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Network Working Group Editors of this version:
Request for Comments: 2578 K. McCloghrie
STD: 58 Cisco Systems
Obsoletes: 1902 D. Perkins
Category: Standards Track SNMPinfo
J. Schoenwaelder
TU Braunschweig
Authors of previous version:
J. Case
SNMP Research
K. McCloghrie
Cisco Systems
M. Rose
First Virtual Holdings
S. Waldbusser
International Network Services
April 1999
Structure of Management Information Version 2 (SMIv2)
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.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Table of Contents
1 Introduction .................................................3
1.1 A Note on Terminology ......................................4
2 Definitions ..................................................4
2.1 The MODULE-IDENTITY macro ..................................5
2.2 Object Names and Syntaxes ..................................5
2.3 The OBJECT-TYPE macro ......................................8
2.5 The NOTIFICATION-TYPE macro ...............................10
2.6 Administrative Identifiers ................................11
3 Information Modules .........................................11
3.1 Macro Invocation ..........................................12
3.1.1 Textual Values and Strings ..............................13
McCloghrie, et al. Standards Track [Page 1]
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3.2 IMPORTing Symbols .........................................14
3.3 Exporting Symbols .........................................14
3.4 ASN.1 Comments ............................................14
3.5 OBJECT IDENTIFIER values ..................................15
3.6 OBJECT IDENTIFIER usage ...................................15
3.7 Reserved Keywords .........................................16
4 Naming Hierarchy ............................................16
5 Mapping of the MODULE-IDENTITY macro ........................17
5.1 Mapping of the LAST-UPDATED clause ........................17
5.2 Mapping of the ORGANIZATION clause ........................17
5.3 Mapping of the CONTACT-INFO clause ........................18
5.4 Mapping of the DESCRIPTION clause .........................18
5.5 Mapping of the REVISION clause ............................18
5.5.1 Mapping of the DESCRIPTION sub-clause ...................18
5.6 Mapping of the MODULE-IDENTITY value ......................18
5.7 Usage Example .............................................18
6 Mapping of the OBJECT-IDENTITY macro ........................19
6.1 Mapping of the STATUS clause ..............................19
6.2 Mapping of the DESCRIPTION clause .........................20
6.3 Mapping of the REFERENCE clause ...........................20
6.4 Mapping of the OBJECT-IDENTITY value ......................20
6.5 Usage Example .............................................20
7 Mapping of the OBJECT-TYPE macro ............................20
7.1 Mapping of the SYNTAX clause ..............................21
7.1.1 Integer32 and INTEGER ...................................21
7.1.2 OCTET STRING ............................................21
7.1.3 OBJECT IDENTIFIER .......................................22
7.1.4 The BITS construct ......................................22
7.1.5 IpAddress ...............................................22
7.1.6 Counter32 ...............................................23
7.1.7 Gauge32 .................................................23
7.1.8 TimeTicks ...............................................24
7.1.9 Opaque ..................................................24
7.1.10 Counter64 ..............................................24
7.1.11 Unsigned32 .............................................25
7.1.12 Conceptual Tables ......................................25
7.1.12.1 Creation and Deletion of Conceptual Rows .............26
7.2 Mapping of the UNITS clause ...............................26
7.3 Mapping of the MAX-ACCESS clause ..........................26
7.4 Mapping of the STATUS clause ..............................27
7.5 Mapping of the DESCRIPTION clause .........................27
7.6 Mapping of the REFERENCE clause ...........................27
7.7 Mapping of the INDEX clause ...............................27
7.8 Mapping of the AUGMENTS clause ............................29
7.8.1 Relation between INDEX and AUGMENTS clauses .............30
7.9 Mapping of the DEFVAL clause ..............................30
7.10 Mapping of the OBJECT-TYPE value .........................31
7.11 Usage Example ............................................32
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8 Mapping of the NOTIFICATION-TYPE macro ......................34
8.1 Mapping of the OBJECTS clause .............................34
8.2 Mapping of the STATUS clause ..............................34
8.3 Mapping of the DESCRIPTION clause .........................35
8.4 Mapping of the REFERENCE clause ...........................35
8.5 Mapping of the NOTIFICATION-TYPE value ....................35
8.6 Usage Example .............................................35
9 Refined Syntax ..............................................36
10 Extending an Information Module ............................37
10.1 Object Assignments .......................................37
10.2 Object Definitions .......................................38
10.3 Notification Definitions .................................39
11 Appendix A: Detailed Sub-typing Rules ......................40
11.1 Syntax Rules .............................................40
11.2 Examples .................................................41
12 Security Considerations ....................................41
13 Editors' Addresses .........................................41
14 References .................................................42
15 Full Copyright Statement ...................................43
1. Introduction
Management information is viewed as a collection of managed objects,
residing in a virtual information store, termed the Management
Information Base (MIB). Collections of related objects are defined
in MIB modules. These modules are written using an adapted subset of
OSI's Abstract Syntax Notation One, ASN.1 (1988) [1]. It is the
purpose of this document, the Structure of Management Information
(SMI), to define that adapted subset, and to assign a set of
associated administrative values.
The SMI is divided into three parts: module definitions, object
definitions, and, notification definitions.
(1) Module definitions are used when describing information modules.
An ASN.1 macro, MODULE-IDENTITY, is used to concisely convey the
semantics of an information module.
(2) Object definitions are used when describing managed objects. An
ASN.1 macro, OBJECT-TYPE, is used to concisely convey the syntax
and semantics of a managed object.
(3) Notification definitions are used when describing unsolicited
transmissions of management information. An ASN.1 macro,
NOTIFICATION-TYPE, is used to concisely convey the syntax and
semantics of a notification.
McCloghrie, et al. Standards Track [Page 3]
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1.1. A Note on Terminology
For the purpose of exposition, the original Structure of Management
Information, as described in RFCs 1155 (STD 16), 1212 (STD 16), and
RFC 1215, is termed the SMI version 1 (SMIv1). The current version
of the Structure of Management Information is termed SMI version 2
(SMIv2).
2. Definitions
SNMPv2-SMI DEFINITIONS ::= BEGIN
-- the path to the root
org OBJECT IDENTIFIER ::= { iso 3 } -- "iso" = 1
dod OBJECT IDENTIFIER ::= { org 6 }
internet OBJECT IDENTIFIER ::= { dod 1 }
directory OBJECT IDENTIFIER ::= { internet 1 }
mgmt OBJECT IDENTIFIER ::= { internet 2 }
mib-2 OBJECT IDENTIFIER ::= { mgmt 1 }
transmission OBJECT IDENTIFIER ::= { mib-2 10 }
experimental OBJECT IDENTIFIER ::= { internet 3 }
private OBJECT IDENTIFIER ::= { internet 4 }
enterprises OBJECT IDENTIFIER ::= { private 1 }
security OBJECT IDENTIFIER ::= { internet 5 }
snmpV2 OBJECT IDENTIFIER ::= { internet 6 }
-- transport domains
snmpDomains OBJECT IDENTIFIER ::= { snmpV2 1 }
-- transport proxies
snmpProxys OBJECT IDENTIFIER ::= { snmpV2 2 }
-- module identities
snmpModules OBJECT IDENTIFIER ::= { snmpV2 3 }
-- Extended UTCTime, to allow dates with four-digit years
-- (Note that this definition of ExtUTCTime is not to be IMPORTed
-- by MIB modules.)
ExtUTCTime ::= OCTET STRING(SIZE(11 | 13))
-- format is YYMMDDHHMMZ or YYYYMMDDHHMMZ
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-- where: YY - last two digits of year (only years
-- between 1900-1999)
-- YYYY - last four digits of the year (any year)
-- MM - month (01 through 12)
-- DD - day of month (01 through 31)
-- HH - hours (00 through 23)
-- MM - minutes (00 through 59)
-- Z - denotes GMT (the ASCII character Z)
--
-- For example, "9502192015Z" and "199502192015Z" represent
-- 8:15pm GMT on 19 February 1995. Years after 1999 must use
-- the four digit year format. Years 1900-1999 may use the
-- two or four digit format.
-- definitions for information modules
MODULE-IDENTITY MACRO ::=
BEGIN
TYPE NOTATION ::=
"LAST-UPDATED" value(Update ExtUTCTime)
"ORGANIZATION" Text
"CONTACT-INFO" Text
"DESCRIPTION" Text
RevisionPart
VALUE NOTATION ::=
value(VALUE OBJECT IDENTIFIER)
RevisionPart ::=
Revisions
| empty
Revisions ::=
Revision
| Revisions Revision
Revision ::=
"REVISION" value(Update ExtUTCTime)
"DESCRIPTION" Text
-- a character string as defined in section 3.1.1
Text ::= value(IA5String)
END
OBJECT-IDENTITY MACRO ::=
BEGIN
TYPE NOTATION ::=
"STATUS" Status
"DESCRIPTION" Text
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ReferPart
VALUE NOTATION ::=
value(VALUE OBJECT IDENTIFIER)
Status ::=
"current"
| "deprecated"
| "obsolete"
ReferPart ::=
"REFERENCE" Text
| empty
-- a character string as defined in section 3.1.1
Text ::= value(IA5String)
END
-- names of objects
-- (Note that these definitions of ObjectName and NotificationName
-- are not to be IMPORTed by MIB modules.)
ObjectName ::=
OBJECT IDENTIFIER
NotificationName ::=
OBJECT IDENTIFIER
-- syntax of objects
-- the "base types" defined here are:
-- 3 built-in ASN.1 types: INTEGER, OCTET STRING, OBJECT IDENTIFIER
-- 8 application-defined types: Integer32, IpAddress, Counter32,
-- Gauge32, Unsigned32, TimeTicks, Opaque, and Counter64
ObjectSyntax ::=
CHOICE {
simple
SimpleSyntax,
-- note that SEQUENCEs for conceptual tables and
-- rows are not mentioned here...
application-wide
ApplicationSyntax
}
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RFC 2578 SMIv2 April 1999
-- built-in ASN.1 types
SimpleSyntax ::=
CHOICE {
-- INTEGERs with a more restrictive range
-- may also be used
integer-value -- includes Integer32
INTEGER (-2147483648..2147483647),
-- OCTET STRINGs with a more restrictive size
-- may also be used
string-value
OCTET STRING (SIZE (0..65535)),
objectID-value
OBJECT IDENTIFIER
}
-- indistinguishable from INTEGER, but never needs more than
-- 32-bits for a two's complement representation
Integer32 ::=
INTEGER (-2147483648..2147483647)
-- application-wide types
ApplicationSyntax ::=
CHOICE {
ipAddress-value
IpAddress,
counter-value
Counter32,
timeticks-value
TimeTicks,
arbitrary-value
Opaque,
big-counter-value
Counter64,
unsigned-integer-value -- includes Gauge32
Unsigned32
}
-- in network-byte order
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RFC 2578 SMIv2 April 1999
-- (this is a tagged type for historical reasons)
IpAddress ::=
[APPLICATION 0]
IMPLICIT OCTET STRING (SIZE (4))
-- this wraps
Counter32 ::=
[APPLICATION 1]
IMPLICIT INTEGER (0..4294967295)
-- this doesn't wrap
Gauge32 ::=
[APPLICATION 2]
IMPLICIT INTEGER (0..4294967295)
-- an unsigned 32-bit quantity
-- indistinguishable from Gauge32
Unsigned32 ::=
[APPLICATION 2]
IMPLICIT INTEGER (0..4294967295)
-- hundredths of seconds since an epoch
TimeTicks ::=
[APPLICATION 3]
IMPLICIT INTEGER (0..4294967295)
-- for backward-compatibility only
Opaque ::=
[APPLICATION 4]
IMPLICIT OCTET STRING
-- for counters that wrap in less than one hour with only 32 bits
Counter64 ::=
[APPLICATION 6]
IMPLICIT INTEGER (0..18446744073709551615)
-- definition for objects
OBJECT-TYPE MACRO ::=
BEGIN
TYPE NOTATION ::=
"SYNTAX" Syntax
UnitsPart
"MAX-ACCESS" Access
"STATUS" Status
"DESCRIPTION" Text
ReferPart
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IndexPart
DefValPart
VALUE NOTATION ::=
value(VALUE ObjectName)
Syntax ::= -- Must be one of the following:
-- a base type (or its refinement),
-- a textual convention (or its refinement), or
-- a BITS pseudo-type
type
| "BITS" "{" NamedBits "}"
NamedBits ::= NamedBit
| NamedBits "," NamedBit
NamedBit ::= identifier "(" number ")" -- number is nonnegative
UnitsPart ::=
"UNITS" Text
| empty
Access ::=
"not-accessible"
| "accessible-for-notify"
| "read-only"
| "read-write"
| "read-create"
Status ::=
"current"
| "deprecated"
| "obsolete"
ReferPart ::=
"REFERENCE" Text
| empty
IndexPart ::=
"INDEX" "{" IndexTypes "}"
| "AUGMENTS" "{" Entry "}"
| empty
IndexTypes ::=
IndexType
| IndexTypes "," IndexType
IndexType ::=
"IMPLIED" Index
| Index
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Index ::=
-- use the SYNTAX value of the
-- correspondent OBJECT-TYPE invocation
value(ObjectName)
Entry ::=
-- use the INDEX value of the
-- correspondent OBJECT-TYPE invocation
value(ObjectName)
DefValPart ::= "DEFVAL" "{" Defvalue "}"
| empty
Defvalue ::= -- must be valid for the type specified in
-- SYNTAX clause of same OBJECT-TYPE macro
value(ObjectSyntax)
| "{" BitsValue "}"
BitsValue ::= BitNames
| empty
BitNames ::= BitName
| BitNames "," BitName
BitName ::= identifier
-- a character string as defined in section 3.1.1
Text ::= value(IA5String)
END
-- definitions for notifications
NOTIFICATION-TYPE MACRO ::=
BEGIN
TYPE NOTATION ::=
ObjectsPart
"STATUS" Status
"DESCRIPTION" Text
ReferPart
VALUE NOTATION ::=
value(VALUE NotificationName)
ObjectsPart ::=
"OBJECTS" "{" Objects "}"
| empty
Objects ::=
Object
McCloghrie, et al. Standards Track [Page 10]
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| Objects "," Object
Object ::=
value(ObjectName)
Status ::=
"current"
| "deprecated"
| "obsolete"
ReferPart ::=
"REFERENCE" Text
| empty
-- a character string as defined in section 3.1.1
Text ::= value(IA5String)
END
-- definitions of administrative identifiers
zeroDotZero OBJECT-IDENTITY
STATUS current
DESCRIPTION
"A value used for null identifiers."
::= { 0 0 }
END
3. Information Modules
An "information module" is an ASN.1 module defining information
relating to network management.
The SMI describes how to use an adapted subset of ASN.1 (1988) to
define an information module. Further, additional restrictions are
placed on "standard" information modules. It is strongly recommended
that "enterprise-specific" information modules also adhere to these
restrictions.
Typically, there are three kinds of information modules:
(1) MIB modules, which contain definitions of inter-related managed
objects, make use of the OBJECT-TYPE and NOTIFICATION-TYPE macros;
(2) compliance statements for MIB modules, which make use of the
MODULE-COMPLIANCE and OBJECT-GROUP macros [2]; and,
(3) capability statements for agent implementations which make use of
the AGENT-CAPABILITIES macros [2].
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This classification scheme does not imply a rigid taxonomy. For
example, a "standard" information module will normally include
definitions of managed objects and a compliance statement.
Similarly, an "enterprise-specific" information module might include
definitions of managed objects and a capability statement. Of
course, a "standard" information module may not contain capability
statements.
The constructs of ASN.1 allowed in SMIv2 information modules include:
the IMPORTS clause, value definitions for OBJECT IDENTIFIERs, type
definitions for SEQUENCEs (with restrictions), ASN.1 type assignments
of the restricted ASN.1 types allowed in SMIv2, and instances of
ASN.1 macros defined in this document and its companion documents [2,
3]. Additional ASN.1 macros must not be defined in SMIv2 information
modules. SMIv1 macros must not be used in SMIv2 information modules.
The names of all standard information modules must be unique (but
different versions of the same information module should have the
same name). Developers of enterprise information modules are
encouraged to choose names for their information modules that will
have a low probability of colliding with standard or other enterprise
information modules. An information module may not use the ASN.1
construct of placing an object identifier value between the module
name and the "DEFINITIONS" keyword. For the purposes of this
specification, an ASN.1 module name begins with an upper-case letter
and continues with zero or more letters, digits, or hyphens, except
that a hyphen can not be the last character, nor can there be two
consecutive hyphens.
All information modules start with exactly one invocation of the
MODULE-IDENTITY macro, which provides contact information as well as
revision history to distinguish between versions of the same
information module. This invocation must appear immediately after
any IMPORTs statements.
3.1. Macro Invocation
Within an information module, each macro invocation appears as:
<descriptor> <macro> <clauses> ::= <value>
where <descriptor> corresponds to an ASN.1 identifier, <macro> names
the macro being invoked, and <clauses> and <value> depend on the
definition of the macro. (Note that this definition of a descriptor
applies to all macros defined in this memo and in [2].)
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RFC 2578 SMIv2 April 1999
For the purposes of this specification, an ASN.1 identifier consists
of one or more letters or digits, and its initial character must be a
lower-case letter. Note that hyphens are not allowed by this
specification (except for use by information modules converted from
SMIv1 which did allow hyphens).
For all descriptors appearing in an information module, the
descriptor shall be unique and mnemonic, and shall not exceed 64
characters in length. (However, descriptors longer than 32
characters are not recommended.) This promotes a common language for
humans to use when discussing the information module and also
facilitates simple table mappings for user-interfaces.
The set of descriptors defined in all "standard" information modules
shall be unique.
Finally, by convention, if the descriptor refers to an object with a
SYNTAX clause value of either Counter32 or Counter64, then the
descriptor used for the object should denote plurality.
3.1.1. Textual Values and Strings
Some clauses in a macro invocation may take a character string as a
textual value (e.g., the DESCRIPTION clause). Other clauses take
binary or hexadecimal strings (in any position where a non-negative
number is allowed).
A character string is preceded and followed by the quote character
("), and consists of an arbitrary number (possibly zero) of:
- any 7-bit displayable ASCII characters except quote ("),
- tab characters,
- spaces, and
- line terminator characters (\n or \r\n).
The value of a character string is interpreted as ASCII.
A binary string consists of a number (possibly zero) of zeros and
ones preceded by a single (') and followed by either the pair ('B) or
('b), where the number is a multiple of eight.
A hexadecimal string consists of an even number (possibly zero) of
hexadecimal digits, preceded by a single (') and followed by either
the pair ('H) or ('h). Digits specified via letters can be in upper
or lower case.
Note that ASN.1 comments can not be enclosed inside any of these
types of strings.
McCloghrie, et al. Standards Track [Page 13]
RFC 2578 SMIv2 April 1999
3.2. IMPORTing Symbols
To reference an external object, the IMPORTS statement must be used
to identify both the descriptor and the module in which the
descriptor is defined, where the module is identified by its ASN.1
module name.
Note that when symbols from "enterprise-specific" information modules
are referenced (e.g., a descriptor), there is the possibility of
collision. As such, if different objects with the same descriptor
are IMPORTed, then this ambiguity is resolved by prefixing the
descriptor with the name of the information module and a dot ("."),
i.e.,
"module.descriptor"
(All descriptors must be unique within any information module.)
Of course, this notation can be used to refer to objects even when
there is no collision when IMPORTing symbols.
Finally, if any of the ASN.1 named types and macros defined in this
document, specifically:
Counter32, Counter64, Gauge32, Integer32, IpAddress, MODULE-
IDENTITY, NOTIFICATION-TYPE, Opaque, OBJECT-TYPE, OBJECT-
IDENTITY, TimeTicks, Unsigned32,
or any of those defined in [2] or [3], are used in an information
module, then they must be imported using the IMPORTS statement.
However, the following must not be included in an IMPORTS statement:
- named types defined by ASN.1 itself, specifically: INTEGER,
OCTET STRING, OBJECT IDENTIFIER, SEQUENCE, SEQUENCE OF type,
- the BITS construct.
3.3. Exporting Symbols
The ASN.1 EXPORTS statement is not allowed in SMIv2 information
modules. All items defined in an information module are
automatically exported.
3.4. ASN.1 Comments
ASN.1 comments can be included in an information module. However, it
is recommended that all substantive descriptions be placed within an
appropriate DESCRIPTION clause.
McCloghrie, et al. Standards Track [Page 14]
RFC 2578 SMIv2 April 1999
ASN.1 comments commence with a pair of adjacent hyphens and end with
the next pair of adjacent hyphens or at the end of the line,
whichever occurs first. Comments ended by a pair of hyphens have the
effect of a single space character.
3.5. OBJECT IDENTIFIER values
An OBJECT IDENTIFIER value is an ordered list of non-negative
numbers. For the SMIv2, each number in the list is referred to as a
sub-identifier, there are at most 128 sub-identifiers in a value, and
each sub-identifier has a maximum value of 2^32-1 (4294967295
decimal).
All OBJECT IDENTIFIER values have at least two sub-identifiers, where
the value of the first sub-identifier is one of the following well-
known names:
Value Name
0 ccitt
1 iso
2 joint-iso-ccitt
(Note that this SMI does not recognize "new" well-known names, e.g.,
as defined when the CCITT became the ITU.)
3.6. OBJECT IDENTIFIER usage
OBJECT IDENTIFIERs are used in information modules in two ways:
(1) registration: the definition of a particular item is registered as
a particular OBJECT IDENTIFIER value, and associated with a
particular descriptor. After such a registration, the semantics
thereby associated with the value are not allowed to change, the
OBJECT IDENTIFIER can not be used for any other registration, and
the descriptor can not be changed nor associated with any other
registration. The following macros result in a registration:
OBJECT-TYPE, MODULE-IDENTITY, NOTIFICATION-TYPE, OBJECT-GROUP,
OBJECT-IDENTITY, NOTIFICATION-GROUP, MODULE-COMPLIANCE,
AGENT-CAPABILITIES.
(2) assignment: a descriptor can be assigned to a particular OBJECT
IDENTIFIER value. For this usage, the semantics associated with
the OBJECT IDENTIFIER value is not allowed to change, and a
descriptor assigned to a particular OBJECT IDENTIFIER value cannot
subsequently be assigned to another. However, multiple descriptors
can be assigned to the same OBJECT IDENTIFIER value. Such
assignments are specified in the following manner:
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RFC 2578 SMIv2 April 1999
mib OBJECT IDENTIFIER ::= { mgmt 1 } -- from RFC1156
mib-2 OBJECT IDENTIFIER ::= { mgmt 1 } -- from RFC1213
fredRouter OBJECT IDENTIFIER ::= { flintStones 1 1 }
barneySwitch OBJECT IDENTIFIER ::= { flintStones bedrock(2) 1 }
Note while the above examples are legal, the following is not:
dinoHost OBJECT IDENTIFIER ::= { flintStones bedrock 2 }
A descriptor is allowed to be associated with both a registration and
an assignment, providing both are associated with the same OBJECT
IDENTIFIER value and semantics.
3.7. Reserved Keywords
The following are reserved keywords which must not be used as
descriptors or module names:
ABSENT ACCESS AGENT-CAPABILITIES ANY APPLICATION AUGMENTS BEGIN
BIT BITS BOOLEAN BY CHOICE COMPONENT COMPONENTS CONTACT-INFO
CREATION-REQUIRES Counter32 Counter64 DEFAULT DEFINED
DEFINITIONS DEFVAL DESCRIPTION DISPLAY-HINT END ENUMERATED
ENTERPRISE EXPLICIT EXPORTS EXTERNAL FALSE FROM GROUP Gauge32
IDENTIFIER IMPLICIT IMPLIED IMPORTS INCLUDES INDEX INTEGER
Integer32 IpAddress LAST-UPDATED MANDATORY-GROUPS MAX MAX-ACCESS
MIN MIN-ACCESS MINUS-INFINITY MODULE MODULE-COMPLIANCE MODULE-
IDENTITY NOTIFICATION-GROUP NOTIFICATION-TYPE NOTIFICATIONS NULL
OBJECT OBJECT-GROUP OBJECT-IDENTITY OBJECT-TYPE OBJECTS OCTET OF
OPTIONAL ORGANIZATION Opaque PLUS-INFINITY PRESENT PRIVATE
PRODUCT-RELEASE REAL REFERENCE REVISION SEQUENCE SET SIZE STATUS
STRING SUPPORTS SYNTAX TAGS TEXTUAL-CONVENTION TRAP-TYPE TRUE
TimeTicks UNITS UNIVERSAL Unsigned32 VARIABLES VARIATION WITH
WRITE-SYNTAX
4. Naming Hierarchy
The root of the subtree administered by the Internet Assigned Numbers
Authority (IANA) for the Internet is:
internet OBJECT IDENTIFIER ::= { iso 3 6 1 }
That is, the Internet subtree of OBJECT IDENTIFIERs starts with the
prefix:
1.3.6.1.
Several branches underneath this subtree are used for network
management:
McCloghrie, et al. Standards Track [Page 16]
RFC 2578 SMIv2 April 1999
mgmt OBJECT IDENTIFIER ::= { internet 2 }
experimental OBJECT IDENTIFIER ::= { internet 3 }
private OBJECT IDENTIFIER ::= { internet 4 }
enterprises OBJECT IDENTIFIER ::= { private 1 }
However, the SMI does not prohibit the definition of objects in other
portions of the object tree.
The mgmt(2) subtree is used to identify "standard" objects.
The experimental(3) subtree is used to identify objects being
designed by working groups of the IETF. If an information module
produced by a working group becomes a "standard" information module,
then at the very beginning of its entry onto the Internet standards
track, the objects are moved under the mgmt(2) subtree.
The private(4) subtree is used to identify objects defined
unilaterally. The enterprises(1) subtree beneath private is used,
among other things, to permit providers of networking subsystems to
register models of their products.
5. Mapping of the MODULE-IDENTITY macro
The MODULE-IDENTITY macro is used to provide contact and revision
history for each information module. It must appear exactly once in
every information module. It should be noted that the expansion of
the MODULE-IDENTITY macro is something which conceptually happens
during implementation and not during run-time.
Note that reference in an IMPORTS clause or in clauses of SMIv2
macros to an information module is NOT through the use of the
'descriptor' of a MODULE-IDENTITY macro; rather, an information
module is referenced through specifying its module name.
5.1. Mapping of the LAST-UPDATED clause
The LAST-UPDATED clause, which must be present, contains the date and
time that this information module was last edited.
5.2. Mapping of the ORGANIZATION clause
The ORGANIZATION clause, which must be present, contains a textual
description of the organization under whose auspices this information
module was developed.
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5.3. Mapping of the CONTACT-INFO clause
The CONTACT-INFO clause, which must be present, contains the name,
postal address, telephone number, and electronic mail address of the
person to whom technical queries concerning this information module
should be sent.
5.4. Mapping of the DESCRIPTION clause
The DESCRIPTION clause, which must be present, contains a high-level
textual description of the contents of this information module.
5.5. Mapping of the REVISION clause
The REVISION clause, which need not be present, is repeatedly used to
describe the revisions (including the initial version) made to this
information module, in reverse chronological order (i.e., most recent
first). Each instance of this clause contains the date and time of
the revision.
5.5.1. Mapping of the DESCRIPTION sub-clause
The DESCRIPTION sub-clause, which must be present for each REVISION
clause, contains a high-level textual description of the revision
identified in that REVISION clause.
5.6. Mapping of the MODULE-IDENTITY value
The value of an invocation of the MODULE-IDENTITY macro is an OBJECT
IDENTIFIER. As such, this value may be authoritatively used when
specifying an OBJECT IDENTIFIER value to refer to the information
module containing the invocation.
Note that it is a common practice to use the value of the MODULE-
IDENTITY macro as a subtree under which other OBJECT IDENTIFIER
values assigned within the module are defined. However, it is legal
(and occasionally necessary) for the other OBJECT IDENTIFIER values
assigned within the module to be unrelated to the OBJECT IDENTIFIER
value of the MODULE-IDENTITY macro.
5.7. Usage Example
Consider how a skeletal MIB module might be constructed: e.g.,
FIZBIN-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE, experimental
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FROM SNMPv2-SMI;
fizbin MODULE-IDENTITY
LAST-UPDATED "199505241811Z"
ORGANIZATION "IETF SNMPv2 Working Group"
CONTACT-INFO
" Marshall T. Rose
Postal: Dover Beach Consulting, Inc.
420 Whisman Court
Mountain View, CA 94043-2186
US
Tel: +1 415 968 1052
Fax: +1 415 968 2510
E-mail: mrose@dbc.mtview.ca.us"
DESCRIPTION
"The MIB module for entities implementing the xxxx
protocol."
REVISION "9505241811Z"
DESCRIPTION
"The latest version of this MIB module."
REVISION "9210070433Z"
DESCRIPTION
"The initial version of this MIB module, published in
RFC yyyy."
-- contact IANA for actual number
::= { experimental xx }
END
6. Mapping of the OBJECT-IDENTITY macro
The OBJECT-IDENTITY macro is used to define information about an
OBJECT IDENTIFIER assignment. All administrative OBJECT IDENTIFIER
assignments which define a type identification value (see
AutonomousType, a textual convention defined in [3]) should be
defined via the OBJECT-IDENTITY macro. It should be noted that the
expansion of the OBJECT-IDENTITY macro is something which
conceptually happens during implementation and not during run-time.
6.1. Mapping of the STATUS clause
The STATUS clause, which must be present, indicates whether this
definition is current or historic.
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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. Mapping of the DESCRIPTION clause
The DESCRIPTION clause, which must be present, contains a textual
description of the object assignment.
6.3. Mapping of the REFERENCE clause
The REFERENCE clause, which need not be present, contains a textual
cross-reference to some other document, either another information
module which defines a related assignment, or some other document
which provides additional information relevant to this definition.
6.4. Mapping of the OBJECT-IDENTITY value
The value of an invocation of the OBJECT-IDENTITY macro is an OBJECT
IDENTIFIER.
6.5. Usage Example
Consider how an OBJECT IDENTIFIER assignment might be made: e.g.,
fizbin69 OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The authoritative identity of the Fizbin 69 chipset."
::= { fizbinChipSets 1 }
7. Mapping of the OBJECT-TYPE macro
The OBJECT-TYPE macro is used to define a type of managed object. It
should be noted that the expansion of the OBJECT-TYPE macro is
something which conceptually happens during implementation and not
during run-time.
For leaf objects which are not columnar objects (i.e., not contained
within a conceptual table), instances of the object are identified by
appending a sub-identifier of zero to the name of that object.
Otherwise, the INDEX clause of the conceptual row object superior to
a columnar object defines instance identification information.
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7.1. Mapping of the SYNTAX clause
The SYNTAX clause, which must be present, defines the abstract data
structure corresponding to that object. The data structure must be
one of the following: a base type, the BITS construct, or a textual
convention. (SEQUENCE OF and SEQUENCE are also possible for
conceptual tables, see section 7.1.12). The base types are those
defined in the ObjectSyntax CHOICE. A textual convention is a
newly-defined type defined as a sub-type of a base type [3].
An extended subset of the full capabilities of ASN.1 (1988) sub-
typing is allowed, as appropriate to the underlying ASN.1 type. Any
such restriction on size, range or enumerations specified in this
clause represents the maximal level of support which makes "protocol
sense". Restrictions on sub-typing are specified in detail in
Section 9 and Appendix A of this memo.
The semantics of ObjectSyntax are now described.
7.1.1. Integer32 and INTEGER
The Integer32 type represents integer-valued information between
-2^31 and 2^31-1 inclusive (-2147483648 to 2147483647 decimal). This
type is indistinguishable from the INTEGER type. Both the INTEGER
and Integer32 types may be sub-typed to be more constrained than the
Integer32 type.
The INTEGER type (but not the Integer32 type) may also be used to
represent integer-valued information as named-number enumerations.
In this case, only those named-numbers so enumerated may be present
as a value. Note that although it is recommended that enumerated
values start at 1 and be numbered contiguously, any valid value for
Integer32 is allowed for an enumerated value and, further, enumerated
values needn't be contiguously assigned.
Finally, a label for a named-number enumeration must consist of one
or more letters or digits, up to a maximum of 64 characters, and the
initial character must be a lower-case letter. (However, labels
longer than 32 characters are not recommended.) Note that hyphens
are not allowed by this specification (except for use by information
modules converted from SMIv1 which did allow hyphens).
7.1.2. OCTET STRING
The OCTET STRING type represents arbitrary binary or textual data.
Although the SMI-specified size limitation for this type is 65535
octets, MIB designers should realize that there may be implementation
and interoperability limitations for sizes in excess of 255 octets.
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7.1.3. OBJECT IDENTIFIER
The OBJECT IDENTIFIER type represents administratively assigned
names. Any instance of this type may have at most 128 sub-
identifiers. Further, each sub-identifier must not exceed the value
2^32-1 (4294967295 decimal).
7.1.4. The BITS construct
The BITS construct represents an enumeration of named bits. This
collection is assigned non-negative, contiguous (but see below)
values, starting at zero. Only those named-bits so enumerated may be
present in a value. (Thus, enumerations must be assigned to
consecutive bits; however, see Section 9 for refinements of an object
with this syntax.)
As part of updating an information module, for an object defined
using the BITS construct, new enumerations can be added or existing
enumerations can have new labels assigned to them. After an
enumeration is added, it might not be possible to distinguish between
an implementation of the updated object for which the new enumeration
is not asserted, and an implementation of the object prior to the
addition. Depending on the circumstances, such an ambiguity could
either be desirable or could be undesirable. The means to avoid such
an ambiguity is dependent on the encoding of values on the wire;
however, one possibility is to define new enumerations starting at
the next multiple of eight bits. (Of course, this can also result in
the enumerations no longer being contiguous.)
Although there is no SMI-specified limitation on the number of
enumerations (and therefore on the length of a value), except as may
be imposed by the limit on the length of an OCTET STRING, MIB
designers should realize that there may be implementation and
interoperability limitations for sizes in excess of 128 bits.
Finally, a label for a named-number enumeration must consist of one
or more letters or digits, up to a maximum of 64 characters, and the
initial character must be a lower-case letter. (However, labels
longer than 32 characters are not recommended.) Note that hyphens
are not allowed by this specification.
7.1.5. IpAddress
The IpAddress type represents a 32-bit internet address. It is
represented as an OCTET STRING of length 4, in network byte-order.
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Note that the IpAddress type is a tagged type for historical reasons.
Network addresses should be represented using an invocation of the
TEXTUAL-CONVENTION macro [3].
7.1.6. Counter32
The Counter32 type represents a non-negative integer which
monotonically increases until it reaches a maximum value of 2^32-1
(4294967295 decimal), when it wraps around and starts increasing
again from zero.
Counters have no defined "initial" value, and thus, a single value of
a Counter has (in general) no information content. Discontinuities
in the monotonically increasing value normally occur at re-
initialization of the management system, and at other times as
specified in the description of an object-type using this ASN.1 type.
If such other times can occur, for example, the creation of an object
instance at times other than re-initialization, then a corresponding
object should be defined, with an appropriate SYNTAX clause, to
indicate the last discontinuity. Examples of appropriate SYNTAX
clause include: TimeStamp (a textual convention defined in [3]),
DateAndTime (another textual convention from [3]) or TimeTicks.
The value of the MAX-ACCESS clause for objects with a SYNTAX clause
value of Counter32 is either "read-only" or "accessible-for-notify".
A DEFVAL clause is not allowed for objects with a SYNTAX clause value
of Counter32.
7.1.7. Gauge32
The Gauge32 type represents a non-negative integer, which may
increase or decrease, but shall never exceed a maximum value, nor
fall below a minimum value. The maximum value can not be greater
than 2^32-1 (4294967295 decimal), and the minimum value can not be
smaller than 0. The value of a Gauge32 has its maximum value
whenever the information being modeled is greater than or equal to
its maximum value, and has its minimum value whenever the information
being modeled is smaller than or equal to its minimum value. If the
information being modeled subsequently decreases below (increases
above) the maximum (minimum) value, the Gauge32 also decreases
(increases). (Note that despite of the use of the term "latched" in
the original definition of this type, it does not become "stuck" at
its maximum or minimum value.)
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7.1.8. TimeTicks
The TimeTicks type represents a non-negative integer which represents
the time, modulo 2^32 (4294967296 decimal), in hundredths of a second
between two epochs. When objects are defined which use this ASN.1
type, the description of the object identifies both of the reference
epochs.
For example, [3] defines the TimeStamp textual convention which is
based on the TimeTicks type. With a TimeStamp, the first reference
epoch is defined as the time when sysUpTime [5] was zero, and the
second reference epoch is defined as the current value of sysUpTime.
The TimeTicks type may not be sub-typed.
7.1.9. Opaque
The Opaque type is provided solely for backward-compatibility, and
shall not be used for newly-defined object types.
The Opaque type supports the capability to pass arbitrary ASN.1
syntax. A value is encoded using the ASN.1 Basic Encoding Rules [4]
into a string of octets. This, in turn, is encoded as an OCTET
STRING, in effect "double-wrapping" the original ASN.1 value.
Note that a conforming implementation need only be able to accept and
recognize opaquely-encoded data. It need not be able to unwrap the
data and then interpret its contents.
A requirement on "standard" MIB modules is that no object may have a
SYNTAX clause value of Opaque.
7.1.10. Counter64
The Counter64 type represents a non-negative integer which
monotonically increases until it reaches a maximum value of 2^64-1
(18446744073709551615 decimal), when it wraps around and starts
increasing again from zero.
Counters have no defined "initial" value, and thus, a single value of
a Counter has (in general) no information content. Discontinuities
in the monotonically increasing value normally occur at re-
initialization of the management system, and at other times as
specified in the description of an object-type using this ASN.1 type.
If such other times can occur, for example, the creation of an object
instance at times other than re-initialization, then a corresponding
object should be defined, with an appropriate SYNTAX clause, to
indicate the last discontinuity. Examples of appropriate SYNTAX
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clause are: TimeStamp (a textual convention defined in [3]),
DateAndTime (another textual convention from [3]) or TimeTicks.
The value of the MAX-ACCESS clause for objects with a SYNTAX clause
value of Counter64 is either "read-only" or "accessible-for-notify".
A requirement on "standard" MIB modules is that the Counter64 type
may be used only if the information being modeled would wrap in less
than one hour if the Counter32 type was used instead.
A DEFVAL clause is not allowed for objects with a SYNTAX clause value
of Counter64.
7.1.11. Unsigned32
The Unsigned32 type represents integer-valued information between 0
and 2^32-1 inclusive (0 to 4294967295 decimal).
7.1.12. Conceptual Tables
Management operations apply exclusively to scalar objects. However,
it is sometimes convenient for developers of management applications
to impose an imaginary, tabular structure on an ordered collection of
objects within the MIB. Each such conceptual table contains zero or
more rows, and each row may contain one or more scalar objects,
termed columnar objects. This conceptualization is formalized by
using the OBJECT-TYPE macro to define both an object which
corresponds to a table and an object which corresponds to a row in
that table. A conceptual table has SYNTAX of the form:
SEQUENCE OF <EntryType>
where <EntryType> refers to the SEQUENCE type of its subordinate
conceptual row. A conceptual row has SYNTAX of the form:
<EntryType>
where <EntryType> is a SEQUENCE type defined as follows:
<EntryType> ::= SEQUENCE { <type1>, ... , <typeN> }
where there is one <type> for each subordinate object, and each
<type> is of the form:
<descriptor> <syntax>
where <descriptor> is the descriptor naming a subordinate object, and
<syntax> has the value of that subordinate object's SYNTAX clause,
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except that both sub-typing information and the named values for
enumerated integers or the named bits for the BITS construct, are
omitted from <syntax>.
Further, a <type> is always present for every subordinate object.
(The ASN.1 DEFAULT and OPTIONAL clauses are disallowed in the
SEQUENCE definition.) The MAX-ACCESS clause for conceptual tables
and rows is "not-accessible".
7.1.12.1. Creation and Deletion of Conceptual Rows
For newly-defined conceptual rows which allow the creation of new
object instances and/or the deletion of existing object instances,
there should be one columnar object with a SYNTAX clause value of
RowStatus (a textual convention defined in [3]) and a MAX-ACCESS
clause value of read-create. By convention, this is termed the
status column for the conceptual row.
7.2. Mapping of the UNITS clause
This UNITS clause, which need not be present, contains a textual
definition of the units associated with that object.
7.3. Mapping of the MAX-ACCESS clause
The MAX-ACCESS clause, which must be present, defines whether it
makes "protocol sense" to read, write and/or create an instance of
the object, or to include its value in a notification. This is the
maximal level of access for the object. (This maximal level of
access is independent of any administrative authorization policy.)
The value "read-write" indicates that read and write access make
"protocol sense", but create does not. The value "read-create"
indicates that read, write and create access make "protocol sense".
The value "not-accessible" indicates an auxiliary object (see Section
7.7). The value "accessible-for-notify" indicates an object which is
accessible only via a notification (e.g., snmpTrapOID [5]).
These values are ordered, from least to greatest: "not-accessible",
"accessible-for-notify", "read-only", "read-write", "read-create".
If any columnar object in a conceptual row has "read-create" as its
maximal level of access, then no other columnar object of the same
conceptual row may have a maximal access of "read-write". (Note that
"read-create" is a superset of "read-write".)
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7.4. Mapping of the STATUS clause
The STATUS clause, which must be present, indicates whether this
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.
7.5. Mapping of the DESCRIPTION clause
The DESCRIPTION clause, which must be present, contains a textual
definition of that object which provides all semantic definitions
necessary for implementation, and should embody any information which
would otherwise be communicated in any ASN.1 commentary annotations
associated with the object.
7.6. Mapping of the REFERENCE clause
The REFERENCE clause, which need not be present, contains a textual
cross-reference to some other document, either another information
module which defines a related assignment, or some other document
which provides additional information relevant to this definition.
7.7. Mapping of the INDEX clause
The INDEX clause, which must be present if that object corresponds to
a conceptual row (unless an AUGMENTS clause is present instead), and
must be absent otherwise, defines instance identification information
for the columnar objects subordinate to that object.
The instance identification information in an INDEX clause must
specify object(s) such that value(s) of those object(s) will
unambiguously distinguish a conceptual row. The objects can be
columnar objects from the same and/or another conceptual table, but
must not be scalar objects. Multiple occurrences of the same object
in a single INDEX clause is strongly discouraged.
The syntax of the objects in the INDEX clause indicate how to form
the instance-identifier:
(1) integer-valued (i.e., having INTEGER as its underlying primitive
type): a single sub-identifier taking the integer value (this
works only for non-negative integers);
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(2) string-valued, fixed-length strings (or variable-length preceded by
the IMPLIED keyword): `n' sub-identifiers, where `n' is the length
of the string (each octet of the string is encoded in a separate
sub-identifier);
(3) string-valued, variable-length strings (not preceded by the IMPLIED
keyword): `n+1' sub-identifiers, where `n' is the length of the
string (the first sub-identifier is `n' itself, following this,
each octet of the string is encoded in a separate sub-identifier);
(4) object identifier-valued (when preceded by the IMPLIED keyword):
`n' sub-identifiers, where `n' is the number of sub-identifiers in
the value (each sub-identifier of the value is copied into a
separate sub-identifier);
(5) object identifier-valued (when not preceded by the IMPLIED
keyword): `n+1' sub-identifiers, where `n' is the number of sub-
identifiers in the value (the first sub-identifier is `n' itself,
following this, each sub-identifier in the value is copied);
(6) IpAddress-valued: 4 sub-identifiers, in the familiar a.b.c.d
notation.
Note that the IMPLIED keyword can only be present for an object
having a variable-length syntax (e.g., variable-length strings or
object identifier-valued objects), Further, the IMPLIED keyword can
only be associated with the last object in the INDEX clause.
Finally, the IMPLIED keyword may not be used on a variable-length
string object if that string might have a value of zero-length.
Since a single value of a Counter has (in general) no information
content (see section 7.1.6 and 7.1.10), objects defined using the
syntax, Counter32 or Counter64, must not be specified in an INDEX
clause. If an object defined using the BITS construct is used in an
INDEX clause, it is considered a variable-length string.
Instances identified by use of integer-valued objects should be
numbered starting from one (i.e., not from zero). The use of zero as
a value for an integer-valued index object should be avoided, except
in special cases.
Objects which are both specified in the INDEX clause of a conceptual
row and also columnar objects of the same conceptual row are termed
auxiliary objects. The MAX-ACCESS clause for auxiliary objects is
"not-accessible", except in the following circumstances:
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(1) within a MIB module originally written to conform to SMIv1, and
later converted to conform to SMIv2; or
(2) a conceptual row must contain at least one columnar object which is
not an auxiliary object. In the event that all of a conceptual
row's columnar objects are also specified in its INDEX clause, then
one of them must be accessible, i.e., have a MAX-ACCESS clause of
"read-only". (Note that this situation does not arise for a
conceptual row allowing create access, since such a row will have a
status column which will not be an auxiliary object.)
Note that objects specified in a conceptual row's INDEX clause need
not be columnar objects of that conceptual row. In this situation,
the DESCRIPTION clause of the conceptual row must include a textual
explanation of how the objects which are included in the INDEX clause
but not columnar objects of that conceptual row, are used in uniquely
identifying instances of the conceptual row's columnar objects.
7.8. Mapping of the AUGMENTS clause
The AUGMENTS clause, which must not be present unless the object
corresponds to a conceptual row, is an alternative to the INDEX
clause. Every object corresponding to a conceptual row has either an
INDEX clause or an AUGMENTS clause.
If an object corresponding to a conceptual row has an INDEX clause,
that row is termed a base conceptual row; alternatively, if the
object has an AUGMENTS clause, the row is said to be a conceptual row
augmentation, where the AUGMENTS clause names the object
corresponding to the base conceptual row which is augmented by this
conceptual row augmentation. (Thus, a conceptual row augmentation
cannot itself be augmented.) Instances of subordinate columnar
objects of a conceptual row augmentation are identified according to
the INDEX clause of the base conceptual row corresponding to the
object named in the AUGMENTS clause. Further, instances of
subordinate columnar objects of a conceptual row augmentation exist
according to the same semantics as instances of subordinate columnar
objects of the base conceptual row being augmented. As such, note
that creation of a base conceptual row implies the correspondent
creation of any conceptual row augmentations.
For example, a MIB designer might wish to define additional columns
in an "enterprise-specific" MIB which logically extend a conceptual
row in a "standard" MIB. The "standard" MIB definition of the
conceptual row would include the INDEX clause and the "enterprise-
specific" MIB would contain the definition of a conceptual row using
the AUGMENTS clause. On the other hand, it would be incorrect to use
the AUGMENTS clause for the relationship between RFC 2233's ifTable
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and the many media-specific MIBs which extend it for specific media
(e.g., the dot3Table in RFC 2358), since not all interfaces are of
the same media.
Note that a base conceptual row may be augmented by multiple
conceptual row augmentations.
7.8.1. Relation between INDEX and AUGMENTS clauses
When defining instance identification information for a conceptual
table:
(1) If there is a one-to-one correspondence between the conceptual rows
of this table and an existing table, then the AUGMENTS clause
should be used.
(2) Otherwise, if there is a sparse relationship between the conceptual
rows of this table and an existing table, then an INDEX clause
should be used which is identical to that in the existing table.
For example, the relationship between RFC 2233's ifTable and a
media-specific MIB which extends the ifTable for a specific media
(e.g., the dot3Table in RFC 2358), is a sparse relationship.
(3) Otherwise, if no existing objects have the required syntax and
semantics, then auxiliary objects should be defined within the
conceptual row for the new table, and those objects should be used
within the INDEX clause for the conceptual row.
7.9. Mapping of the DEFVAL clause
The DEFVAL clause, which need not be present, defines an acceptable
default value which may be used at the discretion of an agent when an
object instance is created. That is, the value is a "hint" to
implementors.
During conceptual row creation, if an instance of a columnar object
is not present as one of the operands in the correspondent management
protocol set operation, then the value of the DEFVAL clause, if
present, indicates an acceptable default value that an agent might
use (especially for a read-only object).
Note that with this definition of the DEFVAL clause, it is
appropriate to use it for any columnar object of a read-create table.
It is also permitted to use it for scalar objects dynamically created
by an agent, or for columnar objects of a read-write table
dynamically created by an agent.
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The value of the DEFVAL clause must, of course, correspond to the
SYNTAX clause for the object. If the value is an OBJECT IDENTIFIER,
then it must be expressed as a single ASN.1 identifier, and not as a
collection of sub-identifiers.
Note that if an operand to the management protocol set operation is
an instance of a read-only object, then the error `notWritable' [6]
will be returned. As such, the DEFVAL clause can be used to provide
an acceptable default value that an agent might use.
By way of example, consider the following possible DEFVAL clauses:
ObjectSyntax DEFVAL clause
---------------- ------------
Integer32 DEFVAL { 1 }
-- same for Gauge32, TimeTicks, Unsigned32
INTEGER DEFVAL { valid } -- enumerated value
OCTET STRING DEFVAL { 'ffffffffffff'H }
DisplayString DEFVAL { "SNMP agent" }
IpAddress DEFVAL { 'c0210415'H } -- 192.33.4.21
OBJECT IDENTIFIER DEFVAL { sysDescr }
BITS DEFVAL { { primary, secondary } }
-- enumerated values that are set
BITS DEFVAL { { } }
-- no enumerated values are set
A binary string used in a DEFVAL clause for an OCTET STRING must be
either an integral multiple of eight or zero bits in length;
similarly, a hexadecimal string must be an even number of hexadecimal
digits. The value of a character string used in a DEFVAL clause must
not contain tab characters or line terminator characters.
Object types with SYNTAX of Counter32 and Counter64 may not have
DEFVAL clauses, since they do not have defined initial values.
However, it is recommended that they be initialized to zero.
7.10. Mapping of the OBJECT-TYPE value
The value of an invocation of the OBJECT-TYPE macro is the name of
the object, which is an OBJECT IDENTIFIER, an administratively
assigned name.
When an OBJECT IDENTIFIER is assigned to an object:
(1) If the object corresponds to a conceptual table, then only a single
assignment, that for a conceptual row, is present immediately
beneath that object. The administratively assigned name for the
conceptual row object is derived by appending a sub-identifier of
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"1" to the administratively assigned name for the conceptual table.
(2) If the object corresponds to a conceptual row, then at least one
assignment, one for each column in the conceptual row, is present
beneath that object. The administratively assigned name for each
column is derived by appending a unique, positive sub-identifier to
the administratively assigned name for the conceptual row.
(3) Otherwise, no other OBJECT IDENTIFIERs which are subordinate to the
object may be assigned.
Note that the final sub-identifier of any administratively assigned
name for an object shall be positive. A zero-valued final sub-
identifier is reserved for future use.
7.11. Usage Example
Consider how one might define a conceptual table and its
subordinates. (This example uses the RowStatus textual convention
defined in [3].)
evalSlot OBJECT-TYPE
SYNTAX Integer32 (0..2147483647)
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The index number of the first unassigned entry in the
evaluation table, or the value of zero indicating that
all entries are assigned.
A management station should create new entries in the
evaluation table using this algorithm: first, issue a
management protocol retrieval operation to determine the
value of evalSlot; and, second, issue a management
protocol set operation to create an instance of the
evalStatus object setting its value to createAndGo(4) or
createAndWait(5). If this latter operation succeeds,
then the management station may continue modifying the
instances corresponding to the newly created conceptual
row, without fear of collision with other management
stations."
::= { eval 1 }
evalTable OBJECT-TYPE
SYNTAX SEQUENCE OF EvalEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
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"The (conceptual) evaluation table."
::= { eval 2 }
evalEntry OBJECT-TYPE
SYNTAX EvalEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"An entry (conceptual row) in the evaluation table."
INDEX { evalIndex }
::= { evalTable 1 }
EvalEntry ::=
SEQUENCE {
evalIndex Integer32,
evalString DisplayString,
evalValue Integer32,
evalStatus RowStatus
}
evalIndex OBJECT-TYPE
SYNTAX Integer32 (1..2147483647)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The auxiliary variable used for identifying instances of
the columnar objects in the evaluation table."
::= { evalEntry 1 }
evalString OBJECT-TYPE
SYNTAX DisplayString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The string to evaluate."
::= { evalEntry 2 }
evalValue OBJECT-TYPE
SYNTAX Integer32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when evalString was last evaluated, or zero if
no such value is available."
DEFVAL { 0 }
::= { evalEntry 3 }
evalStatus OBJECT-TYPE
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SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status column used for creating, modifying, and
deleting instances of the columnar objects in the
evaluation table."
DEFVAL { active }
::= { evalEntry 4 }
8. Mapping of the NOTIFICATION-TYPE macro
The NOTIFICATION-TYPE macro is used to define the information
contained within an unsolicited transmission of management
information (i.e., within either a SNMPv2-Trap-PDU or InformRequest-
PDU). It should be noted that the expansion of the NOTIFICATION-TYPE
macro is something which conceptually happens during implementation
and not during run-time.
8.1. Mapping of the OBJECTS clause
The OBJECTS clause, which need not be present, defines an ordered
sequence of MIB object types. One and only one object instance for
each occurrence of each object type must be present, and in the
specified order, in every instance of the notification. If the same
object type occurs multiple times in a notification's ordered
sequence, then an object instance is present for each of them. An
object type specified in this clause must not have an MAX-ACCESS
clause of "not-accessible". The notification's DESCRIPTION clause
must specify the information/meaning conveyed by each occurrence of
each object type in the sequence. The DESCRIPTION clause must also
specify which object instance is present for each object type in the
notification.
Note that an agent is allowed, at its own discretion, to append as
many additional objects as it considers useful to the end of the
notification (i.e., after the objects defined by the OBJECTS clause).
8.2. Mapping of the STATUS clause
The STATUS clause, which must be present, indicates whether this
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
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interoperability with older/existing implementations.
8.3. Mapping of the DESCRIPTION clause
The DESCRIPTION clause, which must be present, contains a textual
definition of the notification which provides all semantic
definitions necessary for implementation, and should embody any
information which would otherwise be communicated in any ASN.1
commentary annotations associated with the notification. In
particular, the DESCRIPTION clause should document which instances of
the objects mentioned in the OBJECTS clause should be contained
within notifications of this type.
8.4. Mapping of the REFERENCE clause
The REFERENCE clause, which need not be present, contains a textual
cross-reference to some other document, either another information
module which defines a related assignment, or some other document
which provides additional information relevant to this definition.
8.5. Mapping of the NOTIFICATION-TYPE value
The value of an invocation of the NOTIFICATION-TYPE macro is the name
of the notification, which is an OBJECT IDENTIFIER, an
administratively assigned name. In order to achieve compatibility
with SNMPv1 traps, both when converting SMIv1 information modules
to/from this SMI, and in the procedures employed by multi-lingual
systems and proxy forwarding applications, the next to last sub-
identifier in the name of any newly-defined notification must have
the value zero.
Sections 4.2.6 and 4.2.7 of [6] describe how the NOTIFICATION-TYPE
macro is used to generate a SNMPv2-Trap-PDU or InformRequest-PDU,
respectively.
8.6. Usage Example
Consider how a configuration change notification might be described:
entityMIBTraps OBJECT IDENTIFIER ::= { entityMIB 2 }
entityMIBTrapPrefix OBJECT IDENTIFIER ::= { entityMIBTraps 0 }
entConfigChange NOTIFICATION-TYPE
STATUS current
DESCRIPTION
"An entConfigChange trap is sent when the value of
entLastChangeTime changes. It can be utilized by an NMS to
trigger logical/physical entity table maintenance polls.
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An agent must not generate more than one entConfigChange
'trap-event' in a five second period, where a 'trap-event'
is the transmission of a single trap PDU to a list of
trap destinations. If additional configuration changes
occur within the five second 'throttling' period, then
these trap-events should be suppressed by the agent. An
NMS should periodically check the value of
entLastChangeTime to detect any missed entConfigChange
trap-events, e.g. due to throttling or transmission loss."
::= { entityMIBTrapPrefix 1 }
According to this invocation, the notification authoritatively
identified as
{ entityMIBTrapPrefix 1 }
is used to report a particular type of configuration change.
9. Refined Syntax
Some macros have clauses which allows syntax to be refined,
specifically: the SYNTAX clause of the OBJECT-TYPE macro, and the
SYNTAX/WRITE-SYNTAX clauses of the MODULE-COMPLIANCE and AGENT-
CAPABILITIES macros [2]. However, not all refinements of syntax are
appropriate. In particular, the object's primitive or application
type must not be changed.
Further, the following restrictions apply:
Restrictions to Refinement of
object syntax range enumeration size
----------------- ----- ----------- ----
INTEGER (1) (2) -
Integer32 (1) - -
Unsigned32 (1) - -
OCTET STRING - - (3)
OBJECT IDENTIFIER - - -
BITS - (2) -
IpAddress - - -
Counter32 - - -
Counter64 - - -
Gauge32 (1) - -
TimeTicks - - -
where:
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(1) the range of permitted values may be refined by raising the lower-
bounds, by reducing the upper-bounds, and/or by reducing the
alternative value/range choices;
(2) the enumeration of named-values may be refined by removing one or
more named-values (note that for BITS, a refinement may cause the
enumerations to no longer be contiguous); or,
(3) the size in octets of the value may be refined by raising the
lower-bounds, by reducing the upper-bounds, and/or by reducing the
alternative size choices.
No other types of refinements can be specified in the SYNTAX clause.
However, the DESCRIPTION clause is available to specify additional
restrictions which can not be expressed in the SYNTAX clause.
Further details on (and examples of) sub-typing are provided in
Appendix A.
10. Extending an Information Module
As experience is gained with an information module, it may be
desirable to revise that information module. However, changes are
not allowed if they have any potential to cause interoperability
problems "over the wire" between an implementation using an original
specification and an implementation using an updated
specification(s).
For any change, the invocation of the MODULE-IDENTITY macro must be
updated to include information about the revision: specifically,
updating the LAST-UPDATED clause, adding a pair of REVISION and
DESCRIPTION clauses (see section 5.5), and making any necessary
changes to existing clauses, including the ORGANIZATION and CONTACT-
INFO clauses.
Note that any definition contained in an information module is
available to be IMPORT-ed by any other information module, and is
referenced in an IMPORTS clause via the module name. Thus, a module
name should not be changed. Specifically, the module name (e.g.,
"FIZBIN-MIB" in the example of Section 5.7) should not be changed
when revising an information module (except to correct typographical
errors), and definitions should not be moved from one information
module to another.
Also note that obsolete definitions must not be removed from MIB
modules since their descriptors may still be referenced by other
information modules, and the OBJECT IDENTIFIERs used to name them
must never be re-assigned.
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10.1. Object Assignments
If any non-editorial change is made to any clause of a object
assignment, then the OBJECT IDENTIFIER value associated with that
object assignment must also be changed, along with its associated
descriptor.
10.2. Object Definitions
An object definition may be revised in any of the following ways:
(1) A SYNTAX clause containing an enumerated INTEGER may have new
enumerations added or existing labels changed. Similarly, named
bits may be added or existing labels changed for the BITS
construct.
(2) The value of a SYNTAX clause may be replaced by a textual
convention, providing the textual convention is defined to use the
same primitive ASN.1 type, has the same set of values, and has
identical semantics.
(3) A STATUS clause value of "current" may be revised as "deprecated"
or "obsolete". Similarly, a STATUS clause value of "deprecated"
may be revised as "obsolete". When making such a change, the
DESCRIPTION clause should be updated to explain the rationale.
(4) A DEFVAL clause may be added or updated.
(5) A REFERENCE clause may be added or updated.
(6) A UNITS clause may be added.
(7) A conceptual row may be augmented by adding new columnar objects at
the end of the row, and making the corresponding update to the
SEQUENCE definition.
(8) Clarifications and additional information may be included in the
DESCRIPTION clause.
(9) Entirely new objects may be defined, named with previously
unassigned OBJECT IDENTIFIER values.
Otherwise, if the semantics of any previously defined object are
changed (i.e., if a non-editorial change is made to any clause other
than those specifically allowed above), then the OBJECT IDENTIFIER
value associated with that object must also be changed.
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Note that changing the descriptor associated with an existing object
is considered a semantic change, as these strings may be used in an
IMPORTS statement.
10.3. Notification Definitions
A notification definition may be revised in any of the following
ways:
(1) A REFERENCE clause may be added or updated.
(2) A STATUS clause value of "current" may be revised as "deprecated"
or "obsolete". Similarly, a STATUS clause value of "deprecated"
may be revised as "obsolete". When making such a change, the
DESCRIPTION clause should be updated to explain the rationale.
(3) A DESCRIPTION clause may be clarified.
Otherwise, if the semantics of any previously defined notification
are changed (i.e., if a non-editorial change is made to any clause
other those specifically allowed above), then the OBJECT IDENTIFIER
value associated with that notification must also be changed.
Note that changing the descriptor associated with an existing
notification is considered a semantic change, as these strings may be
used in an IMPORTS statement.
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11. Appendix A: Detailed Sub-typing Rules
11.1. Syntax Rules
The syntax rules for sub-typing are given below. Note that while
this syntax is based on ASN.1, it includes some extensions beyond
what is allowed in ASN.1, and a number of ASN.1 constructs are not
allowed by this syntax.
<integerSubType>
::= <empty>
| "(" <range> ["|" <range>]... ")"
<octetStringSubType>
::= <empty>
| "(" "SIZE" "(" <range> ["|" <range>]... ")" ")"
<range>
::= <value>
| <value> ".." <value>
<value>
::= "-" <number>
| <number>
| <hexString>
| <binString>
where:
<empty> is the empty string
<number> is a non-negative integer
<hexString> is a hexadecimal string (e.g., '0F0F'H)
<binString> is a binary string (e.g, '1010'B)
<range> is further restricted as follows:
- any <value> used in a SIZE clause must be non-negative.
- when a pair of values is specified, the first value
must be less than the second value.
- when multiple ranges are specified, the ranges may
not overlap but may touch. For example, (1..4 | 4..9)
is invalid, and (1..4 | 5..9) is valid.
- the ranges must be a subset of the maximum range of the
base type.
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11.2. Examples
Some examples of legal sub-typing:
Integer32 (-20..100)
Integer32 (0..100 | 300..500)
Integer32 (300..500 | 0..100)
Integer32 (0 | 2 | 4 | 6 | 8 | 10)
OCTET STRING (SIZE(0..100))
OCTET STRING (SIZE(0..100 | 300..500))
OCTET STRING (SIZE(0 | 2 | 4 | 6 | 8 | 10))
SYNTAX TimeInterval (0..100)
SYNTAX DisplayString (SIZE(0..32))
(Note the last two examples above are not valid in a TEXTUAL
CONVENTION, see [3].)
Some examples of illegal sub-typing:
Integer32 (150..100) -- first greater than second
Integer32 (0..100 | 50..500) -- ranges overlap
Integer32 (0 | 2 | 0 ) -- value duplicated
Integer32 (MIN..-1 | 1..MAX) -- MIN and MAX not allowed
Integer32 (SIZE (0..34)) -- must not use SIZE
OCTET STRING (0..100) -- must use SIZE
OCTET STRING (SIZE(-10..100)) -- negative SIZE
12. Security Considerations
This document defines a language with which to write and read
descriptions of management information. The language itself has no
security impact on the Internet.
13. Editors' Addresses
Keith McCloghrie
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5260
EMail: kzm@cisco.com
McCloghrie, et al. Standards Track [Page 41]
RFC 2578 SMIv2 April 1999
David Perkins
SNMPinfo
3763 Benton Street
Santa Clara, CA 95051
USA
Phone: +1 408 221-8702
EMail: dperkins@snmpinfo.com
Juergen Schoenwaelder
TU Braunschweig
Bueltenweg 74/75
38106 Braunschweig
Germany
Phone: +49 531 391-3283
EMail: schoenw@ibr.cs.tu-bs.de
14. References
[1] Information processing systems - Open Systems Interconnection -
Specification of Abstract Syntax Notation One (ASN.1),
International Organization for Standardization. International
Standard 8824, (December, 1987).
[2] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M.
and S. Waldbusser, "Conformance Statements for SMIv2", STD 58,
RFC 2580, April 1999.
[3] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M.
and S. Waldbusser, "Textual Conventions for SMIv2", STD 58,
RFC 2579, April 1999.
[4] Information processing systems - Open Systems Interconnection -
Specification of Basic Encoding Rules for Abstract Syntax Notation
One (ASN.1), International Organization for Standardization.
International Standard 8825, (December, 1987).
[5] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M. and
S. Waldbusser, "Management Information Base for Version 2 of the
Simple Network Management Protocol (SNMPv2)", RFC 1907, January
1996.
[6] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M. and
S. Waldbusser, "Protocol Operations for Version 2 of the Simple
Network Management Protocol (SNMPv2)", RFC 1905, January 1996.
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15. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
McCloghrie, et al. Standards Track [Page 43]