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Code Editor : rfc4511.txt

Network Working Group                                J. Sermersheim, Ed.
Request for Comments: 4511                                  Novell, Inc.
Obsoletes: 2251, 2830, 3771                                    June 2006
Category: Standards Track

Lightweight Directory Access Protocol (LDAP): The Protocol

Status of This Memo

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

Abstract

This document describes the protocol elements, along with their
   semantics and encodings, of the Lightweight Directory Access Protocol
   (LDAP).  LDAP provides access to distributed directory services that
   act in accordance with X.500 data and service models.  These protocol
   elements are based on those described in the X.500 Directory Access
   Protocol (DAP).

Table of Contents

1. Introduction ....................................................3
      1.1. Relationship to Other LDAP Specifications ..................3
   2. Conventions .....................................................3
   3. Protocol Model ..................................................4
      3.1. Operation and LDAP Message Layer Relationship ..............5
   4. Elements of Protocol ............................................5
      4.1. Common Elements ............................................5
           4.1.1. Message Envelope ....................................6
           4.1.2. String Types ........................................7
           4.1.3. Distinguished Name and Relative Distinguished Name ..8
           4.1.4. Attribute Descriptions ..............................8
           4.1.5. Attribute Value .....................................8
           4.1.6. Attribute Value Assertion ...........................9
           4.1.7. Attribute and PartialAttribute ......................9
           4.1.8. Matching Rule Identifier ...........................10
           4.1.9. Result Message .....................................10
           4.1.10. Referral ..........................................12

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RFC 4511                         LDAPv3                        June 2006

4.1.11. Controls ..........................................14
      4.2. Bind Operation ............................................16
           4.2.1. Processing of the Bind Request .....................17
           4.2.2. Bind Response ......................................18
      4.3. Unbind Operation ..........................................18
      4.4. Unsolicited Notification ..................................19
           4.4.1. Notice of Disconnection ............................19
      4.5. Search Operation ..........................................20
           4.5.1. Search Request .....................................20
           4.5.2. Search Result ......................................27
           4.5.3. Continuation References in the Search Result .......28
      4.6. Modify Operation ..........................................31
      4.7. Add Operation .............................................33
      4.8. Delete Operation ..........................................34
      4.9. Modify DN Operation .......................................34
      4.10. Compare Operation ........................................36
      4.11. Abandon Operation ........................................36
      4.12. Extended Operation .......................................37
      4.13. IntermediateResponse Message .............................39
           4.13.1. Usage with LDAP ExtendedRequest and
                   ExtendedResponse ..................................40
           4.13.2. Usage with LDAP Request Controls ..................40
      4.14. StartTLS Operation .......................................40
           4.14.1. StartTLS Request ..................................40
           4.14.2. StartTLS Response .................................41
           4.14.3. Removal of the TLS Layer ..........................41
   5. Protocol Encoding, Connection, and Transfer ....................42
      5.1. Protocol Encoding .........................................42
      5.2. Transmission Control Protocol (TCP) .......................43
      5.3. Termination of the LDAP session ...........................43
   6. Security Considerations ........................................43
   7. Acknowledgements ...............................................45
   8. Normative References ...........................................46
   9. Informative References .........................................48
   10. IANA Considerations ...........................................48
   Appendix A. LDAP Result Codes .....................................49
      A.1. Non-Error Result Codes ....................................49
      A.2. Result Codes ..............................................49
   Appendix B. Complete ASN.1 Definition .............................54
   Appendix C. Changes ...............................................60
      C.1. Changes Made to RFC 2251 ..................................60
      C.2. Changes Made to RFC 2830 ..................................66
      C.3. Changes Made to RFC 3771 ..................................66

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RFC 4511                         LDAPv3                        June 2006

1.  Introduction

The Directory is "a collection of open systems cooperating to provide
   directory services" [X.500].  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)).  Clients interact with servers using a directory access
   protocol.

This document details the protocol elements of the Lightweight
   Directory Access Protocol (LDAP), along with their semantics.
   Following the description of protocol elements, it describes the way
   in which the protocol elements are encoded and transferred.

1.1.  Relationship to Other LDAP Specifications

This document is an integral part of the LDAP Technical Specification
   [RFC4510], which obsoletes the previously defined LDAP technical
   specification, RFC 3377, in its entirety.

This document, together with [RFC4510], [RFC4513], and [RFC4512],
   obsoletes RFC 2251 in its entirety.  Section 3.3 is obsoleted by
   [RFC4510].  Sections 4.2.1 (portions) and 4.2.2 are obsoleted by
   [RFC4513].  Sections 3.2, 3.4, 4.1.3 (last paragraph), 4.1.4, 4.1.5,
   4.1.5.1, 4.1.9 (last paragraph), 5.1, 6.1, and 6.2 (last paragraph)
   are obsoleted by [RFC4512].  The remainder of RFC 2251 is obsoleted
   by this document.  Appendix C.1 summarizes substantive changes in the
   remainder.

This document obsoletes RFC 2830, Sections 2 and 4.  The remainder of
   RFC 2830 is obsoleted by [RFC4513].  Appendix C.2 summarizes
   substantive changes to the remaining sections.

This document also obsoletes RFC 3771 in entirety.

2.  Conventions

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

Character names in this document use the notation for code points and
   names from the Unicode Standard [Unicode].  For example, the letter
   "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.

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RFC 4511                         LDAPv3                        June 2006

Note: a glossary of terms used in Unicode can be found in [Glossary].
   Information on the Unicode character encoding model can be found in
   [CharModel].

The term "transport connection" refers to the underlying transport
   services used to carry the protocol exchange, as well as associations
   established by these services.

The term "TLS layer" refers to Transport Layer Security (TLS)
   services used in providing security services, as well as associations
   established by these services.

The term "SASL layer" refers to Simply Authentication and Security
   Layer (SASL) services used in providing security services, as well as
   associations established by these services.

The term "LDAP message layer" refers to the LDAP Message Protocol
   Data Unit (PDU) services used in providing directory services, as
   well as associations established by these services.

The term "LDAP session" refers to combined services (transport
   connection, TLS layer, SASL layer, LDAP message layer) and their
   associations.

See the table in Section 5 for an illustration of these four terms.

3.  Protocol Model

The general model adopted by this protocol is one of clients
   performing protocol operations against servers.  In this model, a
   client transmits a protocol request describing the operation to be
   performed to a server.  The server is then responsible for performing
   the necessary operation(s) in the Directory.  Upon completion of an
   operation, the server typically returns a response containing
   appropriate data to the requesting client.

Protocol operations are generally independent of one another.  Each
   operation is processed as an atomic action, leaving the directory in
   a consistent state.

Although servers are required to return responses whenever such
   responses are defined in the protocol, there is no requirement for
   synchronous behavior on the part of either clients or servers.
   Requests and responses for multiple operations generally may be
   exchanged between a client and server in any order.  If required,
   synchronous behavior may be controlled by client applications.

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RFC 4511                         LDAPv3                        June 2006

The core protocol operations defined in this document can be mapped
   to a subset of the X.500 (1993) Directory Abstract Service [X.511].
   However, there is not a one-to-one mapping between LDAP operations
   and X.500 Directory Access Protocol (DAP) operations.  Server
   implementations acting as a gateway to X.500 directories may need to
   make multiple DAP requests to service a single LDAP request.

3.1.  Operation and LDAP Message Layer Relationship

Protocol operations are exchanged at the LDAP message layer.  When
   the transport connection is closed, any uncompleted operations at the
   LDAP message layer are abandoned (when possible) or are completed
   without transmission of the response (when abandoning them is not
   possible).  Also, when the transport connection is closed, the client
   MUST NOT assume that any uncompleted update operations have succeeded
   or failed.

4.  Elements of Protocol

The protocol is described using Abstract Syntax Notation One
   ([ASN.1]) and is transferred using a subset of ASN.1 Basic Encoding
   Rules ([BER]).  Section 5 specifies how the protocol elements are
   encoded and transferred.

In order to support future extensions to this protocol, extensibility
   is implied where it is allowed per ASN.1 (i.e., sequence, set,
   choice, and enumerated types are extensible).  In addition, ellipses
   (...) have been supplied in ASN.1 types that are explicitly
   extensible as discussed in [RFC4520].  Because of the implied
   extensibility, clients and servers MUST (unless otherwise specified)
   ignore trailing SEQUENCE components whose tags they do not recognize.

Changes to the protocol other than through the extension mechanisms
   described here require a different version number.  A client
   indicates the version it is using as part of the BindRequest,
   described in Section 4.2.  If a client has not sent a Bind, the
   server MUST assume the client is using version 3 or later.

Clients may attempt to determine the protocol versions a server
   supports by reading the 'supportedLDAPVersion' attribute from the
   root DSE (DSA-Specific Entry) [RFC4512].

4.1.  Common Elements

This section describes the LDAPMessage envelope Protocol Data Unit
   (PDU) format, as well as data type definitions, which are used in the
   protocol operations.

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RFC 4511                         LDAPv3                        June 2006

4.1.1.  Message Envelope

For the purposes of protocol exchanges, all protocol operations are
   encapsulated in a common envelope, the LDAPMessage, which is defined
   as follows:

LDAPMessage ::= SEQUENCE {
             messageID       MessageID,
             protocolOp      CHOICE {
                  bindRequest           BindRequest,
                  bindResponse          BindResponse,
                  unbindRequest         UnbindRequest,
                  searchRequest         SearchRequest,
                  searchResEntry        SearchResultEntry,
                  searchResDone         SearchResultDone,
                  searchResRef          SearchResultReference,
                  modifyRequest         ModifyRequest,
                  modifyResponse        ModifyResponse,
                  addRequest            AddRequest,
                  addResponse           AddResponse,
                  delRequest            DelRequest,
                  delResponse           DelResponse,
                  modDNRequest          ModifyDNRequest,
                  modDNResponse         ModifyDNResponse,
                  compareRequest        CompareRequest,
                  compareResponse       CompareResponse,
                  abandonRequest        AbandonRequest,
                  extendedReq           ExtendedRequest,
                  extendedResp          ExtendedResponse,
                  ...,
                  intermediateResponse  IntermediateResponse },
             controls       [0] Controls OPTIONAL }

MessageID ::= INTEGER (0 ..  maxInt)

maxInt INTEGER ::= 2147483647 -- (2^^31 - 1) --

The ASN.1 type Controls is defined in Section 4.1.11.

The function of the LDAPMessage is to provide an envelope containing
   common fields required in all protocol exchanges.  At this time, the
   only common fields are the messageID and the controls.

If the server receives an LDAPMessage from the client in which the
   LDAPMessage SEQUENCE tag cannot be recognized, the messageID cannot
   be parsed, the tag of the protocolOp is not recognized as a request,
   or the encoding structures or lengths of data fields are found to be
   incorrect, then the server SHOULD return the Notice of Disconnection

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RFC 4511                         LDAPv3                        June 2006

described in Section 4.4.1, with the resultCode set to protocolError,
   and MUST immediately terminate the LDAP session as described in
   Section 5.3.

In other cases where the client or server cannot parse an LDAP PDU,
   it SHOULD abruptly terminate the LDAP session (Section 5.3) where
   further communication (including providing notice) would be
   pernicious.  Otherwise, server implementations MUST return an
   appropriate response to the request, with the resultCode set to
   protocolError.

4.1.1.1.  MessageID

All LDAPMessage envelopes encapsulating responses contain the
   messageID value of the corresponding request LDAPMessage.

The messageID of a request MUST have a non-zero value different from
   the messageID of any other request in progress in the same LDAP
   session.  The zero value is reserved for the unsolicited notification
   message.

Typical clients increment a counter for each request.

A client MUST NOT send a request with the same messageID as an
   earlier request in the same LDAP session unless it can be determined
   that the server is no longer servicing the earlier request (e.g.,
   after the final response is received, or a subsequent Bind
   completes).  Otherwise, the behavior is undefined.  For this purpose,
   note that Abandon and successfully abandoned operations do not send
   responses.

4.1.2.  String Types

The LDAPString is a notational convenience to indicate that, although
   strings of LDAPString type encode as ASN.1 OCTET STRING types, the
   [ISO10646] character set (a superset of [Unicode]) is used, encoded
   following the UTF-8 [RFC3629] algorithm.  Note that Unicode
   characters U+0000 through U+007F are the same as ASCII 0 through 127,
   respectively, and have the same single octet UTF-8 encoding.  Other
   Unicode characters have a multiple octet UTF-8 encoding.

LDAPString ::= OCTET STRING -- UTF-8 encoded,
                                    -- [ISO10646] characters

The LDAPOID is a notational convenience to indicate that the
   permitted value of this string is a (UTF-8 encoded) dotted-decimal
   representation of an OBJECT IDENTIFIER.  Although an LDAPOID is

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RFC 4511                         LDAPv3                        June 2006

encoded as an OCTET STRING, values are limited to the definition of
   <numericoid> given in Section 1.4 of [RFC4512].

LDAPOID ::= OCTET STRING -- Constrained to <numericoid>
                                 -- [RFC4512]

For example,

1.3.6.1.4.1.1466.1.2.3

4.1.3.  Distinguished Name and Relative Distinguished Name

An LDAPDN is defined to be the representation of a Distinguished Name
   (DN) after encoding according to the specification in [RFC4514].

LDAPDN ::= LDAPString
                   -- Constrained to <distinguishedName> [RFC4514]

A RelativeLDAPDN is defined to be the representation of a Relative
   Distinguished Name (RDN) after encoding according to the
   specification in [RFC4514].

RelativeLDAPDN ::= LDAPString
                           -- Constrained to <name-component> [RFC4514]

4.1.4.  Attribute Descriptions

The definition and encoding rules for attribute descriptions are
   defined in Section 2.5 of [RFC4512].  Briefly, an attribute
   description is an attribute type and zero or more options.

AttributeDescription ::= LDAPString
                                -- Constrained to <attributedescription>
                                -- [RFC4512]

4.1.5.  Attribute Value

A field of type AttributeValue is an OCTET STRING containing an
   encoded attribute value.  The attribute value is encoded according to
   the LDAP-specific encoding definition of its corresponding syntax.
   The LDAP-specific encoding definitions for different syntaxes and
   attribute types may be found in other documents and in particular
   [RFC4517].

AttributeValue ::= OCTET STRING

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RFC 4511                         LDAPv3                        June 2006

Note that there is no defined limit on the size of this encoding;
   thus, protocol values may include multi-megabyte attribute values
   (e.g., photographs).

Attribute values may be defined that have arbitrary and non-printable
   syntax.  Implementations MUST NOT display or attempt to decode an
   attribute value if its syntax is not known.  The implementation may
   attempt to discover the subschema of the source entry and to retrieve
   the descriptions of 'attributeTypes' from it [RFC4512].

Clients MUST only send attribute values in a request that are valid
   according to the syntax defined for the attributes.

4.1.6.  Attribute Value Assertion

The AttributeValueAssertion (AVA) type definition is similar to the
   one in the X.500 Directory standards.  It contains an attribute
   description and a matching rule ([RFC4512], Section 4.1.3) assertion
   value suitable for that type.  Elements of this type are typically
   used to assert that the value in assertionValue matches a value of an
   attribute.

AttributeValueAssertion ::= SEQUENCE {
             attributeDesc   AttributeDescription,
             assertionValue  AssertionValue }

AssertionValue ::= OCTET STRING

The syntax of the AssertionValue depends on the context of the LDAP
   operation being performed.  For example, the syntax of the EQUALITY
   matching rule for an attribute is used when performing a Compare
   operation.  Often this is the same syntax used for values of the
   attribute type, but in some cases the assertion syntax differs from
   the value syntax.  See objectIdentiferFirstComponentMatch in
   [RFC4517] for an example.

4.1.7.  Attribute and PartialAttribute

Attributes and partial attributes consist of an attribute description
   and attribute values.  A PartialAttribute allows zero values, while
   Attribute requires at least one value.

PartialAttribute ::= SEQUENCE {
             type       AttributeDescription,
             vals       SET OF value AttributeValue }

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RFC 4511                         LDAPv3                        June 2006

Attribute ::= PartialAttribute(WITH COMPONENTS {
             ...,
             vals (SIZE(1..MAX))})

No two of the attribute values may be equivalent as described by
   Section 2.2 of [RFC4512].  The set of attribute values is unordered.
   Implementations MUST NOT rely upon the ordering being repeatable.

4.1.8.  Matching Rule Identifier

Matching rules are defined in Section 4.1.3 of [RFC4512].  A matching
   rule is identified in the protocol by the printable representation of
   either its <numericoid> or one of its short name descriptors
   [RFC4512], e.g., 'caseIgnoreMatch' or '2.5.13.2'.

MatchingRuleId ::= LDAPString

4.1.9.  Result Message

The LDAPResult is the construct used in this protocol to return
   success or failure indications from servers to clients.  To various
   requests, servers will return responses containing the elements found
   in LDAPResult to indicate the final status of the protocol operation
   request.

LDAPResult ::= SEQUENCE {
             resultCode         ENUMERATED {
                  success                      (0),
                  operationsError              (1),
                  protocolError                (2),
                  timeLimitExceeded            (3),
                  sizeLimitExceeded            (4),
                  compareFalse                 (5),
                  compareTrue                  (6),
                  authMethodNotSupported       (7),
                  strongerAuthRequired         (8),
                       -- 9 reserved --
                  referral                     (10),
                  adminLimitExceeded           (11),
                  unavailableCriticalExtension (12),
                  confidentialityRequired      (13),
                  saslBindInProgress           (14),
                  noSuchAttribute              (16),
                  undefinedAttributeType       (17),
                  inappropriateMatching        (18),
                  constraintViolation          (19),
                  attributeOrValueExists       (20),
                  invalidAttributeSyntax       (21),

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RFC 4511                         LDAPv3                        June 2006

-- 22-31 unused --
                  noSuchObject                 (32),
                  aliasProblem                 (33),
                  invalidDNSyntax              (34),
                       -- 35 reserved for undefined isLeaf --
                  aliasDereferencingProblem    (36),
                       -- 37-47 unused --
                  inappropriateAuthentication  (48),
                  invalidCredentials           (49),
                  insufficientAccessRights     (50),
                  busy                         (51),
                  unavailable                  (52),
                  unwillingToPerform           (53),
                  loopDetect                   (54),
                       -- 55-63 unused --
                  namingViolation              (64),
                  objectClassViolation         (65),
                  notAllowedOnNonLeaf          (66),
                  notAllowedOnRDN              (67),
                  entryAlreadyExists           (68),
                  objectClassModsProhibited    (69),
                       -- 70 reserved for CLDAP --
                  affectsMultipleDSAs          (71),
                       -- 72-79 unused --
                  other                        (80),
                  ...  },
             matchedDN          LDAPDN,
             diagnosticMessage  LDAPString,
             referral           [3] Referral OPTIONAL }

The resultCode enumeration is extensible as defined in Section 3.8 of
   [RFC4520].  The meanings of the listed result codes are given in
   Appendix A.  If a server detects multiple errors for an operation,
   only one result code is returned.  The server should return the
   result code that best indicates the nature of the error encountered.
   Servers may return substituted result codes to prevent unauthorized
   disclosures.

The diagnosticMessage field of this construct may, at the server's
   option, be used to return a string containing a textual, human-
   readable diagnostic message (terminal control and page formatting
   characters should be avoided).  As this diagnostic message is not
   standardized, implementations MUST NOT rely on the values returned.
   Diagnostic messages typically supplement the resultCode with
   additional information.  If the server chooses not to return a
   textual diagnostic, the diagnosticMessage field MUST be empty.

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RFC 4511                         LDAPv3                        June 2006

For certain result codes (typically, but not restricted to
   noSuchObject, aliasProblem, invalidDNSyntax, and
   aliasDereferencingProblem), the matchedDN field is set (subject to
   access controls) to the name of the last entry (object or alias) used
   in finding the target (or base) object.  This will be a truncated
   form of the provided name or, if an alias was dereferenced while
   attempting to locate the entry, of the resulting name.  Otherwise,
   the matchedDN field is empty.

4.1.10.  Referral

The referral result code indicates that the contacted server cannot
   or will not perform the operation and that one or more other servers
   may be able to.  Reasons for this include:

- The target entry of the request is not held locally, but the server
     has knowledge of its possible existence elsewhere.

- The operation is restricted on this server -- perhaps due to a
     read-only copy of an entry to be modified.

The referral field is present in an LDAPResult if the resultCode is
   set to referral, and it is absent with all other result codes.  It
   contains one or more references to one or more servers or services
   that may be accessed via LDAP or other protocols.  Referrals can be
   returned in response to any operation request (except Unbind and
   Abandon, which do not have responses).  At least one URI MUST be
   present in the Referral.

During a Search operation, after the baseObject is located, and
   entries are being evaluated, the referral is not returned.  Instead,
   continuation references, described in Section 4.5.3, are returned
   when other servers would need to be contacted to complete the
   operation.

Referral ::= SEQUENCE SIZE (1..MAX) OF uri URI

URI ::= LDAPString     -- limited to characters permitted in
                               -- URIs

If the client wishes to progress the operation, it contacts one of
   the supported services found in the referral.  If multiple URIs are
   present, the client assumes that any supported URI may be used to
   progress the operation.

Clients that follow referrals MUST ensure that they do not loop
   between servers.  They MUST NOT repeatedly contact the same server
   for the same request with the same parameters.  Some clients use a

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counter that is incremented each time referral handling occurs for an
   operation, and these kinds of clients MUST be able to handle at least
   ten nested referrals while progressing the operation.

A URI for a server implementing LDAP and accessible via TCP/IP (v4 or
   v6) [RFC793][RFC791] is written as an LDAP URL according to
   [RFC4516].

Referral values that are LDAP URLs follow these rules:

- If an alias was dereferenced, the <dn> part of the LDAP URL MUST be
     present, with the new target object name.

- It is RECOMMENDED that the <dn> part be present to avoid ambiguity.

- If the <dn> part is present, the client uses this name in its next
     request to progress the operation, and if it is not present the
     client uses the same name as in the original request.

- Some servers (e.g., participating in distributed indexing) may
     provide a different filter in a URL of a referral for a Search
     operation.

- If the <filter> part of the LDAP URL is present, the client uses
     this filter in its next request to progress this Search, and if it
     is not present the client uses the same filter as it used for that
     Search.

- For Search, it is RECOMMENDED that the <scope> part be present to
     avoid ambiguity.

- If the <scope> part is missing, the scope of the original Search is
     used by the client to progress the operation.

- Other aspects of the new request may be the same as or different
     from the request that generated the referral.

Other kinds of URIs may be returned.  The syntax and semantics of
   such URIs is left to future specifications.  Clients may ignore URIs
   that they do not support.

UTF-8 encoded characters appearing in the string representation of a
   DN, search filter, or other fields of the referral value may not be
   legal for URIs (e.g., spaces) and MUST be escaped using the % method
   in [RFC3986].

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4.1.11.  Controls

Controls provide a mechanism whereby the semantics and arguments of
   existing LDAP operations may be extended.  One or more controls may
   be attached to a single LDAP message.  A control only affects the
   semantics of the message it is attached to.

Controls sent by clients are termed 'request controls', and those
   sent by servers are termed 'response controls'.

Controls ::= SEQUENCE OF control Control

Control ::= SEQUENCE {
             controlType             LDAPOID,
             criticality             BOOLEAN DEFAULT FALSE,
             controlValue            OCTET STRING OPTIONAL }

The controlType field is the dotted-decimal representation of an
   OBJECT IDENTIFIER that uniquely identifies the control.  This
   provides unambiguous naming of controls.  Often, response control(s)
   solicited by a request control share controlType values with the
   request control.

The criticality field only has meaning in controls attached to
   request messages (except UnbindRequest).  For controls attached to
   response messages and the UnbindRequest, the criticality field SHOULD
   be FALSE, and MUST be ignored by the receiving protocol peer.  A
   value of TRUE indicates that it is unacceptable to perform the
   operation without applying the semantics of the control.
   Specifically, the criticality field is applied as follows:

- If the server does not recognize the control type, determines that
     it is not appropriate for the operation, or is otherwise unwilling
     to perform the operation with the control, and if the criticality
     field is TRUE, the server MUST NOT perform the operation, and for
     operations that have a response message, it MUST return with the
     resultCode set to unavailableCriticalExtension.

- Regardless of criticality, if a control is applied to an
     operation, it is applied consistently and impartially to the
     entire operation.

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The controlValue may contain information associated with the
   controlType.  Its format is defined by the specification of the
   control.  Implementations MUST be prepared to handle arbitrary
   contents of the controlValue octet string, including zero bytes.  It
   is absent only if there is no value information that is associated
   with a control of its type.  When a controlValue is defined in terms
   of ASN.1, and BER-encoded according to Section 5.1, it also follows
   the extensibility rules in Section 4.

Servers list the controlType of request controls they recognize in
   the 'supportedControl' attribute in the root DSE (Section 5.1 of
   [RFC4512]).

Controls SHOULD NOT be combined unless the semantics of the
   combination has been specified.  The semantics of control
   combinations, if specified, are generally found in the control
   specification most recently published.  When a combination of
   controls is encountered whose semantics are invalid, not specified
   (or not known), the message is considered not well-formed; thus, the
   operation fails with protocolError.  Controls with a criticality of
   FALSE may be ignored in order to arrive at a valid combination.
   Additionally, unless order-dependent semantics are given in a
   specification, the order of a combination of controls in the SEQUENCE
   is ignored.  Where the order is to be ignored but cannot be ignored
   by the server, the message is considered not well-formed, and the
   operation fails with protocolError.  Again, controls with a
   criticality of FALSE may be ignored in order to arrive at a valid
   combination.

This document does not specify any controls.  Controls may be
   specified in other documents.  Documents detailing control extensions
   are to provide for each control:

- the OBJECT IDENTIFIER assigned to the control,

- direction as to what value the sender should provide for the
     criticality field (note: the semantics of the criticality field are
     defined above should not be altered by the control's
     specification),

- whether the controlValue field is present, and if so, the format of
     its contents,

- the semantics of the control, and

- optionally, semantics regarding the combination of the control with
     other controls.

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4.2.  Bind Operation

The function of the Bind operation is to allow authentication
   information to be exchanged between the client and server.  The Bind
   operation should be thought of as the "authenticate" operation.
   Operational, authentication, and security-related semantics of this
   operation are given in [RFC4513].

The Bind request is defined as follows:

BindRequest ::= [APPLICATION 0] SEQUENCE {
             version                 INTEGER (1 ..  127),
             name                    LDAPDN,
             authentication          AuthenticationChoice }

AuthenticationChoice ::= CHOICE {
             simple                  [0] OCTET STRING,
                                     -- 1 and 2 reserved
             sasl                    [3] SaslCredentials,
             ...  }

SaslCredentials ::= SEQUENCE {
             mechanism               LDAPString,
             credentials             OCTET STRING OPTIONAL }

Fields of the BindRequest are:

- version: A version number indicating the version of the protocol to
     be used at the LDAP message layer.  This document describes version
     3 of the protocol.  There is no version negotiation.  The client
     sets this field to the version it desires.  If the server does not
     support the specified version, it MUST respond with a BindResponse
     where the resultCode is set to protocolError.

- name: If not empty, the name of the Directory object that the
     client wishes to bind as.  This field may take on a null value (a
     zero-length string) for the purposes of anonymous binds ([RFC4513],
     Section 5.1) or when using SASL [RFC4422] authentication
     ([RFC4513], Section 5.2).  Where the server attempts to locate the
     named object, it SHALL NOT perform alias dereferencing.

- authentication: Information used in authentication.  This type is
     extensible as defined in Section 3.7 of [RFC4520].  Servers that do
     not support a choice supplied by a client return a BindResponse
     with the resultCode set to authMethodNotSupported.

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Textual passwords (consisting of a character sequence with a known
     character set and encoding) transferred to the server using the
     simple AuthenticationChoice SHALL be transferred as UTF-8 [RFC3629]
     encoded [Unicode].  Prior to transfer, clients SHOULD prepare text
     passwords as "query" strings by applying the SASLprep [RFC4013]
     profile of the stringprep [RFC3454] algorithm.  Passwords
     consisting of other data (such as random octets) MUST NOT be
     altered.  The determination of whether a password is textual is a
     local client matter.

4.2.1.  Processing of the Bind Request

Before processing a BindRequest, all uncompleted operations MUST
   either complete or be abandoned.  The server may either wait for the
   uncompleted operations to complete, or abandon them.  The server then
   proceeds to authenticate the client in either a single-step or
   multi-step Bind process.  Each step requires the server to return a
   BindResponse to indicate the status of authentication.

After sending a BindRequest, clients MUST NOT send further LDAP PDUs
   until receiving the BindResponse.  Similarly, servers SHOULD NOT
   process or respond to requests received while processing a
   BindRequest.

If the client did not bind before sending a request and receives an
   operationsError to that request, it may then send a BindRequest.  If
   this also fails or the client chooses not to bind on the existing
   LDAP session, it may terminate the LDAP session, re-establish it, and
   begin again by first sending a BindRequest.  This will aid in
   interoperating with servers implementing other versions of LDAP.

Clients may send multiple Bind requests to change the authentication
   and/or security associations or to complete a multi-stage Bind
   process.  Authentication from earlier binds is subsequently ignored.

For some SASL authentication mechanisms, it may be necessary for the
   client to invoke the BindRequest multiple times ([RFC4513], Section
   5.2).  Clients MUST NOT invoke operations between two Bind requests
   made as part of a multi-stage Bind.

A client may abort a SASL bind negotiation by sending a BindRequest
   with a different value in the mechanism field of SaslCredentials, or
   an AuthenticationChoice other than sasl.

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If the client sends a BindRequest with the sasl mechanism field as an
   empty string, the server MUST return a BindResponse with the
   resultCode set to authMethodNotSupported.  This will allow the client
   to abort a negotiation if it wishes to try again with the same SASL
   mechanism.

4.2.2.  Bind Response

The Bind response is defined as follows.

BindResponse ::= [APPLICATION 1] SEQUENCE {
             COMPONENTS OF LDAPResult,
             serverSaslCreds    [7] OCTET STRING OPTIONAL }

BindResponse consists simply of an indication from the server of the
   status of the client's request for authentication.

A successful Bind operation is indicated by a BindResponse with a
   resultCode set to success.  Otherwise, an appropriate result code is
   set in the BindResponse.  For BindResponse, the protocolError result
   code may be used to indicate that the version number supplied by the
   client is unsupported.

If the client receives a BindResponse where the resultCode is set to
   protocolError, it is to assume that the server does not support this
   version of LDAP.  While the client may be able proceed with another
   version of this protocol (which may or may not require closing and
   re-establishing the transport connection), how to proceed with
   another version of this protocol is beyond the scope of this
   document.  Clients that are unable or unwilling to proceed SHOULD
   terminate the LDAP session.

The serverSaslCreds field is used as part of a SASL-defined bind
   mechanism to allow the client to authenticate the server to which it
   is communicating, or to perform "challenge-response" authentication.
   If the client bound with the simple choice, or the SASL mechanism
   does not require the server to return information to the client, then
   this field SHALL NOT be included in the BindResponse.

4.3.  Unbind Operation

The function of the Unbind operation is to terminate an LDAP session.
   The Unbind operation is not the antithesis of the Bind operation as
   the name implies.  The naming of these operations are historical.
   The Unbind operation should be thought of as the "quit" operation.

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The Unbind operation is defined as follows:

UnbindRequest ::= [APPLICATION 2] NULL

The client, upon transmission of the UnbindRequest, and the server,
   upon receipt of the UnbindRequest, are to gracefully terminate the
   LDAP session as described in Section 5.3.  Uncompleted operations are
   handled as specified in Section 3.1.

4.4.  Unsolicited Notification

An unsolicited notification is an LDAPMessage sent from the server to
   the client that is not in response to any LDAPMessage received by the
   server.  It is used to signal an extraordinary condition in the
   server or in the LDAP session between the client and the server.  The
   notification is of an advisory nature, and the server will not expect
   any response to be returned from the client.

The unsolicited notification is structured as an LDAPMessage in which
   the messageID is zero and protocolOp is set to the extendedResp
   choice using the ExtendedResponse type (See Section 4.12).  The
   responseName field of the ExtendedResponse always contains an LDAPOID
   that is unique for this notification.

One unsolicited notification (Notice of Disconnection) is defined in
   this document.  The specification of an unsolicited notification
   consists of:

- the OBJECT IDENTIFIER assigned to the notification (to be specified
     in the responseName,

- the format of the contents of the responseValue (if any),

- the circumstances which will cause the notification to be sent, and

- the semantics of the message.

4.4.1.  Notice of Disconnection

This notification may be used by the server to advise the client that
   the server is about to terminate the LDAP session on its own
   initiative.  This notification is intended to assist clients in
   distinguishing between an exceptional server condition and a
   transient network failure.  Note that this notification is not a
   response to an Unbind requested by the client.  Uncompleted
   operations are handled as specified in Section 3.1.

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The responseName is 1.3.6.1.4.1.1466.20036, the responseValue field
   is absent, and the resultCode is used to indicate the reason for the
   disconnection.  When the strongerAuthRequired resultCode is returned
   with this message, it indicates that the server has detected that an
   established security association between the client and server has
   unexpectedly failed or been compromised.

Upon transmission of the Notice of Disconnection, the server
   gracefully terminates the LDAP session as described in Section 5.3.

4.5.  Search Operation

The Search operation is used to request a server to return, subject
   to access controls and other restrictions, a set of entries matching
   a complex search criterion.  This can be used to read attributes from
   a single entry, from entries immediately subordinate to a particular
   entry, or from a whole subtree of entries.

4.5.1.  Search Request

The Search request is defined as follows:

SearchRequest ::= [APPLICATION 3] SEQUENCE {
             baseObject      LDAPDN,
             scope           ENUMERATED {
                  baseObject              (0),
                  singleLevel             (1),
                  wholeSubtree            (2),
                  ...  },
             derefAliases    ENUMERATED {
                  neverDerefAliases       (0),
                  derefInSearching        (1),
                  derefFindingBaseObj     (2),
                  derefAlways             (3) },
             sizeLimit       INTEGER (0 ..  maxInt),
             timeLimit       INTEGER (0 ..  maxInt),
             typesOnly       BOOLEAN,
             filter          Filter,
             attributes      AttributeSelection }

AttributeSelection ::= SEQUENCE OF selector LDAPString
                        -- The LDAPString is constrained to
                        -- <attributeSelector> in Section 4.5.1.8

Filter ::= CHOICE {
             and             [0] SET SIZE (1..MAX) OF filter Filter,
             or              [1] SET SIZE (1..MAX) OF filter Filter,
             not             [2] Filter,

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equalityMatch   [3] AttributeValueAssertion,
             substrings      [4] SubstringFilter,
             greaterOrEqual  [5] AttributeValueAssertion,
             lessOrEqual     [6] AttributeValueAssertion,
             present         [7] AttributeDescription,
             approxMatch     [8] AttributeValueAssertion,
             extensibleMatch [9] MatchingRuleAssertion,
             ...  }

SubstringFilter ::= SEQUENCE {
             type           AttributeDescription,
             substrings     SEQUENCE SIZE (1..MAX) OF substring CHOICE {
                  initial [0] AssertionValue,  -- can occur at most once
                  any     [1] AssertionValue,
                  final   [2] AssertionValue } -- can occur at most once
             }

MatchingRuleAssertion ::= SEQUENCE {
             matchingRule    [1] MatchingRuleId OPTIONAL,
             type            [2] AttributeDescription OPTIONAL,
             matchValue      [3] AssertionValue,
             dnAttributes    [4] BOOLEAN DEFAULT FALSE }

Note that an X.500 "list"-like operation can be emulated by the
   client requesting a singleLevel Search operation with a filter
   checking for the presence of the 'objectClass' attribute, and that an
   X.500 "read"-like operation can be emulated by a baseObject Search
   operation with the same filter.  A server that provides a gateway to
   X.500 is not required to use the Read or List operations, although it
   may choose to do so, and if it does, it must provide the same
   semantics as the X.500 Search operation.

4.5.1.1.  SearchRequest.baseObject

The name of the base object entry (or possibly the root) relative to
   which the Search is to be performed.

4.5.1.2.  SearchRequest.scope

Specifies the scope of the Search to be performed.  The semantics (as
   described in [X.511]) of the defined values of this field are:

baseObject: The scope is constrained to the entry named by
      baseObject.

singleLevel: The scope is constrained to the immediate
      subordinates of the entry named by baseObject.

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wholeSubtree: The scope is constrained to the entry named by
      baseObject and to all its subordinates.

4.5.1.3.  SearchRequest.derefAliases

An indicator as to whether or not alias entries (as defined in
   [RFC4512]) are to be dereferenced during stages of the Search
   operation.

The act of dereferencing an alias includes recursively dereferencing
   aliases that refer to aliases.

Servers MUST detect looping while dereferencing aliases in order to
   prevent denial-of-service attacks of this nature.

The semantics of the defined values of this field are:

neverDerefAliases: Do not dereference aliases in searching or in
      locating the base object of the Search.

derefInSearching: While searching subordinates of the base object,
      dereference any alias within the search scope.  Dereferenced
      objects become the vertices of further search scopes where the
      Search operation is also applied.  If the search scope is
      wholeSubtree, the Search continues in the subtree(s) of any
      dereferenced object.  If the search scope is singleLevel, the
      search is applied to any dereferenced objects and is not applied
      to their subordinates.  Servers SHOULD eliminate duplicate entries
      that arise due to alias dereferencing while searching.

derefFindingBaseObj: Dereference aliases in locating the base
      object of the Search, but not when searching subordinates of the
      base object.

derefAlways: Dereference aliases both in searching and in locating
      the base object of the Search.

4.5.1.4.  SearchRequest.sizeLimit

A size limit that restricts the maximum number of entries to be
   returned as a result of the Search.  A value of zero in this field
   indicates that no client-requested size limit restrictions are in
   effect for the Search.  Servers may also enforce a maximum number of
   entries to return.

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4.5.1.5.  SearchRequest.timeLimit

A time limit that restricts the maximum time (in seconds) allowed for
   a Search.  A value of zero in this field indicates that no client-
   requested time limit restrictions are in effect for the Search.
   Servers may also enforce a maximum time limit for the Search.

4.5.1.6.  SearchRequest.typesOnly

An indicator as to whether Search results are to contain both
   attribute descriptions and values, or just attribute descriptions.
   Setting this field to TRUE causes only attribute descriptions (and
   not values) to be returned.  Setting this field to FALSE causes both
   attribute descriptions and values to be returned.

4.5.1.7.  SearchRequest.filter

A filter that defines the conditions that must be fulfilled in order
   for the Search to match a given entry.

The 'and', 'or', and 'not' choices can be used to form combinations
   of filters.  At least one filter element MUST be present in an 'and'
   or 'or' choice.  The others match against individual attribute values
   of entries in the scope of the Search.  (Implementor's note: the
   'not' filter is an example of a tagged choice in an implicitly-tagged
   module.  In BER this is treated as if the tag were explicit.)

A server MUST evaluate filters according to the three-valued logic of
   [X.511] (1993), Clause 7.8.1.  In summary, a filter is evaluated to
   "TRUE", "FALSE", or "Undefined".  If the filter evaluates to TRUE for
   a particular entry, then the attributes of that entry are returned as
   part of the Search result (subject to any applicable access control
   restrictions).  If the filter evaluates to FALSE or Undefined, then
   the entry is ignored for the Search.

A filter of the "and" choice is TRUE if all the filters in the SET OF
   evaluate to TRUE, FALSE if at least one filter is FALSE, and
   Undefined otherwise.  A filter of the "or" choice is FALSE if all the
   filters in the SET OF evaluate to FALSE, TRUE if at least one filter
   is TRUE, and Undefined otherwise.  A filter of the 'not' choice is
   TRUE if the filter being negated is FALSE, FALSE if it is TRUE, and
   Undefined if it is Undefined.

A filter item evaluates to Undefined when the server would not be
   able to determine whether the assertion value matches an entry.
   Examples include:

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- An attribute description in an equalityMatch, substrings,
     greaterOrEqual, lessOrEqual, approxMatch, or extensibleMatch filter
     is not recognized by the server.

- The attribute type does not define the appropriate matching rule.

- A MatchingRuleId in the extensibleMatch is not recognized by the
     server or is not valid for the attribute type.

- The type of filtering requested is not implemented.

- The assertion value is invalid.

For example, if a server did not recognize the attribute type
   shoeSize, the filters (shoeSize=*), (shoeSize=12), (shoeSize>=12),
   and (shoeSize<=12) would each evaluate to Undefined.

Servers MUST NOT return errors if attribute descriptions or matching
   rule ids are not recognized, assertion values are invalid, or the
   assertion syntax is not supported.  More details of filter processing
   are given in Clause 7.8 of [X.511].

4.5.1.7.1.  SearchRequest.filter.equalityMatch

The matching rule for an equalityMatch filter is defined by the
   EQUALITY matching rule for the attribute type or subtype.  The filter
   is TRUE when the EQUALITY rule returns TRUE as applied to the
   attribute or subtype and the asserted value.

4.5.1.7.2.  SearchRequest.filter.substrings

There SHALL be at most one 'initial' and at most one 'final' in the
   'substrings' of a SubstringFilter.  If 'initial' is present, it SHALL
   be the first element of 'substrings'.  If 'final' is present, it
   SHALL be the last element of 'substrings'.

The matching rule for an AssertionValue in a substrings filter item
   is defined by the SUBSTR matching rule for the attribute type or
   subtype.  The filter is TRUE when the SUBSTR rule returns TRUE as
   applied to the attribute or subtype and the asserted value.

Note that the AssertionValue in a substrings filter item conforms to
   the assertion syntax of the EQUALITY matching rule for the attribute
   type rather than to the assertion syntax of the SUBSTR matching rule
   for the attribute type.  Conceptually, the entire SubstringFilter is
   converted into an assertion value of the substrings matching rule
   prior to applying the rule.

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4.5.1.7.3.  SearchRequest.filter.greaterOrEqual

The matching rule for a greaterOrEqual filter is defined by the
   ORDERING matching rule for the attribute type or subtype.  The filter
   is TRUE when the ORDERING rule returns FALSE as applied to the
   attribute or subtype and the asserted value.

4.5.1.7.4.  SearchRequest.filter.lessOrEqual

The matching rules for a lessOrEqual filter are defined by the
   ORDERING and EQUALITY matching rules for the attribute type or
   subtype.  The filter is TRUE when either the ORDERING or EQUALITY
   rule returns TRUE as applied to the attribute or subtype and the
   asserted value.

4.5.1.7.5.  SearchRequest.filter.present

A present filter is TRUE when there is an attribute or subtype of the
   specified attribute description present in an entry, FALSE when no
   attribute or subtype of the specified attribute description is
   present in an entry, and Undefined otherwise.

4.5.1.7.6.  SearchRequest.filter.approxMatch

An approxMatch filter is TRUE when there is a value of the attribute
   type or subtype for which some locally-defined approximate matching
   algorithm (e.g., spelling variations, phonetic match, etc.) returns
   TRUE.  If a value matches for equality, it also satisfies an
   approximate match.  If approximate matching is not supported for the
   attribute, this filter item should be treated as an equalityMatch.

4.5.1.7.7.  SearchRequest.filter.extensibleMatch

The fields of the extensibleMatch filter item are evaluated as
   follows:

- If the matchingRule field is absent, the type field MUST be
     present, and an equality match is performed for that type.

- If the type field is absent and the matchingRule is present, the
     matchValue is compared against all attributes in an entry that
     support that matchingRule.

- If the type field is present and the matchingRule is present, the
     matchValue is compared against the specified attribute type and its
     subtypes.

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- If the dnAttributes field is set to TRUE, the match is additionally
     applied against all the AttributeValueAssertions in an entry's
     distinguished name, and it evaluates to TRUE if there is at least
     one attribute or subtype in the distinguished name for which the
     filter item evaluates to TRUE.  The dnAttributes field is present
     to alleviate the need for multiple versions of generic matching
     rules (such as word matching), where one applies to entries and
     another applies to entries and DN attributes as well.

The matchingRule used for evaluation determines the syntax for the
   assertion value.  Once the matchingRule and attribute(s) have been
   determined, the filter item evaluates to TRUE if it matches at least
   one attribute type or subtype in the entry, FALSE if it does not
   match any attribute type or subtype in the entry, and Undefined if
   the matchingRule is not recognized, the matchingRule is unsuitable
   for use with the specified type, or the assertionValue is invalid.

4.5.1.8.  SearchRequest.attributes

A selection list of the attributes to be returned from each entry
   that matches the search filter.  Attributes that are subtypes of
   listed attributes are implicitly included.  LDAPString values of this
   field are constrained to the following Augmented Backus-Naur Form
   (ABNF) [RFC4234]:

attributeSelector = attributedescription / selectorspecial

selectorspecial = noattrs / alluserattrs

noattrs = %x31.2E.31 ; "1.1"

alluserattrs = %x2A ; asterisk ("*")

The <attributedescription> production is defined in Section 2.5 of
      [RFC4512].

There are three special cases that may appear in the attributes
      selection list:

1. An empty list with no attributes requests the return of all
         user attributes.

2. A list containing "*" (with zero or more attribute
         descriptions) requests the return of all user attributes in
         addition to other listed (operational) attributes.

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3. A list containing only the OID "1.1" indicates that no
         attributes are to be returned.  If "1.1" is provided with other
         attributeSelector values, the "1.1" attributeSelector is
         ignored.  This OID was chosen because it does not (and can not)
         correspond to any attribute in use.

Client implementors should note that even if all user attributes are
   requested, some attributes and/or attribute values of the entry may
   not be included in Search results due to access controls or other
   restrictions.  Furthermore, servers will not return operational
   attributes, such as objectClasses or attributeTypes, unless they are
   listed by name.  Operational attributes are described in [RFC4512].

Attributes are returned at most once in an entry.  If an attribute
   description is named more than once in the list, the subsequent names
   are ignored.  If an attribute description in the list is not
   recognized, it is ignored by the server.

4.5.2.  Search Result

The results of the Search operation are returned as zero or more
   SearchResultEntry and/or SearchResultReference messages, followed by
   a single SearchResultDone message.

SearchResultEntry ::= [APPLICATION 4] SEQUENCE {
             objectName      LDAPDN,
             attributes      PartialAttributeList }

PartialAttributeList ::= SEQUENCE OF
                             partialAttribute PartialAttribute

SearchResultReference ::= [APPLICATION 19] SEQUENCE
                                  SIZE (1..MAX) OF uri URI

SearchResultDone ::= [APPLICATION 5] LDAPResult

Each SearchResultEntry represents an entry found during the Search.
   Each SearchResultReference represents an area not yet explored during
   the Search.  The SearchResultEntry and SearchResultReference messages
   may come in any order.  Following all the SearchResultReference and
   SearchResultEntry responses, the server returns a SearchResultDone
   response, which contains an indication of success or details any
   errors that have occurred.

Each entry returned in a SearchResultEntry will contain all
   appropriate attributes as specified in the attributes field of the
   Search Request, subject to access control and other administrative
   policy.  Note that the PartialAttributeList may hold zero elements.

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This may happen when none of the attributes of an entry were
   requested or could be returned.  Note also that the partialAttribute
   vals set may hold zero elements.  This may happen when typesOnly is
   requested, access controls prevent the return of values, or other
   reasons.

Some attributes may be constructed by the server and appear in a
   SearchResultEntry attribute list, although they are not stored
   attributes of an entry.  Clients SHOULD NOT assume that all
   attributes can be modified, even if this is permitted by access
   control.

If the server's schema defines short names [RFC4512] for an attribute
   type, then the server SHOULD use one of those names in attribute
   descriptions for that attribute type (in preference to using the
   <numericoid> [RFC4512] format of the attribute type's object
   identifier).  The server SHOULD NOT use the short name if that name
   is known by the server to be ambiguous, or if it is otherwise likely
   to cause interoperability problems.

4.5.3.  Continuation References in the Search Result

If the server was able to locate the entry referred to by the
   baseObject but was unable or unwilling to search one or more non-
   local entries, the server may return one or more
   SearchResultReference messages, each containing a reference to
   another set of servers for continuing the operation.  A server MUST
   NOT return any SearchResultReference messages if it has not located
   the baseObject and thus has not searched any entries.  In this case,
   it would return a SearchResultDone containing either a referral or
   noSuchObject result code (depending on the server's knowledge of the
   entry named in the baseObject).

If a server holds a copy or partial copy of the subordinate naming
   context (Section 5 of [RFC4512]), it may use the search filter to
   determine whether or not to return a SearchResultReference response.
   Otherwise, SearchResultReference responses are always returned when
   in scope.

The SearchResultReference is of the same data type as the Referral.

If the client wishes to progress the Search, it issues a new Search
   operation for each SearchResultReference that is returned.  If
   multiple URIs are present, the client assumes that any supported URI
   may be used to progress the operation.

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Clients that follow search continuation references MUST ensure that
   they do not loop between servers.  They MUST NOT repeatedly contact
   the same server for the same request with the same parameters.  Some
   clients use a counter that is incremented each time search result
   reference handling occurs for an operation, and these kinds of
   clients MUST be able to handle at least ten nested referrals while
   progressing the operation.

Note that the Abandon operation described in Section 4.11 applies
   only to a particular operation sent at the LDAP message layer between
   a client and server.  The client must individually abandon subsequent
   Search operations it wishes to.

A URI for a server implementing LDAP and accessible via TCP/IP (v4 or
   v6) [RFC793][RFC791] is written as an LDAP URL according to
   [RFC4516].

SearchResultReference values that are LDAP URLs follow these rules:

- The <dn> part of the LDAP URL MUST be present, with the new target
     object name.  The client uses this name when following the
     reference.

- Some servers (e.g., participating in distributed indexing) may
     provide a different filter in the LDAP URL.

- If the originating search scope was singleLevel, the <scope> part
     of the LDAP URL will be "base".

- It is RECOMMENDED that the <scope> part be present to avoid
     ambiguity.  In the absence of a <scope> part, the scope of the
     original Search request is assumed.

- Other aspects of the new Search request may be the same as or
     different from the Search request that generated the
     SearchResultReference.

- The name of an unexplored subtree in a SearchResultReference need
     not be subordinate to the base object.

Other kinds of URIs may be returned.  The syntax and semantics of
   such URIs is left to future specifications.  Clients may ignore URIs
   that they do not support.

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UTF-8-encoded characters appearing in the string representation of a
   DN, search filter, or other fields of the referral value may not be
   legal for URIs (e.g., spaces) and MUST be escaped using the % method
   in [RFC3986].

4.5.3.1.  Examples

For example, suppose the contacted server (hosta) holds the entry
   <DC=Example,DC=NET> and the entry <CN=Manager,DC=Example,DC=NET>.  It
   knows that both LDAP servers (hostb) and (hostc) hold
   <OU=People,DC=Example,DC=NET> (one is the master and the other server
   a shadow), and that LDAP-capable server (hostd) holds the subtree
   <OU=Roles,DC=Example,DC=NET>.  If a wholeSubtree Search of
   <DC=Example,DC=NET> is requested to the contacted server, it may
   return the following:

SearchResultEntry for DC=Example,DC=NET
     SearchResultEntry for CN=Manager,DC=Example,DC=NET
     SearchResultReference {
       ldap://hostb/OU=People,DC=Example,DC=NET??sub
       ldap://hostc/OU=People,DC=Example,DC=NET??sub }
     SearchResultReference {
       ldap://hostd/OU=Roles,DC=Example,DC=NET??sub }
     SearchResultDone (success)

Client implementors should note that when following a
   SearchResultReference, additional SearchResultReference may be
   generated.  Continuing the example, if the client contacted the
   server (hostb) and issued the Search request for the subtree
   <OU=People,DC=Example,DC=NET>, the server might respond as follows:

SearchResultEntry for OU=People,DC=Example,DC=NET
     SearchResultReference {
       ldap://hoste/OU=Managers,OU=People,DC=Example,DC=NET??sub }
     SearchResultReference {
       ldap://hostf/OU=Consultants,OU=People,DC=Example,DC=NET??sub }
     SearchResultDone (success)

Similarly, if a singleLevel Search of <DC=Example,DC=NET> is
   requested to the contacted server, it may return the following:

SearchResultEntry for CN=Manager,DC=Example,DC=NET
     SearchResultReference {
       ldap://hostb/OU=People,DC=Example,DC=NET??base
       ldap://hostc/OU=People,DC=Example,DC=NET??base }
     SearchResultReference {
       ldap://hostd/OU=Roles,DC=Example,DC=NET??base }
     SearchResultDone (success)

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If the contacted server does not hold the base object for the Search,
   but has knowledge of its possible location, then it may return a
   referral to the client.  In this case, if the client requests a
   subtree Search of <DC=Example,DC=ORG> to hosta, the server returns a
   SearchResultDone containing a referral.

SearchResultDone (referral) {
       ldap://hostg/DC=Example,DC=ORG??sub }

4.6.  Modify Operation

The Modify operation allows a client to request that a modification
   of an entry be performed on its behalf by a server.  The Modify
   Request is defined as follows:

ModifyRequest ::= [APPLICATION 6] SEQUENCE {
             object          LDAPDN,
             changes         SEQUENCE OF change SEQUENCE {
                  operation       ENUMERATED {
                       add     (0),
                       delete  (1),
                       replace (2),
                       ...  },
                  modification    PartialAttribute } }

Fields of the Modify Request are:

- object: The value of this field contains the name of the entry to
     be modified.  The server SHALL NOT perform any alias dereferencing
     in determining the object to be modified.

- changes: A list of modifications to be performed on the entry.  The
     entire list of modifications MUST be performed in the order they
     are listed as a single atomic operation.  While individual
     modifications may violate certain aspects of the directory schema
     (such as the object class definition and Directory Information Tree
     (DIT) content rule), the resulting entry after the entire list of
     modifications is performed MUST conform to the requirements of the
     directory model and controlling schema [RFC4512].

-  operation: Used to specify the type of modification being
        performed.  Each operation type acts on the following
        modification.  The values of this field have the following
        semantics, respectively:

add: add values listed to the modification attribute,
           creating the attribute if necessary.

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delete: delete values listed from the modification attribute.
           If no values are listed, or if all current values of the
           attribute are listed, the entire attribute is removed.

replace: replace all existing values of the modification
           attribute with the new values listed, creating the attribute
           if it did not already exist.  A replace with no value will
           delete the entire attribute if it exists, and it is ignored
           if the attribute does not exist.

-  modification: A PartialAttribute (which may have an empty SET
        of vals) used to hold the attribute type or attribute type and
        values being modified.

Upon receipt of a Modify Request, the server attempts to perform the
   necessary modifications to the DIT and returns the result in a Modify
   Response, defined as follows:

ModifyResponse ::= [APPLICATION 7] LDAPResult

The server will return to the client a single Modify Response
   indicating either the successful completion of the DIT modification,
   or the reason that the modification failed.  Due to the requirement
   for atomicity in applying the list of modifications in the Modify
   Request, the client may expect that no modifications of the DIT have
   been performed if the Modify Response received indicates any sort of
   error, and that all requested modifications have been performed if
   the Modify Response indicates successful completion of the Modify
   operation.  Whether or not the modification was applied cannot be
   determined by the client if the Modify Response was not received
   (e.g., the LDAP session was terminated or the Modify operation was
   abandoned).

Servers MUST ensure that entries conform to user and system schema
   rules or other data model constraints.  The Modify operation cannot
   be used to remove from an entry any of its distinguished values,
   i.e., those values which form the entry's relative distinguished
   name.  An attempt to do so will result in the server returning the
   notAllowedOnRDN result code.  The Modify DN operation described in
   Section 4.9 is used to rename an entry.

For attribute types that specify no equality matching, the rules in
   Section 2.5.1 of [RFC4512] are followed.

Note that due to the simplifications made in LDAP, there is not a
   direct mapping of the changes in an LDAP ModifyRequest onto the
   changes of a DAP ModifyEntry operation, and different implementations

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of LDAP-DAP gateways may use different means of representing the
   change.  If successful, the final effect of the operations on the
   entry MUST be identical.

4.7.  Add Operation

The Add operation allows a client to request the addition of an entry
   into the Directory.  The Add Request is defined as follows:

AddRequest ::= [APPLICATION 8] SEQUENCE {
             entry           LDAPDN,
             attributes      AttributeList }

AttributeList ::= SEQUENCE OF attribute Attribute

Fields of the Add Request are:

- entry: the name of the entry to be added.  The server SHALL NOT
     dereference any aliases in locating the entry to be added.

- attributes: the list of attributes that, along with those from the
     RDN, make up the content of the entry being added.  Clients MAY or
     MAY NOT include the RDN attribute(s) in this list.  Clients MUST
     NOT supply NO-USER-MODIFICATION attributes such as the
     createTimestamp or creatorsName attributes, since the server
     maintains these automatically.

Servers MUST ensure that entries conform to user and system schema
   rules or other data model constraints.  For attribute types that
   specify no equality matching, the rules in Section 2.5.1 of [RFC4512]
   are followed (this applies to the naming attribute in addition to any
   multi-valued attributes being added).

The entry named in the entry field of the AddRequest MUST NOT exist
   for the AddRequest to succeed.  The immediate superior (parent) of an
   object or alias entry to be added MUST exist.  For example, if the
   client attempted to add <CN=JS,DC=Example,DC=NET>, the
   <DC=Example,DC=NET> entry did not exist, and the <DC=NET> entry did
   exist, then the server would return the noSuchObject result code with
   the matchedDN field containing <DC=NET>.

Upon receipt of an Add Request, a server will attempt to add the
   requested entry.  The result of the Add attempt will be returned to
   the client in the Add Response, defined as follows:

AddResponse ::= [APPLICATION 9] LDAPResult

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A response of success indicates that the new entry has been added to
   the Directory.

4.8.  Delete Operation

The Delete operation allows a client to request the removal of an
   entry from the Directory.  The Delete Request is defined as follows:

DelRequest ::= [APPLICATION 10] LDAPDN

The Delete Request consists of the name of the entry to be deleted.
   The server SHALL NOT dereference aliases while resolving the name of
   the target entry to be removed.

Only leaf entries (those with no subordinate entries) can be deleted
   with this operation.

Upon receipt of a Delete Request, a server will attempt to perform
   the entry removal requested and return the result in the Delete
   Response defined as follows:

DelResponse ::= [APPLICATION 11] LDAPResult

4.9.  Modify DN Operation

The Modify DN operation allows a client to change the Relative
   Distinguished Name (RDN) of an entry in the Directory and/or to move
   a subtree of entries to a new location in the Directory.  The Modify
   DN Request is defined as follows:

ModifyDNRequest ::= [APPLICATION 12] SEQUENCE {
             entry           LDAPDN,
             newrdn          RelativeLDAPDN,
             deleteoldrdn    BOOLEAN,
             newSuperior     [0] LDAPDN OPTIONAL }

Fields of the Modify DN Request are:

- entry: the name of the entry to be changed.  This entry may or may
     not have subordinate entries.

- newrdn: the new RDN of the entry.  The value of the old RDN is
     supplied when moving the entry to a new superior without changing
     its RDN.  Attribute values of the new RDN not matching any
     attribute value of the entry are added to the entry, and an
     appropriate error is returned if this fails.

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- deleteoldrdn: a boolean field that controls whether the old RDN
     attribute values are to be retained as attributes of the entry or
     deleted from the entry.

- newSuperior: if present, this is the name of an existing object
     entry that becomes the immediate superior (parent) of the
     existing entry.

The server SHALL NOT dereference any aliases in locating the objects
   named in entry or newSuperior.

Upon receipt of a ModifyDNRequest, a server will attempt to perform
   the name change and return the result in the Modify DN Response,
   defined as follows:

ModifyDNResponse ::= [APPLICATION 13] LDAPResult

For example, if the entry named in the entry field was <cn=John
   Smith,c=US>, the newrdn field was <cn=John Cougar Smith>, and the
   newSuperior field was absent, then this operation would attempt to
   rename the entry as <cn=John Cougar Smith,c=US>.  If there was
   already an entry with that name, the operation would fail with the
   entryAlreadyExists result code.

The object named in newSuperior MUST exist.  For example, if the
   client attempted to add <CN=JS,DC=Example,DC=NET>, the
   <DC=Example,DC=NET> entry did not exist, and the <DC=NET> entry did
   exist, then the server would return the noSuchObject result code with
   the matchedDN field containing <DC=NET>.

If the deleteoldrdn field is TRUE, the attribute values forming the
   old RDN (but not the new RDN) are deleted from the entry.  If the
   deleteoldrdn field is FALSE, the attribute values forming the old RDN
   will be retained as non-distinguished attribute values of the entry.

Note that X.500 restricts the ModifyDN operation to affect only
   entries that are contained within a single server.  If the LDAP
   server is mapped onto DAP, then this restriction will apply, and the
   affectsMultipleDSAs result code will be returned if this error
   occurred.  In general, clients MUST NOT expect to be able to perform
   arbitrary movements of entries and subtrees between servers or
   between naming contexts.

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4.10.  Compare Operation

The Compare operation allows a client to compare an assertion value
   with the values of a particular attribute in a particular entry in
   the Directory.  The Compare Request is defined as follows:

CompareRequest ::= [APPLICATION 14] SEQUENCE {
             entry           LDAPDN,
             ava             AttributeValueAssertion }

Fields of the Compare Request are:

- entry: the name of the entry to be compared.  The server SHALL NOT
     dereference any aliases in locating the entry to be compared.

- ava: holds the attribute value assertion to be compared.

Upon receipt of a Compare Request, a server will attempt to perform
   the requested comparison and return the result in the Compare
   Response, defined as follows:

CompareResponse ::= [APPLICATION 15] LDAPResult

The resultCode is set to compareTrue, compareFalse, or an appropriate
   error.  compareTrue indicates that the assertion value in the ava
   field matches a value of the attribute or subtype according to the
   attribute's EQUALITY matching rule.  compareFalse indicates that the
   assertion value in the ava field and the values of the attribute or
   subtype did not match.  Other result codes indicate either that the
   result of the comparison was Undefined (Section 4.5.1.7), or that
   some error occurred.

Note that some directory systems may establish access controls that
   permit the values of certain attributes (such as userPassword) to be
   compared but not interrogated by other means.

4.11.  Abandon Operation

The function of the Abandon operation is to allow a client to request
   that the server abandon an uncompleted operation.  The Abandon
   Request is defined as follows:

AbandonRequest ::= [APPLICATION 16] MessageID

The MessageID is that of an operation that was requested earlier at
   this LDAP message layer.  The Abandon request itself has its own
   MessageID.  This is distinct from the MessageID of the earlier
   operation being abandoned.

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There is no response defined in the Abandon operation.  Upon receipt
   of an AbandonRequest, the server MAY abandon the operation identified
   by the MessageID.  Since the client cannot tell the difference
   between a successfully abandoned operation and an uncompleted
   operation, the application of the Abandon operation is limited to
   uses where the client does not require an indication of its outcome.

Abandon, Bind, Unbind, and StartTLS operations cannot be abandoned.

In the event that a server receives an Abandon Request on a Search
   operation in the midst of transmitting responses to the Search, that
   server MUST cease transmitting entry responses to the abandoned
   request immediately, and it MUST NOT send the SearchResultDone.  Of
   course, the server MUST ensure that only properly encoded LDAPMessage
   PDUs are transmitted.

The ability to abandon other (particularly update) operations is at
   the discretion of the server.

Clients should not send Abandon requests for the same operation
   multiple times, and they MUST also be prepared to receive results
   from operations they have abandoned (since these might have been in
   transit when the Abandon was requested or might not be able to be
   abandoned).

Servers MUST discard Abandon requests for messageIDs they do not
   recognize, for operations that cannot be abandoned, and for
   operations that have already been abandoned.

4.12.  Extended Operation

The Extended operation allows additional operations to be defined for
   services not already available in the protocol; for example, to Add
   operations to install transport layer security (see Section 4.14).

The Extended operation allows clients to make requests and receive
   responses with predefined syntaxes and semantics.  These may be
   defined in RFCs or be private to particular implementations.

Each Extended operation consists of an Extended request and an
   Extended response.

ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
             requestName      [0] LDAPOID,
             requestValue     [1] OCTET STRING OPTIONAL }

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The requestName is a dotted-decimal representation of the unique
   OBJECT IDENTIFIER corresponding to the request.  The requestValue is
   information in a form defined by that request, encapsulated inside an
   OCTET STRING.

The server will respond to this with an LDAPMessage containing an
   ExtendedResponse.

ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
             COMPONENTS OF LDAPResult,
             responseName     [10] LDAPOID OPTIONAL,
             responseValue    [11] OCTET STRING OPTIONAL }

The responseName field, when present, contains an LDAPOID that is
   unique for this extended operation or response.  This field is
   optional (even when the extension specification defines an LDAPOID
   for use in this field).  The field will be absent whenever the server
   is unable or unwilling to determine the appropriate LDAPOID to
   return, for instance, when the requestName cannot be parsed or its
   value is not recognized.

Where the requestName is not recognized, the server returns
   protocolError.  (The server may return protocolError in other cases.)

The requestValue and responseValue fields contain information
   associated with the operation.  The format of these fields is defined
   by the specification of the Extended operation.  Implementations MUST
   be prepared to handle arbitrary contents of these fields, including
   zero bytes.  Values that are defined in terms of ASN.1 and BER-
   encoded according to Section 5.1 also follow the extensibility rules
   in Section 4.

Servers list the requestName of Extended Requests they recognize in
   the 'supportedExtension' attribute in the root DSE (Section 5.1 of
   [RFC4512]).

Extended operations may be specified in other documents.  The
   specification of an Extended operation consists of:

- the OBJECT IDENTIFIER assigned to the requestName,

- the OBJECT IDENTIFIER (if any) assigned to the responseName (note
     that the same OBJECT IDENTIFIER may be used for both the
     requestName and responseName),

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- the format of the contents of the requestValue and responseValue
     (if any), and

- the semantics of the operation.

4.13.  IntermediateResponse Message

While the Search operation provides a mechanism to return multiple
   response messages for a single Search request, other operations, by
   nature, do not provide for multiple response messages.

The IntermediateResponse message provides a general mechanism for
   defining single-request/multiple-response operations in LDAP.  This
   message is intended to be used in conjunction with the Extended
   operation to define new single-request/multiple-response operations
   or in conjunction with a control when extending existing LDAP
   operations in a way that requires them to return Intermediate
   response information.

It is intended that the definitions and descriptions of Extended
   operations and controls that make use of the IntermediateResponse
   message will define the circumstances when an IntermediateResponse
   message can be sent by a server and the associated meaning of an
   IntermediateResponse message sent in a particular circumstance.

IntermediateResponse ::= [APPLICATION 25] SEQUENCE {
                responseName     [0] LDAPOID OPTIONAL,
                responseValue    [1] OCTET STRING OPTIONAL }

IntermediateResponse messages SHALL NOT be returned to the client
   unless the client issues a request that specifically solicits their
   return.  This document defines two forms of solicitation: Extended
   operation and request control.  IntermediateResponse messages are
   specified in documents describing the manner in which they are
   solicited (i.e., in the Extended operation or request control
   specification that uses them).  These specifications include:

- the OBJECT IDENTIFIER (if any) assigned to the responseName,

- the format of the contents of the responseValue (if any), and

- the semantics associated with the IntermediateResponse message.

Extensions that allow the return of multiple types of
   IntermediateResponse messages SHALL identify those types using unique
   responseName values (note that one of these may specify no value).

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Sections 4.13.1 and 4.13.2 describe additional requirements on the
   inclusion of responseName and responseValue in IntermediateResponse
   messages.

4.13.1.  Usage with LDAP ExtendedRequest and ExtendedResponse

A single-request/multiple-response operation may be defined using a
   single ExtendedRequest message to solicit zero or more
   IntermediateResponse messages of one or more kinds, followed by an
   ExtendedResponse message.

4.13.2.  Usage with LDAP Request Controls

A control's semantics may include the return of zero or more
   IntermediateResponse messages prior to returning the final result
   code for the operation.  One or more kinds of IntermediateResponse
   messages may be sent in response to a request control.

All IntermediateResponse messages associated with request controls
   SHALL include a responseName.  This requirement ensures that the
   client can correctly identify the source of IntermediateResponse
   messages when:

- two or more controls using IntermediateResponse messages are
     included in a request for any LDAP operation or

- one or more controls using IntermediateResponse messages are
     included in a request with an LDAP Extended operation that uses
     IntermediateResponse messages.

4.14.  StartTLS Operation

The Start Transport Layer Security (StartTLS) operation's purpose is
   to initiate installation of a TLS layer.  The StartTLS operation is
   defined using the Extended operation mechanism described in Section
   4.12.

4.14.1.  StartTLS Request

A client requests TLS establishment by transmitting a StartTLS
   request message to the server.  The StartTLS request is defined in
   terms of an ExtendedRequest.  The requestName is
   "1.3.6.1.4.1.1466.20037", and the requestValue field is always
   absent.

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The client MUST NOT send any LDAP PDUs at this LDAP message layer
   following this request until it receives a StartTLS Extended response
   and, in the case of a successful response, completes TLS
   negotiations.

Detected sequencing problems (particularly those detailed in Section
   3.1.1 of [RFC4513]) result in the resultCode being set to
   operationsError.

If the server does not support TLS (whether by design or by current
   configuration), it returns with the resultCode set to protocolError
   as described in Section 4.12.

4.14.2.  StartTLS Response

When a StartTLS request is received, servers supporting the operation
   MUST return a StartTLS response message to the requestor.  The
   responseName is "1.3.6.1.4.1.1466.20037" when provided (see Section
   4.12).  The responseValue is always absent.

If the server is willing and able to negotiate TLS, it returns the
   StartTLS response with the resultCode set to success.  Upon client
   receipt of a successful StartTLS response, protocol peers may
   commence with TLS negotiation as discussed in Section 3 of [RFC4513].

If the server is otherwise unwilling or unable to perform this
   operation, the server is to return an appropriate result code
   indicating the nature of the problem.  For example, if the TLS
   subsystem is not presently available, the server may indicate this by
   returning with the resultCode set to unavailable.  In cases where a
   non-success result code is returned, the LDAP session is left without
   a TLS layer.

4.14.3.  Removal of the TLS Layer

Either the client or server MAY remove the TLS layer and leave the
   LDAP message layer intact by sending and receiving a TLS closure
   alert.

The initiating protocol peer sends the TLS closure alert and MUST
   wait until it receives a TLS closure alert from the other peer before
   sending further LDAP PDUs.

When a protocol peer receives the initial TLS closure alert, it may
   choose to allow the LDAP message layer to remain intact.  In this
   case, it MUST immediately transmit a TLS closure alert.  Following
   this, it MAY send and receive LDAP PDUs.

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Protocol peers MAY terminate the LDAP session after sending or
   receiving a TLS closure alert.

5.  Protocol Encoding, Connection, and Transfer

This protocol is designed to run over connection-oriented, reliable
   transports, where the data stream is divided into octets (8-bit
   units), with each octet and each bit being significant.

One underlying service, LDAP over TCP, is defined in Section 5.2.
   This service is generally applicable to applications providing or
   consuming X.500-based directory services on the Internet.  This
   specification was generally written with the TCP mapping in mind.
   Specifications detailing other mappings may encounter various
   obstacles.

Implementations of LDAP over TCP MUST implement the mapping as
   described in Section 5.2.

This table illustrates the relationship among the different layers
   involved in an exchange between two protocol peers:

+----------------------+
               |  LDAP message layer  |
               +----------------------+ > LDAP PDUs
               +----------------------+ < data
               |      SASL layer      |
               +----------------------+ > SASL-protected data
               +----------------------+ < data
               |       TLS layer      |
   Application +----------------------+ > TLS-protected data
   ------------+----------------------+ < data
     Transport | transport connection |
               +----------------------+

5.1.  Protocol Encoding

The protocol elements of LDAP SHALL be encoded for exchange using the
   Basic Encoding Rules [BER] of [ASN.1] with the following
   restrictions:

- Only the definite form of length encoding is used.

- OCTET STRING values are encoded in the primitive form only.

- If the value of a BOOLEAN type is true, the encoding of the value
     octet is set to hex "FF".

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- If a value of a type is its default value, it is absent.  Only some
     BOOLEAN and INTEGER types have default values in this protocol
     definition.

These restrictions are meant to ease the overhead of encoding and
   decoding certain elements in BER.

These restrictions do not apply to ASN.1 types encapsulated inside of
   OCTET STRING values, such as attribute values, unless otherwise
   stated.

5.2.  Transmission Control Protocol (TCP)

The encoded LDAPMessage PDUs are mapped directly onto the TCP
   [RFC793] bytestream using the BER-based encoding described in Section
   5.1.  It is recommended that server implementations running over the
   TCP provide a protocol listener on the Internet Assigned Numbers
   Authority (IANA)-assigned LDAP port, 389 [PortReg].  Servers may
   instead provide a listener on a different port number.  Clients MUST
   support contacting servers on any valid TCP port.

5.3.  Termination of the LDAP session

Termination of the LDAP session is typically initiated by the client
   sending an UnbindRequest (Section 4.3), or by the server sending a
   Notice of Disconnection (Section 4.4.1).  In these cases, each
   protocol peer gracefully terminates the LDAP session by ceasing
   exchanges at the LDAP message layer, tearing down any SASL layer,
   tearing down any TLS layer, and closing the transport connection.

A protocol peer may determine that the continuation of any
   communication would be pernicious, and in this case, it may abruptly
   terminate the session by ceasing communication and closing the
   transport connection.

In either case, when the LDAP session is terminated, uncompleted
   operations are handled as specified in Section 3.1.

6.  Security Considerations

This version of the protocol provides facilities for simple
   authentication using a cleartext password, as well as any SASL
   [RFC4422] mechanism.  Installing SASL and/or TLS layers can provide
   integrity and other data security services.

It is also permitted that the server can return its credentials to
   the client, if it chooses to do so.

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Use of cleartext password is strongly discouraged where the
   underlying transport service cannot guarantee confidentiality and may
   result in disclosure of the password to unauthorized parties.

Servers are encouraged to prevent directory modifications by clients
   that have authenticated anonymously [RFC4513].

Security considerations for authentication methods, SASL mechanisms,
   and TLS are described in [RFC4513].

Note that SASL authentication exchanges do not provide data
   confidentiality or integrity protection for the version or name
   fields of the BindRequest or the resultCode, diagnosticMessage, or
   referral fields of the BindResponse, nor for any information
   contained in controls attached to Bind requests or responses.  Thus,
   information contained in these fields SHOULD NOT be relied on unless
   it is otherwise protected (such as by establishing protections at the
   transport layer).

Implementors should note that various security factors (including
   authentication and authorization information and data security
   services) may change during the course of the LDAP session or even
   during the performance of a particular operation.  For instance,
   credentials could expire, authorization identities or access controls
   could change, or the underlying security layer(s) could be replaced
   or terminated.  Implementations should be robust in the handling of
   changing security factors.

In some cases, it may be appropriate to continue the operation even
   in light of security factor changes.  For instance, it may be
   appropriate to continue an Abandon operation regardless of the
   change, or to continue an operation when the change upgraded (or
   maintained) the security factor.  In other cases, it may be
   appropriate to fail or alter the processing of the operation.  For
   instance, if confidential protections were removed, it would be
   appropriate either to fail a request to return sensitive data or,
   minimally, to exclude the return of sensitive data.

Implementations that cache attributes and entries obtained via LDAP
   MUST ensure that access controls are maintained if that information
   is to be provided to multiple clients, since servers may have access
   control policies that prevent the return of entries or attributes in
   Search results except to particular authenticated clients.  For
   example, caches could serve result information only to the client
   whose request caused it to be in the cache.

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Servers may return referrals or Search result references that
   redirect clients to peer servers.  It is possible for a rogue
   application to inject such referrals into the data stream in an
   attempt to redirect a client to a rogue server.  Clients are advised
   to be aware of this and possibly reject referrals when
   confidentiality measures are not in place.  Clients are advised to
   reject referrals from the StartTLS operation.

The matchedDN and diagnosticMessage fields, as well as some
   resultCode values (e.g., attributeOrValueExists and
   entryAlreadyExists), could disclose the presence or absence of
   specific data in the directory that is subject to access and other
   administrative controls.  Server implementations should restrict
   access to protected information equally under both normal and error
   conditions.

Protocol peers MUST be prepared to handle invalid and arbitrary-
   length protocol encodings.  Invalid protocol encodings include: BER
   encoding exceptions, format string and UTF-8 encoding exceptions,
   overflow exceptions, integer value exceptions, and binary mode on/off
   flag exceptions.  The LDAPv3 PROTOS [PROTOS-LDAP] test suite provides
   excellent examples of these exceptions and test cases used to
   discover flaws.

In the event that a protocol peer senses an attack that in its nature
   could cause damage due to further communication at any layer in the
   LDAP session, the protocol peer should abruptly terminate the LDAP
   session as described in Section 5.3.

7.  Acknowledgements

This document is based on RFC 2251 by Mark Wahl, Tim Howes, and Steve
   Kille.  RFC 2251 was a product of the IETF ASID Working Group.

It is also based on RFC 2830 by Jeff Hodges, RL "Bob" Morgan, and
   Mark Wahl.  RFC 2830 was a product of the IETF LDAPEXT Working Group.

It is also based on RFC 3771 by Roger Harrison and Kurt Zeilenga.
   RFC 3771 was an individual submission to the IETF.

This document is a product of the IETF LDAPBIS Working Group.
   Significant contributors of technical review and content include Kurt
   Zeilenga, Steven Legg, and Hallvard Furuseth.

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8.  Normative References

[ASN.1]       ITU-T Recommendation X.680 (07/2002) | ISO/IEC 8824-
                 1:2002 "Information Technology - Abstract Syntax
                 Notation One (ASN.1): Specification of basic notation".

[BER]         ITU-T Rec. X.690 (07/2002) | ISO/IEC 8825-1:2002,
                 "Information technology - ASN.1 encoding rules:
                 Specification of Basic Encoding Rules (BER), Canonical
                 Encoding Rules (CER) and Distinguished Encoding Rules
                 (DER)", 2002.

[ISO10646]    Universal Multiple-Octet Coded Character Set (UCS) -
                 Architecture and Basic Multilingual Plane, ISO/IEC
                 10646-1 : 1993.

[RFC791]      Postel, J., "Internet Protocol", STD 5, RFC 791,
                 September 1981.

[RFC793]      Postel, J., "Transmission Control Protocol", STD 7, RFC
                 793, September 1981.

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

[RFC3454]     Hoffman P. and M. Blanchet, "Preparation of
                 Internationalized Strings ('stringprep')", RFC 3454,
                 December 2002.

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

[RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
                 "Uniform Resource Identifier (URI): Generic Syntax",
                 STD 66, RFC 3986, January 2005.

[RFC4013]     Zeilenga, K., "SASLprep: Stringprep Profile for User
                 Names and Passwords", RFC 4013, February 2005.

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

[RFC4346]     Dierks, T. and E. Rescorla, "The TLS Protocol Version
                 1.1", RFC 4346, March 2006.

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

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

[RFC4512]     Zeilenga, K., Lightweight Directory Access Protocol
                 (LDAP): Directory Information Models", RFC 4512, 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.

[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.

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

[X.511]       ITU-T Rec. X.511, "The Directory: Abstract Service
                 Definition", 1993.

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9.  Informative References

[CharModel]   Whistler, K. and M. Davis, "Unicode Technical Report
                 #17, Character Encoding Model", UTR17,
                 <http://www.unicode.org/unicode/reports/tr17/>, August
                 2000.

[Glossary]    The Unicode Consortium, "Unicode Glossary",
                 <http://www.unicode.org/glossary/>.

[PortReg]     IANA, "Port Numbers",
                 <http://www.iana.org/assignments/port-numbers>.

[PROTOS-LDAP] University of Oulu, "PROTOS Test-Suite: c06-ldapv3"
                 <http://www.ee.oulu.fi/research/ouspg/protos/testing/
                 c06/ldapv3/>.

10.  IANA Considerations

The Internet Assigned Numbers Authority (IANA) has updated the LDAP
   result code registry to indicate that this document provides the
   definitive technical specification for result codes 0-36, 48-54, 64-
   70, 80-90.  It is also noted that one resultCode value
   (strongAuthRequired) has been renamed (to strongerAuthRequired).

The IANA has also updated the LDAP Protocol Mechanism registry to
   indicate that this document and [RFC4513] provides the definitive
   technical specification for the StartTLS (1.3.6.1.4.1.1466.20037)
   Extended operation.

IANA has assigned LDAP Object Identifier 18 [RFC4520] to identify the
   ASN.1 module defined in this document.

Subject: Request for LDAP Object Identifier Registration
        Person & email address to contact for further information:
             Jim Sermersheim <jimse@novell.com>
        Specification: RFC 4511
        Author/Change Controller: IESG
        Comments:
             Identifies the LDAP ASN.1 module

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Appendix A.  LDAP Result Codes

This normative appendix details additional considerations regarding
   LDAP result codes and provides a brief, general description of each
   LDAP result code enumerated in Section 4.1.9.

Additional result codes MAY be defined for use with extensions
   [RFC4520].  Client implementations SHALL treat any result code that
   they do not recognize as an unknown error condition.

The descriptions provided here do not fully account for result code
   substitutions used to prevent unauthorized disclosures (such as
   substitution of noSuchObject for insufficientAccessRights, or
   invalidCredentials for insufficientAccessRights).

A.1.  Non-Error Result Codes

These result codes (called "non-error" result codes) do not indicate
   an error condition:

success (0),
        compareFalse (5),
        compareTrue (6),
        referral (10), and
        saslBindInProgress (14).

The success, compareTrue, and compareFalse result codes indicate
   successful completion (and, hence, are referred to as "successful"
   result codes).

The referral and saslBindInProgress result codes indicate the client
   needs to take additional action to complete the operation.

A.2.  Result Codes

Existing LDAP result codes are described as follows:

success (0)
         Indicates the successful completion of an operation.  Note:
         this code is not used with the Compare operation.  See
         compareFalse (5) and compareTrue (6).

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operationsError (1)
         Indicates that the operation is not properly sequenced with
         relation to other operations (of same or different type).

For example, this code is returned if the client attempts to
         StartTLS [RFC4346] while there are other uncompleted operations
         or if a TLS layer was already installed.

protocolError (2)
         Indicates the server received data that is not well-formed.

For Bind operation only, this code is also used to indicate
         that the server does not support the requested protocol
         version.

For Extended operations only, this code is also used to
         indicate that the server does not support (by design or
         configuration) the Extended operation associated with the
         requestName.

For request operations specifying multiple controls, this may
         be used to indicate that the server cannot ignore the order
         of the controls as specified, or that the combination of the
         specified controls is invalid or unspecified.

timeLimitExceeded (3)
         Indicates that the time limit specified by the client was
         exceeded before the operation could be completed.

sizeLimitExceeded (4)
         Indicates that the size limit specified by the client was
         exceeded before the operation could be completed.

compareFalse (5)
         Indicates that the Compare operation has successfully
         completed and the assertion has evaluated to FALSE or
         Undefined.

compareTrue (6)
         Indicates that the Compare operation has successfully
         completed and the assertion has evaluated to TRUE.

authMethodNotSupported (7)
         Indicates that the authentication method or mechanism is not
         supported.

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strongerAuthRequired (8)
         Indicates the server requires strong(er) authentication in
         order to complete the operation.

When used with the Notice of Disconnection operation, this
         code indicates that the server has detected that an
         established security association between the client and
         server has unexpectedly failed or been compromised.

referral (10)
         Indicates that a referral needs to be chased to complete the
         operation (see Section 4.1.10).

adminLimitExceeded (11)
         Indicates that an administrative limit has been exceeded.

unavailableCriticalExtension (12)
         Indicates a critical control is unrecognized (see Section
         4.1.11).

confidentialityRequired (13)
         Indicates that data confidentiality protections are required.

saslBindInProgress (14)
         Indicates the server requires the client to send a new bind
         request, with the same SASL mechanism, to continue the
         authentication process (see Section 4.2).

noSuchAttribute (16)
         Indicates that the named entry does not contain the specified
         attribute or attribute value.

undefinedAttributeType (17)
         Indicates that a request field contains an unrecognized
         attribute description.

inappropriateMatching (18)
         Indicates that an attempt was made (e.g., in an assertion) to
         use a matching rule not defined for the attribute type
         concerned.

constraintViolation (19)
         Indicates that the client supplied an attribute value that
         does not conform to the constraints placed upon it by the
         data model.

For example, this code is returned when multiple values are
         supplied to an attribute that has a SINGLE-VALUE constraint.

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attributeOrValueExists (20)
         Indicates that the client supplied an attribute or value to
         be added to an entry, but the attribute or value already
         exists.

invalidAttributeSyntax (21)
         Indicates that a purported attribute value does not conform
         to the syntax of the attribute.

noSuchObject (32)
         Indicates that the object does not exist in the DIT.

aliasProblem (33)
         Indicates that an alias problem has occurred.  For example,
         the code may used to indicate an alias has been dereferenced
         that names no object.

invalidDNSyntax (34)
         Indicates that an LDAPDN or RelativeLDAPDN field (e.g., search
         base, target entry, ModifyDN newrdn, etc.) of a request does
         not conform to the required syntax or contains attribute
         values that do not conform to the syntax of the attribute's
         type.

aliasDereferencingProblem (36)
         Indicates that a problem occurred while dereferencing an
         alias.  Typically, an alias was encountered in a situation
         where it was not allowed or where access was denied.

inappropriateAuthentication (48)
         Indicates the server requires the client that had attempted
         to bind anonymously or without supplying credentials to
         provide some form of credentials.

invalidCredentials (49)
         Indicates that the provided credentials (e.g., the user's name
         and password) are invalid.

insufficientAccessRights (50)
         Indicates that the client does not have sufficient access
         rights to perform the operation.

busy (51)
         Indicates that the server is too busy to service the
         operation.

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unavailable (52)
         Indicates that the server is shutting down or a subsystem
         necessary to complete the operation is offline.

unwillingToPerform (53)
         Indicates that the server is unwilling to perform the
         operation.

loopDetect (54)
         Indicates that the server has detected an internal loop (e.g.,
         while dereferencing aliases or chaining an operation).

namingViolation (64)
         Indicates that the entry's name violates naming restrictions.

objectClassViolation (65)
         Indicates that the entry violates object class restrictions.

notAllowedOnNonLeaf (66)
         Indicates that the operation is inappropriately acting upon a
         non-leaf entry.

notAllowedOnRDN (67)
         Indicates that the operation is inappropriately attempting to
         remove a value that forms the entry's relative distinguished
         name.

entryAlreadyExists (68)
         Indicates that the request cannot be fulfilled (added, moved,
         or renamed) as the target entry already exists.

objectClassModsProhibited (69)
         Indicates that an attempt to modify the object class(es) of
         an entry's 'objectClass' attribute is prohibited.

For example, this code is returned when a client attempts to
         modify the structural object class of an entry.

affectsMultipleDSAs (71)
         Indicates that the operation cannot be performed as it would
         affect multiple servers (DSAs).

other (80)
         Indicates the server has encountered an internal error.

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Appendix B.  Complete ASN.1 Definition

This appendix is normative.

Lightweight-Directory-Access-Protocol-V3 {1 3 6 1 1 18}
        -- Copyright (C) The Internet Society (2006).  This version of
        -- this ASN.1 module is part of RFC 4511; see the RFC itself
        -- for full legal notices.
        DEFINITIONS
        IMPLICIT TAGS
        EXTENSIBILITY IMPLIED ::=

BEGIN

MessageID ::= INTEGER (0 ..  maxInt)

maxInt INTEGER ::= 2147483647 -- (2^^31 - 1) --

LDAPString ::= OCTET STRING -- UTF-8 encoded,
                                    -- [ISO10646] characters

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LDAPOID ::= OCTET STRING -- Constrained to <numericoid>
                                 -- [RFC4512]

LDAPDN ::= LDAPString -- Constrained to <distinguishedName>
                              -- [RFC4514]

RelativeLDAPDN ::= LDAPString -- Constrained to <name-component>
                                      -- [RFC4514]

AttributeDescription ::= LDAPString
                                -- Constrained to <attributedescription>
                                -- [RFC4512]

AttributeValue ::= OCTET STRING

AttributeValueAssertion ::= SEQUENCE {
             attributeDesc   AttributeDescription,
             assertionValue  AssertionValue }

AssertionValue ::= OCTET STRING

PartialAttribute ::= SEQUENCE {
             type       AttributeDescription,
             vals       SET OF value AttributeValue }

Attribute ::= PartialAttribute(WITH COMPONENTS {
             ...,
             vals (SIZE(1..MAX))})

MatchingRuleId ::= LDAPString

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noSuchAttribute              (16),
                  undefinedAttributeType       (17),
                  inappropriateMatching        (18),
                  constraintViolation          (19),
                  attributeOrValueExists       (20),
                  invalidAttributeSyntax       (21),
                       -- 22-31 unused --
                  noSuchObject                 (32),
                  aliasProblem                 (33),
                  invalidDNSyntax              (34),
                       -- 35 reserved for undefined isLeaf --
                  aliasDereferencingProblem    (36),
                       -- 37-47 unused --
                  inappropriateAuthentication  (48),
                  invalidCredentials           (49),
                  insufficientAccessRights     (50),
                  busy                         (51),
                  unavailable                  (52),
                  unwillingToPerform           (53),
                  loopDetect                   (54),
                       -- 55-63 unused --
                  namingViolation              (64),
                  objectClassViolation         (65),
                  notAllowedOnNonLeaf          (66),
                  notAllowedOnRDN              (67),
                  entryAlreadyExists           (68),
                  objectClassModsProhibited    (69),
                       -- 70 reserved for CLDAP --
                  affectsMultipleDSAs          (71),
                       -- 72-79 unused --
                  other                        (80),
                  ...  },
             matchedDN          LDAPDN,
             diagnosticMessage  LDAPString,
             referral           [3] Referral OPTIONAL }

Referral ::= SEQUENCE SIZE (1..MAX) OF uri URI

URI ::= LDAPString     -- limited to characters permitted in
                               -- URIs

Controls ::= SEQUENCE OF control Control

Control ::= SEQUENCE {
             controlType             LDAPOID,
             criticality             BOOLEAN DEFAULT FALSE,
             controlValue            OCTET STRING OPTIONAL }

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SaslCredentials ::= SEQUENCE {
             mechanism               LDAPString,
             credentials             OCTET STRING OPTIONAL }

BindResponse ::= [APPLICATION 1] SEQUENCE {
             COMPONENTS OF LDAPResult,
             serverSaslCreds    [7] OCTET STRING OPTIONAL }

UnbindRequest ::= [APPLICATION 2] NULL

AttributeSelection ::= SEQUENCE OF selector LDAPString
                       -- The LDAPString is constrained to
                       -- <attributeSelector> in Section 4.5.1.8

Filter ::= CHOICE {
             and             [0] SET SIZE (1..MAX) OF filter Filter,
             or              [1] SET SIZE (1..MAX) OF filter Filter,
             not             [2] Filter,
             equalityMatch   [3] AttributeValueAssertion,

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substrings      [4] SubstringFilter,
             greaterOrEqual  [5] AttributeValueAssertion,
             lessOrEqual     [6] AttributeValueAssertion,
             present         [7] AttributeDescription,
             approxMatch     [8] AttributeValueAssertion,
             extensibleMatch [9] MatchingRuleAssertion,
             ...  }

SearchResultEntry ::= [APPLICATION 4] SEQUENCE {
             objectName      LDAPDN,
             attributes      PartialAttributeList }

PartialAttributeList ::= SEQUENCE OF
                             partialAttribute PartialAttribute

SearchResultReference ::= [APPLICATION 19] SEQUENCE
                                  SIZE (1..MAX) OF uri URI

SearchResultDone ::= [APPLICATION 5] LDAPResult

ModifyResponse ::= [APPLICATION 7] LDAPResult

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AddRequest ::= [APPLICATION 8] SEQUENCE {
             entry           LDAPDN,
             attributes      AttributeList }

AttributeList ::= SEQUENCE OF attribute Attribute

AddResponse ::= [APPLICATION 9] LDAPResult

DelRequest ::= [APPLICATION 10] LDAPDN

DelResponse ::= [APPLICATION 11] LDAPResult

ModifyDNResponse ::= [APPLICATION 13] LDAPResult

CompareRequest ::= [APPLICATION 14] SEQUENCE {
             entry           LDAPDN,
             ava             AttributeValueAssertion }

CompareResponse ::= [APPLICATION 15] LDAPResult

AbandonRequest ::= [APPLICATION 16] MessageID

ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
             requestName      [0] LDAPOID,
             requestValue     [1] OCTET STRING OPTIONAL }

ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
             COMPONENTS OF LDAPResult,
             responseName     [10] LDAPOID OPTIONAL,
             responseValue    [11] OCTET STRING OPTIONAL }

IntermediateResponse ::= [APPLICATION 25] SEQUENCE {
             responseName     [0] LDAPOID OPTIONAL,
             responseValue    [1] OCTET STRING OPTIONAL }

END

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Appendix C.  Changes

This appendix is non-normative.

This appendix summarizes substantive changes made to RFC 2251, RFC
   2830, and RFC 3771.

C.1.  Changes Made to RFC 2251

This section summarizes the substantive changes made to Sections 1,
   2, 3.1, and 4, and the remainder of RFC 2251.  Readers should
   consult [RFC4512] and [RFC4513] for summaries of changes to other
   sections.

C.1.1.  Section 1 (Status of this Memo)

- Removed IESG note.  Post publication of RFC 2251, mandatory LDAP
     authentication mechanisms have been standardized which are
     sufficient to remove this note.  See [RFC4513] for authentication
     mechanisms.

C.1.2.  Section 3.1 (Protocol Model) and others

- Removed notes giving history between LDAP v1, v2, and v3.  Instead,
     added sufficient language so that this document can stand on its
     own.

C.1.3.  Section 4 (Elements of Protocol)

- Clarified where the extensibility features of ASN.1 apply to the
     protocol.  This change affected various ASN.1 types by the
     inclusion of ellipses (...) to certain elements.
   - Removed the requirement that servers that implement version 3 or
     later MUST provide the 'supportedLDAPVersion' attribute.  This
     statement provided no interoperability advantages.

C.1.4.  Section 4.1.1 (Message Envelope)

- There was a mandatory requirement for the server to return a
     Notice of Disconnection and drop the transport connection when a
     PDU is malformed in a certain way.  This has been updated such that
     the server SHOULD return the Notice of Disconnection, and it MUST
     terminate the LDAP Session.

C.1.5.  Section 4.1.1.1 (Message ID)

- Required that the messageID of requests MUST be non-zero as the
     zero is reserved for Notice of Disconnection.

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- Specified when it is and isn't appropriate to return an already
     used messageID.  RFC 2251 accidentally imposed synchronous server
     behavior in its wording of this.

C.1.6.  Section 4.1.2 (String Types)

- Stated that LDAPOID is constrained to <numericoid> from [RFC4512].

C.1.7.  Section 4.1.5.1 (Binary Option) and others

- Removed the Binary Option from the specification.  There are
     numerous interoperability problems associated with this method of
     alternate attribute type encoding.  Work to specify a suitable
     replacement is ongoing.

C.1.8.  Section 4.1.8 (Attribute)

- Combined the definitions of PartialAttribute and Attribute here,
     and defined Attribute in terms of PartialAttribute.

C.1.9.  Section 4.1.10 (Result Message)

- Renamed "errorMessage" to "diagnosticMessage" as it is allowed to
     be sent for non-error results.
   - Moved some language into Appendix A, and referred the reader there.
   - Allowed matchedDN to be present for other result codes than those
     listed in RFC 2251.
   - Renamed the code "strongAuthRequired" to "strongerAuthRequired" to
     clarify that this code may often be returned to indicate that a
     stronger authentication is needed to perform a given operation.

C.1.10.  Section 4.1.11 (Referral)

- Defined referrals in terms of URIs rather than URLs.
   - Removed the requirement that all referral URIs MUST be equally
     capable of progressing the operation.  The statement was ambiguous
     and provided no instructions on how to carry it out.
   - Added the requirement that clients MUST NOT loop between servers.
   - Clarified the instructions for using LDAPURLs in referrals, and in
     doing so added a recommendation that the scope part be present.
   - Removed imperatives which required clients to use URLs in specific
     ways to progress an operation.  These did nothing for
     interoperability.

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C.1.11.  Section 4.1.12 (Controls)

- Specified how control values defined in terms of ASN.1 are to be
     encoded.
   - Noted that the criticality field is only applied to request
     messages (except UnbindRequest), and must be ignored when present
     on response messages and UnbindRequest.
   - Specified that non-critical controls may be ignored at the
     server's discretion.  There was confusion in the original wording
     which led some to believe that recognized controls may not be
     ignored as long as they were associated with a proper request.
   - Added language regarding combinations of controls and the ordering
     of controls on a message.
   - Specified that when the semantics of the combination of controls
     is undefined or unknown, it results in a protocolError.
   - Changed "The server MUST be prepared" to "Implementations MUST be
     prepared" in paragraph 8 to reflect that both client and server
     implementations must be able to handle this (as both parse
     controls).

C.1.12.  Section 4.2 (Bind Operation)

- Mandated that servers return protocolError when the version is not
     supported.
   - Disambiguated behavior when the simple authentication is used, the
     name is empty, and the password is non-empty.
   - Required servers to not dereference aliases for Bind.  This was
     added for consistency with other operations and to help ensure
     data consistency.
   - Required that textual passwords be transferred as UTF-8 encoded
     Unicode, and added recommendations on string preparation.  This was
     to help ensure interoperability of passwords being sent from
     different clients.

C.1.13.  Section 4.2.1 (Sequencing of the Bind Request)

- This section was largely reorganized for readability, and language
     was added to clarify the authentication state of failed and
     abandoned Bind operations.
   - Removed: "If a SASL transfer encryption or integrity mechanism has
     been negotiated, that mechanism does not support the changing of
     credentials from one identity to another, then the client MUST
     instead establish a new connection."
     If there are dependencies between multiple negotiations of a
     particular SASL mechanism, the technical specification for that
     SASL mechanism details how applications are to deal with them.
     LDAP should not require any special handling.
   - Dropped MUST imperative in paragraph 3 to align with [RFC2119].

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- Mandated that clients not send non-Bind operations while a Bind is
     in progress, and suggested that servers not process them if they
     are received.  This is needed to ensure proper sequencing of the
     Bind in relationship to other operations.

C.1.14.  Section 4.2.3 (Bind Response)

- Moved most error-related text to Appendix A, and added text
     regarding certain errors used in conjunction with the Bind
     operation.
   - Prohibited the server from specifying serverSaslCreds when not
     appropriate.

C.1.15.  Section 4.3 (Unbind Operation)

- Specified that both peers are to cease transmission and terminate
     the LDAP session for the Unbind operation.

C.1.16.  Section 4.4 (Unsolicited Notification)

- Added instructions for future specifications of Unsolicited
     Notifications.

C.1.17.  Section 4.5.1 (Search Request)

- SearchRequest attributes is now defined as an AttributeSelection
     type rather than AttributeDescriptionList, and an ABNF is
     provided.
   - SearchRequest attributes may contain duplicate attribute
     descriptions.  This was previously prohibited.  Now servers are
     instructed to ignore subsequent names when they are duplicated.
     This was relaxed in order to allow different short names and also
     OIDs to be requested for an attribute.
   - The present search filter now evaluates to Undefined when the
     specified attribute is not known to the server.  It used to
     evaluate to FALSE, which caused behavior inconsistent with what
     most would expect, especially when the 'not' operator was used.
   - The Filter choice SubstringFilter substrings type is now defined
     with a lower bound of 1.
   - The SubstringFilter substrings 'initial, 'any', and 'final' types
     are now AssertionValue rather than LDAPString.  Also, added
     imperatives stating that 'initial' (if present) must be listed
     first, and 'final' (if present) must be listed last.
   - Disambiguated the semantics of the derefAliases choices.  There was
     question as to whether derefInSearching applied to the base object
     in a wholeSubtree Search.
   - Added instructions for equalityMatch, substrings, greaterOrEqual,
     lessOrEqual, and approxMatch.

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C.1.18.  Section 4.5.2 (Search Result)

- Recommended that servers not use attribute short names when it
     knows they are ambiguous or may cause interoperability problems.
   - Removed all mention of ExtendedResponse due to lack of
     implementation.

C.1.19.  Section 4.5.3 (Continuation References in the Search Result)

- Made changes similar to those made to Section 4.1.11.

C.1.20.  Section 4.5.3.1 (Example)

- Fixed examples to adhere to changes made to Section 4.5.3.

C.1.21.  Section 4.6 (Modify Operation)

- Replaced AttributeTypeAndValues with Attribute as they are
     equivalent.
   - Specified the types of modification changes that might
     temporarily violate schema.  Some readers were under the impression
     that any temporary schema violation was allowed.

C.1.22.  Section 4.7 (Add Operation)

- Aligned Add operation with X.511 in that the attributes of the RDN
     are used in conjunction with the listed attributes to create the
     entry.  Previously, Add required that the distinguished values be
     present in the listed attributes.
   - Removed requirement that the objectClass attribute MUST be
     specified as some DSE types do not require this attribute.
     Instead, generic wording was added, requiring the added entry to
     adhere to the data model.
   - Removed recommendation regarding placement of objects.  This is
     covered in the data model document.

C.1.23.  Section 4.9 (Modify DN Operation)

- Required servers to not dereference aliases for Modify DN.  This
     was added for consistency with other operations and to help ensure
     data consistency.
   - Allow Modify DN to fail when moving between naming contexts.
   - Specified what happens when the attributes of the newrdn are not
     present on the entry.

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C.1.24.  Section 4.10 (Compare Operation)

- Specified that compareFalse means that the Compare took place and
     the result is false.  There was confusion that led people to
     believe that an Undefined match resulted in compareFalse.
   - Required servers to not dereference aliases for Compare.  This was
     added for consistency with other operations and to help ensure
     data consistency.

C.1.25.  Section 4.11 (Abandon Operation)

- Explained that since Abandon returns no response, clients should
     not use it if they need to know the outcome.
   - Specified that Abandon and Unbind cannot be abandoned.

C.1.26.  Section 4.12 (Extended Operation)

- Specified how values of Extended operations defined in terms of
     ASN.1 are to be encoded.
   - Added instructions on what Extended operation specifications
     consist of.
   - Added a recommendation that servers advertise supported Extended
     operations.

C.1.27.  Section 5.2 (Transfer Protocols)

- Moved referral-specific instructions into referral-related
     sections.

C.1.28.  Section 7 (Security Considerations)

- Reworded notes regarding SASL not protecting certain aspects of
     the LDAP Bind messages.
   - Noted that Servers are encouraged to prevent directory
     modifications by clients that have authenticated anonymously
     [RFC4513].
   - Added a note regarding the possibility of changes to security
     factors (authentication, authorization, and data confidentiality).
   - Warned against following referrals that may have been injected in
     the data stream.
   - Noted that servers should protect information equally, whether in
     an error condition or not, and mentioned matchedDN,
     diagnosticMessage, and resultCodes specifically.
   - Added a note regarding malformed and long encodings.

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C.1.29.  Appendix A (Complete ASN.1 Definition)

- Added "EXTENSIBILITY IMPLIED" to ASN.1 definition.
   - Removed AttributeType.  It is not used.

C.2.  Changes Made to RFC 2830

This section summarizes the substantive changes made to Sections of
   RFC 2830.  Readers should consult [RFC4513] for summaries of changes
   to other sections.

C.2.1.  Section 2.3 (Response other than "success")

- Removed wording indicating that referrals can be returned from
     StartTLS.
   - Removed requirement that only a narrow set of result codes can be
     returned.  Some result codes are required in certain scenarios, but
     any other may be returned if appropriate.
   - Removed requirement that the ExtendedResponse.responseName MUST be
     present.  There are circumstances where this is impossible, and
     requiring this is at odds with language in Section 4.12.

C.2.1.  Section 4 (Closing a TLS Connection)

- Reworded most of this section to align with definitions of the
     LDAP protocol layers.
   - Removed instructions on abrupt closure as this is covered in other
     areas of the document (specifically, Section 5.3)

C.3.  Changes Made to RFC 3771

- Rewrote to fit into this document.  In general, semantics were
     preserved.  Supporting and background language seen as redundant
     due to its presence in this document was omitted.

- Specified that Intermediate responses to a request may be of
     different types, and one of the response types may be specified to
     have no response value.

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Editor's Address

Jim Sermersheim
   Novell, Inc.
   1800 South Novell Place
   Provo, Utah 84606, USA

Phone: +1 801 861-3088
   EMail: jimse@novell.com

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RFC 4511                         LDAPv3                        June 2006

Full Copyright Statement

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