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Network Working Group                                        K. Zeilenga
Request for Comments: 4512                           OpenLDAP Foundation
Obsoletes: 2251, 2252, 2256, 3674                              June 2006
Category: Standards Track

Lightweight Directory Access Protocol (LDAP):
                      Directory Information Models

Status of This Memo

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

Abstract

The Lightweight Directory Access Protocol (LDAP) is an Internet
   protocol for accessing distributed directory services that 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

1. Introduction ....................................................3
      1.1. Relationship to Other LDAP Specifications ..................3
      1.2. Relationship to X.501 ......................................4
      1.3. Conventions ................................................4
      1.4. Common ABNF Productions ....................................4
   2. Model of Directory User Information .............................6
      2.1. The Directory Information Tree .............................7
      2.2. Structure of an Entry ......................................7
      2.3. Naming of Entries ..........................................8
      2.4. Object Classes .............................................9
      2.5. Attribute Descriptions ....................................12
      2.6. Alias Entries .............................................16
   3. Directory Administrative and Operational Information ...........17
      3.1. Subtrees ..................................................17
      3.2. Subentries ................................................18
      3.3. The 'objectClass' attribute ...............................18
      3.4. Operational Attributes ....................................19
   4. Directory Schema ...............................................22
      4.1. Schema Definitions ........................................23
      4.2. Subschema Subentries ......................................32
      4.3. 'extensibleObject' object class ...........................35
      4.4. Subschema Discovery .......................................35
   5. DSA (Server) Informational Model ...............................36
      5.1. Server-Specific Data Requirements .........................36
   6. Other Considerations ...........................................40
      6.1. Preservation of User Information ..........................40
      6.2. Short Names ...............................................41
      6.3. Cache and Shadowing .......................................41
   7. Implementation Guidelines ......................................42
      7.1. Server Guidelines .........................................42
      7.2. Client Guidelines .........................................42
   8. Security Considerations ........................................43
   9. IANA Considerations ............................................43
   10. Acknowledgements ..............................................44
   11. Normative References ..........................................45
   Appendix A. Changes ...............................................47
      A.1. Changes to RFC 2251 .......................................47
      A.2. Changes to RFC 2252 .......................................49
      A.3. Changes to RFC 2256 .......................................50
      A.4. Changes to RFC 3674 .......................................51

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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)
   [RFC4510].

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
   [RFC4510], 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 summarizes changes to
   these sections.  The remainder of RFC 2251 is obsoleted by the
   [RFC4511], [RFC4513], and [RFC4510] documents.

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This document obsoletes RFC 2252, Sections 4, 5, and 7.  Appendix A.2
   summarizes changes to these sections.  The remainder of RFC 2252 is
   obsoleted by [RFC4517].

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 [RFC4519] and [RFC4517].

This document obsoletes RFC 3674 in its entirety.  Appendix A.4
   summarizes changes since RFC 3674.

1.2.  Relationship to X.501

This document includes material, with and without adaptation, from
   [X.501] as necessary to describe this protocol.  These adaptations
   (and any other differences herein) apply to this protocol, and only
   this protocol.

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 [RFC4517].

1.4.  Common ABNF Productions

A number of syntaxes in this document are described using Augmented
   Backus-Naur Form (ABNF) [RFC4234].  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   = %x41-5A / %x61-7A   ; "A"-"Z" / "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 [RFC3629] encoded Unicode [Unicode] 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 (8-bit data unit)

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

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

oid = descr / numericoid

While the <descr> form is generally preferred when the usage is
   restricted to short names referring to object identifiers that
   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
   ambiguous 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 the structure of entries.
   Section 2.3 discusses naming of entries.

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Section 2.4 discusses object classes.
   Section 2.5 discusses attribute descriptions.
   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 that have the same parent are known
         as siblings.

2.2.  Structure of an Entry

An entry consists of a set of attributes that hold information about
   the object that 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 that consists of the attribute
   description 'givenName' (the 'givenName' attribute type [RFC4519] 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.

Attribute values conform to the defined syntax of the attribute type.

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No two values of an attribute may be equivalent.  Two values are
   considered equivalent if and only if they would match according to
   the equality matching rule of the attribute type.  Or, if the
   attribute type is defined with no equality matching rule, two values
   are equivalent if and only if they are identical.  (See 2.5.1 for
   other restrictions.)

For example, a 'givenName' attribute 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.

Additionally, no attribute is to have a value that is not equivalent
   to itself.  For example, the 'givenName' attribute cannot have as a
   value a directory string that includes the REPLACEMENT CHARACTER
   (U+FFFD) code point, as matching involving that directory string is
   Undefined per this attribute's equality matching rule.

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

2.3.  Naming of Entries

2.3.1.  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 to match
   attribute values (each a distinguished value) 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
   [RFC4514]:

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

The last is an example of a multi-valued RDN; that is, an RDN
   composed of multiple AVAs.

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2.3.2.  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
   [RFC4514]:

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

2.3.3.  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.

2.4.  Object Classes

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

As defined in [X.501]:

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

- describing and categorizing 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.

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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
   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 that inherits
   from that abstract class.

Abstract object classes cannot derive from structural or 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.

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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 characterized 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:

- 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 to 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.

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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 that 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).

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 that
   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.

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If no equality matching is specified for the attribute type:

- the attribute (of the type) cannot be used for naming;
      - when adding the attribute (or replacing all values), no two
        values may be equivalent (see 2.2);
      - individual values of a multi-valued attribute are not to be
        independently added or deleted;
      - attribute value assertions (such as matching in search filters
        and comparisons) using values of such a type cannot be
        performed.

Otherwise, the specified equality matching rule is to be used to
   evaluate attribute value assertions concerning the attribute type.
   The specified equality rule is to be transitive and commutative.

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 or not the attribute
   is modifiable by users.  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 refinement of, its
        supertype's syntax, and
      - a subtype must be collective [RFC3671] if its supertype is
        collective.

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.

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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, RFC 4520
   [RFC4520].

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
   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 [RFC4519]).

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.

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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 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 an object or alias entry is of
      precisely one attribute description.  The description is indicated
      when the value is originally added to the entry.

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' or '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"
   (or by "MUST name").

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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.

2.6.  Alias Entries

As adapted from [X.501]:

An alias, or an alias name, for an object is 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
                 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

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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'
        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 [RFC4517].

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.

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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 [RFC4519] (as
   all subentries are named with 'cn').

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 [RFC4517].

The 'objectClass' attribute specifies the object classes of an entry,
   which (among other things) are 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 that 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

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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 that hold values
   administrated by the user (e.g., 'ditContentRules').

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

A DSA-specific operational attribute is used to represent information
   of the DSA Information Model that 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.

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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 that 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 )

The 'distinguishedNameMatch' matching rule and the DistinguishedName
   (1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [RFC4517].

3.4.2.  'createTimestamp'

This attribute appears in entries that 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 [RFC4517].

3.4.3.  'modifiersName'

This attribute appears in entries that have been modified using the
   protocol (e.g., using the 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 )

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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 [RFC4517].

3.4.4.  'modifyTimestamp'

This attribute appears in entries that 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
        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 [RFC4517].

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 [RFC4517].

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 [RFC4517].

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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.

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;

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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
   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*( ALPHA / 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.

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The NAME field provides a set of short names (descriptors) that are
   to 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.

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 (the default is STRUCTURAL);
     MUST and MAY specify the sets of required and allowed attribute
         types, respectively; and
     <extensions> describe extensions.

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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
         [ 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, and SUBSTR 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;
     SINGLE-VALUE indicates attributes of this type are restricted to a
         single value;
     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.

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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 type is a 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
   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 that can be used with that attribute type in an extensibleMatch
   search filter [RFC4511].  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.

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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
   evaluating search filters, determining which individual values are to
   be added or deleted during performance of a Modify operation, and in
   comparing 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

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 attribute types that 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

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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 a string of Unicode [Unicode] characters
   in UTF-8 [RFC3629] form.

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

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 the 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 the rule, the structural object class, or any of the
   allowed auxiliary object classes.

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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 attributes 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 that applies to the structural object class of the
   entry (see Section 2.4.2).  If no active rule is present for the
   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 that entries
         subject to this DIT content rule may belong to;

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MUST, MAY, and NOT specify lists of attribute types that 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
   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.

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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.

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 that 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.

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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 that follow X.500(93) models SHOULD implement subschema using
   the X.500 subschema mechanisms (as detailed in Section 12 of
   [X.501]), 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 that masters entries and permits clients to modify these
   entries SHALL implement and provide access to these subschema
   (sub)entries including providing a 'subschemaSubentry' attribute in
   each modifiable entry.  This is so clients may discover the
   attributes and object classes that 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
        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 [RFC4517].

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.

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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 )

The 'objectIdentifierFirstComponentMatch' matching rule and the
   ObjectClassDescription (1.3.6.1.4.1.1466.115.121.1.37) syntax are
   defined in [RFC4517].

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 [RFC4517].

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 [RFC4517].

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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 [RFC4517].

4.2.5.  'ldapSyntaxes'

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 [RFC4517].

4.2.6.  'dITContentRules'

This attribute lists DIT Content Rules that 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 [RFC4517].

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4.2.7.  'dITStructureRules'

This attribute lists DIT Structure Rules that are 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 [RFC4517].

4.2.8 'nameForms'

This attribute lists Name Forms that are in force.

( 2.5.21.7 NAME 'nameForms'
        EQUALITY objectIdentifierFirstComponentMatch
        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 [RFC4517].

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
   [RFC4511] where baseObject is the DN of the subschema (sub)entry,

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scope is baseObject, filter is "(objectClass=subschema)" [RFC4515],
   and the attributes field lists the names of the desired schema
   attributes (as they are operational).  Note: the
   "(objectClass=subschema)" filter allows LDAP servers that gateway to
   X.500 to detect that subentry information is being requested.

Clients SHOULD NOT assume that a published subschema is complete,
   that the server supports all of the schema elements it publishes, or
   that the server does not support an unpublished element.

5.  DSA (Server) Informational Model

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

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 that 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 [RFC4514] 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 [RFC4511] with
   an empty baseObject, scope of baseObject, the filter

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"(objectClass=*)" [RFC4515], and 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 below.
   Additional attributes may be defined in other documents.

- altServer: alternative servers;

- namingContexts: naming contexts;

- supportedControl: recognized LDAP controls;

- supportedExtension: recognized LDAP extended operations;

- supportedFeatures: recognized LDAP features;

- supportedLDAPVersion: LDAP versions supported; and

- supportedSASLMechanisms: recognized Simple Authentication and
        Security Layers (SASL) [RFC4422] 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 [RFC4513].

The root DSE may also include a 'subschemaSubentry' attribute.  If it
   does, the attribute 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.

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5.1.1.  'altServer'

The 'altServer' attribute lists URIs referring to alternative servers
   that may be contacted when this server becomes unavailable.  URIs for
   servers implementing the LDAP are written according to [RFC4516].
   Other kinds of URIs may be provided.  If the server does not know of
   any other servers that 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
   [RFC4517].

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).
   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 [RFC4517].

5.1.3.  'supportedControl'

The 'supportedControl' attribute lists object identifiers identifying
   the request controls [RFC4511] 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, RFC 4520 [RFC4520].

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( 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 [RFC4517].

5.1.4.  'supportedExtension'

The 'supportedExtension' attribute lists object identifiers
   identifying the extended operations [RFC4511] that 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 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, RFC 4520 [RFC4520].

( 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 [RFC4517].

5.1.5.  'supportedFeatures'

The 'supportedFeatures' attribute lists object identifiers
   identifying elective features that the server supports.  If the
   server does not support any discoverable elective features, this
   attribute will be absent.

( 1.3.6.1.4.1.4203.1.3.5 NAME 'supportedFeatures'
          EQUALITY objectIdentifierMatch
          SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
          USAGE dSAOperation )

Procedures for registering object identifiers used to discovery of
   protocol mechanisms are detailed in BCP 64, RFC 4520 [RFC4520].

The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax and
   objectIdentifierMatch matching rule are defined in [RFC4517].

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5.1.6.  'supportedLDAPVersion'

The 'supportedLDAPVersion' attribute lists the versions of LDAP that
   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 is defined in
   [RFC4517].

5.1.7.  'supportedSASLMechanisms'

The 'supportedSASLMechanisms' attribute lists the SASL mechanisms
   [RFC4422] that the server recognizes and/or supports [RFC4513].  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 [RFC4517].

6.  Other Considerations

6.1.  Preservation of User Information

Syntaxes may be defined that have specific value and/or value form
   (representation) preservation requirements.  For example, a syntax
   containing digitally signed data can mandate that the server preserve
   both the value and form of value presented to ensure that the
   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.

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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 (or the object identifiers they refer to) to the user.
   Instead, they 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, RFC 4520 [RFC4520].

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
   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.

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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 [RFC4519].

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 that 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 or attempt to decode a value as ASN.1 if the
   value's 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 [RFC4511] and [RFC4513].

9.  IANA Considerations

The Internet Assigned Numbers Authority (IANA) has updated the LDAP
   descriptors registry as indicated in 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 4512
      Author/Change Controller: IESG
      Comments:

The following descriptors (short names) has been added to
      the registry.

NAME                         Type OID
        ------------------------     ---- -----------------
        governingStructureRule          A 2.5.21.10
        structuralObjectClass           A 2.5.21.9

The following descriptors (short names) have been 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
        extensibleObject                O 1.3.6.1.4.1.1466.101.120.111
        ldapSyntaxes                    A 1.3.6.1.4.1.1466.101.120.16

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RFC 4512                      LDAP Models                      June 2006

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
        supportedFeatures               A 1.3.6.1.4.1.4203.1.3.5
        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.  Acknowledgements

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.

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RFC 4512                      LDAP Models                      June 2006

11.  Normative References

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

[RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                 10646", STD 63, RFC 3629, November 2003.

[RFC3671]     Zeilenga, K., "Collective Attributes in the Lightweight
                 Directory Access Protocol (LDAP)", RFC 3671, December
                 2003.

[RFC3672]     Zeilenga, K., "Subentries in the Lightweight Directory
                 Access Protocol (LDAP)", RFC 3672, December 2003.

[RFC4234]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
                 Specifications: ABNF", RFC 4234, October 2005.

[RFC4422]     Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
                 Authentication and Security Layer (SASL)", RFC 4422,
                 June 2006.

[RFC4510]     Zeilenga, K., Ed., "Lightweight Directory Access
                 Protocol (LDAP): Technical Specification Road Map", RFC
                 4510, June 2006.

[RFC4511]     Sermersheim, J., Ed., "Lightweight Directory Access
                 Protocol (LDAP): The Protocol", RFC 4511, June 2006.

[RFC4513]     Harrison, R., Ed., "Lightweight Directory Access
                 Protocol (LDAP): Authentication Methods and Security
                 Mechanisms", RFC 4513, June 2006.

[RFC4514]     Zeilenga, K., Ed., "Lightweight Directory Access
                 Protocol (LDAP): String Representation of Distinguished
                 Names", RFC 4514, June 2006.

[RFC4515]     Smith, M., Ed. and T. Howes, "Lightweight Directory
                 Access Protocol (LDAP): String Representation of Search
                 Filters", RFC 4515, June 2006.

[RFC4516]     Smith, M., Ed. and T. Howes, "Lightweight Directory
                 Access Protocol (LDAP): Uniform Resource Locator", RFC
                 4516, June 2006.

[RFC4517]     Legg, S., Ed., "Lightweight Directory Access Protocol
                 (LDAP): Syntaxes and Matching Rules", RFC 4517, June
                 2006.

Zeilenga                    Standards Track                    [Page 45]

RFC 4512                      LDAP Models                      June 2006

[RFC4519]     Sciberras, A., Ed., "Lightweight Directory Access
                 Protocol (LDAP): Schema for User Applications", RFC
                 4519, June 2006.

[RFC4520]     Zeilenga, K., "Internet Assigned Numbers Authority
                 (IANA) Considerations for the Lightweight Directory
                 Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.

[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), 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(2002) (also ISO/IEC 8824-1:2002).

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RFC 4512                      LDAP Models                      June 2006

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
   [RFC4510], [RFC4511], [RFC4517], and [RFC4519] 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, and
   portions of Sections 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
   [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.

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RFC 4512                      LDAP Models                      June 2006

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 this server".  This is inconsistent with the attribute's
     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 terribly 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.1.3.  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);

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RFC 4512                      LDAP Models                      June 2006

- Data model aspects of Attribute Types (from Section 4.1.4),
     Attribute Descriptions (from 4.1.5), Attribute (from 4.1.8),
     Matching Rule Identifier (from 4.1.9); and

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

Clarifications to these portions include:

- Subtyping and AttributeDescriptions with options.

A.1.4.  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 [RFC4234].  The
   string representation of an OBJECT IDENTIFIER was tightened to
   disallow leading zeros as described in RFC 2252.

The <descr> syntax was changed to disallow semicolon (U+003B)
   characters in order 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
   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 in Section 6.2 ("Short Names").

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

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RFC 4512                      LDAP Models                      June 2006

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.

The attribute definition of 'subschemaSubentry' was corrected to list
   the terms SINGLE-VALUE and NO-USER-MODIFICATION in proper order.

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.

The specification of 'extensibleObject' was clarified regarding 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'.

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RFC 4512                      LDAP Models                      June 2006

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.

A.4.  Changes to RFC 3674

This document made no substantive change to the 'supportedFeatures'
   technical specification provided in RFC 3674.

Editor's Address

Kurt D.  Zeilenga
   OpenLDAP Foundation

EMail: Kurt@OpenLDAP.org

Zeilenga                    Standards Track                    [Page 51]

RFC 4512                      LDAP Models                      June 2006

Full Copyright Statement

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Acknowledgement

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Zeilenga                    Standards Track                    [Page 52]

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