RFC 2571 - An Architecture for Describing SNMP Management Frameworks
RFC 2571 (RFC2571)


Network Working Group D. Harrington
Request for Comments: 2571 Cabletron Systems, Inc.
Obsoletes: 2271 R. Presuhn
Category: Standards Track BMC Software, Inc.
B. Wijnen
IBM T. J. Watson Research

An Architecture for Describing
SNMP Management Frameworks

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 ................................................ 4
1.1. Overview .................................................. 4
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 ...................................... 9
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 ............................................ 12
2.8. Protocol Operations ....................................... 12
2.9. Applications .............................................. 13
2.10. Structure of Management Information ...................... 14
2.11. Textual Conventions ...................................... 14
2.12. Conformance Statements ................................... 14
2.13. Management Information Base Modules ...................... 14
2.13.1. SNMP Instrumentation MIBs .............................. 14
2.14. SNMP Framework Documents ................................. 14
3. Elements of the Architecture ................................ 15
3.1. The Naming of Entities .................................... 16
3.1.1. SNMP engine ............................................. 17
3.1.1.1. snmpEngineID .......................................... 17
3.1.1.2. Dispatcher ............................................ 17
3.1.1.3. Message Processing Subsystem .......................... 18
3.1.1.3.1. Message Processing Model ............................ 18
3.1.1.4. Security Subsystem .................................... 18
3.1.1.4.1. Security Model ...................................... 19
3.1.1.4.2. Security Protocol ................................... 19
3.1.2. Access Control Subsystem ................................ 19
3.1.2.1. Access Control Model .................................. 20
3.1.3. Applications ............................................ 20
3.1.3.1. SNMP Manager .......................................... 20
3.1.3.2. SNMP Agent ............................................ 22
3.2. The Naming of Identities .................................. 23
3.2.1. Principal ............................................... 23
3.2.2. securityName ............................................ 23
3.2.3. Model-dependent security ID ............................. 24
3.3. The Naming of Management Information ...................... 25
3.3.1. An SNMP Context ......................................... 26
3.3.2. contextEngineID ......................................... 26
3.3.3. contextName ............................................. 27
3.3.4. scopedPDU ............................................... 27
3.4. Other Constructs .......................................... 27
3.4.1. maxSizeResponseScopedPDU ................................ 27
3.4.2. Local Configuration Datastore ........................... 27
3.4.3. securityLevel ........................................... 27
4. Abstract Service Interfaces ................................. 28
4.1. Dispatcher Primitives ..................................... 28
4.1.1. Generate Outgoing Request or Notification ............... 28
4.1.2. Process Incoming Request or Notification PDU ............ 29
4.1.3. Generate Outgoing Response .............................. 29
4.1.4. Process Incoming Response PDU ........................... 29
4.1.5. Registering Responsibility for Handling SNMP PDUs ....... 30
4.2. Message Processing Subsystem Primitives ................... 30
4.2.1. Prepare Outgoing SNMP Request or Notification Message ... 31
4.2.2. Prepare an Outgoing SNMP Response Message ............... 31

4.2.3. Prepare Data Elements from an Incoming SNMP Message ..... 32
4.3. Access Control Subsystem Primitives ....................... 32
4.4. Security Subsystem Primitives ............................. 33
4.4.1. Generate a Request or Notification Message .............. 33
4.4.2. Process Incoming Message ................................ 33
4.4.3. Generate a Response Message ............................. 34
4.5. Common Primitives ......................................... 34
4.5.1. Release State Reference Information ..................... 35
4.6. Scenario Diagrams ......................................... 36
4.6.1. Command Generator or Notification Originator ............ 36
4.6.2. Scenario Diagram for a Command Responder Application .... 37
5. Managed Object Definitions for SNMP Management Frameworks ... 38
6. IANA Considerations ......................................... 48
6.1. Security Models ........................................... 48
6.2. Message Processing Models ................................. 48
6.3. SnmpEngineID Formats ...................................... 49
7. Intellectual Property ....................................... 49
8. Acknowledgements ............................................ 49
9. Security Considerations ..................................... 51
10. References ................................................. 52
11. Editor's Addresses ......................................... 54
A. Guidelines for Model Designers .............................. 55
A.1. Security Model Design Requirements ........................ 55
A.1.1. Threats ................................................. 55
A.1.2. Security Processing ..................................... 56
A.1.3. Validate the security-stamp in a received message ....... 56
A.1.4. Security MIBs ........................................... 57
A.1.5. Cached Security Data .................................... 57
A.2. Message Processing Model Design Requirements .............. 57
A.2.1. Receiving an SNMP Message from the Network .............. 58
A.2.2. Sending an SNMP Message to the Network .................. 58
A.3. Application Design Requirements ........................... 59
A.3.1. Applications that Initiate Messages ..................... 59
A.3.2. Applications that Receive Responses ..................... 59
A.3.3. Applications that Receive Asynchronous Messages ......... 60
A.3.4. Applications that Send Responses ........................ 60
A.4. Access Control Model Design Requirements .................. 60
B. Full Copyright Statement .................................... 62

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 (elements
of) 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, 9, 10 and 11 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*,
based in turn on SNMPv2p.

- 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
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.

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

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, since they are deemed to be of
lesser importance in this context:

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.

This modularity of specification is not meant to be interpreted as
imposing any specific requirements on implementation.

- Threats
The Security Models in the Security Subsystem SHOULD protect
against the principal and secondary 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 remotely
configurable.

- 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 | | | | | | |
| | +--------------+ +--------------+ +---------------+ | |
| +---------------------------------------------------------------+ |
| |
| +---------------------------------------------------------------+ |
| | MIB Modules written in various formats, e.g.: | |
| | +-------------+ +-------------+ +----------+ +----------+ | |
| | | Standard v1 | | Standard v1 | | Historic | | Draft v2 | | |
| | | RFC 1157 | | RFC 1212 | | RFC 14xx | | RFC 19xx | | |
| | | format | | format | | format | | format | | |
| | +-------------+ +-------------+ +----------+ +----------+ | |
| +---------------------------------------------------------------+ |
| |
+-------------------------------------------------------------------+

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.

An example of such a roadmap is "Introduction to Version 3 of the
Internet-standard Network Management Framework" [RFC2570].

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.

One such coexistance document is [SNMP-COEX], "Coexistence between
Version 1, Version 2, and Version 3 of the Internet-standard Network
Management Framework".

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
keys.

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). SNMP
PDUs define the operations performed by the receiving SNMP engine.
It is the purpose of a Protocol Operations document to define the
operations of the protocol with respect to the processing of the
PDUs. Every PDU belongs to one or more of the PDU classes defined
below:

1) Read Class:

The Read Class contains protocol operations that retrieve
management information. For example, RFC 1905 defines the
following protocol operations for the Read Class: GetRequest-
PDU, GetNextRequest-PDU, and GetBulkRequest-PDU.

2) Write Class:

The Write Class contains protocol operations which attempt to
modify management information. For example, RFC 1905 defines
the following protocol operation for the Write Class:
SetRequest-PDU.

3) Response Class:

The Response Class contains protocol operations which are sent
in response to a previous request. For example, RFC 1905
defines the following for the Response Class: Response-PDU,
Report-PDU.

4) Notification Class:

The Notification Class contains protocol operations which send
a notification to a notification receiver application. For
example, RFC 1905 defines the following operations for the
Notification Class: Trapv2-PDU, InformRequest-PDU.

5) Internal Class:

The Internal Class contains protocol operations which are
exchanged internally between SNMP engines. For example, RFC
1905 defines the following operations for the Internal Class:
Report-PDU.

The preceding five classifications are based on the functional
properties of a PDU. It is also useful to classify PDUs based on
whether a response is expected:

6) Confirmed Class:

The Confirmed Class contains all protocol operations which
cause the receiving SNMP engine to send back a response. For
example, RFC 1905 defines the following operations for the
Confirmed Class: GetRequest-PDU, GetNextRequest-PDU,
GetBulkRequest-PDU, SetRequest-PDU, and InformRequest-PDU.

7) Unconfirmed Class:

The Unconfirmed Class contains all protocol operations which
are not acknowledged. For example, RFC 1905 defines the
following operations for the Unconfirmed Class: Report-PDU,
Trapv2-PDU, and GetResponse-PDU.

An application document defines which Protocol Operations 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 notation 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 be 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 the Conformance Statements document 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 STD 58, RFCs 2578, 2579, 2580,
and RFCs 1905-1907. 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 RFC
1901. 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,

- Access Control, and

- Remote configuration of SNMP parameters.

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 within that
administrative domain. Note that it is possible for SNMP entities in
different administrative domains to have the same value for
snmpEngineID. Federation of administrative domains may necessitate
assignment of new values.

3.1.1.2. Dispatcher

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

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

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

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

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

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

3.1.1.4.2. Security Protocol

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

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, and
user names.

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

3.3. The Naming of Management Information

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

+-----------------------------------------------------------------+
| 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 | | | | some MIB | | | |
| | | +------------+ | | +------------+ | | +------------+ | | |
| | | | | | | | | |
| | | | | | | +------------+ | | |
| | | | | | | | other 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 [RFC2233], 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 that
a PDU's sender would be willing to accept. Note that the size of a
scopedPDU does not include the size of the SNMP message header.

3.4.2. Local Configuration Datastore

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

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

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

3.4.3. securityLevel

This architecture recognizes three levels of security:

- without authentication and without privacy (noAuthNoPriv)

- with authentication but without privacy (authNoPriv)

- with authentication and with privacy (authPriv)

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

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

4. Abstract Service Interfaces

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

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

4.1. Dispatcher Primitives

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

4.1.1. Generate Outgoing Request or Notification

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

statusInformation = -- sendPduHandle if success
-- errorIndication if failure
sendPdu(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
)

4.1.2. Process Incoming Request or Notification PDU

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

processPdu( -- process Request/Notification PDU
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model in use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size of the Response PDU
IN stateReference -- reference to state information
) -- needed when sending a response

4.1.3. Generate Outgoing Response

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

result = -- SUCCESS or FAILURE
returnResponsePdu(
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model in use
IN securityName -- on behalf of this principal
IN securityLevel -- same as on incoming request
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size sender can accept
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
) -- error counter OID/value if error

4.1.4. Process Incoming Response PDU

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

processResponsePdu( -- process Response PDU
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model in use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN statusInformation -- success or errorIndication
IN sendPduHandle -- handle from sendPdu
)

4.1.5. Registering Responsibility for Handling SNMP PDUs

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

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

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

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

4.2. Message Processing Subsystem Primitives

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

4.2.1. Prepare Outgoing SNMP Request or Notification Message

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

statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
IN sendPduHandle -- the handle for matching
-- incoming responses
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)

4.2.2. Prepare an Outgoing SNMP Response Message

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

result = -- SUCCESS or FAILURE
prepareResponseMessage(
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- same as on incoming request
IN securityName -- same as on incoming request
IN securityLevel -- same as on incoming request
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size able to accept
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address

OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)

4.2.3. Prepare Data Elements from an Incoming SNMP Message

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

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

4.3. Access Control Subsystem Primitives

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

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

statusInformation = -- success or errorIndication
isAccessAllowed(
IN securityModel -- Security Model in use
IN securityName -- principal who wants to access
IN securityLevel -- Level of Security
IN viewType -- read, write, or notify view
IN contextName -- context containing variableName
IN variableName -- OID for the managed object
)

4.4. Security Subsystem Primitives

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

4.4.1. Generate a Request or Notification Message

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

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

4.4.2. Process Incoming Message

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

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

4.4.3. Generate a Response Message

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

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

4.5. Common Primitives

These primitive(s) are provided by multiple Subsystems.

4.5.1. Release State Reference Information

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

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

4.6. Scenario Diagrams

4.6.1. Command Generator or Notification Originator

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

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

4.6.2. Scenario Diagram for a Command Responder Application

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

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

5. Managed Object Definitions for SNMP Management Frameworks

SNMP-FRAMEWORK-MIB DEFINITIONS ::= BEGIN

IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY,
snmpModules FROM SNMPv2-SMI
TEXTUAL-CONVENTION FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF;

snmpFrameworkMIB MODULE-IDENTITY
LAST-UPDATED "9901190000Z" -- 19 January 1999
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-EMail: snmpv3@tis.com
Subscribe: majordomo@tis.com
In message body: subscribe snmpv3

Chair: Russ Mundy
TIS Labs at Network Associates
postal: 3060 Washington Rd
Glenwood MD 21738
USA
EMail: mundy@tis.com
phone: +1 301-854-6889

Co-editor Dave Harrington
Cabletron Systems, Inc.
postal: Post Office Box 5005
Mail Stop: Durham
35 Industrial Way
Rochester, NH 03867-5005
USA
EMail: dbh@ctron.com
phone: +1 603-337-7357

Co-editor Randy Presuhn
BMC Software, Inc.
postal: 965 Stewart Drive
Sunnyvale, CA 94086
USA
EMail: randy_presuhn@bmc.com
phone: +1 408-616-3100

Co-editor: Bert Wijnen
IBM T.J. Watson Research
postal: Schagen 33
3461 GL Linschoten

Netherlands
EMail: wijnen@vnet.ibm.com
phone: +31 348-432-794
"
DESCRIPTION "The SNMP Management Architecture MIB"
-- Revision History

REVISION "9901190000Z" -- 19 January 1999
DESCRIPTION "Updated editors' addresses, fixed typos.
Published as RFC2571.
"
REVISION "9711200000Z" -- 20 November 1997
DESCRIPTION "The initial version, published in RFC 2271.
"
::= { snmpModules 10 }

-- Textual Conventions used in the SNMP Management Architecture ***

SnmpEngineID ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "An SNMP engine's administratively-unique identifier.
Objects of this type are for identification, not for
addressing, even though it is possible that an
address may have been used in the generation of
a specific value.

The value for this object may not be all zeros or
all 'ff'H or the empty (zero length) string.

The initial value for this object may be configured
via an operator console entry or via an algorithmic
function. In the latter case, the following
example algorithm is recommended.

In cases where there are multiple engines on the
same system, the use of this algorithm is NOT
appropriate, as it would result in all of those
engines ending up with the same ID value.

1) The very first bit is used to indicate how the
rest of the data is composed.

0 - as defined by enterprise using former methods
that existed before SNMPv3. See item 2 below.

1 - as defined by this architecture, see item 3
below.

Note that this allows existing uses of the
engineID (also known as AgentID [RFC1910]) to
co-exist with any new uses.

2) The snmpEngineID has a length of 12 octets.

The first four octets are set to the binary
equivalent of the agent's SNMP management
private enterprise number as assigned by the
Internet Assigned Numbers Authority (IANA).
For example, if Acme Networks has been assigned
{ enterprises 696 }, the first four octets would
be assigned '000002b8'H.

The remaining eight octets are determined via
one or more enterprise-specific methods. Such
methods must be designed so as to maximize the
possibility that the value of this object will
be unique in the agent's administrative domain.
For example, it may be the IP address of the SNMP
entity, or the MAC address of one of the
interfaces, with each address suitably padded
with random octets. If multiple methods are
defined, then it is recommended that the first
octet indicate the method being used and the
remaining octets be a function of the method.

3) The length of the octet strings varies.

The first four octets are set to the binary
equivalent of the agent's SNMP management
private enterprise number as assigned by the
Internet Assigned Numbers Authority (IANA).
For example, if Acme Networks has been assigned
{ enterprises 696 }, the first four octets would
be assigned '000002b8'H.

The very first bit is set to 1. For example, the
above value for Acme Networks now changes to be
'800002b8'H.

The fifth octet indicates how the rest (6th and
following octets) are formatted. The values for
the fifth octet are:

0 - reserved, unused.

1 - IPv4 address (4 octets)

lowest non-special IP address

2 - IPv6 address (16 octets)
lowest non-special IP address

3 - MAC address (6 octets)
lowest IEEE MAC address, canonical
order

4 - Text, administratively assigned
Maximum remaining length 27

5 - Octets, administratively assigned
Maximum remaining length 27

6-127 - reserved, unused

127-255 - as defined by the enterprise
Maximum remaining length 27
"
SYNTAX OCTET STRING (SIZE(5..32))

SnmpSecurityModel ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "An identifier that uniquely identifies a
securityModel of the Security Subsystem within the
SNMP Management Architecture.

The values for securityModel are allocated as
follows:

- The zero value is reserved.
- Values between 1 and 255, inclusive, are reserved
for standards-track Security Models and are
managed by the Internet Assigned Numbers Authority
(IANA).
- Values greater than 255 are allocated to
enterprise-specific Security Models. An
enterprise-specific securityModel value is defined
to be:

enterpriseID * 256 + security model within
enterprise

For example, the fourth Security Model defined by
the enterprise whose enterpriseID is 1 would be
260.

This scheme for allocation of securityModel
values allows for a maximum of 255 standards-
based Security Models, and for a maximum of
255 Security Models per enterprise.

It is believed that the assignment of new
securityModel values will be rare in practice
because the larger the number of simultaneously
utilized Security Models, the larger the
chance that interoperability will suffer.
Consequently, it is believed that such a range
will be sufficient. In the unlikely event that
the standards committee finds this number to be
insufficient over time, an enterprise number
can be allocated to obtain an additional 255
possible values.

Note that the most significant bit must be zero;
hence, there are 23 bits allocated for various
organizations to design and define non-standard
securityModels. This limits the ability to
define new proprietary implementations of Security
Models to the first 8,388,608 enterprises.

It is worthwhile to note that, in its encoded
form, the securityModel value will normally
require only a single byte since, in practice,
the leftmost bits will be zero for most messages
and sign extension is suppressed by the encoding
rules.

As of this writing, there are several values
of securityModel defined for use with SNMP or
reserved for use with supporting MIB objects.
They are as follows:

0 reserved for 'any'
1 reserved for SNMPv1
2 reserved for SNMPv2c
3 User-Based Security Model (USM)
"
SYNTAX INTEGER(0 .. 2147483647)

SnmpMessageProcessingModel ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "An identifier that uniquely identifies a Message
Processing Model of the Message Processing
Subsystem within a SNMP Management Architecture.

The values for messageProcessingModel are
allocated as follows:

- Values between 0 and 255, inclusive, are
reserved for standards-track Message Processing
Models and are managed by the Internet Assigned
Numbers Authority (IANA).

- Values greater than 255 are allocated to
enterprise-specific Message Processing Models.
An enterprise messageProcessingModel value is
defined to be:

enterpriseID * 256 +
messageProcessingModel within enterprise

For example, the fourth Message Processing Model
defined by the enterprise whose enterpriseID
is 1 would be 260.

This scheme for allocating messageProcessingModel
values allows for a maximum of 255 standards-
based Message Processing Models, and for a
maximum of 255 Message Processing Models per
enterprise.

It is believed that the assignment of new
messageProcessingModel values will be rare
in practice because the larger the number of
simultaneously utilized Message Processing Models,
the larger the chance that interoperability
will suffer. It is believed that such a range
will be sufficient. In the unlikely event that
the standards committee finds this number to be
insufficient over time, an enterprise number
can be allocated to obtain an additional 256
possible values.

Note that the most significant bit must be zero;
hence, there are 23 bits allocated for various
organizations to design and define non-standard
messageProcessingModels. This limits the ability
to define new proprietary implementations of
Message Processing Models to the first 8,388,608
enterprises.

It is worthwhile to note that, in its encoded
form, the messageProcessingModel value will

normally require only a single byte since, in
practice, the leftmost bits will be zero for
most messages and sign extension is suppressed
by the encoding rules.

As of this writing, there are several values of
messageProcessingModel defined for use with SNMP.
They are as follows:

0 reserved for SNMPv1
1 reserved for SNMPv2c
2 reserved for SNMPv2u and SNMPv2*
3 reserved for SNMPv3
"
SYNTAX INTEGER(0 .. 2147483647)

SnmpSecurityLevel ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "A Level of Security at which SNMP messages can be
sent or with which operations are being processed;
in particular, one of:

noAuthNoPriv - without authentication and
without privacy,
authNoPriv - with authentication but
without privacy,
authPriv - with authentication and
with privacy.

These three values are ordered such that
noAuthNoPriv is less than authNoPriv and
authNoPriv is less than authPriv.
"
SYNTAX INTEGER { noAuthNoPriv(1),
authNoPriv(2),
authPriv(3)
}

SnmpAdminString ::= TEXTUAL-CONVENTION
DISPLAY-HINT "255a"
STATUS current
DESCRIPTION "An octet string containing administrative
information, preferably in human-readable form.

To facilitate internationalization, this
information is represented using the ISO/IEC
IS 10646-1 character set, encoded as an octet
string using the UTF-8 transformation format

described in [RFC2279].

Since additional code points are added by
amendments to the 10646 standard from time
to time, implementations must be prepared to
encounter any code point from 0x00000000 to
0x7fffffff. Byte sequences that do not
correspond to the valid UTF-8 encoding of a
code point or are outside this range are
prohibited.

The use of control codes should be avoided.

When it is necessary to represent a newline,
the control code sequence CR LF should be used.

The use of leading or trailing white space should
be avoided.

For code points not directly supported by user
interface hardware or software, an alternative
means of entry and display, such as hexadecimal,
may be provided.

For information encoded in 7-bit US-ASCII,
the UTF-8 encoding is identical to the
US-ASCII encoding.

UTF-8 may require multiple bytes to represent a
single character / code point; thus the length
of this object in octets may be different from
the number of characters encoded. Similarly,
size constraints refer to the number of encoded
octets, not the number of characters represented
by an encoding.

Note that when this TC is used for an object that
is used or envisioned to be used as an index, then
a SIZE restriction MUST be specified so that the
number of sub-identifiers for any object instance
does not exceed the limit of 128, as defined by
[RFC1905].

Note that the size of an SnmpAdminString object is
measured in octets, not characters.
"
SYNTAX OCTET STRING (SIZE (0..255))

-- Administrative assignments ***************************************

snmpFrameworkAdmin
OBJECT IDENTIFIER ::= { snmpFrameworkMIB 1 }
snmpFrameworkMIBObjects
OBJECT IDENTIFIER ::= { snmpFrameworkMIB 2 }
snmpFrameworkMIBConformance
OBJECT IDENTIFIER ::= { snmpFrameworkMIB 3 }

-- the snmpEngine Group ********************************************

snmpEngine OBJECT IDENTIFIER ::= { snmpFrameworkMIBObjects 1 }

snmpEngineID OBJECT-TYPE
SYNTAX SnmpEngineID
MAX-ACCESS read-only
STATUS current
DESCRIPTION "An SNMP engine's administratively-unique identifier.
"
::= { snmpEngine 1 }

snmpEngineBoots OBJECT-TYPE
SYNTAX INTEGER (1..2147483647)
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times that the SNMP engine has
(re-)initialized itself since snmpEngineID
was last configured.
"
::= { snmpEngine 2 }

snmpEngineTime OBJECT-TYPE
SYNTAX INTEGER (0..2147483647)
UNITS "seconds"
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of seconds since the value of
the snmpEngineBoots object last changed.
When incrementing this object's value would
cause it to exceed its maximum,
snmpEngineBoots is incremented as if a
re-initialization had occurred, and this
object's value consequently reverts to zero.
"
::= { snmpEngine 3 }

snmpEngineMaxMessageSize OBJECT-TYPE
SYNTAX INTEGER (484..2147483647)

MAX-ACCESS read-only
STATUS current
DESCRIPTION "The maximum length in octets of an SNMP message
which this SNMP engine can send or receive and
process, determined as the minimum of the maximum
message size values supported among all of the
transports available to and supported by the engine.
"
::= { snmpEngine 4 }

-- Registration Points for Authentication and Privacy Protocols **

snmpAuthProtocols OBJECT-IDENTITY
STATUS current
DESCRIPTION "Registration point for standards-track
authentication protocols used in SNMP Management
Frameworks.
"
::= { snmpFrameworkAdmin 1 }

snmpPrivProtocols OBJECT-IDENTITY
STATUS current
DESCRIPTION "Registration point for standards-track privacy
protocols used in SNMP Management Frameworks.
"
::= { snmpFrameworkAdmin 2 }

-- Conformance information ******************************************

snmpFrameworkMIBCompliances
OBJECT IDENTIFIER ::= {snmpFrameworkMIBConformance 1}
snmpFrameworkMIBGroups
OBJECT IDENTIFIER ::= {snmpFrameworkMIBConformance 2}

-- compliance statements

snmpFrameworkMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines which
implement the SNMP Management Framework MIB.
"
MODULE -- this module
MANDATORY-GROUPS { snmpEngineGroup }

::= { snmpFrameworkMIBCompliances 1 }

-- units of conformance

snmpEngineGroup OBJECT-GROUP
OBJECTS {
snmpEngineID,
snmpEngineBoots,
snmpEngineTime,
snmpEngineMaxMessageSize
}
STATUS current
DESCRIPTION "A collection of objects for identifying and
determining the configuration and current timeliness
values of an SNMP engine.
"
::= { snmpFrameworkMIBGroups 1 }

END

6. IANA Considerations

This document defines three number spaces administered by IANA, one
for security models, another for message processing models, and a
third for SnmpEngineID formats.

6.1. Security Models

The SnmpSecurityModel TEXTUAL-CONVENTION values managed by IANA are
in the range from 0 to 255 inclusive, and are reserved for
standards-track Security Models. If this range should in the future
prove insufficient, an enterprise number can be allocated to obtain
an additional 255 possible values.

As of this writing, there are several values of securityModel defined
for use with SNMP or reserved for use with supporting MIB objects.
They are as follows:
0 reserved for 'any'
1 reserved for SNMPv1
2 reserved for SNMPv2c
3 User-Based Security Model (USM)

6.2. Message Processing Models

The SnmpMessageProcessingModel TEXTUAL-CONVENTION values managed by
IANA are in the range 0 to 255, inclusive. Each value uniquely
identifies a standards-track Message Processing Model of the Message
Processing Subsystem within a SNMP Management Architecture.

Should this range prove insufficient in the future, an enterprise
number may be obtained for the standards committee to get an

additional 256 possible values.

As of this writing, there are several values of
messageProcessingModel defined for use with SNMP. They are as
follows:
0 reserved for SNMPv1
1 reserved for SNMPv2c
2 reserved for SNMPv2u and SNMPv2*
3 reserved for SNMPv3

6.3. SnmpEngineID Formats

The SnmpEngineID TEXTUAL-CONVENTION's fifth octet contains a format
identifier. The values managed by IANA are in the range 6 to 127,
inclusive. Each value uniquely identifies a standards-track
SnmpEngineID format.

7. Intellectual Property

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

8. Acknowledgements

This document is the result of the efforts of the SNMPv3 Working
Group. Some special thanks are in order to the following SNMPv3 WG
members:

Harald Tveit Alvestrand (Maxware)
Dave Battle (SNMP Research, Inc.)
Alan Beard (Disney Worldwide Services)

Paul Berrevoets (SWI Systemware/Halcyon Inc.)
Martin Bjorklund (Ericsson)
Uri Blumenthal (IBM T.J. Watson Research Center)
Jeff Case (SNMP Research, Inc.)
John Curran (BBN)
Mike Daniele (Compaq Computer Corporation)
T. Max Devlin (Eltrax Systems)
John Flick (Hewlett Packard)
Rob Frye (MCI)
Wes Hardaker (U.C.Davis, Information Technology - D.C.A.S.)
David Harrington (Cabletron Systems Inc.)
Lauren Heintz (BMC Software, Inc.)
N.C. Hien (IBM T.J. Watson Research Center)
Michael Kirkham (InterWorking Labs, Inc.)
Dave Levi (SNMP Research, Inc.)
Louis A Mamakos (UUNET Technologies Inc.)
Joe Marzot (Nortel Networks)
Paul Meyer (Secure Computing Corporation)
Keith McCloghrie (Cisco Systems)
Bob Moore (IBM)
Russ Mundy (TIS Labs at Network Associates)
Bob Natale (ACE*COMM Corporation)
Mike O'Dell (UUNET Technologies Inc.)
Dave Perkins (DeskTalk)
Peter Polkinghorne (Brunel University)
Randy Presuhn (BMC Software, Inc.)
David Reeder (TIS Labs at Network Associates)
David Reid (SNMP Research, Inc.)
Aleksey Romanov (Quality Quorum)
Shawn Routhier (Epilogue)
Juergen Schoenwaelder (TU Braunschweig)
Bob Stewart (Cisco Systems)
Mike Thatcher (Independent Consultant)
Bert Wijnen (IBM T.J. Watson Research Center)

The document is based on recommendations of the IETF Security and
Administrative Framework Evolution for SNMP Advisory Team. Members
of that Advisory Team were:

David Harrington (Cabletron Systems Inc.)
Jeff Johnson (Cisco Systems)
David Levi (SNMP Research Inc.)
John Linn (Openvision)
Russ Mundy (Trusted Information Systems) chair
Shawn Routhier (Epilogue)
Glenn Waters (Nortel)
Bert Wijnen (IBM T. J. Watson Research Center)

As recommended by the Advisory Team and the SNMPv3 Working Group
Charter, the design incorporates as much as practical from previous
RFCs and drafts. As a result, special thanks are due to the authors
of previous designs known as SNMPv2u and SNMPv2*:

Jeff Case (SNMP Research, Inc.)
David Harrington (Cabletron Systems Inc.)
David Levi (SNMP Research, Inc.)
Keith McCloghrie (Cisco Systems)
Brian O'Keefe (Hewlett Packard)
Marshall T. Rose (Dover Beach Consulting)
Jon Saperia (BGS Systems Inc.)
Steve Waldbusser (International Network Services)
Glenn W. Waters (Bell-Northern Research Ltd.)

9. Security Considerations

This document describes how an implementation can include a Security
Model to protect management messages and an Access Control Model to
control access to management information.

The level of security provided is determined by the specific Security
Model implementation(s) and the specific Access Control Model
implementation(s) used.

Applications have access to data which is not secured. Applications
SHOULD take reasonable steps to protect the data from disclosure.

It is the responsibility of the purchaser of an implementation to
ensure that:

1) an implementation complies with the rules defined by this
architecture,

2) the Security and Access Control Models utilized satisfy the
security and access control needs of the organization,

3) the implementations of the Models and Applications comply with
the model and application specifications,

4) and the implementation protects configuration secrets from
inadvertent disclosure.

This document also contains a MIB definition module. None of the
objects defined is writable, and the information they represent is
not deemed to be particularly sensitive. However, if they are deemed

sensitive in a particular environment, access to them should be
restricted through the use of appropriately configured Security and
Access Control models.

10. References

[RFC1155] Rose, M. and K. McCloghrie, "Structure and
Identification of Management Information for TCP/IP-
based internets", STD 16, RFC 1155, May 1990.

[RFC1157] Case, J., M. Fedor, M. Schoffstall and J. Davin, "The
Simple Network Management Protocol", STD 15, RFC 1157,
May 1990.

[RFC1212] Rose, M. and K. McCloghrie, "Concise MIB Definitions",
STD 16, RFC 1212, March 1991.

[RFC1901] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Introduction to
Community-based SNMPv2", RFC 1901, January 1996.

[RFC2578] McCloghrie, K., Perkins, D. and J. Schoenwaelder,
"Structure of Management Information Version 2 (SMIv2)",
STD 58, RFC 2578, April 1999.

[RFC2579] McCloghrie, K., Perkins, D. and J. Schoenwaelder,,
"Textual Conventions for SMIv2", STD 58, RFC 2579, April
1999.

[RFC2580] McCloghrie, K., Perkins, D. and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.

[RFC1905] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Protocol Operations for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1905, January 1996.

[RFC1906] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Transport Mappings for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1906, January 1996.

[RFC1907] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Management Information Base
for Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1907 January 1996.

[RFC1908] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Coexistence between Version
1 and Version 2 of the SNMP-standard Network Management
Framework", RFC 1908, January 1996.

[RFC1909] McCloghrie, K., Editor, "An Administrative
Infrastructure for SNMPv2", RFC 1909, February 1996.

[RFC1910] Waters, G., Editor, "User-based Security Model for
SNMPv2", RFC 1910, February 1996.

[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January, 1998.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[BCP-11] Hovey, R. and S. Bradner, "The Organizations Involved in
the IETF Standards Process", BCP 11, RFC 2028, October
1996.

[RFC2233] McCloghrie, K. and F. Kastenholz. "The Interfaces Group
MIB using SMIv2", RFC 2233, November 1997.

[RFC2572] Case, J., Harrington, D., Presuhn, R. and B. Wijnen,
"Message Processing and Dispatching for the Simple
Network Management Protocol (SNMP)", RFC 2572, April
1999.

[RFC2574] Blumenthal, U. and B. Wijnen, "The User-Based Security
Model for Version 3 of the Simple Network Management
Protocol (SNMPv3)", RFC 2574, April 1999.

[RFC2575] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based
Access Control Model for the Simple Network Management
Protocol (SNMP)", RFC 2575, April 1999.

[RFC2573] Levi, D. B., Meyer, P. and B. Stewart, "SNMP
Applications", RFC 2573, April 1999.

[RFC2570] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction to Version 3 of the Internet-standard
Network Management Framework", RFC 2570, April 1999.

[SNMP-COEX] Frye, R., Levi, D. and B. Wijnen, "Coexistence between
Version 1, Version 2, and Version 3 of the Internet-
standard Network Management Framework", Work in
Progress.

11. Editor's Addresses

Bert Wijnen
IBM T.J. Watson Research
Schagen 33
3461 GL Linschoten
Netherlands

Phone: +31 348-432-794
EMail: wijnen@vnet.ibm.com

Dave Harrington
Cabletron Systems, Inc
Post Office Box 5005
Mail Stop: Durham
35 Industrial Way
Rochester, NH 03867-5005
USA

Phone: +1 603-337-7357
EMail: dbh@ctron.com

Randy Presuhn
BMC Software, Inc.
965 Stewart Drive
Sunnyvale, CA 94086
USA

Phone: +1 408-616-3100
Fax: +1 408-616-3101
EMail: randy_presuhn@bmc.com

APPENDIX A

A. Guidelines for Model Designers

This appendix describes guidelines for designers of models which are
expected to fit into the architecture defined in this document.

SNMPv1 and SNMPv2c are two SNMP frameworks which use communities to
provide trivial authentication and access control. SNMPv1 and SNMPv2c
Frameworks can coexist with Frameworks designed according to this
architecture, and modified versions of SNMPv1 and SNMPv2c Frameworks
could be designed to meet the requirements of this architecture, but
this document does not provide guidelines for that coexistence.

Within any subsystem model, there should be no reference to any
specific model of another subsystem, or to data defined by a specific
model of another subsystem.

Transfer of data between the subsystems is deliberately described as
a fixed set of abstract data elements and primitive functions which
can be overloaded to satisfy the needs of multiple model definitions.

Documents which define models to be used within this architecture
SHOULD use the standard primitives between subsystems, possibly
defining specific mechanisms for converting the abstract data
elements into model-usable formats. This constraint exists to allow
subsystem and model documents to be written recognizing common
borders of the subsystem and model. Vendors are not constrained to
recognize these borders in their implementations.

The architecture defines certain standard services to be provided
between subsystems, and the architecture defines abstract service
interfaces to request these services.

Each model definition for a subsystem SHOULD support the standard
service interfaces, but whether, or how, or how well, it performs the
service is dependent on the model definition.

A.1. Security Model Design Requirements

A.1.1. Threats

A document describing a Security Model MUST describe how the model
protects against the threats described under "Security Requirements
of this Architecture", section 1.4.

A.1.2. Security Processing

Received messages MUST be validated by a Model of the Security
Subsystem. Validation includes authentication and privacy processing
if needed, but it is explicitly allowed to send messages which do not
require authentication or privacy.

A received message contains a specified securityLevel to be used
during processing. All messages requiring privacy MUST also require
authentication.

A Security Model specifies rules by which authentication and privacy
are to be done. A model may define mechanisms to provide additional
security features, but the model definition is constrained to using
(possibly a subset of) the abstract data elements defined in this
document for transferring data between subsystems.

Each Security Model may allow multiple security protocols to be used
concurrently within an implementation of the model. Each Security
Model defines how to determine which protocol to use, given the
securityLevel and the security parameters relevant to the message.
Each Security Model, with its associated protocol(s) defines how the
sending/receiving entities are identified, and how secrets are
configured.

Authentication and Privacy protocols supported by Security Models are
uniquely identified using Object Identifiers. IETF standard protocols
for authentication or privacy should have an identifier defined
within the snmpAuthProtocols or the snmpPrivProtocols subtrees.
Enterprise specific protocol identifiers should be defined within the
enterprise subtree.

For privacy, the Security Model defines what portion of the message
is encrypted.

The persistent data used for security should be SNMP-manageable, but
the Security Model defines whether an instantiation of the MIB is a
conformance requirement.

Security Models are replaceable within the Security Subsystem.
Multiple Security Model implementations may exist concurrently within
an SNMP engine. The number of Security Models defined by the SNMP
community should remain small to promote interoperability.

A.1.3. Validate the security-stamp in a received message

A Message Processing Model requests that a Security Model:
- verifies that the message has not been altered,

- authenticates the identification of the principal for whom the
message was generated.
- decrypts the message if it was encrypted.

Additional requirements may be defined by the model, and additional
services may be provided by the model, but the model is constrained
to use the following primitives for transferring data between
subsystems. Implementations are not so constrained.

A Message Processing Model uses the processIncomingMsg primitive as
described in section 4.4.2.

A.1.4. Security MIBs

Each Security Model defines the MIB module(s) required for security
processing, including any MIB module(s) required for the security
protocol(s) supported. The MIB module(s) SHOULD be defined
concurrently with the procedures which use the MIB module(s). The
MIB module(s) are subject to normal access control rules.

The mapping between the model-dependent security ID and the
securityName MUST be able to be determined using SNMP, if the model-
dependent MIB is instantiated and if access control policy allows
access.

A.1.5. Cached Security Data

For each message received, the Security Model caches the state
information such that a Response message can be generated using the
same security information, even if the Local Configuration Datastore
is altered between the time of the incoming request and the outgoing
response.

A Message Processing Model has the responsibility for explicitly
releasing the cached data if such data is no longer needed. To enable
this, an abstract securityStateReference data element is passed from
the Security Model to the Message Processing Model.

The cached security data may be implicitly released via the
generation of a response, or explicitly released by using the
stateRelease primitive, as described in section 4.5.1.

A.2. Message Processing Model Design Requirements

An SNMP engine contains a Message Processing Subsystem which may
contain multiple Message Processing Models.

The Message Processing Model MUST always (conceptually) pass the
complete PDU, i.e., it never forwards less than the complete list of
varBinds.

A.2.1. Receiving an SNMP Message from the Network

Upon receipt of a message from the network, the Dispatcher in the
SNMP engine determines the version of the SNMP message and interacts
with the corresponding Message Processing Model to determine the
abstract data elements.

A Message Processing Model specifies the SNMP Message format it
supports and describes how to determine the values of the abstract
data elements (like msgID, msgMaxSize, msgFlags,
msgSecurityParameters, securityModel, securityLevel etc). A Message
Processing Model interacts with a Security Model to provide security
processing for the message using the processIncomingMsg primitive, as
described in section 4.4.2.

A.2.2. Sending an SNMP Message to the Network

The Dispatcher in the SNMP engine interacts with a Message Processing
Model to prepare an outgoing message. For that it uses the following
primitives:

- for requests and notifications: prepareOutgoingMessage, as
described in section 4.2.1.

- for response messages: prepareResponseMessage, as described in
section 4.2.2.

A Message Processing Model, when preparing an Outgoing SNMP Message,
interacts with a Security Model to secure the message. For that it
uses the following primitives:

- for requests and notifications: generateRequestMsg, as
described in section 4.4.1.

- for response messages: generateResponseMsg as described in
section 4.4.3.

Once the SNMP message is prepared by a Message Processing Model,
the Dispatcher sends the message to the desired address using the
appropriate transport.

A.3. Application Design Requirements

Within an application, there may be an explicit binding to a specific
SNMP message version, i.e., a specific Message Processing Model, and
to a specific Access Control Model, but there should be no reference
to any data defined by a specific Message Processing Model or Access
Control Model.

Within an application, there should be no reference to any specific
Security Model, or any data defined by a specific Security Model.

An application determines whether explicit or implicit access control
should be applied to the operation, and, if access control is needed,
which Access Control Model should be used.

An application has the responsibility to define any MIB module(s)
used to provide application-specific services.

Applications interact with the SNMP engine to initiate messages,
receive responses, receive asynchronous messages, and send responses.

A.3.1. Applications that Initiate Messages

Applications may request that the SNMP engine send messages
containing SNMP commands or notifications using the sendPdu primitive
as described in section 4.1.1.

If it is desired that a message be sent to multiple targets, it is
the responsibility of the application to provide the iteration.

The SNMP engine assumes necessary access control has been applied to
the PDU, and provides no access control services.

The SNMP engine looks at the "expectResponse" parameter, and if a
response is expected, then the appropriate information is cached such
that a later response can be associated to this message, and can then
be returned to the application. A sendPduHandle is returned to the
application so it can later correspond the response with this message
as well.

A.3.2. Applications that Receive Responses

The SNMP engine matches the incoming response messages to outstanding
messages sent by this SNMP engine, and forwards the response to the
associated application using the processResponsePdu primitive, as
described in section 4.1.4.

A.3.3. Applications that Receive Asynchronous Messages

When an SNMP engine receives a message that is not the response to a
request from this SNMP engine, it must determine to which application
the message should be given.

An Application that wishes to receive asynchronous messages registers
itself with the engine using the primitive registerContextEngineID as
described in section 4.1.5.

An Application that wishes to stop receiving asynchronous messages
should unregister itself with the SNMP engine using the primitive
unregisterContextEngineID as described in section 4.1.5.

Only one registration per combination of PDU type and contextEngineID
is permitted at the same time. Duplicate registrations are ignored.
An errorIndication will be returned to the application that attempts
to duplicate a registration.

All asynchronously received messages containing a registered
combination of PDU type and contextEngineID are sent to the
application which registered to support that combination.

The engine forwards the PDU to the registered application, using the
processPdu primitive, as described in section 4.1.2.

A.3.4. Applications that Send Responses

Request operations require responses. An application sends a
response via the returnResponsePdu primitive, as described in section
4.1.3.

The contextEngineID, contextName, securityModel, securityName,
securityLevel, and stateReference parameters are from the initial
processPdu primitive. The PDU and statusInformation are the results
of processing.

A.4. Access Control Model Design Requirements

An Access Control Model determines whether the specified securityName
is allowed to perform the requested operation on a specified managed
object. The Access Control Model specifies the rules by which access
control is determined.

The persistent data used for access control should be manageable
using SNMP, but the Access Control Model defines whether an
instantiation of the MIB is a conformance requirement.

The Access Control Model must provide the primitive isAccessAllowed.

B. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by
the Internet Society.


Comment on RFC 2571

Previous: RFC 2570 - Introduction to Version 3 of the Internet-standard Network Management Framework
Next: RFC 2572 - Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)

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