|Internet-Draft||Parametrized Content-Format for CoAP||October 2022|
|Fossati & Birkholz||Expires 20 April 2023||[Page]|
- Constrained RESTful Environments
- Intended Status:
- Standards Track
Parametrized Content-Format for CoAP
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
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://thomas-fossati.github.io/draft-coap-parametrized-cf/draft-fossati-core-parametrized-cf.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-fossati-core-parametrized-cf/.¶
Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (mailto:email@example.com), which is archived at https://mailarchive.ietf.org/arch/browse/core/. Subscribe at https://www.ietf.org/mailman/listinfo/core/.¶
Source for this draft and an issue tracker can be found at https://github.com/thomas-fossati/draft-coap-parametrized-cf.¶
Status of This Memo
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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 Section 2 of [STD96].) 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 [I-D.lundblade-rats-eat-media-type], 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 [I-D.ietf-core-coral] profiles.¶
To avoid the combinatorial explosion that derives from such premises, this document defines the "parametrized" Content-Format data item (Section 3) 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:¶
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.¶
2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
In this document, the structure of data is specified in CDDL [RFC8610] [RFC9165].¶
The examples in Section 3.2 use CBOR diagnostic notation defined in Section 8 of [STD94] and Appendix G of [RFC8610].¶
3. Parametrized Content-Format
The Parametrized Content-Format is a CBOR [STD94] data item defined by the CDDL [RFC8610] in Figure 1.¶
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.¶
TODO describe use of numeric identifiers for parameter name aliasing (requires a new registry).¶
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.¶
4. Parametrized Content-Format Option
|TBD24||Parametrized Content-Format Option||See Figure 3||none|
The Parametrized Content-Format Option carries a CBOR-encoded Parametrized Content-Format data item.¶
The semantic is identical to the Content-Format Option described in Section 5.10.3 of [RFC7252].¶
5. Parametrized Multi-Valued Accept Option
|TBD13||x||Parametrized Multi-Valued Accept Option||See Figure 4||none|
The Parametrized Multi-Valued Accept Option carries either a single
pa-content-format data item or two or more
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
The semantic is identical to the Accept Option described in Section 5.10.4 of [RFC7252], 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.¶
6. Security Considerations
The security considerations in Section 11.1 of [RFC7252] related to the parsing of protocol elements apply.¶
The security considerations in Section 11.3 of [RFC7252] related to amplification risks apply.¶
7. IANA Considerations
RFC Editor: please replace RFCthis with this RFC number and remove this note.¶
7.1. CoAP Option Numbers Registry
IANA is requested to add the entries from Table 3 to the CoAP Option Numbers sub-registry of the Constrained RESTful Environments (CoRE) Parameters [IANA.core-parameters] registry:¶
|TBD13||Parametrized Multi-Valued Accept Option||Section 5 of RFCthis|
|TBD24||Parametrized Content-Format Option||Section 4 of RFCthis|
This document suggests 13 (TBD13) and 24 (TBD24) as values to be assigned for the new option numbers.¶
8.1. Normative References
- IANA, "Constrained RESTful Environments (CoRE) Parameters", , <https://www.iana.org/assignments/core-parameters>.
- 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/rfc/rfc2119>.
- Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <https://www.rfc-editor.org/rfc/rfc7252>.
- 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/rfc/rfc8174>.
- Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <https://www.rfc-editor.org/rfc/rfc8610>.
- Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, , <https://www.rfc-editor.org/rfc/rfc9165>.
- Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
8.2. Informative References
- Amsüss, C. and T. Fossati, "The Constrained RESTful Application Language (CoRAL)", Work in Progress, Internet-Draft, draft-ietf-core-coral-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-core-coral-05>.
- Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media Types", Work in Progress, Internet-Draft, draft-lundblade-rats-eat-media-type-00, , <https://datatracker.ietf.org/doc/html/draft-lundblade-rats-eat-media-type-00>.
- Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.
Thank you Carsten Bormann, Christian Amsüss, and Marco Tiloca for the useful comments and suggestions.¶