URNBIS P. Saint-Andre
Internet-Draft &yet
Obsoletes: 2141, 3406 (if approved) J. Klensin
Intended status: Standards Track November 1, 2015
Expires: May 4, 2016

Uniform Resource Names (URNs)


A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that is assigned under the "urn" scheme and a particular URN namespace, with the intent that the URN will be either a persistent, location-independent resource identifier or in some cases an abstract designator that is persistent but that does not identify a resource. With regard to URN syntax, this document defines the canonical syntax for URNs (in a way that is consistent with URI syntax), specifies methods for determining URN equivalence, and discusses URI conformance. With regard to URN namespaces, this document specifies a method for defining a URN namespace and associating it with a namespace identifier, and describes procedures for registering namespace identifiers with the Internet Assigned Numbers Authority (IANA). This document obsoletes both RFC 2141 and RFC 3406.

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Table of Contents

1. Introduction

A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) [RFC3986] that is assigned under the "urn" scheme and a particular namespace, with the intent that the URN will be either a persistent, location-independent resource identifier or in some cases an abstract designator that is persistent but that does not identify a resource. A URN namespace is a collection of such identifiers, each of which is (1) unique, (2) assigned in a consistent and managed way, and (3) assigned according to a common definition. (Some URN namespaces create names that exist only as URNs, whereas others create URNs out of names that already exist in other identifier systems, such as ISBNs [RFC3187] and ISSNs [RFC3044].)

The assignment of URNs is done by an organization (or, in some cases, according to an algorithm or other automated process) that has been formally delegated a namespace within the "urn" scheme (e.g., a URN in the 'example' namespace [RFC6963] might be of the form "urn:example:foo").

This document rests on two key assumptions:

  1. Assignment of a URN is a managed process.
  2. The space of URN namespaces is itself managed.

While other URI schemes may allow identifiers to be freely chosen and assigned, such is not the case for URNs. The syntactical correctness of a string starting with "urn:" is not sufficient to make it a URN. In order for the string to be a valid URN, the namespace identifier needs to be registered in accordance with the rules defined here and the remaining parts of the assigned-name portion of the URN needs to be generated in accordance with the rules for the registered namespace.

So that information about both URN syntax and URN namespaces is available in one place, this document does the following:

  1. Defines the canonical syntax for URNs in general (in a way that is consistent with URI syntax), specifies methods for determining URN equivalence, and discusses URI conformance.
  2. Specifies a method for defining a URN namespace and associating it with a namespace identifier, and describes procedures for registering namespace identifiers with the Internet Assigned Numbers Authority (IANA).

For URN syntax and URN namespaces, this document modernizes and replaces the definitions from [RFC2141] and [RFC3406]. These modifications build on the key requirements provided in [RFC1737] and many years of experience with URNs, in both cases attempting to make the smallest reasonable set of changes from the previous definitions. The intent is to define URNs in a consistent manner so that, wherever practical, the parsing, handling, and resolution of URNs can be independent of the namespace within which a given URN is assigned.

This document obsoletes both [RFC2141] and [RFC3406].

2. Terminology

This document uses the terms "resolution" and "resolver" in roughly the sense from [RFC2276], i.e., "resolution" is the act of supplying services related to the identified resource, such as translating the persistent name into one or more current locators for the resource, or delivering metadata about the resource in an appropriate format. At the time of this writing, resolution services are defined in [RFC2483]. In order to underline the difference between the names and locators, this document uses the term Uniform Resource Locator (URL), rather than the generic term Uniform Resource Identifier (URI), to refer to locators; see also Section 1.1.3 of [RFC3986].

If there are or will be resolution services available for a URN, this document calls the URN a resource identifier in roughly the sense from [RFC3986]. If there is no intention to provide any resolution services, this document calls the URN an abstract designator.

Several other important terms used in this document, including some "normalization" operations that are not part of the Unicode Standard [UNICODE], are defined in the URI specification [RFC3986].

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 [RFC2119].

3. URN Syntax

The syntax of URNs as provided in [RFC2141] was defined before the updated specification of URIs in [RFC3986]. The definition of URN syntax is updated in this document to do the following:

In particular, this specification does the following:

However, this specification does not extend the URN syntax to allow characters outside the ASCII range [RFC20], which implies that any such characters need to be percent-encoded as described in Section 2.1 of the URI specification [RFC3986].

The syntax for a URN is defined as follows using the Augmented Backus-Naur Form (ABNF) as specified in [RFC5234]. Rules not defined here (specifically: alphanum, fragment, and pchar) are defined as part of the URI syntax [RFC3986] and used here to point out the syntactic relationship with the terms used there.

   namestring    = assigned-name
                   [ "?" q-component ]
                   [ "??" r-component ]
                   [ "#" f-component ]
   assigned-name = "urn" ":" NID ":" NSS
   NID           = (alphanum) 0*30(ldh) (alphanum)
   ldh           = alphanum / "-"
   NSS           = pchar *(pchar / "/")
   q-component   = pchar *( pchar / "/" / "?" )
   r-component   = pchar *( pchar / "/" / "?" )
   f-component   = fragment

The question mark character "?" can be used without percent-encoding inside q-components, r-components, and f-components. However, it is likely to cause confusion if question marks (especially consecutive ones) are placed inside the r-component, or if a question mark is placed at the start or end of an r-comonents or q-component; therefore such usage is discouraged although not prohibited.

The following sections provide additional information about the syntactic elements of URNs.

3.1. Namespace Identifier

The syntax here is slightly more restrictive than what was defined in [RFC2141], because it prohibits the character "-" at the end of a NID.

Note that NIDs are case insensitive (e.g., "ISBN" and "isbn" are equivalent).

Characters outside the ASCII range [RFC20] are not permitted in NIDs, and no encoding mechanism for such characters is supported.

3.2. Namespace Specific String

The namespace specific string (NSS) is a unique identifier that is assigned and managed in a consistent way and that conforms to the definition of the relevant namespace. The combination of the NID (unique across the entire "urn" scheme) and the NSS (unique within the namespace) ensures that the resulting URN is a globally unique URI.

This document modifies the syntax of the NSS to allow the following characters: "/", "~", and "&". In particular, allowing the "/" character effectively makes it possible to encapsulate hierarchical identifiers from other naming systems. For instance, consider the hypothetical example of a hierarchical naming system in which the identifiers take the form of a series of numbers separated by the "/" character, such as "1/406/47452/2". If the naming authority for such identifiers were to use URNs, it would be natural to place the existing identifiers in the NSS, resulting in URNs such as "urn:example:1/406/47452/2".

However, the foregoing changes to the syntax for the NSS do not modify the encoding rules for URN namespaces that were defined in accordance with [RFC2141]. If any such URN namespace that is used outside of the URN context (i.e., as a standalone identifier space) also allows the use of "/", "~", or "&" in the native form within that identifier space, then the encoding rules for that namespace are not changed by this specification.

Depending on the rules governing a namespace, names that are valid in a namespace might contain characters that are not allowed by the "pchar" production referenced above (e.g., characters outside the ASCII range or, consistent with the restrictions in RFC 3986, the characters "/", "?", "#", "[", and "]"). While such a string might be a valid name, it is not a valid URN until it has been translated into a conformant NSS. In the case of URNs that are formed from names that exist separately in a standalone identifier space, translation of an identifier from its "native" format to URN format is accomplished by using the canonicalization and encoding methods defined for that URN namespace. Software that is not aware of those namespace-specific canonicalization and encoding rules MUST NOT construct URNs from the names in the standalone identifier space.

In order to make URNs as stable and persistent as possible when protocols evolve and the environment around them changes, namespaces SHOULD NOT allow characters outside the basic Latin repertoire [RFC20] unless the nature of the particular namespace makes such characters necessary.

3.3. Optional Components

As compared to [RFC2141], this document extends the URN syntax to permit inclusion of three new types of components: q-component, r-component, and f-component. Because this specification focuses almost exclusively on URN syntax, it does not define detailed semantics for these components for URNs in general. However, each of these components has a distinct role, which is independent of the URN and its namespace, and it is intended that clients will be able to handle these components uniformly for all URNs. In general:

The foregoing generalizations are true regardless of the URN's namespace.

Whenever a URN resolves to a URL which may be used to access the resource, there is a more specific interpretation of q-component and f-component: the q-component is copied verbatim to the query portion of the URL (if that URL scheme supports query), and the f-component is copied verbatim to the fragment portion of the URL. This is necessary, among other reasons, so that interpretation of q-component and f-component, when associated with URNs, will be consistent with the interpretation of relative references containing queries or fragments within documents which are ultimately accessible via URNs. Thus, for URNs which may be resolved to a URL, the semantics of q-component are identical to those for queries to that resource and the semantics of f-components are identical to those of fragments for that resource. The semantics of q-component and f-component for URNs that inherently cannot be resolved to a URL (i.e., for abstract designators) are undefined by this document; however, they SHOULD be consistent with the above roles. It is expected that additional specifications will define the semantics of r-components.

Note: In general, neither the syntax nor the semantics of q-components or f-components are defined by, or dependent on, the namespace of the URN. A particular namespace might, however, define uses of r-components that are specific to its namespace and supported by the resolution services which that namespace operates or recommends.

Any of these components MAY be used with URNs from existing namespaces whether or not the namespace explicitly supports them. As described above, the interpretation of q-component and f-component is namespace-independent. Interpretation of an r-component may be namespace-dependent to some degree if it relies on behavior of a namespace-specific resolution service. If a URN contains r-components that are not defined by the namespace, the meaning of the URN and result of resolution are out of scope for this specification.

3.3.1. q-component

The URN q-component has the same syntax as the URI query component. If a URN resolves to a URL with a scheme that supports a query component, the q-component from the URN is copied verbatim to the query component of the URL. If the URN does not resolve to a URL (i.e., is an abstract designator), the interpretation of the q-component is undefined by this specification.

The q-component is indicated by the first question mark ("?") character and terminated by a double question mark ("??") indicating an r-component, by a number sign ("#") character indicating an f-component, or by the end of the URI. The characters slash ("/") and question mark ("?") may represent data within the q-component. Note that characters outside the ASCII range [RFC20] MUST be percent-encoded using the method defined in Section 2.1 of the generic URI specification [RFC3986].

As described under Section 4, the q-component SHALL NOT be taken into account when determining URN equivalence.

Similarly, the q-component SHALL NOT be taken into account when resolving a URN to a URL and MUST NOT be used to communicate parameters to a resolution service.

Consider the hypothetical example of passing parameters to an application that returns weather reports from different regions and/or for different time periods. This could perhaps be accomplished by specifying lat/long coordinates and datetimes in the URN's q-component, resulting in URNs such as the following.


However, this primary purpose is not intended to forestall other potential uses for q-components for URNs that do not resolve to URLs.

3.3.2. r-component

The URN r-component has no syntactic equivalent in URIs.

The r-component is indicated by a double question mark ("??") and terminated by a number sign ("#") character indicating an f-component or by the end of the URI. The characters slash ("/") and question mark ("?") may represent data within the r-component. Note that characters outside the ASCII range [RFC20] MUST be percent-encoded using the method defined in Section 2.1 of the generic URI specification [RFC3986].

As described under Section 4, the r-component SHALL NOT be taken into account when determining URN equivalence.

However, the r-component SHALL be supplied along with the URN when presenting a request to a URN resolution service.

The r-component is entirely intended for passing information in requests to URN resolution services. The r-component is not intended for passing information to the resources identified by a URN or to applications that manage such resources, since that is the function of the q-component. In addition, the r-component is not intended for passing information to any underlying services that might exist behind a resolver, only to the resolver itself.

This document defines only the syntax of the r-component and reserves it for future use. The exact semantics of the r-component and its use in URN resolution protocols are a matter for potential standardization in separate specifications.

Consider the hypothetical example of passing parameters to resolution service (say, an ISO alpha-2 country code [ISO3166-1] in order to scope down the preferred country in which to search for a physical copy of a book). This could perhaps be accomplished by specifying the country code in the r-component, resulting in URNs such as:


However, this primary purpose is not intended to forestall other potential uses for r-components.

3.3.3. f-component

The URN f-component has the same syntax as the URI fragment component. When a URN containing an f-component resolves to a URL, the f-component from the URN is copied verbatim into the fragment of that URL. If the URN does not resolve to a URL (i.e., is an abstract designator), the interpretation of the f-component is undefined by this specification.

The f-component is indicated by the presence of a number sign ("#") character and terminated by the end of the URI. Note that characters outside the ASCII range [RFC20] MUST be percent-encoded using the method defined in Section 2.1 of the generic URI specification [RFC3986].

As described under Section 4, the f-component SHALL NOT be taken into account when determining URN equivalence.

Similarly, the f-component MUST NOT be passed to resolution servers when querying them for resource locations or metadata.

The f-component is primarily intended to distinguish the constituent parts of resources named by URNs. Thus, for URNs that resolve to URLs, the semantics of an f-component are defined by the media type of those resources, not by the namespace.

Consider the hypothetical example of obtaining resources that are part of a larger entity (say, the chapters of a book). Each part could be specified in the f-component, resulting in URNs such as:


However, this primary purpose is not intended to forestall other potential uses for f-components for URNs that do not resolve to URLs.

4. Equivalence of URNs

4.1. Procedure

For various purposes such as caching, often it is desirable to determine if two URNs are "the same". This is done 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, URNs that this specification considers equal would be rejected. The discussion below applies when the URIs involved are known to be URNs.

Two URNs are equivalent if their <assigned-name> portions are octet-by-octet equal after applying case normalization (as specified in Section 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 of [RFC3986]) MUST NOT be applied.

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

URN namespace definitions MAY include additional rules for 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 equivalent if the procedure here says they are equivalent. For related considerations with regard to NID registration, see below.

4.2. Examples

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

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

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

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

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

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

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 equivalent:

5. URI Conformance

5.1. 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 (e.g., 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.

5.2. Parsing

In part because of the separation of semantics from syntax [I-D.ietf-urnbis-semantics-clarif], 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 which case they may be in a position to invoke scheme-appropriate processing such as by a URN resolver. The URN resolver can either be an external resolver that the URI resolver knows of, or it can 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 5.1.

5.3. URNs and Relative References

[RFC3986] Section 5.2 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 cannot be applied directly to URNs because their syntax does not support the necessary path components. The notion of a URN as a "persistent", "permanent" identifier does not reconcile easily with relative referencing. However, resources named with URNs may contain relative references that do not apply to the URN itself.

Therefore a relative reference SHOULD NOT be evaluated directly with respect to a URN. Instead, a relative reference SHOULD be evaluated indirectly with respect to one of the following:

  1. a base URI (other than a URN) declared by the resource itself; or
  2. a base URI (other than a URN) obtained through the URN resolution process; or
  3. the URL of the resource as obtained through the URN resolution process

(Case 2 permits the resolution process to explicitly supply a base URI if the resource content is supplied directly by the resolution service rather than via an intermediate "location" URI.)

If no such base URI exists, use of a relative reference with respect to a URN is an error. Client behavior in this case is undefined.

Resolution services SHOULD ensure that a base URI is supplied any time they provide resource content directly to a client.

5.4. Transport and Display

When URNs are transported and exchanged, they MUST be represented in the format defined herein. Further, all URN-aware applications MUST offer the option of displaying URNs in this canonical form to allow for direct transcription (for example by cut-and-paste techniques). Such applications might support 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, a URI browser SHOULD display the complete URN (including the "urn" scheme and any components) to ensure that there is no confusion between URN namespace identifiers 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 DOI URI scheme [DOI-URI] is different from, and possibly completely unrelated to, a possible DOI URN namespace.

5.5. URI Design and Ownership

As mentioned, the assignment of URNs is a managed process, as is the assignment of namespaces themselves. Although design of the URNs to be assigned within a given namespace is ceded by this specification to the namespace owner, 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].

6. URN Namespaces

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

  1. The "uniqueness" constraint means that an identifier within the 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 namespace). This holds true even if the identifier itself is deprecated or becomes obsolete.
  2. The "consistent assignment" constraint means that an identifier within the 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 identifiers within the 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 namespace.

With regard to global uniqueness, using different NIDs for different collections of identifiers ensures that no two URNs will be the same for different resources, since each collection is required to uniquely assign each identifier. However, a single resource MAY have more than one URN assigned to it, either in the same namespace (if the namespace permits it) or in different namespaces, and either for similar purposes or different purposes. (For example, if a book were published in a monograph series, it could have both an ISBN [RFC3187] and an ISSN [RFC3044] assigned to it, 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 identifiers specified elsewhere when they are treated as URN namespaces.

With regard to the structure of URNs assigned within a namespace, the development of an identifier structure (and thereby a collection of identifiers) depends on the requirements of the community defining the identifiers, how the identifiers 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 namespace (e.g., the bibliographic and publishing communities in the case of the 'ISBN' and 'ISSN' namespaces, or the developers of extensions to the Extensible Messaging and Presence Protocol in the case of the 'XMPP' namespace).

URN namespaces inherit certain rights and responsibilities by the nature of URNs, e.g.:

  1. They uphold the general principles of a well-managed URN namespace by providing persistent identification of resources and unique assignment of identifier strings in accordance with a common definition.
  2. Optionally, they can be registered in global registration services.

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

Note: [RFC3406] defined a third type of "experimental namespaces", denoted by prefixing the namespace identifier with the string "X-". Consistent with general IETF conclusions about similar approaches [RFC6648], this specification removes the experimental category and syntax. Because experimental namespaces were never registered, removing the experimental category has no impact on the existing registries. However, to avoid the potential for conflict between previously-allowed unregistered experimental namespaces and namespaces registered in the future, no registrations will be accepted for new namespaces beginning with "X-". Because they are not registered, strings that refer to experimental namespaces are not valid URNs. Truly experimental usages MAY, of course, employ the 'example' namespace [RFC6963].

6.1. Formal Namespaces

A formal namespace provides benefit to some subset of users on the Internet. In particular, it would not make sense for a formal 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 NID 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 names assigned within that NID might be small, the potential use of identifiers within that NID is open to any user on the Internet. Formal NIDs might be appropriate even when some aspects are not fully open. For example, a namespace might make use of a fee-based, privately managed, or proprietary registry for assignment of URNs in the namespace. However, it might still benefit some Internet users if the associated services have openly-published identifiers.

An organization that will assign URNs within a formal 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 namespace can remain viable if the organization can no longer maintain the namespace.
  2. Competency in name assignment. This will improve the likelihood of persistence (e.g. to minimize the likelihood of conflicts).
  3. Commitment to not reassigning existing names and to allowing old names to continue to be valid (e.g., if the assignee of a name 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 name is still valid).

A formal 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 namespaces).
  3. It MUST be more than two characters long.
  4. It MUST NOT start with ALPHA ALPHA "-", i.e., any string consisting of two letters followed by one hyphen.
  5. 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].
  6. It MUST NOT start with the string "X-" so that it will not be confused with or conflict any experimental namespace previously permitted by [RFC3406].

All two-letter strings, and all two-letter strings followed by "-" and any sequence of valid NID characters, are reserved for potential use as NIDs based on ISO alpha-2 country codes [ISO3166-1] for eventual national registrations of URN namespaces. The definition and scoping of rules for allocation of responsibility for such country-code-based namespaces is beyond the scope of this document.

Applicants and reviewers considering new NIDs should also be aware that they may be considered as names with semantic implications and hence a source of conflict. Particular attention should be paid to strings that might be construed as names of, 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, 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.

6.2. Informal Namespaces

Informal namespaces are full-fledged URN namespaces, with all the associated rights and responsibilities. Informal namespaces differ from formal namespaces in the process for assigning a NID: for an informal namespace, the registrant does not designate the NID; instead, IANA assigns a 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 namespaces. Thus the syntax of an informal namespace 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, that it (2) not have leading zeros, and that it (3) not cause the NID to exceed the length limitations defined for the URN syntax.

7. Defining and Registering a URN Namespace

7.1. Overview

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

  1. The purpose of the namespace.
  2. The syntax of URNs assigned within the namespace, including whether q-components, r-components, or f-components are allowed.
  3. The process for assigning URNs within the namespace.
  4. The security implications of assigning URNs within the namespace and using the assigned URNs.
  5. Any potential interoperability issues with URNs assigned within the namespace.
  6. Optionally, the process for resolving URNs issued within the namespace.

The section on completing the template (Section 7.4) explains these matters in greater detail. While the registration templates are the same, slightly different procedures are used depending on the source of the registration.

7.2. 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 namespaces or their definitions that are intended to become standards or normative components 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 namespace registration template (see Section 7.4 and Appendix A). This can be done as part of an Internet-Draft or a specification in another series, although that is not necessary.
  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) and IANA will register the requested NID only after receiving directions from that body and a copy of the expert review report.

A 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 namespace management processes.

Experience to date with namespace registration requests has shown that registrants sometimes do not initially understand some of the subtleties of URN namespaces, and that defining the 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 namespaces is Expert Review and a specification is not strictly required, it is RECOMMENDED for registrants to provide a stable specification documenting the namespace definition and expanding upon the issues described herein.

Because naming can be difficult and contentious, 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 NID names and, if such situations arise, to encourage applicants to work out any conflicts among themselves.

7.3. 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 or to reflect knowledge, terminology, or information organization 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-related organization, scientific society, or similar body, a somewhat different procedure is available at the option of that body:

  1. The namespace registration template is filled out and submitted as in steps 1 and 2 above.
  2. A specification is required that reflects of 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, however, 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, they will request that IANA will complete the registration for the NID and IANA will do so.

Decisions about whether to recognize the requesting entity as a standards-related organization, scientific society, or similar body are the responsibility of the IESG.

Note that a similar model has already been defined for recognized standards-related organizations that wish to register Media Types. The document describing that mechanism [RFC6838] provides somewhat more information about the general approach.

7.4. 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.

7.4.1. Purpose

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

  1. The kinds of resources identified by URNs assigned within the namespace.
  2. The scope and applicability of the URNs assigned within the 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 namespace relates to and complements existing URN namespaces, URI schemes, and identifier systems.
  5. The kinds of software applications that can use or resolve the assigned URNs (e.g., by differentiating among disparate namespaces, identifying resources in a persistent fashion, or meaningfully resolving and accessing services associated with the namespace).
  6. Whether resolution services are available or will be available (and, if so, the nature or identity of the services). Examples of q- and r-component semantics and syntax are helpful here, even if they are registered or defined elsewhere later.
  7. Whether the namespace or its definition are expected to become an integral or normative element of a standard being developed in the IETF or some other recognized standards body.

7.4.2. Syntax

The "Syntax" section of the template contains:

  1. A description of the structure of URNs within the namespace, in conformance with the fundamental URN syntax. The structure might be described in terms of a formal definition (e.g., using Augmented BNF for Syntax Specifications (ABNF) as specified in [RFC5234]), an algorithm for generating conformant URNs, or a regular expression for parsing the identifier into components; 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 equivalence between two identifiers in the 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 specific equivalence rules defined in the URI specification [RFC3986]. Examples of equivalence rules include equivalence between uppercase and lowercase characters in the Namespace Specific String, between hyphenated and non-hyphenated groupings in the identifier string, or between single-quotes and double-quotes. (Note that these are not normative statements for any kind of best practice related to handling of equivalences between characters in general; they are statements limited to one particular namespace only.)
  4. Any special considerations necessary for conforming with the URN syntax. This is particularly applicable in the case of existing naming systems that are used in the context of URNs. For example, if a namespace 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].

7.4.3. 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 identifiers or how to request assignment of identifiers from existing assignment authorities.
  2. Methods for ensuring that URNs within the namespace are unique. For example, identifiers 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.

7.4.4. 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 identifiers within the namespace. Examples of such issues include:

7.4.5. Interoperability

The "Interoperability" section MUST specify any potential issues related to interoperability. Examples include possible confusion with other URN namespaces or naming systems 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 an identifier system) SHOULD be resolved as part of the registration process.

7.4.6. Resolution

The "Resolution" section MUST specify whether resolution mechanisms are intended or anticipated for URNs assigned within the namespace (e.g., URNs within some namespaces are intended to act as abstract designators and thus are not intended to be resolved).

If resolution is intended, then this section SHOULD specify whether the organization that assigns URNs within the namespace intends to operate or recommend any resolution services for URNs within that namespace. In addition, if the assigning organization intends to implement registration for publicly advertised resolution services (for example using a system based on principles similar to those described in [RFC2276] and [RFC2483]), then this section SHOULD list or reference the requirements for being publicly advertised by the assigning organization.

8. IANA Considerations

8.1. URI Scheme

This section updates the registration of the 'urn' URI scheme in the Permanent URI Registry [URI-Registry] .

[Note to RFC Editor: please replace "[ this document ]" with "RFC" and the number assigned to this document upon publication.]

URI Scheme Name:
URI Scheme Syntax:
See Section 3 of [ this document ].
URI Scheme Semantics:
The 'urn' scheme identifies Uniform Resource Names, which are persistent, location-independent resource identifiers.
Encoding Considerations:
See Section 3 of [ this document ].
Applications/Protocols That Use This URI Scheme Name:
Uniform Resource Names are used in a wide variety of applications, including bibliographic reference systems and as names for Extensible Markup Language (XML) namespaces.
Interoperability Considerations:
See Section 5 of [ this document ].
Security Considerations:
See Section 7.4.4 and Section 9 of [ this document ].
URNBIS WG [mailto:urn@ietf.org]
Author/Change Controller:
This scheme is registered under the IETF tree. As such, the IETF maintains change control.

8.2. Registration of URN Namespaces

This document outlines the processes for registering URN namespaces, and has implications for the IANA in terms of registries to be maintained (see especially Section 7). In all cases, the IANA ought to assign the appropriate NID (formal or informal) once the procedures outlined in this document have been completed.

9. Security and Privacy Considerations

The definition of a URN namespace needs to account for potential security and privacy issues related to assignment, use, and resolution of identifiers within the namespace (e.g., some namespace resolvers might assign special meaning to certain characters in the Namespace Specific String); see Section 7.4.4 for further discussion.

In most cases, URN namespaces provide a way to declare public information. Nominally, these declarations will have a relatively low security profile, however there is always the danger of "spoofing" and providing misinformation. Information in these declarations ought to be taken as advisory.

10. References

10.1. Normative References

[RFC20] Cerf, V., "ASCII format for network interchange", RFC 20, October 1969.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.

10.2. Informative References

[DOI-URI] Paskin, N., Neylon, E., Hammond, T. and S. Sun, "The "doi" URI Scheme for the Digital Object Identifier (DOI)", June 2003.
[I-D.ietf-urnbis-semantics-clarif] Klensin, J., "URN Semantics Clarification", Internet-Draft draft-ietf-urnbis-semantics-clarif-02, August 2015.
[ISO3166-1] ISO, "Codes for the representation of names of countries and their subdivisions -- Part 1: Country codes", ISO 3166-1:2013, 2013.
[RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for Uniform Resource Names", RFC 1737, December 1994.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2276] Sollins, K., "Architectural Principles of Uniform Resource Name Resolution", RFC 2276, January 1998.
[RFC2483] Mealling, M. and R. Daniel, "URI Resolution Services Necessary for URN Resolution", RFC 2483, January 1999.
[RFC3044] Rozenfeld, S., "Using The ISSN (International Serial Standard Number) as URN (Uniform Resource Names) within an ISSN-URN Namespace", RFC 3044, January 2001.
[RFC3187] Hakala, J. and H. Walravens, "Using International Standard Book Numbers as Uniform Resource Names", RFC 3187, October 2001.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R. and P. Faltstrom, "Uniform Resource Names (URN) Namespace Definition Mechanisms", BCP 66, RFC 3406, October 2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003.
[RFC4854] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace for Extensions to the Extensible Messaging and Presence Protocol (XMPP)", RFC 4854, April 2007.
[RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP)", RFC 5122, February 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010.
[RFC6648] Saint-Andre, P., Crocker, D. and M. Nottingham, "Deprecating the "X-" Prefix and Similar Constructs in Application Protocols", BCP 178, RFC 6648, June 2012.
[RFC6838] Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013.
[RFC6943] Thaler, D., "Issues in Identifier Comparison for Security Purposes", RFC 6943, May 2013.
[RFC6963] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace for Examples", BCP 183, RFC 6963, May 2013.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M. and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, July 2013.
[RFC7282] Resnick, P., "On Consensus and Humming in the IETF", RFC 7282, June 2014.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190, RFC 7320, July 2014.
[UNICODE] The Unicode Consortium, "The Unicode Standard", 2015-.
[URI-Registry] IANA, "Permanent URI Schemes"
[XML-BASE] Marsh, J. and R. Tobin, "XML Base (Second Edition)", World Wide Web Consortium Recommendation REC-xmlbase-20090128, January 2009.
[XML-NAMES] Thompson, H., Hollander, D., Layman, A., Bray, T. and R. Tobin, "Namespaces in XML 1.0 (Third Edition)", World Wide Web Consortium Recommendation REC-xml-names-20091208, December 2009.

Appendix A. Registration Template

Namespace ID:
Requested of IANA (formal) or assigned by IANA (informal).
The version of the registration, starting with 1 and incrementing by 1 with each new version.
The date when the registration is requested of IANA, using the format YYYY-MM-DD.
The person or organization that has registered the NID, including the name and address of the registering organization, as well as the name and contact information (email, phone number, or postal address) of the designated contact person. If the registrant is a recognized standards development organization or scientific society requesting the fact track registration procedure (see Section 7.3), that information should be clearly indicated in this section of the template.
Described under Section 7.4.1 of this document.
Described under Section 7.4.2 of this document. Unless the registration explicitly says otherwise, use of q-components and f-components is not allowed for this namespace.
Described under Section 7.4.3 of this document.
Security and Privacy:
Described under Section 7.4.4 of this document.
Described under Section 7.4.5 of this document.
Described under Section 7.4.6 of this document.
A pointer to an RFC, a specification published by another standards development organization, or another stable document that provides further information about the namespace.
Revision Information:
Description of changes from prior version(s). (Applicable only when earlier registrations have been revised.)

Appendix B. Changes from RFC 2141

This document makes the following substantive changes from [RFC2141]:

Appendix C. Changes from RFC 3406

This document makes the following substantive changes from [RFC3406]:

  1. Relaxes the registration policy for formal namespaces from "IETF Review" to "Expert Review" as discussed in Section 7.2.
  2. Removes the category of experimental namespaces, consistent with [RFC6648].
  3. Simplifies the registration template.

In addition, some of the text has been updated to be consistent with the definition of Uniform Resource Identifiers (URIs) [RFC3986] and the processes for registering information with the IANA [RFC5226], as well as more modern guidance with regard to security [RFC3552] and privacy [RFC6973] issues and identifier comparison [RFC6943].

Appendix D. Contributors

RFC 2141, which provided the basis for the syntax portion of this document, was authored by Ryan Moats.

RFC 3406, which provided the basis for the namespace portion of this document, was authored by Leslie Daigle, Dirk-Willem van Gulik, Renato Iannella, and Patrik Faltstrom.

Their work is gratefully acknowledged.

Appendix E. Acknowledgements

Many thanks to Marc Blanchet, Leslie Daigle, Martin Duerst, Juha Hakala, Ted Hardie, Alfred Hoenes, Paul Jones, Barry Leiba, Sean Leonard, Larry Masinter, Keith Moore, Mark Nottingham, Julian Reschke, Lars Svensson, Henry S. Thompson, Dale Worley, and other participants in the URNBIS WG for their input. Alfred Hoenes in particular edited an earlier version of this document and served as co-chair of the URNBIS WG.

Juha Hakala deserves special recognition for his dedication to successfully completing this work, as do Andrew Newton and Melinda Shore in their roles as working group co-chairs and Barry Leiba in his role as area director.

Appendix F. Change log for versions of draft-ietf-urnbis-rfc2141bis-urn

[[RFC Editor: please remove this appendix before publication.]]

F.1. Changes from -08 to -09

F.2. Changes from -09 to -10

F.3. Changes from -10 to -11

F.4. Changes from -11 to -12

F.5. Changes from -12 to -13

F.6. Changes from -13 to -14

Authors' Addresses

Peter Saint-Andre &yet EMail: peter@andyet.com URI: https://andyet.com/
John C Klensin 1770 Massachusetts Ave, Ste 322 Cambridge, MA 02140 USA Phone: +1 617 245 1457 EMail: john-ietf@jck.com