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3.4.4. Examples of DDI URNs

   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:1.1

   The DDI controlled vocabulary identified by "AggregationMethod" with
   the version "1.1" in the scope of the DDI agency "ddi" and sub-agency
   "cv" in the domain "int" [DDICVAG].

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

    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.

3.10. 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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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.


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