rfc2573.txt

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        unavailable, processing of the management operation is halted,
        no result PDU is generated, the snmpUnavailableContexts counter
        is incremented, and control is passed to step (6) below.

   (6)  The Dispatcher is called to generate a response or report
        message.  The abstract service interface is:

   returnResponsePdu(
     IN   messageProcessingModel   -- typically, SNMP version
     IN   securityModel            -- Security Model in use
     IN   securityName             -- on behalf of this principal
     IN   securityLevel            -- same as on incoming request
     IN   contextEngineID          -- data from/at this SNMP entity
     IN   contextName              -- data from/in this context
     IN   pduVersion               -- the version of the PDU
     IN   PDU                      -- SNMP Protocol Data Unit
     IN   maxSizeResponseScopedPDU -- maximum size of the Response PDU
     IN   stateReference           -- reference to state information
                                   -- as presented with the request
     IN   statusInformation        -- success or errorIndication
          )                        -- error counter OID/value if error




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   Where:

     -  The messageProcessingModel is the value from the processPdu
        call.

     -  The securityModel is the value from the processPdu call.

     -  The securityName is the value from the processPdu call.

     -  The securityLevel is the value from the processPdu call.

     -  The contextEngineID is the value from the processPdu call.

     -  The contextName is the value from the processPdu call.

     -  The pduVersion indicates the version of the PDU to be returned.
        If no result PDU was generated, the pduVersion is an undefined
        value.

     -  The PDU is the result generated in step (5) above.  If no result
        PDU was generated, the PDU is an undefined value.

     -  The maxSizeResponseScopedPDU is a local value indicating the
        maximum size of a ScopedPDU that the application can accept.

     -  The stateReference is the value from the processPdu call.

     -  The statusInformation either contains an indication that no
        error occurred and that a response should be generated, or
        contains an indication that an error occurred along with the OID
        and counter value of the appropriate error counter object.

   Note that a command responder application should always call the
   returnResponsePdu abstract service interface, even in the event of an
   error such as a resource allocation error.  In the event of such an
   error, the PDU value passed to returnResponsePdu should contain
   appropriate values for errorStatus and errorIndex.

   Note that the text above describes situations where the
   snmpUnknownContexts counter is incremented, and where the
   snmpUnavailableContexts counter is incremented.  The difference
   between these is that the snmpUnknownContexts counter is incremented
   when a request is received for a context which unknown to the SNMP
   entity.  The snmpUnavailableContexts counter is incremented when a
   request is received for a context which is known to the SNMP entity,
   but is currently unavailable.  Determining when a context is





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   unavailable is implementation specific, and some implementations may
   never encounter this situation, and so may never increment the
   snmpUnavailableContexts counter.

3.3.  Notification Originator Applications

   A notification originator application generates SNMP messages
   containing Notification-Class PDUs (for example, SNMPv2-Trap PDUs or
   Inform PDUs).  There is no requirement as to what specific types of
   Notification-Class PDUs a particular implementation must be capable
   of generating.

   Notification originator applications require a mechanism for
   identifying the management targets to which notifications should be
   sent.  The particular mechanism used is implementation dependent.
   However, if an implementation makes the configuration of management
   targets SNMP manageable, it MUST use the SNMP-TARGET-MIB module
   described in this document.

   When a notification originator wishes to generate a notification, it
   must first determine in which context the information to be conveyed
   in the notification exists, i.e., it must determine the
   contextEngineID and contextName.  It must then determine the set of
   management targets to which the notification should be sent.  The
   application must also determine, for each management target, what
   specific PDU type the notification message should contain, and if it
   is to contain a Confirmed-Class PDU, the number of retries and
   retransmission algorithm.

   The mechanism by which a notification originator determines this
   information is implementation dependent.  Once the application has
   determined this information, the following procedure is performed for
   each management target:

   (1)  Any appropriate filtering mechanisms are applied to determine
        whether the notification should be sent to the management
        target.  If such filtering mechanisms determine that the
        notification should not be sent, processing continues with the
        next management target.  Otherwise,

   (2)  The appropriate set of variable-bindings is retrieved from local
        MIB instrumentation within the relevant MIB view.  The relevant
        MIB view is determined by the securityLevel, securityModel,
        contextName, and securityName of the management target.  To
        determine whether a particular object instance is within the
        relevant MIB view, the isAccessAllowed abstract service
        interface is used, in the same manner as described in the




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        preceding section.  If the statusInformation returned by
        isAccessAllowed does not indicate accessAllowed, the
        notification is not sent to the management target.

   (3)  The NOTIFICATION-TYPE OBJECT IDENTIFIER of the notification
        (this is the value of the element of the variable bindings whose
        name is snmpTrapOID.0, i.e., the second variable binding) is
        checked using the isAccessAllowed abstract service interface,
        using the same parameters used in the preceding step.  If the
        statusInformation returned by isAccessAllowed does not indicate
        accessAllowed, the notification is not sent to the management
        target.

   (4)  A PDU is constructed using a locally unique request-id value, a
        PDU type as determined by the implementation, an error-status
        and error-index value of 0, and the variable-bindings supplied
        previously in step (2).

   (5)  If the notification contains an Unconfirmed-Class PDU, the
        Dispatcher is called using the following abstract service
        interface:

       statusInformation =              -- sendPduHandle if success
                                        -- errorIndication if failure
         sendPdu(
         IN   transportDomain           -- transport domain to be used
         IN   transportAddress          -- destination network address
         IN   messageProcessingModel    -- typically, SNMP version
         IN   securityModel             -- Security Model to use
         IN   securityName              -- on behalf of this principal
         IN   securityLevel             -- Level of Security requested
         IN   contextEngineID           -- data from/at this entity
         IN   contextName               -- data from/in this context
         IN   pduVersion                -- the version of the PDU
         IN   PDU                       -- SNMP Protocol Data Unit
         IN   expectResponse            -- TRUE or FALSE
              )

   Where:

     -  The transportDomain is that of the management target.

     -  The transportAddress is that of the management target.

     -  The messageProcessingModel is that of the management target.

     -  The securityModel is that of the management target.




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     -  The securityName is that of the management target.

     -  The securityLevel is that of the management target.

     -  The contextEngineID is the value originally determined for the
        notification.

     -  The contextName is the value originally determined for the
        notification.

     -  The pduVersion is the version of the PDU to be sent.

     -  The PDU is the value constructed in step (3) above.

     -  The expectResponse argument indicates that no response is
        expected.

   Otherwise,

   (6)  If the notification contains a Confirmed-Class PDU, then:

        a)  The Dispatcher is called using the sendPdu abstract service
            interface as described in step (4) above, except that the
            expectResponse argument indicates that a response is
            expected.

        b)  The application caches information about the management
            target.

        c)  If a response is received within an appropriate time
            interval from the transport endpoint of the management
            target, the notification is considered acknowledged and the
            cached information is deleted.  Otherwise,

        d)  If a response is not received within an appropriate time
            period, or if a report indication is received, information
            about the management target is retrieved from the cache, and
            steps a) through d) are repeated.  The number of times these
            steps are repeated is equal to the previously determined
            retry count.  If this retry count is exceeded, the
            acknowledgement of the notification is considered to have
            failed, and processing of the notification for this
            management target is halted.  Note that some report
            indications might be considered a failure.  Such report
            indications should be interpreted to mean that the
            acknowledgement of the notification has failed.





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   Responses to Confirmed-Class PDU notifications will be received via
   the processResponsePdu abstract service interface.

   To summarize, the steps that a notification originator follows when
   determining where to send a notification are:

     -  Determine the targets to which the notification should be sent.

     -  Apply any required filtering to the list of targets.

     -  Determine which targets are authorized to receive the
        notification.

3.4.  Notification Receiver Applications

   Notification receiver applications receive SNMP Notification messages
   from the Dispatcher.  Before any messages can be received, the
   notification receiver must register with the Dispatcher using the
   registerContextEngineID abstract service interface.  The parameters
   used are:

     -  The contextEngineID is an undefined 'wildcard' value.
        Notifications are delivered to a registered notification
        receiver regardless of the contextEngineID contained in the
        notification message.

     -  The pduType indicates the type of notifications that the
        application wishes to receive (for example, SNMPv2-Trap PDUs or
        Inform PDUs).

   Once the notification receiver has registered with the Dispatcher,
   messages are received using the processPdu abstract service
   interface.  Parameters are:

     -  The messageProcessingModel indicates which Message Processing
        Model received and processed the message.

     -  The securityModel is the value from the received message.

     -  The securityName is the value from the received message.

     -  The securityLevel is the value from the received message.

     -  The contextEngineID is the value from the received message.

     -  The contextName is the value from the received message.





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     -  The pduVersion indicates the version of the PDU in the received
        message.

     -  The PDU is the value from the received message.

     -  The maxSizeResponseScopedPDU is the maximum allowable size of a
        ScopedPDU containing a Response PDU (based on the maximum
        message size that the originator of the message can accept).

     -  If the message contains an Unconfirmed-Class PDU, the
        stateReference is undefined and unused.  Otherwise, the
        stateReference is a value which references cached information
        about the notification.  This value must be returned to the
        Dispatcher in order to generate a response.