Allow for validation of URN namespaces

mrps.md

@@ -117,6 +117,8 @@ https-scheme URIs are RECOMMENDED to all members.
 
 http-scheme and https-scheme URIs used for entityID values MUST contain a host part whose value is a DNS domain.
 
+The use of urn-scheme URIs for entityID values is NOT RECOMMENDED but MAY be permitted in exceptional circumstances. When permitted, such values MUST be part of a formal namespace registered in terms of [RFC8141]. The Federation Operator MAY further constrain the available urn-scheme namespaces to specific NID prefixes.
+
 The right to use a URI in an entityID SHOULD be established in one of the following ways:
 
 * A Member demonstrates the right to use the host part of a URL by means of domain validation [[#5.5](#55-domain-validation)].
@@ -126,6 +128,7 @@ The right to use a URI in an entityID SHOULD be established in one of the follow
   3. The Tenant's unique identifier can be directly associated with the member in one of the following ways:
      * The solution provider has a lookup or API service that returns either the canonical name of the Member or a domain name the Member has the right to use; or
      * A Registered Representative of the Member attests to the Member’s right to use the entityID; and can demonstrate operational control of the Tenant by means of login to a protected resource that displays both the Tenant’s unique identifier from the entityID, as well as the canonical name of the Member or a domain name the Member has the right to use.
+* A Registered Representative of the Member demonstrates that the specific NSS value of a urn-scheme URI is part of a properly-delegated registry and has been issued to the Member for their use.
 
 #### 5.3 Scope Format
 
@@ -187,6 +190,7 @@ Changes will be communicated to Registered Representatives for the entity.
 * [SAML-Metadata-RPI-V1.0]  SAML V2.0 Metadata Extensions for Registration and Publication Information Version 1.0. 03 April 2012. OASIS Committee Specification 01. http://docs.oasis-open.org/security/saml/Post2.0/saml-metadata-rpi/v1.0/cs01/saml-metadata-rpi-v1.0-cs01.html.
 * [SAML-Metadata-OS] OASIS Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0: http://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf.
 * [DCV] "Validation of Domain Authorization or Control" in "Baseline Requirements for the Issuance and Management of Publicly‐Trusted Certificates", CA/Browser Forum.  https://cabforum.org/baseline-requirements-documents/.
+* [RFC8141] Saint-Andre, P. & Klensin, J., "Uniform Resource Names (URNs)", RFC 8141, ISSN 2070-1721, April 2017.
 
 [REFEDS Metadata Registration Practice Statement template]: https://github.com/REFEDS/MRPS/
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