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Network Working Group A. Farrel Request for Comments: 5511 Old Dog Consulting Category: Standards Track April 2009

       Routing Backus-Naur Form (RBNF): A Syntax Used to Form
     Encoding Rules in Various Routing Protocol Specifications

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.

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 document authors.  All rights reserved.
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 Provisions Relating to IETF Documents in effect on the date of
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 Contributions published or made publicly available before November
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 than English.

Farrel Standards Track [Page 1] RFC 5511 Routing BNF April 2009

Abstract

 Several protocols have been specified in the Routing Area of the IETF
 using a common variant of the Backus-Naur Form (BNF) of representing
 message syntax.  However, there is no formal definition of this
 version of BNF.
 There is value in using the same variant of BNF for the set of
 protocols that are commonly used together.  This reduces confusion
 and simplifies implementation.
 Updating existing documents to use some other variant of BNF that is
 already formally documented would be a substantial piece of work.
 This document provides a formal definition of the variant of BNF that
 has been used (that we call Routing BNF) and makes it available for
 use by new protocols.

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................3
    1.2. Existing Uses ..............................................3
    1.3. Applicability Statement ....................................4
 2. Formal Definitions ..............................................4
    2.1. Rule Definitions ...........................................5
         2.1.1. Rule Name Delimitation ..............................5
         2.1.2. Objects .............................................5
         2.1.3. Constructs ..........................................6
         2.1.4. Messages ............................................6
    2.2. Operators ..................................................6
         2.2.1. Assignment ..........................................6
         2.2.2. Concatenation .......................................7
         2.2.3. Optional Presence ...................................7
         2.2.4. Alternatives ........................................8
         2.2.5. Repetition ..........................................9
         2.2.6. Grouping ...........................................10
    2.3. Editorial Conventions .....................................11
         2.3.1. White Space ........................................11
         2.3.2. Line Breaks ........................................11
         2.3.3. Ordering ...........................................11
    2.4. Precedence ................................................11
 3. Automated Validation ...........................................13
 4. Security Considerations ........................................13
 5. Acknowledgments ................................................13
 6. References .....................................................13
    6.1. Normative References ......................................13
    6.2. Informative References ....................................13

Farrel Standards Track [Page 2] RFC 5511 Routing BNF April 2009

1. Introduction

 Backus-Naur Form (BNF) has been used to specify the message formats
 of several protocols within the Routing Area of the IETF.
 Unfortunately, these specifications are not based on any specific
 formal definition of BNF, and they differ slightly from the
 definitions provided in other places.
 It is clearly valuable to have a formal definition of the syntax-
 defining language that is used.  It would be possible to convert all
 existing specifications to use an established specification of BNF
 (for example, Augmented BNF or ABNF [RFC5234]); however, this would
 require a lot of work.  It should be noted that in ABNF the terminals
 are integers (characters/bytes), while in the BNF form used to define
 message formats, the terminals are "objects" (some kind of message
 elements, but not individual bytes or characters) or entire
 "messages".  This means that converting existing specifications to
 use an established BNF specification would also require extensions to
 that BNF specification.
 On the other hand, the variant of BNF used by the specifications in
 question (which is similar to a subset of Extended BNF [EBNF]) is
 consistent and has only a small number of constructs.  It makes
 sense, therefore, to provide a definition of this variant of BNF to
 allow ease of interpretation of existing documents and to facilitate
 the development of new protocol specifications using the same variant
 of BNF.  A specification will also facilitate automated verification
 of the formal definitions used in future documents.
 This document provides such a specification and names the BNF variant
 Routing BNF (RBNF).

1.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

1.2. Existing Uses

 The first notable use of the variant of BNF that concerns us is in
 the specification of the Resource Reservation Protocol (RSVP)
 [RFC2205].  RSVP has been extended for use in Multiprotocol Label
 Switching (MPLS) networks to provide signaling for Traffic
 Engineering (TE) [RFC3209], and this has been developed for use as
 the signaling protocol in Generalized MPLS (GMPLS) networks
 [RFC3473].

Farrel Standards Track [Page 3] RFC 5511 Routing BNF April 2009

 Each of these three uses of RSVP has given rise to a large number of
 specifications of protocol extensions to provide additional features
 over and above those in the base documents.  Each new feature is
 defined in its own document using the common variant of BNF.
 New protocols have also been specified using the same variant of BNF.
 This has arisen partly because the developers were familiar with the
 BNF used in [RFC2205], etc., but also because of the overlap between
 the protocols, especially with respect to the network objects
 controlled and operated.
 Notable among these additional protocols are the Link Management
 Protocol (LMP) [RFC4204] and the Path Computation Element Protocol
 (PCEP) [RFC5440].  In both cases, further documents that specify
 protocol extensions also use the same variant of BNF.

1.3. Applicability Statement

 RBNF as defined in this document is primarily applicable for the
 protocols listed in the previous section.  The specification may be
 used to facilitate the interpretation of the pre-existing RFCs that
 are referenced.  It should also be used in the specification of
 extensions to those protocols.
 RBNF could also be used for the specification of new protocols.  This
 is most appropriate for the development of new protocols that are
 closely related to those that already use RBNF.  For example, PCEP is
 closely related to RSVP-TE, and when it was developed, the PCE
 working group chose to use the same form of BNF as was already used
 in the RSVP-TE specifications.
 If a wholly new protocol is being developed and is not related to a
 protocol that already uses RBNF, the working group should consider
 carefully whether to use RBNF or to use a more formally specified and
 broader form of BNF such as ABNF [RFC5234].
 The use of RBNF to specify extensions to protocols that do not
 already use RBNF (i.e., that use some other form of BNF) is not
 recommended.

2. Formal Definitions

 The basic building blocks of BNF are rules and operators.  At its
 simplest form, a rule in the context we are defining is a protocol
 object that is traditionally defined by a bit diagram in the protocol
 specification.  Further and more complex rules are constructed by

Farrel Standards Track [Page 4] RFC 5511 Routing BNF April 2009

 combining other rules using operators.  The most complex rule is the
 message that is constructed from an organization of protocol objects
 as specified by the operators.
 An RBNF specification consists of a sequence of rule definitions
 using the operators defined in Section 2.2.  One rule may be
 constructed from a set of other rules using operators.  The order of
 definition of rules does not matter.  That is, the subordinate rules
 MAY be defined first and then used in subsequent definitions of
 further rules, or the top-level rules MAY be defined first followed
 by a set of definitions of the subordinate rules.
 Rule definitions are read left-to-right on any line, and the lines
 are read top-to-bottom on the page.  This becomes particularly
 important when considering sequences of rules and operators.

2.1. Rule Definitions

 No semantics should be assumed from special characters used in rule
 names.  For example, it would be wrong to assume that a rule carries
 a decimal number because the rule name begins or ends with the letter
 "d".  However, individual specifications MAY choose to assign rule
 names in any way that makes the human interpretation of the rule
 easier.

2.1.1. Rule Name Delimitation

 All rule names are enclosed by angle brackets ("<" and ">").  Rule
 names MAY include any printable characters, but MUST NOT include tabs
 or line feeds/breaks.
 Example:
   <Path Message>

2.1.2. Objects

 The most basic (indivisible) rule is termed an object.  The
 definition of an object is derived from its context.
 Objects are typically named in uppercase.  They do not usually use
 spaces within the name, favoring underbars ("_").
 Example:
   <SENDER_TEMPLATE>

Farrel Standards Track [Page 5] RFC 5511 Routing BNF April 2009

2.1.3. Constructs

 Rules that are constructed from other rules using operators are
 termed constructs.
 Constructs are named in lowercase, although capitals are commonly
 used to indicate acronyms.  Spaces and hyphens are used between words
 within names.
 Example:
   <sender descriptor>

2.1.4. Messages

 The final objective is the definition of messages.  These are rules
 that are constructed from objects and constructs using operators.
 The only syntactic difference between a message and a construct is
 that no other rule is typically constructed from a message.
 Messages are typically named in title case.
 Example:
   <Path Message>

2.2. Operators

 Operators are used to build constructs and messages from objects and
 constructs.

2.2.1. Assignment

 Assignment is used to form constructs and messages.
 Meaning:
   The named construct or message on the left-hand side is defined to
   be set equal to the right-hand side of the assignment.
 Encoding:
   colon, colon, equal sign ("::=")
 Example:
   <WF flow descriptor> ::= <FLOWSPEC>
 Note:
   The left-hand side of the assignment and the assignment operator
   MUST be present on the same line.

Farrel Standards Track [Page 6] RFC 5511 Routing BNF April 2009

2.2.2. Concatenation

 Objects and constructs can be combined as a sequence to form a new
 construct or a message.
 Meaning:
   The objects or constructs MUST be present in the order specified.
   The order of reading RBNF is stated in Section 2.
 Encoding:
   A sequence of objects and constructs usually separated by spaces.
   The objects in a sequence MAY be separated by line breaks.
 Example:
   <SE flow descriptor> ::= <FLOWSPEC> <filter spec list>
 Note:
   See Section 2.3.3 for further comments on the ordering of objects
   and constructs.

2.2.3. Optional Presence

 Objects and constructs can be marked as optionally present.
 Meaning:
   The optional objects or constructs MAY be present or absent within
   the assignment.  Unless indicated as optional, objects and
   constructs are mandatory and MUST be present.  The optional
   operator can also be nested to give a hierarchical dependency of
   presence as shown in the example below.
 Encoding:
   Contained in square brackets ("[" and "]").
 Example:
   <PathTear Message> ::= <Common Header> [ <INTEGRITY> ]
                          <SESSION> <RSVP_HOP>
                          [ <sender descriptor> ]
 Example of nesting:
   The optional operator can be nested.  For example,
     <construct> ::= <MAND> [ <OPT_1> [ <OPT_2> ] ]
   In this construction, the object OPT_2 can only be present if OPT_1
   is also present.

Farrel Standards Track [Page 7] RFC 5511 Routing BNF April 2009

 Note:
   The set of objects and constructs within the same pair of square
   brackets is treated as a unit (an unnamed construct).  This means
   that when multiple objects and constructs are included within the
   same pair of square brackets, all MUST be included when one is
   included, unless nested square brackets are used as in the previous
   example.

2.2.4. Alternatives

 Choices can be indicated within assignments.
 Meaning:
   Either one rule or the other MUST be present.
 Encoding:
   The pipe symbol ("|") is used between the objects or constructs
   that are alternatives.
 Example:
   <flow descriptor list> ::= <FF flow descriptor list>
                              | <SE flow descriptor>
 Notes:
   1. Use of explicit grouping (Section 2.2.6) is RECOMMENDED to avoid
      confusion.  Implicit grouping using line breaks (Section 2.3.2)
      is often used, but gives rise to potential misinterpretation and
      SHOULD be avoided in new definitions.
   2. Multiple members of alternate sets can give rise to confusion.
      For example:
      <flow descriptor list> ::=  <empty> |
                           <flow descriptor list> <flow descriptor>
      could be read to mean that an instance of <flow descriptor> must
      be present or that it is optional.
      To avoid this type of issue, explicit grouping (see Section
      2.2.6), or an intermediary MUST be used in all new documents
      (existing uses are not deprecated, and automatic parsers need to
      handle existing RFCs).  See also Section 2.4 for a description
      of precedence rules.
      Thus:
        <construct> ::= <ALT_A> <ALT_B> | <ALT_C> <ALT_D>

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      is not allowed in new documents and MUST be presented using
      grouping or using an intermediary construct.  For example, and
      depending on intended meaning:
        <construct> ::= ( <ALT_A> <ALT_B> ) | ( <ALT_C> <ALT_D> )
        or
        <construct> ::= <ALT_A> ( <ALT_B> | <ALT_C> ) <ALT_D>
      or
        <intermediary X> ::= <ALT_A> <ALT_B>
        <intermediary Y> ::= <ALT_C> <ALT_D>
        <construct> ::= <intermediary X> | <intermediary Y>
        or
        <intermediary Z> ::= <ALT_B> | <ALT_C>
        <construct> ::= <ALT_A> <intermediary Z> <ALT_D>

2.2.5. Repetition

 It could be the case that a sequence of identical objects or
 constructs is required within an assignment.
 Meaning:
   MAY repeat the preceding object, intermediate construct, or
   construct.
 Encoding:
   Three dots ("...").
 Example:
   <Path Message> ::= <Common Header> [ <INTEGRITY> ]
                      <SESSION> <RSVP_HOP>
                      <TIME_VALUES>
                      [ <POLICY_DATA> ... ]
                      [ <sender descriptor> ]
 Notes:
   1. A set of zero or more objects or constructs can be achieved by
      combining with the Optional concept as shown in the example
      above.
   2. Sequences can also be encoded by building a recursive construct
      using the Alternative operator.  For example:

Farrel Standards Track [Page 9] RFC 5511 Routing BNF April 2009

        <sequence> ::= <OBJECT> |
                       ( <OBJECT> <sequence> )
   3. Repetition can also be applied to a component of an assignment
      to indicate the optional repetition of that component.  For
      example, the Notify message in [RFC3473] is defined as follows:
       <Notify message> ::=
                        <Common Header> [<INTEGRITY>]
                        [ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
                        [ <MESSAGE_ID> ]
                        <ERROR_SPEC> <notify session list>
      In this example, there is a sequence of zero or more instances
      of [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>].  One could argue that
      the use of grouping (see Section 2.2.6) or a recursive construct
      (see Note 2, above) would be more clear.

2.2.6. Grouping

 Meaning:
   A group of objects or constructs to be treated together.  This
   notation is not mandatory but is RECOMMENDED for clarity.  See
   Section 2.4 on Precedence.
 Encoding:
   Round brackets ("(" and ")") enclosing a set of objects,
   constructs, and operators.
 Example:
   <group> ::= ( <this> <that> )
 Notes:
   1. The precedence rule in Section 2.4 means that the use of
      grouping is not necessary for the formal interpretation of the
      BNF representation.  However, grouping can make the BNF easier
      to parse unambiguously.  Either grouping or an intermediate
      construct MUST be used for multi-alternates (Section 2.2.4).
   2. Line breaks (Section 2.3.2) are often used to clarify grouping
      as can be seen in the definition of <sequence> in Section 2.2.5,
      but these are open to misinterpretation, and explicit grouping
      is RECOMMENDED.
   3. A practical alternative to grouping is the definition of
      intermediate constructs as illustrated in Note 2 of Section
      2.2.4.

Farrel Standards Track [Page 10] RFC 5511 Routing BNF April 2009

2.3. Editorial Conventions

2.3.1. White Space

 White space (that is space characters) between operators, objects,
 and constructs is ignored but SHOULD be used for readability.

2.3.2. Line Breaks

 Line breaks within an assignment are ignored but SHOULD be used for
 readability.
 Line breaks are often used to imply grouping within the precedence
 rules set out in Section 2.4, but explicit grouping (Section 2.2.6)
 or intermediary constructs (Section 2.2.4) SHOULD be used in new
 definitions.
 A line break MUST NOT be present between the left-hand side of an
 assignment and the assignment operator (see Section 2.2.1).
 New assignments (i.e., new construct or message definitions) MUST
 begin on a new line.

2.3.3. Ordering

 The ordering of objects and constructs in an assignment is explicit.
 Protocol specifications MAY opt to state that ordering is only
 RECOMMENDED.  In this case, elements of a list of objects and
 constructs MAY be received in any order.

2.4. Precedence

 Precedence is the main opportunity for confusion in the use of this
 BNF.  In particular, the use of alternatives mixed with
 concatenations can give rise to different interpretations of the BNF.
 Although precedence can be deduced from a "proper" reading of the BNF
 using the rules defined above and the precedence ordering shown
 below, authors are strongly RECOMMENDED to use grouping (Section
 2.2.6) and ordering (Section 2.3.3) to avoid cases where the reader
 would otherwise be required to understand the precedence rules.
 Automated readers are REQUIRED to parse rules correctly with or
 without this use of grouping.
 The various mechanisms described in the previous sections have the
 following precedence, from highest (binding tightest) at the top, to
 lowest (and loosest) at the bottom:

Farrel Standards Track [Page 11] RFC 5511 Routing BNF April 2009

    objects, constructs
    repetition
    grouping, optional
    concatenation
    alternative
 Note:
   Precedence is the main opportunity for confusion in the use of BNF.
   Authors are strongly RECOMMENDED to use grouping (Section 2.2.6) in
   all places where there is any scope for misinterpretation even when
   the meaning is obvious to the authors.
 Example:
   An example of the confusion in precedence can be found in Section
   3.1.4 of [RFC2205] and is mentioned in Section 2.2.4.
   <flow descriptor list> ::=  <empty> |
                    <flow descriptor list> <flow descriptor>
   The implementer MUST decide which of the following is intended:
   a.  <flow descriptor list> ::= <empty> |
                          ( <flow descriptor list> <flow descriptor> )
   b.  <flow descriptor list> ::= ( <empty> | <flow descriptor list> )
                                  <flow descriptor>
   The line break MAY be interpreted as implying grouping, but that is
   not an explicit rule.  However, the precedence rules say that
   concatenation has higher precedence than the Alternative operator.
   Thus, the text in [RFC2205] SHOULD be interpreted as shown in
   formulation a.
   Similarly (from the same section of [RFC2205]):
     <flow descriptor list> ::=
                      <FLOWSPEC>  <FILTER_SPEC>  |
                      <flow descriptor list> <FF flow descriptor>
   SHALL be interpreted as:
     <flow descriptor list> ::=
                    ( <FLOWSPEC> <FILTER_SPEC> ) |
                    ( <flow descriptor list> <FF flow descriptor> )
   The use of explicit grouping or intermediary constructs is strongly
   RECOMMENDED in new text to avoid confusion.

Farrel Standards Track [Page 12] RFC 5511 Routing BNF April 2009

3. Automated Validation

 RBNF would be appropriate for verification using automated validation
 tools.  Validation tools need to be able to check for close
 conformance to the rules expressed in this document to be useful for
 verifying new documents, but should also be able to parse RBNF as
 used in existing RFCs.  No tools are known at this time.

4. Security Considerations

 This document does not define any network behavior and does not
 introduce or seek to solve any security issues.
 It may be noted that clear and unambiguous protocol specifications
 reduce the likelihood of incompatible or defective implementations
 that might be exploited in security attacks.

5. Acknowledgments

 Thanks to Magnus Westerlund, Nic Neate, Chris Newman, Alfred Hoenes,
 Lou Berger, Julien Meuric, Stuart Venters, Tom Petch, Sam Hartman,
 and Pasi Eronen for review and useful comments.

6. References

6.1. Normative References

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

6.2. Informative References

 [RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
           Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
           Functional Specification", RFC 2205, September 1997.
 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
           and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
           Tunnels", RFC 3209, December 2001.
 [RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
           Switching (GMPLS) Signaling Resource ReserVation Protocol-
           Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
           January 2003.

Farrel Standards Track [Page 13] RFC 5511 Routing BNF April 2009

 [RFC4204] Lang, J., Ed., "Link Management Protocol (LMP)", RFC 4204,
           October 2005.
 [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax
           Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
           Element (PCE) Communication Protocol (PCEP)", RFC 5440,
           March 2009.
 [EBNF]    ISO/IEC 14977, "Information technology -- Syntactic
           metalanguage -- Extended BNF", 1996.

Author's Address

 Adrian Farrel
 Old Dog Consulting
 EMail: adrian@olddog.co.uk

Farrel Standards Track [Page 14]

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