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Networking Working Group                                       A. Farrel
Internet-Draft                                        Old Dog Consulting
Intended Status: Standards Track
Created: September 10, 2008
Expires: March 10, 2009

                  Reduced Backus-Naur Form (RBNF)
          A Syntax Used in Various Protocol Specifications


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   Several protocols have been specified 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 Reduced BNF), and makes it available for
   use by new protocols.

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1. Introduction

   Backus-Naur Form (BNF) has been used to specify the message formats
   of several protocols within the IETF. Unfortunately these
   specifications are not based on any specific formal definition of BNF
   and 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.

   On the other hand, the variant of BNF used by the specifications in
   question 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.

   This document provides such a specification and names the BNF variant
   Reduced BNF (RBNF).

1.1. 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 gone on to be used 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

   Each of these three uses of RSVP has given rise to a considerable
   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 form 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) [PCEP]. Both of these protocols have also given rise to a
   number of protocol extensions that also use the same variant of BNF.

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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 combining other rules using operators. The most
   complex rule is the protocol 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 sub-ordinate
   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 sub-ordinate rules.

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 more

2.1.1. Rule Name Delimitation.

   All rule names are enclosed by angle brackets ("<" and ">").

     <Path Message>

2.1.2. Data Objects

   The most basic (indivisible) rule is termed a data object.

   Data objects are named in upper case. They do not usually use spaces
   within the name, favoring hyphens ("-") or underbars ("_").


2.1.3. Data Constructs

   Rules that are constructed from other rules using operators are
   termed data constructs.

   Data constructs are named in lower case, although capitals may be

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   used to indicate acronyms.

     <sender descriptor>

2.1.4. Protocol Messages

   The final objective is the definition of protocol messages. These are
   constructed from data objects and data constructs using operators.

   Data constructs are named in title case.

     <Path Message>

2.2. Operators

   Operators are used to build data constructs and protocol messages
   from data objects, and from other data constructs.

2.2.1. Assignment

   Assignment is used to form data constructs and protocol messages.

     The named data construct or protocol message on the left-hand side
     is defined to be equivalent to the right-hand side of the

     colon, colon, equal sign

     <WF flow descriptor> ::= <FLOWSPEC>

     The left-hand side of the assignment and the assignment operator
     must be present on the same line.

2.2.2. Sequential Combination

   Data objects and data constructs may be combined as a sequence to
   form a new data construct or protocol message.

     The data objects or data constructs must be present in the order

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     A sequence of data objects and data constructs usually separated
     by spaces. May also be separated by line breaks.

     <SE flow descriptor> ::= <FLOWSPEC> <filter spec list>

     See Section 2.3.3 for comments on ordering of data objects and data

2.2.3. Optional Presence

  Data objects and data constructs may be marked as optionally present.

    The optional data objects or data constructs may be present or
    absent within the assignment. Unless indicated as optional, data
    objects and data constructs are mandatory.

    Contained in square brackets ("[" and "]").

    <PathTear Message> ::= <Common Header> [ <INTEGRITY> ]
                           <SESSION> <RSVP_HOP>
                           [ <sender descriptor> ]

    The optional operator can be nested. For example,

      <construct> ::= <MAND> [ <OPT_1> [ <OPT_2> ] ]

    In this construction, the data object OPT_2 can only be
    present if OPT_1 is also present.

2.2.4. Alternatives

  Choices may be indicated within assignments.

    Either one thing or the other must be present.

    The pipe symbol ("|") is used between the data objects or data
    constructs that are alternatives.

    <flow descriptor list> ::= <FF flow descriptor list>
                               | <SE flow descriptor>

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    Multi-way alternates are not currently common. To avoid confusion,
    grouping should be used (see Section 2.2.6), or an intermediary data
    construct may be created. Thus:

      <construct> ::= <ALT_ONE> | <ALT_TWO> | <ALT_THREE>

    is better presented as

      <construct> ::= ( <ALT_ONE> | <ALT_TWO> ) | <ALT_THREE>

    or as

      <intermediary construct> ::= <ALT_TWO> | <ALT_THREE>
      <construct> ::= <ALT_ONE> | <intermediary construct>

2.2.5. Repetition

  It may be the case that a sequence of identical data objects or data
  constructs is required within an assignment.

    One or more objects or constructs may be present.

    Three dots ("...").

    <Path Message> ::= <Common Header> [ <INTEGRITY> ]
                       <SESSION> <RSVP_HOP>
                       [ <POLICY_DATA> ... ]
                       [ <sender descriptor> ]

    1. A set of zero or more objects or constructs may be achieved by
       combining with the Optional concept as shown in the example

    2. Sequences may also be encoded by building a recursive data
       construct using the Alternative operator. For example:

         <sequence> ::= <OBJECT> |
                        <OBJECT> <sequence>

    3. Repetition may also be applied to a component of an assignment
       to indicate the optional repetition of that component. For

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         <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 data construct
       (see note 2, above) would be more clear.

2.2.6. Grouping

    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.

    Round brackets ("(" and ")") enclosing a set of data objects, data
    constructs, and operators.

    <group> ::= ( <this> <that> )

    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 may make the BNF easier to parse
    unambiguously. Line breaks (Section 2.3.2) are often used to clarify
    grouping as can be seen in the definition of <sequence> in Section

2.3. Editorial Conventions

2.3.1. White Space

   White space (that is space characters) 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. They can be used to enhance readability when the
   precedence rules imply grouping as described in Section 2.2.6 and
   Section 2.4.

   A line break must not be present between the left-hand side of an
   assignment and the assignment operator (see Section 2.2.1).

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   New assignments (i.e., new data construct or protocol message
   definitions) must begin on a new line.

2.3.3. Ordering

   The ordering of data objects and data constructs in an assignment is

   Protocol specifications may opt to state that ordering is only
   recommended. In this case, elements of a list of data objects and
   data constructs may be received in any order.

2.4. Precedence

   Precedence may be deduced from a "proper" reading of the BNF using
   these rules. Grouping and ordering are recommended for clarity.

   The various mechanisms described above have the following precedence,
   from highest (binding tightest) at the top, to lowest and loosest at
   the bottom:

      data objects, data constructs
      grouping, optional

     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.

     An example of the confusion in precedence can be found in Section
     3.1.4 of [RFC2205].

           <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

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     concatenation has higher precedence than the Alternative operators.
     Thus, we should interpret (correctly) the text in [RFC2205] as
     shown in formulation a.

     Similarly (from the same section of [RFC2205]) we should interpret

           <flow descriptor list> ::=
                            <FLOWSPEC>  <FILTER_SPEC>  |
                            <flow descriptor list> <FF flow descriptor>


           <flow descriptor list> ::=
                         ( <FLOWSPEC> <FILTER_SPEC> ) |
                         ( <flow descriptor list> <FF flow descriptor> )

3. Automated Validation

   RBNF would be appropriate for verification using automated validation
   tools. No tools are known at this time.

4. IANA Considerations

   This document makes no requests for IANA action.

5. 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 defective or incompatible implementations
   that might be exploited in security attacks.

6. Acknowledgments

   Thanks to Magnus Westerlund, Nic Neate, Chris Newman, and Alfred
   Hoenes for review and useful comments.

7.  References

7.1.  Normative References


7.2.  Informative References

   [RFC2205] Braden, R. (Ed.), Zhang, L., Berson, S., Herzog, S., and S.
             Jamin, "Resource ReserVation Protocol -- Version 1
             Functional Specification", RFC 2205, September 1997.

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   [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., "Generalized Multi-Protocol Label Switching
             (GMPLS) Signaling Resource ReserVation Protocol-Traffic
             Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC4204] Lang, J., Ed., "The Link Management Protocol (LMP)", RFC
             4204, September 2005.

   [RFC5234] Crocker, D. (Ed.) and Overell, P., "Augmented BNF for
             Syntax Specifications: ABNF", STD 68, RFC 5234, January

   [PCEP]    Vasseur, J.P., and Le Roux, J.-L., "Path Computation
             Element (PCE) Communication Protocol (PCEP) - Version 1",
             draft-ietf-pce-pcep, work in progress.

Author's Address

   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk

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