Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 8141 Filament
Obsoletes: 2141, 3406 J. Klensin
Category: Standards Track April 2017
Uniform Resource Names (URNs)
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
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Appendix A. Registration Template . . . . . . . . . . . . . . . 37Appendix B. Changes from RFC 2141 . . . . . . . . . . . . . . . 38B.1. Syntax Changes from RFC 2141 . . . . . . . . . . . . . . 38B.2. Other Changes from RFC 2141 . . . . . . . . . . . . . . . 39Appendix C. Changes from RFC 3406 . . . . . . . . . . . . . . . 39
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 401. 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
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.
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
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
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.
The following terms are distinguished from each other as described
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
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: 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
This document uses the terms "resource", "identifier", "identify",
"dereference", "representation", and "metadata" roughly as defined in
the URI specification [RFC3986].
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
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
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
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
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
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.
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
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
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 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
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.
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
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].
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
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
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
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
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.
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
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
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.
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
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.
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
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.
If this example resolved to an HTTP URI, the result might look like:
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
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
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: