Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 8141 Filament
Obsoletes: 2141, 3406 J. Klensin
Category: Standards Track April 2017
ISSN: 2070-1721
Uniform Resource Names (URNs)
Abstract
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
that is assigned under the "urn" URI scheme and a particular URN
namespace, with the intent that the URN will be a persistent,
location-independent resource identifier. With regard to URN syntax,
this document defines the canonical syntax for URNs (in a way that is
consistent with URI syntax), specifies methods for determining URN-
equivalence, and discusses URI conformance. With regard to URN
namespaces, this document specifies a method for defining a URN
namespace and associating it with a namespace identifier, and it
describes procedures for registering namespace identifiers with the
Internet Assigned Numbers Authority (IANA). This document obsoletes
both RFCs 2141 and 3406.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8141.
Saint-Andre & Klensin Standards Track [Page 1]
RFC 8141 URNs April 2017
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Saint-Andre & Klensin Standards Track [Page 2]
RFC 8141 URNs April 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Design Trade-offs . . . . . . . . . . . . . . . . . . . . 6
1.2.1. Resolution . . . . . . . . . . . . . . . . . . . . . 8
1.2.2. Character Sets and Encodings . . . . . . . . . . . . 9
2. URN Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Namespace Identifier (NID) . . . . . . . . . . . . . . . 10
2.2. Namespace Specific String (NSS) . . . . . . . . . . . . . 10
2.3. Optional Components . . . . . . . . . . . . . . . . . . . 12
2.3.1. r-component . . . . . . . . . . . . . . . . . . . . . 12
2.3.2. q-component . . . . . . . . . . . . . . . . . . . . . 13
2.3.3. f-component . . . . . . . . . . . . . . . . . . . . . 15
3. URN-Equivalence . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 17
4. URI Conformance . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Use in URI Protocol Slots . . . . . . . . . . . . . . . . 18
4.2. Parsing . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3. URNs and Relative References . . . . . . . . . . . . . . 19
4.4. Transport and Display . . . . . . . . . . . . . . . . . . 19
4.5. URI Design and Ownership . . . . . . . . . . . . . . . . 20
5. URN Namespaces . . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Formal URN Namespaces . . . . . . . . . . . . . . . . . . 22
5.2. Informal URN Namespaces . . . . . . . . . . . . . . . . . 23
6. Defining and Registering a URN Namespace . . . . . . . . . . 24
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 24
6.2. Registration Policy and Process: Community Registrations 25
6.3. Registration Policy and Process: Fast Track for Standards
Development Organizations, Scientific Societies, and
Similar Bodies . . . . . . . . . . . . . . . . . . . . . 26
6.4. Completing the Template . . . . . . . . . . . . . . . . . 27
6.4.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 27
6.4.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 28
6.4.3. Assignment . . . . . . . . . . . . . . . . . . . . . 29
6.4.4. Security and Privacy . . . . . . . . . . . . . . . . 29
6.4.5. Interoperability . . . . . . . . . . . . . . . . . . 30
6.4.6. Resolution . . . . . . . . . . . . . . . . . . . . . 30
6.4.7. Additional Information . . . . . . . . . . . . . . . 30
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
7.1. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . 31
7.2. Registration of URN Namespaces . . . . . . . . . . . . . 31
7.3. Discussion List for New and Updated NID Registrations . . 31
8. Security and Privacy Considerations . . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . 32
Saint-Andre & Klensin Standards Track [Page 3]
RFC 8141 URNs April 2017
Appendix A. Registration Template . . . . . . . . . . . . . . . 37
Appendix B. Changes from RFC 2141 . . . . . . . . . . . . . . . 38
B.1. Syntax Changes from RFC 2141 . . . . . . . . . . . . . . 38
B.2. Other Changes from RFC 2141 . . . . . . . . . . . . . . . 39
Appendix C. Changes from RFC 3406 . . . . . . . . . . . . . . . 39
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
[RFC3986] that is assigned under the "urn" URI scheme and a
particular URN namespace, with the intent that the URN will be a
persistent, location-independent resource identifier. A URN
namespace is a collection of such URNs, each of which is (1) unique,
(2) assigned in a consistent and managed way, and (3) assigned
according to a common definition. (Some URN namespaces create names
that exist only as URNs, whereas others assign URNs based on names
that were already created in non-URN identifier systems, such as
ISBNs [RFC3187], ISSNs [RFC3044], or RFCs [RFC2648].)
The assignment of URNs is done by an organization (or, in some cases,
according to an algorithm or other automated process) that has been
formally delegated a URN namespace within the "urn" scheme (e.g., a
URN in the "example" URN namespace [RFC6963] might be of the form
"urn:example:foo").
This document rests on two key assumptions:
1. Assignment of a URN is a managed process.
2. The space of URN namespaces is itself managed.
While other URI schemes may allow resource identifiers to be freely
chosen and assigned, such is not the case for URNs. The syntactical
correctness of a name starting with "urn:" is not sufficient to make
it a URN. In order for the name to be a valid URN, the namespace
identifier (NID) needs to be registered in accordance with the rules
defined here, and the remaining parts of the assigned-name portion of
the URN need to be generated in accordance with the rules for the
registered URN namespace.
Saint-Andre & Klensin Standards Track [Page 4]
RFC 8141 URNs April 2017
So that information about both URN syntax and URN namespaces is
available in one place, this document does the following:
1. Defines the canonical syntax for URNs in general (in a way that
is consistent with URI syntax), specifies methods for determining
URN-equivalence, and discusses URI conformance.
2. Specifies a method for defining a URN namespace and associating
it with a particular NID, and describes procedures for
registering URN NIDs with the Internet Assigned Numbers Authority
(IANA).
For URN syntax and URN namespaces, this document modernizes and
replaces the original specifications for URN syntax [RFC2141] and for
the definition and registration of URN namespaces [RFC3406]. These
modifications build on the key requirements provided in the original
functional description for URNs [RFC1737] and on the lessons of many
years of experience. In those original documents and in the present
one, the intent is to define URNs in a consistent manner so that,
wherever practical, the parsing, handling, and resolution of URNs can
be independent of the URN namespace within which a given URN is
assigned.
Together with input from several key user communities, the history
and experiences with URNs dictated expansion of the URN definition to
support new functionality, including the use of syntax explicitly
reserved for future standardization in RFC 2141. All URN namespaces
and URNs that were valid under the earlier specifications remain
valid, even though it may be useful to update the definitions of some
URN namespaces to take advantage of new features.
The foregoing considerations, together with various differences
between URNs and URIs that are locators (specifically URLs) as well
as the greater focus on URLs in RFC 3986 as the ultimate successor to
[RFC1738] and [RFC1808], may lead to some interpretations of RFC 3986
and this specification that appear (or perhaps actually are) not
completely consistent, especially with regard to actions or semantics
other than the basic syntax itself. If such situations arise,
discussions of URNs and URN namespaces should be interpreted
according to this document and not by extrapolation from RFC 3986.
Summaries of changes from RFCs 2141 and 3406 appear in Appendices B
and C, respectively. This document obsoletes both [RFC2141] and
[RFC3406]. While it does not explicitly update or replace [RFC1737]
or [RFC2276], the reader who references those documents should be
aware that the conceptual model of URNs in this document is slightly
different from those older specifications.
Saint-Andre & Klensin Standards Track [Page 5]
RFC 8141 URNs April 2017
1.1. Terminology
The following terms are distinguished from each other as described
below:
URN: A URI (as defined in RFC 3986) using the "urn" scheme and with
the properties of a "name" as described in that document as well
as the properties described in this one. The term applies to the
entire URI including its optional components. Note to the reader:
the term "URN" has been used in other contexts to refer to a URN
namespace, the namespace identifier, the assigned-name, and URIs
that do not use the "urn" scheme. All but the last of these is
described using more specific terminology elsewhere in this
document, but, because of those other uses, the term should be
used and interpreted with care.
Locator: An identifier that provides a means of accessing a
resource.
Identifier system: A managed collection of names. This document
refers to identifier systems outside the context of URNs as
"non-URN identifier systems".
URN namespace: An identifier system that is associated with a URN
NID.
NID: The identifier associated with a URN namespace.
NSS: The URN-namespace-specific part of a URN.
Assigned-name: The combination of the "urn:" scheme, the NID, and
the namespace specific string (NSS). An "assigned-name" is
consequently a substring of a URN (as defined above) if that URN
contains any additional components (see Section 2).
The term "name" is deliberately not defined here and should be (and,
in practice, is) used only very informally. RFC 3986 uses the term
as a category of URI distinguished from "locator" (Section 1.1.3) but
also uses it in other contexts. If those uses are treated as
definitional, they would conflict with, e.g., the idea of URN
namespace names (i.e., NIDs) and with terms associated with non-URN
identifier systems.
This document uses the terms "resource", "identifier", "identify",
"dereference", "representation", and "metadata" roughly as defined in
the URI specification [RFC3986].
Saint-Andre & Klensin Standards Track [Page 6]
RFC 8141 URNs April 2017
This document uses the terms "resolution" and "resolver" in roughly
the sense in which they were used in the original discussion of
architectural principles for URNs [RFC2276], i.e., "resolution" is
the act of supplying services related to the identified resource,
such as translating the persistent URN into one or more current
locators for the resource, delivering metadata about the resource in
an appropriate format, or even delivering a representation of the
resource (e.g., a document) without requiring further intermediaries.
At the time of this writing, resolution services are described in
[RFC2483].
On the distinction between representations and metadata, see
Section 1.2.2 of [RFC3986].
Several other terms related to "normalization" operations that are
not part of the Unicode Standard [UNICODE] are also used here as they
are in RFC 3986.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.2. Design Trade-offs
To a degree much greater than when URNs were first considered and
their uses outlined (see [RFC1737]), issues of persistent identifiers
on the Internet involve fundamental design trade-offs that are much
broader than URNs or the URN approach and even touch on open research
questions within the information sciences community. Ideal and
comprehensive specifications about what should be done or required
across the entire universe of URNs would require general agreement
about, and solutions to, a wide range of such issues. Although some
of those issues were introduced by the Internet or computer-age
approaches to character encodings and data abstraction, others
predate the Internet and computer systems by centuries; there is
unlikely to be agreement about comprehensive solutions in the near
future.
Although this specification consequently contains some requirements
and flexibility that would not be present in a more perfect world,
this has been necessary in order to produce a consensus specification
that provides a modernized definition of URNs (the unattractive
alternative would have been to not modernize the definition in spite
of widespread deployment).
The following sub-sections describe two of the relevant issues in
greater detail.
Saint-Andre & Klensin Standards Track [Page 7]
RFC 8141 URNs April 2017
1.2.1. Resolution
One issue that is specific to URNs (as opposed to naming systems in
general) is the fairly difficult topic of "resolution", discussed in
Sections 1.1, 2.3.1, 6.4.6, and elsewhere below.
With traditional Uniform Resource Locators (URLs), i.e., with most
URIs that are locators, resolution is relatively straightforward
because it is used to determine an access mechanism that in turn is
used to dereference the locator by (typically) retrieving a
representation of the associated resource, such as a document (see
Section 1.2.2 of [RFC3986]).
By contrast, resolution for URNs is more flexible and varied.
One important case involves the mapping of a URN to one or more
locators. In this case, the end result is still a matter of
dereferencing the mapped locator(s) to one or more representations.
The primary difference here is persistence: even if a mapped locator
has changed (e.g., a DNS domain name has changed hands and a URL has
not been modified to point to a new location or, in a more extreme
and hypothetical case, the DNS is replaced entirely), a URN user will
be able to obtain the correct representation (e.g., a document) as
long as the resolver has kept its URN-to-locator mappings up to date.
Consequently, the relevant relationships can be defined quite
precisely for URNs that resolve to locators that in turn are
dereferenced to a representation.
However, this specification permits several other cases of URN
resolution as well as URNs for resources that do not involve
information retrieval systems. This is true either individually for
particular URNs or (as defined below) collectively for entire URN
namespaces.
Consider a namespace of URNs that resolve to locators that in turn
are dereferenced only to metadata about resources because the
underlying systems contain no representations of those resources; an
example might be a URN namespace for International Standard Name
Identifiers (ISNIs) as that identifier system is defined in the
relevant standard [ISO.27729.2012], wherein by default a URN would be
resolved only to a metadata record describing the public identity
identified by the ISNI.
Consider also URNs that resolve to representations only if the
requesting entity is authorized to obtain the representation, whereas
other entities can obtain only metadata about the resource; an
example might be documents held within the legal depository
collection of a national library.
Saint-Andre & Klensin Standards Track [Page 8]
RFC 8141 URNs April 2017
Finally, some URNs might not be intended to resolve to locators at
all; examples might include URNs identifying XML namespace names
(e.g., the "dgiwg" URN namespace specified by [RFC6288]), URNs
identifying application features that can be supported within a
communications protocol (e.g., the "alert" URN namespace specified by
[RFC7462]), and URNs identifying enumerated types such as values in a
registry (e.g., a URN namespace could be used to individually
identify the values in all IANA registries, as provisionally proposed
in [IANA-URN]).
Various types of URNs and multiple resolution services that may be
available for them leave the concept of "resolution" more complicated
but also much richer for URNs than the straightforward case of
resolution to a locator that is dereferenced to a representation.
1.2.2. Character Sets and Encodings
A similar set of considerations apply to character sets and
encodings. URNs, especially URNs that will be used as user-facing
identifiers, should be convenient to use in local languages and
writing systems, easily specified with a wide range of keyboards and
local conventions, and unambiguous. There are trade-offs among those
goals, and it is impossible at present to see how a simple and
readily understandable set of rules could be developed that would be
optimal, or even reasonable, for all URNs. The discussion in
Section 2.2 defines an overall framework that should make generalized
parsing and processing possible but also makes recommendations about
rules for individual URN namespaces.
2. URN Syntax
As discussed above, the syntax for URNs in this specification allows
significantly more functionality than was the case in the earlier
specifications, most recently [RFC2141]. It is also harmonized with
the general URI syntax [RFC3986] (which, it must be noted, was
completed after the earlier URN specifications).
However, this specification does not extend the URN syntax to allow
direct use of characters outside the ASCII range [RFC20]. That
restriction implies that any such characters need to be percent-
encoded as described in Section 2.1 of the URI specification
[RFC3986].
The basic syntax for a URN is defined using the Augmented Backus-Naur
Form (ABNF) as specified in [RFC5234]. Rules not defined here
(specifically: alphanum, fragment, and pchar) are defined as part of
the URI syntax [RFC3986] and used here to point out the syntactic
relationship with the terms used there. The definitions of some of
Saint-Andre & Klensin Standards Track [Page 9]
RFC 8141 URNs April 2017
the terms used below are not comprehensive; additional restrictions
are imposed by the prose that can be found in sections of this
document that are specific to those terms (especially r-component in
Section 2.3.1 and q-component in Section 2.3.2).
namestring = assigned-name
[ rq-components ]
[ "#" f-component ]
assigned-name = "urn" ":" NID ":" NSS
NID = (alphanum) 0*30(ldh) (alphanum)
ldh = alphanum / "-"
NSS = pchar *(pchar / "/")
rq-components = [ "?+" r-component ]
[ "?=" q-component ]
r-component = pchar *( pchar / "/" / "?" )
q-component = pchar *( pchar / "/" / "?" )
f-component = fragment
The question mark character "?" can be used without percent-encoding
inside r-components, q-components, and f-components. Other than
inside those components, a "?" that is not immediately followed by
"=" or "+" is not defined for URNs and SHOULD be treated as a syntax
error by URN-specific parsers and other processors.
The following sections provide additional information about the
syntactic elements of URNs.
2.1. Namespace Identifier (NID)
NIDs are case insensitive (e.g., "ISBN" and "isbn" are equivalent).
Characters outside the ASCII range [RFC20] are not permitted in NIDs,
and no encoding mechanism for such characters is supported.
Sections 5.1 and 5.2 impose additional constraints on the strings
that can be used as NIDs, i.e., the syntax shown above is not
comprehensive.
2.2. Namespace Specific String (NSS)
The NSS is a string, unique within a URN namespace, that is assigned
and managed in a consistent way and that conforms to the definition
of the relevant URN namespace. The combination of the NID (unique
across the entire "urn" scheme) and the NSS (unique within the URN
namespace) ensures that the resulting URN is globally unique.
Saint-Andre & Klensin Standards Track [Page 10]
RFC 8141 URNs April 2017
The NSS as specified in this document allows several characters not
permitted by earlier specifications (see Appendix B). In particular,
the "/" character, which is now allowed, effectively makes it
possible to encapsulate hierarchical names from non-URN identifier
systems. For instance, consider the hypothetical example of a
hierarchical identifier system in which the names take the form of a
sequence of numbers separated by the "/" character, such as
"1/406/47452/2". If the authority for such names were to use URNs,
it would be natural to place the existing name in the NSS, resulting
in URNs such as "urn:example:1/406/47452/2".
Those changes to the syntax for the NSS do not modify the encoding
rules for URN namespaces that were defined in accordance with
[RFC2141]. If any such URN namespace whose names are used outside of
the URN context (i.e., in a non-URN identifier system) also allows
the use of "/", "~", or "&" in the native form within that identifier
system, then the encoding rules for that URN namespace are not
changed by this specification.
Depending on the rules governing a non-URN identifier system and its
associated URN namespace, names that are valid in that identifier
system might contain characters that are not allowed by the "pchar"
production referenced above (e.g., characters outside the ASCII range
or, consistent with the restrictions in RFC 3986, the characters "/",
"?", "#", "[", and "]"). While such a name might be valid within the
non-URN identifier system, it is not a valid URN until it has been
translated into an NSS that conforms to the rules of that particular
URN namespace. In the case of URNs that are formed from names that
exist separately in a non-URN identifier system, translation of a
name from its "native" format to a URN format is accomplished by
using the canonicalization and encoding methods defined for URNs in
general or specific rules for that URN namespace. Software that is
not aware of namespace-specific canonicalization and encoding rules
MUST NOT construct URNs from the name in the non-URN identifier
system.
In particular, with regard to characters outside the ASCII range,
URNs that appear in protocols or that are passed between systems MUST
use only Unicode characters encoded in UTF-8 and further encoded as
required by RFC 3986. To the extent feasible and consistent with the
requirements of names defined and standardized elsewhere, as well as
the principles discussed in Section 1.2, the characters used to
represent names SHOULD be restricted to either ASCII letters and
digits or to the characters and syntax of some widely used models
such as those of Internationalizing Domain Names in Applications
(IDNA) [RFC5890], Preparation, Enforcement, and Comparison of
Internationalized Strings (PRECIS) [RFC7613], or the Unicode
Identifier and Pattern Syntax specification [UAX31].
Saint-Andre & Klensin Standards Track [Page 11]
RFC 8141 URNs April 2017
In order to make URNs as stable and persistent as possible when
protocols evolve and the environment around them changes, URN
namespaces SHOULD NOT allow characters outside the ASCII range
[RFC20] unless the nature of the particular URN namespace makes such
characters necessary.
2.3. Optional Components
This specification includes three optional components in the URN
syntax. They are known as r-component, q-component, and f-component
and are described in more detail below. Because this specification
focuses almost exclusively on URN syntax, it does not define detailed
semantics of these components for URNs in general. However, each of
these components has a distinct role that is independent of any given
URN and its URN namespace. It is intended that clients will be able
to handle these components uniformly for all URNs. These components
MAY be used with URNs from existing URN namespaces, whether or not a
URN namespace explicitly supports them. However, consistent with the
approach taken in RFC 3986, the behavior of a URN that contains
components that are undefined or meaningless for a particular URN
namespace or resource is not defined. The following sections
describe these optional components and their interpretation in
greater detail.
2.3.1. r-component
The r-component is intended for passing parameters to URN resolution
services (taken broadly, see Section 1.2) and interpreted by those
services. (By contrast, passing parameters to the resources
identified by a URN, or to applications that manage such resources,
is handled by q-components as described in the next section.)
The URN r-component has no syntactic counterpart in any other known
URI scheme.
The sequence "?+" introduces the r-component. The r-component ends
with a "?=" sequence (which begins a q-component) or a "#" character
(number sign, which begins an f-component). If neither of those
appear, the r-component continues to the end of the URN. Note that
characters outside the ASCII range [RFC20] MUST be percent-encoded
using the method defined in Section 2.1 of the generic URI
specification [RFC3986].
As described in Section 3, the r-component SHALL NOT be taken into
account when determining URN-equivalence. However, the r-component
SHALL be supplied along with the URN when presenting a request to a
URN resolution service.
Saint-Andre & Klensin Standards Track [Page 12]
RFC 8141 URNs April 2017
This document defines only the syntax of the r-component and reserves
it for future use. The exact semantics of the r-component and its
use in URN resolution protocols are a matter for potential
standardization in separate specifications, presumably including
specifications that define conventions and a registry for resolution
service identifiers.
Consider the hypothetical example of passing parameters to a
resolution service (say, an ISO alpha-2 country code [ISO.3166-1] in
order to select the preferred country in which to search for a
physical copy of a book). This could perhaps be accomplished by
specifying the country code in the r-component, resulting in URNs
such as:
urn:example:foo-bar-baz-qux?+CCResolve:cc=uk
While the above should serve as a general explanation and
illustration of the intent for r-components, there are many open
issues with them, including their relationship to resolution
mechanisms associated with the particular URN namespace at
registration time. Thus, r-components SHOULD NOT be used for URNs
before their semantics have been standardized.
2.3.2. q-component
The q-component is intended for passing parameters to either the
named resource or a system that can supply the requested service, for
interpretation by that resource or system. (By contrast, passing
parameters to URN resolution services is handled by r-components as
described in the previous section.)
The URN q-component has the same syntax as the URI query component
but is introduced by "?=", not "?" alone. For a URN that may be
resolved to a URI that is a locator, the semantics of the q-component
are identical to those for the query component of that URI. Thus,
URN resolvers returning a URI that is a locator for a URN with a
q-component do this by copying the q-component from the URN to the
query component of the URI. An example of the copying operation
appears below.
This specification does not specify a required behavior in the case
of URN resolution to a URI that is a locator when the original URN
has a q-component and the URI has a query string. Different
circumstances may require different approaches. Resolvers SHOULD
document their strategy in such cases.
Saint-Andre & Klensin Standards Track [Page 13]
RFC 8141 URNs April 2017
If the URN does not resolve to a URI that is a locator, the
interpretation of the q-component is undefined by this specification.
For URNs that may be resolved to a URI that is a locator, the
semantics of the q-component are identical to those for queries to
the resource located via that URI.
For the sake of consistency with RFC 3986, the general syntax and the
semantics of q-components are not defined by, or dependent on, the
URN namespace of the URN. In parallel with RFC 3986, specifics of
syntax and semantics, e.g., which keywords or terms are meaningful,
of course may depend on a particular URN namespace or even a
particular resource.
The sequence "?=" introduces the q-component. The q-component ends
with a "#" character (number sign, which begins an f-component). If
that character does not appear, the q-component continues to the end
of the URN. The characters slash ("/") and question mark ("?") may
represent data within the q-component. Note that characters outside
the ASCII range [RFC20] MUST be percent-encoded using the method
defined in Section 2.1 of the generic URI specification [RFC3986].
As described in Section 3, the q-component SHALL NOT be taken into
account when determining URN-equivalence.
URN namespaces and associated information placement in syntax SHOULD
be designed to avoid any need for a resolution service to consider
the q-component. Namespace-specific and more generic resolution
systems MUST NOT require that q-component information be passed to
them for processing.
Consider the hypothetical example of passing parameters to an
application that returns weather reports from different regions or
for different time periods. This could perhaps be accomplished by
specifying latitude and longitude coordinates and datetimes in the
URN's q-component, resulting in URNs such as the following.
urn:example:weather?=op=map&lat=39.56
&lon=-104.85&datetime=1969-07-21T02:56:15Z
If this example resolved to an HTTP URI, the result might look like:
https://weatherapp.example?op=map&lat=39.56
&lon=-104.85&datetime=1969-07-21T02:56:15Z
Saint-Andre & Klensin Standards Track [Page 14]
RFC 8141 URNs April 2017
2.3.3. f-component
The f-component is intended to be interpreted by the client as a
specification for a location within, or region of, the named
resource. It distinguishes the constituent parts of a resource named
by a URN. For a URN that resolves to one or more locators that can
be dereferenced to a representation, or where the URN resolver
directly returns a representation of the resource, the semantics of
an f-component are defined by the media type of the representation.
The URN f-component has the same syntax as the URI fragment
component. If a URN containing an f-component resolves to a single
URI that is a locator associated with the named resource, the
f-component from the URN can be applied (usually by the client) as
the fragment of that URI. If the URN does not resolve to a URI that
is a locator, the interpretation of the f-component is undefined by
this specification. Thus, for URNs that may be resolved to a URI
that is a locator, the semantics of f-components are identical to
those of fragments for that resource.
For the sake of consistency with RFC 3986, neither the general syntax
nor the semantics of f-components are defined by, or dependent on,
the URN namespace of the URN. In parallel with RFC 3986, specifics
of syntax and semantics, e.g., which keywords or terms are
meaningful, of course may depend on a particular URN namespace or
even a particular resource.
The f-component is introduced by the number sign ("#") character and
terminated by the end of the URI. Any characters outside the ASCII
range [RFC20] that appear in the f-component MUST be percent-encoded
using the method defined in Section 2.1 of the generic URI
specification [RFC3986].
As described in Section 3, the f-component SHALL NOT be taken into
account when determining URN-equivalence.
Clients SHOULD NOT pass f-components to resolution services unless
those services also perform object retrieval and interpretation
functions.
Consider the hypothetical example of obtaining resources that are
part of a larger entity (say, the chapters of a book). Each part
could be specified in the f-component, resulting in URNs such as:
urn:example:foo-bar-baz-qux#somepart
Saint-Andre & Klensin Standards Track [Page 15]
RFC 8141 URNs April 2017
3. URN-Equivalence
3.1. Procedure
For various purposes such as caching, it is often desirable to
determine if two URNs are "the same". This is done most generally
(i.e., independent of the scheme) by testing for equivalence (see
Section 6.1 of [RFC3986]).
The generic URI specification [RFC3986] is very flexible about
equality comparisons, putting the focus on allowing false negatives
and avoiding false positives. If comparisons are made in a scheme-
independent way, i.e., as URI comparisons only, many URNs that this
specification considers equal would be rejected. The discussion
below applies when the URIs involved are known to be URNs and thus
uses the terms "URN-equivalent" and "URN-equivalence" to refer to
equivalence as specified in this document.
Two URNs are URN-equivalent if their assigned-name portions are
octet-by-octet equal after applying case normalization (as specified
in Section 6.2.2.1 of [RFC3986]) to the following constructs:
1. the URI scheme "urn", by conversion to lower case
2. the NID, by conversion to lower case
3. any percent-encoded characters in the NSS (that is, all character
triplets that match the <pct-encoding> production found in
Section 2.1 of the base URI specification [RFC3986]), by
conversion to upper case for the digits A-F.
Percent-encoded characters MUST NOT be decoded, i.e., percent-
encoding normalization (as specified in Section 6.2.2.2 of [RFC3986])
MUST NOT be applied as part of the comparison process.
If an r-component, q-component, or f-component (or any combination
thereof) is included in a URN, it MUST be ignored for purposes of
determining URN-equivalence.
URN namespace definitions MAY include additional rules for
URN-equivalence, such as case insensitivity of the NSS (or parts
thereof). Such rules MUST always have the effect of eliminating some
of the false negatives obtained by the procedure above and MUST NOT
result in treating two URNs as not "the same" if the procedure here
says they are URN-equivalent. For related considerations with regard
to NID registration, see below.
Saint-Andre & Klensin Standards Track [Page 16]
RFC 8141 URNs April 2017
3.2. Examples
This section shows a variety of URNs (using the "example" NID defined
in [RFC6963]) that highlight the URN-equivalence rules.
First, because the scheme and NID are case insensitive, the following
three URNs are URN-equivalent to each other:
o urn:example:a123,z456
o URN:example:a123,z456
o urn:EXAMPLE:a123,z456
Second, because the r-component, q-component, and f-component are not
taken into account for purposes of testing URN-equivalence, the
following three URNs are URN-equivalent to the first three examples
above:
o urn:example:a123,z456?+abc
o urn:example:a123,z456?=xyz
o urn:example:a123,z456#789
Third, because the "/" character (and anything that follows it) in
the NSS is taken into account for purposes of URN-equivalence, the
following URNs are not URN-equivalent to each other or to the six
preceding URNs:
o urn:example:a123,z456/foo
o urn:example:a123,z456/bar
o urn:example:a123,z456/baz
Fourth, because of percent-encoding, the following URNs are
URN-equivalent only to each other and not to any of those above (note
that, although %2C is the percent-encoded transformation of "," from
the previous examples, such sequences are not decoded for purposes of
testing URN-equivalence):
o urn:example:a123%2Cz456
o URN:EXAMPLE:a123%2cz456
Saint-Andre & Klensin Standards Track [Page 17]
RFC 8141 URNs April 2017
Fifth, because characters in the NSS other than percent-encoded
sequences are treated in a case-sensitive manner (unless otherwise
specified for the URN namespace in question), the following URNs are
not URN-equivalent to the first three URNs:
o urn:example:A123,z456
o urn:example:a123,Z456
Sixth, on casual visual inspection of a URN presented in a human-
oriented interface, the following URN might appear the same as the
first three URNs (because U+0430 CYRILLIC SMALL LETTER A can be
confused with U+0061 LATIN SMALL LETTER A), but it is not
URN-equivalent to the first three URNs:
o urn:example:%D0%B0123,z456
4. URI Conformance
4.1. Use in URI Protocol Slots
Because a URN is, syntactically, a URI under the "urn" scheme, in
theory a URN can be placed in any protocol slot that allows for a URI
(to name just a few, the "href" and "src" attributes in HTML, the
base element in HTML, the "xml:base" attribute in XML [XML-BASE], and
the "xmlns" attribute in XML for XML namespace names [XML-NAMES]).
However, this does not imply that, semantically, it always makes
sense in practice to place a URN in a given URI protocol slot; in
particular, because a URN might not specify the location of a
resource or even point indirectly to one, it might not be appropriate
to place a URN in a URI protocol slot that points to a resource
(e.g., the aforementioned "href" and "src" attributes).
Ultimately, guidelines regarding when it is appropriate to use URIs
under the "urn" scheme (or any other scheme) are the responsibility
of specifications for individual URI protocol slots (e.g., the
specification for the "xml:base" attribute in XML might recommend
that it is inappropriate to use URNs in that protocol slot). This
specification cannot possibly anticipate all of the relevant cases,
and it is not the place of this specification to require or restrict
usage for individual protocol slots.
Saint-Andre & Klensin Standards Track [Page 18]
RFC 8141 URNs April 2017
4.2. Parsing
In part because of the separation of URN semantics from more general
URI syntax, generic URI processors need to pay special attention to
the parsing and analysis rules of RFC 3986 and, in particular, must
treat the URI as opaque unless the scheme and its requirements are
recognized. In the latter case, such processors may be in a position
to invoke scheme-appropriate processing, e.g., by a URN resolver. A
URN resolver can either be an external resolver that the URI resolver
knows of or be functionality built into the URI resolver. Note that
this requirement might impose constraints on the contexts in which
URNs are appropriately used; see Section 4.1.
4.3. URNs and Relative References
Section 5.2 of [RFC3986] describes an algorithm for converting a URI
reference that might be relative to a given base URI into "parsed
components" of the target of that reference, which can then be
recomposed per RFC 3986, Section 5.3 into a target URI. This
algorithm is problematic for URNs because their syntax does not
support the necessary path components. However, if the algorithm is
applied independent of a particular scheme, it should work
predictably for URNs as well, with the following understandings
(syntax production terminology taken from RFC 3986):
1. A system that encounters a <URI-reference> that obeys the syntax
for <relative-ref>, whether it explicitly has the scheme "urn" or
not, will convert it into a target URI as specified in RFC 3986.
2. Because of the persistence and stability expectations of URNs,
authors of documents, etc., that utilize URNs should generally
avoid the use of the "urn" scheme in any <URI-reference> that is
not strictly a <URI> as specified in RFC 3986, specifically
including those that would require processing of <relative-ref>.
4.4. Transport and Display
When URNs are transported and exchanged, they MUST be represented in
the format defined herein. Further, URN-aware applications are
strongly encouraged to offer the option of displaying URNs in this
canonical form to allow for direct transcription (for example by
copy-and-paste techniques). Such applications might support the
display of URNs in a more human-friendly form and might use a
character set that includes characters that are not permitted in URN
syntax as defined in this specification (e.g., when displaying URNs
to humans, such applications might replace percent-encoded strings
with characters from an extended character repertoire such as Unicode
[UNICODE]).
Saint-Andre & Klensin Standards Track [Page 19]
RFC 8141 URNs April 2017
To minimize user confusion, any application displaying URIs SHOULD
display the complete URI (including, for URNs, the "urn" scheme and
any components) to ensure that there is no confusion between URN NIDs
and URI scheme identifiers. For example, a URI beginning with
"urn:xmpp:" [RFC4854] is very different from a URI beginning with
"xmpp:" [RFC5122]. Similarly, a potential Digital Object Identifier
(DOI) URI scheme [DOI-URI] is different from, and possibly completely
unrelated to, a possible DOI URN namespace.
4.5. URI Design and Ownership
As mentioned, the assignment of URNs within a URN namespace is a
managed process, as is the assignment of URN namespaces themselves.
Although design of the URNs to be assigned within a given URN
namespace is ceded by this specification to the URN namespace
manager, doing so in a managed way avoids the problems inherent in
unmanaged generation of URIs as described in the recommendations
regarding URI design and ownership [RFC7320].
5. URN Namespaces
A URN namespace is a collection of names that obey three constraints:
each name is (1) unique, (2) assigned in a consistent way, and (3)
assigned according to a common definition.
1. The "uniqueness" constraint means that a name within the URN
namespace is never assigned to more than one resource and never
reassigned to a different resource (for the kind of "resource"
identified by URNs assigned within the URN namespace). This
holds true even if the name itself is deprecated or becomes
obsolete.
2. The "consistent assignment" constraint means that a name within
the URN namespace is assigned by an organization or created in
accordance with a process or algorithm that is always followed.
3. The "common definition" constraint means that there are clear
definitions for the syntax of names within the URN namespace and
for the process of assigning or creating them.
A URN namespace is identified by a particular NID in order to ensure
the global uniqueness of URNs and, optionally, to provide a cue
regarding the structure of URNs assigned within a URN namespace.
With regard to global uniqueness, using different NIDs for different
collections of names ensures that no two URNs will be the same for
different resources, because each collection is required to uniquely
assign each name. However, a single resource MAY have more than one
Saint-Andre & Klensin Standards Track [Page 20]
RFC 8141 URNs April 2017
URN assigned to it, either in the same URN namespace (if the URN
namespace permits it) or in different URN namespaces, and for either
similar purposes or different purposes. (For example, if a publisher
assigns an ISBN [RFC3187] to an electronic publication and that
publication is later incorporated into a digital long-term archive
operated by a national library, the library might assign the
publication a national bibliography number (NBN) [RFC3188], resulting
in two URNs referring to the same book.) Subject to other
constraints, such as those imposed by the URI syntax [RFC3986], the
rules of the URN scheme are intended to allow preserving the normal
and natural form of names specified in non-URN identifier systems
when they are treated as URNs.
With regard to the structure of names assigned within a URN
namespace, the development of a naming structure (and thereby a
collection of names) depends on the requirements of the community
defining the names, how the names will be assigned and used, etc.
These issues are beyond the scope of URN syntax and the general rules
for URN namespaces, because they are specific to the community
defining a non-URN identifier system or a particular URN namespace
(e.g., the bibliographic and publishing communities in the case of
the "ISBN" URN namespace [RFC3187] and the "ISSN" URN namespace
[RFC3044] or the developers of extensions to the Extensible Messaging
and Presence Protocol [RFC6120] in the case of the "XMPP" URN
namespace [RFC4854]).
Because the colon character (":") is used to separate "urn" from the
NID and the NID from the NSS, it's tempting to think of the entire
URN as being structured by colon characters and to assume that colons
create a structure or hierarchy within the NSS portion of the URN.
Such structure could be specified by a particular NID specification,
but there is no implicit structure. In a URN such as
urn:example:apple:pear:plum:cherry
the NSS string is "apple:pear:plum:cherry" as a whole, and there is
no specific meaning to the colon characters within that NSS string
unless such meaning is described in the specification of the
"example" namespace.
URN namespaces inherit certain rights and responsibilities by the
nature of URNs, in particular:
1. They uphold the general principles of a well-managed URN
namespace by providing persistent identification of resources and
unique assignment of names in accordance with a common
definition.
Saint-Andre & Klensin Standards Track [Page 21]
RFC 8141 URNs April 2017
2. Optionally, they can be registered in global registration
services such as those described in [RFC2483].
There are two types of URN namespaces: formal and informal. These
are distinguished by the expected level of service, the information
needed to define the URN namespace, and the procedures for
registration. Because the majority of the URN namespaces registered
so far have been formal, this document concentrates on formal URN
namespaces.
5.1. Formal URN Namespaces
A formal URN namespace provides benefit to some subset of users on
the Internet. In particular, it would not make sense for a formal
URN namespace to be used only by a community or network that is not
connected to the Internet. For example, it would be inappropriate
for a URN namespace to effectively force someone to use a proprietary
network or service not open to the general Internet user. The intent
is that, while the community of those who might actively use the URNs
assigned within that URN namespace might be small, the potential use
of names within that URN namespace is open to any user on the
Internet. Formal URN namespaces might be appropriate even when some
aspects are not fully open. For example, a URN namespace might make
use of a fee-based, privately managed, or proprietary registry for
assignment of URNs in the URN namespace. However, it might still
benefit some Internet users if the associated services have openly
published names.
An organization that will assign URNs within a formal URN namespace
SHOULD meet the following criteria:
1. Organizational stability and the ability to maintain the URN
namespace for a long time; absent such evidence, it ought to be
clear how the URN namespace can remain viable if the organization
can no longer maintain the URN namespace.
2. Competency in URN assignment. This will improve the likelihood
of persistence (e.g., to minimize the likelihood of conflicts).
3. Commitment to not reassigning existing URNs and to allowing old
URNs to continue to be valid (e.g., if the assignee of a URN is
no longer a member or customer of the assigning organization, if
various information about the assignee or named entity happens to
change, or even if the assignee or the named entity itself is no
longer in existence; in all these cases, the URN is still valid).
Saint-Andre & Klensin Standards Track [Page 22]
RFC 8141 URNs April 2017
A formal URN namespace establishes a particular NID, subject to the
following constraints (above and beyond the syntax rules already
specified):
1. It MUST NOT be an already-registered NID.
2. It MUST NOT start with "urn-" (which is reserved for informal URN
namespaces).
3. It MUST be more than two characters long, and it MUST NOT start
with ALPHA ALPHA "-", i.e., any string consisting of two letters
followed by one hyphen; such strings are reserved for potential
use as NIDs based on ISO alpha-2 country codes [ISO.3166-1] for
eventual national registrations of URN namespaces (however, the
definition and scoping of rules for allocation of responsibility
for such country-code-based URN namespaces are beyond the scope
of this document). As a consequence, it MUST NOT start with the
string "xn--" or any other string consisting of two letters
followed by two hyphens; such strings are reserved for potential
representation of DNS A-labels and similar strings in the future
[RFC5890].
4. It MUST NOT start with the string "X-" so that it will not be
confused with or conflict with any experimental URN namespace
previously permitted by [RFC3406].
Applicants and reviewers considering new NIDs should also be aware
that they may have semantic implications and hence be a source of
conflict. Particular attention should be paid to strings that might
be construed as identifiers for, or registered under the authority
of, countries (including ISO 3166-1 alpha-3 codes) and to strings
that might imply association with existing URI schemes, non-URN
identifier systems, or trademarks. However, in line with traditional
policies, disputes about "ownership" of particular strings are
disagreements among the parties involved; neither IANA nor the IETF
will become involved in such disputes except in response to orders
from a court of competent jurisdiction.
5.2. Informal URN Namespaces
Informal URN namespaces are full-fledged URN namespaces, with all the
associated rights and responsibilities. Informal URN namespaces
differ from formal URN namespaces in the process for assigning the
NID: for an informal URN namespace, the registrant does not designate
the NID; instead, IANA assigns the NID consisting of the string
"urn-" followed by one or more digits (e.g., "urn-7") where the
Saint-Andre & Klensin Standards Track [Page 23]
RFC 8141 URNs April 2017
digits consist of the next available number in the sequence of
positive integers assigned to informal URN namespaces. Thus, the
syntax of an informal URN namespace identifier is:
InformalNamespaceName = "urn-" Number
Number = DigitNonZero 0*Digit
DigitNonZero = "1"/ "2" / "3" / "4"/ "5"
/ "6" / "7" / "8" / "9"
Digit = "0" / DigitNonZero
The only restrictions on <Number> are that it (1) consist strictly of
ASCII digits, (2) not have leading zeros, and (3) not cause the NID
to exceed the length limitations defined for the URN syntax (see
Section 2).
6. Defining and Registering a URN Namespace
6.1. Overview
Because the space of URN namespaces is itself managed, the definition
of a URN namespace SHOULD pay particular attention to:
1. The purpose of the URN namespace.
2. The syntax of URNs assigned within the URN namespace, including
the internal syntax and anticipated effects of r-components or
q-components. (The syntax and interpretation of f-components are
defined in RFC 3986.)
3. The process for assigning URNs within the URN namespace.
4. The security implications of assigning URNs within the URN
namespace and of using the assigned URNs.
5. Any potential interoperability issues with URNs assigned within
the URN namespace.
6. Optionally, the process for resolving URNs assigned within the
URN namespace.
The section on completing the template (Section 6.4) explains these
matters in greater detail. Although the registration templates are
the same in all cases, slightly different procedures are used
depending on the source of the registration.
Saint-Andre & Klensin Standards Track [Page 24]
RFC 8141 URNs April 2017
6.2. Registration Policy and Process: Community Registrations
The basic registration policy for URN namespaces is Expert Review as
defined in the IANA Considerations document [RFC5226]. For URN
namespaces or their definitions that are intended to become standards
or constituent parts of standards, the output of the Expert Review
process is intended to be a report rather than instructions to IANA
to take action (see below). The key steps are:
1. Fill out the URN namespace registration template (see Section 6.4
and Appendix A). This can be done as part of an Internet-Draft
or a specification in another series, although that is not a
requirement.
2. Send the completed template to the urn@ietf.org discussion list
for review.
3. If necessary to address comments received, repeat steps 1 and 2.
4. If the Designated Experts approve the request and no
standardization action is involved, the IANA will register the
requested NID. If standardization is anticipated, the Designated
Experts will prepare a report and forward it to the appropriate
standards approval body (the IESG in the case of the IETF); IANA
will register the requested NID only after receiving directions
from that body and a copy of the Expert Review report.
A URN namespace registration can be revised by updating the
registration template, following the same steps outlined above for
new registrations. A revised registration MUST describe differences
from prior versions and SHOULD make special note of any relevant
changes in the underlying technologies or URN namespace management
processes.
Experience to date with URN namespace registration requests has shown
that registrants sometimes do not initially understand some of the
subtleties of URN namespaces and that defining the URN namespace in
the form of a specification enables the registrants to clearly
formulate their "contract" with the intended user community.
Therefore, although the registration policy for formal URN namespaces
is Expert Review and a specification (as distinct from the
registration template) is not strictly required, registrants SHOULD
provide a stable specification documenting the URN namespace
definition and expanding upon the issues described herein.
Because naming can be difficult and contentious, URN namespace
registrants and the Designated Experts are strongly encouraged to
work together in a spirit of good faith and mutual understanding to
Saint-Andre & Klensin Standards Track [Page 25]
RFC 8141 URNs April 2017
achieve rough consensus (see [RFC7282]) on handling registration
requests. They are also encouraged to bring additional expertise
into the discussion if that would be helpful in providing perspective
or otherwise resolving issues.
Especially when iterations in the registration process are prolonged,
Designated Experts are expected to take reasonable precautions to
avoid "race conditions" on proposed NIDs and, if such situations
arise, to encourage applicants to work out any conflicts among
themselves.
6.3. Registration Policy and Process: Fast Track for Standards
Development Organizations, Scientific Societies, and Similar
Bodies
The IETF recognizes that situations will arise in which URN
namespaces will be created to either embed existing and established
standards, particularly identifier standards, or reflect knowledge,
terminology, or methods of organizing information that lie well
outside the IETF's scope or the likely subject matter knowledge of
its Designated Experts. In situations in which the registration
request originates from, or is authorized by, a recognized standards
development organization, scientific society, or their designees, a
somewhat different procedure is available at the option of that body:
1. The URN namespace registration template is filled out and
submitted as in steps 1 and 2 of Section 6.2.
2. A specification is required that reflects or points to the needed
external standards or specifications. Publication in the RFC
Series or through an IETF process (e.g., posting as an Internet-
Draft) is not expected and would be appropriate only under very
unusual circumstances.
3. The reviews on the discussion list and by the Designated Experts
are strictly advisory, with the decisions about what advice to
accept and the length of time to allocate to the process strictly
under the control of the external body.
4. When that body concludes that the application is sufficiently
mature, its representative(s) will request that IANA complete the
registration for the NID, and IANA will do so.
Decisions about whether to recognize the requesting entity as a
standards development organization or scientific society are the
responsibility of the IESG.
Saint-Andre & Klensin Standards Track [Page 26]
RFC 8141 URNs April 2017
A model similar to this has already been defined for recognized
standards development organizations that wish to register media
types. The document describing that mechanism [RFC6838] provides
somewhat more information about the general approach.
6.4. Completing the Template
A template for defining and registering a URN namespace is provided
in Appendix A. This section describes considerations for completing
the template.
6.4.1. Purpose
The "Purpose" section of the template describes matters such as:
1. The kinds of resources identified by URNs assigned within the URN
namespace.
2. The scope and applicability of the URNs assigned within the URN
namespace; this might include information about the community of
use (e.g., a particular nation, industry, technology, or
organization), whether the assigned URNs will be used on public
networks or private networks, etc.
3. How the intended community (and the Internet community at large)
will benefit from using or resolving the assigned URNs.
4. How the URN namespace relates to and complements existing URN
namespaces, URI schemes, and non-URN identifier systems.
5. The kinds of software applications that can use or resolve the
assigned URNs (e.g., by differentiating among disparate URN
namespaces, identifying resources in a persistent fashion, or
meaningfully resolving and accessing services associated with the
URN namespace).
6. Whether resolution services are available or will be available
(and, if so, the nature or identity of the services). Examples
of q-component and (when they are standardized) r-component
semantics and syntax are helpful here, even if detailed
definitions are provided elsewhere or later.
7. Whether the URN namespace or its definition is expected to become
a constituent part of a standard being developed in the IETF or
some other recognized standards body.
Saint-Andre & Klensin Standards Track [Page 27]
RFC 8141 URNs April 2017
6.4.2. Syntax
The "Syntax" section of the template contains:
1. A description of the structure of URNs within the URN namespace,
in conformance with the fundamental URN syntax. The structure
might be described in terms of a formal definition (e.g., using
ABNF [RFC5234]), an algorithm for generating conformant URNs, or
a regular expression for parsing the name into constituent parts;
alternatively, the structure might be opaque.
2. Any special character encoding rules for assigned URNs (e.g.,
which character ought to always be used for quotes).
3. Rules for determining URN-equivalence between two names in the
URN namespace. Such rules ought to always have the effect of
eliminating false negatives that might otherwise result from
comparison. If it is appropriate and helpful, reference can be
made to particular equivalence rules defined in the URI
specification [RFC3986] or to Section 3 of this document.
Examples of URN-equivalence rules include equivalence between
uppercase and lowercase characters in the NSS, between hyphenated
and non-hyphenated groupings in the name, or between single
quotes and double quotes. There may also be namespace-specific
special encoding considerations, especially for URNs that contain
embedded forms of names from non-URN identifier systems. (Note
that these are not normative statements for any kind of best
practice related to handling of relationships between characters
in general; such statements are limited to one particular URN
namespace only.)
4. Any special considerations necessary for conforming with the URN
syntax. This is particularly applicable in the case of existing,
non-URN identifier systems that are used in the context of URNs.
For example, if a non-URN identifier system is used in contexts
other than URNs, it might make use of characters that are
reserved in the URN syntax. This section ought to note any such
characters and outline necessary mappings to conform to URN
syntax. Normally, this will be handled by percent-encoding the
character as specified in Section 2.1 of the URI specification
[RFC3986] and as discussed in Section 1.2.2 of this
specification.
5. Any special considerations for the meaning of q-components (e.g.,
keywords) or f-components (e.g., predefined terms) in the context
of this URN namespace.
Saint-Andre & Klensin Standards Track [Page 28]
RFC 8141 URNs April 2017
6.4.3. Assignment
The "Assignment" section of the template describes matters such as:
1. Mechanisms or authorities for assigning URNs to resources. It
ought to make clear whether assignment is completely open (e.g.,
following a particular procedure such as first-come, first-served
(FCFS)), completely closed (e.g., for a private organization), or
limited in various ways (e.g., delegated to authorities
recognized by a particular organization); if limited, it ought to
explain how to become an assigner of names or how to request
assignment of names from existing assignment authorities.
2. Methods for ensuring that URNs within the URN namespace are
unique. For example, names might be assigned sequentially or in
accordance with some well-defined process by a single authority,
assignment might be partitioned among delegated authorities that
are individually responsible for respecting uniqueness rules, or
URNs might be created independently following an algorithm that
itself guarantees uniqueness.
6.4.4. Security and Privacy
The "Security and Privacy" section of the template describes any
potential issues related to security and privacy with regard to
assignment, use, and resolution of names within the URN namespace.
Examples of such issues include:
o The consequences of producing false negatives and false positives
during comparison for URN-equivalence (see Section 3.1 of this
specification and "Issues in Identifier Comparison for Security
Purposes" [RFC6943]).
o Leakage of private information when names are communicated on the
public Internet.
o The potential for directory harvesting.
o Various issues discussed in the guidelines for security
considerations in RFCs [RFC3552] and the privacy considerations
for Internet protocols [RFC6973]. In particular, note the privacy
considerations text for the Global System for Mobile
Communications Association (GSMA) / International Mobile station
Equipment Identity (IMEI) namespace [RFC7254], which may provide a
useful model for such cases.
Saint-Andre & Klensin Standards Track [Page 29]
RFC 8141 URNs April 2017
6.4.5. Interoperability
The "Interoperability" section MUST specify any known potential
issues related to interoperability. Examples include possible
confusion with other URN namespaces, non-URN identifier systems, or
URI schemes because of syntax (e.g., percent-encoding of certain
characters) or scope (e.g., overlapping areas of interest). If at
all possible, concerns that arise during the registration of a URN
namespace (e.g., due to the syntax or scope of a non-URN identifier
system) should be resolved as part of or in parallel to the
registration process.
6.4.6. Resolution
The "Resolution" section MUST specify whether resolution mechanisms
are intended or anticipated for URNs assigned within the URN
namespace.
If resolution is intended, then this section SHOULD specify whether
the organization that assigns URNs within the URN namespace intends
to operate or recommend any resolution services for URNs within that
URN namespace. In addition, if the assigning organization intends to
implement registration for publicly advertised resolution services
(for example, using a system developed in the spirit of the original
architectural principles and service descriptions for URN resolution
[RFC2276] [RFC2483]), then this section SHOULD list or reference the
requirements for being publicly advertised by the assigning
organization. In addition, this section SHOULD describe any special
considerations for the handling of r-components in the context of
this URN namespace.
6.4.7. Additional Information
The "Additional Information" section includes information that would
be useful to those trying to understand this registration or its
relationship to other registrations, such as comparisons to existing
URN namespaces that might seem to overlap.
This section of the template is optional.
Saint-Andre & Klensin Standards Track [Page 30]
RFC 8141 URNs April 2017
7. IANA Considerations
7.1. URI Scheme
This section updates the registration of the "urn" URI scheme in the
Permanent URI Registry [URI-Registry].
URI Scheme Name: urn
Status: permanent
URI Scheme Syntax: See Section 2 of RFC 8141.
URI Scheme Semantics: The "urn" scheme identifies Uniform Resource
Names, which are persistent, location-independent resource
identifiers.
Encoding Considerations: See Section 2 of RFC 8141.
Applications/Protocols That Use This URI Scheme Name: Uniform
Resource Names are used in a wide variety of applications,
including bibliographic reference systems and as names for
Extensible Markup Language (XML) namespaces.
Interoperability Considerations: See Section 4 of RFC 8141.
Security Considerations: See Sections 6.4.4 and 8 of RFC 8141.
Contact: URNBIS working group [mailto:urn@ietf.org]
Author/Change Controller: This scheme is registered under the IETF
tree. As such, the IETF maintains change control.
References: None.
7.2. Registration of URN Namespaces
This document outlines the processes for registering URN namespaces
and has implications for the IANA in terms of registries to be
maintained (see especially Section 6). In all cases, the IANA ought
to assign the appropriate NID (formal or informal) once the
procedures outlined in Section 6 have been completed.
7.3. Discussion List for New and Updated NID Registrations
As discussed elsewhere in this document, the discussion list
specified in RFC 3406 (urn-nid@apps.ietf.org) is discontinued and
replaced by the discussion list urn@ietf.org.
Saint-Andre & Klensin Standards Track [Page 31]
RFC 8141 URNs April 2017
8. Security and Privacy Considerations
The definition of a URN namespace needs to account for potential
security and privacy issues related to assignment, use, and
resolution of names within the URN namespace (e.g., some URN
resolvers might assign special meaning to certain characters in the
NSS); see Section 6.4.4 for further discussion.
In most cases, URN namespaces provide a way to declare public
information. Normally, these declarations will have a relatively low
security profile; however, there is always the danger of "spoofing"
and providing misinformation. Information in these declarations
ought to be taken as advisory.
9. References
9.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<http://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
9.2. Informative References
[DOI-URI] Paskin, N., Neylon, E., Hammond, T., and S. Sun, "The
"doi" URI Scheme for the Digital Object Identifier (DOI)",
Work in Progress, draft-paskin-doi-uri-04, June 2003.
Saint-Andre & Klensin Standards Track [Page 32]
RFC 8141 URNs April 2017
[IANA-URN] Saint-Andre, P. and M. Cotton, "A Uniform Resource Name
(URN) Namespace for IANA Registries", Work in Progress,
draft-saintandre-iana-urn-01, February 2013.
[ISO.27729.2012]
ISO, "Information and documentation - International
standard name identifier (ISNI)", ISO 27729:2012,
Technical Committee ISO/TC 46, Information and
documentation, Subcommittee SC 9, Identification and
description, March 2012.
[ISO.3166-1]
ISO, "Codes for the representation of names of countries
and their subdivisions -- Part 1: Country codes",
ISO 3166-1:2013, November 2013.
[RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, DOI 10.17487/RFC1737,
December 1994, <http://www.rfc-editor.org/info/rfc1737>.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
December 1994, <http://www.rfc-editor.org/info/rfc1738>.
[RFC1808] Fielding, R., "Relative Uniform Resource Locators",
RFC 1808, DOI 10.17487/RFC1808, June 1995,
<http://www.rfc-editor.org/info/rfc1808>.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
May 1997, <http://www.rfc-editor.org/info/rfc2141>.
[RFC2276] Sollins, K., "Architectural Principles of Uniform Resource
Name Resolution", RFC 2276, DOI 10.17487/RFC2276, January
1998, <http://www.rfc-editor.org/info/rfc2276>.
[RFC2483] Mealling, M. and R. Daniel, "URI Resolution Services
Necessary for URN Resolution", RFC 2483,
DOI 10.17487/RFC2483, January 1999,
<http://www.rfc-editor.org/info/rfc2483>.
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
DOI 10.17487/RFC2648, August 1999,
<http://www.rfc-editor.org/info/rfc2648>.
[RFC3044] Rozenfeld, S., "Using The ISSN (International Serial
Standard Number) as URN (Uniform Resource Names) within an
ISSN-URN Namespace", RFC 3044, DOI 10.17487/RFC3044,
January 2001, <http://www.rfc-editor.org/info/rfc3044>.
Saint-Andre & Klensin Standards Track [Page 33]
RFC 8141 URNs April 2017
[RFC3187] Hakala, J. and H. Walravens, "Using International Standard
Book Numbers as Uniform Resource Names", RFC 3187,
DOI 10.17487/RFC3187, October 2001,
<http://www.rfc-editor.org/info/rfc3187>.
[RFC3188] Hakala, J., "Using National Bibliography Numbers as
Uniform Resource Names", RFC 3188, DOI 10.17487/RFC3188,
October 2001, <http://www.rfc-editor.org/info/rfc3188>.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition
Mechanisms", BCP 66, RFC 3406, DOI 10.17487/RFC3406,
October 2002, <http://www.rfc-editor.org/info/rfc3406>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>.
[RFC4854] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
for Extensions to the Extensible Messaging and Presence
Protocol (XMPP)", RFC 4854, DOI 10.17487/RFC4854, April
2007, <http://www.rfc-editor.org/info/rfc4854>.
[RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)",
RFC 5122, DOI 10.17487/RFC5122, February 2008,
<http://www.rfc-editor.org/info/rfc5122>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.
[RFC6288] Reed, C., "URN Namespace for the Defence Geospatial
Information Working Group (DGIWG)", RFC 6288,
DOI 10.17487/RFC6288, August 2011,
<http://www.rfc-editor.org/info/rfc6288>.
Saint-Andre & Klensin Standards Track [Page 34]
RFC 8141 URNs April 2017
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648,
DOI 10.17487/RFC6648, June 2012,
<http://www.rfc-editor.org/info/rfc6648>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <http://www.rfc-editor.org/info/rfc6943>.
[RFC6963] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
for Examples", BCP 183, RFC 6963, DOI 10.17487/RFC6963,
May 2013, <http://www.rfc-editor.org/info/rfc6963>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7254] Montemurro, M., Ed., Allen, A., McDonald, D., and P.
Gosden, "A Uniform Resource Name Namespace for the Global
System for Mobile Communications Association (GSMA) and
the International Mobile station Equipment Identity
(IMEI)", RFC 7254, DOI 10.17487/RFC7254, May 2014,
<http://www.rfc-editor.org/info/rfc7254>.
[RFC7282] Resnick, P., "On Consensus and Humming in the IETF",
RFC 7282, DOI 10.17487/RFC7282, June 2014,
<http://www.rfc-editor.org/info/rfc7282>.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<http://www.rfc-editor.org/info/rfc7320>.
[RFC7462] Liess, L., Ed., Jesske, R., Johnston, A., Worley, D., and
P. Kyzivat, "URNs for the Alert-Info Header Field of the
Session Initiation Protocol (SIP)", RFC 7462,
DOI 10.17487/RFC7462, March 2015,
<http://www.rfc-editor.org/info/rfc7462>.
Saint-Andre & Klensin Standards Track [Page 35]
RFC 8141 URNs April 2017
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>.
[UAX31] The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", Unicode 9.0.0,
June 2015, <http://unicode.org/reports/tr31/>.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
[URI-Registry]
IANA, "Uniform Resource Identifier (URI) Schemes",
<http://www.iana.org/assignments/uri-schemes>.
[XML-BASE] Marsh, J. and R. Tobin, "XML Base (Second Edition)", W3C
Recommendation REC-xmlbase-20090128, January 2009,
<http://www.w3.org/TR/2009/REC-xmlbase-20090128>.
[XML-NAMES]
Thompson, H., Hollander, D., Layman, A., Bray, T., and R.
Tobin, "Namespaces in XML 1.0 (Third Edition)", W3C
Recommendation REC-xml-names-20091208, December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
Saint-Andre & Klensin Standards Track [Page 36]
RFC 8141 URNs April 2017
Appendix A. Registration Template
Namespace Identifier: Requested of IANA (formal) or assigned by IANA
(informal).
Version: The version of the registration, starting with 1 and
incrementing by 1 with each new version.
Date: The date when the registration is requested of IANA, using the
format YYYY-MM-DD.
Registrant: The person or organization that has registered the NID,
including the name and address of the registering organization, as
well as the name and contact information (email, phone number, or
postal address) of the designated contact person. If the
registrant is a recognized standards development organization,
scientific society, or similar body requesting the fast-track
registration procedure (see Section 6.3), that information should
be clearly indicated in this section of the template.
Purpose: Described in Section 6.4.1 of this document.
Syntax: Described in Section 6.4.2 of this document. Unless the
registration explicitly describes the semantics of r-components,
q-components, and f-components in the context of this URN
namespace, those semantics are undefined.
Assignment: Described in Section 6.4.3 of this document.
Security and Privacy: Described in Section 6.4.4 of this document.
Interoperability: Described in Section 6.4.5 of this document.
Resolution: Described in Section 6.4.6 of this document.
Documentation: A pointer to an RFC, a specification published by
another standards development organization, or another stable
document that provides further information about this URN
namespace.
Additional Information: Described in Section 6.4.7 of this document.
Revision Information: Description of changes from prior version(s).
(Applicable only when earlier registrations have been revised.)
Saint-Andre & Klensin Standards Track [Page 37]
RFC 8141 URNs April 2017
Appendix B. Changes from RFC 2141
This document makes substantive changes from the syntax and semantics
of [RFC2141]:
B.1. Syntax Changes from RFC 2141
The syntax of URNs as provided in [RFC2141] was defined before the
updated specification of URIs in [RFC3986]. The definition of URN
syntax is updated in this document to do the following:
o Ensure consistency with the URI syntax.
o Facilitate the use of URNs with parameters similar to URI queries
and fragments.
o Permit parameters influencing URN resolution.
o Ease the use of URNs with non-URN identifier systems that include
the "/" character.
In particular, this specification does the following:
o Extends URN syntax to explicitly allow the characters "/", "?",
and "#", which were reserved for future use by RFC 2141. This
change also effectively allows several components of the URI
syntax although without necessarily tying those components to URI
semantics.
o Defines general syntax for an additional component that can be
used in interactions with a URN resolution service.
o Disallows "-" at the end of the NID.
o Allows the "/", "~", and "&" characters in the NSS.
o Makes several smaller syntax adjustments.
Saint-Andre & Klensin Standards Track [Page 38]
RFC 8141 URNs April 2017
B.2. Other Changes from RFC 2141
o Formally registers "urn" as a URI scheme.
o Allows what are now called r-components, q-components, and
f-components.
In addition, some of the text has been updated to be consistent with
the definition of URIs [RFC3986] and the processes for registering
information with the IANA [RFC5226], as well as more modern guidance
with regard to security [RFC3552], privacy [RFC6973], and identifier
comparison [RFC6943].
Appendix C. Changes from RFC 3406
This document makes the following substantive changes from [RFC3406]:
1. Relaxes the registration policy for formal URN namespaces from
"IETF Review" to "Expert Review" as discussed in Section 6.2.
2. Removes the category of experimental URN namespaces, consistent
with [RFC6648]. Experimental URN namespaces were denoted by
prefixing the namespace identifier with the string "X-". Because
experimental URN namespaces were never registered, removing the
experimental category has no impact on the existing registries.
Because experimental URN namespaces are not managed, strings
conforming to URN syntax within experimental URN namespaces are
not valid URNs. Truly experimental usages may, of course, employ
the "example" namespace [RFC6963].
3. Adds some information to, but generally simplifies, the URN
namespace registration template.
Saint-Andre & Klensin Standards Track [Page 39]
RFC 8141 URNs April 2017
Acknowledgements
Many thanks to Marc Blanchet, Leslie Daigle, Martin Duerst, Juha
Hakala, Ted Hardie, Alfred Hoenes, Paul Jones, Barry Leiba, Sean
Leonard, Larry Masinter, Keith Moore, Mark Nottingham, Julian
Reschke, Lars Svensson, Henry S. Thompson, Dale Worley, and other
participants in the URNBIS working group for their input. Alfred
Hoenes in particular edited an earlier draft version of this document
and served as co-chair of the URNBIS working group.
Juha Hakala deserves special recognition for his dedication to
successfully completing this work, as do Andrew Newton and Melinda
Shore in their roles as working group co-chairs and Barry Leiba in
his role as area director and then as co-chair.
Contributors
RFC 2141, which provided the basis for the syntax portion of this
document, was authored by Ryan Moats.
RFC 3406, which provided the basis for the namespace portion of this
document, was authored by Leslie Daigle, Dirk-Willem van Gulik,
Renato Iannella, and Patrik Faltstrom.
Their work is gratefully acknowledged.
Authors' Addresses
Peter Saint-Andre
Filament
P.O. Box 787
Parker, CO 80134
United States of America
Phone: +1 720 256 6756
Email: peter@filament.com
URI: <https://filament.com/>
John C. Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
United States of America
Phone: +1 617 245 1457
Email: john-ietf@jck.com
Saint-Andre & Klensin Standards Track [Page 40]