Parametrized Content-Format for CoAP

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Table of Contents

1. 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 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:¶

• Parametrized-Content-Format (Section 4)¶
• Parametrized-Multi-Valued-Accept (Section 5)¶

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 data item defined by the CDDL 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).¶

[
  65000,
  [ "p1", "a-string-value" ],
  [ "p2", 128 ]
]

4. Parametrized Content-Format Option

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].¶

5. Parametrized Multi-Valued Accept Option

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.¶

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.¶

TODO expand¶

7. IANA Considerations

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

Acknowledgments

Thank you Carsten Bormann, Christian Amsüss, and Marco Tiloca for the useful comments and suggestions.¶

Authors' Addresses