Skip to main content

Updates to the CDDL grammar of RFC 8610
draft-bormann-cbor-update-8610-grammar-00

Document Type Active Internet-Draft (individual)
Author Carsten Bormann
Last updated 2023-03-09
RFC stream (None)
Intended RFC status (None)
Formats
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-bormann-cbor-update-8610-grammar-00
CBOR                                                          C. Bormann
Internet-Draft                                    Universit├Ąt Bremen TZI
Updates: 8610 (if approved)                                 9 March 2023
Intended status: Standards Track                                        
Expires: 10 September 2023

                Updates to the CDDL grammar of RFC 8610
               draft-bormann-cbor-update-8610-grammar-00

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 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

   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/update-8610-grammar/.  Status information for this
   document may be found at https://datatracker.ietf.org/doc/draft-
   bormann-cbor-update-8610-grammar/.

   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/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cbor-wg/update-8610-grammar.

Bormann                 Expires 10 September 2023               [Page 1]
Internet-Draft            CDDL grammar updates                March 2023

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 10 September 2023.

Copyright Notice

   Copyright (c) 2023 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Clarifications and Changes based on Errata Reports  . . . . .   4
     2.1.  Err6527 (text string literals)  . . . . . . . . . . . . .   4
     2.2.  Err6543 (byte string literals)  . . . . . . . . . . . . .   5
       Change proposed by Errata Report 6543 . . . . . . . . . . . .   5
       No change needed after Section 2.1  . . . . . . . . . . . . .   6
   3.  Small Enabling Grammar Changes  . . . . . . . . . . . . . . .   7
     3.1.  Empty data models . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Non-literal Tag Numbers . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9

Bormann                 Expires 10 September 2023               [Page 2]
Internet-Draft            CDDL grammar updates                March 2023

   Appendix A.  Updated Collected ABNF for CDDL  . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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 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 [I-D.bormann-cbor-cddl-2-draft].  Proposals for other
   // additions to the CDDL specification base are in
   // [I-D.draft-bormann-cbor-cddl-freezer].

   Note that the existing extension point "control operator"
   (Section 3.8 of [RFC8610]) can be exercised for new features in
   parallel to the work described here; one set of such proposals is in
   [I-D.bormann-cbor-cddl-more-control].

   The present document, in conjunction with [RFC8610] as well as
   [RFC9165] and [I-D.bormann-cbor-cddl-more-control], is intended to be
   the specification base of what has colloquially been called CDDL 1.1.
   Additional documents describe further work on CDDL.

1.1.  Conventions and Definitions

   The Terminology from [RFC8610] applies.  The grammar in [RFC8610] is
   based on ABNF, which is defined in [RFC5234] and [RFC7405].

Bormann                 Expires 10 September 2023               [Page 3]
Internet-Draft            CDDL grammar updates                March 2023

   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.  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: [Err6527] and [Err6543].
   These are being addressed in this section, updating details of the
   ABNF for these literal syntaxes.  Also, [Err6526] needs to be applied
   (backslashes have been lost during RFC processing in some text
   explaining backslash escaping).

2.1.  Err6527 (text string literals)

   The ABNF used in [RFC8610] for the content of text string literals is
   rather permissive:

   ; RFC 8610 ABNF:
   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 (which should be applying
   here according to Bullet 6 of Section 3.1 of [RFC8610]).  Both can be
   solved by updating the SESC production to:

   ; new rules collectively defining 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

           Figure 1: 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.)

Bormann                 Expires 10 September 2023               [Page 4]
Internet-Draft            CDDL grammar updates                March 2023

   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:

   ; RFC 8610 ABNF:
   bytes = [bsqual] %x27 *BCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
   bsqual = "h" / "b64"

   In BCHAR, the updated version explicitly allows \', which is no
   longer allowed in the updated SESC:

   ; new rule for BCHAR:
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF

                      Figure 2: Updated rule for BCHAR

2.2.  Err6543 (byte string literals)

   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):

   ; RFC 8610 ABNF:
   bytes = [bsqual] %x27 *BCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF

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):

   ; Err6543 proposal:
   bytes = %x27 *BCHAR %x27
         / bsqual %x27 *QCHAR %x27
   BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
   QCHAR = DIGIT / ALPHA / "+" / "/" / "-" / "_" / "=" / WS

    Figure 3: Errata Report 8653 Proposal to Split the Byte String Rules

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

Bormann                 Expires 10 September 2023               [Page 5]
Internet-Draft            CDDL grammar updates                March 2023

   ; RFC 8610 ABNF:
   WS = SP / NL
   SP = %x20
   NL = COMMENT / CRLF
   COMMENT = ";" *PCHAR CRLF
   PCHAR = %x20-7E / %x80-10FFFD
   CRLF = %x0A / %x0D.0A

                  Figure 4: Definition of WS from RFC 8610

   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].

No change needed after Section 2.1

   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 Figure 1
   and Figure 2.

   The rules in Figure 4 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 Section 3 of [RFC9165].  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 Appendix A.1 of
   [I-D.bormann-cbor-cddl-2-draft].

Bormann                 Expires 10 September 2023               [Page 6]
Internet-Draft            CDDL grammar updates                March 2023

3.  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.

3.1.  Empty data models

   _Compatibility_:  backward (not forward)

   [RFC8610] requires a CDDL file to have at least one rule.

   ; RFC 8610 ABNF:
   cddl = S 1*(rule S)

   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:

   ; new top-level rule:
   cddl = S *(rule S)

   and make the existence of at least one rule a semantic constraint, to
   be fulfilled after processing of all directives.

3.2.  Non-literal Tag Numbers

   _Compatibility_:  backward (not forward)

   The CDDL 1.0 syntax for expressing tags in CDDL is (ABNF as in
   [RFC5234]):

   ; extracted from RFC 8610 ABNF:
   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.

Bormann                 Expires 10 September 2023               [Page 7]
Internet-Draft            CDDL grammar updates                March 2023

   CDDL 2.0 extends this to:

   ; new rules collectively defining the tagged case:
   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 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.)

4.  Security Considerations

   The grammar fixes and updates in this document are not believed to
   create additional security considerations.  The security
   considerations in Section 5 of [RFC8610] 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 Section 2.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

Bormann                 Expires 10 September 2023               [Page 8]
Internet-Draft            CDDL grammar updates                March 2023

   [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,
              June 2019, <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, December 2021,
              <https://www.rfc-editor.org/rfc/rfc9165>.

6.2.  Informative References

   [Err6526]  "Errata Report 6526", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6526>.

   [Err6527]  "Errata Report 6527", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6527>.

   [Err6543]  "Errata Report 6543", RFC 8610,
              <https://www.rfc-editor.org/errata/eid6543>.

   [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-01, 5
              March 2023, <https://datatracker.ietf.org/doc/html/draft-
              bormann-cbor-cddl-2-draft-01>.

   [I-D.bormann-cbor-cddl-more-control]
              Bormann, C., "More Control Operators for CDDL", Work in
              Progress, Internet-Draft, draft-bormann-cbor-cddl-more-
              control-00, 25 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              cddl-more-control-00>.

   [I-D.draft-bormann-cbor-cddl-freezer]
              Bormann, C., "A feature freezer for the Concise Data
              Definition Language (CDDL)", Work in Progress, Internet-
              Draft, draft-bormann-cbor-cddl-freezer-10, 24 October
              2022, <https://datatracker.ietf.org/doc/html/draft-
              bormann-cbor-cddl-freezer-10>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

Bormann                 Expires 10 September 2023               [Page 9]
Internet-Draft            CDDL grammar updates                March 2023

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/rfc/rfc7405>.

   [RFC9277]  Richardson, M. and C. Bormann, "On Stable Storage for
              Items in Concise Binary Object Representation (CBOR)",
              RFC 9277, DOI 10.17487/RFC9277, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9277>.

Appendix A.  Updated Collected ABNF for CDDL

   This appendix provides the full ABNF from [RFC8610] with the updates
   applied in the present document.

   cddl = S *(rule S)
   rule = typename [genericparm] S assignt S type
        / groupname [genericparm] S assigng S grpent

   typename = id
   groupname = id

   assignt = "=" / "/="
   assigng = "=" / "//="

   genericparm = "<" S id S *("," S id S ) ">"
   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 = "..." / ".."

Bormann                 Expires 10 September 2023              [Page 10]
Internet-Draft            CDDL grammar updates                March 2023

   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 )

Bormann                 Expires 10 September 2023              [Page 11]
Internet-Draft            CDDL grammar updates                March 2023

   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

                     Figure 5: ABNF for CDDL as updated

Acknowledgments

   TODO acknowledge.

Author's Address

   Carsten Bormann
   Universit├Ąt Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

Bormann                 Expires 10 September 2023              [Page 12]