URNBIS                                                    P. Saint-Andre
Internet-Draft                                                  Filament
Obsoletes: 2141, 3406 (if approved)                           J. Klensin
Intended status: Standards Track                        February 4, 2016
Expires: August 7, 2016

                     Uniform Resource Names (URNs)


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 7, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Specificity and This Standard . . . . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  URN Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Namespace Identifier  . . . . . . . . . . . . . . . . . .   8
     2.2.  Namespace Specific String . . . . . . . . . . . . . . . .   8
     2.3.  Optional Components . . . . . . . . . . . . . . . . . . .   9
       2.3.1.  q-component . . . . . . . . . . . . . . . . . . . . .  10
       2.3.2.  r-component . . . . . . . . . . . . . . . . . . . . .  11
       2.3.3.  f-component . . . . . . . . . . . . . . . . . . . . .  12
   3.  Equivalence of URNs . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Procedure . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  URI Conformance . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Use in URI Protocol Slots . . . . . . . . . . . . . . . .  15
     4.2.  Parsing . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.3.  URNs and Relative References  . . . . . . . . . . . . . .  16
     4.4.  Transport and Display . . . . . . . . . . . . . . . . . .  17
     4.5.  URI Design and Ownership  . . . . . . . . . . . . . . . .  17
   5.  URN Namespaces  . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Formal Namespaces . . . . . . . . . . . . . . . . . . . .  19
     5.2.  Informal Namespaces . . . . . . . . . . . . . . . . . . .  21
   6.  Defining and Registering a URN Namespace  . . . . . . . . . .  21
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  21

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     6.2.  Registration Policy and Process: Community Registrations   22
     6.3.  Registration Policy and Process: Fast Track for Standards
           Development Organizations, Scientific Societies, and
           Similar Bodies  . . . . . . . . . . . . . . . . . . . . .  23
     6.4.  Completing the Template . . . . . . . . . . . . . . . . .  24
       6.4.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . .  24
       6.4.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . .  25
       6.4.3.  Assignment  . . . . . . . . . . . . . . . . . . . . .  25
       6.4.4.  Security and Privacy  . . . . . . . . . . . . . . . .  26
       6.4.5.  Interoperability  . . . . . . . . . . . . . . . . . .  26
       6.4.6.  Resolution  . . . . . . . . . . . . . . . . . . . . .  27
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     7.1.  URI Scheme  . . . . . . . . . . . . . . . . . . . . . . .  27
     7.2.  Registration of URN Namespaces  . . . . . . . . . . . . .  28
   8.  Security and Privacy Considerations . . . . . . . . . . . . .  28
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  29
   Appendix A.  Registration Template  . . . . . . . . . . . . . . .  31
   Appendix B.  Changes from RFC 2141  . . . . . . . . . . . . . . .  32
   Appendix C.  Changes from RFC 3406  . . . . . . . . . . . . . . .  32
   Appendix D.  Contributors . . . . . . . . . . . . . . . . . . . .  33
   Appendix E.  Acknowledgements . . . . . . . . . . . . . . . . . .  33
   Appendix F.  Change log for versions of draft-ietf-urnbis-
                rfc2141bis-urn . . . . . . . . . . . . . . . . . . .  33
     F.1.  Changes from -08 to -09 . . . . . . . . . . . . . . . . .  33
     F.2.  Changes from -09 to -10 . . . . . . . . . . . . . . . . .  34
     F.3.  Changes from -10 to -11 . . . . . . . . . . . . . . . . .  34
     F.4.  Changes from -11 to -12 . . . . . . . . . . . . . . . . .  35
     F.5.  Changes from -12 to -13 . . . . . . . . . . . . . . . . .  35
     F.6.  Changes from -13 to -14 . . . . . . . . . . . . . . . . .  35
     F.7.  Changes from -14 to -15 . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

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

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

   This document rests on two key assumptions:

   1.  Assignment of a URN is a managed process.

   2.  The space of URN namespaces is itself managed.

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

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

   1.  Defines the canonical syntax for URNs in general (in a way that
       is consistent with URI syntax), specifies methods for determining
       URN equivalence, and discusses URI conformance.

   2.  Specifies a method for defining a URN namespace and associating
       it with a namespace identifier, and describes procedures for
       registering namespace identifiers with the Internet Assigned
       Numbers Authority (IANA).

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

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

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1.1.  Specificity and This Standard

   To a degree much greater than when URNs were first considered and
   their uses outlined (Cf.  [RFC1737]) issues of persistent identifiers
   on the Internet involve fundamental design tradeoffs and research
   questions that are much broader that URNs or the URN approach.  Ideal
   and comprehensive specifications about what should be done or
   required across the entire range of URNs would require general
   agreement about those issues and their resolution.  While some of
   them 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.

   Among these general issues, one that is specific to URNs is the
   fairly abstract topic of "resolution", discussed in Section 1.2,
   Section 2.3.2, and elsewhere below.  While it is possible to define
   the relationships quite precisely for a URN that resolves to a URL
   that, in turn, resolves (or locates) to a single target document or
   similar resource, that is only one special case albeit an important
   one.  URNs (either individually or entire namespaces as defined
   below) that do not resolve to URLs at all or that resolve to metadata
   or non-Internet objects are among URN use cases explicitly permitted
   by this specification; each leaves the concept of "resolution"
   somewhat more abstract and difficult than the simply URL resolution

   A similar set of issues arises for 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 tradeoffs 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 namespaces.

   This specification consequently contains some requirements and
   flexibility that would not be present in a more perfect world but
   that are necessary in order to allow producing any consensus
   specification at all rather than just giving up on URNs going

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1.2.  Terminology

   This document uses the terms "resolution" and "resolver" in roughly
   the sense from [RFC2276], i.e., "resolution" is the act of supplying
   services related to the identified resource, such as translating the
   persistent name into one or more current locators for the resource,
   delivering metadata about the resource in an appropriate format, or
   even delivering a document object from a convenient source without
   requiring further intermediaries.  At the time of this writing,
   resolution services are described in [RFC2483].  In order to
   underline the difference between the names and locators, this
   document uses the term Uniform Resource Locator (URL), rather than
   the generic term Uniform Resource Identifier (URI), to refer to
   locators; see also Section 1.1.3 of [RFC3986].

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

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  URN Syntax

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

   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 naming systems that include the '/'

   In particular, this specification does the following:

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   o  Extends URN syntax to explicitly allow the characters '/', "?",
      and "#", which were reserved for future use by [RFC2141]; this
      change effectively also allows several components of the URI
      syntax although without necessarily tying those components to URI

   o  Defines syntax for an additional component that can be used in
      interactions with a URN resolution service.

   o  Makes several smaller syntax adjustments.

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

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

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

   The question mark character "?" can be used without percent-encoding
   inside q-components, r-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.

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2.1.  Namespace Identifier

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

   NIDs are case insensitive (e.g., "ISBN" and "isbn" are equivalent) as
   a consequence of requirements imposed by RFC 3986.

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

2.2.  Namespace Specific String

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

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

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

   Depending on the rules governing a namespace, strings that are valid
   in an NSS associated with a namespace might contain characters that
   are not allowed by the "pchar" production referenced above (e.g.,
   characters outside the ASCII range or, consistent with the
   restrictions in RFC 3986, the characters "/", "?", "#", "[", and
   "]").  While such a string might be a valid name, it is not a valid
   URN until it has been translated into a conformant NSS.  In the case
   of URNs that are formed from names that exist separately in a

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   standalone identifier space, translation of an identifier from its
   "native" format to URN format is accomplished by using the
   canonicalization and encoding methods defined for that URN namespace.
   Software that is not aware of those namespace-specific
   canonicalization and encoding rules MUST NOT construct URNs from the
   names in the standalone identifier space.

   In 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.  To the extent feasible
   consistent with the requirements of identifiers defined and
   standardized elsewhere and the principles discussed in Section 1.1,
   strings SHOULD be restricted to either ASCII letters and digits or to
   the characters and syntax of some widely-used identifier model such
   as those of IDNA [RFC5890], PRECIS [RFC7613], or the Unicode
   Identifier and Pattern Syntax spec [UAX31].

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

2.3.  Optional Components

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

   o  The q-component is intended for parameters to be transmitted to
      either the named resource or a system that can supply the
      requested service, and interpreted by that resource or system.

   o  The r-component is intended for parameters to be transmitted to
      resolution services (taken broadly, see Section 1.1) and
      interpreted by those services.

   o  The f-component is intended to be interpreted by the client as a
      specification for a location within, or region of, the named

   The foregoing generalizations are true independent of the URN's

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

   In general and for consistency with RFC 3986, neither the syntax nor
   the semantics of q-components or f-components are defined by, or
   dependent on, the namespace of the URN.  A particular namespace
   might, however, define uses of r-components that are specific to its
   namespace and supported by the resolution services which that
   namespace operates or recommends.

   Any of these components MAY be used with URNs from existing
   namespaces whether or not the namespace explicitly supports them.  As
   described above, the interpretation of q-component and f-component is
   namespace-independent.  Interpretation of r-components is discused in
   Section 2.3.2 below.

2.3.1.  q-component

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

   The sequence "?=" begins the q-component.  The q-component ends with
   a "?+" sequence (which begins an r-component) or a "#" character
   (number sign, which begins an f-component).  If neither of those
   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

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   [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.

   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

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


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

2.3.2.  r-component

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

   The sequence "?+" begins 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] are not allowed in the
   first token of the r-component (see below) and otherwise MUST be
   percent-encoded using the method defined in Section 2.1 of the
   generic URI specification [RFC3986].

   As described under 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.

   The r-component is primarily intended for passing information in
   requests to URN resolution services.  Although other potential uses
   are also allowed, the r-component MUST NOT be used to pass
   information to the resources identified by a URN or to applications

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   that manage such resources, since that is the function of the

   From the standpoint of syntax, the r-component consists of a
   resolution service identifier and a string.  The resolution service
   identifier must consist of ASCII characters obeying the same syntax
   rules as for the NID (including case-independence).  The string is
   terminated as described above.

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


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

2.3.3.  f-component

   The URN f-component has the same syntax as the URI fragment
   component.  If a URN containing an f-component resolves to a single
   URL associated with the named resource, the f-component from the URN
   can be applied (usually by the client) verbatim as the fragment of
   that URL.

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

   As described under Section 3, the f-component SHALL NOT be taken into
   account when determining URN equivalence.

   Clients SHOULD NOT pass f-components to resolution servers unless
   those servers also perform object retrieval and interpretation

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

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


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

3.  Equivalence of URNs

3.1.  Procedure

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

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

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

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

   2.  the NID, by conversion to lower case

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

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

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

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

3.2.  Examples

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

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

   o  urn:example:a123,z456

   o  URN:example:a123,z456

   o  urn:EXAMPLE:a123,z456

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

   o  urn:example:a123,z456?=abc

   o  urn:example:a123,z456#789

   o  urn:example:a123,z456#abc

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

   o  urn:example:a123,z456/foo

   o  urn:example:a123,z456/bar

   o  urn:example:a123,z456/baz

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

   o  urn:example:a123%2Cz456

   o  URN:EXAMPLE:a123%2cz456

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

   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 equivalent:

   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
   (e.g., the 'href' and 'src' attributes in HTML, the <base/> element
   in HTML, the 'xml:base' attribute in XML [XML-BASE], and the 'xmlns'
   attribute in XML for XML namespace names [XML-NAMES]).

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

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

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   and it is not the place of this specification to require or restrict
   usage for individual protocol slots.

4.2.  Parsing

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

4.3.  URNs and Relative References

   [RFC3986] Section 5.2 describes an algorithm for converting a URI
   reference that might be relative to a given base URI into "parsed
   components" of the target of that reference, which can then be
   recomposed per RFC 3986 Section 5.3 into a target URI.  This
   algorithm cannot be applied directly to URNs because their syntax
   does not support the necessary path components.  The notion of a URN
   as a "persistent", "permanent" identifier does not reconcile easily
   with relative referencing.  However, resources named with URNs may
   contain relative references that do not apply to the URN itself.

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

   1.  a base URI (other than a URN) declared by the resource itself; or

   2.  a base URI (other than a URN) obtained through the URN resolution
       process; or

   3.  the URL of the resource as obtained through the URN resolution

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

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

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

4.4.  Transport and Display

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

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

4.5.  URI Design and Ownership

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

5.  URN Namespaces

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

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

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

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

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

   With regard to global uniqueness, using different NIDs for different
   collections of identifiers ensures that no two URNs will be the same
   for different resources, since each collection is required to
   uniquely assign each identifier.  However, a single resource MAY have
   more than one URN assigned to it, either in the same namespace (if
   the namespace permits it) or in different namespaces, and either for
   similar purposes or different purposes.  (For example, if a book were
   published in a monograph series, it could have both an ISBN [RFC3187]
   and an ISSN [RFC3044] assigned to it, resulting in two URNs referring
   to the same book.)  Subject to other constraints, such as those
   imposed by the URI syntax [RFC3986], the rules of the URN scheme are
   intended to allow preserving the normal and natural form of
   identifiers specified elsewhere when they are treated as URN

   With regard to the structure of URNs assigned within a namespace, the
   development of an identifier structure (and thereby a collection of
   identifiers) depends on the requirements of the community defining
   the identifiers, how the identifiers will be assigned and used, etc.
   These issues are beyond the scope of URN syntax and the general rules
   for URN namespaces, because they are specific to the community
   defining a namespace (e.g., the bibliographic and publishing
   communities in the case of the 'ISBN' and 'ISSN' namespaces, or the
   developers of extensions to the Extensible Messaging and Presence
   Protocol in the case of the 'XMPP' namespace).

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

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

   2.  Optionally, they can be registered in global registration

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

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

5.1.  Formal Namespaces

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

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

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

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

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

   A formal 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

   3.  It MUST be more than two characters long.

   4.  It MUST NOT start with ALPHA ALPHA "-", i.e., any string
       consisting of two letters followed by one hyphen.

   5.  It MUST NOT start with the string "xn--" or any other string
       consisting of two letters followed by two hyphens.  Such strings
       are reserved for potential representation of DNS A-labels and
       similar strings in the future [RFC5890].

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

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

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

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   will become involved in such disputes except in response to orders
   from a court of competent jurisdiction.

5.2.  Informal Namespaces

   Informal namespaces are full-fledged URN namespaces, with all the
   associated rights and responsibilities.  Informal namespaces differ
   from formal namespaces in the process for assigning a NID: for an
   informal namespace, the registrant does not designate the NID;
   instead, IANA assigns a NID consisting of the string 'urn-' followed
   by one or more digits (e.g., "urn-7") where the digits consist of the
   next available number in the sequence of positive integers assigned
   to informal namespaces.  Thus the syntax of an informal namespace is:

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

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

6.  Defining and Registering a URN Namespace

6.1.  Overview

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

   1.  The purpose of the namespace.

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

   3.  The process for assigning URNs within the namespace.

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

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

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   6.  Optionally, the process for resolving URNs issued within the

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

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
   namespaces or their definitions that are intended to become standards
   or normative components of standards, the output of the Expert Review
   process is intended to be a report, rather than instructions to IANA
   to take action (see below).  The key steps are:

   1.  Fill out the namespace registration template (see Section 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 necessary.

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

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

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

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

   Experience to date with namespace registration requests has shown
   that registrants sometimes do not initially understand some of the
   subtleties of URN namespaces, and that defining the namespace in the
   form of a specification enables the registrants to clearly formulate
   their "contract" with the intended user community.  Therefore,
   although the registration policy for formal namespaces is Expert
   Review and a specification is not strictly required, it is
   RECOMMENDED for registrants to provide a stable specification

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   documenting the namespace definition and expanding upon the issues
   described herein.

   Because naming can be difficult and contentious, namespace
   registrants and the designated experts are strongly encouraged to
   work together in a spirit of good faith and mutual understanding to
   achieve rough consensus (see [RFC7282]) on handling registration
   requests.  They are also encouraged to bring additional expertise
   into the discussion if that would be helpful in providing perspective
   or otherwise resolving issues.

   Especially when iterations in the registration process are prolonged,
   designated experts are expected to take reasonable precautions to
   avoid race conditions on proposed NID names and, if such situations
   arise, to encourage applicants to work out any conflicts among

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 or to reflect knowledge, terminology, or information
   organization that lie well outside the IETF's scope or the likely
   subject matter knowledge of its Designated Experts.  In situations in
   which the registration request originates from, or is authorized by,
   a recognized standards-related organization, scientific society, or
   similar body, a somewhat different procedure is available at the
   option of that body:

   1.  The namespace registration template is filled out and submitted
       as in steps 1 and 2 above.

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

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

   4.  When that body concludes that the application is sufficiently
       mature, they will request that IANA will complete the
       registration for the NID and IANA will do so.

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

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

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

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

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

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

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

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

   7.  Whether the namespace or its definition are expected to become an
       integral or normative element of a standard being developed in
       the IETF or some other recognized standards body.

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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 namespace, in
       conformance with the fundamental URN syntax.  The structure might
       be described in terms of a formal definition (e.g., using
       Augmented BNF for Syntax Specifications (ABNF) as specified in
       [RFC5234]), an algorithm for generating conformant URNs, or a
       regular expression for parsing the identifier into components;
       alternatively, the structure might be opaque.

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

   3.  Rules for determining equivalence between two identifiers in the
       namespace.  Such rules ought to always have the effect of
       eliminating false negatives that might otherwise result from
       comparison.  If it is appropriate and helpful, reference can be
       made to specific equivalence rules defined in the URI
       specification [RFC3986].  Examples of equivalence rules include
       equivalence between uppercase and lowercase characters in the
       Namespace Specific String, between hyphenated and non-hyphenated
       groupings in the identifier string, or between single-quotes and
       double-quotes.  (Note that these are not normative statements for
       any kind of best practice related to handling of equivalences
       between characters in general; they are statements limited to one
       particular namespace only.)

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

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

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       recognized by a particular organization); if limited, it ought to
       explain how to become an assigner of identifiers or how to
       request assignment of identifiers from existing assignment

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

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 identifiers within the namespace.
   Examples of such issues include:

   o  The consequences of producing false negatives and false positives
      during comparison for equivalence (see "Issues in Identifier
      Comparison for Security Purposes" [RFC6943])

   o  Leakage of private information when identifiers 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].

6.4.5.  Interoperability

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

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6.4.6.  Resolution

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

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

7.  IANA Considerations

7.1.  URI Scheme

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

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

   URI Scheme Name:  urn

   Status:  permanent

   URI Scheme Syntax:  See Section 2 of [ this document ].

   URI Scheme Semantics:  The 'urn' scheme identifies Uniform Resource
      Names, which are persistent, location-independent resource

   Encoding Considerations:  See Section 2 of [ this document ].

   Applications/Protocols That Use This URI Scheme Name:  Uniform
      Resource Names are used in a wide variety of applications,
      including bibliographic reference systems and as names for
      Extensible Markup Language (XML) namespaces.

   Interoperability Considerations:  See Section 4 of [ this document ].

   Security Considerations:  See Section 6.4.4 and Section 8 of [ this
      document ].

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   Contact:  URNBIS WG [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 this document have been completed.

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 identifiers within the namespace (e.g., some namespace
   resolvers might assign special meaning to certain characters in the
   Namespace Specific String); see Section 6.4.4 for further discussion.

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

9.  References

9.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, 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)",
              June 2003,

              Klensin, J., "URN Semantics Clarification", draft-ietf-
              urnbis-semantics-clarif-03 (work in progress), February

              ISO, "Codes for the representation of names of countries
              and their subdivisions -- Part 1: Country codes",
              ISO 3166-1:2013, 2013.

   [RFC1737]  Sollins, K. and L. Masinter, "Functional Requirements for
              Uniform Resource Names", RFC 1737, December 1994.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2276]  Sollins, K., "Architectural Principles of Uniform Resource
              Name Resolution", RFC 2276, January 1998.

   [RFC2483]  Mealling, M. and R. Daniel, "URI Resolution Services
              Necessary for URN Resolution", RFC 2483, January 1999.

   [RFC3044]  Rozenfeld, S., "Using The ISSN (International Serial
              Standard Number) as URN (Uniform Resource Names) within an
              ISSN-URN Namespace", RFC 3044, January 2001.

   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187, October

   [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", BCP 66, RFC 3406, October 2002.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July

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   [RFC4854]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
              for Extensions to the Extensible Messaging and Presence
              Protocol (XMPP)", RFC 4854, April 2007.

   [RFC5122]  Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 5122, February 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [RFC6963]  Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
              for Examples", BCP 183, RFC 6963, May 2013.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973, July

   [RFC7282]  Resnick, P., "On Consensus and Humming in the IETF",
              RFC 7282, June 2014.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, July 2014.

   [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", June 2015,

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   [UNICODE]  The Unicode Consortium, "The Unicode Standard", 2015,

              IANA, "Permanent URI Schemes",

              Marsh, J. and R. Tobin, "XML Base (Second Edition)", World
              Wide Web Consortium Recommendation REC-xmlbase-20090128,
              January 2009,

              Thompson, H., Hollander, D., Layman, A., Bray, T., and R.
              Tobin, "Namespaces in XML 1.0 (Third Edition)", World Wide
              Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,

Appendix A.  Registration Template

   Namespace ID:  Requested of IANA (formal) or assigned by IANA

   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 or
      scientific society requesting the fact track registration
      procedure (see Section 6.3), that information should be clearly
      indicated in this section of the template.

   Purpose:  Described under Section 6.4.1 of this document.

   Syntax:  Described under Section 6.4.2 of this document.  Unless the
      registration explicitly says otherwise, use of q-components and
      f-components is not allowed for this namespace.

   Assignment:  Described under Section 6.4.3 of this document.

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   Security and Privacy:  Described under Section 6.4.4 of this

   Interoperability:  Described under Section 6.4.5 of this document.

   Resolution:  Described under 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 the namespace.

   Revision Information:  Description of changes from prior version(s).
      (Applicable only when earlier registrations have been revised.)

Appendix B.  Changes from RFC 2141

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

   o  Formally registers 'urn' as a URI scheme.

   o  Disallows "-" at the end of a NID.

   o  Allows the "/", "~", and "&" characters in the namespace-specific
      string (NSS).

   o  Allows q-components, r-components, and f-components.

Appendix C.  Changes from RFC 3406

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

   1.  Relaxes the registration policy for formal namespaces from "IETF
       Review" to "Expert Review" as discussed in Section 6.2.

   2.  Removes the category of experimental namespaces, consistent with

   3.  Simplifies the registration template.

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

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Appendix D.  Contributors

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

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

   Their work is gratefully acknowledged.

Appendix E.  Acknowledgements

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

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

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

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

F.1.  Changes from -08 to -09

   o  Altered the text in Section 4 to reflect list discussions about
      the earlier phrasing.  Also added DOI example and citation to that

   o  Clarified the naming rules for formal namespaces and their
      relationship to ISO 3166, IDNA, etc., reserved strings.

   o  Added an explicit statement about use of URNs in various protocols
      and contexts to Section 4.

   o  Clarified that experimental namespace NIDs, which were explicitly
      not registered, are not valid URNs (in Section 5.

   o  Transformed the partial production in Section 5.2 into valid ABNF.

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   o  Added more text about p-/q-/f-components and recommendations about

   o  Added clarifying note about "?" within q-components and

   o  Added explicit requirement that revisions of existing
      registrations document the changes and added a slot for that
      description to the template.

   o  Many small editorial changes and adjustments including adding
      additional references and cross-references for clarification.

   o  Inserted a placeholder for additional examples.

F.2.  Changes from -09 to -10

   o  Several clarifying editorial changes, most suggested by Ted Hardie
      and Henry S.  Thompson (some of them off-list).

   o  Added a large number of placeholders that identify issues that
      require WG consideration and resolution (or WG delegation to the

F.3.  Changes from -10 to -11

   o  Removed most of the placeholders added in -10.  Supplied new text
      as required or suggested by on-list discussion of those issues.

   o  Replaced the conformance examples Section 3.2 with a more complete
      collection and discussion.

   o  Revised and consolidated the registration procedure, and added
      provisions for NIDs that are the subject of standards and for
      avoiding race conditions about NID strings.

   o  In response to independent comments from Ted Hardie and Henry S.
      Thompson, called attention to the possibility of conflicts between
      NID strings and various claims of national, corporate, and other

   o  Changed the production for assigned-name as suggested by Lars

   o  Several clarifying editorial changes including correcting a glitch
      in instructions to the RFC Editor.

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F.4.  Changes from -11 to -12

   o  Removed p-components as a standalone construct, and instead folded
      them into the NSS.

   o  Defined syntax for r-components as a way to pass information to
      resolvers, but left the semantics for future standardization

   o  Further tuned the discussion of interoperability and related
      registration issues.

   o  Made a number of editorial corrections and reorganized the syntax
      material in Section 2 somewhat to make it internally consistent
      and keep the relationship to RFC 3986 clear.

F.5.  Changes from -12 to -13

   o  More precisely defined the semantics of the optional components.

   o  Defined the term "resolution" and clarified several related
      matters throughout the text.

   o  Clarified terminological relationship to RFC 3986.

   o  Further cleansed the document of p-components.

   o  Corrected several examples to avoid confusion with existing
      identifier systems.

   o  Improved text regarding the purpose of namespaces being

F.6.  Changes from -13 to -14

   o  Reverted the ABNF to what had been defined in version -12.

   o  Added fast-track approval process for standards-related
      organizations, scientific societies, and similar bodies (similar
      to RFC 6838 for Media Types).

F.7.  Changes from -14 to -15

   o  Reorganized the Introduction slightly, adding new subsection 1.1
      and making Terminology (the former Section 2) Section 1.2.

   o  Tightened the discussion of "resolution" somewhat to try to
      mitigate some on-list confusion.

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   o  Added some text about character set choices and repertoires
      (consistent with the Section 1.1 explanation).

   o  Moved away from "?" and "??" for q-component and r-component
      delimiters and went to two-character sequences for each.  This
      includes several changes to the text to remove or modify
      discussions of string termination and the role of a question mark
      not followed by one of the new delimiters.

   o  Redefined r-component to be an ASCII resolver ID and a string.
      Neither is further defined in this specification and text has been
      added to say that.

   o  Several editorial changes to improve clarity, most following up on
      comments made on the list.  These included modifying the table of
      contents so that the subsections on optional components now appear

Authors' Addresses

   Peter Saint-Andre

   Email: peter@filament.com
   URI:   https://filament.com/

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140

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

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