| Internet-Draft | CDDL 2.0 | October 2022 |
| Bormann | Expires 22 April 2023 | [Page] |
The Concise Data Definition Language (CDDL) today is defined by RFC 8610 and RFC 9165. The latter (as well as some more application specific specifications such as RFC 9090) have used the extension point provided in RFC 8610, the control operator.¶
As CDDL is used in larger projects, feature requirements 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 provides a roadmap towards a "CDDL 2.0". It is based on draft-bormann-cbor-cddl-freezer, but is more selective in what potential features it takes up and more detailed in their discussion. It is intended to serve as a basis for prototypical implementations of CDDL 2.0. What specific documents spawn from the present one or whether this document is evolved into a single CDDL 2.0 specification.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-bormann-cbor-cddl-2-draft/.¶
Discussion of this document takes place on the cbor 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/.¶
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This Internet-Draft will expire on 22 April 2023.¶
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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(Please see abstract.)¶
Note that the existing extension point can be exercised for new features in parallel to the work described here.¶
The CDDL 1.0 syntax for expressing tags in CDDL is (ABNF as in [RFC5234]):¶
type2 /= "#" "6" ["." uint] "(" S type S ")"
¶
This means tag numbers can only be given as literal numbers (uints). Some specifications operate on ranges of tag numbers, e.g., [RFC9277] has a range of tag numbers 1668546817 (0x63740101) to 1668612095 (0x6374FFFF) to tag specific content formats. This can currently not be expressed in CDDL.¶
CDDL 2.0 extends this to¶
type2 /= "#" "6" ["." tag-number] "(" S type S ")"
tag-number = uint / ("<" type ">")
¶
So the above range can be expressed in a CDDL fragment such as:¶
ct-tag<content> = #6.<ct-tag-number>(content) ct-tag-number = 1668546817..1668612095 ; or use 0x63740101..0x6374FFFF¶
Note that reuses the angle bracket syntax for generics; this reuse is
innocuous as a generic parameter/argument only ever occurs after a
rule name (id), while it occurs after . here.
(Whether there is potential for human confusion can be debated; the
above example deliberately uses generics as well.)¶
Some CBOR tags often would be most natural to use in a CDDL spec with a literal syntax that is tailored to their semantics instead of their serialization in CBOR. There is currently no way to add such syntaxes, no defined extension point either.¶
The proposal "Application-Oriented Literals in CBOR Extended Diagnostic Notation" [I-D.bormann-cbor-edn-literals] defines application-oriented literals, e.g., of the form¶
dt'2019-07-21T19:53Z'¶
for datetime items. With additional considerations for unambiguous syntax, a similar literal form could be included in CDDL.¶
This proposal opens a name space for the prefix that indicates an
application specific literal.
A registry could be provided to make this name space a genuine
extension point.
(This is currently the production bsqual in Appendix B of [RFC8610].)¶
The syntax provided in [I-D.bormann-cbor-edn-literals] does not enable the use of CDDL types -- it has the same flaw that is being fixed for tag numbers in Section 2.1.¶
A number of errata reports have been made around some details of text string and byte string literal syntax: [Err6527] and [Err6543]. These need to be addressed by re-examining the details of these literal syntaxes. Also, [Err6526] needs to be applied (missing backslashes in text explaining backslash escaping).¶
The ABNF used in [RFC8610] for the content of text string literals is rather permissive:¶
text = %x22 *SCHAR %x22 SCHAR = %x20-21 / %x23-5B / %x5D-7E / %x80-10FFFD / SESC SESC = "\" (%x20-7E / %x80-10FFFD)¶
This allows almost any non-C0 character to be escaped by a backslash,
but critically misses out on the \uXXXX and \uHHHH\uLLLL forms
that JSON allows to specify characters in hex. Both can be solved by
updating the SESC production to:¶
SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\
%x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
(%x75 hexchar) ) ; \u
hexchar = non-surrogate / (high-surrogate "\" %x75 low-surrogate)
non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) /
("D" %x30-37 2HEXDIG )
high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG
low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG
¶
Now that SESC is more restrictively formulated, this also requires an update to the BCHAR production used in the ABNF syntax for byte string literals:¶
bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF bsqual = "h" / "b64"¶
The updated version explicit allows \', which is no longer allowed
in the updated SESC:¶
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF¶
The ABNF used in [RFC8610] for the content of byte string literals lumps together byte strings notated as text with byte strings notated in base16 (hex) or base64 (but see also updated BCHAR production above):¶
bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF¶
Errata report 6543 proposes to handle the two cases in separate productions (where, with an updated SESC, BCHAR obviously needs to be updated as above):¶
bytes = %x27 *BCHAR %x27
/ bsqual %x27 *QCHAR %x27
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
QCHAR = DIGIT / ALPHA / "+" / "/" / "-" / "_" / "=" / WS
¶
This potentially causes a subtle change, which is hidden in the WS production:¶
WS = SP / NL SP = %x20 NL = COMMENT / CRLF COMMENT = ";" *PCHAR CRLF PCHAR = %x20-7E / %x80-10FFFD CRLF = %x0A / %x0D.0A¶
This allows any non-C0 character in a comment, so this fragment becomes possible:¶
foo = h' 43424F52 ; 'CBOR' 0A ; LF, but don't use CR! '¶
The current text is not unambiguously saying whether the three apostrophes
need to be escaped with a \ or not, as in:¶
foo = h' 43424F52 ; \'CBOR\' 0A ; LF, but don\'t use CR! '¶
... which would be supported by the existing ABNF in [RFC8610].¶
The basic (implicit) processing model for CDDL 1.0 applies a CDDL data model to a data item and returns a Boolean that indicates whether the data item matches that model ("validation").¶
Section 4 of [RFC9165] extends this model with named "features". A validation can indicate which features were used. Validation could also be parameterized with information about what features are allowed to be used, enabling variants (see Section 4 of [RFC9165] and [useful] for examples).¶
The cddl tool (Appendix F of [RFC8610]) also supports experimental
forms of "annotating" a validated data item with information about
which rules were used to support validation, currently entirely based on the
information that is in a standard CDDL 1.0 data model.
This leads to a more general concept of "annotation", where the data
model specification supports "annotating" the validated instance by
optionally supplying information in the model.
(The annotated result is a special case of a "post-schema validation
instance" [PSVI], here one where the data item itself is only
augmented, not changed, by the process.)¶
Annotations could in turn provide input to further validation steps, as is often done with Schematron validation in Relax-NG; with an appropriate evaluation language this can be used for checking co-occurrence constraints (Section 5 of [I-D.draft-bormann-cbor-cddl-freezer]).¶
Finally, annotations are a first step to transformation, i.e., describing how a validated data item should be interpreted as a transformed data item by performing certain computations. This generally requires even more support from an evaluation language, simple transformations such as adding in default values may not need much support though.¶
At this time, existing experimental implementations do not lead to a clear choice for what processing model enhancements should be in CDDL 2.0. This document proposes to continue the experimentation and document good approaches.¶
collection of rough ideas with examples¶
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 1.0 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.¶
The knowledge which CDDL snippets need to be concatenated in order to obtain the desired data model lives entirely outside the CDDL snippets in CDDL 1.0. In CDDL 2.0, rules could be packaged as modules and referenced from other modules.¶
There could be some control of namespace pollution, as well as unambiguous referencing into evolving specifications ("versioning") and selection of alternatives (as was emulated with snippets in Section 11 of [RFC8428], although an alternative approach for expressing variants is demonstrated in [useful] based on Section 4 of [RFC9165]).¶
One approach to achieve the module structure that is friendly to existing environments that operate with CDDL 1.0 snippets and CDDL 1.0 implementations is to add a super-syntax (similar to the way pragmas are often added to a language), e.g., by carrying them in what is parsed as comments in CDDL 1.0.¶
each module to be valid CDDL (if missing some rule definitions to be imported).¶
A convention for mapping CDDL-internal names to external ones could be developed, possibly steered by some pragma-like constructs. External names would likely be URI-based, with some conventions as they are used in RDF or Curies. Internal names might look similar to XML QNames. Note that the identifier character set for CDDL deliberately includes $ and @, which could be used in such a convention.¶
Often, a CDDL specification needs to import from specifications in a different language or platform.¶
In many cases, CDDL specifications make use of values that are
specified in IANA registries. The .iana control operator can be
used to reference such a set of values.¶
The reference needs to be able to point to a draft, the registry of which has not been established yet, as well as to an established IANA registry.¶
An example of such a usage might be:¶
cose-algorithm = int .iana ["cose", "algorithms", "value"]¶
Unfortunately, the vocabulary employed in IANA registries has not been designed for machine references. In this case, the potential values would come from applying the XPath expression¶
//iana:registry[@id='algorithms']/iana:record/iana:value¶
to https://www.iana.org/assignments/cose/cose.xml, plus some
filtering on the records returned that only leaves actual allocations.
Additional functionality may be needed for filtering with respect to other
columns of the registry record, e.g., <capabilities> in the case of
this example.¶
This section shows some examples that illustrate potential syntaxes and semantics to be examined.¶
One of the potential objectives here is to keep documents that make use of this extension generally valid as CDDL 1.0 documents, albeit possibly with a need to add further CDDL 1.0 rules to obtain a complete specification.¶
Implicit namespacing might be provided by using a document reference as a namespace tag:¶
RFC8610.int ⬌ int RFC9090.oid ⬌ oid¶
Note that this example establishes a namespace for the prelude
(RFC8610.int); maybe it is worth to do that more explicitly.¶
New syntax for explicitly interacting with namespaces might be but
into RFC 8610 comments, with a specific prefix (and possibly starting
left-aligned). Prefixes proposed include ;;< and ;#; the below
will use ;# even though that probably could pose too many conflicts;
it also might be too inconspicuous.¶
;# export oid, roid, pen as RFC9090 oid = #6.111(bstr) roid = #6.110(bstr) pen = #6.112(bstr)¶
Besides an implicit import such as¶
; unadorned, just import? a = [RFC9090.oid]¶
there also could be an explicit import syntax:¶
;# import oid from RFC9090 a = [oid]¶
Such an explicit syntax might also be able to provide additional parameters such as in the IANA examples above.¶
A convention for establishing RFC references might be easy to establish, but at least Internet-Draft references and IANA registry references should also supported. It is probably worth to add some indirection here, as names of Internet-Drafts might change (including by becoming RFCs).¶
;# include draft-ietf-cbor-time-tag-02.txt as time-tag event-start = time-tag.etime¶
Existing RFCs with CDDL in them could presume an export ...all... as RFCnnnn
(Possibly also per-section exports as in RFC8610.D for the prelude?)¶
Namespace tags for those exports need to be reserved so they cannot be occupied by explicit exporting.¶
New specifications (including RFCs) can "include"/"import" from these namespaces, and maybe "export" their own rules in a more considered way.¶
How to find the document that exports a namespace (IANA? Use by other SDOs? Internal use in an org? How to transition between these states?)¶
Multiple documents exporting into one namespace (Immutable RFC9090 namespace vs. "OID"-namespace? Who manages mutable namespaces?)¶
Updates, revisions, versions, semver:¶
;# insert OID ~> 2.2 ; twiddle-wakka: this version or higher¶
Many of the constructs defined here for CDDL also could be used with ABNF specifications. ABNF would definitely benefit from a standard way to import snippets from existing RFCs. Since CDDL contains ABNF support (Section 3 of [RFC9165]), it would be natural to make some of the functionality discussed in this section available for ABNF as well.¶
(Insert new registry for application specific literals here.)¶