Internet Draft R. Megginson, Editor Document: 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 û (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