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RFC 3413

 
 
 

Simple Network Management Protocol (SNMP) Applications

Part 2 of 4, p. 6 to 28
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3. Elements Of Procedure

   The following sections describe the procedures followed by each type
   of application when generating messages for transmission or when
   processing received messages.  Applications communicate with the
   Dispatcher using the abstract service interfaces defined in
   [RFC3411].

3.1. Command Generator Applications

   A command generator initiates an SNMP request by calling the
   Dispatcher 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 destination of the message.

   - The transportAddress is that of the destination of the message.

   - The messageProcessingModel indicates which Message Processing Model
     the application wishes to use.

   - The securityModel is the security model that the application wishes
     to use.

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   - The securityName is the security model independent name for the
     principal on whose behalf the application wishes the message to be
     generated.

   - The securityLevel is the security level that the application wishes
     to use.

   - The contextEngineID specifies the location of the management
     information it is requesting.  Note that unless the request is
     being sent to a proxy, this value will usually be equal to the
     snmpEngineID value of the engine to which the request is being
     sent.

   - The contextName specifies the local context name for the management
     information it is requesting.

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

   - The PDU is a value constructed by the command generator containing
     the management operation that the command generator wishes to
     perform.

   - The expectResponse argument indicates that a response is expected.

   The result of the sendPdu interface indicates whether the PDU was
   successfully sent.  If it was successfully sent, the returned value
   will be a sendPduHandle.  The command generator should store the
   sendPduHandle so that it can correlate a response to the original
   request.

   The Dispatcher is responsible for delivering the response to a
   particular request to the correct command generator application.  The
   abstract service interface used is:

      processResponsePdu(              -- process Response PDU
        IN   messageProcessingModel    -- typically, SNMP version
        IN   securityModel             -- Security Model in use
        IN   securityName              -- on behalf of this principal
        IN   securityLevel             -- Level of Security
        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   statusInformation         -- success or errorIndication
        IN   sendPduHandle             -- handle from sendPdu
             )

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   Where:

   - The messageProcessingModel is the value from the received response.

   - The securityModel is the value from the received response.

   - The securityName is the value from the received response.

   - The securityLevel is the value from the received response.

   - The contextEngineID is the value from the received response.

   - The contextName is the value from the received response.

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

   - The PDU is the value from the received response.

   - The statusInformation indicates success or failure in receiving the
     response.

   - The sendPduHandle is the value returned by the sendPdu call which
     generated the original request to which this is a response.

   The procedure when a command generator receives a message is as
   follows:

   (1) If the received values of messageProcessingModel, securityModel,
       securityName, contextEngineID, contextName, and pduVersion are
       not all equal to the values used in the original request, the
       response is discarded.

   (2) The operation type, request-id, error-status, error-index, and
       variable-bindings are extracted from the PDU and saved.  If the
       request-id is not equal to the value used in the original
       request, the response is discarded.

   (3) At this point, it is up to the application to take an appropriate
       action.  The specific action is implementation dependent.  If the
       statusInformation indicates that the request failed, an
       appropriate action might be to attempt to transmit the request
       again, or to notify the person operating the application that a
       failure occurred.

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3.2. Command Responder Applications

   Before a command responder application can process messages, it must
   first associate itself with an SNMP engine.  The abstract service
   interface used for this purpose is:

      statusInformation =       -- success or errorIndication
       registerContextEngineID(
       IN   contextEngineID     -- take responsibility for this one
       IN   pduType             -- the pduType(s) to be registered
            )

   Where:

   - The statusInformation indicates success or failure of the
     registration attempt.

   - The contextEngineID is equal to the snmpEngineID of the SNMP engine
     with which the command responder is registering.

   - The pduType indicates a Read-Class and/or Write-Class PDU.

   Note that if another command responder application is already
   registered with an SNMP engine, any further attempts to register with
   the same contextEngineID and pduType will be denied.  This implies
   that separate command responder applications could register
   separately for the various pdu types.  However, in practice this is
   undesirable, and only a single command responder application should
   be registered with an SNMP engine at any given time.

   A command responder application can disassociate with an SNMP engine
   using the following abstract service interface:

      unregisterContextEngineID(
        IN   contextEngineID     -- give up responsibility for this one
        IN   pduType             -- the pduType(s) to be unregistered
             )

   Where:

   - The contextEngineID is equal to the snmpEngineID of the SNMP engine
     with which the command responder is cancelling the registration.

   - The pduType indicates a Read-Class and/or Write-Class PDU.

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   Once the command responder has registered with the SNMP engine, it
   waits to receive SNMP messages.  The abstract service interface used
   for receiving messages is:

   processPdu(                     -- process Request/Notification PDU
     IN   messageProcessingModel   -- typically, SNMP version
     IN   securityModel            -- Security Model in use
     IN   securityName             -- on behalf of this principal
     IN   securityLevel            -- Level of Security
     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
          )                        -- needed when sending a response

   Where:

   - 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.

   - 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).

   - The stateReference is a value which references cached information
     about each received request message.  This value must be returned
     to the Dispatcher in order to generate a response.

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   The procedure when a message is received is as follows:

   (1) The operation type is determined from the ASN.1 tag value
       associated with the PDU parameter.  The operation type should
       always be one of the types previously registered by the
       application.

   (2) The request-id is extracted from the PDU and saved.

   (3) Any PDU type specific parameters are extracted from the PDU and
       saved (for example, if the PDU type is an SNMPv2 GetBulk PDU, the
       non-repeaters and max-repetitions values are extracted).

   (4) The variable-bindings are extracted from the PDU and saved.

   (5) The management operation represented by the PDU type is performed
       with respect to the relevant MIB view within the context named by
       the contextName (for an SNMPv2 PDU type, the operation is
       performed according to the procedures set forth in [RFC1905]).
       The relevant MIB view is determined by the securityLevel,
       securityModel, contextName, securityName, and the class of the
       PDU type.  To determine whether a particular object instance is
       within the relevant MIB view, the following abstract service
       interface is called:

          statusInformation =      -- success or errorIndication
            isAccessAllowed(
            IN   securityModel     -- Security Model in use
            IN   securityName      -- principal who wants to access
            IN   securityLevel     -- Level of Security
            IN   viewType          -- read, write, or notify view
            IN   contextName       -- context containing variableName
            IN   variableName      -- OID for the managed object
                 )

       Where:

       - 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 viewType indicates whether the PDU type is a Read-Class or
         Write-Class operation.

       - The contextName is the value from the received message.

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       - The variableName is the object instance of the variable for
         which access rights are to be checked.

       Normally, the result of the management operation will be a new
       PDU value, and processing will continue in step (6) below.
       However, at any time during the processing of the management
       operation:

       - If the isAccessAllowed ASI returns a noSuchView, noAccessEntry,
         or noGroupName error, processing of the management operation is
         halted, a PDU value is constructed using the values from the
         originally received PDU, but replacing the error-status with an
         authorizationError code, and error-index value of 0, and
         control is passed to step (6) below.

       - If the isAccessAllowed ASI returns an otherError, processing of
         the management operation is halted, a different PDU value is
         constructed using the values from the originally received PDU,
         but replacing the error-status with a genError code and the
         error-index with the index of the failed variable binding, and
         control is passed to step (6) below.

       - If the isAccessAllowed ASI returns a noSuchContext error,
         processing of the management operation is halted, no result PDU
         is generated, the snmpUnknownContexts counter is incremented,
         and control is passed to step (6) below for generation of a
         report message.

       - If the context named by the contextName parameter is
         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 for
         generation of a report message.

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

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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

   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.

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   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 is 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
   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.

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   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 preceding
       section, except that the viewType indicates a Notification-Class
       operation.  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:

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       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.

       - 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 (4) above.

       - The expectResponse argument indicates that no response is
         expected.

       Otherwise,

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   (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 (5) 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,
          and that steps a) through d) need not be repeated.

   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:

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   - 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.

   - 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.

   When an Unconfirmed-Class PDU is delivered to a notification receiver
   application, it first extracts the SNMP operation type, request-id,
   error-status, error-index, and variable-bindings from the PDU.  After
   this, processing depends on the particular implementation.

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   When a Confirmed-Class PDU is received, the notification receiver
   application follows the following procedure:

   (1) The PDU type, request-id, error-status, error-index, and
       variable-bindings are extracted from the PDU.

   (2) A Response-Class PDU is constructed using the extracted
       request-id and variable-bindings, and with error-status and
       error-index both set to 0.

   (3) The Dispatcher is called to generate a response message using the
       returnResponsePdu abstract service interface.  Parameters are:

       - 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.

       - The PDU is the result generated in step (2) above.

       - 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 indicates that no error occurred and that
         a response should be generated.

   (4) After this, processing depends on the particular implementation.

3.5. Proxy Forwarder Applications

   A proxy forwarder application deals with forwarding SNMP messages.
   There are four basic types of messages which a proxy forwarder
   application may need to forward.  These are grouped according to the
   class of PDU type contained in a message.  The four basic types of
   messages are:

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   - Those containing Read-Class or Write-Class PDU types (for example,
     Get, GetNext, GetBulk, and Set PDU types).  These deal with
     requesting or modifying information located within a particular
     context.

   - Those containing Notification-Class PDU types (for example,
     SNMPv2-Trap and Inform PDU types).  These deal with notifications
     concerning information located within a particular context.

   - Those containing a Response-Class PDU type.  Forwarding of
     Response-Class PDUs always occurs as a result of receiving a
     response to a previously forwarded message.

   - Those containing Internal-Class PDU types (for example, a Report
     PDU).  Forwarding of Internal-Class PDU types always occurs as a
     result of receiving an Internal-Class PDU in response to a
     previously forwarded message.

   For the first type, the proxy forwarder's role is to deliver a
   request for management information to an SNMP engine which is
   "closer" or "downstream in the path" to the SNMP engine which has
   access to that information, and to deliver the response containing
   the information back to the SNMP engine from which the request was
   received.  The context information in a request is used to determine
   which SNMP engine has access to the requested information, and this
   is used to determine where and how to forward the request.

   For the second type, the proxy forwarder's role is to determine which
   SNMP engines should receive notifications about management
   information from a particular location.  The context information in a
   notification message determines the location to which the information
   contained in the notification applies.  This is used to determine
   which SNMP engines should receive notification about this
   information.

   For the third type, the proxy forwarder's role is to determine which
   previously forwarded request or notification (if any) the response
   matches, and to forward the response back to the initiator of the
   request or notification.

   For the fourth type, the proxy forwarder's role is to determine which
   previously forwarded request or notification (if any) the Internal-
   Class PDU matches, and to forward the Internal-Class PDU back to the
   initiator of the request or notification.

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   When forwarding messages, a proxy forwarder application must perform
   a translation of incoming management target information into outgoing
   management target information.  How this translation is performed is
   implementation specific.  In many cases, this will be driven by a
   preconfigured translation table.  If a proxy forwarder application
   makes the contents of this table SNMP manageable, it MUST use the
   SNMP-PROXY-MIB module defined in this document.

3.5.1. Request Forwarding

   There are two phases for request forwarding.  First, the incoming
   request needs to be passed through the proxy application.  Then, the
   resulting response needs to be passed back.  These phases are
   described in the following two sections.

3.5.1.1. Processing an Incoming Request

   A proxy forwarder application that wishes to forward request messages
   must first register with the Dispatcher using the
   registerContextEngineID abstract service interface.  The proxy
   forwarder must register each contextEngineID for which it wishes to
   forward messages, as well as for each pduType.  Note that as the
   configuration of a proxy forwarder is changed, the particular
   contextEngineID values for which it is forwarding may change.  The
   proxy forwarder should call the registerContextEngineID and
   unregisterContextEngineID abstract service interfaces as needed to
   reflect its current configuration.

   A proxy forwarder application should never attempt to register a
   value of contextEngineID which is equal to the snmpEngineID of the
   SNMP engine to which the proxy forwarder is associated.

   Once the proxy forwarder has registered for the appropriate
   contextEngineID values, it can start processing messages.  The
   following procedure is used:

   (1) A message is received using the processPdu abstract service
       interface.  The incoming management target information received
       from the processPdu interface is translated into outgoing
       management target information.  Note that this translation may
       vary for different values of contextEngineID and/or contextName.
       The translation should result in a single management target.

   (2) If appropriate outgoing management target information cannot be
       found, the proxy forwarder increments the snmpProxyDrops counter
       [RFC1907], and then calls the Dispatcher using the
       returnResponsePdu abstract service interface.  Parameters are:

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       - 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 is the value from the processPdu call.

       - 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 indicates that an error occurred and
         includes the OID and value of the snmpProxyDrops object.

       Processing of the message stops at this point.  Otherwise,

   (3) A new PDU is constructed.  A unique value of request-id should be
       used in the new PDU (this value will enable a subsequent response
       message to be correlated with this request).  The remainder of
       the new PDU is identical to the received PDU, unless the incoming
       SNMP version and the outgoing SNMP version support different PDU
       versions, in which case the proxy forwarder may need to perform a
       translation on the PDU.  (A method for performing such a
       translation is described in [RFC2576].)

   (4) The proxy forwarder calls the Dispatcher to generate the
       forwarded message, using the sendPdu abstract service interface.
       The parameters are:

       - The transportDomain is that of the outgoing management target.

       - The transportAddress is that of the outgoing management target.

       - The messageProcessingModel is that of the outgoing management
         target.

       - The securityModel is that of the outgoing management target.

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       - The securityName is that of the outgoing management target.

       - The securityLevel is that of the outgoing management target.

       - The contextEngineID is the value from the processPdu call.

       - The contextName is the value from the processPdu call.

       - 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 a response is
         expected.  If the sendPdu call is unsuccessful, the proxy
         forwarder performs the steps described in (2) above.
         Otherwise:

   (5) The proxy forwarder caches the following information in order to
       match an incoming response to the forwarded request:

       - The sendPduHandle returned from the call to sendPdu,

       - The request-id from the received PDU.

       - The contextEngineID,

       - The contextName,

       - The stateReference,

       - The incoming management target information,

       - The outgoing management information,

       - Any other information needed to match an incoming response to
         the forwarded request.

       If this information cannot be cached (possibly due to a lack of
       resources), the proxy forwarder performs the steps described in
       (2) above.  Otherwise:

   (6) Processing of the request stops until a response to the forwarded
       request is received, or until an appropriate time interval has
       expired.  If this time interval expires before a response has
       been received, the cached information about this request is
       removed.

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3.5.1.2. Processing an Incoming Response

       A proxy forwarder follows the following procedure when an
       incoming response is received:

   (1) The incoming response is received using the processResponsePdu
       interface.  The proxy forwarder uses the received parameters to
       locate an entry in its cache of pending forwarded requests.  This
       is done by matching the received parameters with the cached
       values of sendPduHandle, contextEngineID, contextName, outgoing
       management target information, and the request-id contained in
       the received PDU (the proxy forwarder must extract the request-id
       for this purpose).  If an appropriate cache entry cannot be
       found, processing of the response is halted.  Otherwise:

   (2) The cache information is extracted, and removed from the cache.

   (3) A new Response-Class PDU is constructed, using the request-id
       value from the original forwarded request (as extracted from the
       cache).  All other values are identical to those in the received
       Response-Class PDU, unless the incoming SNMP version and the
       outgoing SNMP version support different PDU versions, in which
       case the proxy forwarder may need to perform a translation on the
       PDU.  (A method for performing such a translation is described in
       [RFC2576].)

   (4) The proxy forwarder calls the Dispatcher using the
       returnResponsePdu abstract service interface.  Parameters are:

       - The messageProcessingModel indicates the Message Processing
         Model by which the original incoming message was processed.

       - The securityModel is that of the original incoming management
         target extracted from the cache.

       - The securityName is that of the original incoming management
         target extracted from the cache.

       - The securityLevel is that of the original incoming management
         target extracted from the cache.

       - The contextEngineID is the value extracted from the cache.

       - The contextName is the value extracted from the cache.

       - The pduVersion indicates the version of the PDU to be returned.

       - The PDU is the (possibly translated) Response PDU.

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

       - The stateReference is the value extracted from the cache.

       - The statusInformation indicates that no error occurred and that
         a Response PDU message should be generated.

3.5.1.3. Processing an Incoming Internal-Class PDU

   A proxy forwarder follows the following procedure when an incoming
   Internal-Class PDU is received:

   (1) The incoming Internal-Class PDU is received using the
       processResponsePdu interface.  The proxy forwarder uses the
       received parameters to locate an entry in its cache of pending
       forwarded requests.  This is done by matching the received
       parameters with the cached values of sendPduHandle.  If an
       appropriate cache entry cannot be found, processing of the
       Internal-Class PDU is halted.  Otherwise:

   (2) The cache information is extracted, and removed from the cache.

   (3) If the original incoming management target information indicates
       an SNMP version which does not support Report PDUs, processing of
       the Internal-Class PDU is halted.

   (4) The proxy forwarder calls the Dispatcher using the
       returnResponsePdu abstract service interface.  Parameters are:

       - The messageProcessingModel indicates the Message Processing
         Model by which the original incoming message was processed.

       - The securityModel is that of the original incoming management
         target extracted from the cache.

       - The securityName is that of the original incoming management
         target extracted from the cache.

       - The securityLevel is that of the original incoming management
         target extracted from the cache.

       - The contextEngineID is the value extracted from the cache.

       - The contextName is the value extracted from the cache.

       - The pduVersion indicates the version of the PDU to be returned.

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       - The PDU is unused.

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

       - The stateReference is the value extracted from the cache.

       - The statusInformation contains values specific to the
         Internal-Class PDU type (for example, for a Report PDU, the
         statusInformation contains the contextEngineID, contextName,
         counter OID, and counter value received in the incoming Report
         PDU).

3.5.2. Notification Forwarding

   A proxy forwarder receives notifications in the same manner as a
   notification receiver application, using the processPdu abstract
   service interface.  The following procedure is used when a
   notification is received:

   (1) The incoming management target information received from the
       processPdu interface is translated into outgoing management
       target information.  Note that this translation may vary for
       different values of contextEngineID and/or contextName.  The
       translation may result in multiple management targets.

   (2) If appropriate outgoing management target information cannot be
       found and the notification was an Unconfirmed-Class PDU,
       processing of the notification is halted.  If appropriate
       outgoing management target information cannot be found and the
       notification was a Confirmed-Class PDU, the proxy forwarder
       increments the snmpProxyDrops object, and calls the Dispatcher
       using the returnResponsePdu abstract service interface.  The
       parameters are:

       - 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.

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       - The pduVersion is the value from the processPdu call.

       - The PDU is an undefined and unused 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 indicates that an error occurred and that
         a Report message should be generated.

         Processing of the message stops at this point.  Otherwise,

   (3) The proxy forwarder generates a notification using the procedures
       described in the preceding section on Notification Originators,
       with the following exceptions:

       - The contextEngineID and contextName values from the original
         received notification are used.

       - The outgoing management targets previously determined are used.

       - No filtering mechanisms are applied.

       - The variable-bindings from the original received notification
         are used, rather than retrieving variable-bindings from local
         MIB instrumentation.  In particular, no access-control is
         applied to these variable-bindings, nor to the value of the
         variable-binding containing snmpTrapOID.0.

       - If the original notification contains a Confirmed-Class PDU,
         then any outgoing management targets for which the outgoing
         SNMP version does not support any PDU types that are both
         Notification-Class and Confirmed-Class PDUs will not be used
         when generating the forwarded notifications.

       - If, for any of the outgoing management targets, the incoming
         SNMP version and the outgoing SNMP version support different
         PDU versions, the proxy forwarder may need to perform a
         translation on the PDU.  (A method for performing such a
         translation is described in [RFC2576].)

   (4) If the original received notification contains an
       Unconfirmed-Class PDU, processing of the notification is now
       completed.  Otherwise, the original received notification must
       contain Confirmed-Class PDU, and processing continues.

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   (5) If the forwarded notifications included any Confirmed-Class PDUs,
       processing continues when the procedures described in the section
       for Notification Originators determine that either:

       - None of the generated notifications containing Confirmed-Class
         PDUs have been successfully acknowledged within the longest of
         the time intervals, in which case processing of the original
         notification is halted, or,

       - At least one of the generated notifications containing
         Confirmed-Class PDUs is successfully acknowledged, in which
         case a response to the original received notification
         containing an Confirmed-Class PDU is generated as described in
         the following steps.

   (6) A Response-Class PDU is constructed, using the values of
       request-id and variable-bindings from the original received
       Notification-Class PDU, and error-status and error-index values
       of 0.

   (7) The Dispatcher is called using the returnResponsePdu abstract
       service interface.  Parameters are:

       - 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 constructed in
         step (6) above.

       - The PDU is the value constructed in step (6) above.

       - 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 indicates that no error occurred and that
         a Response-Class PDU message should be generated.


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