One document matched: draft-ietf-ldup-model-02.txt
Differences from draft-ietf-ldup-model-01.txt
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draft-ietf-ldup-model-02.txt
John Merrells
Netscape Communications Corp.
Ed Reed
Novell, Inc.
Uppili Srinivasan
Oracle, Inc.
October 22, 1999
LDAP Replication Architecture
Copyright (C) The Internet Society (1998,1999). All Rights Reserved.
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 made obsolete 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/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This draft, file name draft-ietf-ldup-model-01.txt, is intended to be
become a Proposed Standard RFC, to be published by the IETF Working
Group LDUP. Distribution of this document is unlimited. Comments
should be sent to the LDUP Replication mailing list <ldup@imc.org> or
to the authors.
This Internet-Draft expires on 22 March 2000.
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1. Abstract
This architectural document outlines a suite of schema and protocol
extensions to LDAPv3 that enables the robust, reliable, server-to-
server exchange of directory content and changes.
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 RFC 2119 [RFC2119]. The
sections below reiterate these definitions and include some additional
ones.
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2. Table of Contents
1. Abstract 2
2. Table of Contents 3
3. Introduction 5
3.1 Scope 5
3.2 Document Objectives 6
3.3 Document Non-Objectives 7
3.4 Existing Implementations 7
3.4.1 Replication Log Implementations 8
3.4.2 State-Based Implementations 8
3.5 Terms and Definitions 8
3.6 Consistency Models 9
3.7 LDAP Constraints 10
4. Directory Model 11
4.1 Replica Type 11
4.1.1 Primary Replica 11
4.1.2 Updatable Replica 12
4.1.3 Read-Only Replica 12
4.1.4 Fractional Replicas 12
4.2 Sub-Entries 12
4.3 Glue Entries 12
4.4 Unique Identifiers 12
4.5 Change Sequence Number 13
4.5.1 CSN Composition 13
4.5.2 CSN Representation 13
4.5.3 CSN Generation 14
4.5.3.1 CSN Generation - Log Based Implementation 14
4.5.3.2 CSN Generation - State Based Implementation 14
4.6 State Change Information 15
4.6.1 Entry Change State Storage and Representation 15
4.6.2 Attribute Change State Storage 16
4.6.3 Attribute Value Change State Storage 16
4.7 LDAP Update Operations 16
5. Information Model 16
5.1 Entries, Semantics and Relationships 17
5.2 Root DSE Attributes 17
5.3 Naming Context Auxiliary Object Class and Entries 17
5.4 Replica Object Class and Entries 18
5.5 Lost and Found Entry 18
5.6 Replication Agreement Object Class and Entries 18
5.6.1 Replication Schedule 19
6. Policy Information 20
6.1 Access Control 20
6.2 Schema Knowledge 21
7. LDUP Update Transfer Protocol Framework 21
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7.1 Replication Session Initiation 21
7.1.1 Authentication 22
7.1.2 Consumer Initiated 22
7.1.3 Supplier Initiated 22
7.2 Start Replication Session 22
7.2.1 Start Replication Request 22
7.2.2 Start Replication Response 23
7.2.3 Consumer Initiated, Start Replication Session 23
7.3 Update Transfer 23
7.4 End Replication Session 24
7.4.1 End Replication Request 24
7.4.2 End Replication Response 24
7.5 Integrity & Confidentiality 24
8. LDUP Update Protocols 24
8.1 Replication Updates and Update Primitives 25
8.2 Fractional Updates 25
9. LDUP Full Update Transfer Protocol 25
9.1 Supplier Initiated, Full Update, Start Replication Session 25
9.2 Full Update Transfer 26
9.3 Replication Update Generation 26
9.4 Replication Update Consumption 26
9.5 Full Update, End Replication Session 26
9.6 Interrupted Transmission 27
10. LDUP Incremental Update Transfer Protocol 27
10.1 Update Vector 27
10.2 Supplier Initiated, Incremental Update, Start Replication Session
28
10.3 Replication Update Generation 29
10.3.1 Replication Log Implementation 29
10.3.2 State-Based Implementation 29
10.4 Replication Update Consumption 29
10.5 Update Resolution Procedures 30
10.5.1 URP: Distinguished Names 30
10.5.2 URP: Orphaned Entries 30
10.5.3 URP: Distinguished Not Present 31
10.5.4 URP: Schema - Single Valued Attributes 31
10.5.5 URP: Schema - Required Attributes 31
10.5.6 URP: Schema - Extra Attributes 31
10.5.7 URP: Duplicate Attribute Values 31
10.5.8 URP: Ancestry Graph Cycle 32
10.6 Incremental Update, End Replication Session 32
10.7 Interrupted Transmission 32
11. Purging State Information 33
11.1 Purge Vector 33
11.2 Purging Deleted Entries, Attributes, and Attribute Values 33
12. Replication Configuration and Management 34
13. Time 35
14. Security Considerations 36
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15. Acknowledgements 36
16. References 37
17. Intellectual Property Notice 38
18. Copyright Notice 38
19. Authors' Address 39
20. Appendix B - LDAP Constraints 40
20.1 LDAP Constraints Clauses 40
20.2 LDAP Data Model Constraints 41
20.3 LDAP Operation Behaviour Constraints 42
20.4 New LDAP Constraints 43
20.4.1 New LDAP Data Model Constraints 43
20.4.2 New LDAP Operation Behaviour Constraints 43
3. Introduction
3.1 Scope
This architectural document provides an outline of an LDAP based
replication scheme. Further detailed design documents will draw
guidance from here.
The design proceeds from prior work in the industry, including
concepts from the ITU-T Recommendation X.525 (1993, 1997) Directory
Information Shadowing Protocol (DISP) [X525], experience with widely
deployed distributed directories in network operating systems,
electronic mail address books, and other database technologies. The
emphasis of the design is on:
1. Simplicity of operation.
2. Flexibility of configuration.
3. Manageability of replica operations among mixed heterogeneous
vendor LDAP servers under common administration.
4. Security of content and configuration information when LDAP servers
from more than one administrative authority are interconnected.
A range of deployment scenarios are supported, including multi-master
and single-master topologies. Replication networks may include
transitive and redundant relationships between LDAP servers.
The controlling framework used to define the relationships, types, and
state of replicas of the directory content is defined. In this way the
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directory content can itself be used to monitor and control the
replication network. The directory schema is extended to define object
classes, auxiliary classes, and attributes that describe areas of the
namespace which are replicated, LDAP servers which hold replicas of
various types for the various partitions of the namespace, LDAP Access
Points (network addresses) where such LDAP servers may be contacted,
which namespaces are held on given LDAP servers, and the progress of
replication operations. Among other things, this knowledge of where
directory content is located will provide the basis for dynamic
generation of LDAP referrals. [REF]
An update transfer protocol, which actually brings a replica up to
date with respect to changes in directory content at another replica,
is defined using LDAPv3 protocol extensions. The representation of
directory content and changes will be defined by the LDAP Replication
Update Transfer Protocol sub-team. Incremental and full update
transfer mechanisms are described. Replication protocols are required
to include initial population, change updates, and removal of
directory content.
Security information, including access control policy will be treated
as directory content by the replication protocols. Confidentiality
and integrity of replication information is required to be provided by
lower-level transport/session protocols such as IPSEC and/or TLS.
3.2 Document Objectives
The objectives of this document are:
a) To define the architectural foundations for LDAP Replication, so
that further detailed design documents may be written. For
instance, the Information Model, Update Transfer Protocol, and
Update Resolution Procedures documents.
b) To provide an architectural solution for each clause of the
requirements document [LDUP Requirements].
c) To preserve the LDAP Data Model and Operation Behaviour constraints
defined for LDAP in RFC 2251.
d) To avoid tying the LDUP working group to the schedule of any other
working group.
e) Not to infringe upon known registered intellectual property rights.
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3.3 Document Non-Objectives
This document does not address the following issues, as they are
considered beyond the scope of the Working Group.
a) How LDAP becomes a distributed directory. There are many issues
beyond replication that should be considered. Such as, support for
external references, algorithms for computing referrals from the
distributed directory knowledge, etc.
b) Specifying management protocols to create naming contexts or new
replicas. LDAP may be sufficient for this. The document describes
how new replicas and naming contexts are represented, in the
directory, as entries, attributes, and attribute values.
c) How transactions will be replicated. However, the architecture
should not knowingly prevent or impede them, given the Working
Group's incomplete understanding of the issues at this time.
d) The mapping or merging of disparate Schema definitions.
e) Support of overlapping replicated regions.
f) The case where separate attributes of an entry may be mastered by
different LDAP servers. This might be termed a 'Split Primary'.
Replica roles are defined in section 4.1.
g) The specification of a replication system that supports Sparse
Replication. A Sparse Replica contains a subset of the naming
context entries, being modified by an Entry Selection Filter
criteria associated with the replica. An Entry Selection Filter is
an LDAP filter expression that describes the entries to be
replicated. The design and implementation of this functionality is
not yet well enough understood to specify here.
3.4 Existing Implementations
In order to define a standard replication scheme that may be readily
implemented we must consider the architectures of current LDAP server
implementations. Existing systems currently support proprietary
replication schemes based on one of two general approaches: log-based
or state-based. Some sections of this text may specifically address
the concerns of one approach. They will be clearly marked.
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3.4.1 Replication Log Implementations
Implementations based on the original University of Michigan LDAP
server code record LDAP operations to a operation log. During a
replication session operations are replayed from this log to bring the
Consumer replica up to date. Example implementations of this type at
this time are the Innosoft, Netscape, and Open LDAP Directory Servers.
3.4.2 State-Based Implementations
Directory Server implementations from Novell and Microsoft at this
time do not replay LDAP operations from a operation log. When a
replication session occurs each entry in the Replicated Area is
considered in turn, compared against the update state of the Consumer,
and any resultant changes transmitted. These changes are a set of
assertions about the presence or absence of entries, attributes, and
their values.
3.5 Terms and Definitions
The definitions from the Replication Requirements document have been
copied here and extended.
For brevity, an LDAP server implementation is referred to throughout
as 'the server'.
The LDAP update operations; Add, Delete, Modify, Modify RDN (LDAPv2)
and Modify DN (LDAPv3), are collectively referred to as LDAP Update
Operations.
A Naming Context is a subtree of entries in the Directory Information
Tree (DIT). There may be multiple Naming Contexts stored on a single
server. Naming Contexts are defined in section 17 of [X501].
A Replica is an instance of a replicated Naming Context.
A replicated Naming Context is said to be single-mastered if there is
only one Replica where it may be updated, and multi-mastered if there
is more than one Replica where it may be updated.
A Replication Relationship is established between two or more Replicas
that are hosted on servers that cooperate to service a common area of
the DIT.
A Replication Agreement is defined between two parties of a
Replication Relationship. The properties of the agreement codify the
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Unit of Replication, the Update Transfer Protocol to be used, and the
Replication Schedule of a Replication Session.
A Replication Session is an LDAP session between the two servers
identified by a replication agreement. Interactions occur between the
two servers, resulting in the transfer of updates from the supplier
replica to the consumer replica.
The Initiator of a Replication Session is the initiating server.
A Responder server responds to the replication initiation request from
the Initiator server.
A Supplier server is the source of the updates to be transferred.
A Consumer server is the recipient of the update sequence.
The Update Transfer Protocol is the means by which the Replication
Session proceeds. It defines the protocol for exchanging updates
between the Replication Relationship partners.
A Replication Update is an LDAP Extended Operation that contains
updates to be applied to the DIT. The Update Transfer Protocol carries
a sequence of these messages from the Supplier to the Consumer.
The Update Resolution Procedures repair constraint violations that
occur when updates to a multi-mastered Replica collide.
A Fractional Entry Specification is a list of entry attributes to be
included, or a list of attributes to be excluded in a replica. An
empty specification implies that all entry attributes are included.
A Fractional Entry is an entry that contains only a subset of its
original attributes. It has been modified by a Fractional Entry
Specification.
A Fractional Replica is a replica that holds Fractional Entries of its
naming context.
3.6 Consistency Models
This replication architecture supports a loose consistency model
between replicas of a naming context. It does not attempt to provide
the appearance of a single copy of a replica. The contents of each
replica may be different, but over time they will be converging
towards the same state. This architecture is not intended to support
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LDAP Clients that require a tight consistency model, where the state
of all replicas is always equivalent.
Three levels of consistency are available to LDAP Clients, which are
characterised by their deployment topologies. Single-Server, where
there is just the naming context and no replicas. Single-master, where
there are replicas, but only one may be updated. And, multi-master,
where there is more than one replica to which LDAP update operations
may be directed. The consistency properties of each model are rooted
in their serialization of read and write operations.
1) A single-server deployment of a naming context provides tight
consistency to LDAP applications. LDAP Clients have no choice but to
direct all their operations to a single server, serializing both read
and write operations.
2) A single-mastered deployment of a naming context provides both
tight and loose consistency to LDAP applications. LDAP Clients must
direct all write operations to the single updateable replica, but may
direct their reads to any of the replicas. A client experiences tight
consistency by directing all its operations to the single updatable
replica, and loose consistency by directing any read operations to any
other replica.
3) A multi-mastered deployment of a naming context can provide only
loose consistency to LDAP applications. Across the system writes and
reads are not serialized. An LDAP Client could direct their read and
write operations to a single updatable replica, but they will not
receive tight consistency as interleaved writes could be occurring at
another replica.
Tight consistency can be achieved in a multi-master deployment for a
particular LDAP application if and only if all instances of its client
are directed towards the same updatable replica, and the application
data is not updated by any other LDAP application. Introducing these
constraints to an application and deployment of a naming-context
ensures that writes are serialized providing tight consistency for the
application.
Future work could make use of the architecture proposed in this
document as a basis for allowing clients to request session guarantees
from a server when establishing a connection.
3.7 LDAP Constraints
The LDAP Internet-Draft [LDAPv3] defines a set of Data Model and
Operation Behaviour constraints that a compliant LDAP server must
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enforce. The server must reject an LDAP Update Operation if its
application to the target entry would violate any one of these LDAP
Constraints. [Appendix B contains the original text clauses from RFC
2251, and also a summary.]
In the case of a single-server or single-mastered naming context all
LDAP Constraints are immediately enforced at the single updateable
replica. An error result code is returned to an LDAP Client that
presents an operation that would violate the constraints.
In the case of a multi-mastered naming context not all LDAP
Constraints can be immediately enforced at the updateable replica to
which the LDAP Update Operation is applied. This loosely consistent
replication architecture ensures that at each replica all constraints
are imposed, but as updates are replicated constraint violations arise
that can not be reported to the appropriate client. Any constraint
violations that occur are repaired by a set of update resolution
procedures.
Any LDAP client that has been implemented to expect immediate
enforcement of all LDAP Constraints may not behave as expected
against a multi-mastered naming context.
4. Directory Model
This section describes extensions to the LDAP Directory Model that are
required by this replication architecture.
4.1 Replica Type
Each Replica is characterized with a replica type. This may be
Primary, Updatable, or Read-Only. A Read-Only Replica may be further
defined as being Fractional.
4.1.1 Primary Replica
The Primary Replica is a full copy of the Replica, to which all
applications that require tight consistency should direct their LDAP
Operations. There can be only one Primary Replica within the set of
Replicas of a given Naming Context. It is also permissible for none
of the Replicas to be designated the Primary. The Primary Replica must
not be a Fractional Replica.
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4.1.2 Updatable Replica
An Updatable Replica is a Replica that accepts all the LDAP Update
Operations, but is not the Primary Replica. There could be none, one,
or many Updatable Replicas within the set of Replicas of a given
Naming Context. An Updatable Replica must not be a Fractional Replica.
4.1.3 Read-Only Replica
A Read-Only Replica will accept only non-modifying LDAP operations.
All modification operations shall be referred to an updateable
Replica. The server referred to would usually be a Supplier of this
Replica.
4.1.4 Fractional Replicas
Fractional Replicas must always be Read-Only. All LDAP Update
Operations must be referred to an Updatable Replica. The server
referred to would usually be a Supplier of this Fractional Replica.
4.2 Sub-Entries
Replication management entries are to be stored at the base of the
replicated naming context. They will be of a ‘subentry’ objectclass
to exclude them from regular searches. Entries with the objectclass
subentry are not returned as the result of a search unless the filter
component "(objectclass=subentry)" is included in the search filter.
4.3 Glue Entries
A glue entry is an entry that contains knowledge of its name only. No
other information is held with it. They are distinguished by a
‘glueEntry’ objectclass. Glue entries may be created during a
replication session to repair a constraint violation.
4.4 Unique Identifiers
Distinguished names can change, so are therefore unreliable as
identifiers. A Unique Identifier must therefore be assigned to each
entry as it is created. This identifier will be stored as an
operational attribute of the entry, named ‘entryUUID’. The entryUUID
attribute is single valued. The unique identifier is to be generated
by the UUID (Universally Unique IDentifier) algorithm, also known as
GUID (Globally Unique IDentifier) [UUID]. An example UUID would be,
58DA8D8F-9D6A-101B-AFC0-4210102A8DA7.
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4.5 Change Sequence Number
Change Sequence Numbers (CSNs) are used to impose a total ordering
upon the causal sequence of updates applied to all the replicas of a
naming context. Every LDAP Update Operation is assigned at least one
CSN. A Modify operation may be assigned one CSN per modification.
4.5.1 CSN Composition
A CSN is formed of four components. In order of significance they
are; the time, a change count, a Replica Identifier, and a
modification number. The CSN is composed thus to ensure the uniqueness
of every generated CSN. When CSNs are compared to determine their
ordering they are compared component by component. First the time,
then the change count, then the replica identifier, and finally the
modification number.
The time component is a year-2000-safe representation of the real
world time, with a granularity of one second. Should LDAP Update
Operations occur at different replicas, to the same data, within the
same single second, then the change count is used to further order the
changes.
Because many LDAP Update Operations, at a single replica, may be
applied to the same data in a single second, the change count
component of the CSN is provided to further order the changes. Each
replica maintains a count of LDAP update operations applied against
it. It is reset to zero at the start of each second, and is
monotonically increasing within that second, incremented for each and
every update operation. Should LDAP Update Operations occur at
different replicas, to the same data, within the same single second,
and happen to be assigned the same change count number, then the
Replica Identifier is used to further order the changes.
The Replica Identifier is the value of the RDN attribute on the
Replica Subentry. The Replica Identifier could be assigned
programmatically or administratively, in either case short values are
advised to minimise resource usage. The IA5CaseIgnoreString syntax is
used to compare and order Replica Identifier values.
The fourth and final CSN component, the modification number, is used
for ordering the modifications within an LDAP Modify operation.
4.5.2 CSN Representation
The preferred CSN representation is: yyyy mm dd hh:mi:ssz # 0xSSSS #
replica id # 0xssss
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The ‘z’ in the time stipulates that the time is expressed in GMT
without any daylight savings time offsets permitted, and the 0xssss
represents the hexadecimal representation of an unsigned integer.
Implementations must support 16 bit change counts and should support
longer ones (32, 64, or 128 bits).
An example CSN would be " 1998081018:44:31z#0x000F#1#0x0000 ". The
update assigned this CSN would have been applied at time
1998081018:44:31z happened to be the 16th operation which was applied
in that second, was made against the replica with identifier ‘1’, and
was the first modification of the operation that caused the change.
4.5.3 CSN Generation
Because Change Sequence Numbers are primarily based on timestamps,
clock differences between servers can cause unexpected change
ordering. The synchronization of server clocks is not required, though
it is preferable that clocks are accurate. If timestamps are not
accurate, and a server consistently produces timestamps which are
significantly older than those of other servers, its updates will not
have effect and the real world time ordering of updates will not be
maintained.
However, an implementation may choose to require clock
synchronisation. The Network Time Protocol [NTP] [SNTP] offers a
protocol means by which heterogeneous server hosts may be time
synchronised.
The modifications which made up an LDAP Modify operation are presented
in a sequence. This must be preserved when the resultant changes of
this operation are replicated.
4.5.3.1 CSN Generation - Log Based Implementation
The modification number component may not be required, since the
ordering of the modifications within an LDAP Modify operation have
been preserved in the operation log.
4.5.3.2 CSN Generation - State Based Implementation
The modification number component may be needed to ensure that the
order of the modifications within an LDAP Modify operation are
faithfully replicated.
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4.6 State Change Information
State changes can be introduced via either LDAP Update Operations or
via Replication Updates. A CSN is included with all changes made to an
entry, its attributes, and attribute values. This state information
must be recorded for the entry to enable a total ordering of updates.
The CSN recorded is the CSN assigned to the state change at the server
where the state change was first made. CSNs are only assigned to state
changes that originate from LDAP Update Operations.
Each of the LDAP Update Operations change their target entry in
different ways, and record the CSN of the change differently. The
state information for the resultant state changes are recorded at
three levels. The entry level, attribute level, and attribute value
level. The state change may be shown through.
1) The creation of a deletion CSN for the entry, an attribute, or an
attribute value.
2) In the addition of a new entry, attribute or attribute value, and
its existence CSN.
3) An update to an existing attribute, attribute value, entry
distinguished name, or entry superior name, and its update CSN.
4.6.1 Entry Change State Storage and Representation
When an entry is created, with the LDAP Add operation, the CSN of the
change is added to the entry as the value of an operational attribute
named 'createdEntryCSN', of syntax type LDAPChangeSequenceNumber.
createdEntryCSN ::= csn
Deleted entries are marked as deleted by the addition of the object
class 'deletedEntry'. The attribute 'deletedEntryCSN', of syntax type
LDAP Change Sequence Number, is added to record where and when the
entry was deleted. Deleted entries are not visible to LDAP clients -
they may not be read, they don't appear in lists or search results,
and they may not be changed once deleted. Names of deleted entries
are available for reuse by new entries immediately after the deleted
entry is so marked. It may be desirable to allow deleted entries to be
accessed and manipulated by management and data recovery applications,
but that is outside the scope of this document.
deletedEntryCSN ::= csn
A CSN is recorded for both the RDN, and the Superior DN of the entry.
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4.6.2 Attribute Change State Storage
When all values of an attribute have been deleted, the attribute is
marked as deleted and the CSN of the deletion is recorded. The deleted
state and CSN are stored by the server, but have no representation on
the entry, and may not be the subject of a search operation. This
state information must be stored to enable the Update Resolution
Procedures to be performed.
4.6.3 Attribute Value Change State Storage
The Modification CSN for each value is to be set by the server when it
accepts a modification request to the value, or when a new value with
a later Modification CSN is received via Replication. The modified
value and the Modification CSN changes are required to be atomic, so
that the value and its Modification CSN cannot be out of synch on a
given server. The state information is stored by the server, but it
has no representation on the entry, and may not be the subject of a
search operation.
When the value of an attribute is deleted the state of its deletion
must be recorded, with the CSN of the modifying change. It must be
stored to enable the Update Resolution Procedures to be performed.
4.7 LDAP Update Operations
The server must reject LDAP client update operations with a CSN that
is older than the state information that would be replaced if the
operation were performed. This could occur in a replication topology
where the difference between the clocks of updateable replicas was too
large. Result code 72, serverClocksOutOfSync, is returned to the
client.
5. Information Model
This section describes the object classes of the entries that
represent the replication topology. The where, when and how of Naming
Context replication is administered through these entries. The LDUP
Working Group will publish an Internet Draft to fully detail all these
schema elements. [LDUP Info]
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5.1 Entries, Semantics and Relationships
A hierarchy of entries is defined that describe a Naming Context, its
Replicas, and its Replication Agreements.
The Naming Context Auxiliary Class is added to container entries that
may have separately defined replication policy. [LDUP Info]
Immediately subordinate to a Naming Context entry are the Replica
Subentry container entries that identify its LDAP Access Point, its
Replica Type, and if it is a Fractional Replica, the attributes it
does or does not hold. The attribute value of the entry’s Relative
Distinguished Name (RDN) is termed the Replica Identifier and is used
as a component of each CSN.
Immediately subordinate in the namespace to a Replica Subentry are
Replication Agreement leaf entries which each identify another
Replica, the scheduling policy for replication operations, including
times when replication is to be performed, when it is not to be
performed, or the policies governing event-driven replication
initiation.
5.2 Root DSE Attributes
The Root DSE attribute 'replicaRoot', publishes the names of the
Replicas that are held on that server. Each value of the attribute is
the Distinguished Name of the root entry of the Replicated Area.
5.3 Naming Context Auxiliary Object Class and Entries
Each Naming Context contains attributes which hold common
configuration and policy information for all replicas of the Naming
Context.
A Naming Context Creation attribute records when and where the Naming
Context was created.
The Access Control Policy OID attribute defines the syntax and
semantics of Access Control Information for entries within the Naming
Context.
The Naming Context is based at the entry given the auxiliary class,
and continues down the tree until another Naming Context is
encountered.
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5.4 Replica Object Class and Entries
Each Replica is characterized by a replica type. This may be Primary,
Updatable, or Read-Only. The latter two types may be further defined
as being Fractional. The Replica entry will include a Fractional Entry
Specification for a Fractional Replica.
There is a need to represent network addresses of servers holding
replicas and participating in Replication Agreements. The X.501
Access Point syntax is not sufficient, in that it is tied specifically
to OSI transports. Therefore, a new syntax will be defined for LDAP
which serves the same purpose, but uses IETF-style address
information. [LDUP Info]
An Update Vector describes the point to which the Replica has been
updated, in respect to all the other Replicas of the Naming Context.
The vector is used at the initiation of a replication session to
determine the sequence of updates that should be transferred.
The intent is to enable distributed operations in LDAP with the
replica information stored there, but not to complete the process of
turning LDAP into a fully distributed service.
5.5 Lost and Found Entry
When replicating operations between servers, conflicts may arise that
cause a parent entry to be removed causing its child entries to become
orphaned. In this case the Update Resolution Procedures will make the
Lost and Found Entry the child’s new superior.
Each Replica Entry names it’s Lost and Found Entry, which would
usually be an entry below the Replica Entry itself. This well known
place allows administrators, and their tools, to find and repair
abandoned entries.
5.6 Replication Agreement Object Class and Entries
The Replication Agreement defines:
1. The schedule for Replication Sessions initiation.
2. The server that initiates the Replication Session, either the
Consumer or the Supplier.
3. The authentication credentials that will be presented between
servers.
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4. The network/transport security scheme that will be employed in
order to ensure data confidentiality.
5. The replication protocols and relevant protocol parameters to be
used for Full and Incremental updates. An OID is used to identify
the update transfer protocol, thus allowing for future extensions
or bilaterally agreed upon alternatives.
6. If the Replica is Fractional, the Fractional Entry Specification.
Permission to participate in replication sessions will be controlled,
at least in part, by the presence and content of replica agreements.
The Supplier must be subject to the access control policy enforced by
the Consumer. Since the access control policy information is stored
and replicated as directory content, the access control imposed on the
Supplier by the Consumer must be stored in the Consumer's Replication
Agreement.
5.6.1 Replication Schedule
There are two broad mechanisms for initiating replication sessions:
(1) scheduled event driven and (2) change event driven. The mechanism
used to schedule replication operations between two servers is
determined by the Schedule information that is part of the Replication
Agreement governing the Replicas on those two servers. Because each
Replication Agreement describes the policy for one direction of the
relationship, it is possible that events propagate via scheduled
events in one direction, and by change events in the other.
Change event driven replication sessions are, by their nature,
initiated by suppliers of change information. The server, which the
change is made against, schedules a replication session in response to
the change itself, so that notification of the change is passed on to
other Replicas.
Scheduled event driven replication sessions can be initiated by either
consumers or suppliers of change information. The schedule defines a
calendar of time periods during which Replication Sessions should be
initiated.
Schedule information may include both scheduled and change event
driven mechanisms. For instance, one such policy may be to begin
replication within 15 seconds of any change event, or every 30 minutes
if no change events are received.
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6. Policy Information
Administrative policy information governs the behavior of the server
This policy information needs to be consistently known and applied by
all replicas of a Naming Context. It may be represented in the DIT as
sub-entries, attributes, and attribute values. As such, the Naming
Context Auxiliary Class provides a convenient way to define attributes
which can communicate those policies among all replicas and users of
the directory.
When replicating a naming context that is itself a subtree of another
naming context, there may be policy information stored in its
antecedent entries. The most common examples are prescriptive access
control information and inherited schema definition. Implementations
may also define other policy attributes, or sub-entries, that apply to
a whole subtree. For a naming context to be faithfully reproduced,
this generational information must also be replicated. In all cases
the policy information is transmitted as if it were an element of the
Replica root entry.
Policy information is always replicated in the same manner as any
other entries, attributes, and attribute values.
6.1 Access Control
The Access Control Models supported by a server are identified by the
'accessControlScheme' multi-valued attribute of the Root DSE entry.
Each model is assigned an OID so that Consumers and Suppliers can
determine if their access control policy will be faithfully imposed
when replicated.
An access control policy must be consistently applied by all servers
holding replicas of the same Naming Context. Therefore, the Access
Control Policy attribute is to be an operational attribute of the
Naming Context Auxiliary Class. Thus, any consumer of the directory,
and any server which would replicate a Naming Context, will know that
an Access Control Policy is defined for the Naming Context, and by
reference to the OID value of this attribute, know what policy
mechanism to invoke to enforce that policy. Administrators are
strongly cautioned against placing replicas of naming contexts on
servers that cannot enforce the policy required by the Access Control
Policy OID. Servers should refuse to accept replicas with policies
they are unable to properly interpret.
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6.2 Schema Knowledge
Schema subentries should be subordinate to the naming contexts to
which they apply. Given our model, a single server may hold replicas
of several naming contexts. It is therefore essential that schema
should not be considered to be a server-wide policy, but rather to be
scoped by the namespace to which it applies.
Schema modifications replicate in the same manner as other directory
data. Given the strict ordering of replication events, schema
modifications will naturally be replicated prior to entry creations
which use them, and subsequent to data deletions which eliminate
references to schema elements to be deleted. Servers should not
replicate information about entries which are not defined in the
schema. Servers should not replicate modifications to existing schema
definitions for which there are existing entries and/or attributes
which rely on the schema element.
Should a schema change cause an entry to be in violation of the new
schema, it is recommended that the server preserve the entry for
administrative repair. The server could add a known object class to
make the entry valid and to mark the entry for maintenance.
7. LDUP Update Transfer Protocol Framework
A Replication Session occurs between a Supplier server and Consumer
server over an LDAP connection. This section describes the process by
which a Replication Session is initiated, started and stopped.
The session initiator, termed the Initiator, could be either the
Supplier or Consumer. The Initiator sends an LDAP extended operation
to the Responder identifying the replication agreement being acted on.
The Supplier then sends a sequence of updates to the Consumer.
All transfers are in one direction only. A two way exchange requires
two replication sessions; one session in each direction.
7.1 Replication Session Initiation
The Initiator starts the Replication Session by opening an LDAP
connection to its Responder. The Initiator binds using the
authentication credentials provided in the Replication Agreement. The
extended LDAP operation Start Replication is then sent by the
Initiator to the Responder. This operation identifies which role each
server will perform, and what type of replication is to be performed.
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One server is to be the Consumer, the other the Supplier, and the
replication may be either Full or Incremental. If the Responder does
not support the requested type of replication then an error is
returned.
7.1.1 Authentication
The initiation of a Replication Session is to be restricted to only
permitted clients. The identity and credentials of a connected server
are determined via the bind operation. Access control on the
Replication Agreement determines if the Replication Session may
proceed. Otherwise, the insufficientAccessRights error is returned.
7.1.2 Consumer Initiated
The Consumer binds to the Supplier using the authentication
credentials provided in the Replication Agreement. The Consumer sends
the Start Replication extended request to begin the Replication
Session. The Supplier returns a Start Replication extended response
containing a response code. The Consumer then disconnects from the
Supplier. If the Supplier has agreed to the replication session
initiation, it binds to the Consumer and behaves just as if the
Supplier initiated the replication.
7.1.3 Supplier Initiated
The Supplier binds to the Consumer using the authentication
credentials provided in the Replication Agreement. The Supplier sends
the Start Replication Request extended request to begin the
Replication Session. The Consumer returns a Start Replication extended
response containing a response code, and possibly its Update Vector.
If the Consumer has agreed to the Replication Session initiation, then
the transfer protocol begins.
7.2 Start Replication Session
7.2.1 Start Replication Request
The LDUP Protocol document [LDUP Protocol] defines an LDAP Extended
Request, Start Replication Request, that is sent from the Initiator to
the Responder. The parameters of the Start Replication Request
include: the Distinguished Name of the entry at the root of the Naming
Context, the Replica Identifier of the Initiator, the Update Transfer
Protocol OID, Replica Number Table, and an Ordering flag.
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The DN and Replica Identifier allow the Responder to determine which
Replication Agreement is being acted on.
The Update Transfer Protocol OID identifies the Update Transfer
Protocol that the Initiator wishes to be used. This document defines
two Protocols, one for full update and one for incremental update.
The Replica Number Table provides a mapping from Replica Identifiers
(the RDN of the Replica Sub-Entry) to Replica Numbers (a small
integer). The Supplier sends Replica Numbers instead of Replica
Identifiers to reduce network bandwidth requirements.
The Supplier server uses the Ordering flag to inform the Consumer of
the ordering of the Replication Update sequence transferred during the
Replication Session. The Consumer can make use of this knowledge
should the session be interrupted.
7.2.2 Start Replication Response
The LDUP Protocol document [LDUP Protocol] defines an LDAP Extended
Response, Start Replication Response, that is sent in reply to a Start
Replication Request, from the Responder to the Initiator. The
parameters of the Start Replication Response include an response code,
and an optional Update Vector.
7.2.3 Consumer Initiated, Start Replication Session
The Supplier Responder need not return its Update Vector to the
Consumer Initiator, as it is not needed in this case.
7.3 Update Transfer
Each Update Transfer Protocol is identified by an OID. An LDUP
conformant server implementation must support the two update protocols
defined here, and may support many others. A server will advertise its
protocols in the Root DSE multi-valued attribute
'supportedReplicationProtocols'.
The details of the two mandatory to implement protocols are defined by
the LDUP Protocol Internet Draft [LDUP Protocol]. One protocol
provides a Full Update for initialisation and re-initialisation of a
replica, and the other protocol maintains that replica via an
Incremental Update.
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7.4 End Replication Session
A Replication Session is terminated by the Supplier by sending an End
Replication LDAP extended request, see section 7.4.1. The purpose of
the request and response operations is to carry the Update Vector from
the Supplier to the Consumer in the Full Update case, and to convey
the Update Vector from the Consumer to the Supplier in the Incremental
Update case.
7.4.1 End Replication Request
The LDUP Protocol document [LDUP Protocol] defines an LDAP Extended
Request, End Replication Request, that is sent from the Initiator to
the Responder. The End Replication Request includes a flag for the
Supplier to request the Consumers Update Vector.
When the update has completed the Supplier sends this extended request
to inform the Consumer that all updates have been sent, and to advise
the Consumer if its Update Vector should be returned.
7.4.2 End Replication Response
The LDUP Protocol document [LDUP Protocol] defines an LDAP Extended
Response, End Replication Response, that is sent in reply to an End
Replication Request, from the Responder to the Supplier. The Response
can optionally include an Update Vector.
If the ‘return update vector’ flag in the request was set then the
Consumer should return its Update Vector to the Supplier.
7.5 Integrity & Confidentiality
Data integrity (ie, protection from unintended changes) and
confidentiality (ie, protection from unintended disclosure to
eavesdroppers) SHOULD be provided by appropriate selection of
underlying transports, for instance TLS, or IPSEC. Replication MUST
be supported across TLS LDAP connections. Servers MAY be configured
to refuse replication connections over unprotected TCP connections.
8. LDUP Update Protocols
This Internet-Draft defines two transfer protocols for the supplier to
push changes to the consumer. Other protocols could be defined to
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transfer changes, including those which pull changes from the supplier
to the consumer, but those are left for future work.
8.1 Replication Updates and Update Primitives
Both LDUP Update Protocols define how Replication Updates are
transferred from the Supplier to the Consumer. Each Replication Update
consists of a set of Update Primitives that describe the state changes
that have been made to a single entry. Each Replication Update
contains a single Unique Identifier that addresses the entry to which
the Update Primitives are to be applied.
There are seven types of Update Primitive each of which codifies an
assertion about the state of an entry. They assert the presence or
absence of an entry, the name of the entry, the presence or absence of
its attributes, and the presence or absence its attribute values. An
assertion based approach has been chosen so that the Primitives are
idempotent. Meaning that re-application of a Primitive to an Entry
will cause no change to the entry. This is desirable as it provides
some resilience against some kinds of system failures.
Each Update Primitive contains a CSN that denotes the unique point in
the total ordering of primitives that this primitive appears. The
Supplier maps the Replica Identifier component of the CSN onto a
Replica Number before transmission. The Consumer uses the provided
Replica Number Table to map this back onto the Replica Identifier.
The Update Primitives are fully defined in the LDUP Update
Reconciliation Procedures Internet Draft [LDUP URP].
8.2 Fractional Updates
When fully populating or incrementally bringing up to date a
Fractional Replica each of the Replication Updates must only contain
updates to the attributes in the Fractional Entry Specification.
9. LDUP Full Update Transfer Protocol
9.1 Supplier Initiated, Full Update, Start Replication Session
The Consumer Responder need not return its Update Vector to the
Supplier Initiator, as it is not needed in this case.
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9.2 Full Update Transfer
This Full Update Protocol provides a bulk transfer of the replica
contents for the initial population of new replicas, and the
refreshing of existing replicas.
The Consumer must replace its entire replica contents with that sent
from the Supplier.
The Consumer need not service any requests for this Naming Context
whilst the full update is in progress. The Consumer could return a
referral to another replica, possibly the supplier. [REF]
9.3 Replication Update Generation
The entire state of a Replicated Area can be mapped onto a sequence of
Replication Updates, each of which contains a sequence of Update
Primitives that describe the entire state of a single entry.
The sequence of Replication Updates must be ordered such that no entry
is created before its parent.
9.4 Replication Update Consumption
A Consumer will receive the Replication Updates, extract the sequence
of Update Primitives, and must apply them to the DIB in the order
provided.
9.5 Full Update, End Replication Session
Since the Full Update also replicates the sub-entry that represents
the Supplier Replica the Consumer will have received the Update Vector
that represents the update state of the Consumer.
After a Full Update transfer the Supplier sends the Update Vector that
reflects the update state of the full replica information sent. The
Consumer records this as its Update Vector.
The Supplier could be accepting updates whilst the update is in
progress. Once the Full Update has completed, an Incremental Update
should be performed to transfer these changes.
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9.6 Interrupted Transmission
If the Replication Session terminates before the End Replication
Request is sent, then the Replica is invalid and must be re-
initialised. The Consumer must not permit LDAP Clients to access the
incomplete replica. The Consumer could refer the Client to the
Supplier Replica, or return an error result code.
10. LDUP Incremental Update Transfer Protocol
For efficiency, the Incremental Update Protocol transmits only those
changes that have been made to the Supplier replica that the Consumer
has not already received. In a replication topology with transitive
redundant replication agreements, changes may propagate through the
replica network via different routes.
The Consumer must not support multiple concurrent replication sessions
with more than one Supplier for the same Naming Context. A Supplier
that attempts to initiate a Replication Session with a Consumer
already participating as a Consumer in another Replication Session
will receive the busy error code.
10.1 Update Vector
The Supplier uses the Consumer's Update Vector to determine the
sequence of updates that should be sent to the Consumer.
Each Replica entry includes an Update Vector to record the point to
which the replica has been updated. The vector is a set of CSN values,
one value for each known updateable Replica. Each CSN is the most
recent change, made at that Replica, that has been replicated to this
Replica.
For example, consider two updatable replicas of a naming context, one
is assigned replica identifier ‘1’, the other replica identifier ‘2’.
Each is responsible for maintaining its own update vector, which will
contain two CSNs, one for each replica. So, if both replicas are
identical they will have equivalent update vectors.
Both Update Vectors =
{1998081018:44:31z#0x000F#1#0x0000, 1998081018:51:20z#0x0001#2#0x0000}
Subsequently, at 7pm, an update is applied to replica ‘2’, so its
update vector is updated.
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Replica ‘1’ Update Vector =
{1998081018:44:31z#0x000F#1#0x0000, 1998081018:51:20z#0x0001#2#0x0000}
Replica ‘2’ Update Vector =
{1998081018:44:31z#0x000F#1#0x0000, 1998081019:00:00z#0x0000#2#0x0000}
Since the Update Vector records the state to which the replica has
been updated, a supplier server, during Replication Session
initiation, can determine the sequence of updates that should be sent
to the consumer. From the example above no updates need to be sent
from replica ‘1’ to replica ‘2’, but there is an update pending from
replica ‘2’ to replica ‘1’.
Because the Update Vector embodies knowledge of updates made at all
known replicas it supports replication topologies that include
transitive and redundant connections between replicas. It ensures that
changes are not transferred to a consumer multiple times even though
redundant replication agreements may exist. It also ensures that
updates are passed across the replication network between replicas
that are not directly linked to each other.
It may be the case that a CSN for a given replica is absent, for one
of two reasons.
1. CSNs for Read-Only replicas might be absent because no changes will
have ever been applied to that Replica, so there are no changes to
replicate.
2. CSNs for newly created replicas may be absent because no changes to
that replica have yet been propagated.
An Update Vector might also contain a CSN for a replica that no longer
exists. The replica may have been temporarily taken out of service,
or may have been removed from the replication topology permanently. An
implementation may choose to retire a CSN after some configurable time
period.
10.2 Supplier Initiated, Incremental Update, Start Replication Session
The Consumer Responder must return its Update Vector to the Supplier
Initiator. The Supplier uses this to determine the sequence of
Replication Updates that need to be sent to the Consumer.
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10.3 Replication Update Generation
The Supplier generates a sequence of Replication Updates to be sent to
the consumer. To enforce LDAP Constraint 20.1.6, that the LDAP Modify
must be applied atomically, each Replication Update must contain the
entire sequence of Update Primitives for all the LDAP Operations for
which the Replication Update contains Update Primitives. Stated less
formally, for each primitive the update contains, it must also contain
all the other primitives that came from the same operation.
10.3.1 Replication Log Implementation
A log-based implementation might take the approach of mapping LDAP
Operations onto an equivalent sequence of Update Primitives. A
systematic procedure for achieving this is fully described in the LDUP
Update Reconciliation Procedures Internet Draft [LDUP URP].
The Consumer Update Vector is used to determine the sequence of LDAP
Operations in the operation log that the Consumer has not yet seen.
10.3.2 State-Based Implementation
A state-based implementation might consider each entry of the replica
in turn using the Update Vector of the consumer to find all the state
changes that need to be transferred. Each state change (entry,
attribute, or value - creation, deletion, or update) is mapped onto
the equivalent Update Primitive. All the Update Primitives for a
single entry might be collected into a single Replication Update.
Consequently, it could contain the resultant primitives of many LDAP
operations.
10.4 Replication Update Consumption
A Consumer will receive Replication Updates, extract the sequence of
Update Primitives, and must apply them to the DIB in the order
provided. LDAP Constraint 20.1.6 states that the modifications within
an LDAP Modify operation must be applied in the sequence provided.
Those Update Primitives must be reconciled with the current replica
contents and any previously received updates. In broad outline,
updates are compared to the state information associated with the item
being operated on. If the change has a more recent CSN, then it is
applied to the directory contents. If the change has an older CSN it
is no longer relevant and its change must not be effected.
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If the consumer acts as a supplier to other replicas then the updates
are retained for forwarding.
10.5 Update Resolution Procedures
The LDAP Update Operations must abide by the constraints imposed by
the LDAP Data Model and LDAP Operational Behaviour, Appendix B. An
operation that would violate at least one of these constraints is
rejected with an error result code.
The loose consistency model of this replication architecture and its
support for multiple updateable replicas of a naming context means
that LDAP Update Operations may be accepted at one replica, which
would not be at another. At the time of acceptance, the accepting
replica may not have received other updates that would cause a
constraint to be violated, and the operation to be rejected.
Replication Updates must never be rejected because of a violation of
an LDAP Constraint. If the result of applying the Replication Update
causes a constraint violation to occur, then some remedial action must
be taken to satisfy the constraint. These Update Resolution Procedures
are introduced here, and fully described in the LDAP Update Resolution
Procedures Internet-Draft [LDUP URP].
10.5.1 URP: Distinguished Names
LDAP Constraints 20.1.1 and 20.1.10 ensure that each entry in the
replicated area has a unique DN. A Replication Update could violate
this constraint producing two entries, with different unique
identifiers, but with the same DN. The resolution procedure is to
rename the most recently named entry so that its RDN includes its own
unique identifier. This ensures that the new DN of the entry shall be
unique.
10.5.2 URP: Orphaned Entries
LDAP Constraints 20.1.11 ensures that every entry must have a parent
entry. A Replication Update could violate this constraint producing an
entry with no parent entry. The resolution procedure is to create a
Glue Entry to take the place of the absent parent. The Glue Entry’s
superior will be the Lost and Found Entry. This well known place
allows administrators and their tools to find and repair abandoned
entries.
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10.5.3 URP: Distinguished Not Present
LDAP Constraints 20.1.8 and 20.1.9 ensure that the components of an
RDN appear as attribute values of the entry. A Replication Update
could violate this constraint producing an entry without its
distinguished values. The resolution procedure is to add the missing
attribute values, and mark them as distinguished not present, so that
they can be deleted when the attribute values are no longer
distinguished.
10.5.4 URP: Schema - Single Valued Attributes
LDAP Constraint 20.1.7 enforces the single-valued attribute schema
restriction. A Replication Update could violate this constraint
creating a multi-value single-valued attribute. The resolution
procedure is to consider the value of a single-valued attribute as
always being equal. In this way the most recently added value will be
retained, and the older one discarded.
10.5.5 URP: Schema - Required Attributes
LDAP Constraint 20.1.7 enforces the schema objectclass definitions on
an entry. A Replication Update could violate this constraint creating
an entry that does not have attribute values for required attributes.
The resolution procedure is to ignore the schema violation and mark
the entry for administrative repair.
10.5.6 URP: Schema - Extra Attributes
LDAP Constraint 20.1.3 and 20.1.7 enforces the schema objectclass
definitions on an entry. A Replication Update could violate this
constraint creating an entry that has attribute values not allowed by
the objectclass values of the entry. The resolution procedure is to
ignore the schema violation and mark the entry for administrative
repair.
10.5.7 URP: Duplicate Attribute Values
LDAP Constraint 20.1.5 ensures that the values of an attribute
constitute a set of unique values. A Replication Update could violate
this constraint. The resolution procedure is to enforce this
constraint, recording the most recently assigned CSN with the value.
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10.5.8 URP: Ancestry Graph Cycle
LDAP Constraint 20.4.2.1 prevents against a cycle in the DIT. A
Replication Update could violate this constraint causing an entry to
become it’s own parent, or for it to appear even higher in it’s
ancestry graph. The resolution procedure is to break the cycle by
changing the parent of the entry closest to be the lost and found
entry.
10.6 Incremental Update, End Replication Session
If the Supplier sent none of its own updates to the Consumer, then the
Supplier's CSN within the Supplier's update vector should be updated
with the earliest possible CSN that it could generate, to record the
time of the last successful replication session. The Consumer will
have received the Supplier's Update Vector in the replica sub-entry it
holds for the Supplier replica.
The Consumer's resultant Update Vector CSN values will be at least as
great as the Supplier's Update Vector.
The Supplier may request that the Consumer return its resultant Update
Vector so that the Supplier can update its replica sub-entry for the
Consumer Replica. The Supplier requests this by setting a flag in the
End Replication Request. The default flag value is TRUE meaning the
Consumer Update Vector must be returned.
10.7 Interrupted Transmission
If the Replication Session terminates before the End Replication
Request is sent then the Consumer’s Update Vector may or may not be
updated to reflect the updates received. The Start Replication request
includes a Replication Update Ordering flag which states whether the
updates were sent in CSN order per replica.
If updates are sent in CSN order per replica then it is possible to
update the Consumer Update Vector to reflect that some portion of the
updates to have been sent have been received and successfully applied.
The next Incremental Replication Session will pick up where the failed
session left off.
If updates are not sent in CSN order per replica then the Consumer
Update can not be updated. The next Incremental Replication Session
will begin where the failed session began. Some updates will be
replayed, but because the application of Replication Updates is
idempotent they will not cause any state changes.
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11. Purging State Information
The state information stored with each entry need not be stored
indefinitely. A server implementation may choose to periodically, or
continuously, remove state information that is no longer required. The
mechanism is implementation-dependent, but to ensure interoperability
between implementations, the state information must not be purged
until all known replicas have received and acknowledged the change
associated with a CSN. This is determined from the Purge Vector
[11.1].
All the CSNs stored that are lower than the Purge Vector may be
purged, because no changes with older CSNs can be replicated to this
replica.
11.1 Purge Vector
The Purge Vector is an Update Vector constructed from the Update
Vectors of all known replicas. Each replica has a sub-entry for each
known replica stored below its naming context. Each of those entries
contains the last known update vector for that replica. The lowest CSN
for each replica are taken from these update vectors to form the Purge
Vector. The Purge Vector is used to determine when state information
and updates need no longer be stored.
11.2 Purging Deleted Entries, Attributes, and Attribute Values
The following conditions must hold before an item can be deleted from
the Directory Information Base.
1) The LDAP delete operation has been propagated to all replication
agreement partners.
2) All the updates from all the other replicas with CSNs less than the
CSN on the deletion have been propagated to the server holding the
deleted entry (similarly for deleted attributes and attribute values).
3) The CSN generator of the other Replicas must have advanced beyond
the deletion CSN of the deleted entry. Otherwise, it is possible for
one of those Replicas to generate operations with CSNs earlier than
the deleted entry.
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12. Replication Configuration and Management
Replication management entries, such as replica or replication
agreement entries, can be altered on any updateable replica. These
entries are implicitly included in the directory entries governed by
any agreement associated with this naming context. As a result, all
servers with a replica of a naming context will have access to
information about all other replicas and associated agreements.
The deployment and maintenance of a replicated directory network
involves the creation and management of all the replicas of a naming
context and replication agreements among these replicas. This section
outlines, through an example, the administrative actions necessary to
create a new replica and establish replication agreements. Typically,
administrative tools will guide the administrator and facilitate these
actions. The objective of this example is to illustrate the
architectural relationship among various replication related
operational information.
A copy of an agreement should exist on both the supplier and consumer
side for the replication update transfer protocol to be able to start.
For this purpose, the root of the naming context, replica objects and
the replication agreement objects are created first on one of the
servers. A copy of these objects are then manually created on the
second server associated with the agreement.
The scenario below starts with a server (named DSA1) that holds an
updateable replica of a naming context NC1. Procedures to establish
an updateable replica of the naming context on a second server (DSA2)
are outlined.
On DSA1:
1) Add the context prefix for NC1 to the Root DSE attribute
'replicaRoot' if it does not already exist.
2) Alter the 'ObjectClass' attribute of the root entry of NC1 to
include the "namingContext" auxiliary class.
3) Create a replica object, NC1R1, (as a child of the root of NC1) to
represent the replica on DSA1. The attributes include replica type
(updateable, read-only etc.) and DSA1 access point information.
4) Create a copy of the replica object NC1R2 (after it is created on
DSA2)
5) Create a replication agreement, NC1R1-R2 to represent update
transfer from NC1R1 to NC1R2. This object is a child of NC1R1.
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On DSA2:
1) Add NC1's context prefix to the Root DSE attribute 'replicaRoot'.
2) Create a copy of the root entry of NC1 as a copy of the one in DSA1
(including the namingContext auxiliary class)
3) Create a copy of the replica object NC1R1
4) Create a second replica object, NC1R2 (as a sibling of NC1R1) to
represent the replica on DSA2.
5) Create a copy of the replication agreement, NC1R1-R2
6) Create a replication agreement, NC1R2-R1, to represent update
transfer from NC1R2 to NC1R1. This object is a sibling of NC1R1-
R2.
After these actions update transfer to satisfy either of the two
agreements can commence.
If data already existed in one of the replicas, the update transfer
protocol should perform a complete update of the data associated with
the agreement before normal replication begins.
13. Time
The server assigns a CSN for every LDAP update operation it receives.
Since the CSN is principally based on time, the CSN is susceptible to
the Replica clocks drifting in relation to each other (either forwards
or backwards).
The server must never assign a CSN older than or equal to the last CSN
it assigned.
The server must reject update operations, from any source, which would
result in setting a CSN on an entry or a value which is earlier than
the one that is there. The error code serverClocksOutOfSync (72)
should be returned.
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14. Security Considerations
The preceding architecture discussion covers the server
authentication, session confidentiality, and session integrity in
sections 7.1.1 and 7.5
The internet draft "Authentication Methods" for LDAP, provides a
detailed LDAP security discussion. Its introductory passage is
paraphrased below. [AUTH]
A Replication Session can be protected with the following security
mechanisms.
1) Authentication by means of the SASL mechanism set, possibly backed
by the TLS credentials exchange mechanism,
2) Authorization by means of access control based on the Initiators
authenticated identity,
3) Data integrity protection by means of the TLS protocol or data-
integrity SASL mechanisms,
4) Protection against snooping by means of the TLS protocol or data-
encrypting SASL mechanisms,
The configuration entries that represent Replication Agreements may
contain authentication information. This information must never be
replicated between replicas.
Updates to a multi-mastered entry may collide causing the Update
Resolution Procedures [10.5] to reject or reverse one of the changes
to the entry. The URP algorithms resolve conflicts by using the total
ordering of updates imposed by the assignment of CSNs for every
operation. As a consequence updates originating from system
administrators have no priority over updates originating from regular
system users.
15. Acknowledgements
This document is a product of the LDUP Working Group of the IETF. The
contributions of its members is greatly appreciated.
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16. References
[AUTH] - M. Wahl, H. Alvestrand, J. Hodges, RL "Bob" Morgan,
"Authentication Methods for LDAP", Internet Draft, draft-ietf-ldapext-
authmeth-02.txt, June 1998.
[BCP-11] - R. Hovey, S. Bradner, "The Organizations Involved in the
IETF Standards Process", BCP 11, RFC 2028, October 1996.
[LDAPv3] - M. Wahl, S. Kille, T. Howes, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December1997.
[LDUP Info.] - E. Reed, "LDUP Replication Information Model", Internet
Draft, draft-reed-ldup-infomod-00-1.txt, August 1998.
[LDUP Protocol] - G. Good, E. Stokes 'The LDUP Replication Update
Protocol' , Internet Draft, draft-ietf-ldup-protocol-00.txt, May 1999.
[LDUP Requirements] - R. Weiser, E. Stokes 'LDAP Replication
Requirements', Internet Draft, draft-weiser-replica-req-02.txt, April
1998.
[LDUP URP] - S. Legg 'LDUP Update Reconciliation Procedures',
Internet Draft, draft-legg-ldup-urp-00.txt, February 1999.
[NTP] - D. L. Mills, "Network Time Protocol (Version 3)", RFC 1305,
March, 1992.
[REF] - T. Howes, Mark Wahl, "Referrals and Knowledge References in
LDAP Directories", Internet draft, draft-ietf-ldapext-referral-00.txt,
March 1998.
[RFC2119] - S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119.
[RFC2252] - M. Wahl, A. Coulbeck, T. Howes, S. Kille, 'Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions', RFC
2252, December 1997.
[SNTP] - D. L. Mills, "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 2030, University of Delaware, October
1996.
[TLS] - J. Hodges, R. L. "Bob" Morgan, M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for Transport
Layer Security", Internet draft, draft-ietf-ldapext-ldapv3-tls-01.txt,
June 1998.
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[UUID] - P. Leach, R. Salz, "UUIDs and GUIDs", Internet draft, draft-
leach-uuids-guids-01.txt, February 1998.
[X501] - ITU-T Recommendation X.501 (1993), ) | ISO/IEC 9594-2:1993,
Information Technology - Open Systems Interconnection - The Directory:
Models
[X680] - ITU-T Recommendation X.680 (1994) | ISO/IEC 8824-1:1995,
Information technology - Abstract Syntax Notation One (ASN.1):
Specification of Basic Notation
[X525] - ITU-T Recommendation X.525 (1997) | ISO/IEC 9594-9:1997,
Information Technology - Open Systems Interconnection - The Directory:
Replication
17. Intellectual Property Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it has
made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. [BCP-11]
Copies of claims of rights made available for publication and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
18. Copyright Notice
Copyright (C) The Internet Society (1998,1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
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distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be followed,
or as required to translate it into languages other than English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
19. Authors' Address
John Merrells
Netscape Communications, Inc.
501 East Middlefield Road
Mountain View
CA 94043
E-mail: merrells@netscape.com
Phone: +1 650-937-5739
Edwards E. Reed
Reed-Matthews, Inc.
E-mail: eer@oncalldba.com
Phone: +1 801-785-0315
Uppili Srinivasan
Oracle, Inc.
Redwood Shores
E-mail: usriniva@us.oracle.com
Phone: +1 650 506 3039
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LDUP Engineering Mailing List: ldup-repl@external.cisco.com
LDUP Working Group Mailing List: ietf-ldup@imc.org
20. Appendix B - LDAP Constraints
20.1 LDAP Constraints Clauses
This is an enumeration of the Data Model and Operation Behaviour
constraint clauses defined in RFC 2251. [LDAPv3]
1) Data Model - Entries have names: one or more attribute values from
the entry form its relative distinguished name (RDN), which MUST be
unique among all its siblings. (p5)
2) Data Model - Attributes of Entries - Each entry MUST have an
objectClass attribute. (p6)
3) Data Model - Attributes of Entries - Servers MUST NOT permit
clients to add attributes to an entry unless those attributes are
permitted by the object class definitions. (p6)
4) Relationship to X.500 - This document defines LDAP in terms of
X.500 as an X.500 access mechanism. An LDAP server MUST act in
accordance with the X.500 (1993) series of ITU recommendations when
providing the service. However, it is not required that an LDAP
server make use of any X.500 protocols in providing this service,
e.g. LDAP can be mapped onto any other directory system so long as
the X.500 data and service model as used in LDAP is not violated in
the LDAP interface. (p8)
5) Elements of Protocol - Common Elements - Attribute - Each attribute
value is distinct in the set (no duplicates). (p14)
6) Elements of Protocol - Modify Operation - The entire list of entry
modifications MUST be performed in the order they are listed, as a
single atomic operation. (p33)
7) Elements of Protocol - Modify Operation - While individual
modifications may violate the directory schema, the resulting entry
after the entire list of modifications is performed MUST conform to
the requirements of the directory schema. (p33)
8) Elements of Protocol - Modify Operation - The Modify Operation
cannot be used to remove from an entry any of its distinguished
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values, those values which form the entry's relative distinguished
name. (p34)
9) Elements of Protocol - Add Operation - Clients MUST include
distinguished values (those forming the entry's own RDN) in this
list, the objectClass attribute, and values of any mandatory
attributes of the listed object classes. (p35)
10) Elements of Protocol - Add Operation - The entry named in the
entry field of the AddRequest MUST NOT exist for the AddRequest to
succeed. (p35)
11) Elements of Protocol - Add Operation - The parent of the entry to
be added MUST exist. (p35)
12) Elements of Protocol - Delete Operation - ... only leaf entries
(those with no subordinate entries) can be deleted with this
operation. (p35)
13) Elements of Protocol - Modify DN Operation - If there was already
an entry with that name [the new DN], the operation would fail.
(p36)
14) Elements of Protocol - Modify DN Operation - The server may not
perform the operation and return an error code if the setting of
the deleteoldrdn parameter would cause a schema inconsistency in
the entry. (p36)
20.2 LDAP Data Model Constraints
The LDAP Data Model Constraint clauses as written in RFC 2251 [LDAPv3]
may be summarised as follows.
a) The parent of an entry must exist. (LDAP Constraint 11 & 12.)
b) The RDN of an entry is unique among all its siblings. (LDAP
Constraint 1.)
c) The components of the RDN must appear as attribute values of the
entry. (LDAP Constraint 8 & 9.)
d) An entry must have an objectclass attribute. (LDAP Constraint 2 &
9.)
e) An entry must conform to the schema constraints. (LDAP Constraint
3 & 7.)
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f) Duplicate attribute values are not permitted. (LDAP Constraint 5.)
20.3 LDAP Operation Behaviour Constraints
The LDAP Operation Behaviour Constraint clauses as written in RFC 2251
[LDAPv3] may be summarised as follows.
A) The Add Operation will fail if an entry with the target DN already
exists. (LDAP Constraint 10.)
B) The Add Operation will fail if the entry violates data constraints:
a - The parent of the entry does not exist. (LDAP Constraint 11.)
b - The entry already exists. (LDAP Constraint 10.)
c - The entry RDN components appear as attribute values on the
entry. (LDAP Constraint 9.)
d - The entry has an objectclass attribute. (LDAP Constraint 9.)
e - The entry conforms to the schema constraints. (LDAP
Constraint 9.)
f - The entry has no duplicated attribute values. (LDAP
Constraint 5.)
C) The modifications of a Modify Operation are applied in the order
presented. (LDAP Constraint 6.)
D) The modifications of a Modify Operation are applied atomically.
(LDAP Constraint 6.)
E) A Modify Operation will fail if it results in an entry that
violates data constraints:
c - If it attempts to remove distinguished attribute values.
(LDAP Constraint 8.)
d - If it removes the objectclass attribute. (LDAP Constraint 2.)
e - If it violates the schema constraints. (LDAP Constraint 7.)
f - If it creates duplicate attribute values. (LDAP Constraint
5.)
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F) The Delete Operation will fail if it would result in a DIT that
violates data constraints:
a - The deleted entry must not have any children. (LDAP
Constraint 12.)
G) The ModDN Operation will fail if it would result in a DIT or entry
that violates data constraints:
b - The new Superior entry must exist. (Derived LDAP Data Model
Constraint A)
c - An entry with the new DN must not already exist. (LDAP
Constraint 13.)
c - The new RDN components do not appear as attribute values on
the entry. (LDAP Constraint 1.)
d - If it removes the objectclass attribute. (LDAP Constraint 2.)
e - It is permitted for the operation to result in an entry that
violates the schema constraints. (LDAP Constraint 14.)
20.4 New LDAP Constraints
The introduction of support for multi-mastered entries, by the
replication scheme presented in this document, necessitates the
imposition of new constraints upon the Data Model and LDAP Operation
Behaviour.
20.4.1 New LDAP Data Model Constraints
1) Each entry shall have a unique identifier generated by the UUID
algorithm available through the ‘entryUUID’ operational attribute. The
entryUUID attribute is single valued.
20.4.2 New LDAP Operation Behaviour Constraints
1) The LDAP Data Model Constraints do not prevent cycles in the
ancestry graph. Existing constraints Data Model Constraint - 20.4.1
- (a) and Operation Constraint - 20.4.2 - (B) would prevent this in
the single master case, but not in the presence of multiple
masters.
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2) The LDAP Data Model Constraints state that only the LDAP Modify
Operation is atomic. All other LDAP Update Operations are also
considered to be atomically applied to the DIB.
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