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
draft-farrel-rtg-common-bnf-04.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 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 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
easy.
2.1.1. Rule Name Delimitation.
All rule names are enclosed by angle brackets ("<" and ">").
Example:
<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 ("_").
Example:
<SENDER_TEMPLATE>
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.
Example:
<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.
Example:
<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.
Meaning:
The named data construct or protocol 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.
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.
Meaning:
The data objects or data constructs must be present in the order
specified.
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Encoding:
A sequence of data objects and data 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 comments on ordering of data objects and data
constructs.
2.2.3. Optional Presence
Data objects and data constructs may be marked as optionally present.
Meaning:
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.
Encoding:
Contained in square brackets ("[" and "]").
Example:
<PathTear Message> ::= <Common Header> [ <INTEGRITY> ]
<SESSION> <RSVP_HOP>
[ <sender descriptor> ]
Note:
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.
Meaning:
Either one thing or the other must be present.
Encoding:
The pipe symbol ("|") is used between the data objects or data
constructs that are alternatives.
Example:
<flow descriptor list> ::= <FF flow descriptor list>
| <SE flow descriptor>
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Note:
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.
Meaning:
One or more objects or constructs may be present.
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 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 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
example:
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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
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 data objects, data
constructs, and operators.
Example:
<group> ::= ( <this> <that> )
Note:
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
2.2.5.
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
explicit.
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
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
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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>
as
<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
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.
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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
2008.
[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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