Subject: Revised LDUP "Profile" draft

From: [email-masked] (Richard V Huber)
Date: Tue, 20 Nov 2001 17:45:09 -0500

Internet Drafts Editor -

Please publish the ID below as draft-ietf-ldup-usage-profile-02.txt
(cut at the dotted line).

LDUPers -

The draft below is an updated version of the General Usage Profile.
Changes were mostly editing and clarification.  We expect there will be
a new version after the next revision of the architecture.

Please send comments to the list.

Rick Huber, et al

--- --- --- --- --- --- --- --- --- --- --- --- --- ---

Internet-Draft                                         Richard V. Huber
LDAP Duplication/Replication/Update                 Gerald F. Maziarski
Protocols WG                                          AT&T Laboratories
Intended Category: Informational                          Ryan D. Moats
Expires: May 2002                                        Lemur Networks
                                                          November 2001

              General Usage Profile for LDAPv3 Replication
                  draft-ietf-ldup-usage-profile-02.txt

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
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The list of current Internet-Drafts can be accessed at
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Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

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.
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Table of Contents
1 Introduction........................................................2
2 Meta-data Considerations............................................3
 2.1 Schema Considerations............................................3
 2.2 Replication Agreements...........................................4
 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....................................................9
  4.3.1 Atomicity.....................................................9
    4.3.1.1 Locking...................................................9
    4.3.1.2 Partitioning..............................................9
 4.4 General Principles..............................................10
5 Failover Considerations............................................10
 5.1 Common Issues...................................................11
 5.2 Single Master Issues............................................11
 5.3 Multi-Master Issues.............................................12
6 Other Issues.......................................................13
 6.1 Locking.........................................................13
 6.2 Backup and Restore..............................................13
7 Impact of Non-LDAP Changes/Constraints.............................14
 7.1 Changes Outside of LDAP.........................................14
 7.2 Application Triggers............................................15
 7.3 Policy Conflicts Across Servers.................................15
8 Security Considerations............................................16
9 Acknowledgements...................................................16
10 References........................................................16
Authors' Addresses...................................................17
Full Copyright Statement.............................................18

1  Introduction

As LDAP directories become part of the critical infrastructure for
applications maintaining high reliability and availability is
significant.

Distributed, replicated directories can reduce reliability and capacity
problems.  However, applications that work well with a single,
standalone directory can develop problems in a distributed environment
unless both the applications and the environment are carefully
designed.

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While particular areas of concern depend partly on whether the
distributed directory is a single-master or multi-master system most
concerns that 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.

The current replication framework provides no easily separable subset
of functions for single-master and multi-master replication, therefor
this addresses general issues regarding the deployment of single-master
and multi-master directory systems.  There may be additional drafts in
the future that address specific applications.

2  Meta-data Considerations

Any LDAP directory contains meta-data as well as the user data in the
directory.  Examples of this meta-data include 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 is not an exhaustive list.

This meta-data is stored in the directory itself, frequently accessible
as regular data or as operational attributes.  Issues arise when meta-
data stored in the directory is replicated.  However, not replicating
meta-data also causes issues to arise.

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 homogeneous schema
because of directory vendors' built-in extensions. A given directory
may not utilize all of the elements of its schema, so schema
differences do not always lead to schema mismatches during replication.

Schema mismatch issues are further complicated by the possibility of
replicating the "subschemaSubentry" itself.  Some directories
distribute schema changes through that mechanism.  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
replica.

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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
      class
  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.
      "Strasse")
 13.  ACL format (and consequently, ACL calculus) (Note: this is being
      addressed in the ACL model [ACLModel])

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 [ReplicaReq].  However, not all schema
differences 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 may result in data corruption due to a schema mismatch.

Here are some options for dealing with such potential mismatches:

  -  Use fractional replication to replicate only those attributes that
     do not have differences
  -  Removal of all schema mismatches.
  -  Use the same schema on all systems

The tool described by requirement AM8 of [ReplicaReq] would help
designers detect schema conflicts as early as possible.

2.2  Replication Agreements

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
[ReplicaReq]), critical OID information (from Requirement M6 in
[ReplicaReq]), and replication process parameters (Requirement M7 in
[ReplicaReq]).  Through the use of a standard replication agreement

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schema (Requirement SC2 of [ReplicaReq], [InfoMod]) it is possible to
replicate the replication agreement.

If a replication agreement includes replication credentials, the
agreement should be read 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" problems (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.

2.3  Access Control

The following considerations complicate replication of Access Control
Information:

  -  Access Control Information (ACI) is treated as though it were
     stored as attributes in the directory [ACModel]
  -  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
     changes
  -  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).
  -  Thus, if you aren't careful, you can 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.

Even when ACI is faithfully replicated (with the same transfer format)
among heterogeneous members of a replica group, there is no guarantee

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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.

2.4 Change Logs

Requirement G4 of [ReplicaReq] 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
available.

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], [CIMNames]), 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
directory.

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

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incompatible constraints there is a major problem.  This should be
checked as early in the design phase as possible.

Applications often have their own requirements on naming; in this case
the directory will 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 [ReplicaReq].  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 operations.

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 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

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the master if possible.  Ideally, only sets of related operations that
cannot include a write should go to one of the slave servers.  But load
balancing concerns may make this impractical.

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
closing the session with one server and opening one with another
server.

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 [ReplicaReq] 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
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.

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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. Examples appear below.

4.3.1.1  Locking

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 [ReplicaReq] 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.

4.3.1.2  Partitioning

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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 scoping.

4.4  General Principles

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 are:

  -  Several directory users/applications are changing the same data
  -  They are changing the data at the same time
  -  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

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.

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.

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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.  As there is no current automatic mechanism to
rectify the problem, the administrator should 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 failure.

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 in 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
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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 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

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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 4.3.1.1 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.  The delay due to replication or extended replication intervals
  2.  The out of natural time order arrival of data at a replica
  3.  The 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 preconditions to an LDAP operation
may differ among replicas.  Multi-mastering may exacerbate this
situation, 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 evaluating operations that
place specific preconditions on data to work correctly.  Applications
which depend on such operations may be better suited for transactional
models and/or non-distributed data.

Distributed locking is one operation that depends on strict data
preconditions.  When data preconditions cannot be guaranteed, the lock
is moot.  The same principles hold for "atomic operations", defined
here as any mix of allowable operations contained within the same LDAP
PDU.  RFC2251 requires that they either all fail or are applied as a
unit.  If strict data preconditions cannot be guaranteed, then the
atomic operation may itself result in a further inconsistency requiring
human intervention at one of the consumers.

6.2  Backup and Restore

Backup of a directory server should have the following goals:
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  1.  It can be unambiguously and faithfully restored.
  2.  It is an internally consistent snapshot of an entire replica
      during the time interval it took to make it.  This can only be
      achieved if the server is quiescent.
  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 directory server 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
obtained.

Replicas that have suffered catastrophic data loss may be restored from
backups of working servers temporarily removed from service
specifically to make a copy.  This scenario illustrates the benefit of
having three or more 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 [ReplicaReq] 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

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made.  Since both integrity and security checks are omitted, security
can be adversely affected.

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 copies.

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.

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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Another case arises from the use of authentication levels in the Access
Control Model [ACModel].  The authentication levels represent the
strength of various forms of authentication rather than specific
authentication mechanisms; the association between levels and
mechanisms is left for administrators though some guidance may be
provided [AuthMap].  In a replicated environment, administrators must
be sure that the mappings of all replicating systems are compatible.
If they are not compatible, access controls may have different effects
on different replicas of a partition.

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
directory.

8  Security Considerations

The 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

[ACModel]  E. Stokes, R. Blakeley, R. Byrne, R. Huber, D. Rinkevich,
"Access Control Model for LDAPv3", Internet Draft, draft-ietf-ldapext-
acl-model-08.txt, June 2001.

[AuthMap]  K. Zeilenga, "Authentication Mechanisms Levels", Internet
Draft, draft-zeilenga-auth-lvl-01.txt, April 2001.

[CIMNames]  Desktop Management Task Force, "Guidelines for CIM-to-LDAP
Directory Mappings", DMTF Specification DSP0100, May 2000 (available
online at http://www.dmtf.org/spec/DEN/DSP0100.htm).

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INTERNET DRAFT Gen'l Usage Profile for LDAP Replication   November 2001
[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]  E. Reed, T. Hahn, "LDUP Replication Information Model",
Internet Draft, draft-ietf-ldup-infomod-03.txt, July 2001.

[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.

[ReplicaReq]  E. Stokes, R. Weiser, R. Moats, R. Huber, "LDAPv3
Replication Requirements", Internet Draft, draft-ietf-ldup-replica-req-
08.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 1998.

Authors' Addresses

Richard V. Huber
Room C3-3B30
AT&T Laboratories
200 Laurel Avenue South
Middletown, NJ  07748
USA
E-Mail: [email-masked]
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
USA
E-Mail: [email-masked]
Telephone: +1 732 420 2162
Fax: +1 732 368 1690

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INTERNET DRAFT Gen'l Usage Profile for LDAP Replication   November 2001
Ryan D. Moats
Lemur Networks
15621 Drexel Circle
Omaha, NE  68135
USA
E-Mail: [email-masked]
Telephone: +1 402 894 9456

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