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Internet Draft R. Megginson, Editor
Document: <draft-ietf-ldup-lcup-03.txt> M. Smith
Category: Proposed Standard Netscape
Communications Corp.
O. Natkovich
Yahoo
J. Parham
Microsoft Corporation
June 2002
LDAP Client Update Protocol
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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.
1. Abstract
This document defines the LDAP Client Update Protocol (LCUP). The
protocol is intended to allow an LDAP client to synchronize with the
content of a directory information tree (DIT) stored by an LDAP
server and to be notified about the changes to that content.
2. Conventions used in this document
In the protocol flow definition, the notation C->S and S->C
specifies the direction of the data flow from the client to the
server and from the server to the client respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119
[KEYWORDS].
3. Overview
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 be
optionally notified 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 gets 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.
- 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], 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.
Several features of the protocol distinguish it from LDUP
replication. LCUP is designed such that the server does not need to
maintain state information on behalf of the client. 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.
Megginson, et. al. Proposed Standard - Expires: December 2002 2
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.
4. Protocol Specification
This section describes the protocol elements and the protocol flow.
4.1 Universally Unique Identifiers
Distinguished names can change, so are therefore unreliable
as identifiers. The server SHOULD assign a Universally Unique
Identifier (or UUID for short) to each entry as it is created. This
identifier will be stored as an operational attribute of the entry,
named `entryUUID'. The entryUUID attribute is single valued. A
consistent algorithm for generating such universally unique
identifiers may be standardized at some point in the future.
The definition of the entryUUID attribute type, written using the
BNF form of AttributeDescription described in RFC 2252 [RFC2252] is:
( OID-To-Be-Specified
NAME `entryUUID'
DESC `universally unique entry identifier'
EQUALITY caseIgnoreMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
SINGLE-VALUE
NO-USER-MODIFICATION
USAGE directoryOperation )
4.2 LCUP Cookie Value
The LCUP protocol uses a cookie to hold the state of the client's
data with respect to the server's data. The LCUP Cookie is a value
of the following ASN.1 type:
LCUPCookie ::= SEQUENCE {
scheme LDAPOID,
value OCTET STRING OPTIONAL
}
scheme - this is the OID which identifies the format of the value.
The scheme OID, like 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.
value - 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. The cookie value
MUST be included except when a client has no stored state; i.e.,
when the client is requesting a full synchronization. When the
server sends back a cookie, the cookie value MUST be present.
Megginson, et. al. Proposed Standard - Expires: December 2002 3
Further uses of the LCUP Cookie value are described below.
4.3 Additional LDAP Result Codes defined by LCUP
The LDAP result code names and numbers defined in the following
table are to be replaced with IANA assigned result code names and
numbers per draft-ietf-ldapbis-iana-xx.txt.
lcupResourcesExhausted (TBD) the server is running out of
resources
lcupSecurityViolation (TBD) the client is suspected of malicious
actions
lcupInvalidCookie (TBD) invalid cookie was supplied by the
client - both/either the scheme
and/or the value part was invalid
lcupUnsupportedScheme (TBD) The scheme part of the cookie is a
valid OID but is not supported by
this server
lcupClientDisconnect (TBD) client requested search termination
using the LDAP Cancel extended
operation request [CANCEL]
lcupReloadRequired (TBD) 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.
4.4 Client Update Control Value
A client initiates a synchronization session with a server by
attaching a clientUpdate control to an LDAP searchRequest message.
The client SHOULD specify entryUUID in the attributes list in the
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 clientUpdate
control contains the client's synchronization specification. The
controlType field for the clientUpdate control is
ClientUpdateControlOID (to be assigned). The controlValue is an
OCTET STRING, whose contents are the bytes of the BER encoding of
the following:
Megginson, et. al. Proposed Standard - Expires: December 2002 4
ClientUpdateControlValue ::= SEQUENCE {
updateType ENUMERATED {
synchronizeOnly (0),
synchronizeAndPersist (1),
persistOnly (2) },
sendCookieInterval INTEGER OPTIONAL,
cookie LCUPCookie OPTIONAL
}
updateType - specifies the type of update requested by the client
synchronizeOnly - the server sends all the data needed to
synchronize the client with the server, then closes the
connection
synchronizeAndPersist - the server sends all the data needed to
synchronize the client with the server, then leaves open the
connection, sending to the client any new added, modified, or
deleted entries that satisfy the search criteria.
persistOnly - the server does not synchronize the data with the
client but leaves open the connection and sends over any new
added, modified, or deleted entries that satisfy the search
criteria.
sendCookieInterval <20> (optional) the server SHOULD send the cookie
back in the entryUpdate control value for every
sendCookieInterval number of SearchResultEntry PDUs returned to
the client. For example, if the value is 5, the server SHOULD
send the cookie back in the entryUpdate 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.
cookie - a value that represents the current state of the client's
data. If a cookie is provided, the server MUST use the enclosed
scheme throughout the duration of the LCUP session or until an
LCUP context boundary is crossed, since a new cookie may be
required in that case. If the value or scheme part of the cookie
is invalid, the server MUST return immediately with a
SearchResultDone message with the resultCode set to the value of
lcupInvalidCookie. If the scheme part of the cookie is a valid
OID, but is not supported, the server MUST return immediately
with a SearchResultDone message with the resultCode set to the
value of lcupUnsupportedScheme.
If the cookie is omitted, the server MAY use any scheme it
supports.
4.5 Entry Update Control Value
In response to the client's synchronization request, the server
returns one or more SearchResultEntry PDU that fits the client's
specification. Each SearchResultEntry PDU also contains an
entryUpdateControl that describes the LCUP state of the returned
Megginson, et. al. Proposed Standard - Expires: December 2002 5
entry. To represent a deleted entry, the server attaches an
entryUpdate control to the corresponding SearchResultEntry. The
SearchResultEntry corresponding to a deleted entry MUST contain a
valid DN and SHOULD contain a valid UUID but, to reduce the amount
of data sent to the client, it SHOULD not contain any other
attributes.
Furthermore, the server may elect to periodically return to the
client the cookie that represents the state of the client's data.
This information is useful in case the client crashes or gets
disconnected. The client MAY specify how often to receive the cookie
by the use of the sendCookieInterval in the clientUpdate control
value (see above). If the client does not specify a value, the
server will determine the interval.
The controlType field for the entryUpdate control is
EntryUpdateControlOID (to be assigned). The controlValue is an
OCTET STRING, whose contents are the bytes of the BER encoding of
the following:
EntryUpdateControlValue ::= SEQUENCE {
stateUpdate BOOLEAN,
entryDeleted BOOLEAN,
cookie LCUPCookie OPTIONAL
}
stateUpdate - if set to TRUE, indicates that the entry to which the
control is attached contains no changes and it is sent only to
communicate to the client the new cookie. In this case, the
entryDeleted field MUST be ignored and the cookie field MUST
contain the updated cookie. This feature allows updating the
client's cookie when there are no changes that effect the
client's data store. Note that the control MUST be attached to a
valid SearchResultEntry, which should contain a valid LDAPDN in
the objectName field, and MAY contain an entryUUID attribute, but
SHOULD NOT contain any other attributes. The server MAY send the
entry named by the baseObject from the client's search request.
entryDeleted - if set to TRUE, indicates that the entry to which
the control is attached was deleted. The server MAY also set
this to TRUE if the entry has left the client's search result
set. As far as the client is concerned, a deleted entry is no
different than an entry that has left the result set.
cookie - the LCUP cookie value that represents the current state of
the client's data.
4.6 Client Update Done Control Value
When the server has finished processing the client's request, it
attaches a clientUpdateDone control to the SearchResultDone message
and sends it to the client. However, if the SearchResultDone message
contains a resultCode that is not success or lcupClientDisconnect,
Megginson, et. al. Proposed Standard - Expires: December 2002 6
the clientUpdateDone control MAY be omitted. The controlType field
for the clientUpdateDone control is ClientUpdateDoneControlOID (to
be assigned). The controlValue is an OCTET STRING, whose contents
are the bytes of the BER encoding of the following:
ClientUpdateDoneControlValue ::= SEQUENCE {
cookie LCUPCookie OPTIONAL
}
cookie - the LCUP cookie value that represents the current state of
the client's data. Although this value is OPTIONAL, it MUST be set
in the ClientUpdateDoneControlValue if the SearchResultDone
resultCode is success or lcupClientDisconnect. This provides a
good "checksum" of what the server thinks the state of the client
is. 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 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. Unless the
client sets the updateType field to persistOnly, the server attaches
a clientUpdateDone control that contains the cookie that corresponds
to the current state of the client's data. A server set 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.
4.7 Client Initiated Termination
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 [CANCEL]. The server
responds immediately with a LDAP Cancel response [CANCEL]. The
server MAY send any pending SearchResultEntry 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
SearchResultEntry PDUs as possible. Finally, the server sends the
message SearchResultDone with the clientUpdateDone control attached.
If the client is not interested in the state information, it can
simply abandon the search operation or disconnect from the server.
4.8 Protocol Flow
The client server interaction can proceed in three different ways
depending on the client's requirements. Protocol flows beginning
with an asterisk (*) are optional or conditional.
If the client's intent is not to synchronize data but to trigger
actions in response to directory modifications, the protocol
proceeds as follows:
Megginson, et. al. Proposed Standard - Expires: December 2002 7
C->S Sends a search operation with a clientUpdate control attached.
The search specification determines the part of the DIT the
client wishes to synchronize with and the set of attributes it
is interested in. The updateType field of the control value
should be set to persistOnly.
*S->C If there is an error (invalid search scope, invalid cookie)
the server returns the appropriate error codes and terminates
the request (SearchResultDone message with optional
clientUpdateDone control)
S->C Sends change notification to the client for each change to the
data within the client's search specification. Each
SearchResultEntry may have an entryUpdate control attached.
*S->C If the server starts to run out of resources or the client is
suspected of malicious actions, the server SHOULD terminate
the search operation by sending to the client a
SearchResultDone message with optional clientUpdateDone
control attached. The resultCode in the SearchResultDone
mesasge SHOULD be set to lcupResourcesExhausted or
lcupSecurityViolation depending on the reason for termination.
*C->S If the client receives lcupResourcesExhausted error from the
server, it MUST wait for a while before attempting another
synchronization session with the server. It is RECOMMENDED
that clients use an exponential backoff strategy.
C->S The client terminates the search. The client can do this by
abandoning the search operation, disconnecting from the
server, or by sending an LDAP Cancel operation.
*S->C If the server receives the LDAP Cancel op, it will immediately
send back the LDAP Cancel response
*S->C If the server sent the LDAP Cancel response, the server MAY
send any pending SearchResultEntry PDUs in its outgoing queue
*S->C If the server sent the LDAP Cancel response, after the server
sends the response and any pending SearchResultEntry PDUs, the
server sends the SearchResultDone message with the
clientUpdateDone control attached. The resultCode in the
SearchResultDone message will be either lcupClientDisconnect
or some LDAP error code (not success).
S->C Stops sending changes to the client and closes the connection.
If the client's intent is to synchronize with the server and then
disconnect, the protocol proceeds as follows:
C->S Sends a search operation with the clientUpdate control
attached. The search specification determines the part of the
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. If this is the initial
synchronization session, the client either does not provide a
cookie or provides a cookie with no value; otherwise, the
cookie field of the control is set to the cookie received from
the server at the end of the last synchronization session. If
the scheme field of the cookie was provided, the server MUST
use that scheme throughout the duration of the LCUP session or
until an LCUP boundary is crossed, since the server will
usually require a different cookie in that case anyway. (Note
Megginson, et. al. Proposed Standard - Expires: December 2002 8
that the client can synchronize with different servers during
different synchronization sessions.) The updateType field of
the control value is set to synchronizeOnly.
*S->C If there is an error (invalid search scope, invalid cookie)
the server returns the appropriate error codes and terminates
the request (SearchResultDone message with optional
clientUpdateDone control)
*S->C If no cookie is specified in the clientUpdate control, or if
the value field of the cookie is empty, the server sends all
data that matches the client's search specification followed
by the SearchResultDone message with a clientUpdateDone
control attached. The control contains the cookie that
corresponds to the current state of the client's data. If
synchronization was successful, the resultCode in the
SearchResultDone message should be success.
*S->C If an invalid cookie is specified, the server sends the
SearchResultDone message with the resultCode set to
lcupInvalidCookie.
*S->C If a valid cookie is specified and the data that matches the
search specification has been reloaded or the server does not
contain enough state information to synchronize the client,
the server sends a SearchResultDone message with the
resultCode set to lcupReloadRequired.
*S->C If the cookie is valid and the client is up to date, the
server sends a success response to the client.
S->C If the cookie is valid and there is data to be sent, the
server sends the modified entries to the client. Each
SearchResultEntry contains the attributes requested by the
client in the search specification regardless of whether they
were modified. An entryUpdate control with the entryDeleted
field set to TRUE MUST be attached to every deleted entry. The
server may also periodically attach an entryUpdate control to
the entries sent to the client to indicate the current state
of the client's data. In that case, the cookie field of the
control represents the state of the client's data including
the entry to which the control is attached. Once all the
changes are sent successfully, the server sends a
SearchResultDone with the clientUpdateDone control attached.
The control contains the cookie that represents the current
state of the client's data. The resultCode in the
SearchResultDone message is set to success. If the resultCode
is not success, the server may OPTIONALLY attach the
clientUpdateDone control to the SearchResultDone message.
The client stores the cookie received from the server until
the next synchronization session.
*C->S If the resultCode in the SearchResultDone message is set
lcupReloadRequired, the client clears its data store and
repeats the synchronization process by sending the search
operation with clientUpdate control that contains no cookie,
or that contains a cookie with no value field.
If the client's intent is to be synchronized with the server and
stay notified about data modifications, the protocol proceeds as
follows:
Megginson, et. al. Proposed Standard - Expires: December 2002 9
C->S The client behaves exactly as in the previous case except it
sets the updateType field in the control value to
synchronizeAndPersist.
S->C The server behaves exactly as in the previous case except the
connection is kept open after the initial set of changes is
sent to the client. A SearchResultDone message is not sent to
the client; instead, the server keeps sending changes to the
client.
*S->C If the server starts to run out of resources or the client is
suspected of malicious actions, the server SHOULD terminate
the search operation by sending to the client a
SearchResultDone message with the resultCode set to
lcupResourcesExhausted or lcupSecurityViolation depending on
the reason for termination.
*C->S If the client receives lcupResourcesExhausted error from the
server, it MUST wait for a while before attempting another
synchronization session with the server. We recommend
exponential backoff strategy.
C->S Sends an LDAP Cancel operation to the server to terminate the
synchronization session.
S->C Responds with an LDAP Cancel response, followed optionally by
SearchResultEntry PDUs, followed by a SearchResultDone with
the clientUpdateDone control optionally attached. If the
control is present, it contains the cookie that represents the
current state of the client's data. The value of the
resultCode in the SearchResultDone message will be either
lcupClientDisconnect or some other LDAPResult resultCode (not
success). The control may not be present if some error
occurred.
4.9 Size and Time Limits
The search request size or the time limits can only be imposed for
non-persistent operations, those that set the updateType field of
the ClientUpdateControlValue to synchronizeOnly (for the entire
operation) or synchronizeAndPersist (for the initial synchronization
phase only). All other operations MUST set both limits to 0. The
server SHOULD ignore the limits set for persistent operations.
4.10 Changes vs. Operations
A server that supports UUIDs SHOULD communicate a modifyDN
operation by sending the client the current form of the entry (with
its new DN) along with an entryUUID attribute. A server that does
not support UUIDs SHOULD communicate a modifyDN operation by sending
the client a deletion for the previous DN followed by an entry for
the new DN. Note that for servers that do not support UUIDs, no
guarantees are made about the correctness of the client state in the
presence of modifyDN operations.
Communicating modifyDN operations by sending a delete of the old DN
followed by an entry with the new DN makes it impossible for an LCUP
client to distinguish between a modifyDN operation, which is one
Megginson, et. al. Proposed Standard - Expires: December 2002 10
atomic operation, and an delete operation followed by an add of a
new entry. The loss of information about atomicity may cause
problems for some LCUP clients. For example, when an entry is
renamed, a client that manages resources such as a person's mailbox
might delete the mailbox and everything in it instead of merely
changing the name associated with the mailbox.
Also note that regardless of how a modifyDN operation is
communicated to the client, if the client state shows that the
object that underwent the modifyDN operation was the root of a
subtree, the client MUST infer that the DNs of all objects in the
subtree have changed such that they reflect the new DN of the
subtree root.
4.11 Operations on the Same Connection
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.
4.12 Interactions with Other LDAP Search and Response Controls
LCUP defines neither restrictions nor guarantees about the ability
to use the LDAP client update control 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 ignore the LCUP 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.
5. Additional Features
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. This section
is intended to open a discussion on the merits of including and
approaches to implementing these features.
5.1 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
draft 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,
Megginson, et. al. Proposed Standard - Expires: December 2002 11
- 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.
5.2 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
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 entryDeleted value of the
entryUpdateControl will be TRUE.
5.3 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.
Megginson, et. al. Proposed Standard - Expires: December 2002 12
5.4 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.
5.5 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.
6. Client Side Considerations
Clients SHOULD always specify entryUUID in the SearchRequest
attribute list.
The cookie received from the server after a synchronization session
can only be used with the same or more restrictive search
specification than the search that generated the cookie. The server
will reject the search operation with a cookie that does not satisfy
this condition. This is because the client can end up with an
incomplete data store otherwise. A more restrictive search
specification is the one that generates 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 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
Megginson, et. al. Proposed Standard - Expires: December 2002 13
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.
For alias dereferencing, the server will behave as if the client had
requested neverDerefAliases or derefFindingBaseObj as the
derefAliases field in the search request [RFC2251, Section 4.5.1].
If the client specifies a value other than neverDerefAliases or
derefFindingBaseObj, the server will return protocolError to the
client.
Changes to data (e.g., that might affect the LCUP client's filter or
scope) or meta-data (e.g., that might affect the client's read
access) may affect the presence of entries in the search set.
Servers MAY notify LCUP clients of changes to the search set that
result from such changes, but an LCUP client MUST NOT assume that
such notification will occur. Therefore, in the case where a client
is maintaining a cache of entries using LCUP, the data held by the
client may be a superset or a subset of the entries that would be
returned by a new search request. For example, if access control
meta information is changed to deny access to particular entries in
the search result set, and the access control information is outside
of the search scope (e.g., in a parent entry), the client may have
entries stored locally which are no longer part of its desired
search set. Similarly, if entries are added to the search result
set due to changes in meta-data, the client's cache of entries may
not include these entries.
Some clients may wish to perform an initial synchronization in order
to prime a cache or establish a baseline set of entries, then look
for changes after that. The recommended way to do this is to first
issue an LCUP search with the updateType field of the clientUpdate
control value set to synchronizeOnly, then after that search
successfully completes, immediately issue an LCUP search with the
updateType field of the clientUpdate control value set to
synchronizeAndPersist.
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 entryUpdate control
value, so that if they have to reconnect, they do not have to
Megginson, et. al. Proposed Standard - Expires: December 2002 14
process many redundant entries. These clients should set the
sendCookieInterval in the clientUpdate control value to a low
number, perhaps even 1. Also, 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 clientUpdateDone control value upon successful completion).
These clients should set the sendCookieInterval in the clientUpdate
control value to a high number.
7. Server Implementation Considerations
Servers SHOULD support UUIDs. Otherwise, it will be very difficult
to support modifyDN operations. Adding support for UUIDs should be
seen as a necessary component of LCUP.
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, the 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.
A server may support one or more LCUP cookie schemes. It is
expected that schemes will be published along with their OIDs as
RFCs. If a client initiates an LCUP session with a particular
scheme, the server MUST use that same scheme throughout the LCUP
session, or until an LCUP context boundary is crossed, in which case
the server will usually require a different cookie anyway.
In addition, 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 can only be used with the search specification
that is equally or more restrictive than the one for which the
cookie was generated.
An implementation must make sure that it can correctly update the
client's cookie when there is a size limit imposed on the search
results by either the client's request or by the server's
configuration. 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.
Megginson, et. al. Proposed Standard - Expires: December 2002 15
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 be managed
by LCUP and some may not. A part of the DIT which is enabled for
LCUP is referred to as an LCUP Context. The server SHOULD send a
SearchResultReference [RFC2251, SECTION 4.5.3] when the LCUP Context
for a returned entry changes. The server SHOULD return all entries
for a particular LCUP Context before returning a reference to other
LCUP Contexts or non-LCUP enabled parts of the DIT, in order to
minimize the processing burden on the clients. 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.
An implementation SHOULD notify the client about all entries deleted
from the search set since the client's last session, but an LCUP
client MUST NOT assume that such notification will occur. For
example, the server might not notify the client of the deletion of
an object if the object left the search set following the client's
last synchronization and prior to the object's deletion. An LDUP
compliant implementation can achieve this through the use of entry
tombstones. The implementation should avoid aggressive tombstone
purging since lack of tombstones would cause client's data to be
reloaded. We suggest that only the tombstone content be removed
during the regular trimming cycle while tombstones themselves are
discarded much less frequently.
The specification makes no guarantees about how soon a server should
send notification of a changed entry to the client when the
connection between the client and the server is kept open. This is
intentional as any specific maximum delay would be impossible to
meet in a distributed directory service implementation. Server
implementors are encouraged to minimize the delay before sending
notifications to ensure that clients' needs for timeliness of change
notification are met.
Implementors 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 implementors are also
encouraged to support a large number of client connections if they
need to support large numbers of persistent operations.
8. Synchronizing Heterogeneous Data Stores
Megginson, et. al. Proposed Standard - Expires: December 2002 16
Clients, like a meta directory join engine, synchronizing multiple
writable data stores will only work correctly if each piece of
information is single mastered (for instance, only by an LDUP
compliant directory). 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.
9. Security Considerations
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 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.
Access control on the data can be modified in such a way that the
data is no longer visible to the client. The specification does not
specify how the server should handle this condition. Moreover, data
consistency is not guaranteed if access control is changed from a
more restrictive to a less restrictive one. This is because access
control can be considered as an additional filter on the search
specification and the protocol does not support going from a more to
a less restrictive search specification. See Client Side
Considerations Section for more detailed explanation of the problem.
10. Normative References
[KEYWORDS] S. Bradner, "Keywords for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2251] M. Wahl, T. Howes, S. Kille "Lightweight Directory
Access Protocol", 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.
[CANCEL] K. Zeilenga, "LDAP Cancel Extended Operation",
draft-zeilenga-ldap-cancel-xx.txt, a work in progress.
Megginson, et. al. Proposed Standard - Expires: December 2002 17
11. Acknowledgements
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.
12. Author's Addresses
Rich Megginson
Netscape Communications Corp.
901 San Antonio Rd.
Palo Alto, CA 94303-4900
Mail Stop SCA17 - 201
Phone: +1 505 797-7762
Email: richm@netscape.com
Olga Natkovich
Yahoo, Inc.
701 First Ave.
Sunnyvale, CA 94089
Phone: +1 408 349-6153
Email: olgan@yahoo-inc.com
Mark Smith
Netscape Communications Corp.
901 San Antonio Rd.
Palo Alto, CA 94303-4900
Mail Stop SCA17 - 201
Phone: +1 650 937-3477
Email: mcs@netscape.com
Jeff Parham
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Phone: +1 425 882-8080
Email: jeffparh@microsoft.com
13. Full Copyright Statement
"Copyright (C) The Internet Society (date). 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
Megginson, et. al. Proposed Standard - Expires: December 2002 18
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
14. Appendix A - Summary of Changes
Changes new to version 03:
Emphasized the use of UUIDs throughout the document. Implementers
are strongly encouraged to use UUIDs as a necessary component of
the protocol.
Removed the LCUP Cancel extended operation in favor of the new
LDAP Cancel operation [CANCEL].
Got rid of the lcupSuccess result code. All result codes will be
added to the IANA LDAP result code registry as part of the LDAP
standard. Also removed the result code and text from the client
update done control value.
Changed any and all wording suggesting an LCUP Context is related
to a naming context. New text says an LCUP Context is a part of
the DIT that supports LCUP, and that a server may have one or more
LCUP Contexts.
Removed Old Section 4.2: lcupCookieScheme
We decided that LCUP did not need a discovery mechanism. The
controls and extended operations will be published in the root DSE
as per the LDAP standards.
Changed references to "Unique Identifier" to either "Universally
Unique Identifier" or "UUID".
Added this text to section 6 "Client Side Considerations":
"- 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."
Added a note to section 6 "Client Side Considerations" about how
to establish a baseline set of entries or entry cache.
Added the field sendCookieInterval to the clientUpdate control
value.
Added a note to section 6 "Client Side Considerations" explaining
possible uses of the sendCookieInterval.
Megginson, et. al. Proposed Standard - Expires: December 2002 19
Changes new to version 02:
Section 4.2: The lcupCookieScheme operational attribute MUST be
present in the root DSE, and MAY be present in entries. Each
value of the attribute in the root DSE will be a list of OIDs of
cookie schemes followed by the DN of the LCUP context which
supports the schemes. The attribute value in the DIT entries will
be the list of OIDs followed by the DN of the LCUP context.
section 4.5 - the entry uuid is now MAY instead of MUST - if
implementers do not wish to identify entries by a unique ID other
than DN (which may not be unique), then so be it. For returned
SearchResultEntry PDUs other than deleted entries, the client MAY
request that the Unique Identifier attribute be returned by
specifying it in the attribute list to be returned by the search
request.
section 4.5 - added "or the base DN of the client's search
request." to the phrase. "The server MAY send the entry at the
root of the client's tree, or the base DN of the client's search
request." I think this clarifies which entry the client may
search for.
section 4.6 - the clientUpdateDone control is now optional for
error conditions. Also, the cookie value of the control is now
optional for lcup error conditions (e.g. not lcupSuccess or
lcupClientDisconnect).
Added section 4.12 - Interactions with Other LDAP Search and
Response Controls
Added blurb about alias dereferencing back to section 6:
"For alias dereferencing, the server will behave as if the client
had requested neverDerefAliases or derefFindingBaseObj as the
derefAliases field in the search request [RFC2251, Section 4.5.1].
If the client specifies a value other than neverDerefAliases or
derefFindingBaseObj, the server will return protocolError to the
client."
Changed this in section 6:
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"
Changes new to version 01:
The opaque cookie has been split into two parts - a scheme which
is an OID, and a value. The value may or may not have a format
known to the client, depending on the specified scheme. Section
4.2 describes the new cookie format and defines the LCUP Cookie
Value.
Megginson, et. al. Proposed Standard - Expires: December 2002 20
Added new section 4.3 - the lcupCookieScheme operational
attribute.
Changes new to version 00:
Added the definition for Unique Identifier (basically copied from
the LDUP model doc http://search.ietf.org/internet-drafts/draft-
ietf-ldup-model-06.txt. I needed to add the definition here
because LCUP needs a Unique Identifier but should not be dependent
on LDUP.
Removed all normative references to LDUP. I've left the
implementation suggestions that refer to LDUP, but LCUP should not
be dependent on LDUP.
Cleaned up the protocol flows.
Removed this text from section 4.8: "Clients MUST NOT issue
multiple synchronization requests on the same connection. This is
because the protocol includes an extended operation and it would
be impossible to decide which synchronization session it belongs
to." - This is no longer true, since the extended operation now
includes the message ID of the search request.
"Client Side Consideration" section - the client will never
receive a referral or continuation reference
Added section 12. Acknowledgements
Removed normative references to documents not depended on.
Removed explicit references to software vendors.
Section 4.1 - Changed ClientUpdateControlValue to remove the
keepConnection and changesOnly fields and replace them with
updateType which is an ENUMERATED with three values:
synchronizeOnly, synchronizeAndPersist, and persistOnly.
Section 4.2 - The EntryUpdateControlValue fields stateUpdate and
entryDeleted no longer have DEFAULT values, they must be specified
- this eliminates any potential ambiguity.
Added this text to the description of the entryDeleted field
(section 4.2): "The server SHOULD also set this to TRUE if the
entry has left the clients search result set. As far as the client
is concerned, a deleted entry is no different than an entry which
has left the result set."
Section 4.2 - Added an explanation of the concept and requirement
for the Unique Identifier.
Section 4.4 - Added to the extended operation a request value
containing the message id of the operation to stop.
Megginson, et. al. Proposed Standard - Expires: December 2002 21
Updated contact information for Olga.
Removed Michael Armijo and added Jeff Parham as an author.
Changes new to previous version:
"Authors" section - added Rich Megginson as the new editor.
"Client Side Consideration" section - added a note and a question
concerning referral and continuation reference handling.
"Client Update Control Value" section (4.1) - clarified the meaning
of keepConnection and added a table summarizing the effects of
different values of keepConnection and changesOnly.
"Stop Client Update Request and Response" - added section 4.4
describing this extended operation.
Megginson, et. al. Proposed Standard - Expires: December 2002 22
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