Networking Working Group A. Farrel
Internet-Draft Old Dog Consulting
Intended Status: Standards Track
Created: September 12, 2008
Expires: March 12, 2009
Reduced Backus-Naur Form (RBNF)
A Syntax Used in Various Protocol Specifications
draft-farrel-rtg-common-bnf-05.txt
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Abstract
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 (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.
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
[RFC3473].
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 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
large 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
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
easy.
2.1.1. Rule Name Delimitation.
All rule names are enclosed by angle brackets ("<" and ">"). Rules
names may include any printable characters, but not 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 upper case. They do not usually use
spaces within the name, favoring underbars ("_").
Example:
<SENDER_TEMPLATE>
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2.1.3. Constructs
Rules that are constructed from other rules using operators are
termed constructs.
Constructs are named in lower case, 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 equivalent 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.
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2.2.2. Concatenation
Objects and constructs may be combined as a sequence to form a new
construct or message.
Meaning:
The objects or constructs must be present in the order specified.
Encoding:
A sequence of objects and constructs usually separated by spaces.
May also be separated by line breaks.
Example:
<SE flow descriptor> ::= <FLOWSPEC> <filter spec list>
Note:
See Section 2.3.3 for further comments on ordering of objects and
constructs.
2.2.3. Optional Presence
Objects and constructs may 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.
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.
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
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example.
2.2.4. Alternatives
Choices may 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. Multi-way alternates are not common. To avoid confusion,
explicit grouping (see Section 2.2.6), or an intermediary must
be used. Thus:
<construct> ::= <ALT_ONE> | <ALT_TWO> | <ALT_THREE>
is not allowed and must be 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 objects or constructs
is required within an assignment.
Meaning:
One or more objects or constructs may be present.
Encoding:
Three dots ("...").
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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 may be achieved by
combining with the Optional concept as shown in the example
above.
2. Sequences may also be encoded by building a recursive 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
example:
<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
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BNF representation. However, grouping may make the BNF easier to
parse unambiguously and is recommended as a solution 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 grouping is
recommended.
3. A practical alternative to grouping is he definition of
intermediate constructs as illustrated in Note 2 of Section
2.2.4.
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 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,
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from highest (binding tightest) at the top, to lowest and loosest at
the bottom:
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].
<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 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>
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.
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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, Alfred Hoenes,
and Lou Berger for review and useful comments.
7. References
7.1. Normative References
None
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.
[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
2008.
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[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.
[EBNF] ISO/IEC 14977, "Information technology -- Syntactic
metalanguage -- Extended BNF", 1996
Author's Address
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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