rfc1212.txt

开发snmp的开发包有两个开放的SNMP开发库

                              | empty

          END

          IndexSyntax ::=
              CHOICE {
                  number
                      INTEGER (0..MAX),
                  string
                      OCTET STRING,
                  object
                      OBJECT IDENTIFIER,
                  address
                      NetworkAddress,
                  ipAddress
                      IpAddress
              }


4.1.  Mapping of the OBJECT-TYPE macro

   It should be noted that the expansion of the OBJECT-TYPE macro is
   something which conceptually happens during implementation and not
   during run-time.

4.1.1.  Mapping of the SYNTAX clause

   The SYNTAX clause, which must be present, defines the abstract data
   structure corresponding to that object type.  The ASN.1 language [6]
   is used for this purpose.  However, the SMI purposely restricts the
   ASN.1 constructs which may be used.  These restrictions are made
   expressly for simplicity.





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4.1.2.  Mapping of the ACCESS clause

   The ACCESS clause, which must be present, defines the minimum level
   of support required for that object type.  As a local matter,
   implementations may support other access types (e.g., an
   implementation may elect to permitting writing a variable marked as
   read-only).  Further, protocol-specific "views" (e.g., those
   indirectly implied by an SNMP community) may make further
   restrictions on access to a variable.

4.1.3.  Mapping of the STATUS clause

   The STATUS clause, which must be present, defines the implementation
   support required for that object type.

4.1.4.  Mapping of the DESCRIPTION clause

   The DESCRIPTION clause, which need not be present, contains a textual
   definition of that object type 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.  Note that, in
   order to conform to the ASN.1 syntax, the entire value of this clause
   must be enclosed in double quotation marks, although the value may be
   multi-line.

   Further, note that if the MIB module does not contain a textual
   description of the object type elsewhere then the DESCRIPTION clause
   must be present.

4.1.5.  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 MIB module.  This
   is useful when de-osifying a MIB produced by some other organization.

4.1.6.  Mapping of the INDEX clause

   The INDEX clause, which may be present only if that object type
   corresponds to a conceptual row, defines instance identification
   information for that object type.  (Historically, each MIB definition
   contained a section entitled "Identification of OBJECT instances for
   use with the SNMP".  By using the INDEX clause, this section need no
   longer occur as this clause concisely captures the precise semantics
   needed for instance identification.)

   If the INDEX clause is not present, and the object type corresponds
   to a non-columnar object, then instances of the object are identified



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   by appending a sub-identifier of zero to the name of that object.
   Further, note that if the MIB module does not contain a textual
   description of how instance identification information is derived for
   columnar objects, then the INDEX clause must be present.

   To define the instance identification information, determine which
   object value(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: `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: `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: `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);

          (5)  NetworkAddress-valued: `n+1' sub-identifiers, where `n'
               depends on the kind of address being encoded (the first
               sub-identifier indicates the kind of address, value 1
               indicates an IpAddress); or,

          (6)  IpAddress-valued: 4 sub-identifiers, in the familiar
               a.b.c.d notation.

   Note that if an "indextype" value is present (e.g., INTEGER rather
   than ifIndex), then a DESCRIPTION clause must be present; the text
   contained therein indicates the semantics of the "indextype" value.












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   By way of example, in the context of MIB-II [7], the following INDEX
   clauses might be present:

                 objects under         INDEX clause
               -----------------       ------------
               ifEntry                 { ifIndex }
               atEntry                 { atNetIfIndex,
                                         atNetAddress }
               ipAddrEntry             { ipAdEntAddr }
               ipRouteEntry            { ipRouteDest }
               ipNetToMediaEntry       { ipNetToMediaIfIndex,
                                         ipNetToMediaNetAddress }
               tcpConnEntry            { tcpConnLocalAddress,
                                         tcpConnLocalPort,
                                         tcpConnRemoteAddress,
                                         tcpConnRemotePort }
               udpEntry                { udpLocalAddress,
                                         udpLocalPort }
               egpNeighEntry           { egpNeighAddr }


4.1.7.  Mapping of the DEFVAL clause

   The DEFVAL clause, which need not be present, defines an acceptable
   default value which may be used when an object instance is created at
   the discretion of the agent acting in conformance with the third
   paradigm described in Section 4.2 above.

   During conceptual row creation, if an instance of a columnar object
   is not present as one of the operands in the correspondent SNMP set
   operation, then the value of the DEFVAL clause, if present, indicates
   an acceptable default value that the agent might use.

   The value of the DEFVAL clause must, of course, correspond to the
   SYNTAX clause for the object.  Note that if an operand to the SNMP
   set operation is an instance of a read-only object, then the error
   noSuchName will be returned.  As such, the DEFVAL clause can be used
   to provide an acceptable default value that the agent might use.

   It is possible that no acceptable default value may exist for any of
   the columnar objects in a conceptual row for which the creation of
   new object instances is allowed.  In this case, the objects specified
   in the INDEX clause must have a corresponding ACCESS clause value of
   read-write.







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   By way of example, consider the following possible DEFVAL clauses:

       ObjectSyntax            DEFVAL clause
       -----------------       ------------
       INTEGER                 1 -- same for Counter, Gauge, TimeTicks
       OCTET STRING            'ffffffffffff'h
       DisplayString           "any NVT ASCII string"
       OBJECT IDENTIFIER       sysDescr
       OBJECT IDENTIFIER       { system 2 }
       NULL                    NULL
       NetworkAddress          { internet 'c0210415'h }
       IpAddress               'c0210415'h -- 192.33.4.21


4.1.8.  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.

4.2.  Usage Example

   Consider how the ipNetToMediaTable from MIB-II might be fully
   described:

          -- the IP Address Translation tables

          -- The Address Translation tables contain IpAddress to
          -- "physical" address equivalences.  Some interfaces do not
          -- use translation tables for determining address equivalences
          -- (e.g., DDN-X.25 has an algorithmic method); if all
          -- interfaces are of this type, then the Address Translation
          -- table is empty, i.e., has zero entries.

          ipNetToMediaTable OBJECT-TYPE
              SYNTAX  SEQUENCE OF IpNetToMediaEntry
              ACCESS  not-accessible
              STATUS  mandatory
              DESCRIPTION
                      "The IP Address Translation table used for mapping
                      from IP addresses to physical addresses."
              ::= { ip 22 }

          ipNetToMediaEntry OBJECT-TYPE
              SYNTAX  IpNetToMediaEntry
              ACCESS  not-accessible
              STATUS  mandatory
              DESCRIPTION
                      "Each entry contains one IpAddress to 'physical'



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                      address equivalence."
              INDEX   { ipNetToMediaIfIndex,
                        ipNetToMediaNetAddress }
              ::= { ipNetToMediaTable 1 }

          IpNetToMediaEntry ::=
              SEQUENCE {
                  ipNetToMediaIfIndex
                      INTEGER,
                  ipNetToMediaPhysAddress
                      OCTET STRING,
                  ipNetToMediaNetAddress
                      IpAddress,
                  ipNetoToMediaType
                      INTEGER
              }

          ipNetToMediaIfIndex OBJECT-TYPE
              SYNTAX  INTEGER
              ACCESS  read-write
              STATUS  mandatory
              DESCRIPTION
                      "The interface on which this entry's equivalence
                      is effective.  The interface identified by a
                      particular value of this index is the same
                      interface as identified by the same value of
                      ifIndex."
              ::= { ipNetToMediaEntry 1 }

          ipNetToMediaPhysAddress OBJECT-TYPE
              SYNTAX  OCTET STRING
              ACCESS  read-write
              STATUS  mandatory
              DESCRIPTION
                      "The media-dependent 'physical' address."
              ::= { ipNetToMediaEntry 2 }

          ipNetToMediaNetAddress OBJECT-TYPE
              SYNTAX  IpAddress
              ACCESS  read-write
              STATUS  mandatory
              DESCRIPTION
                      "The IpAddress corresponding to the media-
                      dependent 'physical' address."
              ::= { ipNetToMediaEntry 3 }

          ipNetToMediaType OBJECT-TYPE
              SYNTAX  INTEGER {



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                          other(1),   -- none of the following
                          invalid(2), -- an invalidated mapping
                          dynamic(3),
                          static(4)
                      }
              ACCESS  read-write
              STATUS  mandatory
              DESCRIPTION
                      "The type of mapping.

                      Setting this object to the value invalid(2) has
                      the effect of invalidating the corresponding entry
                      in the ipNetToMediaTable.  That is, it effectively
                      disassociates the interface identified with said
                      entry from the mapping identified with said entry.
                      It is an implementation-specific matter as to
                      whether the agent removes an invalidated entry
                      from the table.  Accordingly, management stations
                      must be prepared to receive tabular information
                      from agents that corresponds to entries not
                      currently in use.  Proper interpretation of such
                      entries requires examination of the relevant
                      ipNetToMediaType object."
                  ::= { ipNetToMediaEntry 4 }


5.  Appendix: DE-osifying MIBs

   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