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
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  (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 [email protected] 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: [email protected]


  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: [email protected]

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