RFC 9423 CoRE Target Attributes Registry March 2024
Bormann Informational [Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9423
Category:
Informational
Published:
ISSN:
2070-1721
Author:
C. Bormann
Universität Bremen TZI

RFC 9423

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.

Table of Contents

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.

Table 1: Initial Entries in the Target Attributes Registry
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].

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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288]
Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, , <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, , <https://www.rfc-editor.org/info/rfc20>.

4.2. Informative References

[RFC5988]
Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, , <https://www.rfc-editor.org/info/rfc5988>.
[RFC6690]
Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, , <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, , <https://www.rfc-editor.org/info/rfc7252>.
[RFC7641]
Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, , <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, , <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, , <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, , <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

Jaime provided the list of initial registrations.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany