| RFC 9423 | CoRE Target Attributes Registry | April 2024 |
| Bormann | Informational | [Page] |
The Constrained RESTful Environments (CoRE) specifications apply web technologies to constrained environments. One such important technology is Web Linking (RFC 8288), which CoRE specifications use as the basis for a number of discovery protocols, such as the Link Format (RFC 6690) in the Constrained Application Protocol's (CoAP's) resource discovery process (Section 7.2 of RFC 7252) and the Resource Directory (RD) (RFC 9176).¶
Web Links can have target attributes, the names of which are not generally coordinated by the Web Linking specification (Section 2.2 of RFC 8288). This document introduces an IANA registry for coordinating names of target attributes when used in CoRE. It updates the "RD Parameters" IANA registry created by RFC 9176 to coordinate with this registry.¶
This document is not an Internet Standards Track specification; it is published for informational purposes.¶
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). Not all documents approved by the IESG are candidates for any level of Internet Standard; see 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 https://www.rfc-editor.org/info/rfc9423.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The Constrained RESTful Environments (CoRE) specifications apply web technologies to constrained environments. One such important technology is Web Linking [RFC8288], which CoRE specifications use as the basis for a number of discovery protocols, such as the Link Format [RFC6690] in the Constrained Application Protocol's (CoAP's) resource discovery process (Section 7.2 of [RFC7252]) and the Resource Directory (RD) [RFC9176].¶
Web Links can have target attributes. The original Web Linking specification (Section 3 of [RFC5988]) did not attempt to coordinate names of target attributes except for providing common target attributes for use in the Link HTTP header. The current revision of that specification (Section 2.2 of [RFC8288]) clarifies as follows:¶
This specification does not attempt to coordinate the name of target attributes, their cardinality, or use. Those creating and maintaining serialisations SHOULD coordinate their target attributes to avoid conflicts in semantics or syntax and MAY define their own registries of target attributes.¶
This document introduces an IANA registry for coordinating names of target attributes when used in CoRE, with specific instructions for the designated expert for this registry (Section 2.1). It updates the "RD Parameters" IANA registry created by [RFC9176] to coordinate with this registry.¶
With this registry now available, registration of target attributes is strongly encouraged. The incentive is that an unregistered attribute name might be registered with a different meaning at any time.¶
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Per this specification, IANA has created a new "Target Attributes" registry in the "Constrained RESTful Environments (CoRE) Parameters" registry group [IANA.core-parameters], with the policy "Expert Review" (Section 4.5 of RFC 8126 [BCP26]).¶
The expert is requested to guide the registrant towards reasonably short target attribute names where the shortness will help conserve resources in constrained systems, but to also be frugal in the allocation of very short names, keeping them in reserve for applications that are likely to enjoy wide use and can make good use of their shortness.¶
The expert is also instructed to direct the registrant to provide a specification (Section 4.6 of RFC 8126 [BCP26]) but can make exceptions -- for instance, when a specification is not available at the time of registration but is likely forthcoming.¶
Any questions or issues that might interest a wider audience might be raised by the expert on the core-parameters@ietf.org mailing list for a time-limited discussion. This might include security considerations, or opportunities for orchestration, e.g., when different names with similar intent are being or could be registered.¶
If the expert becomes aware of target attributes that are deployed and in use, they may also initiate a registration on their own if they deem that such a registration can avert potential future collisions.¶
Each entry in the registry must include the following:¶
A lowercase ASCII string [STD80] that starts with a letter and can
contain digits and hyphen-minus characters afterward
([a-z][-a-z0-9]*).
(Note that [RFC8288] requires target attribute names to be
interpreted in a case-insensitive way; the restriction to lowercase
here ensures that they are registered in a predictable form.)¶
A brief description.¶
A reference document that provides a description of the target attribute, including the semantics for when the target attribute appears more than once in a link.¶
Initial entries in this registry are listed in Table 1.¶
| Attribute Name | Brief Description | Change Controller | Reference |
|---|---|---|---|
| href | reserved (not useful as target attribute name) | IETF | [RFC6690] |
| anchor | reserved (not useful as target attribute name) | IETF | [RFC6690] |
| rel | reserved (not useful as target attribute name) | IETF | [RFC6690] |
| rev | reserved (not useful as target attribute name) | IETF | [RFC6690] |
| hreflang | (Web Linking) | IETF | [RFC8288] |
| media | (Web Linking) | IETF | [RFC8288] |
| title | (Web Linking) | IETF | [RFC8288] |
| type | (Web Linking) | IETF | [RFC8288] |
| rt | resource type | IETF | Section 3.1 of [RFC6690] |
| if | interface description | IETF | Section 3.2 of [RFC6690] |
| sz | maximum size estimate | IETF | Section 3.3 of [RFC6690] |
| ct | Content-Format hint | IETF | Section 7.2.1 of [RFC7252] |
| obs | observable resource | IETF | Section 6 of [RFC7641] |
| hct | HTTP-CoAP URI mapping template | IETF | Section 5.5 of [RFC8075] |
| osc | hint: resource only accessible using OSCORE | IETF | Section 9 of [RFC8613] |
| ep | Endpoint Name (with rt="core.rd-ep") | IETF | Section 9.3 of [RFC9176] |
| d | Sector (with rt="core.rd-ep") | IETF | Section 9.3 of [RFC9176] |
| base | Registration Base URI (with rt="core.rd-ep") | IETF | Section 9.3 of [RFC9176] |
| et | Endpoint Type (with rt="core.rd-ep") | IETF | Section 9.3 of [RFC9176] |
A number of names are reserved, as they are used for parameters in links other than target attributes. A further set of target attributes is predefined in [RFC8288] and is imported into this registry.¶
Section 9.3 of [RFC9176] created the "RD Parameters" IANA registry. Per this document, IANA has added the following note to that registry:¶
Note: In accordance with RFC 9423, all entries with the "A" flag set, including new ones, MUST also be registered in the "Target Attributes" registry [IANA.core-parameters].¶
The security considerations of [RFC8288] apply, as do those of the discovery specifications [RFC6690], [RFC7252], and [RFC9176].¶
The CoRE Working Group had been discussing setting up a registry for target attributes since the final touches were made on [RFC6690]. The update of the Web Linking specification to [RFC8288] provided the formal setting, but it took until Jaime Jiménez provided the set of initial registrations to generate a first draft version of this specification. The current document addresses additional input and Working Group Last Call comments by Esko Dijk, Marco Tiloca, Thomas Fossati, and Mohamed Boucadair, as well as Area Director review comments from Rob Wilton.¶
Jaime provided the list of initial registrations.¶