|CDDL Control Operators
The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point.¶
The present document defines a number of control operators that were
not yet ready at the time RFC 8610 was completed:
.det for the construction of constants;
.abnfb for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and
.feature for indicating the use of a non-basic feature in an instance.¶
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.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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|String concatenation, pre-dedenting
|ABNF in CDDL (text strings)
|ABNF in CDDL (byte strings)
|Indicates name of feature used (extension point)
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.¶
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 "ABNF" in this specification stands for the combination of [RFC5234] and [RFC7405]; i.e., the ABNF control operators defined by this document allow use of the case-sensitive extensions defined in [RFC7405].¶
CDDL as defined in [RFC8610] does not have any mechanisms to compute
literals. To cover a large part of the use cases, this specification adds three control
.plus for numeric addition,
.cat for string
.det for string concatenation with dedenting of
both sides (target and controller).¶
For these operators, as with all control operators, targets and controllers are types. The resulting type is therefore formally a function of the elements of the cross-product of the two types. Not all tools may be able to work with non-unique targets or controllers.¶
In many cases, numbers are needed relative to a
base number in a specification. The
.plus control identifies a number that is
constructed by adding the numeric values of the target and the
The target and controller both MUST be numeric. If the target is a floating point number and the controller an integer number, or vice versa, the sum is converted into the type of the target; converting from a floating point number to an integer selects its floor (the largest integer less than or equal to the floating point number, i.e., rounding towards negative infinity).¶
The example in Figure 1 contains the generic definition of a CDDL
interval that gives a lower and upper bound and, optionally,
The parameter BASE allows the non-conflicting use of a multiple of these
interval groups in one map by assigning different labels to the
entries of the interval.
rect combines two of these interval groups into a map, one group for
the X dimension (using 0, 1, and 2 as labels) and one for the Y dimension
(using 3, 4, and 5 as labels).¶
It is often useful to be able to compose string literals out of component literals defined in different places in the specification.¶
.cat control identifies a string that is built from a
concatenation of the target and the controller.
The target and controller both MUST be strings.
The result of the operation has the same type as the target.
The concatenation is performed on the bytes in both strings.
If the target is a text string, the result of that concatenation MUST
be valid UTF-8.¶
The example in Figure 2
builds a text string named
c from concatenating the target text string
and the controller byte string entered in a text form byte string literal.
(This particular idiom is useful when the text string contains
newlines, which, as shown in the example for
b, may be harder to
read when entered in the format that the pure CDDL text string
notation inherits from JSON.)¶
Multi-line string literals for various applications, including embedded ABNF (Section 3), need to be set flush left, at least partially. Often, having some indentation in the source code for the literal can promote readability, as in Figure 3.¶
The control operator
.det works like
.cat, except that both
arguments (target and controller) are independently dedented before
the concatenation takes place.¶
For the purposes of this specification, we define "dedenting" as:¶
.det is a shortcut for "dedenting cat".
The maybe more obvious name
.dedcat has not been chosen
as it is longer and may invoke unpleasant images.)¶
Occasionally, dedenting of only a single item is needed.
This can be achieved by using this operator with an empty string,
"" .det rhs or
lhs .det "", which can in turn be combined
.cat: in the construct
lhs .cat ("" .det rhs), only
Many IETF protocols define allowable values for their text strings in ABNF [RFC5234] [RFC7405]. It is often desirable to define a text string type in CDDL by employing existing ABNF embedded into the CDDL specification. Without specific ABNF support in CDDL, that ABNF would usually need to be translated into a regular expression (if that is even possible).¶
ABNF is added to CDDL in the same way that regular
expressions were added: by defining a
.abnf control operator.
The target is usually
text or some restriction on it, and the controller
is the text of an ABNF specification.¶
There are several small issues; the solutions are given here:¶
.abnfb for ABNF
denoting byte sequences and
.abnf for denoting sequences of
Unicode scalar values (code points) represented as UTF-8 text strings.
Both control operators can be applied to targets of either string
type; the ABNF is applied to the sequence of bytes in the string
and interprets it as a sequence of bytes (
.abnfb) or as a sequence
of code points represented as an UTF-8 text string (
The controller string MUST be a string. When a byte string, it MUST be valid UTF-8 and is interpreted as the text string that has the same sequence of bytes.¶
.cat operator introduced in
Section 2.2 and the syntax for text in byte strings.
As is customary for ABNF, the syntax of ABNF itself (not the syntax
expressed in ABNF!) is relaxed to allow a single line feed as a
CRLF = %x0A / %x0D.0A¶
operator and/or by
.det if there is indentation to be disposed of.¶
These points are combined into an example in Figure 5, which uses ABNF from [RFC3339] to specify one of each of the Concise Binary Object Representation (CBOR) tags defined in [RFC8943] and [RFC8949].¶
Commonly, the kind of validation enabled by languages such as CDDL provides a Boolean result: valid or invalid.¶
In rapidly evolving environments, this is too simplistic. The data models described by a CDDL specification may continually be enhanced by additional features, and it would be useful even for a specification that does not yet describe a specific future feature to identify the extension point the feature can use and accept such extensions while marking them as extensions.¶
.feature control annotates the target as making use of the
feature named by the controller. The latter will usually be a string.
A tool that validates an instance against that specification may mark
the instance as using a feature that is annotated by the
More specifically, the tool's diagnostic output might contain the controller (right-hand side) as a feature name and the target (left-hand side) as a feature detail. However, in some cases, the target has too much detail, and the specification might want to hint to the tool that more limited detail is appropriate. In this case, the controller should be an array, with the first element being the feature name (that would otherwise be the entire controller) and the second element being the detail (usually another string), as illustrated in Figure 6.¶
Figure 7 shows what could be the definition of a person, with
potential extensions beyond
organization being marked
Extensions that are known at the time this definition is written can be
$$person-extensions. However, future extensions
would be deemed invalid unless the wildcard at the end of the map is
These extensions could then be specifically examined by a user or a
tool that makes use of the validation result; the label (map key)
actually used makes a fine feature detail for the tool's diagnostic
Leaving out the entire extension point would mean that instances that
make use of an extension would be marked as wholesale invalid, making
the entire validation approach much less useful.
Leaving the extension point in but not marking its use as special
would render mistakes (such as using the label "
organisation" instead of
A CDDL tool may simply report the set of features being used; the control then only provides information to the process requesting the validation. One could also imagine a tool that takes arguments, allowing the tool to accept certain features and reject others (enable/disable). The latter approach could, for instance, be used for a JSON/CBOR switch, as illustrated in Figure 9, using Sensor Measurement Lists (SenML) [RFC8428] as the example data model used with both JSON and CBOR.¶
It remains to be seen if the enable/disable approach can lead to new idioms of using CDDL. The language currently has no way to enforce mutually exclusive use of features, as would be needed in this example.¶
While both [RFC5234] and [RFC7405] state that security is truly believed to be irrelevant to the respective document, the use of formal description techniques cannot only simplify but sometimes also complicate a specification. This can lead to security problems in implementations and in the specification itself. As with CDDL itself, ABNF should be judiciously applied, and overly complex (or "cute") constructions should be avoided.¶
Jim Schaad suggested several improvements.
.feature feature was developed out of a discussion with Henk Birkholz.
Paul Kyzivat helped isolate the need for
.det is an abbreviation for "dedenting cat", but Det is also the name of a German TV cartoon character created in the 1960s.¶