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URN Namespace Definition Mechanisms

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2611.
Authors Dirk-Willem van Gulik , Leslie Daigle
Last updated 2013-03-02 (Latest revision 1999-04-01)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
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IESG IESG state RFC 2611 (Best Current Practice)
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Internet Draft                               Leslie L. Daigle
March 31, 1999                               Bunyip Information Systems
draft-ietf-urn-nid-req-08.txt                Dirk-Willem van Gulik
                                             ISIS/CEO, JRC Ispra
                                             Renato Iannella
                                             DSTC Pty Ltd
                                             Patrik Faltstrom

      URN Namespace Definition Mechanisms

Status of This Document

        This document is an Internet-Draft and is in full conformance
        with all provisions of Section 10 of RFC2026.

        Internet-Drafts are working documents of the Internet Engineering
        Task Force (IETF), its areas, and its working groups.  Note that
        other groups may also distribute working documents as
        Internet-Drafts are draft documents valid for a maximum of six
        months and may be updated, replaced, or obsoleted by other
        documents at any time.  It is inappropriate to use Internet-
        Drafts as reference material or to cite them other than as
        "work in progress."
        The list of current Internet-Drafts can be accessed at
        The list of Internet-Draft Shadow Directories can be accessed at

0.0 Abstract

The URN WG has defined a syntax for Uniform Resource Names
(URNs) [RFC2141], as well as some proposed mechanisms for their
resolution and use in Internet applications ([RFC2168, RFC2169]).    
The whole rests on the concept of individual "namespaces" within the
URN structure.  Apart from  proof-of-concept namespaces, the use
of existing identifiers in URNs has been discussed ([RFC2288]),
and this document lays out general definitions of and 
mechanisms for establishing URN "namespaces". 

1.0 Introduction

Uniform Resource Names (URNs) are resource identifiers with the
specific requirements for enabling location independent
identification of a resource, as well as longevity of reference.
There are 2 assumptions that are key to this document:

Assumption #1:

   Assignment of a URN is a managed process.

   I.e., not all strings that conform to URN syntax are necessarily
   valid URNs.  A URN is assigned according to the rules of a 
   particular namespace (in terms of syntax, semantics, and process).

Assumption #2:

   The space of URN namespaces is managed.
   I.e., not all syntactically correct URN namespaces (per the URN
   syntax definition)  are valid URN namespaces.  A URN namespace
   must have a recognized definition in order to be valid.

The purpose of this document is to outline a mechanism and provide a
template for explicit namespace definition, along with the mechanism
for associating an identifier (called a "Namespace ID", or NID) which
is registered with the Internet Assigned Number Authority, IANA.

Note that this document restricts itself to the description of
processes for the creation of URN namespaces.  If "resolution" of any
so-created URN identifiers is desired, a separate process of
registration in a global NID directory, such as that provided by the
NAPTR system [RFC2168], is necessary.  See [NAPTR-REG] for information
on obtaining registration in the NAPTR global NID directory.

2.0 What is a URN Namespace?

For the purposes of URNs, a "namespace" is a collection of
uniquely-assigned identifiers.  A URN namespace itself has an
identifier in order to

        . ensure global uniqueness of URNs
        . (where desired) provide a cue for the structure of the

For example, ISBNs and ISSNs are both collections of identifiers used
in the traditional publishing world; while there may be some number (or
numbers) that is both a valid ISBN identifier and ISSN identifier,
using different designators for the two collections ensures that no
two URNs will be the same for different resources.

The development of an identifier structure, and thereby a collection
of identifiers, is a process that is inherently dependent on the 
requirements of the community defining the identifier, how they will 
be assigned, and the uses to which they will be put.  All of these 
issues are specific to the individual community seeking to define a 
namespace (e.g., publishing community, association of booksellers, 
protocol developers, etc); they are beyond the scope of the IETF 
URN work.

This document outlines the processes by which a collection of
identifiers satisfying certain constraints (uniqueness of assignment,
etc) can become a bona fide URN namespace by obtaining a NID.  In a
nutshell, a template for the definition of the namespace is completed
for deposit with IANA, and a NID is assigned.  The details of the
process and possibilities for NID strings are outlined below; first, a
template for the definition is provided.

3.0 URN Namespace Definition Template

Definition of a URN namespace is accomplished by completing the
following information template.  Apart from providing a mechanism
for disclosing structure of the URN namespace, this information
is designed to be useful for

        . entities seeking to have a URN assigned in a namespace
          (if applicable)
        . entities seeking to provide URN resolvers for a namespace 
          (if applicable)

This is particularly important for communities evaluating the
possibility of using a portion of an existing URN namespace rather 
than creating their own.

Information in the template is as follows:

Namespace ID:
        Assigned by IANA.  In some contexts, a particular one
        may be requested (see below).

Registration Information:

        This is information to identify the particular version of
        registration information:

        . registration version number: starting with 1, incrementing by 1
                with each new version
        . registration date: date submitted to the IANA, using
                the format 
                as outlined in [ISO8601].

Declared registrant of the namespace:  

        Required: Name and e-mail address.
        Recommended:  Affiliation, address, etc.

Declaration of syntactic structure:

        This section should outline any structural features of
        identifiers in this namespace.  At the very least, this
        description may be used to introduce terminology used in
        other sections.  This structure may also be used for
        determining realistic caching/shortcuts approaches; suitable 
        caveats should be provided.  If there are any specific
        character encoding rules (e.g., which character should
        always be used for single-quotes), these should be listed

        Answers might include, but are not limited to:

        . the structure is opaque (no exposition)
        . a regular expression for parsing the identifier into
          components, including naming authorities

Relevant ancillary documentation:

        This section should list any RFCs, standards, or other published
        documentation that defines or explains all or part of the
        namespace structure.

        Answers might include, but are not limited to:
        . RFCs outlining syntax of the namespace
        . Other of the defining community's (e.g., ISO) documents 
          outlining syntax of the identifiers in the namespace
        . Explanatory material introducing the namespace

Identifier uniqueness considerations:
        This section should address the requirement that 
        URN identifiers be assigned uniquely -- they are assigned
        to at most one resource, and are not reassigned.

        (Note that the definition of "resource" is fairly
        broad; for example, information on "Today's Weather" might 
        be considered a single resource, although the content is

        Possible answers include, but are not limited to:

        . exposition of the structure of the identifiers, and
          partitioning of the space of identifiers amongst 
          assignment authorities which are individually responsible
          for respecting uniqueness rules
        . identifiers are assigned sequentially
        . information is withheld; the namespace is opaque

Identifier persistence considerations:

        Although non-reassignment of URN identifiers ensures
        that a URN will persist in identifying a particular
        resource even after the "lifetime of the resource", 
        some consideration should be given to the persistence
        of the usability of the URN.  This is particularly
        important in the case of URN namespaces providing 
        global resolution.

        Possible answers include, but are not limited to:

        . quality of service considerations

Process of identifier assignment:

        This section should detail the mechanisms and/or authorities
        for assigning URNs to resources.  It should make clear whether
        assignment is completely open, or if limited, how
        to become an assigner of identifiers, and/or get one
        assigned by existing assignment authorities.  Answers
        could include, but are not limited to:

        . assignment is completely open, following a particular
        . assignment is delegated to authorities recognized by
          a particular organization (e.g., the Digital Object
          Identifier Foundation controls the DOI assignment space and 
          its delegation)
        . assignment is completely closed (e.g., for a private

Process for identifier resolution:

        If a namespace is intended to be accessible for global
        resolution, it must be registerd in an RDS (Resolution
        Discovery System, see [RFC2276]) such as NAPTR.  Resolution
        then proceeds according to standard URI resolution processes,
        and the mechanisms of the RDS.  What this section should
        outline is the requirements for becoming a recognized resolver 
        of URNs in this namespace (and being so-listed in the RDS

        Answers may include, but are not limited to:

        . the namespace is not listed with an RDS; this is not
        . resolution mirroring is completely open, with a mechanism
          for updating an appropriate RDS
        . resolution is controlled by entities to which assignment
          has been delegated

Rules for Lexical Equivalence:

        If there are particular algorithms for determining
        equivalence between two identifiers in the underlying
        namespace (hence, in the URN string itself), rules can 
        be provided here.  

        Some examples include:
        . equivalence between hyphenated and non-hyphenated
          groupings in the identifier string
        . equivalence between single-quotes and double-quotes
        . Namespace-defined equivalences between specific 
          characters, such as "character X with or without
          diacritic marks".

        Note that these are not normative statements for any kind of 
        best practice for handling equivalences between characters; 
        they are statements limited to reflecting the namespace's 
        own rules.

Conformance with URN Syntax:

        This section should outline any special considerations
        required for conforming with the URN syntax.  This is
        particularly applicable in the case of legacy naming
        systems that are used in the context of URNs.

        For example, if a namespace is used in contexts other 
        than URNs, it may make use of characters that are reserved
        in the URN syntax.  This section should flag any such
        characters, and outline necessary mappings to conform to 
        URN syntax.  Normally, this will be handled by hex encoding
        the symbol.

        For example, see the section on SICIs in [RFC2288].

Validation mechanism:

        Apart from attempting resolution of a URN, a URN namespace
        may provide mechanism for "validating" a URN -- i.e., 
        determining whether a given string is currently a
        validly-assigned URN.  For example, even if an ISBN
        URN namespace is created, it is not clear that
        all ISBNs will translate directly into "assigned URNs".

        A validation mechanims might be:

        . a syntax grammar
        . an on-line service
        . an off-line service

        This section should outline the scope of the use of the
        identifiers in this namespace.  Apart from considerations
        of private vs. public namespaces, this section is critical
        in evaluating the applicability of a requested NID.  For
        example, a namespace claiming to deal in "social security
        numbers" should have a global scope and address all 
        social security number structures (unlikely).  On the
        other hand, at a national level, it is reasonable to
        propose a URN namespace for "this nation's social security

4.0 URN Namespace Registration, Update,  and NID Assignment Process

Different levels of disclosure are expected/defined for namespaces.
According to the level of open-forum  discussion surrounding
the disclosure, a URN namespace may be assigned or may request a
particular identifier.  The [IANA-CONSIDERATIONS] document suggests
the need to specify update mechanisms for registrations -- who 
is given the authority to do so, from time to time, and what are
the processes.  Since URNs are meant to be persistently useful, few
(if any) changes should be made to the structural interpretation of
URN strings (e.g., adding or removing rules for lexical equivalence that
might affect the interpretation of URN IDs already assigned).  However, it 
may be important to introduce clarifications, expand the list of
authorized URN assigners, etc, over the natural course of a namespace's
lifetime.  Specific processes are outlined below.

There are 3 categories of URN namespaces defined here, distinguished
by expected level of service and required procedures for registration.
Furthermore, registration maintenance procedures vary slightly from
one category to another.

          I. Experimental: These are not explicitly registered with IANA. They
                take the form


                No provision is made for avoiding collision of experimental
                NIDs; they are intended for use within internal or limited 
                experimental contexts.

                As there is no registration, no registration maintenance
                procedures are needed.

         II. Informal:  These are registered with IANA and are assigned a 
                number sequence as an identifier, in the format:
                        "urn-" <number>

                where <number> is chosen by the IANA on a First Come First
                Served basis (see [IANA-CONSIDERATIONS]).

                Registrants should send a copy of the registration
                template (see section 3.0), duly completed, to the 


                mailing and allow for a 2 week discussion period for
                clarifying the expression of the registration information
                and suggestions for improvements to the namespace proposal.  
                After suggestions for clarification of the registration 
                information have been incorporated, the template may be 
                submitted to:

                for assignment of a NID.

                The only restrictions on <number> are that it consist
                strictly of digits and that it not cause the NID to exceed
                length limitations outlined in the URN syntax ([RFC2168]).

                Registrations may be updated by the original registrant,
                or an entity designated by the registrant, by updating
                the registration template, submitting it to the discussion
                list for a further 2 week discussion period, and finally
                resubmitting it to IANA, as described above.

        III. Formal:  These are processed through an RFC review
                process.  The RFC need not be standards-track.  The
                template defined in section 3.0 may be included as part
                of an RFC defining some other aspect of the namespace,
                or it may be put forward as an RFC in its own right.
                The proposed template should be sent to the


                mailing list to allow for a 2 week discussion period  for
                clarifying the expression of the registration information,
                before the IESG progresses the document to RFC status.

                A particular NID string is requested, and is assigned by IETF
                consensus (as defined in [IANA-CONSIDERATIONS]), with
                the additional constraints that the NID string must 

                        . not be an already-registered NID
                        . not start with "x-" (see Type I above) 
                        . not start with "urn-" (see Type II above)
                        . not start with "XY-", where XY is any
                          combination of 2 ASCII letters  (see NOTE, below)
                        . be more than 2 letters long

                NOTE: ALL two-letter combinations, and two-letter combinations
                followed by "-" and any sequence of valid NID characters,  are 
                reserved for potential use as countrycode-based  NIDs for 
                eventual national registrations of URN namespaces.   The
                definition and scoping of rules for allocation of responsibility
                for such namespaces is beyond the scope of this document.

                Registrations may be updated by updating the RFC through 
                standard IETF RFC update mechanisms.  Thus, proposals for 
                updates may be made by the original authors, other IETF 
                participants, or the IESG.  In any case, the proposed 
                updated template must be circulated on the urn-nid 
                discussion list, allowing for a 2 week review period. 

URN namespace registrations will be posted in the anonymous FTP directory

5.0 Example

The following example is provided for the purposes of illustration of
the URN NID template described in section 3.0.  Although it is based on
a hypothetical "generic Internet namespace" that has been discussed informally
within the URN WG, there are still technical and infrastructural issues
that would have to be resolved before such a namespace could be properly
and completely described.  

Namespace ID:
        To be assigned

Registration Information:

        Version 1
        Date: <when submitted>

Declared registrant of the namespace:

        Required: Name and e-mail address.
        Recommended:  Affiliation, address, etc.

Declared registrant of the namespace:  

        Name:           T. Cat
        Affiliation:    Thinking Cat Enterprises
        Address:        1 ThinkingCat Way
                        Trupville, NewCountry

Declaration of structure:

        The identifier structure is as follows:
        URN:<assigned number>:<FQDN>:<assigned US-ASCII string>

        where FQDN is a fully-qualified domain name, and the
        assigned string is conformant to URN syntax requirements.

Relevant ancillary documentation:

        Definition of domain names, found in:

        RFC1035, November 1987.

Identifier uniqueness considerations:
        Uniqueness is guaranteed as long as the assigned
        string is never reassigned for a given FQDN, and that the FQDN
        is never reassigned.    
        N.B.:  operationally, there is nothing that prevents a domain
        name from being reassigned;  indeed, it is not an uncommon 
        occurrence.  This is one of the reasons that this example
        makes a poor URN namespace in practice, and is therefore not
        seriously being proposed as it stands.

Identifier persistence considerations:

        Persistence of identifiers is dependent upon suitable
        delegation of resolution at the level of "FQDN"s, and persistence
        of FQDN assignment.
        Same note as above.

Process of identifier assignment:

        Assignment of these URNs delegated to individual domain
        name holders (for FQDNs).  The holder of the FQDN registration
        is required to maintain an entry (or delegate it) in the
        NAPTR RDS.  Within each of these delegated name partitions,
        the string may be assigned per local requirements.

        e.g.  urn:<assigned number>

Process for identifier resolution:

        Domain name holders are responsible for operating or
        delegating resolution servers for the FQDN in which they
        have assigned URNs.

Rules for Lexical Equivalence:

        FQDNs are case-insensitive.  Thus, the portion of the URN

                urn:<assigned number>:<FQDN>:

        is case-insenstive for matches.  The remainder of the identifier
        must be considered case-sensitve.

Conformance with URN Syntax:

        No special considerations.

Validation mechanism:

        None specified.


6.0 Security Considerations

This document largely focuses on providing mechanisms for the
declaration of public information.  Nominally, these declarations
should be of relatively low security profile, however there is
always the danger of "spoofing" and providing mis-information.
Information in these declarations should be taken as advisory.

7.0 References

[RFC2168] Ron Daniel & Michael Mealling, "Resolution of Uniform 
    Resource Identifiers using the Domain Name System", RFC 2168,
    June 1997.

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

[ISO8601] ISO 8601 : 1988 (E), "Data elements and interchange formats - 
    Information interchange - Representation of dates and times"

[RFC2288] C. Lynch, C. Preston & R. Daniel, "Using Existing 
    Bibliographic Identifiers as Uniform Resource Names", RFC 2288,
    February 1998.

[NAPTR-REG] M. Mealling, "Assignment Procedures for the URI Resolution 
    using DNS (RFC2168)", draft-ietf-urn-net-procedures-00.txt.

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

[IANA-CONSIDERATIONS] T. Narten and H. Alvestrand, "Guidelines for
    Writing an IANA Considerations Section in RFCs", 

[RFC1737] Karen R Sollins & Larry Masinter, "Functional Requirements 
    for Uniform Resource Names", RFC1737, December 1994

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

8.0 Authors' Addresses

Leslie L. Daigle
Bunyip Information Systems Inc
147 St. Paul St. West
Suite 200
Montreal, Quebec, CANADA
H2Y 1Z5
voice: +1 514 285-0088
fax:   +1 514 285-4515

Dirk-Willem van Gulik
Joint Research Centre Ispra
21020 Ispra (Va)
voice: +39 332 78 9549 or 5044  
fax:   +39 332 78 9185

Renato Iannella
DSTC Pty Ltd
Gehrmann Labs, The Uni of Queensland
voice:  +61 7 3365 4310
fax:    +61 7 3365 4311

Patrik Faltstrom
Borgarfjordsgatan 16
P.O. Box 62
S-164 94 Kista
voice:  +46-5626 4000
fax:    +46-5626 4200