The LCUP protocol is intended to allow LDAP clients to synchronize
with the content stored by LDAP servers.
The problem areas addressed by the protocol include:
- Mobile clients that maintain a local read-only copy of the
directory data. While off-line, the client uses the local copy of
the data. When the client connects to the network, it
synchronizes with the current directory content and can optionally
receive notification about the changes that occur while it is on-
line. For example, a mail client can maintain a local copy of the
corporate address book that it synchronizes with the master copy
whenever the client is connected to the corporate network.
- Applications intending to synchronize heterogeneous data stores.
A meta directory application, for instance, would periodically
retrieve a list of modified entries from the directory, construct
the changes and apply them to a foreign data store.
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RFC 3928 LDAP Client Update Protocol October 2004
- Clients that need to take certain actions when a directory entry
is modified. For instance, an electronic mail repository may want
to perform a "create mailbox" task when a new person entry is
added to an LDAP directory and a "delete mailbox" task when a
person entry is removed.
The problem areas not being considered:
- Directory server to directory server synchronization. The IETF is
developing a LDAP replication protocol, called LDUP [RFC3384],
which is specifically designed to address this problem area.
There are currently several protocols in use for LDAP client server
synchronization. While each protocol addresses the needs of a
particular group of clients (e.g., on-line clients or off-line
clients), none satisfies the requirements of all clients in the
target group. For instance, a mobile client that was off-line and
wants to become up to date with the server and stay up to date while
connected can't be easily supported by any of the existing protocols.
LCUP is designed such that the server does not need to maintain state
information specific to individual clients. The server may need to
maintain additional state information about attribute modifications,
deleted entries, and moved/renamed entries. The clients are
responsible for storing the information about how up to date they are
with respect to the server's content. LCUP design avoids the need
for LCUP-specific update agreements to be made between client and
server prior to LCUP use. The client decides when and from where to
retrieve the changes. LCUP design requires clients to initiate the
update session and "pull" the changes from server.
LCUP operations are subject to administrative and access control
policies enforced by the server.
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, RFC 2119
[RFC2119].
LCUP will work best if the following conditions are met:
1) The server stores some degree of historical state or change
information to reduce the amount of wire traffic required for
incremental synchronizations. The optimal balance between server
state and wire traffic varies amongst implementations and usage
scenarios, and is therefore left in the hands of implementers.
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2) The client cannot be assumed to understand the physical
information model (virtual attributes, operational attributes,
subentries, etc.) implemented by the server. Optimizations would
be possible if such assumptions could be made.
3) Meta data changes and renames and deletions of large subtrees are
very infrequent. LCUP makes these assumptions in order to reduce
client complexity required to deal with these special operations,
though when they do occur they may result in a large number of
incremental update messages or a full resync.
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]. All ASN.1 in this document uses implicit tags.
Distinguished names can change, so are therefore unreliable as
identifiers. A Universally Unique Identifier (or UUID for short)
MUST be used to uniquely identify entries used with LCUP. The UUID
is part of the Sync Update control value (see below) returned with
each search result. The server SHOULD provide the UUID as a single
valued operational attribute of the entry (e.g., "entryUUID"). We
RECOMMEND that the server provides a way to do efficient (i.e.,
indexed) searches for values of UUID, e.g., by using a search filter
like (entryUUID=<some UUID value>) to quickly search for and retrieve
an entry based on its UUID. Servers SHOULD use a UUID format as
specified in [UUID]. The UUID used by LCUP is a value of the
following ASN.1 type:
LCUPUUID ::= OCTET STRING
The LCUP protocol uses a cookie to hold the state of the client's
data with respect to the server's data. Each cookie format is
uniquely identified by its scheme. The LCUP Scheme is a value of the
following ASN.1 type:
LCUPScheme ::= LDAPOID
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This is the OID which identifies the format of the LCUP Cookie value.
The scheme OID, as all object identifiers, MUST be unique for a given
cookie scheme. The cookie value may be opaque or it may be exposed
to LCUP clients. For cookie schemes that expose their value, the
preferred form of documentation is an RFC. It is expected that there
will be one or more standards track cookie schemes where the value
format is exposed and described in detail.
The LCUP Cookie is a value of the following ASN.1 type:
LCUPCookie ::= OCTET STRING
This is the actual data describing the state of the client's data.
This value may be opaque, or its value may have some well-known
format, depending on the scheme.
Further uses of the LCUP Cookie value are described below.
A part of the DIT which is enabled for LCUP is referred to as an LCUP
Context. A server may support one or more LCUP Contexts. For
example, a server with two naming contexts may support LCUP in one
naming context but not the other, or support different LCUP cookie
schemes in each naming context. Each LCUP Context MAY use a
different cookie scheme. An LCUP search will not cross an LCUP
Context boundary, but will instead return a SearchResultReference
message, with the LDAP URL specifying the same host and port as
currently being searched, and with the baseDN set to the baseDN of
the new LCUP Context. The client is then responsible for issuing
another search using the new baseDN, and possibly a different cookie
if that LCUP Context uses a different cookie. The client is
responsible for maintaining a mapping of the LDAP URL to its
corresponding cookie.
Implementations of this specification SHALL recognize the following
additional resultCode values. The LDAP result code names and numbers
defined in the following table have been assigned by IANA per RFC
3383 [RFC3383].
lcupResourcesExhausted (113) the server is running out of resources
lcupSecurityViolation (114) the client is suspected of malicious
actions
lcupInvalidData (115) invalid scheme or cookie was supplied
by the client
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lcupUnsupportedScheme (116) The cookie scheme is a valid OID but
is not supported by this server
lcupReloadRequired (117) indicates that client data needs to be
reinitialized. This reason is
returned if the server does not
contain sufficient information to
synchronize the client or if the
server's data was reloaded since the
last synchronization session
The uses of these codes are described below.
The Sync Request Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier 1.3.6.1.1.7.1 and the
controlValue, an OCTET STRING, contains a BER-encoded
syncRequestControlValue.
syncRequestControlValue ::= SEQUENCE {
updateType ENUMERATED {
syncOnly (0),
syncAndPersist (1),
persistOnly (2) },
sendCookieInterval [0] INTEGER OPTIONAL,
scheme [1] LCUPScheme OPTIONAL,
cookie [2] LCUPCookie OPTIONAL
}
sendCookieInterval - the server SHOULD send the cookie back in the
Sync Update control value (defined below) for every
sendCookieInterval number of SearchResultEntry and
SearchResultReference PDUs returned to the client. For example, if
the value is 5, the server SHOULD send the cookie back in the Sync
Update control value for every 5 search results returned to the
client. If this value is absent, zero or less than zero, the server
chooses the interval.
The Sync Request Control is only applicable to the searchRequest
message. Use of this control is described below.
The Sync Update Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier 1.3.6.1.1.7.2 and the
controlValue, an OCTET STRING, contains a BER-encoded
syncUpdateControlValue.
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RFC 3928 LDAP Client Update Protocol October 2004
syncUpdateControlValue ::= SEQUENCE {
stateUpdate BOOLEAN,
entryUUID [0] LCUPUUID OPTIONAL, -- REQUIRED for entries --
UUIDAttribute [1] AttributeType OPTIONAL,
entryLeftSet [2] BOOLEAN,
persistPhase [3] BOOLEAN,
scheme [4] LCUPScheme OPTIONAL,
cookie [5] LCUPCookie OPTIONAL
}
The field UUIDAttribute contains the name or OID of the attribute
that the client should use to perform searches for entries based on
the UUID. The client should be able to use it in an equality search
filter, e.g., "(<uuid attribute>=<entry UUID value>)" and should be
able to use it in the attribute list of the search request to return
its value. The UUIDAttribute field may be omitted if the server does
not support searching on the UUID values.
The Sync Update Control is only applicable to SearchResultEntry and
SearchResultReference messages. Although entryUUID is OPTIONAL, it
MUST be used with SearchResultEntry messages. Use of this control is
described below.
The Sync Done Control is an LDAP Control [RFC2251, Section 4.1.2]
where the controlType is the object identifier 1.3.6.1.1.7.3 and the
controlValue contains a BER-encoded syncDoneValue.
syncDoneValue ::= SEQUENCE {
scheme [0] LCUPScheme OPTIONAL,
cookie [1] LCUPCookie OPTIONAL
}
The Sync Done Control is only applicable to SearchResultDone message.
Use of this control is described below.
A client initiates a synchronization or persistent search session
with a server by attaching a Sync Request control to an LDAP
searchRequest message. The search specification determines the part
of the directory information tree (DIT) the client wishes to
synchronize with, the set of attributes it is interested in and the
amount of data the client is willing to receive. The Sync Request
control contains the client's request specification.
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If there is an error condition, the server MUST immediately return a
SearchResultDone message with the resultCode set to an error code.
This table maps a condition to its corresponding behavior and
resultCode.
Condition Behavior or resultCode
Sync Request Control is not Server behaves as [RFC2251, Section
supported 4.1.2] - specifically, if the
criticality of the control is FALSE,
the server will process the request
as a normal search request
Scheme is not supported lcupUnsupportedScheme
A control value field is lcupInvalidData
invalid (e.g., illegal
updateType, or the scheme is
not a valid OID, or the cookie
is invalid)
Server is running out of lcupResourcesExhausted
resources
Server suspects client of lcupSecurityViolation
malicious behavior (frequent
connects/disconnects, etc.)
The server cannot bring the lcupReloadRequired
client up to date (server data
has been reloaded, or other
changes prevent
convergence)
For an initial synchronization or full resync, the fields of the Sync
Request control MUST be specified as follows:
updateType - MUST be set to syncOnly or syncAndPersist
sendCookieInterval - MAY be set
scheme - MAY be set - if set, the server MUST use this
specified scheme or return lcupUnsupportedScheme
(see above) - if not set, the server MAY use any
scheme it supports.
cookie - MUST NOT be set
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If the request was successful, the client will receive results as
described in the section "LCUP Search Responses" below.
For an incremental or update synchronization, the fields of the Sync
Request control MUST be specified as follows:
updateType - MUST be set to syncOnly or syncAndPersist
sendCookieInterval - MAY be set
scheme - MUST be set
cookie - MUST be set
The client SHOULD always use the latest cookie it received from the
server.
If the request was successful, the client will receive results as
described in the section "LCUP Search Responses" below.
For persistent only search request, the fields of the Sync Request
MUST be specified as follows:
updateType - MUST be set to persistOnly
sendCookieInterval - MAY be set
scheme - MAY be set - if set, the server MUST use this
specified scheme or return
lcupUnsupportedScheme (see above) - if not set,
the server MAY use any scheme it supports.
cookie - MAY be set, but the server MUST ignore it
If the request was successful, the client will receive results as
described in the section "LCUP Search Responses" below.
In response to the client's LCUP request, the server returns zero or
more SearchResultEntry or SearchResultReference PDUs that fit the
client's specification, followed by a SearchResultDone PDU. The
behavior is as specified in [RFC2251 Section 4.5]. Each
SearchResultEntry or SearchResultReference PDU also contains a Sync
Update control that describes the LCUP state of the returned entry.
The SearchResultDone PDU contains a Sync Done control. The following
sections specify behaviors in addition to [RFC2251 Section 4.5].
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The server may use the Sync Update control to return information not
related to a particular entry. It MAY do this at any time to return
a cookie to the client, or to inform the client that the sync phase
of a syncAndPersist search is complete and the persist phase has
begun. It MAY do this during the persist phase even though no entry
has changed that would have normally triggered a response. In order
to do this, it is REQUIRED to return the following:
- A SearchResultEntry PDU with the objectName field set to the DN of
the baseObject of the search request and with an empty attribute
list.
- A Sync Update control value with the fields set to the following:
stateUpdate - MUST be set to TRUE
entryUUID - SHOULD be set to the UUID of the baseObject of the
search request
entryLeftSet - MUST be set to FALSE
persistPhase - MUST be FALSE if the search is in the sync phase of a
request, and MUST be TRUE if the search is in the
persist phase
UUIDAttribute - SHOULD only be set if this is either the first result
returned or if the attribute has changed
scheme - MUST be set if the cookie is set and the cookie
format has changed; otherwise, it MAY be omitted
cookie - SHOULD be set
If the server merely wants to return a cookie to the client, it
should return as above with the cookie field set.
During a syncAndPersist request, the server MUST return (as above)
immediately after the last entry of the sync phase has been sent and
before the first entry of the persist phase has been sent. In this
case, the persistPhase field MUST be set to TRUE. This allows the
client to know that the sync phase is complete and the persist phase
is starting.
The cookie field of the Sync Update control value MAY be set in any
returned result, during both the sync phase and the persist phase.
The server should return the cookie to the client often enough for
the client to resync in a reasonable period of time in case the
search is disconnected or otherwise terminated. The
sendCookieInterval field in the Sync Request control is a suggestion
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RFC 3928 LDAP Client Update Protocol October 2004
to the server of how often to return the cookie in the Sync Update
control. The server SHOULD respect this value.
The scheme field of the Sync Update control value MUST be set if the
cookie is set and the cookie format has changed; otherwise, it MAY be
omitted.
Some clients may have unreliable connections, for example, a wireless
device or a WAN connection. These clients may want to insure that
the cookie is returned often in the Sync Update control value, so
that if they have to reconnect, they do not have to process many
redundant entries. These clients should set the sendCookieInterval
in the Sync Request control value to a low number, perhaps even 1.
Some clients may have a limited bandwidth connection, and may not
want to receive the cookie very often, or even at all (however, the
cookie is always sent back in the Sync Done control value upon
successful completion). These clients should set the
sendCookieInterval in the Sync Request control value to a high
number.
A reasonable behavior of the server is to return the cookie only when
data in the LCUP context has changed, even if the client has
specified a frequent sendCookieInterval. If nothing has changed, the
server can probably save some bandwidth by not returning the cookie.
An entry SHALL BE considered to have entered the client's search
result set if one of the following conditions is met:
- During the sync phase for an incremental sync operation, the entry
is present in the search result set but was not present before;
this can be due to the entry being added via an LDAP Add
operation, or by the entry being moved into the result set by an
LDAP Modify DN operation, or by some modification to the entry
that causes it to enter the result set (e.g., adding an attribute
value that matches the clients search filter), or by some meta-
data change that causes the entry to enter the result set (e.g.,
relaxing of some access control that permits the entry to be
visible to the client).
- During the persist phase for a persistent search operation, the
entry enters the search result set; this can be due to the entry
being added via an LDAP Add operation, or by the entry being moved
into the result set by an LDAP Modify DN operation, or by some
modification to the entry that causes it to enter the result set
(e.g., adding an attribute value that matches the clients search
filter), or by some meta-data change that causes the entry to
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RFC 3928 LDAP Client Update Protocol October 2004
enter the result set (e.g., relaxing of some access control that
permits the entry to be visible to the client).
An entry SHALL BE considered to be changed if one or more of the
attributes in the attribute list in the search request have been
modified. For example, if the search request listed the attributes
"cn sn uid", and there is an entry in the client's search result set
with the "cn" attribute that has been modified, the entry is
considered to be modified. The modification may be due to an LDAP
Modify operation or by some change to the meta-data for the entry
(e.g., virtual attributes) that causes some change to the value of
the specified attributes.
The converse of this is that an entry SHALL NOT BE considered to be
changed if none of the attributes in the attribute list of the search
request are modified attributes of the entry. For example, if the
search request listed the attributes "cn sn uid", and there is an
entry in the client's search result set with the "foo" attribute that
has been modified, and none of the "cn" or "sn" or "uid" attributes
have been modified, the entry is NOT considered to be changed.
An entry SHALL BE considered to have left the client's search result
set if one of the following conditions is met:
- During the sync phase for an incremental sync operation, the entry
is not present in the search result set but was present before;
this can be due to the entry being deleted via an LDAP Delete
operation, or by the entry leaving the result set via an LDAP
Modify DN operation, or by some modification to the entry that
causes it to leave the result set (e.g., changing/removing an
attribute value so that it no longer matches the client's search
filter), or by some meta-data change that causes the entry to
leave the result set (e.g., adding of some access control that
denies the entry to be visible to the client).
- During the persist phase for a persistent search operation, the
entry leaves the search result set; this can be due to the entry
being deleted via an LDAP Delete operation, or by the entry
leaving the result set via an LDAP Modify DN operation, or by some
modification to the entry that causes it to leave the result set
(e.g., changing/removing an attribute value so that it no longer
matches the client's search filter), or by some meta-data change
Megginson, et al. Standards Track [Page 13]
RFC 3928 LDAP Client Update Protocol October 2004
that causes the entry to leave the result set (e.g., adding of
some access control that denies the entry to be visible to the
client).
An entry SHOULD be returned as present under the following
conditions:
- The request is an initial synchronization or full resync request
and the entry is present in the client's search result set
- The request is an incremental synchronization and the entry has
changed or entered the result set since the last sync
- The search is in the persist phase and the entry enters the result
set or changes
For a SearchResultEntry return, the fields of the Sync Update control
value MUST be set as follows:
stateUpdate - MUST be set to FALSE
entryUUID - MUST be set to the UUID of the entry
entryLeftSet - MUST be set to FALSE
persistPhase - MUST be set to FALSE if during the sync phase or TRUE
if during the persist phase
UUIDAttribute - SHOULD only be set if this is either the first result
returned or if the attribute has changed
scheme - as above
cookie - as above
The searchResultReference return will look the same, except that the
entryUUID is not required. If it is specified, it MUST contain the
UUID of the DSE holding the reference knowledge.
An entry SHOULD be returned as having left the result set under the
following conditions:
- The request is an incremental synchronization during the sync
phase and the entry has left the result set
- The search is in the persist phase and the entry has left the
result set
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RFC 3928 LDAP Client Update Protocol October 2004
- The entry has left the result set as a result of an LDAP Delete or
LDAP Modify DN operation against the entry itself (i.e., not as a
result of an operation against its parent or ancestor)
For a SearchResultEntry return where the entry has left the result
set, the fields of the Sync Update control value MUST be set as
follows:
stateUpdate - MUST be set to FALSE
entryUUID - MUST be set to the UUID of the entry that left the
result set
entryLeftSet - MUST be set to TRUE
persistPhase - MUST be set to FALSE if during the sync phase or TRUE
if during the persist phase
UUIDAttribute - SHOULD only be set if this is either the first result
returned or if the attribute has changed
scheme - as above
cookie - as above
The searchResultReference return will look the same, except that the
entryUUID is not required. If it is specified, it MUST contain the
UUID of the DSE holding the reference knowledge.
Some server implementations keep track of deleted entries using a
tombstone - a hidden entry that keeps track of the state, but not all
of the data, of an entry that has been deleted. In this case, the
tombstone may not contain all of the original attributes of the
entry, and therefore it may be impossible for the server to determine
if an entry should be removed from the result set based on the
attributes in the client's search request. Servers SHOULD keep
enough information about the attributes in the deleted entries to
determine if an entry should be removed from the result set. Since
this may not be possible, the server MAY return an entry as having
left the result set even if it is not or never was in the client's
result set. Clients MUST ignore these notifications.
During the persistent phase, the server SHOULD return the changed
entries to the client as quickly as possible.
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RFC 3928 LDAP Client Update Protocol October 2004
During a sync phase, the server MUST NOT return any entries with the
persistPhase flag set to TRUE, and during the persist phase, all
entries returned MUST have the persistPhase flag set to TRUE. The
server MUST NOT mix and match sync phase entries with persist phase
entries. If there are any sync phase entries to return, they MUST be
returned before any persist phase entries are returned.
There may be updates to the entries in the result set of a sync phase
search during the actual search operation. If the DSA is under a
heavy update load, and it attempts to send all of those updated
entries to the client in addition to the other updates it was already
planning to send for the sync phase, the server may never get to the
end of the sync phase. Therefore, it is left up to the discretion of
the server implementation to decide when the client is "in sync" -
that is, when to end a syncOnly request, or when to send the Sync
Update Informational Response between the sync phase and the persist
phase of a syncAndPersist request. The server MAY send the same
entry multiple times during the sync phase if the entry changes
during the sync phase.
A reasonable behavior is for the server to generate a cookie based on
the server state at the time the client initiated the LCUP request,
and only send entries up to that point during the sync phase. Entries
updated after that point will be returned only during the persist
phase of a syncAndPersist request, or only upon an incremental
synchronization.
An operational attribute SHOULD be returned if it is specified in the
attributes list and would normally be returned as subject to the
constraints of [RFC2251 Section 4.5]. If the server does not support
syncing of operational attributes, the server MUST return a
SearchResultDone message with a resultCode of unwillingToPerform.
LDAP Subentries [RFC3672] SHOULD be returned if they would normally
be returned by the search request. If the server does not support
syncing of LDAP Subentries, and the server can determine from the
search request that the client has requested LDAP Subentries to be
returned (e.g., search control or search filter), the server MUST
return a SearchResultDone message with a resultCode of
unwillingToPerform. Otherwise, the server MAY simply omit returning
LDAP Subentries.
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An entry may have attributes whose presence in the entry, or presence
of values of the attribute, is generated on the fly, possibly by some
mechanism outside of the entry, elsewhere in the DIT. An example of
this is collective attributes [RFC3671]. These attributes shall be
referred to in this document as virtual attributes.
LCUP treats these attributes the same way as normal, non-virtual
attributes. A virtual attribute SHOULD be returned if it is
specified in the attributes list and would normally be returned as
subject to the constraints of [RFC2251 Section 4.5]. If the server
does not support syncing of virtual attributes, the server MUST
return a SearchResultDone message with a resultCode of
unwillingToPerform.
One consequence of this is that if you change the definition of a
virtual attribute such that it makes the value of that attribute
change in many entries in the client's search scope, this means that
a server may have to return many entries to the client as a result of
that one change. It is not anticipated that this will be a frequent
occurrence, and the server has the option to simply force the client
to resync if necessary.
It is also possible that a future LDAP control will allow the client
to request only virtual or only non-virtual attributes.
There is a special case where a Modify DN or a Delete operation is
applied to the base entry of a subtree, and either that base entry or
entries in the subtree are within the scope of an LCUP search
request. In this case, all of the entries in the subtree are
implicitly renamed or removed.
In either of these cases, the server MUST do one of the following:
- treat all of these entries as having been renamed or removed and
return each entry to the client as such
- decide that this would be prohibitively expensive, and force the
client to resync
If the search base object has been renamed, and the client has
received a noSuchObject as the result of a search request, the client
MAY use the entryUUID and UUIDAttribute to locate the new DN that is
the result of the modify DN operation.
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If at any time during an LCUP search, either during the sync phase or
the persist phase, the server determines that it cannot guarantee
that it can bring the client's copy of the data to eventual
convergence, it SHOULD immediately terminate the LCUP search request
and return a SearchResultDone message with a resultCode of
lcupReloadRequired. This can also happen at the beginning of an
incremental synchronization request, if the client presents a cookie
that is out of date or otherwise unable to be processed. The client
should then issue an initial synchronization request.
This can happen, for example, if the data on the server is reloaded,
or if there has been some change to the meta-data that makes it
impossible for the server to determine if a particular entry should
or should not be part of the search result set, or if the meta-data
change makes it too resource intensive for the server to calculate
the proper result set.
The server can also return lcupReloadRequired if it determines that
it would be more efficient for the client to perform a reload, for
example, if too many entries have changed and a simple reload would
be much faster.
When the server has successfully finished processing the client's
request, it attaches a Sync Done control to the SearchResultDone
message and sends it to the client. However, if the SearchResultDone
message contains a resultCode that is not success or canceled, the
Sync Done control MAY be omitted. Although the LCUP cookie is
OPTIONAL in the Sync Done control value, it MUST be set if the
SearchResultDone resultCode is success or canceled. The server
SHOULD also set the cookie if the resultCode is
lcupResourcesExhausted, timeLimitExceeded, sizeLimitExceeded, or
adminLimitExceeded. This allows the client to more easily resync
later. If some error occurred, either an LDAP search error (e.g.,
insufficientAccessRights) or an LCUP error (e.g.,
lcupUnsupportedScheme), the cookie MAY be omitted. If the cookie is
set, the scheme MUST be set also if the cookie format has changed,
otherwise, it MAY be omitted.
If server resources become tight, the server can terminate one or
more search operations by sending a SearchResultDone message to the
client(s) with a resultCode of lcupResourcesExhausted. The server
SHOULD attach a Sync Done control with the cookie set. A server side
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RFC 3928 LDAP Client Update Protocol October 2004
policy is used to decide which searches to terminate. This can also
be used as a security mechanism to disconnect clients that are
suspected of malicious actions, but if the server can infer that the
client is malicious, the server SHOULD return lcupSecurityViolation
instead.
If the client needs to terminate the synchronization process and it
wishes to obtain the cookie that represents the current state of its
data, it issues an LDAP Cancel operation [RFC3909]. The server
responds immediately with a LDAP Cancel response [RFC3909]. The
server MAY send any pending SearchResultEntry or
SearchResultReference PDUs if the server cannot easily abort or
remove those search results from its outgoing queue. The server
SHOULD send as few of these remaining messages as possible. Finally,
the server sends the message SearchResultDone with the Sync Done
control attached. If the search was successful up to that point, the
resultCode field of the SearchResultDone message MUST be canceled
[RFC3909], and the cookie MUST be set in the Sync Done control. If
there is an error condition, the server MAY return as described in
section 4.4.1 above, or MAY return as described in [RFC3909].
If the client is not interested in the state information, it can
simply abandon the search operation or disconnect from the server.
The server SHALL support size and time limits as specified in
[RFC2251, Section 5]. The server SHOULD ensure that if the operation
is terminated due to these conditions, the cookie is sent back to the
client.
It is permissible for the client to issue other LDAP operations on
the connection used by the protocol. Since each LDAP
request/response carries a message id there will be no ambiguity
about which PDU belongs to which operation. By sharing the
connection among multiple operations, the server will be able to
conserve its resources.
LCUP defines neither restrictions nor guarantees about the ability to
use the controls defined in this document in conjunction with other
LDAP controls, except for the following: A server MAY ignore non-
critical controls supplied with the LCUP control. A server MAY
Megginson, et al. Standards Track [Page 19]
RFC 3928 LDAP Client Update Protocol October 2004
ignore an LCUP defined control if it is non-critical and it is
supplied with other critical controls. If a server receives a
critical LCUP control with another critical control, and the server
does not support both controls at the same time, the server SHOULD
return unavailableCriticalExtension.
It is up to the server implementation to determine if the server
supports controls such as the Sort or VLV or similar controls that
change the order of the entries sent to the client. But note that it
may be difficult or impossible for a server to perform an incremental
synchronization in the presence of such controls, since the cookie
will typically be based off a change number, or Change Sequence
Number (CSN), or timestamp, or some criteria other than an
alphabetical order.
Use of an LCUP cookie with multiple DSAs in a replicated environment
is not defined by LCUP. An implementation of LCUP may support
continuation of an LCUP session with another DSA holding a replica of
the LCUP context. Clients MAY submit cookies returned by one DSA to
a different DSA; it is up to the server to determine if a cookie is
one they recognize or not and to return an appropriate result code if
not.
The cookie received from the server after a synchronization session
SHOULD only be used with the same search specification as the search
that generated the cookie. Some servers MAY allow the cookie to be
used with a more restrictive search specification than the search
that generated the cookie. If the server does not support the
cookie, it MUST return lcupInvalidCookie. This is because the client
can end up with an incomplete data store otherwise. A more
restrictive search specification is one that would generate a subset
of the data produced by the original search specification.
Because an LCUP client specifies the area of the tree with which it
wishes to synchronize through the standard LDAP search specification,
the client can be returned noSuchObject error if the root of the
synchronization area was renamed between the synchronization sessions
or during a synchronization session. If this condition occurs, the
client can attempt to locate the root by using the root's UUID saved
in client's local data store. It then can repeat the synchronization
Megginson, et al. Standards Track [Page 20]
RFC 3928 LDAP Client Update Protocol October 2004
request using the new search base. In general, a client can detect
that an entry was renamed and apply the changes received to the right
entry by using the UUID rather than DN based addressing.
Each active persistent operation requires that an open TCP connection
be maintained between an LDAP client and an LDAP server that might
not otherwise be kept open. Therefore, client implementors are
encouraged to avoid using persistent operations for non-essential
tasks and to close idle LDAP connections as soon as practical. The
server may close connections if server resources become tight.
The client MAY receive a continuation reference
(SearchResultReference [RFC2251 SECTION 4.5.3]) if the search request
spans multiple parts of the DIT, some of which may require a
different LCUP cookie, some of which may not even be managed by LCUP.
The client SHOULD maintain a cache of the LDAP URLs returned in the
continuation references and the cookies associated with them. The
client is responsible for performing another LCUP search to follow
the references, and SHOULD use the cookie corresponding to the LDAP
URL for that reference (if it has a cookie).
The client may receive a referral (Referral [RFC2251 SECTION 4.1.11])
when the search base is a subordinate reference, and this will end
the operation.
The server MAY send the same entry multiple times during a sync phase
if the entry changes during the sync phase. The client SHOULD use
the last sent copy of the entry as the current one.
If the client receives an lcupResourcesExhausted or
lcupSecurityViolation resultCode, the client SHOULD wait at least 5
seconds before attempting another operation. It is RECOMMENDED that
the client use an exponential backoff strategy, but different clients
may want to use different backoff strategies.
Megginson, et al. Standards Track [Page 21]
RFC 3928 LDAP Client Update Protocol October 2004
Servers MUST support UUIDs. UUIDs are required in the Sync Update
control. Additionally, server implementers SHOULD make the UUID
values for the entries available as an attribute of the entry, and
provide indexing or other mechanisms to allow clients to search for
an entry using the UUID attribute in the search filter. The
syncUpdate control provides a field UUIDAttribute to allow the server
to let the client know the name or OID of the attribute to use to
search for an entry by UUID.
By design, the protocol supports multiple cookie schemes. This is to
allow different implementations the flexibility of storing any
information applicable to their environment. A reasonable
implementation for an LDUP compliant server would be to use the
Replica Update Vector (RUV). For each master, RUV contains the
largest CSN seen from this master. In addition, RUV implemented by
some directory servers (not yet in LDUP) contains replica generation
- an opaque string that identifies the replica's data store. The
replica generation value changes whenever the replica's data is
reloaded. Replica generation is intended to signal the
replication/synchronization peers that the replica's data was
reloaded and that all other replicas need to be reinitialized. RUV
satisfies the three most important properties of the cookie: (1) it
uniquely identifies the state of client's data, (2) it can be used to
synchronize with multiple servers, and (3) it can be used to detect
that the server's data was reloaded. If RUV is used as the cookie,
entries last modified by a particular master must be sent to the
client in the order of their last modified CSN. This ordering
guarantees that the RUV can be updated after each entry is sent.
A server may support one or more LCUP cookie schemes. It is expected
that schemes will be published along with their OIDs as RFCs. The
server's DIT may be partitioned into different sections which may
have different cookies associated with them. For example, some
servers may use some sort of replication mechanism to support LCUP.
If so, the DIT may be partitioned into multiple replicas. A client
may send an LCUP search request that spans multiple replicas. Some
parts of the DIT spanned by the search request scope may support LCUP
and some may not. The server MUST send a SearchResultReference
Megginson, et al. Standards Track [Page 22]
RFC 3928 LDAP Client Update Protocol October 2004
[RFC2251, SECTION 4.5.3] when the LCUP Context for a returned entry
changes. The server SHOULD send all references to other LCUP
Contexts in the search scope first, in order to allow the clients to
process these searches in parallel. The LDAP URL(s) returned MUST
contain the DN(s) of the base of another section of the DIT (however
the server implementation has partitioned the DIT). The client will
then issue another LCUP search using the LDAP URL returned. Each
section of the DIT MAY require a different cookie value, so the
client SHOULD maintain a cache, mapping the different LDAP URL values
to different cookies. If the cookie changes, the scheme may change
as well, but the cookie scheme MUST be the same within a given LCUP
Context.
The cookie must contain enough information to allow the server to
determine whether the cookie can be safely used with the search
specification it is attached to. As discussed earlier in the
document, the cookie SHOULD only be used with the search
specification that is equal to the one for which the cookie was
generated, but some servers MAY support using a cookie with a search
specification that is more restrictive than the one used to generate
the cookie.
The specification makes no guarantees about how soon a server should
send notification of a changed entry to the client during the persist
phase. This is intentional as any specific maximum delay would be
impossible to meet in a distributed directory service implementation.
Server implementers are encouraged to minimize the delay before
sending notifications to ensure that clients' needs for timeliness of
change notification are met.
Implementers of servers that support the mechanism described in this
document should ensure that their implementation scales well as the
number of active persistent operations and the number of changes made
in the directory increases. Server implementers are also encouraged
to support a large number of client connections if they need to
support large numbers of persistent operations.
Megginson, et al. Standards Track [Page 23]
RFC 3928 LDAP Client Update Protocol October 2004
LCUP design does not consider issues associated with alias
dereferencing in search. Clients MUST specify derefAliases as either
neverDerefAliases or derefFindingBaseObj. Servers are to return
protocolError if the client specifies either derefInSearching or
derefAlways.
Clients, like a meta directory join engine, synchronizing multiple
writable data stores, will only work correctly if each piece of
information comes from a single authoritative data source. In a
replicated environment, an LCUP Context should employ the same
conflict resolution scheme across all its replicas. This is because
different systems have different notions of time and different update
resolution procedures. As a result, a change applied on one system
can be discarded by the other, thus preventing the data stores from
converging.
This document lists several values that have been registered by the
IANA. The following LDAP result codes have been assigned by IANA as
described in section 3.6 of [RFC3383]:
lcupResourcesExhausted 113
lcupSecurityViolation 114
lcupInvalidData 115
lcupUnsupportedScheme 116
lcupReloadRequired 117
The three controls defined in this document have been registered as
LDAP Protocol Mechanisms as described in section 3.2 of [RFC3383].
One OID, 1.3.6.1.1.7, has been assigned by IANA as described in
section 3.1 of [RFC3383]. The OIDs for the controls defined in this
document are derived as follows from the one assigned by IANA:
LCUP Sync Request Control 1.3.6.1.1.7.1
LCUP Sync Update Control 1.3.6.1.1.7.2
LCUP Sync Done Control 1.3.6.1.1.7.3
In some situations, it may be important to prevent general exposure
of information about changes that occur in an LDAP server. Therefore,
servers that implement the mechanism described in this document
SHOULD provide a means to enforce access control on the entries
Megginson, et al. Standards Track [Page 24]
RFC 3928 LDAP Client Update Protocol October 2004
returned and MAY also provide specific access control mechanisms to
control the use of the controls and extended operations defined in
this document.
As with normal LDAP search requests, a malicious client can initiate
a large number of persistent search requests in an attempt to consume
all available server resources and deny service to legitimate
clients. The protocol provides the means to stop malicious clients
by disconnecting them from the server. The servers that implement
the mechanism SHOULD provide the means to detect the malicious
clients. In addition, the servers SHOULD provide the means to limit
the number of resources that can be consumed by a single client.
[RFC3384] Stokes, E., Weiser, R., Moats, R., and R. Huber,
"Lightweight Directory Access Protocol (version 3)
Replication Requirements", RFC 3384, October 2002.
[RFC3671] Zeilenga, K., "Collective Attributes in the Lightweight
Directory Access Protocol (LDAP)", RFC 3671, December
2003.
[RFC3672] Zeilenga, K. and S. Legg, "Subentries in the Lightweight
Directory Access Protocol (LDAP)", RFC 3672, December
2003.
The LCUP protocol is based in part on the Persistent Search Change
Notification Mechanism defined by Mark Smith, Gordon Good, Tim Howes,
and Rob Weltman, the LDAPv3 Triggered Search Control defined by Mark
Wahl, and the LDAP Control for Directory Synchronization defined by
Michael Armijo. The members of the IETF LDUP working group made
significant contributions to this document.
Megginson, et al. Standards Track [Page 26]
RFC 3928 LDAP Client Update Protocol October 2004
Appendix - Features Left Out of LCUP
There are several features present in other protocols or considered
useful by clients that are currently not included in the protocol
primarily because they are difficult to implement on the server.
These features are briefly discussed in this section.
Triggered Search Change Type
This feature is present in the Triggered Search specification. A
flag is attached to each entry returned to the client indicating the
reason why this entry is returned. The possible reasons from the
document are:
- notChange: the entry existed in the directory and matched the
search at the time the operation is being performed,
- enteredSet: the entry entered the result,
- leftSet: the entry left the result,
- modified: the entry was part of the result set, was modified or
renamed, and still is in the result set.
The leftSet feature is particularly useful because it indicates to
the client that an entry is no longer within the client's search
specification and the client can remove the associated data from its
data store. Ironically, this feature is the hardest to implement on
the server because the server does not keep track of the client's
state and has no easy way of telling which entries moved out of scope
between synchronization sessions with the client. A compromise could
be reached by only providing this feature for the operations that
occur while the client is connected to the server. This is easier to
accomplish because the decision about the change type can be made
based only on the change without need for any historical information.
This, however, would add complexity to the protocol.
Persistent Search Change Type
This feature is present in the Persistent Search specification.
Persistent search has the notion of changeTypes. The client
specifies which type of updates will cause entries to be returned,
and optionally whether the server tags each returned entry with the
type of change that caused that entry to be returned.
For LCUP, the intention is full synchronization, not partial. Each
entry returned by an LCUP search will have some change associated
with it that may concern the client. The client may have to have a
Megginson, et al. Standards Track [Page 27]
RFC 3928 LDAP Client Update Protocol October 2004
local index of entries by DN or UUID to determine if the entry has
been added or just modified. It is easy for clients to determine if
the entry has been deleted because the entryLeftSet value of the Sync
Update control will be TRUE.
Sending Changes
Some earlier synchronization protocols sent the client(s) only the
modified attributes of the entry rather than the entire entry. While
this approach can significantly reduce the amount of data returned to
the client, it has several disadvantages. First, unless a separate
mechanism (like the change type described above) is used to notify
the client about entries moving into the search scope, sending only
the changes can result in the client having an incomplete version of
the data. Let's consider an example. An attribute of an entry is
modified. As a result of the change, the entry enters the scope of
the client's search. If only the changes are sent, the client would
never see the initial data of the entry. Second, this feature is
hard to implement since the server might not contain sufficient
information to construct the changes based solely on the server's
state and the client's cookie. On the other hand, this feature can
be easily implemented by the client assuming that the client has the
previous version of the data and can perform value by value
comparisons.
Data Size Limits
Some earlier synchronization protocols allowed clients to control the
amount of data sent to them in the search response. This feature was
intended to allow clients with limited resources to process
synchronization data in batches. However, an LDAP search operation
already provides the means for the client to specify the size limit
by setting the sizeLimit field in the SearchRequest to the maximum
number of entries the client is willing to receive. While the
granularity is not the same, the assumption is that regular LDAP
clients that can deal with the limitations of the LDAP protocol will
implement LCUP.
Data Ordering
Some earlier synchronization protocols allowed a client to specify
that parent entries should be sent before the children for add
operations and children entries sent before their parents during
delete operations. This ordering helps clients to maintain a
hierarchical view of the data in their data store. While possibly
useful, this feature is relatively hard to implement and is expensive
to perform.
Megginson, et al. Standards Track [Page 28]
RFC 3928 LDAP Client Update Protocol October 2004
Authors' Addresses
Rich Megginson
Netscape Communications Corp., an America Online company.
360 W. Caribbean Drive
Sunnyvale, CA 94089
USA
Phone: +1 505 797-7762
EMail: rmegginson0224@aol.com
Olga Natkovich
Yahoo, Inc.
701 First Ave.
Sunnyvale, CA 94089
USA
Phone: +1 408 349-6153
EMail: olgan@yahoo-inc.com
Mark Smith
Pearl Crescent, LLC
447 Marlpool Drive
Saline, MI 48176
USA
Phone: +1 734 944-2856
EMail: mcs@pearlcrescent.com
Jeff Parham
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
USA
Phone: +1 425 882-8080
EMail: jeffparh@microsoft.com
Megginson, et al. Standards Track [Page 29]
RFC 3928 LDAP Client Update Protocol October 2004
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Megginson, et al. Standards Track [Page 30]