[RFC3410] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC3410, December 2002.
Appendix A
A. Guidelines for Model Designers
This appendix describes guidelines for designers of models which are
expected to fit into the architecture defined in this document.
SNMPv1 and SNMPv2c are two SNMP frameworks which use communities to
provide trivial authentication and access control. SNMPv1 and
SNMPv2c Frameworks can coexist with Frameworks designed according to
this architecture, and modified versions of SNMPv1 and SNMPv2c
Frameworks could be designed to meet the requirements of this
architecture, but this document does not provide guidelines for that
coexistence.
Within any subsystem model, there should be no reference to any
specific model of another subsystem, or to data defined by a specific
model of another subsystem.
Transfer of data between the subsystems is deliberately described as
a fixed set of abstract data elements and primitive functions which
can be overloaded to satisfy the needs of multiple model definitions.
Documents which define models to be used within this architecture
SHOULD use the standard primitives between subsystems, possibly
defining specific mechanisms for converting the abstract data
elements into model-usable formats. This constraint exists to allow
subsystem and model documents to be written recognizing common
borders of the subsystem and model. Vendors are not constrained to
recognize these borders in their implementations.
The architecture defines certain standard services to be provided
between subsystems, and the architecture defines abstract service
interfaces to request these services.
Each model definition for a subsystem SHOULD support the standard
service interfaces, but whether, or how, or how well, it performs the
service is dependent on the model definition.
A.1. Security Model Design Requirements
A.1.1. Threats
A document describing a Security Model MUST describe how the model
protects against the threats described under "Security Requirements
of this Architecture", section 1.4.
A.1.2. Security Processing
Received messages MUST be validated by a Model of the Security
Subsystem. Validation includes authentication and privacy processing
if needed, but it is explicitly allowed to send messages which do not
require authentication or privacy.
A received message contains a specified securityLevel to be used
during processing. All messages requiring privacy MUST also require
authentication.
A Security Model specifies rules by which authentication and privacy
are to be done. A model may define mechanisms to provide additional
security features, but the model definition is constrained to using
(possibly a subset of) the abstract data elements defined in this
document for transferring data between subsystems.
Each Security Model may allow multiple security protocols to be used
concurrently within an implementation of the model. Each Security
Model defines how to determine which protocol to use, given the
securityLevel and the security parameters relevant to the message.
Each Security Model, with its associated protocol(s) defines how the
sending/receiving entities are identified, and how secrets are
configured.
Authentication and Privacy protocols supported by Security Models are
uniquely identified using Object Identifiers. IETF standard
protocols for authentication or privacy should have an identifier
defined within the snmpAuthProtocols or the snmpPrivProtocols
subtrees. Enterprise specific protocol identifiers should be defined
within the enterprise subtree.
For privacy, the Security Model defines what portion of the message
is encrypted.
The persistent data used for security should be SNMP-manageable, but
the Security Model defines whether an instantiation of the MIB is a
conformance requirement.
Security Models are replaceable within the Security Subsystem.
Multiple Security Model implementations may exist concurrently within
an SNMP engine. The number of Security Models defined by the SNMP
community should remain small to promote interoperability.
A.1.3. Validate the security-stamp in a received message
A Message Processing Model requests that a Security Model:
- verifies that the message has not been altered,
- authenticates the identification of the principal for whom the
message was generated.
- decrypts the message if it was encrypted.
Additional requirements may be defined by the model, and additional
services may be provided by the model, but the model is constrained
to use the following primitives for transferring data between
subsystems. Implementations are not so constrained.
A Message Processing Model uses the processIncomingMsg primitive as
described in section 4.4.2.
A.1.4. Security MIBs
Each Security Model defines the MIB module(s) required for security
processing, including any MIB module(s) required for the security
protocol(s) supported. The MIB module(s) SHOULD be defined
concurrently with the procedures which use the MIB module(s). The
MIB module(s) are subject to normal access control rules.
The mapping between the model-dependent security ID and the
securityName MUST be able to be determined using SNMP, if the model-
dependent MIB is instantiated and if access control policy allows
access.
A.1.5. Cached Security Data
For each message received, the Security Model caches the state
information such that a Response message can be generated using the
same security information, even if the Local Configuration Datastore
is altered between the time of the incoming request and the outgoing
response.
A Message Processing Model has the responsibility for explicitly
releasing the cached data if such data is no longer needed. To
enable this, an abstract securityStateReference data element is
passed from the Security Model to the Message Processing Model.
The cached security data may be implicitly released via the
generation of a response, or explicitly released by using the
stateRelease primitive, as described in section 4.5.1.
A.2. Message Processing Model Design Requirements
An SNMP engine contains a Message Processing Subsystem which may
contain multiple Message Processing Models.
The Message Processing Model MUST always (conceptually) pass the
complete PDU, i.e., it never forwards less than the complete list of
varBinds.
A.2.1. Receiving an SNMP Message from the Network
Upon receipt of a message from the network, the Dispatcher in the
SNMP engine determines the version of the SNMP message and interacts
with the corresponding Message Processing Model to determine the
abstract data elements.
A Message Processing Model specifies the SNMP Message format it
supports and describes how to determine the values of the abstract
data elements (like msgID, msgMaxSize, msgFlags,
msgSecurityParameters, securityModel, securityLevel etc). A Message
Processing Model interacts with a Security Model to provide security
processing for the message using the processIncomingMsg primitive, as
described in section 4.4.2.
A.2.2. Sending an SNMP Message to the Network
The Dispatcher in the SNMP engine interacts with a Message Processing
Model to prepare an outgoing message. For that it uses the following
primitives:
- for requests and notifications: prepareOutgoingMessage, as
described in section 4.2.1.
- for response messages: prepareResponseMessage, as described in
section 4.2.2.
A Message Processing Model, when preparing an Outgoing SNMP Message,
interacts with a Security Model to secure the message. For that it
uses the following primitives:
- for requests and notifications: generateRequestMsg, as
described in section 4.4.1.
- for response messages: generateResponseMsg as described in
section 4.4.3.
Once the SNMP message is prepared by a Message Processing Model, the
Dispatcher sends the message to the desired address using the
appropriate transport.
A.3. Application Design Requirements
Within an application, there may be an explicit binding to a specific
SNMP message version, i.e., a specific Message Processing Model, and
to a specific Access Control Model, but there should be no reference
to any data defined by a specific Message Processing Model or Access
Control Model.
Within an application, there should be no reference to any specific
Security Model, or any data defined by a specific Security Model.
An application determines whether explicit or implicit access control
should be applied to the operation, and, if access control is needed,
which Access Control Model should be used.
An application has the responsibility to define any MIB module(s)
used to provide application-specific services.
Applications interact with the SNMP engine to initiate messages,
receive responses, receive asynchronous messages, and send responses.
A.3.1. Applications that Initiate Messages
Applications may request that the SNMP engine send messages
containing SNMP commands or notifications using the sendPdu primitive
as described in section 4.1.1.
If it is desired that a message be sent to multiple targets, it is
the responsibility of the application to provide the iteration.
The SNMP engine assumes necessary access control has been applied to
the PDU, and provides no access control services.
The SNMP engine looks at the "expectResponse" parameter, and if a
response is expected, then the appropriate information is cached such
that a later response can be associated to this message, and can then
be returned to the application. A sendPduHandle is returned to the
application so it can later correspond the response with this message
as well.
A.3.2. Applications that Receive Responses
The SNMP engine matches the incoming response messages to outstanding
messages sent by this SNMP engine, and forwards the response to the
associated application using the processResponsePdu primitive, as
described in section 4.1.4.
A.3.3. Applications that Receive Asynchronous Messages
When an SNMP engine receives a message that is not the response to a
request from this SNMP engine, it must determine to which application
the message should be given.
An Application that wishes to receive asynchronous messages registers
itself with the engine using the primitive registerContextEngineID as
described in section 4.1.5.
An Application that wishes to stop receiving asynchronous messages
should unregister itself with the SNMP engine using the primitive
unregisterContextEngineID as described in section 4.1.5.
Only one registration per combination of PDU type and contextEngineID
is permitted at the same time. Duplicate registrations are ignored.
An errorIndication will be returned to the application that attempts
to duplicate a registration.
All asynchronously received messages containing a registered
combination of PDU type and contextEngineID are sent to the
application which registered to support that combination.
The engine forwards the PDU to the registered application, using the
processPdu primitive, as described in section 4.1.2.
A.3.4. Applications that Send Responses
Request operations require responses. An application sends a
response via the returnResponsePdu primitive, as described in section
4.1.3.
The contextEngineID, contextName, securityModel, securityName,
securityLevel, and stateReference parameters are from the initial
processPdu primitive. The PDU and statusInformation are the results
of processing.
A.4. Access Control Model Design Requirements
An Access Control Model determines whether the specified securityName
is allowed to perform the requested operation on a specified managed
object. The Access Control Model specifies the rules by which access
control is determined.
The persistent data used for access control should be manageable
using SNMP, but the Access Control Model defines whether an
instantiation of the MIB is a conformance requirement.
The Access Control Model must provide the primitive isAccessAllowed.
Editors' Addresses
Bert Wijnen
Lucent Technologies
Schagen 33
3461 GL Linschoten
Netherlands
Phone: +31 348-680-485
EMail: bwijnen@lucent.com
David Harrington
Enterasys Networks
Post Office Box 5005
35 Industrial Way
Rochester, New Hampshire 03866-5005
USA
Phone: +1 603-337-2614
EMail: dbh@enterasys.com
Randy Presuhn
BMC Software, Inc.
2141 North First Street
San Jose, California 95131
USA
Phone: +1 408-546-1006
Fax: +1 408-965-0359
EMail: randy_presuhn@bmc.com
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