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Network Working Group SNMPv2 Working Group Request for Comments: 1902 J. Case Obsoletes: 1442 SNMP Research, Inc. Category: Standards Track K. McCloghrie

                                                   Cisco Systems, Inc.
                                                               M. Rose
                                          Dover Beach Consulting, Inc.
                                                         S. Waldbusser
                                        International Network Services
                                                          January 1996
                Structure of Management Information
                        for Version 2 of the
            Simple Network Management Protocol (SNMPv2)

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.

1. Introduction

 A management system contains:  several (potentially many) nodes, each
 with a processing entity, termed an agent, which has access to
 management instrumentation; at least one management station; and, a
 management protocol, used to convey management information between
 the agents and management stations.  Operations of the protocol are
 carried out under an administrative framework which defines
 authentication, authorization, access control, and privacy policies.
 Management stations execute management applications which monitor and
 control managed elements.  Managed elements are devices such as
 hosts, routers, terminal servers, etc., which are monitored and
 controlled via access to their management information.
 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) [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.

SNMPv2 Working Group Standards Track [Page 1] RFC 1902 SMI for SNMPv2 January 1996

 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.

1.1. A Note on Terminology

 For the purpose of exposition, the original Internet-standard Network
 Management Framework, as described in RFCs 1155 (STD 16), 1157 (STD
 15), and 1212 (STD 16), is termed the SNMP version 1 framework
 (SNMPv1).  The current framework is termed the SNMP version 2
 framework (SNMPv2).

2. Definitions

SNMPv2-SMI DEFINITIONS ::= BEGIN

– the path to the root

org OBJECT IDENTIFIER ::= { iso 3 } 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 Working Group Standards Track [Page 2] RFC 1902 SMI for SNMPv2 January 1996

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 }

– definitions for information modules

MODULE-IDENTITY MACRO ::= BEGIN

  TYPE NOTATION ::=
                "LAST-UPDATED" value(Update UTCTime)
                "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 UTCTime)
                "DESCRIPTION" Text
  1. - uses the NVT ASCII character set

Text ::= """" string """" END

OBJECT-IDENTITY MACRO ::= BEGIN

  TYPE NOTATION ::=
                "STATUS" Status
                "DESCRIPTION" Text
                ReferPart

SNMPv2 Working Group Standards Track [Page 3] RFC 1902 SMI for SNMPv2 January 1996

  VALUE NOTATION ::=
                value(VALUE OBJECT IDENTIFIER)
  Status ::=
                "current"
              | "deprecated"
              | "obsolete"
  ReferPart ::=
              "REFERENCE" Text
            | empty
  Text ::= """" string """"

END

– names of objects

ObjectName ::=

  OBJECT IDENTIFIER

NotificationName ::=

  OBJECT IDENTIFIER

– syntax of objects

ObjectSyntax ::=

  CHOICE {
      simple
          SimpleSyntax,
  1. - note that SEQUENCEs for conceptual tables and
  2. - 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),

SNMPv2 Working Group Standards Track [Page 4] RFC 1902 SMI for SNMPv2 January 1996

  1. - OCTET STRINGs with a more restrictive size
  2. - 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 ::=

  [UNIVERSAL 2]
      IMPLICIT 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 ::=

SNMPv2 Working Group Standards Track [Page 5] RFC 1902 SMI for SNMPv2 January 1996

  [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 ::=

SNMPv2 Working Group Standards Track [Page 6] RFC 1902 SMI for SNMPv2 January 1996

                type(ObjectSyntax)
              | "BITS" "{" Kibbles "}"
  Kibbles ::=
                Kibble
              | Kibbles "," Kibble
  Kibble ::=
               identifier "(" nonNegativeNumber ")"
  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(Indexobject ObjectName)
  Entry ::=
                  -- use the INDEX value of the
                  -- correspondent OBJECT-TYPE invocation
                value(Entryobject ObjectName)
  DefValPart ::=

SNMPv2 Working Group Standards Track [Page 7] RFC 1902 SMI for SNMPv2 January 1996

                "DEFVAL" "{" value(Defval Syntax) "}"
              | empty
  1. - uses the NVT ASCII character set

Text ::= """" string """" 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(Name ObjectName)
  Status ::=
                "current"
              | "deprecated"
              | "obsolete"
  ReferPart ::=
              "REFERENCE" Text
            | empty
  1. - uses the NVT ASCII character set

Text ::= """" string """" END

– definitions of administrative identifiers

zeroDotZero OBJECT-IDENTITY

  STATUS     current
  DESCRIPTION

SNMPv2 Working Group Standards Track [Page 8] RFC 1902 SMI for SNMPv2 January 1996

          "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 a subset of ASN.1 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].
 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 SNMPv2 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
 SNMPv2, and instances of ASN.1 macros defined in this document and in
 other documents [2, 3] of the SNMPv2 framework.  Additional ASN.1
 macros may not be defined in SNMPv2 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

SNMPv2 Working Group Standards Track [Page 9] RFC 1902 SMI for SNMPv2 January 1996

 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.
 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].)
 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, even though hyphen is allowed by [1].  This
 restriction enables arithmetic expressions in languages which use the
 minus sign to reference these descriptors without ambiguity.)
 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 Clauses

 Some clauses in a macro invocation may take a textual value (e.g.,
 the DESCRIPTION clause).  Note that, in order to conform to the ASN.1
 syntax, the entire value of these clauses must be enclosed in double
 quotation marks, and therefore cannot itself contain double quotation

SNMPv2 Working Group Standards Track [Page 10] RFC 1902 SMI for SNMPv2 January 1996

 marks, although the value may be multi-line.

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 even when there is no collision
 when IMPORTing symbols.
 Finally, the IMPORTS statement may not be used to import an ASN.1
 named type which corresponds to either the SEQUENCE or SEQUENCE OF
 type.

3.3. Exporting Symbols

 The ASN.1 EXPORTS statement is not allowed in SNMPv2 information
 modules.  All items defined in an information module are
 automatically exported.

3.4. ASN.1 Comments

 Comments in ASN.1 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.

3.5. OBJECT IDENTIFIER values

 An OBJECT IDENTIFIER value is an ordered list of non-negative
 numbers.  For the SNMPv2 framework, 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:

SNMPv2 Working Group Standards Track [Page 11] RFC 1902 SMI for SNMPv2 January 1996

   Value   Name
     0     ccitt
     1     iso
     2     joint-iso-ccitt

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:
   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.

SNMPv2 Working Group Standards Track [Page 12] RFC 1902 SMI for SNMPv2 January 1996

 Note that reference in an IMPORTS clause or in clauses of SNMPv2
 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.  The date and time
 are represented in UTC Time format (see Appendix B).

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 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.  The date and time are represented in UTC Time format
 (see Appendix B).

5.5.1. Mapping of the DESCRIPTION sub-clause

 The DESCRIPTION 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

SNMPv2 Working Group Standards Track [Page 13] RFC 1902 SMI for SNMPv2 January 1996

 specifying an OBJECT IDENTIFIER value to refer to the information
 module containing the invocation.

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 "9505241811Z"
  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."

– contact IANA for actual number

  ::= { experimental xx }

END

SNMPv2 Working Group Standards Track [Page 14] RFC 1902 SMI for SNMPv2 January 1996

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.
 The values "current", and "obsolete" are self-explanatory.  The
 "deprecated" value indicates that the definition is obsolete, but
 that an implementor may wish to support it to foster interoperability
 with older 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 an object assignment defined in some other
 information module.

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 }

SNMPv2 Working Group Standards Track [Page 15] RFC 1902 SMI for SNMPv2 January 1996

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.

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].
 A extended subset of the full capabilities of ASN.1 sub-typing is
 allowed, as appropriate to the underingly ASN.1 type.  Any such
 restriction on size, range, enumerations or repertoire 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 C 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 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.

SNMPv2 Working Group Standards Track [Page 16] RFC 1902 SMI for SNMPv2 January 1996

 Finally, a label for a named-number enumeration must consist of one
 or more letters or digits (no hyphens), 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.)

7.1.2. OCTET STRING

 The OCTET STRING type represents arbitrary binary or textual data.
 Although there is no SMI-specified size limitation for this type, 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 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.)
 Although there is no SMI-specified limitation on the number of
 enumerations (and therefore on the length of a value), 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 (no hyphens), 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.)

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].

SNMPv2 Working Group Standards Track [Page 17] RFC 1902 SMI for SNMPv2 January 1996

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 a SYNTAX clause value of TimeStamp (a
 textual convention defined in [3]) indicating the time of the last
 discontinuity.
 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.  The
 maximum value can not be greater than 2^32-1 (4294967295 decimal).
 The value of a Gauge has its maximum value whenever the information
 being modeled is greater or equal to that maximum value; if the
 information being modeled subsequently decreases below the maximum
 value, the Gauge also decreases.

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.

SNMPv2 Working Group Standards Track [Page 18] RFC 1902 SMI for SNMPv2 January 1996

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 a SYNTAX clause value of TimeStamp (a
 textual convention defined in [3]) indicating the time of the last
 discontinuity.
 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.

SNMPv2 Working Group Standards Track [Page 19] RFC 1902 SMI for SNMPv2 January 1996

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,
 normally omitting the sub-typing information.  Further, these ASN.1
 types are always present (the 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.

SNMPv2 Working Group Standards Track [Page 20] RFC 1902 SMI for SNMPv2 January 1996

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".)

7.4. Mapping of the STATUS clause

 The STATUS clause, which must be present, indicates whether this
 definition is current or historic.
 The values "current", and "obsolete" are self-explanatory.  The
 "deprecated" value indicates that the definition is obsolete, but
 that an implementor may wish to support that object to foster
 interoperability with older 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.

SNMPv2 Working Group Standards Track [Page 21] RFC 1902 SMI for SNMPv2 January 1996

7.6. Mapping of the REFERENCE clause

 The REFERENCE clause, which need not be present, contains a textual
 cross-reference to an object defined in some other information
 module.  This is useful when de-osifying a MIB module produced by
 some other organization.

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 syntax of those
 objects indicate how to form the instance-identifier:

(1) integer-valued: a single sub-identifier taking the integer value

   (this works only for non-negative integers);

(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

SNMPv2 Working Group Standards Track [Page 22] RFC 1902 SMI for SNMPv2 January 1996

 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.
 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:

(1) within a MIB module originally written to conform to the SNMPv1

   framework, and later converted to conform to the SNMPv2 framework;
   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

SNMPv2 Working Group Standards Track [Page 23] RFC 1902 SMI for SNMPv2 January 1996

 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 1573's ifTable
 and the many media-specific MIBs which extend it for specific media
 (e.g., the dot3Table in RFC 1650), 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 1573's ifTable and a
   media-specific MIB which extends the ifTable for a specific media
   (e.g., the dot3Table in RFC 1650), 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 a SNMPv2 entity
 acting in an agent role when an object instance is created.

SNMPv2 Working Group Standards Track [Page 24] RFC 1902 SMI for SNMPv2 January 1996

 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 a SNMPv2 entity
 acting in an agent role might use.
 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 a SNMPv2 entity acting in an agent
 role 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 }
   OBJECT IDENTIFIER  DEFVAL { sysDescr }
   BITS               DEFVAL { { primary, secondary } }
                      -- enumerated values that are set
   IpAddress          DEFVAL { 'c0210415'H } -- 192.33.4.21
 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
   "1" to the administratively assigned name for the conceptual table.

SNMPv2 Working Group Standards Track [Page 25] RFC 1902 SMI for SNMPv2 January 1996

(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.
 Further note that although conceptual tables and rows are given
 administratively assigned names, these conceptual objects may not be
 manipulated in aggregate form by the management protocol.

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      INTEGER
  MAX-ACCESS  read-only
  STATUS      current
  DESCRIPTION
          "The index number of the first unassigned entry in the
          evaluation table.
          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

SNMPv2 Working Group Standards Track [Page 26] RFC 1902 SMI for SNMPv2 January 1996

          "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
  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 executed."
  DEFVAL  { 0 }
      ::= { evalEntry 3 }

evalStatus OBJECT-TYPE

  SYNTAX      RowStatus

SNMPv2 Working Group Standards Track [Page 27] RFC 1902 SMI for SNMPv2 January 1996

  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 the ordered
 sequence of MIB object types which are contained within every
 instance of the notification.  An object type specified in this
 clause may not have an MAX-ACCESS clause of "not-accessible".

8.2. Mapping of the STATUS clause

 The STATUS clause, which must be present, indicates whether this
 definition is current or historic.
 The values "current", and "obsolete" are self-explanatory.  The
 "deprecated" value indicates that the definition is obsolete, but
 that an implementor may wish to support the notification to foster
 interoperability with older 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.

SNMPv2 Working Group Standards Track [Page 28] RFC 1902 SMI for SNMPv2 January 1996

8.4. Mapping of the REFERENCE clause

 The REFERENCE clause, which need not be present, contains a textual
 cross-reference to a notification defined in some other information
 module.  This is useful when de-osifying a MIB module produced by
 some other organization.

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 the procedures employed by proxy agents (see Section 3.1.2 of
 [7]), 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 linkUp trap might be described:

linkUp NOTIFICATION-TYPE

  OBJECTS { ifIndex }
  STATUS  current
  DESCRIPTION
          "A linkUp trap signifies that the SNMPv2 entity, acting in
          an agent role, recognizes that one of the communication
          links represented in its configuration has come up."
  ::= { snmpTraps 4 }

According to this invocation, the trap authoritatively identified as

   { snmpTraps 4 }

is used to report a link coming up.

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.

SNMPv2 Working Group Standards Track [Page 29] RFC 1902 SMI for SNMPv2 January 1996

 Further, the following restrictions apply:
                          Restrictions to Refinement on
object syntax         range   enumeration     size    repertoire
-----------------     -----   -----------     ----    ----------
          INTEGER      (1)        (2)           -         -
        Integer32      (1)         -            -         -
       Unsigned32      (1)         -            -         -
     OCTET STRING       -          -           (3)       (4)
OBJECT IDENTIFIER       -          -            -         -
             BITS       -         (2)           -         -
        IpAddress       -          -            -         -
        Counter32       -          -            -         -
        Counter64       -          -            -         -
          Gauge32      (1)         -            -         -
        TimeTicks       -          -            -         -

where:

(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);

(3) the size in characters of the value may be refined by raising the

   lower-bounds, by reducing the upper-bounds, and/or by reducing the
   alternative size choices; or,

(4) the repertoire of characters in the value may be reduced by further

   sub-typing.
 Otherwise no refinements are possible.  Further details on sub-typing
 are provided in Appendix C.

10. Extending an Information Module

 As experience is gained with a published information module, it may
 be desirable to revise that information module.
 To begin, the invocation of the MODULE-IDENTITY macro should be
 updated to include information about the revision.  Usually, this
 consists of updating the LAST-UPDATED clause and adding a pair of
 REVISION and DESCRIPTION clauses.  However, other existing clauses in
 the invocation may be updated.

SNMPv2 Working Group Standards Track [Page 30] RFC 1902 SMI for SNMPv2 January 1996

 Note that the module's label (e.g., "FIZBIN-MIB" from the example in
 Section 5.8), is not changed when the information module is revised.

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.

(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".

(3) A DEFVAL clause may be added or updated.

(4) A REFERENCE clause may be added or updated.

(5) A UNITS clause may be added.

(6) A conceptual row may be augmented by adding new columnar objects at

   the end of the row.

(7) 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
 those specifically allowed above), then the OBJECT IDENTIFIER value
 associated with that object must also be changed.
 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.
 Finally, note that if an object has the value of its STATUS clause
 changed, then the value of its DESCRIPTION clause should be updated
 accordingly.

SNMPv2 Working Group Standards Track [Page 31] RFC 1902 SMI for SNMPv2 January 1996

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.
 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.
 Finally, note that if an object has the value of its STATUS clause
 changed, then the value of its DESCRIPTION clause should be updated
 accordingly.

SNMPv2 Working Group Standards Track [Page 32] RFC 1902 SMI for SNMPv2 January 1996

11. Appendix A: de-OSIfying a MIB module

 There has been an increasing amount of work recently on taking MIBs
 defined by other organizations (e.g., the IEEE) and de-osifying them
 for use with the Internet-standard network management framework.  The
 steps to achieve this are straight-forward, though tedious.  Of
 course, it is helpful to already be experienced in writing MIB
 modules for use with the Internet-standard network management
 framework.
 The first step is to construct a skeletal MIB module, as shown
 earlier in Section 5.8.  The next step is to categorize the objects
 into groups.  Optional objects are not permitted.  Thus, when a MIB
 module is created, optional objects must be placed in a additional
 groups, which, if implemented, all objects in the group must be
 implemented.  For the first pass, it is wisest to simply ignore any
 optional objects in the original MIB:  experience shows it is better
 to define a core MIB module first, containing only essential objects;
 later, if experience demands, other objects can be added.

11.1. Managed Object Mapping

 Next for each managed object class, determine whether there can exist
 multiple instances of that managed object class.  If not, then for
 each of its attributes, use the OBJECT-TYPE macro to make an
 equivalent definition.
 Otherwise, if multiple instances of the managed object class can
 exist, then define a conceptual table having conceptual rows each
 containing a columnar object for each of the managed object class's
 attributes.  If the managed object class is contained within the
 containment tree of another managed object class, then the assignment
 of an object is normally required for each of the "distinguished
 attributes" of the containing managed object class.  If they do not
 already exist within the MIB module, then they can be added via the
 definition of additional columnar objects in the conceptual row
 corresponding to the contained managed object class.
 In defining a conceptual row, it is useful to consider the
 optimization of network management operations which will act upon its
 columnar objects.  In particular, it is wisest to avoid defining more
 columnar objects within a conceptual row, than can fit in a single
 PDU.  As a rule of thumb, a conceptual row should contain no more
 than approximately 20 objects.  Similarly, or as a way to abide by
 the "20 object guideline", columnar objects should be grouped into
 tables according to the expected grouping of network management
 operations upon them.  As such, the content of conceptual rows should
 reflect typical access scenarios, e.g., they should be organized

SNMPv2 Working Group Standards Track [Page 33] RFC 1902 SMI for SNMPv2 January 1996

 along functional lines such as one row for statistics and another row
 for parameters, or along usage lines such as commonly-needed objects
 versus rarely-needed objects.
 On the other hand, the definition of conceptual rows where the number
 of columnar objects used as indexes outnumbers the number used to
 hold information, should also be avoided.  In particular, the
 splitting of a managed object class's attributes into many conceptual
 tables should not be used as a way to obtain the same degree of
 flexibility/complexity as is often found in MIBs with a myriad of
 optionals.

11.1.1. Mapping to the SYNTAX clause

 When mapping to the SYNTAX clause of the OBJECT-TYPE macro:

(1) An object with BOOLEAN syntax becomes a TruthValue [3].

(2) An object with INTEGER syntax becomes an Integer32.

(3) An object with ENUMERATED syntax becomes an INTEGER with

   enumerations, taking any of the values given which can be
   represented with an Integer32.

(4) An object with BIT STRING syntax having enumerations becomes a BITS

   construct.

(5) An object with BIT STRING syntax but no enumerations becomes an

   OCTET STRING.

(6) An object with a character string syntax becomes either an OCTET

   STRING, or a DisplayString [3], depending on the repertoire of the
   character string.

(7) A non-tabular object with a complex syntax, such as REAL or

   EXTERNAL, must be decomposed, usually into an OCTET STRING (if
   sensible).  As a rule, any object with a complicated syntax should
   be avoided.

(8) Tabular objects must be decomposed into rows of columnar objects.

11.1.2. Mapping to the UNITS clause

 If the description of this managed object defines a unit-basis, then
 mapping to this clause is straight-forward.

SNMPv2 Working Group Standards Track [Page 34] RFC 1902 SMI for SNMPv2 January 1996

11.1.3. Mapping to the MAX-ACCESS clause

 This is straight-forward.

11.1.4. Mapping to the STATUS clause

 This is straight-forward.

11.1.5. Mapping to the DESCRIPTION clause

 This is straight-forward:  simply copy the text, making sure that any
 embedded double quotation marks are sanitized (i.e., replaced with
 single-quotes or removed).

11.1.6. Mapping to the REFERENCE clause

 This is straight-forward:  simply include a textual reference to the
 object being mapped, the document which defines the object, and
 perhaps a page number in the document.

11.1.7. Mapping to the INDEX clause

 If necessary, decide how instance-identifiers for columnar objects
 are to be formed and define this clause accordingly.

11.1.8. Mapping to the DEFVAL clause

 Decide if a meaningful default value can be assigned to the object
 being mapped, and if so, define the DEFVAL clause accordingly.

11.2. Action Mapping

 Actions are modeled as read-write objects, in which writing a
 particular value results in a state change.  (Usually, as a part of
 this state change, some action might take place.)

11.2.1. Mapping to the SYNTAX clause

 Usually the Integer32 syntax is used with a distinguished value
 provided for each action that the object provides access to.  In
 addition, there is usually one other distinguished value, which is
 the one returned when the object is read.

11.2.2. Mapping to the MAX-ACCESS clause

 Always use read-write or read-create.

SNMPv2 Working Group Standards Track [Page 35] RFC 1902 SMI for SNMPv2 January 1996

11.2.3. Mapping to the STATUS clause

 This is straight-forward.

11.2.4. Mapping to the DESCRIPTION clause

 This is straight-forward:  simply copy the text, making sure that any
 embedded double quotation marks are sanitized (i.e., replaced with
 single-quotes or removed).

11.2.5. Mapping to the REFERENCE clause

 This is straight-forward:  simply include a textual reference to the
 action being mapped, the document which defines the action, and
 perhaps a page number in the document.

11.3. Event Mapping

 Events are modeled as SNMPv2 notifications using NOTIFICATION-TYPE
 macro.  However, recall that SNMPv2 emphasizes trap-directed polling.
 As such, few, and usually no, notifications, need be defined for any
 MIB module.

11.3.1. Mapping to the STATUS clause

 This is straight-forward.

11.3.2. Mapping to the DESCRIPTION clause

 This is straight-forward:  simply copy the text, making sure that any
 embedded double quotation marks are sanitized (i.e., replaced with
 single-quotes or removed).

11.3.3. Mapping to the REFERENCE clause

 This is straight-forward:  simply include a textual reference to the
 notification being mapped, the document which defines the
 notification, and perhaps a page number in the document.

SNMPv2 Working Group Standards Track [Page 36] RFC 1902 SMI for SNMPv2 January 1996

12. Appendix B: UTC Time Format

 Several clauses defined in this document use the UTC Time format:
   YYMMDDHHMMZ
   where: YY - last two digits of year
          MM - month (01 through 12)
          DD - day of month (01 through 31)
          HH - hours (00 through 23)
          MM - minutes (00 through 59)
           Z - the character "Z" denotes Greenwich Mean Time (GMT).
 For example, "9502192015Z" represents 8:15pm GMT on 19 February 1995.

13. Appendix C: Detailed Sub-typing Rules

13.1. 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 (i.e. 'xxxx'H)
       <binString> is a binary string (i.e. 'xxxx'B)

SNMPv2 Working Group Standards Track [Page 37] RFC 1902 SMI for SNMPv2 January 1996

       <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.

13.2. 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))

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

13.3. Rules for Textual Conventions

 Sub-typing of Textual Conventions (see [3]) is allowed but must be
 valid.  In particular, each range specified for the textual
 convention must be a subset of a range specified for the base type.
 For example,
   Tc1 ::= INTEGER (1..10 | 11..20)
   Tc2 ::= Tc1 (2..10 | 12..15)       -- is valid
   Tc3 ::= Tc1 (4..8)                 -- is valid
   Tc4 ::= Tc1 (8..12)                -- is invalid

14. Security Considerations

 Security issues are not discussed in this memo.

SNMPv2 Working Group Standards Track [Page 38] RFC 1902 SMI for SNMPv2 January 1996

15. Editor's Address

 Keith McCloghrie
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134-1706
 US
 Phone: +1 408 526 5260
 EMail: kzm@cisco.com

16. Acknowledgements

 This document is the result of significant work by the four major
 contributors:
 Jeffrey D. Case (SNMP Research, case@snmp.com)
 Keith McCloghrie (Cisco Systems, kzm@cisco.com)
 Marshall T. Rose (Dover Beach Consulting, mrose@dbc.mtview.ca.us)
 Steven Waldbusser (International Network Services, stevew@uni.ins.com)
 In addition, the contributions of the SNMPv2 Working Group are
 acknowledged.  In particular, a special thanks is extended for the
 contributions of:
   Alexander I. Alten (Novell)
   Dave Arneson (Cabletron)
   Uri Blumenthal (IBM)
   Doug Book (Chipcom)
   Kim Curran (Bell-Northern Research)
   Jim Galvin (Trusted Information Systems)
   Maria Greene (Ascom Timeplex)
   Iain Hanson (Digital)
   Dave Harrington (Cabletron)
   Nguyen Hien (IBM)
   Jeff Johnson (Cisco Systems)
   Michael Kornegay (Object Quest)
   Deirdre Kostick (AT&T Bell Labs)
   David Levi (SNMP Research)
   Daniel Mahoney (Cabletron)
   Bob Natale (ACE*COMM)
   Brian O'Keefe (Hewlett Packard)
   Andrew Pearson (SNMP Research)
   Dave Perkins (Peer Networks)
   Randy Presuhn (Peer Networks)
   Aleksey Romanov (Quality Quorum)
   Shawn Routhier (Epilogue)
   Jon Saperia (BGS Systems)

SNMPv2 Working Group Standards Track [Page 39] RFC 1902 SMI for SNMPv2 January 1996

   Bob Stewart (Cisco Systems, bstewart@cisco.com), chair
   Kaj Tesink (Bellcore)
   Glenn Waters (Bell-Northern Research)
   Bert Wijnen (IBM)

17. 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] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and

   S. Waldbusser, "Conformance Statements for Version 2 of the Simple
   Network Management Protocol (SNMPv2)", RFC 1904, January 1996.

[3] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and

   S. Waldbusser, "Textual Conventions for Version 2 of the Simple
   Network Management Protocol (SNMPv2)", RFC 1903, January 1996.

[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] 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] 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.

[7] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and

   S. Waldbusser, "Coexistence between Version 1 and Version 2 of the
   Internet-standard Network Management Framework", RFC 1908,
   January 1996.

SNMPv2 Working Group Standards Track [Page 40]

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