RFC 3780
SMIng - Next Generation Structure of Management Information
Pages: 64
Experimental
Experimental
Part 2 of 3 – Pages 20 to 41
4. The SMIng File Structure
The topmost container of SMIng information is a file. An SMIng file may contain zero, one or more modules. It is RECOMMENDED that modules be stored into separate files by their module names, where possible. However, for dedicated purposes, it may be reasonable to collect several modules in a single file. The top level SMIng construct is the `module' statement (Section 5) that defines a single SMIng module. A module contains a sequence of sections in an obligatory order with different kinds of definitions. Whether these sections contain statements or remain empty mainly depends on the purpose of the module.4.1. Comments
Comments can be included at any position in an SMIng file, except between the characters of a single token like those of a quoted string. However, it is RECOMMENDED that all substantive descriptions be placed within an appropriate description clause, so that the information is available to SMIng parsers.
Comments commence with a pair of adjacent slashes `//' and end at the end of the line.4.2. Textual Data
Some statements, namely `organization', `contact', `description', `reference', `abnf', `format', and `units', get a textual argument. This text, as well as representations of OctetString values, have to be enclosed in double quotes. They may contain arbitrary characters with the following exceptional encoding rules: A backslash character introduces a special character, which depends on the character that immediately follows the backslash: \n new line \t a tab character \" a double quote \\ a single backslash If the text contains a line break followed by whitespace which is used to indent the text according to the layout in the SMIng file, this prefixing whitespace is stripped from the text.4.3. Statements and Arguments
SMIng has a very small set of basic grammar rules based on the concept of statements. Each statement starts with a lower-case keyword identifying the statement, followed by a number (possibly zero) of arguments. An argument may be quoted text, an identifier, a value of any base type, a list of identifiers enclosed in parenthesis `( )', or a statement block enclosed in curly braces `{ }'. Since statement blocks are valid arguments, it is possible to nest statement sequences. Each statement is terminated by a semicolon `;'. The core set of statements may be extended using the SMIng `extension' statement. See Sections 6 and 11 for details. At places where a statement is expected, but an unknown lower-case word is read, those statements MUST be skipped up to the proper semicolon, including nested statement blocks.5. The module Statement
The `module' statement is used as a container of all definitions of a single SMIng module. It gets two arguments: an upper-case module name and a statement block that contains mandatory and optional statements and sections of statements in an obligatory order:
module <MODULE-NAME> {
<optional import statements>
<organization statement>
<contact statement>
<description statement>
<optional reference statement>
<at least one revision statement>
<optional extension statements>
<optional typedef statements>
<optional identity statements>
<optional class statements>
};
The optional `import' statements (Section 5.1) are followed by the
mandatory `organization' (Section 5.2), `contact' (Section 5.3), and
`description' (Section 5.4) statements and the optional `reference'
statement (Section 5.5), which in turn are followed by at least one
mandatory `revision' statement (Section 5.6). The part up to this
point defines the module's meta information, i.e., information that
describes the whole module but does not define any items used by
applications in the first instance. This part of a module is
followed by its main definitions, namely SMIng extensions (Section
6), derived types (Section 7), identities (Section 8), and classes
(Section 9).
See the `moduleStatement' rule of the SMIng grammar (Appendix B) for
the formal syntax of the `module' statement.
5.1. The module's import Statement
The optional module's `import' statement is used to import
identifiers from external modules into the local module's namespace.
It gets two arguments: the name of the external module and a comma-
separated list of one or more identifiers to be imported enclosed in
parenthesis.
Multiple `import' statements for the same module but with disjoint
lists of identifiers are allowed, though NOT RECOMMENDED. The same
identifier from the same module MUST NOT be imported multiple times.
To import identifiers with the same name from different modules might
be necessary and is allowed. To distinguish
them in the local module, they have to be referred by qualified names. Importing identifiers not used in the local module is NOT RECOMMENDED. See the `importStatement' rule of the SMIng grammar (Appendix B) for the formal syntax of the `import' statement.5.2. The module's organization Statement
The module's `organization' statement, which must be present, gets one argument which is used to specify a textual description of the organization(s) under whose auspices this module was developed.5.3. The module's contact Statement
The module's `contact' statement, which must be present, gets one argument which is used to specify the name, postal address, telephone number, and electronic mail address of the person to whom technical queries concerning this module should be sent.5.4. The module's description Statement
The module's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the contents of this module.5.5. The module's reference Statement
The module's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related management information, or some other document which provides additional information relevant to this module.5.6. The module's revision Statement
The module's `revision' statement is repeatedly used to specify the editorial revisions of the module, including the initial revision. It gets one argument which is a statement block that holds detailed information in an obligatory order. A module MUST have at least one initial `revision' statement. For every editorial change, a new one MUST be added in front of the revisions sequence, so that all revisions are in reverse chronological order. See the `revisionStatement' rule of the SMIng grammar (Appendix B) for the formal syntax of the `revision' statement.
5.6.1. The revision's date Statement
The revision's `date' statement, which must be present, gets one argument which is used to specify the date and time of the revision in the format `YYYY-MM-DD HH:MM' or `YYYY-MM-DD' which implies the time `00:00'. The time is always given in UTC. See the `date' rule of the SMIng grammar (Appendix B) for the formal syntax of the revision's `date' statement.5.6.2. The revision's description Statement
The revision's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the revision.5.7. Usage Example
Consider how a skeletal module might be constructed: module ACME-MIB { import NMRG-SMING (DisplayString); organization "IRTF Network Management Research Group (NMRG)"; contact "IRTF Network Management Research Group (NMRG) http://www.ibr.cs.tu-bs.de/projects/nmrg/ Joe L. User ACME, Inc. 42 Anywhere Drive Nowhere, CA 95134 USA Phone: +1 800 555 0815 EMail: joe@acme.example.com"; description "The module for entities implementing the ACME protocol. Copyright (C) The Internet Society (2004). All Rights Reserved. This version of this MIB module is part of RFC 3780, see the RFC itself for legal notices.";
revision {
date "2003-12-16";
description
"Initial revision, published as RFC 3780.";
};
// ... further definitions ...
}; // end of module ACME-MIB.
6. The extension Statement
The `extension' statement defines new statements to be used in the
local module following this extension statement definition or in
external modules that may import this extension statement definition.
The `extension' statement gets two arguments: a lower-case extension
statement identifier and a statement block that holds detailed
extension information in an obligatory order.
Extension statement identifiers SHOULD NOT contain any upper-case
characters.
Note that the SMIng extension feature does not allow the formal
specification of the context, or argument syntax and semantics of an
extension. Its only purpose is to declare the existence of an
extension and to allow a unique reference to an extension. See
Section 11 for detailed information on extensions and [RFC3781] for
mappings of SMIng definitions to SNMP, which is formally defined as
an extension.
See the `extensionStatement' rule of the SMIng grammar (Appendix B)
for the formal syntax of the `extension' statement.
6.1. The extension's status Statement
The extension's `status' statement, which must be present, gets one
argument which is used to specify whether this extension definition
is current or historic. The value `current' means that the
definition is current and valid. The value `obsolete' means the
definition is obsolete and should not be implemented and/or can be
removed if previously implemented. While the value `deprecated' also
indicates an obsolete definition, it permits new/continued
implementation in order to foster interoperability with older/
existing implementations.
6.2. The extension's description Statement
The extension's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the extension statement. It is RECOMMENDED that information on the extension's context, its semantics, and implementation conditions be included. See also Section 11.6.3. The extension's reference Statement
The extension's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related extension definitions, or some other document which provides additional information relevant to this extension.6.4. The extension's abnf Statement
The extension's `abnf' statement, which need not be present, gets one argument which is used to specify a formal ABNF [RFC2234] grammar definition of the extension. This grammar can reference rule names from the core SMIng grammar (Appendix B). Note that the `abnf' statement should contain only pure ABNF and no additional text, though comments prefixed by a semicolon are allowed but should probably be moved to the description statement. Note that double quotes within the ABNF grammar have to be represented as `\"' according to Section 4.2.6.5. Usage Example
extension severity { status current; description "The optional severity extension statement can only be applied to the statement block of an SMIng class' event definition. If it is present it denotes the severity level of the event in a range from 0 (emergency) to 7 (debug)."; abnf "severityStatement = severityKeyword sep number optsep \";\" severityKeyword = \"severity\""; };
7. The typedef Statement
The `typedef' statement defines new data types to be used in the local module or in external modules. It gets two arguments: an upper-case type identifier and a statement block that holds detailed type information in an obligatory order. Type identifiers SHOULD NOT consist of all upper-case characters and SHOULD NOT contain hyphens. See the `typedefStatement' rule of the SMIng grammar (Appendix B) for the formal syntax of the `typedef' statement.7.1. The typedef's type Statement
The typedef's `type' statement, which must be present, gets one argument which is used to specify the type from which this type is derived. Optionally, type restrictions may be applied to the new type by appending subtyping information according to the rules of the base type. See Section 3 for SMIng base types and their type restrictions.7.2. The typedef's default Statement
The typedef's `default' statement, which need not be present, gets one argument which is used to specify an acceptable default value for attributes of this type. A default value may be used when an attribute instance is created. That is, the value is a "hint" to implementors. The value of the `default' statement must, of course, correspond to the (probably restricted) type specified in the typedef's `type' statement. The default value of a type may be overwritten by a default value of an attribute of this type. Note that for some types, default values make no sense.7.3. The typedef's format Statement
The typedef's `format' statement, which need not be present, gets one argument which is used to give a hint as to how the value of an instance of an attribute of this type might be displayed. See Section 3.13 for a description of format specifications.
If no format is specified, it is inherited from the type given in the `type' statement. On the other hand, the format specification of a type may be semantically refined by a format specification of an attribute of this type.7.4. The typedef's units Statement
The typedef's `units' statement, which need not be present, gets one argument which is used to specify a textual definition of the units associated with attributes of this type. If no units are specified, they are inherited from the type given in the `type' statement. On the other hand, the units specification of a type may be semantically refined by a units specification of an attribute of this type. The units specification has to be appropriate for values displayed according to the typedef's format specification, if present. For example, if the type defines frequency values of type Unsigned64 measured in thousands of Hertz, the format specification should be `d-3' and the units specification should be `Hertz' or `Hz'. If the format specification would be omitted, the units specification should be `Milli-Hertz' or `mHz'. Authors of SMIng modules should pay attention to keep format and units specifications in sync. Application implementors MUST NOT implement units specifications without implementing format specifications.7.5. The typedef's status Statement
The typedef's `status' statement, which must be present, gets one argument which is used to specify whether this type definition is current or historic. The value `current' means that the definition is current and valid. The value `obsolete' means the definition is obsolete and should not be implemented and/or can be removed if previously implemented. While the value `deprecated' also indicates an obsolete definition, it permits new/continued implementation in order to foster interoperability with older/existing implementations. Derived types SHOULD NOT be defined as `current' if their underlying type is `deprecated' or `obsolete'. Similarly, they SHOULD NOT be defined as `deprecated' if their underlying type is `obsolete'. Nevertheless, subsequent revisions of the underlying type cannot be avoided, but SHOULD be taken into account in subsequent revisions of the local module.
7.6. The typedef's description Statement
The typedef's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the newly defined type. It is RECOMMENDED that all semantic definitions necessary for implementation, and to embody any information which would otherwise be communicated in any commentary annotations associated with this type definition be included.7.7. The typedef's reference Statement
The typedef's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related type definitions, or some other document which provides additional information relevant to this type definition.7.8. Usage Examples
typedef RptrOperStatus { type Enumeration (other(1), ok(2), rptrFailure(3), groupFailure(4), portFailure(5), generalFailure(6)); default other; // undefined by default. status deprecated; description "A type to indicate the operational state of a repeater."; reference "[IEEE 802.3 Mgt], 30.4.1.1.5, aRepeaterHealthState."; }; typedef SnmpTransportDomain { type Pointer (snmpTransportDomain); status current; description "A pointer to an SNMP transport domain identity."; }; typedef DateAndTime { type OctetString (8 | 11); format "2d-1d-1d,1d:1d:1d.1d,1a1d:1d"; status current; description "A date-time specification. ...
Note that if only local time is known, then timezone
information (fields 8-10) is not present.";
reference
"RFC 2579, SNMPv2-TC.DateAndTime.";
};
typedef Frequency {
type Unsigned64;
format "d-3"
units "Hertz";
status current;
description
"A wide-range frequency specification measured
in thousands of Hertz.";
};
8. The identity Statement
The `identity' statement is used to define a new abstract and untyped
identity. Its only purpose is to denote its name, semantics, and
existence. An identity can be defined either from scratch or derived
from a parent identity. The `identity' statement gets the following
two arguments: The first argument is a lower-case identity
identifier. The second argument is a statement block that holds
detailed identity information in an obligatory order.
See the `identityStatement' rule of the SMIng grammar (Appendix B)
for the formal syntax of the `identity' statement.
8.1. The identity's parent Statement
The identity's `parent' statement must be present for a derived
identity and must be absent for an identity defined from scratch. It
gets one argument which is used to specify the parent identity from
which this identity shall be derived.
8.2. The identity's status Statement
The identity's `status' statement, which must be present, gets one
argument which is used to specify whether this identity definition is
current or historic. The value `current' means that the definition
is current and valid. The value `obsolete' means the definition is
obsolete and should not be implemented and/or can be removed if
previously implemented. While the value `deprecated' also indicates
an obsolete definition, it permits new/continued implementation in
order to foster interoperability with older/existing implementations.
Derived identities SHOULD NOT be defined as `current' if their parent identity is `deprecated' or `obsolete'. Similarly, they SHOULD NOT be defined as `deprecated' if their parent identity is `obsolete'. Nevertheless, subsequent revisions of the parent identity cannot be avoided, but SHOULD be taken into account in subsequent revisions of the local module.8.3. The identity' description Statement
The identity's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the newly defined identity. It is RECOMMENDED that all semantic definitions necessary for implementation, and to embody any information which would otherwise be communicated in any commentary annotations associated with this identity definition be included.8.4. The identity's reference Statement
The identity's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related identity definitions, or some other document which provides additional information relevant to this identity definition.8.5. Usage Examples
identity null { status current; description "An identity used to represent null pointer values."; }; identity snmpTransportDomain { status current; description "A generic SNMP transport domain identity."; }; identity snmpUDPDomain { parent snmpTransportDomain; status current; description "The SNMP over UDP transport domain."; };
9. The class Statement
The `class' statement is used to define a new class that represents a container of related attributes and events (Section 9.2, Section 9.4). A class can be defined either from scratch or derived from a parent class. A derived class inherits all attributes and events of the parent class and can be extended by additional attributes and events. The `class' statement gets the following two arguments: The first argument is an upper-case class identifier. The second argument is a statement block that holds detailed class information in an obligatory order. See the `classStatement' rule of the SMIng grammar (Appendix B) for the formal syntax of the `class' statement.9.1. The class' extends Statement
The class' `extends' statement must be present for a class derived from a parent class and must be absent for a class defined from scratch. It gets one argument which is used to specify the parent class from which this class shall be derived.9.2. The class' attribute Statement
The class' `attribute' statement, which can be present zero, one or multiple times, gets two arguments: the attribute name and a statement block that holds detailed attribute information in an obligatory order.9.2.1. The attribute's type Statement
The attribute's `type' statement must be present. It gets at least one argument which is used to specify the type of the attribute: either a type name or a class name. In case of a type name, it may be restricted by a second argument according to the restriction rules described in Section 3.9.2.2. The attribute's access Statement
The attribute's `access' statement must be present for attributes typed by a base type or derived type, and must be absent for attributes typed by a class. It gets one argument which is used to specify whether it makes sense to read and/or write an instance of the attribute, or to include its value in an event. This is the maximal level of access for the attribute. This maximal level of access is independent of any administrative authorization policy.
The value `readwrite' indicates that read and write access makes sense. The value `readonly' indicates that read access makes sense, but write access is never possible. The value `eventonly' indicates an object which is accessible only via an event. These values are ordered, from least to greatest access level: `eventonly', `readonly', `readwrite'.9.2.3. The attribute's default Statement
The attribute's `default' statement need not be present for attributes typed by a base type or derived type, and must be absent for attributes typed by a class. It gets one argument which is used to specify an acceptable default value for this attribute. A default value may be used when an attribute instance is created. That is, the value is a "hint" to implementors. The value of the `default' statement must, of course, correspond to the (probably restricted) type specified in the attribute's `type' statement. The attribute's default value overrides the default value of the underlying type definition if both are present.9.2.4. The attribute's format Statement
The attribute's `format' statement need not be present for attributes typed by a base type or derived type, and must be absent for attributes typed by a class. It gets one argument which is used to give a hint as to how the value of an instance of this attribute might be displayed. See Section 3.13 for a description of format specifications. The attribute's format specification overrides the format specification of the underlying type definition if both are present.9.2.5. The attribute's units Statement
The attribute's `units' statement need not be present for attributes typed by a base type or derived type, and must be absent for attributes typed by a class. It gets one argument which is used to specify a textual definition of the units associated with this attribute. The attribute's units specification overrides the units specification of the underlying type definition if both are present.
The units specification has to be appropriate for values displayed according to the attribute's format specification if present. For example, if the attribute represents a frequency value of type Unsigned64 measured in thousands of Hertz, the format specification should be `d-3' and the units specification should be `Hertz' or `Hz'. If the format specification would be omitted, the units specification should be `Milli-Hertz' or `mHz'. Authors of SMIng modules should pay attention to keep format and units specifications of type and attribute definitions in sync. Application implementors MUST NOT implement units specifications without implementing format specifications.9.2.6. The attribute's status Statement
The attribute's `status' statement must be present. It gets one argument which is used to specify whether this attribute definition is current or historic. The value `current' means that the definition is current and valid. The value `obsolete' means the definition is obsolete and should not be implemented and/or can be removed if previously implemented. While the value `deprecated' also indicates an obsolete definition, it permits new/continued implementation in order to foster interoperability with older/ existing implementations. Attributes SHOULD NOT be defined as `current' if their type or their containing class is `deprecated' or `obsolete'. Similarly, they SHOULD NOT be defined as `deprecated' if their type or their containing class is `obsolete'. Nevertheless, subsequent revisions of used type definition cannot be avoided, but SHOULD be taken into account in subsequent revisions of the local module.9.2.7. The attribute's description Statement
The attribute's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of this attribute. It is RECOMMENDED that all semantic definitions necessary for the implementation of this attribute be included.9.2.8. The attribute's reference Statement
The attribute's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related attribute definitions, or some other document which provides additional information relevant to this attribute definition.
9.3. The class' unique Statement
The class' `unique' statement, which need not be present, gets one argument that specifies a comma-separated list of attributes of this class, enclosed in parenthesis. If present, this list of attributes makes up a unique identification of all possible instances of this class. It can be used as a unique key in underlying protocols. If the list is empty, the class should be regarded as a scalar class with only a single instance. If the `unique' statement is not present, the class is not meant to be instantiated directly, but to be contained in other classes or the parent class of other refining classes. If present, the attribute list MUST NOT contain any attribute more than once and the attributes should be ordered where appropriate so that the attributes that are most significant in most situations appear first.9.4. The class' event Statement
The class' `event' statement is used to define an event related to an instance of this class that can occur asynchronously. It gets two arguments: a lower-case event identifier and a statement block that holds detailed information in an obligatory order. See the `eventStatement' rule of the SMIng grammar (Appendix B) for the formal syntax of the `event' statement.9.4.1. The event's status Statement
The event's `status' statement, which must be present, gets one argument which is used to specify whether this event definition is current or historic. The value `current' means that the definition is current and valid. The value `obsolete' means the definition is obsolete and should not be implemented and/or can be removed if previously implemented. While the value `deprecated' also indicates an obsolete definition, it permits new/continued implementation in order to foster interoperability with older/existing implementations.9.4.2. The event's description Statement
The event's `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of this event.
It is RECOMMENDED that all semantic definitions necessary for the implementation of this event be included. In particular, which instance of the class is associated with an event of this type SHOULD be documented.9.4.3. The event's reference Statement
The event's `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related event definitions, or some other document which provides additional information relevant to this event definition.9.5. The class' status Statement
The class' `status' statement, which must be present, gets one argument which is used to specify whether this class definition is current or historic. The value `current' means that the definition is current and valid. The value `obsolete' means the definition is obsolete and should not be implemented and/or can be removed if previously implemented. While the value `deprecated' also indicates an obsolete definition, it permits new/continued implementation in order to foster interoperability with older/existing implementations. Derived classes SHOULD NOT be defined as `current' if their parent class is `deprecated' or `obsolete'. Similarly, they SHOULD NOT be defined as `deprecated' if their parent class is `obsolete'. Nevertheless, subsequent revisions of the parent class cannot be avoided, but SHOULD be taken into account in subsequent revisions of the local module.9.6. The class' description Statement
The class' `description' statement, which must be present, gets one argument which is used to specify a high-level textual description of the newly defined class. It is RECOMMENDED that all semantic definitions necessary for implementation, and to embody any information which would otherwise be communicated in any commentary annotations associated with this class definition be included.
9.7. The class' reference Statement
The class' `reference' statement, which need not be present, gets one argument which is used to specify a textual cross-reference to some other document, either another module which defines related class definitions, or some other document which provides additional information relevant to this class definition.9.8. Usage Example
Consider how an event might be described that signals a status change of an interface: class Interface { // ... attribute speed { type Gauge32; access readonly; units "bps"; status current; description "An estimate of the interface's current bandwidth in bits per second."; }; // ... attribute adminStatus { type AdminStatus; access readwrite; status current; description "The desired state of the interface."; }; attribute operStatus { type OperStatus; access readonly; status current; description "The current operational state of the interface."; }; event linkDown { status current; description "A linkDown event signifies that the ifOperStatus attribute for this interface instance is about to enter the down state from some other state (but not from the notPresent state). This other state is indicated by the included value of ifOperStatus.";
};
status current;
description
"A physical or logical network interface.";
};
10. Extending a Module
As experience is gained with a module, it may be desirable to revise
that module. However, changes are not allowed if they have any
potential to cause interoperability problems between an
implementation using an original specification and an implementation
using an updated specification(s).
For any change, some statements near the top of the module MUST be
updated to include information about the revision: specifically, a
new `revision' statement (Section 5.6) must be included in front of
the `revision' statements. Furthermore, any necessary changes MUST
be applied to other statements, including the `organization' and
`contact' statements (Section 5.2, Section 5.3).
Note that any definition contained in a module is available to be
imported by any other module, and is referenced in an `import'
statement via the module name. Thus, a module name MUST NOT be
changed. Specifically, the module name (e.g., `ACME-MIB' in the
example of Section 5.7) MUST NOT be changed when revising a module
(except to correct typographical errors), and definitions MUST NOT be
moved from one module to another.
Also note that obsolete definitions MUST NOT be removed from modules
since their identifiers may still be referenced by other modules.
A definition may be revised in any of the following ways:
o In `typedef' statement blocks, a `type' statement containing an
`Enumeration' or `Bits' type may have new named numbers added.
o In `typedef' statement blocks, the value of a `type' statement may
be replaced by another type if the new type is derived (directly
or indirectly) from the same base type, has the same set of
values, and has identical semantics.
o In `attribute' statements where the `type' sub-statement specifies
a class, the class may be replaced by another class if the new
class is derived (directly or indirectly) from the base class and
both classes have identical semantics.
o In `attribute' statements where the `type' sub-statement specifies
a base type, a defined type, or an implicitly derived type (i.e.,
not a class), that type may be replaced by another type if the new
type is derived (directly or indirectly) from the same base type,
has the same set of values, and has identical semantics.
o In any statement block, a `status' statement value of `current'
may be revised as `deprecated' or `obsolete'. Similarly, a
`status' statement value of `deprecated' may be revised as
`obsolete'. When making such a change, the `description'
statement SHOULD be updated to explain the rationale.
o In `typedef' and `attribute' statement blocks, a `default'
statement may be added or updated.
o In `typedef' and `attribute' statement blocks, a `units' statement
may be added.
o A class may be augmented by adding new attributes.
o In any statement block, clarifications and additional information
may be included in the `description' statement.
o In any statement block, a `reference' statement may be added or
updated.
o Entirely new extensions, types, identities, and classes may be
defined, using previously unassigned identifiers.
Otherwise, if the semantics of any previous definition are changed
(i.e., if a non-editorial change is made to any definition other than
those specifically allowed above), then this MUST be achieved by a
new definition with a new identifier. In case of a class where the
semantics of any attributes are changed, the new class can be defined
by derivation from the old class and refining the changed attributes.
Note that changing the identifier associated with an existing
definition is considered a semantic change, as these strings may be
used in an `import' statement.
11. SMIng Language Extensibility
While the core SMIng language has a well defined set of statements
(Section 5 through Section 9.4) that are used to specify those
aspects of management information commonly regarded as necessary
without management protocol specific information, there may be
further information people wish to express. Describing additional information informally in description statements has a disadvantage in that this information cannot be parsed by any program. SMIng allows modules to include statements that are unknown to a parser but fulfil some core grammar rules (Section 4.3). Furthermore, additional statements may be defined by the `extension' statement (Section 6). Extensions can be used in the local module or in other modules that import the extension. This has some advantages: o A parser can differentiate between statements known as extensions and unknown statements. This enables the parser to complain about unknown statements, e.g., due to typos. o If an extension's definition contains a formal ABNF grammar definition and a parser is able to interpret this ABNF definition, this enables the parser to also complain about the wrong usage of an extension. o Since there might be some common need for extensions, there is a relatively high probability of extension name collisions originated by different organizations, as long as there is no standardized extension for that purpose. The requirement to explicitly import extension statements allows those extensions to be distinguished. o The supported extensions of an SMIng implementation, e.g., an SMIng module compiler, can be clearly expressed. The only formal effect of an extension statement definition is to declare its existence and status, and optionally its ABNF grammar. All additional aspects SHOULD be described in the `description' statement: o The detailed semantics of the new statement SHOULD be described. o The contexts in which the new statement can be used SHOULD be described, e.g., a new statement may be designed to be used only in the statement block of a module, but not in other nested statement blocks. Others may be applicable in multiple contexts. In addition, the point in the sequence of an obligatory order of other statements, where the new statement may be inserted, might be prescribed. o The circumstances that make the new statement mandatory or optional SHOULD be described.
o The syntax of the new statement SHOULD at least be described
informally, if not supplied formally in an `abnf' statement.
o It might be reasonable to give some suggestions under which
conditions the implementation of the new statement is adequate and
how it could be integrated into existent implementations.
Some possible extension applications are:
o The formal mapping of SMIng definitions into the SNMP [RFC3781]
framework is defined as an SMIng extension. Other mappings may
follow in the future.
o Inlined annotations to definitions. For example, a vendor may
wish to describe additional information to class and attribute
definitions in private modules. An example are severity levels of
events in the statement block of an `event' statement.
o Arbitrary annotations to external definitions. For example, a
vendor may wish to describe additional information to definitions
in a "standard" module. This allows a vendor to implement
"standard" modules as well as additional private features, without
redundant module definitions, but on top of "standard" module
definitions.
(page 41 continued on part 3)