Constrained RESTful Environments (CoRE) Target Attributes Registry
A registry for names that describe links in constrained IoT systems.
Bormann, C., "Constrained RESTful Environments (CoRE) Target Attributes Registry", RFC 9423, DOI 10.17487/RFC9423, April 2024, <https://www.rfc-editor.org/info/rfc9423>.
@misc{rfc9423,
series = {Request for Comments},
number = 9423,
howpublished = {RFC 9423},
publisher = {RFC Editor},
doi = {10.17487/RFC9423},
url = {https://www.rfc-editor.org/info/rfc9423},
author = {C. Bormann},
title = {{Constrained RESTful Environments (CoRE) Target Attributes Registry}},
pagetotal = 8,
year = 2024,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Bormann Request for Comments: 9423 Universität Bremen TZI Category: Informational April 2024 ISSN: 2070-1721 Constrained RESTful Environments (CoRE) Target Attributes Registry Abstract 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. Status of This Memo 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 Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction 1.1. Terminology 2. IANA Considerations 2.1. Instructions for the Designated Expert 2.2. Structure of Entries 2.3. Initial Entries 3. Security Considerations 4. References 4.1. Normative References 4.2. Informative References Acknowledgements Contributors Author's Address 1. Introduction 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. 1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. IANA Considerations 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]). 2.1. 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 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. 2.2. Structure of Entries Each entry in the registry must include the following: Attribute Name: 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.) Brief Description: A brief description. Change Controller: See Section 2.3 of RFC 8126 [BCP26]. 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. 2.3. Initial Entries Initial entries in this registry are listed in Table 1. +===========+=========================+============+===============+ | Attribute | Brief Description | Change | Reference | | Name | | Controller | | +===========+=========================+============+===============+ | href | reserved (not useful as | IETF | [RFC6690] | | | target attribute name) | | | +-----------+-------------------------+------------+---------------+ | anchor | reserved (not useful as | IETF | [RFC6690] | | | target attribute name) | | | +-----------+-------------------------+------------+---------------+ | rel | reserved (not useful as | IETF | [RFC6690] | | | target attribute name) | | | +-----------+-------------------------+------------+---------------+ | rev | reserved (not useful as | IETF | [RFC6690] | | | target attribute name) | | | +-----------+-------------------------+------------+---------------+ | 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 | IETF | Section 5.5 | | | template | | of [RFC8075] | +-----------+-------------------------+------------+---------------+ | osc | hint: resource only | IETF | Section 9 of | | | accessible using OSCORE | | [RFC8613] | +-----------+-------------------------+------------+---------------+ | ep | Endpoint Name (with | IETF | Section 9.3 | | | rt="core.rd-ep") | | of [RFC9176] | +-----------+-------------------------+------------+---------------+ | d | Sector (with | IETF | Section 9.3 | | | rt="core.rd-ep") | | of [RFC9176] | +-----------+-------------------------+------------+---------------+ | base | Registration Base URI | IETF | Section 9.3 | | | (with rt="core.rd-ep") | | of [RFC9176] | +-----------+-------------------------+------------+---------------+ | et | Endpoint Type (with | IETF | Section 9.3 | | | rt="core.rd-ep") | | of [RFC9176] | +-----------+-------------------------+------------+---------------+ Table 1: Initial Entries in the Target Attributes Registry 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]. 3. Security Considerations The security considerations of [RFC8288] apply, as do those of the discovery specifications [RFC6690], [RFC7252], and [RFC9176]. 4. References 4.1. Normative References [BCP26] Best Current Practice 26, <https://www.rfc-editor.org/info/bcp26>. At the time of writing, this BCP comprises the following: Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>. [IANA.core-parameters] IANA, "Constrained RESTful Environments (CoRE) Parameters", <https://www.iana.org/assignments/core-parameters>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017, <https://www.rfc-editor.org/info/rfc8288>. [STD80] Internet Standard 80, <https://www.rfc-editor.org/info/std80>. At the time of writing, this STD comprises the following: Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, <https://www.rfc-editor.org/info/rfc20>. 4.2. Informative References [RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010, <https://www.rfc-editor.org/info/rfc5988>. [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, <https://www.rfc-editor.org/info/rfc6690>. [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/info/rfc7252>. [RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015, <https://www.rfc-editor.org/info/rfc7641>. [RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and E. Dijk, "Guidelines for Mapping Implementations: HTTP to the Constrained Application Protocol (CoAP)", RFC 8075, DOI 10.17487/RFC8075, February 2017, <https://www.rfc-editor.org/info/rfc8075>. [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, <https://www.rfc-editor.org/info/rfc8613>. [RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and P. van der Stok, "Constrained RESTful Environments (CoRE) Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April 2022, <https://www.rfc-editor.org/info/rfc9176>. Acknowledgements 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. Contributors Jaime Jiménez Ericsson Email: jaime@iki.fi Jaime provided the list of initial registrations. Author's Address Carsten Bormann Universität Bremen TZI Postfach 330440 D-28359 Bremen Germany Phone: +49-421-218-63921 Email: cabo@tzi.org
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§1Web Linking in CoRE
Constrained RESTful Environments (CoRE) applies web technologies to resource-constrained systems. One key technology is Web Linking (RFC 8288), which CoRE specifications use as the foundation for discovery protocols such as the Link Format used in the Constrained Application Protocol's (CoAP) resource discovery process and the Resource Directory (RD).
Web Links can carry target attributes—names that describe properties of the linked resource, such as its resource type (rt), interface description (if), content format (ct), or maximum size (sz). The Web Linking specification itself does not coordinate the names of target attributes, leaving implementers free to define their own but at the risk of creating conflicts when different systems choose the same name for different purposes.
When a tiny device (like a sensor) tells other devices what it is, it uses special labels. This RFC creates a list of approved labels so everyone uses the same ones.
§2Why a Registry is Needed
The CoRE Working Group has been discussing setting up a registry for target attribute names since the work on CoRE Link Format (RFC 6690). The update of Web Linking to RFC 8288 provided the formal setting and acknowledgment that while Web Linking does not coordinate attribute names, individual serializations (like CoRE's) should do so.
Without a registry, an unregistered attribute name might be registered later with a different meaning, causing incompatibilities. By creating a registry now, systems implementing CoRE can register their attribute names, ensuring that each name has a consistent meaning across all CoRE-based protocols and implementations. The registry also guides developers toward reasonably short names (important for conserving resources in constrained systems) while preserving very short names for attributes that are likely to be widely used.
Different groups might invent their own labels without knowing what others chose. A central list prevents confusion and helps new devices pick good names that everyone agrees on.
§3How the Registry Works
RFC 9423 creates a new "Target Attributes" registry under the Constrained RESTful Environments (CoRE) Parameters registry group at IANA. The registry uses an "Expert Review" policy, meaning that each new registration is reviewed by a designated expert who guides registrants toward appropriate names and ensures quality specifications.
Each registry entry must include an attribute name (lowercase ASCII starting with a letter, with optional digits and hyphens), a brief description, a change controller, and a reference document describing the attribute and its semantics. The expert is instructed to encourage reasonably short names and to direct registrants to provide full specifications—though exceptions can be made when a specification is not available yet but is likely forthcoming.
When someone wants to register a label, they describe it and an expert checks that it makes sense, does not conflict with other labels, and is documented well.
§4Initial Attributes
The registry is seeded with initial entries from existing CoRE and Web Linking standards. Entries from RFC 6690 (CoRE Link Format) include rt (resource type), if (interface description), sz (maximum size estimate), and ct (Content-Format hint). Additional entries come from CoAP specifications: obs (observable resource), hct (HTTP-CoAP URI mapping), and osc (OSCORE-only hint).
From the Resource Directory (RFC 9176) come ep (Endpoint Name), d (Sector), base (Registration Base URI), and et (Endpoint Type). From Web Linking (RFC 8288) come hreflang, media, title, and type. Certain names like href, anchor, rel, and rev are reserved as not useful for target attributes.
The list starts with labels that CoRE systems already use, like rt for what kind of resource it is, and if for how to talk to it.
§5Coordination with Related Registries
RFC 9423 updates the "RD Parameters" IANA registry created by RFC 9176 to coordinate with the new Target Attributes registry. The RD Parameters registry marks certain entries with an "A" flag, indicating they are target attributes. Per this document, all entries with the "A" flag, including new ones, must also be registered in the Target Attributes registry. This ensures a single source of truth for attribute definitions across CoRE specifications.
The Resource Directory has its own list of labels. This RFC makes sure that any label used in both places is defined in the same central registry.