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INTERNET-DRAFT                                      Kurt D. Zeilenga
Intended Category: Experimental                  OpenLDAP Foundation
Expires in six months                                 Jong Hyuk Choi
                                                     IBM Corporation

                                                   27 September 2004




                The LDAP Content Synchronization Operation
                    <draft-zeilenga-ldup-sync-06.txt>




Status of this Memo

  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDUP Working Group mailing list
  at <ietf-ldup@imc.org>.  Please send editorial comments directly to
  the document editor at <Kurt@OpenLDAP.org>.

  By submitting this Internet-Draft, I accept the provisions of Section
  4 of RFC 3667.  By submitting this Internet-Draft, I certify that any
  applicable patent or other IPR claims of which I am aware have been
  disclosed, or will be disclosed, and any of which I become aware will
  be disclosed, in accordance with RFC 3668.

  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups. Note that other
  groups may also distribute working documents as Internet-Drafts.

  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time. It is inappropriate to use Internet-Drafts as reference material
  or to cite them other than as "work in progress."

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

  Copyright (C) The Internet Society (2004).  All Rights Reserved.

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




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Abstract

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


Table of Contents

  Status of this Memo                                          1
  Abstract                                                     2
  Table of Contents
  1.   Introduction                                            3
  1.1.     Background
  1.2.     Intended Usage                                      4
  1.3.     Overview                                            5
  1.4.     Conventions                                         8
  2.   Elements of the Sync Operation
  2.1.     Common ASN.1 Elements                               9
  2.2.     Sync Request Control
  2.3.     Sync State Control                                 10
  2.4.     Sync Done Control
  2.5.     Sync Info Message
  2.6.     Sync Result Codes                                  11
  3.   Content Synchronization
  3.1.     Synchronization Session                            12
  3.2.     Content Determination
  3.3.     refreshOnly Mode                                   13
  3.4.     refreshAndPersist Mode                             16
  3.5.     Search Request Parameters                          17
  3.6.     objectName Issues                                  18
  3.7.     Canceling the Sync Operation                       19
  3.8.     Refresh Required
  3.9.     Chattiness Considerations                          20
  3.10.    Operation Multiplexing                             21
  4.   Meta Information Considerations                        22
  4.1.     Entry DN
  4.2.     Operational Attributes
  4.3.     Collective Attributes                              23
  4.4.     Access and Other Administrative Controls
  5.   Interaction with Other Controls                        24
  5.1.     ManageDsaIT Control
  5.2.     Subentries Control
  6.   Shadowing Considerations
  7.   Security Considerations                                25
  8.   IANA Considerations                                    26



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  8.1.     Object Identifier
  8.2.     LDAP Protocol Mechanism
  8.3.     LDAP Result Codes
  9.   Acknowledgments
  10.  Normative References                                   27
  11.  Informative References                                 28
  12.  Authors' Addresses                                     29
  Appendix A.  CSN-based Implementation Considerations
  Intellectual Property Rights                                31
  Full Copyright                                              32


1. Introduction

  The Lightweight Directory Access Protocol (LDAP) [RFC3377] provides a
  mechanism, the search operation [RFC2251], which allows a client to
  request directory content matching a complex set of assertions and for
  the server to 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 met 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 which 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:

    - fail to ensure a reasonable level of convergence;
    - fail 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.




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  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.

  The Sync Operation does not require servers to maintain nor to 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 in 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 servers to 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 in the case that 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



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  copy is an accurate reflection of the content held by the server.
  Transient inconsistencies will be resolved by subsequent
  synchronization operations.

  Possible uses include:
    - White page service applications may use the Sync Operation to
      maintain current copy of a DIT fragment.  For example, a mail user
      agent which 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)



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  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 the entry
  [EntryUUID].  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 was 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 which
  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 which 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 which 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



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  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 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 which 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 the
  server to 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



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  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.  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 [CANCEL] 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 [RFC2251].


2. Elements of the Sync Operation

  The Sync Operation is defined as an extension to the LDAP Search
  Operation [RFC2251] 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) responses 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.




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  To allow clients to discover support for this operation, servers
  implementing this operation SHOULD publish the
  1.3.6.1.4.1.4203.1.9.1.1 as a value of 'supportedControl' attribute
  [RFC2252] of the root DSA-specific entry (DSE).  A server MAY choose
  to advertise this extension only when the client is authorized to use
  it.


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 which 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 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 [RFC2251, Section 4.1.2]
  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)



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          },
          cookie     syncCookie OPTIONAL,
          reloadHint BOOLEAN DEFAULT FALSE
      }

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


2.3 Sync State Control

  The Sync State Control is an LDAP Control [RFC2251, Section 4.1.2]
  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 [RFC2251, Section 4.1.2]
  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 SearchResultDone Message.


2.5 Sync Info Message




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  The Sync Info Message is an LDAP Intermediate Response Message
  [LDAPIRM] 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 [RFC2251] is defined:

      e-syncRefreshRequired (IANA-ASSIGNED-CODE)


3. Content Synchronization

  The Sync Operation is invoked by the client sending 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
  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 Section 3.3 and Section
  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 are 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 [RFC2251], 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
  which would be returned by a normal search operation nor, for those
  entries included, not 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, entryUUID containing the entry's UUID, and no cookie.  Each
  SearchResultReference Message SHALL include a Sync State Control of
  state add, 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 the operation successfully
  completed.  Otherwise, the result code indicates the nature of
  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




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

  The server is REQUIRED to either:
      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 which 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, 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 which has not been changed since
  the previous Sync Operation, an empty SearchResultEntry is returned
  whose objectName reflects the entry's current DN, the attributes field
  is empty, and a Sync State Control consisting of state present,
  entryUUID containing the UUID of the entry, and no cookie.  For each
  reference which 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, entryUUID containing the UUID of the entry, and no cookie.
  No messages are sent for entries or references which 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 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, the attributes field is empty, and a Sync
  State Control consisting of state delete, 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, 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 which has been deleted from the
  content since the last Sync Operation.  syncUUIDs may be empty.  The
  Sync Info Message of syncIdSet may contain 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 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 ended by a SearchResultDone Message with
  Sync Done Control which SHOULD contain 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 are marked ended at the end of the delete
  phase by a SearchResultDone Message with Sync Done Control which
  SHOULD contain 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 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 excepting
  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 which represents the entry
  as it appears in the content.  The message SHALL include a Sync State
  Control with state of add, 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 which represents the
  reference in the content.  The message SHALL include a Sync State



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  Control with state of add, entryUUID containing the UUID associated
  with the reference, and an optional cookie.

  Where DIT updates cause an entry to be modified within the content,
  the server provides a SearchResultEntry Message which represents the
  entry as it appears in the content.  The message SHALL include a Sync
  State Control with state of modify, 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 SearchResultEntry Message which represents the
  reference in the content.  The message SHALL include a Sync State
  Control with state of modify, 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 SearchResultReference Message with an empty SEQUENCE
  OF LDAPURL.  The message SHALL include a Sync State Control with state
  of delete, entryUUID containing the UUID associated with the
  reference, and an optional cookie.

  Where DIT updates cause a reference 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,
  entryUUID containing the entry's UUID, 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 which has been deleted from the
  content.  The Sync Info Message of syncIdSet may contain 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 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 to no longer 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 which 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



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  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.

  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).

  It should also be noted 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 [CANCEL] Operation and support
  cancellation of outstanding Sync Operations as described here.

  To cancel an outstanding Sync Operation, the client issues an LDAP
  Cancel [CANCEL] 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 the persist
  stage by returning a 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



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  achieve the eventual convergence.

  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.

  It is noted that the client may receive one or more of
  SearchResultEntry, SearchResultReference, and/or Sync Info Messages
  before it receives 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.



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  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
  refresh was provided and, 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 which
  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
  which 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 syncIdSet Sync Info Message when there are
  multiple delete or present messages to reduce the amount of
  synchronization traffic.

  It is also noted that there may be many clients interested in a
  particular directory change, and 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 [RFC2251] allows operations to be multiplexed
  over a single LDAP session.  Clients SHOULD NOT maintain multiple LDAP



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  sessions with the same server.  Servers SHOULD ensure that responses
  from concurrently processed operations are interleaved fairly.

  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 with each other.  This ensures that an event
  requiring a 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 [RFC2251] to TRUE if
        matching a value of an attribute of the entry and 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 is 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 which implement 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



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  in subschema subentries.

  As a counter example, servers which implement aliases [RFC2256][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 [RFC2252].  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 [RFC3371].

  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 which the modification affected) to be
  transmitted.

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





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5. Interaction with Other Controls

  The Sync Operation may be used with:

      - ManageDsaIT Control [RFC3296]
      - 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 appear in the content or not.


6. Shadowing Considerations

  As noted in [RFC2251], some servers may hold shadow copies of entries
  which 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 which are equally capable
  of being used to obtain particular directory content from, a client
  SHOULD NOT assume that each of these server 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 which are equally capable in
  regards to their support for the Sync operation and which hold equally
  complete copies of the entries should be made to appear as one logical



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  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.

  It is noted 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 which allow shadowing events
  to be coalesced.  Where so, 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 which 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 which 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 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.

  It is noted that even small updates to the directory may cause
  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



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  update operations are not abused.

  Implementors of this (or any) LDAP extension should be familiar with
  general LDAP security considerations [RFC3377].


8. IANA Considerations

  Registration of the following values is requested.

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


8.2.  LDAP Protocol Mechanism

  It is requested that IANA register 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 XXXX
      Author/Change Controller: IESG
      Comments: none


8.3.  LDAP Result Codes

  It is requested that IANA register 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 (IANA-ASSIGNED-CODE)
      Specification: RFC XXXX
      Author/Change Controller: IESG
      Comments:  none


9. Acknowledgments

  This document borrows significantly from the LDAP Client Update



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  Protocol [LCUP], 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].


10. Normative References

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

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

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

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

  [RFC3377]     Hodges, J. and R. Morgan, "Lightweight Directory Access
                Protocol (v3): Technical Specification", RFC 3377,
                September 2002.

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

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

  [CANCEL]      Zeilenga, K., "LDAP Cancel Extended Operation",
                draft-zeilenga-ldap-cancel-xx.txt, a work in progress.
  [EntryUUID]   Zeilenga, K., "The LDAP EntryUUID Operational
                Attribute", draft-zeilenga-ldap-uuid-xx.txt, a work in
                progress.

  [LDAPIRM]     Harrison, R. and Zeilenga, K., "LDAP Intermediate
                Response",
                draft-rharrison-ldap-intermediate-resp-00.txt, a work in
                progress.

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




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  [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).

  [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

  [RFC2256]     Wahl, M., "A Summary of the X.500(96) User Schema for
                use with LDAPv3", RFC 2256, December 1997.

  [RFC3383]     Zeilenga, K., "IANA Considerations for LDAP", BCP 64
                (also RFC 3383), September 2002.

  [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.511]       International Telecommunication Union -
                Telecommunication Standardization Sector, "The
                Directory: Abstract Service Definition", X.511(1993).

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

  [UUIDinfo]    The Open Group, "Universally Unique Identifier" appendix
                of the CAE Specification "DCE 1.1: Remote Procedure
                Calls", Document Number C706,
                <http://www.opengroup.org/products/publications/catalog/c706.htm>
                (appendix available at:
                <http://www.opengroup.org/onlinepubs/9629399/apdxa.htm>),
                August 1997.




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  [DIRSYNC]     Armijo, M., "Microsoft LDAP Control for Directory
                Synchronization", draft-armijo-ldap-dirsync-xx.txt, a
                work in progress.

  [LCUP]        Megginson, R., et. al., "LDAP Client Update Protocol",
                draft-ietf-ldup-lcup-xx.txt, a work in progress.

  [PSEARCH]     Smith, M., et. al., "Persistent Search: A Simple LDAP
                Change Notification Mechanism",
                draft-ietf-ldapext-psearch-xx.txt, a work in progress.

  [TSEARCH]     Wahl, M., "LDAPv3 Triggered Search Control",
                draft-ietf-ldapext-trigger-xx.txt, a work in progress.


12. Authors' Addresses

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

  Jong Hyuk Choi
  IBM Corporation
  <jongchoi@us.ibm.com>



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 a Change Sequence Number
  based approaches.

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

  A Change Sequence Number is effectively a time stamp which has
  sufficient granularity to ensure that the precedence relationship in
  time of two updates to the same object can be determined.  Change



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  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.  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 which we
  will call the context CSN.  The context CSN is the greatest committed
  entry CSN which 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
  all relevant changes 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 number is
  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 changes whose CSN is
  greater than the context CSN after search processing.

  A simple server implementation could use 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 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, which affects 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 cookie CSN, the server
  should ignore the cookie and treat the request as an initial request
  for content.




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  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 are 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.




Intellectual Property Rights

  The IETF takes no position regarding the validity or scope of any
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  pertain to the implementation or use of the technology described in
  this document or the extent to which any license under such rights
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  on the procedures with respect to rights in RFC documents can be found
  in BCP 78 and BCP 79.

  Copies of IPR disclosures made to the IETF Secretariat and any
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  attempt made to obtain a general license or permission for the use of
  such proprietary rights by implementers or users of this specification
  can be obtained from the IETF on-line IPR repository at
  http://www.ietf.org/ipr.

  The IETF invites any interested party to bring to its attention any
  copyrights, patents or patent applications, or other proprietary
  rights that may cover technology that may be required to implement
  this standard.  Please address the information to the IETF at
  ietf-ipr@ietf.org.





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Full Copyright

  Copyright (C) The Internet Society (2004).  This document is subject
  to the rights, licenses and restrictions contained in BCP 78, and
  except as set forth therein, the authors retain all their rights.

  This document and the information contained herein are provided on an
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