A feature freezer for the Concise Data Definition Language (CDDL)

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Table of Contents

1. Introduction

In defining the Concise Data Definition Language (CDDL), some features have turned up that would be nice to have. In the interest of completing this specification in a timely manner, the present document was started to collect nice-to-have features that did not make it into the first RFC for CDDL [RFC8610], or the specifications exercising its extension points, such as [RFC9165].¶

Significant parts of this draft have now moved over to [I-D.bormann-cbor-cddl-2-draft], which in turn references [I-D.ietf-cbor-update-8610-grammar], [I-D.ietf-cbor-cddl-more-control], [I-D.ietf-cbor-cddl-modules]. The remaining items in Sections 3 to 4 of this draft are not directly related to the CDDL 2.0 effort. Section 5 might turn into a part of CDDL 2.5.¶

The remaining sections are not proposing to change CDDL, but are ancillary developments: Section 6 is more interesting for the ecosystem of tools around CDDL. Section 7 examines extending the area of application of CDDL beyond CBOR and JSON.¶

There is always a danger for a document like this to become a shopping list; the intention is to develop this document further based on the rapidly growing real-world experience with the first CDDL standard. Some sections are labeled WONTFIX, reflecting an assumption that the specific extension objective will not be addressed or will be addressed in a different way.¶

2. Base language features

3. Literal syntax

Literal syntax is also discussed in Appendix A.1 of [I-D.bormann-cbor-cddl-2-draft], which might provide another approach to Section 3.1. This appendix is in turn based on ideas in [I-D.ietf-cbor-edn-literals].¶

4. Controls

Controls are the main extension point of the CDDL language. It is relatively painless to add controls to CDDL; this mechanism has been exercised in [RFC9090] for SDNV [RFC6256] and ASN.1 OID related byte strings, and in [RFC9165] for more generally applicable controls, including an interface to ABNF [RFC5234] [RFC7405]. A more recent collection of additions that is ready for standardization is specified in [I-D.ietf-cbor-cddl-more-control].¶

Several further candidates have been identified that aren't quite ready for adoption, of which a few shall be listed here.¶

Field = uint .bitfield Fieldbits

Fieldbits = [
  flag1: [1, bool],
  val: [4, Vals],
  flag2: [1, bool],
]

Vals = &(A: 0, B: 1, C: 2, D: 3)

5. Co-occurrence Constraints

While there are no co-occurrence constraints in CDDL, many actual use cases can be addressed by using the fact that a group is a grammar:¶

postal = {
  ( street: text,
    housenumber: text) //
  ( pobox: text .regexp "[0-9]+" )
}

However, constraints that are not just structural/tree-based but are predicates combining parts of the structure cannot be expressed:¶

session = {
  timeout: uint,
}

other-session = {
  timeout: uint  .lt [somehow refer to session.timeout],
}

As a minimum, this requires the ability to reach over to other parts of the tree in a control. Compare JSON Pointer [RFC6901] and JSON Relative Pointer [I-D.handrews-relative-json-pointer]. Stefan Gössner's jsonpath is a JSON variant of XPath that is now undergoing standardization [jsonpath].¶

More generally, something akin to what Schematron is to Relax-NG may be needed.¶

6. Alternative Representations

For CDDL, alternative representations e.g. in JSON (and thus in YAML) could be defined, similar to the way YANG defines an XML-based serialization called YIN in Section 11 of [RFC6020]. One proposal for such a syntax is provided by the cddlc tool [cddlc], which is reproduced below. This could be written up in more detail and agreed upon. (Since cddlc version 0.1.8, the "mem"-labeled array includes information about the presence of a cut, see Section 3.5.4 of [RFC8610].)¶

cddlj = ["cddl", +rule]
rule = ["=" / "/=" / "//=", namep, type]
namep = ["name", id] / ["gen", id, +id]
id = text .regexp "[A-Za-z@_$](([-.])*[A-Za-z0-9@_$])*"
op = ".." / "..." /
  text .regexp "\\.[A-Za-z@_$](([-.])*[A-Za-z0-9@_$])*"
namea = ["name", id] / ["gen", id, +type]
type = value / namea / ["op", op, type, type] /
  ["map", group] / ["ary", group] / ["tcho", 2*type] /
  ["unwrap", namea] / ["enum", group / namea] /
  ["prim", ?((6, type/uint, ?type) // (0..7, ?uint))]
group = ["mem", bool, null/type, type] /
  ["rep", uint, uint/false, group] /
  ["seq", 2*group] / ["gcho", 2*group]
value = ["number"/"text"/"bytes", text]

The "prim"-labeled array includes support for non-literal tag numbers (Section 3.2 of [I-D.ietf-cbor-update-8610-grammar]).¶

More recently, a variant of this format has been used for easier processing. It collects rules in a map (JSON object) and binds generic parameters to argument positions. This variant will be described in a further revision of this document.¶

7. Other target formats

CDDL has originally been designed to describe CBOR and JSON data. One format of interest is comma-separated values, CSV [RFC4180]. [I-D.bormann-cbor-cddl-csv] is a draft for using CDDL models with CSV.¶

8. IANA Considerations

This document makes no requests of IANA.¶

9. Security considerations

The security considerations of [RFC8610] apply.¶

Acknowledgements

Before [RFC8610] was finally published, many people have asked for CDDL to be completed, soon. These are usually also the people who have brought up observations that led to the proposals discussed here. Sean Leonard has campaigned for a regexp literal syntax.¶

Author's Address