]> Constrained RESTful Environments (CoRE) Target Attributes Registry Universität Bremen TZI
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art core CoAP Web Linking Resource Discovery 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.
Introduction The Constrained RESTful Environments (CoRE) specifications apply web technologies to constrained environments. One such important technology is Web Linking , which CoRE specifications use as the basis for a number of discovery protocols, such as the Link Format in the Constrained Application Protocol's (CoAP's) resource discovery process () and the Resource Directory (RD) . Web Links can have target attributes. The original Web Linking specification () 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 () 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 (). It updates the "RD Parameters" IANA registry created by 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.
Terminology 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 when, and only when, they appear in all capitals, as shown here.
IANA Considerations Per this specification, IANA has created a new "Target Attributes" registry in the "Constrained RESTful Environments (CoRE) Parameters" registry group , with the policy "Expert Review" (Section of RFC 8126 ).
Instructions for the Designated Expert 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 of RFC 8126 ) 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.
Structure of Entries Each entry in the registry must include the following:
Attribute Name:
A lowercase ASCII string that starts with a letter and can contain digits and hyphen-minus characters afterward ([a-z][-a-z0-9]*). (Note that 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.)
Brief Description:
A brief description.
Change Controller:
See Section of RFC 8126 .
Reference:
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 Initial entries in this registry are listed in . Initial Entries in the Target Attributes Registry
Attribute Name Brief Description Change Controller Reference
href reserved (not useful as target attribute name) IETF
anchor reserved (not useful as target attribute name) IETF
rel reserved (not useful as target attribute name) IETF
rev reserved (not useful as target attribute name) IETF
hreflang (Web Linking) IETF
media (Web Linking) IETF
title (Web Linking) IETF
type (Web Linking) IETF
rt resource type IETF
if interface description IETF
sz maximum size estimate IETF
ct Content-Format hint IETF
obs observable resource IETF
hct HTTP-CoAP URI mapping template IETF
osc hint: resource only accessible using OSCORE IETF
ep Endpoint Name (with rt="core.rd-ep") IETF
d Sector (with rt="core.rd-ep") IETF
base Registration Base URI (with rt="core.rd-ep") IETF
et Endpoint Type (with rt="core.rd-ep") IETF
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 and is imported into this registry. 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 .
Security Considerations The security considerations of apply, as do those of the discovery specifications , , and .
References Normative References Constrained RESTful Environments (CoRE) Parameters IANA Informative References
Acknowledgements The CoRE Working Group had been discussing setting up a registry for target attributes since the final touches were made on . The update of the Web Linking specification to provided the formal setting, but it took until 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 , , , and , as well as Area Director review comments from .
Contributors Ericsson
jaime@iki.fi
Jaime provided the list of initial registrations.