Constrained RESTful Environments (CoRE) Target Attributes Registry Universität Bremen TZI
Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org
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.
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 .
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Table of Contents
  • .  Introduction
    • .  Terminology
  • .  IANA Considerations
    • .  Instructions for the Designated Expert
    • .  Structure of Entries
    • .  Initial Entries
  • .  Security Considerations
  • .  References
    • .  Normative References
    • .  Informative References
  • Acknowledgements
  • Contributors
  • Author's Address
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 Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Constrained RESTful Environments (CoRE) Parameters IANA Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Web Linking This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field. ASCII format for network interchange Informative References Web Linking This document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header field. [STANDARDS-TRACK] Constrained RESTful Environments (CoRE) Link Format This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Observing Resources in the Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. Guidelines for Mapping Implementations: HTTP to the Constrained Application Protocol (CoAP) This document provides reference information for implementing a cross-protocol network proxy that performs translation from the HTTP protocol to the Constrained Application Protocol (CoAP). This will enable an HTTP client to access resources on a CoAP server through the proxy. This document describes how an HTTP request is mapped to a CoAP request and how a CoAP response is mapped back to an HTTP response. This includes guidelines for status code, URI, and media type mappings, as well as additional interworking advice. Object Security for Constrained RESTful Environments (OSCORE) This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. Constrained RESTful Environments (CoRE) Resource Directory In many Internet of Things (IoT) applications, direct discovery of resources is not practical due to sleeping nodes or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which contains information about resources held on other servers, allowing lookups to be performed for those resources. The input to an RD is composed of links, and the output is composed of links constructed from the information stored in the RD. This document specifies the web interfaces that an RD supports for web servers to discover the RD and to register, maintain, look up, and remove information on resources. Furthermore, new target attributes useful in conjunction with an RD are defined.
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.
Author's Address Universität Bremen TZI
Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org