Internet-Draft CDDL control operators February 2024
Bormann Expires 29 August 2024 [Page]
Network Working Group
Intended Status:
Standards Track
C. Bormann
Universität Bremen TZI

More Control Operators for CDDL


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 both in 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, JSON, Printf-style formatting), operations on text, and deterministic encoding.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at Status information for this document may be found at

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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 both in an application-specific [RFC9090] and a more general [RFC9165] way.

The present document defines a number of additional generally applicable control operators:

Table 1: New control operators in this document
Name Purpose
.b64u, .b64c Base64 representation of byte strings
.b64u-sloppy, .b64c-sloppy (sloppy-tolerant variants of the above)
.hex, .hexlc, .hexuc Base16 representation of byte strings
.b32, .h32 Base32 representation of byte strings
.b45 Base45 representation of byte strings
.decimal Text representation of integer numbers
.printf Printf-formatted text representation of data items
.json Text representation of JSON values
.join Building text from array of components
.cbordet, .cborseqdet deterministically encoded CBOR data items, CBOR sequences

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".

2. Text Conversion

2.1. Byte Strings: Base16 (Hex), Base32, Base45, Base64

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 is complicated by the large number of transformations in use. Inspired by Section 8 of RFC 8949 [STD94], we use representations defined in [RFC4648] with the following names:

Table 2: Control Operators for Text Conversion of byte strings
name meaning reference
.b64u Base64URL, no padding Section 5 of [RFC4648]
.b64u-sloppy Base64URL, no padding, sloppy Section 5 of [RFC4648]
.b64c Base64 classic, padding Section 4 of [RFC4648]
.b64c-sloppy Base64 classic, padding, sloppy Section 4 of [RFC4648]
.b32 Base32, no padding Section 6 of [RFC4648]
.h32 Base32/hex alphabet, no padding Section 7 of [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]

Note that this specification is somewhat opinionated here: It does not provide base64url, base32 or base32hex 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.

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 behind table 1 in Section 4 of [RFC4648]. Note that the present specification is opinionated again in not specifying a sloppy variant of base32 or base32/hex, as no legacy use of sloppy base32(/hex) was known at the time of writing. Base45 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 at least strongly specified not to allow sloppy forms of encoding).

2.2. Numbers

Table 3: Control Operator for Text Conversion of Integers
name meaning reference
.decimal Decimal Integer ---

The control operator .decimal allows the modeling of text strings that carry numeric information in decimal form, such as in the uint64/int64 formats of YANG-JSON [RFC7951].

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

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

2.3. Printf-style Formatting

Table 4: Control Operator for Printf-formatting of data item(s)
name meaning reference
.printf Printf-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 of [C]).

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 is given the format string and the data items following it in the array.

From the printf specification in the C language, 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 for example in:

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 into text strings, the JSON value for which can easily be defined in CDDL. This is supported by a control operator similar to .cbor in Section 3.8.4 of [RFC8610].

Table 5: Control Operator for Text Conversion of JSON values
name meaning reference
.json JSON [STD90]
embedded-claims = text .json claims
claims = {iss: text, exp: text}

Note that a .jsonseq is not provided, as no use case is known yet. There is no way to constrain the use of blank space in data items to be validated; variants (e.g, not providing for any blank space) could be defined.

3. Text Processing

3.1. Join

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

Table 6: Control Operator for Text Generation from Arrays
name meaning reference
.join concatenate elements of an array ---

In general, this control operator is hard to validate as it would require full parser functionality. It is therefore recommended to only use it in simple cases, and leave full parsing to ABNF (see Section 3 of [RFC9165]) or similar.

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

4. Deterministic Encoding

[RFC8610] and [RFC8742] specify the control operators .cbor and .cborseq to indicate that the value of a byte string should be an encoded CBOR data item or a CBOR sequence.

This specification provides complementary control operators .cbordet and .cborseqdet that indicate that these data items/sequences need to be encoded in accordance to Sections 4.2.1 and 4.2.2 of RFC 8949 [STD94].

Table 7: Control Operator for Deterministically Encoded Data Items and Sequences
name meaning reference
.cbordet deterministically encoded CBOR data item [RFC8610]
.cborseqdet CBOR sequence made from deterministically encoded CBOR data items [RFC8742]

Note that considerations of deterministic representation at the application level can often be expressed in the CDDL definition of the right-hand side and then do not need additional control operators.

5. IANA Considerations

RFC Editor: please replace RFC-XXXX with the RFC number of this RFC and remove this note.

This document requests IANA to register the contents of Table 8 into the registry "CDDL Control Operators" of [IANA.cddl]:

Table 8: New control operators to be registered
Name Reference
.b64u [RFC-XXXX]
.b64u-sloppy [RFC-XXXX]
.b64c [RFC-XXXX]
.b64c-sloppy [RFC-XXXX]
.b45 [RFC-XXXX]
.b32 [RFC-XXXX]
.h32 [RFC-XXXX]
.hex [RFC-XXXX]
.hexlc [RFC-XXXX]
.hexuc [RFC-XXXX]
.decimal [RFC-XXXX]
.printf [RFC-XXXX]
.json [RFC-XXXX]
.join [RFC-XXXX]
.cbordet [RFC-XXXX]
.cborseqdet [RFC-XXXX]

6. Implementation Status

This section is to be removed before publishing as an RFC.

In the CDDL tool described in Appendix F of [RFC8610], the control operators defined in the present revision of this specification are implemented as of version 0.10.4.

7. Security considerations

The security considerations of [RFC8610] apply.

8. References

8.1. Normative References

International Organization for Standardization, "Information technology — Programming languages — C", Fourth Edition, ISO/IEC 9899:2018, , <>. Technically equivalent specification text is available at
IANA, "Concise Data Definition Language (CDDL)", <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
Bormann, C., "Concise Binary Object Representation (CBOR) Sequences", RFC 8742, DOI 10.17487/RFC8742, , <>.
Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, , <>.
Fältström, P., Ljunggren, F., and D.W. van Gulik, "The Base45 Data Encoding", RFC 9285, DOI 10.17487/RFC9285, , <>.
Bormann, C. and T. Bray, "I-Regexp: An Interoperable Regular Expression Format", RFC 9485, DOI 10.17487/RFC9485, , <>.
Internet Standard 90, <>.
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, , <>.
Internet Standard 94, <>.
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, , <>.

8.2. Informative References

Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, , <>.
Bormann, C., "Concise Binary Object Representation (CBOR) Tags for Object Identifiers", RFC 9090, DOI 10.17487/RFC9090, , <>.


Henk Birkholz suggested the need for many of the control operators defined here.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen