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RFC 2261


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An Architecture for Describing SNMP Management Frameworks

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Network Working Group                                      D. Harrington
Request for Comments: 2261                       Cabletron Systems, Inc.
Category: Standards Track                                     R. Presuhn
                                                      BMC Software, Inc.
                                                               B. Wijnen
                                               IBM T. J. Watson Research
                                                            January 1998


                     An Architecture for Describing
                       SNMP Management Frameworks


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 (1997).  All Rights Reserved.

Abstract

   This document describes an architecture for describing SNMP
   Management Frameworks.  The architecture is designed to be modular to
   allow the evolution of the SNMP protocol standards over time.  The
   major portions of the architecture are an SNMP engine containing a
   Message Processing Subsystem, a Security Subsystem and an Access
   Control Subsystem, and possibly multiple SNMP applications which
   provide specific functional processing of management data.

Table of Contents

   1. Introduction ................................................    3
   1.1. Overview ..................................................    3
   1.2. SNMP ......................................................    4
   1.3. Goals of this Architecture ................................    5
   1.4. Security Requirements of this Architecture ................    6
   1.5. Design Decisions ..........................................    7
   2. Documentation Overview ......................................    8
   2.1. Document Roadmap ..........................................   10
   2.2. Applicability Statement ...................................   10
   2.3. Coexistence and Transition ................................   10
   2.4. Transport Mappings ........................................   11

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   2.5. Message Processing ........................................   11
   2.6. Security ..................................................   11
   2.7. Access Control ............................................   11
   2.8. Protocol Operations .......................................   12
   2.9. Applications ..............................................   12
   2.10. Structure of Management Information ......................   12
   2.11. Textual Conventions ......................................   13
   2.12. Conformance Statements ...................................   13
   2.13. Management Information Base Modules ......................   13
   2.13.1. SNMP Instrumentation MIBs ..............................   13
   2.14. SNMP Framework Documents .................................   13
   3. Elements of the Architecture ................................   14
   3.1. The Naming of Entities ....................................   14
   3.1.1. SNMP engine .............................................   15
   3.1.1.1. snmpEngineID ..........................................   16
   3.1.1.2. Dispatcher ............................................   16
   3.1.1.3. Message Processing Subsystem ..........................   16
   3.1.1.3.1. Message Processing Model ............................   17
   3.1.1.4. Security Subsystem ....................................   17
   3.1.1.4.1. Security Model ......................................   17
   3.1.1.4.2. Security Protocol ...................................   18
   3.1.2. Access Control Subsystem ................................   18
   3.1.2.1. Access Control Model ..................................   18
   3.1.3. Applications ............................................   18
   3.1.3.1. SNMP Manager ..........................................   19
   3.1.3.2. SNMP Agent ............................................   20
   3.2. The Naming of Identities ..................................   21
   3.2.1. Principal ...............................................   21
   3.2.2. securityName ............................................   21
   3.2.3. Model-dependent security ID .............................   22
   3.3. The Naming of Management Information ......................   22
   3.3.1. An SNMP Context .........................................   23
   3.3.2. contextEngineID .........................................   24
   3.3.3. contextName .............................................   24
   3.3.4. scopedPDU ...............................................   25
   3.4. Other Constructs ..........................................   25
   3.4.1. maxSizeResponseScopedPDU ................................   25
   3.4.2. Local Configuration Datastore ...........................   25
   3.4.3. securityLevel ...........................................   25
   4. Abstract Service Interfaces .................................   26
   4.1. Dispatcher Primitives .....................................   26
   4.1.1. Generate Outgoing Request or Notification ...............   26
   4.1.2. Process Incoming Request or Notification PDU ............   26
   4.1.3. Generate Outgoing Response ..............................   27
   4.1.4. Process Incoming Response PDU ...........................   27
   4.1.5. Registering Responsibility for Handling SNMP PDUs .......   28
   4.2. Message Processing Subsystem Primitives ...................   28
   4.2.1. Prepare Outgoing SNMP Request or Notification Message ...   28

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   4.2.2. Prepare an Outgoing SNMP Response Message ...............   29
   4.2.3. Prepare Data Elements from an Incoming SNMP Message .....   29
   4.3. Access Control Subsystem Primitives .......................   30
   4.4. Security Subsystem Primitives .............................   30
   4.4.1. Generate a Request or Notification Message ..............   30
   4.4.2. Process Incoming Message ................................   31
   4.4.3. Generate a Response Message .............................   31
   4.5. Common Primitives .........................................   32
   4.5.1. Release State Reference Information .....................   32
   4.6. Scenario Diagrams .........................................   32
   4.6.1. Command Generator or Notification Originator ............   32
   4.6.2. Scenario Diagram for a Command Responder Application ....   33
   5. Managed Object Definitions for SNMP Management Frameworks ...   35
   6. Intellectual Property .......................................   44
   7. Acknowledgements ............................................   45
   8. Security Considerations .....................................   46
   9. References ..................................................   46
   10. Editors' Addresses .........................................   48
   A. Guidelines for Model Designers ..............................   49
   A.1. Security Model Design Requirements ........................   49
   A.1.1. Threats .................................................   49
   A.1.2. Security Processing .....................................   50
   A.1.3. Validate the security-stamp in a received message .......   51
   A.1.4. Security MIBs ...........................................   51
   A.1.5. Cached Security Data ....................................   51
   A.2. Message Processing Model Design Requirements ..............   52
   A.2.1. Receiving an SNMP Message from the Network ..............   52
   A.2.2. Sending an SNMP Message to the Network ..................   52
   A.3. Application Design Requirements ...........................   53
   A.3.1. Applications that Initiate Messages .....................   53
   A.3.2. Applications that Receive Responses .....................   54
   A.3.3. Applications that Receive Asynchronous Messages .........   54
   A.3.4. Applications that Send Responses ........................   54
   A.4. Access Control Model Design Requirements ..................   55
   B. Full Copyright Statement ....................................   56

1.  Introduction

1.1.  Overview

   This document defines a vocabulary for describing SNMP Management
   Frameworks, and an architecture for describing the major portions of
   SNMP Management Frameworks.

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   This document does not provide a general introduction to SNMP. Other
   documents and books can provide a much better introduction to SNMP.
   Nor does this document provide a history of SNMP. That also can be
   found in books and other documents.

   Section 1 describes the purpose, goals, and design decisions of this
   architecture.

   Section 2 describes various types of documents which define SNMP
   Frameworks, and how they fit into this architecture. It also provides
   a minimal road map to the documents which have previously defined
   SNMP frameworks.

   Section 3 details the vocabulary of this architecture and its pieces.
   This section is important for understanding the remaining sections,
   and for understanding documents which are written to fit within this
   architecture.

   Section 4 describes the primitives used for the abstract service
   interfaces between the various subsystems, models and applications
   within this architecture.

   Section 5 defines a collection of managed objects used to instrument
   SNMP entities within this architecture.

   Sections 6, 7, 8, and 9 are administrative in nature.

   Appendix A contains guidelines for designers of Models which are
   expected to fit within this architecture.

   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].

1.2.  SNMP

   An SNMP management system contains:

      -  several (potentially many) nodes, each with an SNMP entity
         containing command responder and notification originator
         applications, which have access to management instrumentation
         (traditionally called agents);

      -  at least one SNMP entity containing command generator and/or
         notification receiver applications (traditionally called a
         manager) and,

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      -  a management protocol, used to convey management information
         between the SNMP entities.

   SNMP entities executing command generator and notification receiver
   applications monitor and control managed elements.  Managed elements
   are devices such as hosts, routers, terminal servers, etc., which are
   monitored and controlled via access to their management information.

   It is the purpose of this document to define an architecture which
   can evolve to realize effective management in a variety of
   configurations and environments. The architecture has been designed
   to meet the needs of implementations of:

      -  minimal SNMP entities with command responder and/or
         notification originator applications (traditionally called SNMP
         agents),

      -  SNMP entities with proxy forwarder applications (traditionally
         called SNMP proxy agents),

      -  command line driven SNMP entities with command generator and/or
         notification receiver applications (traditionally called SNMP
         command line managers),

      -  SNMP entities with  command generator and/or notification
         receiver, plus command responder and/or notification originator
         applications (traditionally called SNMP mid-level managers or
         dual-role entities),

      -  SNMP entities with command generator and/or notification
         receiver and possibly other types of applications for managing
         a potentially very large number of managed nodes (traditionally
         called (network) management stations).

1.3.  Goals of this Architecture

   This architecture was driven by the following goals:

      -  Use existing materials as much as possible. It is heavily based
         on previous work, informally known as SNMPv2u and SNMPv2*.

      -  Address the need for secure SET support, which is considered
         the most important deficiency in SNMPv1 and SNMPv2c.

      -  Make it possible to move portions of the architecture forward
         in the standards track, even if consensus has not been reached
         on all pieces.

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      -  Define an architecture that allows for longevity of the SNMP
         Frameworks that have been and will be defined.

      -  Keep SNMP as simple as possible.

      -  Make it relatively inexpensive to deploy a minimal conforming
         implementation.

      -  Make it possible to upgrade portions of SNMP as new approaches
         become available, without disrupting an entire SNMP framework.

      -  Make it possible to support features required in large
         networks, but make the expense of supporting a feature directly
         related to the support of the feature.

1.4.  Security Requirements of this Architecture

   Several of the classical threats to network protocols are applicable
   to the management problem and therefore would be applicable to any
   Security Model used in an SNMP Management Framework. Other threats
   are not applicable to the management problem.  This section discusses
   principal threats, secondary threats, and threats which are of lesser
   importance.

   The principal threats against which any Security Model used within
   this architecture SHOULD provide protection are:

   Modification of Information
      The modification threat is the danger that some unauthorized SNMP
      entity may alter in-transit SNMP messages generated on behalf of
      an authorized principal in such a way as to effect unauthorized
      management operations, including falsifying the value of an
      object.

   Masquerade
      The masquerade threat is the danger that management operations not
      authorized for some principal may be attempted by assuming the
      identity of another principal that has the appropriate
      authorizations.

   Message Stream Modification
      The SNMP protocol is typically based upon a connectionless
      transport service which may operate over any subnetwork service.
      The re-ordering, delay or replay of messages can and does occur
      through the natural operation of many such subnetwork services.
      The message stream modification threat is the danger that messages

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      may be maliciously re-ordered, delayed or replayed to an extent
      which is greater than can occur through the natural operation of a
      subnetwork service, in order to effect unauthorized management
      operations.

   Disclosure
      The disclosure threat is the danger of eavesdropping on the
      exchanges between SNMP engines.  Protecting against this threat
      may be required as a matter of local policy.

   There are at least two threats against which a Security Model within
   this architecture need not protect.

   Denial of Service
      A Security Model need not attempt to address the broad range of
      attacks by which service on behalf of authorized users is denied.
      Indeed, such denial-of-service attacks are in many cases
      indistinguishable from the type of network failures with which any
      viable management protocol must cope as a matter of course.

   Traffic Analysis
      A Security Model need not attempt to address traffic analysis
      attacks.  Many traffic patterns are predictable - entities may be
      managed on a regular basis by a relatively small number of
      management stations - and therefore there is no significant
      advantage afforded by protecting against traffic analysis.

1.5.  Design Decisions

   Various design decisions were made in support of the goals of the
   architecture and the security requirements:

      - Architecture
         An architecture should be defined which identifies the
         conceptual boundaries between the documents. Subsystems should
         be defined which describe the abstract services provided by
         specific portions of an SNMP framework. Abstract service
         interfaces, as described by service primitives, define the
         abstract boundaries between documents, and the abstract
         services that are provided by the conceptual subsystems of an
         SNMP framework.

      - Self-contained Documents
         Elements of procedure plus the MIB objects which are needed for
         processing for a specific portion of an SNMP framework should
         be defined in the same document, and as much as possible,
         should not be referenced in other documents. This allows pieces
         to be designed and documented as independent and self-contained

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         parts, which is consistent with the general SNMP MIB module
         approach.  As portions of SNMP change over time, the documents
         describing other portions of SNMP are not directly impacted.
         This modularity allows, for example, Security Models,
         authentication and privacy mechanisms, and message formats to
         be upgraded and supplemented as the need arises. The self-
         contained documents can move along the standards track on
         different time-lines.

      - Threats
         The Security Models in the Security Subsystem SHOULD protect
         against the principal threats: modification of information,
         masquerade, message stream modification and disclosure.  They
         do not need to protect against denial of service and traffic
         analysis.

      - Remote Configuration
         The Security and Access Control Subsystems add a whole new set
         of SNMP configuration parameters.  The Security Subsystem also
         requires frequent changes of secrets at the various SNMP
         entities. To make this deployable in a large operational
         environment, these SNMP parameters must be able to be remotely
         configured.

      - Controlled Complexity
         It is recognized that producers of simple managed devices want
         to keep the resources used by SNMP to a minimum.  At the same
         time, there is a need for more complex configurations which can
         spend more resources for SNMP and thus provide more
         functionality.  The design tries to keep the competing
         requirements of these two environments in balance and allows
         the more complex environments to logically extend the simple
         environment.

2.  Documentation Overview

   The following figure shows the set of documents that fit within the
   SNMP Architecture.

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   +------------------------- Document Set ----------------------------+
   |                                                                   |
   | +------------+            +-----------------+  +----------------+ |
   | | Document * |            | Applicability * |  | Coexistence  * | |
   | | Roadmap    |            | Statement       |  | & Transition   | |
   | +------------+            +-----------------+  +----------------+ |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | Message Handling                                              | |
   | | +----------------+  +-----------------+  +-----------------+  | |
   | | | Transport      |  | Message         |  | Security        |  | |
   | | | Mappings       |  | Processing and  |  |                 |  | |
   | | |                |  | Dispatcher      |  |                 |  | |
   | | +----------------+  +-----------------+  +-----------------+  | |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | PDU Handling                                                  | |
   | | +----------------+  +-----------------+  +-----------------+  | |
   | | | Protocol       |  | Applications    |  | Access          |  | |
   | | | Operations     |  |                 |  | Control         |  | |
   | | +----------------+  +-----------------+  +-----------------+  | |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | Information Model                                             | |
   | | +--------------+   +--------------+    +---------------+      | |
   | | | Structure of |   | Textual      |    | Conformance   |      | |
   | | | Management   |   | Conventions  |    | Statements    |      | |
   | | | Information  |   |              |    |               |      | |
   | | +--------------+   +--------------+    +---------------+      | |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | MIBs                                                          | |
   | | +-------------+ +-------------+ +----------+ +----------+     | |
   | | | Standard v1 | | Standard v1 | | Historic | | Draft v2 |     | |
   | | | RFC1157     | | RFC1212     | | RFC14XX  | | RFC19XX  |     | |
   | | | format      | | format      | | format   | | format   |     | |
   | | +-------------+ +-------------+ +----------+ +----------+     | |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   +-------------------------------------------------------------------+

   Note: RFC14XX means RFCs 1442, 1443, and 1444.  RFC19XX means RFCs
   1902, 1903, and 1904.

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   Those marked with an asterisk (*) are expected to be written in the
   future. Each of these documents may be replaced or supplemented.
   This Architecture document specifically describes how new documents
   fit into the set of documents in the area of Message and PDU
   handling.

2.1.  Document Roadmap

   One or more documents may be written to describe how sets of
   documents taken together form specific Frameworks. The configuration
   of document sets might change over time, so the "road map" should be
   maintained in a document separate from the standards documents
   themselves.

2.2.  Applicability Statement

   SNMP is used in networks that vary widely in size and complexity, by
   organizations that vary widely in their requirements of management.
   Some models will be designed to address specific problems of
   management, such as message security.

   One or more documents may be written to describe the environments to
   which certain versions of SNMP or models within SNMP would be
   appropriately applied, and those to which a given model might be
   inappropriately applied.

2.3.  Coexistence and Transition

   The purpose of an evolutionary architecture is to permit new models
   to replace or supplement existing models. The interactions between
   models could result in incompatibilities, security "holes", and other
   undesirable effects.

   The purpose of Coexistence documents is to detail recognized
   anomalies and to describe required and recommended behaviors for
   resolving the interactions between models within the architecture.

   Coexistence documents may be prepared separately from model
   definition documents, to describe and resolve interaction anomalies
   between a model definition and one or more other model definitions.

   Additionally, recommendations for transitions between models may also
   be described, either in a coexistence document or in a separate
   document.

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2.4.  Transport Mappings

   SNMP messages are sent over various transports. It is the purpose of
   Transport Mapping documents to define how the mapping between SNMP
   and the transport is done.

2.5.  Message Processing

   A Message Processing Model document defines a message format, which
   is typically identified by a version field in an SNMP message header.
   The document may also define a MIB module for use in message
   processing and for instrumentation of version-specific interactions.

   An SNMP engine includes one or more Message Processing Models, and
   thus may support sending and receiving multiple versions of SNMP
   messages.

2.6.  Security

   Some environments require secure protocol interactions. Security is
   normally applied at two different stages:

      -  in the transmission/receipt of messages, and

      -  in the processing of the contents of messages.

   For purposes of this document, "security" refers to message-level
   security; "access control" refers to the security applied to protocol
   operations.

   Authentication, encryption, and timeliness checking are common
   functions of message level security.

   A security document describes a Security Model, the threats against
   which the model protects, the goals of the Security Model, the
   protocols which it uses to meet those goals, and it may define a MIB
   module to describe the data used during processing, and to allow the
   remote configuration of message-level security parameters, such as
   passwords.

   An SNMP engine may support multiple Security Models concurrently.

2.7.  Access Control

   During processing, it may be required to control access to managed
   objects for operations.

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   An Access Control Model defines mechanisms to determine whether
   access to a managed object should be allowed.  An Access Control
   Model may define a MIB module used during processing and to allow the
   remote configuration of access control policies.

2.8.  Protocol Operations

   SNMP messages encapsulate an SNMP Protocol Data Unit (PDU). It is the
   purpose of a Protocol Operations document to define the operations of
   the protocol with respect to the processing of the PDUs.

   An application document defines which Protocol Operations documents
   are supported by the application.

2.9.  Applications

   An SNMP entity normally includes a number of applications.
   Applications use the services of an SNMP engine to accomplish
   specific tasks. They coordinate the processing of management
   information operations, and may use SNMP messages to communicate with
   other SNMP entities.

   Applications documents describe the purpose of an application, the
   services required of the associated SNMP engine, and the protocol
   operations and informational model that the application uses to
   perform management operations.

   An application document defines which set of documents are used to
   specifically define the structure of management information, textual
   conventions, conformance requirements, and operations supported by
   the application.

2.10.  Structure of Management Information

   Management information is viewed as a collection of managed objects,
   residing in a virtual information store, termed the Management
   Information Base (MIB). Collections of related objects are defined in
   MIB modules.

   It is the purpose of a Structure of Management Information document
   to establish the syntax for defining objects, modules, and other
   elements of managed information.

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2.11.  Textual Conventions

   When designing a MIB module, it is often useful to define new types
   similar to those defined in the SMI, but with more precise semantics,
   or which have special semantics associated with them. These newly
   defined types are termed textual conventions, and may defined in
   separate documents, or within a MIB module.

2.12.  Conformance Statements

   It may be useful to define the acceptable lower-bounds of
   implementation, along with the actual level of implementation
   achieved. It is the purpose of Conformance Statements to define the
   notation used for these purposes.

2.13.  Management Information Base Modules

   MIB documents describe collections of managed objects which
   instrument some aspect of a managed node.

2.13.1.  SNMP Instrumentation MIBs

   An SNMP MIB document may define a collection of managed objects which
   instrument the SNMP protocol itself. In addition, MIB modules may be
   defined within the documents which describe portions of the SNMP
   architecture, such as the documents for Message processing Models,
   Security Models, etc. for the purpose of instrumenting those Models,
   and for the purpose of allowing remote configuration of the Model.

2.14.  SNMP Framework Documents

   This architecture is designed to allow an orderly evolution of
   portions of SNMP Frameworks.

   Throughout the rest of this document, the term "subsystem" refers to
   an abstract and incomplete specification of a portion of a Framework,
   that is further refined by a model specification.

   A "model" describes a specific design of a subsystem, defining
   additional constraints and rules for conformance to the model.  A
   model is sufficiently detailed to make it possible to implement the
   specification.

   An "implementation" is an instantiation of a subsystem, conforming to
   one or more specific models.

   SNMP version 1 (SNMPv1), is the original Internet-standard Network
   Management Framework, as described in RFCs 1155, 1157, and 1212.

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   SNMP version 2 (SNMPv2), is the SNMPv2 Framework as derived from the
   SNMPv1 Framework. It is described in RFCs 1902-1907. SNMPv2 has no
   message definition.

   The Community-based SNMP version 2 (SNMPv2c), is an experimental SNMP
   Framework which supplements the SNMPv2 Framework, as described in
   RFC1901. It adds the SNMPv2c message format, which is similar to the

   SNMPv1 message format.

   SNMP version 3 (SNMPv3), is an extensible SNMP Framework which
   supplements the SNMPv2 Framework, by supporting the following:

      -  a new SNMP message format,

      -  Security for Messages, and

      -  Access Control.

   Other SNMP Frameworks, i.e., other configurations of implemented
   subsystems, are expected to also be consistent with this
   architecture.

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.

   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.

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   +-------------------------------------------------------------------+
   |  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   |  |              |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   +-------------------------------------------------------------------+

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,

Top      ToC       Page 16 
      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.

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.

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.

Top      ToC       Page 17 
   +------------------------------------------------------------------+
   |                                                                  |
   |  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.

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 defines 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.

Top      ToC       Page 18 
3.1.1.4.2.  Security Protocol

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

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.

Top      ToC       Page 19 
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.  * One or more
   models may be present.

                       (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 RFC1906)     | |  +->| otherMP  * |<--->| +------------+ | |
   | +-------------------+ |     +------------+  |  |                | |
   |          ^            +---------------------+  +----------------+ |
   |          |                                                        |
   |          v                                                        |
   +-------------------------------------------------------------------+
   +-----+ +-----+       +-------+
   | UDP | | IPX | . . . | other |
   +-----+ +-----+       +-------+
      ^       ^              ^
      |       |              |
      v       v              v
   +------------------------------+
   |           Network            |
   +------------------------------+

Top      ToC       Page 20 
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.
   +------------------------------+
   |           Network            |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v
   +-----+ +-----+       +-------+
   | UDP | | IPX | . . . | other |
   +-----+ +-----+       +-------+              (traditional SNMP agent)
   +-------------------------------------------------------------------+
   |              ^                                                    |
   |              |        +---------------------+  +----------------+ |
   |              |        | Message Processing  |  | Security       | |
   | Dispatcher   v        | Subsystem           |  | Subsystem      | |
   | +-------------------+ |     +------------+  |  |                | |
   | | Transport         | |  +->| v1MP     * |<--->| +------------+ | |
   | | Mapping           | |  |  +------------+  |  | | Other      | | |
   | | (e.g. RFC1906)    | |  |  +------------+  |  | | 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 |
   +-------------------------------------------------------------------+

Top      ToC       Page 21 
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.

Top      ToC       Page 22 
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, user
   names, and parties.

   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.

Top      ToC       Page 23 
   +-----------------------------------------------------------------+
   |  SNMP entity (identified by snmpEngineID, example: abcd)        |
   |                                                                 |
   |  +------------------------------------------------------------+ |
   |  | SNMP engine (identified by snmpEngineID)                   | |
   |  |                                                            | |
   |  | +-------------+ +------------+ +-----------+ +-----------+ | |
   |  | |             | |            | |           | |           | | |
   |  | | Dispatcher  | | Message    | | Security  | | Access    | | |
   |  | |             | | Processing | | Subsystem | | Control   | | |
   |  | |             | | Subsystem  | |           | | Subsystem | | |
   |  | |             | |            | |           | |           | | |
   |  | +-------------+ +------------+ +-----------+ +-----------+ | |
   |  |                                                            | |
   |  +------------------------------------------------------------+ |
   |                                                                 |
   |  +------------------------------------------------------------+ |
   |  |  Command Responder Application                             | |
   |  |  (contextEngineID, example: abcd)                          | |
   |  |                                                            | |
   |  |  example contextNames:                                     | |
   |  |                                                            | |
   |  |  "bridge1"          "bridge2"            "" (default)      | |
   |  |  ---------          ---------            ------------      | |
   |  |      |                  |                   |              | |
   |  +------|------------------|-------------------|--------------+ |
   |         |                  |                   |                |
   |  +------|------------------|-------------------|--------------+ |
   |  |  MIB | instrumentation  |                   |              | |
   |  |  +---v------------+ +---v------------+ +----v-----------+  | |
   |  |  | context        | | context        | | context        |  | |
   |  |  |                | |                | |                |  | |
   |  |  | +------------+ | | +------------+ | | +------------+ |  | |
   |  |  | | bridge MIB | | | | bridge MIB | | | | other MIB  | |  | |
   |  |  | +------------+ | | +------------+ | | +------------+ |  | |
   |  |  |                | |                | |                |  | |
   |  |  |                | |                | | +------------+ |  | |
   |  |  |                | |                | | | some  MIB  | |  | |
   |  |  |                | |                | | +------------+ |  | |
   |  |  |                | |                | |                |  | |
   +-----------------------------------------------------------------+

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.

Top      ToC       Page 24 
   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 [RFC1573], 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.

3.3.3.  contextName

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

Top      ToC       Page 25 
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, RFC1905 for more information about
   SNMP PDUs.

3.4.  Other Constructs

3.4.1.  maxSizeResponseScopedPDU

   The maxSizeResponseScopedPDU is the maximum size of a scopedPDU to be
   included in a response message.  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)

   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.


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