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Versions: 00 01 02 03 04 05 06 07 08 09                  Standards Track
Internet-Draft                                 D. Boreham, Bozeman Pass
LDAPext Working Group                            J. Sermersheim, Novell
Intended Category: Standards Track                  A. Kashi, Microsoft
Expires: Jun 2003                                              Nov 2002

       LDAP Extensions for Scrolling View Browsing of Search Results

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 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

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

   This document is intended to be submitted, after review and revision,
   as a Standards Track document. Distribution of this memo is
   Please send comments to the authors.

2. Abstract

   This document describes a Virtual List View extension for the
   Lightweight Directory Access Protocol (LDAP) Search operation. This
   extension is designed to allow the "virtual list box" feature, common
   in existing commercial e-mail address book applications, to be
   supported efficiently by LDAP servers. LDAP servers' inability to
   support this client feature is a significant impediment to LDAP
   replacing proprietary protocols in commercial e-mail systems.

   The extension allows a client to specify that the server return, for
   a given LDAP search with associated sort keys, a contiguous subset of
   the search result set. This subset is specified in terms of offsets
   into the ordered list, or in terms of a greater than or equal
   comparison value.

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3. Conventions used in this document
   The key words "MUST", "SHALL", "SHOULD", "SHOULD NOT", and "MAY" in
   this document are to be interpreted as described in RFC 2119

   Protocol elements are described using ASN.1 [X.680].  The term "BER-
   encoded" means the element is to be encoded using the Basic Encoding
   Rules [X.690] under the restrictions detailed in Section 5.1 of

   The phrase "subsequent virtual list request" is used in this document
   to describe a search request accompanied by a VirtualListViewRequest
   control, where the search base, scope, and filter are the same as a
   previous search request also accompanied by a VirtualListViewRequest
   control, and where the contextID of the subsequent
   VirtualListViewRequest control, is set to that of the contextID in
   the VirtualListViewResponse control that accompanied the previous
   search response.

   The phrase "contiguous virtual list request" is used to describe a
   subsequent virtual list request which is requesting search results
   adjoining or overlapping the result returned from the prior virtual
   list request.

4. Background

   A Virtual List is a graphical user interface technique employed where
   ordered lists containing a large number of entries need to be
   displayed. A window containing a small number of visible list entries
   is drawn. The visible portion of the list may be relocated to
   different points within the list by means of user input. This input
   can be to a scroll bar slider; from cursor keys; from page up/down
   keys; from alphanumeric keys for "typedown". The user is given the
   impression that they may browse the complete list at will, even
   though it may contain millions of entries. It is the fact that the
   complete list contents are never required at any one time that
   characterizes Virtual List View. Rather than fetch the complete list
   from wherever it is stored (typically from disk or a remote server),
   only that information which is required to display the part of the
   list currently in view is fetched. The subject of this document is
   the interaction between client and server required to implement this
   functionality in the context of the results from an ordered [SSS]
   Lightweight Directory Access Protocol (LDAP) search operation

   For example, suppose an e-mail address book application displays a
   list view onto the list containing the names of all the holders of e-
   mail accounts at a large university. The list is ordered
   alphabetically. While there may be tens of thousands of entries in
   this list, the address book list view displays only 20 such accounts

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   at any one time. The list has an accompanying scroll bar and text
   input window for type-down. When first displayed, the list view shows
   the first 20 entries in the list, and the scroll bar slider is
   positioned at the top of its range. Should the user drag the slider
   to the bottom of its range, the displayed contents of the list view
   should be updated to show the last 20 entries in the list. Similarly,
   if the slider is positioned somewhere in the middle of its travel,
   the displayed contents of the list view should be updated to contain
   the 20 entries located at that relative position within the complete
   list. Starting from any display point, if the user uses the cursor
   keys or clicks on the scroll bar to request that the list be scrolled
   up or down by one entry, the displayed contents should be updated to
   reflect this. Similarly the list should be displayed correctly when
   the user requests a page scroll up or down. Finally, when the user
   types characters in the type-down window, the displayed contents of
   the list should "jump" or "seek" to the appropriate point within the
   list. For example, if the user types "B", the displayed list could
   center around the first user with a name beginning with the letter
   "B". When this happens, the scroll bar slider should also be updated
   to reflect the new relative location within the list.

   This document defines a request control which extends the LDAP search
   operation. Always used in conjunction with the server side sorting
   control [SSS], this allows a client to retrieve selected portions of
   large search result set in a fashion suitable for the implementation
   of a virtual list view.

5. Client-Server Interaction

   The Virtual List View control extends a regular LDAP Search operation
   which MUST also include a server-side sorting control [SSS]. Rather
   than returning the complete set of appropriate SearchResultEntry
   messages, the server is instructed to return a contiguous subset of
   those entries, taken from the ordered result set, centered around a
   particular target entry. Henceforth, in the interests of brevity, the
   ordered search result set will be referred to as "the list".

   The sort control may contain any sort specification valid for the
   server. The attributeType field in the first SortKeyList sequence
   element has special significance for "typedown". The Virtual List
   View control acts upon a set of ordered entries and this order must
   be repeatable for all subsequent virtual list requests. The server-
   side sorting control is intended to aid in this ordering, but other
   mechanisms may need to be employed to produce a repeatable order--
   especially for entries that don't have a value of the sort key.

   The desired target entry and the number of entries to be returned,
   both before and after that target entry in the list, are determined
   by the client's VirtualListViewRequest control.

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   When the server returns the set of entries to the client, it attaches
   a VirtualListViewResponse control to the SearchResultDone message.
   The server returns in this control: its current estimate for the list
   content count, the location within the list corresponding to the
   target entry, any error codes, and optionally a context identifier.

   The target entry is specified in the VirtualListViewRequest control
   by one of two methods. The first method is for the client to indicate
   the target entry's offset within the list. The second way is for the
   client to supply an attribute assertion value. The value is compared
   against the values of the attribute specified as the primary sort key
   in the sort control attached to the search operation. The first sort
   key in the SortKeyList is the primary sort key. The target entry is
   the first entry in the list with value greater than or equal to (in
   the primary sort order), the presented value. The order is determined
   by rules defined in [SSS]. Selection of the target entry by this
   means is designed to implement "typedown". Note that it is possible
   that no entry satisfies these conditions, in which case there is no
   target entry. This condition is indicated by the server returning the
   special value contentCount + 1 in the target position field.

   Because the server may not have an accurate estimate of the number of
   entries in the list, and to take account of cases where the list size
   is changing during the time the user browses the list, and because
   the client needs a way to indicate specific list targets "beginning"
   and "end", offsets within the list are transmitted between client and
   server as ratios---offset to content count. The server sends its
   latest estimate as to the number of entries in the list (content
   count) to the client in every response control. The client sends its
   assumed value for the content count in every request control. The
   server examines the content count and offsets presented by the client
   and computes the corresponding offsets within the list, based on its
   own idea of the content count.

        Si = Sc * (Ci / Cc)

        Si is the actual list offset used by the server
        Sc is the server's estimate for content count
        Ci is the client's submitted offset
        Cc is the client's submitted content count
        The result is rounded to the nearest integer.

   If the content count is stable, and the client returns to the server
   the content count most recently received, Cc = Sc and the offsets
   transmitted become the actual server list offsets.

   The following special cases exist when the client is specifying the
   offset and content count:
   - an offset of one and a content count of non-one (Ci = 1, Cc != 1)
     indicates that the target is the first entry in the list.

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   - equivalent values (Ci = Cc) indicate that the target is the last
     entry in the list.
   - a content count of zero (Cc = 0, Ci != 0) means the client has no
     idea what the content count is, the server MUST use its own
     content count estimate in place of the client's.

   Because the server always returns contentCount and targetPosition,
   the client can always determine which of the returned entries is the
   target entry. Where the number of entries returned is the same as the
   number requested, the client is able to identify the target by simple
   arithmetic. Where the number of entries returned is not the same as
   the number requested (because the requested range crosses the
   beginning or end of the list, or both), the client MUST use the
   target position and content count values returned by the server to
   identify the target entry. For example, suppose that 10 entries
   before and 10 after the target were requested, but the server returns
   13 entries, a content count of 100 and a target position of 3. The
   client can determine that the first entry must be entry number 1 in
   the list, therefore the 13 entries returned are the first 13 entries
   in the list, and the target is the third one.

   A server-generated contextID MAY be returned to clients. A client
   receiving a contextID MUST return it unchanged or not return it at
   all, in a subsequent request which relates to the same list. The
   purpose of this interaction is to maintain state information between
   the client and server.

6. The Controls

   Support for the virtual list view control extension is indicated by
   the presence of the OID "2.16.840.1.113730.3.4.9" in the
   supportedControl attribute of a server's root DSE.

6.1. Request Control

   This control is included in the SearchRequest message as part of the
   controls field of the LDAPMessage, as defined in Section 4.1.12 of
   [LDAPPROT]. The controlType is set to "2.16.840.1.113730.3.4.9". If
   this control is included in a SearchRequest message, a Server Side
   Sorting request control [SSS] MUST also be present in the message.
   The controlValue, an OCTET STRING, is the BER-encoding of the
   following SEQUENCE:

   VirtualListViewRequest ::= SEQUENCE {
          beforeCount    INTEGER (0..maxInt),
          afterCount     INTEGER (0..maxInt),
          target       CHOICE {
                         byOffset        [0] SEQUENCE {
                              offset          INTEGER (1 .. maxInt),
                              contentCount    INTEGER (0 .. maxInt) },

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                         greaterThanOrEqual [1] AssertionValue },
          contextID     OCTET STRING OPTIONAL }

   beforeCount indicates how many entries before the target entry the
   client wants the server to send.

   afterCount indicates the number of entries after the target entry the
   client wants the server to send.

   offset and contentCount identify the target entry as detailed in
   section 5.

   greaterThanOrEqual is a matching rule assertion value defined in
   [LDAPPROT]. The assertion value is encoded according to the ORDERING
   matching rule for the attributeDescription in the sort control [SSS].
   If present, the value supplied in greaterThanOrEqual is used to
   determine the target entry by comparison with the values of the
   attribute specified as the primary sort key. The first list entry
   who's value is no less than (less than or equal to when the sort
   order is reversed) the supplied value is the target entry.

   If present, the contextID field contains the value of the most
   recently received contextID field from a VirtualListViewResponse
   control for the same list view. If the contextID is not known because
   no contextID has been sent by the server in a VirtualListViewResponse
   control, it SHALL be omitted. If the server receives a contextID that
   is invalid, it SHALL fail the search operation and indicate the
   failure with a protocolError (3) value in the virtualListViewResult
   field of the VirtualListViewResponse. The contextID provides state
   information between the client and server. This state information is
   used by the server to ensure continuity contiguous virtual list
   requests. When a server receives a VirtualListViewRequest control
   that includes a contextID, it SHALL determine whether the client has
   sent a contiguous virtual list request and SHALL provide contiguous
   entries if possible. If a valid contextID is sent, and the server is
   unable to determine whether contiguous data is requested, or is
   unable to provide requested contiguous data, it SHALL fail the search
   operation and indicate the failure with an unwillingToPerform (53)
   value in the virtualListViewResult field of the
   VirtualListViewResponse. contextID values have no validity outside
   the connection and query with which they were received. A client MUST
   NOT submit a contextID which it received from a different connection,
   a different query, or a different server.

   The type AssertionValue and value maxInt are defined in [LDAPPROT].

6.2. Response Control

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   If the request control is serviced, this response control is included
   in the SearchResultDone message as part of the controls field of the
   LDAPMessage, as defined in Section 4.1.12 of [LDAPPROT].

   The controlType is set to "2.16.840.1.113730.3.4.10". The
   controlValue, an OCTET STRING, is the BER-encoding of the following

   VirtualListViewResponse ::= SEQUENCE {
          targetPosition    INTEGER (0 .. maxInt),
          contentCount     INTEGER (0 .. maxInt),
          virtualListViewResult ENUMERATED {
               success (0),
               operationsError (1),
               protocolError (3),
               unwillingToPerform (53),
               insufficientAccessRights (50),
               timeLimitExceeded (3),
               adminLimitExceeded (11),
               innapropriateMatching (18),
               sortControlMissing (60),
               offsetRangeError (61),
               ... },
          contextID     OCTET STRING OPTIONAL }

   targetPosition gives the list offset for the target entry.

   contentCount gives the server's estimate of the current number of
   entries in the list. Together these give sufficient information for
   the client to update a list box slider position to match the newly
   retrieved entries and identify the target entry. The contentCount
   value returned SHOULD be used in a subsequent VirtualListViewRequest

   contextID is a server-defined octet string. If present, the contents
   of the contextID field SHOULD be returned to the server by a client
   in a subsequent virtual list request. The presence of a contextID
   here indicates that the server is willing to return contiguous data
   from a subsequent search request which uses the same search criteria,
   accompanied by a VirtualListViewRequest which indicates that the
   client wishes to receive an adjoining page of data.

   The virtualListViewResult codes which are common to the LDAP
   searchResultDone (adminLimitExceeded, timeLimitExceeded,
   operationsError, unwillingToPerform, insufficientAccessRights,
   success, other) have the same meanings as defined in [LDAPPROT], but
   they pertain specifically to the VLV operation. For example, the
   server could exceed a VLV-specific administrative limit while
   processing a SearchRequest with a VirtualListViewRequest control.
   Obviously, the same administrative limit would not be exceeded should

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   the same SearchRequest be submitted by the client without the
   VirtualListViewRequest control. In this case, the client can
   determine that the administrative limit has been exceeded in
   servicing the VLV request, and can if it chooses resubmit the
   SearchRequest without the VirtualListViewRequest control, or with
   different parameters.

   insufficientAccessRights means that the server denied the client
   permission to perform the VLV operation.

   If the server determines that the results of the search presented
   exceed the range specified in INTEGER values, or if the client
   specifies an invalid offset or contentCount, the server MUST set the
   virtualListViewResult value to offsetRangeError.

6.2.1 virtualListViewError

   A new LDAP error is introduced called virtualListViewError. Its value
   is 76. This error indicates that the search operation failed due to
   the inclusion of the VirtualListViewRequest control.

   If the resultCode in the SearchResultDone message is set to
   virtualListViewError (76), then the virtualListViewResult value MUST
   NOT be success (as virtualListViewResult indicates the specific error
   condition). If resultCode in the SearchResultDone message is not set
   to virtualListViewError (76), then the virtualListViewResult value
   SHOULD be success (0) and its value MUST be ignored.

7. Protocol Example

   Here we walk through the client-server interaction for a specific
   virtual list view example: The task is to display a list of all 78564
   persons in the US company "Ace Industry". This will be done by
   creating a graphical user interface object to display the list
   contents, and by repeatedly sending different versions of the same
   virtual list view search request to the server. The list view
   displays 20 entries on the screen at a time.

   We form a search with baseObject of "o=Ace Industry,c=us"; scope of
   wholeSubtree; and filter of "(objectClass=person)". We attach a
   server-side sort control [SSS] to the search request, specifying
   ascending sort on attribute "cn". To this search request, we attach a
   virtual list view request control with contents determined by the
   user activity and send the search request to the server. We display
   the results from each search result entry in the list window and
   update the slider position.

   When the list view is first displayed, we want to initialize the
   contents showing the beginning of the list. Therefore, we set
   beforeCount to 0, afterCount to 19, contentCount to 0, offset to 1
   and send the request to the server. The server duly returns the first

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   20 entries in the list, plus a content count of 78564 and
   targetPosition of 1. We therefore leave the scroll bar slider at its
   current location (the top of its range).

   Say that next the user drags the scroll bar slider down to the bottom
   of its range. We now wish to display the last 20 entries in the list,
   so we set beforeCount to 19, afterCount to 0, contentCount to 78564,
   offset to 78564 and send the request to the server. The server
   returns the last 20 entries in the list, plus a content count of
   78564 and a targetPosition of 78564.

   Next the user presses a page up key. Our page size is 20, so we set
   beforeCount to 0, afterCount to 19, contentCount to 78564, offset to
   78564-19-20 and send the request to the server. The server returns
   the preceding 20 entries in the list, plus a content count of 78564
   and a targetPosition of 78525.

   Now the user grabs the scroll bar slider and drags it to 68% of the
   way down its travel. 68% of 78564 is 53424 so we set beforeCount to
   9, afterCount to 10, contentCount to 78564, offset to 53424 and send
   the request to the server. The server returns the preceding 20
   entries in the list, plus a content count of 78564 and a
   targetPosition of 53424.

   Lastly, the user types the letter "B". We set beforeCount to 9,
   afterCount to 10 and greaterThanOrEqual to "B". The server finds the
   first entry in the list not less than "B", let's say "Babs Jensen",
   and returns the nine preceding entries, the target entry, and the
   proceeding 10 entries. The server returns a content count of 78564
   and a targetPosition of 5234 and so the client updates its scroll bar
   slider to 6.7% of full scale.

8. Notes for Implementers

   While the feature is expected to be generally useful for arbitrary
   search and sort specifications, it is specifically designed for those
   cases where the result set is very large. The intention is that this
   feature be implemented efficiently by means of pre-computed indices
   pertaining to a set of specific cases. For example, an offset
   relating to "all the employees in the local organization, sorted by
   surname" would be a common case.

   The intention for client software is that the feature should fit
   easily with the host platform's graphical user interface facilities
   for the display of scrolling lists. Thus the task of the client
   implementers should be one of reformatting up the requests for
   information received from the list view code to match the format of
   the virtual list view request and response controls.

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   Client implementers MUST be aware that any offset value returned by
   the server might be approximate. Do not design clients that only
   operate correctly when offsets are exact. However, if contextIDs are
   used, and adjoining pages of information are requested, the server
   will return contiguous data.

   Server implementers using indexing technology which features
   approximate positioning should consider returning contextIDs to
   clients. The use of a contextID will allow the server to distinguish
   between client requests which relate to different displayed lists on
   the client. Consequently the server can decide more intelligently
   whether to reposition an existing database cursor accurately to
   within a short distance of its current position, or to reposition to
   an approximate position. Thus the client will see precise offsets for
   "short" repositioning (e.g. paging up or down), but approximate
   offsets for a "long" reposition (e.g. a slider movement).

   Server implementers are free to return an LDAP result code of
   virtualListViewError and a virtualListViewResult of
   unwillingToPerform should their server be unable to service any
   particular VLV search. This might be because the resolution of the
   search is computationally infeasible, or because excessive server
   resources would be required to service the search.

   Client implementers should note that this control is only defined on
   a client interaction with a single server. If a search scope spans
   multiple naming contexts that are not held locally, search result
   references will be returned, and may occur at any point in the search
   operation. The client is responsible for deciding when and how to
   apply this control to the referred-to servers, and how to collate the
   results from multiple servers.

9. Relationship to "Simple Paged Results"

   These controls are designed to support the virtual list view, which
   has proved hard to implement with the Simple Paged Results mechanism
   [SPaged]. However, the controls described here support any operation
   possible with the Simple Paged Results mechanism. The two mechanisms
   are not complementary; rather one has a superset of the other's
   features. One area where the mechanism presented here is not a strict
   superset of the Simple Paged Results scheme is that here we require a
   sort order to be specified. No such requirement is made for paged

10. Security Considerations

   Server implementers may wish to consider whether clients are able to
   consume excessive server resources in requesting virtual list
   operations. Access control to the feature itself; configuration

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   options limiting the feature's use to certain predetermined search
   base DNs and filters; throttling mechanisms designed to limit the
   ability for one client to soak up server resources, may be

   Consideration should be given as to whether a client will be able to
   retrieve the complete contents, or a significant subset of the
   complete contents of the directory using this feature. This may be
   undesirable in some circumstances and consequently it may be
   necessary to enforce some access control or administrative limit.

   Clients can, using this control, determine how many entries match a
   particular filter, before the entries are returned to the client.
   This may require special processing in servers which perform access
   control checks on entries to determine whether the existence of the
   entry can be disclosed to the client.

   Server implementers should exercise caution concerning the content of
   the contextID. Should the contextID contain internal server state, it
   may be possible for a malicious client to use that information to
   gain unauthorized access to information.

11. IANA Considerations

11.1 Request for LDAP Result Code

   In accordance with section 3.6 of [LDAPIANA], it is requested that
   IANA register the LDAP result code virtualListViewError (76) upon
   Standards Action by the IESG. The value 76 has been suggested by
   experts, had expert review, and is currently being used by some
   implementations. If 76 is unavailable on not chosen, the value in the
   paragraphs in Section 6.2.1 will need to be updated. The following
   registration template is suggested:

   Subject: LDAP Result Code Registration
   Person & email address to contact for further information: Jim
   Result Code Name: virtualListViewError
   Specification: RFCXXXX
   Author/Change Controller: IESG
   Comments:  request LDAP result codes be assigned

12. Acknowledgements

   Chris Weider, Anoop Anantha, and Michael Armijo of Microsoft co-
   authored previous versions of this document.

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13. Normative References

   [X.680]    ITU-T Rec. X.680, "Abstract Syntax Notation One (ASN.1) -
              Specification of Basic Notation", 1994.

   [X.690]    ITU-T Rec. X.690, "Specification of ASN.1 encoding rules:
              Basic, Canonical, and Distinguished Encoding Rules",

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

   [SSS]       Wahl, M., Herron, A. and T. Howes, "LDAP Control
               Extension for Server Side Sorting of Search Results",
               RFC 2891, August, 2000.

   [Bradner97] Bradner, S., "Key Words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [LDAPIANA] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
              Considerations for the Lightweight Directory Access
              Protocol (LDAP)", RFC 3383, September 2002.

14. Informative References

   [SPaged]    Weider, C., Herron, A., Anantha, A. and T. Howes, "LDAP
               Control Extension for Simple Paged Results Manipulation",
               RFC2696, September 1999.

15. Authors' Addresses

        David Boreham
        Bozeman Pass, Inc
        +1 406 222 7093

        Jim Sermersheim
        1800 South Novell Place
        Provo, Utah 84606, USA

        Asaf Kashi
        Microsoft Corporation
        1 Microsoft Way
        Redmond, WA 98052, USA
        +1 425 882-8080

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16. Full Copyright Statement

   Copyright (C) The Internet Society (2002). All Rights Reserved.
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   or assist in its implementation may be prepared, copied, published
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   not be revoked by the Internet Society or its successors or assigns.
   This document and the information contained herein is provided on an

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