GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc4234

Network Working Group D. Crocker, Ed. Request for Comments: 4234 Brandenburg InternetWorking Obsoletes: 2234 P. Overell Category: Standards Track THUS plc.

                                                          October 2005
           Augmented BNF for Syntax Specifications: ABNF

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 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.

Crocker & Overell Standards Track [Page 1] RFC 4234 ABNF October 2005

Table of Contents

 1. INTRODUCTION ....................................................2
 2. RULE DEFINITION .................................................3
    2.1. Rule Naming ................................................3
    2.2. Rule Form ..................................................3
    2.3. Terminal Values ............................................4
    2.4. External Encodings .........................................5
 3. OPERATORS .......................................................6
    3.1. Concatenation:  Rule1 Rule2 ................................6
    3.2. Alternatives:  Rule1 / Rule2 ...............................6
    3.3. Incremental Alternatives: Rule1 =/ Rule2 ...................7
    3.4. Value Range Alternatives:  %c##-## .........................7
    3.5. Sequence Group:  (Rule1 Rule2) .............................8
    3.6. Variable Repetition:  *Rule ................................8
    3.7. Specific Repetition:  nRule ................................9
    3.8. Optional Sequence:  [RULE] .................................9
    3.9. Comment:  ; Comment ........................................9
    3.10. Operator Precedence .......................................9
 4. ABNF DEFINITION OF ABNF ........................................10
 5. SECURITY CONSIDERATIONS ........................................11
 6. References .....................................................11
    6.1. Normative References ......................................11
    6.2. Informative References ....................................11
 Appendix A.  ACKNOWLEDGEMENTS .....................................13
 Appendix B.  APPENDIX - CORE ABNF OF ABNF .........................13
    B.1.  Core Rules ...............................................13
    B.2.  Common Encoding ..........................................14

1. INTRODUCTION

 Internet technical specifications often need to define a formal
 syntax and are free to employ whatever notation their authors deem
 useful.  Over the years, a modified version of Backus-Naur Form
 (BNF), called Augmented BNF (ABNF), has been popular among many
 Internet specifications.  It balances compactness and simplicity,
 with reasonable representational power.  In the early days of the
 Arpanet, each specification contained its own definition of ABNF.
 This included the email specifications, [RFC733] and then [RFC822],
 which came to be the common citations for defining ABNF.  The current
 document separates those definitions to permit selective reference.
 Predictably, it also provides some modifications and enhancements.
 The differences between standard BNF and ABNF involve naming rules,
 repetition, alternatives, order-independence, and value ranges.
 Appendix B supplies rule definitions and encoding for a core lexical
 analyzer of the type common to several Internet specifications.  It
 is provided as a convenience and is otherwise separate from the meta

Crocker & Overell Standards Track [Page 2] RFC 4234 ABNF October 2005

 language defined in the body of this document, and separate from its
 formal status.
 Changes since [RFC2234]:
    In Section 3.7, the phrase: "That is, exactly <N> occurrences of
    <element>." was corrected to: "That is, exactly <n> occurrences of
    <element>."
    Some continuation comment lines needed to be corrected to begin
    with comment character (";").

2. RULE DEFINITION

2.1. Rule Naming

 The name of a rule is simply the name itself; that is, a sequence of
 characters, beginning with an alphabetic character, and followed by a
 combination of alphabetics, digits, and hyphens (dashes).
 NOTE:
    Rule names are case-insensitive
 The names <rulename>, <Rulename>, <RULENAME>, and <rUlENamE> all
 refer to the same rule.
 Unlike original BNF, angle brackets ("<", ">") are not required.
 However, angle brackets may be used around a rule name whenever their
 presence facilitates in discerning the use of a rule name.  This is
 typically restricted to rule name references in free-form prose, or
 to distinguish partial rules that combine into a string not separated
 by white space, such as shown in the discussion about repetition,
 below.

2.2. Rule Form

 A rule is defined by the following sequence:
       name =  elements crlf
 where <name> is the name of the rule, <elements> is one or more rule
 names or terminal specifications, and <crlf> is the end-of-line
 indicator (carriage return followed by line feed).  The equal sign
 separates the name from the definition of the rule.  The elements
 form a sequence of one or more rule names and/or value definitions,
 combined according to the various operators defined in this document,
 such as alternative and repetition.

Crocker & Overell Standards Track [Page 3] RFC 4234 ABNF October 2005

 For visual ease, rule definitions are left aligned.  When a rule
 requires multiple lines, the continuation lines are indented.  The
 left alignment and indentation are relative to the first lines of the
 ABNF rules and need not match the left margin of the document.

2.3. Terminal Values

 Rules resolve into a string of terminal values, sometimes called
 characters.  In ABNF, a character is merely a non-negative integer.
 In certain contexts, a specific mapping (encoding) of values into a
 character set (such as ASCII) will be specified.
 Terminals are specified by one or more numeric characters, with the
 base interpretation of those characters indicated explicitly.  The
 following bases are currently defined:
       b           =  binary
       d           =  decimal
       x           =  hexadecimal
 Hence:
       CR          =  %d13
       CR          =  %x0D
 respectively specify the decimal and hexadecimal representation of
 [US-ASCII] for carriage return.
 A concatenated string of such values is specified compactly, using a
 period (".") to indicate a separation of characters within that
 value.  Hence:
       CRLF        =  %d13.10
 ABNF permits the specification of literal text strings directly,
 enclosed in quotation-marks.  Hence:
       command     =  "command string"
 Literal text strings are interpreted as a concatenated set of
 printable characters.

Crocker & Overell Standards Track [Page 4] RFC 4234 ABNF October 2005

 NOTE:
    ABNF strings are case-insensitive and the character set for these
    strings is us-ascii.
 Hence:
       rulename = "abc"
 and:
       rulename = "aBc"
 will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and
 "ABC".
    To specify a rule that IS case SENSITIVE, specify the characters
    individually.
 For example:
       rulename    =  %d97 %d98 %d99
 or
       rulename    =  %d97.98.99
 will match only the string that comprises only the lowercased
 characters, abc.

2.4. External Encodings

 External representations of terminal value characters will vary
 according to constraints in the storage or transmission environment.
 Hence, the same ABNF-based grammar may have multiple external
 encodings, such as one for a 7-bit US-ASCII environment, another for
 a binary octet environment, and still a different one when 16-bit
 Unicode is used.  Encoding details are beyond the scope of ABNF,
 although Appendix A (Core) provides definitions for a 7-bit US-ASCII
 environment as has been common to much of the Internet.
 By separating external encoding from the syntax, it is intended that
 alternate encoding environments can be used for the same syntax.

Crocker & Overell Standards Track [Page 5] RFC 4234 ABNF October 2005

3. OPERATORS

3.1. Concatenation: Rule1 Rule2

 A rule can define a simple, ordered string of values (i.e., a
 concatenation of contiguous characters) by listing a sequence of rule
 names.  For example:
       foo         =  %x61           ; a
       bar         =  %x62           ; b
       mumble      =  foo bar foo
 So that the rule <mumble> matches the lowercase string "aba".
 LINEAR WHITE SPACE: Concatenation is at the core of the ABNF parsing
 model.  A string of contiguous characters (values) is parsed
 according to the rules defined in ABNF.  For Internet specifications,
 there is some history of permitting linear white space (space and
 horizontal tab) to be freely and implicitly interspersed around major
 constructs, such as delimiting special characters or atomic strings.
 NOTE:
    This specification for ABNF does not provide for implicit
    specification of linear white space.
 Any grammar that wishes to permit linear white space around
 delimiters or string segments must specify it explicitly.  It is
 often useful to provide for such white space in "core" rules that are
 then used variously among higher-level rules.  The "core" rules might
 be formed into a lexical analyzer or simply be part of the main
 ruleset.

3.2. Alternatives: Rule1 / Rule2

 Elements separated by a forward slash ("/") are alternatives.
 Therefore,
       foo / bar
 will accept <foo> or <bar>.

Crocker & Overell Standards Track [Page 6] RFC 4234 ABNF October 2005

 NOTE:
    A quoted string containing alphabetic characters is a special form
    for specifying alternative characters and is interpreted as a
    non-terminal representing the set of combinatorial strings with
    the contained characters, in the specified order but with any
    mixture of upper and lower case.

3.3. Incremental Alternatives: Rule1 =/ Rule2

 It is sometimes convenient to specify a list of alternatives in
 fragments.  That is, an initial rule may match one or more
 alternatives, with later rule definitions adding to the set of
 alternatives.  This is particularly useful for otherwise, independent
 specifications that derive from the same parent rule set, such as
 often occurs with parameter lists.  ABNF permits this incremental
 definition through the construct:
       oldrule     =/ additional-alternatives
 So that the rule set
       ruleset     =  alt1 / alt2
       ruleset     =/ alt3
       ruleset     =/ alt4 / alt5
 is the same as specifying
       ruleset     =  alt1 / alt2 / alt3 / alt4 / alt5

3.4. Value Range Alternatives: %c##-##

 A range of alternative numeric values can be specified compactly,
 using dash ("-") to indicate the range of alternative values.  Hence:
       DIGIT       =  %x30-39
 is equivalent to:
       DIGIT       =  "0" / "1" / "2" / "3" / "4" / "5" / "6" /
                      "7" / "8" / "9"
 Concatenated numeric values and numeric value ranges cannot be
 specified in the same string.  A numeric value may use the dotted
 notation for concatenation or it may use the dash notation to specify

Crocker & Overell Standards Track [Page 7] RFC 4234 ABNF October 2005

 one value range.  Hence, to specify one printable character between
 end of line sequences, the specification could be:
       char-line = %x0D.0A %x20-7E %x0D.0A

3.5. Sequence Group: (Rule1 Rule2)

 Elements enclosed in parentheses are treated as a single element,
 whose contents are STRICTLY ORDERED.  Thus,
       elem (foo / bar) blat
 matches (elem foo blat) or (elem bar blat), and
       elem foo / bar blat
 matches (elem foo) or (bar blat).
 NOTE:
    It is strongly advised that grouping notation be used, rather than
    relying on the proper reading of "bare" alternations, when
    alternatives consist of multiple rule names or literals.
 Hence, it is recommended that the following form be used:
      (elem foo) / (bar blat)
 It will avoid misinterpretation by casual readers.
 The sequence group notation is also used within free text to set off
 an element sequence from the prose.

3.6. Variable Repetition: *Rule

 The operator "*" preceding an element indicates repetition.  The full
 form is:
       <a>*<b>element
 where <a> and <b> are optional decimal values, indicating at least
 <a> and at most <b> occurrences of the element.
 Default values are 0 and infinity so that *<element> allows any
 number, including zero; 1*<element> requires at least one;
 3*3<element> allows exactly 3 and 1*2<element> allows one or two.

Crocker & Overell Standards Track [Page 8] RFC 4234 ABNF October 2005

3.7. Specific Repetition: nRule

 A rule of the form:
       <n>element
 is equivalent to
       <n>*<n>element
 That is, exactly <n> occurrences of <element>.  Thus, 2DIGIT is a 2-
 digit number, and 3ALPHA is a string of three alphabetic characters.

3.8. Optional Sequence: [RULE]

 Square brackets enclose an optional element sequence:
       [foo bar]
 is equivalent to
  • 1(foo bar).

3.9. Comment: ; Comment

 A semi-colon starts a comment that continues to the end of line.
 This is a simple way of including useful notes in parallel with the
 specifications.

3.10. Operator Precedence

 The various mechanisms described above have the following precedence,
 from highest (binding tightest) at the top, to lowest (loosest) at
 the bottom:
       Strings, Names formation
       Comment
       Value range
       Repetition
       Grouping, Optional
       Concatenation
       Alternative

Crocker & Overell Standards Track [Page 9] RFC 4234 ABNF October 2005

 Use of the alternative operator, freely mixed with concatenations,
 can be confusing.
    Again, it is recommended that the grouping operator be used to
    make explicit concatenation groups.

4. ABNF DEFINITION OF ABNF

 NOTES:
    1. This syntax requires a formatting of rules that is relatively
       strict.  Hence, the version of a ruleset included in a
       specification might need preprocessing to ensure that it can be
       interpreted by an ABNF parser.
    2. This syntax uses the rules provided in Appendix B (Core).
       rulelist       =  1*( rule / (*c-wsp c-nl) )
       rule           =  rulename defined-as elements c-nl
                              ; continues if next line starts
                              ;  with white space
       rulename       =  ALPHA *(ALPHA / DIGIT / "-")
       defined-as     =  *c-wsp ("=" / "=/") *c-wsp
                              ; basic rules definition and
                              ;  incremental alternatives
       elements       =  alternation *c-wsp
       c-wsp          =  WSP / (c-nl WSP)
       c-nl           =  comment / CRLF
                              ; comment or newline
       comment        =  ";" *(WSP / VCHAR) CRLF
       alternation    =  concatenation
                         *(*c-wsp "/" *c-wsp concatenation)
       concatenation  =  repetition *(1*c-wsp repetition)
       repetition     =  [repeat] element
       repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)

Crocker & Overell Standards Track [Page 10] RFC 4234 ABNF October 2005

       element        =  rulename / group / option /
                         char-val / num-val / prose-val
       group          =  "(" *c-wsp alternation *c-wsp ")"
       option         =  "[" *c-wsp alternation *c-wsp "]"
       char-val       =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
                              ; quoted string of SP and VCHAR
                              ;  without DQUOTE
       num-val        =  "%" (bin-val / dec-val / hex-val)
       bin-val        =  "b" 1*BIT
                         [ 1*("." 1*BIT) / ("-" 1*BIT) ]
                              ; series of concatenated bit values
                              ;  or single ONEOF range
       dec-val        =  "d" 1*DIGIT
                         [ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]
       hex-val        =  "x" 1*HEXDIG
                         [ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ]
       prose-val      =  "<" *(%x20-3D / %x3F-7E) ">"
                              ; bracketed string of SP and VCHAR
                              ;  without angles
                              ; prose description, to be used as
                              ;  last resort

5. SECURITY CONSIDERATIONS

 Security is truly believed to be irrelevant to this document.

6. References

6.1. Normative References

 [US-ASCII] American National Standards Institute, "Coded Character
            Set -- 7-bit American Standard Code for Information
            Interchange", ANSI X3.4, 1986.

6.2. Informative References

 [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234, November 1997.

Crocker & Overell Standards Track [Page 11] RFC 4234 ABNF October 2005

 [RFC733]   Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
            "Standard for the format of ARPA network text messages",
            RFC 733, November 1977.
 [RFC822]   Crocker, D., "Standard for the format of ARPA Internet
            text messages", STD 11, RFC 822, August 1982.

Crocker & Overell Standards Track [Page 12] RFC 4234 ABNF October 2005

Appendix A. ACKNOWLEDGEMENTS

 The syntax for ABNF was originally specified in RFC 733.  Ken L.
 Harrenstien, of SRI International, was responsible for re-coding the
 BNF into an augmented BNF that makes the representation smaller and
 easier to understand.
 This recent project began as a simple effort to cull out the portion
 of RFC 822 that has been repeatedly cited by non-email specification
 writers, namely the description of augmented BNF.  Rather than simply
 and blindly converting the existing text into a separate document,
 the working group chose to give careful consideration to the
 deficiencies, as well as benefits, of the existing specification and
 related specifications made available over the last 15 years, and
 therefore to pursue enhancement.  This turned the project into
 something rather more ambitious than was first intended.
 Interestingly, the result is not massively different from that
 original, although decisions, such as removing the list notation,
 came as a surprise.
 This "separated" version of the specification was part of the DRUMS
 working group, with significant contributions from Jerome Abela,
 Harald Alvestrand, Robert Elz, Roger Fajman, Aviva Garrett, Tom
 Harsch, Dan Kohn, Bill McQuillan, Keith Moore, Chris Newman, Pete
 Resnick, and Henning Schulzrinne.
 Julian Reschke warrants a special thanks for converting the Draft
 Standard version to XML source form.

Appendix B. APPENDIX - CORE ABNF OF ABNF

 This Appendix is provided as a convenient core for specific grammars.
 The definitions may be used as a core set of rules.

B.1. 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

Crocker & Overell Standards Track [Page 13] RFC 4234 ABNF October 2005

       CR             =  %x0D
                              ; carriage return
       CRLF           =  CR LF
                              ; Internet standard newline
       CTL            =  %x00-1F / %x7F
                              ; controls
       DIGIT          =  %x30-39
                              ; 0-9
       DQUOTE         =  %x22
                              ; " (Double Quote)
       HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
       HTAB           =  %x09
                              ; horizontal tab
       LF             =  %x0A
                              ; linefeed
       LWSP           =  *(WSP / CRLF WSP)
                              ; linear white space (past newline)
       OCTET          =  %x00-FF
                              ; 8 bits of data
       SP             =  %x20
       VCHAR          =  %x21-7E
                              ; visible (printing) characters
       WSP            =  SP / HTAB
                              ; white space

B.2. Common Encoding

 Externally, data are represented as "network virtual ASCII" (namely,
 7-bit US-ASCII in an 8-bit field), with the high (8th) bit set to
 zero.  A string of values is in "network byte order", in which the
 higher-valued bytes are represented on the left-hand side and are
 sent over the network first.

Crocker & Overell Standards Track [Page 14] RFC 4234 ABNF October 2005

Authors' Addresses

 Dave Crocker (editor)
 Brandenburg InternetWorking
 675 Spruce Dr.
 Sunnyvale, CA  94086
 US
 Phone: +1.408.246.8253
 EMail: dcrocker@bbiw.net
 Paul Overell
 THUS plc.
 1/2 Berkeley Square
 99 Berkeley Street
 Glasgow
 G3 7HR
 UK
 EMail: paul.overell@thus.net

Crocker & Overell Standards Track [Page 15] RFC 4234 ABNF October 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Crocker & Overell Standards Track [Page 16]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4234.txt · Last modified: 2005/10/03 18:29 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki