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✏️ Editing: rfc4533.txt

Network Working Group                                        K. Zeilenga
Request for Comments: 4533                           OpenLDAP Foundation
Category: Experimental                                         J.H. Choi
                                                         IBM Corporation
                                                               June 2006

The Lightweight Directory Access Protocol (LDAP)
                   Content Synchronization Operation

Status of This Memo

This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG Note

The IESG notes that this work was originally discussed in the LDUP
   working group.  The group came to consensus on a different approach,
   documented in RFC 3928; that document is on the standards track and
   should be reviewed by those considering implementation of this
   proposal.

Abstract

This specification describes the Lightweight Directory Access
   Protocol (LDAP) Content Synchronization Operation.  The operation
   allows a client to maintain a copy of a fragment of the Directory
   Information Tree (DIT).  It supports both polling for changes and
   listening for changes.  The operation is defined as an extension of
   the LDAP Search Operation.

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Table of Contents

1. Introduction ....................................................3
      1.1. Background .................................................3
      1.2. Intended Usage .............................................4
      1.3. Overview ...................................................5
      1.4. Conventions ................................................8
   2. Elements of the Sync Operation ..................................8
      2.1. Common ASN.1 Elements ......................................9
      2.2. Sync Request Control .......................................9
      2.3. Sync State Control ........................................10
      2.4. Sync Done Control .........................................10
      2.5. Sync Info Message .........................................11
      2.6. Sync Result Codes .........................................11
   3. Content Synchronization ........................................11
      3.1. Synchronization Session ...................................12
      3.2. Content Determination .....................................12
      3.3. refreshOnly Mode ..........................................13
      3.4. refreshAndPersist Mode ....................................16
      3.5. Search Request Parameters .................................17
      3.6. objectName ................................................18
      3.7. Canceling the Sync Operation ..............................19
      3.8. Refresh Required ..........................................19
      3.9. Chattiness Considerations .................................20
      3.10. Operation Multiplexing ...................................21
   4. Meta Information Considerations ................................22
      4.1. Entry DN ..................................................22
      4.2. Operational Attributes ....................................22
      4.3. Collective Attributes .....................................23
      4.4. Access and Other Administrative Controls ..................23
   5. Interaction with Other Controls ................................23
      5.1. ManageDsaIT Control .......................................24
      5.2. Subentries Control ........................................24
   6. Shadowing Considerations .......................................24
   7. Security Considerations ........................................25
   8. IANA Considerations ............................................26
      8.1. Object Identifier .........................................26
      8.2. LDAP Protocol Mechanism ...................................26
      8.3. LDAP Result Codes .........................................26
   9. Acknowledgements ...............................................26
   10. Normative References ..........................................27
   11. Informative References ........................................28
   Appendix A.  CSN-based Implementation Considerations ..............29

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1.  Introduction

The Lightweight Directory Access Protocol (LDAP) [RFC4510] provides a
   mechanism, the search operation [RFC4511], that allows a client to
   request directory content matching a complex set of assertions and to
   request that the server return this content, subject to access
   control and other restrictions, to the client.  However, LDAP does
   not provide (despite the introduction of numerous extensions in this
   area) an effective and efficient mechanism for maintaining
   synchronized copies of directory content.  This document introduces a
   new mechanism specifically designed to meet the content
   synchronization requirements of sophisticated directory applications.

This document defines the LDAP Content Synchronization Operation, or
   Sync Operation for short, which allows a client to maintain a
   synchronized copy of a fragment of a Directory Information Tree
   (DIT).  The Sync Operation is defined as a set of controls and other
   protocol elements that extend the Search Operation.

1.1.  Background

Over the years, a number of content synchronization approaches have
   been suggested for use in LDAP directory services.  These approaches
   are inadequate for one or more of the following reasons:

-  failure to ensure a reasonable level of convergence;

-  failure to detect that convergence cannot be achieved (without
         reload);

-  require pre-arranged synchronization agreements;

-  require the server to maintain histories of past changes to DIT
         content and/or meta information;

-  require the server to maintain synchronization state on a per-
         client basis; and/or

-  are overly chatty.

The Sync Operation provides eventual convergence of synchronized
   content when possible and, when not, notification that a full reload
   is required.

The Sync Operation does not require pre-arranged synchronization
   agreements.

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The Sync Operation does not require that servers maintain or use any
   history of past changes to the DIT or to meta information.  However,
   servers may maintain and use histories (e.g., change logs,
   tombstones, DIT snapshots) to reduce the number of messages generated
   and to reduce their size.  As it is not always feasible to maintain
   and use histories, the operation may be implemented using purely
   (current) state-based approaches.  The Sync Operation allows use of
   either the state-based approach or the history-based approach on an
   operation-by-operation basis to balance the size of history and the
   amount of traffic.  The Sync Operation also allows the combined use
   of the state-based and the history-based approaches.

The Sync Operation does not require that servers maintain
   synchronization state on a per-client basis.  However, servers may
   maintain and use per-client state information to reduce the number of
   messages generated and the size of such messages.

A synchronization mechanism can be considered overly chatty when
   synchronization traffic is not reasonably bounded.  The Sync
   Operation traffic is bounded by the size of updated (or new) entries
   and the number of unchanged entries in the content.  The operation is
   designed to avoid full content exchanges, even when the history
   information available to the server is insufficient to determine the
   client's state.  The operation is also designed to avoid transmission
   of out-of-content history information, as its size is not bounded by
   the content and it is not always feasible to transmit such history
   information due to security reasons.

This document includes a number of non-normative appendices providing
   additional information to server implementors.

1.2.  Intended Usage

The Sync Operation is intended to be used in applications requiring
   eventually-convergent content synchronization.  Upon completion of
   each synchronization stage of the operation, all information to
   construct a synchronized client copy of the content has been provided
   to the client or the client has been notified that a complete content
   reload is necessary.  Except for transient inconsistencies due to
   concurrent operation (or other) processing at the server, the client
   copy is an accurate reflection of the content held by the server.
   Transient inconsistencies will be resolved by subsequent
   synchronization operations.

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Possible uses include the following:

-  White page service applications may use the Sync Operation to
         maintain a current copy of a DIT fragment, for example, a mail
         user agent that uses the sync operation to maintain a local
         copy of an enterprise address book.

-  Meta-information engines may use the Sync Operation to maintain
         a copy of a DIT fragment.

-  Caching proxy services may use the Sync Operation to maintain a
         coherent content cache.

-  Lightweight master-slave replication between heterogeneous
         directory servers.  For example, the Sync Operation can be used
         by a slave server to maintain a shadow copy of a DIT fragment.
         (Note: The International Telephone Union (ITU) has defined the
         X.500 Directory [X.500] Information Shadowing Protocol (DISP)
         [X.525], which may be used for master-slave replication between
         directory servers.  Other experimental LDAP replication
         protocols also exist.)

This protocol is not intended to be used in applications requiring
   transactional data consistency.

As this protocol transfers all visible values of entries belonging to
   the content upon change instead of change deltas, this protocol is
   not appropriate for bandwidth-challenged applications or deployments.

1.3.  Overview

This section provides an overview of basic ways the Sync Operation
   can be used to maintain a synchronized client copy of a DIT fragment.

-  Polling for changes: refreshOnly mode

-  Listening for changes: refreshAndPersist mode

1.3.1.  Polling for Changes (refreshOnly)

To obtain its initial client copy, the client issues a Sync request:
   a search request with the Sync Request Control with mode set to
   refreshOnly.  The server, much like it would with a normal search
   operation, returns (subject to access controls and other
   restrictions) the content matching the search criteria (baseObject,
   scope, filter, attributes).  Additionally, with each entry returned,
   the server provides a Sync State Control indicating state add.  This
   control contains the Universally Unique Identifier (UUID) [UUID] of

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the entry [RFC4530].  Unlike the Distinguished Name (DN), which may
   change over time, an entry's UUID is stable.  The initial content is
   followed by a SearchResultDone with a Sync Done Control.  The Sync
   Done Control provides a syncCookie.  The syncCookie represents
   session state.

To poll for updates to the client copy, the client reissues the Sync
   Operation with the syncCookie previously returned.  The server, much
   as it would with a normal search operation, determines which content
   would be returned as if the operation were a normal search operation.
   However, using the syncCookie as an indicator of what content the
   client was sent previously, the server sends copies of entries that
   have changed with a Sync State Control indicating state add.  For
   each changed entry, all (modified or unmodified) attributes belonging
   to the content are sent.

The server may perform either or both of the two distinct
   synchronization phases that are distinguished by how to synchronize
   entries deleted from the content: the present and the delete phases.
   When the server uses a single phase for the refresh stage, each phase
   is marked as ended by a SearchResultDone with a Sync Done Control.  A
   present phase is identified by a FALSE refreshDeletes value in the
   Sync Done Control.  A delete phase is identified by a TRUE
   refreshDeletes value.  The present phase may be followed by a delete
   phase.  The two phases are delimited by a refreshPresent Sync Info
   Message having a FALSE refreshDone value.  In the case that both the
   phases are used, the present phase is used to bring the client copy
   up to the state at which the subsequent delete phase can begin.

In the present phase, the server sends an empty entry (i.e., no
   attributes) with a Sync State Control indicating state present for
   each unchanged entry.

The delete phase may be used when the server can reliably determine
   which entries in the prior client copy are no longer present in the
   content and the number of such entries is less than or equal to the
   number of unchanged entries.  In the delete mode, the server sends an
   empty entry with a Sync State Control indicating state delete for
   each entry that is no longer in the content, instead of returning an
   empty entry with state present for each present entry.

The server may send syncIdSet Sync Info Messages containing the set
   of UUIDs of either unchanged present entries or deleted entries,
   instead of sending multiple individual messages.  If refreshDeletes
   of syncIdSet is set to FALSE, the UUIDs of unchanged present entries
   are contained in the syncUUIDs set; if refreshDeletes of syncIdSet is
   set to TRUE, the UUIDs of the entries no longer present in the
   content are contained in the syncUUIDs set.  An optional cookie can

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be included in the syncIdSet to represent the state of the content
   after synchronizing the presence or the absence of the entries
   contained in the syncUUIDs set.

The synchronized copy of the DIT fragment is constructed by the
   client.

If refreshDeletes of syncDoneValue is FALSE, the new copy includes
   all changed entries returned by the reissued Sync Operation, as well
   as all unchanged entries identified as being present by the reissued
   Sync Operation, but whose content is provided by the previous Sync
   Operation.  The unchanged entries not identified as being present are
   deleted from the client content.  They had been either deleted,
   moved, or otherwise scoped-out from the content.

If refreshDeletes of syncDoneValue is TRUE, the new copy includes all
   changed entries returned by the reissued Sync Operation, as well as
   all other entries of the previous copy except for those that are
   identified as having been deleted from the content.

The client can, at some later time, re-poll for changes to this
   synchronized client copy.

1.3.2.  Listening for Changes (refreshAndPersist)

Polling for changes can be expensive in terms of server, client, and
   network resources.  The refreshAndPersist mode allows for active
   updates of changed entries in the content.

By selecting the refreshAndPersist mode, the client requests that the
   server send updates of entries that are changed after the initial
   refresh content is determined.  Instead of sending a SearchResultDone
   Message as in polling, the server sends a Sync Info Message to the
   client indicating that the refresh stage is complete and then enters
   the persist stage.  After receipt of this Sync Info Message, the
   client will construct a synchronized copy as described in Section
   1.3.1.

The server may then send change notifications as the result of the
   original Sync search request, which now remains persistent in the
   server.  For entries to be added to the returned content, the server
   sends a SearchResultEntry (with attributes) with a Sync State Control
   indicating state add.  For entries to be deleted from the content,
   the server sends a SearchResultEntry containing no attributes and a
   Sync State Control indicating state delete.  For entries to be
   modified in the return content, the server sends a SearchResultEntry
   (with attributes) with a Sync State Control indicating state modify.

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Upon modification of an entry, all (modified or unmodified)
   attributes belonging to the content are sent.

Note that renaming an entry of the DIT may cause an add state change
   where the entry is renamed into the content, a delete state change
   where the entry is renamed out of the content, and a modify state
   change where the entry remains in the content.  Also note that a
   modification of an entry of the DIT may cause an add, delete, or
   modify state change to the content.

Upon receipt of a change notification, the client updates its copy of
   the content.

If the server desires to update the syncCookie during the persist
   stage, it may include the syncCookie in any Sync State Control or
   Sync Info Message returned.

The operation persists until canceled [RFC3909] by the client or
   terminated by the server.  A Sync Done Control shall be attached to
   SearchResultDone Message to provide a new syncCookie.

1.4.  Conventions

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

Protocol elements are described using ASN.1 [X.680] with implicit
   tags.  The term "BER-encoded" means the element is to be encoded
   using the Basic Encoding Rules [X.690] under the restrictions
   detailed in Section 5.1 of [RFC4511].

2.  Elements of the Sync Operation

The Sync Operation is defined as an extension to the LDAP Search
   Operation [RFC4511] where the directory user agent (DUA or client)
   submits a SearchRequest Message with a Sync Request Control and the
   directory system agent (DSA or server) responds with zero or more
   SearchResultEntry Messages, each with a Sync State Control; zero or
   more SearchResultReference Messages, each with a Sync State Control;
   zero or more Sync Info Intermediate Response Messages; and a
   SearchResultDone Message with a Sync Done Control.

To allow clients to discover support for this operation, servers
   implementing this operation SHOULD publish 1.3.6.1.4.1.4203.1.9.1.1
   as a value of the 'supportedControl' attribute [RFC4512] of the root
   DSA-specific entry (DSE).  A server MAY choose to advertise this
   extension only when the client is authorized to use it.

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2.1.  Common ASN.1 Elements

2.1.1.  syncUUID

The syncUUID data type is an OCTET STRING holding a 128-bit
   (16-octet) Universally Unique Identifier (UUID) [UUID].

syncUUID ::= OCTET STRING (SIZE(16))
           -- constrained to UUID

2.1.2.  syncCookie

The syncCookie is a notational convenience to indicate that, while
   the syncCookie type is encoded as an OCTET STRING, its value is an
   opaque value containing information about the synchronization session
   and its state.  Generally, the session information would include a
   hash of the operation parameters that the server requires not be
   changed and the synchronization state information would include a
   commit (log) sequence number, a change sequence number, or a time
   stamp.  For convenience of description, the term "no cookie" refers
   either to a null cookie or to a cookie with pre-initialized
   synchronization state.

syncCookie ::= OCTET STRING

2.2.  Sync Request Control

The Sync Request Control is an LDAP Control [RFC4511] where the
   controlType is the object identifier 1.3.6.1.4.1.4203.1.9.1.1 and the
   controlValue, an OCTET STRING, contains a BER-encoded
   syncRequestValue.  The criticality field is either TRUE or FALSE.

syncRequestValue ::= SEQUENCE {
          mode ENUMERATED {
              -- 0 unused
              refreshOnly       (1),
              -- 2 reserved
              refreshAndPersist (3)
          },
          cookie     syncCookie OPTIONAL,
          reloadHint BOOLEAN DEFAULT FALSE
      }

The Sync Request Control is only applicable to the SearchRequest
   Message.

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2.3.  Sync State Control

The Sync State Control is an LDAP Control [RFC4511] where the
   controlType is the object identifier 1.3.6.1.4.1.4203.1.9.1.2 and the
   controlValue, an OCTET STRING, contains a BER-encoded syncStateValue.
   The criticality is FALSE.

syncStateValue ::= SEQUENCE {
          state ENUMERATED {
              present (0),
              add (1),
              modify (2),
              delete (3)
          },
          entryUUID syncUUID,
          cookie    syncCookie OPTIONAL
      }

The Sync State Control is only applicable to SearchResultEntry and
   SearchResultReference Messages.

2.4.  Sync Done Control

The Sync Done Control is an LDAP Control [RFC4511] where the
   controlType is the object identifier 1.3.6.1.4.1.4203.1.9.1.3 and the
   controlValue contains a BER-encoded syncDoneValue.  The criticality
   is FALSE (and hence absent).

syncDoneValue ::= SEQUENCE {
          cookie          syncCookie OPTIONAL,
          refreshDeletes  BOOLEAN DEFAULT FALSE
      }

The Sync Done Control is only applicable to the SearchResultDone
   Message.

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2.5.  Sync Info Message

The Sync Info Message is an LDAP Intermediate Response Message
   [RFC4511] where responseName is the object identifier
   1.3.6.1.4.1.4203.1.9.1.4 and responseValue contains a BER-encoded
   syncInfoValue.  The criticality is FALSE (and hence absent).

syncInfoValue ::= CHOICE {
          newcookie      [0] syncCookie,
          refreshDelete  [1] SEQUENCE {
              cookie         syncCookie OPTIONAL,
              refreshDone    BOOLEAN DEFAULT TRUE
          },
          refreshPresent [2] SEQUENCE {
              cookie         syncCookie OPTIONAL,
              refreshDone    BOOLEAN DEFAULT TRUE
          },
          syncIdSet      [3] SEQUENCE {
              cookie         syncCookie OPTIONAL,
              refreshDeletes BOOLEAN DEFAULT FALSE,
              syncUUIDs      SET OF syncUUID
          }
      }

2.6.  Sync Result Codes

The following LDAP resultCode [RFC4511] is defined:

e-syncRefreshRequired (4096)

3.  Content Synchronization

The Sync Operation is invoked when the client sends a SearchRequest
   Message with a Sync Request Control.

The absence of a cookie or an initialized synchronization state in a
   cookie indicates a request for initial content, while the presence of
   a cookie representing a state of a client copy indicates a request
   for a content update.  Synchronization Sessions are discussed in
   Section 3.1.  Content Determination is discussed in Section 3.2.

The mode is either refreshOnly or refreshAndPersist.  The refreshOnly
   and refreshAndPersist modes are discussed in Sections 3.3 and 3.4,
   respectively.  The refreshOnly mode consists only of a refresh stage,
   while the refreshAndPersist mode consists of a refresh stage and a
   subsequent persist stage.

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3.1.  Synchronization Session

A sequence of Sync Operations where the last cookie returned by the
   server for one operation is provided by the client in the next
   operation is said to belong to the same Synchronization Session.

The client MUST specify the same content-controlling parameters (see
   Section 3.5) in each Search Request of the session.  The client
   SHOULD also issue each Sync request of a session under the same
   authentication and authorization associations with equivalent
   integrity and protections.  If the server does not recognize the
   request cookie or the request is made under different associations or
   non-equivalent protections, the server SHALL return the initial
   content as if no cookie had been provided or return an empty content
   with the e-syncRefreshRequired LDAP result code.  The decision
   between the return of the initial content and the return of the empty
   content with the e-syncRefreshRequired result code MAY be based on
   reloadHint in the Sync Request Control from the client.  If the
   server recognizes the request cookie as representing empty or initial
   synchronization state of the client copy, the server SHALL return the
   initial content.

A Synchronization Session may span multiple LDAP sessions between the
   client and the server.  The client SHOULD issue each Sync request of
   a session to the same server.  (Note: Shadowing considerations are
   discussed in Section 6.)

3.2.  Content Determination

The content to be provided is determined by parameters of the Search
   Request, as described in [RFC4511], and possibly other controls.  The
   same content parameters SHOULD be used in each Sync request of a
   session.  If different content is requested and the server is
   unwilling or unable to process the request, the server SHALL return
   the initial content as if no cookie had been provided or return an
   empty content with the e-syncRefreshRequired LDAP result code.  The
   decision between the return of the initial content and the return of
   the empty content with the e-syncRefreshRequired result code MAY be
   based on reloadHint in the Sync Request Control from the client.

The content may not necessarily include all entries or references
   that would be returned by a normal search operation, nor, for those
   entries included, all attributes returned by a normal search.  When
   the server is unwilling or unable to provide synchronization for any
   attribute for a set of entries, the server MUST treat all filter
   components matching against these attributes as Undefined and MUST
   NOT return these attributes in SearchResultEntry responses.

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Servers SHOULD support synchronization for all non-collective user-
   application attributes for all entries.

The server may also return continuation references to other servers
   or to itself.  The latter is allowed as the server may partition the
   entries it holds into separate synchronization contexts.

The client may chase all or some of these continuations, each as a
   separate content synchronization session.

3.3.  refreshOnly Mode

A Sync request with mode refreshOnly and with no cookie is a poll for
   initial content.  A Sync request with mode refreshOnly and with a
   cookie representing a synchronization state is a poll for content
   update.

3.3.1.  Initial Content Poll

Upon receipt of the request, the server provides the initial content
   using a set of zero or more SearchResultEntry and
   SearchResultReference Messages followed by a SearchResultDone
   Message.

Each SearchResultEntry Message SHALL include a Sync State Control of
   state add, an entryUUID containing the entry's UUID, and no cookie.
   Each SearchResultReference Message SHALL include a Sync State Control
   of state add, an entryUUID containing the UUID associated with the
   reference (normally the UUID of the associated named referral
   [RFC3296] object), and no cookie.  The SearchResultDone Message SHALL
   include a Sync Done Control having refreshDeletes set to FALSE.

A resultCode value of success indicates that the operation
   successfully completed.  Otherwise, the result code indicates the
   nature of the failure.  The server may return e-syncRefreshRequired
   result code on the initial content poll if it is safe to do so when
   it is unable to perform the operation due to various reasons.
   reloadHint is set to FALSE in the SearchRequest Message requesting
   the initial content poll.

If the operation is successful, a cookie representing the
   synchronization state of the current client copy SHOULD be returned
   for use in subsequent Sync Operations.

3.3.2.  Content Update Poll

Upon receipt of the request, the server provides the content refresh
   using a set of zero or more SearchResultEntry and

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SearchResultReference Messages followed by a SearchResultDone
   Message.

The server is REQUIRED to:

a) provide the sequence of messages necessary for eventual
         convergence of the client's copy of the content to the server's
         copy,

b) treat the request as an initial content request (e.g., ignore
         the cookie or the synchronization state represented in the
         cookie),

c) indicate that the incremental convergence is not possible by
         returning e-syncRefreshRequired,

d) return a resultCode other than success or e-
         syncRefreshRequired.

A Sync Operation may consist of a single present phase, a single
   delete phase, or a present phase followed by a delete phase.

In each phase, for each entry or reference that has been added to the
   content or been changed since the previous Sync Operation indicated
   by the cookie, the server returns a SearchResultEntry or
   SearchResultReference Message, respectively, each with a Sync State
   Control consisting of state add, an entryUUID containing the UUID of
   the entry or reference, and no cookie.  Each SearchResultEntry
   Message represents the current state of a changed entry.  Each
   SearchResultReference Message represents the current state of a
   changed reference.

In the present phase, for each entry that has not been changed since
   the previous Sync Operation, an empty SearchResultEntry is returned
   whose objectName reflects the entry's current DN, whose attributes
   field is empty, and whose Sync State Control consists of state
   present, an entryUUID containing the UUID of the entry, and no
   cookie.  For each reference that has not been changed since the
   previous Sync Operation, an empty SearchResultReference containing an
   empty SEQUENCE OF LDAPURL is returned with a Sync State Control
   consisting of state present, an entryUUID containing the UUID of the
   entry, and no cookie.  No messages are sent for entries or references
   that are no longer in the content.

Multiple empty entries with a Sync State Control of state present
   SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
   value with refreshDeletes set to FALSE.  syncUUIDs contain a set of
   UUIDs of the entries and references unchanged since the last Sync

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Operation.  syncUUIDs may be empty.  The Sync Info Message of
   syncIdSet may contain a cookie to represent the state of the content
   after performing the synchronization of the entries in the set.

In the delete phase, for each entry no longer in the content, the
   server returns a SearchResultEntry whose objectName reflects a past
   DN of the entry or is empty, whose attributes field is empty, and
   whose Sync State Control consists of state delete, an entryUUID
   containing the UUID of the deleted entry, and no cookie.  For each
   reference no longer in the content, a SearchResultReference
   containing an empty SEQUENCE OF LDAPURL is returned with a Sync State
   Control consisting of state delete, an entryUUID containing the UUID
   of the deleted reference, and no cookie.

Multiple empty entries with a Sync State Control of state delete
   SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
   value with refreshDeletes set to TRUE.  syncUUIDs contain a set of
   UUIDs of the entries and references that have been deleted from the
   content since the last Sync Operation.  syncUUIDs may be empty.  The
   Sync Info Message of syncIdSet may contain a cookie to represent the
   state of the content after performing the synchronization of the
   entries in the set.

When a present phase is followed by a delete phase, the two phases
   are delimited by a Sync Info Message containing syncInfoValue of
   refreshPresent, which may contain a cookie representing the state
   after completing the present phase.  The refreshPresent contains
   refreshDone, which is always FALSE in the refreshOnly mode of Sync
   Operation because it is followed by a delete phase.

If a Sync Operation consists of a single phase, each phase and hence
   the Sync Operation are marked as ended by a SearchResultDone Message
   with Sync Done Control, which SHOULD contain a cookie representing
   the state of the content after completing the Sync Operation.  The
   Sync Done Control contains refreshDeletes, which is set to FALSE for
   the present phase and set to TRUE for the delete phase.

If a Sync Operation consists of a present phase followed by a delete
   phase, the Sync Operation is marked as ended at the end of the delete
   phase by a SearchResultDone Message with Sync Done Control, which
   SHOULD contain a cookie representing the state of the content after
   completing the Sync Operation.  The Sync Done Control contains
   refreshDeletes, which is set to TRUE.

The client can specify whether it prefers to receive an initial
   content by supplying reloadHint of TRUE or to receive a e-
   syncRefreshRequired resultCode by supplying reloadHint of FALSE
   (hence absent), in the case that the server determines that it is

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impossible or inefficient to achieve the eventual convergence by
   continuing the current incremental synchronization thread.

A resultCode value of success indicates that the operation is
   successfully completed.  A resultCode value of e-syncRefreshRequired
   indicates that a full or partial refresh is needed.  Otherwise, the
   result code indicates the nature of failure.  A cookie is provided in
   the Sync Done Control for use in subsequent Sync Operations for
   incremental synchronization.

3.4.  refreshAndPersist Mode

A Sync request with mode refreshAndPersist asks for initial content
   or content update (during the refresh stage) followed by change
   notifications (during the persist stage).

3.4.1.  refresh Stage

The content refresh is provided as described in Section 3.3, except
   that the successful completion of content refresh is indicated by
   sending a Sync Info Message of refreshDelete or refreshPresent with a
   refreshDone value set to TRUE instead of a SearchResultDone Message
   with resultCode success.  A cookie SHOULD be returned in the Sync
   Info Message to represent the state of the content after finishing
   the refresh stage of the Sync Operation.

3.4.2.  persist Stage

Change notifications are provided during the persist stage.

As updates are made to the DIT, the server notifies the client of
   changes to the content.  DIT updates may cause entries and references
   to be added to the content, deleted from the content, or modified
   within the content.  DIT updates may also cause references to be
   added, deleted, or modified within the content.

Where DIT updates cause an entry to be added to the content, the
   server provides a SearchResultEntry Message that represents the entry
   as it appears in the content.  The message SHALL include a Sync State
   Control with state of add, an entryUUID containing the entry's UUID,
   and an optional cookie.

Where DIT updates cause a reference to be added to the content, the
   server provides a SearchResultReference Message that represents the
   reference in the content.  The message SHALL include a Sync State
   Control with state of add, an entryUUID containing the UUID
   associated with the reference, and an optional cookie.

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Where DIT updates cause an entry to be modified within the content,
   the server provides a SearchResultEntry Message that represents the
   entry as it appears in the content.  The message SHALL include a Sync
   State Control with state of modify, an entryUUID containing the
   entry's UUID, and an optional cookie.

Where DIT updates cause a reference to be modified within the
   content, the server provides a SearchResultReference Message that
   represents the reference in the content.  The message SHALL include a
   Sync State Control with state of modify, an entryUUID containing the
   UUID associated with the reference, and an optional cookie.

Where DIT updates cause an entry to be deleted from the content, the
   server provides a SearchResultEntry Message with no attributes.  The
   message SHALL include a Sync State Control with state of delete, an
   entryUUID containing the entry's UUID, and an optional cookie.

Where DIT updates cause a reference to be deleted from the content,
   the server provides a SearchResultReference Message with an empty
   SEQUENCE OF LDAPURL.  The message SHALL include a Sync State Control
   with state of delete, an entryUUID containing the UUID associated
   with the reference, and an optional cookie.

Multiple empty entries with a Sync State Control of state delete
   SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
   value with refreshDeletes set to TRUE. syncUUIDs contain a set of
   UUIDs of the entries and references that have been deleted from the
   content.  The Sync Info Message of syncIdSet may contain a cookie to
   represent the state of the content after performing the
   synchronization of the entries in the set.

With each of these messages, the server may provide a new cookie to
   be used in subsequent Sync Operations.  Additionally, the server may
   also return Sync Info Messages of choice newCookie to provide a new
   cookie.  The client SHOULD use the newest (last) cookie it received
   from the server in subsequent Sync Operations.

3.5.  Search Request Parameters

As stated in Section 3.1, the client SHOULD specify the same
   content-controlling parameters in each Search Request of the session.
   All fields of the SearchRequest Message are considered content-
   controlling parameters except for sizeLimit and timeLimit.

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3.5.1.  baseObject

As with the normal search operation, the refresh and persist stages
   are not isolated from DIT changes.  It is possible that the entry
   referred to by the baseObject is deleted, renamed, or moved.  It is
   also possible that the alias object used in finding the entry
   referred to by the baseObject is changed such that the baseObject
   refers to a different entry.

If the DIT is updated during processing of the Sync Operation in a
   manner that causes the baseObject no longer to refer to any entry or
   in a manner that changes the entry the baseObject refers to, the
   server SHALL return an appropriate non-success result code, such as
   noSuchObject, aliasProblem, aliasDereferencingProblem, referral, or
   e-syncRefreshRequired.

3.5.2.  derefAliases

This operation does not support alias dereferencing during searching.
   The client SHALL specify neverDerefAliases or derefFindingBaseObj for
   the SearchRequest derefAliases parameter.  The server SHALL treat
   other values (e.g., derefInSearching, derefAlways) as protocol
   errors.

3.5.3.  sizeLimit

The sizeLimit applies only to entries (regardless of their state in
   Sync State Control) returned during the refreshOnly operation or the
   refresh stage of the refreshAndPersist operation.

3.5.4.  timeLimit

For a refreshOnly Sync Operation, the timeLimit applies to the whole
   operation.  For a refreshAndPersist operation, the timeLimit applies
   only to the refresh stage including the generation of the Sync Info
   Message with a refreshDone value of TRUE.

3.5.5.  filter

The client SHOULD avoid filter assertions that apply to the values of
   the attributes likely to be considered by the server as ones holding
   meta-information.  See Section 4.

3.6.  objectName

The Sync Operation uses entryUUID values provided in the Sync State
   Control as the primary keys to entries.  The client MUST use these
   entryUUIDs to correlate synchronization messages.

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In some circumstances, the DN returned may not reflect the entry's
   current DN.  In particular, when the entry is being deleted from the
   content, the server may provide an empty DN if the server does not
   wish to disclose the entry's current DN (or, if deleted from the DIT,
   the entry's last DN).

Also note that the entry's DN may be viewed as meta information (see
   Section 4.1).

3.7.  Canceling the Sync Operation

Servers MUST implement the LDAP Cancel [RFC3909] Operation and
   support cancellation of outstanding Sync Operations as described
   here.

To cancel an outstanding Sync Operation, the client issues an LDAP
   Cancel [RFC3909] Operation.

If at any time the server becomes unwilling or unable to continue
   processing a Sync Operation, the server SHALL return a
   SearchResultDone with a non-success resultCode indicating the reason
   for the termination of the operation.

Whether the client or the server initiated the termination, the
   server may provide a cookie in the Sync Done Control for use in
   subsequent Sync Operations.

3.8.  Refresh Required

In order to achieve the eventually-convergent synchronization, the
   server may terminate the Sync Operation in the refresh or persist
   stages by returning an e-syncRefreshRequired resultCode to the
   client.  If no cookie is provided, a full refresh is needed.  If a
   cookie representing a synchronization state is provided in this
   response, an incremental refresh is needed.

To obtain a full refresh, the client then issues a new
   synchronization request with no cookie.  To obtain an incremental
   reload, the client issues a new synchronization with the provided
   cookie.

The server may choose to provide a full copy in the refresh stage
   (e.g., ignore the cookie or the synchronization state represented in
   the cookie) instead of providing an incremental refresh in order to
   achieve the eventual convergence.

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The decision between the return of the initial content and the return
   of the e-syncRefreshRequired result code may be based on reloadHint
   in the Sync Request Control from the client.

In the case of persist stage Sync, the server returns the resultCode
   of e-syncRefreshRequired to the client to indicate that the client
   needs to issue a new Sync Operation in order to obtain a synchronized
   copy of the content.  If no cookie is provided, a full refresh is
   needed.  If a cookie representing a synchronization state is
   provided, an incremental refresh is needed.

The server may also return e-syncRefreshRequired if it determines
   that a refresh would be more efficient than sending all the messages
   required for convergence.

Note that the client may receive one or more of SearchResultEntry,
   SearchResultReference, and/or Sync Info Messages before it receives a
   SearchResultDone Message with the e-syncRefreshRequired result code.

3.9.  Chattiness Considerations

The server MUST ensure that the number of entry messages generated to
   refresh the client content does not exceed the number of entries
   presently in the content.  While there is no requirement for servers
   to maintain history information, if the server has sufficient history
   to allow it to reliably determine which entries in the prior client
   copy are no longer present in the content and the number of such
   entries is less than or equal to the number of unchanged entries, the
   server SHOULD generate delete entry messages instead of present entry
   messages (see Section 3.3.2).

When the amount of history information maintained in the server is
   not enough for the clients to perform infrequent refreshOnly Sync
   Operations, it is likely that the server has incomplete history
   information (e.g., due to truncation) by the time those clients
   connect again.

The server SHOULD NOT resort to full reload when the history
   information is not enough to generate delete entry messages.  The
   server SHOULD generate either present entry messages only or present
   entry messages followed by delete entry messages to bring the client
   copy to the current state.  In the latter case, the present entry
   messages bring the client copy to a state covered by the history
   information maintained in the server.

The server SHOULD maintain enough (current or historical) state
   information (such as a context-wide last modify time stamp) to
   determine if no changes were made in the context since the content

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refresh was provided and, when no changes were made, generate zero
   delete entry messages instead of present messages.

The server SHOULD NOT use the history information when its use does
   not reduce the synchronization traffic or when its use can expose
   sensitive information not allowed to be received by the client.

The server implementor should also consider chattiness issues that
   span multiple Sync Operations of a session.  As noted in Section 3.8,
   the server may return e-syncRefreshRequired if it determines that a
   reload would be more efficient than continuing under the current
   operation.  If reloadHint in the Sync Request is TRUE, the server may
   initiate a reload without directing the client to request a reload.

The server SHOULD transfer a new cookie frequently to avoid having to
   transfer information already provided to the client.  Even where DIT
   changes do not cause content synchronization changes to be
   transferred, it may be advantageous to provide a new cookie using a
   Sync Info Message.  However, the server SHOULD avoid overloading the
   client or network with Sync Info Messages.

During persist mode, the server SHOULD coalesce multiple outstanding
   messages updating the same entry.  The server MAY delay generation of
   an entry update in anticipation of subsequent changes to that entry
   that could be coalesced.  The length of the delay should be long
   enough to allow coalescing of update requests issued back to back but
   short enough that the transient inconsistency induced by the delay is
   corrected in a timely manner.

The server SHOULD use the syncIdSet Sync Info Message when there are
   multiple delete or present messages to reduce the amount of
   synchronization traffic.

Also note that there may be many clients interested in a particular
   directory change, and that servers attempting to service all of these
   at once may cause congestion on the network.  The congestion issues
   are magnified when the change requires a large transfer to each
   interested client.  Implementors and deployers of servers should take
   steps to prevent and manage network congestion.

3.10.  Operation Multiplexing

The LDAP protocol model [RFC4511] allows operations to be multiplexed
   over a single LDAP session.  Clients SHOULD NOT maintain multiple
   LDAP sessions with the same server.  Servers SHOULD ensure that
   responses from concurrently processed operations are interleaved
   fairly.

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Clients SHOULD combine Sync Operations whose result set is largely
   overlapping.  This avoids having to return multiple messages, once
   for each overlapping session, for changes to entries in the overlap.

Clients SHOULD NOT combine Sync Operations whose result sets are
   largely non-overlapping.  This ensures that an event requiring an
   e-syncRefreshRequired response can be limited to as few result sets
   as possible.

4.  Meta Information Considerations

4.1.  Entry DN

As an entry's DN is constructed from its relative DN (RDN) and the
   entry's parent's DN, it is often viewed as meta information.

While renaming or moving to a new superior causes the entry's DN to
   change, that change SHOULD NOT, by itself, cause synchronization
   messages to be sent for that entry.  However, if the renaming or the
   moving could cause the entry to be added or deleted from the content,
   appropriate synchronization messages should be generated to indicate
   this to the client.

When a server treats the entry's DN as meta information, the server
   SHALL either

-  evaluate all MatchingRuleAssertions [RFC4511] to TRUE if
         matching a value of an attribute of the entry, otherwise
         Undefined, or

-  evaluate all MatchingRuleAssertion with dnAttributes of TRUE as
         Undefined.

The latter choice is offered for ease of server implementation.

4.2.  Operational Attributes

Where values of an operational attribute are determined by values not
   held as part of the entry it appears in, the operational attribute
   SHOULD NOT support synchronization of that operational attribute.

For example, in servers that implement the X.501 subschema model
   [X.501], servers should not support synchronization of the
   subschemaSubentry attribute as its value is determined by values held
   and administrated in subschema subentries.

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As a counter example, servers that implement aliases [RFC4512][X.501]
   can support synchronization of the aliasedObjectName attribute as its
   values are held and administrated as part of the alias entries.

Servers SHOULD support synchronization of the following operational
   attributes: createTimestamp, modifyTimestamp, creatorsName,
   modifiersName [RFC4512].  Servers MAY support synchronization of
   other operational attributes.

4.3.  Collective Attributes

A collective attribute is "a user attribute whose values are the same
   for each member of an entry collection" [X.501].  Use of collective
   attributes in LDAP is discussed in [RFC3671].

Modification of a collective attribute generally affects the content
   of multiple entries, which are the members of the collection.  It is
   inefficient to include values of collective attributes visible in
   entries of the collection, as a single modification of a collective
   attribute requires transmission of multiple SearchResultEntry (one
   for each entry of the collection that the modification affected).

Servers SHOULD NOT synchronize collective attributes appearing in
   entries of any collection.  Servers MAY support synchronization of
   collective attributes appearing in collective attribute subentries.

4.4.  Access and Other Administrative Controls

Entries are commonly subject to access and other administrative
   Controls.  While portions of the policy information governing a
   particular entry may be held in the entry, policy information is
   often held elsewhere (in superior entries, in subentries, in the root
   DSE, in configuration files, etc.).  Because of this, changes to
   policy information make it difficult to ensure eventual convergence
   during incremental synchronization.

Where it is impractical or infeasible to generate content changes
   resulting from a change to policy information, servers may opt to
   return e-syncRefreshRequired or to treat the Sync Operation as an
   initial content request (e.g., ignore the cookie or the
   synchronization state represented in the cookie).

5.  Interaction with Other Controls

The Sync Operation may be used with:

- ManageDsaIT Control [RFC3296]

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RFC 4533         LDAP Content Synchronization Operation        June 2006

- Subentries Control [RFC3672]

as described below.  The Sync Operation may be used with other LDAP
   extensions as detailed in other documents.

5.1.  ManageDsaIT Control

The ManageDsaIT Control [RFC3296] indicates that the operation acts
   upon the DSA Information Tree and causes referral and other special
   entries to be treated as object entries with respect to the
   operation.

5.2.  Subentries Control

The Subentries Control is used with the search operation "to control
   the visibility of entries and subentries which are within scope"
   [RFC3672].  When used with the Sync Operation, the subentries control
   and other factors (search scope, filter, etc.) are used to determine
   whether an entry or subentry appears in the content.

6.  Shadowing Considerations

As noted in [RFC4511], some servers may hold shadow copies of entries
   that can be used to answer search and comparison queries.  Such
   servers may also support content synchronization requests.  This
   section discusses considerations for implementors and deployers for
   the implementation and deployment of the Sync operation in shadowed
   directories.

While a client may know of multiple servers that are equally capable
   of being used to obtain particular directory content from, a client
   SHOULD NOT assume that each of these servers is equally capable of
   continuing a content synchronization session.  As stated in Section
   3.1, the client SHOULD issue each Sync request of a Sync session to
   the same server.

However, through domain naming or IP address redirection or other
   techniques, multiple physical servers can be made to appear as one
   logical server to a client.  Only servers that are equally capable in
   regards to their support for the Sync operation and that hold equally
   complete copies of the entries should be made to appear as one
   logical server.  In particular, each physical server acting as one
   logical server SHOULD be equally capable of continuing a content
   synchronization based upon cookies provided by any of the other
   physical servers without requiring a full reload.  Because there is
   no standard LDAP shadowing mechanism, the specification of how to
   independently implement equally capable servers (as well as the
   precise definition of "equally capable") is left to future documents.

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Note that it may be difficult for the server to reliably determine
   what content was provided to the client by another server, especially
   in the shadowing environments that allow shadowing events to be
   coalesced.  For these servers, the use of the delete phase discussed
   in Section 3.3.2 may not be applicable.

7.  Security Considerations

In order to maintain a synchronized copy of the content, a client is
   to delete information from its copy of the content as described
   above.  However, the client may maintain knowledge of information
   disclosed to it by the server separate from its copy of the content
   used for synchronization.  Management of this knowledge is beyond the
   scope of this document.  Servers should be careful not to disclose
   information for content the client is not authorized to have
   knowledge of and/or about.

While the information provided by a series of refreshOnly Sync
   Operations is similar to that provided by a series of Search
   Operations, persist stage may disclose additional information.  A
   client may be able to discern information about the particular
   sequence of update operations that caused content change.

Implementors should take precautions against malicious cookie
   content, including malformed cookies or valid cookies used with
   different security associations and/or protections in an attempt to
   obtain unauthorized access to information.  Servers may include a
   digital signature in the cookie to detect tampering.

The operation may be the target of direct denial-of-service attacks.
   Implementors should provide safeguards to ensure the operation is not
   abused.  Servers may place access control or other restrictions upon
   the use of this operation.

Note that even small updates to the directory may cause a significant
   amount of traffic to be generated to clients using this operation.  A
   user could abuse its update privileges to mount an indirect denial of
   service to these clients, other clients, and/or portions of the
   network.  Servers should provide safeguards to ensure that update
   operations are not abused.

Implementors of this (or any) LDAP extension should be familiar with
   general LDAP security considerations [RFC4510].

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8.  IANA Considerations

Registration of the following values have been completed by the IANA
   [RFC4520].

8.1.  Object Identifier

The OID arc 1.3.6.1.4.1.4203.1.9.1 was assigned [ASSIGN] by the
   OpenLDAP Foundation, under its IANA-assigned private enterprise
   allocation [PRIVATE], for use in this specification.

8.2.  LDAP Protocol Mechanism

The IANA has registered the LDAP Protocol Mechanism described in this
   document.

Subject: Request for LDAP Protocol Mechanism Registration
      Object Identifier: 1.3.6.1.4.1.4203.1.9.1.1
      Description: LDAP Content Synchronization Control
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@openldap.org>
      Usage: Control
      Specification: RFC 4533
      Author/Change Controller: Kurt D. Zeilenga, Jong Hyuk Choi
      Comments: none

8.3.  LDAP Result Codes

The IANA has registered the LDAP Result Code described in this
   document.

Subject: LDAP Result Code Registration
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@OpenLDAP.org>
      Result Code Name: e-syncRefreshRequired (4096)
      Specification: RFC 4533
      Author/Change Controller: Kurt D. Zeilenga, Jong Hyuk Choi
      Comments:  none

9.  Acknowledgements

This document borrows significantly from the LDAP Client Update
   Protocol [RFC3928], a product of the IETF LDUP working group.  This
   document also benefited from Persistent Search [PSEARCH], Triggered
   Search [TSEARCH], and Directory Synchronization [DIRSYNC] works.
   This document also borrows from "Lightweight Directory Access
   Protocol (v3)" [RFC2251].

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10.  Normative References

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

[RFC3296]   Zeilenga, K., "Named Subordinate References in
               Lightweight Directory Access Protocol (LDAP)
               Directories", RFC 3296, July 2002.

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

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

[RFC3909]   Zeilenga, K., "Lightweight Directory Access Protocol
               (LDAP) Cancel Operation", RFC 3909, October 2004.

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

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

[RFC4512]   Zeilenga, K., "Lightweight Directory Access Protocol
               (LDAP): Directory Information Models", RFC 4512, June
               2006.

[RFC4530]   Zeilenga, K., "Lightweight Directory Access Protocol
               (LDAP) entryUUID Operational Attribute", RFC 4530, June
               2006.

[UUID]      International Organization for Standardization (ISO),
               "Information technology - Open Systems Interconnection -
               Remote Procedure Call", ISO/IEC 11578:1996

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

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

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RFC 4533         LDAP Content Synchronization Operation        June 2006

[X.690]     International Telecommunication Union - Telecommunication
               Standardization Sector, "Specification of ASN.1 encoding
               rules: Basic Encoding Rules (BER), Canonical Encoding
               Rules (CER), and Distinguished Encoding Rules (DER)",
               X.690(1997) (also ISO/IEC 8825-1:1998).

11.  Informative References

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

[RFC3928]   Megginson, R., Ed., Smith, M., Natkovich, O., and J.
               Parham, "Lightweight Directory Access Protocol (LDAP)
               Client Update Protocol (LCUP)", RFC 3928, October 2004.

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

[PRIVATE]   IANA, "Private Enterprise Numbers",
               http://www.iana.org/assignments/enterprise-numbers.

[ASSIGN]    OpenLDAP Foundation, "OpenLDAP OID Delegations",
               http://www.openldap.org/foundation/oid-delegate.txt.

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

[X.525]     International Telecommunication Union - Telecommunication
               Standardization Sector, "The Directory: Replication",
               X.525(1993).

[DIRSYNC]   Armijo, M., "Microsoft LDAP Control for Directory
               Synchronization", Work in Progress.

[PSEARCH]   Smith, M., et al., "Persistent Search: A Simple LDAP
               Change Notification Mechanism", Work in Progress.

[TSEARCH]   Wahl, M., "LDAPv3 Triggered Search Control", Work in
               Progress.

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RFC 4533         LDAP Content Synchronization Operation        June 2006

Appendix A.  CSN-based Implementation Considerations

This appendix is provided for informational purposes only; it is not
   a normative part of the LDAP Content Synchronization Operation's
   technical specification.

This appendix discusses LDAP Content Synchronization Operation server
   implementation considerations associated with Change Sequence Number
   based approaches.

Change Sequence Number based approaches are targeted for use in
   servers that do not maintain history information (e.g., change logs,
   state snapshots) about changes made to the Directory and hence, must
   rely on current directory state and minimal synchronization state
   information embedded in Sync Cookie.  Servers that maintain history
   information should consider other approaches that exploit the history
   information.

A Change Sequence Number is effectively a time stamp that has
   sufficient granularity to ensure that the precedence relationship in
   time of two updates to the same object can be determined.  Change
   Sequence Numbers are not to be confused with Commit Sequence Numbers
   or Commit Log Record Numbers.  A Commit Sequence Number allows one to
   determine how two commits (to the same object or different objects)
   relate to each other in time.  A Change Sequence Number associated
   with different entries may be committed out of order.  In the
   remainder of this Appendix, the term CSN refers to a Change Sequence
   Number.

In these approaches, the server not only maintains a CSN for each
   directory entry (the entry CSN) but also maintains a value that we
   will call the context CSN.  The context CSN is the greatest committed
   entry CSN that is not greater than any outstanding (uncommitted)
   entry CSNs for all entries in a directory context.  The values of
   context CSN are used in syncCookie values as synchronization state
   indicators.

As search operations are not isolated from individual directory
   update operations and individual update operations cannot be assumed
   to be serialized, one cannot assume that the returned content
   incorporates each relevant change whose change sequence number is
   less than or equal to the greatest entry CSN in the content.  The
   content incorporates all the relevant changes whose change sequence
   numbers are less than or equal to context CSN before search
   processing.  The content may also incorporate any subset of the
   changes whose change sequence number is greater than context CSN
   before search processing but less than or equal to the context CSN
   after search processing.  The content does not incorporate any of the

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RFC 4533         LDAP Content Synchronization Operation        June 2006

changes whose CSN is greater than the context CSN after search
   processing.

A simple server implementation could use the value of the context CSN
   before search processing to indicate state.  Such an implementation
   would embed this value into each SyncCookie returned.  We'll call
   this the cookie CSN.  When a refresh was requested, the server would
   simply generate "update" messages for all entries in the content
   whose CSN is greater than the supplied cookie CSN and generate
   "present" messages for all other entries in the content.  However, if
   the current context CSN is the same as the cookie CSN, the server
   should instead generate zero "updates" and zero "delete" messages and
   indicate a refreshDeletes of TRUE, as the directory has not changed.

The implementation should also consider the impact of changes to meta
   information, such as access controls, that affect content
   determination.  One approach is for the server to maintain a
   context-wide meta information CSN or meta CSN.  This meta CSN would
   be updated whenever meta information affecting content determination
   was changed.  If the value of the meta CSN is greater than the cookie
   CSN, the server should ignore the cookie and treat the request as an
   initial request for content.

Additionally, servers may want to consider maintaining some per-
   session history information to reduce the number of messages needed
   to be transferred during incremental refreshes.  Specifically, a
   server could record information about entries as they leave the scope
   of a disconnected sync session and later use this information to
   generate delete messages instead of present messages.

When the history information is truncated, the CSN of the latest
   truncated history information entry may be recorded as the truncated
   CSN of the history information.  The truncated CSN may be used to
   determine whether a client copy can be covered by the history
   information by comparing it to the synchronization state contained in
   the cookie supplied by the client.

When there is a large number of sessions, it may make sense to
   maintain such history only for the selected clients.  Also, servers
   taking this approach need to consider resource consumption issues to
   ensure reasonable server operation and to protect against abuse.  It
   may be appropriate to restrict this mode of operation by policy.

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RFC 4533         LDAP Content Synchronization Operation        June 2006

Authors' Addresses

Kurt D. Zeilenga
   OpenLDAP Foundation

EMail: Kurt@OpenLDAP.org

Jong Hyuk Choi
   IBM Corporation

EMail: jongchoi@us.ibm.com

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RFC 4533         LDAP Content Synchronization Operation        June 2006

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