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A Uniform Resource Name (URN) Namespace for the Data Documentation Initiative (DDI)
draft-urn-ddi-05

Document Type Active Internet-Draft (individual)
Author Joachim Wackerow
Last updated 2023-05-17 (Latest revision 2023-05-14)
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draft-urn-ddi-05
Network Working Group                                  Joachim Wackerow
Internet Draft                                             DDI Alliance
Intended status: Informational                             May 14, 2023
Expires: November 2023

            A Uniform Resource Name (URN) Namespace for the
                  Data Documentation Initiative (DDI)
                          draft-urn-ddi-05.txt

Abstract

   This document describes the Namespace Identifier (NID) "ddi" for
   Uniform Resource Names (URNs) used to identify resources that
   conform to the standards published by the Data Documentation
   Initiative (DDI) Alliance (https://ddialliance.org/).

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF). Note that
   other groups may also distribute working documents as Internet-
   Drafts. The list of current Internet-Drafts is at
   http://datatracker.ietf.org/drafts/current/.

   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."

   This document is published for informational purposes. It is an
   independent submission. It is not an Internet Standard and does
   not have rough consensus of the IETF community.

   This Internet-Draft will expire in Novemboer 2023.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document.

   Code Components extracted from this document must include
   Simplified BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................4
   3. Specification..................................................5
   3.1. Declaration of Syntactic Structure...........................5
   3.1.1. Description................................................5
   3.1.2. ABNF Grammar...............................................6
   3.1.3. Regular Expression.........................................8
   3.1.4. Examples of DDI URNs.......................................9
   3.2. Relevant Ancillary Documentation.............................9
   3.3. Identifier Uniqueness Considerations........................10
   3.4. Identifier Persistence Considerations.......................10
   3.5. Process of Identifier Assignment............................10
   3.6. Process for Identifier Resolution...........................10
   3.7. Rules for Lexical Equivalence...............................11
   3.8. Conformance with URN Syntax.................................12
   3.9. Validation Mechanism........................................12
   3.10. Scope......................................................12
   4. Namespace Considerations......................................12
   4.1. URN Assignment Procedures...................................12
   4.2. URN Resolution/Delegation...................................12
   4.3. Type of Resources to be Identified..........................12
   4.4. Type of Services............................................12
   5. Community Considerations......................................13
   5.1. Open Assignment and Use of Identifiers......................13
   5.2. Open Operation of Resolution Servers........................13
   5.3. Creation of Software for Service Discovery..................13
   6. IANA Considerations...........................................14
   7. Security Considerations.......................................14
   8. References....................................................14
   8.1. Normative References........................................14
   8.2. Informative References......................................16
   9. Acknowledgments...............................................19
   Appendix A. Example DNS Records..................................20
   A.1. Delegation of the URN Namespace "ddi".......................20
   A.2. Delegation of DDI Agencies..................................20
   A.3. DDI Services................................................20
   Appendix B. Algorithm for DDI Service Discovery..................22

1. Introduction

   This document registers a formal namespace identifier (NID) for
   Uniform Resource Names [RFC8141] associated with DDI resources in
   accordance with the process defined in [RFC8141].

   The DDI Alliance is an international collaboration dedicated to
   establishing metadata standards and semantic products for
   describing social science data, data covering human activity, and
   other data based on observational methods. DDI specifications are
   free standards that can document and manage different stages in

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   the research data lifecycle, such as conceptualization,
   collection, processing, distribution, discovery, and archiving.
   Documenting data with DDI facilitates understanding,
   interpretation, and use -- by people, software systems, and
   computer networks.

   The specifications DDI Codebook [DDIC] and DDI Lifecycle [DDIL]
   are expressed in XML Schema, DDI XKOS - Extended Knowledge
   Organization System [DDIXKOS] in OWL/RDF, SDTL - Structured Data
   Transformation Language [SDTL] in JSON Schema, and the upcoming
   DDI - Cross Domain Integration (DDI-CDI) in UML. DDI is aligned
   with other metadata standards like Dublin Core Metadata Initiative
   [DUBLINC], Statistical Data and Metadata Exchange [SDMX] for
   exchanging aggregate data, ISO/IEC 11179 [IS11179] for building
   metadata registries such as question, variable, and concept banks,
   and ISO 19115 [IS19115] for supporting geographic information
   systems.

   DDI URNs support reusability of DDI resources inside a single DDI
   instance and in a distributed network of DDI instances.

   The DDI specification is developed and maintained by the DDI
   Alliance [DDIALL]. The DDI Alliance is a self-sustaining
   membership organization whose over 40 member institutions have a
   voice in the development of the DDI specifications. This memo
   describing the ddi URN is an informational specification. It is
   not a standard and is not the product of the IETF.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to
   be interpreted as carrying RFC 2119 significance.

   "DDI": Data Documentation Initiative. The single term is often
   used as a synonym for the DDI specification.

   "DDI agency": An organization which maintains DDI resources.

   "DDI Alliance": Alliance for the Data Documentation Initiative
   (DDI).

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3. Specification

   This section provides the information required to register a
   formal namespace according to the registration procedure defined
   in [RFC8141]. The URNs conform to the syntax defined in [RFC8141].

3.1. Declaration of Syntactic Structure

3.1.1. Description

   The Namespace Specific String (NSS) of all URNs using the "ddi"
   NID is a globally unique identifier consisting of the DDI agency
   identifier (registration authority identifier), the identifier of
   the DDI resource (data identifier), and the version of the
   resource (version identifier) [DDIID]. This structure is according
   to the International Registration Data Identifier (IRDI) defined
   in ISO/IEC 11179 Information technology - Metadata registries
   (MDR) - Part 6: Registration, Annex A [IS11179].

   A description of the DDI resource identification is available in
   the DDI Lifecycle (3.3) Technical Guide, section Identification
   [DDIID].

   The DDI NSS has the following structure:

      <agency-identifier>:<resource-identifier>:<version-identifier>

   agency-identifier is the identifier of a DDI agency that maintains
   DDI resources. This identifier basically follows the rules of
   reversed domain names and is case-insensitive. This way, the DNS
   resolution of DDI agency identifiers is supported. The hierarchy
   of domains descends from the left to the right label in the name;
   each label to the right specifies a subdivision, or subdomain of
   the domain to the left. The left-most label of agency-identifier
   conveys the top-level domain. It SHALL be a country code
   corresponding to ISO 3166 alpa-2 codes [ISO3166] or another top-
   level domain maintained by IANA [TLD]. All two-letter top-level
   domains are reserved for current and future ISO 3166 codes.
   Assignment of identifiers for DDI agencies in the requested
   namespace is managed by the DDI Alliance (see section 3.8. on
   "Process of Identifier Assignment"). The next subdomain identifies
   the agency within that top-level domain. Further optional
   subdomains can follow. The top-level domain and possible
   subdomains are separated by the full stop character. The full stop
   character is not allowed within top-level domain names or
   subdomain names. The top-level domain and subdomains are composed
   from the limited set of characters for the preferred form of a DNS
   label ([RFC1035] section 2.3.1). The length of the label and the
   full name are restricted by DNS rules ([RFC2181] section 11). The
   agency identifier is case insensitive ([RFC4343] section 2).

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   resource-identifier is the identifier of a DDI resource of a DDI
   agency. The value MUST be unique in the scope of this DDI agency.
   The resource identifier is case sensitive.

   version-identifier is the version of a DDI resource of a DDI
   agency. The value MUST be unique in the scope of this resource.
   The resource version is case sensitive.

3.1.2. ABNF Grammar

   The following syntax specification for the complete URN uses the
   augmented Backus-Naur Form (ABNF) as described in [RFC5234].

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     ABNF Grammar

     ; Rules are case-sensitive, if not stated otherwise.
     ddi-urn              = urn separator ddi separator ddi-irdi
     ; urn is case-insensitive, see [RFC8141].
     urn                  = "urn"
     ; ddi is the URN namespace identifier.
     ; ddi is case-insensitive, see [RFC8141] section 2.1.
     ddi                  = "ddi"
     ; ddi-irdi is the namespace specific string (NSS).
     ; ddi-irdi - international registration data identifier,
     ; see [IS11179] Annex A.2.
     ddi-irdi             = agency-identifier separator
                            resource-identifier separator
                            version-identifier
     ; agency-identifier is case-insensitive. See [RFC4343] section
     2.
     ; For allowed characters see [RFC1035] section 2.3.1.
     ; For length restrictions see [RFC2181] section 11.
     agency-identifier    = top-level-domain
                              sub-separator ddi-authority-id
                              *(sub-separator ddi-sub-authority-id)
                            ; length limit is 255 characters
                            ; see section 11 of [RFC2181]
     top-level-domain     = dns-label
     ddi-authority-id     = dns-label
     ddi-sub-authority-id = dns-label
     dns-label            = (ALPHA / DIGIT)
                              [ *(ALPHA / DIGIT / "-")
                                (ALPHA / DIGIT) ]
                            ; length limit is 63 characters
                            ; see section 11 of [RFC2181]
     resource-identifier  = restricted-string
                            *("/" restricted-string)
     version-identifier   = restricted-string
                            *("/" restricted-string)
     restricted-string    = 1*(unreserved / sub-delims / "@")
     ; Definitions for unreserved and sub-delims from [RFC3986] 2.2.
     unreserved           = ALPHA / DIGIT / "-" / "." / "_" / "~"
     sub-delims           = "!" / "$" / "&" / "'" / "(" / ")" /
                            "*" / "+" / "," / ";" / "="
     separator            = ":"
     sub-separator        = "."
     ; ALPHA and DIGIT are actually defined in the ABNF
     specification.
     ; They are declared here for convenience purposes.
     ALPHA                = %x41-5A /  ; uppercase letters
                            %x61-7A    ; lowercase letters
     DIGIT                = %x30-39    ; digits

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3.1.3. Regular Expression

   The used syntax is the XML Schema flavor which can be easily used
   in other flavors. These regular expressions implicitly anchor at
   the head and tail. The following regular expression syntax uses
   components (component names indicated by angle brackets, i.e.
   <component>) and is written in free-spacing mode for easier
   reading (the XML Schema flavor does not support that). Please note
   that use of multiple quantifiers in regular expressions can result
   in false outcomes due to so-called greediness. Therefore, there
   are separate regular expressions for the length restriction and
   other purposes for the components agency-identifier and dns-label.

     ddi-urn              := [Uu][Rr][Nn] : [Dd][Dd][Ii] :
                               <agency-identifier> :
                               <resource-identifier> :
                               <version-identifier>

     agency-identifier    := <top-level-domain> \.
                               <ddi-authority-id>
                               (\. <ddi-sub-authority-id>)*

     agency-identifier    := .{1,255}

     top-level-domain     := <dns-label>

     ddi-authority-id     := <dns-label>

     ddi-sub-authority-id := <dns-label>

     dns-label            := [A-Za-z0-9]([-A-Za-z0-9]*[A-Za-z0-9])?

     dns-label            := .{1,63}

     resource-identifier  := <restricted-string>
                             (/ <restricted-string>)*

     version-identifier   := <restricted-string>
                             (/ <restricted-string>)*

     restricted-string    := [A-Za-z0-9-._~!$&'()*+,;=@]+

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3.1.4. Examples of DDI URNs

   The examples are taken from the DDI Lifecycle 3.3. documentation.
   Please note that the resource identifiers are simplified. In real
   applications, they are much longer for unique identification
   purposes. They don't relate to DDI types like the examples might
   suggest.

   URN of a Represented Variable

      urn:ddi:us.ddia1:R-V1:1

   The DDI represented variable identified by "R-V1" with the version
   "1" of the DDI agency "ddia1" located in the domain "us"
   [DDIEXRV].

   URN of a Question Item

      urn:ddi:us.ddia1:PISA-QS.QI-2:1

   The DDI question item identified by "PISA-QS.QI-2" with the
   version "1" of the DDI agency "ddia1" in the domain "us"
   [DDIEXQU].

   URN as Reference to a Controlled Vocabulary

      urn:ddi:int.ddi.cv:AggregationMethod:1.0

   The DDI controlled vocabulary identified by "AggregationMethod"
   with the version "1.0" in the scope of the DDI agency "ddi" and
   sub-agency "cv" in the domain "int" [DDICVAG].

3.2. Relevant Ancillary Documentation

   An introductory article on DDI can be found at [DDIINTR].

   Information on the DDI specifications (DDI-C, DDI-L, XKOS,
   Controlled Vocabularies, and SDTL) can be found in the standards
   section of the DDI Alliance website [DDIALL].

   Information on domain names can be found in the relevant RFCs.

   o  For an overview, see [RFC1034].

   o  Regarding case insensitivity, see [RFC1035] section 2.3.3.

   o  Regarding syntax, see [RFC952] section "Grammatical Host Table
      Specification" B. and [RFC1123] section 2.1.

   o  Regarding size limits, see [RFC1123] section 2.1 and [RFC1035]
      section 2.3.4.

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3.3. Identifier Uniqueness Considerations

   Assignment of identifiers for DDI agencies in the requested
   namespace will be managed by the DDI Alliance, which will ensure
   that the assigned DDI agency identifiers are consistent with the
   directives for unique identification of DDI agencies.

   Assignment of URNs for resources of a DDI agency in the requested
   namespace will be managed by the respective DDI agency, which
   ensures that the assigned URNs are unique for the scope of the
   agency.

3.4. Identifier Persistence Considerations

   Persistence of identifiers is dependent upon suitable delegation
   of resolution at the level of the DDI agencies, and persistence of
   DDI agency assignment. The persistence of the referenced resource
   is also the responsibility of the DDI agency.

3.5. Process of Identifier Assignment

   Assignment of identifiers for DDI agencies in the requested
   namespaceis managed by the DDI Alliance. A registry for DDI agency
   identifiers ensures through an approval process that the syntax of
   agency identifiers complies with the associated rules [DDIREGI].

   Assignment of URNs for resources of a DDI agency and sub-agencies
   of a DDI agency in the requested namespace will be managed by the
   respective DDI agency.

3.6. Process for Identifier Resolution

   The DDI Alliance will promote a service discovery system for
   identifying available services connected to DDI agencies using the
   Domain Name System (DNS). A DNS request for a DDI agency within
   the domain ddi.urn.arpa is delegated by the DNS servers of the DDI
   Alliance to the DNS servers of the relevant DDI agency. The
   response is a list of available DDI services for the agency
   identifier under which the agency has assigned URNs. The approach
   is based on the Dynamic Delegation Discovery System (DDDS)
   [RFC3401] and especially the straightforward URI-enabled NAPTR (U-
   NAPTR) [RFC4848].

   The DDI Alliance is responsible for operating or delegating
   resolution requests to the resolution servers of the relevant DDI
   agencies. DDI agencies are responsible for operating or delegating
   resolution servers for the agency identifier under which they have
   assigned URNs.

   Sample Sequence Diagram for receiving a list of DDI services from
   the example DDI agency "ddia1".

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   Client  NS for       NS for             NS for         DDI
   services
           urn.arpa     ddialliance.org    example1.edu   for
   us.ddia1
      |       |               |               |               |
    1 |------>|               |               |               |
    2 |<------|               |               |               |
    3 |---------------------->|               |               |
    4 |<----------------------|               |               |
    5 |-------------------------------------->|               |
    6 |<--------------------------------------|               |
    7 |------------------------------------------------------>|
    8 |<------------------------------------------------------|

     1.  The name server (NS) of IANA for the domain "urn.arpa."
           is reached with the request "ddia1.us.ddi.urn.arpa." for
           the DDI agency "us.ddia1".

     2./3. The request is delegated to the name server for
           "ddialliance.org".

     4./5. The request is delegated to the name server for
           "example1.edu" (domain of the DDI agency "us.ddia1").

     6.  The server responds with a list of NAPTR records
           [RFC3403] pointing to available DDI services for the DDI
           agency "us.ddia1".

     7.  The client selects an appropriate DDI service and sends a
           request for a DDI URN to this service.

     8.  The DDI service responds for example with a DDI object
           identified by the requested DDI URN.

   See Appendix A for examples of name server records.

3.7. Rules for Lexical Equivalence

   The DDI agency identifier basically follows the rules of domain
   names. Domain names are case-insensitive. Thus, the portion of the
   URN

      urn:ddi:<agency-id>:

   is case-insensitive for matches. The remainder of the identifier
   MUST be considered case-sensitive.

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3.8. Conformance with URN Syntax

   The Namespace Specific String conforms to the related section in
   [RFC8141]. It is composed from the limited set of characters for
   an URN NSS [RFC8141]. Percent encoding is not used.

3.9. Validation Mechanism

   The DDI Alliance will promote development of software for
   validation purposes.

3.10. Scope

   The scope is global.

4. Namespace Considerations

   There is no available namespace that will allow one to uniquely
   identify and access DDI resources.

4.1. URN Assignment Procedures

   See also above section on Process of Identifier Assignment.

4.2. URN Resolution/Delegation

   See also above section on Process for Identifier Resolution.

   It is RECOMMENDED to use sub-agencies for flexible administration.
   For example, delegation of URNs of a sub-agency to different
   servers would be easily possible.

4.3. Type of Resources to be Identified

   The DDI specifications define resources at a granular level, many
   of which can be identified by a DDI URN.

4.4. Type of Services

   Examples of potential services are listed below. The services and
   appropriate service tags need to be defined in future. The
   mentioned service tags are from [RFC2483].

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   o  DDI repository

       o I2R (URI to Resource): given a DDI URN return one instance
          of the resource identified by that URN.

   o  DDI registry

       o I2C (URI to URC, Uniform Resource Characteristics are
          descriptions of resources): given a DDI URN return a
          description or a summary of that resource.

   o  DDI URN resolution

       o I2L (URI to URL): given a DDI URN return one URL that
          identifies a location where the identified DDI resource can
          be found.

       o I2Ls (URI to URLs): given a DDI URN return one or more URLs
          that identify multiple locations of the identified DDI
          resource.

5. Community Considerations

5.1. Open Assignment and Use of Identifiers

   DDI agency identifiers can be registered at the DDI Alliance. The
   DDI Alliance will maintain a registry of the assigned values for
   the DDI agency identifier used in the NSS. Information may be
   obtained from the following address: secretariat@ddialliance.org.

   DDI agencies assign URNs and potential sub-agencies within the
   scope of the assigned DDI agency identifiers.

   See also above section on Identifier Uniqueness Considerations.

5.2. Open Operation of Resolution Servers

   The DDI Alliance operates publicly accessible name servers for the
   delegation of DNS requests within the domain ddi.urn.arpa to DNS
   servers of DDI agencies.

5.3. Creation of Software for Service Discovery

   The DDI Alliance will promote software for the resolution of DDI
   agency identifiers and service discovery. See also Appendix B for
   the resolution algorithm. A basic resolver library is available
   [DDIRESO].

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6. IANA Considerations

   IANA is requested to update the "ddi" entry in the Universal
   Resource Names registry to reference this specification.

   The registration for "ddi" in the "URN.ARPA" zone is approved.
   Requests for the domain ddi.urn.arpa will be delegated to the name
   servers of the DDI Alliance.

7. Security Considerations

   URN:DDI identifiers are assigned to resources that are public
   information, and therefore resolving these identifiers has low
   security profile.

   Registration of DDI agencies is approved by the DDI Alliance.
   Assignment and resolution of URN:DDI identifiers are controlled by
   the DDI Alliance and approved DDI agencies. The DDI Alliance SHALL
   have in place control mechanisms in order to make sure that DDI
   Agency applications from malicious 3rd parties will not be
   accepted. URN:DDI resolvers will be protected against
   eavesdropping and attacks with appropriate tools.

   This document introduces no additional technical security
   considerations beyond those associated with the use and resolution
   of URNs in general.

   The security of the DNS-based resolution of DDI agency identifiers
   is only as good as the security of DNS queries in general. A full
   discussion of the security threats pertaining to DNS and possible
   solutions can be found in [RFC3833]. Further information on
   security considerations regarding U-NAPTR can be found in
   [RFC4848] section 6. DNS over HTTPS (DoH) [RFC8484] could be used
   to increase security by preventing eavesdropping and manipulation
   of DNS data by man-in-the-middle attacks. The HTTPS protocol
   encrypts the data between the DoH client and the DoH-based DNS
   resolver.

8. References

8.1. Normative References

   [DDIC]    DDI Codebook, DDI Alliance 2000-2014,
             <https://ddialliance.org/Specification/DDI-Codebook/>.

   [DDIL]    DDI Lifecycle, DDI Alliance 2009-2020,
             <https://ddialliance.org/Specification/DDI-Lifecycle/>.

   [DDIXKOS] DDI XKOS - Extended Knowledge Organization System, DDI
             Alliance 2020,
             <https://ddialliance.org/Specification/RDF/XKOS>.

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   [DDIID]   DDI Lifecycle (3.3) Technical Guide, Identification,
             <https://ddi-lifecycle-technical-
             guide.readthedocs.io/en/latest/General%20Structures/Iden
             tification.html>.

   [RFC952]  Harrenstien, K., Stahl, M., and E. Feinler "DoD Internet
             Host Table Specification", RFC 952, October 1985,
             <https://www.rfc-editor.org/info/rfc952>.

   [RFC1034] Mockapetris, P., "Domain Names - Concepts and
             Facilities", RFC 1034, November 1987,
             <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035] Mockapetris, P., "Domain Names - Implementation and
             Specification", RFC 1035, November 1987,
             <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1123] Braden, R., Ed. "Requirements for Internet Hosts --
             Application and Support", RFC 1123, October 1989,
             <https://www.rfc-editor.org/info/rfc1123>.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
             3", BCP 9, RFC 2026, October 1996,
             <https://www.rfc-editor.org/info/rfc2026>.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997,
             <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
             Specification", RFC 2181, July 1997,
             <https://www.rfc-editor.org/info/rfc2181>.

   [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifier (URI): Generic Syntax", RFC 3986,
             <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4343] Eastlake, D. 3rd, "Domain Name System (DNS) Case
             Insensitivity Clarification", RFC 4343, January 2006,
             <https://www.rfc-editor.org/info/rfc4343>.

   [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
             Syntax Specifications: ABNF", STD 68, RFC 5234, January
             2008, <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5378] Bradner, S. and J. Contreras, "Rights Contributors
             Provide to the IETF Trust", BCP 78, RFC 5378, November
             2008, <https://www.rfc-editor.org/info/rfc5378>.

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   [RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
             (URNs)", RFC 8141, April 2017,
             <https://www.rfc-editor.org/info/rfc8141>.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", RFC 8174, May 2017, <https://www.rfc-
             editor.org/info/rfc8174>.

   [RFC8179] Bradner, S. and J. Contreras, "Intellectual Property
             Rights in IETF Technology", BCP 79, RFC 8179, May 2017,
             <https://www.rfc-editor.org/info/rfc8179>.

   [SDTL]   SDTL - Structured Data Transformation Language - Version
             1.0, <https://ddialliance.org/products/sdtl/1.0/>.

   [TLD]    IANA Database of Top Level Domains,
             <https://www.iana.org/domains/root/db>.

8.2. Informative References

   [ABNF2RS] ABNF to REGEX Regular Expression Generator, Michael R.
             Sweet, <https://www.msweet.org/abnf/>.

   [ABNFGEN] abnfgen, Jutta Degener, <http://www.quut.com/abnfgen/>.

   [ABNFPFE] ABNF Parser, Bill Fenner,
             <http://tools.ietf.org/tools/bap/abnf.cgi>.

   [DDIALL]  DDI Alliance, <https://ddialliance.org/>.

   [DDICVAG] DDI Controlled Vocabulary for Aggregation Method,
             <https://ddialliance.org/Specification/DDI-
             CV/AggregationMethod_1.0.html>.

   [DDIEXQU] DDI Lifecycle 3.3 Technical Guide, Examples, Questions,
             <https://ddi-lifecycle-technical-
             guide.readthedocs.io/en/latest/Examples/Questions.html>.

   [DDIEXRV] DDI Lifecycle 3.3 Technical Guide, Examples, Represented
             Variable, <https://ddi-lifecycle-technical-
             guide.readthedocs.io/en/latest/Examples/RepresentedVaria
             ble.html>.

   [DDIINTR] Vardigan, M., Heus, P., and W. Thomas. "Data
             Documentation Initiative: Toward a Standard for the
             Social Sciences." The International Journal of Digital
             Curation 3, 1 (2008),
             <http://www.ijdc.net/article/view/66>.

   [DDIREGI] DDI Agency Registry,
             <https://registry.ddialliance.org/>.

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   [DDIRESO] DDI Agency Registry Tools,
             <https://registry.ddialliance.org/Home/Tools>.

   [DUBLINC] Dublin Core Metadata Initiative,
             <https://www.dublincore.org/>.

   [ISO3166] ISO 3166 Country Codes,
             <https://www.iso.org/iso-3166-country-codes.html>.

   [IS11179] ISO/IEC 11179 Information technology - Metadata
             registries (MDR) - Part 6: Registration,
             <http://metadata-standards.org/11179/>.

   [IS19115] ISO 19115, Geographic information -- Metadata,
             <https://www.iso.org/standard/26020.html>.

   [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
             specifying the location of services (DNS SRV)", RFC
             2782, February 2000, <https://www.rfc-
             editor.org/info/rfc2782>.

   [RFC2843] Mealling, M., Daniel, R., "URI Resolution Services
             Necessary for URN Resolution", RFC 2483, January 1999,
             <https://www.rfc-editor.org/info/rfc2483>.

   [RFC3401] Mealling, M., "Dynamic Delegation Discovery System
             (DDDS) - Part Two: The Algorithm", RFC 3401, October
             2002, <https://www.rfc-editor.org/info/rfc3401>.

   [RFC3402] Mealling, M., "Dynamic Delegation Discovery System
             (DDDS) - Part Two: The Algorithm", RFC 3402, October
             2002, <https://www.rfc-editor.org/info/rfc3402>.

   [RFC3403] Mealling, M., "Dynamic Delegation Discovery System
             (DDDS) - Part Three: The Domain Name System (DNS)
             Database", RFC 3403, October 2002,
             <https://www.rfc-editor.org/info/rfc3403>.

   [RFC3833] Atkins, D. and R. Austein, "Threat Analysis Of The
             Domain Name System", RFC 3833, August 2004,
             <https://www.rfc-editor.org/info/rfc3833>.

   [RFC3958] Daigle, L., Newton, A., "Domain-Based Application
             Service Location Using SRV RRs and the Dynamic
             Delegation Discovery Service (DDDS)", RFC 3958, January
             2005,
             <https://www.rfc-editor.org/info/rfc3958>.

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   [RFC4848] Daigle, L., "Domain-Based Application Service Location
             Using URIs and the Dynamic Delegation Discovery Service
             (DDDS)", RFC 4848, April 2007,
             <https://www.rfc-editor.org/info/rfc4848>.

   [RFC5385] Touch, J, "Version 2.0 Microsoft Word Template for
             Creating Internet Drafts and RFCs", RFC 5385, February
             2010, <https://www.rfc-editor.org/info/rfc5385>.

   [RFC8484] Hoffman, P., McManus, P., "DNS Queries over HTTPS
             (DoH)", RFC 8484, October 2018, <https://www.rfc-
             editor.org/rfc/rfc8484>.

   [SDMX]    SDMX -- Statistical Data and Metadata Exchange,
             <https://sdmx.org/>.

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9. Acknowledgments

   Many thanks to Arofan Gregory, Dan Smith, and Wendy Thomas from
   the DDI Alliance Technical Committee, and Peter Koch from DENIC
   (German Network Information Center) for discussion and input that
   led to this document.

   The following software tools have been helpful in evaluating the
   ABNF grammar and the regular expressions: an ABNF parser
   [ABNFPFE], a tool that creates regular expressions from an ABNF
   grammar [ABNF2RS], and a tool that generates random strings that
   match an ABNF grammar [ABNFGEN].

   This document was prepared using the Word template
   2-Word-v2.0.template.dot [RFC5385].

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Appendix A.                 Example DNS Records

   The examples use NAPTR [RFC3403] and SRV [RFC2782][RFC3958]
   records. The values for the services and flags fields of the NAPTR
   records will be determined by the DDI application ([RFC3403]
   section 9.).

A.1. Delegation of the URN Namespace "ddi"

   Example records defined at a.iana-servers.net and other
   authoritative name servers for the domain urn.arpa.

   The empty flag indicates that the lookup is not terminal and the
   next probe to DNS is for more NAPTR records where the new domain
   is "dns.ddialliance.org".

   ; Delegation to name servers of ddialliance.org
   ;           order pref  flag  service  regexp replacement
   ddi.urn.arpa.
      IN NAPTR 100   10    ""    ""       ""     dns.ddialliance.org.

A.2. Delegation of DDI Agencies

   Example records defined at dns.ddialliance.org for ddi.urn.arpa.

   The empty flag indicates that the lookup is not terminal and the
   next probe to DNS is for more NAPTR records where the new domain
   is the DNS server of the relevant DDI agency.

   ; Delegation to name servers of subdomains in ddi.urn.arpa, i.e.
   ; DDI agencies.
   ;           order pref  flag  service  regexp replacement
   ddia1.us.ddi.urn.arpa.
      IN NAPTR 100   10    ""    ""       ""     dns.example1.edu.
   ddia2.de.ddi.urn.arpa.
      IN NAPTR 100   10    ""    ""       ""     dns.example2.org.
   ddia3.gb.ddi.urn.arpa.
      IN NAPTR 100   10    ""    ""       ""     dns.example3.ac.uk.

A.3. DDI Services

   Example records defined at dns.example2.org for ddi.urn.arpa.

   The "u" flag states that the rule is terminal and that the output
   is a URI which contains the information needed to contact that DDI
   service. The "s" flag states that the rule is terminal and that
   the output of the rewrite will be a domain-name for which an SRV
   record SHOULD be queried. See also [RFC4848] section 4.4.

   The service N2R returns one instance of the resource identified by
   the given URN. That service is a repository of DDI resources

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   available at http://repos.example2.org/N2R/; possibly a REST-based
   service. The service N2C returns a description of the resource
   identified by the given URN. That service is a registry of DDI
   resources available at registry-udp.example2.org port 10060.

   U-NAPTR permits regular expressions of a form that does a complete
   replacement of the matched string with a URI, expressed as a
   constant string. With this limited form of regular expression
   ([RFC4848] section 2.2.), applications using NAPTR need not
   implement full regular expression parsers.

   ddia2.de.ddi.urn.arpa.
   ;         order pref flag
     IN NAPTR 100  10  "u"  "N2R+http"                   ( ; service
                            "!.*!http://repos.example2.org/N2R/!"
   ;regex
                            .                              ;
   replacement
                                                         )
     IN NAPTR 100  10  "s"  "N2C+udp"                    ( ; service
                            ""                             ; regex
                            _registry._udp.example2.org.   ;
   replacement
                                                         )
   ; all subdomains in ddia2.de.ddi.urn.arpa.
   *.ddia2.de.ddi.urn.arpa.
   ddia2.de.ddi.urn.arpa.
   ;         order pref flag
     IN NAPTR 100  10  "u"  "N2R+http"                   ( ; service
                            "!.*!http://repos.example2.org/N2R/!"
   ;regex
                            .                              ;
   replacement
                                                         )
     IN NAPTR 100  10  "s"  "N2C+udp"                    ( ; service
                            ""                             ; regex
                            _registry._udp.example2.org.   ;
   replacement
                                                         )
   ;_service._protocol.name
   ; TTL    class  SRV  priority weight port  target
   _registry._udp.example2.org
     14400  IN     SRV  0        0      10060 registry-
   udp.example2.org.

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Appendix B.                 Algorithm for DDI Service Discovery

   The description is based on the Dynamic Delegation Discovery
   System (DDDS) algorithm [RFC3402].

   Application Unique String

     The Application Unique String is a DDI URN.

   First Well Known Rule

     1. Extracting the characters between the second and third colon
        (the agency identifier).
     2. Normalizing case of that string.
     3. Reversing the order of the substrings separated by dots.
     4. Appending the string ".ddi.urn.arpa" to the end to get a
        domain name.

   Valid Databases

     The DNS is specified as a DDDS Database for this application
     which uses the NAPTR DNS resource records to contain the
     rewrite rules for service discovery.

     The DNS is queried for NAPTR records for the domain name which
     is the output of the First Well Known Rule.

   Expected Output

     The expected output is the information necessary to connect to
     authoritative server(s) (host, port, protocol, or URL) for an
     application service within a given DDI agency. The result is a
     list of terminal NAPTR records pointing to services available
     for the relevant DDI agency.

   The application selects the appropriate service and contacts the
   service for the given URN.

   The process can be optimized by an application cache for the NAPTR
   records of already requested DDI agencies.

Author's Address

   Joachim Wackerow
   c/o The Data Documentation Initiative Alliance (DDI Alliance)
   ICPSR, University of Michigan
   PO Box 1248
   Ann Arbor, MI 48106-1248
   USA
   Email:   joachim.wackerow@posteo.de, secretariat@ddialliance.org
   Website: ddialliance.org

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