path: root/doc/rfc/rfc8141.txt

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)

Abstract

   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
   http://www.rfc-editor.org/info/rfc8141.














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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
   (http://trustee.ietf.org/license-info) 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.





































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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



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   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
   "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 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.









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   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
       (IANA).

   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
   assigned.

   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.




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1.1.  Terminology

   The following terms are distinguished from each other as described
   below:

   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
      resource.

   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.

   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].





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   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
   [RFC2483].

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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
   future.

   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.



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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
   namespaces.

   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.



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   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
   [RFC3986].

   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



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   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
   comprehensive.

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.






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   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
   system.

   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].



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   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.




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   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:

      urn:example:foo-bar-baz-qux?+CCResolve:cc=uk

   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.






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   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.

      urn:example:weather?=op=map&lat=39.56
         &lon=-104.85&datetime=1969-07-21T02:56:15Z

   If this example resolved to an HTTP URI, the result might look like:

      https://weatherapp.example?op=map&lat=39.56
         &lon=-104.85&datetime=1969-07-21T02:56:15Z








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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
   functions.

   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:

   urn:example:foo-bar-baz-qux#somepart






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3.  URN-Equivalence

3.1.  Procedure

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

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

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

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

   2.  the NID, by conversion to lower case

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

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

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

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





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3.2.  Examples

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

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

   o  urn:example:a123,z456

   o  URN:example:a123,z456

   o  urn:EXAMPLE:a123,z456

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

   o  urn:example:a123,z456?+abc

   o  urn:example:a123,z456?=xyz

   o  urn:example:a123,z456#789

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

   o  urn:example:a123,z456/foo

   o  urn:example:a123,z456/bar

   o  urn:example:a123,z456/baz

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

   o  urn:example:a123%2Cz456

   o  URN:EXAMPLE:a123%2cz456






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

   o  urn:example:A123,z456

   o  urn:example:a123,Z456

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

   o  urn:example:%D0%B0123,z456

4.  URI Conformance

4.1.  Use in URI Protocol Slots

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

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

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









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4.2.  Parsing

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

4.3.  URNs and Relative References

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

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

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

4.4.  Transport and Display

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



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

4.5.  URI Design and Ownership

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

5.  URN Namespaces

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

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

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

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

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

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



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

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

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

      urn:example:apple:pear:plum:cherry

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

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

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




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

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

5.1.  Formal URN Namespaces

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

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

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

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

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






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

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

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

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

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

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

5.2.  Informal URN Namespaces

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





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

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

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

6.  Defining and Registering a URN Namespace

6.1.  Overview

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

   1.  The purpose of the URN namespace.

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

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

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

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

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

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







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6.2.  Registration Policy and Process: Community Registrations

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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

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

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





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

6.4.  Completing the Template

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

6.4.1.  Purpose

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

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

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

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

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

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

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

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






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6.4.2.  Syntax

   The "Syntax" section of the template contains:

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

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

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

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

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





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6.4.3.  Assignment

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

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

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

6.4.4.  Security and Privacy

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

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

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

   o  The potential for directory harvesting.

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






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6.4.5.  Interoperability

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

6.4.6.  Resolution

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

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

6.4.7.  Additional Information

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

   This section of the template is optional.












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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
      identifiers.

   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 [mailto:urn@ietf.org]

   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 (urn-nid@apps.ietf.org) is discontinued and
   replaced by the discussion list urn@ietf.org.



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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,
              <http://www.rfc-editor.org/info/rfc20>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc3986>.

   [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,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

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.




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   [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.27729.2012]
              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.3166-1]
              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, <http://www.rfc-editor.org/info/rfc1737>.

   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
              December 1994, <http://www.rfc-editor.org/info/rfc1738>.

   [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",
              RFC 1808, DOI 10.17487/RFC1808, June 1995,
              <http://www.rfc-editor.org/info/rfc1808>.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
              May 1997, <http://www.rfc-editor.org/info/rfc2141>.

   [RFC2276]  Sollins, K., "Architectural Principles of Uniform Resource
              Name Resolution", RFC 2276, DOI 10.17487/RFC2276, January
              1998, <http://www.rfc-editor.org/info/rfc2276>.

   [RFC2483]  Mealling, M. and R. Daniel, "URI Resolution Services
              Necessary for URN Resolution", RFC 2483,
              DOI 10.17487/RFC2483, January 1999,
              <http://www.rfc-editor.org/info/rfc2483>.

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
              DOI 10.17487/RFC2648, August 1999,
              <http://www.rfc-editor.org/info/rfc2648>.

   [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, <http://www.rfc-editor.org/info/rfc3044>.



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   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187,
              DOI 10.17487/RFC3187, October 2001,
              <http://www.rfc-editor.org/info/rfc3187>.

   [RFC3188]  Hakala, J., "Using National Bibliography Numbers as
              Uniform Resource Names", RFC 3188, DOI 10.17487/RFC3188,
              October 2001, <http://www.rfc-editor.org/info/rfc3188>.

   [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, <http://www.rfc-editor.org/info/rfc3406>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [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, <http://www.rfc-editor.org/info/rfc4854>.

   [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,
              <http://www.rfc-editor.org/info/rfc5122>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <http://www.rfc-editor.org/info/rfc6120>.

   [RFC6288]  Reed, C., "URN Namespace for the Defence Geospatial
              Information Working Group (DGIWG)", RFC 6288,
              DOI 10.17487/RFC6288, August 2011,
              <http://www.rfc-editor.org/info/rfc6288>.








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   [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,
              <http://www.rfc-editor.org/info/rfc6648>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <http://www.rfc-editor.org/info/rfc6943>.

   [RFC6963]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
              for Examples", BCP 183, RFC 6963, DOI 10.17487/RFC6963,
              May 2013, <http://www.rfc-editor.org/info/rfc6963>.

   [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,
              <http://www.rfc-editor.org/info/rfc6973>.

   [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,
              <http://www.rfc-editor.org/info/rfc7254>.

   [RFC7282]  Resnick, P., "On Consensus and Humming in the IETF",
              RFC 7282, DOI 10.17487/RFC7282, June 2014,
              <http://www.rfc-editor.org/info/rfc7282>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,
              <http://www.rfc-editor.org/info/rfc7320>.

   [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,
              <http://www.rfc-editor.org/info/rfc7462>.






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   [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,
              <http://www.rfc-editor.org/info/rfc7613>.

   [UAX31]    The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax", Unicode 9.0.0,
              June 2015, <http://unicode.org/reports/tr31/>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

   [URI-Registry]
              IANA, "Uniform Resource Identifier (URI) Schemes",
              <http://www.iana.org/assignments/uri-schemes>.

   [XML-BASE] Marsh, J. and R. Tobin, "XML Base (Second Edition)", W3C
              Recommendation REC-xmlbase-20090128, January 2009,
              <http://www.w3.org/TR/2009/REC-xmlbase-20090128>.

   [XML-NAMES]
              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,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

























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Appendix A.  Registration Template

   Namespace Identifier:  Requested of IANA (formal) or assigned by IANA
      (informal).

   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
      namespace.

   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.)







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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
      semantics.

   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.














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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
      f-components.

   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.



















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RFC 8141                          URNs                        April 2017


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 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.

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.

Authors' Addresses

   Peter Saint-Andre
   Filament
   P.O. Box 787
   Parker, CO  80134
   United States of America

   Phone: +1 720 256 6756
   Email: peter@filament.com
   URI:   <https://filament.com/>


   John C. Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   United States of America

   Phone: +1 617 245 1457
   Email: john-ietf@jck.com





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