Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                          26 December 2021
Expires: 29 June 2022


   A feature freezer for the Concise Data Definition Language (CDDL)
                   draft-bormann-cbor-cddl-freezer-09

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.

   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

   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 29 June 2022.

Copyright Notice

   Copyright (c) 2021 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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 . . . . . . . . . . . . . . .   4
     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 . . . . . . . . . . . . . . . . . . . .   7
   5.  Co-occurrence Constraints . . . . . . . . . . . . . . . . . .   7
   6.  Module superstructure . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Namespacing . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Cross-universe references . . . . . . . . . . . . . . . .   8
       6.2.1.  IANA references . . . . . . . . . . . . . . . . . . .   8
     6.3.  Potential examples  . . . . . . . . . . . . . . . . . . .   9
       6.3.1.  How name spaces might look like . . . . . . . . . . .   9
       6.3.2.  Explicitly interacting with namespaces  . . . . . . .   9
       6.3.3.  Document references . . . . . . . . . . . . . . . . .  10
       6.3.4.  Add retroactive exporting to RFCs . . . . . . . . . .  10
       6.3.5.  Operations  . . . . . . . . . . . . . . . . . . . . .  10
       6.3.6.  To be discussed . . . . . . . . . . . . . . . . . . .  11
   7.  Alternative Representations . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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].



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

   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

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.

   Based on the CoRAL work [I-D.ietf-core-coral], the proposal
   "Application-Oriented Literals in CBOR Extended Diagnostic Notation"
   [I-D.bormann-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.











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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.

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"




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

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

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:







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

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

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.

   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.)



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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:

   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 yet [jsonpath].





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   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).

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.

6.2.  Cross-universe references

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

6.2.1.  IANA references

   In many cases, CDDL specifications make use of values that are
   specified in IANA registries.  The .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

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




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   to https://www.iana.org/assignments/cose/cose.xml, plus some
   filtering on the records returned that only leaves actual
   allocations.  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.

6.3.  Potential examples

   This section shows some examples that illustrate potential syntaxes
   and semantics to be examined.

   One of the potential objectives here is to keep documents that make
   use of this extension generally valid as CDDL 1.0 documents, albeit
   possibly with a need to add further CDDL 1.0 rules to obtain a
   complete specification.

6.3.1.  How name spaces might look like

   Implicit namespacing might be provided by using a document reference
   as a namespace tag:

   RFC8610.int ⬌ int
   RFC9090.oid ⬌ oid

   Note that this example establishes a namespace for the prelude
   (RFC8610.int); maybe it is worth to do that more explicitly.

6.3.2.  Explicitly interacting with namespaces

   New syntax for explicitly interacting with namespaces might be but
   into RFC 8610 comments, with a specific prefix (and possibly starting
   left-aligned).  Prefixes proposed include ;;< and ;#; the below will
   use ;# even though that probably could pose too many conflicts; it
   also might be too inconspicuous.

   ;# export oid, roid, pen as RFC9090
   oid = #6.111(bstr)
   roid = #6.110(bstr)
   pen = #6.112(bstr)

   Besides an implicit import such as

   ; unadorned, just import?
   a = [RFC9090.oid]

   there also could be an explicit import syntax:





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   ;# import oid from RFC9090
   a = [oid]

   Such an explicit syntax might also be able to provide additional
   parameters such as in the IANA examples above.

6.3.3.  Document references

   A convention for establishing RFC references might be easy to
   establish, but at least Internet-Draft references and IANA registry
   references should also supported.  It is probably worth to add some
   indirection here, as names of Internet-Drafts might change (including
   by becoming RFCs).

6.3.4.  Add retroactive exporting to RFCs

   Existing RFCs with CDDL in them could presume an export ...all... as
   RFCnnnn (Possibly also per-section exports as in RFC8610.D for the
   prelude?)

   Namespace tags for those exports need to be reserved so they cannot
   be occupied by explicit exporting.

   New specifications (including RFCs) can "include"/"import" from these
   namespaces, and maybe "export" their own rules in a more considered
   way.

6.3.5.  Operations

   *  "export":

      1.  prefix: add a namespace name to "local" rulenames:

         `oid` ➔ `RFC9090.oid`

      2.  make that namespace available to other specs

   *  "import": include (prefixed) definitions from a source

      1.  use as is: RFC9090.oid

      2.  unprefix: oid

      Example: prelude processing -- include+unprefix from Appendix D of
      RFC8610.

   *  "include": find files, turn into namespaces to import from




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6.3.6.  To be discussed

   How to find the document that exports a namespace (IANA?  Use by
   other SDOs?  Internal use in an org?  How to transition between these
   states?)

   Multiple documents exporting into one namespace (_Immutable_ RFC9090
   namespace vs. "OID"-namespace?  Who manages _mutable_ namespaces?)

   Updates, revisions, versions, semver:

   ;# insert OID ~> 2.2   ; twiddle-wakka: this version or higher

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], which is
   reproduced below.  This could be written up in more detail 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




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10.1.  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>.

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.bormann-cbor-edn-literals]
              Bormann, C., "Application-Oriented Literals in CBOR
              Extended Diagnostic Notation", Work in Progress, Internet-
              Draft, draft-bormann-cbor-edn-literals-00, 6 October 2021,
              <https://www.ietf.org/archive/id/draft-bormann-cbor-edn-
              literals-00.txt>.

   [I-D.draft-bormann-jsonpath-iregexp]
              Bormann, C., "I-Regexp: An Interoperable Regexp Format",
              Work in Progress, Internet-Draft, draft-bormann-jsonpath-
              iregexp-01, 13 November 2021,
              <https://www.ietf.org/archive/id/draft-bormann-jsonpath-
              iregexp-01.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>.




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   [I-D.ietf-core-coral]
              Amsüss, C. and T. Fossati, "The Constrained RESTful
              Application Language (CoRAL)", Work in Progress, Internet-
              Draft, draft-ietf-core-coral-04, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-coral-
              04.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>.

   [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]     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>.







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Internet-Draft            CDDL feature freezer             December 2021


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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