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INTERNET-DRAFT                                   David Boreham, Netscape
                                                 Jim Sermersheim, Novell
                                                Anoop Anantha, Microsoft
                                               Michael Armijo, Microsoft
ldapext Working Group                                      6 April, 2000

     LDAP Extensions for Scrolling View Browsing of Search Results

                This document expires on 5 October 2000

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

2.  Abstract

This document describes a Virtual List View control  extension  for  the
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.

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.  Background

A Virtual List is a graphical user interface  technique  employed  where

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RFC DRAFT                                                     April 2000

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 con-
tents 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 appropri-
ate 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.

The key words "MUST", "SHOULD", and "MAY" used in this document  are  to

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be interpreted as described in [Bradner97].

4.  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 ele-
ment 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  con-
tent  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  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 esti-
mate as to the number of entries in the  list  (content  count)  to  the

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RFC DRAFT                                                     April 2000

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 con-
tent 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 transmit-
ted 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 arith-
metic. 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 tar-
get  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 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.

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RFC DRAFT                                                     April 2000

5.  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.

5.1.  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 cri-
ticality   SHOULD be set to TRUE. If this control is included in a Sear-
chRequest 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 con-
textID 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
client should not submit a contextID which it received from another con-
nection, a connection now closed, or a different server.

5.2.  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

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RFC DRAFT                                                     April 2000


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 Vir-
tualListViewRequest control.

The virtualListViewResult codes which  are  common  to  the  LDAP  sear-
chResponse  (adminLimitExceeded,  timeLimitExceeded, busy, operationsEr-
ror, unwillingToPerform, insufficientAccessRights) have the  same  mean-
ings  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. How-
ever, the same administration limit would not  be  exceeded  should  the
same  SearchRequest  be submitted by the client without the VirtualList-
ViewRequest control.  In this case, the client  can  determine  that  an
administration limit has been exceeded in servicing the VLV request, and
can if it chooses resubmit the SearchRequest  without  the  VirtualList-
ViewRequest control.

insufficientAccessRights means that the server denied the client permis-
sion to perform the VLV operation.

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RFC DRAFT                                                     April 2000

If the server determines that the results of the search presented exceed
the  range  provided  by  the  32-bit  offset  values,  it  MUST  return

6.  Protocol Example

Here we walk through the client-server interaction for a  specific  vir-
tual list view example:  The task is to display a list of all 78564 peo-
ple 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  sub-
tree;  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  con-
tents 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

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 =

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  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, after-
Count = 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.

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RFC DRAFT                                                     April 2000

Lastly, the user types the letter "B". We set beforeCount  =  9,  after-
Count  =  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

7.  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 per-
taining 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 dis-
tance of its current position, or to reposition to an approximate  posi-
tion. Thus the client will see precise offsets for "short" repositioning
(e.g. paging up or down), but approximate offsets for a  "long"  reposi-
tion (e.g. a slider movement).

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

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RFC DRAFT                                                     April 2000

as a part of its response to the search request, the client is responsi-
ble  for  deciding when and how to apply this control to the referred-to
servers, and how to collate the results from multiple servers.

8.  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.

9.  Security Considerations

Server implementers may wish to consider whether  clients  are  able  to
consume  excessive  server  resources  in requesting virtual list opera-
tions. Access control to the feature itself; configuration options  lim-
iting  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 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  con-
tained  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.

10.  Acknowledgements

Chris Weider of Microsoft co-authored a previous version of  this  docu-

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RFC DRAFT                                                     April 2000

11.  References

     Wahl, M, S. Kille and T. Howes, "Lightweight Directory Access  Pro-
     tocol (v3)", Internet Standard, December, 1997. RFC2251.

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

[SSS]Wahl, M, A. Herron and T. Howes, "LDAP Control Extension for Server
     Side  Sorting  of  Search  Results",  Internet  Draft, April, 1999.
     Available as draft-ietf-asid-ldapv3-sorting-02.txt.

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

12.  Authors' Addresses

   David Boreham
   iPlanet e-commerce solutions
   501 E. Middlefield Road
   Mountain View, CA 94043, USA
   +1 650 937-5206

   Jim Sermersheim
   122 East 1700 South
   Provo, Utah 84606, USA

   Anoop Anantha
   Microsoft Corp.
   1 Microsoft Way
   Redmond, WA 98052, USA
   +1 425 882-8080

   Michael Armijo
   Microsoft Corp.
   1 Microsoft Way
   Redmond, WA 98052, USA
   +1 425 882-8080
   This document expires on 5 October 2000

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RFC DRAFT                                                     April 2000

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