CDDL control operators: Concise Data Definition Language (CDDL): Additional Control Operators for the Conversion and Processing of Text

Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 9741                        Universität Bremen TZI
Category: Standards Track                                     March 2025
ISSN: 2070-1721


 Concise Data Definition Language (CDDL): Additional Control Operators
               for the Conversion and Processing of Text

Abstract

   The Concise Data Definition Language (CDDL), standardized in RFC
   8610, provides "control operators" as its main language extension
   point.  RFCs have added to this extension point in both an
   application-specific and a more general way.

   The present document defines a number of additional generally
   applicable control operators for text conversion (bytes, integers,
   printf-style formatting, and JSON) and for an operation on text.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9741.

Copyright Notice

   Copyright (c) 2025 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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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Text Conversion
     2.1.  Byte Strings: Base 16 (Hex), Base 32, Base 45, and Base 64
     2.2.  Numerals
     2.3.  Printf-Style Formatting
     2.4.  JSON Values
   3.  Text Processing
     3.1.  Join
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   List of Figures
   List of Tables
   Acknowledgements
   Author's Address

1.  Introduction

   The Concise Data Definition Language (CDDL), standardized in
   [RFC8610], provides "control operators" as its main language
   extension point (Section 3.8 of [RFC8610]).  RFCs have added to this
   extension point in both an application-specific [RFC9090] and a more
   general [RFC9165] way.

   The present document defines a number of additional generally
   applicable control operators.  In Table 1, the column marked t is for
   "target type" (left-hand side), and the column marked c is for
   "controller type" (right-hand side).

    +===============+=========+=======+==============================+
    | Name          | t       | c     | Purpose                      |
    +===============+=========+=======+==============================+
    | .b64u, .b64c  | text    | bytes | Base64 representation of     |
    |               |         |       | byte strings                 |
    +---------------+---------+-------+------------------------------+
    | .b64u-sloppy, | text    | bytes | Sloppy-tolerant variants of  |
    | .b64c-sloppy  |         |       | the above                    |
    +---------------+---------+-------+------------------------------+
    | .hex, .hexlc, | text    | bytes | Base16 representation of     |
    | .hexuc        |         |       | byte strings                 |
    +---------------+---------+-------+------------------------------+
    | .b32, .h32    | text    | bytes | Base32 representation of     |
    |               |         |       | byte strings                 |
    +---------------+---------+-------+------------------------------+
    | .b45          | text    | bytes | Base45 representation of     |
    |               |         |       | byte strings                 |
    +---------------+---------+-------+------------------------------+
    | .base10       | text    | int   | Text representation of       |
    |               |         |       | integer numbers              |
    +---------------+---------+-------+------------------------------+
    | .printf       | text    | array | Printf-formatted text        |
    |               |         |       | representation of data items |
    +---------------+---------+-------+------------------------------+
    | .json         | text    | any   | Text representation of JSON  |
    |               |         |       | values                       |
    +---------------+---------+-------+------------------------------+
    | .join         | text or | array | Build text or byte string    |
    |               | bytes   |       | from array of components     |
    +---------------+---------+-------+------------------------------+

        Table 1: Summary of New Control Operators in This Document

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Regular expressions mentioned in the text are as defined in
   [RFC9485].

   This specification uses terminology from [RFC8610].  In particular,
   with respect to control operators, "target" refers to the left-hand-
   side operand and "controller" to the right-hand-side operand.  "Tool"
   refers to tools along the lines of that described in Appendix F of
   [RFC8610].  Note also that the data model underlying CDDL provides
   for text strings as well as byte strings as two separate types, which
   are then collectively referred to as "strings".

   The term "opinionated" is used in this document to explain that the
   selection of operators included is somewhat frugal, based on opinions
   about what the preferred (and likely) usage scenarios will be.
   Specifically, not including a potential choice doesn't by itself
   intend to express that the choice is unacceptable; it might still be
   added in a future registration if these opinions evolve.

2.  Text Conversion

2.1.  Byte Strings: Base 16 (Hex), Base 32, Base 45, and Base 64

   A CDDL model often defines data that are byte strings in essence but
   need to be transported in various encoded forms, such as base64 or
   hex.  This section defines a number of control operators to model
   these conversions.

   The control operators generally are of a form that could be used like
   this:

   signature-for-json = text .b64u signature
   signature = bytes .cbor COSE_Sign1

   The specification of these control operators cannot provide full
   coverage of the large number of transformations in use; it focuses on
   [RFC4648] and additionally [RFC9285], as shown in Table 2.  For the
   representations defined in [RFC4648], this specification uses names
   as inspired by Section 8 of RFC 8949 [STD94]:

   +==============+===========================+========================+
   | Name         | Meaning                   | Reference              |
   +==============+===========================+========================+
   | .b64u        | Base64url, no padding     | Section 5 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .b64u-sloppy | Base64url, no padding,    | Section 5 of           |
   |              | sloppy                    | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .b64c        | Base64 classic, padding   | Section 4 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .b64c-sloppy | Base64 classic, padding,  | Section 4 of           |
   |              | sloppy                    | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .b32         | Base32, no padding        | Section 6 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .h32         | Base32 with "Extended     | Section 7 of           |
   |              | Hex" alphabet, no padding | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .hex         | Base16 (hex), either case | Section 8 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .hexlc       | Base16 (hex), lower case  | Section 8 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .hexuc       | Base16 (hex), upper case  | Section 8 of           |
   |              |                           | [RFC4648]              |
   +--------------+---------------------------+------------------------+
   | .b45         | Base45                    | [RFC9285]              |
   +--------------+---------------------------+------------------------+

       Table 2: Control Operators for Text Conversion of Byte Strings

   Note that this specification is somewhat opinionated here: It does
   not provide base64url or base32(hex) encoding with padding or base64
   classic without padding.  Experience indicates that these
   combinations only ever occur in error, so the usability of CDDL is
   increased by not providing them in the first place.  Also, adding "c"
   makes sure that any decision for classic base64 is actively taken.

   These control operators are "strict" in their matching, i.e., they
   only match base encodings that conform to the mandates of their
   defining documents.  Note that this also means that .b64u and .b64c
   only match text strings composed of the set of characters defined for
   each of them, respectively.  (This is perhaps worth pointing out
   explicitly as it contrasts with the "b64" literal prefix that can be
   used to notate byte strings in CDDL source code, which simply accepts
   characters from either alphabet.  This behavior is different from the
   matching behavior of the four base64 control operators defined here.)

   The additional designation "sloppy" indicates that the text string is
   not validated for any additional bits being zero, in variance to what
   is specified in the paragraph that follows Table 1 in Section 4 of
   [RFC4648].  Note that the present specification is opinionated again
   in not specifying a sloppy variant of base32 or base32hex, as no
   legacy use of sloppy base32(hex) was known at the time of writing.
   Base45 [RFC9285] is known to be suboptimal for use in environments
   with limited data transparency (such as URLs) but is included because
   of its close relationship to QR codes and its wide use in health
   informatics (note that base45 is strongly specified not to allow
   sloppy forms of encoding).

2.2.  Numerals

           +=========+============================+===========+
           | Name    | Meaning                    | Reference |
           +=========+============================+===========+
           | .base10 | Base-ten (decimal) integer | ---       |
           +---------+----------------------------+-----------+

             Table 3: Control Operator for Text Conversion of
                                 Integers

   The control operator .base10 allows the modeling of text strings that
   carry an integer number in decimal form (as a text string with digits
   in the usual base-ten positional numeral system), such as in the
   uint64/int64 formats of YANG-JSON [RFC7951].

   yang-json-sid = text .base10 (0..9223372036854775807)

   Again, the specification is opinionated by only providing for integer
   numbers represented without leading zeros, i.e., the decimal integer
   numerals match the regular expression 0|-?[1-9][0-9]* (of course,
   this is further restricted by the control type).  See Section 2.3 for
   more flexibility and for other numeric bases such as octal,
   hexadecimal, or binary conversions.

   Note that this control operator governs text representations of
   integers and should not be confused with the control operators
   governing text representations of byte strings (such as .b64u).  This
   contrast is somewhat reinforced by spelling out "base" in the name
   .base10 as opposed to those of the byte string operators.

2.3.  Printf-Style Formatting

     +=========+=========================================+===========+
     | Name    | Meaning                                 | Reference |
     +=========+=========================================+===========+
     | .printf | Printf-style formatting of data item(s) | ---       |
     +---------+-----------------------------------------+-----------+

       Table 4: Control Operator for Printf-Style Formatting of Data
                                  Item(s)

   The control operator .printf allows the modeling of text strings that
   carry various formatted information, as long as the format can be
   represented in printf-style formatting strings as they are used in
   the C language (see Section 7.23.6.1 of [C]; note that the "C23"
   standard includes %b and %B for formatting into binary digits).

   The controller (right-hand side) of the .printf control is an array
   of one printf-style format string and zero or more data items that
   fit the individual conversion specifications in the format string.
   The construct matches a text string representing the textual output
   of an equivalent C-language printf function call that receives as
   arguments the format string and the data items following it in the
   array.

   Out of the functionality described for printf formatting in
   Section 7.23.6.1 of the C language specification [C], length
   modifiers (paragraph 7) are not used and MUST NOT be included in the
   format string.  The "s" conversion specifier (paragraph 8) is used to
   interpolate a text string in UTF-8 form.  The "c" conversion
   specifier (paragraph 8) represents a single Unicode scalar value as a
   UTF-8 character.  The "p" and "n" conversion specifiers (paragraph 8)
   are not used and MUST NOT be included in the format string.

   In the following example, my_alg_19 matches the text string "0x0013":

   my_alg_19 = hexlabel<19>
   hexlabel<K> = text .printf (["0x%04x", K])

   The data items in the controller array do not need to be literals, as
   in the following example:

   any_alg = hexlabel<1..20>
   hexlabel<K> = text .printf (["0x%04x", K])

   Here, any_alg matches the text strings "0x0013" or "0x0001" but not
   "0x1234".

2.4.  JSON Values

   Some applications store complete JSON texts [STD90] into text
   strings.  The JSON value of these can easily be defined in CDDL by
   using the default JSON-to-CBOR conversion rules provided in
   Section 6.2 of RFC 8949 [STD94].  This is supported by a control
   operator similar to .cbor as defined in Section 3.8.4 of [RFC8610].

                      +=======+=========+===========+
                      | Name  | Meaning | Reference |
                      +=======+=========+===========+
                      | .json | JSON    | [STD90]   |
                      +-------+---------+-----------+

                         Table 5: Control Operator
                        for Text Conversion of JSON
                                   Values

   embedded-claims = text .json claims
   claims = {iss: text, exp: text}

   Notes:

   *  JSON has known interoperability problems [RFC7493].  While
      Section 4 of [RFC7493] probably is not relevant to this
      specification, Section 2 of [RFC7493] provides requirements that
      need to be followed to make use of the generic data model
      underlying CDDL.  Note that the intention of Section 2.2 of
      [RFC7493] is directly supported by Section 6.2 of RFC 8949
      [STD94].  The recommendation to use text strings for representing
      numbers outside JSON's interoperable range is a requirement on the
      application data model and therefore needs to be reflected on the
      right-hand side of the .json control operator.

   *  This control operator provides no way to constrain the use of
      blank space or other serialization variants in the JSON
      representation of the data items; restrictions on the
      serialization to specific variants (e.g., not providing for the
      addition of any insignificant blank space and prescribing an order
      in which map entries are serialized) could be defined in future
      control operators.

   *  A .jsonseq is not provided in this document for JSON text
      sequences [RFC7464], as no use case for inclusion in CDDL is known
      at the time of writing; again, future control operators could
      address this use case.

3.  Text Processing

3.1.  Join

   Often, text strings need to be constructed out of parts that can best
   be modeled as an array.

         +=======+==================================+===========+
         | Name  | Meaning                          | Reference |
         +=======+==================================+===========+
         | .join | Concatenate elements of an array | ---       |
         +-------+----------------------------------+-----------+

            Table 6: Control Operator for Text Generation from
                                  Arrays

   For example, an IPv4 address in dotted-decimal might be modeled as in
   Figure 1.

   legacy-ip-address = text .join legacy-ip-address-elements
   legacy-ip-address-elements = [bytetext, ".", bytetext, ".",
                                 bytetext, ".", bytetext]
   bytetext = text .base10 byte
   byte = 0..255

      Figure 1: Using the .join Operator to Build Dotted-Decimal IPv4
                                 Addresses

   The elements of the controller array need to be strings (text or byte
   strings).  The control operator matches a data item if that data item
   is also a string, built by concatenating the strings in the array.
   The result of this concatenation is of the same kind of string (text
   or bytes) as the first element of the array.  (If there is no element
   in the array, the .join construct matches either kind of empty
   string, obviously further constrained by the control operator
   target.)  The concatenation is performed on the sequences of bytes in
   the strings.  If the result of the concatenation is a text string,
   the resulting sequence of bytes only matches the target data item if
   that result is a valid text string (i.e., valid UTF-8).  Note that in
   contrast to the algorithm used in Section 3.2.3 of RFC 8949 [STD94],
   there is no need for all individual byte sequences going into the
   concatenation to constitute valid text strings.

   Note that this control operator is hard to validate in the most
   general case, as this would require full parser functionality.
   Simple implementation strategies will use array elements with
   constant values as guideposts ("markers", such as the "." in
   Figure 1) for isolating the variable elements that need further
   validation at the CDDL data model level.  Therefore, it is
   recommended to limit the use of .join to simple arrangements where
   the array elements are laid out explicitly and there are no adjacent
   variable elements without intervening constant values, and where
   these constant values do not occur within the text described by the
   variable elements.  If more complex parsing functionality is
   required, the ABNF control operators (see Section 3 of [RFC9165]) may
   be useful; however, these cannot reach back into CDDL-specified
   elements like .join can.

      |  Implementation note: A validator implementation can use the
      |  marker elements to scan the text and isolate the variable
      |  elements.  It also can build a parsing regexp from the elements
      |  of the controller array, with capture groups for each element,
      |  and validate the captures against the elements of the array.
      |  (For more about parsing regexps, see Section 6 of [RFC9485];
      |  see also Section 8 of [RFC9485] for security considerations
      |  related to regexps.)  In the most general case, these
      |  implementation strategies can exhibit false negatives, where
      |  the implementation cannot find the structure that would be
      |  successfully validated using the controller; it is RECOMMENDED
      |  that implementations provide full coverage at least for the
      |  marker-based subset outlined in the previous paragraph.

4.  IANA Considerations

   IANA has registered the contents of Table 7 into the "CDDL Control
   Operators" registry of [IANA.cddl]:

                       +==============+===========+
                       | Name         | Reference |
                       +==============+===========+
                       | .b64u        | RFC 9741  |
                       +--------------+-----------+
                       | .b64u-sloppy | RFC 9741  |
                       +--------------+-----------+
                       | .b64c        | RFC 9741  |
                       +--------------+-----------+
                       | .b64c-sloppy | RFC 9741  |
                       +--------------+-----------+
                       | .b45         | RFC 9741  |
                       +--------------+-----------+
                       | .b32         | RFC 9741  |
                       +--------------+-----------+
                       | .h32         | RFC 9741  |
                       +--------------+-----------+
                       | .hex         | RFC 9741  |
                       +--------------+-----------+
                       | .hexlc       | RFC 9741  |
                       +--------------+-----------+
                       | .hexuc       | RFC 9741  |
                       +--------------+-----------+
                       | .base10      | RFC 9741  |
                       +--------------+-----------+
                       | .printf      | RFC 9741  |
                       +--------------+-----------+
                       | .json        | RFC 9741  |
                       +--------------+-----------+
                       | .join        | RFC 9741  |
                       +--------------+-----------+

                           Table 7: New Control
                                Operators

5.  Security Considerations

   The security considerations in Section 5 of [RFC8610] apply.  In
   addition, for the control operators defined in Section 2.1, the
   security considerations in Section 12 of [RFC4648] apply.

6.  References

6.1.  Normative References

   [C]        International Organization for Standardization,
              "Information technology - Programming languages - C",
              Fourth Edition, ISO/IEC 9899:2024, October 2024,
              <https://www.iso.org/standard/82075.html>.  Technically
              equivalent specification text is available at
              <https://www.open-std.org/jtc1/sc22/wg14/www/docs/
              n3220.pdf>.

   [IANA.cddl]
              IANA, "Concise Data Definition Language (CDDL)",
              <https://www.iana.org/assignments/cddl>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [RFC9285]  Fältström, P., Ljunggren, F., and D.W. van Gulik, "The
              Base45 Data Encoding", RFC 9285, DOI 10.17487/RFC9285,
              August 2022, <https://www.rfc-editor.org/info/rfc9285>.

   [RFC9485]  Bormann, C. and T. Bray, "I-Regexp: An Interoperable
              Regular Expression Format", RFC 9485,
              DOI 10.17487/RFC9485, October 2023,
              <https://www.rfc-editor.org/info/rfc9485>.

   [STD90]    Internet Standard 90,
              <https://www.rfc-editor.org/info/std90>.
              At the time of writing, this STD comprises the following:

              Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [STD94]    Internet Standard 94,
              <https://www.rfc-editor.org/info/std94>.
              At the time of writing, this STD comprises the following:

              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

6.2.  Informative References

   [RFC7464]  Williams, N., "JavaScript Object Notation (JSON) Text
              Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,
              <https://www.rfc-editor.org/info/rfc7464>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <https://www.rfc-editor.org/info/rfc7951>.

   [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>.

List of Figures

   Figure 1: Using the .join Operator to Build Dotted-Decimal IPv4
   Addresses

List of Tables

   Table 1: Summary of New Control Operators in This Document

   Table 2: Control Operators for Text Conversion of Byte Strings

   Table 3: Control Operator for Text Conversion of Integers

   Table 4: Control Operator for Printf-Style Formatting of Data Item(s)

   Table 5: Control Operator for Text Conversion of JSON Values

   Table 6: Control Operator for Text Generation from Arrays

   Table 7: New Control Operators

Acknowledgements

   Henk Birkholz suggested the need for many of the control operators
   defined here.  The author would like to thank Laurence Lundblade and
   Jeremy O'Donoghue for sharpening some of the mandates, Mikolai
   Gütschow for improvements to some examples, A.J. Stein for serving as
   shepherd for this document and for his shepherd review, the IESG and
   Directorate reviewers (notably Ari Keränen, Darrel Miller, and Éric
   Vyncke), and Orie Steele for serving as responsible AD and for
   providing a detailed AD review.

Author's Address

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