INTERNET-DRAFT Kurt D. Zeilenga Intended Category: BCP OpenLDAP Foundation Expires in six months 3 March 2003 Considerations for LDAP Extensions Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. This document is intended to be, after appropriate review and revision, submitted to the RFC Editor as a Best Current Practices document. Distribution of this memo is unlimited. Technical discussion of this document will take place on the IETF LDAP Extensions Working Group mailing list . Please send editorial comments directly to the author . Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at . The list of Internet-Draft Shadow Directories can be accessed at . Copyright 2003, The Internet Society. All Rights Reserved. Please see the Copyright section near the end of this document for more information. Abstract The Lightweight Directory Access Protocol (LDAP) is extensible. It provides mechanisms for adding new operations, extending existing operations, and expanding user and system schema. This document discusses considerations for designers of LDAP extensions. Zeilenga LDAP Extensions [Page 1] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119]. In this case, "the specification" as used by BCP 14 refers to the processing of protocols being submitted to the IETF standards process. The term "Request Control" refers to a control attached to client generated message sent to a server. The term "Response Control" refers to a control attached to server generated message sent to a client. 1. Background and Intended Use The Lightweight Directory Access Protocol (LDAP) [RFC3377] is an extensible protocol. LDAP allows for new operations to be added and existing operations to be enhanced [RFC2251]. LDAP allows additional schema to be defined [RFC2252]. This can include additional object classes, attribute types, matching rules, additional syntaxes, and other elements of schema. LDAP supports a Simple Authentication and Security Layer (SASL) authentication method [RFC2829]. SASL [RFC2222] is extensible. LDAP supports establishment of Transport Layer Security (TLS) [RFC2830]. TLS [RFC2246] is extensible. LDAP provides an ability to extend attribute types with options [RFC2251]. LDAP defines one option, ;binary). Other options can be defined in other documents. LDAP has an extensible URL format [RFC2255]. Lastly, LDAP allows for certain extensions to the protocol's ASN.1 [X.680] definition to be made. This allows a wide range of changes. For example, new result codes needed to support extensions to be added. This document describes practices which engineers MUST consider when designing extensions to LDAP. 2. General Considerations Zeilenga LDAP Extensions [Page 2] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 2.1 Scope of Extension Mutually agreeing peers may, within the confines of an extension, agree to significant changes in protocol semantics. However, designers MUST consider the impact of an extension upon protocol peers which have not agreed to implement, or otherwise recognize and support, the extension. 2.2. Discovery Mechanism Extensions SHOULD provide an adequate discover mechanism. Generally, a client discovers the capabilities of the server by querying the root DSE and/or other DSEs containing operational information (such as subschema subentries). The 'supportedControl' attribute is used to advertised supported controls. The 'supportedExtension' attribute is used to advertised supported extended operations. The 'supportedFeatures' is used to advertise features [FEATURES]. Other root DSE attributes MAY be defined to advertise other capabilities. As LDAP design is based upon the client-request/server-response paradigm, no mechanism is provided for a server to discover the capabilities of a client. 2.3. Interaction between extensions Designers SHOULD consider how extensions they engineer interaction with other extensions. Designers SHOULD consider extensibility of extensions they specify. Extensions to LDAP SHOULD themselves be extensible. 2.4. Internationalization Considerations LDAP is designed to support the full ISO 10646-1 (a superset of Unicode) Universal Character Set (UCS) [ISOUCS]. Extensions SHOULD avoid unnecessarily restricting applications to subsets of UCS (e.g., Basic Multilingual Plane, ISO 8859-1, ASCII, Printable String). LDAP Language Tag Options [RFC2596] provides a mechanism for tagging text (and other) values with language information. Extensions which define attribute types SHOULD allow use of language tags with these attributes. Zeilenga LDAP Extensions [Page 3] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 2.5. IANA Considerations Designers SHALL register protocol values of their LDAP extensions in accordance with BCP 64 [RFC3383]. Specifications which create new extensible protocol elements SHALL extended existing or establish new registries for values of these elements in accordance with BCP 26 [RFC2434]. 2.6. Use of Formal Languages LDAP is described in terms of ASN.1 [X.680] and encoded using a particular subset [RFC2251, Section 5.1] of the Basic Encoding Rules [X.690]. To allow reuse of parsers/generators used in implementing the LDAP "core" technical specification [RFC3377], it is generally RECOMMENDED that protocol elements (e.g. extension specific contains of controlValue, requestValue, response fields, and value syntax) specified by extensions be described and encoded similarly. Formal languages SHOULD be used in specifications in accordance with IESG guidelines [FORMAL]. 3. LDAP Operation Extensions Extensions SHOULD use controls to define extensions which complement existing operations. Where the extension to be defined does not complement the existing operation, designers SHOULD consider defining an extended operation instead. For example, a subtree delete operation could be designed as either an extension of the delete operation or as a new operation. As the feature complements the existing delete operation, use of the control mechanism to extend the delete operation is likely more appropriate. As a counter (and contrived) example, a locate services [LOCATE] operation (an operation which would return for a DN a set of LDAP URLs which hold this DN) could be designed as either a search operation or a new operation. As the feature doesn't complement the search operation (e.g., the operation is not searching for entries held in the Directory Information Tree), it is likely more appropriate to define a new operation using the extended operation mechanism. 3.1. Controls Controls [RFC2251, Section 4.1.12] is the RECOMMENDED mechanism for extending existing operations. The existing operation can be a base Zeilenga LDAP Extensions [Page 4] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 operation defined in RFC 2251 (e.g., search, modify, etc.) or an extended operation (e.g. Start TLS [RFC2830], Password Modify [RFC3062]), or a operation defined as an extension to a base or extended operation. Extensions SHOULD NOT return Response controls unless unless the server has specific knowledge that the client can make use of the control. Generally, the client requests the return of a particular response control by providing a related request control. An existing operation MAY be extended to return IntermediateResponse messages [LDAPIMR]. Specifications of controls SHALL NOT attach additional semantics to criticality of controls beyond those defined in RFC 2251, Section 4.1.12. A specification MAY mandate the criticality take on a particular value (e.g., TRUE or FALSE) where appropriate. 3.2. Extended Operations Extended Operations [RFC2251, Section 4.12] is the RECOMMENDED mechanism for defining new operations. An extended operation consists of an ExtendedRequest message, zero or more IntermediateResponse messages [LDAPIMR], and an ExtendedResponse message. 3.3. Intermediate Responses Extensions SHALL use Intermediate Responses [LDAPIMR] instead of ExtendedResponse [RFC2251] messages to return partial results. 3.4. Unsolicited Notifications Unsolicited notifications [RFC2251, Section 4.4] offer a capability for the server to notify the client of events not associated with operation currently being processed. Extensions SHOULD be designed such that unsolicited notifications are not be returned unless the server has specific knowledge that the client can make use of the notification. Generally, the client requests the return of particular unsolicited notification by performing a related extended operation. For example, a time hack extension could be designed to return unsolicited notifications at regular intervals which were enabled by an extended operation (which possibly specified the desired interval). Zeilenga LDAP Extensions [Page 5] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 4. Extending LDAP ASN.1 definition LDAP allows limited extension of the LDAPv3 ASN.1 definition [RFC2251, Appendix A] to made. 4.1. Result Codes Extensions which specify new operations or enhance existing operations often need to define new result codes. The extension SHOULD be designed such that a client has a reasonably clear indication of the nature of the successful or non-successful result. Extensions SHOULD use existing result codes to indicate conditions which are consistent with each codes intended meaning [RFC2251][X.511]. Extensions MAY introduce new result codes where no existing result code provides an adequate indication of the nature of the result. Extensions SHALL NOT disallow nor otherwise restrict the return of general service result codes, especially those reporting a protocol, service, or security problem, indicating server is unable or unwilling to complete the operation. 4.2. LDAP Message Types While extensions can specify new types of LDAP messages by extending the protocolOp CHOICE of the LDAPMessage SEQUENCE, this is generally unnecessary and inappropriate. Existing operation extension mechanisms (e.g., extended operations and unsolicited notifications) SHOULD be used instead. However, there may be cases where an extension does not fit well into these mechanisms. In such cases, a new extension mechanism SHOULD be defined which can be used by multiple extensions which have similar needs. 4.3. Authentication Methods The Bind operation currently supports two authentication methods, simple and SASL. SASL [RFC2222] is an extensible authentication framework used by multiple application level protocols (e.g. BEEP, SMTP, IMAP). It is RECOMMENDED that new authentication processes be defined as SASL mechanisms. New LDAP authentication methods MAY be added to support new authentication frameworks. The Bind operation primary function is to establish the LDAP association [RFC2829]. No other operation SHALL be defined (or extended) to establish the LDAP association [RFC2829]. However, other Zeilenga LDAP Extensions [Page 6] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 operations MAY be defined to establish other security associations (e.g., IPSEC). 4.4. Elements of SEQUENCE extensions Section 4 of [RFC2251] states: In order to support future extensions to this protocol, clients and servers MUST ignore elements of SEQUENCE encodings whose tags they do not recognize. This mechanism is NOT RECOMMENDED. That is, designers SHOULD consider other mechanisms before resorting inserting new elements in existing SEQUENCE encodings. 5. Schema Extensions Extensions defining LDAP schema elements SHALL provide schema definitions conformant with syntaxes defined in [RFC2252]. Definitions provided MAY be reformatted (line wrapped) for readability but this SHALL be noted in the extension specification. For definitions allow a NAME field, new schema elements SHOULD provide one and only one name. The name SHOULD be short. Each schema definition allows a DESC field. The DESC field, if provided, SHOULD contain a short descriptive phrase. The DESC field SHALL NOT contain information imperative to the implementor. That is, the interpretation of the specification SHALL NOT be different if all DESC fields were removed. The extension SHALL NOT mandate that implementations provide the same DESC field in schema they publish. Implementators MAY replace or remove the DESC field. Published schema elements SHALL NOT be redefined. Replacement schema elements (new OIDs, new NAMEs) SHOULD be defined as needed. Schema designers SHOULD reuse existing schema elements where appropriate. 5.1. LDAP Syntaxes Each LDAP syntax is defined in terms of ASN.1 [X.680]. Each extension detailing an LDAP syntax MUST specify the ASN.1 data definition associated with the syntax. A distinct LDAP syntax SHOULD be created for each distinct ASN.1 data definition (including constraints). Zeilenga LDAP Extensions [Page 7] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 Each LDAP syntax SHOULD have a string encoding defined for it. It is RECOMMENDED that this string encoding be restricted to a UTF-8 encoded UCS characters. Use of general purpose string encoding rules (such as Generic String Encoding Rules [GSER]) to provide string encodings for complex ASN.1 data definitions is RECOMMENDED. Otherwise, it is RECOMMENDED that the string encoding be described using a formal language (e.g., ABNF [RFC2234]). Formal languages SHOULD be used in specifications in accordance with IESG guidelines [FORMAL]. If no string encoding is defined, the extension SHALL specify how the transfer encoding is to be indicated. Generally, the extension SHOULD mandate use of ;binary or other transfer encoding option. 5.2. Matching Rules Three basic kinds of matching rules (e.g., EQUALITY, ORDERING, and SUBSTRING) may be associated with an attribute type. In addition, LDAP provides an extensible matching rule mechanism. The matching rule specification SHOULD detail which kind of matching rule it is and SHOULD describe which kinds of values it can be used with. 5.3. Attribute Types Designers SHOULD carefully consider how the structure of values is to restricted. Designers SHOULD consider that servers will only enforce constraints of the attribute's syntax. That is, an attribute intended to hold URIs, but which has directoryString syntax, is not restricted to values which are URIs. Designers SHOULD carefully consider which matching rules, if any, are appropriate for the attribute type. Matching rules specified for an attribute type MUST be compatible with the attribute type's syntax. Extensions specifying operational attributes MUST detail how servers are to maintain and/or utilize values of each operational attribute. 5.4. Object Classes Designers SHOULD carefully consider whether each attributes of an object class is required ("MUST") or allowed ("MAY"). Extensions specifying object classes which allow (or require) operational attributes MUST specify how servers are to maintain and/or Zeilenga LDAP Extensions [Page 8] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 utilize entries belonging to these object classes. 6. Other Extension Mechanisms 6.1. Attribute Description Options Each option is identified by a string of letters, numbers, and hyphen. This string SHOULD be short. 7. Other Considerations 7.1. Examples Example DN strings SHOULD conform to the syntax defined in [RFC2253]. Example LDAP filter strings SHOULD conform to the syntax defined in [RFC2254]. Example LDAP URLs SHOULD conform to the syntax defined in [RFC2255]. Entries SHOULD be represented using LDIF [RFC2849]. 7.2. Authorization Identities Extensions interacting with authorization identities SHALL support the LDAP authzId format [RFC2829, Section 9]. 8. Security Considerations LDAP is designed not to limit the kinds of extensions which can be implemented. This section attempts to outline some specific issues which designers need to consider, it is not (and cannot be) all encompassing. Designers MUST do their own evaluation of the security considerations applicable to their extension. Designers MUST NOT the assume that the LDAP "core" technical specification [RFC3377] adequately addresses the specific concerns surrounding their extension nor assume that their extension has no specific concerns. 8.1 Extending Bind Operation with Controls Controls attached to the request and response messages of a Bind Operation [RFC2251][RFC2829] are not protected by the security layers established by the Bind operation. Specifications detailing controls extending the Bind operation SHALL Zeilenga LDAP Extensions [Page 9] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 detail that the Bind negotiated security layers do not protect the information contained in these controls and SHALL detail why this protection is not needed. It is RECOMMENDED that designers consider alternative mechanisms for providing the function. For example, an extended operation issued subsequent to the Bind operation, hence protected by the security layers negotiated by the Bind operation, might be used to provided the desired function. 8.2 Extending the Start TLS Operation with Controls Controls attached to the request and response messages of a Start TLS Operation [RFC2830] are not protected by the security layers established by the Start TLS operation. Specifications detailing controls extending the Start TLS operation SHALL detail that the Start TLS negotiated security layers do not protect the information contained in these controls and SHALL detail why this protection is not needed. It is RECOMMENDED that designers consider alternative mechanisms for providing the function. For example, an extended operation issued subsequent to the Start TLS operation, hence protected by the security layers negotiated by the Start TLS operation, might be used to provided the desired function. 9. Acknowledgment The author thanks the thoughtful comments of members of the IETF LDAPbis and LDAPext working groups. This work builds upon "LDAP Extension Style Guide" [GUIDE] by Bruce Greenblatt. Some text was borrowed from "Guidelines for Writing an IANA Considerations Section in RFCs" [RFC2434] by Thomas Narten and Harald Alvestrand. 10. Author's Address Kurt D. Zeilenga OpenLDAP Foundation Zeilenga LDAP Extensions [Page 10] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 11. References 11.1. Normative References [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14 (also RFC 2119), March 1997. [RFC2222] J. Myers, "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access Protocol (v3)", RFC 2251, December 1997. [RFC2252] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight Directory Access Protocol (v3): Attribute Syntax Definitions", RFC 2252, December 1997. [RFC2253] M. Wahl, S. Kille, T. Howes, "Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names", RFC 2253, December 1997. [RFC2254] T. Howes, "A String Representation of LDAP Search Filters", RFC 2254, December 1997. [RFC2255] T. Howes, M. Smith, "The LDAP URL Format", RFC 2255, December, 1997. [RFC2256] Wahl, M., "A Summary of the X.500(96) User Schema for use with LDAPv3", RFC 2256, December 1997. [RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26 (also RFC 2434), October 1998. [RFC2596] M. Wahl, T. Howes, "Use of Language Codes in LDAP", RFC 2596, May 1999. [RFC2829] M. Wahl, H. Alvestrand, J. Hodges, R. Morgan, "Authentication Methods for LDAP", RFC 2829, May 2000. [RFC2830] J. Hodges, R. Morgan, and M. Wahl, "Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security", RFC 2830, May 2000. [RFC2849] G. Good, "The LDAP Data Interchange Format (LDIF) - Zeilenga LDAP Extensions [Page 11] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 Technical Specification", RFC 2849, June 2000. [ISOUCS] "Universal Multiple-Octet Coded Character Set (UCS) - Architecture and Basic Multilingual Plane", ISO/IEC 10646-1 : 1993. [RFC3377] J. Hodges, R. Morgan, "Lightweight Directory Access Protocol (v3): Technical Specification", RFC3377, September 2002. [RFC3383] K. Zeilenga, "IANA Considerations for LDAP", BCP 64 (also RFC 3383), September 2002. [LDAPIMR] R. Harrison, K. Zeilenga, "LDAP Intermediate Response", draft-rharrison-ldap-intermediate-resp-00.txt (a work in progress). [FEATURES] K. Zeilenga, "Feature Discovery in LDAP", draft-zeilenga-ldap-features-xx.txt (a work in progress). [FORMAL] IESG, "Guidelines for the use of formal languages in IETF specifications", , 2001. [X.680] ITU-T, "Abstract Syntax Notation One (ASN.1) - Specification of Basic Notation", X.680, 1994. [X.690] ITU-T, "Specification of ASN.1 encoding rules: Basic, Canonical, and Distinguished Encoding Rules", X.690, 1994. 11.2. Informative References [RFC2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC3062] K. Zeilenga, "LDAP Password Modify Extended Operation", RFC 3062, February 2000. [GSER] S. Legg, "Generic String Encoding Rules for ASN.1 Types", draft-legg-ldapext-gser-xx.txt, a work in progress. [LOCATE] IETF LDAPext WG, "Discovering LDAP Services with DNS", draft-ietf-ldapext-locate-xx.txt (work in progress). [GUIDE] B. Greenblatt, "LDAP Extension Style Guide", draft-greenblatt-ldapext-style-xx.txt (work in progress). Zeilenga LDAP Extensions [Page 12] INTERNET-DRAFT draft-zeilenga-ldap-ext-03 3 March 2003 Copyright 2003, The Internet Society. 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