A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cbor-cddl-freezer-07

Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                             21 April 2021
Expires: 23 October 2021

   A feature freezer for the Concise Data Definition Language (CDDL)
                   draft-bormann-cbor-cddl-freezer-07

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.

   It is now time to discuss thawing some of the concepts discussed
   here.  A number of additional proposals have been added.

Status of This Memo

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Base language features  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Cuts  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Literal syntax  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Tag-oriented Literals . . . . . . . . . . . . . . . . . .   3
     3.2.  Regular Expression Literals . . . . . . . . . . . . . . .   3
     3.3.  Clarifications  . . . . . . . . . . . . . . . . . . . . .   4
       3.3.1.  Err6527 . . . . . . . . . . . . . . . . . . . . . . .   4
       3.3.2.  Err6543 . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Controls  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Control operator .pcre  . . . . . . . . . . . . . . . . .   6
     4.2.  Endianness in .bits . . . . . . . . . . . . . . . . . . .   6
     4.3.  .bitfield control . . . . . . . . . . . . . . . . . . . .   6
   5.  Co-occurrence Constraints . . . . . . . . . . . . . . . . . .   7
   6.  Module superstructure . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Namespacing . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Alternative Representations . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security considerations . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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].

   It is now time to discuss thawing some of the concepts discussed
   here.  A number of additional proposals have been added.

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   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 real-world experience with the first CDDL standard.

2.  Base language features

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.  Tag-oriented Literals

   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 their
   serialization in CBOR.  There is currently no way to add such
   syntaxes, no defined extension point either.

   The text form of CoRAL [I-D.ietf-core-coral] defines literals of the
   form

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

   for datetime items.  (Similar advances should then probably be made
   in diagnostic notation.)

3.2.  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.

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3.3.  Clarifications

   A number of errata reports have been made around some details of text
   string and byte string literal syntax: [Err6527] and [Err6543].
   These need to be addressed by re-examining the details of these
   literal syntaxes.  Also, [Err6526] needs to be applied.

3.3.1.  Err6527

   The ABNF used in [RFC8610] for the content of text string literals is
   rather permissive:

   text = %x22 *SCHAR %x22
   SCHAR = %x20-21 / %x23-5B / %x5D-7E / %x80-10FFFD / SESC
   SESC = "\" (%x20-7E / %x80-10FFFD)

   This allows almost any non-C0 character to be escaped by a backslash,
   but critically misses out on the "\uXXXX" and "\uHHHH\uLLLL" forms
   that JSON allows to specify characters in hex.  Both can be solved by
   updating the SESC production to:

   SESC = "\" ( %x22 / "/" / "\" /                 ; \" \/ \\
                %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
                (%x75 hexchar) )                   ; \u
   hexchar = non-surrogate / (high-surrogate "\" %x75 low-surrogate)
   non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) /
                  ("D" %x30-37 2HEXDIG )
   high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG
   low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG

   Now that SESC is more restrictively formulated, this also requires an
   update to the BCHAR production used in the ABNF syntax for byte
   string literals:

   bytes = [bsqual] %x27 *BCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
   bsqual = "h" / "b64"

   The updated version explicit allows "\'", which is no longer allowed
   in the updated SESC:

   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF

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3.3.2.  Err6543

   The ABNF used in [RFC8610] for the content of byte string literals
   lumps together byte strings notated as text with byte strings notated
   in base16 (hex) or base64 (but see also updated BCHAR production
   above):

   bytes = [bsqual] %x27 *BCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF

   Errata report 6543 proposes to handle the two cases in separate
   productions (where, with an updated SESC, BCHAR obviously needs to be
   updated as above):

   bytes = %x27 *BCHAR %x27
         / bsqual %x27 *QCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
   QCHAR = DIGIT / ALPHA / "+" / "/" / "-" / "_" / "=" / WS

   This potentially causes a subtle change, which is hidden in the WS
   production:

   WS = SP / NL
   SP = %x20
   NL = COMMENT / CRLF
   COMMENT = ";" *PCHAR CRLF
   PCHAR = %x20-7E / %x80-10FFFD
   CRLF = %x0A / %x0D.0A

   This allows any non-C0 character in a comment, so this fragment
   becomes possible:

   foo = h'
      43424F52 ; 'CBOR'
      0A       ; LF, but don't use CR!
   '

   The current text is not unambiguously saying whether the three
   apostrophes need to be escaped with a "\" or not, as in:

   foo = h'
      43424F52 ; \'CBOR\'
      0A       ; LF, but don\'t use CR!
   '

   ... which would be supported by the existing ABNF in [RFC8610].

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4.  Controls

   Controls are the main extension point of the CDDL language.  It is
   relatively painless to add controls to CDDL.  Several candidates have
   been identified that aren't quite ready for adoption, of which one
   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.

   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.

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)

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   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:

   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 has not been
   formally standardized [jsonpath].

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

6.  Module superstructure

   CDDL rules could be packaged as modules and referenced from other
   modules.  There could be some control of namespace pollution, as well
   as unambiguous referencing ("versioning").

   This is probably best achieved by a pragma-like syntax which could be
   carried in CDDL comments, leaving each module to be valid CDDL (if
   missing some rule definitions to be imported).

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6.1.  Namespacing

   A convention for mapping CDDL-internal names to external ones could
   be developed, possibly steered by some pragma-like constructs.
   External names would likely be URI-based, with some conventions as
   they are used in RDF or Curies.  Internal names might look similar to
   XML QNames.  Note that the identifier character set for CDDL
   deliberately includes $ and @, which could be used in such a
   convention.

7.  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]; this could
   be written up and agreed upon.

   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", ?(0..7, ?uint)]
   group = ["mem", null/type, type] /
     ["rep", uint, uint/false, group] /
     ["seq", 2*group] / ["gcho", 2*group]
   value = ["number"/"text"/"bytes", text]

8.  IANA Considerations

   This document makes no requests of IANA.

9.  Security considerations

   The security considerations of [RFC8610] apply.

10.  References

10.1.  Normative References

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   [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>.

10.2.  Informative References

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

   [Err6526]  "Errata Report 6526", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6526>.

   [Err6527]  "Errata Report 6527", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6527>.

   [Err6543]  "Errata Report 6543", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6543>.

   [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>.

   [I-D.ietf-core-coral]
              Hartke, K., "The Constrained RESTful Application Language
              (CoRAL)", Work in Progress, Internet-Draft, draft-ietf-
              core-coral-03, 9 March 2020,
              <https://www.ietf.org/archive/id/draft-ietf-core-coral-
              03.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/>.

   [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>.

   [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>.

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   [XSD2]     Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium Recommendation 
              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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