An Architecture for Describing
SNMP Management Frameworks
1 August 1997
D. Harrington
Cabletron Systems, Inc.
dbh@cabletron.com
B. Wijnen
IBM T.J. Watson Research
wijnen@vnet.ibm.com
<draft-ietf-snmpv3-next-gen-arch-04.txt>
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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 network management data.
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0. Issues
0.1. Resolved Issues
. contextEngineID in reportPDU = snmpEngineID of report generator
. returnResponsePDU - are all parameters needed? overrides allowed?
all parameters kept for future flexibility
overrides not supported by SNMPv3
. use of IN/OUT indicators in primitives accepted
. NT/Unix-like access control - can be defined as future model
. user-friendly names? yes, but with limits
. SnmpAdminString as index? yes, but restrict sizes
. need both MMS and maxSizeResponseScopedPDU? yes.
. synchronous vs. asynchronous primitives? synchronous preferred
. should we change MIB naming? no, it is acceptable
. is it ok that USM is bound to SNMPv3? while undesirable, it is
acceptable. A cleaner model may be defined in the future.
. should securityModel "any" be supported? for ACM use, not SNMPv3
. what defines SNMPv3? a document will be published after Munich
. Is an application-level handle needed for request/response matching?
yes. create sendPduhandle
. Is wildcard contextEngineID/pduType registration needed? No. This is
an internal interface, and wildcarding can be supported by an
implementation, but is not required in the standard.
. Should indices be integers or SnmpAdminStrings? SnmpAdminStrings
is the consensus.
. Should protocols be identified as OIDs or Integers? OIDs
. terminology:
securityLevel rather than LoS
msgXXXX to identify message fields in SNMPv3
. OID or Integer for auth/priv protocol identifiers
Consensus: use OID
. Is Glossary needed to describe primitive parameters, or is the
expanded template adequate for this purpose?
Consensus: Terms are basically all defined in section 3.
. state_reference releases
Consensus: documents checked; we think it is OK now
. new SnmpEngineID format rules to be discussed yet.
Consensus: Limit size to be 1..32
. needs changes to meet STDGUIDE guidelines
We think we're meeting them now
. we punted snmpEngineMaxMessageSize at 2nd interim because that
info travels in each SNMPv3 message. However, we may want to
re-introduce it so that SNMPv1/v2c managers can learn the value!!
Consensus: Nobody picked up on this, so it seems not needed.
. Do we need a mechanism to discover securityModels supported
Can be decided after Munich
. add a "Decision History" section (as an appendix?)
Can be decided after Munich
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0.1.1. Issues discussed at second Interim Meeting:
. A "readable" introduction supplement may be done after Munich.
. Applications are responsible for retries, but implementations may
differ.
. TCs should not be defined just to describe primitive parameters.
If they cannot be described adequately in text, they can be defined
in a Glossary. Avoid describing implementation details.
. Is SnmpAdminString appropriate for all strings, such as
securityIdentifier and context and group? These had different
sizes and semantics. size and semantics may be defined in
syntax and description of OBJECT
. AdminString has size (0..255); revisit for utf8 discussions
. securityModel #s - 00 for IETF standards; from v2* documents
. protocol IDs - integer or OID? voted 13-0 for OID.
. uniqueness of securityName
. mapping between principal and securityName is outside scope of WG.
. principals may have more than one securityName in an entity
. mappings may exist between many types of MDID and a single
securityName
. mappings may exist between different (model, Name) and the same
securityName by varying the model or the Name.
. the securityName and a MDID may be identical. This can be defined
by the Security Model.
(user,"public") may map to securityName "public"
. [securityName, securityModel] yields zero or one MDName, with
exceptions for backwards compatibility. The exception is defined
by the model, and the problems are the province of the model to
resolve.
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0.2. Change Log
[version 4.14]
. formatting
. pagination
[version 4.13]
. new acknowledgements
. updated references
. updated issues list
. ordered security, editors, acknowledgements, references sections
. checked line lengths
[version 4.12]
. cleanup
. added expectResponse to processIncomingMsg to address Levi-raised
concern
. acknowledgements
. MIB checked by SMICng
. post to snmpv3 mailing list
[version 4.11]
. Change Primitives between MP and SEC to try and address the issue
of architectural binding to message format.
. Added securityName and securityLevel to the returnResponsePdu
primitive so that architecturally it could be different for a
request and a response.
. Rename processMsg primitive to processIncomingMsg
[version 4.10]
. spell check
[version 4.9]
. editorial changes
. fix SnmpEngineID TC
. add a note to SnmpAdminString
. rename title of section 1.1
. expand description of Dispatcher a bit
[version 4.8]
. Added parameter pduVersion on primitives:
sendPdu
processPdu
returnResponsePdu
processResponsePdu
prepareDataElements
prepareOutgoingMessage
prepareResponseMessage
. Added parameter messageProcessingModel on the primitive:
processPdu
processResponsePdu
returnResponsePdu
. Removed messageProcessingModel parameter from primitives:
registerContextEngineID
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unregisterContextEngineID
. Renamed SNMP Version Multiplexer to Dispatcher
. Renamed Version Multiplexer to Message Multiplexer
. Renamed Application Multiplexer to PDU Dispatcher
. Rearranged some parameters in various Primitives so the sequence
of parameters is now more consistent.
[version 4.7]
. editorial cleanup
. changed asterisk text
. modified snmpv3 framework description to eliminate dependencies
. reorder 4.2.x to reflect transaction order
. changed SnmpEngineID size to 1..32
[version 4.6]
. Changes to use synchronous primitives where possible
. Changes to describe SNMP Version Multiplexer
. Remove (empty) glossary
. Redraw documentation figure
. Redraw Operational Overview Figure
. Remove old section 4 (Architectural Elements of Procedure)
These moved to the MP document into the SNMP Version Multiplexer
section.
. Move Overview of all primitives from Appendix to Section 4.
. Simplify Appendix A to just described Model Designer Guidelines
and refer back to section 4 for specific primitives
. Remove Appendix B (An Evolutionary Architecture - Design Goals)
. added design decision regarding security
. Included latest Snmp SecurityModel TC (as it was actually posted
to the SNMPv3 mailing list).
[version 4.5]
. start with <draft-ietf-snmpv3-next-gen-arch-03.txt>
. change vendor to implementor
. change LoS to securityLevel
. remove mention of enterprise
. change Internet Management Framework to SNMP Management Framework
. modify usage of "frameworks" to improve internal consistency.
. change Message Processing Abstract Service Interface to
Application Multiplexor
. change description of SNMP engine
. moved "one-to-one association" for entity and engine to discussion
of engine.
. changed distributing to dispatching
. added asterisks to indicate v3* items are also not required.
. changed "community access control" to "other access control"
. added TC for SnmpMessageProcessingModel
. modified Security Considerations
. modified acknowledgements
[version 4.4]
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. Fixed one error in the MIB (found with SMICng)
. Reformatted text for SnmpAdminString, no change in text.
. Changed text for SnmpEngineID.. this is still under discussion.
But this new text seems to be getting close to what we want.
. Added an issue w.r.t. snmpEngineMaxMessageSize
. adapt Primitive names and parameters to very latest (july 11) names
. removed blank lines before the .p page controls.
. publish as <draft-ietf-snmpv3-next-gen-arch-03.txt>
[version 4.3]
. some minor editing adjustments
[version 4.2]
. modify abstract so there is no requirement for one entity
to contain both a command generator and a notification receiver.
. modify Introduction list of entities which are meant to be
supported
. reorganized sections 1 through 4 for more consistency in contents.
. described section contents in Introduction:Target Audience
. move documentation descriptions to section 2
. rewrite section 4 to be more like a real elements of procedure.
. modified SnmpSecurityModel and SnmpEngineID definitions
. replaced MIB with Bert's replacement
. added Randy's TC for SnmpAdminString
. modified the example algorithm text for SnmpEngineID
. rewrote security considerations for brevity.
. modified "context" description
. moved "Threats" to Goals/Requirements
. eliminated snmpEngineMaxMessageSize object
. posted to snmpv3 (by DBH)
[version 4.1]
. Adopt "abstract" to new terminology
. Addressed all comments I (BW) made to DBH in an earlier email
. Changed Introduction section to new terminology
. changed wording for "implementation" to indicate it may contain
multiple models.
. Section 2. Started some wording on Goals and Design decisions
. Added the overview picture of a traditional agent and a
traditional manager. This is in section 2.
. Changed overview figure in section 3. to address the comments
by Dave Levi. It now lists the type of applications
. At various places ensure that text (easily) fits within 72
columns as required by RFC-editors Guidelines document.
. Section 2.3 (new section) has the documents set overview.
I verified the claims about standards. Not sure I worded the
SNMPv2 std correctly,. We'll hear it if we did it wrong.
. Section 2.4 (new section) gives overview of SNMP entities based
on modified Dave Levi figure. I (Bert) wonder however if it would
not be better to move it to after section 3.1.13
. Section 3. Added more figures... please let us know if you find
then useful and/or helpful. We could also move these back to
section 2 if such makes more sense.
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. Added a picture in section 3.2.
It also shows some of access control, so not sure it really fits
here, although it does map principal to model dependent security
ID to securityName
. Replace "<" with "is lower than" in section 3.4.3 which seems
better in a text document.
. Renamed section 4.1 to "SNMP engine processing" instead of
"The Message Processing Subsystem" because the transport
mappings, mpc multiplexor and such is done in ARCH document so
it is done "in general in the engine" and it passes a specific
message to a Message Processing Subsystem.
. "bulletized" some stuff in section 4.2 and 4.3.
Dave, this is just how I (Bert) like it better. Feel free to
undo it if you strongly disagree
. Section 4.3 changed "initiate a transaction" to "originate a
notification".
. Inserted title line for section 4.4 (I think it was missing)
I have named it "Information Model" in accordance with the change
I made (after Russ's comments) in the document figure to lump SMI,
TC and Conformance together.
. Inserted a title for section 4.5 named "Operational Model" to
get in sync with the the lumping together of ProtoOps and Transport
Mappings in document overview
. Renumber section 4.4.4 to 4,5,1 and added 4.5.2 to follow the
document overview figure. If we really want to follow it, then
maybe we should also reorder some of these sections. Like
Access Control seems specifically misplaced. So I decided to move
it before applications as section 4.3, so the 4.x above should
all be read as 4.x+1
. Removed size from SnmpEngineID TC... why did you limit it to
(0..2048). Did we not decide to leave it open?
. Should we not remove snmpEngineMaxMessageSize from the MIB.
That was agreed at 2nd interim. It travels in every message and so
seems to be useless. However, I think it could indeed still help
SNMPv1 or SNMPv2c managers.
. I kept your definitions of registration-points for auth and priv
protocols, but my recollection is that they would be completely
removed from ARCH and that it would all be done in SEC document.
. Modified Security Considerations. Was still talking about LPM.
. Appendix. I am still wondering if we need to use capitals for
things like "Security Model" "Subsystem" and such. This is only
an appendix... but we better be consistent, no? Anyway
I changed it so it is consistent (at least I tried).
. Appendix, renamed imf to snmpFramework
. Appendix, changed state_reference and state_release to
stateReference and stateRelease to be consistent with other names
for abstract data and primitives.
. A.2 changed MessageEngine to SNMP engine
. Fixed ASI primitives to be in sync with SEC document.
I also thought that our ARCH document-outline wanted to at least
have the primitives listed within the main body of the text, no?
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. Adapted send_pdu to sendPdu primitive as reconciled by Randy
In fact I made sure all primitives are in-line with current
agreement on names and parameters.
. Rename title of A.2.4 and A.2.5 so it fits on 1 line in contents
. I did not look at appendix B. That is your (DBH) specialty is it
not ? ;-).
. Quick simple spell check done with "spell" on AIX
[version 4.0]
. move section 7 - Model Requirements to an appendix
. move Section 3 - Design Goals to an appendix
. modified Section 5 - Naming
. remove "possibly multiple"
. moved Section 5 to Section 3
. change orangelets to applications
. modify description of applications
. change scopedPDU-MMS and PDU-MMS to maxSizeResponseScopedPDU
. change Scoped-PDU and ScopedPDU to scopedPDU (no dash, lower case S)
. change imfxxx to snmpFrameworkxxx
. change security-entity to principal
. change securityIdentity to securityName
. change MIID to securityName
. eliminate all reference to groupName or group
. LoS ordering noAuthNoPriv < authNoPriv < authPriv
. Los TC naming - noAuthNoPriv(1), authNoPriv(2), authPriv(3)
. remove TCs not used in MIBs - securityIdentity TC etc
. changed Message Processing and Control to Message Processing
. changed future tense to present tense
. eliminate messageEngine
. added/updated primitives
. addressed issues raised on the mailing list
[version 3.1]
. change securityIdentity to MIID
. write text to explain the differences between security-identities,
model-dependent identifiers, and model-independent identifiers.
. write text to explain distinction within the LCD of the security
data, the access control data, and the orangelet data.
. identify issues
. publish as <draft-ietf-snmpv3-next-gen-arch-02.txt>
[version 3.0]
. add section on threats for message security
. add section on threats for access control
. change application to orangelet
. remove references to F-Ts
. change securityIdentity to security-identity
. change securityCookie to securityIdentity
. the format of securityIdentity is defined by the model
. add securityModel to passed parameters as needed
. eliminate group from passed parameters
. remove unused IMPORTS
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. add glossary section with initial set of words to define
. differentiate the messageEngine from the contextEngine
. eliminate the term SNMPng
. rewrote 1.1. A Note on Terminology
. eliminated assumptions about SNMP processing always being
message related
. rewrote 4.x to reflect new thinking
. rewrote 5.x to reflect new thinking
. rewrote 6.x (the MIB) to reflect new thinking
. added MIB objects at this level (previously only TCs)
. rewrote 7.x
. sent to v3edit list
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1. Introduction
1.1. Overview
This document assumes an audience with varying levels of technical
understanding of SNMP.
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.
This document defines a vocabulary for describing SNMP Management
Frameworks, and an architecture for describing the major portions of
SNMP Management Frameworks.
Section 1 describes the purpose, goals, and design decisions of
this architecture.
Section 2 describes various types of documents which define SNMP
Frameworks, and how they fit into this architecture. It also provides
a minimal roadmap to the documents which have previously defined
SNMP frameworks.
Section 3 details the vocabulary of this architecture and its pieces.
This section is important for understanding the remaining sections,
and for understanding documents which are written to fit within this
architecture.
Section 4 describes the primitives used for the abstract service
interfaces between the various subsystems, models and applications
within this architecture.
Section 5 defines a collection of managed objects used to instrument
SNMP entities within this architecture.
Sections 6, 7, 8, and 9 are administrative in nature.
Appendix A contains guidelines for designers of Models which are
expected to fit within this architecture.
1.2. SNMP Management Systems
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;
- at least one SNMP entity containing command generator and/or
notification receiver applications; and,
- a management protocol, used to convey management information
between the SNMP entities.
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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 network 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 agent),
- command line driven SNMP entities with command generator and/or
notification receiver applications (traditionally called SNMP
command line managers),
- SNMP entities with command generator and/or notification
receiver, plus command responder and/or notification originator
applications (traditionally called SNMP mid-level managers or
dual-role entities),
- SNMP entities with command generator and/or notification
receiver and possibly other types of applications for managing
a potentially very large number of managed nodes (traditionally
called network management stations).
1.3. Goals of this Architecture
This architecture was driven by the following goals:
- Use existing materials as much as possible. It is heavily based
on previous work, informally known as SNMPv2u and SNMPv2*.
- Address the need for secure SET support, which is considered
the most important deficiency in SNMPv1 and SNMPv2c.
- Make it possible to move portions of the architecture forward
in the standards track, even if consensus has not been reached
on all pieces.
- Define an architecture that allows for longevity of the SNMP
Frameworks that have been and will be defined.
- Keep SNMP as simple as possible.
- Make it relatively inexpensive to deploy a minimal conformant
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.
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1.4. Security Requirements of this Architecture
Several of the classical threats to network protocols are applicable
to the network management problem and therefore would be applicable
to any Security Model used in an SNMP Management Framework. Other
threats are not applicable to the network management problem. This
section discusses principal threats, secondary threats, and threats
which are of lesser importance.
The principal threats against which any Security Model used within
this architecture SHOULD provide protection are:
Modification of Information
The modification threat is the danger that some unauthorized SNMP
entity may alter in-transit SNMP messages generated on behalf of
an authorized principal in such a way as to effect unauthorized
management operations, including falsifying the value of an object.
Masquerade
The masquerade threat is the danger that management operations
not authorized for some principal may be attempted by assuming
the identity of another principal that has the appropriate
authorizations.
Message Stream Modification
The SNMP protocol is typically based upon a connectionless
transport service which may operate over any subnetwork service.
The re-ordering, delay or replay of messages can and does occur
through the natural operation of many such subnetwork services.
The message stream modification threat is the danger that messages
may be maliciously re-ordered, delayed or replayed to an extent
which is greater than can occur through the natural operation of
a subnetwork service, in order to effect unauthorized management
operations.
Disclosure
The disclosure threat is the danger of eavesdropping on the
exchanges between SNMP engines. Protecting against this threat
may be required as a matter of local policy.
There are at least two threats against which a Security Model within
this architecture need not protect.
Denial of Service
A Security Model need not attempt to address the broad range of
attacks by which service on behalf of authorized users is denied.
Indeed, such denial-of-service attacks are in many cases
indistinguishable from the type of network failures with which any
viable network management protocol must cope as a matter of course.
Traffic Analysis
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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 designs decision 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.
- The Security Models in the Security Subsystem SHOULD protect
against the principal threats: modification of information,
masquerade, message stream modification and disclosure.
They do not need to protect against denial of service and
traffic analysis.
- Remote Configuration
The Security and Access Control Subsystems add a whole new set
of SNMP configuration parameters. The Security Subsystem also
requires frequent changes of secrets at the various SNMP
entities. To make this deployable in a large operational
environment, these SNMP parameters must be able to be remotely
configured.
- Controlled Complexity
It is recognized that simple managed devices want to keep the
resources used by SNMP to a minimum. At the same time, there
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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.
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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 | | | | |
| | | | | Dispatching | | | | |
| | +-----------------+ +-----------------+ +-----------------+ | |
| +----------------------------------------------------------------+ |
| |
| +----------------------------------------------------------------+ |
| | PDU Handling | |
| | +-----------------+ +-----------------+ +-----------------+ | |
| | | Protocol | | Applications | | Access | | |
| | | Operations | | | | Control | | |
| | +-----------------+ +-----------------+ +-----------------+ | |
| +----------------------------------------------------------------+ |
| |
| +----------------------------------------------------------------+ |
| | Information Model | |
| | +--------------+ +--------------+ +---------------+ | |
| | | Structure of | | Textual | | Conformance | | |
| | | Management | | Conventions | | Statements | | |
| | | Information | | | | | | |
| | +--------------+ +--------------+ +---------------+ | |
| +----------------------------------------------------------------+ |
| |
| +----------------------------------------------------------------+ |
| | MIBs | |
| | +-------------+ +-------------+ +----------+ +----------+ | |
| | | Standard v1 | | Standard v1 | | Historic | | Draft v2 | | |
| | | RFC1157 | | RFC1212 | | RFC14xx | | RFC19xx | | |
| | | format | | format | | format | | format | | |
| | +-------------+ +-------------+ +----------+ +----------+ | |
| +----------------------------------------------------------------+ |
| |
+--------------------------------------------------------------------+
Those marked with an asterisk (*) are expected to be written in the
future. Each of these documents may be replaced or supplemented.
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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 "roadmap" should be maintained in
a document separate from the standards documents themselves.
2.2. Applicability Statement
SNMP is used in networks that vary widely in size and complexity,
by organizations that vary widely in their requirements of network
management. Some models will be designed to address specific problems
of network 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.
It would be very difficult to document all the possible interactions
between a model and all other previously existing models while in the
process of developing a new model.
Coexistence documents are therefore expected to be prepared separately
from model definition documents, to describe and resolve interaction
anomalies between a model definition and one or more other model
definitions.
Additionally, recommendations for transitions between models may
also be described, either in a coexistence document or in a separate
document.
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2.4. Transport Mappings
SNMP messages are sent over various transports. It is the purpose of
Transport Mapping documents to define how the mapping between SNMP
and the transport is done.
2.5. Message Processing
A Message Processing Model document defines a message format, which is
typically identified by a version field in an SNMP message header.
The document may also define a MIB module for use in message
processing and for instrumentation of version-specific interactions.
An SNMP engine includes one or more Message Processing Models, and thus
may support sending and receiving multiple versions of SNMP messages.
2.6. Security
Some environments require secure protocol interactions. Security is
normally applied at two different stages:
- in the transmission/receipt of messages, and
- in the processing of the contents of messages.
For purposes of this document, "security" refers to message-level
security; "access control" refers to the security applied to protocol
operations.
Authentication, encryption, and timeliness checking are common
functions of message level security.
A security document describes a Security Model, the threats against
which the model protects, the goals of the Security Model, the
protocols which it uses to meet those goals, and it may define a MIB
module to describe the data used during processing, and to allow the
remote configuration of message-level security parameters, such as
passwords.
An SNMP engine may support multiple Security Models concurrently.
2.7. Access Control
During processing, it may be required to control access to certain
instrumentation for certain operations. An Access Control Model
determines whether access to an object should be allowed. The
mechanism by which access control is checked is defined by the
Access Control Model.
An Access Control Model document defines the mechanisms used to
determine whether access to a managed object should be allowed,
and may define a MIB module used during processing, and to allow
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the remote configuration of access control policies.
2.8. Protocol Operations
SNMP messages encapsulate an SNMP Protocol Data Unit (PDU). It is the
purpose of a Protocol Operations document to define the operations
of the protocol with respect to the processing of the PDUs.
An application document defines which Protocol Operations documents
are supported by the application.
2.9. Applications
An SNMP entity normally includes a number of applications.
Applications use the services of an SNMP engine to accomplish
specific tasks. They coordinate the processing of management
information operations, and may use SNMP messages to communicate
with other SNMP entities.
Applications documents describe the purpose of an application, the
services required of the associated SNMP engine, and the protocol
operations and informational model that the application uses to
perform network management operations.
An application document defines which set of documents are used to
specifically define the structure of management information, textual
conventions, conformance requirements, and operations supported by
the application.
2.10. Structure of Management Information
Management information is viewed as a collection of managed objects,
residing in a virtual information store, termed the Management
Information Base (MIB). Collections of related objects are defined
in MIB modules.
It is the purpose of a Structure of Management Information document
to establish the syntax for defining objects, modules, and other
elements of managed information.
2.11. Textual Conventions
When designing a MIB module, it is often useful to define new types
similar to those defined in the SMI, but with more precise semantics,
or which have special semantics associated with them. These newly
defined types are termed textual conventions, and may defined in
separate documents, or within a MIB module.
2.12. Conformance Statements
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It may be useful to define the acceptable lower-bounds of
implementation, along with the actual level of implementation
achieved. It is the purpose of Conformance Statements to define
the notation used for these purposes.
2.13. Management Information Base Modules
MIB documents describe collections of managed objects which instrument
some aspect of a managed node.
2.13.1. SNMP Instrumentation MIBs
An SNMP MIB document may define a collection of managed objects which
instrument the SNMP protocol itself. In addition, MIB modules may be
defined within the documents which describe portions of the SNMP
architecture, such as the documents for Message processing Models,
Security Models, etc. for the purpose of instrumenting those Models,
and for the purpose of allowing remote configuration of the Model.
2.14. SNMP Framework Documents
This architecture is designed to allow an orderly evolution of
portions of SNMP Frameworks.
Throughout the rest of this document, the term "subsystem" refers
to an abstract and incomplete specification of a portion of
a Framework, that is further refined by a model specification.
A "model" describes a specific design of a subsystem, defining
additional constraints and rules for conformance to the model.
A model is sufficiently detailed to make it possible to implement
the specification.
An "implementation" is an instantiation of a subsystem, conforming
to one or more specific models.
SNMP version 1 (SNMPv1), is the original Internet-standard Network
Management Framework, as described in RFCs 1155, 1157, and 1212.
SNMP version 2 (SNMPv2), is the SNMPv2 Framework as derived from the
SNMPv1 Framework. It is described in RFCs 1902-1907. SNMPv2 has no
message definition.
Community-based SNMP version 2 (SNMPv2c), is an experimental SNMP
Framework which supplements the SNMPv2 Framework, as described in
RFC1901. It adds the SNMPv2c message format 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,
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- Security for Messages, and
- Access Control.
Other SNMP Frameworks, i.e. other configurations of implemented
subsystems, are expected to also be consistent with this architecture.
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2.15. Operational Overview
The following pictures show two communicating SNMP entities using
the conceptual modularity described by this SNMP Architecture.
The pictures represent SNMP entities that have traditionally been
called SNMP manager and SNMP agent respectively.
* 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 |
+------------------------------+
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+------------------------------+
| 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 | | |
| | | | | +------------+ | | | Model | | |
| | PDU Dispatcher | | +->| otherMP * |<--->| +-------------+ | |
| +-------------------+ | +------------+ | | | |
| ^ +---------------------+ +-----------------+ |
| | |
| v |
| +-------+-------------------------+----------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY * | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+--------------------------------------------------------------------+
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3. 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
The following picture shows detail about an SNMP entity and how
components within it are named.
+--------------------------------------------------------------------+
| SNMP entity |
| |
| +--------------------------------------------------------------+ |
| | SNMP engine (identified by snmpEngineID) | |
| | | |
| | +-------------+ +------------+ +-----------+ +-----------+ | |
| | | | | | | | | | | |
| | | Dispatcher | | Message | | Security | | Access | | |
| | | | | Processing | | Subsystem | | Control | | |
| | | | | Subsystem | | | | Subsystem | | |
| | | | | | | | | | | |
| | +-------------+ +------------+ +-----------+ +-----------+ | |
| | | |
| +--------------------------------------------------------------+ |
| |
| +--------------------------------------------------------------+ |
| | Application(s) | |
| | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | Command | | Notification | | Proxy | | |
| | | Generator | | Receiver | | Forwarder | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | Command | | Notification | | Other | | |
| | | Responder | | Originator | | | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | |
| +--------------------------------------------------------------+ |
| |
+--------------------------------------------------------------------+
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3.1.1. SNMP entity
An SNMP entity is an implementation of this architecture. Each such
SNMP entity consists of an SNMP engine and one or more associated
applications.
3.1.2. 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.
3.1.3. snmpEngineID
Within an administrative domain, an snmpEngineID is the unique
and unambiguous identifier of an SNMP engine. Since there is a
one-to-one association between SNMP engines and SNMP entities,
it also uniquely and unambiguously identifies the SNMP entity.
3.1.4. 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 interact
with the corresponding Message Processing Model,
- providing an abstract interface to SNMP applications for
dispatching a PDU to an application.
- providing an abstract interface for SNMP applications that
allows them to send a PDU to a remote SNMP entity.
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3.1.5. 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 picture.
* 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 | |
| | | | | | | | | |
| +------------+ +------------+ +------------+ +------------+ |
| |
+------------------------------------------------------------------+
3.1.6. 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 messages.
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3.1.7. 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 next picture.
* One or more Security Models may be present.
+------------------------------------------------------------------+
| |
| Security Subsystem |
| |
| +----------------+ +-----------------+ +-------------------+ |
| | * | | * | | * | |
| | User-Based | | Other | | Other | |
| | Security | | Security | | Security | |
| | Model | | Model | | Model | |
| | | | | | | |
| +----------------+ +-----------------+ +-------------------+ |
| |
+------------------------------------------------------------------+
3.1.8. Security Model
A Security Model defines the threats against which it protects,
the goals of its services, and the security protocols used to provide
security services such as authentication and privacy.
3.1.9. Security Protocol
A Security Protocol defines the mechanisms, procedures, and MIB
data used to provide a security service such as authentication
or privacy.
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3.1.10. 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 | |
| | | | | | | |
| +---------------+ +-----------------+ +------------------+ |
| |
+------------------------------------------------------------------+
3.1.11. Access Control Model
An Access Control Model defines a particular access decision function
in order to support decisions regarding access rights.
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3.1.12. 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.
3.1.13. 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.
3.1.14. 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.
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3.2. The Naming of Identities
principal <---------------------------------+
|
+-------------------------------------|-----+
| SNMP engine | |
| | |
| +-----------------------+ | |
| | Security Model | | |
| | +-------------+ | | |
wire | | | Model | +------------+--+ |
<----------->| Dependent |<-->| | securityName| |
| | | Security ID | +---------------+ |
| | +-------------+ | |
| | | |
| +-----------------------+ |
| |
| |
+-------------------------------------------+
3.2.1. Principal
A principal is the "who" on whose behalf services are provided
or processing takes place.
A principal can be, among other things, an individual acting in
a particular role; a set of individuals, with each acting in a
particular role; an application; or a set of applications;
and combinations thereof.
3.2.2. securityName
A securityName is a human readable string representing a principal.
It has a model independent format, and can be used outside a
particular Security Model.
3.2.3. Model dependent security ID
A model dependent security ID is the model specific representation
of a securityName within a particular Security Model.
Model dependent security IDs may or may not be human readable, and
have a model dependent syntax. Examples include community names,
user names, and parties.
The transformation of model dependent security IDs into securityNames
and vice versa is the responsibility of the relevant Security Model.
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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. Such a Command Responder application uses a contextEngineID
equal to the snmpEngineID of its associated SNMP engine.
+-----------------------------------------------------------------+
| SNMP entity (identified by snmpEngineID, example: abcd) |
| |
| +------------------------------------------------------------+ |
| | SNMP engine (identified by snmpEngineID) | |
| | | |
| | +-------------+ +------------+ +-----------+ +-----------+ | |
| | | | | | | | | | | |
| | | Dispatcher | | Message | | Security | | Access | | |
| | | | | Processing | | Subsystem | | Control | | |
| | | | | Subsystem | | | | Subsystem | | |
| | | | | | | | | | | |
| | +-------------+ +------------+ +-----------+ +-----------+ | |
| | | |
| +------------------------------------------------------------+ |
| |
| +------------------------------------------------------------+ |
| | Command Responder Application | |
| | (contextEngineID, example: abcd) | |
| | | |
| | example contextNames: | |
| | | |
| | "bridge1" "bridge2" "" (default) | |
| | --------- --------- ------------ | |
| | | | | | |
| +------|------------------|-------------------|--------------+ |
| | | | |
| +------|------------------|-------------------|--------------+ |
| | MIB | instrumentation | | | |
| | +---v------------+ +---v------------+ +----v-----------+ | |
| | | context | | context | | context | | |
| | | | | | | | | |
| | | +------------+ | | +------------+ | | +------------+ | | |
| | | | bridge MIB | | | | bridge MIB | | | | other MIB | | | |
| | | +------------+ | | +------------+ | | +------------+ | | |
| | | | | | | | | |
| | | | | | | +------------+ | | |
| | | | | | | | some MIB | | | |
| | | | | | | +------------+ | | |
| | | | | | | | | |
+-----------------------------------------------------------------+
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3.3.1. An SNMP Context
An SNMP context, or just "context" for short, is a collection of
management information accessible by an SNMP entity. An item of
management information may exist in more than one context. An SNMP
engine potentially has access to many contexts.
Typically, there are many instances of each managed object type within
a management domain. For simplicity, the method for identifying
instances specified by the MIB module does not allow each instance to
be distinguished amongst the set of all instances within a management
domain; rather, it allows each instance to be identified only within
some scope or "context", where there are multiple such contexts within
the management domain. Often, a context is a physical device, or
perhaps, a logical device, although a context can also encompass
multiple devices, or a subset of a single device, or even a subset of
multiple devices, but a context is always defined as a subset of a
single SNMP entity. Thus, in order to identify an individual item of
management information within the management domain, its contextName
and contextEngineID must be identified in addition to its object type
and its instance.
For example, the managed object type ifDescr [RFC1573], is defined as
the description of a network interface. To identify the description
of device-X's first network interface, four pieces of information are
needed: the snmpEngineID of the SNMP entity which provides access to
the management information at device-X, the contextName (device-X),
the managed object type (ifDescr), and the instance ("1").
Each context has (at least) one unique identification within the
management domain. The same item of management information can exist
in multiple contexts. So, an item of management information can have
multiple unique identifications, either because it exists in multiple
contexts, and/or because each such context has multiple unique
identifications.
The combination of a contextEngineID and a contextName unambiguously
identifies a context within an administrative domain.
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.
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3.3.4. scopedPDU
A scopedPDU is a block of data containing a contextEngineID,
a contextName, and a PDU.
The PDU is an SNMP Protocol Data Unit containing information
named in the context which is unambiguously identified within
an administrative domain by the combination of the contextEngineID
and the contextName. See, for example, RFC1905 for more information
about SNMP PDUs.
3.4. Other Constructs
3.4.1. maxSizeResponseScopedPDU
The maxSizeResponseScopedPDU is the maximum size of a scopedPDU to
be included in a response message, making allowance for the message
header.
3.4.2. Local Configuration Datastore
The subsystems, models, and applications within an SNMP entity may
need to retain their own sets of configuration information.
Portions of the configuration information may be accessible as
managed objects.
The collection of these sets of information is referred to
as an entity's Local Configuration Datastore (LCD).
3.4.3. securityLevel
This architecture recognizes three levels of security:
- without authentication and without privacy (noAuthNoPriv)
- with authentication but without privacy (authNoPriv)
- with authentication and with privacy (authPriv)
These three values are ordered such that noAuthNoPriv is less than
authNoPriv and authNoPriv is less than authPriv.
Every message has an associated securityLevel. All Subsystems (Message
Processing, Security, Access Control) and applications are required
to either supply a value of securityLevel or to abide by the supplied
value of securityLevel while processing the message and its contents.
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4. Abstract Service Interfaces.
Abstract service interfaces have been defined to describe the
conceptual interfaces between the various subsystems within an SNMP
entity.
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. Common Primitives
These primitive(s) are provided by multiple Subsystems.
4.1.1. 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
)
4.2. 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.
4.2.1. 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
)
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4.2.2. 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
4.2.3. Generate Outgoing Response
The PDU Dispatcher provides the following primitive for an application
to return an SNMP Response PDU to the PDU Dispatcher:
returnResponsePdu(
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model in use
IN securityName -- on behalf of this principal
IN securityLevel -- same as on incoming request
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size of the Response PDU
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
) -- error counter OID/value if error
4.2.4. 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
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IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN statusInformation -- success or errorIndication
IN sendPduHandle -- handle from sendPDU
)
4.2.5. Registering Responsibility for Handling SNMP PDUs.
Applications can register/unregister responsibility for a specific
contextEngineID, for specific pduTypes, with the PDU Dispatcher
according to these primitives:
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
)
4.3. 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.
4.3.1. Prepare an 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
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OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)
4.3.2. 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 of the Response PDU
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)
4.3.3. 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
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OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for a possible Response
)
4.4. 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
)
4.5. Security Subsystem Primitives
The Message Processing Subsystem is the typical client of the services
of the Security Subsystem.
4.5.1. 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
)
4.5.2. Process Incoming Message
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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 of the Response PDU
OUT securityStateReference -- reference to security state
) -- information, needed for response
4.5.3. 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
)
4.6. User Based Security Model Internal Primitives
4.6.1. User-based Security Model Primitives for Authentication
The User-based Security Model provides the following internal
primitives to pass data back and forth between the Security Model
itself and the authentication service:
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statusInformation =
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)
statusInformation =
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)
4.6.2. User-based Security Model Primitives for Privacy
The User-based Security Model provides the following internal
primitives to pass data back and forth between the Security Model
itself and the privacy service:
statusInformation =
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)
statusInformation =
decryptData(
IN decryptKey -- secret key for decrypting
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
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4.7. Scenario Diagrams
4.7.1. Command Generator or Notification Originator Application
This diagram shows how a Command Generator or Notification Originator
application requests that a PDU be sent, and how the response is
returned (asynchronously) to that application.
Command Dispatcher Message Security
Generator | Processing Model
| | Model |
| | | |
| sendPdu | | |
|------------------->| | |
| | prepareOutgoingMessage | |
: |------------------------->| |
: | | generateRequestMsg |
: | |-------------------->|
: | | |
: | |<--------------------|
: | | |
: |<-------------------------| |
: | | |
: |------------------+ | |
: | Send SNMP | | |
: | Request Message | | |
: | to Network | | |
: | v | |
: : : : :
: : : : :
: : : : :
: | | | |
: | | | |
: | Receive SNMP | | |
: | Response Message | | |
: | from Network | | |
: |<-----------------+ | |
: | | |
: | prepareDataElements | |
: |------------------------->| |
: | | processIncomingMsg |
: | |-------------------->|
: | | |
: | |<--------------------|
: | | |
: |<-------------------------| |
| processResponsePdu | | |
|<-------------------| | |
| | | |
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4.7.2. 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 | |
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5. Definition of Managed Objects for SNMP Management Frameworks
SNMP-FRAMEWORK-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY,
snmpModules, Unsigned32, Integer32 FROM SNMPv2-SMI
TEXTUAL-CONVENTION FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF;
snmpFrameworkMIB MODULE-IDENTITY
LAST-UPDATED "9707260000Z" -- 26 July 1997, midnight
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-email: snmpv3@tis.com
Subscribe: majordomo@tis.com
In message body: subscribe snmpv3
Chair: Russ Mundy
Trusted Information Systems
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
MailStop: Durham
35 Industrial Way
Rochester NH 03867-5005
USA
email: dbh@cabletron.com
phone: +1-603-337-7357
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"
::= { snmpModules 7 } -- DBH: check if this number is indeed OK
-- Textual Conventions used in the SNMP Management Architecture ***
SnmpEngineID ::= TEXTUAL-CONVENTION
STATUS current
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DESCRIPTION "An SNMP engine's administratively-unique identifier.
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.
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 network 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 network management
private enterprise number as assigned by the
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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(1..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
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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.
The eight bits allow a maximum of 255 (256-1 reserved)
standards based Security Models. Similarly, they
allow 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 'none'
1 reserved for SNMPv1
2 reserved for SNMPv2c
3 User-Base Security Model (USM)
255 reserved for 'any'
"
SYNTAX INTEGER(0..2147483647)
SnmpMessageProcessingModel ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "An identifier that uniquely identifies a Message
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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.
The eight bits allow a maximum of 256 standards based
Message Processing Models. Similarly, they allow a
maximum 256 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 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
messageProcessingModel defined for use with SNMP.
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They are as follows:
0 reserved for SNMPv1
1 reserved for SNMPv2c
2 reserved for SNMPv2u
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
character encoding scheme described in RFC 2044.
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.
The use of control codes should be avoided.
For code points not directly supported by user
interface hardware or software, an alternative means
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of entry and display, such as hexadecimal, may be
provided.
For information encoded in 7-bit US-ASCII, the UTF-8
representation is identical to the US-ASCII 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
sub-identifiers for any object instance do not exceed
the limit of 128, as defined by [RFC1905].
"
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 Unsigned32 -- (1..4294967295)
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times that the SNMP engine has
(re-)initialized itself since its initial
configuration.
"
::= { snmpEngine 2 }
snmpEngineTime OBJECT-TYPE
SYNTAX Integer32 (0..2147483647)
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of seconds since the SNMP engine last
incremented the snmpEngineBoots object.
"
::= { snmpEngine 3 }
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-- 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
}
STATUS current
DESCRIPTION "A collection of objects for identifying and
determining the configuration and current timeliness
values of an SNMP engine.
"
::= { snmpFrameworkMIBGroups 1 }
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END
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6. Security Considerations
This document describes how an implementation can include a Security
Model to protect network 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.
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7. Editor's Addresses
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
Co-editor Dave Harrington
Cabletron Systems, Inc
postal: Post Office Box 5005
MailStop: Durham
35 Industrial Way
Rochester NH 03867-5005
USA
email: dbh@cabletron.com
phone: +1-603-337-7357
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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:
Dave Battle (SNMP Research, Inc.)
Uri Blumenthal (IBM T.J. Watson Research Center)
Jeff Case (SNMP Research, Inc.)
John Curran (BBN)
T. Max Devlin (Hi-TECH Connections)
John Flick (Hewlett Packard)
David Harrington (Cabletron Systems Inc.)
N.C. Hien (IBM T.J. Watson Research Center)
Dave Levi (SNMP Research, Inc.)
Louis A Mamakos (UUNET Technologies Inc.)
Paul Meyer (Secure Computing Corporation)
Keith McCloghrie (Cisco Systems)
Russ Mundy (Trusted Information Systems, Inc.)
Bob Natale (ACE*COMM Corporation)
Mike O'Dell (UUNET Technologies Inc.)
Dave Perkins (DeskTalk)
Peter Polkinghorne (Brunel University)
Randy Presuhn (BMC Software, Inc.)
David Reid (SNMP Research, Inc.)
Shawn Routhier (Epilogue)
Juergen Schoenwaelder (TU Braunschweig)
Bob Stewart (Cisco Systems)
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)
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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.)
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9. 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,
University of Tennessee at Knoxville, Performance Systems s
International, Performance International, and the MIT Laboratory
for Computer Science, 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.
[RFC1902] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M., and S., Waldbusser, "Structure of Management
Information for Version 2 of the Simple Network Management
Protocol (SNMPv2)", RFC 1902, January 1996.
[RFC1903] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M.,
and S. Waldbusser, "Textual Conventions for Version 2 of the Simple
Network Management Protocol (SNMPv2)", RFC 1903, January 1996.
[RFC1904] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M.,
and S., Waldbusser, "Conformance Statements for Version 2 of the
Simple Network Management Protocol (SNMPv2)", RFC 1904,
January 1996.
[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
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Framework", RFC 1908, January 1996.
[RFC1909] McCloghrie, K., Editor, "An Administrative Infrastructure
for SNMPv2", RFC1909, February 1996
[RFC1910] Waters, G., Editor, "User-based Security Model for SNMPv2",
RFC1910, February 1996
[SNMP-MPD] The SNMPv3 Working Group, Case, J., Harrington, D.,
Wijnen, B., "Message Processing and Dispatching for the Simple
Network Management Protocol (SNMP)",
draft-ietf-snmpv3-mpc-03.txt, August 1997
[SNMP-USM] The SNMPv3 Working Group, Blumenthal, U., Wijnen, B.,
"The User-Based Security Model for Version 3 of the Simple
Network Management Protocol (SNMPv3)",
draft-ietf-snmpv3-usm-01.txt, August 1997.
[SNMP-ACM] The SNMPv3 Working Group, Wijnen, B., Presuhn, R.,
McCloghrie, K., "View-based Access Control Model for the Simple
Network Management Protocol (SNMP)",
draft-ietf-snmpv3-acm-02.txt, August 1997.
[SNMP-APPL] The SNMPv3 Working Group, Levi, D. B., Meyer, P.,
Stewart, B., "SNMPv3 Applications",
<draft-ietf-snmpv3-appl-01.txt>, August 1997
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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
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if needed, but it is explicitly allowed to send messages which do not
require authentication or privacy.
A received message contains a specified Level of Security 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
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subsystems. Implementations are not so constrained.
A Message Processing Model uses the processMsg primitive as
described in section 4.5.
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 4.1.
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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 processMsg primitive, as described in section 4.5.
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 4.4
- for response messages:
prepareResponseMessage, as described in section 4.4
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 4.5.
- for response messages:
generateResponseMsg as described in section 4.5.
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
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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 4.2.
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 4.2.
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.
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An Application that wishes to receive asynchronous messages registers
itself with the engine using the primitive registerContextEngineID
as described in section 4.2.
An Application that wishes to stop receiving asynchronous messages
should unregister itself with the SNMP engine using the primitive
unregisterContextEngineID as described in section 4.2.
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 4.2.
A.3.4. Applications that Send Responses
Request operations require responses. An application sends
a response via the returnResponsePdu primitive, as described in
section 4.2.
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
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Table of Contents
0. Issues 2
0.1. Resolved Issues 2
0.1.1. Issues discussed at second Interim Meeting: 3
0.2. Change Log 4
1. Introduction 10
1.1. Overview 10
1.2. SNMP Management Systems 10
1.3. Goals of this Architecture 11
1.4. Security Requirements of this Architecture 12
1.5. Design Decisions 13
2. Documentation Overview 15
2.1. Document Roadmap 16
2.2. Applicability Statement 16
2.3. Coexistence and Transition 16
2.4. Transport Mappings 17
2.5. Message Processing 17
2.6. Security 17
2.7. Access Control 17
2.8. Protocol Operations 18
2.9. Applications 18
2.10. Structure of Management Information 18
2.11. Textual Conventions 18
2.12. Conformance Statements 18
2.13. Management Information Base Modules 19
2.13.1. SNMP Instrumentation MIBs 19
2.14. SNMP Framework Documents 19
2.15. Operational Overview 21
3. Elements of the Architecture 23
3.1. The Naming of Entities 23
3.1.1. SNMP entity 24
3.1.2. SNMP engine 24
3.1.3. snmpEngineID 24
3.1.4. Dispatcher 24
3.1.5. Message Processing Subsystem 25
3.1.6. Message Processing Model 25
3.1.7. Security Subsystem 26
3.1.8. Security Model 26
3.1.9. Security Protocol 26
3.1.10. Access Control Subsystem 27
3.1.11. Access Control Model 27
3.1.12. Applications 28
3.1.13. SNMP Agent 28
3.1.14. SNMP Manager 28
3.2. The Naming of Identities 29
3.2.1. Principal 29
3.2.2. securityName 29
3.2.3. Model dependent security ID 29
3.3. The Naming of Management Information 30
3.3.1. An SNMP Context 31
3.3.2. contextEngineID 31
3.3.3. contextName 31
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3.3.4. scopedPDU 32
3.4. Other Constructs 32
3.4.1. maxSizeResponseScopedPDU 32
3.4.2. Local Configuration Datastore 32
3.4.3. securityLevel 32
4. Abstract Service Interfaces. 33
4.1. Common Primitives 33
4.1.1. Release State Reference Information 33
4.2. Dispatcher Primitives 33
4.2.1. Generate Outgoing Request or Notification 33
4.2.2. Process Incoming Request or Notification PDU 34
4.2.3. Generate Outgoing Response 34
4.2.4. Process Incoming Response PDU 34
4.2.5. Registering Responsibility for Handling SNMP PDUs. 35
4.3. Message Processing Subsystem Primitives 35
4.3.1. Prepare an Outgoing SNMP Request or Notification Message 35
4.3.2. Prepare an Outgoing SNMP Response Message 36
4.3.3. Prepare Data Elements from an Incoming SNMP Message 36
4.4. Access Control Subsystem Primitives 37
4.5. Security Subsystem Primitives 37
4.5.1. Generate a Request or Notification Message 37
4.5.2. Process Incoming Message 37
4.5.3. Generate a Response Message 38
4.6. User Based Security Model Internal Primitives 38
4.6.1. User-based Security Model Primitives for Authentication 38
4.6.2. User-based Security Model Primitives for Privacy 39
4.7. Scenario Diagrams 40
4.7.1. Command Generator or Notification Originator Application 40
4.7.2. Scenario Diagram for a Command Responder Application 41
5. Definition of Managed Objects for SNMP Management Frameworks 42
6. Security Considerations 51
7. Editor's Addresses 52
8. Acknowledgements 53
9. References 55
A. Guidelines for Model Designers 57
A.1. Security Model Design Requirements 57
A.1.1. Threats 57
A.1.2. Security Processing 57
A.1.3. Validate the security-stamp in a received message 58
A.1.4. Security MIBs 59
A.1.5. Cached Security Data 59
A.2. Message Processing Model Design Requirements 60
A.2.1. Receiving an SNMP Message from the Network 60
A.2.2. Sending an SNMP Message to the Network 60
A.3. Application Design Requirements 60
A.3.1. Applications that Initiate Messages 61
A.3.2. Applications that Receive Responses 61
A.3.3. Applications that Receive Asynchronous Messages 61
A.3.4. Applications that Send Responses 62
A.4. Access Control Model Design Requirements 62
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