RFC 2271
An Architecture for Describing SNMP Management Frameworks
Network Working Group D. Harrington Request for Comments: 2271 Cabletron Systems, Inc. Obsoletes: 2261 R. Presuhn Category: Standards Track 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 (1998). All Rights Reserved. IANA Note Due to a clerical error in the assignment of the snmpModules in this memo, this RFC provides the corrected number assignment for this protocol. This memo obsoletes RFC 2261. 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 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 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.
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,
- 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.
- 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
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
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.
+------------------------- 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.
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.
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.
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.
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.
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.
+-------------------------------------------------------------------+ | 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,
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.
+------------------------------------------------------------------+ | | | 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.
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.
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 | +------------------------------+
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 | +-------------------------------------------------------------------+
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.
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.
+-----------------------------------------------------------------+ | 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.
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.
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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