Internet-Draft D. Boreham, Bozeman Pass
LDAPext Working Group J. Sermersheim, Novell
Intended Category: Standards Track A. Kashi, Microsoft
<draft-ietf-ldapext-ldapv3-vlv-06.txt>
Expires: Nov 2002 May 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.
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This document is intended to be submitted, after review and revision,
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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.
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.
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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 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.
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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, 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"
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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 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.
- equivalent values (Ci = Cc) indicate that the target is the last
entry in the list.
- a content count of zero, and a non-zero offset (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 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
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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 client should not submit a contextID
which it received from another connection, a connection now closed,
or a different server.
6.2. Response Control
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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 chooses
resubmit the SearchRequest without the VirtualListViewRequest
control.
insufficientAccessRights means that the server denied the client
permission to perform the VLV operation.
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If the server determines that the results of the search presented
exceed the range specified in INTEGER values, it MUST return
offsetRangeError.
6.2.1 virtualListViewError
A new LDAP error is introduced called virtualListViewError. Its value
is 76.
[Note to the IESG/IANA/RFC Editor: the value 76 has been suggested by
experts, had expert review, and is currently being used by some
implementations. The intent is to have this number designated as an
official IANA assigned LDAP Result Code (see draft-ietf-ldapbis-iana-
xx.txt, Section 3.5)]
If the server returns any code other than success (0) for
virtualListViewResult, then the server SHOULD return
virtualListViewError as the resultCode of the SearchResultDone
message.
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
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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 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
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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 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 feature� 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.
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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.
11. Acknowledgements
Chris Weider, Anoop Anantha, and Michael Armijo of Microsoft co-
authored previous versions 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.
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13. Authors' Addresses
David Boreham
Bozeman Pass, Inc
+1 406 222 7093
david@bozemanpass.com
Jim Sermersheim
Novell, Inc
1800 South Novell Place
Provo, Utah 84606, USA
jimse@novell.com
Asaf Kashi
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052, USA
+1 425 882-8080
asafk@microsoft.com
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