INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Experimental OpenLDAP Foundation
Expires in six months Jong Hyuk Choi
IBM Corporation
3 February 2004
The LDAP Content Synchronization Operation
<draft-zeilenga-ldup-sync-05.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Distribution of this memo is unlimited. Technical discussion of this
document will take place on the IETF LDUP Working Group mailing list
at <[email protected]>. Please send editorial comments directly to
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Please see the Full Copyright section near the end of this document
for more information.
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Abstract
This specification describes the LDAP (Lightweight Directory Access
Protocol) Content Synchronization Operation. The operation allows a
client to maintain a copy of a fragment of directory information tree.
It supports both polling for changes and listening for changes. The
operation is defined as an extension of the LDAP Search Operation.
Table of Contents
Status of this Memo 1
Abstract 2
Table of Contents
1. Introduction 3
1.1. Background
1.2. Intended Usage 4
1.3. Overview 5
1.4. Conventions
2. Elements of the Sync Operation 8
2.1. Common ASN.1 Elements 9
2.2. Sync Request Control
2.3. Sync State Control
2.4. Sync Done Control 10
2.5. Sync Info Message
2.6. Sync Result Codes 11
3. Content Synchronization
3.1. Synchronization Session
3.2. Content Determination 12
3.3. refreshOnly Mode 13
3.4. refreshAndPersist Mode 16
3.5. Search Request Parameters 17
3.6. objectName Issues 18
3.7. Canceling the Sync Operation 19
3.8. Refresh Required
3.9. Chattiness Considerations 20
3.10. Operation Multiplexing 21
4. Meta Information Considerations 22
4.1. Entry DN
4.2. Operational Attributes
4.3. Collective Attributes 23
4.4. Access and Other Administrative Controls
5. Interaction with Other Controls
5.1. ManageDsaIT Control 24
5.2. Subentries Control
6. Shadowing Considerations
7. Security Considerations 25
8. IANA Considerations
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8.1. Object Identifier 26
8.2. LDAP Protocol Mechanism
8.3. LDAP Result Codes
9. Acknowledgments
10. Normative References 27
11. Informative References 28
12. Authors' Addresses 29
Appendix A. CSN-based Implementation Considerations
Intellectual Property Rights 31
Full Copyright
1. Introduction
The Lightweight Directory Access Protocol (LDAP) [RFC3377] provides a
mechanism, the search operation [RFC2251], which allows a client to
request directory content matching a complex set of assertions and for
the server to return this content, subject to access control and other
restrictions, to the client. However, LDAP does not provide (despite
the introduction of numerous extensions in this area) an effective and
efficient mechanism for maintaining synchronized copies of directory
content. This document introduces a new mechanism specifically
designed to met the content synchronization requirements of
sophisticated directory applications.
This document defines the LDAP Content Synchronization Operation, or
Sync Operation for short, which allows a client to maintain a
synchronized copy of a fragment of a Directory Information Tree (DIT).
The Sync Operation is defined as a set of controls and other protocol
elements which extend the Search Operation.
1.1. Background
Over the years, a number of content synchronization approaches have
been suggested for use in LDAP directory services. These approaches
are inadequate for one or more of the following reasons:
- fail to ensure a reasonable level of convergence;
- fail to detect that convergence cannot be achieved (without
reload);
- require pre-arranged synchronization agreements;
- require the server to maintain histories of past changes to DIT
content and/or meta information;
- require the server to maintain synchronization state on a per
client basis; and/or
- are overly chatty.
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The Sync Operation provides eventual convergence of synchronized
content when possible and, when not, notification that a full reload
is required.
The Sync Operation does not require pre-arranged synchronization
agreements.
The Sync Operation does not require servers to maintain nor to use any
history of past changes to the DIT or to meta information. However,
servers may maintain and use histories (e.g., change logs, tombstones,
DIT snapshots) to reduce the number of messages generated and to
reduce their size. As it is not always feasible to maintain and use
histories, the operation may be implemented using purely (current)
state-based approaches. The Sync Operation allows use of either the
state-based approach or the history-based approach in an operation by
operation basis to balance the size of history and the amount of
traffic. The Sync Operation also allows the combined use of the
state-based and the history-based approaches.
The Sync Operation does not require servers to maintain
synchronization state on a per client basis. However, servers may
maintain and use per client state information to reduce the number of
messages generated and the size of such messages.
A synchronization mechanism can be considered overly chatty when
synchronization traffic is not reasonably bounded. The Sync Operation
traffic is bounded by the size of updated (or new) entries and the
number of unchanged entries in the content. The operation is designed
to avoid full content exchanges even in the case that the history
information available to the server is insufficient to determine the
client's state. The operation is also designed to avoid transmission
of out-of-content history information, as its size is not bounded by
the content and it is not always feasible to transmit such history
information due to security reasons.
This document includes a number of non-normative appendices providing
additional information to server implementors.
1.2. Intended Usage
The Sync Operation is intended to be used in applications requiring
eventually-convergent content synchronization. Upon completion of
each synchronization stage of the operation, all information to
construct a synchronized client copy of the content has been provided
to the client or the client has been notified that a complete content
reload is necessary. Except for transient inconsistencies due to
concurrent operation (or other) processing at the server, the client
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copy is an accurate reflection of the content held by the server.
Transient inconsistencies will be resolved by subsequent
synchronization operations.
Possible uses include:
- White page service applications may use the Sync Operation to
maintain current copy of a DIT fragment. For example, a mail user
agent which uses the sync operation to maintain a local copy of an
enterprise address book.
- Meta-information engines may use the Sync Operation to maintain a
copy of a DIT fragment.
- Caching proxy services may use the Sync Operation to maintain a
coherent content cache.
- Lightweight master-slave replication between heterogeneous
directory servers. For example, the Sync Operation can be used by
a slave server to maintain a shadow copy of a DIT fragment.
(Note: The International Telephone Union (ITU) has defined the
X.500 Directory [X.500] Information Shadowing Protocol (DISP)
[X.525] which may be used for master-slave replication between
directory servers. Other experimental LDAP replication protocols
also exist.)
This protocol is not intended to be used in applications requiring
transactional data consistency.
As this protocol transfers all visible values of entries belonging to
the content upon change instead of change deltas, this protocol is not
appropriate for bandwidth-challenged applications or deployments.
1.3. Overview
This section provides an overview of basic ways the Sync Operation can
be used to maintain a synchronized client copy of a DIT fragment.
- Polling for Changes: refreshOnly mode
- Listening for Changes: refreshAndPersist mode
1.3.1. Polling for Changes (refreshOnly)
To obtain its initial client copy, the client issues a Sync request: a
search request with the Sync Request Control with mode set to
refreshOnly. The server, much like it would with a normal search
operation, returns (subject to access controls and other restrictions)
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the content matching the search criteria (baseObject, scope, filter,
attributes). Additionally, with each entry returned, the server
provides a Sync State Control indicating state add. This control
contains the Universally Unique Identifier (UUID) [UUID] of the entry
[EntryUUID]. Unlike the Distinguished Name (DN), which may change
over time, an entry's UUID is stable. The initial content is followed
by a SearchResultDone with a Sync Done Control. The Sync Done Control
provides a syncCookie. The syncCookie represents session state.
To poll for updates to the client copy, the client reissues the Sync
Operation with the syncCookie previously returned. The server, much
as it would with a normal search operation, determines which content
would be returned as if the operation was a normal search operation.
However, using the syncCookie as an indicator of what content the
client was sent previously, the server sends copies of entries which
have changed with a Sync State Control indicating state add. For each
changed entry, all (modified or unmodified) attributes belonging to
the content are sent.
The server may perform either or both of the two distinct
synchronization phases which are distinguished by how to synchronize
entries deleted from the content: the present and the delete phases.
When the server uses a single phase for the refresh stage, each phase
is marked as ended by a SearchResultDone with a Sync Done Control. A
present phase is identified by a FALSE refreshDeletes value in the
Sync Done Control. A delete phase is identified by a TRUE
refreshDeletes value. The present phase may be followed by a delete
phase. The two phases are delimited by a refreshPresent Sync Info
Message having a FALSE refreshDone value. In the case that both the
phases are used, the present phase is used to bring the client copy up
to the state at which the subsequent delete phase can begin.
In the present phase, the server sends an empty entry (i.e., no
attributes) with a Sync State Control indicating state present for
each unchanged entry.
The delete phase may be used when the server can reliably determine
which entries in the prior client copy are no longer present in the
content and the number of such entries is less than or equal to the
number of unchanged entries. In the delete mode, the server sends an
empty entry with a Sync State Control indicating state delete for each
entry which is no longer in the content, instead of returning an empty
entry with state present for each present entry.
The server may send syncIdSet Sync Info Messages containing the set of
UUIDs of either unchanged present entries or deleted entries, instead
of sending multiple individual messages. If refreshDeletes of
syncIdSet is set to FALSE, the UUIDs of unchanged present entries are
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contained in the syncUUIDs set; if refreshDeletes of syncIdSet is set
to TRUE, the UUIDs of the entries no longer present in the content are
contained in the syncUUIDs set. An optional cookie can be included in
the syncIdSet to represent the state of the content after
synchronizing the presence or the absence of the entries contained in
the syncUUIDs set.
The synchronized copy of the DIT fragment is constructed by the
client.
If refreshDeletes of syncDoneValue is FALSE, the new copy includes all
changed entries returned by the reissued Sync Operation as well as all
unchanged entries identified as being present by the reissued Sync
Operation, but whose content is provided by the previous Sync
Operation. The unchanged entries not identified as being present are
deleted from the client content. They had been either deleted, moved,
or otherwise scoped-out from the content.
If refreshDeletes of syncDoneValue is TRUE, the new copy includes all
changed entries returned by the reissued Sync Operation as well as all
other entries of the previous copy except for those which are
identified as having been deleted from the content.
The client can, at some later time, re-poll for changes to this
synchronized client copy.
1.3.2. Listening for Changes (refreshAndPersist)
Polling for changes can be expensive in terms of server, client, and
network resources. The refreshAndPersist mode allows for active
updates of changed entries in the content.
By selecting the refreshAndPersist mode, the client requests the
server to send updates of entries that are changed after the initial
refresh content is determined. Instead of sending a SearchResultDone
Message as in polling, the server sends a Sync Info Message to the
client indicating that the refresh stage is complete and then enters
the persist stage. After receipt of this Sync Info Message, the
client will construct a synchronized copy as described in Section
1.3.1.
The server may then send change notifications as the result of the
original Sync search request which now remains persistent in the
server. For entries to be added to the returned content, the server
sends a SearchResultEntry (with attributes) with a Sync State Control
indicating state add. For entries to be deleted from the content, the
server sends a SearchResultEntry containing no attributes and a Sync
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State Control indicating state delete. For entries to be modified in
the return content, the server sends a SearchResultEntry (with
attributes) with a Sync State Control indicating state modify. Upon
modification of an entry, all (modified or unmodified) attributes
belonging to the content are sent.
Note that renaming an entry of the DIT may cause an add state change
where the entry is renamed into the content, a delete state change
where the entry is renamed out of the content, and a modify state
change where the entry remains in the content. Also note that a
modification of an entry of the DIT may cause an add, delete, or
modify state change to the content.
Upon receipt of a change notification, the client updates its copy of
the content.
If the server desires to update the syncCookie during the persist
stage, it may include the syncCookie in any Sync State Control or Sync
Info Message returned.
The operation persists until canceled [CANCEL] by the client or
terminated by the server. A Sync Done Control shall be attached to
SearchResultDone Message to provide a new syncCookie.
1.4. Conventions
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 BCP 14 [RFC2119].
Protocol elements are described using ASN.1 [X.680] with implicit
tags. The term "BER-encoded" means the element is to be encoded using
the Basic Encoding Rules [X.690] under the restrictions detailed in
Section 5.1 of [RFC2251].
2. Elements of the Sync Operation
The Sync Operation is defined as an extension to the LDAP Search
Operation [RFC2251] where the directory user agent (DUA or client)
submits a SearchRequest Message with a Sync Request Control and the
directory system agent (DSA or server) responses with zero or more
SearchResultEntry Messages, each with a Sync State Control; zero or
more SearchResultReference Messages, each with a Sync State Control;
zero or more Sync Info Intermediate Response Messages; and a
SearchResultDone Message with a Sync Done Control.
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To allow clients to discover support for this operation, servers
implementing this operation SHOULD publish the
1.3.6.1.4.1.4203.1.9.1.1 as a value of 'supportedControl' attribute
[RFC2252] of the root DSA-specific entry (DSE). A server MAY choose
to advertise this extension only when the client is authorized to use
it.
2.1 Common ASN.1 Elements
2.1.1 syncUUID
The syncUUID data type is an OCTET STRING holding a 128-bit (16-octet)
Universally Unique Identifier (UUID) [UUID].
syncUUID ::= OCTET STRING (SIZE(16))
-- constrained to UUID
2.1.2 syncCookie
The syncCookie is a notational convenience to indicate that, while the
syncCookie type is encoded as an OCTET STRING, its value is an opaque
value containing information about the synchronization session and its
state. Generally, the session information would include a hash of the
operation parameters which the server requires not be changed and the
synchronization state information would include a commit (log)
sequence number, a change sequence number, or a time stamp. For
convenience of description, the term no cookie refers either to null
cookie or to a cookie with pre-initialized synchronization state.
syncCookie ::= OCTET STRING
2.2 Sync Request Control
The Sync Request Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier
1.3.6.1.4.1.4203.1.9.1.1 and the controlValue, an OCTET STRING,
contains a BER-encoded syncRequestValue. The criticality field is
either TRUE or FALSE.
syncRequestValue ::= SEQUENCE {
mode ENUMERATED {
-- 0 unused
refreshOnly (1),
-- 2 reserved
refreshAndPersist (3)
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},
cookie syncCookie OPTIONAL,
reloadHint BOOLEAN DEFAULT FALSE
}
The Sync Request Control is only applicable to the SearchRequest
Message.
2.3 Sync State Control
The Sync State Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier
1.3.6.1.4.1.4203.1.9.1.2 and the controlValue, an OCTET STRING,
contains a BER-encoded syncStateValue. The criticality is FALSE.
syncStateValue ::= SEQUENCE {
state ENUMERATED {
present (0),
add (1),
modify (2),
delete (3)
},
entryUUID syncUUID,
cookie syncCookie OPTIONAL
}
The Sync State Control is only applicable to SearchResultEntry and
SearchResultReference Messages.
2.4 Sync Done Control
The Sync Done Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier
1.3.6.1.4.1.4203.1.9.1.3 and the controlValue contains a BER-encoded
syncDoneValue. The criticality is FALSE (and hence absent).
syncDoneValue ::= SEQUENCE {
cookie syncCookie OPTIONAL,
refreshDeletes BOOLEAN DEFAULT FALSE
}
The Sync Done Control is only applicable to SearchResultDone Message.
2.5 Sync Info Message
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The Sync Info Message is an LDAP Intermediate Response Message
[LDAPIRM] where responseName is the object identifier
1.3.6.1.4.1.4203.1.9.1.4 and responseValue contains a BER-encoded
syncInfoValue. The criticality is FALSE (and hence absent).
syncInfoValue ::= CHOICE {
newcookie [0] syncCookie,
refreshDelete [1] SEQUENCE {
cookie syncCookie OPTIONAL,
refreshDone BOOLEAN DEFAULT TRUE
},
refreshPresent [2] SEQUENCE {
cookie syncCookie OPTIONAL,
refreshDone BOOLEAN DEFAULT TRUE
},
syncIdSet [3] SEQUENCE {
cookie syncCookie OPTIONAL,
refreshDeletes BOOLEAN DEFAULT FALSE,
syncUUIDs SET OF syncUUID
}
}
2.6 Sync Result Codes
The following LDAP resultCode [RFC2251] is defined:
e-syncRefreshRequired (IANA-ASSIGNED-CODE)
3. Content Synchronization
The Sync Operation is invoked by the client sending a SearchRequest
Message with a Sync Request Control.
The absence of a cookie or an initialized synchronization state in a
cookie indicates a request for initial content while the presence of a
cookie representing a state of a client copy indicates a request for
content update. Synchronization Sessions are discussed in Section
3.1. Content Determination is discussed in Section 3.2.
The mode is either refreshOnly or refreshAndPersist. The refreshOnly
and refreshAndPersist modes are discussed in Section 3.3 and Section
3.4, respectively. The refreshOnly mode consists only of a refresh
stage, while the refreshAndPersist mode consists of a refresh stage
and a subsequent persist stage.
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3.1. Synchronization Session
A sequence of Sync Operations where the last cookie returned by the
server for one operation is provided by the client in the next
operation are said to belong to the same Synchronization Session.
The client MUST specify the same content controlling parameters (see
Section 3.5) in each Search Request of the session. The client SHOULD
also issue each Sync request of a session under the same
authentication and authorization associations with equivalent
integrity and protections. If the server does not recognize the
request cookie or the request is made under different associations or
non-equivalent protections, the server SHALL return the initial
content as if no cookie had been provided or return an empty content
with the e-syncRefreshRequired LDAP result code. The decision between
the return of the initial content and the return of the empty content
with the e-syncRefreshRequired result code MAY be based on reloadHint
in the Sync Request Control from the client. If the server recognizes
the request cookie as representing empty or initial synchronization
state of the client copy, the server SHALL return the initial content.
A Synchronization Session may span multiple LDAP sessions between the
client and the server. The client SHOULD issue each Sync request of a
session to the same server. (Note: Shadowing considerations are
discussed in Section 6.)
3.2. Content Determination
The content to be provided is determined by parameters of the Search
Request, as described in [RFC2251], and possibly other controls. The
same content parameters SHOULD be used in each Sync request of a
session. If different content is requested and the server is
unwilling or unable to process the request, the server SHALL return
the initial content as if no cookie had been provided or return an
empty content with the e-syncRefreshRequired LDAP result code. The
decision between the return of the initial content and the return of
the empty content with the e-syncRefreshRequired result code MAY be
based on reloadHint in the Sync Request Control from the client.
The content may not necessarily include all entries or references
which would be returned by a normal search operation nor, for those
entries included, not all attributes returned by a normal search.
When the server is unwilling or unable to provide synchronization for
any attribute for a set of entries, the server MUST treat all filter
components matching against these attributes as Undefined and MUST NOT
return these attributes in SearchResultEntry responses.
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Servers SHOULD support synchronization for all non-collective
user-application attributes for all entries.
The server may also return continuation references to other servers or
to itself. The latter is allowed as the server may partition the
entries it holds into separate synchronization contexts.
The client may chase all or some of these continuations, each as a
separate content synchronization session.
3.3. refreshOnly Mode
A Sync request with mode refreshOnly and with no cookie is a poll for
initial content. A Sync request with mode refreshOnly and with a
cookie representing a synchronization state is a poll for content
update.
3.3.1. Initial Content Poll
Upon receipt of the request, the server provides the initial content
using a set of zero or more SearchResultEntry and
SearchResultReference Messages followed by a SearchResultDone Message.
Each SearchResultEntry Message SHALL include a Sync State Control of
state add, entryUUID containing the entry's UUID, and no cookie. Each
SearchResultReference Message SHALL include a Sync State Control of
state add, entryUUID containing the UUID associated with the reference
(normally the UUID of the associated named referral [RFC3296] object),
and no cookie. The SearchResultDone Message SHALL include a Sync Done
Control having refreshDeletes set to FALSE.
A resultCode value of success indicates the operation successfully
completed. Otherwise, the result code indicates the nature of
failure. The server may return e-syncRefreshRequired result code on
the initial content poll if it is safe to do so when it is unable to
perform the operation due to various reasons. reloadHint is set to
FALSE in the SearchRequest Message requesting the initial content
poll.
If the operation is successful, a cookie representing the
synchronization state of the current client copy SHOULD be returned
for use in subsequent Sync Operations.
3.3.2. Content Update Poll
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Upon receipt of the request the server provides the content refresh
using a set of zero or more SearchResultEntry and
SearchResultReference Messages followed by a SearchResultDone Message.
The server is REQUIRED to either:
a) provide the sequence of messages necessary for eventual
convergence of the client's copy of the content to the server's
copy,
b) treat the request as an initial content request (e.g., ignore
the cookie or the synchronization state represented in the
cookie),
c) indicate that the incremental convergence is not possible by
returning e-syncRefreshRequired,
d) return a resultCode other than success or
e-syncRefreshRequired.
A Sync Operation may consist of a single present phase, a single
delete phase, or a present phase followed by a delete phase.
In each phase, for each entry or reference which has been added to the
content or been changed since the previous Sync Operation indicated by
the cookie, the server returns a SearchResultEntry or
SearchResultReference Message, respectively, each with a Sync State
Control consisting of state add, entryUUID containing the UUID of the
entry or reference, and no cookie. Each SearchResultEntry Message
represents the current state of a changed entry. Each
SearchResultReference Message represents the current state of a
changed reference.
In the present phase, for each entry which has not been changed since
the previous Sync Operation, an empty SearchResultEntry is returned
whose objectName reflects the entry's current DN, the attributes field
is empty, and a Sync State Control consisting of state present,
entryUUID containing the UUID of the entry, and no cookie. For each
reference which has not been changed since the previous Sync
Operation, an empty SearchResultReference containing an empty SEQUENCE
OF LDAPURL is returned with a Sync State Control consisting of state
present, entryUUID containing the UUID of the entry, and no cookie.
No messages are sent for entries or references which are no longer in
the content.
Multiple empty entries with a Sync State Control of state present
SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
value with refreshDeletes set to FALSE. syncUUIDs contain a set of
UUIDs of the entries and references unchanged since the last Sync
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Operation. syncUUIDs may be empty. The Sync Info Message of
syncIdSet may contain cookie to represent the state of the content
after performing the synchronization of the entries in the set.
In the delete phase, for each entry no longer in the content, the
server returns a SearchResultEntry whose objectName reflects a past DN
of the entry or is empty, the attributes field is empty, and a Sync
State Control consisting of state delete, entryUUID containing the
UUID of the deleted entry, and no cookie. For each reference no
longer in the content, a SearchResultReference containing an empty
SEQUENCE OF LDAPURL is returned with a Sync State Control consisting
of state delete, entryUUID containing the UUID of the deleted
reference, and no cookie.
Multiple empty entries with a Sync State Control of state delete
SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
value with refreshDeletes set to TRUE. syncUUIDs contain a set of
UUIDs of the entries and references which has been deleted from the
content since the last Sync Operation. syncUUIDs may be empty. The
Sync Info Message of syncIdSet may contain cookie to represent the
state of the content after performing the synchronization of the
entries in the set.
When a present phase is followed by a delete phase, the two phases are
delimited by a Sync Info Message containing syncInfoValue of
refreshPresent, which may contain cookie representing the state after
completing the present phase. The refreshPresent contains refreshDone
which is always FALSE in the refreshOnly mode of Sync Operation
because it is followed by a delete phase.
If a Sync Operation consists of a single phase, each phase and hence
the Sync Operation are marked ended by a SearchResultDone Message with
Sync Done Control which SHOULD contain cookie representing the state
of the content after completing the Sync Operation. The Sync Done
Control contains refreshDeletes which is set to FALSE for the present
phase and set to TRUE for the delete phase.
If a Sync Operation consists of a present phase followed by a delete
phase, the Sync Operation are marked ended at the end of the delete
phase by a SearchResultDone Message with Sync Done Control which
SHOULD contain cookie representing the state of the content after
completing the Sync Operation. The Sync Done Control contains
refreshDeletes which is set to TRUE.
The client can specify whether it prefers to receive an initial
content by supplying reloadHint of TRUE or to receive a
e-syncRefreshRequired resultCode by supplying reloadHint of FALSE
(hence absent), in the case that the server determines that it is
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impossible or inefficient to achieve the eventual convergence by
continuing the current incremental synchronization thread.
A resultCode value of success indicates the operation is successfully
completed. A resultCode value of e-syncRefreshRequired indicates that
a full or partial refresh is needed. Otherwise, the result code
indicates the nature of failure. A cookie is provided in the Sync
Done Control for use in subsequent Sync Operations for incremental
synchronization.
3.4. refreshAndPersist Mode
A Sync request with mode refreshAndPersist asks for initial content or
content update (during the refresh stage) followed by change
notifications (during the persist stage).
3.4.1. refresh Stage
The content refresh is provided as described in Section 3.3 excepting
that the successful completion of content refresh is indicated by
sending a Sync Info Message of refreshDelete or refreshPresent with a
refreshDone value set to TRUE instead of a SearchResultDone Message
with resultCode success. A cookie SHOULD be returned in the Sync Info
Message to represent the state of the content after finishing the
refresh stage of the Sync Operation.
3.4.2. persist Stage
Change notifications are provided during the persist stage.
As updates are made to the DIT the server notifies the client of
changes to the content. DIT updates may cause entries and references
to be added to the content, deleted from the content, or modified
within the content. DIT updates may also cause references to be
added, deleted, or modified within the content.
Where DIT updates cause an entry to be added to the content, the
server provides a SearchResultEntry Message which represents the entry
as it appears in the content. The message SHALL include a Sync State
Control with state of add, entryUUID containing the entry's UUID, and
an optional cookie.
Where DIT updates cause a reference to be added to the content, the
server provides a SearchResultReference Message which represents the
reference in the content. The message SHALL include a Sync State
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Control with state of add, entryUUID containing the UUID associated
with the reference, and an optional cookie.
Where DIT updates cause an entry to be modified within the content,
the server provides a SearchResultEntry Message which represents the
entry as it appears in the content. The message SHALL include a Sync
State Control with state of modify, entryUUID containing the entry's
UUID, and an optional cookie.
Where DIT updates cause a reference to be modified within the content,
the server provides a SearchResultEntry Message which represents the
reference in the content. The message SHALL include a Sync State
Control with state of modify, entryUUID containing the UUID associated
with the reference, and an optional cookie.
Where DIT updates cause an entry to be deleted from the content, the
server provides a SearchResultReference Message with an empty SEQUENCE
OF LDAPURL. The message SHALL include a Sync State Control with state
of delete, entryUUID containing the UUID associated with the
reference, and an optional cookie.
Where DIT updates cause a reference to be deleted from the content,
the server provides a SearchResultEntry Message with no attributes.
The message SHALL include a Sync State Control with state of delete,
entryUUID containing the entry's UUID, and an optional cookie.
Multiple empty entries with a Sync State Control of state delete
SHOULD be coalesced into one or more Sync Info Messages of syncIdSet
value with refreshDeletes set to TRUE. syncUUIDs contain a set of
UUIDs of the entries and references which has been deleted from the
content. The Sync Info Message of syncIdSet may contain cookie to
represent the state of the content after performing the
synchronization of the entries in the set.
With each of these messages, the server may provide a new cookie to be
used in subsequent Sync Operations. Additionally, the server may also
return Sync Info Messages of choice newCookie to provide a new cookie.
The client SHOULD use the newest (last) cookie it received from the
server in subsequent Sync Operations.
3.5. Search Request Parameters
As stated in Section 3.1, the client SHOULD specify the same content
controlling parameters in each Search Request of the session. All
fields of the SearchRequest Message are considered content controlling
parameters except for sizeLimit and timeLimit.
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3.5.1. baseObject
As with the normal search operation, the refresh and persist stages
are not isolated from DIT changes. It is possible that the entry
referred to by the baseObject is deleted, renamed, or moved. It is
also possible that alias object used in finding the entry referred to
by the baseObject is changed such that the baseObject refers to a
different entry.
If the DIT is updated during processing of the Sync Operation in a
manner that causes the baseObject to no longer refer to any entry or
in a manner that changes the entry the baseObject refers to, the
server SHALL return an appropriate non-success result code such as
noSuchObject, aliasProblem, aliasDereferencingProblem, referral, or
e-syncRefreshRequired.
3.5.2. derefAliases
This operation does not support alias dereferencing during searching.
The client SHALL specify neverDerefAliases or derefFindingBaseObj for
the SearchRequest derefAliases parameter. The server SHALL treat
other values (e.g., derefInSearching, derefAlways) as protocol errors.
3.5.3. sizeLimit
The sizeLimit applies only to entries (regardless of their state in
Sync State Control) returned during the refreshOnly operation or the
refresh stage of the refreshAndPersist operation.
3.5.4. timeLimit
For a refreshOnly Sync Operation, the timeLimit applies to the whole
operation. For a refreshAndPersist operation, the timeLimit applies
only to the refresh stage including the generation of the Sync Info
Message with a refreshDone value of TRUE.
3.5.5. filter
The client SHOULD avoid filter assertions which apply to the values of
the attributes likely to be considered by the server as ones holding
meta-information. See Section 4.
3.6. objectName
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The Sync Operation uses entryUUID values provided in the Sync State
Control as the primary keys to entries. The client MUST use these
entryUUIDs to correlate synchronization messages.
In some circumstances the DN returned may not reflect the entry's
current DN. In particular, when the entry is being deleted from the
content, the server may provide an empty DN if the server does not
wish to disclose the entry's current DN (or, if deleted from the DIT,
the entry's last DN).
It should also be noted that the entry's DN may be viewed as meta
information (see Section 4.1).
3.7. Canceling the Sync Operation
Servers MUST implement the LDAP Cancel [CANCEL] Operation and support
cancellation of outstanding Sync Operations as described here.
To cancel an outstanding Sync Operation, the client issues an LDAP
Cancel [CANCEL] Operation.
If at any time the server becomes unwilling or unable to continue
processing a Sync Operation, the server SHALL return a
SearchResultDone with a non-success resultCode indicating the reason
for the termination of the operation.
Whether the client or the server initiated the termination, the server
may provide a cookie in the Sync Done Control for use in subsequent
Sync Operations.
3.8. Refresh Required
In order to achieve the eventually-convergent synchronization, the
server may terminate the Sync Operation in the refresh or the persist
stage by returning a e-syncRefreshRequired resultCode to the client.
If no cookie is provided, a full refresh is needed. If a cookie
representing a synchronization state is provided in this response, an
incremental refresh is needed.
To obtain a full refresh, the client then issues a new synchronization
request with no cookie. To obtain an incremental reload, the client
issues a new synchronization with the provided cookie.
The server may choose to provide a full copy in the refresh stage
(e.g., ignore the cookie or the synchronization state represented in
the cookie) instead of providing an incremental refresh in order to
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achieve the eventual convergence.
The decision between the return of the initial content and the return
of the e-syncRefreshRequired result code may be based on reloadHint in
the Sync Request Control from the client.
In the case of persist stage Sync, the server returns the resultCode
of e-syncRefreshRequired to the client to indicate that the client
needs to issue a new Sync Operation in order to obtain a synchronized
copy of the content. If no cookie is provided, a full refresh is
needed. If a cookie representing a synchronization state is provided,
an incremental refresh is needed.
The server may also return e-syncRefreshRequired if it determines that
a refresh would be more efficient than sending all the messages
required for convergence.
It is noted that the client may receive one or more of
SearchResultEntry, SearchResultReference, and/or Sync Info Messages
before it receives SearchResultDone Message with the
e-syncRefreshRequired result code.
3.9. Chattiness Considerations
The server MUST ensure that the number of entry messages generated to
refresh the client content does not exceed the number of entries
presently in the content. While there is no requirement for servers
to maintain history information, if the server has sufficient history
to allow it to reliably determine which entries in the prior client
copy are no longer present in the content and the number of such
entries is less than or equal to the number of unchanged entries, the
server SHOULD generate delete entry messages instead of present entry
messages (see Section 3.3.2).
When the amount of history information maintained in the server is not
enough for the clients to perform infrequent refreshOnly Sync
Operations, it is likely that the server has incomplete history
information (e.g. due to truncation) by the time those clients connect
again.
The server SHOULD NOT resort to full reload when the history
information is not enough to generate delete entry messages. The
server SHOULD generate either present entry messages only or present
entry messages followed by delete entry messages to bring the client
copy to the current state. In the latter case, the present entry
messages bring the client copy to a state covered by the history
information maintained in the server.
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The server SHOULD maintain enough (current or historical) state
information (such as a context-wide last modify time stamp) to
determine if no changes were made in the context since the content
refresh was provided and, and when no changes were made, generate zero
delete entry messages instead of present messages.
The server SHOULD NOT use the history information when its use does
not reduce the synchronization traffic or when its use can expose
sensitive information not allowed to be received by the client.
The server implementor should also consider chattiness issues which
span multiple Sync Operations of a session. As noted in Section 3.8,
the server may return e-syncRefreshRequired if it determines that a
reload would be more efficient than continuing under the current
operation. If reloadHint in the Sync Request is TRUE, the server may
initiate a reload without directing the client to request a reload.
The server SHOULD transfer a new cookie frequently to avoid having to
transfer information already provided to the client. Even where DIT
changes do not cause content synchronization changes to be
transferred, it may be advantageous to provide a new cookie using a
Sync Info Message. However, the server SHOULD avoid overloading the
client or network with Sync Info Messages.
During persist mode, the server SHOULD coalesce multiple outstanding
messages updating the same entry. The server MAY delay generation of
an entry update in anticipation of subsequent changes to that entry
which could be coalesced. The length of the delay should be long
enough to allow coalescing of update requests issued back to back but
short enough that the transient inconsistency induced by the delay is
corrected in a timely manner.
The server SHOULD use syncIdSet Sync Info Message when there are
multiple delete or present messages to reduce the amount of
synchronization traffic.
It is also noted that there may be many clients interested in a
particular directory change, and servers attempting to service all of
these at once may cause congestion on the network. The congestion
issues are magnified when the change requires a large transfer to each
interested client. Implementors and deployers of servers should take
steps to prevent and manage network congestion.
3.10. Operation Multiplexing
The LDAP protocol model [RFC2251] allows operations to be multiplexed
over a single LDAP session. Clients SHOULD NOT maintain multiple LDAP
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sessions with the same server. Servers SHOULD ensure that responses
from concurrently processed operations are interleaved fairly.
Clients SHOULD combine Sync Operations whose result set is largely
overlapping. This avoids having to return multiple messages, once for
each overlapping session, for changes to entries in the overlap.
Clients SHOULD NOT combine Sync Operations whose result sets are
largely non-overlapping with each other. This ensures that an event
requiring a e-syncRefreshRequired response can be limited to as few
result sets as possible.
4. Meta Information Considerations
4.1. Entry DN
As an entry's DN is constructed from its relative DN (RDN) and the
entry's parent's DN, it is often viewed as meta information.
While renaming or moving to a new superior causes the entry's DN to
change, that change SHOULD NOT, by itself, cause synchronization
messages to be sent for that entry. However, if the renaming or the
moving could cause the entry to be added or deleted from the content,
appropriate synchronization messages should be generated to indicate
this to the client.
When a server treats the entry's DN as meta information, the server
SHALL either
- evaluate all MatchingRuleAssertions [RFC2251] to TRUE if
matching a value of an attribute of the entry and otherwise
Undefined, or
- evaluate all MatchingRuleAssertion with dnAttributes of TRUE as
Undefined.
The latter choice is offered for ease of server implementation.
4.2. Operational Attributes
Where values of an operational attribute is determined by values not
held as part of the entry it appears in, the operational attribute
SHOULD NOT support synchronization of that operational attribute.
For example, in servers which implement X.501 subschema model [X.501],
servers should not support synchronization of the subschemaSubentry
attribute as its value is determined by values held and administrated
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in subschema subentries.
As a counter example, servers which implement aliases [RFC2256][X.501]
can support synchronization of the aliasedObjectName attribute as its
values are held and administrated as part of the alias entries.
Servers SHOULD support synchronization of the following operational
attributes: createTimestamp, modifyTimestamp, creatorsName,
modifiersName [RFC2252]. Servers MAY support synchronization of other
operational attributes.
4.3. Collective Attributes
A collective attribute is "a user attribute whose values are the same
for each member of an entry collection" [X.501]. Use of collective
attributes in LDAP is discussed in [RFC3371].
Modification of a collective attribute generally affects the content
of multiple entries, which are the members of the collection. It is
inefficient to include values of collective attributes visible in
entries of the collection, as a single modification of a collective
attribute requires transmission of multiple SearchResultEntry (one for
each entry of the collection which the modification affected) to be
transmitted.
Servers SHOULD NOT synchronize collective attributes appearing in
entries of any collection. Servers MAY support synchronization of
collective attributes appearing in collective attribute subentries.
4.4. Access and Other Administrative Controls
Entries are commonly subject to access and other administrative
Controls. While portions of the policy information governing a
particular entry may be held in the entry, policy information is often
held elsewhere (in superior entries, in subentries, in the root DSE,
in configuration files etc.). Because of this, changes to policy
information make it difficult to ensure eventual convergence during
incremental synchronization.
Where it is impractical or infeasible to generate content changes
resulting from a change to policy information, servers may opt to
return e-syncRefreshRequired or treat the Sync Operation as an initial
content request (e.g., ignore the cookie or the synchronization state
represented in the cookie).
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5. Interaction with Other Controls
The Sync Operation may be used with:
- ManageDsaIT Control [RFC3296]
- Subentries Control [RFC3672]
as described below. The Sync Operation may be used with other LDAP
extensions as detailed in other documents.
5.1. ManageDsaIT Control
The ManageDsaIT Control [RFC3296] indicates that the operation acts
upon the DSA Information Tree and causes referral and other special
entries to be treated as object entries with respect to the operation.
5.2. Subentries Control
The Subentries Control is used with the search operation "to control
the visibility of entries and subentries which are within scope"
[RFC3672]. When used with the Sync Operation, the subentries control
and other factors (search scope, filter, etc.) are used to determine
whether an entry or subentry appear in the content or not.
6. Shadowing Considerations
As noted in [RFC2251], some servers may hold shadow copies of entries
which can be used to answer search and comparison queries. Such
servers may also support content synchronization requests. This
section discusses considerations for implementors and deployers for
the implementation and deployment of the Sync operation in shadowed
directories.
While a client may know of multiple servers which are equally capable
of being used to obtain particular directory content from, a client
SHOULD NOT assume that each of these server is equally capable of
continuing a content synchronization session. As stated in Section
3.1, the client SHOULD issue each Sync request of a Sync session to
the same server.
However, through domain naming or IP address redirection or other
techniques, multiple physical servers can be made to appear as one
logical server to a client. Only servers which are equally capable in
regards to their support for the Sync operation and which hold equally
complete copies of the entries should be made to appear as one logical
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server. In particular, each physical server acting as one logical
server SHOULD be equally capable of continuing a content
synchronization based upon cookies provided by any of the other
physical servers without requiring a full reload. Because there is no
standard LDAP shadowing mechanism, the specification of how to
independently implement equally capable servers (as well as the
precise definition of "equally capable") is left to future documents.
It is noted that it may be difficult for the server to reliably
determine what content was provided to the client by another server,
especially in the shadowing environments which allow shadowing events
to be coalesced. Where so, the use of the delete phase discussed in
Section 3.3.2 may not be applicable.
7. Security Considerations
In order to maintain a synchronized copy of the content, a client is
to delete information from its copy of the content as described above.
However, the client may maintain knowledge of information disclosed to
it by the server separate from its copy of the content used for
synchronization. Management of this knowledge is beyond the scope of
this document. Servers should be careful not to disclose information
for content which the client is not authorized to have knowledge of
and/or about.
While the information provided by a series of refreshOnly Sync
Operations is similar to that provided by a series of Search
Operations, persist stage may disclose additional information. A
client may be able to discern information about the particular
sequence of update operations which caused content change.
Implementors should take precautions against malicious cookie content,
including malformed cookies or valid cookies used with different
security associations and/or protections in attempt to obtain
unauthorized access to information. Servers may include a digital
signature in the cookie to detect tampering.
The operation may be the target of direct denial of service attacks.
Implementors should provide safeguards to ensure the operation is not
abused. Servers may place access control or other restrictions upon
the use of this operation.
It is noted that even small updates to the directory may cause
significant amount of traffic to be generated to clients using this
operation. A user could abuse its update privileges to mount an
indirect denial of service to these clients, other clients, and/or
portions of the network. Servers should provide safeguards to ensure
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update operations are not abused.
Implementors of this (or any) LDAP extension should be familiar with
general LDAP security considerations [RFC3377].
8. IANA Considerations
Registration of the following values is requested.
The OID arc 1.3.6.1.4.1.4203.1.9.1 was assigned [ASSIGN] by OpenLDAP
Foundation, under its IANA-assigned private enterprise allocation
[PRIVATE], for use in this specification.
8.2. LDAP Protocol Mechanism
It is requested that IANA register the LDAP Protocol Mechanism
described in this document.
Subject: Request for LDAP Protocol Mechanism Registration
Object Identifier: 1.3.6.1.4.1.4203.1.9.1.1
Description: LDAP Content Synchronization Control
Person & email address to contact for further information:
Kurt Zeilenga <[email protected]>
Usage: Control
Specification: RFC XXXX
Author/Change Controller: IESG
Comments: none
8.3. LDAP Result Codes
It is requested that IANA register the LDAP Result Code described in
this document.
Subject: LDAP Result Code Registration
Person & email address to contact for further information:
Kurt Zeilenga <[email protected]>
Result Code Name: e-syncRefreshRequired (IANA-ASSIGNED-CODE)
Specification: RFC XXXX
Author/Change Controller: IESG
Comments: none
9. Acknowledgments
This document borrows significantly from the LDAP Client Update
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Protocol [LCUP], a product of the IETF LDUP working group. This
document also benefited from Persistent Search [PSEARCH], Triggered
Search [TSEARCH], and Directory Synchronization [DIRSYNC] works. This
document also borrows from "Lightweight Directory Access Protocol
(v3)" [RFC2251].
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC2251] Wahl, M., T. Howes and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[RFC2252] Wahl, M., A. Coulbeck, T. Howes, and S. Kille,
"Lightweight Directory Access Protocol (v3): Attribute
Syntax Definitions", RFC 2252, December 1997.
[RFC3296] Zeilenga, K., "Named Subordinate References in
Lightweight Directory Access Protocol (LDAP)
Directories", RFC 3296, July 2002.
[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC 3377,
September 2002.
[RFC3671] Zeilenga, K., "Collective Attributes in LDAP", RFC 3671,
December 2003.
[RFC3672] Zeilenga, K. and S. Legg, "Subentries in LDAP", RFC
3672, December 2003.
[CANCEL] Zeilenga, K., "LDAP Cancel Extended Operation",
draft-zeilenga-ldap-cancel-xx.txt, a work in progress.
[EntryUUID] Zeilenga, K., "The LDAP EntryUUID Operational
Attribute", draft-zeilenga-ldap-uuid-xx.txt, a work in
progress.
[LDAPIRM] Harrison, R. and Zeilenga, K., "LDAP Intermediate
Response",
draft-rharrison-ldap-intermediate-resp-00.txt, a work in
progress.
[UUID] International Organization for Standardization (ISO),
"Information technology - Open Systems Interconnection -
Remote Procedure Call", ISO/IEC 11578:1996
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[X.680] International Telecommunication Union -
Telecommunication Standardization Sector, "Abstract
Syntax Notation One (ASN.1) - Specification of Basic
Notation", X.680(1997) (also ISO/IEC 8824-1:1998).
[X.690] International Telecommunication Union -
Telecommunication Standardization Sector, "Specification
of ASN.1 encoding rules: Basic Encoding Rules (BER),
Canonical Encoding Rules (CER), and Distinguished
Encoding Rules (DER)", X.690(1997) (also ISO/IEC
8825-1:1998).
11. Informative References
[RFC2256] Wahl, M., "A Summary of the X.500(96) User Schema for
use with LDAPv3", RFC 2256, December 1997.
[RFC3383] Zeilenga, K., "IANA Considerations for LDAP", BCP 64
(also RFC 3383), September 2002.
[PRIVATE] IANA, "Private Enterprise Numbers",
http://www.iana.org/assignments/enterprise-numbers.
[ASSIGN] OpenLDAP Foundation, "OpenLDAP OID Delegations",
http://www.openldap.org/foundation/oid-delegate.txt.
[X.500] International Telecommunication Union -
Telecommunication Standardization Sector, "The Directory
-- Overview of concepts, models and services,"
X.500(1993) (also ISO/IEC 9594-1:1994).
[X.511] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory: Abstract Service Definition", X.511(1993).
[X.525] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory: Replication", X.525(1993).
[UUIDinfo] The Open Group, "Universally Unique Identifier" appendix
of the CAE Specification "DCE 1.1: Remote Procedure
Calls", Document Number C706,
<http://www.opengroup.org/products/publications/
catalog/c706.htm> (appendix available at:
<http://www.opengroup.org/onlinepubs/9629399/
apdxa.htm>), August 1997.
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[DIRSYNC] Armijo, M., "Microsoft LDAP Control for Directory
Synchronization", draft-armijo-ldap-dirsync-xx.txt, a
work in progress.
[LCUP] Megginson, R., et. al., "LDAP Client Update Protocol",
draft-ietf-ldup-lcup-xx.txt, a work in progress.
[PSEARCH] Smith, M., et. al., "Persistent Search: A Simple LDAP
Change Notification Mechanism",
draft-ietf-ldapext-psearch-xx.txt, a work in progress.
[TSEARCH] Wahl, M., "LDAPv3 Triggered Search Control",
draft-ietf-ldapext-trigger-xx.txt, a work in progress.
12. Authors' Addresses
Kurt D. Zeilenga
OpenLDAP Foundation
<[email protected]>
Jong Hyuk Choi
IBM Corporation
<[email protected]>
Appendix A. CSN-based Implementation Considerations
This appendix is provided for informational purposes only, it is not a
normative part of the LDAP Content Synchronization Operation's
technical specification.
This appendix discusses LDAP Content Synchronization Operation server
implementation considerations associated with a Change Sequence Number
based approaches.
Change Sequence Number based approaches are targeted for use in
servers which do not maintain history information (e.g., change logs,
state snapshots, etc.) about changes made to the Directory and hence,
must rely on current directory state and minimal synchronization state
information embedded in Sync Cookie. Servers which maintain history
information should consider other approaches which exploit the history
information.
A Change Sequence Number is effectively a time stamp which has
sufficient granularity to ensure that the precedence relationship in
time of two updates to the same object can be determined. Change
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Sequence Numbers are not to be confused with Commit Sequence Numbers
or Commit Log Record Numbers. A Commit Sequence Number allows one to
determine how two commits (to the same object or different objects)
relate to each other in time. Change Sequence Number associated with
different entries may be committed out of order. In the remainder of
this Appendix, the term CSN refers to a Change Sequence Number.
In these approaches, the server not only maintains a CSN for each
directory entry (the entry CSN), but also maintains a value which we
will call the context CSN. The context CSN is the greatest committed
entry CSN which is not greater than any outstanding (uncommitted)
entry CSNs for all entries in a directory context. The values of
context CSN are used in syncCookie values as synchronization state
indicators.
As search operations are not isolated from individual directory update
operations and individual update operations cannot be assumed to be
serialized, one cannot assume that the returned content incorporates
all relevant changes whose change sequence number is less than or
equal to the greatest entry CSN in the content. The content
incorporates all the relevant changes whose change sequence number is
less than or equal to context CSN before search processing. The
content may also incorporate any subset of the changes whose change
sequence number is greater than context CSN before search processing
but less than or equal to the context CSN after search processing.
The content does not incorporate any of the changes whose CSN is
greater than the context CSN after search processing.
A simple server implementation could use value of the context CSN
before search processing to indicate state. Such an implementation
would embed this value into each SyncCookie returned. We'll call this
the cookie CSN. When a refresh was requested, the server would simply
generate "update" messages for all entries in the content whose CSN is
greater than the supplied cookie CSN and generate "present" messages
for all other entries in the content. However, if the current context
CSN is the same as the cookie CSN, the server should instead generate
zero "updates" and zero "delete" messages, and indicate refreshDeletes
of TRUE as the directory has not changed.
The implementation should also consider the impact of changes to meta
information, such as access controls, which affects content
determination. One approach is for the server to maintain a context
wide meta information CSN or meta CSN. This meta CSN would be updated
whenever meta information affecting content determination was changed.
If the value of the meta CSN is greater than cookie CSN, the server
should ignore the cookie and treat the request as an initial request
for content.
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Additionally, servers may want to consider maintaining some
per-session history information to reduce the number of messages
needed to be transferred during incremental refreshes. Specifically,
a server could record information about entries as they leave the
scope of a disconnected sync session and later use this information to
generate delete messages instead of present messages.
When the history information is truncated, the CSN of the latest
truncated history information entry may be recorded as the truncated
CSN of the history information. The truncated CSN may be used to
determine whether a client copy can be covered by the history
information by comparing it to the synchronization state contained in
the cookie supplied by the client.
When there are a large number of sessions, it may make sense to
maintain such history only for the selected clients. Also, servers
taking this approach need to consider resource consumption issues to
ensure reasonable server operation and to protect against abuse. It
may be appropriate to restrict this mode of operation by policy.
Intellectual Property Rights
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. 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.
Full Copyright
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INTERNET-DRAFT draft-zeilenga-ldup-sync-05 3 February 2004
Copyright (C) The Internet Society (2004). 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
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.
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