Network Working Group                                   M. Smith, Editor
INTERNET-DRAFT                             Netscape Communications Corp.
Intended Category: Informational                                 G. Good
Expires: May 2001                                               T. Howes
                                                         Loudcloud, Inc.
                                                              R. Weltman

                                                        15 November 2000

     Persistent Search: A Simple LDAP Change Notification Mechanism

1.  Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.  Internet-Drafts are working docu-
ments 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

The list of Internet-Draft Shadow Directories can be accessed at

This draft document will be submitted to the RFC Editor as an Informa-
tional document. Distribution of this memo is unlimited.  Technical dis-
cussion of this document will take place on the IETF LDAP Extension
Working Group mailing list <>.  Please send
editorial comments directly to the editor <>.

Copyright (C) The Internet Society (1997-2000). All Rights Reserved.

Please see the Copyright section near the end of this document for more

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2.  Abstract

This document defines two controls that extend the LDAPv3 [LDAP] search
operation to provide a simple mechanism by which an LDAP client can
receive notification of changes that occur in an LDAP server.  The
mechanism is designed to be very flexible yet easy for clients and
servers to implement.  Since the IETF is likely to pursue a different,
more comprehensive solution in this area, this document will eventually
be published with Informational status in order to document an existing

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",  and "MAY" in this document are
to be interpreted as described in RFC 2119 [KEYWORDS].

3.  General Approach

The approach taken by the Persistent Search mechanism described in this
document is to alter the standard LDAP search operation so that it does
not end after the initial set of entries matching the search criteria
are returned.  Instead, LDAP servers keep the search operation going.
This provides clients and servers participating in Persistent Search
with an active channel through which entries that change (and additional
information about the changes that occur) can be communicated.

4.  Persistent Search Control

This control may be included in the Controls portion of an LDAPv3 Sear-
chRequest message.  The controlType is "2.16.840.1.113730.3.4.3".

           PersistentSearch ::= SEQUENCE {
                   changeTypes INTEGER,
                   changesOnly BOOLEAN,
                   returnECs BOOLEAN

Upon receiving this control, a server that supports it MUST process this
as a standard LDAPv3 search with the following exceptions:

   a)   If changesOnly is TRUE, the server MUST NOT return any existing
        entries that match the search criteria.  Entries are only
        returned when they are changed (added, modified, deleted, or
        subject to a modifyDN operation).

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   b)   The server MUST NOT return a SearchResultDone message.  Instead,
        the search operation MUST be kept active until it is abandoned
        by the client or until the client unbinds.

   c)   As changes are made to the server, the effected entries MUST be
        returned to the client if they match the standard search cri-
        teria and if the operation that caused the change is included in
        the changeTypes field.  The changeTypes field is the logical OR
        of one or more of these values: add (1), delete (2), modify (4),
        modDN (8).

   d)   If returnECs is TRUE, the server MUST return an Entry Change
        Notification control with each entry returned as the result of
        changes.  This control is described in the next section.

5.  Entry Change Notification Control

This control provides additional information about the change the caused
a particular entry to be returned as the result of a persistent search.
The controlType is "2.16.840.1.113730.3.4.7".  If the client set the
returnECs boolean to TRUE in the PersistentSearch control, servers MUST
include an EntryChangeNotification control in the Controls portion of
each SearchResultEntry that is returned due to an entry being added,
deleted, or modified.

           EntryChangeNotification ::= SEQUENCE {
                     changeType ENUMERATED {
                             add             (1),
                             delete          (2),
                             modify          (4),
                             modDN           (8)
                     previousDN   LDAPDN OPTIONAL,     -- modifyDN ops. only
                     changeNumber INTEGER OPTIONAL     -- if supported

changeType indicates what LDAP operation caused the entry to be

previousDN is present only for modifyDN operations and gives the DN of
the entry before it was renamed and/or moved.  Servers MUST include this
optional field only when returning change notifications as a result of
modifyDN operations.

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changeNumber is the change number [CHANGELOG] assigned by a server for
the change.  If a server supports an LDAP Change Log it SHOULD include
this field.

6.  Intended Use

Some of the scenarios that the Persistent Search mechanism described in
this document is designed to support are described in this section.
Other uses of the mechanism are possible as well, but please refer to
the "Implementation Considerations" section for some issues to consider.

6.1.  Cache Consistency

An LDAP client application with high performance needs may want to main-
tain a temporary, local cache of information obtained through LDAP
search, compare, or bind operations.  To improve performance, the local
cache is always consulted before sending a request to an LDAP server.
The client application can use Persistent Search(es) against the change-
log [CHANGELOG] (if one is available) or against one or more subtrees
within the LDAP server to enable it to maintain consistency between the
data in its local cache and the data stored in the LDAP server.  A Per-
sistent Search request where the changesOnly flag is FALSE can be used
if it is desirable to prime the cache; otherwise changesOnly would typi-
cally be set to TRUE in the request.

Caches are used for reasons other than performance improvement as well.
In some cases, they arise naturally out of a particular application's
design.  For example, an LDAP client designed for administration of
information held in LDAP servers will undoubtedly generate screen
displays that show information gleaned from an LDAP server.  The screen
display is a cache that is active and visible until the user of the
application takes some action that causes different information to be
displayed.  A refresh button or similar control may be provided to the
user to allow them to update the cached display.  A Persistent Search
request can be used instead by the administrative application to
automatically refresh the screen display as soon as the underlying LDAP
information changes.

6.2.  Synchronization

Some LDAP clients such as those that execute on a portable computer may
maintain a partial or complete offline copy of the entries stored in an
LDAP server.  While connected to the network, such a client can direct
all queries to the copy of data it holds and use a Persistent Search to

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actively maintain the contents of the offline copy (alternatively, the
client could direct requests to the LDAP server that is the source of
the data).  While disconnected from the network, the client must satisfy
all queries using its offline copy of the data.  When the client recon-
nects to the network, it can synchronize its own copy of the data with
the one stored on the LDAP server and proceed to actively maintain its
offline copy by issuing a Persistent Search with the changesOnly flag
set to FALSE against the server's changelog [CHANGELOG].  A search
filter like "(changeNumber>=NUM)" where NUM is an integer one greater
than the last change the client processed would be used to limit the
entries returned to the set of changes the client has not yet seen.

6.3.  Triggered Actions

An LDAP client application may want to take some action when an entry in
the directory is changed.  A Persistent Search request can be used to
proactively monitor one or more LDAP servers for interesting changes
that in turn cause specific actions to be taken by an application.  For
example, 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 deleted from an LDAP

7.  Implementation Considerations

Implementors of servers that support the mechanism described in this
document should ensure that their implementation scales well as the
number of active Persistent Search requests increases and as the number
of changes made in the directory increases.

Each active Persistent Search request requires that an open TCP connec-
tion 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 Search for non-essential tasks and
to close idle LDAP connections as soon as practical.  Server implemen-
tors are encouraged to support a large number of client connections if
they need to support large numbers of Persistent Search clients.

This specification makes no guarantees about how soon a server should
send notification of a changed entry to a Persistent Search client.
This is intentional as any specific maximum delay would be impossible to
meet in a distributed directory service implementation.  Server imple-
mentors are encouraged to minimize the delay before sending notifica-
tions to ensure that clients' needs for timeliness of change

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notification are met.

8.  Limitations of the Persistent Search Mechanism

The mechanism described in the document has some limitations that make
it unsuitable as a comprehensive LDAP change notification solution.

8.1.  No Notification When Disconnected

Changes that occur in the LDAP data store while a client is not con-
nected cannot be detected using Persistent Search.  Therefore, discon-
nected clients that also require accurate synchronization must use
another mechanism such as [CHANGELOG] in conjunction with Persistent
Search.  This is inconvenient and it introduces an undesireable depen-
dency on another mechanism.

8.2.  No "Enter Set" or "Leave Set" Notifications

No notification is provided when an entry enters or leaves a result set
(the set of entries specified by the client using the search base,
scope, and filter).  This kind of indication would be useful, for exam-
ple, when an entry enters or leaves the scope of the result set due to a
ModifyDN operation or when a Modify operation acts on a attribute value
that is used in the search filter.  Without this kind of notification,
it is difficult for clients to maintain an accurate cache of the entries
they wish to monitor.

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 PersistentSearch and EntryChangeNotification controls.

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.  For
this reason, servers that implement the mechanism described in the docu-
ment SHOULD provide a means to limit the number of resources that can be
consumed by a single client.

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10.  Copyright

Copyright (C) The Internet Society (1997-2000). All Rights Reserved.

This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and dis-
tributed, 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 Stan-
dards process must be followed, or as required to translate it into
languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS

11.  Bibliography

[KEYWORDS]   S. Bradner, "Key words for use in RFCs to Indicate Require-
             ment Levels", RFC 2119, March 1997.

[LDAP]       M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
             Protocol (v3)", RFC 2251, December 1997.

[CHANGELOG]  G. Good, "Definition of an Object Class to Hold LDAP Change
             Record", INTERNET-DRAFT <draft-ietf-asid-changelog-01.txt>,
             July 1997.

[PSEARCHAPI] M. Smith, "LDAP C API Extensions for Persistent Search",
             INTERNET-DRAFT <draft-ietf-ldapext-c-api-psearch-00.txt>,
             March 1998.

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12.  Authors' Addresses

   Mark Smith
   Netscape Communications Corp.
   901 San Antonio Rd.
   Palo Alto, CA  94303-4900
   Mail Stop SCA17 - 201
   +1 650 937-3477

   Gordon Good
   Loudcloud, Inc.
   599 N. Mathilda Avenue
   Sunnyvale, CA 94085
   +1 408 744-7300

   Rob Weltman
   +1 650 949-5279

   Tim Howes
   Loudcloud, Inc.
   599 N. Mathilda Avenue
   Sunnyvale, CA 94085
   +1 408 744-7300

13.  Appendix A:  Changes since draft-ietf-ldapext-psearch-02.txt

   "Persistent Search Control" section: replaced 'SearchResult message'
   with 'SearchResultDone message' to match RFC 2251 terminology.

   Added new section "Limitations of the Persistent Search Mechanism."

   Updated Author's Addresses.

14.  Appendix B:  Changes since draft-ietf-ldapext-psearch-01.txt

   "Status of this Memo" section: changed "Intended Category" to Infor-
   mational.  Also updated boilerplate text to reflect current I-D
   guidelines and updated copyright to include the year "2000."

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   "Abstract" section: added sentence that says why this will be pub-
   lished as Informational.

   "Entry Change Notification Control" section: added the word "only" to
   clarify that the previousDN field is only returned for modifyDN

   "Authors' Addresses" section: updated Tim Howes' information.

Smith, et. al.      Intended Category: Informational            [Page 9]

1.     Status of this Memo............................................1
2.     Abstract.......................................................2
3.     General Approach...............................................2
4.     Persistent Search Control......................................2
5.     Entry Change Notification Control..............................3
6.     Intended Use...................................................4
6.1.      Cache Consistency...........................................4
6.2.      Synchronization.............................................4
6.3.      Triggered Actions...........................................5
7.     Implementation Considerations..................................5
8.     Limitations of the Persistent Search Mechanism.................6
8.1.      No Notification When Disconnected...........................6
8.2.      No "Enter Set" or "Leave Set" Notifications.................6
9.     Security Considerations........................................6
10.    Copyright......................................................7
11.    Bibliography...................................................7
12.    Authors' Addresses.............................................8
13.    Appendix A:  Changes since draft-ietf-ldapext-psearch-02.txt...8
14.    Appendix B:  Changes since draft-ietf-ldapext-psearch-01.txt...8