]> Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

ART CBOR Concise Data Definition Language 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 updates errata and makes other small fixes for the ABNF grammar defined for CDDL in RFC 8610. Previous versions of the changes in this document were part of draft-bormann-cbor-cddl-2-draft and previously draft-bormann-cbor-cddl-freezer. This submission extracts out those grammar changes that are ready for WG adoption and publication. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the CBOR Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

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 updates errata and makes other small fixes for the ABNF grammar defined for CDDL in RFC 8610. Previous versions of the changes in this document were part of draft-bormann-cbor-cddl-2-draft and previously draft-bormann-cbor-cddl-freezer. This submission extracts out those grammar changes that are ready for WG adoption and publication. Proposals for grammar and other changes that need more work can be found in . Proposals for other additions to the CDDL specification base are in . Note that the existing extension point "control operator" () can be exercised for new features in parallel to the work described here; one set of such proposals is in . The present document, in conjunction with as well as and , is intended to be the specification base of what has colloquially been called CDDL 1.1. Additional documents describe further work on CDDL.

Conventions and Definitions The Terminology from applies. The grammar in is based on ABNF, which is defined in and . 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 when, and only when, they appear in all capitals, as shown here.

Clarifications and Changes based on Errata Reports

Compatibility:

errata fix (targets 1.0 and 2.0)

A number of errata reports have been made around some details of text string and byte string literal syntax: and . These are being addressed in this section, updating details of the ABNF for these literal syntaxes. Also, needs to be applied (backslashes have been lost during RFC processing in some text explaining backslash escaping).

Err6527 (text string literals) The ABNF used in for the content of text string literals is rather permissive: 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 (which should be applying here according to Bullet 6 of ). Both can be solved by updating the SESC production to:

Updated string parsing to allow hex escapes

(Notes: In ABNF, strings such as "A", "B" etc. are case-insensitive, as is intended here. We could have written %x62 as %s"b", but didn't, in order to maximize ABNF tool compatibility.) 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: In BCHAR, the updated version explicitly allows \', which is no longer allowed in the updated SESC:

Updated rule for BCHAR

Err6543 (byte string literals) The ABNF used in 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):

Change proposed by Errata Report 6543 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):

Errata Report 8653 Proposal to Split the Byte String Rules

This potentially causes a subtle change, which is hidden in the WS production:

Definition of WS from RFC 8610

This allows any non-C0 character in a comment, so this fragment becomes possible: The current text is not unambiguously saying whether the three apostrophes need to be escaped with a \ or not, as in: ... which would be supported by the existing ABNF in .

No change needed after This document takes the simpler approach of leaving the processing of the content of the byte string literal to a semantic step after processing the syntax of the bytes/BCHAR rules as updated by and . The rules in are therefore applied to the result of this processing where bsqual is given as h or b64. Note that this approach also works well with the use of byte strings in . It does require some care when copy-pasting into CDDL models from ABNF that contains single quotes (which may also hide as apostrophes in comments); these need to be escaped or possibly replaced by %x27. Finally, our approach may leave the door open wider to extending bsqual as proposed in .

Small Enabling Grammar Changes The two subsections in this section specify two small changes to the grammar that are intended to enable certain kinds of specifications.

Empty data models

Compatibility:

backward (not forward)

requires a CDDL file to have at least one rule. This makes sense when the file has to stand alone, as a CDDL data model needs to have at least one rule to provide an entry point (start rule). With CDDL 2.0, CDDL files can also include directives, and these might be the source of all the rules that ultimately make up the module created by the file. Any other rule content in the file has to be available for directive processing, making the requirement for at least one rule cumbersome. Therefore, we extend the grammar as follows: and make the existence of at least one rule a semantic constraint, to be fulfilled after processing of all directives.

Non-literal Tag Numbers

Compatibility:

backward (not forward)

The CDDL 1.0 syntax for expressing tags in CDDL is (ABNF as in ): This means tag numbers can only be given as literal numbers (uints). Some specifications operate on ranges of tag numbers, e.g., 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: ") ]]> So the above range can be expressed in a CDDL fragment such as: = #6.(content) ct-tag-number = 1668546817..1668612095 ; or use 0x63740101..0x6374FFFF ]]> Note that this syntax 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.)

Security Considerations The grammar fixes and updates in this document are not believed to create additional security considerations. The security considerations in do apply, and specifically the potential for confusion is increased in an environment that uses a combination of CDDL tools some of which have been updated and some of which have not been, in particular based on .

IANA Considerations This document has no IANA actions.

References Normative References This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. 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: , , and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document extends the base definition of ABNF (Augmented Backus-Naur Form) to include a way to specify US-ASCII string literals that are matched in a case-sensitive manner. Universität Bremen TZI 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 application control operators for text conversion (Bytes, Integers, JSON) and operations on text. Universität Bremen TZI In defining the Concise Data Definition Language (CDDL), some features have turned up that would be nice to have. In the interest of completing this specification in a timely manner, the present document was started to collect nice-to-have features that did not make it into the first RFC for CDDL, RFC 8610, or the specifications exercising its extension points, such as RFC 9165. Significant parts of this draft have now moved over to the CDDL 2.0 project, described in draft-bormann-cbor-cddl-2-draft. The remaining items in this draft are not directly related to the CDDL 2.0 effort. Universität Bremen TZI 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 document defines a stored ("file") format for Concise Binary Object Representation (CBOR) data items that is friendly to common systems that recognize file types, such as the Unix file(1) command.

Updated Collected ABNF for CDDL This appendix provides the full ABNF from with the updates applied in the present document.

ABNF for CDDL as updated " genericarg = "<" S type1 S *("," S type1 S ) ">" type = type1 *(S "/" S type1) type1 = type2 [S (rangeop / ctlop) S type2] ; space may be needed before the operator if type2 ends in a name type2 = value / typename [genericarg] / "(" S type S ")" / "{" S group S "}" / "[" S group S "]" / "~" S typename [genericarg] / "&" S "(" S group S ")" / "&" S groupname [genericarg] / "#" "6" ["." tag-number] "(" S type S ")" / "#" DIGIT ["." uint] ; major/ai / "#" ; any tag-number = uint / ("<" type ">") rangeop = "..." / ".." ctlop = "." id group = grpchoice *(S "//" S grpchoice) grpchoice = *(grpent optcom) grpent = [occur S] [memberkey S] type / [occur S] groupname [genericarg] ; preempted by above / [occur S] "(" S group S ")" memberkey = type1 S ["^" S] "=>" / bareword S ":" / value S ":" bareword = id optcom = S ["," S] occur = [uint] "*" [uint] / "+" / "?" uint = DIGIT1 *DIGIT / "0x" 1*HEXDIG / "0b" 1*BINDIG / "0" value = number / text / bytes int = ["-"] uint ; This is a float if it has fraction or exponent; int otherwise number = hexfloat / (int ["." fraction] ["e" exponent ]) hexfloat = ["-"] "0x" 1*HEXDIG ["." 1*HEXDIG] "p" exponent fraction = 1*DIGIT exponent = ["+"/"-"] 1*DIGIT text = %x22 *SCHAR %x22 SCHAR = %x20-21 / %x23-5B / %x5D-7E / %x80-10FFFD / SESC SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\ %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t (%x75 hexchar) ) ; \uXXXX 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 bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF bsqual = "h" / "b64" id = EALPHA *(*("-" / ".") (EALPHA / DIGIT)) ALPHA = %x41-5A / %x61-7A EALPHA = ALPHA / "@" / "_" / "$" DIGIT = %x30-39 DIGIT1 = %x31-39 HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" BINDIG = %x30-31 S = *WS WS = SP / NL SP = %x20 NL = COMMENT / CRLF COMMENT = ";" *PCHAR CRLF PCHAR = %x20-7E / %x80-10FFFD CRLF = %x0A / %x0D.0A ]]>

Acknowledgments TODO acknowledge.