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INTERNET-DRAFT Russel F. Weiser
Informational Draft Digital Signature Trust Co.
Expires 21 April 2000 Ellen Stokes
IBM
21 October 1999
LDAP V3 Replication Requirements
<draft-ietf-ldup-replica-req-02.txt>
Status of this Memo
This document is am 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
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Drafts Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document discusses the fundamental requirements for replication
of data accessible via the LDAPv3 [RFC2251] protocol. It is intended
to be a gathering place for general replication requirements needed
to provide interoperability between informational directories.
The key words MUST, MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Weiser & Stokes 21 April 2000 [PAGE 1]
INTERNET-DRAFT LDAP Replication Requirements 21 October 1999
Table of Contents
1.Introduction.....................................................3
2. Terminology.....................................................3
3. Objective.......................................................5
4. Applicability Statement.........................................5
5. Replication Model..............................................10
6. Replication Protocol...........................................12
7. Schema.........................................................13
8. Administration and Management Considerations...................13
9. Acknowledgement................................................14
10. References....................................................15
11. Author's Address..............................................15
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1. Introduction
The ability to distribute directory information throughout the
network provides a two fold benefit to the network: (1) increasing
the reliability of the directory through fault tolerance, and
(2) brings the directory content closer to the clients using the
data. LDAP<41>s acceptance as an access protocol for directory
information is driving the need to distribute LDAP directory content
among servers within enterprise and Internet. Currently LDAP does
not define a replication mechanism and only generally mentions LDAP
shadow servers (see [RFC2251] and [Changelog]) in passing. The
requirements for replication are critical to the successful
deployment and acceptance of LDAP in the market place.
2. Terminology
For the purposes of this document, the following terminology
definitions are used:
Area of replication - A whole or portion of a directory tree(DIT)
making up a distinct unit of data to be replicated. This may also be
known as "unit of replication".
Atomic operation - The ability to treat and contain several updates
or attribute changes as a single operation for replication purposes
to guarantee that the several updates or attribute changes are
propagated to a replica as a single unit.
Authoritative Master Replica - The Primary updateable replica of the
replicated information.
Conflict resolution - Deterministic procedures within replication
protocols, utilized to resolve change information conflicts that may
arise due to conflicting changes affecting a directory entry.
Fractional replication - The capability to replicate a subset of
attributes of any given entry.
Incremental Update - The process of updating a replica, or copy, of
a naming context, by updating only those fields or objects which
have changed.
Master Slave, or Single Master Replication - Replication model that
assumes only one server, the master, allows write access to the
replicated data. Note that Master-Slave replication can be
considered a proper subset of multi-master replication.
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Multi-Master Replication - A replication model where entries can be
written and updated on any of several updateable replica copies
without requiring communication with other updateable replicas
before the write or update is performed.
Naming Context - Suffix of a Sub-tree. A sub-tree of entries held in
a single server [X.500].
One-way Replication - The process of synchronization in a single
direction where the authoritative source information is provided to
a replica.
Partial Replication - The capability to replicate some subset of
entries in a naming context.
Propagation behavior - The general behavior of the actual
synchronization process between a consumer and a provider of
replication information.
Read-only Replica - A read-only copy of a replicated directory. A
read-only replica is assumed to be a slave replica of master slave
or single master replication definition.
Replica - A single instance of a whole or portion of the Directory
tree (DIT) as defined by area of replication.
Replica Ring - A set of servers, which hold in common the same DIT
information as, defined by <20>Area of replication<6F>. These servers may
be managed under a single replication agreement that handles all
members of the set of servers as a group.
Replica Cycle - When a change or groups of changes need to be
propagated to the other member of a replica ring. The process of
contacting a replica member would be considered the beginning of a
replication cycle; the termination of communications with a replica
is the end of the cycle whether its due to an error or successful
exchange of update records.
Replication - The process of copying portions of naming context
information and content between multiple LDAP servers, such that
certain predefined portions of the information are available from
different servers. Replication can occur between either homogeneous
implementations across heterogeneous platforms (operating systems)
or heterogeneous implementations supporting identical replication
across heterogeneous platforms (operating systems).
Sparse Replica - A incomplete copy of a sub-tree which maybe
inclusive with updateable, or Read-only. See Partial replication and
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Fractional replication.
Topology - Refers to the shape of the directed graph describing the
relationships between replicas, as in the replicated directory
topology.
Two-way Replication - The process of synchronization where change
information may flow bi-directionally between two replica.
Update Propagation - Protocol-based process by which directory
replicas are reconciled.
Updateable Replica - A Non-authoritative read-writeable copy of the
replicated information. Such that during conflict resolution a
authoritative master takes precedents in resolving conflicts.
3. Objective
The major objective is to provide an interoperable LDAP V3 directory
synchronization protocol which is simple, highly efficient and
flexible enough to support both multi-master and master-slave
replication operations to meet the needs of both the internet and
enterprise environments.
4. Applicability Statement
Generally replication can be characterized by looking at data
consistency models across existing technologies. This may provide
insight to LDAP v3 replication requirements. The following is a
brief examination of the following data models.
Model 1: Tight Consistency -- Includes environments where all
replicas must always contain exactly the same directory content. Two
phase commit transaction models may be used to preserve transaction
consistency.
Model 2: Eventual Consistency or Transient Consistency -- Includes
X.500 Directories, Bayou [XEROX], and NDS (Novell Directory
Services) names service where definite knowledge of the global
replica topology is provided through predetermined replication
agreements. Such that every update propagates to every replica that
it can reach via a path of stepwise eventual connectivity.
Transaction consistency is preserved for transactions directed at
the master server in X.500 implementations. NDS additionally
provides deterministic consistency over time to all replicas due to
its inherent replication policies.
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Model 3: Limited Effort Eventual Consistency -- Includes Xerox
Clearinghouse [XEROX] that provides a statistical probability of
convergence with global knowledge of replica topology. Similar to
"Eventual Consistency", except where replicas may purge updates
therefore dropping propagation changes when some replica time
boundary is exceeded, thus leaving some changes replicated to a
portion of the replica topology. Transactional consistency is not
preserved, though some weaker constraints on consistency are
available.
Model 4: Loosest Consistency -- Includes opportunistic or simple
cache where information is provided from the cache until stale.
Model 5: Ad hoc -- A copy of a date store where no follow up checks
are made for the accuracy/freshness of the data.
Consistency models 2, and 3 involve the use of prearranged
replication agreements or "Predefined Replication Agreements"
between cooperating servers. The complexity of Model 1's use of 2-
phase commit adds additional overhead that should not considered at
this time. Models 4 and 5 involve unregistered replicas which
"pull" updates from another directory server without that server's
knowledge. These models can be considered to violate a directory's
security policies. Therefore models 1, 4, and 5 are declared to be
out of scope of this working group.
So through further review of these consistency models two
application areas can then be derived with even further
characterizations of the data types usages.
Eventual Consistency or Transient Consistency (Model 2) - This model
provides policy configuration through security management
parameters; the data is more dynamic and utilizes dynamic address
information.
Limited Effort Eventual Consistency (Model 3) - This model matches a
white-pages environment which contains fairly static data and
address information. This model mainly replicates message
attributes.
Therefore it is believed an LDAP replication should be flexible
enough to cover the above range of capabilities. The generalized use
of LDUP replication environment is to provide for the distribution
of LDAP directory information in order to improve accessibility and
consistency of the information held by the directory.
4.1 Replication Scenarios
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The following directory deployment examples are intended to
substantiate and validate our replication requirements. It is
assumed in all cases that directory implementations from different
vendors are involved.
4.1.1 Extranet Example
A company has a trading partner to whom it wishes to provide
directory information. This information may be as simple as a
corporate telephone directory, or as complex as an extranet work
flow application. For performance reasons the company may wish to
have a replica of its directory within the Partner Company, rather
than simply exposed beyond its firewall.
The requirements, which 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.
4.1.2 Consolidation Example
Company A acquires company B. In the transition period, whilst the
organizations are merged, both directory services must coexist.
Company A may wish to attach company B's directory to its own.
The requirements, which 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.
4.1.3 Replication Heterogeneous Deployment Example
An organization may deliberately deploy multiple directory services
within their enterprise to employ the differing benefits of each
service. In this case multi-master replication will be required to
ensure that the multiple updateable replicas of the DIT are
synchronized. Some vendors may provide directory clients, which are
tied to their own directory service.
The requirements, which follow from this scenario, are:
- Multi-Master replication
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- Common access control model and Access control model
identification.
- Secure transmission of updates.
- Replication between DITs with potentially differing schemas.
4.1.4 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, which follow from this scenario, are:
- Multi-Master replication.
- Common access control model and Access control model
identification.
- Secure transmission of updates.
4.1.5 Supplier Initiated Replication
A single master environment, which maintains a number of replicas of
the DIT by pushing changes, based on a defined schedule.
The requirements, which follow from this scenario, are:
- Single-master environment.
- Supplier-initiated replication.
- Secure transmission of updates.
4.1.6 Consumer Initiated Replication
Again a single mastered replication topology, but the 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, which follow from this scenario, are:
- Single-master environment.
- Consumer initiated replication.
- Open scheduling (anytime).
4.1.7 Prioritized attribute replication
The password attribute can provide an example of the requirement for
prioritized attribute replication. A user is working in Utah and the
administrator resides in California. The user has forgotten his
password. So the user calls or emails the administrator to request a
new password. The administrator provides the updated password (a
change). Policy states that this attribute is critical and must be
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INTERNET-DRAFT LDAP Replication Requirements 21 October 1999
available to the user for login immediately (e.g. shortly) after the
administrator changed it. Replication needs to occur immediately for
critical attributes/objects.
The requirements, which follow from this scenario, are:
- Incremental replication of changes.
- Automatic replication on change of certain attributes.
- Replicate based on time/attribute semantics.
4.1.8 Bandwidth issues
The replication of Server (A) R/W replica (a) in Katmandu 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.
The requirements, which 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
can not be reached.
- Allowances for consumer initiated or supplier initiated
replication.
4.1.9 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 message informs him. The administrator uses his
$19.95 RepCo LDAP directory replication diagnostics tools to look at
Root DSE replica knowledge on server 17 and determines that server
42 made by LDAP<41>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, which follow from this scenario, are:
- Provides replication audit history.
- Provisions for managing conflict resolution.
- Provide LDAP access to predetermined agreements, topology and
policy attributes.
- Provide operations for comparing replica<63>s content for validity.
- Provide LDAP access to status and audit information.
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4.1.10 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.
The requirements, which follow from this scenario, are:
- Predefined replication agreements that manage more than a single
area of replication that is held on numerous servers.
- Common support of replication management knowledge across vendor
implementation.
- Rescheduling and continuation of a replication cycle when one
server in a replica ring is busy and/or unavailable.
5. Replication Model
5.1 LDAP Replication MUST be allowed to span different vendors
directory services in order to provide interoperability.
5.2 All replicas MUST eventually be updated with the changed
information, if specified by the replication policy.
5.3 Replication schedules MUST be configurable to allow for
periodic replication, with the replication period determined by
administrator of the replicated system.
5.4 Replication Model MUST enable replication cycle to be initiated
on change or based on the number of pending changes.
5.5 The replication model MUST allow for administrative
initiation of replication cycle for any replica that may have
just come back online or was unavailable during previous
replication cycles.
5.6 The replication model MUST support both master-slave and
authoritative multi-updateable replica relationships.
5.7 All replicated information between the master database and its
replica databases MUST be identical including all non-user
modify operational attributes such as time stamps. Note this
does not imply that the entire database is identical from
replica to replica, but that the subset of data, chosen to
replicate is identical from replica to replica. Some
operational attributes may be dynamically evaluated; these
attributes will not necessarily appear to be identical.
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5.8 In distributed multi-vendor environment, LDAP replication MUST
NOT require all copies of the replicated information be
complete copies of the replicated object.
5.9 LDAP replication MUST encompass common schema objects and
attributes, access control, and name space information.
5.10 Sub-tree Replication MUST be defined to allow for greater
flexibility in replication topologies of the DIT as defined by
the area of replication called partial replication.
5.11 Replication of critical values MUST be synchronized and have
priority over non-critical values. An example of a critical
value might be a password or certificate value.
5.12 Replication activities MUST occur within the context of a
predefined replication agreement that addresses proper
knowledge of access requirements and credentials between the
synchronizing directories. Currently X.525 DISP [X.525]
discusses this as a shadowing agreement including such
information as unit of replication, update mode, and access
point defining many of the policies between the master and a
replica.
5.13 The acceptance and usage of the Internet requires that LDAP
replication be available across disparate vendor directory
services.
5.14 LDAP replication MUST provide scalability to both enterprise
and Internet environments, e.g. an LDAP server may provide
replication services to replicas within an enterprise as well
as across the Internet.
5.15 The replication model MUST define deterministic policy such
that replication cycle startup time conflicts between two or
more competing master replicas may be resolved
programmatically. An example might be automatic submission and
rescheduling by one of the masters. In such a case, these
replication "conflicts" MUST be resolved by the replication
policy.
5.16 Any replication capable LDAP server MUST allow replication
where the 2 replicating servers agree they can replicate. This
may be accomplished through administrative agreements assuming
compatible access control model and common schema are provided.
5.17 The replication model MUST be able to handle convergence and
resurrection of attributes and objects. This is a consequence
of delete and move with respect to the replication process.
Weiser & Stokes 21 April 2000 [Page 11]
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5.18 It is not realistic to assume that all vendors have cooperating
schemas, but that replication may be allowed between diverse
schema. The Model MAY allow for replication between divergent
schema of objects.
6. Replication Protocol
6.1 The act of replication SHOULD have minimal impact on both the
system and network performance.
6.2 The replica synchronization SHOULD be handled in such a manner
as to not saturate network with repetitive entry replication
from multiple synchronization providers points.
6.3 Replication MUST only be allowed after the authentication and
verification of authorization of both the replica and the
source directory.
6.4 The transport for LDAP synchronization MUST allow for the
integrity and confidentiality of each replicated server.
6.5 Replicated data MUST be transferable in a secure manner.
6.6 Replication protocol MUST provide for recovery and rescheduling
of a replication cycle due to a replication initiation
conflicts (e.g. consumer busy replicating with other servers)
and or loss of connection(e.g. supplier cannot reach a
replica). The replication protocol MUST include restarting at
the last acknowledged update prior to interruption rather than
re-sending updates it had already sent to a consuming replica.
6.7 LDAP replication MUST allow for full update to facilitate
replica initialization and reset loading utilizing a
standardized format such as LDIF [LDIF] format.
6.8 The replication standard SHOULD NOT limit the size of a
replica. The area of replication is defined to be a whole or
portion of a DIT, also allowing a portion of a naming context
to be replicated. Incremental replication SHOULD be allowed.
6.9 The replication agreements MUST accommodate multiple servers
receiving the same replica under a single predefined agreement.
6.10 The replication protocol MUST allow either a master or replica
to initiate the replication process.
6.11 Additionally the initiator MUST be allowed to determine
whether it will become a consumer or supplier during the
synchronization startup process. This would allow a replica to
Weiser & Stokes 21 April 2000 [Page 12]
INTERNET-DRAFT LDAP Replication Requirements 21 October 1999
be periodically connected and synchronized from remote sites at
the local administrator's discretion.
6.12 Multiple LDAP changes to a single server: If transactional
consistency is propagated during replication, then multiple LDAP
changes submitted to a single server SHOULD BE treated as a
single 'atomic unit of work'.
6.13 An LDAP Replication Standard SHOULD NOT limit the transaction
rate of a replication session.
6.14 Entry change information MUST be purged or discarded in a
timely manner when change information becomes outdated due to
propagated to all replica members.
7. Schema
7.1 Replica knowledge MUST be provided as DSE attributes.
7.2 The Replication Protocol documents MUST define standard schema
for representing replication agreements, and MUST define the
semantics associated with modifying the attributes of
replication agreements. The documents MUST also define a
standard method for determining the location of these
agreements accessible utilizing LDAP.
7.3 The Replication Protocol documents MUST define standard schema
for publishing state information about a given replica, and
MUST define a standard method for determining the location of
this information.
7.4 A location independent management point MUST be defined to
provide authorized administrators with well known access to the
replication policies, regardless of network location.
7.5 Replication agreements of all servers containing replicated
information MUST be accessible via LDAP.
7.6 All objects MUST be uniquely identifiable throughout the object
lifetime .
8. Administration and Management Considerations
8.1 Replication policies MUST allow replication of changed
information to be administratively postponed to a more
Weiser & Stokes 21 April 2000 [Page 13]
INTERNET-DRAFT LDAP Replication Requirements 21 October 1999
convenient period.
8.2 Allowance for non-scheduled replication of a replica MUST be
provided upon request such that the replica server has been
down or unconnected for a period of time.
8.3 Each copy of a replica MUST maintain audit history information
of which servers it has replicated with and which servers have
replicated with it.
8.4 A replica MUST store conflicted versions of the replicated
object to allow optional human review and intervention.
8.5 Access to replication predetermined agreements, topologies, and
policies attributes MUST be provided through LDAP access.
8.6 The capability to check the differences between two replicas
for the same information SHOULD be provided for. This should
entail a client invoking an operation at some server, which
causes 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.
8.7 Authenticated access SHOULD be provided so that Administrative
LDAP clients may query a server for the current state and
replication history for each replica that the server maintains
replication agreements with.
8.8 The ability to view replication conflicts, and override the
resolution derived by the replication policy MUST be provided.
8.9 The deletion of sensitive data MUST be handled in an orderly
manner so that at no time will that data be available without
proper access control. That is, access control information
(ACI) associated with sensitive data must be deleted after or
simultaneously with the delete of the sensitive data. Likewise,
when adding sensitive data, ACI MUST be added first or
simultaneously with the addition of that data.
9. Acknowledgement
This document is based on input from IETF members interested in LDUP
Replication.
Weiser & Stokes 21 April 2000 [Page 14]
INTERNET-DRAFT LDAP Replication Requirements 21 October 1999
10. References
[RFC2251] M. Wahl, T. Howes, S. Kille "Lightweight Directory Access
Protocal", RFC 2251.
[RFC2119] S.Bradner, " Key words for use in RFCs to indicate
Requirement Levels", RFC 2119.
[LDIF] Gordon Good, "The LDAP Data Interchange Format (LDIF)",
Internet draft, draft-ietf-asid-ldif-00.txt, November 1996.
[Changelog] Gordon Good, "Definitions of an Object Class to Hold
LDAP Change records", Internet Draft, draft-ietf-asid-changelog-
00.txt, November 1996.
[X.501] ITU-T Recommendation X.501 (1993), | ISO/IEC 9594-2: 1993,
Information Technology - Open Systems Interconnection - The
Directory: Models
[XEROX] Hauser, C. "Managing update conflicts in Bayou, a weakly
connected replicated storage system". Palo Alto, CA: Xerox PARC,
Computer Science Laboratory; 1995 August; CSL-95-4. [CSL-95-04]
11. Author's Address
Russel F. Weiser
Digital Signature Trust Co.
One South Main Street
Salt Lake City, Utah 84111
USA
E-mail: rweiser@digsigtrust.com
Telephone: +1-801-983-4415
Fax +1-801-983-4408
Ellen J. Stokes
IBM
11400 Burnet Rd.
Austin, Texas 78758
USA
E-mail: stokes@austin.ibm.com
Telephone: +1-512-838-3725
Fax: +1-512-838-0156
Weiser & Stokes 21 April 2000 [Page 15]
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