URNBIS P. Saint-Andre
Internet-Draft Filament
Obsoletes: 2141, 3406 (if approved) J. Klensin
Intended status: Standards Track April 16, 2016
Expires: October 18, 2016
Uniform Resource Names (URNs)
draft-ietf-urnbis-rfc2141bis-urn-16
Abstract
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on October 18, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
Saint-Andre & Klensin Expires October 18, 2016 [Page 1]
Internet-Draft URNs April 2016
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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 (NID) . . . . . . . . . . . . . . . 7
2.2. Namespace Specific String (NSS) . . . . . . . . . . . . . 8
2.3. Optional Components . . . . . . . . . . . . . . . . . . . 9
2.3.1. q-component . . . . . . . . . . . . . . . . . . . . . 9
2.3.2. r-component . . . . . . . . . . . . . . . . . . . . . 10
2.3.3. f-component . . . . . . . . . . . . . . . . . . . . . 11
3. Equivalence of URNs . . . . . . . . . . . . . . . . . . . . . 12
3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 13
4. URI Conformance . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Use in URI Protocol Slots . . . . . . . . . . . . . . . . 15
4.2. Parsing . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. URNs and Relative References . . . . . . . . . . . . . . 15
4.4. Transport and Display . . . . . . . . . . . . . . . . . . 16
4.5. URI Design and Ownership . . . . . . . . . . . . . . . . 17
5. URN Namespaces . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Formal Namespaces . . . . . . . . . . . . . . . . . . . . 18
5.2. Informal Namespaces . . . . . . . . . . . . . . . . . . . 20
6. Defining and Registering a URN Namespace . . . . . . . . . . 20
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
Saint-Andre & Klensin Expires October 18, 2016 [Page 2]
Internet-Draft URNs April 2016
6.2. Registration Policy and Process: Community Registrations 21
6.3. Registration Policy and Process: Fast Track for Standards
Development Organizations, Scientific Societies, and
Similar Bodies . . . . . . . . . . . . . . . . . . . . . 22
6.4. Completing the Template . . . . . . . . . . . . . . . . . 23
6.4.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 23
6.4.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 24
6.4.3. Assignment . . . . . . . . . . . . . . . . . . . . . 25
6.4.4. Security and Privacy . . . . . . . . . . . . . . . . 25
6.4.5. Interoperability . . . . . . . . . . . . . . . . . . 26
6.4.6. Resolution . . . . . . . . . . . . . . . . . . . . . 26
6.4.7. Additional Information . . . . . . . . . . . . . . . 26
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
7.1. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . 27
7.2. Registration of URN Namespaces . . . . . . . . . . . . . 27
8. Security and Privacy Considerations . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Registration Template . . . . . . . . . . . . . . . 31
Appendix B. Changes from RFC 2141 . . . . . . . . . . . . . . . 32
B.1. Syntax changes from RFC 2141 . . . . . . . . . . . . . . 32
B.2. Other changes from RFC 2141 . . . . . . . . . . . . . . . 33
Appendix C. Changes from RFC 3406 . . . . . . . . . . . . . . . 33
Appendix D. Contributors . . . . . . . . . . . . . . . . . . . . 33
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 33
Appendix F. Change log for versions of draft-ietf-urnbis-
rfc2141bis-urn . . . . . . . . . . . . . . . . . . . 34
F.1. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 34
F.2. Changes from -09 to -10 . . . . . . . . . . . . . . . . . 35
F.3. Changes from -10 to -11 . . . . . . . . . . . . . . . . . 35
F.4. Changes from -11 to -12 . . . . . . . . . . . . . . . . . 35
F.5. Changes from -12 to -13 . . . . . . . . . . . . . . . . . 36
F.6. Changes from -13 to -14 . . . . . . . . . . . . . . . . . 36
F.7. Changes from -14 to -15 . . . . . . . . . . . . . . . . . 36
F.8. Changes from -15 (2016-02-04) to -16 . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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
Saint-Andre & Klensin Expires October 18, 2016 [Page 3]
Internet-Draft URNs April 2016
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].)
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
"urn:example:foo").
This document rests on two key assumptions:
1. Assignment of a URN is a managed process.
2. The space of URN namespaces is itself managed.
While other URI schemes may allow 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
namespace.
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 the original URN syntax [RFC2141] and
namespace definition and registration [RFC3406] specifications.
These modifications build on the key requirements provided in the
original functional description for URN [RFC1737] and many years of
experience. In both those original documents and the present one,
the intent is to define URNs in a consistent manner so that, wherever
practical, the parsing, handling, and resolution of URNs can be
independent of the namespace within which a given URN is assigned.
Saint-Andre & Klensin Expires October 18, 2016 [Page 4]
Internet-Draft URNs April 2016
Together with input from several key user communities, the history
and experiences dictated expansion of the URN definition to support
new functionality, including the use of syntax explicitly reserved
for future standardization in RFC 2141. All namespaces and URNs that
were valid under the earlier specifications remain valid even though
it may be useful to update some of them to take advantage of new
features.
Summaries of changes from RFC 2141 and RFC 3406 appear in Appendix B
and Appendix C respectively. This document obsoletes both [RFC2141]
and [RFC3406]. While it does not explicitly update or replace
[RFC1737] or [RFC2276] the reader who references those documents
should be aware that the conceptual model of URNs in this document is
slightly different from those older specifications.
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 simple case of
resolution to a URL.
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-
Saint-Andre & Klensin Expires October 18, 2016 [Page 5]
Internet-Draft URNs April 2016
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
forward.
1.2. Terminology
This document uses the terms "resolution" and "resolver" in roughly
the sense in which they were used in the original discussion of
architectural principles for URNs [RFC2276], i.e., "resolution" is
the act of supplying services related to the identified resource,
such as translating the persistent 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
that term is used in [RFC3986]. If there is no intention to provide
any resolution services, and the distinction is important, 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2. URN Syntax
As discussed above, the syntax for URNs in this specification allows
significantly more functionality than was the case in the earlier
specifications. It is also harmonized with the general URI syntax
Saint-Andre & Klensin Expires October 18, 2016 [Page 6]
Internet-Draft URNs April 2016
[RFC3986]. That syntax definition was completed after the earlier
URN specifications.
However, this specification does not extend the URN syntax to allow
direct use of characters outside the ASCII range [RFC20]. That
prohibition implies that any such characters need to be percent-
encoded as described in Section 2.1 of the URI specification
[RFC3986].
The basic syntax for a URN is defined using the Augmented Backus-Naur
Form (ABNF) as specified in [RFC5234]. Rules not defined here
(specifically: alphanum, fragment, and pchar) are defined as part of
the URI syntax [RFC3986] and used here to point out the syntactic
relationship with the terms used there. The definitions of some of
the terms used below are not complete; additional restrictions are
imposed sections of the document that are specific to those terms.
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 = pchar *( pchar / "/" / "?" )
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.
2.1. Namespace Identifier (NID)
Namespace Identifiers (NIDs) are case insensitive (e.g., "ISBN" and
"isbn" are equivalent).
Characters outside the ASCII range [RFC20] are not permitted in NIDs,
and no encoding mechanism for such characters is supported.
Saint-Andre & Klensin Expires October 18, 2016 [Page 7]
Internet-Draft URNs April 2016
Section 5.1 and Section 5.2 impose additional constraints on the
strings that can be used as NIDs, i.e., the syntax shown above is not
comprehensive.
2.2. Namespace Specific String (NSS)
The namespace specific string (NSS) is a unique identifier within a
namespace 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 URI.
The NSS specified in this document allows characters not permitted by
earlier specifications (see Appendix B. In particular, the "/"
character, which is now allowed, 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 sequence 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 "urn:example:1/406/47452/2".
The 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, non-embedded, 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 that 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
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 8]
Internet-Draft URNs April 2016
In particular, with regard to characters outside the ASCII range,
URNs that appear in protocols or that are passed between systems MUST
use only Unicode characters encoded in UTF-8 and further encoded as
required by RFC 3986. To the extent feasible 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
This specification includes three optional components in the URN
syntax. They are known as q-component, r-component, and f-component
and are described in more detail below. Because this specification
focuses almost exclusively on URN syntax, it does not define detailed
semantics of these components for URNs in general. However, each of
these components has a distinct role that is independent of the URN
and its namespace. It is intended that clients will be able to
handle these components uniformly for all URNs. These components MAY
be used with URNs from existing namespaces, whether or not a
namespace explicitly supports them. However, consistent with the
approach taken in RFC 3986, the behavior of a URN that contains
components that are undefined or meaningless for a particular
namespace or resource is not defined. The following sections
describe these optional components and their interpretation in
greater detail.
2.3.1. q-component
The q-component is intended for passing parameters to either the
named resource or a system that can supply the requested service, for
interpretation by that resource or system. (By contrast, passing
parameters to URN resolution services is handled by r-components as
described in the next section.)
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, 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 or resolves directly to an object or a non-URL
Saint-Andre & Klensin Expires October 18, 2016 [Page 9]
Internet-Draft URNs April 2016
resource designator), the interpretation of the q-component is
undefined by this specification. Thus, for URNs which may be
resolved to a URL, the semantics of q-component are identical to
those for queries to the resource located via that URL.
For the sake of consistency with RFC 3986, neither the general syntax
nor the semantics of q-components are defined by, or dependent on,
the namespace of the URN. In parallel with RFC 3896, specifics of
syntax and semantics, e.g., which keywords or terms are meaningful,
of course may depend on a particular namespace or even a particular
resource.
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
[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
processing.
Consider the hypothetical example of passing parameters to an
application that returns weather reports from different regions or
for different time periods. This could perhaps be accomplished by
specifying latitude and longitude coordinates and datetimes in the
URN's q-component, resulting in URNs such as the following.
urn:example:weather?=op=map&lat=39.56
&lon=-104.85&datetime=1969-07-21T02:56:15Z
2.3.2. r-component
The r-component is intended for passing parameters to URN resolution
services (taken broadly, see Section 1.1) and interpreted by those
services. (By contrast, passing parameters to the resources
identified by a URN, or to applications that manage such resources,
is handled by q-components as described in the previous section.)
Saint-Andre & Klensin Expires October 18, 2016 [Page 10]
Internet-Draft URNs April 2016
The URN r-component has no syntactic equivalent in URLs.
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] 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.
This document defines only the syntax of the r-component and reserves
it for future use. The exact semantics of the r-component and its
use in URN resolution protocols are a matter for potential
standardization in separate specifications, presumably including
specifications that define conventions and a registry for resolution
service identifiers.
Consider the hypothetical example of passing parameters to a
resolution service (say, an ISO alpha-2 country code [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:
urn:example:foo-bar-baz-qux?+CCResolve: cc=uk
While the above should serve as a general explanation and
illustration of the intent for r-components, there are many
unresolved issues with them, including their relationship to
resolution mechanisms associated with the particular NID and
namespace at registration time. Thus r-components SHOULD NOT be used
for actual URNs until additional development and standardization work
is complete, including specification of any necessary registration
mechanisms.
2.3.3. f-component
The f-component is intended to be interpreted by the client as a
specification for a location within, or region of, the named
resource.
The URN f-component has the same syntax as the URI fragment
component. If a URN containing an f-component resolves to a single
Saint-Andre & Klensin Expires October 18, 2016 [Page 11]
Internet-Draft URNs April 2016
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. If the URN does not resolve to a URL (e.g., is an abstract
designator), the interpretation of the f-component is undefined by
this specification. Thus, for URNs which may be resolved to a URL,
the semantics of f-components are identical to those of fragments for
that resource.
For the sake of consistency with RFC 3986, neither the general syntax
nor the semantics of f-components are defined by, or dependent on,
the namespace of the URN. In parallel with RFC 3896, specifics of
syntax and semantics, e.g., which keywords or terms are meaningful,
of course may depend on a particular namespace or even a particular
resource.
The f-component is indicated by the presence of a number sign ("#")
character and terminated by the end of the URI. Any characters
outside the ASCII range [RFC20] that appear in the f-component MUST
be percent-encoded using the method defined in Section 2.1 of the
generic URI specification [RFC3986].
As described under Section 3, the f-component SHALL NOT be taken into
account when determining URN equivalence.
Clients SHOULD NOT pass f-components to resolution services unless
those services also perform object retrieval and interpretation
functions.
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:
urn:example:foo-bar-baz-qux#somepart
3. Equivalence of URNs
3.1. Procedure
For various purposes such as caching, it is often desirable to
determine if two URNs are "the same". This is done by testing for
equivalence (see Section 6.1 of [RFC3986]).
Saint-Andre & Klensin Expires October 18, 2016 [Page 12]
Internet-Draft URNs April 2016
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 6.2.2.1 of [RFC3986]) to the following constructs:
1. the URI scheme "urn", by conversion to lower case
2. the NID, by conversion to lower case
3. any percent-encoded characters in the NSS (that is, all character
triplets that match the <pct-encoding> production found in
Section 2.1 of the base URI specification [RFC3986]), by
conversion to upper case for the digits A-F.
Percent-encoded characters MUST NOT be decoded, i.e., percent-
encoding normalization (as specified in Section 6.2.2.2 of [RFC3986])
MUST NOT be applied as part of the comparison process.
If 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
Saint-Andre & Klensin Expires October 18, 2016 [Page 13]
Internet-Draft URNs April 2016
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
URNs:
o urn:example:a123,z456/foo
o urn:example:a123,z456/bar
o urn:example:a123,z456/baz
Fourth, because of percent-encoding, the following URNs are
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
Saint-Andre & Klensin Expires October 18, 2016 [Page 14]
Internet-Draft URNs April 2016
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,
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 URN semantics from more general
URI 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 the latter case, such
processors 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
Section 5.2 of [RFC3986] describes an algorithm for converting a URI
reference that might be relative to a given base URI into "parsed
components" of the target of that reference, which can then be
recomposed per RFC 3986 Section 5.3 into a target URI. This
algorithm cannot be applied directly to URNs because their syntax
does not support the necessary path components. Whenever a URN
Saint-Andre & Klensin Expires October 18, 2016 [Page 15]
Internet-Draft URNs April 2016
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. Even though the notion
of a URN as a "persistent", "permanent" identifier does not reconcile
easily with relative referencing, resources named with URNs may
contain relative references that do not apply to the URN itself.
Given the foregoing, 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
process
(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.
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:"
Saint-Andre & Klensin Expires October 18, 2016 [Page 16]
Internet-Draft URNs April 2016
[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
obsolete.
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 publisher
assigns an ISBN to an electronic publication and that publication is
later incorporated into a digital long term archive operated by a
Saint-Andre & Klensin Expires October 18, 2016 [Page 17]
Internet-Draft URNs April 2016
national library, the library might assign the publication a NBN,
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 namespaces.
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
services such as those described in [RFC2483].
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.
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,
Saint-Andre & Klensin Expires October 18, 2016 [Page 18]
Internet-Draft URNs April 2016
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).
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
valid).
A formal namespace establishes a particular NID, subject to the
following constraints (above and beyond the syntax rules already
specified):
1. It MUST NOT be an already-registered NID.
2. It MUST NOT start with "urn-" (which is reserved for informal
namespaces).
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
Saint-Andre & Klensin Expires October 18, 2016 [Page 19]
Internet-Draft URNs April 2016
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
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
syntax.
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 20]
Internet-Draft URNs April 2016
2. The syntax of URNs assigned within the namespace, including the
internal syntax and anticipated effects of q-components or
r-components. (The syntax and interpretation of f-components are
defined 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.
6. Optionally, the process for resolving URNs issued within the
namespace.
The section on completing the template (Section 6.4) explains these
matters in greater detail. Although the registration templates are
the same in all cases, slightly different procedures are used
depending on the source of the registration.
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); IANA
will register the requested NID only after receiving directions
from that body and a copy of the expert review report.
Saint-Andre & Klensin Expires October 18, 2016 [Page 21]
Internet-Draft URNs April 2016
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, registrants
SHOULD provide a stable specification 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
themselves.
6.3. Registration Policy and Process: Fast Track for Standards
Development Organizations, Scientific Societies, and Similar
Bodies
The IETF recognizes that situations will arise in which URN
namespaces will be created to either embed existing and established
standards, particularly identifier standards, or to reflect
knowledge, terminology, or methods of organizing information that lie
well outside the IETF's scope or the likely subject matter knowledge
of its Designated Experts. In situations in which the registration
request originates from, or is authorized by, a recognized standards-
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 22]
Internet-Draft URNs April 2016
2. A specification is required that reflects or points to the needed
external standards or specifications. Publication in the RFC
Series or through an IETF process (e.g., posting as an Internet
Draft) is not expected and would be appropriate only under very
unusual circumstances.
3. The reviews on the discussion list and by the designated experts
are strictly advisory, with the decisions about what advice to
accept and the length of time to allocate to the process strictly
under the control of the external body.
4. When that body concludes that the application is sufficiently
mature, its represenative(s) will request that IANA complete the
registration for the NID, and IANA will do so.
Decisions about whether to recognize the requesting entity as a
standards-related organization, scientific society, or similar body
are the responsibility of the IESG.
A model similar to this 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
namespace.
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 23]
Internet-Draft URNs April 2016
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
namespace).
6. Whether resolution services are available or will be available
(and, if so, the nature or identity of the services). Examples
of q-component and, when they are standardized, r-component,
semantics and syntax are helpful here, even if detailed
definitions are provided elsewhere later.
7. Whether the namespace or its definition is expected to become an
integral or normative element of a standard being developed in
the IETF or some other recognized standards body.
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. There may also be namespace-specific special
encoding considerations, especially for URNs that contain
embedded forms of other types of identifiers. (Note that these
are not normative statements for any kind of best practice
related to handling of equivalences between characters in
general; such statements are limited to one particular namespace
only.)
Saint-Andre & Klensin Expires October 18, 2016 [Page 24]
Internet-Draft URNs April 2016
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].
5. Any special considerations for the meaning of q-components (e.g.,
keywords) or f-components (e.g., predefined terms) in the context
of this namespace.
6.4.3. Assignment
The "Assignment" section of the template describes matters such as:
1. Mechanisms or authorities for assigning URNs to resources. It
ought to make clear whether assignment is completely open (e.g.,
following a particular procedure such as first-come, first-served
(FCFS)), completely closed (e.g., for a private organization), or
limited in various ways (e.g., delegated to authorities
recognized by a particular organization); if limited, it ought to
explain how to become an assigner of identifiers or how to
request assignment of identifiers from existing assignment
authorities.
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])
Saint-Andre & Klensin Expires October 18, 2016 [Page 25]
Internet-Draft URNs April 2016
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 known 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 or in parallel to the registration process.
6.4.6. Resolution
The "Resolution" section MUST specify whether resolution mechanisms
are intended or anticipated for URNs assigned within the 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. In addition, this section SHOULD describe
any special considerations for the handilng of r-components in the
context of this namespace.
6.4.7. Additional Information
Please include any additional information that would be useful to
those trying to understand this registration or its relationship to
others, such as comparisons to existing namespaces that might seem to
overlap.
This section of the template is optional.
Saint-Andre & Klensin Expires October 18, 2016 [Page 26]
Internet-Draft URNs April 2016
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
identifiers.
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 ].
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 27]
Internet-Draft URNs April 2016
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. Normally, these declarations will have a relatively low
security profile, however there is always the danger of "spoofing"
and providing misinformation. Information in these declarations
ought to be taken as advisory.
9. References
9.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", 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.
[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,
<http://tools.ietf.org/id/draft-paskin-doi-uri-04.txt>.
[I-D.ietf-urnbis-semantics-clarif]
Klensin, J., "URN Semantics Clarification", draft-ietf-
urnbis-semantics-clarif-03 (work in progress), February
2016.
Saint-Andre & Klensin Expires October 18, 2016 [Page 28]
Internet-Draft URNs April 2016
[ISO3166-1]
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
2001.
[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
2003.
[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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 29]
Internet-Draft URNs April 2016
[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
2013.
[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,
<http://www.rfc-editor.org/info/rfc7613>.
[UAX31] The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", June 2015,
<http://unicode.org/reports/tr31/>.
[UNICODE] The Unicode Consortium, "The Unicode Standard", 2015,
<http://www.unicode.org/versions/latest/>.
[URI-Registry]
IANA, "Permanent URI Schemes",
<http://www.iana.org/assignments/uri-schemes/
uri-schemes.xhtml#uri-schemes-1>.
[XML-BASE]
Marsh, J. and R. Tobin, "XML Base (Second Edition)", World
Wide Web Consortium Recommendation REC-xmlbase-20090128,
January 2009,
<http://www.w3.org/TR/2009/REC-xmlbase-20090128>.
Saint-Andre & Klensin Expires October 18, 2016 [Page 30]
Internet-Draft URNs April 2016
[XML-NAMES]
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,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
Appendix A. Registration Template
Namespace ID: Requested of IANA (formal) or assigned by IANA
(informal).
Version: The version of the registration, starting with 1 and
incrementing by 1 with each new version.
Date: The date when the registration is requested of IANA, using the
format YYYY-MM-DD.
Registrant: The person or organization that has registered the NID,
including the name and address of the registering organization, as
well as the name and contact information (email, phone number, or
postal address) of the designated contact person. If the
registrant is a recognized standards development organization 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.
Security and Privacy: Described under Section 6.4.4 of this
document.
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.
Additional Information Described under Section 6.4.7 of this
document.
Saint-Andre & Klensin Expires October 18, 2016 [Page 31]
Internet-Draft URNs April 2016
Revision Information: Description of changes from prior version(s).
(Applicable only when earlier registrations have been revised.)
Additional Information: Any additional information that would be
useful to the reader or those trying to understand the
registration, perhaps in context with other work. May be a
reference to another document or omitted if not needed.
Appendix B. Changes from RFC 2141
This document makes substantive changes from the syntax and semantics
of [RFC2141]:
B.1. Syntax changes from RFC 2141
The syntax of URNs as provided in [RFC2141] was defined before the
updated specification of URIs in [RFC3986]. The definition of URN
syntax is updated in this document to do the following:
o Ensure consistency with the URI syntax.
o Facilitate the use of URNs with parameters similar to URI queries
and fragments.
o Permit parameters influencing URN resolution.
o Ease the use of URNs with naming systems that include the '/'
character.
In particular, this specification does the following:
o Extends URN syntax to explicitly allow the characters '/', "?",
and "#", which were reserved for future use by RFC 2141. As
described below, this change effectively also allows several
components of the URI syntax although without necessarily tying
those components to URI semantics.
o Defines general syntax for an additional component that can be
used in interactions with a URN resolution service.
o Disallows "-" at the end of a NID.
o Allows the "/", "~", and "&" characters in the namespace-specific
string (NSS).
o Makes several smaller syntax adjustments.
Saint-Andre & Klensin Expires October 18, 2016 [Page 32]
Internet-Draft URNs April 2016
B.2. Other changes from RFC 2141
o Formally registers 'urn' as a URI scheme.
o Allows what are now called q-components, r-components, and
f-components.
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].
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
[RFC6648]. Experimental namespaces were denoted by prefixing the
namespace identifier with the string "X-". Because experimental
namespaces were never registered, removing the experimental
category has no impact on the existing registries. 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].
3. Adds some information too, but generally simplifies, the
registration template.
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
Saint-Andre & Klensin Expires October 18, 2016 [Page 33]
Internet-Draft URNs April 2016
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 and then as co-chair.
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
section.
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.
o Added more text about p-/q-/f-components and recommendations about
use.
o Added clarifying note about "?" within q-components and
f-components.
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 34]
Internet-Draft URNs April 2016
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
editors).
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
perogatives.
o Changed the production for assigned-name as suggested by Lars
Svensson.
o Several clarifying editorial changes including correcting a glitch
in instructions to the RFC Editor.
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
efforts.
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.
Saint-Andre & Klensin Expires October 18, 2016 [Page 35]
Internet-Draft URNs April 2016
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
registered.
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.
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
Saint-Andre & Klensin Expires October 18, 2016 [Page 36]
Internet-Draft URNs April 2016
contents so that the subsections on optional components now appear
there.
F.8. Changes from -15 (2016-02-04) to -16
o Rewrote the introductory material to make the relationship to
other specifications more clear and allow removing or altering
text that was stated in terms of changes from 2141. The
specification is now self-contained with regard to the earlier
definitions and descriptions of URNs.
o Removed the parts of Section 2 that were really a description of
changes from RFC 2141 to Appendix B, where such changes are
enumerated. Similarly, removed most material describing changes
from RFC 3406 to Appendix C.
o Replaced one example.
o Rearranged and rewrote text to improve clarity and relationships
to other documents and to reduce redundant material.
o Made it more clear that r-components, despite the partial syntax
specification, are reserved for future standardization.
o Clarified that there can be URNs that neither resolve to URLs nor
are abstract designators.
o Added pointers to make it clear that the Syntax material in
Section 2 is not self-contained, e.g., that its subsections and
other sections further restrict strings that can be used for NIDs
and so on.
o Added an "Additional Information" section to the registration
template. See list discussion on and about 2016-03-18.
o Minor editorial/ typographic fixes (per comment from Lars).
Authors' Addresses
Peter Saint-Andre
Filament
Email: peter@filament.com
URI: https://filament.com/
Saint-Andre & Klensin Expires October 18, 2016 [Page 37]
Internet-Draft URNs April 2016
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john-ietf@jck.com
Saint-Andre & Klensin Expires October 18, 2016 [Page 38]