Internet DRAFT - draft-bormann-cbor-update-8610-grammar
draft-bormann-cbor-update-8610-grammar
CBOR C. Bormann
Internet-Draft Universität 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.
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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
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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.
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Please review these documents carefully, as they describe your rights
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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
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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].
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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.)
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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:
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; 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].
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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.
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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>.
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[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>.
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[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 = "..." / ".."
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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 )
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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
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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