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Versions: (draft-bormann-cddl-freezer) 00 01

Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                           August 10, 2018
Expires: February 11, 2019


   A feature freezer for the Concise Data Definition Language (CDDL)
                   draft-bormann-cbor-cddl-freezer-01

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 collects nice-to-have features that did not make it into the
   first RFC for CDDL.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 11, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Cuts  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Literal syntax  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Computed Literals . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Tag-oriented Literals . . . . . . . . . . . . . . . . . .   3
     3.3.  Regular Expression Literals . . . . . . . . . . . . . . .   3
   4.  Embedded ANBF . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Controls  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Control operator .pcre  . . . . . . . . . . . . . . . . .   4
     5.2.  Endianness in .bits . . . . . . . . . . . . . . . . . . .   4
     5.3.  .bitfield control . . . . . . . . . . . . . . . . . . . .   4
   6.  Co-occurrence Constraints . . . . . . . . . . . . . . . . . .   5
   7.  Module superstructure . . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   (TO DO: Insert an extended form of the abstract first here, expanding
   the reference to [I-D.ietf-cbor-cddl].)

   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.  Cuts

   Section 3.5.3 of [I-D.ietf-cbor-cddl] 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 [I-D.ietf-cbor-cddl], before this possibly
   becomes a mainline language feature in a future version of CDDL.








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3.  Literal syntax

3.1.  Computed Literals

   CDDL cannot compute.

   This is a liability in at least two situations:

   o  integers often need to be defined relative to an offset.  It might
      be desirable to be able to write something like:

                              base = 16777216
                              a = base + 1
                              b = base + 2

   o  some string literals (in particular, complex regular expressions)
      would best be composed from components.  This could be done by
      adding a concatenation operator (maybe even "+" as used for
      addition above), or by adding string interpolation to the string
      literal syntax.

3.2.  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.

3.3.  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 5.1 then raises the question of how to indicate the regular
   expression flavor.

4.  Embedded ANBF

   It would often be desirable to define a text string type by employing
   ABNF [RFC5234] [RFC7405] embedded into the CDDL specification.
   Currently, that ABNF would usually need to be translated into a
   regular expression (if that is even possible).

   Note that some applications of computed literals for strings could be
   covered by such a feature (or partially vice versa).




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5.  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.

5.1.  Control operator .pcre

   There are many variants of regular expression languages.
   Section 3.8.3 of [I-D.ietf-cbor-cddl] 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.)

5.2.  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.)

5.3.  .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).

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

7.  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).

8.  IANA Considerations

   This document makes no requests of IANA.






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9.  Security considerations

   The security considerations of [I-D.ietf-cbor-cddl] apply.

10.  References

10.1.  Normative References

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-ietf-cbor-cddl-03
              (work in progress), July 2018.

10.2.  Informative References

   [I-D.handrews-relative-json-pointer]
              Luff, G. and H. Andrews, "Relative JSON Pointers", draft-
              handrews-relative-json-pointer-01 (work in progress),
              January 2018.

   [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>.

   [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>.

   [XSD2]     Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium Recommendation
              REC-xmlschema-2-20041028, October 2004,
              <http://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




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   the proposals discussed here.  Sean Leonard has campaigned for a
   regexp syntax.

Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org






































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