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RFC 3411

 
 
 

An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks

Part 2 of 3, p. 16 to 39
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3.  Elements of the Architecture

   This section describes the various elements of the architecture and
   how they are named.  There are three kinds of naming:

      1) the naming of entities,

      2) the naming of identities, and

      3) the naming of management information.

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   This architecture also defines some names for other constructs that
   are used in the documentation.

3.1.  The Naming of Entities

   An SNMP entity is an implementation of this architecture.  Each such
   SNMP entity consists of an SNMP engine and one or more associated
   applications.

   The following figure shows details about an SNMP entity and the
   components within it.

   +-------------------------------------------------------------------+
   |  SNMP entity                                                      |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  SNMP engine (identified by snmpEngineID)                   |  |
   |  |                                                             |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  |  |            | |            | |           | |           |  |  |
   |  |  | Dispatcher | | Message    | | Security  | | Access    |  |  |
   |  |  |            | | Processing | | Subsystem | | Control   |  |  |
   |  |  |            | | Subsystem  | |           | | Subsystem |  |  |
   |  |  |            | |            | |           | |           |  |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  |                                                             |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  Application(s)                                             |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Proxy        |        |  |
   |  |  | Generator   |  | Receiver     |  | Forwarder    |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Other        |        |  |
   |  |  | Responder   |  | Originator   |  |              |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   +-------------------------------------------------------------------+

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3.1.1.  SNMP engine

   An SNMP engine provides services for sending and receiving messages,
   authenticating and encrypting messages, and controlling access to
   managed objects.  There is a one-to-one association between an SNMP
   engine and the SNMP entity which contains it.

   The engine contains:

      1) a Dispatcher,

      2) a Message Processing Subsystem,

      3) a Security Subsystem, and

      4) an Access Control Subsystem.

3.1.1.1.  snmpEngineID

   Within an administrative domain, an snmpEngineID is the unique and
   unambiguous identifier of an SNMP engine.  Since there is a one-to-
   one association between SNMP engines and SNMP entities, it also
   uniquely and unambiguously identifies the SNMP entity within that
   administrative domain.  Note that it is possible for SNMP entities in
   different administrative domains to have the same value for
   snmpEngineID.  Federation of administrative domains may necessitate
   assignment of new values.

3.1.1.2.  Dispatcher

   There is only one Dispatcher in an SNMP engine.  It allows for
   concurrent support of multiple versions of SNMP messages in the SNMP
   engine.  It does so by:

      -  sending and receiving SNMP messages to/from the network,

      -  determining the version of an SNMP message and interacting with
         the corresponding Message Processing Model,

      -  providing an abstract interface to SNMP applications for
         delivery of a PDU to an application.

      -  providing an abstract interface for SNMP applications that
         allows them to send a PDU to a remote SNMP entity.

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3.1.1.3.  Message Processing Subsystem

   The Message Processing Subsystem is responsible for preparing
   messages for sending, and extracting data from received messages.

   The Message Processing Subsystem potentially contains multiple
   Message Processing Models as shown in the next figure.

   * One or more Message Processing Models may be present.

   +------------------------------------------------------------------+
   |                                                                  |
   |  Message Processing Subsystem                                    |
   |                                                                  |
   |  +------------+  +------------+  +------------+  +------------+  |
   |  |          * |  |          * |  |          * |  |          * |  |
   |  | SNMPv3     |  | SNMPv1     |  | SNMPv2c    |  | Other      |  |
   |  | Message    |  | Message    |  | Message    |  | Message    |  |
   |  | Processing |  | Processing |  | Processing |  | Processing |  |
   |  | Model      |  | Model      |  | Model      |  | Model      |  |
   |  |            |  |            |  |            |  |            |  |
   |  +------------+  +------------+  +------------+  +------------+  |
   |                                                                  |
   +------------------------------------------------------------------+

3.1.1.3.1.  Message Processing Model

   Each Message Processing Model defines the format of a particular
   version of an SNMP message and coordinates the preparation and
   extraction of each such version-specific message format.

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3.1.1.4.  Security Subsystem

   The Security Subsystem provides security services such as the
   authentication and privacy of messages and potentially contains
   multiple Security Models as shown in the following figure

   * One or more Security Models may be present.

   +------------------------------------------------------------------+
   |                                                                  |
   |  Security Subsystem                                              |
   |                                                                  |
   |  +----------------+  +-----------------+  +-------------------+  |
   |  |              * |  |               * |  |                 * |  |
   |  | User-Based     |  | Other           |  | Other             |  |
   |  | Security       |  | Security        |  | Security          |  |
   |  | Model          |  | Model           |  | Model             |  |
   |  |                |  |                 |  |                   |  |
   |  +----------------+  +-----------------+  +-------------------+  |
   |                                                                  |
   +------------------------------------------------------------------+

3.1.1.4.1.  Security Model

   A Security Model specifies the threats against which it protects, the
   goals of its services, and the security protocols used to provide
   security services such as authentication and privacy.

3.1.1.4.2.  Security Protocol

   A Security Protocol specifies the mechanisms, procedures, and MIB
   objects used to provide a security service such as authentication or
   privacy.

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3.1.2.  Access Control Subsystem

   The Access Control Subsystem provides authorization services by means
   of one or more (*) Access Control Models.

   +------------------------------------------------------------------+
   |                                                                  |
   |  Access Control Subsystem                                        |
   |                                                                  |
   |  +---------------+   +-----------------+   +------------------+  |
   |  |             * |   |               * |   |                * |  |
   |  | View-Based    |   | Other           |   | Other            |  |
   |  | Access        |   | Access          |   | Access           |  |
   |  | Control       |   | Control         |   | Control          |  |
   |  | Model         |   | Model           |   | Model            |  |
   |  |               |   |                 |   |                  |  |
   |  +---------------+   +-----------------+   +------------------+  |
   |                                                                  |
   +------------------------------------------------------------------+

3.1.2.1.  Access Control Model

   An Access Control Model defines a particular access decision function
   in order to support decisions regarding access rights.

3.1.3.  Applications

   There are several types of applications, including:

      -  command generators, which monitor and manipulate management
         data,

      -  command responders, which provide access to management data,

      -  notification originators, which initiate asynchronous messages,

      -  notification receivers, which process asynchronous messages,

      and

      -  proxy forwarders, which forward messages between entities.

   These applications make use of the services provided by the SNMP
   engine.

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3.1.3.1.  SNMP Manager

   An SNMP entity containing one or more command generator and/or
   notification receiver applications (along with their associated SNMP
   engine) has traditionally been called an SNMP manager.

                       (traditional SNMP manager)
   +-------------------------------------------------------------------+
   | +--------------+  +--------------+  +--------------+  SNMP entity |
   | | NOTIFICATION |  | NOTIFICATION |  |   COMMAND    |              |
   | |  ORIGINATOR  |  |   RECEIVER   |  |  GENERATOR   |              |
   | | applications |  | applications |  | applications |              |
   | +--------------+  +--------------+  +--------------+              |
   |         ^                ^                 ^                      |
   |         |                |                 |                      |
   |         v                v                 v                      |
   |         +-------+--------+-----------------+                      |
   |                 ^                                                 |
   |                 |     +---------------------+  +----------------+ |
   |                 |     | Message Processing  |  | Security       | |
   | Dispatcher      v     | Subsystem           |  | Subsystem      | |
   | +-------------------+ |     +------------+  |  |                | |
   | | PDU Dispatcher    | |  +->| v1MP     * |<--->| +------------+ | |
   | |                   | |  |  +------------+  |  | | Other      | | |
   | |                   | |  |  +------------+  |  | | Security   | | |
   | |                   | |  +->| v2cMP    * |<--->| | Model      | | |
   | | Message           | |  |  +------------+  |  | +------------+ | |
   | | Dispatcher  <--------->+                  |  |                | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | |                   | |  +->| v3MP     * |<--->| | User-based | | |
   | | Transport         | |  |  +------------+  |  | | Security   | | |
   | | Mapping           | |  |  +------------+  |  | | Model      | | |
   | | (e.g., RFC 3417)  | |  +->| otherMP  * |<--->| +------------+ | |
   | +-------------------+ |     +------------+  |  |                | |
   |          ^            +---------------------+  +----------------+ |
   |          |                                                        |
   |          v                                                        |
   +-------------------------------------------------------------------+
   +-----+ +-----+       +-------+
   | UDP | | IPX | . . . | other |
   +-----+ +-----+       +-------+
      ^       ^              ^
      |       |              |      * One or more models may be present.
      v       v              v
   +------------------------------+
   |           Network            |
   +------------------------------+

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3.1.3.2.  SNMP Agent

   An SNMP entity containing one or more command responder and/or
   notification originator applications (along with their associated
   SNMP engine) has traditionally been called an SNMP agent.

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   * One or more models may be present.

   +------------------------------+
   |           Network            |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v
   +-----+ +-----+       +-------+
   | UDP | | IPX | . . . | other |
   +-----+ +-----+       +-------+              (traditional SNMP agent)
   +-------------------------------------------------------------------+
   |              ^                                                    |
   |              |        +---------------------+  +----------------+ |
   |              |        | Message Processing  |  | Security       | |
   | Dispatcher   v        | Subsystem           |  | Subsystem      | |
   | +-------------------+ |     +------------+  |  |                | |
   | | Transport         | |  +->| v1MP     * |<--->| +------------+ | |
   | | Mapping           | |  |  +------------+  |  | | Other      | | |
   | | (e.g., RFC 3417)  | |  |  +------------+  |  | | Security   | | |
   | |                   | |  +->| v2cMP    * |<--->| | Model      | | |
   | | Message           | |  |  +------------+  |  | +------------+ | |
   | | Dispatcher  <--------->|  +------------+  |  | +------------+ | |
   | |                   | |  +->| v3MP     * |<--->| | User-based | | |
   | |                   | |  |  +------------+  |  | | Security   | | |
   | | PDU Dispatcher    | |  |  +------------+  |  | | Model      | | |
   | +-------------------+ |  +->| otherMP  * |<--->| +------------+ | |
   |              ^        |     +------------+  |  |                | |
   |              |        +---------------------+  +----------------+ |
   |              v                                                    |
   |      +-------+-------------------------+---------------+          |
   |      ^                                 ^               ^          |
   |      |                                 |               |          |
   |      v                                 v               v          |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   | |   COMMAND   |   | ACCESS  |   | NOTIFICATION |  |    PROXY    | |
   | |  RESPONDER  |<->| CONTROL |<->|  ORIGINATOR  |  |  FORWARDER  | |
   | | application |   |         |   | applications |  | application | |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   |      ^                                 ^                          |
   |      |                                 |                          |
   |      v                                 v                          |
   | +----------------------------------------------+                  |
   | |             MIB instrumentation              |      SNMP entity |
   +-------------------------------------------------------------------+

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3.2.  The Naming of Identities

                            principal
                                ^
                                |
                                |
   +----------------------------|-------------+
   | SNMP engine                v             |
   |                    +--------------+      |
   |                    |              |      |
   |  +-----------------| securityName |---+  |
   |  | Security Model  |              |   |  |
   |  |                 +--------------+   |  |
   |  |                         ^          |  |
   |  |                         |          |  |
   |  |                         v          |  |
   |  |  +------------------------------+  |  |
   |  |  |                              |  |  |
   |  |  | Model                        |  |  |
   |  |  | Dependent                    |  |  |
   |  |  | Security ID                  |  |  |
   |  |  |                              |  |  |
   |  |  +------------------------------+  |  |
   |  |                         ^          |  |
   |  |                         |          |  |
   |  +-------------------------|----------+  |
   |                            |             |
   |                            |             |
   +----------------------------|-------------+
                                |
                                v
                             network

3.2.1.  Principal

   A principal is the "who" on whose behalf services are provided or
   processing takes place.

   A principal can be, among other things, an individual acting in a
   particular role; a set of individuals, with each acting in a
   particular role; an application or a set of applications; and
   combinations thereof.

3.2.2.  securityName

   A securityName is a human readable string representing a principal.
   It has a model-independent format, and can be used outside a
   particular Security Model.

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3.2.3.  Model-dependent security ID

   A model-dependent security ID is the model-specific representation of
   a securityName within a particular Security Model.

   Model-dependent security IDs may or may not be human readable, and
   have a model-dependent syntax.  Examples include community names, and
   user names.

   The transformation of model-dependent security IDs into securityNames
   and vice versa is the responsibility of the relevant Security Model.

3.3.  The Naming of Management Information

   Management information resides at an SNMP entity where a Command
   Responder Application has local access to potentially multiple
   contexts.  This application uses a contextEngineID equal to the
   snmpEngineID of its associated SNMP engine.

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   +-----------------------------------------------------------------+
   |  SNMP entity (identified by snmpEngineID, for example:          |
   |  '800002b804616263'H (enterpise 696, string "abc")              |
   |                                                                 |
   |  +------------------------------------------------------------+ |
   |  | SNMP engine (identified by snmpEngineID)                   | |
   |  |                                                            | |
   |  | +-------------+ +------------+ +-----------+ +-----------+ | |
   |  | |             | |            | |           | |           | | |
   |  | | Dispatcher  | | Message    | | Security  | | Access    | | |
   |  | |             | | Processing | | Subsystem | | Control   | | |
   |  | |             | | Subsystem  | |           | | Subsystem | | |
   |  | |             | |            | |           | |           | | |
   |  | +-------------+ +------------+ +-----------+ +-----------+ | |
   |  |                                                            | |
   |  +------------------------------------------------------------+ |
   |                                                                 |
   |  +------------------------------------------------------------+ |
   |  |  Command Responder Application                             | |
   |  |  (contextEngineID, example: '800002b804616263'H)           | |
   |  |                                                            | |
   |  |  example contextNames:                                     | |
   |  |                                                            | |
   |  |  "bridge1"          "bridge2"            "" (default)      | |
   |  |  ---------          ---------            ------------      | |
   |  |      |                  |                   |              | |
   |  +------|------------------|-------------------|--------------+ |
   |         |                  |                   |                |
   |  +------|------------------|-------------------|--------------+ |
   |  |  MIB | instrumentation  |                   |              | |
   |  |  +---v------------+ +---v------------+ +----v-----------+  | |
   |  |  | context        | | context        | | context        |  | |
   |  |  |                | |                | |                |  | |
   |  |  | +------------+ | | +------------+ | | +------------+ |  | |
   |  |  | | bridge MIB | | | | bridge MIB | | | | some  MIB  | |  | |
   |  |  | +------------+ | | +------------+ | | +------------+ |  | |
   |  |  |                | |                | |                |  | |
   |  |  |                | |                | | +------------+ |  | |
   |  |  |                | |                | | | other MIB  | |  | |
   |  |  |                | |                | | +------------+ |  | |
   |  |  |                | |                | |                |  | |
   +-----------------------------------------------------------------+

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3.3.1.  An SNMP Context

   An SNMP context, or just "context" for short, is a collection of
   management information accessible by an SNMP entity.  An item of
   management information may exist in more than one context.  An SNMP
   entity potentially has access to many contexts.

   Typically, there are many instances of each managed object type
   within a management domain.  For simplicity, the method for
   identifying instances specified by the MIB module does not allow each
   instance to be distinguished amongst the set of all instances within
   a management domain; rather, it allows each instance to be identified
   only within some scope or "context", where there are multiple such
   contexts within the management domain.  Often, a context is a
   physical device, or perhaps, a logical device, although a context can
   also encompass multiple devices, or a subset of a single device, or
   even a subset of multiple devices, but a context is always defined as
   a subset of a single SNMP entity.  Thus, in order to identify an
   individual item of management information within the management
   domain, its contextName and contextEngineID must be identified in
   addition to its object type and its instance.

   For example, the managed object type ifDescr [RFC2863], is defined as
   the description of a network interface.  To identify the description
   of device-X's first network interface, four pieces of information are
   needed: the snmpEngineID of the SNMP entity which provides access to
   the management information at device-X, the contextName (device-X),
   the managed object type (ifDescr), and the instance ("1").

   Each context has (at least) one unique identification within the
   management domain.  The same item of management information can exist
   in multiple contexts.  An item of management information may have
   multiple unique identifications.  This occurs when an item of
   management information exists in multiple contexts, and this also
   occurs when a context has multiple unique identifications.

   The combination of a contextEngineID and a contextName unambiguously
   identifies a context within an administrative domain; note that there
   may be multiple unique combinations of contextEngineID and
   contextName that unambiguously identify the same context.

3.3.2.  contextEngineID

   Within an administrative domain, a contextEngineID uniquely
   identifies an SNMP entity that may realize an instance of a context
   with a particular contextName.

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3.3.3.  contextName

   A contextName is used to name a context.  Each contextName MUST be
   unique within an SNMP entity.

3.3.4.  scopedPDU

   A scopedPDU is a block of data containing a contextEngineID, a
   contextName, and a PDU.

   The PDU is an SNMP Protocol Data Unit containing information named in
   the context which is unambiguously identified within an
   administrative domain by the combination of the contextEngineID and
   the contextName.  See, for example, RFC 3416 for more information
   about SNMP PDUs.

3.4.  Other Constructs

3.4.1.  maxSizeResponseScopedPDU

   The maxSizeResponseScopedPDU is the maximum size of a scopedPDU that
   a PDU's sender would be willing to accept.  Note that the size of a
   scopedPDU does not include the size of the SNMP message header.

3.4.2.  Local Configuration Datastore

   The subsystems, models, and applications within an SNMP entity may
   need to retain their own sets of configuration information.

   Portions of the configuration information may be accessible as
   managed objects.

   The collection of these sets of information is referred to as an
   entity's Local Configuration Datastore (LCD).

3.4.3.  securityLevel

   This architecture recognizes three levels of security:

      -  without authentication and without privacy (noAuthNoPriv)

      -  with authentication but without privacy (authNoPriv)

      -  with authentication and with privacy (authPriv)

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   These three values are ordered such that noAuthNoPriv is less than
   authNoPriv and authNoPriv is less than authPriv.

   Every message has an associated securityLevel.  All Subsystems
   (Message Processing, Security, Access Control) and applications are
   REQUIRED to either supply a value of securityLevel or to abide by the
   supplied value of securityLevel while processing the message and its
   contents.

4.  Abstract Service Interfaces

   Abstract service interfaces have been defined to describe the
   conceptual interfaces between the various subsystems within an SNMP
   entity.  The abstract service interfaces are intended to help clarify
   the externally observable behavior of SNMP entities, and are not
   intended to constrain the structure or organization of
   implementations in any way.  Most specifically, they should not be
   interpreted as APIs or as requirements statements for APIs.

   These abstract service interfaces are defined by a set of primitives
   that define the services provided and the abstract data elements that
   are to be passed when the services are invoked.  This section lists
   the primitives that have been defined for the various subsystems.

4.1.  Dispatcher Primitives

   The Dispatcher typically provides services to the SNMP applications
   via its PDU Dispatcher.  This section describes the primitives
   provided by the PDU Dispatcher.

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4.1.1.  Generate Outgoing Request or Notification

   The PDU Dispatcher provides the following primitive for an
   application to send an SNMP Request or Notification to another SNMP
   entity:

   statusInformation =              -- sendPduHandle if success
                                    -- errorIndication if failure
     sendPdu(
     IN   transportDomain           -- transport domain to be used
     IN   transportAddress          -- transport address to be used
     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
          )

4.1.2.  Process Incoming Request or Notification PDU

   The PDU Dispatcher provides the following primitive to pass an
   incoming SNMP PDU to an application:

   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

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4.1.3.  Generate Outgoing Response

   The PDU Dispatcher provides the following primitive for an
   application to return an SNMP Response PDU to the PDU Dispatcher:

   result =                         -- SUCCESS or FAILURE
   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 sender can accept
     IN   stateReference            -- reference to state information
                                    -- as presented with the request
     IN   statusInformation         -- success or errorIndication
          )                         -- error counter OID/value if error

4.1.4.  Process Incoming Response PDU

   The PDU Dispatcher provides the following primitive to pass an
   incoming SNMP Response PDU to an application:

   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
          )

4.1.5.  Registering Responsibility for Handling SNMP PDUs

   Applications can register/unregister responsibility for a specific
   contextEngineID, for specific pduTypes, with the PDU Dispatcher
   according to the following primitives.  The list of particular
   pduTypes that an application can register for is determined by the
   Message Processing Model(s) supported by the SNMP entity that
   contains the PDU Dispatcher.

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

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

   Note that realizations of the registerContextEngineID and
   unregisterContextEngineID abstract service interfaces may provide
   implementation-specific ways for applications to register/deregister
   responsibility for all possible values of the contextEngineID or
   pduType parameters.

4.2.  Message Processing Subsystem Primitives

   The Dispatcher interacts with a Message Processing Model to process a
   specific version of an SNMP Message.  This section describes the
   primitives provided by the Message Processing Subsystem.

4.2.1.  Prepare Outgoing SNMP Request or Notification Message

   The Message Processing Subsystem provides this service primitive for
   preparing an outgoing SNMP Request or Notification Message:

   statusInformation =              -- success or errorIndication
     prepareOutgoingMessage(
     IN   transportDomain           -- transport domain to be used
     IN   transportAddress          -- transport address to be used
     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
     IN   sendPduHandle             -- the handle for matching
                                    -- incoming responses
     OUT  destTransportDomain       -- destination transport domain
     OUT  destTransportAddress      -- destination transport address
     OUT  outgoingMessage           -- the message to send
     OUT  outgoingMessageLength     -- its length
          )

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4.2.2.  Prepare an Outgoing SNMP Response Message

   The Message Processing Subsystem provides this service primitive for
   preparing an outgoing SNMP Response Message:

   result =                         -- SUCCESS or FAILURE
     prepareResponseMessage(
     IN   messageProcessingModel    -- typically, SNMP version
     IN   securityModel             -- same as on incoming request
     IN   securityName              -- same as on incoming request
     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 able to accept
     IN   stateReference            -- reference to state information
                                    -- as presented with the request
     IN   statusInformation         -- success or errorIndication
                                    -- error counter OID/value if error
     OUT  destTransportDomain       -- destination transport domain
     OUT  destTransportAddress      -- destination transport address
     OUT  outgoingMessage           -- the message to send
     OUT  outgoingMessageLength     -- its length
          )

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4.2.3.  Prepare Data Elements from an Incoming SNMP Message

   The Message Processing Subsystem provides this service primitive for
   preparing the abstract data elements from an incoming SNMP message:

   result =                         -- SUCCESS or errorIndication
     prepareDataElements(
     IN   transportDomain           -- origin transport domain
     IN   transportAddress          -- origin transport address
     IN   wholeMsg                  -- as received from the network
     IN   wholeMsgLength            -- as received from the network
     OUT  messageProcessingModel    -- typically, SNMP version
     OUT  securityModel             -- Security Model to use
     OUT  securityName              -- on behalf of this principal
     OUT  securityLevel             -- Level of Security requested
     OUT  contextEngineID           -- data from/at this entity
     OUT  contextName               -- data from/in this context
     OUT  pduVersion                -- the version of the PDU
     OUT  PDU                       -- SNMP Protocol Data Unit
     OUT  pduType                   -- SNMP PDU type
     OUT  sendPduHandle             -- handle for matched request
     OUT  maxSizeResponseScopedPDU  -- maximum size sender can accept
     OUT  statusInformation         -- success or errorIndication
                                    -- error counter OID/value if error
     OUT  stateReference            -- reference to state information
                                    -- to be used for possible Response
          )

4.3.  Access Control Subsystem Primitives

   Applications are the typical clients of the service(s) of the Access
   Control Subsystem.

   The following primitive is provided by the Access Control Subsystem
   to check if access is allowed:

   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
          )

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4.4.  Security Subsystem Primitives

   The Message Processing Subsystem is the typical client of the
   services of the Security Subsystem.

4.4.1.  Generate a Request or Notification Message

   The Security Subsystem provides the following primitive to generate a
   Request or Notification message:

   statusInformation =
     generateRequestMsg(
     IN   messageProcessingModel    -- typically, SNMP version
     IN   globalData                -- message header, admin data
     IN   maxMessageSize            -- of the sending SNMP entity
     IN   securityModel             -- for the outgoing message
     IN   securityEngineID          -- authoritative SNMP entity
     IN   securityName              -- on behalf of this principal
     IN   securityLevel             -- Level of Security requested
     IN   scopedPDU                 -- message (plaintext) payload
     OUT  securityParameters        -- filled in by Security Module
     OUT  wholeMsg                  -- complete generated message
     OUT  wholeMsgLength            -- length of the generated message
          )

4.4.2.  Process Incoming Message

   The Security Subsystem provides the following primitive to process an
   incoming message:

   statusInformation =              -- errorIndication or success
                                    -- error counter OID/value if error
     processIncomingMsg(
     IN   messageProcessingModel    -- typically, SNMP version
     IN   maxMessageSize            -- of the sending SNMP entity
     IN   securityParameters        -- for the received message
     IN   securityModel             -- for the received message
     IN   securityLevel             -- Level of Security
     IN   wholeMsg                  -- as received on the wire
     IN   wholeMsgLength            -- length as received on the wire
     OUT  securityEngineID          -- authoritative SNMP entity
     OUT  securityName              -- identification of the principal
     OUT  scopedPDU,                -- message (plaintext) payload
     OUT  maxSizeResponseScopedPDU  -- maximum size sender can handle
     OUT  securityStateReference    -- reference to security state
          )                         -- information, needed for response

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4.4.3.  Generate a Response Message

   The Security Subsystem provides the following primitive to generate a
   Response message:

   statusInformation =
     generateResponseMsg(
     IN   messageProcessingModel    -- typically, SNMP version
     IN   globalData                -- message header, admin data
     IN   maxMessageSize            -- of the sending SNMP entity
     IN   securityModel             -- for the outgoing message
     IN   securityEngineID          -- authoritative SNMP entity
     IN   securityName              -- on behalf of this principal
     IN   securityLevel             -- for the outgoing message
     IN   scopedPDU                 -- message (plaintext) payload
     IN   securityStateReference    -- reference to security state
                                    -- information from original request
     OUT  securityParameters        -- filled in by Security Module
     OUT  wholeMsg                  -- complete generated message
     OUT  wholeMsgLength            -- length of the generated message
          )

4.5.  Common Primitives

   These primitive(s) are provided by multiple Subsystems.

4.5.1.  Release State Reference Information

   All Subsystems which pass stateReference information also provide a
   primitive to release the memory that holds the referenced state
   information:

   stateRelease(
     IN   stateReference       -- handle of reference to be released
          )

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4.6.  Scenario Diagrams

4.6.1.  Command Generator or Notification Originator

   This diagram shows how a Command Generator or Notification Originator
   application requests that a PDU be sent, and how the response is
   returned (asynchronously) to that application.

   Command           Dispatcher               Message           Security
   Generator            |                     Processing           Model
   |                    |                     Model                    |
   |      sendPdu       |                        |                     |
   |------------------->|                        |                     |
   |                    | prepareOutgoingMessage |                     |
   :                    |----------------------->|                     |
   :                    |                        | generateRequestMsg  |
   :                    |                        |-------------------->|
   :                    |                        |                     |
   :                    |                        |<--------------------|
   :                    |                        |                     |
   :                    |<-----------------------|                     |
   :                    |                        |                     |
   :                    |------------------+     |                     |
   :                    | Send SNMP        |     |                     |
   :                    | Request Message  |     |                     |
   :                    | to Network       |     |                     |
   :                    |                  v     |                     |
   :                    :                  :     :                     :
   :                    :                  :     :                     :
   :                    :                  :     :                     :
   :                    |                  |     |                     |
   :                    | Receive SNMP     |     |                     |
   :                    | Response Message |     |                     |
   :                    | from Network     |     |                     |
   :                    |<-----------------+     |                     |
   :                    |                        |                     |
   :                    |   prepareDataElements  |                     |
   :                    |----------------------->|                     |
   :                    |                        | processIncomingMsg  |
   :                    |                        |-------------------->|
   :                    |                        |                     |
   :                    |                        |<--------------------|
   :                    |                        |                     |
   :                    |<-----------------------|                     |
   | processResponsePdu |                        |                     |
   |<-------------------|                        |                     |
   |                    |                        |                     |

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4.6.2.  Scenario Diagram for a Command Responder Application

   This diagram shows how a Command Responder or Notification Receiver
   application registers for handling a pduType, how a PDU is dispatched
   to the application after an SNMP message is received, and how the
   Response is (asynchronously) send back to the network.

   Command               Dispatcher            Message          Security
   Responder                 |                 Processing          Model
   |                         |                 Model                   |
   |                         |                    |                    |
   | registerContextEngineID |                    |                    |
   |------------------------>|                    |                    |
   |<------------------------|              |     |                    |
   |                         | Receive SNMP |     |                    |
   :                         | Message      |     |                    |
   :                         | from Network |     |                    |
   :                         |<-------------+     |                    |
   :                         |                    |                    |
   :                         |prepareDataElements |                    |
   :                         |------------------->|                    |
   :                         |                    | processIncomingMsg |
   :                         |                    |------------------->|
   :                         |                    |                    |
   :                         |                    |<-------------------|
   :                         |                    |                    |
   :                         |<-------------------|                    |
   |     processPdu          |                    |                    |
   |<------------------------|                    |                    |
   |                         |                    |                    |
   :                         :                    :                    :
   :                         :                    :                    :
   |    returnResponsePdu    |                    |                    |
   |------------------------>|                    |                    |
   :                         | prepareResponseMsg |                    |
   :                         |------------------->|                    |
   :                         |                    |generateResponseMsg |
   :                         |                    |------------------->|
   :                         |                    |                    |
   :                         |                    |<-------------------|
   :                         |                    |                    |
   :                         |<-------------------|                    |
   :                         |                    |                    |
   :                         |--------------+     |                    |
   :                         | Send SNMP    |     |                    |
   :                         | Message      |     |                    |
   :                         | to Network   |     |                    |
   :                         |              v     |                    |


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