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].

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].