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Internet-Draft D. Boreham, Bozeman Pass LDAPext Working Group J. Sermersheim, Novell Intended Category: Standards Track A. Kashi, Microsoft <draft-ietf-ldapext-ldapv3-vlv-09.txt> 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 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. 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. Boreham et al Internet-Draft 1  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 [Bradner97]. 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 [LDAPPROT]. 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 [LDAPPROT]. 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 Boreham et al Internet-Draft 2  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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. Boreham et al Internet-Draft 3  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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) 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. Boreham et al Internet-Draft 4  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results - 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) }, Boreham et al Internet-Draft 5  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 Boreham et al Internet-Draft 6  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 SEQUENCE: 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), 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 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 Boreham et al Internet-Draft 7  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 Boreham et al Internet-Draft 8  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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. Boreham et al Internet-Draft 9  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 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 Boreham et al Internet-Draft 10  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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 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 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 Sermersheim 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. Boreham et al Internet-Draft 11  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 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", 1994. [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 david@bozemanpass.com Jim Sermersheim Novell 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 Boreham et al Internet-Draft 12  LDAP Extensions for Scrolling View Nov 2002 Browsing of Search Results 16. Full Copyright Statement Copyright (C) The Internet Society (2002). 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 copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS 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 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Boreham et al Internet-Draft 13