RFC 2573
SNMP Applications
Network Working Group D. Levi Request for Comments: 2573 SNMP Research, Inc. Obsoletes: 2273 P. Meyer Category: Standards Track Secure Computing Corporation B. Stewart Cisco Systems April 1999 SNMP Applications Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved.Abstract
This memo describes five types of SNMP applications which make use of an SNMP engine as described in [RFC2571]. The types of application described are Command Generators, Command Responders, Notification Originators, Notification Receivers, and Proxy Forwarders. This memo also defines MIB modules for specifying targets of management operations, for notification filtering, and for proxy forwarding.Table Of Contents
1 Overview ..................................................... 2 1.1 Command Generator Applications ............................. 3 1.2 Command Responder Applications ............................. 3 1.3 Notification Originator Applications ....................... 3 1.4 Notification Receiver Applications ......................... 3 1.5 Proxy Forwarder Applications ............................... 4 2 Management Targets ........................................... 5 3 Elements Of Procedure ........................................ 6 3.1 Command Generator Applications ............................. 6 3.2 Command Responder Applications ............................. 9 3.3 Notification Originator Applications ....................... 14 3.4 Notification Receiver Applications ......................... 17
3.5 Proxy Forwarder Applications ............................... 19 3.5.1 Request Forwarding ....................................... 20 3.5.1.1 Processing an Incoming Request ......................... 20 3.5.1.2 Processing an Incoming Response ........................ 23 3.5.1.3 Processing an Incoming Internal-Class PDU .............. 24 3.5.2 Notification Forwarding .................................. 25 4 The Structure of the MIB Modules ............................. 28 4.1 The Management Target MIB Module ........................... 28 4.1.1 Tag Lists ................................................ 29 4.1.2 Definitions .............................................. 30 4.2 The Notification MIB Module ................................ 43 4.2.1 Definitions .............................................. 43 4.3 The Proxy MIB Module ....................................... 55 4.3.1 Definitions .............................................. 55 5 Identification of Management Targets in Notification Originators ............................................... 61 6 Notification Filtering ....................................... 62 7 Management Target Translation in Proxy Forwarder Applica- tions ..................................................... 63 7.1 Management Target Translation for Request Forwarding ....... 63 7.2 Management Target Translation for Notification Forwarding ........................................................... 64 8 Intellectual Property ........................................ 65 9 Acknowledgments .............................................. 66 10 Security Considerations ..................................... 67 11 References .................................................. 67 12 Editors' Addresses........................................... 69 A. Trap Configuration Example .................................. 70 B. Full Copyright Statement .................................... 721. Overview
This document describes five types of SNMP applications: - Applications which initiate SNMP Read-Class, and/or Write-Class requests, called 'command generators.' - Applications which respond to SNMP Read-Class, and/or Write-Class requests, called 'command responders.' - Applications which generate SNMP Notification-Class PDUs, called 'notification originators.' - Applications which receive SNMP Notification-Class PDUs, called 'notification receivers.' - Applications which forward SNMP messages, called 'proxy forwarders.'
Note that there are no restrictions on which types of applications may be associated with a particular SNMP engine. For example, a single SNMP engine may, in fact, be associated with both command generator and command responder applications. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].1.1. Command Generator Applications
A command generator application initiates SNMP Read-Class and/or Write-Class requests, as well as processing the response to a request which it generated.1.2. Command Responder Applications
A command responder application receives SNMP Read-Class and/or Write-Class requests destined for the local system as indicated by the fact that the contextEngineID in the received request is equal to that of the local engine through which the request was received. The command responder application will perform the appropriate protocol operation, using access control, and will generate a response message to be sent to the request's originator.1.3. Notification Originator Applications
A notification originator application conceptually monitors a system for particular events or conditions, and generates Notification-Class messages based on these events or conditions. A notification originator must have a mechanism for determining where to send messages, and what SNMP version and security parameters to use when sending messages. A mechanism and MIB module for this purpose is provided in this document. Note that Notification-Class PDUs generated by a notification originator may be either Confirmed-Class or Unconfirmed-Class PDU types.1.4. Notification Receiver Applications
A notification receiver application listens for notification messages, and generates response messages when a message containing a Confirmed-Class PDU is received.
1.5. Proxy Forwarder Applications
A proxy forwarder application forwards SNMP messages. Note that implementation of a proxy forwarder application is optional. The sections describing proxy (4.5, 5.3, and 8) may be skipped for implementations that do not include a proxy forwarder application. The term "proxy" has historically been used very loosely, with multiple different meanings. These different meanings include (among others): (1) the forwarding of SNMP requests to other SNMP entities without regard for what managed object types are being accessed; for example, in order to forward an SNMP request from one transport domain to another, or to translate SNMP requests of one version into SNMP requests of another version; (2) the translation of SNMP requests into operations of some non- SNMP management protocol; and (3) support for aggregated managed objects where the value of one managed object instance depends upon the values of multiple other (remote) items of management information. Each of these scenarios can be advantageous; for example, support for aggregation of management information can significantly reduce the bandwidth requirements of large-scale management activities. However, using a single term to cover multiple different scenarios causes confusion. To avoid such confusion, this document uses the term "proxy" with a much more tightly defined meaning. The term "proxy" is used in this document to refer to a proxy forwarder application which forwards either SNMP messages without regard for what managed objects are contained within those messages. This definition is most closely related to the first definition above. Note, however, that in the SNMP architecture [RFC2571], a proxy forwarder is actually an application, and need not be associated with what is traditionally thought of as an SNMP agent. Specifically, the distinction between a traditional SNMP agent and a proxy forwarder application is simple:
- a proxy forwarder application forwards SNMP messages to other
SNMP engines according to the context, and irrespective of the
specific managed object types being accessed, and forwards the
response to such previously forwarded messages back to the SNMP
engine from which the original message was received;
- in contrast, the command responder application that is part of
what is traditionally thought of as an SNMP agent, and which
processes SNMP requests according to the (names of the)
individual managed object types and instances being accessed, is
NOT a proxy forwarder application from the perspective of this
document.
Thus, when a proxy forwarder application forwards a request or
notification for a particular contextEngineID / contextName pair, not
only is the information on how to forward the request specifically
associated with that context, but the proxy forwarder application has
no need of a detailed definition of a MIB view (since the proxy
forwarder application forwards the request irrespective of the
managed object types).
In contrast, a command responder application must have the detailed
definition of the MIB view, and even if it needs to issue requests to
other entities, via SNMP or otherwise, that need is dependent on the
individual managed object instances being accessed (i.e., not only on
the context).
Note that it is a design goal of a proxy forwarder application to act
as an intermediary between the endpoints of a transaction. In
particular, when forwarding Confirmed Notification-Class messages,
the associated response is forwarded when it is received from the
target to which the Notification-Class message was forwarded, rather
than generating a response immediately when the Notification-Class
message is received.
2. Management Targets
Some types of applications (notification generators and proxy
forwarders in particular) require a mechanism for determining where
and how to send generated messages. This document provides a
mechanism and MIB module for this purpose. The set of information
that describes where and how to send a message is called a '
Management Target', and consists of two kinds of information:
- Destination information, consisting of a transport domain and a
transport address. This is also termed a transport endpoint.
- SNMP parameters, consisting of message processing model,
security model, security level, and security name information.
The SNMP-TARGET-MIB module described later in this document contains
one table for each of these types of information. There can be a
many-to-many relationship in the MIB between these two types of
information. That is, there may be multiple transport endpoints
associated with a particular set of SNMP parameters, or a particular
transport endpoint may be associated with several sets of SNMP
parameters.
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
[RFC2571].
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 is
to be generated.
- The securityLevel is the security level that the application
wishes to use.
- The contextEngineID is provided by the command generator if it
wishes to explicitly specify the location of the management
information it is requesting.
- The contextName is provided by the command generator if it
wishes to explicitly specify 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 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.
- 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.
(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 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. 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 (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.
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.
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 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 [COEX].)
(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 originally received.
- The contextName is the value originally received.
- 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 [COEX].)
(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 received value.
- The securityModel is the received value.
- The securityName is the received value.
- The securityLevel is the received value.
- The contextEngineID is the received value.
- The contextName is the received value.
- The pduVersion is the received value.
- 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 received value.
- 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.
- If the original notification contains a Confirmed-Class PDU,
then any outgoing management targets, for which the outgoing
SNMP version does not support and PDU types which 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 [COEX].)
(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 a
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 originally received value.
- The securityModel is the originally received value.
- The securityName is the originally received value.
- The securityLevel is the originally received value.
- The contextEngineID is the originally received value.
- The contextName is the originally received value.
- 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 originally received value.
- The statusInformation indicates that no error occurred and that
a Response-Class PDU message should be generated.
(page 28 continued on part 2)
