CDDL 2.0 and beyond -- a draft plan
draft-bormann-cbor-cddl-2-draft-08

CBOR Working group                                            C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                              1 March 2026
Expires: 2 September 2026

                   CDDL 2.0 and beyond — a draft plan
                   draft-bormann-cbor-cddl-2-draft-08

Abstract

   The Concise Data Definition Language (CDDL) today is defined by
   RFC 8610, RFC 9165, RFC 9682, and RFC 9741).  RFC 9165 and the latter
   (as well as some more application specific specifications such as
   RFC 9090) have used the extension point provided in RFC 8610, the
   control operator.

   As CDDL is used in larger projects, feature requirements become known
   that cannot be easily mapped into this single extension point.
   Hence, there is a need for evolution of the base CDDL specification
   itself.

   The present document provides a roadmap towards a "CDDL 2.0"; it is
   intended to serve as a basis for implementations that evolve with the
   concept of CDDL 2.0.  It is based on draft-bormann-cbor-cddl-freezer,
   but is more selective in what potential features it takes up and more
   detailed in their discussion.  This document is intended to evolve
   over time; it might spawn specific documents and then retire, or it
   might eventually be published as a roadmap document.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-bormann-cbor-cddl-2-draft/.

   Discussion of this document takes place on the cbor Working Group
   mailing list (mailto:cbor@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/cbor/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/cbor/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cbor-wg/cddl-2.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  CDDL 1.1 + 2 plan (standards track) . . . . . . . . . . .   3
     1.2.  Other documents . . . . . . . . . . . . . . . . . . . . .   4
   2.  Mending syntax deficits . . . . . . . . . . . . . . . . . . .   5
     2.1.  Tag-oriented Literals . . . . . . . . . . . . . . . . . .   5
   3.  Processing model: Beyond Validation . . . . . . . . . . . . .   5
     3.1.  Annotations . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Transformation  . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Next Steps  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Module superstructure . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Cross-universe references . . . . . . . . . . . . . . . .   6
     4.2.  ABNF is a lot like CDDL . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7

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     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Fridge . . . . . . . . . . . . . . . . . . . . . . .   9
     A.1.  Tag-oriented Literals . . . . . . . . . . . . . . . . . .   9
     A.2.  Cross-universe references . . . . . . . . . . . . . . . .  10
       A.2.1.  IANA references . . . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   (Please see abstract.)

   Note that the existing extension point can be exercised for new
   features in parallel to the work described here.  [RFC9741] forms
   part of the first set of specifications going forward from the CDDL-2
   project together with [RFC9682].

   The rest of this introduction gives a rough overview over what could
   be the development plan for CDDL 1.1, 2.0, 2.5.

1.1.  CDDL 1.1 + 2 plan (standards track)

   This section is completed.

   CDDL 1.1 milestone

   *  "CDDL 1.1": [RFC9682], _Grammar_ fixes: Empty files (enabling CDDL
      2), non-literal tags, errata fixes.

   *  Parallel to CDDL 1.1: More _control_ operators [RFC9741]:
      Additional control operators, another iteration like [RFC9165]
      before.

   CDDL 2.0 work:

   *  Technically complete before *IETF 119*: CDDL 2.0:
      [I-D.ietf-cbor-cddl-modules] (import/include directives,
      implemented).  Feedback is available from IETF 119, one remaining
      technical issue (sockets).

   *  Potentially, further directives to be added.  No proposals are
      ripe for specification.

   "CDDL 2.5":

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   *  Being prepared: CDDL 2.5: Section 3 of the present document
      ("_annotations_", plus some functionality enabled by that).  The
      requirements are reasonably well-understood; the specific form
      this takes needs to be worked out.  Enables, e.g., Section 5 of
      [I-D.bormann-cbor-cddl-freezer] (co-occurrence).

1.2.  Other documents

   Not on the main line of development, but important ancillary work:

   *  (Informational, implemented): Section 6 (alternative
      representations) of [I-D.bormann-cbor-cddl-freezer]: CDDL-in-JSON
      format(s) for interchange of CDDL model information between tools.

   *  (Informational, companion to [I-D.ietf-cbor-cddl-modules]):
      [I-D.bormann-cbor-rfc-cddl-models] (builds standard collection of
      referenceable models).

   *  (BCP?  Informational?): [I-D.bormann-cbor-draft-numbers] (BCP for
      handling assigned numbers during draft stage; can stay
      informational as the work described is completed and any reference
      to the document erased before a specification using it would be
      published).

   *  Application-oriented literal e'' [I-D.ietf-cbor-edn-e-ref] makes
      use of [I-D.ietf-cbor-edn-literals] so that diagnostic notation
      can refer to named numbers that are specified in CDDL.
      Implemented, see [enum-literals] for an introduction.

   *  Further proposals to improve the integration between CDDL and EDN
      are likely; see [I-D.bormann-cbor-edn-mapkey] for a work in
      progress.

   More explorative at this point:

   *  (Standards-Track?)  The remaining Section 2 of this document:
      application-oriented literals in CDDL mirroring the work in
      [I-D.ietf-cbor-edn-literals].

   *  (Informational or Standards-Track?): [I-D.bormann-cbor-cddl-csv]
      (using CDDL to model CSV documents).

   Important CBOR work that may be reflected in some CDDL extensions:

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   *  Evolving Extended Diagnostic Notation
      [I-D.ietf-cbor-edn-literals].  While EDN and CDDL are independent
      languages (with EDN rooted in JSON and CDDL in ABNF and Relax-NG),
      they are often used together, and developments in one may spawn
      parallel work in the other.

   *  Common Deterministic Encoding (CDE) [I-D.ietf-cbor-cde] and
      related documents.  These do define CDDL operators already, which
      may be sufficient for initial use; this might be extended once
      more experience has been gained.

   *  Packed CBOR [I-D.ietf-cbor-packed].  CDDL already can be used to
      describe the original data item represented in a packed data item.
      Requirements for describing the latter have not yet been
      collected; there is some relation to transformation (Section 3.2)
      that might need to be explored.

2.  Mending syntax deficits

   The previous content of this section formed the basis for [RFC9682],
   except for Section 2.1.

2.1.  Tag-oriented Literals

   Incomplete, see Appendix A.1.

3.  Processing model: Beyond Validation

   _Proposal Status_:  experiments with implementations ongoing
   _Compatibility_:  backwards compatible

   The basic (implicit) processing model for CDDL 1.0 applies a CDDL
   data model to a data item and returns a Boolean that indicates
   whether the data item matches that model ("_validation_").

   Section 4 of [RFC9165] extends this model with named "_features_".  A
   validation can indicate which features were used.  Validation could
   also be parameterized with information about what features are
   allowed to be used, enabling variants (see Section 4 of [RFC9165] and
   [useful] for examples).

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3.1.  Annotations

   The cddl tool (Appendix F of [RFC8610]) also supports experimental
   forms of "annotating" a validated data item with information about
   which rules were used to support validation, currently entirely based
   on the information that is in a standard CDDL 1.0 data model.  This
   leads to a more general concept of "_annotation_", where the data
   model specification supports "annotating" the validated instance by
   optionally supplying information in the model.  (The annotated result
   is a special case of a "post-schema validation instance" [PSVI], here
   one where the data item itself is only augmented, not changed, by the
   process.)

   Annotations could in turn provide input to further validation steps,
   as is often done with Schematron validation in Relax-NG; with an
   appropriate evaluation language this can be used for checking co-
   occurrence constraints (Section 5 of
   [I-D.bormann-cbor-cddl-freezer]).

3.2.  Transformation

   Finally, annotations are a first step to _transformation_, i.e.,
   describing how a validated data item should be interpreted as a
   transformed data item by performing certain computations.  This
   generally requires even more support from an evaluation language,
   simple transformations such as adding in default values may not need
   much support though.

3.3.  Next Steps

   At this time, existing experimental implementations do not lead to a
   clear choice for what processing model enhancements should be in
   CDDL 2.0 follow-ons.  This document proposes to continue the
   experimentation and document good approaches.

4.  Module superstructure

   The previous content of this section formed the basis for
   [I-D.ietf-cbor-cddl-modules].  Additional work might be started on
   the ideas outlined in the subsections of this section.

4.1.  Cross-universe references

   See Appendix A.2.

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4.2.  ABNF is a lot like CDDL

   Many of the constructs defined here for CDDL also could be used with
   ABNF specifications.  ABNF would definitely benefit from a standard
   way to import snippets from existing RFCs.  Since CDDL contains ABNF
   support (Section 3 of [RFC9165]), it would be natural to make some of
   the functionality discussed in this section available for ABNF as
   well.

5.  IANA Considerations

   This document makes no requests of IANA.

6.  Security considerations

   The security considerations of [RFC8610] apply.

7.  References

7.1.  Normative References

   [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>.

7.2.  Informative References

   [enum-literals]
              "[Cbor] Getting diagnostic notation examples in drafts
              under control", 26 February 2024,
              <https://mailarchive.ietf.org/arch/msg/cbor/
              D8h_0Egog89GaRLFNwb1VfKlHI4>.

   [EXTRACT-RB]
              "extract.rb — extract CDDL from an enum-style IANA
              registry", n.d., <https://github.com/cabo/common-
              cddl/blob/main/extract.rb>.

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   [I-D.bormann-cbor-cddl-csv]
              Bormann, C. and H. Birkholz, "Using CDDL for CSVs", Work
              in Progress, Internet-Draft, draft-bormann-cbor-cddl-csv-
              08, 1 March 2026, <https://datatracker.ietf.org/doc/html/
              draft-bormann-cbor-cddl-csv-08>.

   [I-D.bormann-cbor-cddl-freezer]
              Bormann, C., "A feature freezer for the Concise Data
              Definition Language (CDDL)", Work in Progress, Internet-
              Draft, draft-bormann-cbor-cddl-freezer-16, 30 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              cddl-freezer-16>.

   [I-D.bormann-cbor-draft-numbers]
              Bormann, C., "Managing CBOR codepoints in Internet-
              Drafts", Work in Progress, Internet-Draft, draft-bormann-
              cbor-draft-numbers-07, 12 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              draft-numbers-07>.

   [I-D.bormann-cbor-edn-mapkey]
              Bormann, C. and C. Amsüss, "CBOR: Generating Numeric Map
              Labels from Textual EDN", Work in Progress, Internet-
              Draft, draft-bormann-cbor-edn-mapkey-01, 1 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              edn-mapkey-01>.

   [I-D.bormann-cbor-rfc-cddl-models]
              Bormann, C., "CDDL models for some existing RFCs", Work in
              Progress, Internet-Draft, draft-bormann-cbor-rfc-cddl-
              models-07, 22 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              rfc-cddl-models-07>.

   [I-D.ietf-cbor-cddl-modules]
              Bormann, C. and B. Moran, "CDDL Module Structure", Work in
              Progress, Internet-Draft, draft-ietf-cbor-cddl-modules-06,
              1 March 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cbor-cddl-modules-06>.

   [I-D.ietf-cbor-cde]
              Bormann, C., "CBOR Common Deterministic Encoding (CDE)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-cde-13,
              13 October 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cbor-cde-13>.

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   [I-D.ietf-cbor-edn-e-ref]
              Bormann, C., "External References to Values in CBOR
              Diagnostic Notation (EDN)", Work in Progress, Internet-
              Draft, draft-ietf-cbor-edn-e-ref-02, 2 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-e-ref-02>.

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
              literals-19, 16 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-19>.

   [I-D.ietf-cbor-packed]
              Bormann, C. and M. Gütschow, "Packed CBOR", Work in
              Progress, Internet-Draft, draft-ietf-cbor-packed-19, 2
              February 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cbor-packed-19>.

   [PSVI]     "Use Cases for XML Schema PSVI API", 24 June 2002,
              <https://www.w3.org/XML/2002/05/psvi-use-cases>.

   [RFC9682]  Bormann, C., "Updates to the Concise Data Definition
              Language (CDDL) Grammar", RFC 9682, DOI 10.17487/RFC9682,
              November 2024, <https://www.rfc-editor.org/rfc/rfc9682>.

   [RFC9741]  Bormann, C., "Concise Data Definition Language (CDDL):
              Additional Control Operators for the Conversion and
              Processing of Text", RFC 9741, DOI 10.17487/RFC9741, March
              2025, <https://www.rfc-editor.org/rfc/rfc9741>.

   [useful]   "Useful CDDL", n.d.,
              <https://github.com/cbor-wg/cddl/wiki/Useful-CDDL>.

Appendix A.  Fridge

   This appendix contains sections that may not make it to a 2.0
   milestone, but might be part of a followup.

A.1.  Tag-oriented Literals

   _Proposal Status_:  rough idea, porting from EDN
   _Compatibility_:  backward (not forward)

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   Some CBOR tags often would be most natural to use in a CDDL spec with
   a literal syntax that is tailored to their semantics instead of the
   serialization of their tag content in CBOR.  There is currently no
   way to add such syntaxes, no defined extension point either.

   The specification "CBOR Extended Diagnostic Notation (EDN):
   Application-Oriented Literals, ABNF, and Media Type"
   [I-D.ietf-cbor-edn-literals] defines application-oriented literals,
   e.g., of the form

      dt'2019-07-21T19:53Z'

   for datetime items.  With additional considerations for unambiguous
   syntax, a similar literal form could be included in CDDL.

   This proposal opens a namespace for the prefix that indicates an
   application specific literal.  A registry could be provided to turn
   this namespace into a genuine extension point.  (This is currently
   the production bsqual in Appendix B of [RFC8610].)

   The syntax provided in [I-D.ietf-cbor-edn-literals] does not enable
   the use of named CDDL rules — using it directly in CDDL would have
   the same flaw that is being fixed for tag numbers in Section 3.2 of
   [RFC9682].

A.2.  Cross-universe references

   Often, a CDDL specification needs to import from specifications in a
   different language or platform.

A.2.1.  IANA references

   In many cases, CDDL specifications make use of values that are
   specified in IANA registries.  The proposed .iana control operator
   can be used to reference such a set of values.

   The reference needs to be able to point to a draft, the registry of
   which has not been established yet, as well as to an established IANA
   registry.

   An example of such a usage might be:

   cose-algorithm = int .iana ["cose", "algorithms", "value"]

   Unfortunately, the vocabulary employed in IANA registries has not
   been designed for machine references.  In this case, the potential
   values would come from applying the XPath expression

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   //iana:registry[@id='algorithms']/iana:record/iana:value

   to https://www.iana.org/assignments/cose/cose.xml, plus some
   filtering on the records returned that only leaves actual
   allocations.  Section 3.1 of [I-D.bormann-cbor-rfc-cddl-models]
   contains an example of a CDDL module that is automatically generated
   from those assignments.  (The code for this extraction is available
   in the document source repository of
   [I-D.bormann-cbor-rfc-cddl-models] as [EXTRACT-RB].)

   Additional functionality may be needed for filtering with respect to
   other columns of the registry record, e.g., <capabilities> in the
   case of this example.

Acknowledgements

   TBD

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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