IETF URNbis WG                                            A. Hoenes, Ed.
Internet-Draft                                                    TR-Sys
Obsoletes: 2141 (if approved)                             March 12, 2012
Intended status: Standards Track
Expires: September 13, 2012

                   Uniform Resource Name (URN) Syntax


   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers.  This document serves as
   the foundation of the 'urn' URI Scheme according to RFC 3986 and sets
   forward the canonical syntax for URNs, which subdivides URNs into
   "namespaces".  A discussion of both existing legacy and new
   namespaces and requirements for URN presentation and transmission are
   presented.  Finally, there is a discussion of URN equivalence and how
   to determine it.  This document supersedes RFC 2141.

   The requirements and procedures for URN Namespace registration
   documents are set forth in BCP 66, for which RFC 3406bis is the
   companion revised specification document replacing RFC 3406.


   Comments are welcome on the mailing list (or sent to the
   document editor).  The home page of the URNbis WG is located at

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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 13, 2012.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Historical Perspective and Motivation  . . . . . . . . . .  4
     1.2.  Background on Properties of URNs . . . . . . . . . . . . .  6
     1.3.  Objective of this Memo . . . . . . . . . . . . . . . . . .  7
     1.4.  Requirement Language . . . . . . . . . . . . . . . . . . .  8
   2.  URN Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Namespace Identifier (NID) Syntax  . . . . . . . . . . . . 13
     2.2.  Namespace Specific String (NSS) Syntax . . . . . . . . . . 15
     2.3.  Special and Reserved Characters  . . . . . . . . . . . . . 15
       2.3.1.  Delimiter Characters . . . . . . . . . . . . . . . . . 16
       2.3.2.  The Percent Character, Percent-Encoding  . . . . . . . 16
       2.3.3.  Other Excluded Characters  . . . . . . . . . . . . . . 17
   3.  Support of Existing Legacy Naming Systems and New Naming
       Systems  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.  URN Presentation and Transport . . . . . . . . . . . . . . . . 18
   5.  Lexical Equivalence of URNs  . . . . . . . . . . . . . . . . . 18
     5.1.  Examples of Lexical Equivalence  . . . . . . . . . . . . . 19
   6.  Functional Equivalence of URNs . . . . . . . . . . . . . . . . 19
   7.  The 'urn' URI Scheme . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  Registration of URI Scheme 'urn' . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     11.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  Handling of URNs by URL Resolvers/Browsers  . . . . . 26
   Appendix B.  Collected ABNF (Informative)  . . . . . . . . . . . . 26
   Appendix C.  Breakdown of NSS Syntax Evolution since RFC 2141
                (Informative) . . . . . . . . . . . . . . . . . . . . 27
   Appendix D.  Changes since RFC 2141 (Informative)  . . . . . . . . 29
     D.1.  Essential Changes from RFC 2141  . . . . . . . . . . . . . 29
     D.2.  Changes from RFC 2141 to Individual Draft -00  . . . . . . 29
     D.3.  Changes from Individual Draft -00 to -02 . . . . . . . . . 30
     D.4.  Changes from Individual Draft -02 to WG Draft -00  . . . . 30
     D.5.  Changes from WG Draft -00 to WG Draft -01  . . . . . . . . 30
     D.6.  Changes from WG Draft -01 to WG Draft -02  . . . . . . . . 31
   Appendix E.  How to Locate IETF Documents (Informative)  . . . . . 32

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1.  Introduction

   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers and are designed to make
   it easy to map other namespaces (that share the properties of URNs)
   into URI-space.  Therefore, the URN syntax provides a means to encode
   character data in a form that can be sent in existing protocols,
   transcribed on most keyboards, etc.

   To this end, URNs are designed as an intrinsic part of the more
   general framework of Uniform Resource Identifiers (URIs); 'urn' is a
   particular URI Scheme (according to STD 66, RFC 3986 [RFC3986] and
   BCP 35, RFC 4395 [RFC4395]) that is dedicated to forming a
   hierarchical framework for persistent identifiers.

   The first level of hierarchy is given by the classification of URIs
   into "URI Schemes", and for URNs, the second level is organized into
   "URN Namespaces".  Henceforth both terms are used in this
   capitalization to distinguish them from the more general common
   meaning of "scheme" and "namespace".

   It is an explicit design goal that pre-existing systems of persistent
   identifiers are mapped into the URN framework.  Ordinarily, each such
   traditional identifier system (namespace) -- standard or otherwise --
   will occupy its own URN Namespace.  However, shared URN Namespaces
   are possible (and in fact, already exist), but the identifier-driven
   mechanisms needed to distinguish the originating namespaces make
   registration and maintenance of such URN Namespaces more complicated.

   URN (as a URI Scheme) as such does not have a specific scope.  The
   applicability of the URN system, that is, the totality of the
   resources that URNs can be assigned to, is the union of all
   identifier systems that have an associated registered URN Namespace.
   Ideally every new namespace will thus extend the URN applicability.

1.1.  Historical Perspective and Motivation

   Since this RFC will be of particular interest for groups and
   individuals that are interested in persistent identifiers in general
   and not in continuous contact with the IETF and the RFC series, this
   section gives a brief outline of the evolution of the matter over
   time.  Appendix E gives hints on how to obtain RFCs and related

   Attempts to define generally applicable identifiers for network
   resources go back to the mid-1970s.  Among the applicable RFCs is RFC
   615 [RFC0615], which subsequently has been obsoleted by RFC 645

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   The seminal document in the RFC series regarding URIs (Uniform
   Resource Identifiers) for use with the World Wide Web (WWW) was RFC
   1630 [RFC1630], published in 1994.  In the same year, the general
   concept or Uniform Resource Names has been laid down in RFC 1737
   [RFC1737] and that of Uniform Resource Locators in RFC 1736

   The original formal specification of URN Syntax, RFC 2141 [RFC2141]
   was adopted in 1997.  That document was based on the original
   specification of URLs (Uniform Resource Locators) in RFC 1738
   [RFC1738] and RFC 1808 [RFC1808], which later on, in 1998, was
   generalized and consolidated in the Generic URI specification,
   RFC 2396 [RFC2396].  Most parts of these URI/URL documents were
   superseded in 2005 by STD 66, RFC 3986 [RFC3986].  Notably, RFC 2141
   makes (essentially normative) reference to a draft version of
   RFC 2396.

   Over time, the terms "URI", "URL", and "URN" have been refined and
   slightly shifted according to emerging insight and use.  This has
   been clarified in a joint effort of the IETF and the World Wide Web
   Council, published 2002 for the IETF in RFC 3305 [RFC3305].

   The wealth of URI Schemes and URN Namespaces needs to be organized in
   a persistent way, in order to guide application developers and users
   to the standardized top level branches and the related
   specifications.  These registries are maintained by the Internet
   Assigned Numbers Authority (IANA) [IANA] at [IANA-URI] and
   [IANA-URN], respectively.  Registration procedures for URI Schemes
   originally had been laid down in RFC 2717 [RFC2717] and guidelines
   for the related specification documents were given in RFC 2718
   [RFC2718].  These documents have been obsoleted and consolidated into
   BCP 35, RFC 4395 [RFC4395], which is based on, and aligned with,
   RFC 3986.

      Note that RFC 2141 predates RFC 2717 and, although the 'urn' URI
      scheme traditionally was listed in [IANA-URI] with a pointer to
      RFC 2141, this registration has never been performed formally.

   Similarly, the URN Namespace definition and registration mechanisms
   originally have been specified in RFC 2611 [RFC2611], which has been
   obsoleted by BCP 66, RFC 3406 [RFC3406].  Guidelines for documents
   prescribing IANA procedures have been revised as well over the years,
   and at the time of this writing, BCP 26, RFC 5226 [RFC5226] is the
   normative document.  Neither RFC 4395 nor RFC 3406 conform to
   RFC 5226.

   Early documents specifying URI and URN syntax, including RFC 2141,
   made use of an ad-hoc variant of the original Backus-Naur Form (BNF)
   that never has been formally specified.

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   Over the years, the IETF has shifted to the use of a predominant
   formal language used to define the syntax of textual protocol
   elements, dubbed "Augmented Backus-Naur Form" (ABNF).  The
   specification of ABNF also has evolved, and now STD 68, RFC 5234
   [RFC5234] is the normative document for it (that also will be used in
   this RFC).

1.2.  Background on Properties of URNs

   This section aims at quoting requirements as identified in the past;
   it does not attempt to revise or redefine these requirements, but it
   gives some hints where more than a decade of experience with URNs has
   shed a different light on past views.  The citations below are given
   here to make this document self-contained and avoid normative down-
   references to old work.

   RFC 1738 [RFC1738] defined the purpose of URNs as follows:

   o  The purpose or function of a URN is to provide a globally unique,
      persistent identifier used for recognition, for access to
      characteristics of the resource, or for access to the resource

   Section 2 of RFC 1738 [RFC1738] listed the functional requirements
   for URNs (quote slightly edited to reflect the time passed since that
   RFC was written and the actual definition of the URN scheme that has

   o  Global scope: A URN is a name with global scope which does not
      imply a location.  It has the same meaning everywhere.

   o  Global uniqueness: The same URN will never be assigned to two
      different resources.

   o  Persistence: It is intended that the lifetime of a URN be
      permanent.  That is, the URN will be globally unique forever, and
      may well be used as a reference to a resource well beyond the
      lifetime of the resource it identifies or of any naming authority
      involved in the assignment of its name.

   o  Scalability: URNs can be assigned to any resource that might
      conceivably be available on the network, for hundreds of years.

   o  Legacy support: The URN scheme permits the support of existing
      legacy naming systems, insofar as they satisfy the other
      requirements described here. [...]

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   o  Extensibility: The URN scheme permits future extensions.

   o  Independence: It is solely the responsibility of a name issuing
      authority to determine the conditions under which it will issue a

   o  Resolution: URNs will not impede resolution. [...]

   The URN syntax described below also accommodates the fundamental
   "Requirements for URN Encoding" in Section 3 of RFC 1738 [RFC1738],
   as far as experience gained has not lead to relax unrealistical
   detail requirements:

   o  Single encoding: The encoding for presentation for people in clear
      text, electronic mail and the like is the same as the encoding in
      other transmissions.

   o  Simple comparison: A comparison algorithm for URNs is simple,
      local, and deterministic. [...]

   o  Human transcribability: For URNs to be easily transcribable by
      humans without error, they need to be short, use a minimum of
      special characters, and be case insensitive. [...]

         In particular practice gained with active URN Namespaces has
         shown that this former goal is rather unrealistic, since
         usually preference is given to 1:1 usage of existing
         namespaces, which might not have this property.  However, we
         hold that, at least, the rough kind of resource identified by a
         URN should be easily recognizable for humans.

   o  Transport friendliness: A URN can be transported unmodified in the
      common Internet protocols, such as TCP, SMTP, FTP, Telnet, etc.,
      as well as printed paper.

   o  Machine consumption: A URN can be parsed by a computer.

   o  Text recognition: The encoding of a URN needs to enhance the
      ability to find and parse URNs in free text.

1.3.  Objective of this Memo

   RFC 2141 does not seamlessly match current Internet Standards.  The
   primary objective of this document is the alignment with the URI
   standard [RFC3986] and URI Scheme guidelines [RFC4395], the ABNF
   standard [RFC5234] and the current IANA Guidelines [RFC5226] in

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   Further, experience from emerging international efforts to establish
   a general, distributed, stable URN resolution service have been taken
   into account during the draft stage of this document.

   For advancing the URN specification on the Internet Standards-Track,
   it needs to be based on documents of comparable maturity.  Therefore,
   to further advancements of the formal maturity level of this RFC, it
   deliberately makes normative references only to documents at Full
   Standard or Best Current Practice level.

   Thus, this replacement document for RFC 2141 should make it possible
   to advance the URN framework on the Internet Standard maturity
   ladder.  All other related documents depend on it; therefore this is
   the first step to undertake.

   Out of scope for this document is a revision of the URN Namespace
   Definition Mechanisms document, BCP 66.  This is being undertaken in
   a companion document, RFC 3406bis

1.4.  Requirement Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14 [RFC2119].

2.  URN Syntax

   This document defines the URI Scheme 'urn'.  Hence, URNs are specific
   URIs as specified in STD 66 [RFC3986].  The formal syntax definitions
   below are given in ABNF according to STD 68 [RFC5234] and make use of
   some "Core Rules" specified in Appendix B of that Standard and
   several generic rules defined in Appendix A of RFC 3986.

   The syntax definitions below do, and syntax definitions in dependent
   documents MUST, conform to the URI syntax specified in RFC 3986, in
   the sense that additional syntax rules must only constrain the
   general rules from RFC 3986.  In other words: a general URI parser
   based on RFC 3986 MUST be able to parse any legal URN, and specific
   semantics can be obtained from URN-specific parsing.

   URNs conform to the <path-rootless> variant of the general URI syntax
   specified in Section 3 of [RFC3986], reproduced here informally:

      URI = scheme ":" path-rootless [ "?" query ] [ "#" fragment ]

      path-rootless = segment-nz *( "/" segment )

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      segment-nz    = 1*pchar
      segment       = *pchar

      pchar = unreserved / pct-encoded / sub-delims / ":" / "@"

   In the case of URNs, we have:

      scheme     = "urn"

   and for <path-rootless>, only a single segment is used, but the
   following additional syntax rule is superimposed on <path-rootless>
   to establish a level of hierarchy called "Namespace":

      urn-path   = NID ":" NSS

   Here "urn" is the URI scheme name, <NID> is the Namespace Identifier,
   and <NSS> is the Namespace Specific String.  The colons are REQUIRED
   separator characters.

      Note that it is common practise in several existing URN Namespaces
      (and fully supported by this syntax) to use additional colon(s) as
      separator character(s) in order to introduce further level(s) of
      hierarchy into the NSS syntax, where needed.  (See also
      Section 2.3.1 below.)

   Per RFC 3986, the URN Scheme name (here "urn") is case-insensitive.

   The Namespace ID (also a case-insensitive string) determines the
   syntactic structure and the semantic interpretation of the Namespace
   Specific String.  Details on NID syntax can be found below in
   Section 2.1, and the NSS syntax is elaborated upon in Section 2.2.

   Each particular URN Namespace is based on a specific document that
   must normatively describe (among other things) the details of the
   <NSS> values allowed in conjunction with the respective <NID>.  The
   syntax and semantics of these <NSS> values are ordinarily specified
   by an existing persistent identifier system (namespace); for
   instance, in the 'ISBN' URN Namespace, each NSS must be a valid ISBN.
   Some URN Namespaces may have strict rules for well formed NSSs, while
   some others may be far more relaxed.  There may also be significant
   differences regarding the identifier assignment process.  The overall
   specification requirements and registration procedures for URN
   Namespaces are the subject of a dedicated companion document, BCP 66,
   which has been updated for conformance to BCP 26 and alignment with
   implementation experience RFC 3406bis

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      RFC 2141 was published before the URI Generic Syntax was finalized
      and therefore had to defer the decision on whether <query> and
      <fragment> components are applicable to URNs.  RFC 2141 therefore
      has reserved the use of bare (unencoded) question mark ("?") and
      hash ("#") characters in URNs for future usage in conformance with
      the generic URI syntax.

      URNs have been in use for more than a decade.  Some user
      communities want to be able to use these components (which are
      split off by the high-level parsing rules of RFC 3986), or at
      least the <fragment> component, in the context of their focal
      URNs.  Therefore, this document allows the designers of selected
      URN Namespaces to specify the use of the <fragment> component with
      URNs belonging these Namespaces, whereas the specification of
      usage of the <query> component is set aside to future
      standardization efforts for URN resolution.  Thus, this draft
      allows both of these components in the general syntax.


      Regarding fragment identifiers, Section 3.5, para 1 of RFC 3986,
      indicates that "The fragment identifier ... allows indirect
      identification of a secondary resource by reference to a primary
      resource and additional identifying information.  The identified
      secondary resource may be some portion or subset of the primary
      resource, some view on representations of the primary resource, or
      some other resource defined or described by those
      representations."  RFC 3986 continues in specifying that the
      details of the interpretation of fragment identifiers are specific
      to the media types returned upon resolution of an URI.  The
      entirety of the purposes mentioned in the above quote obviously
      only can be achieved fully if the "consumer" of the URI becomes
      aware of the fragment identifier as part of the requested URI,
      since, e.g., secondary resources might consist in representations
      might only be available in particular media types.  However, RFC
      3986 subsequently (in the penultimate paragraph of Section 3.5)
      specifies that the evaluation of fragment identifiers be a client-
      side matter and browsers are to strip them from request URIs sent
      in information retrieval protocols.
      Based on this, contemporary web browsers do not communicate
      fragment identifiers to the web server but perform fragment
      selection locally on the returned (HTML) resource.  To make things
      even more complicated, the most popular media type (HTML) does
      only allow to set markers (which are anchor points in the
      serialized media stream and used by browsers to identify a
      specific position in the content) and does not allow browsers to

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      regularly identify actual, conceptional fragments of the media
      delivered -- like, e.g., the "proper content" of a web page,
      excluding navigation bars etc. -- so that in practice users have
      got accustomed to understanding a "fragment" as actually
      designating a *position* in the media, not a *part* of it.

      Therefore, potential usage of <fragment> components in URNs is
      rather limited and has to be considered very seriously by
      designers of URN Namespaces that would liek to make use of them.
      URN Namespaces that rely on (unmodified) browser resolution via
      HTTP/HTML cannot rely on the usage of fragment identifiers to
      steer the resolution process.  Thus, the use of fragment
      identifiers only seems to be useful for URN Namespaces that are
      intended to either (a) exclusively make use of resolution systems
      / clients that can cope with handing off a full-featured URN
      (including a possible fragment identifier) to the resolution
      service, or (b) exclusively employ HTML/HTTP based resolution
      systems / clients, i.e., where the resolution results are returned
      as HTML such that web browsers can perform the fragment selection,
      or as some other media type that better supports the
      identification and actual selection of embedded fragments, even in
      off-the-shelf web browsers -- perhaps possible for certain
      variants of XML-based media types.

   The syntax of <query> and <fragment> are defined in RFC 3986.
   Question mark and hash sign remain reserved as separator characters
   for these URI components and therefore MUST NOT appear unencoded in a
   NSS.  This rule guarantees backwards compatibility with existing URN
   Namespaces and improves the compatibility of URN syntax with general
   URI parsers.

   The <query> part MUST NOT be present in any *assigned* URN.  This
   specification reserves its use for future standardization related to
   URN services and resolution.  A <query> part can only be added to an
   assigned URN and appear in a URI *reference* [RFC3986] to a URN that
   is intended to be used with URN resolution services, and, in
   accordance with the general specification of this part in RFC 3986,
   its purpose is restricted to indicate the requested URN resolution
   service and/or particular service aspects of the intended resolution
   response, e.g., to select the kind of metadata sought about the given
   object that is identified by the basic, assigned URN.

   The <fragment> part is not generally allowed in URNs.  It is only
   applicable to URN Namespaces that specifically opt to support its
   usage.  Thus, a URN Namespace registration document MAY specify the
   usage of <fragment> with URNs of that particular URN Namespace.
   Absent a registered namespace definition based on this document and

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   RFC 3406bis that explicitly specifies its usage, URNs within a
   particular URN Namespace MUST NOT contain a fragment identifier.

   The use of fragment identifiers may be useful if the URN Namespace is
   based on an existing identifier scheme that designates objects of
   reasonable complexity such that there is a need to make reference of
   parts of such resources in typical network access environments
   without incurring the effort to assign and maintain different
   (assigned) NSSs in such cases.

   URN Namespaces will deal with various kinds of fragments.  For
   instance, publications can be divided into smaller parts -- journals
   consist of volumes, issues and articles, and books may contain
   chapters.  These logical fragments are usually not fragments in the
   sense of the deliberations in the URI Generic Syntax, and if so,
   <fragment> MUST NOT be used.  However, namespaces MAY have internal
   means for identification of logical fragments such as journal
   articles.  For instance, the ISBN (International Standard Book
   Number) system allows assignment of ISBN numbers to book chapters if
   they are available as separate items.  Namespace specific fragment
   identification practices are beyond the scope of this document, since
   they do not rely on URI Generic Syntax, and their application is the
   primary RECOMMENDED way to deal with fragment identification.  If a
   namespace lacks this possibility, a URN Namespace definition SHOULD
   define syntactical parts of its NSSs that amend the original
   identifiers of the underlying namespace in a readily parseable way
   and serve to allow assignment of URNs in that namespace to the
   intended abstract fragments.  A URN Namespace registration MAY forbid
   all kind of fragment identification (even if it were possible on the
   basis of URI Generic Syntax), if the application rules and syntax of
   the identifier does not allow identification of fragments.  ISSN
   (International Standard Serial Number) is an example of this kind of
   identifier / namespace.

   The use of <fragment> as specified in RFC 3986 is possible if and
   only if (a) the URN Namespace is based on an existing identifier
   scheme that designates objects of reasonable complexity that there is
   a need to make reference of parts of such resources in typical
   network access environments; and (b) these parts will be identified
   in the canonical manner of the media type(s) delivered upon URN
   resolution.  Direct resolution to them SHOULD be possible and

   If in a given namespace URNs are never assigned to a particular
   manifestation of a resource (for instance, a PDF version of a book),
   but can be transferred from one manifestation to the next or apply to
   all of them, <fragment> usage is forbidden.  This applies also to the
   situation when identified resources are works (without any references
   to physical embodiments of the work).

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   The use of <fragment> SHOULD NOT be opted for if the underlying
   namespace provides for the intrinsic possibility to identify such
   parts or if there is a readily usable method to construct NSSs by
   combining the existing identifiers with a component (or components)
   to identify such parts in an easily discernable manner.

   Whether the URI Generic Syntax is applied or not, there are various
   ways in which fragment identifiers can be generated:

   (a)  Fragment identifiers (if any) are assigned individually to the
        relevant fragments of a larger entity during the URN assignment
        process.  If a URN Namespace opts for this model, its
        specification SHOULD describe the additional syntax restrictions
        to be adhered to and the particulars of the (per-URN) assignment

   (b)  A specific set of fragment identifiers is generally applicable
        to all resources targeted by URNs of the specific URN Namespace.
        In this case, the specification document MUST specify a finite
        set of <fragment> values, or precise, generic rules for the
        automated formation of syntactically valid fragment identifiers
        for the particular URN Namespace.  The specification SHOULD
        indicate the treatment of syntactically valid <fragment> values
        in case they are not semantically valid for a given base URN.
        Absent such specification, the default is to ignore such
        fragment identifiers.

   URN resolver clients SHOULD pass a given <fragment> part of a URN
   unchanged to the resolver service.  The default URN resolution
   behavior is to ignore any <fragment> part if either the applicable
   URN Namespace definition did not specify its use, or if no specific
   related information was available for the basic resource in case (b)
   above, or if that basic URN plus fragment identifier has not been
   assigned in case (a) above.

2.1.  Namespace Identifier (NID) Syntax

   The following is the syntax for the Namespace Identifier.  To (i) be
   consistent with all potential resolution schemes and (ii) not put any
   undue constraints on any potential resolution scheme, Namespace
   Identifiers are ASCII strings with the syntax:

      NID = (ALPHA / DIGIT) 0*30(ALPHA / DIGIT / "-") (ALPHA / DIGIT)

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      The above definition is slightly more restrictive than it was in
      RFC 2141, to better reflect common practice for "handle"-like
      identifiers in other IETF protocols (a.k.a.  "LDH" syntax) and
      requirements from RFC 3406bis.  RFC 3406bis contains further
      syntax restrictions on NID strings.

      The above rule still allows NIDs that contain multiple adjacent
      hyphens or have the form of decimal numbers or decimal number

      Should this be further restricted _in this document_ or is it
      sufficient to defer to the additional (NID kind specific) rules in
      RFC 3406bis and the common sense of URN Namespace authors and the
      designated IANA experts?
      Anyhow, such restrictions would be fully backward compatible -- as
      is the above tightened rule -- because no NIDs have been defined
      so far that would violate these restrictions.  Hyphens have been
      used only in the naming pattern for "Informal Namespace IDs" per
      RFC 3406[bis].

      The document editor senses the low level of discussion of this
      issue as an indication that this Issue can be closed.

   Namespace Identifiers are case-insensitive, so that for instance
   "ISBN" and "isbn" refer to the same namespace.

   To avoid confusion with the URI Scheme name "urn", the NID "urn" is
   permanently reserved by this RFC and MUST NOT be used or registered.

      This reservation is carried over unchanged from RFC 2141, for
      historical reasons.

      Further possible reservations and/or details are out of scope for
      this document, but might be within the scope of RFC 3406bis.
      It has been suggested that no additional reservations should be
      codified and the final decision in any case should be left to the
      common sense of URN Namespace authors and the designated IANA

      The document editor senses the low level of discussion of this
      issue as an indication that this Issue can be closed.

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2.2.  Namespace Specific String (NSS) Syntax

   As already required since RFC 1737, there is a single canonical
   representation of the NSS portion of an URN.

   The format of this single canonical form follows:

         NSS   = 1*pchar   ; or equivalent:    NSS   = segment-nz

   (<pchar> and <segment-nz> are defined in Section 3.3 of RFC 3986.)

      Note: The informational Appendix C expands on the evolution of the
      NSS syntax specification since RFC 2141.

   ISSUE (for the record):
      In comparison to RFC 2141, essentially now "&" and "~" are allowed
      in the NSS syntax, in full conformance with the generic URI
      syntax.  On the other hand, the <reserved> characters are no more
      part of the formal syntax -- unfortunately (or erroneously) these
      were included in the formal syntax rules of RFC 2141 and only
      exluded after that fact in the prose, which at least in one
      instance has lead to a URN Namespace definition document that
      allows <reserved> in the formal NSS syntax but does _not_ properly
      exclude their use in the prose.  The interpretation of "%" was
      ambiguous in RFC 2141; it is now only allowed (in the formal
      syntax and in the prose) in <pct-encoded> constructs.

      The document editor senses that this change of the NSS syntax has
      found consensus and that hence this Issue is regarded as closed.

   Depending on the rules governing a namespace, valid identifiers in a
   namespace might contain characters that are not members of the URN
   character repertoire above (<pchar>).  In order to achieve
   conformance with this NSS specification, such strings MUST be
   translated into canonical NSS format before embedding them into a
   URN, using them as protocol elements, or otherwise passing them on to
   other applications.  Translation is done by encoding each character
   outside the URN character repertoire as a sequence of octets using
   UTF-8 encoding (STD 63 [RFC3629]), and the "percent-encoding" of each
   of those octets as "%" followed by two <HEXDIG> characters.  The
   latter two characters form the hexadecimal representation of that
   octet.  (See Section 2.3.2 below for more details.)

2.3.  Special and Reserved Characters

   The remaining printable characters not included in the <pchar>
   repertoire comprise the generic delimiters and the reserved
   characters, which are restricted for special use only.  These

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   characters are discussed below, giving the specifics of why each
   character is special or reserved.

2.3.1.  Delimiter Characters

   RFC 3986 [RFC3986] defines the general delimiter characters used in

      gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"

   From among the <gen-delims>, ":" and "@" are also included in the
   <pchar> rule and hence allowed in the path components of URIs.

   The at-character ("@") in generic URIs only has a specific meaning
   when contained in the <authority> part, which is absent in URNs.
   Hence, "@" is available in the <NSS> part of URNs.

   With URNs, the colon (":") is used as a delimiter character not only
   between the scheme name ("urn") and the <NID>, but also between the
   latter and the <NSS>, and many existing URN Namespaces additionally
   use ":" to further subdivide a single RFC 3986 path segment in the
   <NSS> in a hierarchical manner.

   Note: Using ":" as a sub-delimiter in the path in favor of "/" is
   attractive because it avoids possible complications that could arise
   from accidental inappropriate use of relative URI references
   [RFC3986] for URNs.

   The characters "/", "?", and "#" separate path components and the
   <query> and <fragment> parts in the generic URI syntax; they are
   restricted to this role in URNs as well, although the <path> in URNs
   only admits a single <segment> and hence "/" is not allowed.
   Therefore, these characters MUST NOT appear literally in the <NSS>
   part of a URN in unencoded form.  Namespaces that need these
   characters MUST employ in their URNs the appropriate percent-encoding
   for each such character.

   The square brackets ("[" and "]") also play a particular role when
   contained in the <authority> part, which is absent in URNs.  However,
   for conformance with the generic URI syntax, they are not allowed
   literally in the <NSS> component of URNs.  If a specific URN
   Namespace reflects semantics that require these characters, they MUST
   be percent-encoded in the respective URNs.

2.3.2.  The Percent Character, Percent-Encoding

   The percent character ("%") is reserved in the URN syntax for
   introducing the escape sequence for an octet that is either not a

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   printable ASCII character or reserved for special purposes, as
   described in this section.  The presence of a "%" character in a URN
   MUST always be followed by two <HEXDIG> characters, which three
   characters together semantically form an abstract <pct-encoded>
   octet.  Literal use of the "%" character in an underlying namespace
   MUST therefore be encoded as "%25" in URNs for that namespace.

   Namespaces MAY designate one or more characters from the URN
   character repertoire as having special meaning for that namespace.
   If the namespace also uses that character in a literal sense as well,
   the character used in a literal sense MUST be encoded with "%"
   followed by the hexadecimal representation of that octet.  Further, a
   character MUST NOT be percent-encoded if the character is not a
   reserved character.  Therefore, the process of registering a
   namespace identifier shall include publication of a definition of
   which characters have a special meaning to that namespace -- cf. RFC
   3406bis [I-D.ietf-urnbis-rfc3406bis-urn-ns-reg].

2.3.3.  Other Excluded Characters

   The following list is included only for the sake of completeness.  It
   includes the characters discussed in Sections 2.3.1 and 2.3.2.  Any
   octets/characters on this list are explicitly NOT part of the URN
   <NSS> character repertoire, and if used in an URN, MUST be percent-

              excluded = CTL / SP        ; control characters and space
                       / DQUOTE          ; "
                       / "#"             ; from <gen-delims>
                       / "%"             ; see above
                       / "/"             ; from <gen-delims>
                       / "<" / ">"
                       / "?"             ; from <gen-delims>
                       / "["             ; from <gen-delims>
                       / "\"
                       / "]"             ; from <gen-delims>
                       / "^"
                       / "`"
                       / "{" / "|" / "}"
                       / %x7F            ; DEL (control character)
                       / %x80-FF         ; non-ASCII

   The NUL octet (0 hex) is renowned for a long history of trouble in
   implementations.  It MUST NOT be used in URNs, in either unencoded or
   percent-encoded form.

   In a textual context for a URN, the NSS part ends when an octet/
   character from the excluded character set (<excluded>) is

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   encountered.  The character from the excluded character set is NOT
   part of the NSS.

   The more general issue of discerning URNs in non-structured text is
   not specific to URNs, but a general issue for recognizing URIs (by
   humans or automata), and hence out of scope of this document.

3.  Support of Existing Legacy Naming Systems and New Naming Systems

   Any identifier to be used as a URN MUST be expressed in conformance
   with the URI and URN syntax specifications ([RFC3986], this
   document).  If names from (existing or newly devised) namespaces
   contain characters other than those defined for the URN character
   set, they MUST be translated into canonical form as discussed in
   Section 2.2.

   On the other hand, every namespace specific string in a given URN
   Namespace MUST be based on an identifier that conforms to the
   requirements of the identifier system to which the URN Namespace is
   assigned; in the simplest form, if the syntactical rules admit, the
   NSS can be the original identifier.  For instance, every legal NSS in
   the ISBN Namespace must be a valid ISBN.

4.  URN Presentation and Transport

   The URN syntax defines the canonical format for URNs and all URN
   transport and interchanges MUST take place in this format.  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 MAY support display
   of URNs in a more human-friendly form and may use a character set
   that includes characters that aren't permitted in URN syntax as
   defined in this RFC (that is, they may replace %-notation by
   characters in some extended character set in display to humans).

   Note:  Such transformation for the purpose of presentation, if done
      blindly without NID-specific knowledge of special character usage,
      might introduce ambiguity, because in the cases described above in
      the second paragraph of Section 2.3.2, the unescaped and percent-
      escaped form of the same character might carry different semantics
      in NSSs of some URN Namespaces.

5.  Lexical Equivalence of URNs

   For various purposes such as caching, it is often desirable to
   determine whether two URNs are the same without resolving them.  The
   general-purpose means of doing so is by testing for "lexical
   equivalence" as defined below.

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   Two URNs are lexically equivalent if they are octet-by-octet equal
   after the following preprocessing:
      1. normalize the case of the leading "urn" scheme name;
      2. normalize the case of the NID;
      3. normalize the case of any percent-encoding;
      4. remove the <query> part of the URI, if present.

   Note that percent-encoding MUST NOT be removed.  It is an
   implementation detail not affecting interoperability whether a URN
   comparison function internally prefers normalization (in the above 3
   steps) to lower or to upper case.  Note also that <fragment> MUST NOT
   be removed, since there is no lexical equivalence between the "base"
   URN and one which uses <fragment> -- the former identifies the
   resource as the whole; the latter just a part of it.

   Some namespaces may define additional lexical equivalences, such as
   case-insensitivity of the NSS (or parts thereof).  Additional lexical
   equivalences MUST be documented as part of Namespace registration,
   MUST always only have the effect of eliminating some of the false
   negatives obtained by the procedure above, i.e. they MUST NOT say
   that two URNs are not equivalent if the procedure above says they are

5.1.  Examples of Lexical Equivalence

   The following hypothetical URN comparisons highlight the lexical
   equivalence definitions:

      1- URN:foo:a123,456
      2- urn:foo:a123,456
      3- urn:FOO:a123,456
      4- urn:foo:A123,456
      5- urn:foo:a123%2C456
      6- URN:FOO:a123%2c456
      7- urn:foo:a123,456?xyz
      8- urn:foo:a123,456#xyz

   URNs 1, 2, 3, and 7 are all lexically equivalent.  URN 4 is not
   lexically equivalent to any of the other URNs of the above set.  The
   same holds for URN 8.
   URNs 5 and 6 are only lexically equivalent to each other.

6.  Functional Equivalence of URNs

   Functional equivalence is determined by practice within a given
   namespace and managed by resolvers for that namespace.  Thus, it is
   beyond the scope of this document.  Namespace registrations must
   include guidance on how to determine functional equivalence for that
   URN Namespace, i.e., when two URNs are identical within a namespace.

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   On the other hand, it is permissible to have two different URNs --
   even from different URN Namespaces -- be assigned to a particular
   resource.  This can only be detected by resolving the URNs and
   analysis of the resolution responses; hence, this is out of scope for
   this memo.

7.  The 'urn' URI Scheme

   At the time of publication of RFC 2141, no formal registration
   procedure for URI Schemes had been established yet, and so IANA only
   informally has registered the 'urn' URI Scheme with a reference to

   Section 7.1 below contains the URI scheme registration template for
   the 'urn' scheme, in accordance with RFC 4395 [RFC4395].

      Note: In order to be usable as a standalone text (after being
      extracted from this RFC), the template below does not contain
      formal anchors to the references listed in Section 11, but instead
      gives the common document designations in prose.  However, for
      compliance with editorial policy, it needs to be noted here:

   This registration template refers to RFCs 2196, 2276, 2608, 3401
   through 3404, 3406bis, 3629 (STD 63), and 3986 (STD 66) ([RFC2169]
   [RFC2276] [RFC2608] [RFC3401] [RFC3402] [RFC3403] [RFC3404]
   [I-D.ietf-urnbis-rfc3406bis-urn-ns-reg] [RFC3629] [RFC3986]).

7.1.  Registration of URI Scheme 'urn'

   [ RFC Editor: Please replace "XXXX" in all instances of "RFC XXXX"
   below by the RFC number assigned to this document. ]

   URI scheme name:  urn

   Status:  permanent

   URI scheme syntax:

      See Section 2 of RFC XXXX.

   URI scheme semantics:

      'urn' URIs, known as Universal Resource Names (URNs), serve as
      persistent, location-independent, resource identifiers for
      concrete and abstract objects that have network accessible
      instances and/or metadata.

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      URNs are structured hierarchically into URN Namespaces, the
      management of which is delegated to namespace-specific
      authorities.  Each such URN Namespace is founded in an independent
      specification and registered with IANA, following the guidelines
      and procedures of BCP 66 (at the time of this registration: RFC
      3406, an update is in progress as RFC 3406bis

   Encoding considerations:

      All URNs are ASCII strings conforming to the general URI syntax
      from STD 66.  As described in Sections 2.2 and 2.3.2 of RFC XXXX,
      there may be characters allowed by the syntax and semantics of the
      identifier system underlying the URN Namespace but not contained
      in the US-ASCII charset.  Such characters MUST first be
      represented in Unicode and encoded in UTF-8 according to STD 63.
      Any octets outside the allowed character set MUST then be percent-

      Note that it is perfectly possible that the syntax and semantics
      of an underlying identifier system does not admit specific
      characters allowed by the syntax rules in RFC XXXX.

   Applications/protocols that use this URI scheme:

      URNs that serve to identify abstract resources for protocol
      purposes are expected to be recognized directly by the
      implementations of these portocols.

      In general, resolution systems for URNs are specified on a per-
      namespace basis.  If appropriate for the namespace, these systems
      resolve URNs to (possibly multiple) URIs that allow the network
      access to the identified object or metadata on it.

      "Architectural Principles of Uniform Resource Name Resolution"
      (RFC 2276) explains the basic concepts.  Some resolution systems
      laid down in IETF specifications are:

      *  Trivial HTTP-based URN Resolution (RFC 2169)

      *  Dynamic Delegation Discovery System (DDDS, RFCs 3401-3404)

      *  Service Location Protocol (SLPv2, RFC 2608)

   Interoperability Considerations:

      Persistence and stability of URNs require appropriate resolution

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   Security Considerations:

      See Section 8 of RFC XXXX.


      The IETF URNbis working group.
      This registration will be discussed on the following IETF lists:
      urn and uri-review (AT

   Author / Change controller:

      The authors of RFC XXXX.
      Change control is with the IESG.


      RFC XXXX.

      Procedures for the specification and registration of URN
      Namespaces are detailed in BCP 66 (at the time of this writing:
      RFC 3406; an update is in progress in the URNbis WG as RFC 3406bis

8.  Security Considerations

   This document specifies the syntax and general requirements for URNs,
   which are the specific URIs that use the 'urn' URI scheme.  As such,
   the general security considerations of STD 66 [RFC3986] apply.
   However, each URN Namespace will have specific security
   considerations, according to the semantics and usage of the
   underlying namespace.  While some namespaces may assign special
   meaning to particular characters generically allowed in the Namespace
   Specific String, any security considerations resulting from such
   assignment are outside the scope of this document.  It is REQUIRED by
   BCP 66 (currently [RFC3406], to be replaced by RFC 3406bis
   [I-D.ietf-urnbis-rfc3406bis-urn-ns-reg]) that the process of
   registering a namespace identifier include any such considerations.

9.  IANA Considerations

   IANA is asked to update the existing informal registration of the
   'urn' URI Scheme by the template in Section 7.1 above and list this
   RFC as the current normative reference in [IANA-URI].

   IANA is asked to add a note to [IANA-URN] that 'urn' is a permanently
   reserved formal namespace identifier string that cannot be
   registered, in order to avoid confusion with the 'urn' URI scheme.

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   IANA is asked to again make available the URN Namespace Registry
   [IANA-URN] in a generic form (i.e.  HTML) at the generic URI given in
   the Reference, and to make the XML and TXT versions available from
   that HTML version.  (This state already had been achieved, but
   something seems to have been lost in 2011.)

10.  Acknowledgements

   This document is heavily based on RFC 2141, the author of which has
   laid the foundation for this work; that RFC contained the following

      Thanks to various members of the URN working group for comments on
      earlier drafts of this document.  This document is partially
      supported by the National Science Foundation, Cooperative
      Agreement NCR-9218179.

   This document also heavily relies on and acknowledges the work done
   for STD 66 [RFC3986] and earlier RFCs that are being quoted
   informally, in particular RFC 1737 [RFC1737].  The experiences
   gathered during the first (more than a) decade of URN usage were also
   helpful, so individuals and organizations which have implemented and
   used URNs are also acknowledged.

   Many individuals in the URNbis working group have participated in the
   detailed discussion of this memo.  Particular thanks for detailed
   review comments and text suggestions go to Juha Hakala and Mykyta

11.  References

11.1.  Normative References

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

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

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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11.2.  Informative References

              Hoenes, A., "Uniform Resource Name (URN) Namespace
              Definition Mechanisms",
              draft-ietf-urnbis-rfc3406bis-urn-ns-reg-02 (work in
              progress), March 2012.

   [IANA]     IANA, "The Internet Assigned Numbers Authority",

              IANA, "URI Schemes Registry",

              IANA, "URN Namespace Registry",

   [RFC0615]  Crocker, D., "Proposed Network Standard Data Pathname
              syntax", RFC 615, March 1974.

   [RFC0645]  Crocker, D., "Network Standard Data Specification syntax",
              RFC 645, June 1974.

   [RFC1630]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
              Unifying Syntax for the Expression of Names and Addresses
              of Objects on the Network as used in the World-Wide Web",
              RFC 1630, June 1994.

   [RFC1736]  Kunze, J., "Functional Recommendations for Internet
              Resource Locators", RFC 1736, February 1995.

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

   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, December 1994.

   [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",
              RFC 1808, June 1995.

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

   [RFC2169]  Daniel, R., "A Trivial Convention for using HTTP in URN
              Resolution", RFC 2169, June 1997.

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

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   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2608]  Guttman, E., Perkins, C., Veizades, J., and M. Day,
              "Service Location Protocol, Version 2", RFC 2608,
              June 1999.

   [RFC2611]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "URN Namespace Definition Mechanisms", BCP 33, RFC 2611,
              June 1999.

   [RFC2717]  Petke, R. and I. King, "Registration Procedures for URL
              Scheme Names", BCP 35, RFC 2717, November 1999.

   [RFC2718]  Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
              "Guidelines for new URL Schemes", RFC 2718, November 1999.

   [RFC3305]  Mealling, M. and R. Denenberg, "Report from the Joint W3C/
              IETF URI Planning Interest Group: Uniform Resource
              Identifiers (URIs), URLs, and Uniform Resource Names
              (URNs): Clarifications and Recommendations", RFC 3305,
              August 2002.

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401, October 2002.

   [RFC3402]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Two: The Algorithm", RFC 3402, October 2002.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3404]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Four: The Uniform Resource Identifiers (URI)",
              RFC 3404, October 2002.

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

   [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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Appendix A.  Handling of URNs by URL Resolvers/Browsers

   The URN syntax has been defined so that URNs can be used in places
   where URLs are expected.  A resolver that conforms to the current URI
   syntax specification [RFC3986] will extract a scheme value of "urn"
   rather than a scheme value of "urn:<nid>".

   An URN MUST be considered an opaque URI by URL resolvers and passed
   (with the "urn:" tag) to a URN resolver for resolution.  The URN
   resolver can either be an external resolver that the URL resolver
   knows of, or it can be functionality built into the URL resolver.

   To avoid confusion of users, a URL browser SHOULD display the
   complete URN (including the "urn:" tag) to ensure that there is no
   confusion between URN Namespace identifiers and URI Scheme names.

Appendix B.  Collected ABNF (Informative)

   As a service to implementers specifically interested in URN syntax,
   the complete ABNF for URNs is collected here, including the
   referenced rules from [RFC5234] and [RFC3986].  In case of
   (unexpected) inconsistencies, these documents remain normative for
   the respective productions.

   URNs conform to the <path-rootless> variant of the general URI syntax
   specified in Section 3 of [RFC3986] :

       URI    = scheme ":" path-rootless [ "?" query ] [ "#" fragment ]

       scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
       path-rootless = segment-nz *( "/" segment )
       query         = *( pchar / "/" / "?" )
       fragment      = *( pchar / "/" / "?" )

       segment-nz    = 1*pchar
       segment       = *pchar
       pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

       unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
       pct-encoded   = "%" HEXDIG HEXDIG
       sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
                     / "*" / "+" / "," / ";" / "="

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   In the case of URNs, the above rules are subject to more specific

         scheme        = "urn"
                          ; specific, fixed (assigned) value

         urn-path      =  NID ":" NSS
                          ; to be superimposed on <path-rootless>

         NID           = ( ALPHA / DIGIT ) 1*31( ALPHA / DIGIT / "-" )
                         ; RFC 3406[bis] contains more specific rules

         NSS           = 1*pchar
                         ; or equivalent:    NSS   = segment-nz

   The above rules make use of the following "Core Rules" from Appendix
   B.1 of [RFC5234] :

         ALPHA         =  %x41-5A / %x61-7A   ; A-Z / a-z
         DIGIT         =  %x30-39             ; 0-9
         HEXDIG        =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

Appendix C.  Breakdown of NSS Syntax Evolution since RFC 2141

   In order to make visible the detailed migration path from RFC 2141
   and the influence of the evolution of URI syntax from RFC 2396 to RFC
   3986 on it, this appendix provides a highly annotated and expanded
   version of the NSS syntax provided in Section 2.2:

         NSS           = 1*pchar   ; or equivalent:   NSS   = segment-nz

   In particular, the breakdown below serves to provide evidence of that
   this syntax correctly reflects the addition of "&" and "~" to the
   repertoire of characters allowed in the NSS portion of URNs
   previously allowed by RFC 2141; it expands on the syntax specified in
   RFC 2141 after translation to standard ABNF.

         NSS         = 1*URN-char

         URN-char    = trans / pct-encoded
    ; Note that <pct-encoded> from RFC 3986 here replaces the
    ; explicit, expanded form used in RFC 2141.

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         trans       = ALPHA / DIGIT / u-other
    ; Note that RFC 2141's <other> has been disambiguated here
    ; into <u-other>.
    ; RFC 2141 also said:
    ;                / reserved
    ; This caused an ambiguity in RFC 2141 with respect to "%", which
    ; now is resolved here by omission of this dangling alternative.
    ; After adoption of the generic URI syntax from RFC 3986, there
    ; is no more need to deal here with the higher-level separator
    ; characters "/", "?", and "#" contained in <reserved>
    ; (beyond "%", which is fully taken care of by <pct-encoded>),
    ; which are part of RFC 3986's <gen-delims>, as shown below.

    ; From RFC 2141:
    ;    reserved  = '%" / "/" / "?" / "#"         ; SIC!
    ;                ^ ^

         u-other     = ":" / "@"
                       ; those from RFC 3986 <gen-delims>
                       ; specifically allowed in <pchar>.
    ; From RFC 3986:
    ;    gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"

                     / "!" / "$" /       "'" / "(" / ")"
                     / "*" / "+" / "," / ";" / "="
                       ; this is RFC 3986 <sub-delims> except "&".
    ; From RFC 3986:
    ;    sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
    ;                / "*" / "+" / "," / ";" / "="
    ; The URNbis WG arrived at unanimous consensus that "&" can be
    ; allowed without harm to backward compatibility for existing
    ; URN Namespaces.

                     / "-" / "." / "_"   ; <unreserved> except "~"
    ; From RFC 3986:
    ;    unreserved  = ALPHA / DIGIT
    ;                / "-" / "." / "_" / "~"
    ; The URNbis WG arrived at unanimous consensus that "~" can be
    ; allowed without harm to backward compatibility for existing
    ; URN Namespaces.

    ; Since we now allow "&" and "~" , <trans> becomes <pchar> ,
    ; greatly simplifying the syntax rules and parsers!

    ; From RFC 3986:
    ;    segment-nz  = 1*pchar
    ;    pchar       = unreserved / pct-encoded / sub-delims / ":" / "@"

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Appendix D.  Changes since RFC 2141 (Informative)

D.1.  Essential Changes from RFC 2141

   [ RFC Editor: please remove the Appendix D.1 headline and all
   subsequent subsections starting with Appendix D.2. ]

   T.B.D. (after consolidation of this memo)

D.2.  Changes from RFC 2141 to Individual Draft -00

   Abstract amended: URI scheme, replacement for 2141, point to 3406.
   Use contemporary boilerplate.  Added transient "Discussion" section.

   s1: added new 1st para (URI scheme) and 3rd para (hierarchy).
   s1.1 (Historical Perspective) added for background & motivation.
   s1.2 (Objective) added.
   s1.3 (2119 keywords) added -- used now throughout normative text.

   s2 (URN Syntax): Shifted from BNF to ABNF; explain relationship to
   3986 and gaps, how the gaps could be bridged, distinguish between URI
   generics and URN specifics; got rid of references to immature
   documents (1630, 1737).
   s2.1 (NID syntax): Use ABNF and RFC 5234 terminals (core rules);
   removed reference to an old draft of 2396; clarified prohibition to
   use "urn" as NID.
   s2.2 (NSS syntax): Shifted from BNF to ABNF; made ABNF consistent
   with subsequent textual description; exposition much expanded,
   showing relationship with 3986 and resulting incompatibilities;
   proposed how to bridge gaps, to make parsing more uniform among URIs;
   updated i18n considerations and pointer to UTF-8 specification.
   s.2.3, s2.3.*: reworked and much expanded, along the grouping of
   delimiter characters from 3986 in new s2.3.1 (including old s.2.3.2);
   made text fully consistent with ABNF in s2.2; consistent usage of
   term "percent-encoded"; old s.2.3.1 became s2.3.2; old s3.4 became

   s3.3.3, providing complete, annotated list of excluded characters,
   ordered by ascending code point; and restating design decisions
   needed to be made to close gaps to 3986.

   s3 through s6: only minor editorial changes.

   s7: formal registration of 'urn' URI scheme added, using 4395

   s8: Security Cons. slightly amended.

   s9: new: IANA Cons. added wrt s7.1 and prohibition of NID "urn".

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   s10: Acknowledgments amended.

   s11: References split into Normative and Informative; updated refs
   and added many; only FS and BCP allowed as Normative Refs to further
   promotion of document.

   Added Appendices A through D.

D.3.  Changes from Individual Draft -00 to -02

   Updated "Discussion" on front page to point to dedicated urn list.

   Numerous editorial improvements and additions for clarification, in
   particular in the Introduction.  No technical changes.

   More Informative References; missing details supplied in D.2.

D.4.  Changes from Individual Draft -02 to WG Draft -00

   Added new s1.2 to Introduction, with excerpts from RFC 1737 to
   provide background on URN functional and syntax requirements.
   Renumbered previous s1.2 and s1.3 to s1.3 and s1.4, respectively.

   Supplied text in s2 regarding the envisioned use of query and
   fragment parts, based on various discussion -- including a
   preliminary evaluation in PersID.

   Changed "SHOULD never" to "MUST NOT" for NUL character in NSS.

   Various editorial and grammar fixes; corrected STD / BCP numbers.

D.5.  Changes from WG Draft -00 to WG Draft -01

   Reflect WG consensus on adding "&" and "~" to the set of characters
   allowed in the NSS part of URNs, thus aligning URN syntax with
   generic URI syntax from RFC 3986.

   Moved breakdown of NSS syntax evolution from s2.2 to new Appendix C.

   Avoid "[URN] character set" in favor of "character repertoire" to
   minimize potential clashes with IETF terminology on charsets.

   s2.3.3: URN recognition in text documents is regarded out of scope.

   The previous version was ambiguous on whether eventual query and/or
   fragment parts were regarded as part of the NSS; after closer
   inspection of the syntax, clarification has been added that the <urn-
   path> syntax is indeed superimposed on the <segment-nz> ABNF rule for

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   URNs, and hence does not cover the trailing higher level parts
   (query, fragment) according to the URI syntax.

   Filled in Appendix B contents.

   Numerous editorial and grammar improvements.

D.6.  Changes from WG Draft -01 to WG Draft -02

   Added note at the beginning of Section 1.2 highlighting the purpose
   of this section.  The URNbis charter excludes a revision of RFC 1738,
   and hence the changes suggested on the list to alter and update this
   section have been dismissed.

   Added hint to URN Namespace designers in Section 2 that ":" is
   customarily used in URN Namespaces to provide further level(s) of
   hierarchical subdivision of NSSs.

   Reworked text on fragment identification issues and resulting
   specification, mostly based on Juha Hakala's evaluation of the
   consensus evolving from the list discussion.

   Modified ABNF rule for NIDs to better align it with rules for similar
   identifiers used in IETF protocols.  The new rule now prohibits a
   trailing hyphen, but defers further restricting rules on NID syntax
   (based on the kind of NID) to RFC 3406bis.

   More clearly documented and marked (still open / already closed)
   ISSUES.  The related text will be removed in the next draft version,
   whence it should have been transferred into the IETF issue tracking

   Text of Section 3 revised, based on Juha's suggestion.

   In Section 5, added removal of <query> part (but not <fragment> part)
   to canonicalization steps for the purpose of determining lexical
   equivalence of URNs (Juha's comment).  Also added examples showing

   Elaborated a bit more on Encoding Consideration in the URI Scheme
   registration template (Juha's comments).

   Numerous editorial corrections and improvements.

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Appendix E.  How to Locate IETF Documents (Informative)

   Request For Comments (RFCs) are available from the RFC Editor site
   using the canonical URIs <>
   or <> (where 'NNNN' is
   the serial number of the RFC), and from numerous mirror sites.
   Additional metadata for any RFC, including possible Errata, are
   available from <> (where 'NNNN'
   again is the serial number of the RFC).  A HTML-ized version and a
   PDF facsimile of each RFC are available from the IETF Tools site at
   <> and
   <>, respectively.

   Current Internet Draft documents are available via the search engines
   at <> and
   <>; archival copies of older
   IETF documents can be found at <>.

Author's Address

   Alfred Hoenes (editor)
   Gerlinger Str. 12
   Ditzingen  D-71254


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