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Parametrized Content-Format for CoAP
draft-fossati-core-parametrized-cf-01

Document Type Active Internet-Draft (individual)
Authors Thomas Fossati , Henk Birkholz
Last updated 2022-10-17
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draft-fossati-core-parametrized-cf-01
Constrained RESTful Environments                              T. Fossati
Internet-Draft                                                       arm
Intended status: Standards Track                             H. Birkholz
Expires: 20 April 2023                                    Fraunhofer SIT
                                                         17 October 2022

                  Parametrized Content-Format for CoAP
                 draft-fossati-core-parametrized-cf-01

Abstract

   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: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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Parametrized Content-Format . . . . . . . . . . . . . . . . .   3
     3.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Parametrized Content-Format Option  . . . . . . . . . . . . .   5
   5.  Parametrized Multi-Valued Accept Option . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.

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   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 [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.

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   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
   '

             Figure 1: CDDL for the Parametrized Content-Format

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

3.1.  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.

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

3.2.  Examples

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

                Figure 2: Content-Format with two paramters

4.  Parametrized Content-Format Option

   +========+=+===+===+===+================+========+========+=========+
   | Number |C| U | N | R | Name           |Format  | Length | Default |
   +========+=+===+===+===+================+========+========+=========+
   | TBD24  | |   |   |   | Parametrized   |See     |        | none    |
   |        | |   |   |   | Content-Format |Figure 3|        |         |
   |        | |   |   |   | Option         |        |        |         |
   +--------+-+---+---+---+----------------+--------+--------+---------+

                Table 1: 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

            Figure 3: Parametrized Content-Format Option Format

   The semantic is identical to the Content-Format Option described in
   Section 5.10.3 of [RFC7252].

5.  Parametrized Multi-Valued Accept Option

   +========+===+===+===+===+==============+========+========+=========+
   | Number | C | U | N | R |Name          |Format  | Length | Default |
   +========+===+===+===+===+==============+========+========+=========+
   | TBD13  | x |   |   |   |Parametrized  |See     |        | none    |
   |        |   |   |   |   |Multi-Valued  |Figure 4|        |         |
   |        |   |   |   |   |Accept Option |        |        |         |
   +--------+---+---+---+---+--------------+--------+--------+---------+

              Table 2: Parametrized Multi-Valued Accept Option

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   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>

          Figure 4: Parametrized Multi-Valued Accept Option 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.

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:

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     +========+=========================================+============+
     | Number | Name                                    | Reference  |
     +========+=========================================+============+
     | TBD13  | Parametrized Multi-Valued Accept Option | Section 5  |
     |        |                                         | of RFCthis |
     +--------+-----------------------------------------+------------+
     | TBD24  | Parametrized Content-Format Option      | Section 4  |
     |        |                                         | of RFCthis |
     +--------+-----------------------------------------+------------+

                            Table 3: New Options

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

8.  References

8.1.  Normative References

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters", 8 June 2012,
              <https://www.iana.org/assignments/core-parameters>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8610]  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,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [RFC9165]  Bormann, C., "Additional Control Operators for the Concise
              Data Definition Language (CDDL)", RFC 9165,
              DOI 10.17487/RFC9165, December 2021,
              <https://www.rfc-editor.org/rfc/rfc9165>.

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   [STD94]    Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

8.2.  Informative References

   [I-D.ietf-core-coral]
              Amsüss, C. and T. Fossati, "The Constrained RESTful
              Application Language (CoRAL)", Work in Progress, Internet-
              Draft, draft-ietf-core-coral-05, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coral-05>.

   [I-D.lundblade-rats-eat-media-type]
              Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media
              Types", Work in Progress, Internet-Draft, draft-lundblade-
              rats-eat-media-type-00, 26 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-lundblade-
              rats-eat-media-type-00>.

   [STD96]    Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9052>.

Acknowledgments

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

Authors' Addresses

   Thomas Fossati
   arm
   Email: thomas.fossati@arm.com

   Henk Birkholz
   Fraunhofer SIT
   Email: henk.birkholz@sit.fraunhofer.de

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