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Versions: 00 01 02 03 04 05 06 RFC 4533

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

                                                          5 May 2003




                  LDAP Content Synchronization Operation
                    <draft-zeilenga-ldup-sync-02.txt>




1. 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 2003, The Internet Society.  All Rights Reserved.

  Please see the 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.


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].  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].


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.


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



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       reload);
    3) require pre-arranged synchronization agreements;
    4) require the server to maintain synchronization state on a per
       client basis;
    5) require the server to maintain histories of past changes to DIT
       content and/or meta information; and/or
    6) are overly chatty.

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

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

  The Sync operation does not require servers to maintain
  synchronization state on a per user basis.

  The Sync operation does not require servers to maintain any history of
  past changes to the DIT or to meta information.  While histories
  (e.g., change logs, tombstones, DIT snapshots) may be used in the
  implementation of the Sync operation, the operation may be implemented
  using purely state-based approaches.

  As the Sync operation does not require servers to maintain any
  histories of past changes, it can be implemented in environments where
  it is not feasible to maintain such histories.  Histories, if
  available, may be used by the server to reduce the number of messages
  generated and reduce their size.

  The Sync operation chattiness is reasonably bound.


1.2. Intended Usage

  The Sync operation is intended to be used in applications requiring
  eventual-convergent content synchronization.  Upon completion of each
  synchronization phase of the operation, all information to construct
  an 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



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      maintain current shadow copy of a DIT fragment.  For example, an
      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 Synchronization Protocol [X.525] which may be used for
        master-slave replication between LDAP servers.  Other
        experimental LDAP replication protocols exist.  The Sync
        operation should be viewed as complementary to these replication
        protocols.

  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 basis 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).
  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



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  Distinguished Names (DNs), which may change over time, an 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
  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)

  Polling for changes can be expensive in terms of server, client, and



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  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 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 phase is complete
  and then enters persist phase.  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 with 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 a 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 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



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  SearchResultEntry messages, each with a Sync State control; zero or
  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 root DSE attribute.


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; the
  synchronization state information includes a commit (log) sequence
  number, a change sequence number, or a time stamp; and a digital
  signature for detection of tampering.

      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 {
          mode ENUMERATED {
              -- 0 unused
              refreshOnly       (1),
              -- 2 reserved



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              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.


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
  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 resultCodes [RFC2251] are 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 a
  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



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  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.


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.
  Where the server is unwilling or unable to provide synchronization for
  an attribute for a set of entries, the server MUST treat all filter
  components matching against these attribute as Undefined and MUST NOT
  return the attribute 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 in a
  separate LDAP 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.


3.3.1.  Initial Content Poll

  Upon receipt of the request, the server provides the initial content



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  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 referral [RFC3296] object's entryUUID), 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
  state of a changed reference.

  For each entry which has not been changed since the previous Sync



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  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.  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.


3.4.  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 with state refreshDone message instead of a
  SearchResultDone message with resultCode success.  A cookie SHOULD be
  returned for use in subsequent Sync operations.


3.4.2. persist stage

  Change notifications are provided during the persist stage.



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  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.
  The client SHOULD use newest (last) cookie it received from the server
  in subsequent Sync operations.





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3.5.    Search Request Parameters

  As stated in Section 3.1, the client SHOULD specify the same content
  controlling parameters (see Section 3.5) 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 phases
  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
  to processing up to and including generating the Sync Info with state
  refreshDone message.





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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
  Cancel operation [CANCEL]....


3.7. Refresh Required

  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.  The client
  may then request a full reload (e.g., no cookie) instead of
  incremental synchronization in order to obtain a new copy of the
  content.  In case that the client issues incremental synchronization
  requests between the issue of a syncRefreshRequired and that of a full
  reload, the server should send a syncRefreshRequired response again,
  but the client may receive one or more searchResultEntry responses
  before it receives the syncRefreshRequired response.

  The server may also 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.



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  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 full reload operation (e.g., no cookie) in order to obtain
  a synchronized copy of the content.

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


3.8. 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 equal 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.

  The server implementor should also consider chattiness issues which
  span multiple Sync operations of a session.  As noted in Section 3.7,
  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.



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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 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 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 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.  Synchronization of operational attributes is
  discussed in Section 4.1.


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 detailed 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 require 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. 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.

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

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


7. IANA Considerations

  Registration of the following values is requested.


7.1.  Object Identifier




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

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


7.2.  LDAP Protocol Mechanism

  It is requested that IANA register upon Standards Action 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: RFCXXXX
      Author/Change Controller: IESG
      Comments: none


7.3.  LDAP Result Codes

  It is requested that IANA register upon Standards Action 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: RFCXXXX
      Author/Change Controller: IESG
      Comments:  none


8. Acknowledgment

  This work borrows significantly from the LDAP Client Update Protocol
  [LCUP].  This work also benefited Persistent Search [PSEARCH],
  Triggered Search [TSEARCH], and Directory Synchronization [DIRSYNC]



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  efforts.  This work also borrows from "Lightweight Directory Access
  Protocol (v3)" [RFC2251].


9. Normative References

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

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

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

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

  [RFC2830]    J. Hodges, R. Morgan, and M. Wahl, "Lightweight Directory
               Access Protocol (v3): Extension for Transport Layer
               Security", RFC 2830, May 2000.

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

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

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

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

  [X.680]      ITU-T, "Abstract Syntax Notation One (ASN.1) -
               Specification of Basic Notation", X.680, 1994.

  [X.690]      ITU-T, "Specification of ASN.1 encoding rules:  Basic,
               Canonical, and Distinguished Encoding Rules", X.690,
               1994.

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




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


10. Informative References

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

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

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

  [X.525]      ITU, "The Directory: Replication", ITU-T Rec. X.525,
               1993.

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

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

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

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

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

  [UUID-CSN]   K. Zeilenga, J. Choi, "LDAP UUID and CSN Operational
               Attributes", draft-zeilenga-ldap-uuid-csn-xx.txt, a work
               (not yet) in progress.


10. Authors' Address

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




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


Full Copyright

  Copyright 2003, The Internet Society.  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 implementation 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
  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.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE AUTHORS, THE INTERNET SOCIETY, AND THE INTERNET
  ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
  INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
  INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
  WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Appendix - 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 some of the implementation considerations
  associated with a Change Sequence Number [UUID-CSN] based approaches
  to supporting the LDAP Content Synchronization Operation.

  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



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  which maintain historical information should consider an 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 an entry CSN
  operational attribute for each directory entry (as discussed in [UUID-
  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.

  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



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  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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