One document matched: draft-ietf-isms-tmsm-06.txt
Differences from draft-ietf-isms-tmsm-05.txt
Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Updates: 3411,3412,3414,3417 J. Schoenwaelder
(if approved) International University Bremen
Intended status: Standards Track February 5, 2007
Expires: August 9, 2007
Transport Subsystem for the Simple Network Management Protocol (SNMP)
draft-ietf-isms-tmsm-06
Status of This Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document defines a Transport Subsystem, extending the Simple
Network Management Protocol (SNMP) architecture defined in RFC 3411.
This document defines a subsystem to contain Transport Models,
comparable to other subsystems in the RFC3411 architecture. As work
is being done to expand the transport to include secure transport
such as SSH and TLS, using a subsystem will enable consistent design
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and modularity of such Transport Models. This document identifies
and describes some key aspects that need to be considered for any
Transport Model for SNMP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Where this Extension Fits . . . . . . . . . . . . . . . . 4
1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . . 6
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements of a Transport Model . . . . . . . . . . . . . . 8
3.1. Message Security Requirements . . . . . . . . . . . . . . 8
3.1.1. Security Protocol Requirements . . . . . . . . . . . . 8
3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Architectural Modularity Requirements . . . . . . . . 9
3.2.2. Access Control Requirements . . . . . . . . . . . . . 13
3.2.3. Security Parameter Passing Requirements . . . . . . . 14
3.2.4. Separation of Authentication and Authorization . . . . 15
3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 16
3.3.1. Session Establishment Requirements . . . . . . . . . . 17
3.3.2. Session Maintenance Requirements . . . . . . . . . . . 18
3.3.3. Message security versus session security . . . . . . . 18
4. Scenario Diagrams for the Transport Subsystem . . . . . . . . 19
4.1. Command Generator or Notification Originator . . . . . . . 19
4.2. Command Responder . . . . . . . . . . . . . . . . . . . . 21
5. Cached Information and References . . . . . . . . . . . . . . 22
5.1. securityStateReference . . . . . . . . . . . . . . . . . . 23
5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 24
6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 24
6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 24
6.2. Other Outgoing ASIs . . . . . . . . . . . . . . . . . . . 25
6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 26
6.4. Other Incoming ASIs . . . . . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Parameter Table . . . . . . . . . . . . . . . . . . . 31
A.1. ParameterList.csv . . . . . . . . . . . . . . . . . . . . 31
Appendix B. Why tmStateReference? . . . . . . . . . . . . . . . . 33
B.1. Define an Abstract Service Interface . . . . . . . . . . . 33
B.2. Using an Encapsulating Header . . . . . . . . . . . . . . 33
B.3. Modifying Existing Fields in an SNMP Message . . . . . . . 34
B.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 34
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 34
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
This document defines a Transport Subsystem, extending the Simple
Network Management Protocol (SNMP) architecture defined in [RFC3411].
This document identifies and describes some key aspects that need to
be considered for any Transport Model for SNMP.
1.1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [RFC3410].
1.2. Where this Extension Fits
It is expected that readers of this document will have read RFC3410
and RFC3411, and have a general understanding of the functionality
defined in RFCs 3412-3418.
The "Transport Subsystem" is an additional component for the SNMP
Engine depicted in RFC3411, section 3.1.
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The following diagram depicts its place in the RFC3411 architecture.:
+-------------------------------------------------------------------+
| SNMP entity |
| |
| +-------------------------------------------------------------+ |
| | SNMP engine (identified by snmpEngineID) | |
| | | |
| | +------------+ | |
| | | Transport | | |
| | | Subsystem | | |
| | +------------+ | |
| | | |
| | +------------+ +------------+ +-----------+ +-----------+ | |
| | | Dispatcher | | Message | | Security | | Access | | |
| | | | | Processing | | Subsystem | | Control | | |
| | | | | Subsystem | | | | Subsystem | | |
| | +------------+ +------------+ +-----------+ +-----------+ | |
| +-------------------------------------------------------------+ |
| |
| +-------------------------------------------------------------+ |
| | Application(s) | |
| | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | Command | | Notification | | Proxy | | |
| | | Generator | | Receiver | | Forwarder | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | |
| | +-------------+ +--------------+ +--------------+ | |
| | | Command | | Notification | | Other | | |
| | | Responder | | Originator | | | | |
| | +-------------+ +--------------+ +--------------+ | |
| +-------------------------------------------------------------+ |
| |
+-------------------------------------------------------------------+
The transport mappings defined in RFC3417 do not provide lower-layer
security functionality, and thus do not provide transport-specific
security parameters. This document updates RFC3411 and RFC3417 by
defining an architectural extension and ASIs that transport mappings
(models) can use to pass transport-specific security parameters to
other subsystems, including transport-specific security parameters
translated into the transport-independent securityName and
securityLevel.
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1.3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The key words "must", "must not", "required", "shall", "shall not",
"should", "should not", "recommended", "may", and "optional" in this
document are not to be interpreted as described in RFC2119. They
will usually, but not always, be used in a context relating to
compatibility with the RFC3411 architecture or the subsystem defined
here, but which might have no impact on on-the-wire compatibility.
These terms are used as guidance for designers of proposed IETF
models to make the designs compatible with RFC3411 subsystems and
Abstract Service Interfaces (see section 3.2). Implementers are free
to implement differently. Some usages of these lowercase terms are
simply normal English usage.
2. Motivation
Just as there are multiple ways to secure one's home or business, in
a continuum of alternatives, there are multiple ways to secure a
network management protocol. Let's consider three general
approaches.
In the first approach, an individual could sit on his front porch
waiting for intruders. In the second approach, he could hire an
employee , schedule the employee, position the employee to guard what
he wants protected, hire a second guard to cover if the first gets
sick, and so on. In the third approach, he could hire a security
company, tell them what he wants protected, and they could hire
employees, train them, position the guards, schedule the guards, send
a replacement when a guard cannot make it, etc., thus providing the
desired security, with no significant effort on his part other than
identifying requirements and verifying the quality of the service
being provided.
The User-based Security Model (USM) as defined in [RFC3414] largely
uses the first approach - it provides its own security. It utilizes
existing mechanisms (e.g., SHA), but provides all the coordination.
USM provides for the authentication of a principal, message
encryption, data integrity checking, timeliness checking, etc.
USM was designed to be independent of other existing security
infrastructures. USM therefore requires a separate principal and key
management infrastructure. Operators have reported that deploying
another principal and key management infrastructure in order to use
SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use
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external mechanisms to handle the distribution of keys for use by
USM. The more important issue is that operators wanted to leverage a
single user base that wasn't specific to SNMP.
A solution based on the second approach might use a USM-compliant
architecture, but combine the authentication mechanism with an
external mechanism, such as RADIUS [RFC2865], to provide the
authentication service. It might be possible to utilize an external
protocol to encrypt a message, to check timeliness, to check data
integrity, etc. It is difficult to cobble together a number of
subcontracted services and coordinate them however, because it is
difficult to build solid security bindings between the various
services, and potential for gaps in the security is significant.
A solution based on the third approach might utilize one or more
lower-layer security mechanisms to provide the message-oriented
security services required. These would include authentication of
the sender, encryption, timeliness checking, and data integrity
checking. There are a number of IETF standards available or in
development to address these problems through security layers at the
transport layer or application layer, among them TLS [RFC4366], SASL
[RFC4422], and SSH [RFC4251].
From an operational perspective, it is highly desirable to use
security mechanisms that can unify the administrative security
management for SNMPv3, command line interfaces (CLIs) and other
management interfaces. The use of security services provided by
lower layers is the approach commonly used for the CLI, and is also
the approach being proposed for NETCONF [RFC4741].
This document defines a Transport Subsystem extension to the RFC3411
architecture based on the third approach. This extension specifies
how other lower layer protocols with common security infrastructures
can be used underneath the SNMP protocol and the desired goal of
unified administrative security can be met.
This extension allows security to be provided by an external protocol
connected to the SNMP engine through an SNMP Transport Model
[RFC3417]. Such a Transport Model would then enable the use of
existing security mechanisms such as (TLS) [RFC4366] or SSH [RFC4251]
within the RFC3411 architecture.
There are a number of Internet security protocols and mechanisms that
are in wide spread use. Many of them try to provide a generic
infrastructure to be used by many different application layer
protocols. The motivation behind the Transport Subsystem is to
leverage these protocols where it seems useful.
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There are a number of challenges to be addressed to map the security
provided by a secure transport into the SNMP architecture so that
SNMP continues to provide interoperability with existing
implementations. These challenges are described in detail in this
document. For some key issues, design choices are described that
might be made to provide a workable solution that meets operational
requirements and fits into the SNMP architecture defined in
[RFC3411].
3. Requirements of a Transport Model
3.1. Message Security Requirements
Transport security protocols SHOULD provide protection against the
following message-oriented threats [RFC3411]:
1. modification of information
2. masquerade
3. message stream modification
4. disclosure
These threats are described in section 1.4 of [RFC3411]. It is not
required to protect against denial of service or traffic analysis,
but it should not make those threats significantly worse.
3.1.1. Security Protocol Requirements
There are a number of standard protocols that could be proposed as
possible solutions within the Transport Subsystem. Some factors
SHOULD be considered when selecting a protocol.
Using a protocol in a manner for which it was not designed has
numerous problems. The advertised security characteristics of a
protocol might depend on it being used as designed; when used in
other ways, it might not deliver the expected security
characteristics. It is recommended that any proposed model include a
description of the applicability of the Transport Model.
A Transport Model SHOULD require no modifications to the underlying
protocol. Modifying the protocol might change its security
characteristics in ways that would impact other existing usages. If
a change is necessary, the change SHOULD be an extension that has no
impact on the existing usages. Any Transport Model SHOULD include a
description of potential impact on other usages of the protocol.
Transport Models MUST be able to coexist with each other.
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3.2. SNMP Requirements
3.2.1. Architectural Modularity Requirements
SNMP version 3 (SNMPv3) is based on a modular architecture (defined
in [RFC3411] section 3) to allow the evolution of the SNMP protocol
standards over time, and to minimize side effects between subsystems
when changes are made.
The RFC3411 architecture includes a Security Subsystem for enabling
different methods of providing security services, a Message
Processing Subsystem permitting different message versions to be
handled by a single engine, Applications(s) to support different
types of application processors, and an Access Control Subsystem for
allowing multiple approaches to access control. The RFC3411
architecture does not include a subsystem for Transport Models,
despite the fact there are multiple transport mappings already
defined for SNMP. This document addresses the need for a Transport
Subsystem compatible with the RFC3411 architecture. As work is being
done to expand the transport to include secure transport such as SSH
and TLS, using a subsystem will enable consistent design and
modularity of such Transport Models.
The design of this Transport Subsystem accepts the goals of the
RFC3411 architecture defined in section 1.5 of [RFC3411]. This
Transport Subsystem uses a modular design that will permit Transport
Models to be advanced through the standards process independently of
other Transport Models, and independent of other modular SNMP
components as much as possible.
Parameters have been added to the ASIs to pass model-independent
transport address information.
IETF standards typically require one mandatory to implement solution,
with the capability of adding new mechanisms in the future. Part of
the motivation of developing Transport Models is to develop support
for secure transport protocols, such as a Transport Model that
utilizes the Secure Shell protocol. Any Transport Model SHOULD
define one minimum-compliance security mechanism, such as
certificates, to ensure a basic level of interoperability, but should
also be able to support additional existing and new mechanisms.
The Transport Subsystem permits multiple transport protocols to be
"plugged into" the RFC3411 architecture, supported by corresponding
Transport Models, including models that are security-aware.
The RFC3411 architecture and the Security Subsystem assume that a
Security Model is called by a Message Processing Model and will
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perform multiple security functions within the Security Subsystem. A
Transport Model that supports a secure transport protocol might
perform similar security functions within the Transport Subsystem. A
Transport Model might perform the translation of transport security
parameters to/from security-model-independent parameters.
To accommodate this, an implementation-specific cache of transport-
specific information will be described (not shown), and the data
flows between the Transport Subsystem and the Transport Dispatch,
between the Message Dispatch and the Message Processing Subsystem,
and between the Message Processing Subsystem and the Security
Subsystem will be extended to pass security-model-independent values.
New Security Models may also be defined that understand how to work
with the modified ASIs and the cache. One such Security Mode, the
Transport Security Model, is defined in
The following diagram depicts the SNMPv3 architecture including the
new Transport Subsystem defined in this document, and a new Transport
Security Model defined in [I-D.ietf-isms-transport-security-model].
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+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-------------------------------------------------------------------+
| +--------------------------------------------------+ |
| | Transport Subsystem | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | |
| | | UDP | | TCP | | SSH | | TLS | . . . | other | | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | |
| +--------------------------------------------------+ |
| ^ |
| | |
| Dispatcher v |
| +-------------------+ +---------------------+ +----------------+ |
| | Transport | | Message Processing | | Security | |
| | Dispatch | | Subsystem | | Subsystem | |
| | | | +------------+ | | +------------+ | |
| | | | +->| v1MP |<--->| | USM | | |
| | | | | +------------+ | | +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | | | +->| v2cMP |<--->| | Transport | | |
| | Message | | | +------------+ | | | Security | | |
| | Dispatch <--------->| +------------+ | | | Model | | |
| | | | +->| v3MP |<--->| +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | PDU Dispatch | | | +------------+ | | | Other | | |
| +-------------------+ | +->| otherMP |<--->| | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| 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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3.2.1.1. Processing Differences between USM and Secure Transport
USM and secure transports differ is the processing order and
responsibilities within the RFC3411 architecture. While the steps
are the same, they occur in a different order, and may be done by
different subsystems. The following lists illustrate the difference
in the flow and the responsibility for different processing steps for
incoming messages when using USM and when using a secure transport.
(Note that these lists are simplified for illustrative purposes, and
do not represent all details of processing. Transport Models must
provide the detailed elements of procedure.)
With USM and other Security Models, security processing starts when
the Message Processing Model decodes portions of the ASN.1 message to
extract an opaque block of security parameters and header parameters
that identify which Security Model should process the message to
perform authentication, decryption, timeliness checking, integrity
checking, and translation of parameters to model-independent
parameters. A secure transport performs those security functions on
the message, before the ASN.1 is decoded.
Step 6 cannot occur until after decryption occurs. Step 6 and beyond
are the same for USM and a secure transport.
3.2.1.1.1. USM and the RFC3411 Architecture
1) decode the ASN.1 header (Message Processing Model)
2) determine the SNMP Security Model and parameters (Message
Processing Model)
3) verify securityLevel. [Security Model]
4) translate parameters to model-independent parameters (Security
Model)
5) authenticate and decrypt message. [Security Model]
6) determine the pduType in the decrypted portions (Message
Processing Model), and
7) pass on the decrypted portions with model-independent parameters.
3.2.1.2. Transport Subsystem and the RFC3411 Architecture
1) authenticate and decrypt message. [Transport Model]
2) translate parameters to model-independent parameters (Transport
Model)
3) decode the ASN.1 header (Message Processing Model)
4) determine the SNMP Security Model and parameters (Message
Processing Model)
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5) verify securityLevel [Security Model]
6) determine the pduType in the decrypted portions (Message
Processing Model), and
7) pass on the decrypted portions with model-independent security
parameters
If a message is secured using a secure transport layer, then the
Transport Model should provide the translation from the authenticated
identity (e.g., an SSH user name) to the securityName in step 3.
3.2.1.3. Passing Information between Engines
A secure Transport Model will establish an authenticated and/or
encrypted tunnel between the Transport Models of two SNMP engines.
After a transport layer tunnel is established, then SNMP messages can
be sent through the tunnel from one SNMP engine to the other SNMP
engine. Transport Models MAY support sending multiple SNMP messages
through the same tunnel.
3.2.2. Access Control Requirements
RFC3411 made some design decisions related to the support of an
Access Control Subsystem. These include a securityName and
securityLevel mapping, the separation of Authentication and
Authorization, and the passing of model-independent security
parameters.
3.2.2.1. securityName and securityLevel Mapping
For SNMP access control to function properly, Security Models MUST
establish a securityLevel and a securityName, which is the security-
model-independent identifier for a principal. The Message Processing
Subsystem relies on a Security Model, such as USM, to play a role in
security that goes beyond protecting the message - it provides a
mapping between the security-model-specific principal to a security-
model independent securityName which can be used for subsequent
processing, such as for access control.
The securityName MUST be mapped from the mechanism-specific
authenticated identity, and this mapping must be done for incoming
messages before the Security Model passes securityName to the Message
Processing Model via the processIncoming ASI. This translation from
a mechanism-specific authenticated identity to a securityName might
be done by the Transport Model, and the securityName is then provided
to the Security Model via the tmStateReference to be passed to the
Message Processing Model.
If the type of authentication provided by the transport layer (e.g.,
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TLS) is considered adequate to secure and/or encrypt the message, but
inadequate to provide the desired granularity of access control
(e.g., user-based), then a second authentication (e.g., one provided
via a RADIUS server) MAY be used to provide the authentication
identity which is mapped to the securityName. This approach would
require a good analysis of the potential for man-in-the-middle
attacks or masquerade possibilities.
3.2.3. Security Parameter Passing Requirements
RFC3411 section 4 describes abstract data flows between the
subsystems, models and applications within the architecture.
Abstract Service Interfaces describe the flow of data, passing model-
independent information between subsystems within an engine. The
RFC3411 architecture has no ASI parameters for passing security
information between the Transport Subsystem and the dispatcher, or
between the dispatcher and the Message Processing Model. This
document defines or modifies ASIs for this purpose.
The security parameters include a model-independent identifier of the
security "principal" (the securityName), the Security Model used to
perform the authentication, and which authentication and privacy
services were (should be) applied to the message (securityLevel).
A Message Processing Model might unpack SNMP-specific security
parameters from an incoming message before calling a specific
Security Model to authenticate and decrypt an incoming message,
perform integrity checking, and translate security-model-specific
parameters into model-independent parameters. When using a secure
Transport Model, security parameters might be provided through means
other than carrying them in the SNMP message; the parameters for
incoming messages might be extracted from the transport layer by the
Transport Model before the message is passed to the Message
Processing Subsystem.
This document describes a cache mechanism (see Section 5), into which
the Transport Model puts information about the transport and security
parameters applied to a transport connection or an incoming message,
and a Security Model may extract that information from the cache. A
tmStateReference is passed as an extra parameter in the ASIs of the
Transport Subsystem and the Message Processing and Security
Subsystems, to identify the relevant cache. This approach of passing
a model-independent reference is consistent with the
securityStateReference cache already being passed around in the
RFC3411 ASIs.
For outgoing messages, even when a secure Transport Model will
provide the security services, a Message Processing Model might have
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a Security Model actually create the message from its component
parts. Whether there are any security services provided by the
Security Model for an outgoing message is security-model-dependent.
For incoming messages, even when a secure Transport Model provides
security services, a Security Model might provide some security
functionality that can only be provided after the message version or
other parameters are extracted from the message.
3.2.4. Separation of Authentication and Authorization
The RFC3411 architecture defines a separation of authentication and
authorization (access control), and a Transport Model that provides
security services should take care to not violate this separation. A
Transport Model must not specify how the securityModel and
securityName could be dynamically mapped to an access control
mechanism, such as a VACM-style groupName.
The RECOMMENDED approach is to pass the model-independent security
parameters via the isAccessAllowed ASI, and perform the mapping from
the model-independent security parameters to an access-control-model-
dependent policy within the Access Control Model. The
isAccessAllowed ASI is used for passing the securityModel,
securityName, and securityLevel parameters that are independent of
any specific security model and any specific access control model to
the Access Control Subsystem.
The mapping of (securityModel, securityName, securityLevel) to an
access-control-model-specific policy should be handled within a
specific access control model. This mapping should not be done in
the Transport or Security Subsystems, to be consistent with the
modularity of the RFC3411 architecture. This separation was a
deliberate decision of the SNMPv3 WG, to allow support for
authentication protocols which did not provide authorization (access
control) capabilities, and to support authorization schemes, such as
VACM, that do not perform their own authentication.
The View-based Access Control Model uses the securityModel and the
securityName as inputs to check for access rights. It determines the
groupName as a function of securityModel and securityName. Providing
a binding outside the Access Control Subsystem might create
dependencies that could make it harder to develop alternate models of
access control, such as one built on UNIX groups or Windows domains.
To provide support for protocols which simultaneously send
information for authentication and authorization (access control),
such as RADIUS [RFC2865], access-control-model-specific information
might be cached or otherwise made available to the Access Control
Subsystem, e.g., via a MIB table similar to the
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vacmSecurityToGroupTable, so the Access Control Subsystem can create
an appropriate binding between the access-control-model-independent
securityModel and securityName and an access-control-model-specific
policy. This would be highly undesirable, however, if it creates a
dependency between a Transport Model or a Security Model and an
Access Control Model.
3.3. Session Requirements
Some secure transports might have a notion of sessions, while other
secure transports might provide channels or other session-like
mechanism. Throughout this document, the term session is used in a
broad sense to cover sessions, channels, and session-like mechanisms.
Session refers to an association between two SNMP engines that
permits the transmission of one or more SNMP messages within the
lifetime of the session. How the session is actually established,
opened, closed, or maintained is specific to a particular Transport
Model.
Sessions are not part of the SNMP architecture defined in [RFC3411],
but are considered desirable because the cost of authentication can
be amortized over potentially many transactions.
The architecture defined in [RFC3411] does not include a session
selector in the Abstract Service Interfaces, and neither is that done
for the Transport Subsystem, so an SNMP application has no mechanism
to select a session using the ASIs except by passing a unique
combination of transportDomain, transportAddress, securityName,
securityModel, and securityLevel. Implementers, of course, might
provide non-standard mechanisms to select sessions. The
transportDomain and transportAddress identify the transport
connection to a remote network node; the securityName identifies
which security principal to communicate with at that address (e.g.,
different NMS applications), and the securityModel and securityLevel
might permit selection of different sets of security properties for
different purposes (e.g., encrypted SETs vs. non-encrypted GETs).
All Transport Models should discuss the impact of sessions on SNMP
usage, including how to establish/open a transport session (i.e., how
it maps to the concepts of session-like mechanisms of the underlying
protocol), how to behave when a session cannot be established, how to
close a session properly, how to behave when a session is closed
improperly, the session security properties, session establishment
overhead, and session maintenance overhead.
To reduce redundancy, this document describes aspects that are
expected to be common to all Transport Model sessions.
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3.3.1. Session Establishment Requirements
SNMP applications must provide the transportDomain, transportAddress,
securityName, securityModel, and securityLevel to be used for a
session.
SNMP Applications might have no knowledge of whether the session that
will be used to carry commands was initially established as a
notification session, or a request-response session, and SHOULD NOT
make any assumptions based on knowing the direction of the session.
If an administrator or Transport Model designer wants to
differentiate a session established for different purposes, such as a
notification session versus a request-response session, the
application can use different securityNames or transport addresses
(e.g., port 161 vs. port 162) for different purposes.
An SNMP engine containing an application that initiates
communication, e.g., a Command Generator or Notification Originator,
must be able to attempt to establish a session for delivery if a
session does not yet exist. If a session cannot be established then
the message is discarded.
Sessions are usually established by the Transport Model when no
appropriate session is found for an outgoing message, but sessions
might be established in advance to support features such as
notifications. How sessions are established in advance is beyond the
scope of this document.
Sessions are initiated by notification originators when there is no
currently established connection that can be used to send the
notification. For a client-server security protocol, this might
require provisioning authentication credentials on the agent, either
statically or dynamically, so the client/agent can successfully
authenticate to a notification receiver.
A Transport Model must be able to determine whether a session does or
does not exist, and must be able to determine which session has the
appropriate security characteristics (transportDomain,
transportAddress, securityName, securityModel, and securityLevel) for
an outgoing message.
A Transport Model implementation MAY reuse an already established
session with the appropriate transportDomain, transportAddress,
securityName, securityModel, and securityLevel characteristics for
delivery of a message containing a different pduType than originally
caused the session to be created. For example, an implementation
that has an existing session originally established to receive a
request MAY use that session to send an outgoing notification, and
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MAY use a session that was originally established to send a
notification to send a request. Responses SHOULD be returned using
the same session that carried the corresponding request message.
Reuse of sessions is not required for conformance.
If a session can be reused for a different pduType, but a receiver is
not prepared to accept different pduTypes over the same session, then
the message MAY be dropped by the receiver.
3.3.2. Session Maintenance Requirements
A Transport Model can tear down sessions as needed. It might be
necessary for some implementations to tear down sessions as the
result of resource constraints, for example.
The decision to tear down a session is implementation-dependent.
While it is possible for an implementation to automatically tear down
each session once an operation has completed, this is not recommended
for anticipated performance reasons. How an implementation
determines that an operation has completed, including all potential
error paths, is implementation-dependent.
The elements of procedure describe when cached information can be
discarded, in some circumstances, and the timing of cache cleanup
might have security implications, but cache memory management is an
implementation issue.
If a Transport Model defines MIB module objects to maintain session
state information, then the Transport Model MUST define what SHOULD
happen to the objects when a related session is torn down, since this
will impact interoperability of the MIB module.
3.3.3. Message security versus session security
A Transport Model session is associated with state information that
is maintained for its lifetime. This state information allows for
the application of various security services to multiple messages.
Cryptographic keys established at the beginning of the session SHOULD
be used to provide authentication, integrity checking, and encryption
services for data that is communicated during the session. The
cryptographic protocols used to establish keys for a Transport Model
session SHOULD ensure that fresh new session keys are generated for
each session. In addition sequence information might be maintained
in the session which can be used to prevent the replay and reordering
of messages within a session. If each session uses new keys, then a
cross-session replay attack will be unsuccessful; that is, an
attacker cannot successfully replay on one session a message he
observed from another session. A good security protocol will also
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protect against replay attacks _within_ a session; that is, an
attacker cannot successfully replay a message observed earlier in the
same session.
A Transport Model session will have a single transportDomain,
transportAddress, securityModel, securityName and securityLevel
associated with it. If an exchange between communicating engines
requires a different securityLevel or is on behalf of a different
securityName, or uses a different securityModel, then another session
would be needed. An immediate consequence of this is that
implementations SHOULD be able to maintain some reasonable number of
concurrent sessions.
For Transport Models, securityName should be specified during session
setup, and associated with the session identifier.
SNMPv3 was designed to support multiple levels of security,
selectable on a per-message basis by an SNMP application, because,
for example, there is not much value in using encryption for a
Commander Generator to poll for potentially non-sensitive performance
data on thousands of interfaces every ten minutes; the encryption
might add significant overhead to processing of the messages.
Some Transport Models might support only specific authentication and
encryption services, such as requiring all messages to be carried
using both authentication and encryption, regardless of the security
level requested by an SNMP application. A Transport Model may
upgrade the requested security level, i.e. noAuthNoPriv and
authNoPriv MAY be sent over an authenticated and encrypted session.
4. Scenario Diagrams for the Transport Subsystem
RFC3411 section 4.6 provides scenario diagrams to illustrate how an
outgoing message is created, and how an incoming message is
processed. Both diagrams are incomplete, however. In section 4.6.1,
the diagram doesn't show an ASI for sending an SNMP request to the
network or for receiving an SNMP response message from the network.
In section 4.6.2, the diagram doesn't show the ASIs to receive an
SNMP message from the network, or to send an SNMP message to the
network.
4.1. Command Generator or Notification Originator
This diagram from RFC3411 4.6.1 shows how a Command Generator or
Notification Originator application [RFC3413] requests that a PDU be
sent, and how the response is returned (asynchronously) to that
application.
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This document defines a sendMessage ASI to send SNMP messages to the
network, and a receiveMessage ASI to receive SNMP messages from the
network.
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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4.2. Command Responder
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 an SNMP message is received, and how the
Response is (asynchronously) sent back to the network.
This document defines the sendMessage and receiveMessage ASIs for
this purpose.
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Command Dispatcher Message Security
Responder | Processing Model
| | Model |
| | | |
| registerContextEngineID | | |
|------------------------>| | |
|<------------------------| | | |
| | Receive SNMP | | |
: | Message | | |
: | from Network | | |
: |<-------------+ | |
: | | |
: |prepareDataElements | |
: |------------------->| |
: | | processIncomingMsg |
: | |------------------->|
: | | |
: | |<-------------------|
: | | |
: |<-------------------| |
| processPdu | | |
|<------------------------| | |
| | | |
: : : :
: : : :
| returnResponsePdu | | |
|------------------------>| | |
: | prepareResponseMsg | |
: |------------------->| |
: | |generateResponseMsg |
: | |------------------->|
: | | |
: | |<-------------------|
: | | |
: |<-------------------| |
: | | |
: |--------------+ | |
: | Send SNMP | | |
: | Message | | |
: | to Network | | |
: | v | |
5. Cached Information and References
The RFC3411 architecture uses caches to store dynamic model-specific
information, and uses references in the ASIs to indicate in a model-
independent manner which cached information flows between subsystems.
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There are two levels of state that might need to be maintained: the
security state in a request-response pair, and potentially long-term
state relating to transport and security.
This state is maintained in caches. To simplify the elements of
procedure, the release of state information is not always explicitly
specified. As a general rule, if state information is available when
a message being processed gets discarded, the state related to that
message should also be discarded, and if state information is
available when a relationship between engines is severed, such as the
closing of a transport session, the state information for that
relationship might also be discarded.
This document differentiates the tmStateReference from the
securityStateReference. This document does not specify an
implementation strategy, only an abstract description of the data
that flows between subsystems. An implementation might use one cache
and one reference to serve both functions, but an implementer must be
aware of the cache-release issues to prevent the cache from being
released before a security or Transport Model has had an opportunity
to extract the information it needs.
5.1. securityStateReference
From RFC3411: "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." To enable this, an abstract
securityStateReference data element, defined in RFC3411 section
A.1.5, is passed from the Security Model to the Message Processing
Model.
The information saved should include the model-independent parameters
(transportDomain, transportAddress, securityName, securityModel, and
securityLevel), related security parameters, and other information
needed to match the response with the request. The related security
parameters may include transport-specific security information.
The Message Processing Model has the responsibility for explicitly
releasing the securityStateReference when such data is no longer
needed. The securityStateReference cached data may be implicitly
released via the generation of a response, or explicitly released by
using the stateRelease ASI, as defined in RFC 3411 section 4.5.1."
If the Transport Model connection is closed between the time a
Request is received and a Response message is being prepared, then
the Response message MAY be discarded.
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5.2. tmStateReference
For each message or transport session, information about the message
security is stored in a cache, which may include model- and
mechanism-specific parameters. The tmStateReference is passed
between subsystems to provide a handle for the cache. A Transport
Model may store transport-specific parameters in the cache for
subsequent usage. Since the contents of a cache are meaningful only
within an implementation, and not on-the-wire, the format of the
cache is implementation-specific.
The state referenced by tmStateReference might be saved in a Local
Configuration Datastore (LCD) to make it available across multiple
messages, as compared to securityStateReference which is designed to
be saved only for the life of a request-response pair of messages.
It is expected that an LCD will allow lookup based on the combination
of transportDomain, transportAddress, securityName, securityModel,
and securityLevel, and that the cache contain these values to
reference entries in the LCD.
6. Abstract Service Interfaces
Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an
SNMP entity, and to help keep the subsystems independent of each
other except for the common parameters.
This document follows the example of RFC3411 regarding the release of
state information, and regarding error indications.
1) The release of state information is not always explicitly
specified in a transport model. As a general rule, if state
information is available when a message gets discarded, the message-
state information should also be released, and if state information
is available when a session is closed, the session state information
should also be released.
2) An error indication in statusInformation may include an OID and
value for an incremented counter and a value for securityLevel, and
values for contextEngineID and contextName for the counter, and the
securityStateReference if the information is available at the point
where the error is detected.
6.1. sendMessage ASI
The sendMessage ASI is used to pass a message from the Dispatcher to
the appropriate Transport Model for sending.
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If present and valid, the tmStateReference refers to a cache
containing transport-model-specific parameters for the transport and
transport security. How the information in the cache is used is
transport-model-dependent and implementation-dependent. How a
tmStateReference is determined to be present and valid is
implementation-dependent.
This may sound underspecified, but keep in mind that a transport
model might be something like SNMP over UDP over IPv6, where no
security is provided, so it might have no mechanisms for utilizing a
securityName and securityLevel.
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
6.2. Other Outgoing ASIs
A tmStateReference parameter has been added to the
prepareOutgoingMessage, generateRequestMsg, and generateResponseMsg
ASIs as an OUT parameter.
statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
IN sendPduHandle -- the handle for matching
incoming responses
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
OUT tmStateReference -- (NEW) reference to transport state
)
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The tmStateReference parameter of generateRequestMsg or
generateResponseMsg is passed in the return parameters of the
Security Subsystem to the Message Processing Subsystem. If a cache
exists for a session identifiable from transportDomain,
transportAddress, securityModel, securityName, and securityLevel,
then an appropriate Security Model might create a tmStateReference to
the cache and pass that as an OUT parameter.
If one does not exist, the Security Model might create a cache
referenced by tmStateReference. This information might include
transportDomain, transportAddress, the securityModel, the
securityLevel, and the securityName, plus any model or mechanism-
specific details. The contents of the cache may be incomplete until
the Transport Model has established a session. What information is
passed, and how this information is determined, is implementation and
security-model-specific.
The prepareOutgoingMessage ASI passes tmStateReference from the
Message Processing Subsystem to the dispatcher. How or if the
Message Processing Subsystem modifies or utilizes the contents of the
cache is message-processing-model-specific.
This may sound underspecified, but keep in mind that a message
processing model might have access to all the information from the
cache and from the message, and have no need to call a Security Model
to do any processing; an application might choose a Security Model
such as USM to authenticate and secure the SNMP message, but also
utilize a secure transport such as that provided by the SSH Transport
Model to send the message to its destination.
6.3. The receiveMessage ASI
If one does not exist, the Transport Model might create a cache
referenced by tmStateReference. If present, this information might
include transportDomain, transportAddress, securityLevel, and
securityName, plus model or mechanism-specific details. How this
information is determined is implementation and transport-model-
specific.
This may sound underspecified, but keep in mind that a transport
model might be something like SNMP over UDP over IPv6, where no
security is provided, so it might have no mechanisms for determining
a securityName and securityLevel.
The Transport Model does not know the securityModel for an incoming
message; this will be determined by the Message Processing Model in a
message-processing-model-dependent manner.
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The receiveMessage ASI is used to pass a message from the Transport
Subsystem to the Dispatcher.
statusInformation =
receiveMessage(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN incomingMessage -- the message received
IN incomingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
6.4. Other Incoming ASIs
To support the Transport Subsystem, the tmStateReference is added to
the prepareDataElements ASI (from the Dispatcher to the Message
Processing Subsystem), and to the processIncomingMsg ASI (from the
Message Processing Subsystem to the Security Model Subsystem). How
or if a Message Processing Model or Security Model uses
tmStateReference is message-processing-model-dependent and security-
model-dependent.
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
IN tmStateReference -- (NEW) from the Transport Model
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- the version of the PDU
OUT PDU -- SNMP Protocol Data Unit
OUT pduType -- SNMP PDU type
OUT sendPduHandle -- handle for matched request
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for possible Response
)
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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
IN tmStateReference -- (NEW) from the Transport Model
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
) -- information, needed for response
The tmStateReference parameter of prepareDataElements is passed from
the dispatcher to the Message Processing Subsystem. How or if the
Message Processing Subsystem modifies or utilizes the contents of the
cache is message-processing-model-specific.
The processIncomingMessage ASI passes tmStateReference from the
Message Processing Subsystem to the Security Subsystem.
If tmStateReference is present and valid, an appropriate Security
Model might utilize the information in the cache. How or if the
Security Subsystem utilizes the information in the cache is security-
model-specific.
This may sound underspecified, but keep in mind that a message
processing model might have access to all the information from the
cache and from the message, and have no need to call a Security Model
to do any processing. The Message Processing Model might determine
that the USM Security Model is specified in an SNMPv3 message header;
the USM Security Model has no need of values in the tmStateReference
cache to authenticate and secure the SNMP message, but an application
might have chosen to use a secure transport such as that provided by
the SSH Transport Model to send the message to its destination.
7. Security Considerations
This document defines an architectural approach that permits SNMP to
utilize transport layer security services. Each proposed Transport
Model should discuss the security considerations of the Transport
Model.
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It is considered desirable by some industry segments that SNMP
Transport Models should utilize transport layer security that
addresses perfect forward secrecy at least for encryption keys.
Perfect forward secrecy guarantees that compromise of long term
secret keys does not result in disclosure of past session keys. Each
proposed Transport Model should include a discussion in its security
considerations of whether perfect forward security is appropriate for
the Transport Model.
Since the cache and LCD will contain security-related parameters,
implementers should store this information (in memory or in
persistent storage) in a manner to protect it from unauthorized
disclosure and/or modification.
Care must be taken to ensure that a SNMP engine is sending packets
out over a transport using credentials that are legal for that engine
to use on behalf of that user. Otherwise an engine that has multiple
transports open might be "tricked" into sending a message through the
wrong transport.
8. IANA Considerations
This document requires no action by IANA.
9. Acknowledgments
The Integrated Security for SNMP WG would like to thank the following
people for their contributions to the process:
The authors of submitted Security Model proposals: Chris Elliot, Wes
Hardaker, David Harrington, Keith McCloghrie, Kaushik Narayan, David
Perkins, Joseph Salowey, and Juergen Schoenwaelder.
The members of the Protocol Evaluation Team: Uri Blumenthal,
Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla.
WG members who committed to and performed detailed reviews: Jeffrey
Hutzelman
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for
use in RFCs to Indicate
Requirement Levels",
BCP 14, RFC 2119,
March 1997.
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[RFC3411] Harrington, D., Presuhn,
R., and B. Wijnen, "An
Architecture for Describing
Simple Network Management
Protocol (SNMP) Management
Frameworks", STD 62,
RFC 3411, December 2002.
[RFC3412] Case, J., Harrington, D.,
Presuhn, R., and B. Wijnen,
"Message Processing and
Dispatching for the Simple
Network Management Protocol
(SNMP)", STD 62, RFC 3412,
December 2002.
[RFC3414] Blumenthal, U. and B.
Wijnen, "User-based
Security Model (USM) for
version 3 of the Simple
Network Management Protocol
(SNMPv3)", STD 62,
RFC 3414, December 2002.
[RFC3417] Presuhn, R., "Transport
Mappings for the Simple
Network Management Protocol
(SNMP)", STD 62, RFC 3417,
December 2002.
10.2. Informative References
[RFC2865] Rigney, C., Willens, S.,
Rubens, A., and W. Simpson,
"Remote Authentication Dial
In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3410] Case, J., Mundy, R.,
Partain, D., and B.
Stewart, "Introduction and
Applicability Statements
for Internet-Standard
Management Framework",
RFC 3410, December 2002.
[RFC3413] Levi, D., Meyer, P., and B.
Stewart, "Simple Network
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Management Protocol (SNMP)
Applications", STD 62,
RFC 3413, December 2002.
[RFC4366] Blake-Wilson, S., Nystrom,
M., Hopwood, D., Mikkelsen,
J., and T. Wright,
"Transport Layer Security
(TLS) Extensions",
RFC 4366, April 2006.
[RFC4422] Melnikov, A. and K.
Zeilenga, "Simple
Authentication and Security
Layer (SASL)", RFC 4422,
June 2006.
[RFC4251] Ylonen, T. and C. Lonvick,
"The Secure Shell (SSH)
Protocol Architecture",
RFC 4251, January 2006.
[RFC4741] Enns, R., "NETCONF
Configuration Protocol",
RFC 4741, December 2006.
[I-D.ietf-isms-transport-security-model] Harrington, D., "Transport
Security Model for SNMP", d
raft-ietf-isms-transport-
security-model-02 (work in
progress), January 2007.
Appendix A. Parameter Table
Following is a Comma-separated-values (CSV) formatted matrix useful
for tracking data flows into and out of the dispatcher, Transport,
Message Processing, and Security Subsystems. This will be of most
use to designers of models, to understand what information is
available at which points in the processing, following the RFC3411
architecture (and this subsystem). Import this into your favorite
spreadsheet or other CSV compatible application. You will need to
remove lines feeds from the second, third, and fourth lines, which
needed to be wrapped to fit into RFC line lengths.
A.1. ParameterList.csv
,Dispatcher,,,,Messaging,,,Security,,,Transport,
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,sendPDU,returnResponse,processPDU,processResponse,
prepareOutgoingMessage,prepareResponseMessage,prepareDataElements,
generateRequest,processIncoming,generateResponse,
sendMessage,receiveMessage
transportDomain,In,,,,In,,In,,,,,In
transportAddress,In,,,,In,,In,,,,,In
destTransportDomain,,,,,Out,Out,,,,,In,
destTransportAddress,,,,,Out,Out,,,,,In,
messageProcessingModel,In,In,In,In,In,In,Out,In,In,In,,
securityModel,In,In,In,In,In,In,Out,In,In,In,,
securityName,In,In,In,In,In,In,Out,In,Out,In,,
securityLevel,In,In,In,In,In,In,Out,In,In,In,,
contextEngineID,In,In,In,In,In,In,Out,,,,,
contextName,In,In,In,In,In,In,Out,,,,,
expectResponse,In,,,,In,,,,,,,
PDU,In,In,In,In,In,In,Out,,,,,
pduVersion,In,In,In,In,In,In,Out,,,,,
statusInfo,Out,In,,In,,In,Out,Out,Out,Out,,
errorIndication,Out,Out,,,,,Out,,,,,
sendPduHandle,Out,,,In,In,,Out,,,,,
maxSizeResponsePDU,,In,In,,,In,Out,,Out,,,
stateReference,,In,In,,,In,Out,,,,,
wholeMessage,,,,,Out,Out,In,Out,In,Out,In,In
messageLength,,,,,Out,Out,In,Out,In,Out,In,In
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maxMessageSize,,,,,,,,In,In,In,,
globalData,,,,,,,,In,,In,,
securityEngineID,,,,,,,,In,Out,In,,
scopedPDU,,,,,,,,In,Out,In,,
securityParameters,,,,,,,,Out,In,Out,,
securityStateReference,,,,,,,,,Out,In,,
pduType,,,,,,,Out,,,,,
tmStateReference,,,,,Out,Out,In,,In,,In,In
Appendix B. Why tmStateReference?
This appendix considers why a cache-based approach was selected for
passing parameters.
There are four approaches that could be used for passing information
between the Transport Model and a Security Model.
1. one could define an ASI to supplement the existing ASIs, or
2. one could add a header to encapsulate the SNMP message,
3. one could utilize fields already defined in the existing SNMPv3
message, or
4. one could pass the information in an implementation-specific
cache or via a MIB module.
B.1. Define an Abstract Service Interface
Abstract Service Interfaces (ASIs) are defined by a set of primitives
that specify the services provided and the abstract data elements
that are to be passed when the services are invoked. Defining
additional ASIs to pass the security and transport information from
the Transport Subsystem to Security Subsystem has the advantage of
being consistent with existing RFC3411/3412 practice, and helps to
ensure that any Transport Model proposals pass the necessary data,
and do not cause side effects by creating model-specific dependencies
between itself and other models or other subsystems other than those
that are clearly defined by an ASI.
B.2. Using an Encapsulating Header
A header could encapsulate the SNMP message to pass necessary
information from the Transport Model to the dispatcher and then to a
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Message Processing Model. The message header would be included in
the wholeMessage ASI parameter, and would be removed by a
corresponding Message Processing Model. This would imply the (one
and only) messaging dispatcher would need to be modified to determine
which SNMP message version was involved, and a new Message Processing
Model would need to be developed that knew how to extract the header
from the message and pass it to the Security Model.
B.3. Modifying Existing Fields in an SNMP Message
[RFC3412] defines the SNMPv3 message, which contains fields to pass
security related parameters. The Transport Subsystem could use these
fields in an SNMPv3 message, or comparable fields in other message
formats to pass information between Transport Models in different
SNMP engines, and to pass information between a Transport Model and a
corresponding Message Processing Model.
If the fields in an incoming SNMPv3 message are changed by the
Transport Model before passing it to the Security Model, then the
Transport Model will need to decode the ASN.1 message, modify the
fields, and re-encode the message in ASN.1 before passing the message
on to the message dispatcher or to the transport layer. This would
require an intimate knowledge of the message format and message
versions so the Transport Model knew which fields could be modified.
This would seriously violate the modularity of the architecture.
B.4. Using a Cache
This document describes a cache, into which the Transport Model puts
information about the security applied to an incoming message, and a
Security Model can extract that information from the cache. Given
that there might be multiple TM-security caches, a tmStateReference
is passed as an extra parameter in the ASIs between the Transport
Subsystem and the Security Subsystem, so the Security Model knows
which cache of information to consult.
This approach does create dependencies between a specific Transport
Model and a corresponding specific Security Model. However, the
approach of passing a model-independent reference to a model-
dependent cache is consistent with the securityStateReference already
being passed around in the RFC3411 ASIs.
Appendix C. Open Issues
NOTE to RFC editor: If this section is empty, then please remove this
open issues section before publishing this document as an RFC. (If
it is not empty, please send it back to the editor to resolve.
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o MUST responses go back on the same session?
o How should we describe the case where a management system wants to
keep session info available for inspection after a session has
closed? see "Abstract Service Interfaces"
o Do Informs work correctly?
o How does a Transport Model know whether a message is a
notification or a request/response?
o cache contents - do we define this?
Appendix D. Change Log
NOTE to RFC editor: Please remove this change log before publishing
this document as an RFC.
Changes from revision -05- to -06-
mostly editorial changes
removed some paragraphs considered unnecessary
added Updates to header
modified some text to get the security details right
modified text re: ASIs so they are not API-like
cleaned up some diagrams
cleaned up RFC2119 language
added section numbers to citations to RFC3411
removed gun for political correctness
Changes from revision -04- to -05-
removed all objects from the MIB module.
changed document status to "Standard" rather than the xml2rfc
default of informational.
changed mention of MD5 to SHA
moved addressing style to TDomain and TAddress
modified the diagrams as requested
removed the "layered stack" diagrams that compared USM and a
Transport Model processing
removed discussion of speculative features that might exist in
future Transport Models
removed openSession and closeSession ASIs, since those are model-
dependent
removed the MIB module
removed the MIB boilerplate intro (this memo defines a SMIv2 MIB
...)
removed IANA considerations related to the now-gone MIB module
removed security considerations related to the MIB module
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removed references needed for the MIB module
changed receiveMessage ASI to use origin transport domain/address
updated Parameter CSV appendix
Changes from revision -03- to -04-
changed title from Transport Mapping Security Model Architectural
Extension to Transport Subsystem
modified the abstract and introduction
changed TMSM to TMS
changed MPSP to simply Security Model
changed SMSP to simply Security Model
changed TMSP to Transport Model
removed MPSP and TMSP and SMSP from Acronyms section
modified diagrams
removed most references to dispatcher functionality
worked to remove dependencies between transport and security
models.
defined snmpTransportModel enumeration similar to
snmpSecurityModel, etc.
eliminated all reference to SNMPv3 msgXXXX fields
changed tmSessionReference back to tmStateReference
Changes from revision -02- to -03-
o removed session table from MIB module
o removed sessionID from ASIs
o reorganized to put ASI discussions in EOP section, as was done in
SSHSM
o changed user auth to client auth
o changed tmStateReference to tmSessionReference
o modified document to meet consensus positions published by JS
o
* authoritative is model-specific
* msgSecurityParameters usage is model-specific
* msgFlags vs. securityLevel is model/implementation-specific
* notifications must be able to cause creation of a session
* security considerations must be model-specific
* TDomain and TAddress are model-specific
* MPSP changed to SMSP (Security Model security processing)
Changes from revision -01- to -02-
o wrote text for session establishment requirements section.
o wrote text for session maintenance requirements section.
o removed section on relation to SNMPv2-MIB
o updated MIB module to pass smilint
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o Added Structure of the MIB module, and other expected MIB-related
sections.
o updated author address
o corrected spelling
o removed msgFlags appendix
o Removed section on implementation considerations.
o started modifying the security boilerplate to address TMS and MIB
security issues
o reorganized slightly to better separate requirements from proposed
solution. This probably needs additional work.
o removed section with sample protocols and sample
tmSessionReference.
o Added section for acronyms
o moved section comparing parameter passing techniques to appendix.
o Removed section on notification requirements.
Changes from revision -00-
o changed SSH references from I-Ds to RFCs
o removed parameters from tmSessionReference for DTLS that revealed
lower layer info.
o Added TMS-MIB module
o Added Internet-Standard Management Framework boilerplate
o Added Structure of the MIB Module
o Added MIB security considerations boilerplate (to be completed)
o Added IANA Considerations
o Added ASI Parameter table
o Added discussion of Sessions
o Added Open issues and Change Log
o Rearranged sections
Authors' Addresses
David Harrington
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: dharrington@huawei.com
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Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
EMail: j.schoenwaelder@iu-bremen.de
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