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RFC 8141

 
 
 

Uniform Resource Names (URNs)

Part 2 of 3, p. 16 to 30
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3.  URN-Equivalence

3.1.  Procedure

   For various purposes such as caching, it is often desirable to
   determine if two URNs are "the same".  This is done most generally
   (i.e., independent of the scheme) by testing for equivalence (see
   Section 6.1 of [RFC3986]).

   The generic URI specification [RFC3986] is very flexible about
   equality comparisons, putting the focus on allowing false negatives
   and avoiding false positives.  If comparisons are made in a scheme-
   independent way, i.e., as URI comparisons only, many URNs that this
   specification considers equal would be rejected.  The discussion
   below applies when the URIs involved are known to be URNs and thus
   uses the terms "URN-equivalent" and "URN-equivalence" to refer to
   equivalence as specified in this document.

   Two URNs are URN-equivalent if their assigned-name portions are
   octet-by-octet equal after applying case normalization (as specified
   in Section 6.2.2.1 of [RFC3986]) to the following constructs:

   1.  the URI scheme "urn", by conversion to lower case

   2.  the NID, by conversion to lower case

   3.  any percent-encoded characters in the NSS (that is, all character
       triplets that match the <pct-encoding> production found in
       Section 2.1 of the base URI specification [RFC3986]), by
       conversion to upper case for the digits A-F.

   Percent-encoded characters MUST NOT be decoded, i.e., percent-
   encoding normalization (as specified in Section 6.2.2.2 of [RFC3986])
   MUST NOT be applied as part of the comparison process.

   If an r-component, q-component, or f-component (or any combination
   thereof) is included in a URN, it MUST be ignored for purposes of
   determining URN-equivalence.

   URN namespace definitions MAY include additional rules for
   URN-equivalence, such as case insensitivity of the NSS (or parts
   thereof).  Such rules MUST always have the effect of eliminating some
   of the false negatives obtained by the procedure above and MUST NOT
   result in treating two URNs as not "the same" if the procedure here
   says they are URN-equivalent.  For related considerations with regard
   to NID registration, see below.

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3.2.  Examples

   This section shows a variety of URNs (using the "example" NID defined
   in [RFC6963]) that highlight the URN-equivalence rules.

   First, because the scheme and NID are case insensitive, the following
   three URNs are URN-equivalent to each other:

   o  urn:example:a123,z456

   o  URN:example:a123,z456

   o  urn:EXAMPLE:a123,z456

   Second, because the r-component, q-component, and f-component are not
   taken into account for purposes of testing URN-equivalence, the
   following three URNs are URN-equivalent to the first three examples
   above:

   o  urn:example:a123,z456?+abc

   o  urn:example:a123,z456?=xyz

   o  urn:example:a123,z456#789

   Third, because the "/" character (and anything that follows it) in
   the NSS is taken into account for purposes of URN-equivalence, the
   following URNs are not URN-equivalent to each other or to the six
   preceding URNs:

   o  urn:example:a123,z456/foo

   o  urn:example:a123,z456/bar

   o  urn:example:a123,z456/baz

   Fourth, because of percent-encoding, the following URNs are
   URN-equivalent only to each other and not to any of those above (note
   that, although %2C is the percent-encoded transformation of "," from
   the previous examples, such sequences are not decoded for purposes of
   testing URN-equivalence):

   o  urn:example:a123%2Cz456

   o  URN:EXAMPLE:a123%2cz456

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   Fifth, because characters in the NSS other than percent-encoded
   sequences are treated in a case-sensitive manner (unless otherwise
   specified for the URN namespace in question), the following URNs are
   not URN-equivalent to the first three URNs:

   o  urn:example:A123,z456

   o  urn:example:a123,Z456

   Sixth, on casual visual inspection of a URN presented in a human-
   oriented interface, the following URN might appear the same as the
   first three URNs (because U+0430 CYRILLIC SMALL LETTER A can be
   confused with U+0061 LATIN SMALL LETTER A), but it is not
   URN-equivalent to the first three URNs:

   o  urn:example:%D0%B0123,z456

4.  URI Conformance

4.1.  Use in URI Protocol Slots

   Because a URN is, syntactically, a URI under the "urn" scheme, in
   theory a URN can be placed in any protocol slot that allows for a URI
   (to name just a few, the "href" and "src" attributes in HTML, the
   base element in HTML, the "xml:base" attribute in XML [XML-BASE], and
   the "xmlns" attribute in XML for XML namespace names [XML-NAMES]).

   However, this does not imply that, semantically, it always makes
   sense in practice to place a URN in a given URI protocol slot; in
   particular, because a URN might not specify the location of a
   resource or even point indirectly to one, it might not be appropriate
   to place a URN in a URI protocol slot that points to a resource
   (e.g., the aforementioned "href" and "src" attributes).

   Ultimately, guidelines regarding when it is appropriate to use URIs
   under the "urn" scheme (or any other scheme) are the responsibility
   of specifications for individual URI protocol slots (e.g., the
   specification for the "xml:base" attribute in XML might recommend
   that it is inappropriate to use URNs in that protocol slot).  This
   specification cannot possibly anticipate all of the relevant cases,
   and it is not the place of this specification to require or restrict
   usage for individual protocol slots.

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4.2.  Parsing

   In part because of the separation of URN semantics from more general
   URI syntax, generic URI processors need to pay special attention to
   the parsing and analysis rules of RFC 3986 and, in particular, must
   treat the URI as opaque unless the scheme and its requirements are
   recognized.  In the latter case, such processors may be in a position
   to invoke scheme-appropriate processing, e.g., by a URN resolver.  A
   URN resolver can either be an external resolver that the URI resolver
   knows of or be functionality built into the URI resolver.  Note that
   this requirement might impose constraints on the contexts in which
   URNs are appropriately used; see Section 4.1.

4.3.  URNs and Relative References

   Section 5.2 of [RFC3986] describes an algorithm for converting a URI
   reference that might be relative to a given base URI into "parsed
   components" of the target of that reference, which can then be
   recomposed per RFC 3986, Section 5.3 into a target URI.  This
   algorithm is problematic for URNs because their syntax does not
   support the necessary path components.  However, if the algorithm is
   applied independent of a particular scheme, it should work
   predictably for URNs as well, with the following understandings
   (syntax production terminology taken from RFC 3986):

   1.  A system that encounters a <URI-reference> that obeys the syntax
       for <relative-ref>, whether it explicitly has the scheme "urn" or
       not, will convert it into a target URI as specified in RFC 3986.

   2.  Because of the persistence and stability expectations of URNs,
       authors of documents, etc., that utilize URNs should generally
       avoid the use of the "urn" scheme in any <URI-reference> that is
       not strictly a <URI> as specified in RFC 3986, specifically
       including those that would require processing of <relative-ref>.

4.4.  Transport and Display

   When URNs are transported and exchanged, they MUST be represented in
   the format defined herein.  Further, URN-aware applications are
   strongly encouraged to offer the option of displaying URNs in this
   canonical form to allow for direct transcription (for example by
   copy-and-paste techniques).  Such applications might support the
   display of URNs in a more human-friendly form and might use a
   character set that includes characters that are not permitted in URN
   syntax as defined in this specification (e.g., when displaying URNs
   to humans, such applications might replace percent-encoded strings
   with characters from an extended character repertoire such as Unicode
   [UNICODE]).

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   To minimize user confusion, any application displaying URIs SHOULD
   display the complete URI (including, for URNs, the "urn" scheme and
   any components) to ensure that there is no confusion between URN NIDs
   and URI scheme identifiers.  For example, a URI beginning with
   "urn:xmpp:" [RFC4854] is very different from a URI beginning with
   "xmpp:" [RFC5122].  Similarly, a potential Digital Object Identifier
   (DOI) URI scheme [DOI-URI] is different from, and possibly completely
   unrelated to, a possible DOI URN namespace.

4.5.  URI Design and Ownership

   As mentioned, the assignment of URNs within a URN namespace is a
   managed process, as is the assignment of URN namespaces themselves.
   Although design of the URNs to be assigned within a given URN
   namespace is ceded by this specification to the URN namespace
   manager, doing so in a managed way avoids the problems inherent in
   unmanaged generation of URIs as described in the recommendations
   regarding URI design and ownership [RFC7320].

5.  URN Namespaces

   A URN namespace is a collection of names that obey three constraints:
   each name is (1) unique, (2) assigned in a consistent way, and (3)
   assigned according to a common definition.

   1.  The "uniqueness" constraint means that a name within the URN
       namespace is never assigned to more than one resource and never
       reassigned to a different resource (for the kind of "resource"
       identified by URNs assigned within the URN namespace).  This
       holds true even if the name itself is deprecated or becomes
       obsolete.

   2.  The "consistent assignment" constraint means that a name within
       the URN namespace is assigned by an organization or created in
       accordance with a process or algorithm that is always followed.

   3.  The "common definition" constraint means that there are clear
       definitions for the syntax of names within the URN namespace and
       for the process of assigning or creating them.

   A URN namespace is identified by a particular NID in order to ensure
   the global uniqueness of URNs and, optionally, to provide a cue
   regarding the structure of URNs assigned within a URN namespace.

   With regard to global uniqueness, using different NIDs for different
   collections of names ensures that no two URNs will be the same for
   different resources, because each collection is required to uniquely
   assign each name.  However, a single resource MAY have more than one

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   URN assigned to it, either in the same URN namespace (if the URN
   namespace permits it) or in different URN namespaces, and for either
   similar purposes or different purposes.  (For example, if a publisher
   assigns an ISBN [RFC3187] to an electronic publication and that
   publication is later incorporated into a digital long-term archive
   operated by a national library, the library might assign the
   publication a national bibliography number (NBN) [RFC3188], resulting
   in two URNs referring to the same book.)  Subject to other
   constraints, such as those imposed by the URI syntax [RFC3986], the
   rules of the URN scheme are intended to allow preserving the normal
   and natural form of names specified in non-URN identifier systems
   when they are treated as URNs.

   With regard to the structure of names assigned within a URN
   namespace, the development of a naming structure (and thereby a
   collection of names) depends on the requirements of the community
   defining the names, how the names will be assigned and used, etc.
   These issues are beyond the scope of URN syntax and the general rules
   for URN namespaces, because they are specific to the community
   defining a non-URN identifier system or a particular URN namespace
   (e.g., the bibliographic and publishing communities in the case of
   the "ISBN" URN namespace [RFC3187] and the "ISSN" URN namespace
   [RFC3044] or the developers of extensions to the Extensible Messaging
   and Presence Protocol [RFC6120] in the case of the "XMPP" URN
   namespace [RFC4854]).

   Because the colon character (":") is used to separate "urn" from the
   NID and the NID from the NSS, it's tempting to think of the entire
   URN as being structured by colon characters and to assume that colons
   create a structure or hierarchy within the NSS portion of the URN.
   Such structure could be specified by a particular NID specification,
   but there is no implicit structure.  In a URN such as

      urn:example:apple:pear:plum:cherry

   the NSS string is "apple:pear:plum:cherry" as a whole, and there is
   no specific meaning to the colon characters within that NSS string
   unless such meaning is described in the specification of the
   "example" namespace.

   URN namespaces inherit certain rights and responsibilities by the
   nature of URNs, in particular:

   1.  They uphold the general principles of a well-managed URN
       namespace by providing persistent identification of resources and
       unique assignment of names in accordance with a common
       definition.

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   2.  Optionally, they can be registered in global registration
       services such as those described in [RFC2483].

   There are two types of URN namespaces: formal and informal.  These
   are distinguished by the expected level of service, the information
   needed to define the URN namespace, and the procedures for
   registration.  Because the majority of the URN namespaces registered
   so far have been formal, this document concentrates on formal URN
   namespaces.

5.1.  Formal URN Namespaces

   A formal URN namespace provides benefit to some subset of users on
   the Internet.  In particular, it would not make sense for a formal
   URN namespace to be used only by a community or network that is not
   connected to the Internet.  For example, it would be inappropriate
   for a URN namespace to effectively force someone to use a proprietary
   network or service not open to the general Internet user.  The intent
   is that, while the community of those who might actively use the URNs
   assigned within that URN namespace might be small, the potential use
   of names within that URN namespace is open to any user on the
   Internet.  Formal URN namespaces might be appropriate even when some
   aspects are not fully open.  For example, a URN namespace might make
   use of a fee-based, privately managed, or proprietary registry for
   assignment of URNs in the URN namespace.  However, it might still
   benefit some Internet users if the associated services have openly
   published names.

   An organization that will assign URNs within a formal URN namespace
   SHOULD meet the following criteria:

   1.  Organizational stability and the ability to maintain the URN
       namespace for a long time; absent such evidence, it ought to be
       clear how the URN namespace can remain viable if the organization
       can no longer maintain the URN namespace.

   2.  Competency in URN assignment.  This will improve the likelihood
       of persistence (e.g., to minimize the likelihood of conflicts).

   3.  Commitment to not reassigning existing URNs and to allowing old
       URNs to continue to be valid (e.g., if the assignee of a URN is
       no longer a member or customer of the assigning organization, if
       various information about the assignee or named entity happens to
       change, or even if the assignee or the named entity itself is no
       longer in existence; in all these cases, the URN is still valid).

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   A formal URN namespace establishes a particular NID, subject to the
   following constraints (above and beyond the syntax rules already
   specified):

   1.  It MUST NOT be an already-registered NID.

   2.  It MUST NOT start with "urn-" (which is reserved for informal URN
       namespaces).

   3.  It MUST be more than two characters long, and it MUST NOT start
       with ALPHA ALPHA "-", i.e., any string consisting of two letters
       followed by one hyphen; such strings are reserved for potential
       use as NIDs based on ISO alpha-2 country codes [ISO.3166-1] for
       eventual national registrations of URN namespaces (however, the
       definition and scoping of rules for allocation of responsibility
       for such country-code-based URN namespaces are beyond the scope
       of this document).  As a consequence, it MUST NOT start with the
       string "xn--" or any other string consisting of two letters
       followed by two hyphens; such strings are reserved for potential
       representation of DNS A-labels and similar strings in the future
       [RFC5890].

   4.  It MUST NOT start with the string "X-" so that it will not be
       confused with or conflict with any experimental URN namespace
       previously permitted by [RFC3406].

   Applicants and reviewers considering new NIDs should also be aware
   that they may have semantic implications and hence be a source of
   conflict.  Particular attention should be paid to strings that might
   be construed as identifiers for, or registered under the authority
   of, countries (including ISO 3166-1 alpha-3 codes) and to strings
   that might imply association with existing URI schemes, non-URN
   identifier systems, or trademarks.  However, in line with traditional
   policies, disputes about "ownership" of particular strings are
   disagreements among the parties involved; neither IANA nor the IETF
   will become involved in such disputes except in response to orders
   from a court of competent jurisdiction.

5.2.  Informal URN Namespaces

   Informal URN namespaces are full-fledged URN namespaces, with all the
   associated rights and responsibilities.  Informal URN namespaces
   differ from formal URN namespaces in the process for assigning the
   NID: for an informal URN namespace, the registrant does not designate
   the NID; instead, IANA assigns the NID consisting of the string
   "urn-" followed by one or more digits (e.g., "urn-7") where the

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   digits consist of the next available number in the sequence of
   positive integers assigned to informal URN namespaces.  Thus, the
   syntax of an informal URN namespace identifier is:

       InformalNamespaceName = "urn-" Number
       Number                = DigitNonZero 0*Digit
       DigitNonZero          = "1"/ "2" / "3" / "4"/ "5"
                             / "6" / "7" / "8" / "9"
       Digit                 = "0" / DigitNonZero

   The only restrictions on <Number> are that it (1) consist strictly of
   ASCII digits, (2) not have leading zeros, and (3) not cause the NID
   to exceed the length limitations defined for the URN syntax (see
   Section 2).

6.  Defining and Registering a URN Namespace

6.1.  Overview

   Because the space of URN namespaces is itself managed, the definition
   of a URN namespace SHOULD pay particular attention to:

   1.  The purpose of the URN namespace.

   2.  The syntax of URNs assigned within the URN namespace, including
       the internal syntax and anticipated effects of r-components or
       q-components.  (The syntax and interpretation of f-components are
       defined in RFC 3986.)

   3.  The process for assigning URNs within the URN namespace.

   4.  The security implications of assigning URNs within the URN
       namespace and of using the assigned URNs.

   5.  Any potential interoperability issues with URNs assigned within
       the URN namespace.

   6.  Optionally, the process for resolving URNs assigned within the
       URN namespace.

   The section on completing the template (Section 6.4) explains these
   matters in greater detail.  Although the registration templates are
   the same in all cases, slightly different procedures are used
   depending on the source of the registration.

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6.2.  Registration Policy and Process: Community Registrations

   The basic registration policy for URN namespaces is Expert Review as
   defined in the IANA Considerations document [RFC5226].  For URN
   namespaces or their definitions that are intended to become standards
   or constituent parts of standards, the output of the Expert Review
   process is intended to be a report rather than instructions to IANA
   to take action (see below).  The key steps are:

   1.  Fill out the URN namespace registration template (see Section 6.4
       and Appendix A).  This can be done as part of an Internet-Draft
       or a specification in another series, although that is not a
       requirement.

   2.  Send the completed template to the urn@ietf.org discussion list
       for review.

   3.  If necessary to address comments received, repeat steps 1 and 2.

   4.  If the Designated Experts approve the request and no
       standardization action is involved, the IANA will register the
       requested NID.  If standardization is anticipated, the Designated
       Experts will prepare a report and forward it to the appropriate
       standards approval body (the IESG in the case of the IETF); IANA
       will register the requested NID only after receiving directions
       from that body and a copy of the Expert Review report.

   A URN namespace registration can be revised by updating the
   registration template, following the same steps outlined above for
   new registrations.  A revised registration MUST describe differences
   from prior versions and SHOULD make special note of any relevant
   changes in the underlying technologies or URN namespace management
   processes.

   Experience to date with URN namespace registration requests has shown
   that registrants sometimes do not initially understand some of the
   subtleties of URN namespaces and that defining the URN namespace in
   the form of a specification enables the registrants to clearly
   formulate their "contract" with the intended user community.
   Therefore, although the registration policy for formal URN namespaces
   is Expert Review and a specification (as distinct from the
   registration template) is not strictly required, registrants SHOULD
   provide a stable specification documenting the URN namespace
   definition and expanding upon the issues described herein.

   Because naming can be difficult and contentious, URN namespace
   registrants and the Designated Experts are strongly encouraged to
   work together in a spirit of good faith and mutual understanding to

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   achieve rough consensus (see [RFC7282]) on handling registration
   requests.  They are also encouraged to bring additional expertise
   into the discussion if that would be helpful in providing perspective
   or otherwise resolving issues.

   Especially when iterations in the registration process are prolonged,
   Designated Experts are expected to take reasonable precautions to
   avoid "race conditions" on proposed NIDs and, if such situations
   arise, to encourage applicants to work out any conflicts among
   themselves.

6.3.  Registration Policy and Process: Fast Track for Standards
      Development Organizations, Scientific Societies, and Similar
      Bodies

   The IETF recognizes that situations will arise in which URN
   namespaces will be created to either embed existing and established
   standards, particularly identifier standards, or reflect knowledge,
   terminology, or methods of organizing information that lie well
   outside the IETF's scope or the likely subject matter knowledge of
   its Designated Experts.  In situations in which the registration
   request originates from, or is authorized by, a recognized standards
   development organization, scientific society, or their designees, a
   somewhat different procedure is available at the option of that body:

   1.  The URN namespace registration template is filled out and
       submitted as in steps 1 and 2 of Section 6.2.

   2.  A specification is required that reflects or points to the needed
       external standards or specifications.  Publication in the RFC
       Series or through an IETF process (e.g., posting as an Internet-
       Draft) is not expected and would be appropriate only under very
       unusual circumstances.

   3.  The reviews on the discussion list and by the Designated Experts
       are strictly advisory, with the decisions about what advice to
       accept and the length of time to allocate to the process strictly
       under the control of the external body.

   4.  When that body concludes that the application is sufficiently
       mature, its representative(s) will request that IANA complete the
       registration for the NID, and IANA will do so.

   Decisions about whether to recognize the requesting entity as a
   standards development organization or scientific society are the
   responsibility of the IESG.

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   A model similar to this has already been defined for recognized
   standards development organizations that wish to register media
   types.  The document describing that mechanism [RFC6838] provides
   somewhat more information about the general approach.

6.4.  Completing the Template

   A template for defining and registering a URN namespace is provided
   in Appendix A.  This section describes considerations for completing
   the template.

6.4.1.  Purpose

   The "Purpose" section of the template describes matters such as:

   1.  The kinds of resources identified by URNs assigned within the URN
       namespace.

   2.  The scope and applicability of the URNs assigned within the URN
       namespace; this might include information about the community of
       use (e.g., a particular nation, industry, technology, or
       organization), whether the assigned URNs will be used on public
       networks or private networks, etc.

   3.  How the intended community (and the Internet community at large)
       will benefit from using or resolving the assigned URNs.

   4.  How the URN namespace relates to and complements existing URN
       namespaces, URI schemes, and non-URN identifier systems.

   5.  The kinds of software applications that can use or resolve the
       assigned URNs (e.g., by differentiating among disparate URN
       namespaces, identifying resources in a persistent fashion, or
       meaningfully resolving and accessing services associated with the
       URN namespace).

   6.  Whether resolution services are available or will be available
       (and, if so, the nature or identity of the services).  Examples
       of q-component and (when they are standardized) r-component
       semantics and syntax are helpful here, even if detailed
       definitions are provided elsewhere or later.

   7.  Whether the URN namespace or its definition is expected to become
       a constituent part of a standard being developed in the IETF or
       some other recognized standards body.

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6.4.2.  Syntax

   The "Syntax" section of the template contains:

   1.  A description of the structure of URNs within the URN namespace,
       in conformance with the fundamental URN syntax.  The structure
       might be described in terms of a formal definition (e.g., using
       ABNF [RFC5234]), an algorithm for generating conformant URNs, or
       a regular expression for parsing the name into constituent parts;
       alternatively, the structure might be opaque.

   2.  Any special character encoding rules for assigned URNs (e.g.,
       which character ought to always be used for quotes).

   3.  Rules for determining URN-equivalence between two names in the
       URN namespace.  Such rules ought to always have the effect of
       eliminating false negatives that might otherwise result from
       comparison.  If it is appropriate and helpful, reference can be
       made to particular equivalence rules defined in the URI
       specification [RFC3986] or to Section 3 of this document.
       Examples of URN-equivalence rules include equivalence between
       uppercase and lowercase characters in the NSS, between hyphenated
       and non-hyphenated groupings in the name, or between single
       quotes and double quotes.  There may also be namespace-specific
       special encoding considerations, especially for URNs that contain
       embedded forms of names from non-URN identifier systems.  (Note
       that these are not normative statements for any kind of best
       practice related to handling of relationships between characters
       in general; such statements are limited to one particular URN
       namespace only.)

   4.  Any special considerations necessary for conforming with the URN
       syntax.  This is particularly applicable in the case of existing,
       non-URN identifier systems that are used in the context of URNs.
       For example, if a non-URN identifier system is used in contexts
       other than URNs, it might make use of characters that are
       reserved in the URN syntax.  This section ought to note any such
       characters and outline necessary mappings to conform to URN
       syntax.  Normally, this will be handled by percent-encoding the
       character as specified in Section 2.1 of the URI specification
       [RFC3986] and as discussed in Section 1.2.2 of this
       specification.

   5.  Any special considerations for the meaning of q-components (e.g.,
       keywords) or f-components (e.g., predefined terms) in the context
       of this URN namespace.

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6.4.3.  Assignment

   The "Assignment" section of the template describes matters such as:

   1.  Mechanisms or authorities for assigning URNs to resources.  It
       ought to make clear whether assignment is completely open (e.g.,
       following a particular procedure such as first-come, first-served
       (FCFS)), completely closed (e.g., for a private organization), or
       limited in various ways (e.g., delegated to authorities
       recognized by a particular organization); if limited, it ought to
       explain how to become an assigner of names or how to request
       assignment of names from existing assignment authorities.

   2.  Methods for ensuring that URNs within the URN namespace are
       unique.  For example, names might be assigned sequentially or in
       accordance with some well-defined process by a single authority,
       assignment might be partitioned among delegated authorities that
       are individually responsible for respecting uniqueness rules, or
       URNs might be created independently following an algorithm that
       itself guarantees uniqueness.

6.4.4.  Security and Privacy

   The "Security and Privacy" section of the template describes any
   potential issues related to security and privacy with regard to
   assignment, use, and resolution of names within the URN namespace.
   Examples of such issues include:

   o  The consequences of producing false negatives and false positives
      during comparison for URN-equivalence (see Section 3.1 of this
      specification and "Issues in Identifier Comparison for Security
      Purposes" [RFC6943]).

   o  Leakage of private information when names are communicated on the
      public Internet.

   o  The potential for directory harvesting.

   o  Various issues discussed in the guidelines for security
      considerations in RFCs [RFC3552] and the privacy considerations
      for Internet protocols [RFC6973].  In particular, note the privacy
      considerations text for the Global System for Mobile
      Communications Association (GSMA) / International Mobile station
      Equipment Identity (IMEI) namespace [RFC7254], which may provide a
      useful model for such cases.

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6.4.5.  Interoperability

   The "Interoperability" section MUST specify any known potential
   issues related to interoperability.  Examples include possible
   confusion with other URN namespaces, non-URN identifier systems, or
   URI schemes because of syntax (e.g., percent-encoding of certain
   characters) or scope (e.g., overlapping areas of interest).  If at
   all possible, concerns that arise during the registration of a URN
   namespace (e.g., due to the syntax or scope of a non-URN identifier
   system) should be resolved as part of or in parallel to the
   registration process.

6.4.6.  Resolution

   The "Resolution" section MUST specify whether resolution mechanisms
   are intended or anticipated for URNs assigned within the URN
   namespace.

   If resolution is intended, then this section SHOULD specify whether
   the organization that assigns URNs within the URN namespace intends
   to operate or recommend any resolution services for URNs within that
   URN namespace.  In addition, if the assigning organization intends to
   implement registration for publicly advertised resolution services
   (for example, using a system developed in the spirit of the original
   architectural principles and service descriptions for URN resolution
   [RFC2276] [RFC2483]), then this section SHOULD list or reference the
   requirements for being publicly advertised by the assigning
   organization.  In addition, this section SHOULD describe any special
   considerations for the handling of r-components in the context of
   this URN namespace.

6.4.7.  Additional Information

   The "Additional Information" section includes information that would
   be useful to those trying to understand this registration or its
   relationship to other registrations, such as comparisons to existing
   URN namespaces that might seem to overlap.

   This section of the template is optional.


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