| RFC 9741 | CDDL: More Control Operators for Text | March 2025 |
| Bormann | Standards Track | [Page] |
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.¶
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 (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 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 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.¶
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, .b64c-sloppy
|
text | bytes | Sloppy-tolerant variants of the above |
.hex, .hexlc, .hexuc
|
text | bytes | Base16 representation of 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 bytes | array | Build text or byte string from array of components |
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.¶
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, 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 with "Extended 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 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).¶
| Name | Meaning | Reference |
|---|---|---|
.base10
|
Base-ten (decimal) integer | --- |
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.¶
| Name | Meaning | Reference |
|---|---|---|
.printf
|
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".¶
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] |
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.¶
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 | --- |
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
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.¶
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 |
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.¶
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¶
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.¶