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]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 2]
RFC 2578 SMIv2 April 1999 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]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 4]
RFC 2578 SMIv2 April 1999 -- 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 McCloghrie, et al. Standards Track [Page 5]
RFC 2578 SMIv2 April 1999 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 } McCloghrie, et al. Standards Track [Page 6]
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 McCloghrie, et al. Standards Track [Page 7]
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 McCloghrie, et al. Standards Track [Page 8]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 9]
RFC 2578 SMIv2 April 1999 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]
RFC 2578 SMIv2 April 1999 | 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]. McCloghrie, et al. Standards Track [Page 11]
RFC 2578 SMIv2 April 1999 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].) McCloghrie, et al. Standards Track [Page 12]
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: McCloghrie, et al. Standards Track [Page 15]
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. McCloghrie, et al. Standards Track [Page 17]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 18]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 19]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 20]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 21]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 22]
RFC 2578 SMIv2 April 1999 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.) McCloghrie, et al. Standards Track [Page 23]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 24]
RFC 2578 SMIv2 April 1999 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, McCloghrie, et al. Standards Track [Page 25]
RFC 2578 SMIv2 April 1999 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".) McCloghrie, et al. Standards Track [Page 26]
RFC 2578 SMIv2 April 1999 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); McCloghrie, et al. Standards Track [Page 27]
RFC 2578 SMIv2 April 1999 (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: McCloghrie, et al. Standards Track [Page 28]
RFC 2578 SMIv2 April 1999 (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 McCloghrie, et al. Standards Track [Page 29]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 30]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 31]
RFC 2578 SMIv2 April 1999 "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 McCloghrie, et al. Standards Track [Page 32]
RFC 2578 SMIv2 April 1999 "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 McCloghrie, et al. Standards Track [Page 33]
RFC 2578 SMIv2 April 1999 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 McCloghrie, et al. Standards Track [Page 34]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 35]
RFC 2578 SMIv2 April 1999 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: McCloghrie, et al. Standards Track [Page 36]
RFC 2578 SMIv2 April 1999 (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. McCloghrie, et al. Standards Track [Page 37]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 38]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 39]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 40]
RFC 2578 SMIv2 April 1999 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. McCloghrie, et al. Standards Track [Page 42]
RFC 2578 SMIv2 April 1999 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]