Uniform Resource Names (URNs)
draft-ietf-urnbis-rfc2141bis-urn-09
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8141.
|
|
|---|---|---|---|
| Authors | Peter Saint-Andre , Dr. John C. Klensin | ||
| Last updated | 2015-02-06 | ||
| Replaces | draft-ah-rfc2141bis-urn, draft-saintandre-urnbis-2141bis, draft-ietf-urnbis-rfc3406bis-urn-ns-reg | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 8141 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-urnbis-rfc2141bis-urn-09
URNBIS P. Saint-Andre
Internet-Draft &yet
Obsoletes: 2141, 3406 (if approved) J. Klensin
Intended status: Standards Track
Expires: August 10, 2015 February 6, 2015
Uniform Resource Names (URNs)
draft-ietf-urnbis-rfc2141bis-urn-09
Abstract
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
that is assigned under the "urn" scheme and a particular URN
namespace, typically with the intent that the URN will be a
persistent, location-independent resource identifier or abstract
designator. With regard to URN syntax, this document defines the
canonical syntax for URNs (in a way that is consistent with URI
syntax), specifies methods for determining URN equivalence, and
discusses URI conformance. With regard to URN namespaces, this
document specifies a method for defining a URN namespace and
associating it with a namespace identifier, and describes procedures
for registering namespace identifiers with the Internet Assigned
Numbers Authority (IANA). This document obsoletes both RFC 2141 and
RFC 3406.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 10, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. URN Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Namespace Identifier Syntax . . . . . . . . . . . . . . . 5
3.2. Namespace Specific String Syntax . . . . . . . . . . . . 5
3.3. p-component, q-component, and f-component . . . . . . . . 6
4. Equivalence of URNs . . . . . . . . . . . . . . . . . . . . . 7
4.1. Procedure . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
5. URI Conformance . . . . . . . . . . . . . . . . . . . . . . . 9
6. URN Namespaces . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Formal Namespaces . . . . . . . . . . . . . . . . . . . . 12
6.2. Informal Namespaces . . . . . . . . . . . . . . . . . . . 13
7. Defining a URN Namespace . . . . . . . . . . . . . . . . . . 13
7.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Assignment . . . . . . . . . . . . . . . . . . . . . . . 15
7.4. Security and Privacy . . . . . . . . . . . . . . . . . . 16
7.5. Resolution . . . . . . . . . . . . . . . . . . . . . . . 16
8. Registering a URN Namespace . . . . . . . . . . . . . . . . . 17
8.1. Formal Namespaces . . . . . . . . . . . . . . . . . . . . 17
8.2. Informal Namespaces . . . . . . . . . . . . . . . . . . . 17
9. Guidelines for Designated Experts . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
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10.1. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . 18
10.2. Registration of URN Namespaces . . . . . . . . . . . . . 19
11. Security and Privacy Considerations . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Registration Template . . . . . . . . . . . . . . . 22
A.1. Namespace ID . . . . . . . . . . . . . . . . . . . . . . 22
A.2. Version . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.3. Date . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.4. Registrant . . . . . . . . . . . . . . . . . . . . . . . 22
A.5. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.7. Assignment . . . . . . . . . . . . . . . . . . . . . . . 22
A.8. Resolution . . . . . . . . . . . . . . . . . . . . . . . 22
A.9. Documentation . . . . . . . . . . . . . . . . . . . . . . 23
A.10. Revision Information . . . . . . . . . . . . . . . . . . 23
Appendix B. Changes from RFC 2141 . . . . . . . . . . . . . . . 23
Appendix C. Changes from RFC 3406 . . . . . . . . . . . . . . . 23
Appendix D. Contributors . . . . . . . . . . . . . . . . . . . . 23
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Appendix F. Change log for versions of draft-ietf-urnbis-
rfc2141bis-urn . . . . . . . . . . . . . . . . . . . 24
F.1. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI)
[RFC3986] that is assigned under the "urn" scheme and a particular
namespace, typically with the intent that the URN will be a
persistent, location-independent resource identifier or abstract
designator.
The assignment of URNs is done by an organization (or, in some cases,
according to an algorithm or other automated process) that has been
formally delegated a namespace within the "urn" scheme (e.g., a URN
in the 'example' namespace [RFC6963] might be of the form
"urn:example:foo").
This document rests on two key assumptions:
1. Assignment of a URN is a managed process.
A string that conforms to the URN syntax is not necessarily a
valid URN, because a URN needs to be assigned according to the
rules of a particular namespace (in terms of syntax, semantics,
and process).
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2. The space of URN namespaces is itself managed.
A string in the namespace identifier slot of the URN syntax is
not necessarily a valid URN namespace identifier, because in
order to be valid a namespace needs to be defined and registered
in accordance with the rules specified in this document.
So that information about both URN syntax and URN namespaces is
available in one place, this document does the following:
1. Defines the canonical syntax for URNs in general (in a way that
is consistent with URI syntax), specifies methods for determining
URN equivalence, and discusses URI conformance.
2. Specifies a method for defining a URN namespace and associating
it with a namespace identifier, and describes procedures for
registering namespace identifiers with the Internet Assigned
Numbers Authority (IANA).
For URN syntax and URN namespaces, this document modernizes and
replaces the definitions from [RFC2141] and [RFC3406]. These
modifications build on the requirements provided in [RFC1737] and
many years of experience with URNs, in both cases attempting to make
the smallest reasonable set of changes from the previous definitions.
This document obsoletes both [RFC2141] and [RFC3406].
2. Terminology
Several important terms used in this document are defined in the URI
specification [RFC3986].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. URN Syntax
The syntax of URNs as provided in [RFC2141] was defined before the
updated specification of URIs in [RFC3986]. To ensure consistency
with the URI syntax as well as semantic flexibility in the use of
URNs within particular applications (see
[I-D.ietf-urnbis-semantics-clarif] for further discussion), this
specification extends the syntax of URNs to explicitly allow several
characters (and thus URI components) that were not allowed by
[RFC2141], and also makes several smaller syntax adjustments.
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As a result, the syntax for a URN is defined as follows using the
Augmented Backus-Naur Form (ABNF) as specified in [RFC5234]. Rules
not defined below (i.e., alphanum, pchar, path-absolute, query, and
fragment) are defined in [RFC3986].
namestring = assigned-name
[ p-component ]
[ q-component ]
[ f-component ]
assigned-name = "urn" ":" NID ":" NSS
; the URI scheme ("urn") is case insensitive
NID = (alphanum) 0*30(ldh) (alphanum)
ldh = alphanum / "-"
NSS = 1*(pchar)
p-component = "/" path-absolute
q-component = "?" query
f-component = "#" fragment
Note that "?" can be used without %-encoding inside q-components and
f-components.
The following sections provide additional information about these
rules.
3.1. Namespace Identifier Syntax
The syntax here is slightly more restrictive than what was defined in
[RFC2141], since it forbids the character "-" at the end of a NID.
NIDs are case insensitive (e.g., "ISBN" and "isbn" are equivalent).
3.2. Namespace Specific String Syntax
Depending on the rules governing a namespace, names that are valid in
a namespace might contain characters that are not allowed in URNs
according to the "pchar" rule (e.g., characters outside the ASCII
range or characters that are reserved in URIs, such as "/", "?", and
"#"). Such a string MUST be translated into a conformant NSS before
using it as a protocol element or otherwise passing it on to other
applications. Translation is done by percent-encoding each
disallowed character using the method defined in Section 2.1 of
[RFC3986]. Note that the "%" character is allowed only for the
purpose of percent-encoding.
In order to make URNs as stable and persistent as possible when
protocols evolve and the environment around them changes, namespaces
SHOULD NOT allow characters outside the basic Latin repertoire
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[RFC20] unless the nature of the particular namespace makes such
characters necessary.
If a namespace designates one or more characters conforming to the
"pchar" rule as having special meaning for that namespace (e.g., "@")
and the namespace also uses that character in a literal sense, when
used in a literal sense the character MUST be percent-encoded (e.g.,
"%40"). For related considerations with regard to NID registration,
see below.
3.3. p-component, q-component, and f-component
The p-component, q-component, and f-component are optional components
that follow the assigned-name. In terms of URI syntax these
components are essentially equivalent to the URI "path-absolute",
"query", and "fragment" constructions, respectively. However, the
URN p-component, q-component, and f-component need not be
semantically equivalent to the URI path component, query component,
and fragment component; therefore they are called by different names
in this specification.
Unless specifically defined for a particular namespace after
publication of this document, use of these components is disallowed,
thereby maintaining strict backward compatibility with namespaces
defined in accordance with [RFC2141] and registered in accordance
with [RFC3406].
This specification does not define the semantics of the p-component,
q-component, and f-component for URNs in general. Instead,
additional specifications might establish these matters for URN-
related services (such as URN resolution) or for individual URN
namespaces (e.g., to handle extended information about the resource
identified by a URN). For example, it is possible that the
q-component might be used in requests to URN resolution services, or
that the f-component might be used to distinguish the integral parts
of resources named by URNs in particular namespaces (say, the
chapters of a book). However, defining such usage is the
responsibility of specifications for URN resolution services,
namespace registration requests and specifications for individual
namespaces, and other appropriate documentation (such as policy
documents governing the management of a given URN namespace).
As general guidance that might not apply to all cases, it would be
inappropriate for namespaces that do not intend to support resolution
services to allow q-components. Namespaces which deal with digital
manifestations might be able to support f-components.
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3.3.1. p-component
The only formal restriction placed upon a p-component by this
specification is that the syntax SHALL adhere to the "path-absolute"
rule from [RFC3986]. The inner syntax of a p-component is to be
defined by the specification for a particular namespace or URN-
related service. (For example, a namespace specification might
define a character such as "~" or "@" as a delimiter inside
p-components assigned within that namespace.)
As described under Section 4, the p-component SHALL be taken into
account when determining URN equivalence.
3.3.2. q-component
The only formal restriction placed upon a q-component by this
specification is that the syntax SHALL adhere to the "query" rule
from [RFC3986] (prepended by the "?" character). The inner syntax of
a q-component is to be defined by the specification for a particular
namespace. (For example, a namespace specification might define a
character such as ";" or "=" as a delimiter inside q-components
assigned within that namespace.)
As described under Section 4, the q-component SHALL NOT be taken into
account when determining URN equivalence.
3.3.3. f-component
The only formal restriction placed upon an f-component by this
specification is that the syntax SHALL adhere to the "fragment" rule
from [RFC3986] (prepended by the "#" character). The inner syntax of
an f-component is to be defined by the specification for a particular
namespace. (For example, a namespace specification might define a
character such as "&" or "+" as a delimiter inside f-components
assigned within that namespace.)
As described under Section 4, the f-component SHALL NOT be taken into
account when determining URN equivalence.
4. Equivalence of URNs
4.1. Procedure
For various purposes such as caching, often it is desirable to
determine if two URNs are "the same". This is done by testing for
equivalence (see Section 6.1 of [RFC3986]).
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Note that [RFC3986] is very flexible about equality comparisons,
putting the focus on allowing false negatives and avoiding false
positives. If comparisons are made in a scheme-independent way,
i.e., as URI comparisons only, URNs that this specification considers
equal would be rejected. The discussion below applies when the URI
is known to be a URN.
Two URNs are equivalent if they 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"
2. the NID
3. any percent-encoded characters (see Section 2.1 of the base URI
specification [RFC3986]) in the NSS
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.
If a q-component or f-component (or both) are included in a URN, they
MUST be ignored for purposes of determining equivalence.
URN namespaces MAY define additional rules for equivalence, such as
case-insensitivity of the NSS (or parts thereof). Such rules MUST
always have the effect of eliminating some of the false negatives
obtained by the procedure above and MUST NOT result in treating two
URNs as not equivalent if the procedure here says they are
equivalent. For related considerations with regard to NID
registration, see below.
4.2. Examples
The following six URN comparisons (which use the "example" NID
defined in [RFC6963]) highlight the equivalence rules:
1. URN:example:a123,456
2. urn:example:a123,456
3. urn:EXAMPLE:a123,456
4. urn:example:a123%2C456
5. URN:EXAMPLE:a123%2c456
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6. urn:example:A123,456
7. urn:example:a123,456/789
8. urn:example:a123,456/abc
9. urn:example:a123,456?789
10. urn:example:a123,456?abc
11. urn:example:a123,456#789
12. urn:example:a123,456#abc
URNs 1, 2, 3, 9, 10, 11, and 12 are equivalent. URNs 4 and 5 are
equivalent only to each other. URNs 6, 7, and 8 are not equivalent
to any of the others.
5. URI Conformance
Because a URN is, syntactically, a URI under the "urn" scheme, in
theory a URN can be placed in any protocol slot that allows for a URI
(e.g., an XML namespace name [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 (examples include the 'href' and 'src'
attributes and the <base/> element in HTML, as well as the 'xml:base'
attribute in XML [XML-BASE]). Ultimately, specifications of where it
is appropriate to use URNs, or URNs created within particular URN
namespaces, are the responsibility of descriptions of individual URI
schemes and contexts; this specification cannot possibly anticipate
all of the relevant cases.
Despite the fact that URNs are not hierarchical and are not
appropriate for use as a base URI (see Section 5.1 of [RFC3986]), the
relative resolution algorithm specified in Section 5.2 of [RFC3986]
still applies to the "urn" URI scheme; implementers need to be aware,
however, that running the algorithm against URNs will lead to results
that might be unexpected or not useful.
A resolver that conforms to the URI specification [RFC3986] will
extract a scheme of "urn" rather than a scheme value of "urn:<nid>".
A URN MUST be considered an opaque URI by URI resolvers and passed
(with the "urn" scheme) to a URN resolver for resolution. The URN
resolver can either be an external resolver that the URI resolver
knows of, or it can be functionality built into the URI resolver.
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Note that this requirement MAY impose constraints on the contexts in
which URNs are appropriately used; see the previous section.
To minimize user confusion, a URI browser SHOULD display the complete
URN (including the "urn" scheme and any components) to ensure that
there is no confusion between URN namespace identifiers and URI
scheme identifiers. For example, a URI beginning with "urn:xmpp:"
[RFC4854] is very different from a URI beginning with "xmpp:"
[RFC5122]. Similarly, a potential DOI scheme [DOI-URI] is different
from, and possibly completely unrelated to, a possible DOI URN
namespace.
When URNs are transported and exchanged, they MUST be represented in
this format. Further, all URN-aware applications MUST offer the
option of displaying URNs in this canonical form to allow for direct
transcription (for example by cut and paste techniques). Such
applications might support display of URNs in a more human-friendly
form and might use a character set that includes characters that are
not permitted in URN syntax as defined in this specification (e.g.,
when displaying URNs to humans, such applications might replace
percent-encoded strings with characters from an extended character
repertoire such as that of [UNICODE]).
As mentioned, the assignment of URNs is a managed process, as is the
assignment of namespaces themselves. Although design of the URNs to
be assigned within a given namespace is ceded by this specification
to the namespace owner, doing so in a managed way avoids the problems
inherent in unmanaged generation of URIs as described in the
recommendations regarding URI design and ownership [RFC7320].
6. URN Namespaces
A URN namespace is a collection of identifiers that obey three
constraints. Such a namespace is (1) unique, (2) assigned in a
consistent way, and (3) assigned according to a common definition.
1. The "uniqueness" constraint means that an identifier within the
namespace is never assigned to more than one resource and never
reassigned to a different resource, even if the identifier itself
is deprecated or becomes obsolete.
2. The "consistent assignment" constraint means that an identifier
within the namespace is assigned by an organization or created in
accordance with a process or algorithm that is always followed.
3. The "common definition" constraint means that there are clear
definitions for the syntax of identifiers within the namespace
and for the process of assigning or creating them.
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A URN namespace is identified by a particular NID in order to ensure
the global uniqueness of URNs and, optionally, to provide a cue
regarding the structure of URNs assigned within a namespace.
With regard to global uniqueness, using different NIDs for different
collections of identifiers ensures that no two URNs will be the same
for different resources, since each collection is required to
uniquely assign each identifier. However, a single resource can have
more than one URN assigned to it for different purposes (e.g., some
numbers might be valid identifiers in two different identifier
systems, where the namespace identifier differentiates between the
resulting URNs). Subject to other constraints, such as those imposed
by the URI syntax [RFC3986], the rules of the URN scheme are intended
to allow preserving the normal and natural form of identifiers
specified elsewhere and treated as URN namespaces.
With regard to the structure of URNs assigned within a namespace, the
development of an identifier structure (and thereby a collection of
identifiers) depends on the requirements of the community defining
the identifiers, how the identifiers will be assigned and used, etc.
These issues are beyond the scope of URN syntax and the general rules
for URN namespaces, because they are specific to the community
defining a namespace (e.g., the bibliographic and publishing
communities in the case of the 'ISBN' and 'ISSN' namespaces, or the
developers of extensions to the Extensible Messaging and Presence
Protocol in the case of the 'XMPP' namespace).
URN namespaces inherit certain rights and responsibilities by the
nature of URNs, e.g.:
1. They uphold the general principles of a well-managed URN
namespace by providing persistent identification of resources and
unique assignment of identifier strings.
2. They can be registered in global registration services.
There are two types of URN namespace: formal and informal. These are
distinguished by the expected level of service, the information
needed to define the namespace, and the procedures for registration.
Because the majority of the namespaces registered so far have been
formal, this document concentrates on formal namespaces.
Note: [RFC3406] defined a third type of "experimental namespaces",
denoted by prefixing the namespace identifier with the string "X-".
Consistent with [RFC6648], this specification removes the
experimental category. Because experimental namespaces were never
registered, removing the experimental category has no impact on the
existing registries or future registration procedures. Because they
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are not registered, strings that refer to existing experimental
namespaces are not valid URNs. Truly experimental usages can, of
course, employ the 'example' namespace [RFC6963].
6.1. Formal Namespaces
A formal namespace provides benefit to some subset of users on the
Internet. In particular, it would not make sense for a formal
namespace to be used only by a community or network that is not
connected to the Internet. For example, it would be inappropriate
for a NID to effectively force someone to use a proprietary network
or service not open to the general Internet user. The intent is
that, while the community of those who might actively use the names
assigned within that NID might be small, the potential use of
identifiers within that NID is open to any user on the Internet.
Formal NIDs might be appropriate even when some aspects are not fully
open. For example, a namespace might make use of a fee-based,
privately managed, or proprietary registry for assignment of URNs in
the namespace. However, it might still benefit some Internet users
if the associated services have openly-published access protocols.
An organization that will assign URNs within a formal namespace ought
to meet the following criteria:
1. Organizational stability and the ability to maintain the URN
namespace for a long time; absent such evidence, it ought to be
clear how the namespace can remain viable if the organization can
no longer maintain the namespace.
2. Competency in name assignment. This will improve the likelihood
of persistence (e.g. to minimize the likelihood of conflicts).
3. Commitment to not reassigning existing names and to allowing old
names to continue to be valid, even if the owners or assignees of
those names are no longer members or customers of that
organization. With regard to URN resolution [RFC2276], this does
not mean that there needs to be resolution of such names, only
that the names will not resolve to false or stale information.
A formal namespace establishes a particular NID, subject to the
following constraints (above and beyond the syntax rules already
specified):
1. It MUST NOT be an already-registered NID.
2. It MUST NOT start with "urn-" (which is reserved for informal
namespaces).
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3. It MUST be more than two characters long.
4. It MUST NOT start with "aa-", where "aa" is any combination of
two ASCII letters and the hyphen is followed by something other
than another hyphen.
5. It MUST NOT start with the string "xn--" or any other string
consisting of two letters followed by two hyphens. Those strings
are reserved for potential representation of DNS A-labels and
similar strings in the future [RFC5890].
All two-letter strings, and all two-letter strings followed by "-"
and any sequence of valid NID characters, are reserved for potential
use as country-code-based NIDs for eventual national registrations of
URN namespaces. The definition and scoping of rules for allocation
of responsibility for such country-code-based namespaces is beyond
the scope of this document.
6.2. Informal Namespaces
Informal namespaces are full-fledged URN namespaces, with all the
associated rights and responsibilities. Informal namespaces differ
from formal namespaces in the process for assigning a NID: for an
informal namespace, the registrant does not designate the NID;
instead, IANA assigns a NID consisting of the string 'urn-' followed
by one or more digits (e.g., "urn-7") where the digits consist of the
next available number in the sequence of positive integers assigned
to informal namespaces. Thus the syntax of an informal namespace is:
InformalNamespaceName = "urn-" Number
Number = DigitNonZero 0*Digit
DigitNonZero = "1"/ "2" / "3" / "4"/ "5"
/ "6" / "7" / "8" / "9"
Digit = "0" / DigitNonZero
The only restrictions on <Number> are that it (1) consist strictly of
ASCII digits, that it (2) not have leading zeros, and that it (3) not
cause the NID to exceed the length limitations defined for the URN
syntax.
7. Defining a URN Namespace
The definition of a formal namespace ought to pay particular
attention to:
1. The purpose of the namespace.
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2. The syntax of URNs assigned within the namespace, including
whether p-, q-, and/or f-components are allowed.
3. The process for assigning URNs within the namespace.
4. The security implications of assigning URNs within the namespace
and using the assigned URNs.
5. Optionally, the process for resolving URNs issued within the
namepace.
The following sections explain these matters in greater detail. For
convenience, a template for defining and registering a URN namespace
is provided under Appendix A. This information can be especially
helpful to entities that wish to request assignment of a URN in a
namespace and to entities that wish to provide URN resolution for a
namespace.
7.1. Purpose
The "Purpose" section of the template describes matters such as:
1. The kinds of resources identified by URNs assigned within the
namespace.
2. Why it is preferable to use URNs rather than some other
technology (e.g., separate URI schemes or URIs in existing
schemes) and why no existing URN namespace is a good fit.
3. The kinds of software applications that can use or resolve the
assigned URNs (e.g., by differentiating among disparate
namespaces, identifying resources in a persistent fashion, or
meaningfully resolving and accessing services associated with the
namespace).
4. The scope of the namespace (public vs. private, global vs. local
to a particular organization, nation, or industry). For example,
a namespace claiming to deal in "national identification numbers"
ought to have a global scope and address all identity number
structures, whereas a URN scheme for a particular national
identification number system would need to handle only the
structure for that nation's identity numbers.
5. How the intended community (and the Internet community at large)
will benefit from using or resolving the assigned URNs.
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7.2. Syntax
The "Syntax" section of the template contains:
1. A description of the structure of URNs within the namespace, in
conformance with the fundamental URN syntax. The structure might
be described in terms of a formal definition (e.g., using
Augmented BNF for Syntax Specifications (ABNF) as specified in
[RFC5234]), an algorithm for generating conformant URNs, or a
regular expression for parsing the identifier into components;
alternatively, the structure might be opaque.
2. Any special character encoding rules for assigned URNs (e.g.,
which character ought to always be used for single-quotes).
3. If p-components, q-components, and/or f-components are allowed
for the namespace, a discussion of how they are used.
4. Rules for determining equivalence between two identifiers in the
namespace. Such rules ought to always have the effect of
eliminating false negatives that might otherwise result from
comparison. If it is appropriate and helpful, reference can be
made to the equivalence rules defined in the URI specification
[RFC3986]. Examples of equivalence rules include equivalence
between uppercase and lowercase characters in the Namespace
Specific String, between hyphenated and non-hyphenated groupings
in the identifier string, or between single-quotes and double-
quotes. (Note that these are not normative statements for any
kind of best practice related to handling of equivalences between
characters in general; they are statements limited to one
particular namespace only.)
5. Any special considerations necessary for conforming with the URN
syntax. This is particularly applicable in the case of existing
naming systems that are used in the context of URNs. For
example, if a namespace is used in contexts other than URNs, it
might make use of characters that are reserved in the URN syntax.
This section ought to note any such characters, and outline
necessary mappings to conform to URN syntax. Normally, this will
be handled by percent-encoding the character as specified in the
URI specification [RFC3986].
7.3. 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.,
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following a particular procedure such as first-come, first-served
(FCFS)), completely closed (e.g., for a private organization), or
limited in various ways (e.g., delegated to authorities
recognized by a particular organization); if limited, it ought to
explain how to become an assigner of identifiers or how to
request assignment of identifiers from existing assignment
authorities.
2. Methods for ensuring that URNs within the namespace are unique.
For example, identifiers might be assigned sequentially or in
accordance with some well-defined process by a single authority,
assignment might be partitioned among delegated authorities that
are individually responsible for respecting uniqueness rules, or
URNs might be created independently following an algorithm that
itself guarantees uniqueness.
7.4. Security and Privacy
The "Security" section of the template describes any potential issues
related to security and privacy with regard to assignment, use, and
resolution of identifiers within the namespace. Examples of such
issues include:
o The consequences of producing false negatives and false positives
during comparison for equivalence (see "Issues in Identifier
Comparison for Security Purposes" [RFC6943])
o Leakage of private information when identifiers are communicated
on the public Internet
o The potential for directory harvesting
o Various issues discussed in the guidelines for security
considerations in RFCs [RFC3552] and the privacy considerations
for Internet protocols [RFC6973].
7.5. Resolution
The "Resolution" section specifies the rules for resolution of URNs
assigned within the namespace. If such URNs are intended to be
resolvable, the namespace needs to be registered in a Resolution
Discovery System (RDS, see [RFC2276]) such as DDDS. Resolution then
proceeds according to standard URI resolution processes, as well as
the mechanisms of the RDS. This section ought to list the
requirements for becoming a recognized resolver of URNs in the
relevant namespace (and being so listed in the RDS registry).
Answers might include, but are not limited to:
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1. The namespace is not listed with an RDS; therefore this section
is not applicable.
2. Resolution mirroring is completely open, with a mechanism for
updating an appropriate RDS.
3. Resolution is controlled by entities to which assignment has been
delegated.
8. Registering a URN Namespace
8.1. Formal Namespaces
The registration policy for formal namespaces is Expert Review as
defined in the "IANA Considerations" document [RFC5226] and in
Section 9 below. The key steps for registration of a formal
namespace are:
1. Fill out the namespace registration template (see Appendix A).
This can be done as part of an Internet-Draft or a specification
in another series, although that is not necessary.
2. Send the completed template to the urn-nid@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, the IANA will
register the requested NID.
A formal namespace registration can be revised by updating the
registration template, following the same steps outlined above for
new registrations. A revised registration MUST describe differences
from prior versions and SHOULD make special note of any relevant
changes in the underlying technologies or namespace management
processes.
8.2. Informal Namespaces
The registration policy for informal namespaces is First Come First
Served [RFC5226]. The key steps for registration of an informal
namespace are:
1. Write a completed namespace definition template (see Appendix A).
2. Send it to the urn-nid@ietf.org discussion list for feedback.
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3. Once the review period has expired, send the final template to
IANA (via the iana@iana.org email address).
An informal namespace registration can be revised by updating the
registration template, following the same steps outlined above for
new registrations.
9. Guidelines for Designated Experts
Experience to date with NID registration requests has shown that
registrants sometimes do not initially understand some of the
subtleties of URN namespaces, and that defining the namespace in the
form of a specification enables the registrants to clearly formulate
their "contract" with the intended user community. Therefore,
although the registration policy for formal namespaces is Expert
Review and a stable specification is not strictly required, the
designated experts for NID registration requests ought to encourage
applicants to provide a stable specification documenting the
namespace definition.
Naming can be difficult and contentious; the designated experts and
applicants are strongly encouraged to work together in a spirit of
good faith and mutual understanding to achieve rough consensus on
progressing registrations through the process. They are also
encouraged to bring additional expertise into the discussion if that
would be helpful in adding perspective or otherwise resolving issues.
10. IANA Considerations
10.1. URI Scheme
This section updates the registration of the 'urn' URI scheme in the
Permanent URI Registry [URI-Registry] .
[Note to RFC Editor: please replace "XXXX" with the number assigned
to this document upon publication.]
URI Scheme Name: urn
Status: permanent
URI Scheme Syntax: See Section 3 of [ this document ].
URI Scheme Semantics: The 'urn' scheme identifies Uniform Resource
Names, which are persistent, location-independent resource
identifiers.
Encoding Considerations: See Section 3.2 of [ this document ].
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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 5 of [ this document ].
Security Considerations: See Section 7.4 and Section 11 of [ this
document ].
Contact: URNBIS WG [mailto:urn@ietf.org]
Author/Change Controller: This scheme is registered under the IETF
tree. As such, the IETF maintains change control.
References None.
10.2. 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. In all cases, the IANA ought to assign the appropriate
NID (formal or informal) once the procedures outlined in this
document have been completed.
11. Security and Privacy Considerations
The definition of a URN namespace needs to account for potential
security and privacy issues related to assignment, use, and
resolution of identifiers within the namespace (e.g., some namespace
resolvers might assign special meaning to certain characters in the
Namespace Specific String); see Section 7.4 for further discussion.
In most cases, URN namespaces provide a way to declare public
information. Nominally, 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.
12. References
12.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[URI-Registry]
IANA, "Permanent URI Schemes",
<http://www.iana.org/assignments/uri-schemes/
uri-schemes.xhtml#uri-schemes-1>.
12.2. Informative References
[I-D.ietf-urnbis-semantics-clarif]
Klensin, J., "URN Semantics Clarification", draft-ietf-
urnbis-semantics-clarif-00 (work in progress), August
2014.
[DOI-URI] Paskin, N., Neylon, E., Hammond, T., and S. Sun, "The
"doi" URI Scheme for the Digital Object Identifier (DOI)",
June 2003,
<http://tools.ietf.org/id/draft-paskin-doi-uri-04.txt>.
[RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2276] Sollins, K., "Architectural Principles of Uniform Resource
Name Resolution", RFC 2276, January 1998.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition
Mechanisms", BCP 66, RFC 3406, October 2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
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[RFC4854] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
for Extensions to the Extensible Messaging and Presence
Protocol (XMPP)", RFC 4854, April 2007.
[RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)", RFC
5122, February 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
[RFC6943] Thaler, D., "Issues in Identifier Comparison for Security
Purposes", RFC 6943, May 2013.
[RFC6963] Saint-Andre, P., "A Uniform Resource Name (URN) Namespace
for Examples", BCP 183, RFC 6963, May 2013.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190, RFC
7320, July 2014.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
6.3", 2013,
<http://www.unicode.org/versions/Unicode6.3.0/>.
[XML-BASE]
Marsh, J. and R. Tobin, "XML Base (Second Edition)", World
Wide Web Consortium Recommendation REC-xmlbase-20090128,
January 2009,
<http://www.w3.org/TR/2009/REC-xmlbase-20090128>.
[XML-NAMES]
Thompson, H., Hollander, D., Layman, A., Bray, T., and R.
Tobin, "Namespaces in XML 1.0 (Third Edition)", World Wide
Web Consortium Recommendation REC-xml-names-20091208,
December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
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Appendix A. Registration Template
A.1. Namespace ID
Requested of IANA (formal) or assigned by IANA (informal).
A.2. Version
The version of the registration, starting with 1 and incrementing by
1 with each new version.
A.3. Date
The date when the registration is requested of IANA, using the format
YYYY-MM-DD.
A.4. Registrant
The person or organization that has registered the NID, including the
following information:
o The name and address of the registering organization.
o The name and contact information (email, phone number, and/or
postal address) of the designated contact person.
A.5. Purpose
Described under Section 7.1 of this document.
A.6. Syntax
Described under Section 7.2 of this document. Unless the
registration explicitly says otherwise, use of p-, q-, and/or
f-components is not allowed for this namespace.
A.7. Assignment
Described under Section 7.3 of this document.
A.8. Resolution
Described under Section 7.5 of this document.
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A.9. Documentation
A pointer to an RFC, a specification published by another standards
development organization, or another stable document that provides
further information about the namespace.
A.10. Revision Information
(Applicable only when earlier registrations have been revised.)
Description of changes from prior version(s).
Appendix B. Changes from RFC 2141
This document makes the following substantive changes from [RFC2141]:
o Allows p-components, q-components, and f-components.
o Disallows "-" at the end of a NID.
o Allows the "~" and "&" characters in an NSS.
o Formally registers 'urn' as a URI scheme.
Appendix C. Changes from RFC 3406
This document makes the following substantive changes from [RFC3406]:
1. Relaxes the registration policy for formal namespaces from "IETF
Review" to "Expert Review" as discussed in Section 8.1 and
Section 9.
2. Removes the category of experimental namespaces, consistent with
[RFC6648].
3. Simplifies the registration template.
In addition, some of the text has been updated to be consistent with
the definition of Uniform Resource Identifiers (URIs) [RFC3986] and
the processes for registering information with the IANA [RFC5226], as
well as more modern guidance with regard to security issues [RFC3552]
and identifier comparison [RFC6943].
Appendix D. Contributors
RFC 2141, which provided the basis for the syntax portion of this
document, was authored by Ryan Moats.
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RFC 3406, which provided the basis for the namespace portion of this
document, was authored by Leslie Daigle, Dirk-Willem van Gulik,
Renato Iannella, and Patrik Faltstrom.
Their work is gratefully acknowledged.
Appendix E. Acknowledgements
Many thanks to Marc Blanchet, Leslie Daigle, Martin Duerst, Juha
Hakala, Ted Hardie, Alfred Hoenes, Paul Jones, Barry Leiba, Sean
Leonard, Larry Masinter, Keith Moore, Mark Nottingham, Julian
Reschke, Lars Svensson, Dale Worley, and other participants in the
URNBIS WG for their input. Alfred Hoenes in particular edited an
earlier version of this document and served as co-chair of the URNBIS
WG.
Juha Hakala deserves special recognition for his dedication to
successfully completing this work, as do Andrew Newton in his role as
working group chair and Barry Leiba in his role as area director.
Appendix F. Change log for versions of draft-ietf-urnbis-rfc2141bis-urn
[[RFC Editor: please remove this appendix before publication.]]
F.1. Changes from -08 to -09
o Altered the text in Section 5 to reflect list discussions about
the earlier phrasing. Also added DOI example and citation to that
section.
o Clarified the naming rules for formal namespaces and their
relationship to ISO 3166, IDNA, etc., reserved strings.
o Added an explicit statement about use of URNs in various protocols
and contexts to Section 5.
o Clarified that experimental namespace NIDs, which were explicitly
not registered, are not valid URNs (in Section 6.
o Transformed the partial production in Section 6.2 into valid ABNF.
o Added more text about p-/q-/f-components and recommendations about
use.
o Added clarifying note about "?" within q-components and
f-components.
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o Added explicit requirement that revisions of existing
registrations document the changes and added a slot for that
description to the template.
o Many small editorial changes and adjustments including adding
additional references and cross-references for clarification.
o Inserted a placeholder for additional examples.
Authors' Addresses
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john-ietf@jck.com
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