is the time
  the entry was last modified.

      ( 2.5.18.2 NAME 'modifyTimestamp'
        EQUALITY generalizedTimeMatch
        ORDERING generalizedTimeOrderingMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.24



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        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )

  The 'generalizedTimeMatch' and 'generalizedTimeOrderingMatch' matching
  rules and the GeneralizedTime (1.3.6.1.4.1.1466.115.121.1.24) syntax
  are defined in [Syntaxes].


3.4.5. 'structuralObjectClass'

  This attribute indicates the structural object class of the entry.

      ( 2.5.21.9 NAME 'structuralObjectClass'
        EQUALITY objectIdentifierMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )

  The 'objectIdentifierMatch' matching rule and OBJECT IDENTIFIER
  (1.3.6.1.4.1.1466.115.121.1.38) syntax is defined in [Syntaxes].


3.4.6. 'governingStructureRule'

  This attribute indicates the structure rule governing the entry.

      ( 2.5.21.10 NAME 'governingStructureRule'
        EQUALITY integerMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
        SINGLE-VALUE NO-USER-MODIFICATION
        USAGE directoryOperation )

  The 'integerMatch' matching rule and INTEGER
  (1.3.6.1.4.1.1466.115.121.1.27) syntax is defined in [Syntaxes].


4. Directory Schema

  As defined in [X.501]:

      The Directory Schema is a set of definitions and constraints
      concerning the structure of the DIT, the possible ways entries are
      named, the information that can be held in an entry, the
      attributes used to represent that information and their
      organization into hierarchies to facilitate search and retrieval
      of the information and the ways in which values of attributes may
      be matched in attribute value and matching rule assertions.




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      NOTE 1 - The schema enables the Directory system to, for example:

      - prevent the creation of subordinate entries of the wrong
        object-class (e.g. a country as a subordinate of a person);

      - prevent the addition of attribute-types to an entry
        inappropriate to the object-class (e.g. a serial number to a
        person's entry);

      - prevent the addition of an attribute value of a syntax not
        matching that defined for the attribute-type (e.g. a printable
        string to a bit string).

        Formally, the Directory Schema comprises a set of:

      a) Name Form definitions that define primitive naming relations
         for structural object classes;

      b) DIT Structure Rule definitions that define the names that
         entries may have and the ways in which the entries may be
         related to one another in the DIT;

      c) DIT Content Rule definitions that extend the specification of
         allowable attributes for entries beyond those indicated by the
         structural object classes of the entries;

      d) Object Class definitions that define the basic set of mandatory
         and optional attributes that shall be present, and may be
         present, respectively, in an entry of a given class, and which
         indicate the kind of object class that is being defined;

      e) Attribute Type definitions that identify the object identifier
         by which an attribute is known, its syntax, associated matching
         rules, whether it is an operational attribute and if so its
         type, whether it is a collective attribute, whether it is
         permitted to have multiple values and whether or not it is
         derived from another attribute type;

      f) Matching Rule definitions that define matching rules.

  And in LDAP:

      g) LDAP Syntax definitions that define encodings used in LDAP.


4.1. Schema Definitions

  Schema definitions in this section are described using ABNF and rely



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  on the common productions specified in Section 1.2 as well as these:

      noidlen = numericoid [ LCURLY len RCURLY ]
      len = number

      oids = oid / ( LPAREN WSP oidlist WSP RPAREN )
      oidlist = oid *( WSP DOLLAR WSP oid )

      extensions = *( SP xstring SP qdstrings )
      xstring = "X" HYPHEN 1*( UALPHA / HYPHEN / USCORE )

      qdescrs = qdescr / ( LPAREN WSP qdescrlist WSP RPAREN )
      qdescrlist = [ qdescr *( SP qdescr ) ]
      qdescr = SQUOTE descr SQUOTE

      qdstrings = qdstring / ( LPAREN WSP qdstringlist WSP RPAREN )
      qdstringlist = [ qdstring *( SP qdstring ) ]
      qdstring = SQUOTE dstring SQUOTE
      dstring = 1*( QS / QQ / QUTF8 )   ; escaped UTF-8 string

      QQ =  ESC %x32 %x37 ; "\27"
      QS =  ESC %x35 ( %x43 / %x63 ) ; "\5C" / "\5c"

      ; Any UTF-8 encoded Unicode character
      ; except %x27 ("'") and %x5C ("\")
      QUTF8    = QUTF1 / UTFMB

      ; Any ASCII character except %x27 ("'") and %x5C ("\")
      QUTF1    = %x00-26 / %x28-5B / %x5D-7F

  Schema definitions in this section also share a number of common
  terms.

  The NAME field provides a set of short names (descriptors) which are
  be used as aliases for the OID.

  The DESC field optionally allows a descriptive string to be provided
  by the directory administrator and/or implementor.  While
  specifications may suggest a descriptive string, there is no
  requirement that the suggested (or any) descriptive string be used.

  The OBSOLETE field, if present, indicates the element is not active.

  Implementors should note that future versions of this document may
  expand these definitions to include additional terms.  Terms whose
  identifier begins with "X-" are reserved for private experiments, and
  are followed by <SP> and <qdstrings> tokens.




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4.1.1. Object Class Definitions

  Object Class definitions are written according to the ABNF:

    ObjectClassDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        [ SP "SUP" SP oids ]       ; superior object classes
        [ SP kind ]                ; kind of class
        [ SP "MUST" SP oids ]      ; attribute types
        [ SP "MAY" SP oids ]       ; attribute types
        extensions WSP RPAREN

    kind = "ABSTRACT" / "STRUCTURAL" / "AUXILIARY"

  where:
    <numericoid> is object identifier assigned to this object class;
    NAME <qdescrs> are short names (descriptors) identifying this object
        class;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this object class is not active;
    SUP <oids> specifies the direct superclasses of this object class;
    the kind of object class is indicated by one of ABSTRACT,
        STRUCTURAL, or AUXILIARY, default is STRUCTURAL;
    MUST and MAY specify the sets of required and allowed attribute
        types, respectively; and
    <extensions> describe extensions.


4.1.2. Attribute Types

  Attribute Type definitions are written according to the ABNF:

    AttributeTypeDescription = LPAREN WSP
        numericoid                    ; object identifier
        [ SP "NAME" SP qdescrs ]      ; short names (descriptors)
        [ SP "DESC" SP qdstring ]     ; description
        [ SP "OBSOLETE" ]             ; not active
        [ SP "SUP" SP oid ]           ; supertype
        [ SP "EQUALITY" SP oid ]      ; equality matching rule
        [ SP "ORDERING" SP oid ]      ; ordering matching rule
        [ SP "SUBSTR" SP oid ]        ; substrings matching rule
        [ SP "SYNTAX" SP noidlen ]    ; value syntax
        [ SP "SINGLE-VALUE" ]         ; single-value
        [ SP "COLLECTIVE" ]           ; collective
        [ SP "NO-USER-MODIFICATION" ] ; not user modifiable



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        [ SP "USAGE" SP usage ]       ; usage
        extensions WSP RPAREN         ; extensions

    usage = "userApplications"     /  ; user
            "directoryOperation"   /  ; directory operational
            "distributedOperation" /  ; DSA-shared operational
            "dSAOperation"            ; DSA-specific operational

  where:
    <numericoid> is object identifier assigned to this attribute type;
    NAME <qdescrs> are short names (descriptors) identifying this
        attribute type;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this attribute type is not active;
    SUP oid specifies the direct supertype of this type;
    EQUALITY, ORDERING, SUBSTRING provide the oid of the equality,
        ordering, and substrings matching rules, respectively;
    SYNTAX identifies value syntax by object identifier and may suggest
        a minimum upper bound;
    COLLECTIVE indicates this attribute type is collective
        [X.501][RFC3671];
    NO-USER-MODIFICATION indicates this attribute type is not user
        modifiable;
    USAGE indicates the application of this attribute type; and
    <extensions> describe extensions.

  Each attribute type description must contain at least one of the SUP
  or SYNTAX fields.  If no SYNTAX field is provided, the attribute type
  description takes its value from the supertype.

  If SUP field is provided, the EQUALITY, ORDERING, and SUBSTRING
  fields, if not specified, take their value from the supertype.

  Usage of userApplications, the default, indicates that attributes of
  this type represent user information.  That is, they are user
  attributes.

  A usage of directoryOperation, distributedOperation, or dSAOperation
  indicates that attributes of this type represent operational and/or
  administrative information.  That is, they are operational attributes.

  directoryOperation usage indicates that the attribute of this type is
  a directory operational attribute.  distributedOperation usage
  indicates that the attribute of this DSA-shared usage operational
  attribute.  dSAOperation usage indicates that the attribute of this
  type is a DSA-specific operational attribute.

  COLLECTIVE requires usage userApplications.  Use of collective



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  attribute types in LDAP is discussed in [RFC3671].

  NO-USER-MODIFICATION requires an operational usage.

  Note that the <AttributeTypeDescription> does not list the matching
  rules which can be used with that attribute type in an extensibleMatch
  search filter [Protocol].  This is done using the 'matchingRuleUse'
  attribute described in Section 4.1.4.

  This document refines the schema description of X.501 by requiring
  that the SYNTAX field in an <AttributeTypeDescription> be a string
  representation of an object identifier for the LDAP string syntax
  definition with an optional indication of the suggested minimum bound
  of a value of this attribute.

  A suggested minimum upper bound on the number of characters in a value
  with a string-based syntax, or the number of bytes in a value for all
  other syntaxes, may be indicated by appending this bound count inside
  of curly braces following the syntax's OBJECT IDENTIFIER in an
  Attribute Type Description.  This bound is not part of the syntax name
  itself.  For instance, "1.3.6.4.1.1466.0{64}" suggests that server
  implementations should allow a string to be 64 characters long,
  although they may allow longer strings.  Note that a single character
  of the Directory String syntax may be encoded in more than one octet
  since UTF-8 [RFC3629] is a variable-length encoding.


4.1.3. Matching Rules

  Matching rules are used in performance of attribute value assertions,
  such as in performance of a Compare operation.  They are also used in
  evaluation of a Search filters, in determining which individual values
  are be added or deleted during performance of a Modify operation, and
  used in comparison of distinguished names

  Each matching rule is identified by an object identifier (OID) and,
  optionally, one or more short names (descriptors).

  Matching rule definitions are written according to the ABNF:

    MatchingRuleDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        SP "SYNTAX" SP numericoid  ; assertion syntax
        extensions WSP RPAREN      ; extensions




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  where:
    <numericoid> is object identifier assigned to this matching rule;
    NAME <qdescrs> are short names (descriptors) identifying this
        matching rule;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this matching rule is not active;
    SYNTAX identifies the assertion syntax (the syntax of the assertion
        value) by object identifier; and
    <extensions> describe extensions.


4.1.4. Matching Rule Uses

  A matching rule use lists the attributes which are suitable for use
  with an extensibleMatch search filter.

  Matching rule use descriptions are written according to the following
  ABNF:

    MatchingRuleUseDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        SP "APPLIES" SP oids       ; attribute types
        extensions WSP RPAREN      ; extensions

  where:
    <numericoid> is the object identifier of the matching rule
        associated with this matching rule use description;
    NAME <qdescrs> are short names (descriptors) identifying this
        matching rule use;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this matching rule use is not active;
    APPLIES provides a list of attribute types the matching rule applies
        to; and
    <extensions> describe extensions.


4.1.5. LDAP Syntaxes

  LDAP Syntaxes of (attribute and assertion) values are described in
  terms of ASN.1 [X.680] and, optionally, have an octet string encoding
  known as the LDAP-specific encoding.  Commonly, the LDAP-specific
  encoding is constrained to string of Unicode [Unicode] characters in
  UTF-8 [RFC3629] form.

  Each LDAP syntax is identified by an object identifier (OID).



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  LDAP syntax definitions are written according to the ABNF:

    SyntaxDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "DESC" SP qdstring ]  ; description
        extensions WSP RPAREN      ; extensions

  where:
    <numericoid> is object identifier assigned to this LDAP syntax;
    DESC <qdstring> is a short descriptive string; and
    <extensions> describe extensions.


4.1.6. DIT Content Rules

  A DIT content rule is a "rule governing the content of entries of a
  particular structural object class" [X.501].

  For DIT entries of a particular structural object class, a DIT content
  rule specifies which auxiliary object classes the entries are allowed
  to belong to and which additional attributes (by type) are required,
  allowed or not allowed to appear in the entries.

  The list of precluded attributes cannot include any attribute listed
  as mandatory in rule, the structural object class, or any of the
  allowed auxiliary object classes.

  Each content rule is identified by the object identifier, as well as
  any short names (descriptors), of the structural object class it
  applies to.

  An entry may only belong to auxiliary object classes listed in the
  governing content rule.

  An entry must contain all attributes required by the object classes
  the entry belongs to as well as all attributed required by the
  governing content rule.

  An entry may contain any non-precluded attributes allowed by the
  object classes the entry belongs to as well as all attributes allowed
  by the governing content rule.

  An entry cannot include any attribute precluded by the governing
  content rule.

  An entry is governed by (if present and active in the subschema) the
  DIT content rule which applies to the structural object class of the
  entry (see Section 2.4.2).  If no active rule is present for the



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  entry's structural object class, the entry's content is governed by
  the structural object class (and possibly other aspects of user and
  system schema).  DIT content rules for superclasses of the structural
  object class of an entry are not applicable to that entry.

  DIT content rule descriptions are written according to the ABNF:

    DITContentRuleDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        [ SP "AUX" SP oids ]       ; auxiliary object classes
        [ SP "MUST" SP oids ]      ; attribute types
        [ SP "MAY" SP oids ]       ; attribute types
        [ SP "NOT" SP oids ]       ; attribute types
        extensions WSP RPAREN      ; extensions

  where:
    <numericoid> is the object identifier of the structural object class
        associated with this DIT content rule;
    NAME <qdescrs> are short names (descriptors) identifying this DIT
        content rule;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this DIT content rule use is not active;
    AUX specifies a list of auxiliary object classes which entries
        subject to this DIT content rule may belong to;
    MUST, MAY, and NOT specify lists of attribute types which are
        required, allowed, or precluded, respectively, from appearing in
        entries subject to this DIT content rule; and
    <extensions> describe extensions.


4.1.7. DIT Structure Rules and Name Forms

  It is sometimes desirable to regulate where object and alias entries
  can be placed in the DIT and how they can be named based upon their
  structural object class.


4.1.7.1. DIT Structure Rules

  A DIT structure rule is a "rule governing the structure of the DIT by
  specifying a permitted superior to subordinate entry relationship.  A
  structure rule relates a name form, and therefore a structural object
  class, to superior structure rules.  This permits entries of the
  structural object class identified by the name form to exist in the
  DIT as subordinates to entries governed by the indicated superior



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  structure rules" [X.501].

  DIT structure rule descriptions are written according to the ABNF:

    DITStructureRuleDescription = LPAREN WSP
        ruleid                     ; rule identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        SP "FORM" SP oid           ; NameForm
        [ SP "SUP" ruleids ]       ; superior rules
        extensions WSP RPAREN      ; extensions

    ruleids = ruleid / ( LPAREN WSP ruleidlist WSP RPAREN )
    ruleidlist = ruleid *( SP ruleid )
    ruleid = number

  where:
    <ruleid> is the rule identifier of this DIT structure rule;
    NAME <qdescrs> are short names (descriptors) identifying this DIT
        structure rule;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this DIT structure rule use is not active;
    FORM is specifies the name form associated with this DIT structure
        rule;
    SUP identifies superior rules (by rule id); and
    <extensions> describe extensions.

  If no superior rules are identified, the DIT structure rule applies
  to an autonomous administrative point (e.g. the root vertex of the
  subtree controlled by the subschema) [X.501].


4.1.7.2. Name Forms

  A name form "specifies a permissible RDN for entries of a particular
  structural object class.  A name form identifies a named object
  class and one or more attribute types to be used for naming (i.e.
  for the RDN).  Name forms are primitive pieces of specification
  used in the definition of DIT structure rules" [X.501].

  Each name form indicates the structural object class to be named,
  a set of required attribute types, and a set of allowed attribute
  types.  A particular attribute type cannot be in both sets.

  Entries governed by the form must be named using a value from each
  required attribute type and zero or more values from the allowed
  attribute types.



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  Each name form is identified by an object identifier (OID) and,
  optionally, one or more short names (descriptors).

  Name form descriptions are written according to the ABNF:

    NameFormDescription = LPAREN WSP
        numericoid                 ; object identifier
        [ SP "NAME" SP qdescrs ]   ; short names (descriptors)
        [ SP "DESC" SP qdstring ]  ; description
        [ SP "OBSOLETE" ]          ; not active
        SP "OC" SP oid             ; structural object class
        SP "MUST" SP oids          ; attribute types
        [ SP "MAY" SP oids ]       ; attribute types
        extensions WSP RPAREN      ; extensions

  where:
    <numericoid> is object identifier which identifies this name form;
    NAME <qdescrs> are short names (descriptors) identifying this name
        form;
    DESC <qdstring> is a short descriptive string;
    OBSOLETE indicates this name form is not active;
    OC identifies the structural object class this rule applies to,
    MUST and MAY specify the sets of required and allowed, respectively,
        naming attributes for this name form; and
    <extensions> describe extensions.

  All attribute types in the required ("MUST") and allowed ("MAY") lists
  shall be different.


4.2. Subschema Subentries

  Subschema (sub)entries are used for administering information about
  the directory schema.  A single subschema (sub)entry contains all
  schema definitions (see Section 4.1) used by entries in a particular
  part of the directory tree.

  Servers which follow X.500(93) models SHOULD implement subschema using
  the X.500 subschema mechanisms (as detailed in Section 12 of [X.501]),
  and so these are not ordinary object entries but subentries (see
  Section 3.2).  LDAP clients SHOULD NOT assume that servers implement
  any of the other aspects of X.500 subschema.

  Servers MAY allow subschema modification.  Procedures for subschema
  modification are discussed in Section 14.5 of [X.501].

  A server which masters entries and permits clients to modify these
  entries SHALL implement and provide access to these subschema



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  (sub)entries including providing a 'subschemaSubentry' attribute in
  each modifiable entry.  This so clients may discover the attributes
  and object classes which are permitted to be present.  It is strongly
  RECOMMENDED that all other servers implement this as well.

  The value of the 'subschemaSubentry' attribute is the name of the
  subschema (sub)entry holding the subschema controlling the entry.

      ( 2.5.18.10 NAME 'subschemaSubentry'
        EQUALITY distinguishedNameMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
        NO-USER-MODIFICATION SINGLE-VALUE
        USAGE directoryOperation )

  The 'distinguishedNameMatch' matching rule and the DistinguishedName
  (1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [Syntaxes].

  Subschema is held in (sub)entries belonging to the subschema auxiliary
  object class.

      ( 2.5.20.1 NAME 'subschema' AUXILIARY
        MAY ( dITStructureRules $ nameForms $ ditContentRules $
          objectClasses $ attributeTypes $ matchingRules $
          matchingRuleUse ) )

  The 'ldapSyntaxes' operational attribute may also be present in
  subschema entries.

  Servers MAY provide additional attributes (described in other
  documents) in subschema (sub)entries.

  Servers SHOULD provide the attributes 'createTimestamp' and
  'modifyTimestamp' in subschema (sub)entries, in order to allow clients
  to maintain their caches of schema information.

  The following subsections provide attribute type definitions for each
  of schema definition attribute types.


4.2.1. 'objectClasses'

  This attribute holds definitions of object classes.

      ( 2.5.21.6 NAME 'objectClasses'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.37
        USAGE directoryOperation )




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  The 'objectIdentifierFirstComponentMatch' matching rule and the
  ObjectClassDescription (1.3.6.1.4.1.1466.115.121.1.37) syntax are
  defined in [Syntaxes].


4.2.2. 'attributeTypes'

  This attribute holds definitions of attribute types.

      ( 2.5.21.5 NAME 'attributeTypes'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.3
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  AttributeTypeDescription (1.3.6.1.4.1.1466.115.121.1.3) syntax are
  defined in [Syntaxes].


4.2.3. 'matchingRules'

  This attribute holds definitions of matching rules.

      ( 2.5.21.4 NAME 'matchingRules'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.30
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  MatchingRuleDescription (1.3.6.1.4.1.1466.115.121.1.30) syntax are
  defined in [Syntaxes].


4.2.4 'matchingRuleUse'

  This attribute holds definitions of matching rule uses.

      ( 2.5.21.8 NAME 'matchingRuleUse'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.31
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  MatchingRuleUseDescription (1.3.6.1.4.1.1466.115.121.1.31) syntax are
  defined in [Syntaxes].


4.2.5. 'ldapSyntaxes'



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  This attribute holds definitions of LDAP syntaxes.

      ( 1.3.6.1.4.1.1466.101.120.16 NAME 'ldapSyntaxes'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.54
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  SyntaxDescription (1.3.6.1.4.1.1466.115.121.1.54) syntax are defined
  in [Syntaxes].


4.2.6. 'dITContentRules'

  This attribute lists DIT Content Rules which are present in the
  subschema.

      ( 2.5.21.2 NAME 'dITContentRules'
        EQUALITY objectIdentifierFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.16
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  DITContentRuleDescription (1.3.6.1.4.1.1466.115.121.1.16) syntax are
  defined in [Syntaxes].


4.2.7. 'dITStructureRules'

  This attribute lists DIT Structure Rules which present in the
  subschema.

      ( 2.5.21.1 NAME 'dITStructureRules'
        EQUALITY integerFirstComponentMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.17
        USAGE directoryOperation )

  The 'integerFirstComponentMatch' matching rule and the
  DITStructureRuleDescription (1.3.6.1.4.1.1466.115.121.1.17) syntax are
  defined in [Syntaxes].


4.2.8 'nameForms'

  This attribute lists Name Forms which are in force.

      ( 2.5.21.7 NAME 'nameForms'
        EQUALITY objectIdentifierFirstComponentMatch



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        SYNTAX 1.3.6.1.4.1.1466.115.121.1.35
        USAGE directoryOperation )

  The 'objectIdentifierFirstComponentMatch' matching rule and the
  NameFormDescription (1.3.6.1.4.1.1466.115.121.1.35) syntax are defined
  in [Syntaxes].


4.3. 'extensibleObject' object class

  The 'extensibleObject' auxiliary object class allows entries that
  belong to it to hold any user attribute.  The set of allowed attribute
  types of this object class is implicitly the set of all attribute
  types of userApplications usage.

      ( 1.3.6.1.4.1.1466.101.120.111 NAME 'extensibleObject'
        SUP top AUXILIARY )

  The mandatory attributes of the other object classes of this entry are
  still required to be present and any precluded attributes are still
  not allowed to be present.



4.4. Subschema Discovery

  To discover the DN of the subschema (sub)entry holding the subschema
  controlling a particular entry, a client reads that entry's
  'subschemaSubentry' operational attribute.  To read schema attributes
  from the subschema (sub)entry, clients MUST issue a Search operation
  [Protocol] where baseObject is the DN of the subschema (sub)entry,
  scope is baseObject, filter is "(objectClass=subschema)" [Filters],
  and attributes field lists the names of the desired schema attributes
  (as they are operational).  Note: the "(objectClass=subschema)" filter
  allows LDAP servers which gateway to X.500 to detect that subentry
  information is being requested.

  Clients SHOULD NOT assume a published subschema is complete nor assume
  the server supports all of the schema elements it publishes nor assume
  the server does not support an unpublished element.


5. DSA (Server) Informational Model

  The LDAP protocol assumes there are one or more servers which jointly
  provide access to a Directory Information Tree (DIT).  The server
  holding the original information is called the "master" (for that
  information).  Servers which hold copies of the original information



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  are referred to as "shadowing" or "caching" servers.

  As defined in [X.501]:

      context prefix: The sequence of RDNs leading from the Root of the
          DIT to the initial vertex of a naming context; corresponds to
          the distinguished name of that vertex.

  and:

      naming context: A subtree of entries held in a single master DSA.

  That is, a naming context is the largest collection of entries,
  starting at an entry that is mastered by a particular server, and
  including all its subordinates and their subordinates, down to the
  entries which are mastered by different servers.  The context prefix
  is the name of the initial entry.

  The root of the DIT is a DSA-specific Entry (DSE) and not part of any
  naming context (or any subtree); each server has different attribute
  values in the root DSE.


5.1. Server-specific Data Requirements

  An LDAP server SHALL provide information about itself and other
  information that is specific to each server.  This is represented as a
  group of attributes located in the root DSE, which is named with the
  DN with zero RDNs (whose [LDAPDN] representation is as the zero-length
  string).

  These attributes are retrievable, subject to access control and other
  restrictions, if a client performs a Search operation [Protocol] with
  an empty baseObject, scope of baseObject, the filter "(objectClass=*)"
  [Filters], and with the attributes field listing the names of the
  desired attributes.  It is noted that root DSE attributes are
  operational, and like other operational attributes, are not returned
  in search requests unless requested by name.

  The root DSE SHALL NOT be included if the client performs a subtree
  search starting from the root.

  Servers may allow clients to modify attributes of the root DSE where
  appropriate.

  The following attributes of the root DSE are defined in [Syntaxes].
  Additional attributes may be defined in other documents.




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    - altServer: alternative servers;

    - namingContexts: naming contexts;

    - supportedControl: recognized LDAP controls;

    - supportedExtension: recognized LDAP extended operations;

    - supportedLDAPVersion: LDAP versions supported; and

    - supportedSASLMechanisms: recognized Simple Authentication and
      Security Layers (SASL) [SASL] mechanisms.

  The values provided for these attributes may depend on
  session-specific and other factors.  For example, a server supporting
  the SASL EXTERNAL mechanism might only list "EXTERNAL" when the
  client's identity has been established by a lower level.  See
  [AuthMeth].

  The root DSE may also include a 'subschemaSubentry' attribute.  If so,
  it refers to the subschema (sub)entry holding the schema controlling
  the root DSE.  Clients SHOULD NOT assume that this subschema
  (sub)entry controls other entries held by the server.  General
  subschema discovery procedures are provided in Section 4.4.


5.1.1. 'altServer'

  The 'altServer' attribute lists URIs referring to alternative servers
  which may be contacted when this server becomes unavailable.  URIs for
  servers implementing the LDAP are written according to [LDAPURL].
  Other kinds of URIs may be provided.  If the server does not know of
  any other servers which could be used this attribute will be absent.
  Clients may cache this information in case their preferred server
  later becomes unavailable.

      ( 1.3.6.1.4.1.1466.101.120.6 NAME 'altServer'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.26
        USAGE dSAOperation )

  The IA5String (1.3.6.1.4.1.1466.115.121.1.26) syntax is defined in
  [Syntaxes].


5.1.2. 'namingContexts'

  The 'namingContexts' attribute lists the context prefixes of the
  naming contexts the server masters or shadows (in part or in whole).



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  If the server is a first-level DSA [X.501], it should list (in
  addition) an empty string (indicating the root of the DIT).  If the
  server does not master or shadow any information (e.g. it is an LDAP
  gateway to a public X.500 directory) this attribute will be absent.
  If the server believes it masters or shadows the entire directory, the
  attribute will have a single value, and that value will be the empty
  string (indicating the root of the DIT).

  This attribute may be used, for example, to select a suitable entry
  name for subsequent operations with this server.

      ( 1.3.6.1.4.1.1466.101.120.5 NAME 'namingContexts'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
        USAGE dSAOperation )

  The DistinguishedName (1.3.6.1.4.1.1466.115.121.1.12) syntax is
  defined in [Syntaxes].


5.1.3. 'supportedControl'

  The 'supportedControl' attribute lists object identifiers identifying
  the request controls [Protocol] the server supports.  If the server
  does not support any request controls, this attribute will be absent.
  Object identifiers identifying response controls need not be listed.

  Procedures for registering object identifiers used to discovery of
  protocol mechanisms are detailed in BCP 64 [BCP64bis].

      ( 1.3.6.1.4.1.1466.101.120.13 NAME 'supportedControl'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
        USAGE dSAOperation )

  The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax is
  defined in [Syntaxes].


5.1.4. 'supportedExtension'

  The 'supportedExtension' attribute lists object identifiers
  identifying the extended operations [Protocol] which the server
  supports.  If the server does not support any extended operations,
  this attribute will be absent.

  An extended operation generally consists of an extended request and an
  extended response but may also include other protocol data units (such
  as intermediate responses).  The object identifier assigned to the
  extended request is used to identify the extended operation.  Other



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  object identifiers used in the extended operation need not be listed
  as values of this attribute.

  Procedures for registering object identifiers used to discovery of
  protocol mechanisms are detailed in BCP 64 [BCP64bis].

      ( 1.3.6.1.4.1.1466.101.120.7 NAME 'supportedExtension'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
        USAGE dSAOperation )

  The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax is
  defined in [Syntaxes].


5.1.5. 'supportedLDAPVersion'

  The 'supportedLDAPVersion' attribute lists the versions of LDAP which
  the server supports.

      ( 1.3.6.1.4.1.1466.101.120.15 NAME 'supportedLDAPVersion'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
        USAGE dSAOperation )

  The INTEGER (1.3.6.1.4.1.1466.115.121.1.27) syntax are defined in
  [Syntaxes].


5.1.6. 'supportedSASLMechanisms'

  The 'supportedSASLMechanisms' attribute lists the SASL mechanisms
  [SASL] which the server recognizes and/or supports [AuthMeth].  The
  contents of this attribute may depend on the current session state.
  If the server does not support any SASL mechanisms this attribute will
  not be present.

      ( 1.3.6.1.4.1.1466.101.120.14 NAME 'supportedSASLMechanisms'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
        USAGE dSAOperation )

  The Directory String (1.3.6.1.4.1.1466.115.121.1.15) syntax is defined
  in [Syntaxes].


6. Other Considerations

6.1. Preservation of User Information

  Syntaxes may be defined which have specific value and/or value form



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  (representation) preservation requirements.  For example, a syntax
  containing digitally signed data can mandate the server preserve both
  the value and form of value presented to ensure signature is not
  invalidated.

  Where such requirements have not been explicitly stated, servers
  SHOULD preserve the value of user information but MAY return the value
  in a different form.  And where a server is unable (or unwilling) to
  preserve the value of user information, the server SHALL ensure that
  an equivalent value (per Section 2.3) is returned.


6.2. Short Names

  Short names, also known as descriptors, are used as more readable
  aliases for object identifiers and are used to identify various schema
  elements.  However, it is not expected that LDAP implementations with
  human user interface would display these short names (nor the object
  identifiers they refer to) to the user, but would most likely be
  performing translations (such as expressing the short name in one of
  the local national languages).  For example, the short name "st"
  (stateOrProvinceName) might be displayed to a German-speaking user as
  "Land".

  The same short name might have different meaning in different
  subschemas and, within a particular subschema, the same short name
  might refer to different object identifiers each identifying a
  different kind of schema element.

  Implementations MUST be prepared that the same short name might be
  used in a subschema to refer to the different kinds of schema
  elements.  That is, there might be an object class 'x-fubar' and an
  attribute type 'x-fubar' in a subschema.

  Implementations MUST be prepared that the same short name might be
  used in the different subschemas to refer to the different schema
  elements.  That is, there might be two matching rules 'x-fubar', each
  in different subschemas.

  Procedures for registering short names (descriptors) are detailed in
  BCP 64 [BCP64bis].


6.3. Cache and Shadowing

  Some servers may hold cache or shadow copies of entries, which can be
  used to answer search and comparison queries, but will return
  referrals or contact other servers if modification operations are



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  requested.  Servers that perform shadowing or caching MUST ensure that
  they do not violate any access control constraints placed on the data
  by the originating server.


7. Implementation Guidelines

7.1 Server Guidelines

  Servers MUST recognize all names of attribute types and object classes
  defined in this document but, unless stated otherwise, need not
  support the associated functionality.  Servers SHOULD recognize all
  the names of attribute types and object classes defined in Section 3
  and 4, respectively, of [Schema].

  Servers MUST ensure that entries conform to user and system schema
  rules or other data model constraints.

  Servers MAY support DIT Content Rules.  Servers MAY support DIT
  Structure Rules and Name Forms.

  Servers MAY support alias entries.

  Servers MAY support the 'extensibleObject' object class.

  Servers MAY support subentries.  If so, they MUST do so in accordance
  with [RFC3672].  Servers which do not support subentries SHOULD use
  object entries to mimic subentries as detailed in Section 3.2.

  Servers MAY implement additional schema elements.  Servers SHOULD
  provide definitions of all schema elements they support in subschema
  (sub)entries.


7.2 Client Guidelines

  In the absence of prior agreements with servers, clients SHOULD NOT
  assume that servers support any particular schema elements beyond
  those referenced in Section 7.1.  The client can retrieve subschema
  information as described in Section 4.4.

  Clients MUST NOT display nor attempt to decode as ASN.1, a value if
  its syntax is not known.   Clients MUST NOT assume the LDAP-specific
  string encoding is restricted to a UTF-8 encoded string of Unicode
  characters or any particular subset of Unicode (such as a printable
  subset) unless such restriction is explicitly stated.  Clients SHOULD
  NOT send attribute values in a request that are not valid according to
  the syntax defined for the attributes.



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8. Security Considerations

  Attributes of directory entries are used to provide descriptive
  information about the real-world objects they represent, which can be
  people, organizations or devices.  Most countries have privacy laws
  regarding the publication of information about people.

  General security considerations for accessing directory information
  with LDAP are discussed in [Protocol] and [AuthMeth].


9. IANA Considerations

  It is requested that the Internet Assigned Numbers Authority (IANA)
  update the LDAP descriptors registry as indicated the following
  template:

      Subject: Request for LDAP Descriptor Registration Update
      Descriptor (short name): see comment
      Object Identifier: see comment
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@OpenLDAP.org>
      Usage: see comment
      Specification: RFC XXXX
      Author/Change Controller: IESG
      Comments:

      The following descriptors (short names) should be added to
      the registry.

        NAME                         Type OID
        ------------------------     ---- -----------------
        governingStructureRule          A 2.5.21.10
        structuralObjectClass              A 2.5.21.5

      The following descriptors (short names) should be updated to
      refer to this RFC.

        NAME                         Type OID
        ------------------------     ---- -----------------
        alias                           O 2.5.6.1
        aliasedObjectName               A 2.5.4.1
        altServer                       A 1.3.6.1.4.1.1466.101.120.6
        attributeTypes                  A 2.5.21.5
        createTimestamp                 A 2.5.18.1
        creatorsName                    A 2.5.18.3
        dITContentRules                 A 2.5.21.2
        dITStructureRules               A 2.5.21.1



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        extensibleObject                O 1.3.6.1.4.1.1466.101.120.111
        ldapSyntaxes                    A 1.3.6.1.4.1.1466.101.120.16
        matchingRuleUse                 A 2.5.21.8
        matchingRules                   A 2.5.21.4
        modifiersName                   A 2.5.18.4
        modifyTimestamp                 A 2.5.18.2
        nameForms                       A 2.5.21.7
        namingContexts                  A 1.3.6.1.4.1.1466.101.120.5
        objectClass                     A 2.5.4.0
        objectClasses                   A 2.5.21.6
        subschema                       O 2.5.20.1
        subschemaSubentry               A 2.5.18.10
        supportedControl                A 1.3.6.1.4.1.1466.101.120.13
        supportedExtension              A 1.3.6.1.4.1.1466.101.120.7
        supportedLDAPVersion            A 1.3.6.1.4.1.1466.101.120.15
        supportedSASLMechanisms         A 1.3.6.1.4.1.1466.101.120.14
        top                             O 2.5.6.0


10. Acknowledgments

  This document is based in part on RFC 2251 by M. Wahl, T.  Howes, and
  S. Kille; RFC 2252 by M. Wahl, A. Coulbeck, T. Howes, S.  Kille; and
  RFC 2556 by M. Wahl, all products of the IETF Access, Searching and
  Indexing of Directories (ASID) Working Group.  This document is also
  based in part on "The Directory: Models" [X.501], a product of the
  International Telephone Union (ITU).  Additional text was borrowed
  from RFC 2253 by M. Wahl, T. Howes, and S. Kille.

  This document is a product of the IETF LDAP Revision (LDAPBIS) Working
  Group.


11. Editor's Address

  Kurt Zeilenga
  E-mail: <kurt@openldap.org>


12. References

12.1. Normative References

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

  [RFC2234]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 2234, November 1997.



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  [RFC3639]     Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3639 (also STD 63), November 2003.

  [RFC3671]     Zeilenga, K., "Collective Attributes in LDAP", RFC 3671,
                December 2003.

  [RFC3672]     Zeilenga, K. and S. Legg, "Subentries in LDAP", RFC
                3672, December 2003.

  [BCP64bis]    Zeilenga, K., "IANA Considerations for LDAP", draft-
                ietf-ldapbis-bcp64-xx.txt, a work in progress.

  [Roadmap]     Zeilenga, K. (editor), "LDAP: Technical Specification
                Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in
                progress.

  [Protocol]    Sermersheim, J. (editor), "LDAP: The Protocol",
                draft-ietf-ldapbis-protocol-xx.txt, a work in progress.

  [AuthMeth]    Harrison, R. (editor), "LDAP: Authentication Methods and
                Connection Level Security Mechanisms",
                draft-ietf-ldapbis-authmeth-xx.txt, a work in progress.

  [Filters]     Smith, M. (editor), LDAPbis WG, "LDAP: String
                Representation of Search Filters",
                draft-ietf-ldapbis-filter-xx.txt, a work in progress.

  [LDAPDN]      Zeilenga, K. (editor), "LDAP: String Representation of
                Distinguished Names", draft-ietf-ldapbis-dn-xx.txt, a
                work in progress.

  [LDAPURL]     Smith, M. (editor), "LDAP: Uniform Resource Locator",
                draft-ietf-ldapbis-url-xx.txt, a work in progress.

  [SASL]        Melnikov, A. (Editor), "Simple Authentication and
                Security Layer (SASL)",
                draft-ietf-sasl-rfc2222bis-xx.txt, a work in progress.

  [Syntaxes]    Legg, S. (editor), "LDAP: Syntaxes and Matching Rules",
                draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.

  [Schema]      Dally, K. (editor), "LDAP: User Schema",
                draft-ietf-ldapbis-user-schema-xx.txt, a work in
                progress.

  [Unicode]     The Unicode Consortium, "The Unicode Standard, Version
                3.2.0" is defined by "The Unicode Standard, Version 3.0"
                (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),



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                as amended by the "Unicode Standard Annex #27: Unicode
                3.1" (http://www.unicode.org/reports/tr27/) and by the
                "Unicode Standard Annex #28: Unicode 3.2"
                (http://www.unicode.org/reports/tr28/).

  [X.500]       International Telecommunication Union -
                Telecommunication Standardization Sector, "The Directory
                -- Overview of concepts, models and services,"
                X.500(1993) (also ISO/IEC 9594-1:1994).

  [X.501]       International Telecommunication Union -
                Telecommunication Standardization Sector, "The Directory
                -- Models," X.501(1993) (also ISO/IEC 9594-2:1994).

  [X.680]       International Telecommunication Union -
                Telecommunication Standardization Sector, "Abstract
                Syntax Notation One (ASN.1) - Specification of Basic
                Notation", X.680(1997) (also ISO/IEC 8824-1:1998).


12.2. Informative References

  None.


Appendix A.  Changes

  This appendix is non-normative.

  This document amounts to nearly a complete rewrite of portions of RFC
  2251, RFC 2252, and RFC 2256.  This rewrite was undertaken to improve
  overall clarity of technical specification.  This appendix provides a
  summary of substantive changes made to the portions of these documents
  incorporated into this document.  Readers should consult [Roadmap],
  [Protocol], [Syntaxes], and [Schema] for summaries of remaining
  portions of these documents.


A.1 Changes to RFC 2251

  This document incorporates from RFC 2251 sections 3.2 and 3.4,
  portions of Section 4 and 6 as summarized below.


A.1.1 Section 3.2 of RFC 2251

  Section 3.2 of RFC 2251 provided a brief introduction to the X.500
  data model, as used by LDAP.  The previous specification relied on



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  [X.501] but lacked clarity in how X.500 models are adapted for use by
  LDAP.  This document describes the X.500 data models, as used by LDAP
  in greater detail, especially in areas where adaptation is needed.

  Section 3.2.1 of RFC 2251 described an attribute as "a type with one
  or more associated values."  In LDAP, an attribute is better described
  as an attribute description, a type with zero or more options, and one
  or more associated values.

  Section 3.2.2 of RFC 2251 mandated that subschema subentries contain
  objectClasses and attributeTypes attributes, yet X.500(93) treats
  these attributes as optional.  While generally all implementations
  that support X.500(93) subschema mechanisms will provide both of these
  attributes, it is not absolutely required for interoperability that
  all servers do.  The mandate was removed for consistency with
  X.500(93).   The subschema discovery mechanism was also clarified to
  indicate that subschema controlling an entry is obtained by reading
  the (sub)entry referred to by that entry's 'subschemaSubentry'
  attribute.


A.1.2 Section 3.4 of RFC 2251

  Section 3.4 of RFC 2251 provided "Server-specific Data Requirements".
  This material, with changes, was incorporated in Section 5.1 of this
  document.

  Changes:

  - Clarify that attributes of the root DSE are subject to "other
    restrictions" in addition to access controls.

  - Clarify that only recognized extended requests need to be enumerated
    'supportedExtension'.

  - Clarify that only recognized request controls need to be enumerated
    'supportedControl'.

  - Clarify that root DSE attributes are operational and, like other
    operational attributes, will not be returned in search requests
    unless requested by name.

  - Clarify that not all root DSE attributes are user modifiable.

  - Remove inconsistent text regarding handling of the
    'subschemaSubentry' attribute within the root DSE.  The previous
    specification stated that the 'subschemaSubentry' attribute held in
    the root DSE referred to "subschema entries (or subentries) known by



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    this server."  This is inconsistent with the attribute intended use
    as well as its formal definition as a single valued attribute
    [X.501].  It is also noted that a simple (possibly incomplete) list
    of subschema (sub)entries is not terrible useful.  This document (in
    section 5.1) specifies that the 'subschemaSubentry' attribute of the
    root DSE refers to the subschema controlling the root DSE.  It is
    noted that the general subschema discovery mechanism remains
    available (see Section 4.4 of this document).


A.1.2 Section 4 of RFC 2251

  Portions of Section 4 of RFC 2251 detailing aspects of the information
  model used by LDAP were incorporated in this document, including:

  - Restriction of distinguished values to attributes whose descriptions
    have no options (from Section 4.1.3).

  - Data model aspects of Attribute Types (from Section 4.1.4),
    Attribute Descriptions (from 4.1.4), Attribute (from 4.1.8),
    Matching Rule Identifier (from 4.1.9).

  - User schema requirements (from Section 4.1.6, 4.5.1, and 4.7).


A.1.3 Section 6 of RFC 2251

    The Section 6.1 and the second paragraph of Section 6.2 of RFC 2251
    where incorporated into this document.


A.2 Changes to RFC 2252

    This document incorporates Sections 4, 5 and 7 from RFC 2252.


A.2.1 Section 4 of RFC 2252

    The specification was updated to use Augmented BNF [RFC2234].  The
    string representation of an OBJECT IDENTIFIER was tighten to
    disallow leading zeros as described in RFC 2252 text.

    The <descr> syntax was changed to disallow semicolon (U+003B)
    characters to appear to be consistent its natural language
    specification "descr is the syntactic representation of an object
    descriptor, which consists of letters and digits, starting with a
    letter."  In a related change, the statement "an
    AttributeDescription can be used as the value in a NAME part of an



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    AttributeTypeDescription" was deleted.  RFC 2252 provided no
    specification of the semantics of attribute options appearing in
    NAME fields.

    RFC 2252 stated that the <descr> form of <oid> SHOULD be preferred
    over the <numericoid> form.  However, <descr> form can be ambiguous.
    To address this issue, the imperative was replaced with a statement
    (in Section 1.4) that while the <descr> form is generally preferred,
    <numericoid> should be used where an unambiguous <descr> is not
    available.  Additionally, an expanded discussion of descriptor
    issues is discussed in Section 6.2 (Short Names).

    The ABNF for a quoted string (qdstring) was updated to reflect
    support for the escaping mechanism described in 4.3 of RFC 2252.


A.2.2 Section 5 of RFC 2252

    Definitions of operational attributes provided in Section 5 of RFC
    2252 where incorporated into this document.

    The 'namingContexts' description was clarified.  A first-level DSA
    should publish, in addition to other values, "" indicating the root
    of the DIT.

    The 'altServer' description was clarified.  It may hold any URI.

    The 'supportedExtension' description was clarified.  A server need
    only list the OBJECT IDENTIFIERs associated with the extended
    requests of the extended operations it recognizes.

    The 'supportedControl' description was clarified.  A server need
    only list the OBJECT IDENTIFIERs associated with the request
    controls it recognizes.

    Descriptions for the 'structuralObjectClass' and
    'governingStructureRule' operational attribute types were added.


A.2.3 Section 7 of RFC 2252

    Section 7 of RFC 2252 provides definitions of the 'subschema' and
    'extensibleObject' object classes.  These definitions where
    integrated into Section 4.2 and Section 4.3 of this document,
    respectively.  Section 7 of RFC 2252 also contained the object class
    implementation requirement.  This was incorporated into Section 7 of
    this document.




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    The specification of 'extensibleObject' was clarified of how it
    interacts with precluded attributes.


A.3 Changes to RFC 2256

    This document incorporates Sections 5.1, 5.2, 7.1, and 7.2 of RFC
    2256.

    Section 5.1 of RFC 2256 provided the definition of the 'objectClass'
    attribute type.  This was integrated into Section 2.4.1 of this
    document.  The statement "One of the values is either 'top' or
    'alias'" was replaced with statement that one of the values is 'top'
    as entries belonging to 'alias' also belong to 'top'.

    Section 5.2 of RFC 2256 provided the definition of the
    'aliasedObjectName' attribute type.  This was integrated into
    Section 2.6.2 of this document.

    Section 7.1 of RFC 2256 provided the definition of the 'top' object
    class.  This was integrated into Section 2.4.1 of this document.

    Section 7.2 of RFC 2256 provided the definition of the 'alias'
    object class.  This was integrated into Section 2.6.1 of this
    document.



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  rights which may cover technology that may be required to practice
  this standard.  Please address the information to the IETF Executive



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  Director.



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  This document and translations of it may be copied and furnished to
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