Network Working Group S. Leonard
Internet-Draft Penango, Inc.
Updates: 5234 (if approved) September 20, 2016
Intended Status: Standards Track
Expires: March 24, 2017
Comprehensive Core Rules and References for ABNF
draft-seantek-abnf-more-core-rules-06
Abstract
This document extends the base definition of ABNF (Augmented Backus-
Naur Form) to include comprehensive support for certain symbols
related to ASCII, and defines a reference syntax.
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
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at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on March 24, 2017.
Copyright Notice
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document authors. All rights reserved.
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1. Introduction
Augmented Backus-Naur Form (ABNF) [RFC5234] is a formal syntax that
is popular among many Internet specifications. Many Internet
documents employ this syntax along with the Core Rules defined in
Appendix B.1 of [RFC5234]. However, the Core Rules do not specify
many symbols in the ASCII range that are also needed by these relying
documents, forcing document authors to define them as local rules.
Sometimes different documents define these common symbols in
different ways, resulting in confusion or incompatibility when the
rules are misread or are combined with other sets of rules.
Furthermore, [RFC5234] does not clarify whether referencing [RFC5234]
for ABNF automatically defines its Core Rules.
[RFC5234] also lacks a syntax for referring to rules from other
specifications. Instead, authors have been required to name the rules
and sources in the specification prose. While this method has served
authors well, it has hampered machine-readable ABNF efforts for
services such as syntax highlighting, automatic grammar checking, and
compiling into target computer languages.
2. Comprehensive Core Rule Update
This document provides Core Rules that include comprehensive support
for certain symbols, namely DELETE (DEL) and the C0 controls in
[ASCII86], which for purposes of this document is equivalent to
[RFC0020].
3. Reference Syntax
To reference a rule in another ABNF grammar, use the syntax
rulename@REF. The referenced rule resolves to terminal values in the
context of the referenced ABNF grammar, essentially replacing the
verbiage: "{RULE} is taken from [{Section X} of] {[RFCXXXX]}" in
prose specification text that introduces the ABNF. The following
enhancement to [RFC5234] permits this referenced-rule syntax as a
change to the <element> production:
element = rulename [ "@" ruleref ] / group / option
char-val / num-val / prose-val
ruleref = ref-doc / ref-path
ref-doc = "[" 1*(%x20-5A / %x5C / %x5E-7E / ref-doc) "]"
ref-path = "<" 1*(%x20-3B / "<" / "=" / ">" / %x3F-7E) ">"
In a referenced-rule production the <rulename> production preceding
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the "@" specifies the name of the rule in the reference containing
ABNF. The <ruleref> production following the "@" specifies the
reference containing the rule. This specification does not define the
semantics if a rule is found in a grammar that is not ABNF. (This
limitation is because rule names in ABNF are case-insensitive and
drawn from a limited character repertoire. Some rule names in other
BNFs may be unreachable or ambiguous, even though the productions
named by the rules are linguistically compatible.)
The <ref-doc> production is a document reference of a resource
containing ABNF. The term "document reference" refers to "the
document containing this ABNF (i.e., the instance of these production
rules)". In IETF-related publications, ref-doc conveniently is of the
same form as document references, such as "[RFC1605]". [[NB: in this
draft:]] Square brackets "[" and "]" are allowed to be inside ref-
doc, as long as the brackets are balanced.
The <ref-path> production is a path to a resource containing ABNF.
The ABNF might be in a text file or MIME entity, for example. The
intent of this production is to accommodate file paths and Uniform
Resource Identifiers [RFC3986] (including fragment identifier
components), but this specification imposes no requirement to
validate conformance to those syntaxes. If the characters "<" or ">"
are present in the path, they are syntactically distinguishable from
the ref-path terminators by being escaped with a preceding backslash.
The assumption is that ref-doc rather than ref-path productions will
be used in published standards documents.
Stylistically, authors are encouraged to put reference syntax at the
top of a list of rules, and to limit usage of the reference syntax to
the single element of a rule definition. For example:
You = Edward@[FFIV]
spoony = spoony@[FFIV]
bard = bard@[FF-JOB-CLASS]
chara = Tellah@[FFIV]
insult = chara ":" You spoony bard "!"
Formally, this document updates [RFC5234] but does not modify it in
situ. Authors need to reference this document if they want to include
these enhancements; bare references to [RFC5234] do not include this
specification (or, for that matter, [RFC7405]). This directive
follows a model whereby document authors can choose whether to invoke
particular enhancements to ABNF. As time goes on, the IETF can
determine how often these enhancements are invoked, and can decide
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whether to include them as part of a revision to the base [RFC5234].
A bare reference to this document invokes the reference syntax
enhancement and the Core Rules of Appendix A (i.e., the Core Rules do
not have to use reference syntax). Nevertheless, document authors are
free to qualify a reference to this document to invoke each feature
selectively.
Appendix A of this document is meant to mirror Appendix B.1 of
[RFC5234]; therefore, concurrently referencing Appendix B.1 of
[RFC5234] is superfluous. Document authors who reference this
document should use the rules of Appendix A, and should not attempt
to redefine or provide incremental alternatives to them (except for
backwards compatibility with prior documents).
6. IANA Considerations This document implies no IANA considerations.
7. Security Considerations
While the Core Rules themselves may not be security-relevant, the use
of such control characters could very well be security-relevant,
because they may trigger special functions on various devices, while
being invisible in other contexts.
Unfortunately security is relevant to the reference syntax in this
document. Using the reference syntax facilitates automated processing
of ABNF. A malicious source could supply different ABNF as an attack
vector on a compiled program. Furthermore, referring to a mutable
resource (e.g., a document series such as BCP) permits the resource
to change its contained ABNF, which may be well-intentioned but have
side-effects when combined with the referring ABNF. Authors should
stick to persistent, durable references, whose integrity can be
validated easily.
8. Acknowledgements
The author wishes to thank Paul Kyzivat and Chris Newman for ongoing
discussion and comments during the development of this draft.
9. References
9.1. Normative References
[ASCII86] American National Standards Institute, "Coded Character Set
-- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
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[RFC0020] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
9.2. Informative References
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
9.0.0", The Unicode Consortium, August 2016.
[RFC1345] Simonsen, K., "Character Mnemonics and Character Sets",
RFC 1345, June 1992.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
Appendix A. Comprehensive Core Rules
Certain basic rules are in uppercase, such as SP, HTAB, CRLF, DIGIT,
ALPHA, etc.
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
BIT = "0" / "1"
CHAR = %x01-7F
; any 7-bit US-ASCII character,
; excluding NUL
CR = %x0D
; carriage return
CRLF = CR LF
; Internet standard newline
CTL = %x00-1F / %x7F
; controls
DIGIT = %x30-39
; 0-9
DQUOTE = %x22
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; " (Double Quote)
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
HTAB = %x09
; horizontal tab
LF = %x0A
; linefeed
LWSP = *(WSP / CRLF WSP)
; Use of this linear-white-space rule
; permits lines containing only white
; space that are no longer legal in
; mail headers and have caused
; interoperability problems in other
; contexts.
; Do not use when defining mail
; headers and use with caution in
; other contexts.
OCTET = %x00-FF
; 8 bits of data
SP = %x20
VCHAR = %x21-7E
; visible (printing) characters
WSP = SP / HTAB
; white space
NUL = %d0
SOH = %d1
STX = %d2
ETX = %d3
EOT = %d4
ENQ = %d5
ACK = %d6
BEL = %d7
BS = %d8
HT = %d9 ; also defined as HTAB
VT = %d11
FF = %d12 ; (literally used in every RFC)
SO = %d14
SI = %d15
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DLE = %d16
DC1 = %d17
DC2 = %d18
DC3 = %d19
DC4 = %d20
NAK = %d21
SYN = %d22
ETB = %d23
CAN = %d24
EM = %d25
SUB = %d26
ESC = %d27
FS = %d28
GS = %d29
RS = %d30
US = %d31
DEL = %d127
ASCII = %x00-7F
C0 = %x00-1F
G0 = VCHAR ; 94-set
Appendix B. Guidance for Automated Referenced Rule Conversion
ABNF is a formal notation for describing the syntax of languages used
in (Internet-connected) computing. Emphasis is therefore placed on
human interpretation of ABNF grammars in the context of prose
specifications, over formal computer languages that require machine
tools to interpret. Nevertheless, as a formal syntactic metalanguage,
tools can interpret ABNF grammars and validate conformance of
grammars to ABNF as well as conformance of language instances to
ABNF-defined grammars. This informative appendix provides guidance on
how an automated tool might convert between referenced rules and
terminal values.
[[TODO: Discuss and put content here.]]
Assume the existence of an "ABNF extractor", a tool that takes as
input a document, and provides as output a stream of ABNF conforming
to the <rulelist> production of ABNF.
Extract the document reference from the <refrule>.
Match the document reference to a reference in the References section
of an RFC or conforming Internet-Draft.
Parse the reference for an identifier that can be dereferenced, e.g.,
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a file path or URI.
Dereference the identifier.
Use the ABNF extractor to extract ABNF from the dereferenced
document.
Identify the <rulename> that matches the <rulename> from the
<refrule>.
If the ABNF in the dereferenced document is resolved to terminal
values, it is resolved in its own context, not in the context of the
original <refrule>'s ABNF.
Author's Address
Sean Leonard
Penango, Inc.
5900 Wilshire Boulevard
21st Floor
Los Angeles, CA 90036
USA
EMail: dev+ietf@seantek.com
URI: http://www.penango.com/
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