]> arm

thomas.fossati@arm.com

Fraunhofer SIT

henk.birkholz@sit.fraunhofer.de

Applications and Real-Time Constrained RESTful Environments This document specifies a "parametrized" CoAP Content-Format data item that allows supplementing a Content-Format with additional media type parameters. This document also defines two new CoAP Options, Parmetrized-Content-Format and Parametrized-Multi-Valued-Accept, that build upon the "parametrized" Content-Format data item to work around some of the limitations of the existing Accept and Content-Format Options. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction CoAP squashes the combination of a media type, media type parameters and content coding into a single Content-Format number. (For an example, see Table 2 in .) This number is carried in the Content-Format and Accept Options. Such compression strategy is ideal in cases where the set of possible parameters combinations is known upfront and has small cardinality. However, it lacks the flexibility to deal smoothly with situations where the number of combinations can grow unbounded. An example is , in which the "profile" media type parameter can carry a number of different values that are constantly minted through a loosely regulated process. Another example is content negotiation of CoRAL profiles. To avoid the combinatorial explosion that derives from such premises, this document defines the "parametrized" Content-Format data item () as a mechanism to enrich a given Content-Format with additional media type parameters. Two new CoAP Options that build upon such data item are also defined:

• Parametrized-Content-Format ()
• Parametrized-Multi-Valued-Accept ()

The latter also works around the limited content negotiation capabilities of the CoAP Accept Option by allowing to accept more than one Content-Format per request.

Conventions and Definitions 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. In this document, the structure of data is specified in CDDL . The examples in use CBOR diagnostic notation defined in and .

Parametrized Content-Format The Parametrized Content-Format is a CBOR data item defined by the CDDL in . The first element in the tuple is the Content-Format identifier, followed by one or more name-value pairs representing the additional media type parameters.

CDDL for the Parametrized Content-Format

TODO describe use of numeric identifiers for parameter name aliasing (requires a new registry).

Requirements The list that follows details the semantic requirements that a Parametrized Content-Format data item must satisfy:

• The intersection between the media parameters already encoded in the Content-Format identifier and the set of parameters carried in the name-value pairs of the Parametrized Content-Format MUST be empty.
• Each name-value pair MUST be a registered parameter for the media type.

If any of the conditions listed above is not met, the entire data item is considered invalid and MUST NOT be processed further.

Examples

Content-Format with two paramters

Parametrized Content-Format Option Parametrized Content-Format Option

Number	C	U	N	R	Name	Format	Length	Default
TBD24					Parametrized Content-Format Option	See		none

The Parametrized Content-Format Option carries a CBOR-encoded Parametrized Content-Format data item.

Parametrized Content-Format Option Format

The semantic is identical to the Content-Format Option described in .

Parametrized Multi-Valued Accept Option Parametrized Multi-Valued Accept Option

Number	C	U	N	R	Name	Format	Length	Default
TBD13	x				Parametrized Multi-Valued Accept Option	See		none

The Parametrized Multi-Valued Accept Option carries either a single CBOR-encoded pa-content-format data item or two or more pa-content-format items wrapped in a CBOR array. In turn, each pa-content-format can be either a plain Content-Format or a Parametrized Content-Format as described in .

Parametrized Multi-Valued Accept Option Format = T / [ 2* T ] pmva-option-fmt = bytes .cbor one-or-more ]]>

The semantic is identical to the Accept Option described in , except for the ability to list more than one acceptable (parametrized) Content-Format, which is key to enable finer-grained content negotiation. The Content-Formats are listed in order of preference. If more than one match is found, the entry with the lowest index in the array MUST be selected.

Security Considerations The security considerations in related to the parsing of protocol elements apply. The security considerations in related to amplification risks apply. TODO expand

IANA Considerations RFC Editor: please replace RFCthis with this RFC number and remove this note.

CoAP Option Numbers Registry IANA is requested to add the entries from to the CoAP Option Numbers sub-registry of the Constrained RESTful Environments (CoRE) Parameters registry: New Options

Number	Name	Reference
TBD13	Parametrized Multi-Valued Accept Option	of RFCthis
TBD24	Parametrized Content-Format Option	of RFCthis

This document suggests 13 (TBD13) and 24 (TBD24) as values to be assigned for the new option numbers.

References Normative References 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. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed: , , and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Internet Assigned Numbers Authority 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. 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. Informative References Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. ARM The Constrained RESTful Application Language (CoRAL) defines a data model and interaction model as well as a compact serialization formats for the description of typed connections between resources on the Web ("links"), possible operations on such resources ("forms"), and simple resource metadata. Security Theory LLC Fraunhofer Institute for Secure Information Technology arm Payloads used in Remote Attestation Procedures may require an associated media type for their conveyance, for example when used in RESTful APIs. This memo defines media types to be used for Entity Attestation Tokens (EAT).

Acknowledgments Thank you Carsten Bormann, , and Marco Tiloca for the useful comments and suggestions.