]> 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 . 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 .) This number is carried in the Content-Format and Accept Options. This compression strategy is ideal in cases where the set of possible 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. 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.

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 zero or more name-value pairs representing the additional media type parameters. The name-value pairs are optional to support the case where the Parametrized Content-Format is used in Parametrized Multi-Valued Accept Option ().

CDDL for the Parametrized Content-Format

TODO describe use of numeric identifiers as alias for parameter names (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

Example #1

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 a single CBOR-encoded Parametrized Content-Format data item or two or more Parametrized Content-Format data items as a CBOR array.

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 TODO Security

IANA Considerations TODO IANA

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 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. 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 for the ability to have basic security services defined 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. 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 TODO acknowledge.