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rfc:rfc3384

Network Working Group E. Stokes Request for Comments: 3384 IBM Category: Informational R. Weiser

                                               Digital Signature Trust
                                                              R. Moats
                                                        Lemur Networks
                                                              R. Huber
                                                     AT&T Laboratories
                                                          October 2002
         Lightweight Directory Access Protocol (version 3)
                     Replication Requirements

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This document discusses the fundamental requirements for replication
 of data accessible via the Lightweight Directory Access Protocol
 (version 3) (LDAPv3).  It is intended to be a gathering place for
 general replication requirements needed to provide interoperability
 between informational directories.

Table of Contents

 1    Introduction...................................................2
 2    Terminology....................................................3
 3    The Models.....................................................5
 4    Requirements...................................................7
 4.1  General........................................................7
 4.2  Model..........................................................8
 4.3  Protocol.......................................................9
 4.4  Schema........................................................10
 4.5  Single Master.................................................10
 4.6  Multi-Master..................................................11
 4.7  Administration and Management.................................11
 4.8  Security......................................................12
 5    Security Considerations.......................................13
 6    Acknowledgements..............................................13

Stokes, et. al. Informational [Page 1] RFC 3384 LDAPv3 Replication Requirements October 2002

 7    References....................................................13
 A    Appendix A - Usage Scenarios..................................15
 A.1  Extranet Example..............................................15
 A.2  Consolidation Example.........................................15
 A.3  Replication Heterogeneous Deployment Example..................16
 A.4  Shared Name Space Example.....................................16
 A.5  Supplier Initiated Replication................................16
 A.6  Consumer Initiated Replication................................17
 A.7  Prioritized attribute replication.............................17
 A.8  Bandwidth issues..............................................17
 A.9  Interoperable Administration and Management...................18
 A.10 Enterprise Directory Replication Mesh.........................18
 A.11 Failure of the Master in a Master-Slave Replicated Directory..19
 A.12 Failure of a Directory Holding Critical Service Information...19
 B    Appendix B - Rationale........................................20
 B.1  Meta-Data Implications........................................20
 B.2  Order of Transfer for Replicating Data........................20
 B.3  Schema Mismatches and Replication.............................21
 B.4  Detecting and Repairing Inconsistencies Among Replicas........22
 B.5  Some Test Cases for Conflict Resolution in Multi-Master
      Replication...................................................23
 B.6  Data Confidentiality and Data Integrity During Replication....27
 B.7  Failover in Single-Master Systems.............................27
 B.8  Including Operational Attributes in Atomic Operations.........29
      Authors' Addresses............................................30
      Full Copyright Statement......................................31

1 Introduction

 Distributing directory information throughout the network provides a
 two-fold benefit: (1) it increases the reliability of the directory
 through fault tolerance, and (2) it brings the directory content
 closer to the clients using the data.  LDAP's success as an access
 protocol for directory information is driving the need to distribute
 LDAP directory content within the enterprise and Internet.
 Currently, LDAP does not define a replication mechanism, and mentions
 LDAP shadow servers (see [RFC2251]) in passing.  A standard mechanism
 for directory replication in a multi-vendor environment is critical
 to the continued success of LDAP in the market place.
 This document sets out the requirements for replication between
 multiple LDAP servers.  While RFC 2251 and RFC 2252 [RFC2252] set
 forth the standards for communication between LDAP clients and
 servers there are additional requirements for server-to-server
 communication.  Some of these are covered here.
 This document first introduces the terminology to be used, then
 presents the different replication models being considered.

Stokes, et. al. Informational [Page 2] RFC 3384 LDAPv3 Replication Requirements October 2002

 Requirements follow, along with security considerations.  The
 reasoning that leads to the requirements is presented in the
 Appendices.  This was done to provide a clean separation of the
 requirements from their justification.
 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 [RFC2119].

2 Terminology

 The following terms are used in this document:
 Anonymous Replication - Replication where the endpoints are
 identified to each other but not authenticated.  Also known as
 "unauthenticated replication".
 Area of replication - A whole or portion of a Directory Information
 Tree (DIT) that makes up a distinct unit of data to be replicated.
 An area of replication is defined by a replication base entry and
 includes all or some of the depending entries contained therein on a
 single server.  It divides directory data into partitions whose
 propagation behavior may be independently configured from other
 partitions.  Areas of replication may overlap or be nested.  This is
 a subset of the definition of a "replicated area" in X.525 [X.525].
 Atomic operation - A set of changes to directory data which the LDAP
 standards guarantee will be treated as a unit; all changes will be
 made or all the changes will fail.
 Atomicity Information - Information about atomic operations passed as
 part of replication.
 Conflict - A situation that arises when changes are made to the same
 directory data on different directory servers before replication can
 synchronize the data on the servers.  When the servers do
 synchronize, they have inconsistent data - a conflict.
 Conflict resolution - Deterministic procedures used to resolve change
 information conflicts that may arise during replication.
 Critical OID - Attributes or object classes defined in the
 replication agreement as being critical to the operation of the
 system.  Changes affecting critical OIDs cause immediate initiation
 of a replica cycle.  An example of a critical OID might be a password
 or certificate.

Stokes, et. al. Informational [Page 3] RFC 3384 LDAPv3 Replication Requirements October 2002

 Fractional replication - The capability to filter a subset of
 attributes for replication.
 Incremental Update - An update that contains only those attributes or
 entries that have changed.
 Master Replica - A replica that may be directly updated via LDAP
 operations.  In a Master-Slave Replication system, the Master Replica
 is the only directly updateable replica in the replica-group.
 Master-Slave, or Single Master Replication - A replication model that
 assumes only one server, the master, allows LDAP write access to the
 replicated data.  Note that Master-Slave replication can be
 considered a proper subset of multi-master replication.
 Meta-Data - Data collected by the replication system that describes
 the status/state of replication.
 Multi-Master Replication - A replication model where entries can be
 written and updated on any of several master replica copies without
 requiring communication with other master replicas before the write
 or update is performed.
 One-way Replication  - The process of synchronization in a single
 direction where the authoritative source information is provided to a
 replica.
 Partial Replication - Partial Replication is Fractional Replication,
 Sparse Replication, or both.
 Propagation Behavior - The behavior of the synchronization process
 between a consumer and a supplier.
 Replica - An instance of an area of replication on a server.
 Replica-Group - The servers that hold instances of a particular area
 of replication.  A server may be part of several replica-groups.
 Replica (or Replication) Cycle - The interval during which update
 information is exchanged between two or more replicas.  It begins
 during an attempt to push data to, or pull data from, another replica
 or set of replicas, and ends when the data has successfully been
 exchanged or an error is encountered.
 Replication - The process of synchronizing data distributed across
 directory servers and rectifying update conflicts.

Stokes, et. al. Informational [Page 4] RFC 3384 LDAPv3 Replication Requirements October 2002

 Replication Agreement - A collection of information describing the
 parameters of replication between two or more servers in a replica-
 group.
 Replication Base Entry - The distinguished name of the root vertex of
 a replicated area.
 Replication Initiation Conflict - A Replication Initiation Conflict
 is a situation where two sources want to update the same replica at
 the same time.
 Replication Session - A session set up between two servers in a
 replica-group to pass update information as part of a replica cycle.
 Slave (or Read-Only) Replica - A replica that cannot be directly
 updated via LDAP requests.  Changes may only be made via replication
 from a master replica.  Read-only replicas may occur in both single-
 and multi-master systems.
 Sparse Replication - The capability to filter some subset of entries
 (other than a complete collection) of an area of replication.
 Topology - The shape of the directed graph describing the
 relationships between replicas.
 Two-way Replication - The process of synchronization where change
 information flows bi-directionally between two replicas.
 Unauthenticated Replication - See Anonymous Replication.
 Update Propagation - Protocol-based process by which directory
 replicas are reconciled.

3 The Models

 The objective is to provide an interoperable, LDAPv3 directory
 synchronization protocol that is simple, efficient and flexible;
 supporting both multi-master and master-slave replication.  The
 protocol must meet the needs of both the Internet and enterprise
 environments.
 There are five data consistency models.
 Model 1: Transactional Consistency -- Environments that exhibit all
 four of the ACID properties (Atomicity, Consistency, Isolation,
 Durability) [ACID].

Stokes, et. al. Informational [Page 5] RFC 3384 LDAPv3 Replication Requirements October 2002

 Model 2: Eventual (or Transient) Consistency -- Environments where
 definite knowledge of the topology is provided through predetermined
 replication agreements.  Examples include X.500 Directories (the
 X.500 model is single-master only) [X.501, X.525], Bayou [XEROX], and
 NDS (Novell Directory Services) [NDS].  In this model, every update
 propagates to every replica that it can reach via a path of stepwise
 eventual connectivity.
 Model 3: Limited Effort Eventual (or Probabilistic) Consistency --
 Environments that provide a statistical probability of convergence
 with knowledge of topology.  An example is the Xerox Clearinghouse
 [XEROX2].  This model is similar to "Eventual Consistency", except
 where replicas may purge updates.  Purging drops propagation changes
 when some replica time boundary is exceeded, thus leaving some
 changes replicated to only a portion of the topology.  Transactional
 consistency is not preserved, though some weaker constraints on
 consistency are available.
 Model 4: Loosest Consistency -- Environments where information is
 provided from an opportunistic or simple cache until stale.  Complete
 knowledge of topology may not be shared among all replicas.
 Model 5: Ad hoc -- A copy of a data store where no follow up checks
 are made for the accuracy/freshness of the data.
 Consistency models 1, 2 and 3 involve the use of prearranged
 replication agreements among servers.  While model 1 may simplify
 support for atomicity in multi-master systems, the added complexity
 of the distributed 2-phase commit required for Model 1 is
 significant; therefor, model 1 will not be considered at this time.
 Models 4 and 5 involve unregistered replicas that "pull" updates from
 another directory server without that server's knowledge.  These
 models violate a directory's security policies.
 Models 2 and 3 illustrate two replication scenarios that must be
 handled:  policy configuration through security management parameters
 (model 2), and hosting relatively static data and address information
 as in white-pages applications (model 3).  Therefore, replication
 requirements are presented for models 2 and 3.
 Interoperability among directories using LDAP replication may be
 limited for implementations that add semantics beyond those specified
 by the LDAP core documents (RFC 2251-2256, 2829, 2830).  In addition,
 the "core" specifications include numerous features which are not
 mandatory-to-implement (e.g., RECOMMENDED or OPTIONAL).  There are
 also numerous elective extensions.  Thus LDAP replication
 interoperability between independent implementations of LDAP which
 support different options may be limited.  Use of applicability

Stokes, et. al. Informational [Page 6] RFC 3384 LDAPv3 Replication Requirements October 2002

 statements to improve interoperability in particular application
 spaces is RECOMMENDED.

4 Requirements

4.1 General

 G1.  LDAP Replication MUST support models 2 (Eventual Consistency)
      and 3 (Limited Effort Eventual Consistency) above.
 G2.  LDAP Replication SHOULD NOT preclude support for model 1
      (Transactional Consistency) in the future.
 G3.  LDAP replication SHOULD have minimal impact on system
      performance.
 G4.  The LDAP Replication Standard SHOULD NOT limit the replication
      transaction rate.
 G5.  The LDAP replication standard SHOULD NOT limit the size of an
      area of replication or a replica.
 G6.  Meta-data collected by the LDAP replication mechanism MUST NOT
      grow without bound.
 G7.  All policy and state data pertaining to replication MUST be
      accessible via LDAP.
 G8.  LDAP replication MUST be capable of replicating the following:
  1. all userApplication attribute types
  1. all directoryOperation and distributedOperation attribute

types defined in the LDAP "core" specifications (RFCs 2251-

        2256, 2829-2830)
  1. attribute subtypes
  1. attribute description options (e.g., ";binary" and Language

Tags [RFC2596])

 G9.  LDAP replication SHOULD support replication of
      directoryOperation and distributedOperation attribute types
      defined in standards track LDAP extensions.
 G10. LDAP replication MUST NOT support replication of dsaOperation
      attribute types as such attributes are DSA-specific.

Stokes, et. al. Informational [Page 7] RFC 3384 LDAPv3 Replication Requirements October 2002

 G11. The LDAP replication system should limit impact on the network
      by minimizing the number of messages and the amount of traffic
      sent.

4.2 Model

 M1.  The model MUST support the following triggers for initiation of
      a replica cycle:
      a) A configurable set of scheduled times
      b) Periodically, with a configurable period between replica
         cycles
      c) A configurable maximum amount of time between replica cycles
      d) A configurable number of accumulated changes
      e) Change in the value of a critical OID
      f) As the result of an automatic rescheduling after a
         replication initiation conflict
      g) A manual request for immediate replication
      With the exception of manual request, the specific trigger(s)
      and related parameters for a given server MUST be identified in
      a well-known place defined by the standard, e.g., the
      Replication Agreement(s).
 M2.  The replication model MUST support both master-slave and multi-
      master relationships.
 M3.  An attribute in an entry MUST eventually converge to the same
      set of values in every replica holding that entry.
 M4.  LDAP replication MUST encompass schema definitions, attribute
      names and values, access control information, knowledge
      information, and name space information.
 M5.  LDAP replication MUST NOT require that all copies of the
      replicated information be complete, but MAY require that at
      least one copy be complete.  The model MUST support Partial
      Replicas.
 M6.  The determination of which OIDs are critical MUST be
      configurable in the replication agreement.

Stokes, et. al. Informational [Page 8] RFC 3384 LDAPv3 Replication Requirements October 2002

 M7.  The parameters of the replication process among the members of
      the replica-group, including access parameters, necessary
      authentication credentials, assurances of confidentiality
      (encryption), and area(s) of replication MUST be defined in a
      standard location (e.g., the replication agreements).
 M8.  The replication agreements SHOULD accommodate multiple servers
      receiving the same area of replication under a single predefined
      agreement.
 M9.  LDAP replication MUST provide scalability to both enterprise and
      Internet environments, e.g., an LDAP server must be able to
      provide replication services to replicas within an enterprise as
      well as across the Internet.
 M10. While different directory implementations can support
      different/extended schema, schema mismatches between two
      replicating servers MUST be handled.  One way of handling such
      mismatches might be to raise an error condition.
 M11. There MUST be a facility that can update, or totally refresh, a
      replica-group from a standard data format, such as LDIF format
      [RFC2849].
 M12. An update received by a consumer more than once MUST NOT produce
      a different outcome than if the update were received only once.

4.3 Protocol

 P1.  The replication protocol MUST provide for recovery and
      rescheduling of a replication session due to replication
      initiation conflicts (e.g., consumer busy replicating with other
      servers) and or loss of connection (e.g., supplier cannot reach
      a replica).
 P2.  LDUP replication SHOULD NOT send an update to a consumer if the
      consumer has previously acknowledged that update.
 P3.  The LDAP replication protocol MUST allow for full update to
      facilitate replica initialization and reset loading utilizing a
      standardized format such as LDIF [RFC2849] format.
 P4.  Incremental replication MUST be allowed.
 P5.  The replication protocol MUST allow either a master or slave
      replica to initiate the replication process.

Stokes, et. al. Informational [Page 9] RFC 3384 LDAPv3 Replication Requirements October 2002

 P6.  The protocol MUST preserve atomicity of LDAP operations as
      defined in RFC2251 [RFC2251].  In a multi-master environment
      this may lead to an unresolvable conflict.  MM5 and MM6 discuss
      how to handle this situation.
 P7.  The protocol MUST support a mechanism to report schema
      mismatches between replicas discovered during a replication
      session.

4.4 Schema

 SC1.  A standard way to determine what replicas are held on a server
       MUST be defined.
 SC2.  A standard schema for representing replication agreements MUST
       be defined.
 SC3.  The semantics associated with modifying the attributes of
       replication agreements MUST be defined.
 SC4.  A standard method for determining the location of replication
       agreements MUST be defined.
 SC5.  A standard schema for publishing state information about a
       given replica MUST be defined.
 SC6.  A standard method for determining the location of replica state
       information MUST be defined.
 SC7.  It MUST be possible for appropriately authorized
       administrators, regardless of their network location, to access
       replication agreements in the DIT.
 SC8.  Replication agreements of all servers containing replicated
       information MUST be accessible via LDAP.
 SC9.  An entry MUST be uniquely identifiable throughout its lifetime.

4.5 Single Master

 SM1.  A Single Master system SHOULD provide a fast method of
       promoting a slave replica to become the master replica.

Stokes, et. al. Informational [Page 10] RFC 3384 LDAPv3 Replication Requirements October 2002

 SM2.  The master replica in a Single Master system SHOULD send all
       changes to read-only replicas in the order in which the master
       applied them.

4.6 Multi-Master

 MM1.  The replication protocol SHOULD NOT saturate the network with
       redundant or unnecessary entry replication.
 MM2.  The initiator MUST be allowed to determine whether it will
       become a consumer or supplier during the synchronization
       startup process.
 MM3.  During a replica cycle, it MUST be possible for the two servers
       to switch between the consumer and supplier roles.
 MM4.  When multiple master replicas want to start a replica cycle
       with the same replica at the same time, the model MUST have an
       automatic and deterministic mechanism for resolving or avoiding
       replication initiation conflict.
 MM5.  Multi-master replication MUST NOT lose information during
       replication.  If conflict resolution would result in the loss
       of directory information, the replication process MUST store
       that information, notify the administrator of the nature of the
       conflict and the information that was lost, and provide a
       mechanism for possible override by the administrator.
 MM6.  Multi-master replication MUST support convergence of the values
       of attributes and entries.  Convergence may result in an event
       as described in MM5.
 MM7.  Multi-master conflict resolution MUST NOT depend on the in-
       order arrival of changes at a replica to assure eventual
       convergence.
 MM8.  Multi-master replication MUST support read-only replicas as
       well as read-write replicas.

4.7 Administration and Management

 AM1.  Replication agreements MUST allow the initiation of a replica
       cycle to be administratively postponed to a more convenient
       period.
 AM2.  Each copy of a replica MUST maintain audit history information
       of which servers it has replicated with and which servers have
       replicated with it.

Stokes, et. al. Informational [Page 11] RFC 3384 LDAPv3 Replication Requirements October 2002

 AM3.  Access to replication agreements, topologies, and policy
       attributes MUST be provided through LDAP.
 AM4.  The capability to check the differences between two replicas
       for the same information SHOULD be provided.
 AM5.  A mechanism to fix differences between replicas without
       triggering new replica cycles SHOULD be provided.
 AM6.  The sequence of updates to access control information (ACI) and
       the data controlled by that ACI MUST be maintained by
       replication.
 AM7.  It MUST be possible to add a 'blank' replica to a replica-
       group, and force a full update from (one of) the Master(s), for
       the purpose of adding a new directory server to the system.
 AM8.  Vendors SHOULD provide tools to audit schema compatibility
       within a potential replica-group.

4.8 Security

 The terms "data confidentiality" and "data integrity" are defined in
 the Internet Security Glossary [RFC2828].
 S1.  The protocol MUST support mutual authentication of the source
      and the replica directories during initialization of a
      replication session.
 S2.  The protocol MUST support mutual verification of authorization
      of the source to send and the replica to receive replicated data
      during initialization of a replication session.
 S3.  The protocol MUST also support the initialization of anonymous
      replication sessions.
 S4.  The replication protocol MUST support transfer of data with data
      integrity and data confidentiality.
 S5.  The replication protocol MUST support the ability during
      initialization of a replication session for an authenticated
      source and replica to mutually decide to disable data integrity
      and data confidentiality within the context of and for the
      duration of that particular replication session.
 S6.  To promote interoperability, there MUST be a mandatory-to-
      implement data confidentiality mechanism.

Stokes, et. al. Informational [Page 12] RFC 3384 LDAPv3 Replication Requirements October 2002

 S7.  The transport for administrative access MUST permit assurance of
      the integrity and confidentiality of all data transferred.
 S8.  To support data integrity, there must be a mandatory-to-
      implement data integrity mechanism.

5 Security Considerations

 This document includes security requirements (listed in section 4.8
 above) for the replication model and protocol.  As noted in Section
 3, interoperability may be impacted when replicating among servers
 that implement non-standard extensions to basic LDAP semantics.
 Security-related and general LDAP interoperability will be
 significantly impacted by the degree of consistency with which
 implementations support existing and future standards detailing LDAP
 security models, such as a future standard LDAP access control model.

6 Acknowledgements

 This document is based on input from IETF members interested in LDUP
 Replication.

7 References

 [ACID]    T. Haerder, A. Reuter, "Principles of Transaction-Oriented
           Database Recovery", Computing Surveys, Vol. 15, No. 4
           (December 1983), pp. 287-317.
 [NDS]     Novell, "NDS Technical Overview", 104-000223-001,
           http://developer.novell.com/ndk/doc/ndslib/dsov_enu/data/
           h6tvg4z7.html, September, 2000.
 [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
           Access Protocol", RFC 2251, December 1997.
 [RFC2252] Wahl, M., Coulbeck, A., Howes, T. and S. Kille,
           "Lightweight Directory Access Protocol (v3): Attribute
           Syntax Definitions", RFC 2252, December 1997.
 [RFC2253] Kille, S., Wahl, M. and T. Howes, "Lightweight Directory
           Access Protocol (v3): UTF-8 String Representation of
           Distinguished Names", RFC 2253, December 1997.
 [RFC2254] Howes, T., "The String Representation of LDAP Search
           Filters", RFC 2254, December 1997.

Stokes, et. al. Informational [Page 13] RFC 3384 LDAPv3 Replication Requirements October 2002

 [RFC2255] Howes, T. and M. Smith, "The LDAP URL Format", RFC 2255,
           December 1997.
 [RFC2256] Wahl, M., "A Summary of the X.500(96) User Schema for use
           with LDAPv3", RFC 2256, December 1997.
 [RFC2596] Wahl, M. and T. Howes, "Use of Language Codes in LDAP", RFC
           2596, May 1999.
 [RFC2828] Shirey, R. "Internet Security Glossary", FYI 36, RFC 2828,
           May 2000.
 [RFC2829] Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
           "Authentication Methods for LDAP", RFC 2829, May 2000.
 [RFC2830] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory
           Access Protocol (v3): Extension for Transport Layer
           Security", RFC 2830, May 2000.
 [RFC2849] Good, G., "The LDAP Data Interchange Format (LDIF)", RFC
           2849, June 2000.
 [X.501]   ITU-T Recommendation X.501 (1993), | ISO/IEC 9594-2: 1993,
           Information Technology - Open Systems Interconnection - The
           Directory: Models.
 [X.525]   ITU-T Recommendation X.525 (1997), | ISO/IEC 9594-9: 1997,
           Information Technology - Open Systems Interconnection - The
           Directory: Replication.
 [XEROX]   C. Hauser, "Managing update conflicts in Bayou, a weakly
           connected replicated storage system". Palo Alto, CA: Xerox
           PARC, Computer Science Laboratory; 1995 August; CSL-95-4.
 [XEROX2]  Alan D. Demers, Mark Gealy, Daniel Greene, Carl Hauser,
           Wesley Irish, John Larson, Sue Manning, Scott Shenker,
           Howard Sturgis, Daniel Swinehart, Douglas Terry, Don Woods,
           "Epidemic Algorithms for Replicated Database Maintenance".
           Palo Alto, CA, Xerox PARC, January 1989.

Stokes, et. al. Informational [Page 14] RFC 3384 LDAPv3 Replication Requirements October 2002

A. APPENDIX A - Usage Scenarios

 The following directory deployment examples are intended to validate
 our replication requirements.  A heterogeneous set of directory
 implementations is assumed for all the cases below.  This material is
 intended as background; no requirements are presented in this
 Appendix.

A.1. Extranet Example

 A company has a trading partner with whom it wishes to share
 directory information.  This information may be as simple as a
 corporate telephone directory, or as complex as an extranet workflow
 application.  For performance reasons, the company wishes to place a
 replica of its directory within the Partner Company, rather than
 exposing its directory beyond its firewall.
 The requirements that follow from this scenario are:
  1. One-way replication, single mastered.
  1. Authentication of clients.
  1. Common access control and access control identification.
  1. Secure transmission of updates.
  1. Selective attribute replication (Fractional Replication), so that

only partial entries can be replicated.

A.2. Consolidation Example

 Company A acquires company B.  Each company has an existing
 directory.
 During the transition period, as the organizations are merged, both
 directory services must coexist.  Company A may wish to attach
 company B's directory to its own.
 The requirements that follow from this scenario are:
  1. Multi-Master replication.
  1. Common access control model. Access control model identification.
  1. Secure transmission of updates.
  1. Replication between DITs with potentially differing schema.

Stokes, et. al. Informational [Page 15] RFC 3384 LDAPv3 Replication Requirements October 2002

A.3. Replication Heterogeneous Deployment Example

 An organization may choose to deploy directory implementations from
 multiple vendors, to enjoy the distinguishing benefits of each.
 In this case, multi-master replication is required to ensure that the
 multiple replicas of the DIT are synchronized.  Some vendors may
 provide directory clients, which are tied to their own directory
 service.
 The requirements that follow from this scenario are:
  1. Multi-Master replication
  1. Common access control model and access control model

identification.

  1. Secure transmission of updates.
  1. Replication among DITs with potentially differing schemas.

A.4. Shared Name Space Example

 Two organizations may choose to cooperate on some venture and need a
 shared name space to manage their operation.  Both organizations will
 require administrative rights over the shared name space.
 The requirements that follow from this scenario are:
  1. Multi-Master replication.
  1. Common access control model and access control model

identification.

  1. Secure transmission of updates.

A.5. Supplier Initiated Replication

 This is a single master environment that maintains a number of
 replicas of the DIT by pushing changes based on a defined schedule.
 The requirements that follow from this scenario are:
  1. Single-master environment.
  1. Supplier-initiated replication.
  1. Secure transmission of updates.

Stokes, et. al. Informational [Page 16] RFC 3384 LDAPv3 Replication Requirements October 2002

A.6. Consumer Initiated Replication

 Again a single mastered replication topology, but the slave replica
 initiates the replication exchange rather than the master.  An
 example of this is a replica that resides on a laptop computer that
 may run disconnected for a period of time.
 The requirements that follow from this scenario are:
  1. Single-master environment.
  1. Consumer initiated replication.
  1. Open scheduling (anytime).

A.7. Prioritized attribute replication

 The password attribute can provide an example of the requirement for
 prioritized attribute replication.  A user is working in Utah and the
 administrator resides in California.  The user has forgotten his
 password.  So the user calls or emails the administrator to request a
 new password.  The administrator provides the updated password (a
 change).
 Under normal conditions, the directory replicates to a number of
 different locations overnight.  But corporate security policy states
 that passwords are critical and the new value must be available
 immediately (e.g., shortly) after any change.  Replication needs to
 occur immediately for critical attributes/entries.
 The requirements that follow from this scenario are:
  1. Incremental replication of changes.
  1. Immediate replication on change of certain attributes.
  1. Replicate based on time/attribute semantics.

A.8. Bandwidth issues

 The replication of Server (A) R/W replica (a) in Kathmandu is handled
 via a dial up phone link to Paris where server (B) R/W replica of (a)
 resides.  Server (C) R/W replica of (a) is connected by a T1
 connection to server (B).  Each connection has a different
 performance characteristic.

Stokes, et. al. Informational [Page 17] RFC 3384 LDAPv3 Replication Requirements October 2002

 The requirements that follow from this scenario are:
  1. Minimize repetitive updates when replicating from multiple

replication paths.

  1. Incremental replication of changes.
  1. Provide replication cycles to delay and/or retry when connections

cannot be reached.

  1. Allowances for consumer initiated or supplier initiated

replication.

A.9. Interoperable Administration and Management

 The administrator with administrative authority of the corporate
 directory which is replicated by numerous geographically dispersed
 LDAP servers from different vendors notices that the replication
 process is not completing correctly as the change log is continuing
 to grow and/or error messages inform him.  The administrator uses his
 $19.95 RepCo LDAP directory replication diagnostic tools to look at
 Root DSE replica knowledge on server 17 and determines that server 42
 made by LDAP'RUS Inc. is not replicating properly due to an object
 conflict.  Using his Repco Remote repair tools he connects to server
 42 and resolves the conflict on the remote server.
 The requirements that follow from this scenario are:
  1. Provide replication audit history.
  1. Provide mechanisms for managing conflict resolution.
  1. Provide LDAP access to predetermined agreements, topology and

policy attributes.

  1. Provide operations for comparing replica's content for validity.
  1. Provide LDAP access to status and audit information.

A.10. Enterprise Directory Replication Mesh

 A Corporation builds a mesh of directory servers within the
 enterprise utilizing LDAP servers from various vendors.  Five servers
 are holding the same area of replication.  The predetermined
 replication agreement(s) for the enterprise mesh are under a single
 management, and the security domain allows a single predetermined
 replication agreement to manage the 5 servers' replication.

Stokes, et. al. Informational [Page 18] RFC 3384 LDAPv3 Replication Requirements October 2002

 The requirements that follow from this scenario are:
  1. One predefined replication agreement that manages a single area of

replication that is held on numerous servers.

  1. Common support of replication management knowledge across vendor

implementation.

  1. Rescheduling and continuation of a replication cycle when one

server in a replica-group is busy and/or unavailable.

A.11. Failure of the Master in a Master-Slave Replicated Directory

 A company has a corporate directory that is used by the corporate
 email system.  The directory is held on a mesh of servers from
 several vendors.  A corporate relocation results in the closing of
 the location where the master copy of the directory is located.
 Employee information (such as mailbox locations and employee
 certificate information) must be kept up to date or mail cannot be
 delivered.
 The requirements that follow from this scenario are:
  1. An existing slave replica must be "promote-able" to become the new

master.

  1. The "promotion" must be done without significant downtime, since

updates to the directory will continue.

A.12. Failure of a Directory Holding Critical Service Information

 An ISP uses a policy management system that uses a directory as the
 policy data repository.  The directory is replicated in several
 different sites on different vendors' products to avoid single points
 of failure.  It is imperative that the directory be available and be
 updateable even if one site is disconnected from the network.
 Changes to the data must be traceable, and it must be possible to
 determine how changes made from different sites interacted.
 The requirements that follow from this scenario are:
  1. Multi-master replication.
  1. Ability to reschedule replication sessions.
  1. Support for manual review and override of replication conflict

resolution.

Stokes, et. al. Informational [Page 19] RFC 3384 LDAPv3 Replication Requirements October 2002

B. APPENDIX B - Rationale

 This Appendix gives some of the background behind the requirements.
 It is included to help the protocol designers understand the thinking
 behind some of the requirements and to present some of the issues
 that should be considered during design.  With the exception of
 section B.8, which contains a suggested requirement for the update to
 RFC 2251, this Appendix does not state any formal requirements.

B.1. Meta-Data Implications

 Requirement G4 states that meta-data must not grow without bound.
 This implies that meta-data must, at some point, be purged from the
 system.  This, in turn, raises concerns about stability.  Purging
 meta-data before all replicas have been updated may lead to
 incomplete replication of change information and inconsistencies
 among replicas.  Therefore, care must be taken setting up the rules
 for purging meta-data from the system while still ensuring that
 meta-data will not grow forever.

B.2. Order of Transfer for Replicating Data

 Situations may arise where it would be beneficial to replicate data
 out-of-order (e.g., send data to consumer replicas in a different
 order than it was processed at the supplier replica).  One such case
 might occur if a large bulk load was done on the master server in a
 single-master environment and then a single change to a critical OID
 (a password change, for example) was then made.  Rather than wait for
 all the bulk data to be sent to the replicas, the password change
 might be moved to the head of the queue and be sent before all the
 bulk data was transferred.  Other cases where this might be
 considered are schema changes or changes to critical policy data
 stored in the directory.
 While there are practical benefits to allowing out-of-order transfer,
 there are some negative consequences as well.  Once out-of-order
 transfers are permitted, all receiving replicas must be prepared to
 deal with data and schema conflicts that might arise.
 As an example, assume that schema changes are critical and must be
 moved to the front of the replication queue.  Now assume that a
 schema change deletes an attribute for some object class.  It is
 possible that some of the operations ahead of the schema change in
 the queue are operations to delete values of the soon-to-be-deleted

Stokes, et. al. Informational [Page 20] RFC 3384 LDAPv3 Replication Requirements October 2002

 attribute so that the schema change can be done with no problems.  If
 the schema change moves to the head of the queue, the consumer
 servers might have to delete an attribute that still has values, and
 then receive requests to delete the values of an attribute that is no
 longer defined.
 In the multi-master case, similar situations can arise when
 simultaneous changes are made to different replicas.  Thus, multi-
 master systems must have conflict resolution algorithms in place to
 handle such situations.  But in the single-master case conflict
 resolution is not needed unless the master is allowed to send data
 out-of-order.  This is the reasoning behind requirement SM2, which
 recommends that data always be sent in order in single-master
 replication.
 Note that even with this restriction, the concept of a critical OID
 is still useful in single-master replication.  An example of its
 utility can be found in section A.7.

B.3. Schema Mismatches and Replication

 Multi-vendor environments are the primary area of interest for LDAP
 replication standards.  Some attention must thus be paid to the issue
 of schema mismatches, since they can easily arise when vendors
 deliver slightly different base schema with their directory products.
 Even when both products meet the requirements of the standards
 [RFC2252], the vendors may have included additional attributes or
 object classes with their products.  When two different vendors'
 products attempt to replicate, these additions can cause schema
 mismatches.  Another potential cause of schema mismatches is
 discussed in section A.3.
 There are only a few possible responses when a mismatch is
 discovered.
  1. Raise an error condition and ignore the data. This should always

be allowed and is the basis for requirement P8 and the comment on

   M10.
  1. Map/convert the data to the form required by the consuming replica.

A system may choose this course; requirement M10 is intended to

   allow this option.  The extent of the conversion is up to the
   implementation; in the extreme it could support use of the
   replication protocol in meta-directories.
  1. Quietly ignore (do not store on the consumer replica and do not

raise an error condition) any data that does not conform to the

   schema at the consumer.

Stokes, et. al. Informational [Page 21] RFC 3384 LDAPv3 Replication Requirements October 2002

 Requirement M10 is intended to exclude the last option.
 Requirement AM8 suggests that vendors should provide tools to help
 discover schema mismatches when replication is being set up.  But
 schema will change after the initial setup, so the replication system
 must be prepared to handle unexpected mismatches.
 Normal IETF practice in protocol implementation suggests that one be
 strict in what one sends and be flexible in what one receives.  The
 parallel in this case is that a supplier should be prepared to
 receive an error notification for any schema mismatch, but a consumer
 may choose to do a conversion instead.
 The other option that can be considered in this situation is the use
 of fractional replication.  If replication is set up so only the
 common attributes are replicated, mismatches can be avoided.
 One additional consideration here is replication of the schema
 itself.  M4 requires that it be possible to replicate schema.  If a
 consumer replica is doing conversion, extreme care should be taken if
 schema elements are replicated since some attributes are intended to
 have different definitions on different replicas.
 For fractional replication, the protocol designers and implementors
 should give careful consideration to the way they handle schema
 replication.  Some options for schema replication include:
  1. All schema elements are replicated.
  1. Schema elements are replicated only if they are used by attributes

that are being replicated.

  1. Schema are manually configured on the servers involved in

fractional replication; schema elements are not replicated via the

   protocol.

B.4. Detecting and Repairing Inconsistencies Among Replicas

 Despite the best efforts of designers, implementors, and operators,
 inconsistencies will occasionally crop up among replicas in
 production directories.  Tools will be needed to detect and to
 correct these inconsistencies.

Stokes, et. al. Informational [Page 22] RFC 3384 LDAPv3 Replication Requirements October 2002

 A special client may accomplish detection through periodic
 comparisons of replicas.  This client would typically read two
 replicas of the same replication base entry and compare the answers,
 possibly by BINDing to each of the two replicas to be compared and
 reading them both.  In cases where the directory automatically
 reroutes some requests (e.g., chaining), mechanisms to force access
 to a particular replica should be supplied.
 Alternatively, the server could support a special request to handle
 this situation.  A client would invoke an operation at some server.
 It would cause that server to extract the contents from some other
 server it has a replication agreement with and report the differences
 back to the client as the result.
 If an inconsistency is found, it needs to be repaired.  To determine
 the appropriate repair, the administrator will need access to the
 replication history to figure out how the inconsistency occurred and
 what the correct repair should be.
 When a repair is made, it should be restricted to the replica that
 needs to be fixed; the repair should not cause new replication events
 to be started.  This may require special tools to change the local
 data store without triggering replication.
 Requirements AM2, AM4, and AM5 address these needs.

B.5. Some Test Cases for Conflict Resolution in Multi-Master Replication

 Use of multi-master replication inevitably leads to the possibility
 that incompatible changes will be made simultaneously on different
 servers.  In such cases, conflict resolution algorithms must be
 applied.
 As a guiding principle, conflict resolution should avoid surprising
 the user.  One way to do this is to adopt the principle that, to the
 extent possible, conflict resolution should mimic the situation that
 would happen if there were a single server where all the requests
 were handled.
 While this is a useful guideline, there are some situations where it
 is impossible to implement.  Some of these cases are examined in this
 section.  In particular, there are some cases where data will be
 "lost" in multi-master replication that would not be lost in a
 single-server configuration.

Stokes, et. al. Informational [Page 23] RFC 3384 LDAPv3 Replication Requirements October 2002

 In the examples below, assume that there are three replicas, A, B,
 and C.  All three replicas are updateable.  Changes are made to
 replicas A and B before replication allows either replica to see the
 change made on the other.  In discussion of the multi-master cases,
 we assume that the change to A takes precedence using whatever rules
 are in force for conflict resolution.

B.5.1. Create-Create

 A user creates a new entry with distinguished name DN on A.  At the
 same time, a different user adds an entry with the same distinguished
 name on B.
 In the single-server case, one of the create operations would have
 occurred before the other, and the second request would have failed.
 In the multi-master case, each create was successful on its
 originating server.  The problem is not detected until replication
 takes place.  When a replication request to create a DN that already
 exists arrives at one of the servers, conflict resolution is invoked.
 (Note that the two requests can be distinguished even though they
 have the same DN because every entry has some sort of unique
 identifier per requirement SC9.)
 As noted above, in these discussions we assume that the change from
 replica A has priority based on the conflict resolution algorithm.
 Whichever change arrives first, requirement MM6 says that the values
 from replica A must be those in place on all replicas at the end of
 the replication cycle.  Requirement MM5 states that the system cannot
 quietly ignore the values from replica B.
 The values from replica B might be logged with some notice to the
 administrators, or they might be added to the DIT with a machine
 generated DN (again with notice to the administrators).  If they are
 stored with a machine generated DN, the same DN must be used on all
 servers in the replica-group (otherwise requirement M3 would be
 violated).  Note that in the case where the entry in question is a
 container, storage with a machine generated DN provides a place where
 descendent entries may be stored if any descendents were generated
 before the replication cycle was completed.
 In any case, some mechanism must be provided to allow the
 administrator to reverse the conflict resolution algorithm and force
 the values originally created on B into place on all replicas if
 desired.

Stokes, et. al. Informational [Page 24] RFC 3384 LDAPv3 Replication Requirements October 2002

B.5.2. Rename-Rename

 On replica A, an entry with distinguished name DN1 is renamed to DN.
 At the same time on replica B, an entry with distinguished name DN2
 is renamed to DN.
 In the single-server case, one rename operation would occur before
 the other and the second would fail since the target name already
 exists.
 In the multi-master case, each rename was successful on its
 originating server.  Assuming that the change on A has priority in
 the conflict resolution sense, DN will be left with the values from
 DN1 in all replicas and DN1 will no longer exist in any replica.  The
 question is what happens to DN2 and its original values.
 Requirement MM5 states that these values must be stored somewhere.
 They might be logged, they might be left in the DIT as the values of
 DN2, or they might be left in the DIT as the values of some machine
 generated DN.  Leaving them as the values of DN2 is attractive since
 it is the same as the single-server case, but if a new DN2 has
 already been created before the replica cycle finishes, there are
 some very complex cases to resolve.  Any of the solutions described
 in this paragraph would be consistent with requirement MM5.

B.5.3. Locking Based on Atomicity of ModifyRequest

 There is an entry with distinguished name DN that contains attributes
 X, Y, and Z.  The value of X is 1.  On replica A, a ModifyRequest is
 processed which includes modifications to change that value of X from
 1 to 0 and to set the value of Y to "USER1".  At the same time,
 replica B processes a ModifyRequest which includes modifications to
 change the value of X from 1 to 0 and to set the value of Y to
 "USER2" and the value of Z to 42.  The application in this case is
 using X as a lock and is depending on the atomic nature of
 ModifyRequests to provide mutual exclusion for lock access.
 In the single-server case, the two operations would have occurred
 sequentially.  Since a ModifyRequest is atomic, the entire first
 operation would succeed.  The second ModifyRequest would fail, since
 the value of X would be 0 when it was attempted, and the modification
 changing X from 1 to 0 would thus fail.  The atomicity rule would
 cause all other modifications in the ModifyRequest to fail as well.
 In the multi-master case, it is inevitable that at least some of the
 changes will be reversed despite the use of the lock.  Assuming the
 changes from A have priority per the conflict resolution algorithm,
 the value of X should be 0 and the value of Y should be "USER1" The

Stokes, et. al. Informational [Page 25] RFC 3384 LDAPv3 Replication Requirements October 2002

 interesting question is the value of Z at the end of the replication
 cycle.  If it is 42, the atomicity constraint on the change from B
 has been violated.  But for it to revert to its previous value,
 grouping information must be retained and it is not clear when that
 information can be safely discarded.  Thus, requirement G6 may be
 violated.

B.5.4. General Principles

 With multi-master replication there are a number of cases where a
 user or application will complete a sequence of operations with a
 server but those actions are later "undone" because someone else
 completed a conflicting set of operations at another server.
 To some extent, this can happen in any multi-user system.  If a user
 changes the value of an attribute and later reads it back,
 intervening operations by another user may have changed the value.
 In the multi-master case, the problem is worsened, since techniques
 used to resolve the problem in the single-server case won't work as
 shown in the examples above.
 The major question here is one of intended use.  In LDAP standards
 work, it has long been said that replication provides "loose
 consistency" among replicas.  At several IETF meetings and on the
 mailing list, usage examples from finance where locking is required
 have been declared poor uses for LDAP.  Requirement G1 is consistent
 with this history.  But if loose consistency is the goal, the locking
 example above is an inappropriate use of LDAP, at least in a
 replicated environment.

B.5.5. Avoiding the Problem

 The examples above discuss some of the most difficult problems that
 can arise in multi-master replication.  While they can be dealt with,
 dealing with them is difficult and can lead to situations that are
 quite confusing to the application and to users.
 The common characteristics of the examples are:
  1. Several directory users/applications are changing the same data.
  1. They are changing the data before previous changes have replicated.
  1. They are using different directory servers to make these changes.
  1. They are changing data that are parts of a distinguished name or

they are using ModifyRequest to both read and write a given

   attribute value in a single atomic request.

Stokes, et. al. Informational [Page 26] RFC 3384 LDAPv3 Replication Requirements October 2002

 If any one of these conditions is reversed, the types of problems
 described above will not occur.  There are many useful applications
 of multi-master directories where at least one of the above
 conditions does not occur.  For cases where all four do occur,
 application designers should be aware of the possible consequences.

B.6. Data Confidentiality and Data Integrity During Replication

 Directories will frequently hold proprietary information.  Policy
 information, name and address information, and customer lists can be
 quite proprietary and are likely to be stored in directories.  Such
 data must be protected against intercept or modification during
 replication.
 In some cases, the network environment (e.g., a private network) may
 provide sufficient data confidentiality and integrity for the
 application.  In other cases, the data in the directory may be public
 and not require protection.  For these reasons data confidentiality
 and integrity were not made requirements for all replication
 sessions.  But there are a substantial number of applications that
 will need data confidentiality and integrity for replication, so
 there is a requirement (S4) that the protocol allow for data
 confidentiality and integrity in those cases where they are needed.
 Typically, the policy on the use of confidentiality and integrity
 measures would be held in the replication agreement per requirement
 M7.
 This leaves the question of what mechanism(s) to use.  While this is
 ultimately a design/implementation decision, replication across
 different vendors' directory products is an important goal of the
 LDAP replication work at the IETF.  If different vendors choose to
 support different data confidentiality and integrity mechanisms, the
 advantages of a standard replication protocol would be lost.  Thus
 there is a requirement (S6) for mandatory-to-implement data
 confidentiality and integrity mechanisms.
 Anonymous replication (requirement S3) is supported since it may be
 useful in the same sorts of situations where data integrity and data
 confidentiality protection are not needed.

B.7. Failover in Single-Master Systems

 In a single-master system, all modifications must originate at the
 master.  The master is therefore a single point of failure for
 modifications.  This can cause concern when high availability is a
 requirement for the directory system.

Stokes, et. al. Informational [Page 27] RFC 3384 LDAPv3 Replication Requirements October 2002

 One way to reduce the problem is to provide a failover process that
 converts a slave replica to master when the original master fails.
 The time required to execute the failover process then becomes a
 major factor in availability of the system as a whole.
 Factors that designers and implementors should consider when working
 on failover include:
  1. If the master replica contains control information or meta-data

that is not part of the slave replica(s), this information will

   have to be inserted into the slave that is being "promoted" to
   master as part of the failover process.  Since the old master is
   presumably unavailable at this point, it may be difficult to obtain
   this data.  For example, if the master holds the status information
   of all replicas, but each slave replica only holds its own status
   information, failover would require that the new master get the
   status of all existing replicas, presumably from those replicas.
   Similar issues could arise for replication agreements if the master
   is the only system that holds a complete set.
  1. If data privacy mechanisms (e.g., encryption) are in use during

replication, the new master would need to have the necessary key

   information to talk to all of the slave replicas.
  1. It is not only the new master that needs to be reconfigured. The

slaves also need to have their configurations updated so they know

   where updates should come from and where they should refer
   modifications.
  1. The failover mechanism should be able to handle a situation where

the old master is "broken" but not "dead". The slave replicas

   should ignore updates from the old master after failover is
   initiated.
  1. The old master will eventually be repaired and returned to the

replica-group. It might join the group as a slave and pick up the

   changes it has "missed" from the new master, or there might be some
   mechanism to bring it into sync with the new master and then let it
   take over as master.  Some resynchronization mechanism will be
   needed.
  1. Availability would be maximized if the whole failover process could

be automated (e.g., failover is initiated by an external system

   when it determines that the original master is not functioning
   properly).

Stokes, et. al. Informational [Page 28] RFC 3384 LDAPv3 Replication Requirements October 2002

B.8. Including Operational Attributes in Atomic Operations

 LDAPv3 [RFC2251] declares that some operations are atomic (e.g., all
 of the modifications in a single ModifyRequest).  It also defines
 several operational attributes that store information about when
 changes are made to the directory (createTimestamp, etc.) and which
 ID was responsible for a given change (modifiersName, etc.).
 Currently, there is no statement in RFC2251 requiring that changes to
 these operational attributes be atomic with the changes to the data.
 It is RECOMMENDED that this requirement be added during the revision
 of RFC2251.  In the interim, replication SHOULD treat these
 operations as though such a requirement were in place.

Stokes, et. al. Informational [Page 29] RFC 3384 LDAPv3 Replication Requirements October 2002

Authors' Addresses

 Russel F. Weiser
 Digital Signature Trust Co.
 1095 East 2100 South
 Suite #201
 Salt Lake City, UT 84106
 Phone: +1 801 326 5421
 Fax:  +1 801 326 5421
 EMail: rweiser@trustdst.com
 Ellen J. Stokes
 IBM
 11400 Burnet Rd.
 Austin, TX  78758
 Phone: +1 512 436 9098
 Fax: +1 512 436 1193
 EMail: stokese@us.ibm.com
 Ryan D. Moats
 Lemur Networks
 15621 Drexel Circle
 Omaha, NE  68135
 Phone: +1 402 894 9456
 EMail: rmoats@lemurnetworks.net
 Richard V. Huber
 Room C3-3B30
 AT&T Laboratories
 200 Laurel Avenue South
 Middletown, NJ  07748
 Phone: +1 732 420 2632
 Fax: +1 732 368 1690
 EMail: rvh@att.com

Stokes, et. al. Informational [Page 30] RFC 3384 LDAPv3 Replication Requirements October 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Stokes, et. al. Informational [Page 31]

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