RFC 2578
Structure of Management Information Version 2 (SMIv2)
Structure of Management Information Version 2 (SMIv2) 💖
okay bestie, so this is RFC 2578 🎀, and it's the one that defines the SMIv2 (Structure of Management Information Version 2). It's a Standards Track document with STD 58 status, published in April 1999. It obsoletes RFC 1902, so that older version is officially replaced. The editors are K. McCloghrie (Cisco Systems), D. Perkins (SNMPinfo), and J. Schoenwaelder (TU Braunschweig) 👑. The authors of the previous version were J. Case (SNMP Research), K. McCloghrie (Cisco Systems), M. Rose (First Virtual Holdings), and S. Waldbusser (International Network Services) 🫶.
Status of this Memo ✨
so this doc is an Internet standards track protocol for the whole Internet community, and it's basically asking everyone for discussion and suggestions to make it better 🦋. If you want to know where this protocol actually stands in terms of standardization, check the current edition of the "Internet Official Protocol Standards" (STD 1) 📚. Distribution of this memo is unlimited, so share it with everyone 💅.
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
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
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 ...................................431. Introduction 🦋
okay so management information is basically a big collection of managed objects that live in a virtual information store called the Management Information Base (MIB) 🗂️. Related objects get grouped together into MIB modules, and those modules are written in an adapted subset of OSI's Abstract Syntax Notation One, ASN.1 (1988) [1] 🧚. This document, the Structure of Management Information (SMI), is where that adapted subset gets defined, and it also hands out a set of associated administrative values 💜.
The SMI has three parts, and honestly they're giving very clean organization 🍓:
(1) Module definitions describe information modules. They use an ASN.1 macro called MODULE-IDENTITY to concisely convey the semantics of an information module ✨.
(2) Object definitions describe managed objects. They use an ASN.1 macro called OBJECT-TYPE to concisely convey the syntax and semantics of a managed object 🌈.
(3) Notification definitions describe unsolicited transmissions of management information. They use an ASN.1 macro called NOTIFICATION-TYPE to concisely convey the syntax and semantics of a notification 🔥.
1.1. A Note on Terminology
so for clarity, the original Structure of Management Information, as described in RFCs 1155 (STD 16), 1212 (STD 16), and RFC 1215, gets called the SMI version 1 (SMIv1) 🙈. The current version is called SMI version 2 (SMIv2), which is the one this whole doc is about 💅.
2. Definitions 🌈
this is the part where the actual ASN.1 definitions live, so buckle up, bestie 🎀. Here's the root of the whole naming tree and the ASN.1 macros:
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
-- 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
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 objectsok that's the first chunk of the definitions, and the syntax of objects is up next 👀✨ stay tuned, we're just getting started 💖
-- 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
}
-- 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
-- (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
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
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
| 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 }
END3. Information Modules ✨
okay so an "information module" is basically an ASN.1 module that defines info for network management, and it's giving organised 💅
The SMI tells you how to use an adapted subset of ASN.1 (1988) to define one of these. On top of that, "standard" information modules have extra restrictions. It is strongly recommended that "enterprise-specific" information modules follow these restrictions too, bestie 🫶
There are typically three kinds of information modules:
(1) MIB modules, which contain definitions of inter-related managed objects and 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].
ngl this classification is not 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 ASN.1 constructs 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. Not allowed, periodt 🙅♀️
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 pick names that 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 this spec, an ASN.1 module name begins with an upper-case letter and continues with zero or more letters, digits, or hyphens. The only catch: a hyphen can not be the last character, and there can not be two consecutive hyphens 💀
All information modules start with exactly one invocation of the MODULE-IDENTITY macro. This gives contact info and revision history so people can tell versions of the same information module apart 🎀 This invocation must appear immediately after any IMPORTs statements.
3.1. Macro Invocation
Within an information module, each macro invocation looks like this:
<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].) For this spec, an ASN.1 identifier consists of one or more letters or digits, and its initial character must be a lower-case letter. Hyphens are not allowed by this specification, except for 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 discussing the information module and makes simple table mappings for user interfaces easier, which is so iconic 👑
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. Counters are plural vibes only 💜
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 (
\nor\r\n).
The value of a character string is interpreted as ASCII. Simple, clean, no notes 🌈
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. Don't put comments in there, bestie, it'll break things 😭
3.2. IMPORTing Symbols 📥
Okay so, to reference an external object, you have to use the IMPORTS statement to name both the descriptor and the module it lives in, and the module gets identified by its ASN.1 module name 💅
Heads up: when symbols from "enterprise-specific" information modules get referenced (like a descriptor), there's a chance of collision 😭 If different objects with the same descriptor get IMPORTed, the ambiguity gets fixed by prefixing the descriptor with the name of the information module and a dot, like this:
"module.descriptor"(All descriptors must be unique within any information module.)
Of course, you can use this notation to refer to objects even when there's no collision while IMPORTing, so it's always an option 🫶
Finally, if any of the ASN.1 named types and macros defined in this document get used in an information module, they must be imported using the IMPORTS statement. That's the list:
Counter32, Counter64, Gauge32, Integer32, IpAddress, MODULE-
IDENTITY, NOTIFICATION-TYPE, Opaque, OBJECT-TYPE, OBJECT-
IDENTITY, TimeTicks, Unsigned32,The same goes for anything defined in [2] or [3]. BUT the following must not be included in an IMPORTS statement, so don't sneak them in 🙈
- 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. Everything defined in an information module gets automatically exported, so you don't have to do anything extra ✨
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, so put the good stuff there 🍓
ASN.1 comments start with a pair of adjacent hyphens and end at the next pair of adjacent hyphens or at the end of the line, whichever comes first. Comments ended by a pair of hyphens act like 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 called 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 first sub-identifier is one of these 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 get used in information modules in two ways:
(1) registration: the definition of a particular item gets registered as a particular OBJECT IDENTIFIER value, and associated with a particular descriptor. After that registration, the semantics tied to 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 or associated with any other registration. These 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 tied to the OBJECT IDENTIFIER value are not allowed to change, and a descriptor assigned to a particular OBJECT IDENTIFIER value cannot later be assigned to another one. Multiple descriptors can be assigned to the same OBJECT IDENTIFIER value, though 💜 Assignments get specified like this:
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, so skip it 🚫
dinoHost OBJECT IDENTIFIER ::= { flintStones bedrock 2 }A descriptor is allowed to be associated with both a registration and an assignment, as long as both are tied to the same OBJECT IDENTIFIER value and semantics 🎀
3.7. Reserved Keywords 🚧
These are the reserved keywords which must not be used as descriptors or module names. Don't say I didn't warn you 😤
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-SYNTAX4. 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 }So the Internet subtree of OBJECT IDENTIFIERs starts with the prefix 1.3.6.1.
Several branches underneath this subtree are used for network management:
mgmt OBJECT IDENTIFIER ::= { internet 2 }
experimental OBJECT IDENTIFIER ::= { internet 3 }
private OBJECT IDENTIFIER ::= { internet 4 }
enterprises OBJECT IDENTIFIER ::= { private 1 }The SMI does not prohibit defining objects in other portions of the object tree, though, so there's room to roam 🌈
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 get moved under the mgmt(2) subtree. That's the glow-up arc 🦋
The private(4) subtree is used to identify objects defined unilaterally. The enterprises(1) subtree beneath private is used, among other things, to let providers of networking subsystems 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 conceptually happens during implementation, not during run-time ⏳
Note that a reference in an IMPORTS clause, or in clauses of SMIv2 macros, to an information module is NOT made through the 'descriptor' of a MODULE-IDENTITY macro. Instead, an information module is referenced by 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 🤝
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 about 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 used repeatedly 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. So this value can be authoritatively used when specifying an OBJECT IDENTIFIER value to refer to the information module containing the invocation 🎯
It's 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. Flexible queen behavior 👑
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
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 }
END6. 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 conceptually happens during implementation, not during run-time 💖
6.1. Mapping of the STATUS clause 💅
okay so the STATUS clause must be present, and its whole job is telling you whether a definition is current or historic 🫶
The value "current" means the definition is current and valid, no cap ✨ The value "obsolete" means the definition is obsolete, so it should not be implemented, and/or it can be removed if it was implemented before 💀 "deprecated" also marks something as obsolete, but it still permits new or continued implementation so things keep working with older, existing implementations 🦋
6.2. Mapping of the DESCRIPTION clause 🎀
The DESCRIPTION clause must be present. It's the spot for a textual description of the object assignment, so this is where the vibe check goes 📝
6.3. Mapping of the REFERENCE clause 🌸
The REFERENCE clause need not be present, but if you add it, it's a textual cross-reference to some other document. That could be another information module that defines a related assignment, or some other document that gives extra info 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 🍓
Let's look at 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. Heads up, bestie: expanding the OBJECT-TYPE macro is something that conceptually happens during implementation, not during run-time ⏰
For leaf objects that aren't columnar objects (so, not inside 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 is what defines instance identification info 🔑
7.1. Mapping of the SYNTAX clause 💖
The SYNTAX clause must be present, and it defines the abstract data structure that matches 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 the ones defined in the ObjectSyntax CHOICE. A textual convention is a newly-defined type that's a sub-type of a base type [3] 🧩
An extended subset of the full capabilities of ASN.1 (1988) sub-typing is allowed, as fits the underlying ASN.1 type. Any restriction on size, range or enumerations specified in this clause represents the maximal level of support that 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 that are 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 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. The SMI-specified size limitation for this type is 65535 octets, but MIB designers should realize that there may be implementation and interoperability limitations for sizes in excess of 255 octets 🥹
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 that are 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, that ambiguity could either be desirable or undesirable. The means to avoid it depends on how values are encoded on the wire. One possibility, however, 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 🧭
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 that 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, so 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 clauses 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 the use of the term "latched" in the original definition of this type, it does not become "stuck" at its maximum or minimum value.) 📈
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 that 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, so 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 clauses 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 🔢✨
okay so Unsigned32 is the type for integer-valued info that goes from 0 all the way up to 2^32-1 inclusive (aka 0 to 4294967295 decimal) 💅 no negatives allowed, bestie 🙈
7.1.12. Conceptual Tables 🧁🗂️
okay this one is so iconic 😭 management operations only ever apply to scalar objects, but sometimes it's way easier for management app developers to imagine a little table structure over an ordered group of objects in the MIB. so each of these conceptual tables has zero or more rows, and each row can hold one or more scalar objects, which we call columnar objects 🌸 we formalize this by using the OBJECT-TYPE macro to define BOTH an object for the table and an object for a row in that table. the table's SYNTAX looks like this:
SEQUENCE OF <EntryType>where <EntryType> points to the SEQUENCE type of its subordinate conceptual row. and the row's SYNTAX looks like this:
<EntryType>where <EntryType> is a SEQUENCE type defined like this:
<EntryType> ::= SEQUENCE { <type1>, ... , <typeN> }there's one <type> for each subordinate object, and each <type> looks like this:
<descriptor> <syntax>the <descriptor> is the name of a subordinate object, and <syntax> is that subordinate object's SYNTAX clause value, EXCEPT the sub-typing info and the named values for enumerated integers (or the named bits for the BITS construct) get left out of it 🦋
also, a <type> is ALWAYS there for every subordinate object 💖 (the ASN.1 DEFAULT and OPTIONAL clauses are a no-go in the SEQUENCE definition, periodt). and 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 that let you create new object instances and/or delete existing ones, 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 we call this the status column for the conceptual row, and it's giving main character energy 👑
7.2. Mapping of the UNITS clause 📏
this UNITS clause doesn't have to be there, and it holds a textual definition of the units that go with that object. simple and sweet 🍓
7.3. Mapping of the MAX-ACCESS clause 🔐
the MAX-ACCESS clause MUST be present... wait, correction, it's must in the original, lowercase, so it's a MUST-be-there vibe in the spec's own words 🫶 it 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, and it's totally independent of any administrative authorization policy 💅
the value "read-write" means read and write access make "protocol sense", but create does not. the value "read-create" means read, write AND create access all make "protocol sense" ✨ the value "not-accessible" means it's an auxiliary object (see Section 7.7). and the value "accessible-for-notify" means the object is only accessible through 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" 📈
⚠️ bestie, read this part carefully: if ANY columnar object in a conceptual row has"read-create"as its maximal level of access, then NO other columnar object in that same conceptual row can have a maximal access of"read-write". (note that"read-create"is a superset of"read-write", so it's the bigger one 💜)
7.4. Mapping of the STATUS clause 🚦
the STATUS clause must be present, and it says whether the definition is current or historic 🕰️
the value "current" means the definition is current and valid ✅ the value "obsolete" means the definition is obsolete and should not be implemented, and/or it can be removed if it was implemented before 🗑️ and the value "deprecated" ALSO means obsolete, but it still allows new or continued implementation so things stay interoperable with older, existing implementations 🤝 it's the gentle fade-out, not the cancel 🌈
7.5. Mapping of the DESCRIPTION clause 📝
the DESCRIPTION clause must be present 💖 it holds a textual definition of the object that gives ALL the semantic definitions needed for implementation, and it should carry any info that would otherwise be shared in ASN.1 commentary annotations attached to the object 🧚
7.6. Mapping of the REFERENCE clause 🔗
the REFERENCE clause doesn't have to be there. it's a textual cross-reference to some other document, either another information module that defines a related assignment, or some other doc that gives extra info relevant to this definition 📚✨
7.7. Mapping of the INDEX clause 🪪
the INDEX clause must be present if the object corresponds to a conceptual row (unless an AUGMENTS clause is there instead), and it must be absent otherwise 🙅♀️ it defines the instance identification info for the columnar objects under that object 💜
the instance identification info in an INDEX clause has to name object(s) where their value(s) will unambiguously tell apart a conceptual row. the objects can be columnar objects from the same and/or another conceptual table, but they must NOT be scalar objects 🚫 having the same object listed more than once in a single INDEX clause is strongly discouraged, so maybe don't 😭
the syntax of each INDEX object tells you how to build the instance-identifier 🧩
- 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);
- string-valued, fixed-length strings (or variable-length preceded by the IMPLIED keyword):
n' sub-identifiers, wheren' is the length of the string (each octet of the string is encoded in a separate sub-identifier); - string-valued, variable-length strings (not preceded by the IMPLIED keyword):
n+1' sub-identifiers, wheren' 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); - object identifier-valued (when preceded by the IMPLIED keyword):
n' sub-identifiers, wheren' is the number of sub-identifiers in the value (each sub-identifier of the value is copied into a separate sub-identifier); - object identifier-valued (when not preceded by the IMPLIED keyword):
n+1' sub-identifiers, wheren' 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); - 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 with a variable-length syntax (e.g., variable-length strings or object identifier-valued objects), 🫠 and the IMPLIED keyword can only be attached to 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 with the syntax Counter32 or Counter64 must not be specified in an INDEX clause 🙅♀️ and if an object defined with the BITS construct is used in an INDEX clause, it's treated as a variable-length string 🎀
instances identified by integer-valued objects should be numbered starting from one (i.e., not from zero) 1️⃣ using zero as a value for an integer-valued index object should be avoided, except in special cases 🙈
objects that are BOTH specified in the INDEX clause of a conceptual row AND columnar objects of that same row are called auxiliary objects 🦋 their MAX-ACCESS clause is "not-accessible", EXCEPT in these situations:
- within a MIB module originally written to conform to SMIv1, and later converted to conform to SMIv2; or
- a conceptual row must contain at least one columnar object that is NOT an auxiliary object. if ALL of a conceptual row's columnar objects are also 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 doesn't happen for a conceptual row that allows create access, since that row will have a status column which won't be an auxiliary object 💅)
note that objects in a conceptual row's INDEX clause don't have to be columnar objects of that row 🌈 in that case, the DESCRIPTION clause of the conceptual row must include a textual explanation of how the INDEX objects that aren't columnar objects of that row are used to uniquely identify instances of the row's columnar objects 📖
7.8. Mapping of the AUGMENTS clause 🧩✨
the AUGMENTS clause must NOT be present unless the object corresponds to a conceptual row, and it's an alternative to the INDEX clause 🙃 every object corresponding to a conceptual row has either an INDEX clause or an AUGMENTS clause, no in-between 💜
if an object corresponding to a conceptual row has an INDEX clause, that row is called a base conceptual row 👑 if the object has an AUGMENTS clause instead, the row is a conceptual row augmentation, and the AUGMENTS clause names the object for the base conceptual row that this augmentation extends. (so a conceptual row augmentation can NEVER itself be augmented, lol 💀) instances of the subordinate columnar objects of a conceptual row augmentation are identified according to the INDEX clause of the base conceptual row for the object named in the AUGMENTS clause 🫶 and they exist under the SAME semantics as the subordinate columnar objects of the base row being augmented. so creating a base conceptual row also implies the correspondent creation of any conceptual row augmentations 🌸
for example, a MIB designer might want to add extra columns in an "enterprise-specific" MIB that logically extend a conceptual row in a "standard" MIB. the "standard" MIB definition of the row would have the INDEX clause, and the "enterprise-specific" MIB would define the row using the AUGMENTS clause 🎀 but on the flip side, it would be INCORRECT to use the AUGMENTS clause for the relationship between RFC 2233's ifTable and the many media-specific MIBs that extend it for specific media (e.g., the dot3Table in RFC 2358), because not all interfaces are the same media 🙅♀️
note that a base conceptual row can be augmented by multiple conceptual row augmentations 🦋
7.8.1. Relation between INDEX and AUGMENTS clauses 🔍
when you're defining instance identification info for a conceptual table:
- if there's a one-to-one correspondence between the conceptual rows of this table and an existing table, then the AUGMENTS clause should be used 💖
- otherwise, if there's a sparse relationship between the conceptual rows of this table and an existing table, then an INDEX clause should be used that is identical to the one in the existing table. for example, the relationship between RFC 2233's ifTable and a media-specific MIB that extends the ifTable for a specific media (e.g., the dot3Table in RFC 2358) is a sparse relationship 🌈
- Okayyy, so if NO existing objects have the right syntax AND semantics 😭, then auxiliary objects should be defined inside the conceptual row for the new table, and those objects should be used in the INDEX clause for that row 🎀
7.9. Mapping of the DEFVAL clause 💖
The DEFVAL clause is totally optional (it need not be present) and it's basically an acceptable default value 💅 that an agent may use at its own discretion when an object instance gets created. Think of it as a "hint" for implementors 🫶
When a conceptual row is being created and an instance of a columnar object isn't one of the operands in the correspondent management protocol set operation, the DEFVAL clause (if there is one) tells the agent an acceptable default it could use, especially for a read-only object ✨
It's giving flexibility, bestie 🦋 Using DEFVAL is appropriate for any columnar object of a read-create table. It's also permitted for scalar objects that an agent dynamically creates, or for columnar objects of a read-write table that an agent dynamically creates 🌈
The value of the DEFVAL clause must, of course, match the SYNTAX clause for the object. If the value is an OBJECT IDENTIFIER, it must be written as a single ASN.1 identifier, NOT as a collection of sub-identifiers 🙈
Heads up: if an operand to the management protocol set operation is an instance of a read-only object, you'll get the error notWritable' [6] 💀 That's exactly why DEFVAL is such a clutch, because it gives the agent a safe default to fall back on 👑
Here's the vibe check with some example 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 setA 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 🧚 And a character string used in a DEFVAL clause must not contain tab characters or line terminator characters 🙅♀️
Counter32 and Counter64 objects are a special case 💜 Object types with SYNTAX of Counter32 and Counter64 may not have DEFVAL clauses, since they don't 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. That name is an OBJECT IDENTIFIER, an administratively assigned name 🎀
When an OBJECT IDENTIFIER is assigned to an object:
- If the object corresponds to a conceptual table, then only a single assignment (the one 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 "1" to the administratively assigned name for the conceptual table 🦋
- If the object corresponds to a conceptual row, then at least one assignment is present beneath that object, one for each column in the conceptual row. 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 💅
- 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 🧁
Let's see 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
"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
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 inside an unsolicited transmission of management information (i.e., inside either a SNMPv2-Trap-PDU or an InformRequest-PDU) 💌 Heads up, the expansion of the NOTIFICATION-TYPE macro is something that conceptually happens during implementation, NOT at 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, 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 a MAX-ACCESS clause of "not-accessible" 🙅♀️
The notification's DESCRIPTION clause must specify the information and 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, says whether this definition is current or historic 💅
The value "current" means 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 it was previously implemented 🗑️ And while "deprecated" also indicates an obsolete definition, it permits new or continued implementation to foster interoperability with older and existing implementations 🫶
8.3. Mapping of the DESCRIPTION clause 📖
The DESCRIPTION clause, which must be present, has a textual definition of the notification. It gives all the semantic definitions needed for implementation, and should embody any information that would otherwise be communicated in 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, is a textual cross-reference to some other document. That could be another information module that defines a related assignment, or some other document that gives 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. That name is an OBJECT IDENTIFIER, an administratively assigned name. To stay compatible with SNMPv1 traps, both when converting SMIv1 information modules to and from this SMI and in the procedures used 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 🎀
okay so picture a config change notification, bestie 🧚 this is the example where the MIB stuff finally clicks:
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.
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 }so like, this notification is literally the "something changed in the config" ping 🔔 and the DESCRIPTION is doing all the heavy lifting. the agent is not allowed to spam more than one entConfigChange trap-event in a five second window (rude, but make sense 💅), and if more changes happen during that throttling period, the extra trap-events get suppressed. the NMS is supposed to keep an eye on entLastChangeTime so it doesn't miss anything that got throttled or lost in transit 👀
and according to this invocation, the notification authoritatively identified as
{ entityMIBTrapPrefix 1 }is used to report a particular type of configuration change. main character energy 👑
9. Refined Syntax ✨
okay now we get into refining syntax, which is basically letting you narrow down the vibe of a type without changing its whole personality 🦋 some macros have clauses that let you do this: the SYNTAX clause of the OBJECT-TYPE macro, and the SYNTAX/WRITE-SYNTAX clauses of the MODULE-COMPLIANCE and AGENT-CAPABILITIES macros [2]. but not every refinement is appropriate. the object's primitive or application type MUST NOT be changed. that is a hard no, periodt 🙅♀️
here are the restrictions, the table is the cheat sheet 🍓
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 these are the rules, lowkey the core of the whole section 🫶
(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. BUT the DESCRIPTION clause is there for the extra restrictions that can't fit in SYNTAX 🎀 and if you want more details (and examples!) on sub-typing, Appendix A has you covered, iykyk 💖
10. Extending an Information Module 🌈
so the thing about info modules is that once people are using them, you can't just yolo changes whenever. 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). that's the whole drama 😭
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. your paperwork is part of the glow-up 💅
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. so 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. don't rename your whole identity, bestie 🙈
also, 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. ghosted, not deleted 👻
10.1. Object Assignments 🍓
if any non-editorial change is made to any clause of an object assignment, then the OBJECT IDENTIFIER value associated with that object assignment MUST also be changed, along with its associated descriptor. real change means new ID, no exceptions 🔥
10.2. Object Definitions 🦋
an object definition MAY be revised in any of the following ways, and this list is the good stuff:
(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. identical semantics, not vibes 🧐
(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. explain yourself, it's giving honesty 💜
(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. tell the people what's up 🗣️
(9) entirely new objects MAY be defined, named with previously unassigned OBJECT IDENTIFIER values. fresh start, new ID 🌸
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. semantic change = new ID, that's the rule, no cap 🧢
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. so don't touch the name, slay 💅
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 than 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. same energy as objects, the name is sacred 🫶
11. Appendix A: Detailed Sub-typing Rules 🎀
11.1. Syntax Rules 💖
the syntax rules for sub-typing are given below. the syntax is based on ASN.1, but it adds some extras on top, and a bunch of ASN.1 constructs are NOT allowed here, so don't bring them 🙅♀️
<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.11.2. Examples 🦋
some examples of legal sub-typing, these are the good ones ✨
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].) so careful with those, bestie 🙈
some examples of illegal sub-typing, the ones that get you ratio'd 💀
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 SIZE12. 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. ✨ literally nothing to panic about here, the whole vibe is "it's just a language" 🫶
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
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.de14. 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.
15. Full Copyright Statement 🎀
Okay, legal stuff time, bestie 🫶 It's a bit dry, but the copyright lines are super important, so I'm keeping the actual wording exactly as written.
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.
And that's a wrap on RFC 2578! 🦋 The whole spec, start to finish, no cap 💅✨
📦 where this page comes from
- ietf: rfc2578 1999-04
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms