RFC 3413
Simple Network Management Protocol (SNMP) Applications
Pages: 74
Internet Standard: 62
→ Errata
STD 62 is also: 3411 3412 3414 3415 3416 3417 3418
Obsoletes: 2573
Internet Standard: 62
→ Errata
STD 62 is also: 3411 3412 3414 3415 3416 3417 3418
Obsoletes: 2573
Part 2 of 4 – Pages 6 to 28
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.
- 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
)
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.
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.
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.
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.
- 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:
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.
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.
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:
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,
(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:
- 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.
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:
- 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.
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:
- 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.
- 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.
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.
- 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.
- 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.
- 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.
(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.
(next page on part 3)