Internet-Draft CDDL Module Structure December 2023
Bormann Expires 21 June 2024 [Page]
Workgroup:
CBOR Maintenance and Extensions
Internet-Draft:
draft-ietf-cbor-cddl-modules-01
Published:
Intended Status:
Standards Track
Expires:
Author:
C. Bormann
Universität Bremen TZI

CDDL Module Structure

Abstract

At the time of writing, the Concise Data Definition Language (CDDL) is defined by RFC 8610 and RFC 9165. The latter has used the extension point provided in RFC 8610, the control operator.

As CDDL is being used in larger projects, the need for corrections and additional features has become known that cannot be easily mapped into this single extension point. Hence, there is a need for evolution of the base CDDL specification itself.

The present document defines a backward- and forward-compatible way to add a module structure to CDDL.

About This Document

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

The latest revision of this draft can be found at https://cbor-wg.github.io/cddl-modules/draft-ietf-cbor-cddl-modules.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-cbor-cddl-modules/.

Discussion of this document takes place on the CBOR Maintenance and Extensions Working Group mailing list (mailto:cbor@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at https://www.ietf.org/mailman/listinfo/cbor/.

Source for this draft and an issue tracker can be found at https://github.com/cbor-wg/cddl-modules.

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 21 June 2024.

1. Introduction

At the time of writing, the Concise Data Definition Language (CDDL) is defined by RFC 8610 and RFC 9165. The latter has used the extension point provided in RFC 8610, the control operator.

As CDDL is being used in larger projects, the need for corrections and additional features has become known that cannot be easily mapped into this single extension point. Hence, there is a need for evolution of the base CDDL specification itself.

The present document defines a backward- and forward-compatible way to add a module structure to CDDL.

The present document is intended to be the specification base of what has colloquially been called CDDL 2.0, a term that is now focusing on module structure (other documents make up what is now sometimes called CDDL 1.1). Additional documents describe further work on CDDL.

1.1. Conventions and Definitions

The Terminology from [RFC8610] applies.

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.

2. Module superstructure

Compatibility:

bidirectional (both backward and forward)

Originally, CDDL was used for small data models that could be expressed in a few lines. As the size of data models that need to be expressed in CDDL has increased, the need to modularize and re-use components is increasing.

CDDL as documented in [RFC8610] (basic CDDL) has been designed with a crude form of composition: Concatenating a number of CDDL snippets creates a valid CDDL data model unless there is a name collision (identical redefinition is allowed to facilitate this approach). With larger models, managing the name space to avoid collisions becomes more pressing.

In CDDL's original composition model, the knowledge which CDDL snippets need to be concatenated in order to obtain the desired data model lives entirely outside the CDDL snippets. With the module structure defined in the present document, rules are packaged as modules and referenced from other modules, providing methods for control of namespace pollution.

Further work may be expended on unambiguous referencing into evolving specifications ("versioning") and selection of alternatives (as was emulated with snippets in Section 11 of [RFC8428]. Note that one approach for expressing variants is demonstrated in [useful] based on Section 4 of [RFC9165]). Potential further work is outlined in Sections 4 and A.2 of [I-D.bormann-cbor-cddl-2-draft].

2.1. Compatibility

To achieve the module structure in a way that is friendly to existing environments that operate with CDDL and basic CDDL implementations, we add a super-syntax (similar to the way pragmas are often added to a language), by carrying them in what is parsed as comments in basic CDDL.

This enables each module source file to be basic CDDL on its own (possibly needing to add some rule definitions imported from other source files).

2.2. Namespacing

When importing rules from other modules, there is the potential for name collisions. This is exacerbated when the modules evolve, which may lead to the introduction of a name into an imported module that is also used (likely in a different way) in the importing module.

To be able to manage names in such a way that collisions can be avoided, we introduce means to prepend a prefix to the names of rules being imported: the "as" clause.

2.3. "Directives", the "module"

This specification introduces directives into CDDL. A single CDDL file becomes a module by processing the (zero or more) directives in it.

The semantics of the module are independent of the module(s) using it, however, importing a module may involve transforming its rule names into a new namespace (Section 2.2).

Directives look like comments in basic CDDL, so they do not interfere with forward compatibility.

In the CDDL module structure, lines starting with the prefix ;# are parsed as directives.

2.4. Naming and Finding Modules

We assume that module names are filenames taken from one of several source directories available to the CDDL module structure processor via the environment. This avoids the need to nail down brittle pathnames or (partial?) URIs into the CDDL files.

The exact way how these source directories and possibly a precedence between them are established is intentionally not fully defined in this specification; it is expected that this will be specified in the context of the models just as the way they are intended to be used will be. (A more formal structure may follow later.)

In the module structure implementation that is part of the CDDL Tool described in Appendix B, the set of sources is determined from an environment variable, CDDL_INCLUDE_PATH, which is modeled after usual command-line search paths. It is a colon-separated list of pathnames to directories, with one special feature: an empty element points to the tool's own collection. This collection contains fragments of extracted CDDL from published RFCs, using names such as rfc9052.

(Future versions might augment this with Web extractors and/or ways to extract CDDL modules from github and from Internet-Drafts; see Appendix A.2 of [I-D.bormann-cbor-cddl-2-draft] for some design considerations.)

The default CDDL_INCLUDE_PATH is:

.:

That is: files are found in the current directory (.) and, if not found there, cddlc’s collection.

In the examples following, a cddlc command line will be shown (starting with an isolated $ sign) with the module-structured CDDL input; the resulting basic CDDL will be shown separately.

2.5. Basic Set of Directives

Two groups of directives are defined at this point:

  • include, which includes all the rules from a module (which includes the ones imported/included there, transitively), or specific explicitly selected rules (clause ending in "from");

  • import, which includes only those rules from the module that are referenced, implicitly or explicitly (clause ending in "from"), including the rules that are referenced from these rules, transitively.

The include function is more useful for composing a single model from parts controlled by one author, while the import function is more about treating a module as a library:

The way an import works is shown by this simple example:

$ cddlc -2tcddl -
start = COSE_Key
;# import rfc9052

This results in the following CDDL 1.0 specification:

start = COSE_Key
COSE_Key = {
  1 => tstr / int,
  ? 2 => bstr,
  ? 3 => tstr / int,
  ? 4 => [+ tstr / int],
  ? 5 => bstr,
  * label => values,
}
label = int / tstr
values = any

This is appropriate for using libraries that are well known to the importing specification. However, if it is not acceptable that the library can pollute the namespace of the importing module, the import directive can specify a namespace prefix ("as" clause):

$ cddlc -2tcddl -
start = cose.COSE_Key
;# import rfc9052 as cose

This results in the following CDDL 1.0 specification:

start = cose.COSE_Key
cose.COSE_Key = {
  1 => tstr / int,
  ? 2 => bstr,
  ? 3 => tstr / int,
  ? 4 => [+ tstr / int],
  ? 5 => bstr,
  * cose.label => cose.values,
}
cose.label = int / tstr
cose.values = any

Note how the imported names are prefixed with cose. as specified in the import directive, but CDDL prelude (Appendix D of [RFC8610]) names such as tstr and any are not.

2.6. Explicit selection of names

Both import and include directives can be augmented by an explicit mentioning of rule names (clause ending in "from").

include

Starting with include:

$ cddlc -2tcddl -
mydata = {* label => values}
;# include label, values from rfc9052

With include, only exactly the rules mentioned are included:

mydata = {* label => values}
label = int / tstr
values = any

The module from which rules are explicitly imported can be namespaced:

$ cddlc -2tcddl -
mydata = {* label => values}
;# include cose.label, cose.values from rfc9052 as cose

Again, only exactly the rules mentioned are included:

mydata = {* label => values}
cose.label = int / tstr
cose.values = any

import

Both examples would work exactly the same with import, as the included rules do not reference anything else from the included module.

An import however also draws in the transitive closure of the rules referenced:

$ cddlc -2tcddl -
mydata = {Fritz: cose.empty_or_serialized_map}
;# import cose.empty_or_serialized_map from rfc9052 as cose

The transitive closure of the rules mentioned is included:

mydata = {"Fritz" => cose.empty_or_serialized_map}
cose.empty_or_serialized_map = bstr .cbor cose.header_map / bstr .size 0
cose.header_map = {
  cose.Generic_Headers,
  * cose.label => cose.values,
}
cose.Generic_Headers = (
  ? 1 => int / tstr,
  ? 2 => [+ cose.label],
  ? 3 => tstr / int,
  ? 4 => bstr,
  ? (5 => bstr // 6 => bstr),
  )
cose.label = int / tstr
cose.values = any

The import statement can also request an alias for an imported name:

$ cddlc -2tcddl -
mydata = {Fritz: cose.empty_or_serialized_map}
;# import empty_or_serialized_map from rfc9052 as cose

Note how an additional rule provides an alias for empty_or_serialized_map that does not have the namespace prefix:

mydata = {"Fritz" => cose.empty_or_serialized_map}
empty_or_serialized_map = cose.empty_or_serialized_map
cose.empty_or_serialized_map = bstr .cbor cose.header_map / bstr .size 0
cose.header_map = {
  cose.Generic_Headers,
  * cose.label => cose.values,
}
cose.Generic_Headers = (
  ? 1 => int / tstr,
  ? 2 => [+ cose.label],
  ? 3 => tstr / int,
  ? 4 => bstr,
  ? (5 => bstr // 6 => bstr),
  )
cose.label = int / tstr
cose.values = any

2.7. Tool Support for Command-Line Control

Tools may provide a convenient way to initiate the processing of directives from the command line.

A tool may provide a way to root the module tree from the command line:

$ cddlc -2tcddl -icose=rfc9052 -scose.COSE_Key

The command line argument -icose=rfc9052 is a shortcut for

;# import rfc9052 as cose

Together with the start rule name, cose.COSE_Key, this results in the following CDDL 1.0 specification:

$.start.$ = cose.COSE_Key
cose.COSE_Key = {
  1 => tstr / int,
  ? 2 => bstr,
  ? 3 => tstr / int,
  ? 4 => [+ tstr / int],
  ? 5 => bstr,
  * cose.label => cose.values,
}
cose.label = int / tstr
cose.values = any

In other words, the module synthesized from the command line had an empty CDDL file, which therefore was not provided (no on the command line).

3. Security Considerations

The module structure specified in this document is not believed to create additional security considerations beyond the general security considerations in Section 5 of [RFC8610].

Implementations that employ the module structure defined in this document need to ascertain the provenance of the modules they combine into the CDDL models they employ operationally. This specification does not define how the source directories accessed via the CDDL_INCLUDE_PATH are populated; this process needs to undergo the same care and scrutiny as any other introduction of source code into a build environment.

4. IANA Considerations

This document has no IANA actions.

5. References

5.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC7405]
Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, , <https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC9165]
Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, , <https://www.rfc-editor.org/rfc/rfc9165>.
[STD68]
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, .

5.2. Informative References

[cddlc]
"CDDL conversion utilities", n.d., <https://github.com/cabo/cddlc>.
[I-D.bormann-cbor-cddl-2-draft]
Bormann, C., "CDDL 2.0 -- a draft plan", Work in Progress, Internet-Draft, draft-bormann-cbor-cddl-2-draft-03, , <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-cddl-2-draft-03>.
[RFC8428]
Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, , <https://www.rfc-editor.org/rfc/rfc8428>.
[useful]
"Useful CDDL", n.d., <https://github.com/cbor-wg/cddl/wiki/Useful-CDDL>.

Appendix A. ABNF Specification

This section specifies the grammar employed by the module structure directives using the ABNF language defined in [STD68] and [RFC7405].

directive = ";#" RS (%s"import" / %s"include") RS [from-clause]
                    filename [as-clause] CRLF
from-clause = 1*(id [","] RS) %s"from" RS
as-clause = RS %s"as" RS id
filename = 1*("-" / "." / %x30-39 / %x41-5a / "_" / %x61-7a)
id = ("$" / %x40-5a / "_" / %x61-7a)
     *("$" / %x30-39 / %x40-5a / "_" / %x61-7a)
RS = 1*WS
WS = SP
SP = %x20
CRLF = %x0A / %x0D.0A

Appendix B. A CDDL Tool that Implements CDDL Module Structure

This appendix is for information only.

A rough CDDL tool is available [cddlc]. It can process module-structured CDDL models into basic CDDL models that can then be processed by the original CDDL tool described in Appendix F of [RFC8610].

A typical command line involving both tools might be:

cddlc -2 -tcddl mytestfile.cddl | cddl - gp 10

Install on a system with a modern Ruby (Ruby version ≥ 3.0) via:

gem install cddlc

The present document assumes the use of cddlc of at least version 0.1.8.

Acknowledgments

TODO acknowledge.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany