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Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 8141 Filament Obsoletes: 2141, 3406 J. Klensin Category: Standards Track April 2017 ISSN: 2070-1721

                   Uniform Resource Names (URNs)


 A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
 that is assigned under the "urn" URI scheme and a particular URN
 namespace, with the intent that the URN will be a persistent,
 location-independent resource identifier.  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 it
 describes procedures for registering namespace identifiers with the
 Internet Assigned Numbers Authority (IANA).  This document obsoletes
 both RFCs 2141 and 3406.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Saint-Andre & Klensin Standards Track [Page 1] RFC 8141 URNs April 2017

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 ( in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Saint-Andre & Klensin Standards Track [Page 2] RFC 8141 URNs April 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   1.2.  Design Trade-offs . . . . . . . . . . . . . . . . . . . .   6
     1.2.1.  Resolution  . . . . . . . . . . . . . . . . . . . . .   8
     1.2.2.  Character Sets and Encodings  . . . . . . . . . . . .   9
 2.  URN Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   2.1.  Namespace Identifier (NID)  . . . . . . . . . . . . . . .  10
   2.2.  Namespace Specific String (NSS) . . . . . . . . . . . . .  10
   2.3.  Optional Components . . . . . . . . . . . . . . . . . . .  12
     2.3.1.  r-component . . . . . . . . . . . . . . . . . . . . .  12
     2.3.2.  q-component . . . . . . . . . . . . . . . . . . . . .  13
     2.3.3.  f-component . . . . . . . . . . . . . . . . . . . . .  15
 3.  URN-Equivalence . . . . . . . . . . . . . . . . . . . . . . .  16
   3.1.  Procedure . . . . . . . . . . . . . . . . . . . . . . . .  16
   3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  17
 4.  URI Conformance . . . . . . . . . . . . . . . . . . . . . . .  18
   4.1.  Use in URI Protocol Slots . . . . . . . . . . . . . . . .  18
   4.2.  Parsing . . . . . . . . . . . . . . . . . . . . . . . . .  19
   4.3.  URNs and Relative References  . . . . . . . . . . . . . .  19
   4.4.  Transport and Display . . . . . . . . . . . . . . . . . .  19
   4.5.  URI Design and Ownership  . . . . . . . . . . . . . . . .  20
 5.  URN Namespaces  . . . . . . . . . . . . . . . . . . . . . . .  20
   5.1.  Formal URN Namespaces . . . . . . . . . . . . . . . . . .  22
   5.2.  Informal URN Namespaces . . . . . . . . . . . . . . . . .  23
 6.  Defining and Registering a URN Namespace  . . . . . . . . . .  24
   6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  24
   6.2.  Registration Policy and Process: Community Registrations   25
   6.3.  Registration Policy and Process: Fast Track for Standards
         Development Organizations, Scientific Societies, and
         Similar Bodies  . . . . . . . . . . . . . . . . . . . . .  26
   6.4.  Completing the Template . . . . . . . . . . . . . . . . .  27
     6.4.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . .  27
     6.4.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . .  28
     6.4.3.  Assignment  . . . . . . . . . . . . . . . . . . . . .  29
     6.4.4.  Security and Privacy  . . . . . . . . . . . . . . . .  29
     6.4.5.  Interoperability  . . . . . . . . . . . . . . . . . .  30
     6.4.6.  Resolution  . . . . . . . . . . . . . . . . . . . . .  30
     6.4.7.  Additional Information  . . . . . . . . . . . . . . .  30
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   7.1.  URI Scheme  . . . . . . . . . . . . . . . . . . . . . . .  31
   7.2.  Registration of URN Namespaces  . . . . . . . . . . . . .  31
   7.3.  Discussion List for New and Updated NID Registrations . .  31
 8.  Security and Privacy Considerations . . . . . . . . . . . . .  32
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  32
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  32

Saint-Andre & Klensin Standards Track [Page 3] RFC 8141 URNs April 2017

 Appendix A.  Registration Template  . . . . . . . . . . . . . . .  37
 Appendix B.  Changes from RFC 2141  . . . . . . . . . . . . . . .  38
   B.1.  Syntax Changes from RFC 2141  . . . . . . . . . . . . . .  38
   B.2.  Other Changes from RFC 2141 . . . . . . . . . . . . . . .  39
 Appendix C.  Changes from RFC 3406  . . . . . . . . . . . . . . .  39
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  40
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  40
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1. Introduction

 A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
 [RFC3986] that is assigned under the "urn" URI scheme and a
 particular URN namespace, with the intent that the URN will be a
 persistent, location-independent resource identifier.  A URN
 namespace is a collection of such URNs, 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 assign URNs based on names
 that were already created in non-URN identifier systems, such as
 ISBNs [RFC3187], ISSNs [RFC3044], or RFCs [RFC2648].)
 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 URN namespace within the "urn" scheme (e.g., a
 URN in the "example" URN namespace [RFC6963] might be of the form
 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 resource identifiers to be freely
 chosen and assigned, such is not the case for URNs.  The syntactical
 correctness of a name starting with "urn:" is not sufficient to make
 it a URN.  In order for the name to be a valid URN, the namespace
 identifier (NID) needs to be registered in accordance with the rules
 defined here, and the remaining parts of the assigned-name portion of
 the URN need to be generated in accordance with the rules for the
 registered URN namespace.

Saint-Andre & Klensin Standards Track [Page 4] RFC 8141 URNs April 2017

 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 particular NID, and describes procedures for
     registering URN NIDs with the Internet Assigned Numbers Authority
 For URN syntax and URN namespaces, this document modernizes and
 replaces the original specifications for URN syntax [RFC2141] and for
 the definition and registration of URN namespaces [RFC3406].  These
 modifications build on the key requirements provided in the original
 functional description for URNs [RFC1737] and on the lessons of many
 years of experience.  In those original documents and in the present
 one, 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 URN namespace within which a given URN is
 Together with input from several key user communities, the history
 and experiences with URNs dictated expansion of the URN definition to
 support new functionality, including the use of syntax explicitly
 reserved for future standardization in RFC 2141.  All URN namespaces
 and URNs that were valid under the earlier specifications remain
 valid, even though it may be useful to update the definitions of some
 URN namespaces to take advantage of new features.
 The foregoing considerations, together with various differences
 between URNs and URIs that are locators (specifically URLs) as well
 as the greater focus on URLs in RFC 3986 as the ultimate successor to
 [RFC1738] and [RFC1808], may lead to some interpretations of RFC 3986
 and this specification that appear (or perhaps actually are) not
 completely consistent, especially with regard to actions or semantics
 other than the basic syntax itself.  If such situations arise,
 discussions of URNs and URN namespaces should be interpreted
 according to this document and not by extrapolation from RFC 3986.
 Summaries of changes from RFCs 2141 and 3406 appear in Appendices B
 and C, respectively.  This document obsoletes both [RFC2141] and
 [RFC3406].  While it does not explicitly update or replace [RFC1737]
 or [RFC2276], the reader who references those documents should be
 aware that the conceptual model of URNs in this document is slightly
 different from those older specifications.

Saint-Andre & Klensin Standards Track [Page 5] RFC 8141 URNs April 2017

1.1. Terminology

 The following terms are distinguished from each other as described
 URN:  A URI (as defined in RFC 3986) using the "urn" scheme and with
    the properties of a "name" as described in that document as well
    as the properties described in this one.  The term applies to the
    entire URI including its optional components.  Note to the reader:
    the term "URN" has been used in other contexts to refer to a URN
    namespace, the namespace identifier, the assigned-name, and URIs
    that do not use the "urn" scheme.  All but the last of these is
    described using more specific terminology elsewhere in this
    document, but, because of those other uses, the term should be
    used and interpreted with care.
 Locator:  An identifier that provides a means of accessing a
 Identifier system:  A managed collection of names.  This document
    refers to identifier systems outside the context of URNs as
    "non-URN identifier systems".
 URN namespace:  An identifier system that is associated with a URN
 NID:  The identifier associated with a URN namespace.
 NSS:  The URN-namespace-specific part of a URN.
 Assigned-name:  The combination of the "urn:" scheme, the NID, and
    the namespace specific string (NSS).  An "assigned-name" is
    consequently a substring of a URN (as defined above) if that URN
    contains any additional components (see Section 2).
 The term "name" is deliberately not defined here and should be (and,
 in practice, is) used only very informally.  RFC 3986 uses the term
 as a category of URI distinguished from "locator" (Section 1.1.3) but
 also uses it in other contexts.  If those uses are treated as
 definitional, they would conflict with, e.g., the idea of URN
 namespace names (i.e., NIDs) and with terms associated with non-URN
 identifier systems.
 This document uses the terms "resource", "identifier", "identify",
 "dereference", "representation", and "metadata" roughly as defined in
 the URI specification [RFC3986].

Saint-Andre & Klensin Standards Track [Page 6] RFC 8141 URNs April 2017

 This document uses the terms "resolution" and "resolver" in roughly
 the sense in which they were used in the original discussion of
 architectural principles for URNs [RFC2276], i.e., "resolution" is
 the act of supplying services related to the identified resource,
 such as translating the persistent URN into one or more current
 locators for the resource, delivering metadata about the resource in
 an appropriate format, or even delivering a representation of the
 resource (e.g., a document) without requiring further intermediaries.
 At the time of this writing, resolution services are described in
 On the distinction between representations and metadata, see
 Section 1.2.2 of [RFC3986].
 Several other terms related to "normalization" operations that are
 not part of the Unicode Standard [UNICODE] are also used here as they
 are in RFC 3986.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "OPTIONAL" in this document are to be interpreted as described in

1.2. Design Trade-offs

 To a degree much greater than when URNs were first considered and
 their uses outlined (see [RFC1737]), issues of persistent identifiers
 on the Internet involve fundamental design trade-offs that are much
 broader than URNs or the URN approach and even touch on open research
 questions within the information sciences community.  Ideal and
 comprehensive specifications about what should be done or required
 across the entire universe of URNs would require general agreement
 about, and solutions to, a wide range of such issues.  Although some
 of those issues were introduced by the Internet or computer-age
 approaches to character encodings and data abstraction, others
 predate the Internet and computer systems by centuries; there is
 unlikely to be agreement about comprehensive solutions in the near
 Although this specification consequently contains some requirements
 and flexibility that would not be present in a more perfect world,
 this has been necessary in order to produce a consensus specification
 that provides a modernized definition of URNs (the unattractive
 alternative would have been to not modernize the definition in spite
 of widespread deployment).
 The following sub-sections describe two of the relevant issues in
 greater detail.

Saint-Andre & Klensin Standards Track [Page 7] RFC 8141 URNs April 2017

1.2.1. Resolution

 One issue that is specific to URNs (as opposed to naming systems in
 general) is the fairly difficult topic of "resolution", discussed in
 Sections 1.1, 2.3.1, 6.4.6, and elsewhere below.
 With traditional Uniform Resource Locators (URLs), i.e., with most
 URIs that are locators, resolution is relatively straightforward
 because it is used to determine an access mechanism that in turn is
 used to dereference the locator by (typically) retrieving a
 representation of the associated resource, such as a document (see
 Section 1.2.2 of [RFC3986]).
 By contrast, resolution for URNs is more flexible and varied.
 One important case involves the mapping of a URN to one or more
 locators.  In this case, the end result is still a matter of
 dereferencing the mapped locator(s) to one or more representations.
 The primary difference here is persistence: even if a mapped locator
 has changed (e.g., a DNS domain name has changed hands and a URL has
 not been modified to point to a new location or, in a more extreme
 and hypothetical case, the DNS is replaced entirely), a URN user will
 be able to obtain the correct representation (e.g., a document) as
 long as the resolver has kept its URN-to-locator mappings up to date.
 Consequently, the relevant relationships can be defined quite
 precisely for URNs that resolve to locators that in turn are
 dereferenced to a representation.
 However, this specification permits several other cases of URN
 resolution as well as URNs for resources that do not involve
 information retrieval systems.  This is true either individually for
 particular URNs or (as defined below) collectively for entire URN
 Consider a namespace of URNs that resolve to locators that in turn
 are dereferenced only to metadata about resources because the
 underlying systems contain no representations of those resources; an
 example might be a URN namespace for International Standard Name
 Identifiers (ISNIs) as that identifier system is defined in the
 relevant standard [ISO.27729.2012], wherein by default a URN would be
 resolved only to a metadata record describing the public identity
 identified by the ISNI.
 Consider also URNs that resolve to representations only if the
 requesting entity is authorized to obtain the representation, whereas
 other entities can obtain only metadata about the resource; an
 example might be documents held within the legal depository
 collection of a national library.

Saint-Andre & Klensin Standards Track [Page 8] RFC 8141 URNs April 2017

 Finally, some URNs might not be intended to resolve to locators at
 all; examples might include URNs identifying XML namespace names
 (e.g., the "dgiwg" URN namespace specified by [RFC6288]), URNs
 identifying application features that can be supported within a
 communications protocol (e.g., the "alert" URN namespace specified by
 [RFC7462]), and URNs identifying enumerated types such as values in a
 registry (e.g., a URN namespace could be used to individually
 identify the values in all IANA registries, as provisionally proposed
 in [IANA-URN]).
 Various types of URNs and multiple resolution services that may be
 available for them leave the concept of "resolution" more complicated
 but also much richer for URNs than the straightforward case of
 resolution to a locator that is dereferenced to a representation.

1.2.2. Character Sets and Encodings

 A similar set of considerations apply to character sets and
 encodings.  URNs, especially URNs that will be used as user-facing
 identifiers, should be convenient to use in local languages and
 writing systems, easily specified with a wide range of keyboards and
 local conventions, and unambiguous.  There are trade-offs among those
 goals, and it is impossible at present to see how a simple and
 readily understandable set of rules could be developed that would be
 optimal, or even reasonable, for all URNs.  The discussion in
 Section 2.2 defines an overall framework that should make generalized
 parsing and processing possible but also makes recommendations about
 rules for individual URN namespaces.

2. URN Syntax

 As discussed above, the syntax for URNs in this specification allows
 significantly more functionality than was the case in the earlier
 specifications, most recently [RFC2141].  It is also harmonized with
 the general URI syntax [RFC3986] (which, it must be noted, was
 completed after the earlier URN specifications).
 However, this specification does not extend the URN syntax to allow
 direct use of characters outside the ASCII range [RFC20].  That
 restriction implies that any such characters need to be percent-
 encoded as described in Section 2.1 of the URI specification
 The basic syntax for a URN is defined 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.  The definitions of some of

Saint-Andre & Klensin Standards Track [Page 9] RFC 8141 URNs April 2017

 the terms used below are not comprehensive; additional restrictions
 are imposed by the prose that can be found in sections of this
 document that are specific to those terms (especially r-component in
 Section 2.3.1 and q-component in Section 2.3.2).
    namestring    = assigned-name
                    [ rq-components ]
                    [ "#" f-component ]
    assigned-name = "urn" ":" NID ":" NSS
    NID           = (alphanum) 0*30(ldh) (alphanum)
    ldh           = alphanum / "-"
    NSS           = pchar *(pchar / "/")
    rq-components = [ "?+" r-component ]
                    [ "?=" q-component ]
    r-component   = pchar *( pchar / "/" / "?" )
    q-component   = pchar *( pchar / "/" / "?" )
    f-component   = fragment
 The question mark character "?" can be used without percent-encoding
 inside r-components, q-components, and f-components.  Other than
 inside those components, a "?" that is not immediately followed by
 "=" or "+" is not defined for URNs and SHOULD be treated as a syntax
 error by URN-specific parsers and other processors.
 The following sections provide additional information about the
 syntactic elements of URNs.

2.1. Namespace Identifier (NID)

 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.
 Sections 5.1 and 5.2 impose additional constraints on the strings
 that can be used as NIDs, i.e., the syntax shown above is not

2.2. Namespace Specific String (NSS)

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

Saint-Andre & Klensin Standards Track [Page 10] RFC 8141 URNs April 2017

 The NSS as specified in this document allows several characters not
 permitted by earlier specifications (see Appendix B).  In particular,
 the "/" character, which is now allowed, effectively makes it
 possible to encapsulate hierarchical names from non-URN identifier
 systems.  For instance, consider the hypothetical example of a
 hierarchical identifier system in which the names take the form of a
 sequence of numbers separated by the "/" character, such as
 "1/406/47452/2".  If the authority for such names were to use URNs,
 it would be natural to place the existing name in the NSS, resulting
 in URNs such as "urn:example:1/406/47452/2".
 Those 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 whose names are used outside of
 the URN context (i.e., in a non-URN identifier system) also allows
 the use of "/", "~", or "&" in the native form within that identifier
 system, then the encoding rules for that URN namespace are not
 changed by this specification.
 Depending on the rules governing a non-URN identifier system and its
 associated URN namespace, names that are valid in that identifier
 system 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 name might be valid within the
 non-URN identifier system, it is not a valid URN until it has been
 translated into an NSS that conforms to the rules of that particular
 URN namespace.  In the case of URNs that are formed from names that
 exist separately in a non-URN identifier system, translation of a
 name from its "native" format to a URN format is accomplished by
 using the canonicalization and encoding methods defined for URNs in
 general or specific rules for that URN namespace.  Software that is
 not aware of namespace-specific canonicalization and encoding rules
 MUST NOT construct URNs from the name in the non-URN identifier
 In particular, with regard to characters outside the ASCII range,
 URNs that appear in protocols or that are passed between systems MUST
 use only Unicode characters encoded in UTF-8 and further encoded as
 required by RFC 3986.  To the extent feasible and consistent with the
 requirements of names defined and standardized elsewhere, as well as
 the principles discussed in Section 1.2, the characters used to
 represent names SHOULD be restricted to either ASCII letters and
 digits or to the characters and syntax of some widely used models
 such as those of Internationalizing Domain Names in Applications
 (IDNA) [RFC5890], Preparation, Enforcement, and Comparison of
 Internationalized Strings (PRECIS) [RFC7613], or the Unicode
 Identifier and Pattern Syntax specification [UAX31].

Saint-Andre & Klensin Standards Track [Page 11] RFC 8141 URNs April 2017

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

2.3. Optional Components

 This specification includes three optional components in the URN
 syntax.  They are known as r-component, q-component, and f-component
 and are described in more detail below.  Because this specification
 focuses almost exclusively on URN syntax, it does not define detailed
 semantics of these components for URNs in general.  However, each of
 these components has a distinct role that is independent of any given
 URN and its URN namespace.  It is intended that clients will be able
 to handle these components uniformly for all URNs.  These components
 MAY be used with URNs from existing URN namespaces, whether or not a
 URN namespace explicitly supports them.  However, consistent with the
 approach taken in RFC 3986, the behavior of a URN that contains
 components that are undefined or meaningless for a particular URN
 namespace or resource is not defined.  The following sections
 describe these optional components and their interpretation in
 greater detail.

2.3.1. r-component

 The r-component is intended for passing parameters to URN resolution
 services (taken broadly, see Section 1.2) and interpreted by those
 services.  (By contrast, passing parameters to the resources
 identified by a URN, or to applications that manage such resources,
 is handled by q-components as described in the next section.)
 The URN r-component has no syntactic counterpart in any other known
 URI scheme.
 The sequence "?+" introduces the r-component.  The r-component ends
 with a "?=" sequence (which begins a q-component) or a "#" character
 (number sign, which begins an f-component).  If neither of those
 appear, the r-component continues to the end of the URN.  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 in Section 3, 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.

Saint-Andre & Klensin Standards Track [Page 12] RFC 8141 URNs April 2017

 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, presumably including
 specifications that define conventions and a registry for resolution
 service identifiers.
 Consider the hypothetical example of passing parameters to a
 resolution service (say, an ISO alpha-2 country code [ISO.3166-1] in
 order to select 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:
 While the above should serve as a general explanation and
 illustration of the intent for r-components, there are many open
 issues with them, including their relationship to resolution
 mechanisms associated with the particular URN namespace at
 registration time.  Thus, r-components SHOULD NOT be used for URNs
 before their semantics have been standardized.

2.3.2. q-component

 The q-component is intended for passing parameters to either the
 named resource or a system that can supply the requested service, for
 interpretation by that resource or system.  (By contrast, passing
 parameters to URN resolution services is handled by r-components as
 described in the previous section.)
 The URN q-component has the same syntax as the URI query component
 but is introduced by "?=", not "?" alone.  For a URN that may be
 resolved to a URI that is a locator, the semantics of the q-component
 are identical to those for the query component of that URI.  Thus,
 URN resolvers returning a URI that is a locator for a URN with a
 q-component do this by copying the q-component from the URN to the
 query component of the URI.  An example of the copying operation
 appears below.
 This specification does not specify a required behavior in the case
 of URN resolution to a URI that is a locator when the original URN
 has a q-component and the URI has a query string.  Different
 circumstances may require different approaches.  Resolvers SHOULD
 document their strategy in such cases.

Saint-Andre & Klensin Standards Track [Page 13] RFC 8141 URNs April 2017

 If the URN does not resolve to a URI that is a locator, the
 interpretation of the q-component is undefined by this specification.
 For URNs that may be resolved to a URI that is a locator, the
 semantics of the q-component are identical to those for queries to
 the resource located via that URI.
 For the sake of consistency with RFC 3986, the general syntax and the
 semantics of q-components are not defined by, or dependent on, the
 URN namespace of the URN.  In parallel with RFC 3986, specifics of
 syntax and semantics, e.g., which keywords or terms are meaningful,
 of course may depend on a particular URN namespace or even a
 particular resource.
 The sequence "?=" introduces the q-component.  The q-component ends
 with a "#" character (number sign, which begins an f-component).  If
 that character does not appear, the q-component continues to the end
 of the URN.  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 in Section 3, the q-component SHALL NOT be taken into
 account when determining URN-equivalence.
 URN namespaces and associated information placement in syntax SHOULD
 be designed to avoid any need for a resolution service to consider
 the q-component.  Namespace-specific and more generic resolution
 systems MUST NOT require that q-component information be passed to
 them for processing.
 Consider the hypothetical example of passing parameters to an
 application that returns weather reports from different regions or
 for different time periods.  This could perhaps be accomplished by
 specifying latitude and longitude coordinates and datetimes in the
 URN's q-component, resulting in URNs such as the following.
 If this example resolved to an HTTP URI, the result might look like:

Saint-Andre & Klensin Standards Track [Page 14] RFC 8141 URNs April 2017

2.3.3. f-component

 The f-component is intended to be interpreted by the client as a
 specification for a location within, or region of, the named
 resource.  It distinguishes the constituent parts of a resource named
 by a URN.  For a URN that resolves to one or more locators that can
 be dereferenced to a representation, or where the URN resolver
 directly returns a representation of the resource, the semantics of
 an f-component are defined by the media type of the representation.
 The URN f-component has the same syntax as the URI fragment
 component.  If a URN containing an f-component resolves to a single
 URI that is a locator associated with the named resource, the
 f-component from the URN can be applied (usually by the client) as
 the fragment of that URI.  If the URN does not resolve to a URI that
 is a locator, the interpretation of the f-component is undefined by
 this specification.  Thus, for URNs that may be resolved to a URI
 that is a locator, the semantics of f-components are identical to
 those of fragments for that resource.
 For the sake of consistency with RFC 3986, neither the general syntax
 nor the semantics of f-components are defined by, or dependent on,
 the URN namespace of the URN.  In parallel with RFC 3986, specifics
 of syntax and semantics, e.g., which keywords or terms are
 meaningful, of course may depend on a particular URN namespace or
 even a particular resource.
 The f-component is introduced by the number sign ("#") character and
 terminated by the end of the URI.  Any characters outside the ASCII
 range [RFC20] that appear in the f-component MUST be percent-encoded
 using the method defined in Section 2.1 of the generic URI
 specification [RFC3986].
 As described in Section 3, the f-component SHALL NOT be taken into
 account when determining URN-equivalence.
 Clients SHOULD NOT pass f-components to resolution services unless
 those services also perform object retrieval and interpretation
 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:

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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 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 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
 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

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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
 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

Saint-Andre & Klensin Standards Track [Page 20] RFC 8141 URNs April 2017

 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
 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

Saint-Andre & Klensin Standards Track [Page 21] RFC 8141 URNs April 2017

 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

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
 1.  It MUST NOT be an already-registered NID.
 2.  It MUST NOT start with "urn-" (which is reserved for informal URN
 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
 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

Saint-Andre & Klensin Standards Track [Page 23] RFC 8141 URNs April 2017

 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
 2.  Send the completed template to the 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
 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

Saint-Andre & Klensin Standards Track [Page 25] RFC 8141 URNs April 2017

 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

6.3. Registration Policy and Process: Fast Track for Standards

    Development Organizations, Scientific Societies, and Similar
 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.

Saint-Andre & Klensin Standards Track [Page 26] RFC 8141 URNs April 2017

 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
 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.

Saint-Andre & Klensin Standards Track [Page 27] RFC 8141 URNs April 2017

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
 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.

Saint-Andre & Klensin Standards Track [Page 28] RFC 8141 URNs April 2017

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.

Saint-Andre & Klensin Standards Track [Page 29] RFC 8141 URNs April 2017

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
 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.

Saint-Andre & Klensin Standards Track [Page 30] RFC 8141 URNs April 2017

7. IANA Considerations

7.1. URI Scheme

 This section updates the registration of the "urn" URI scheme in the
 Permanent URI Registry [URI-Registry].
 URI Scheme Name:  urn
 Status:  permanent
 URI Scheme Syntax:  See Section 2 of RFC 8141.
 URI Scheme Semantics:  The "urn" scheme identifies Uniform Resource
    Names, which are persistent, location-independent resource
 Encoding Considerations:  See Section 2 of RFC 8141.
 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 4 of RFC 8141.
 Security Considerations:  See Sections 6.4.4 and 8 of RFC 8141.
 Contact:  URNBIS working group []
 Author/Change Controller:  This scheme is registered under the IETF
    tree.  As such, the IETF maintains change control.
 References:  None.

7.2. 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 6).  In all cases, the IANA ought
 to assign the appropriate NID (formal or informal) once the
 procedures outlined in Section 6 have been completed.

7.3. Discussion List for New and Updated NID Registrations

 As discussed elsewhere in this document, the discussion list
 specified in RFC 3406 ( is discontinued and
 replaced by the discussion list

Saint-Andre & Klensin Standards Track [Page 31] RFC 8141 URNs April 2017

8. 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 names within the URN namespace (e.g., some URN
 resolvers might assign special meaning to certain characters in the
 NSS); see Section 6.4.4 for further discussion.
 In most cases, URN namespaces provide a way to declare public
 information.  Normally, 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.

9. References

9.1. Normative References

 [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
            RFC 20, DOI 10.17487/RFC0020, October 1969,
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,

9.2. Informative References

 [DOI-URI]  Paskin, N., Neylon, E., Hammond, T., and S. Sun, "The
            "doi" URI Scheme for the Digital Object Identifier (DOI)",
            Work in Progress, draft-paskin-doi-uri-04, June 2003.

Saint-Andre & Klensin Standards Track [Page 32] RFC 8141 URNs April 2017

 [IANA-URN] Saint-Andre, P. and M. Cotton, "A Uniform Resource Name
            (URN) Namespace for IANA Registries", Work in Progress,
            draft-saintandre-iana-urn-01, February 2013.
            ISO, "Information and documentation - International
            standard name identifier (ISNI)", ISO 27729:2012,
            Technical Committee ISO/TC 46, Information and
            documentation, Subcommittee SC 9, Identification and
            description, March 2012.
            ISO, "Codes for the representation of names of countries
            and their subdivisions -- Part 1: Country codes",
            ISO 3166-1:2013, November 2013.
 [RFC1737]  Sollins, K. and L. Masinter, "Functional Requirements for
            Uniform Resource Names", RFC 1737, DOI 10.17487/RFC1737,
            December 1994, <>.
 [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
            Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
            December 1994, <>.
 [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",
            RFC 1808, DOI 10.17487/RFC1808, June 1995,
 [RFC2141]  Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
            May 1997, <>.
 [RFC2276]  Sollins, K., "Architectural Principles of Uniform Resource
            Name Resolution", RFC 2276, DOI 10.17487/RFC2276, January
            1998, <>.
 [RFC2483]  Mealling, M. and R. Daniel, "URI Resolution Services
            Necessary for URN Resolution", RFC 2483,
            DOI 10.17487/RFC2483, January 1999,
 [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
            DOI 10.17487/RFC2648, August 1999,
 [RFC3044]  Rozenfeld, S., "Using The ISSN (International Serial
            Standard Number) as URN (Uniform Resource Names) within an
            ISSN-URN Namespace", RFC 3044, DOI 10.17487/RFC3044,
            January 2001, <>.

Saint-Andre & Klensin Standards Track [Page 33] RFC 8141 URNs April 2017

 [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
            Book Numbers as Uniform Resource Names", RFC 3187,
            DOI 10.17487/RFC3187, October 2001,
 [RFC3188]  Hakala, J., "Using National Bibliography Numbers as
            Uniform Resource Names", RFC 3188, DOI 10.17487/RFC3188,
            October 2001, <>.
 [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
            "Uniform Resource Names (URN) Namespace Definition
            Mechanisms", BCP 66, RFC 3406, DOI 10.17487/RFC3406,
            October 2002, <>.
 [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
            Text on Security Considerations", BCP 72, RFC 3552,
            DOI 10.17487/RFC3552, July 2003,
 [RFC4854]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
            for Extensions to the Extensible Messaging and Presence
            Protocol (XMPP)", RFC 4854, DOI 10.17487/RFC4854, 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, DOI 10.17487/RFC5122, February 2008,
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
 [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
            March 2011, <>.
 [RFC6288]  Reed, C., "URN Namespace for the Defence Geospatial
            Information Working Group (DGIWG)", RFC 6288,
            DOI 10.17487/RFC6288, August 2011,

Saint-Andre & Klensin Standards Track [Page 34] RFC 8141 URNs April 2017

 [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
            "Deprecating the "X-" Prefix and Similar Constructs in
            Application Protocols", BCP 178, RFC 6648,
            DOI 10.17487/RFC6648, June 2012,
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13,
            RFC 6838, DOI 10.17487/RFC6838, January 2013,
 [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
            Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
            2013, <>.
 [RFC6963]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
            for Examples", BCP 183, RFC 6963, DOI 10.17487/RFC6963,
            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,
            DOI 10.17487/RFC6973, July 2013,
 [RFC7254]  Montemurro, M., Ed., Allen, A., McDonald, D., and P.
            Gosden, "A Uniform Resource Name Namespace for the Global
            System for Mobile Communications Association (GSMA) and
            the International Mobile station Equipment Identity
            (IMEI)", RFC 7254, DOI 10.17487/RFC7254, May 2014,
 [RFC7282]  Resnick, P., "On Consensus and Humming in the IETF",
            RFC 7282, DOI 10.17487/RFC7282, June 2014,
 [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
            RFC 7320, DOI 10.17487/RFC7320, July 2014,
 [RFC7462]  Liess, L., Ed., Jesske, R., Johnston, A., Worley, D., and
            P. Kyzivat, "URNs for the Alert-Info Header Field of the
            Session Initiation Protocol (SIP)", RFC 7462,
            DOI 10.17487/RFC7462, March 2015,

Saint-Andre & Klensin Standards Track [Page 35] RFC 8141 URNs April 2017

 [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
            Enforcement, and Comparison of Internationalized Strings
            Representing Usernames and Passwords", RFC 7613,
            DOI 10.17487/RFC7613, August 2015,
 [UAX31]    The Unicode Consortium, "Unicode Standard Annex #31:
            Unicode Identifier and Pattern Syntax", Unicode 9.0.0,
            June 2015, <>.
 [UNICODE]  The Unicode Consortium, "The Unicode Standard",
            IANA, "Uniform Resource Identifier (URI) Schemes",
 [XML-BASE] Marsh, J. and R. Tobin, "XML Base (Second Edition)", W3C
            Recommendation REC-xmlbase-20090128, January 2009,
            Thompson, H., Hollander, D., Layman, A., Bray, T., and R.
            Tobin, "Namespaces in XML 1.0 (Third Edition)", W3C
            Recommendation REC-xml-names-20091208, December 2009,

Saint-Andre & Klensin Standards Track [Page 36] RFC 8141 URNs April 2017

Appendix A. Registration Template

 Namespace Identifier:  Requested of IANA (formal) or assigned by IANA
 Version:  The version of the registration, starting with 1 and
    incrementing by 1 with each new version.
 Date:  The date when the registration is requested of IANA, using the
    format YYYY-MM-DD.
 Registrant:  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,
    scientific society, or similar body requesting the fast-track
    registration procedure (see Section 6.3), that information should
    be clearly indicated in this section of the template.
 Purpose:  Described in Section 6.4.1 of this document.
 Syntax:  Described in Section 6.4.2 of this document.  Unless the
    registration explicitly describes the semantics of r-components,
    q-components, and f-components in the context of this URN
    namespace, those semantics are undefined.
 Assignment:  Described in Section 6.4.3 of this document.
 Security and Privacy:  Described in Section 6.4.4 of this document.
 Interoperability:  Described in Section 6.4.5 of this document.
 Resolution:  Described in Section 6.4.6 of this document.
 Documentation:  A pointer to an RFC, a specification published by
    another standards development organization, or another stable
    document that provides further information about this URN
 Additional Information:  Described in Section 6.4.7 of this document.
 Revision Information:  Description of changes from prior version(s).
    (Applicable only when earlier registrations have been revised.)

Saint-Andre & Klensin Standards Track [Page 37] RFC 8141 URNs April 2017

Appendix B. Changes from RFC 2141

 This document makes substantive changes from the syntax and semantics
 of [RFC2141]:

B.1. Syntax Changes from RFC 2141

 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:
 o  Ensure consistency with the URI syntax.
 o  Facilitate the use of URNs with parameters similar to URI queries
    and fragments.
 o  Permit parameters influencing URN resolution.
 o  Ease the use of URNs with non-URN identifier systems that include
    the "/" character.
 In particular, this specification does the following:
 o  Extends URN syntax to explicitly allow the characters "/", "?",
    and "#", which were reserved for future use by RFC 2141.  This
    change also effectively allows several components of the URI
    syntax although without necessarily tying those components to URI
 o  Defines general syntax for an additional component that can be
    used in interactions with a URN resolution service.
 o  Disallows "-" at the end of the NID.
 o  Allows the "/", "~", and "&" characters in the NSS.
 o  Makes several smaller syntax adjustments.

Saint-Andre & Klensin Standards Track [Page 38] RFC 8141 URNs April 2017

B.2. Other Changes from RFC 2141

 o  Formally registers "urn" as a URI scheme.
 o  Allows what are now called r-components, q-components, and
 In addition, some of the text has been updated to be consistent with
 the definition of URIs [RFC3986] and the processes for registering
 information with the IANA [RFC5226], as well as more modern guidance
 with regard to security [RFC3552], privacy [RFC6973], and identifier
 comparison [RFC6943].

Appendix C. Changes from RFC 3406

 This document makes the following substantive changes from [RFC3406]:
 1.  Relaxes the registration policy for formal URN namespaces from
     "IETF Review" to "Expert Review" as discussed in Section 6.2.
 2.  Removes the category of experimental URN namespaces, consistent
     with [RFC6648].  Experimental URN namespaces were denoted by
     prefixing the namespace identifier with the string "X-".  Because
     experimental URN namespaces were never registered, removing the
     experimental category has no impact on the existing registries.
     Because experimental URN namespaces are not managed, strings
     conforming to URN syntax within experimental URN namespaces are
     not valid URNs.  Truly experimental usages may, of course, employ
     the "example" namespace [RFC6963].
 3.  Adds some information to, but generally simplifies, the URN
     namespace registration template.

Saint-Andre & Klensin Standards Track [Page 39] RFC 8141 URNs April 2017


 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 working group for their input.  Alfred
 Hoenes in particular edited an earlier draft version of this document
 and served as co-chair of the URNBIS working group.
 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 and then as co-chair.


 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.

Authors' Addresses

 Peter Saint-Andre
 P.O. Box 787
 Parker, CO  80134
 United States of America
 Phone: +1 720 256 6756
 URI:   <>
 John C. Klensin
 1770 Massachusetts Ave, Ste 322
 Cambridge, MA  02140
 United States of America
 Phone: +1 617 245 1457

Saint-Andre & Klensin Standards Track [Page 40]

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