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 188.8.131.52 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 184.108.40.206 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.
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
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.
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]).
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
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.
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).
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
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.
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 email@example.com 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
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.
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.
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.
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.
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.