Internet-Draft | Parametrized Content-Format for CoAP | October 2022 |
Fossati & Birkholz | Expires 20 April 2023 | [Page] |
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.¶
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:core@ietf.org), 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.¶
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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.¶
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].¶
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.¶
parametrized-content-format = [ content-format, + [ parameter-name, parameter-value ] ] content-format = 0..65535 parameter-name = textual / numeric parameter-value = any textual = text .abnf ("parameter-name" .det RFC6838-parameter-name) numeric = int RFC6838-parameter-name = ' parameter-name = restricted-name restricted-name = restricted-name-first *126restricted-name-chars restricted-name-first = ALPHA / DIGIT restricted-name-chars = ALPHA / DIGIT / "!" / "#" / "$" / "&" / "-" / "^" / "_" restricted-name-chars =/ "." ; Characters before first dot always ; specify a facet name restricted-name-chars =/ "+" ; Characters after last plus always ; specify a structured syntax suffix ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9 '
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:¶
If any of the conditions listed above is not met, the entire data item is considered invalid and MUST NOT be processed further.¶
[ 65000, [ "p1", "a-string-value" ], [ "p2", 128 ] ]
Number | C | U | N | R | Name | Format | Length | Default |
---|---|---|---|---|---|---|---|---|
TBD24 | Parametrized Content-Format Option | See Figure 3 | none |
The Parametrized Content-Format Option carries a CBOR-encoded Parametrized Content-Format data item.¶
pcf-option-fmt = bytes .cbor parametrized-content-format
The semantic is identical to the Content-Format Option described in Section 5.10.3 of [RFC7252].¶
Number | C | U | N | R | Name | Format | Length | Default |
---|---|---|---|---|---|---|---|---|
TBD13 | x | Parametrized Multi-Valued Accept Option | See Figure 4 | 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
Figure 4.¶
pa-content-format = content-format / parametrized-content-format one-or-more<T> = T / [ 2* T ] pmva-option-fmt = bytes .cbor one-or-more<pa-content-format>
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.¶
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.¶
TODO expand¶
RFC Editor: please replace RFCthis with this RFC number and remove this note.¶
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:¶
Number | Name | Reference |
---|---|---|
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.¶
Thank you Carsten Bormann, Christian Amsüss, and Marco Tiloca for the useful comments and suggestions.¶