Uniform Resource Name (URN) Syntax
draft-ietf-urnbis-rfc2141bis-urn-04

Abstract

A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that is intended to serve as a persistent, location-independent resource identifier. The general class of URNs is differentiated from all other URIs through the use of the 'urn' URI scheme. This document defines the canonical syntax for URNs, guidelines for URN namespaces, requirements for URN presentation and transmission, and methods for determining URN equivalence. This document obsoletes RFC 2141.

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Table of Contents

• 1. Introduction
• 2. Discussion Venue
• 3. Terminology
• 4. Requirements
• 5. URN Syntax
• 5.1. Namespace Identifier Syntax
• 5.2. Namespace Specific String Syntax
• 6. URN Presentation and Transport
• 7. Lexical Equivalence in URNs
• 7.1. Procedure
• 7.2. Examples
• 8. Functional Equivalence in URNs
• 9. Handling of URNs by URI Processors
• 10. Security Considerations
• 11. IANA Considerations
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Appendix A. Changes from RFC 2141
• Appendix B. Acknowledgements
• Authors' Addresses

1. Introduction

A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) [RFC3986] that is intended to serve as a persistent, location-independent resource identifier. The general class of URNs is differentiated from all other URIs through the use of the 'urn' URI scheme. This document defines the canonical syntax for URNs, guidelines for URN namespaces, requirements for URN presentation and transmission, and methods for determining URN equivalence.

URNs were originally defined in [RFC2141]. The goal of this document is to specify URNs with the smallest reasonable set of changes from the original definition while ensuring consistency with the updated specification of URIs in [RFC3986]. If approved, this document will obsolete RFC 2141.

The discussion venue for this specification is the mailing list of the URNBIS Working Group; further information can be found at https://www.ietf.org/mailman/listinfo/urn.

2. Discussion Venue

The discussion venue for this document is mailing list of the URNBIS WG; visit https://www.ietf.org/mailman/listinfo/urn for subscription and archive information.

3. 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].

4. Requirements

The requirements for URNs are specified in [RFC1737]. This document does not modify or update those requirements.

5. URN Syntax

The syntax for a URN is defined as follows using the Augmented Backus-Naur Form (ABNF) as specified in [RFC5234].

   URN       = "urn" ":" NID ":" NSS
             ;
             ; the URI scheme ("urn") is case-insensitive
             ;
   NID       = (alphanum) 0*30(ldh) (alphanum)
             ;
             ; alphanum is defined in RFC 3986
             ;
   ldh       = alphanum / "-"
   NSS       = 1*(pchar)
             ;
             ; pchar is defined in RFC 3986
             ;
        

The following sections describe provide additional information about these rules.

5.1. Namespace Identifier Syntax

The syntax here is slightly more restrictive than what was defined in [RFC2141], since it forbids the hyphen character "-" at the end of a NID.

NIDs are case insensitive (e.g., "ISBN" and "isbn" identify the same namespace).

5.2. 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 urnchar 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 [RFC3986].

The "%" character is allowed only for the purpose of percent-encoding.

If a namespace designates one or more characters conforming to the urnchar 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 [RFC3406].

6. URN Presentation and Transport

The URN syntax defines the canonical format for URNs. All URN transport and interchanges MUST take place 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 RFC (i.e., when displaying URNs to humans, such applications might replace percent-encoded strings with characters in an extended character set such as Unicode).

7. Lexical Equivalence in URNs

7.1. 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 "lexical equivalence".

Two URNs are lexically equivalent if they are octet-by-octet equal after the following preprocessing rules:

1. normalize the case of the URI scheme "urn"
2. normalize the case of the NID
3. normalize the case of any percent-encoding

Note: Percent-encoded characters MUST NOT be decoded.

URN namespaces MAY define additional rules for lexical 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 [RFC3406].

7.2. Examples

The following URN comparisons highlight the lexical equivalence rules:

1. URN:foo:a123,456
2. urn:foo:a123,456
3. urn:FOO:a123,456
4. urn:foo:A123,456
5. urn:foo:a123%2C456
6. URN:FOO:a123%2c456

URNs 1, 2, and 3 are lexically equivalent. URN 4 is not lexically equivalent to any of the other URNs in the above set. URNs 5 and 6 are lexically equivalent only to each other.

8. Functional Equivalence in URNs

Functional equivalence is determined within a given namespace and managed by resolvers for that namespace, and thus is beyond the scope of this document. For related considerations with regard to NID registration, see [RFC3406].

9. Handling of URNs by URI Processors

The URN syntax has been defined so that URNs can be used in places where URIs are expected. 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-in to the URI resolver.

To minimize user confusion, a URI browser SHOULD display the complete URN (including the "urn" scheme) to ensure that there is no confusion between URN namespace identifiers and URL scheme identifiers.

10. Security Considerations

This document specifies the syntax for URNs. While some namespaces resolvers may assign special meaning to certain of the characters of the Namespace Specific String, any security consideration resulting from such assignment are outside the scope of this document. For related considerations with regard to NID registration, see [RFC3406].

11. IANA Considerations

This section formally registers a URI scheme of 'urn'.

[Note to RFC Editor: please change "XXXX" to the number assigned to this document upon publication.]

URI Scheme Name:

urn

Status:

permanent

URI Scheme Syntax:

See Section 4 of RFCXXXX.

URI Scheme Semantics:

The 'urn' scheme identifies Uniform Resource Names, which are persistent, location-independent resource identifiers.

Encoding Considerations:

See Section 4.2 of RFCXXXX.

Applications/Protocols That Use This URI Scheme Name:

Uniform Resource Names are used in a wide variety of applications, including bibliographical reference systems and as names for Extensible Markup Language (XML) namespaces.

Interoperability Considerations:

There are no known interoperability concerns related to use of the 'urn' URI scheme.

Security Considerations:

See Section 9 of RFCXXXX.

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.

12. References

12.1. Normative References

12.2. Informative References

Appendix A. Changes from RFC 2141

This document makes the following substantive changes from [RFC2141]:

• Disallowed "-" at the end of a NID.
• Allowed the "~" and "&" characters in an NSS.
• Formally registered 'urn' as a URI scheme.

Appendix B. Acknowledgements

RFC 2141, which provided the basis for this document, was authored by Ryan Moats.

Thanks to Julian Reschke for his corrections to the ABNF.

Authors' Addresses