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

                                                        18 June 2003




                The LDAP Content Synchronization Operation
                    <draft-zeilenga-ldup-sync-03.txt>




Status of this Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  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>.

  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 (2003).  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 shadow 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
  2.   Elements of the Sync Operation                          7
  2.1.     Common ASN.1 elements                               8
  2.2.     Sync Request Control
  2.3.     Sync State Control                                  9
  2.4.     Sync Done Control
  2.5.     Sync Info Message                                  10
  2.6.     Sync Result Codes
  3.   Content Synchronization
  3.1.     Synchronization Session
  3.2.     Content Determination                              11
  3.3.     refreshOnly mode
  3.4.     refreshAndPersist mode                             13
  3.5.     Search Request Parameters                          15
  3.6.     objectName Issues                                  16
  3.7.     Canceling the Sync Operation
  3.8.     Refresh Required
  3.9.     Chattiness Considerations                          17
  3.10.    Operation Multiplexing                             18
  4.   Meta Information Considerations
  4.1.     Entry DN                                           19
  4.2.     Operational Attributes
  4.3.     Collective Attributes
  4.4.     Access and other administrative controls           20
  5.   Interaction with other controls
  5.1.     ManageDsaIT control
  5.2.     Subentries control                                 21
  6.   Shadowing Considerations
  7.   Security Considerations                                22
  8.   IANA Considerations



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  8.1.     Object Identifier
  8.2.     LDAP Protocol Mechanism                            23
  8.3.     LDAP Result Codes
  9.   Acknowledgments
  10.  Normative References                                   24
  11.  Informative References                                 25
  12.  Authors' Addresses
  Appendix A.  CSN-based Implementation Considerations        26
  Full Copyright                                              27


1. Introduction

  The Lightweight Directory Access Protocol (LDAP) [RFC3377] provides a
  mechanism, the search operation [RFC2251], to allow a client to
  request the return of 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, short of repeating a
  search operation each time a new copy needed, LDAP does not provide an
  effective and efficient mechanism for maintaining synchronized copies
  of directory content.

  This document defines the LDAP Content Synchronization operation, or
  Sync operation for short, which allows a client to maintain a
  synchronized shadow 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.

  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].


1.1. Background

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

    1) do not ensure a reasonable level of convergence;
    2) fail to detect that convergence cannot be achieved (without
       reload);
    3) require pre-arranged synchronization agreements;
    4) require the server to maintain histories of past changes to DIT
       content and/or meta information;
    5) require the server to maintain synchronization state on a per
       client basis; and/or
    6) 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 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 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 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 their size.

  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 where the history information
  available to the server is insufficient to determine the client's
  state.

  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
  eventual-convergent content synchronization.  Upon completion of each
  synchronization stage of the operation, all information to construct a
  synchronized shadow copy of the content has been provided to the
  client or the client has been notified that a complete content reload
  is necessary.  Excepting for transient inconsistencies due to
  concurrent operation (or other) processing at the server, the shadow
  copy is an accurate reflection of the content held by the server.
  Each inconsistency is transient in that it will be corrected during
  subsequent synchronization requests.

  Possible uses include:
    - White page service applications may use the Sync operation to
      maintain current shadow copy of a DIT fragment.  For example, a



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      mail user agent which use the sync operation to maintain a local
      copy of an enterprise address book.

    - Meta-information engines may use the Sync operation to maintain a
      shadow 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 Synchronization Protocol (DISP)
      [X.525] which may be used for master-slave replication between
      LDAP servers.  Other experimental LDAP replication protocols also
      exist.)  The technical specification and applicability statement
      of Sync operation based LDAP replication is left to future
      documents.

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

  As this protocol transfers all visible values of entries 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 shadow 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 shadow 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 the entry.
  Unlike Distinguished Names (DNs), which may change over time, an



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  entry's UUIDs are 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 shadow 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.  For each unchanged entry, the server sends an
  empty entry (e.g., no attributes) with a Sync State control indicating
  state present.  The set of updates is followed by a searchResultDone
  with a Sync Done control.

  If the server can reliably determine which entries in the prior shadow
  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 may, instead of returning an empty entry with state present for
  each present entry, send an empty entry with state delete for each
  entry which is no longer in the content.  Also, the Sync Done control
  refreshDeletes is set to TRUE to indicate to the client that this
  method was used.  This field is FALSE otherwise.

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

  If refreshDeletes 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
  shadow content.  They had been either deleted, moved, or otherwise
  scoped-out from the content.

  If refreshDeletes 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 those which were identified as having been
  deleted from the content.

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


1.3.2. Listening for Changes (refreshAndPersist)




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  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 described above, the server sends a Sync Info message to
  the client indicating that refresh stage is complete and then enters
  persist stage.  After receipt of this Sync Info message, the client
  will have a synchronized shadow copy as described above.

  The server may then send change notifications.  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.  To modify entries 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 may be attached to
  searchResultDone message to provide a new syncCookie.


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



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  more SearchResultReference messages, each with a Sync State control;
  zero or more Sync 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 the IANA-ASSIGNED-OID.1 as
  a value of 'supportedControl' attribute [RFC2252] of the root DSA-
  specific entry (DSE).

  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].


2.1 Common ASN.1 elements

2.1.1 syncUUID

  The syncUUID is a notational convenience to indicate that, while the
  syncUUID type is encoded as an OCTET STRING, its value is restricted
  to the string representation of an Universally Unique Identifier
  (UUID) defined in [UUID].

      syncUUID ::= OCTET STRING


  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 includes a commit (log) sequence
  number, a change sequence number, or a time stamp.

      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 IANA-ASSIGNED-OID.1 and
  the controlValue, an OCTET STRING, contains a BER-encoded
  syncRequestValue.  The criticality field is either TRUE or FALSE.

      syncRequestValue ::= SEQUENCE {



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          mode ENUMERATED {
              -- 0 unused
              refreshOnly       (1),
              -- 2 reserved
              refreshAndPersist (3)
          },
          cookie syncCookie OPTIONAL
      }

  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 IANA-ASSIGNED-OID.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 IANA-ASSIGNED-OID.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.



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2.5 Sync Info Message

  The Sync Info Message is an LDAP Intermediate Response Message
  [LDAPIRM] where responseName is the object identifier
  IANA-ASSIGNED-OID.4 and responseValue contains a BER-encoded
  syncInfoValue.  The criticality is FALSE (and hence absent).

      syncInfoValue ::= CHOICE {
           newcookie [0] syncCookie,
           refreshDone [1] SEQUENCE {
           cookie syncCookie OPTIONAL,
           refreshDeletes BOOLEAN DEFAULT FALSE
           }
      }


2.6 Sync Result Codes

  The following LDAP resultCode [RFC2251] is defined:

      syncRefreshRequired (IANA-ASSIGNED-CODE-0)


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 indicates a request for initial content while
  the presence of a cookie 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 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.


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



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  also issue each Sync request of a session under the same
  authentication and authorization associations with equivalent
  integrity and confidential protections.  If the server does not
  recognize the request cookie or the request is made under different
  associations or inequivalent protections, the server SHALL process the
  request as if no cookie had been provided.

  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 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 process the request as if no
  cookie had been provided.

  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.

  Servers SHOULD support synchronization for all non-collective
  user-applications 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 no cookie is a poll for
  initial content.  A Sync request with mode refreshOnly and cookie is a
  poll for content update.





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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 referral [RFC3296] object), and
  no cookie.  The searchResultDone message SHALL include a Sync Done
  control.  The refreshDeletes SHALL be FALSE.

  A resultCode value of success indicates the operation successfully
  completed.  Otherwise, the result code indicates the nature of
  failure.

  If the operation is successful, a cookie 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
  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),

      c) indicate that convergence is not possible by returning
         syncRefreshRequired,

      d) return a resultCode other than success or syncRefreshRequired.

  For each entry or reference added to the content or was changed since
  the previous Sync operation indicated by the cookie, the server
  returns a searchResultEntry or searchResultReference message,
  respectively, each with a Sync State cookie 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



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  state of a changed reference.

  For each entry which has not been changed since the previous Sync
  operation, a searchResultEntry is returned whose objectName reflects
  the entry's current DN, the attributes field is empty, and a Sync
  State control 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, a searchResultReference containing
  an empty SEQUENCE OF LDAPURL is returned with a Sync State control 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 content.

  As an alternative to sending messages for each entry and reference
  which has not been changed, the server may instead return the
  following.  For each entry no longer in content, return a
  searchResultEntry whose objectName reflects a past DN of the entry or
  is empty, the attributes field is empty, and a Sync State control of
  state delete, entryUUID containing the UUID of the deleted entry, and
  no cookie.  For each reference no longer in content, a
  searchResultReference containing an empty SEQUENCE OF LDAPURL is
  returned with a a Sync State control of state delete, entryUUID
  containing the UUID of the deleted reference, and no cookie.

  A resultCode value of success indicates the operation successfully
  completed.  A resultCode value of 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 when incremental
  sychronization is possible.


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 successful completion of content refresh is indicated by sending
  a Sync Info message with a refreshDone value instead of a
  SearchResultDone message with resultCode success.  A cookie SHOULD be
  returned in the refreshDone value for use in subsequent Sync
  operations.




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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 references to
  be added to the content, deleted from the content, or modify entries
  in 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
  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 in 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 in 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.

  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.



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  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.


3.5.1.  baseObject Issues

  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 be the baseObject be 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 refers to any entry or
  changes which entry the baseObject refers to, the server SHALL return
  an appropriate non-success result code such as noSuchObject,
  aliasProblem, aliasDereferencingProblem, referral, or
  syncRefreshRequired.


3.5.2.  derefAliases Issues

  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 Issues

  The sizeLimit applies only to entries (regardless of their syncState)
  returned during refreshOnly processing or the refresh stage of the
  refreshAndPersist processing.


3.5.4.  timeLimit Issues

  For a refreshOnly Sync operation, the timeLimit applies to the whole
  operation.  For a refreshAndPersist operation, the timeLimit applies



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  to processing up to and including generating the Sync Info message
  with a refreshDone value.


3.5.5.  filter Issues

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


3.6.  objectName Issues

  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 SHOULD implement the LDAP Cancel [CANCEL] operation and
  support cancellation of outstanding Sync operations as described here.

  To cancel an outstanding Sync Operation, the client SHOULD issue a
  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 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




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  In order to achieve the eventual-convergent synchronization, the
  server may terminate the Sync operation in refresh or persist stage by
  returning a syncRefreshRequired resultCode to the client.  If a cookie
  is not provided in this response, a full refresh is needed.  If a
  cookie 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) instead of providing an incremental refresh
  in order to achieve the eventual convergence.

  In the case of persist stage Sync, the server returns the resultCode
  of 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 a cookie is not provided, a full refresh is needed.
  If a cookie is provided, an incremental refresh is needed.

  The server may also return 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
  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 historical information, if the server has sufficient
  history to allow it to reliably determine which entries in the prior
  shadow 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).

  The server SHOULD maintain enough (current or historical) state
  information (such as a context-wide last modify time stamp), to
  determine that no changes were made in the context since the content
  to refresh was provided and, and when no changes were made, generate
  zero delete entry messages instead of present messages.




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  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 syncRefreshRequired if it determines that a
  refresh would be more efficient than continuing under the current
  operation.

  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.

  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 transfers to
  each interested client.  Implementors and deployers of servers should
  take steps 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
  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 which are largely
  non-overlapping result sets.  This ensures that an event requiring an
  syncRefreshRequired response can be limited to as few as possible
  result sets.


4. Meta Information Considerations




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

  Where 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
  in subschema subentries.

  For 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




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  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 [COLLECTIVE].

  Modification of a collective attribute generally affects the content
  of multiple entries, each a member 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 of
  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, ...).  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 syncRefreshRequired or treat the Sync Operation as an initial
  content request (e.g., ignore the cookie).


5. Interaction with other controls

  The Sync Operation may be used with:

      - ManageDsaIT Control [RFC3296]
      - Subentries Control [SUBENTRY]

  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



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  objects to be treated as normal objects 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"
  [SUBENTRY].  When used with the Sync Operation, the subentries control
  and other factors (search scope, filter, etc.) are used to determining
  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
  technics, 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
  server.  In particular, each physical server acting as one logical
  server SHOULD be equally capable of continuing a content
  synchronization based upon cookies provided 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 content poll alternative approach
  (sending of entry delete messages instead of entry present messages)
  discussed in Section 3.3.2 may not be applicable.




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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.

  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.  An user could abuse its update priviledges to mount an
  indirect denial of service to these clients, other clients, and/or
  portions of the network.  Servers should provide safeguards to ensure
  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.


8.1.  Object Identifier

  It is requested that IANA register an LDAP Object Identifier to
  identify elements of the LDAP Content Synchronization Operation as
  defined in this document.




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      Subject: Request for LDAP Object Identifier Registration
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@OpenLDAP.org>
      Specification: RFC XXXX
      Author/Change Controller: IESG
      Comments:
          Identifies elements of the LDAP Content Synchronization Operation


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: IANA-ASSIGNED-OID
      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 Codes 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: syncRefreshRequired (IANA-ASSIGNED-CODE-0)
      Specification: RFC XXXX
      Author/Change Controller: IESG
      Comments:  none


9. Acknowledgments

  This document borrows significantly from the LDAP Client Update
  Protocol [LCUP] work.  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 (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.

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

  [SUBENTRY]    Zeilenga, K. and S. Legg, "Subentries in LDAP",
                draft-zeilenga-ldap-subentry-xx.txt, a work in progress.

  [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).

  [CANCEL]      Zeilenga, K., "LDAP Cancel Extended Operation",
                draft-zeilenga-ldap-cancel-xx.txt, a work in progress.
  [COLLECTIVE]  Zeilenga, K., "Collective Attributes in LDAP",
                draft-zeilenga-ldap-collective-xx.txt, a work in
                progress.

  [UUID]        International Organization for Standardization (ISO),
                "Information technology - Open Systems Interconnection -



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                Remote Procedure Call", ISO/IEC 11578:1996


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.

  [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).

  [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>

  Jonghyuk Choi
  IBM Corporation
  <jongchoi@us.ibm.com>



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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 targetted for use in
  servers which do not maintain historical 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 historical information should consider other approaches
  which exploit the historical information.

  A Change Sequence Number is, effectively a time stamp has sufficient
  granularity to ensure that 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 allow one to determine how to 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 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 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 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.




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  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
  entry "update" messages for all entries in the content whose CSN is
  greater than the cookie CSN and entry "present" messages for all other
  entries in the content.  However, if the current context CSN is same
  as the cookie CSN, the server should instead generate zero "updates",
  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.

  Additionally, servers may want to consider maintaining some
  per-session historical state information to reduce the number of
  messages in 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 "update"+"delete" messages instead of
  "update"+"present" messages.  When there are large number of sessions,
  it may only make sense to maintain such history for clients where it
  of most value.  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.




Full Copyright

  Copyright (C) The Internet Society (2003). All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implmentation may be prepared, copied, published and
  distributed, in whole or in part, without restriction of any kind,
  provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other



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  Internet organizations, except as needed for the  purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be followed,
  or as required to translate it into languages other than English.















































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