Internet-Draft                                         Richard V. Huber
 LDAP Duplication/Replication/Update                 Gerald F. Maziarski
 Protocols WG                                          AT&T Laboratories
 Intended Category: Informational                          Ryan D. Moats
 Expires: March 2004                                      Lemur Networks
                                                          September 2003
              General Usage Profile for LDAPv3 Replication
 Status of This Memo
 This document is an Internet-Draft and is in full conformance with all
 provisions of Section 10 of RFC2026.
 Internet-Drafts are working documents of the Internet Engineering Task
 Force (IETF), its areas, and its working groups.  Note that other
 groups may also distribute working documents as Internet-Drafts.
 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time.  It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."
 The list of current Internet-Drafts can be accessed at
 The list of Internet-Drafts Shadow Directories can be accessed at
 Copyright Notice
 Copyright (C) The Internet Society (2001). All Rights Reserved.
 Support for replication in LDAP directory systems is often one of the
 key factors in the decision to deploy them.  But replication brings
 design constraints along with its benefits.
 We discuss some of the factors that should be taken into consideration
 when designing a replicated directory system.  Both programming and
 architectural/operational concerns are addressed and both single- and
 multi-master directories are considered.
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 Table of Contents
 1 Introduction........................................................3
 2 Meta-data Considerations............................................3
  2.1 Schema Considerations............................................3
  2.2 Replication Agreements...........................................5
  2.3 Access Control...................................................5
  2.4 Change Logs......................................................6
 3 Naming Considerations...............................................6
 4 Conflict Resolution Considerations..................................7
  4.1 Consistent Access after Changes..................................7
  4.2 Conflict Resolution in Single-Master Systems.....................8
  4.3 Problem Cases....................................................8
   4.3.1 Atomicity.....................................................8 Locking...................................................8 Partitioning..............................................9
  4.4 General Principles...............................................9
 5 Failover Considerations............................................10
  5.1 Common Issues...................................................10
  5.2 Single Master Issues............................................11
  5.3 Multi-Master Issues.............................................12
 6 Other Issues.......................................................12
  6.1 Locking.........................................................12
  6.2 Backup and Restore..............................................13
 7 Impact of Non-LDAP Changes/Constraints.............................13
  7.1 Changes Outside of LDAP.........................................13
  7.2 Application Triggers............................................14
  7.3 Policy Conflicts Across Servers.................................14
 8 Security Considerations............................................15
 9 Acknowledgements...................................................15
 10 References........................................................15
 Authors' Addresses...................................................16
 Full Copyright Statement.............................................16
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 1  Introduction
 As applications come to rely on LDAP directories as part of their
 mission-critical infrastructure, the need for highly reliable and
 highly available LDAP systems will increase.
 Distributed, replicated directories can increase reliability and
 reduce capacity problems.  Nevertheless, applications which work well
 with a single, standalone directory may develop problems in a
 distributed environment unless both the applications and the
 environment are designed with data distribution as one of the
 While the detailed design criteria will depend partly on whether the
 distributed directory is a single-master or multi-master system many
 concerns are common to both.  This document flags some issues as being
 specific to either single-master or multi-master directories;
 unflagged issues pertain to both.
 Any given class of directory applications (e.g. white pages, policy,
 authentication and authorization) is not inherently single- or multi-
 master.  This choice will depend on the requirements
 (reliability/availability, update procedures, performance, etc.) of a
 specific instance of the application.  Therefore, this document
 addresses general issues regarding the deployment of single- and
 multi-master directory systems.  There may be future documents that
 address specific applications.
 2  Meta-data Considerations
 Any LDAP directory contains meta-data as well as the user data in the
 directory.  A non-exhaustive list of meta-data includes descriptions
 of the data in the directory (e.g. schema), policies for use of the
 data (e.g. access controls), and configuration/status information
 (e.g. replication agreements).
 This meta-data is stored in the directory itself, accessible as
 regular data or as operational attributes.  Issues may arise when
 meta-data stored in the directory is replicated.  However, not
 replicating meta-data may also be problematic.
 This section examines some of the potential problems.
 2.1  Schema Considerations
 If the schema of one or more of the copies of a replica differs from
 the schema of the other replicas, then there is a possibility of
 schema mismatch when data is exchanged between them.  The schema
 extensibility feature of LDAP nearly guarantees that replica groups
 comprised of a heterogeneous mix of systems will not contain
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 homogeneous schema because of directory vendors' built-in extensions.
 A given directory may not use all of the elements of its schema, so
 schema differences do not always lead to schema mismatches during
 Schema mismatch issues are further complicated by the possibility of
 replicating the "subschemaSubentry" itself.  Some directories use this
 technique to distribute schema changes.  Currently there is no
 standard for LDAP schema representation within the subschemaSubentry.
 In the absence of such a standard, full schema interoperability is not
 possible in the IETF sense.  Directory designers should establish
 common schema on all servers holding a common replica, and should
 avoid use of vendor-specific attributes.
 The following is a partial list of possible schema mismatches:
   1.  Object class not defined
   2.  Structure Rule of an object class
   3.  Structural vs. Auxiliary in an object class
   4.  Optional vs. Mandatory attribute in an object class
   5.  Object identifiers differ on an attribute type or on an object
   6.  Type and number of attributes defined in a class
   7.  Attribute type not defined
   8.  Base syntax of an attribute type
   9.  Multi-valued vs. single-valued attribute types
  10.  Matching rule of an attribute type
  11.  Naming collisions of attribute type names
  12.  Attribute name aliasing ("street" vs. "streetAddress" vs.
  13.  ACL format (and consequently, ACL calculus)
 Schema mismatches that cause data corruption in one or more of the
 replicas must result in meta-data (e.g. log entries) in order to
 comply with Requirement P7 of [RFC3384].  Note that schema differences
 do not produce corruption in all circumstances.  Some schema
 differences may have little or no impact on the proper storage of
 replicated data.  However, any time data is added to the directory,
 replication between heterogeneous schemas may result in data
 corruption due to a schema mismatch.
 Options for dealing with such potential mismatches include:
   -  Use fractional replication to replicate only those attributes
      that do not have differences
   -  Remove all schema mismatches
   -  Use the same schema on all systems
 The tool described by requirement AM8 of [RFC3384] would help
 designers detect schema conflicts as early as possible.
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 Replication Agreements are central to replication, as they allow
 configuration of most of the aspects of the replication process,
 including the triggers for replica cycles (from Requirement M1 in
 [RFC3384]), critical OID information (from Requirement M6 in
 [RFC3384]), and replication process parameters (Requirement M7 in
 [RFC3384]).  Through the use of a standard replication agreement
 schema (Requirement SC2 of [RFC3384], [InfoMod]) it is possible to
 replicate the replication agreement.
 If a replication agreement includes replication credentials, the
 agreement should be read and write protected in the directory, and
 transport of the replication agreement should be encrypted.
 When replication agreements are themselves distributed via
 replication, they are subject to same "loose consistency" issues (due
 to replication delay and deferred conflict resolution) as other data.
 Even a temporary inconsistency among replication agreements may cause
 unbalanced replication and further inconsistency.  As "multi-
 mastering" complicates "loose consistency" issues, avoidance of these
 issues by making all replication agreement changes through the same
 master (see Sections 4 and 5) is strongly advised.
 2.2  Access Control
 The following considerations complicate replication of Access Control
   -  Access Control Information (ACI) is treated as though it were
      stored as attributes in the directory [RFC2820]
   -  LDAP [RFC2251] declares that changes resulting from a single LDAP
      MODIFY are atomic (but see caveats for multi-master replication
      in Sections 3 and 4)
   -  The ACI affecting a given entry may not be part of that entry (it
      could be part of a group entry or part of an ancestor of the
      entry in question)
   -  The ACI cannot always be changed atomically with associated data
   -  The interaction of replication and partitioning is still unclear
      (i.e. what happens when access control policy is inherited from
      an area of replication that is not held locally)
 Do not leave windows where data is unprotected.
 To reduce risk:
   -  In all environments, access control changes should be made before
      adds and after deletes
   -  In multi-master environments, access control changes and the
      associated data changes should be made on same system.
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 Even when ACI is faithfully replicated (with the same transfer format)
 among heterogeneous members of a replica group, there is no guarantee
 that an ACI change is expressed similarly everywhere in the group.
 This caveat is partly due to the open issues with respect to
 partitioning mentioned above, and partly due to vendor differences
 with regard to the expression of security policy.
 2.3 Change Logs
 Requirement G4 of [RFC3384] states that meta-data must not grow
 without bound.  Since it is unrealistic to assume that meta-data won't
 be needed during replication, designers must consider how and when
 meta-data can be purged.
 Replicas that use connections with intermittent quality should use
 explicit replica cycle scheduling.  Since the systems know when
 replication should have occurred, delayed replication can be detected
 and manual intervention initiated before the meta-data grows without
 bound.  In extreme cases, it may be necessary to remove a replica from
 the replication group and restore it once better connectivity is
 In a multi-master system, it is possible for a consumer to receive
 changes that cannot be applied.  For example, a modify request for an
 entry may arrive before the add request that creates that entry.  The
 replication system will typically queue this change and wait for
 additional changes (see Section 3.3).
 3  Naming Considerations
 A number of naming models have been proposed for directories
 ([RFC1255], [RFC2377]), and many others have been implemented on an ad
 hoc basis.  Each of these models specifies the naming attributes to be
 used and provides rules for using them which may also include
 containment rules.
 The naming plan applies to the directory as a whole, not the
 individual servers holding replicas.  Therefore, in a heterogeneous
 replicated environment, all of the replicating servers must be capable
 of supporting all of the rules for the naming plan in use for that
 Some directory implementations have naming constraints (e.g.
 containment rules, restrictions on attributes that can be used for
 naming).  If such an implementation is part of a replicated directory,
 those constraints will have to be observed by all participating
 directories.  If the environment contains implementations with
 incompatible constraints there is a major problem.  This should be
 checked as early in the design phase as possible.
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 Applications often have their own requirements on naming; if a
 directory supports multiple applications it may have to support
 multiple naming schemes.  Thus, if two independent applications start
 sharing previously separate directory information, care should be
 taken that the naming is consistent across the applications.   A
 difference in name form may be accepted through LDUP without
 constraint violation, but nevertheless result in unexpected behavior
 from a cross-application perspective.  Consistent naming is not only
 important to the directory, but to the applications that consume
 directory information as well.
 4  Conflict Resolution Considerations
 4.1  Consistent Access after Changes
 Many operations on a directory are done as a set of steps.  For
 example, a new object may be created by one operation, and its values
 may be filled in as part of a separate LDAP operation.  An
 administrator may add a user to a directory, and that user may then
 try to log in using the new entry.
 Replicated LDAP directories provide loose consistency [RFC3384].  A
 new entry or a change to an existing entry will not reach all replicas
 immediately; there will be some delay before changes are available on
 all replicas.  Changes made (e.g. adding a new user) on one physical
 system may appear to be "lost" if checked on another physical system
 before replication is complete.
 In general, LDAP applications should be prepared to operate correctly
 in the face of replication delays.  In some cases, this means
 designing to allow for delay.  In the case of the newly created user,
 it should be standard practice to ask the user to wait a while before
 trying to use the entry.  In the case where the new object must be
 filled in, the application should make appropriate use of LDAP
 sessions to make sure that the same server is reached for both
 As a general rule, an LDAP application should bind once and not unbind
 until a complete set of related operations have been performed. To
 achieve load balancing of write operations in a multi-master
 environment, balancing the write-enabled connections is recommended
 over balancing the individual LDAP write operations.
 In the single-master case, all write requests go to one server.  If a
 set of related reads and writes are done, they should all be done on
 the master if possible.
 In some cases, related requests will deal with data in different
 partitions that are not all available on a single server.  In this
 case, it is safer to keep sessions open to all servers rather than
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 closing the session with one server and opening one with another
 It may not always be obvious to clients that they are using different
 servers.  If a load distribution system is used between the client and
 the server, the client may find that a change request and a subsequent
 lookup are directed to different physical servers even though the
 original requests were sent to the same server name and/or address.
 Since LDAP is session oriented, any load distribution system used
 should take sessions into account.  Thus, keeping all related read and
 write requests within a single bind/unbind session should be the goal
 in this situation as well.
 4.2  Conflict Resolution in Single-Master Systems
 It is possible that resolution conflicts could occur in a single
 master replication system.  Because requirement SM2 of [RFC3384] is a
 "SHOULD" and not a "MUST", it is possible for implementers to reorder
 changes.  If changes are reordered, it is quite possible for a
 conflict to occur.  Consider a case where schema changes are declared
 critical and must be moved to the front of the replication queue.
 Then the consumer servers might have to delete an attribute that still
 has values, and later process requests to delete the values of that
 now undefined attribute.
 However, directory administrators may have scenarios where re-ordering
 of replication information is desirable.  On a case-by-case basis, the
 directory administrator should make such decisions.
 Many vendors may not implement conflict resolution for single-master
 replication.  If such a system receives out-of-order changes from a
 system that does support them, replication errors will almost
 certainly occur.  Designers should be aware that mismatches in the
 capabilities of replicating single-master directories could cause
 problems.  Designs should not permit the master to re-order changes
 unless all slave copies are known to handle the situation correctly.
 4.3  Problem Cases
 4.3.1  Atomicity
 The fact that replication does not guarantee the time order arrival of
 changes at a consumer allows situations where changes that were
 applied successfully at the supplier may fail in part when an attempt
 is made to apply the same change at the consumer.  Some examples
 appear below; additional examples are given in Appendix B.5 of
 [RFC3384].  Locking
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 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" But
 what  is the value of Z at the end of the replication cycle?  If it is
 42, then 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. Therefore, it is not clear when such
 information may be safely discarded.  Thus, requirement G6 in
 [RFC3384] may be violated.
 The utility of locking mechanisms cannot be guaranteed with multi-
 master replication, and therefore results are likely to be misleading.
 As discussed further in section 6.1 below, its use in multi-master
 environments should be deprecated.  Partitioning
 Partitioning (design of replica groups) also adds complexity.  For
 example, suppose two servers, A and B, are members of a replica-group
 for area of replication X while servers B and C are members of
 replica-group for area Y.  It is possible to issue a ModifyRDN
 operation on server B that moves an entry from area X to area Y.
 Replication in area X would delete the entry on server A while
 replication in area Y would add the entry to server C.  However, if
 another change on server C prevented the add operation from working
 (e.g. an entry with the same RDN but a different GUID exists there
 already), then the change on server A is inconsistent and will need to
 be reversed.  Other examples of cases of this class include group
 membership modification and access control scope.
 4.4  General Principles
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 The examples above discuss some of the most difficult problems that
 can arise in multi-master replication.  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 [RFC3384] are:
   1. Several directory users/applications are changing the same data
   2. They are changing the data at the same time
   3. They are using different directory servers to make these changes
   4. 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.
 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 or where careful design can reverse one of the
 conditions.  If, for example, all atomic read/modify requests for a
 given object can be directed to the same server, condition 3 will not
 occur.  For cases where all four conditions do occur, application
 designers should be aware of the possible consequences.
 5  Failover Considerations
 One of the major reasons to use directory replication is to improve
 reliability of the directory system as a whole.  Replication permits
 hot- and warm-standby configurations to be built easily.
 But there are some issues that must be considered during design.  In
 this situation, single-master systems actually raise more concerns
 than multi-master.  Both are addressed below.
 5.1  Common Issues
 In both the single- and multi-master cases, clients must be able to
 find an alternate quickly when a server fails.  Some possible ways to
 do this are detailed in [FindingLDAP] and [LDAPinDNS].  If all else
 fails, a list of possible servers can be built into client
 applications.  Designers should consider how clients are notified that
 the server is again available.
 When the failed server comes back up, it is brought back into
 synchronization with the other servers and is ready to take requests.
 It is always possible that the failed server, if it was acting as a
 supplier, was unable to completely distribute its pending changes
 before removal from service, leaving its consumers in an inconsistent
 state.  During the period between its removal from service and its
 eventual return, the inconsistency may have been compounded by further
 application activity.  At the time of publication there is no standard
 automatic mechanism to rectify the problem, so the administrator must
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 use whatever mechanism is available to compare the replicas for
 consistency as soon after the event as is reasonable.
 Note that the process used to bring a failed server back into
 replication can also be used to add a server to a set of replicating
 servers.  In this case, the new server might be initialized from a
 backed-up copy of the directory or it may acquire the entire DIB via
 replication.  The former method is usually preferable when the
 directory is large.
 5.2  Single Master Issues
 In a single-master system, the master is a single point of failure, as
 all modification has to originate at the master server.  When high
 availability is a requirement, a quick, automated failover process for
 converting a slave replica to a new master is desirable, as the
 failover time becomes a major factor in determining system
 availability.  The considerations in section 5.1 apply here; clients
 must know how to find the new master or a new slave in case of
 To aid in promotion of a slave replica, the master could replicate
 control information and meta-data (including replication credentials)
 so that this information is available during failover promotion.  This
 data may either be replicated on a single "failover designate" slave
 (which would become the master during failover) or it could be
 replicated to all slaves.  The first possibility has the advantage of
 minimizing the amount of extra replication while the second more
 robustly handles multiple failovers (i.e. failover of the newly
 promoted master to another slave before the original master has been
 restored).  If this method is followed, data privacy mechanisms should
 be used to protect the replication session.
 If data privacy mechanisms (e.g. encryption) are used to protect the
 replication session, the new master must have the necessary key
 information.  Further this key information should be independent of
 the master that is using it (i.e. not tied to the IP address of the
 master server).  If it is not independent, slave replicas could be
 pre-configured with the keys for all possible masters to reduce
 failover time.
 Restoration of the failed or broken master can be handled in one of
 two ways:
   -  It could join the replica group and function as a slave.
   -  It could join the replica group and negotiate with the new master
      to synchronize and then take over as master.
 In either case, clients need a way to know that a new server is
 available.  If the broken master is returned to service as a slave,
 then the administrator must, external to LDUP, distribute and resolve
 whatever pending changes remained undistributed and unresolved from
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 the time immediately before it was removed from service. If the broken
 master is returned as a new master, then care must be taken with its
 replacement master to ensure that all of its pending changes are
 distributed and resolved before it is returned to duty as a slave.
 The slave replicas may also use the replication agreement to filter
 which master is allowed to submit changes.  Such a model allows the
 slave servers to function correctly when the master server is "broken"
 and sending out incorrect updates.  However, then it is necessary to
 update the replication agreement during the fail over process so that
 the slaves will accept updates from the new master.  This is the case
 for both the original failure and the restoration of the restored
 master if that is how the restored master rejoins the replica group.
 5.3  Multi-Master Issues
 Typically, a multi-master configuration is used when high availability
 is required for writes as well as reads in the directory.   Because
 there are multiple active servers prepared to take write requests,
 there is no "switchover" time in this case.  But clients still need to
 be able to find an alternate server, so the considerations of Section
 5.1 apply here.
 6  Other Issues
 6.1  Locking
 Section discussed the problems that can arise when the
 "modify" command in LDAP is used for locking in a multi-master
 environment.  There are more general principles at work there.  LDAP
 is a distributed and replicated directory service that is typically
 described as "loosely consistent".
 In loose consistency, the data should eventually converge among the
 replicas, but at any given instant, replicas may be in disagreement.
 This stipulation is the general result of:
   1.  Delay due to replication intervals
   2.  Out of natural time order arrival of data at a replica
   3.  Temporary isolation of distributed systems from one another
   4.  Failure to accept a change due to conflict resolution failure on
       a replica
 Because of loose consistency, data encountered by an LDAP operation
 may differ among replicas.  Multi-mastering may exacerbate loose
 consistency, but single-mastering will not totally eliminate it if
 out-of-order replication is allowed (see Section 4.2).  One must
 carefully assess the effect of loose consistency when considering the
 use of distributed LDAP servers as a data store.  Applications which
 depend on synchronous consistency may be better suited for
 transactional models and/or non-distributed data.
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 6.2  Backup and Restore
 Backup of a directory server should have the following goals:
   1.  The directory (or the replica) can be unambiguously and
       faithfully restored from the backup.
   2.  The backup is an internally consistent snapshot of an entire
       replica during the time interval it took to make it.
   3.  Replication can resume on a machine restored from that backup
       without information loss.
 Backup and restore of a single, operating directory server (rather
 than the entire directory system) presents its own challenges. "Loose
 consistency" works against the probability of achieving a loss-free
 copy of all the data in the directory, except under ideal conditions.
 Backup and restore of distributed directories is a decidedly easier
 task when the constraint of continuous availability is removed.  In
 most cases, the removal of entire directory systems from write service
 is impossible, even for small periods of time.  It is more practical
 to remove a single replica from service to achieve a condition of
 quiescence.  Once all write load is removed, including write load due
 to replication, an internally consistent copy of the data may be
 Replicas that have suffered catastrophic data loss may be restored
 from backups of working ones temporarily removed from service
 specifically to make a copy.  This scenario illustrates the benefit of
 having three or more master replicas in the system: no single point of
 write failure in the event that one of the replicas must be restored
 from a copy of another.
 The M11 requirement from [RFC3384] allows an empty replica to be
 brought up to date through replication.  This feature duplicates, but
 does not make entirely unnecessary, backup procedures on directory
 servers. Backups are still needed to recover data that has been lost
 to all replicas, either through normal LDAP updates or through some
 catastrophic event.
 7  Impact of Non-LDAP Changes/Constraints
 7.1  Changes Outside of LDAP
 LDAP directories are typically built on top of some database or file
 system.  Thus there are ways to change the data that do not go through
 the normal LDAP change mechanisms (e.g. ModifyRequest).  If the data
 is modified outside of LDAP, the changes will not be checked for
 schema conformance nor will access controls be checked as the changes
 are made.  Since both integrity and security checks are omitted,
 security can be adversely affected.
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 Also, many systems use the normal LDAP modification mechanisms to
 trigger replication.  Changes made using non-LDAP mechanisms may not
 be replicated at all, leading to inconsistencies between replica
 7.2  Application Triggers
 Directory servers commonly integrate one or more specific
 applications. To achieve this integration the directory server may
 intercept updates and run application-specific "trigger" code.  Such
 triggers enforce directory invariants that cannot be expressed by the
 LDAP schema.
 A simple trigger example is password policy enforcement. A directory
 server might interpret a request to replace the current value of the
 userPassword attribute with some new value as a request to first check
 that the new value conforms to the server's password policy (e.g. the
 value is sufficiently long and complex) before storing the new value.
 Using this trigger the directory server voids the security risk
 associated with passwords that are easy to attack.
 A more complex trigger example is password hashing.  A directory
 server might interpret a request to replace the current value of the
 userPassword attribute with some new value as a request to compute one
 or more secure hashes of the new value and store these hashes in one
 or more attributes, storing no value in the userPassword attribute.
 Using this trigger the directory server avoids the security exposure
 of storing the plaintext password.
 Replication between directory servers with different application
 triggers will compromise directory integrity.
 Similarly, one cannot extend the directory with stored procedures that
 execute on access - such as scripts, programs or controls which change
 the data - because the expression of such mechanisms may not be
 guaranteed to be consistent among heterogeneous servers.
 7.3  Policy Conflicts Across Servers
 In addition to the discussions of ACI in Section 2.3 and triggering in
 section 7.2, LDUP replication can not (by its definition) handle
 replication of information that makes use of policy not expressible in
 the LDAP protocol.  A prime example of this is security encoding of
 attributes (e.g. userPassword).  This encoding is typically
 implementation specific and is not easily expressible via the LDAP
 protocol.  Therefore replication of userPassword attributes between
 directory servers that use different encoding schemes will impede
 replication in a way that is not describable as schema or syntax
 mismatch.  This is because of the bind-time policy semantics that are
 the true point of conflict.
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 In general, any attribute with semantics that are outside the scope of
 what is expressible by the LDAP protocol could result in strange
 replication errors.  Therefore, distributed directory implementers
 should (in the absence of a way to express such semantics) either
 strive for a homogeneous set of servers or ensure during acceptance
 testing that a new server can support the existing semantics of their
 8  Security Considerations
 This document discusses issues that arise in replication.  Some of
 these issues are security related (e.g. replication of access control
 information) and the security implications are discussed in the
 relevant sections.
 9  Acknowledgements
 This document owes a lot to discussions on the LDUP mailing list.  In
 particular, the authors would like to thank Ed Reed, whose email to
 the mailing list drove much of section 6.1, and Mark Brown for
 identifying and generating text on the issues discussed in section 7.
 10  References
 [FindingLDAP]  R. Moats, R. Hedberg, "A Taxonomy of Methods for LDAP
 Clients Finding Servers", Internet Draft, draft-ietf-ldapext-ldap-
 taxonomy-05.txt, July 2001.
 [InfoMod]  R. Moats, R. Huber, J. McMeeking, "LDUP Replication
 Information Model", Internet Draft, draft-ietf-ldup-infomod-07.txt,
 June 2003.
 [LDAPinDNS]  M. Armijo, L. Esibov, P. Leach, R. L. Morgan,
 "Discovering LDAP Services with DNS", Internet Draft, draft-ietf-
 ldapext-locate-05.txt, March 2001.
 [RFC1255]  The North American Directory Forum, "A Naming Scheme for
 c=US", RFC 1255, September 1991.
 [RFC2251]  M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
 Protocol", RFC 2251, December 1997.
 [RFC2377]  A. Grimstad, R. Huber, S. Sataluri, M. Wahl, "Naming Plan
 for Internet Directory-Enabled Applications", RFC 2377, September
 [RFC2820]  E. Stokes, D. Byrne, B. Blakley, P. Behara, ôAccess Control
 Requirements for LDAPö, RFC2820. May 2000.
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 INTERNET DRAFT Gen'l Usage Profile for LDAP Replication  September 2003
 [RFC3384]  E. Stokes, R. Weiser, R. Moats, R. Huber, "LDAPv3
 Replication Requirements", RFC 3384, October 2002.
 Authors' Addresses
 Richard V. Huber
 Room C3-3B30
 AT&T Laboratories
 200 Laurel Avenue South
 Middletown, NJ  07748
 Telephone: +1 732 420 2632
 Fax: +1 732 368 1690
 Gerald F. Maziarski
 Room C3-3Z01
 AT&T Laboratories
 200 Laurel Avenue South
 Middletown, NJ  07748
 Telephone: +1 732 420 2162
 Fax: +1 732 368 1690
 Ryan D. Moats
 Lemur Networks
 15621 Drexel Circle
 Omaha, NE  68135
 Telephone: +1 402 894 9456
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 translate it into languages other than English.
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 INTERNET DRAFT Gen'l Usage Profile for LDAP Replication  September 2003
 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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