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   INTERNET-DRAFT                              D. Boreham, Bozeman Pass
   LDAPext Working Group                         J. Sermersheim, Novell
   Category: Standards Track                      A. Anantha, Microsoft
   <draft-ietf-ldapext-ldapv3-vlv-05.txt>          M. Armijo, Microsoft
   Expires: May 2002                                A. Kashi, Microsoft
                                                          November 2001



      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
   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.

   This document is intended to be submitted, after review and
   revision, as a Standards Track document.  Distribution of this memo
   is unlimited.  It is filed as <draft-ietf-ldapext-ldapv3-vlv-
   05.txt>, and expires May, 2002.

   Please send comments to the authors.


2. Abstract

   This document describes a Virtual List View control extension for
   the Lightweight Directory Access Protocol (LDAP) Search operation.
   This control 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.


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   The control 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.

3. Conventions used in this document
   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 [Bradner97].


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 a
   sorted LDAP search request.

   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 sorted
   alphabetically.  While there may be tens of thousands of entries in
   this list, the address book list view displays only 20 such accounts
   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

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   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 sorted result
   set, centered around a particular target entry. Henceforth, in the
   interests of brevity, the sorted 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 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.

   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, and any error codes.

   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

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   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)

        Where:
        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 are allowed:  a  client  sending  a
   content count  of zero (Cc = 0) means "client has no idea what the
   content count is, server MUST use its own content  count  estimate
   in  place  of  the client's".  An offset value of one (Ci = 1)
   always means that the target is the first entry in the list. Client
   specifying an offset which equals the content count specified in the
   same request control (Ci = Cc) means that the target is the last
   entry in the list.  Ci may only equal zero when Cc is also zero.
   This signifies the last entry in the list.

   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

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   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 context identifier MAY be returned to clients.  A
   client receiving a context identifier SHOULD return it unchanged in
   a subsequent request which relates to the same list.  The purpose of
   this interaction is to enhance the performance and effectiveness of
   servers which employ approximate positioning.


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
   [LDAPv3].  The controlType is set to "2.16.840.1.113730.3.4.9". The
   criticality SHOULD be set to TRUE.  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 is an OCTET
   STRING whose value is the BER-encoding of the following SEQUENCE:

             VirtualListViewRequest ::= SEQUENCE {
                     beforeCount    INTEGER (0..maxInt),
                     afterCount     INTEGER (0..maxInt),
                     CHOICE {
                             byoffset [0] SEQUENCE {
                              offset          INTEGER (0 .. maxInt),
                              contentCount    INTEGER (0 .. maxInt) },
                             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 4.  greaterThanOrEqual is an attribute assertion value
   defined in [LDAPv3].  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. The type AssertionValue and value
   maxInt are defined in [LDAPv3].  contextID values  have  no validity
   outwith the connection on which they were received.  That is, a

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   client should not submit a contextID which it received from another
   connection, a connection now closed, or a different server.


6.2. Response Control

   This control is included in the SearchResultDone message as part of
   the controls field of the LDAPMessage, as defined in Section 4.1.12
   of [LDAPv3].

   The controlType is set to "2.16.840.1.113730.3.4.10". The
   criticality is FALSE (MAY be absent).  The controlValue is an OCTET
   STRING, whose value is the BER encoding of a value of the following
   SEQUENCE:

        VirtualListViewResponse ::= SEQUENCE {
                targetPosition    INTEGER (0 .. maxInt),
                contentCount     INTEGER (0 .. maxInt),
                virtualListViewResult ENUMERATED {
                success (0),
                operationsError (1),
                unwillingToPerform (53),
                insufficientAccessRights (50),
                busy (51),
                timeLimitExceeded (3),
                adminLimitExceeded (11),
                sortControlMissing (60),
                offsetRangeError (61),
                other (80) },
                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
   control.  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 VirtualListViewRequest control.

   The virtualListViewResult codes which  are  common  to  the  LDAP
   searchResponse (adminLimitExceeded, timeLimitExceeded, busy,
   operationsError, unwillingToPerform, insufficientAccessRights) have
   the same meanings as defined in [LDAPv3], but they pertain
   specifically to the VLV operation.  For example, the server could
   exceed an administration limit processing a SearchRequest with a
   VirtualListViewRequest control. However, the same administration
   limit would not be exceeded should the same SearchRequest be
   submitted by the client without the VirtualListViewRequest control.
   In this case, the client can determine that an administration limit
   has been exceeded in servicing the VLV request, and can if it

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   chooses resubmit the SearchRequest without the
   VirtualListViewRequest control.

   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 provided by the 32-bit offset values, it MUST
   return offsetRangeError.

   If the server returns any code other then success (0) for
   virtualListViewResult, then the server MUST return controlError (76)
   as the resultCode of the SearchResultDone message. [ctrlErr]


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 people 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 baseDN "o=Ace Industry, c=us"; search scope
   subtree; filter "objectClass=inetOrgPerson". We attach a server sort
   order control to the search, specifying ascending sort on attribute
   "cn".  To this base search, we attach a virtual list view request
   control with contents determined by the user activity and send the
   search to the server.  We display the results from each search 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 = 0, afterCount = 19, contentCount = 0, offset = 1 and
   send the request to the server.  The server duly returns the first
   20 entries in the list, plus the content count = 78564 and
   targetPosition = 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 = 19, afterCount = 0, contentCount =
   78564, offset = 78564 and send the request to the server. The server
   returns the last 20 entries in the list, plus the content count =
   78564 and targetPosition = 78564.

   Next the user presses a page up key. Our page size  is  20,  so  we
   set beforeCount  =  0,  afterCount  =  19,  contentCount  =  78564,
   offset = 78564-19-20 and send the request to the server. The server

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   returns the preceding 20 entries in the list, plus the content count
   = 78564 and targetPosition = 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 =
   9, afterCount = 10, contentCount = 78564, offset = 53424 and send
   the request to the server.  The server returns the preceding 20
   entries in the list, plus the content count = 78564 and
   targetPosition = 53424.

   Lastly, the user types the letter "B". We set beforeCount = 9,
   afterCount = 10 and greaterThanOrEqual = "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 content count = 78564 and
   targetPosition = 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.

   Client implementers should note that any offset value returned by
   the server may be approximate. Do not design clients > which only
   operate correctly when offsets are exact.

   Server implementers using indexing technology which features
   approximate positioning should consider returning context
   identifiers to clients.  The use of a context identifier 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).


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   Server implementers are free to return status code
   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 server returns
   referrals as a part of its response to the search request, 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 results.


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
   options limiting the featureÆs 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
   appropriate.

   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.

   Clients can, using this control, determine how many entries are
   contained within a portion of the DIT. This may constitute a
   security hazard. Again, access controls may be appropriate.

   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.


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11. Acknowledgements

   Chris Weider of Microsoft co-authored a previous version of this
   document.


12. References


   [LDAPv3]    Wahl, M., Kille, S. and T. Howes, "Lightweight Directory
               Access Protocol (v3)", Internet Standard, RFC 2251,
               December, 1997.

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

   [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.

   [ctrlErr]   Armijo, M. and A. Kashi, ôResult Code for LDAP
               Controlsö, Internet-Draft, September, 2001.
               Work in progress published as:
               <draft-armijo-ldap-control-error-02.txt>


13. Authors' Addresses

        David Boreham
        Bozeman Pass, Inc
        +1 406 222 7093
        david@bozemanpass.com

        Jim Sermersheim
        Novell
        122 East 1700 South
        Provo, Utah 84606, USA
        jimse@novell.com

        Anoop Anantha
        Microsoft Corporation
        1 Microsoft Way
        Redmond, WA 98052, USA
        +1 425 882-8080
        anoopa@microsoft.com

        Michael Armijo

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        Microsoft Corporation
        1 Microsoft Way
        Redmond, WA 98052, USA
        +1 425 882-8080
        micharm@microsoft.com

        Asaf Kashi
        Microsoft Corporation
        1 Microsoft Way
        Redmond, WA 98052, USA
        +1 425 882-8080
        asafk@microsoft.com


14. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  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
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   followed, or as required to translate it into languages other than
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   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
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."





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