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INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires in six months Jonghyuk Choi
IBM Corporation
5 May 2003
LDAP Content Synchronization Operation
<draft-zeilenga-ldup-sync-02.txt>
1. 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 <ietf-ldup@imc.org>. Please send editorial comments directly to
the document editor at <Kurt@OpenLDAP.org>.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
<http://www.ietf.org/ietf/1id-abstracts.txt>. The list of
Internet-Draft Shadow Directories can be accessed at
<http://www.ietf.org/shadow.html>.
Copyright 2003, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Zeilenga LDAP Content Sync Operation [Page 1]
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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 shadow 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.
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]. 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].
1. Introduction
The Lightweight Directory Access Protocol (LDAP) [RFC3377] provides a
mechanism, the search operation [RFC2251], to allow a client to
request the return of 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, short of repeating a
search operation each time a new copy needed, LDAP does not provide an
effective and efficient mechanism for maintaining synchronized copies
of directory content.
This document defines the LDAP Content Synchronization operation, or
Sync operation for short, which allows a client to maintain a
synchronized shadow 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 directory synchronization approaches have
been suggested. These approaches are inadequate for one or more of
the following reasons:
1) do not ensure a reasonable level of convergence;
2) fail to detect that convergence cannot be achieved (without
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reload);
3) require pre-arranged synchronization agreements;
4) require the server to maintain synchronization state on a per
client basis;
5) require the server to maintain histories of past changes to DIT
content and/or meta information; and/or
6) are overly chatty.
The Sync operation provides eventual convergence of synchronized
content when possible and, when not, notification that content reload
is required.
The Sync operation does not require pre-arranged synchronization
agreements.
The Sync operation does not require servers to maintain
synchronization state on a per user basis.
The Sync operation does not require servers to maintain any history of
past changes to the DIT or to meta information. While histories
(e.g., change logs, tombstones, DIT snapshots) may be used in the
implementation of the Sync operation, the operation may be implemented
using purely state-based approaches.
As the Sync operation does not require servers to maintain any
histories of past changes, it can be implemented in environments where
it is not feasible to maintain such histories. Histories, if
available, may be used by the server to reduce the number of messages
generated and reduce their size.
The Sync operation chattiness is reasonably bound.
1.2. Intended Usage
The Sync operation is intended to be used in applications requiring
eventual-convergent content synchronization. Upon completion of each
synchronization phase of the operation, all information to construct
an synchronized shadow copy of the content has been provided to the
client or the client has been notified that a complete content reload
is necessary. Excepting for transient inconsistencies due to
concurrent operation (or other) processing at the server, the shadow
copy is an accurate reflection of the content held by the server.
Each inconsistency is transient in that it will be corrected during
subsequent synchronization requests.
Possible uses include:
- White page service applications may use the Sync operation to
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maintain current shadow copy of a DIT fragment. For example, an
mail user agent which use the sync operation to maintain a local
copy of an enterprise address book.
- Meta-information engines may use the Sync operation to maintain a
shadow 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 Synchronization Protocol [X.525] which may be used for
master-slave replication between LDAP servers. Other
experimental LDAP replication protocols exist. The Sync
operation should be viewed as complementary to these replication
protocols.
This protocol is not intended to be used in applications requiring
transactional data consistency.
As this protocol transfers all visible values of entries 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 basis ways the Sync operation can
be used to maintain a synchronized shadow 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 shadow 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)
the content matching the search criteria (baseObject, scope, filter).
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. Unlike
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Distinguished Names (DNs), which may change over time, an entry's
UUIDs are 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 shadow 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
unchanged entry, the server sends an empty entry (e.g., no attributes)
with a Sync State control indicating state present. The set of
updates is followed by a searchResultDone with a Sync Done control.
If the server can reliably determine which entries in the prior shadow
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 may, instead of returning an empty entry with state present for
each present entry, send an empty entry with state delete for each
entry which is no longer in the content. Also, the Sync Done control
refreshDeletes is set to TRUE to indicate to the client that this
method was used. This field is FALSE otherwise.
The synchronized shadow copy of the DIT fragment is constructed by the
client.
If refreshDeletes 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
shadow content. They had been either deleted, moved, or otherwise
scoped-out from the content.
If refreshDeletes 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 those which were identified as having been
deleted from the content.
The client can, at some later time, re-poll for changes to this
synchronized shadow copy.
1.3.2. Listening for Changes (refreshAndPersist)
Polling for changes can be expensive in terms of server, client, and
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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 the
initial refresh content is determined. Instead of sending a
searchResultDone message as described above, the server sends a Sync
Info message to the client indicating that refresh phase is complete
and then enters persist phase. After receipt of this Sync Info
message, the client will have a synchronized shadow copy as described
above.
The server may then send change notifications. 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 with no attributes and a Sync State
control indicating state delete. To modify entries 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 a 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 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 may be attached to
searchResultDone message to provide a new syncCookie.
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
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SearchResultEntry messages, each with a Sync State control; zero or
more SearchResultReference messages, each with a Sync State control;
zero or more Sync Intermediate Response messages; and a
searchResultDone message with a Sync Done control.
To allow clients to discover support for this operation, servers
implementing this operation SHOULD publish the IANA-ASSIGNED-OID.1 as
a value of supportedControl root DSE attribute.
2.1 Common ASN.1 elements
2.1.1 syncUUID
The syncUUID is a notational convenience to indicate that, while the
syncUUID type is encoded as an OCTET STRING, its value is restricted
to the string representation of an Universally Unique Identifier
(UUID) defined in [UUID].
syncUUID ::= OCTET STRING
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; the
synchronization state information includes a commit (log) sequence
number, a change sequence number, or a time stamp; and a digital
signature for detection of tampering.
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 IANA-ASSIGNED-OID.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
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refreshAndPersist (3)
},
cookie syncCookie OPTIONAL
}
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 IANA-ASSIGNED-OID.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 IANA-ASSIGNED-OID.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
IANA-ASSIGNED-OID.4 and responseValue contains a BER-encoded
syncInfoValue. The criticality is FALSE (and hence absent).
syncInfoValue ::= CHOICE {
newcookie [0] syncCookie,
refreshDone [1] SEQUENCE {
cookie syncCookie OPTIONAL,
refreshDeletes BOOLEAN DEFAULT FALSE
}
}
2.6 Sync Result Codes
The following LDAP resultCodes [RFC2251] are defined:
syncRefreshRequired (IANA-ASSIGNED-CODE-0)
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 indicates a request for initial content while
the presence of a cookie 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 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.
3.1. Synchronization Session
A sequence of Sync Operations where the last cookie returned by a
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
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integrity and confidential protections. If the server does not
recognize the request cookie or the request is made under different
associations or inequivalent protections, the server SHALL process the
request as if no cookie had been provided.
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.
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 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 process the request as if no
cookie had been provided.
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.
Where the server is unwilling or unable to provide synchronization for
an attribute for a set of entries, the server MUST treat all filter
components matching against these attribute as Undefined and MUST NOT
return the attribute in searchResultEntry responses.
Servers SHOULD support synchronization for all non-collective
user-applications 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 in a
separate LDAP session.
3.3. refreshOnly mode
A Sync request with mode refreshOnly and no cookie is a poll for
initial content. A Sync request with mode refreshOnly and cookie is a
poll for content update.
3.3.1. Initial Content Poll
Upon receipt of the request, the server provides the initial content
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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 referral [RFC3296] object's entryUUID), and no cookie.
The searchResultDone message SHALL include a Sync Done control. The
refreshDeletes SHALL be FALSE.
A resultCode value of success indicates the operation successfully
completed. Otherwise, the result code indicates the nature of
failure.
If the operation is successful, a cookie SHOULD be returned for use in
subsequent Sync operations.
3.3.2. Content Update Poll
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),
c) indicate that convergence is not possible by returning
syncRefreshRequired,
d) return a resultCode other than success or syncRefreshRequired.
For each entry or reference added to the content or was changed since
the previous Sync operation indicated by the cookie, the server
returns a searchResultEntry or searchResultReference message,
respectively, each with a Sync State cookie 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.
For each entry which has not been changed since the previous Sync
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operation, a searchResultEntry is returned whose objectName reflects
the entry's current DN, the attributes field is empty, and a Sync
State control 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, a searchResultReference containing
an empty SEQUENCE OF LDAPURL is returned with a Sync State control 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 content.
As an alternative to sending messages for each entry and reference
which has not been changed, the server may instead return the
following. For each entry no longer in content, return a
searchResultEntry whose objectName reflects a past DN of the entry or
is empty, the attributes field is empty, and a Sync State control of
state delete, entryUUID containing the UUID of the deleted entry, and
no cookie. For each reference no longer in content, a
searchResultReference containing an empty SEQUENCE OF LDAPURL is
returned with a a Sync State control of state delete, entryUUID
containing the UUID of the deleted reference, and no cookie.
A resultCode value of success indicates the operation successfully
completed. Otherwise, the result code indicates the nature of
failure.
If the operation is successful, a cookie SHOULD be returned for use in
subsequent Sync operations.
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 successful completion of content refresh is indicated by sending
a Sync Info with state refreshDone message instead of a
SearchResultDone message with resultCode success. A cookie SHOULD be
returned for use in subsequent Sync operations.
3.4.2. persist stage
Change notifications are provided during the persist stage.
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As updates are made to the DIT the server notifies the client of
changes to the content. DIT updates may cause entries references to
be added to the content, deleted from the content, or modify entries
in 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
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 in 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 in 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.
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 newest (last) cookie it received from the server
in subsequent Sync operations.
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3.5. Search Request Parameters
As stated in Section 3.1, the client SHOULD specify the same content
controlling parameters (see Section 3.5) in each Search Request of the
session. All fields of the SearchRequest message are considered
content controlling parameters except for sizeLimit and timeLimit.
3.5.1. baseObject Issues
As with the normal search operation, the refresh and persist phases
are not isolated from DIT changes. It is possible that the entry
referred to be the baseObject be 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 refers to any entry or
changes which entry the baseObject refers to, the server SHALL return
an appropriate non-success result code such as noSuchObject,
aliasProblem, aliasDereferencingProblem, referral, or
syncRefreshRequired.
3.5.2. derefAliases Issues
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 Issues
The sizeLimit applies only to entries (regardless of their syncState)
returned during refreshOnly processing or the refresh stage of the
refreshAndPersist processing.
3.5.4. timeLimit Issues
For a refreshOnly Sync operation, the timeLimit applies to the whole
operation. For a refreshAndPersist operation, the timeLimit applies
to processing up to and including generating the Sync Info with state
refreshDone message.
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3.5.5. filter Issues
The client SHOULD avoid filter assertions which apply to values of
attributes likely to be considered by the server as holding meta-
information. See section 4.
3.6. objectName Issues
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 SHOULD implement the LDAP Cancel [CANCEL] operation and
support cancellation of outstanding Sync operations as described here.
To cancel an outstanding Sync Operation, the client SHOULD issue a
Cancel operation [CANCEL]....
3.7. Refresh Required
In order to achieve the eventual-convergent synchronization, the
server may terminate the Sync operation in refresh or persist stage by
returning a syncRefreshRequired resultCode to the client. The client
may then request a full reload (e.g., no cookie) instead of
incremental synchronization in order to obtain a new copy of the
content. In case that the client issues incremental synchronization
requests between the issue of a syncRefreshRequired and that of a full
reload, the server should send a syncRefreshRequired response again,
but the client may receive one or more searchResultEntry responses
before it receives the syncRefreshRequired response.
The server may also choose to provide a full copy in the refresh stage
(e.g., ignore the cookie) instead of providing an incremental refresh
in order to achieve the eventual convergence.
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In the case of persist stage Sync, the server returns the resultCode
of syncRefreshRequired to the client to indicate that the client needs
to issue a full reload operation (e.g., no cookie) in order to obtain
a synchronized copy of the content.
The server may also return syncRefreshRequired if it determines that a
refresh would be more efficient than sending all the messages required
for convergence.
3.8. 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 historical information, if the server has sufficient
history to allow it to reliably determine which entries in the prior
shadow copy are no longer present in the content and the number of
such entries is less than equal the number of unchanged entries, the
server SHOULD generate delete entry messages instead of present entry
messages (see Section 3.3.2).
The server SHOULD maintain enough (current or historical) state
information (such as a context-wide last modify time stamp), to
determine that no changes were made in the context since the content
to refresh was provided and, and when no changes were made, generate
zero delete entry messages instead of present messages.
The server implementor should also consider chattiness issues which
span multiple Sync operations of a session. As noted in Section 3.7,
the server may return syncRefreshRequired if it determines that a
refresh would be more efficient than continuing under the current
operation.
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.
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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 a superior to an entry causes the entry's DN
to change, that change SHOULD NOT, by itself, cause synchronization
message to be sent for that entry. However, if renaming or moving of
a superior could cause the entry to added or deleted from the content
and, if so, appropriate synchronization messages should be generated
to indicate this to the client.
Where a server treats the entry's DN as meta information, the server
SHALL either
- evaluate all MatchingRuleAssertions 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
in subschema subentries.
For 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. Synchronization of operational attributes is
discussed in Section 4.1.
4.3. Collective Attributes
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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 detailed in [COLLECTIVE].
Modification of a collective attribute generally affects the content
of multiple entries, each a member 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 require transmission of multiple SearchResultEntry (one of
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, ...). 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 syncRefreshRequired or treat the Sync Operation as an initial
content request (e.g., ignore the cookie).
5. Interaction with other controls
The Sync Operation may be used with:
- ManageDsaIT Control [RFC3296]
- Subentries Control [SUBENTRY]
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
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objects to be treated as normal objects 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"
[SUBENTRY]. When used with the Sync Operation, the subentries control
and other factors (search scope, filter, etc.) are used to determining
whether an entry or subentry appear in the content or not.
6. 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.
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.
The Sync operation may be the target of denial of service attacks.
Implementors should provide safeguards to ensure these mechanisms are
not abused. Servers may place access control or other restrictions
upon the use of this operation.
Implementors of this (or any) LDAP extension should be familiar with
general LDAP security considerations [RFC3377].
7. IANA Considerations
Registration of the following values is requested.
7.1. Object Identifier
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It is requested that IANA register upon Standards Action an LDAP
Object Identifier to identify elements of the LDAP Content
Synchronization Operation as defined in this document.
Subject: Request for LDAP Object Identifier Registration
Person & email address to contact for further information:
Kurt Zeilenga <kurt@OpenLDAP.org>
Specification: RFCXXXX
Author/Change Controller: IESG
Comments:
Identifies elements of the LDAP Content Synchronization Operation
7.2. LDAP Protocol Mechanism
It is requested that IANA register upon Standards Action the LDAP
Protocol Mechanism described in this document.
Subject: Request for LDAP Protocol Mechanism Registration
Object Identifier: IANA-ASSIGNED-OID
Description: LDAP Content Synchronization Control
Person & email address to contact for further information:
Kurt Zeilenga <kurt@openldap.org>
Usage: Control
Specification: RFCXXXX
Author/Change Controller: IESG
Comments: none
7.3. LDAP Result Codes
It is requested that IANA register upon Standards Action the LDAP
Result Codes described in this document.
Subject: LDAP Result Code Registration
Person & email address to contact for further information:
Kurt Zeilenga <kurt@OpenLDAP.org>
Result Code Name: syncRefreshRequired (IANA-ASSIGNED-CODE-0)
Specification: RFCXXXX
Author/Change Controller: IESG
Comments: none
8. Acknowledgment
This work borrows significantly from the LDAP Client Update Protocol
[LCUP]. This work also benefited Persistent Search [PSEARCH],
Triggered Search [TSEARCH], and Directory Synchronization [DIRSYNC]
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efforts. This work also borrows from "Lightweight Directory Access
Protocol (v3)" [RFC2251].
9. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[RFC2252] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax
Definitions", RFC 2252, December 1997.
[RFC2256] M. Wahl, "A Summary of the X.500(96) User Schema for use
with LDAPv3", RFC 2256, December 1997.
[RFC2830] J. Hodges, R. Morgan, and M. Wahl, "Lightweight Directory
Access Protocol (v3): Extension for Transport Layer
Security", RFC 2830, May 2000.
[RFC3296] K. Zeilenga, "Named Subordinate References in Lightweight
Directory Access Protocol (LDAP) Directories", RFC 3296,
July 2002.
[RFC3377] J. Hodges, R.L. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC 3377,
September 2002.
[LDAPIRM] R. Harrison, K. Zeilenga, "LDAP Intermediate Response
Message", draft-rharrison-ldap-intermediate-resp-xx.txt
(a work in progress).
[SUBENTRY] K. Zeilenga, S. Legg, "Subentries in LDAP",
draft-zeilenga-ldap-subentry-xx.txt, a work in progress.
[X.680] ITU-T, "Abstract Syntax Notation One (ASN.1) -
Specification of Basic Notation", X.680, 1994.
[X.690] ITU-T, "Specification of ASN.1 encoding rules: Basic,
Canonical, and Distinguished Encoding Rules", X.690,
1994.
[CANCEL] K. Zeilenga, "LDAP Cancel Extended Operation",
draft-zeilenga-ldap-cancel-xx.txt, a work in progress.
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[UUID] International Organization for Standardization (ISO),
"Information technology - Open Systems Interconnection -
Remote Procedure Call", ISO/IEC 11578:1996.
10. Informative References
[RFC3383] K. Zeilenga, "IANA Considerations for LDAP", BCP 64 (also
RFC 3383), September 2002.
[X.500] ITU-T Rec. X.500, "The Directory: Overview of Concepts,
Models and Service", 1993.
[X.511] ITU, "The Directory: Abstract Service Definition", ITU-T
Rec. X.511, 1993.
[X.525] ITU, "The Directory: Replication", ITU-T Rec. X.525,
1993.
[COLLECTIVE] K. Zeilenga, "Collective Attributes in LDAP",
draft-zeilenga-ldap-collective-xx.txt, a work in
progress.
[DIRSYNC] M. Armijo, "Microsoft LDAP Control for Directory
Synchronization", draft-armijo-ldap-dirsync-xx.txt, a
work in progress.
[LCUP] R. Megginson, et. al., "LDAP Client Update Protocol",
draft-ietf-ldup-lcup-xx.txt, a work in progress.
[PSEARCH] M. Smith, et. al., "Persistent Search: A Simple LDAP
Change Notification Mechanism",
draft-ietf-ldapext-psearch-xx.txt, a work in progress.
[TSEARCH] M. Wahl, "LDAPv3 Triggered Search Control",
draft-ietf-ldapext-trigger-xx.txt, a work in progress.
[UUID-CSN] K. Zeilenga, J. Choi, "LDAP UUID and CSN Operational
Attributes", draft-zeilenga-ldap-uuid-csn-xx.txt, a work
(not yet) in progress.
10. Authors' Address
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
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Jonghyuk Choi
IBM Corporation
<jongchoi@us.ibm.com>
Full Copyright
Copyright 2003, The Internet Society. 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.
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 AUTHORS, 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.
Appendix - 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 some of the implementation considerations
associated with a Change Sequence Number [UUID-CSN] based approaches
to supporting the LDAP Content Synchronization Operation.
Change Sequence Number-based approaches are targetted for use in
servers which do not maintain historical 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
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which maintain historical information should consider an other
approaches which exploit the historical information.
A Change Sequence Number is, effectively a time stamp has sufficient
granularity to ensure that relationship in time of two updates to the
same object can be determined. Change Sequence Numbers are not to be
confused with Commit Sequence Numbers or Commit Log Record Numbers. A
Commit Sequence Number allow one to determine how to 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 an entry CSN
operational attribute for each directory entry (as discussed in [UUID-
CSN], but 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 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 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
entry "update" messages for all entries in the content whose CSN is
greater than the cookie CSN and entry "present" messages for all other
entries in the content. However, if the current context CSN is same
as the cookie CSN, the server should instead generate zero "updates",
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
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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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