A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cbor-cddl-freezer-10

Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                           24 October 2022
Expires: 27 April 2023

   A feature freezer for the Concise Data Definition Language (CDDL)
                   draft-bormann-cbor-cddl-freezer-10

Abstract

   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, RFC 8610, or the specifications
   exercising its extension points, such as RFC 9165.

   Significant parts of this draft have now moved over to the CDDL 2.0
   project, described in draft-bormann-cbor-cddl-2-draft.  The remaining
   items in this draft are not directly related to the CDDL 2.0 effort.

Status of This Memo

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Copyright Notice

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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Base language features  . . . . . . . . . . . . . . . . . . .   2
     2.1.  Cuts  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Literal syntax  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Regular Expression Literals . . . . . . . . . . . . . . .   3
   4.  Controls  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Control operator .pcre  . . . . . . . . . . . . . . . . .   3
     4.2.  Endianness in .bits . . . . . . . . . . . . . . . . . . .   4
     4.3.  .bitfield control . . . . . . . . . . . . . . . . . . . .   4
   5.  Co-occurrence Constraints . . . . . . . . . . . . . . . . . .   4
   6.  Alternative Representations . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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].  The remaining items in this draft
   are not directly related to the CDDL 2.0 effort.

   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.

2.  Base language features

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2.1.  Cuts

   Section 3.5.4 of [RFC8610] alludes to a new language feature, _cuts_,
   and defines it in a fashion that is rather focused on a single
   application in the context of maps and generating better diagnostic
   information about them.

   The present document is expected to grow a more complete definition
   of cuts, with the expectation that it will be upwards-compatible to
   the existing one in [RFC8610], before this possibly becomes a
   mainline language feature in a future version of CDDL.

3.  Literal syntax

3.1.  Regular Expression Literals

   Regular expressions currently are notated as strings in CDDL, with
   all the string escaping rules applied once.  It might be convenient
   to have a more conventional literal format for regular expressions,
   possibly also providing a place to add modifiers such as /i.  This
   might also imply text .regexp ..., which with the proposal in
   Section 4.1 then raises the question of how to indicate the regular
   expression flavor.

   (With the support for ABNF in [RFC9165], the need for this is
   reduced.  Also, the proliferation of regular expression flavors is
   hard to address with a single syntax.)

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

4.1.  Control operator .pcre

   There are many variants of regular expression languages.
   Section 3.8.3 of [RFC8610] defines the .regexp control, which is
   based on XSD [XSD2] regular expressions.  As discussed in that
   section, the most desirable form of regular expressions in many cases
   is the family called "Perl-Compatible Regular Expressions" ([PCRE]);
   however, no formally stable definition of PCRE is available at this
   time for normatively referencing it from an RFC.

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   The present document defines the control operator .pcre, which is
   similar to .regexp, but uses PCRE2 regular expressions.  More
   specifically, a .pcre control indicates that the text string given as
   a target needs to match the PCRE regular expression given as a value
   in the control type, where that regular expression is anchored on
   both sides.  (If anchoring is not desired for a side, .* needs to be
   inserted there.)

   Similarly, .es2018re could be defined for ECMAscript 2018 regular
   expressions with anchors added.

   See also [I-D.draft-bormann-jsonpath-iregexp], which could be
   specifically called out via .iregexp (even though .regexp as per
   Section 3.8.3 of [RFC8610] would also have the same semantics, except
   for a wider range of regexps).

4.2.  Endianness in .bits

   How useful would it be to have another variant of .bits that counts
   bits like in RFC box notation?  (Or at least per-byte?  32-bit words
   don't always perfectly mesh with byte strings.)

4.3.  .bitfield control

   Provide a way to specify bitfields in byte strings and uints to a
   higher level of detail than is possible with .bits.  Strawman:

   Field = uint .bitfield Fieldbits

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

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

   Note that the group within the controlling array can have choices,
   enabling the whole power of a context-free grammar (but not much
   more).

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:

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   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
   Goessner'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.

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   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", 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 2.1 of [I-D.bormann-cbor-cddl-2-draft]).

7.  IANA Considerations

   This document makes no requests of IANA.

8.  Security considerations

   The security considerations of [RFC8610] apply.

9.  References

9.1.  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/info/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/info/rfc9165>.

9.2.  Informative References

   [cddlc]    "CDDL conversion utilities", n.d.,
              <https://github.com/cabo/cddlc>.

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   [I-D.bormann-cbor-cddl-2-draft]
              Bormann, C., "CDDL 2.0 -- a draft plan", Work in Progress,
              Internet-Draft, draft-bormann-cbor-cddl-2-draft-00, 19
              October 2022, <https://www.ietf.org/archive/id/draft-
              bormann-cbor-cddl-2-draft-00.txt>.

   [I-D.draft-bormann-jsonpath-iregexp]
              Bormann, C. and T. Bray, "I-Regexp: An Interoperable
              Regexp Format", Work in Progress, Internet-Draft, draft-
              bormann-jsonpath-iregexp-04, 25 April 2022,
              <https://www.ietf.org/archive/id/draft-bormann-jsonpath-
              iregexp-04.txt>.

   [I-D.handrews-relative-json-pointer]
              Luff, G. and H. Andrews, "Relative JSON Pointers", Work in
              Progress, Internet-Draft, draft-handrews-relative-json-
              pointer-02, 18 September 2019,
              <https://www.ietf.org/archive/id/draft-handrews-relative-
              json-pointer-02.txt>.

   [jsonpath] "jsonpath online evaluator", n.d., <https://jsonpath.com>.

   [PCRE]     "Perl-compatible Regular Expressions (revised API:
              PCRE2)", n.d., <http://pcre.org/current/doc/html/>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6256]  Eddy, W. and E. Davies, "Using Self-Delimiting Numeric
              Values in Protocols", RFC 6256, DOI 10.17487/RFC6256, May
              2011, <https://www.rfc-editor.org/info/rfc6256>.

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

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   [RFC9090]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Tags for Object Identifiers", RFC 9090,
              DOI 10.17487/RFC9090, July 2021,
              <https://www.rfc-editor.org/info/rfc9090>.

   [XSD2]     Malhotra, A., Ed. and P. V. Biron, Ed., "XML Schema Part
              2: Datatypes Second Edition", W3C REC REC-xmlschema-
              2-20041028, W3C REC-xmlschema-2-20041028, 28 October 2004,
              <https://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.

Acknowledgements

   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

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

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