RFC3384: Lightweight Directory Access Protocol (version 3) Replication Requirements

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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.


   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

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   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
   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.

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   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",
   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.

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   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

   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.

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   Replication Agreement - A collection of information describing the
   parameters of replication between two or more servers in a replica-

   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

   There are five data consistency models.

   Model 1: Transactional Consistency -- Environments that exhibit all
   four of the ACID properties (Atomicity, Consistency, Isolation,
   Durability) [ACID].

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   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

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   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

   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:

        - all userApplication attribute types

        - all directoryOperation and distributedOperation attribute
          types defined in the LDAP "core" specifications (RFCs 2251-
          2256, 2829-2830)

        - attribute subtypes

        - 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.

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

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

        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

   M6.  The determination of which OIDs are critical MUST be
        configurable in the replication agreement.

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   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

   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

   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.

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   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

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.

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   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

   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

   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.

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   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

   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.

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   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

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,
             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.

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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.

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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

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:

   - One-way replication, single mastered.

   - Authentication of clients.

   - Common access control and access control identification.

   - Secure transmission of updates.

   - 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

   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:

   - Multi-Master replication.

   - Common access control model. Access control model identification.

   - Secure transmission of updates.

   - Replication between DITs with potentially differing schema.

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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

   The requirements that follow from this scenario are:

   - Multi-Master replication

   - Common access control model and access control model

   - Secure transmission of updates.

   - 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:

   - Multi-Master replication.

   - Common access control model and access control model

   - 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:

   - Single-master environment.

   - Supplier-initiated replication.

   - Secure transmission of updates.

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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:

   - Single-master environment.

   - Consumer initiated replication.

   - 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

   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:

   - Incremental replication of changes.

   - Immediate replication on change of certain attributes.

   - 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.

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   The requirements that follow from this scenario are:

   - Minimize repetitive updates when replicating from multiple
     replication paths.

   - Incremental replication of changes.

   - Provide replication cycles to delay and/or retry when connections
     cannot be reached.

   - Allowances for consumer initiated or supplier initiated

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:

   - Provide replication audit history.

   - Provide mechanisms for managing conflict resolution.

   - Provide LDAP access to predetermined agreements, topology and
     policy attributes.

   - Provide operations for comparing replica's content for validity.

   - 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.

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   The requirements that follow from this scenario are:

   - One predefined replication agreement that manages a single area of
     replication that is held on numerous servers.

   - Common support of replication management knowledge across vendor

   - 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

   The requirements that follow from this scenario are:

   - An existing slave replica must be "promote-able" to become the new

   - 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:

   - Multi-master replication.

   - Ability to reschedule replication sessions.

   - Support for manual review and override of replication conflict

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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

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   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

   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

   - Raise an error condition and ignore the data.  This should always
     be allowed and is the basis for requirement P8 and the comment on

   - 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.

   - 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.

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   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:

   - All schema elements are replicated.

   - Schema elements are replicated only if they are used by attributes
     that are being replicated.

   - Schema are manually configured on the servers involved in
     fractional replication; schema elements are not replicated via the

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.

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   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

   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.

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   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

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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

   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

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   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

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:

   - Several directory users/applications are changing the same data.

   - They are changing the data before previous changes have replicated.

   - They are using different directory servers to make these changes.

   - 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.

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   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

   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

   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.

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   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:

   - 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.

   - 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.

   - 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

   - 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

   - 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

   - 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

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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.

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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
   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

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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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   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


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

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