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Network Working Group D. Harrington
Request for Comments: 5590 Huawei Technologies (USA)
Updates: 3411, 3412, 3414, 3417 J. Schoenwaelder
Category: Standards Track Jacobs University Bremen
June 2009
Transport Subsystem for the Simple Network Management Protocol (SNMP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
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than English.
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 that is
comparable to other subsystems in the RFC 3411 architecture. As work
is being done to expand the transports to include secure transports,
such as the Secure Shell (SSH) Protocol and Transport Layer Security
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RFC 5590 SNMP Transport Subsystem June 2009
(TLS), using a subsystem will enable consistent design 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Internet-Standard Management Framework . . . . . . . . 3
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Where This Extension Fits . . . . . . . . . . . . . . . . 4
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements of a Transport Model . . . . . . . . . . . . . . 7
3.1. Message Security Requirements . . . . . . . . . . . . . . 7
3.1.1. Security Protocol Requirements . . . . . . . . . . . . 7
3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Architectural Modularity Requirements . . . . . . . . 8
3.2.2. Access Control Requirements . . . . . . . . . . . . . 11
3.2.3. Security Parameter Passing Requirements . . . . . . . 12
3.2.4. Separation of Authentication and Authorization . . . . 12
3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 13
3.3.1. No SNMP Sessions . . . . . . . . . . . . . . . . . . . 13
3.3.2. Session Establishment Requirements . . . . . . . . . . 14
3.3.3. Session Maintenance Requirements . . . . . . . . . . . 15
3.3.4. Message Security versus Session Security . . . . . . . 15
4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 16
5. Cached Information and References . . . . . . . . . . . . . . 17
5.1. securityStateReference . . . . . . . . . . . . . . . . . . 17
5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 17
5.2.1. Transport Information . . . . . . . . . . . . . . . . 18
5.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 19
5.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 20
5.2.4. Session Information . . . . . . . . . . . . . . . . . 20
6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 21
6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 21
6.2. Changes to RFC 3411 Outgoing ASIs . . . . . . . . . . . . 22
6.2.1. Message Processing Subsystem Primitives . . . . . . . 22
6.2.2. Security Subsystem Primitives . . . . . . . . . . . . 23
6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 24
6.4. Changes to RFC 3411 Incoming ASIs . . . . . . . . . . . . 25
6.4.1. Message Processing Subsystem Primitive . . . . . . . . 25
6.4.2. Security Subsystem Primitive . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7.1. Coexistence, Security Parameters, and Access Control . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
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RFC 5590 SNMP Transport Subsystem June 2009
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Why tmStateReference? . . . . . . . . . . . . . . . . 32
A.1. Define an Abstract Service Interface . . . . . . . . . . . 32
A.2. Using an Encapsulating Header . . . . . . . . . . . . . . 32
A.3. Modifying Existing Fields in an SNMP Message . . . . . . . 32
A.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 33
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. 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].
Lowercase versions of the keywords should be read as in normal
English. They will usually, but not always, be used in a context
that relates to compatibility with the RFC 3411 architecture or the
subsystem defined here but that 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 RFC 3411
subsystems and Abstract Service Interfaces (ASIs). Implementers are
free to implement differently. Some usages of these lowercase terms
are simply normal English usage.
For consistency with SNMP-related specifications, this document
favors terminology as defined in STD 62, rather than favoring
terminology that is consistent with non-SNMP specifications that use
different variations of the same terminology. This is consistent
with the IESG decision to not require the SNMPv3 terminology be
modified to match the usage of other non-SNMP specifications when
SNMPv3 was advanced to Full Standard.
This document discusses an extension to the modular RFC 3411
architecture; this is not a protocol document. An architectural
"MUST" is a really sharp constraint; to allow for the evolution of
technology and to not unnecessarily constrain future models, often a
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RFC 5590 SNMP Transport Subsystem June 2009
"SHOULD" or a "should" is more appropriate than a "MUST" in an
architecture. Future models MAY express tighter requirements for
their own model-specific processing.
1.3. Where This Extension Fits
It is expected that readers of this document will have read RFCs 3410
and 3411, 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 RFC 3411, Section 3.1.
The following diagram depicts its place in the RFC 3411 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 | | | | |
| | +-------------+ +--------------+ +--------------+ | |
| +-------------------------------------------------------------+ |
| |
+-------------------------------------------------------------------+
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RFC 5590 SNMP Transport Subsystem June 2009
The transport mappings defined in RFC 3417 do not provide lower-layer
security functionality, and thus do not provide transport-specific
security parameters. This document updates RFC 3411 and RFC 3417 by
defining an architectural extension and modifying the ASIs that
transport mappings (hereafter called "Transport Models") can use to
pass transport-specific security parameters to other subsystems,
including transport-specific security parameters that are translated
into the transport-independent securityName and securityLevel
parameters.
The Transport Security Model [RFC5591] and the Secure Shell Transport
Model [RFC5592] utilize the Transport Subsystem. The Transport
Security Model is an alternative to the existing SNMPv1 Security
Model [RFC3584], the SNMPv2c Security Model [RFC3584], and the User-
based Security Model [RFC3414]. The Secure Shell Transport Model is
an alternative to existing transport mappings as described in
[RFC3417].
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 leave the details to
them. Considerations of hiring and training employees, positioning
and scheduling the guards, arranging for cover, etc., are the
responsibility of the security company. The individual therefore
achieves the desired security, with significantly less effort on his
part except for identifying requirements and verifying the quality of
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 uses a separate principal and key
management infrastructure. Operators have reported that deploying
another principal and key management infrastructure in order to use
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RFC 5590 SNMP Transport Subsystem June 2009
SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use
external mechanisms to handle the distribution of keys for use by
USM. The more important issue is that operators wanted to leverage
existing user management infrastructures that were not specific to
SNMP.
A USM-compliant architecture might combine the authentication
mechanism with an external mechanism, such as RADIUS [RFC2865], to
provide the authentication service. Similarly, it might be possible
to utilize an external protocol to encrypt a message, to check
timeliness, to check data integrity, etc. However, this corresponds
to the second approach -- requiring the coordination of a number of
differently subcontracted services. Building solid security between
the various services is difficult, and there is a significant
potential for gaps in security.
An alternative approach might be to 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. This
corresponds to the third approach described above. 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 are TLS [RFC5246], Simple
Authentication and Security Layer (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 other network management protocols,
such as syslog [RFC5424] and NETCONF [RFC4741].
This document defines a Transport Subsystem extension to the RFC 3411
architecture that is 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 [RFC5246] or SSH [RFC4251],
within the RFC 3411 architecture.
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There are a number of Internet security protocols and mechanisms that
are in widespread 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.
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:
1. modification of information
2. masquerade
3. message stream modification
4. disclosure
These threats are described in Section 1.4 of [RFC3411]. The
security requirements outlined there do not require protection
against denial of service or traffic analysis; however, transport
security protocols 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.
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RFC 5590 SNMP Transport Subsystem June 2009
A Transport Model SHOULD NOT require modifications to the underlying
protocol. Modifying the protocol might change its security
characteristics in ways that could 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 specification
should include a description of potential impact on other usages of
the protocol.
Since multiple Transport Models can exist simultaneously within the
Transport Subsystem, Transport Models MUST be able to coexist with
each other.
3.2. SNMP Requirements
3.2.1. Architectural Modularity Requirements
SNMP version 3 (SNMPv3) is based on a modular architecture (defined
in Section 3 of [RFC3411]) to allow the evolution of the SNMP
protocol standards over time and to minimize the side effects between
subsystems when changes are made.
The RFC 3411 architecture includes a Message Processing Subsystem for
permitting different message versions to be handled by a single
engine, a Security Subsystem for enabling different methods of
providing security services, Applications to support different types
of Application processors, and an Access Control Subsystem for
allowing multiple approaches to access control. The RFC 3411
architecture does not include a subsystem for Transport Models,
despite the fact there are multiple transport mappings already
defined for SNMP [RFC3417]. This document describes a Transport
Subsystem that is compatible with the RFC 3411 architecture. As work
is being done to use secure transports 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 RFC
3411 architecture that are defined in Section 1.5 of [RFC3411]. This
Transport Subsystem uses a modular design that permits Transport
Models (which might or might not be security-aware) to be "plugged
into" the RFC 3411 architecture. Such Transport Models would be
independent of other modular SNMP components as much as possible.
This design also permits Transport Models to be advanced through the
standards process independently of other Transport Models.
The following diagram depicts the SNMPv3 architecture, including the
new Transport Subsystem defined in this document and a new Transport
Security Model defined in [RFC5591].
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RFC 5590 SNMP Transport Subsystem June 2009
+------------------------------+
| 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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RFC 5590 SNMP Transport Subsystem June 2009
3.2.1.1. Changes to the RFC 3411 Architecture
The RFC 3411 architecture and the Security Subsystem assume that a
Security Model is called by a Message Processing Model and will
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,
including 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 on this path will be extended to pass Security-Model-
independent values. This document amends some of the ASIs defined in
RFC 3411; these changes are covered in Section 6 of this document.
New Security Models might be defined that understand how to work with
these modified ASIs and the transport-information cache. One such
Security Model, the Transport Security Model, is defined in
[RFC5591].
3.2.1.2. Changes to RFC 3411 Processing
The introduction of secure transports affects the responsibilities
and order of processing within the RFC 3411 architecture. While the
steps are the same, they might occur in a different order, and might
be done by different subsystems. With the existing RFC 3411
architecture, security processing starts when the Message Processing
Model decodes portions of the encoded message to extract parameters
that identify which Security Model MUST handle the security-related
tasks.
A secure transport performs those security functions on the message,
before the message is decoded. Some of these functions might then be
repeated by the selected Security Model.
3.2.1.3. Passing Information between SNMP Engines
A secure Transport Model will establish an authenticated and possibly
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. While
the Community Security Models [RFC3584] and the User-based Security
Model establish a security association for each SNMP message, newer
Transport Models MAY support sending multiple SNMP messages through
the same tunnel to amortize the costs of establishing a security
association.
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3.2.2. Access Control Requirements
RFC 3411 made some design decisions related to the support of an
Access Control Subsystem. These include establishing and passing in
a model-independent manner the securityModel, securityName, and
securityLevel parameters, and separating message authentication from
data-access authorization.
3.2.2.1. securityName and securityLevel Mapping
SNMP data-access controls are expected to work on the basis of who
can perform what operations on which subsets of data, and based on
the security services that will be provided to secure the data in
transit. The securityModel and securityLevel parameters establish
the protections for transit -- whether authentication and privacy
services will be or have been applied to the message. The
securityName is a model-independent identifier of the security
"principal".
A Security Model plays a role in security that goes beyond protecting
the message -- it provides a mapping between the Security-Model-
specific principal for an incoming message to a Security-Model
independent securityName that can be used for subsequent processing,
such as for access control. The securityName is mapped from a
mechanism-specific identity, and this mapping must be done for
incoming messages by the Security Model before it passes securityName
to the Message Processing Model via the processIncoming ASI.
A Security Model is also responsible to specify, via the
securityLevel parameter, whether incoming messages have been
authenticated and encrypted, and to ensure that outgoing messages are
authenticated and encrypted based on the value of securityLevel.
A Transport Model MAY provide suggested values for securityName and
securityLevel. A Security Model might have multiple sources for
determining the principal and desired security services, and a
particular Security Model might or might not utilize the values
proposed by a Transport Model when deciding the value of securityName
and securityLevel.
Documents defining a new transport domain MUST define a prefix that
MAY be prepended to all securityNames passed by the Security Model.
The prefix MUST include one to four US-ASCII alpha-numeric
characters, not including a ":" (US-ASCII 0x3a) character. If a
prefix is used, a securityName is constructed by concatenating the
prefix and a ":" (US-ASCII 0x3a) character, followed by a non-empty
identity in an snmpAdminString-compatible format. The prefix can be
used by SNMP Applications to distinguish "alice" authenticated by SSH
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RFC 5590 SNMP Transport Subsystem June 2009
from "alice" authenticated by TLS. Transport domains and their
corresponding prefixes are coordinated via the IANA registry "SNMP
Transport Domains".
3.2.3. Security Parameter Passing Requirements
A Message Processing Model might unpack SNMP-specific security
parameters from an incoming message before calling a specific
Security Model to handle the security-related processing of the
message. When using a secure Transport Model, some security
parameters 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;
a Security Model might extract that information from the cache. A
tmStateReference is passed as an extra parameter in the ASIs between
the Transport Subsystem and the Message Processing and Security
Subsystems in order 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 RFC
3411 ASIs.
3.2.4. Separation of Authentication and Authorization
The RFC 3411 architecture defines a separation of authentication and
the authorization to access and/or modify MIB data. A set of model-
independent parameters (securityModel, securityName, and
securityLevel) are passed between the Security Subsystem, the
Applications, and the Access Control Subsystem.
This separation was a deliberate decision of the SNMPv3 WG, in order
to allow support for authentication protocols that do not provide
data-access authorization capabilities, and in order to support data-
access authorization schemes, such as the View-based access Control
Model (VACM), that do not perform their own authentication.
A Message Processing Model determines which Security Model is used,
either based on the message version (e.g., SNMPv1 and SNMPv2c) or
possibly by a value specified in the message (e.g., msgSecurityModel
field in SNMPv3).
The Security Model makes the decision which securityName and
securityLevel values are passed as model-independent parameters to an
Application, which then passes them via the isAccessAllowed ASI to
the Access Control Subsystem.
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RFC 5590 SNMP Transport Subsystem June 2009
An Access Control Model performs the mapping from the model-
independent security parameters to a policy within the Access Control
Model that is Access-Control-Model-dependent.
A Transport Model does not know which Security Model will be used for
an incoming message, and so cannot know how the securityName and
securityLevel parameters will be determined. It can propose an
authenticated identity (via the tmSecurityName field), but there is
no guarantee that this value will be used by the Security Model. For
example, non-transport-aware Security Models will typically determine
the securityName (and securityLevel) based on the contents of the
SNMP message itself. Such Security Models will simply not know that
the tmStateReference cache exists.
Further, even if the Transport Model can influence the choice of
securityName, it cannot directly determine the authorization allowed
to this identity. If two different Transport Models each
authenticate a transport principal that are then both mapped to the
same securityName, then these two identities will typically be
afforded exactly the same authorization by the Access Control Model.
The only way for the Access Control Model to differentiate between
identities based on the underlying Transport Model would be for such
transport-authenticated identities to be mapped to distinct
securityNames. How and if this is done is Security-Model-dependent.
3.3. Session Requirements
Some secure transports have a notion of sessions, while other secure
transports provide channels or other session-like mechanisms.
Throughout this document, the term "session" is used in a broad sense
to cover transport sessions, transport channels, and other transport-
layer, session-like mechanisms. Transport-layer sessions that can
secure multiple SNMP messages within the lifetime of the session are
considered desirable because the cost of authentication can be
amortized over potentially many transactions. How a transport
session is actually established, opened, closed, or maintained is
specific to a particular Transport Model.
To reduce redundancy, this document describes aspects that are
expected to be common to all Transport Model sessions.
3.3.1. No SNMP Sessions
The architecture defined in [RFC3411] and the Transport Subsystem
defined in this document do not support SNMP sessions or include a
session selector in the Abstract Service Interfaces.
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The Transport Subsystem might support transport sessions. However,
the Transport Subsystem does not have access to the pduType (i.e.,
the SNMP operation type), and so cannot select a given transport
session for particular types of traffic.
Certain parameters of the Abstract Service Interfaces might be used
to guide the selection of an appropriate transport session to use for
a given request by an Application.
The transportDomain and transportAddress identify the transport
connection to a remote network node. Elements of the transport
address (such as the port number) might be used by an Application to
send a particular PDU type to a particular transport address. For
example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are
used to configure notification originators with the destination port
to which SNMPv2-Trap PDUs or Inform PDUs are to be sent, but the
Transport Subsystem never looks inside the PDU.
The securityName identifies which security principal to communicate
with at that address (e.g., different Network Management System (NMS)
applications), and the securityLevel might permit selection of
different sets of security properties for different purposes (e.g.,
encrypted SET vs. non-encrypted GET operations).
However, because the handling of transport sessions is specific to
each Transport Model, some Transport Models MAY restrict selecting a
particular transport session. A user application might use a unique
combination of transportDomain, transportAddress, securityModel,
securityName, and securityLevel to try to force the selection of a
given transport session. This usage is NOT RECOMMENDED because it is
not guaranteed to be interoperable across implementations and across
models.
Implementations SHOULD be able to maintain some reasonable number of
concurrent transport sessions, and MAY provide non-standard internal
mechanisms to select transport sessions.
3.3.2. Session Establishment Requirements
SNMP Applications provide the transportDomain, transportAddress,
securityName, and securityLevel to be used to create a new session.
If the Transport Model cannot provide at least the requested level of
security, the Transport Model should discard the message and should
notify the Dispatcher that establishing a session and sending the
message failed. Similarly, if the session cannot be established,
then the message should be discarded and the Dispatcher notified.
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Transport session establishment might require provisioning
authentication credentials at an engine, either statically or
dynamically. How this is done is dependent on the Transport Model
and the implementation.
3.3.3. 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. How
an implementation determines that an operation has completed is
implementation-dependent. While it is possible to tear down each
transport session after processing for each message has completed,
this is not recommended for performance reasons.
The elements of procedure describe when cached information can be
discarded, 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 happens
to the objects when a related session is torn down, since this will
impact the interoperability of the MIB module.
3.3.4. 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 associated with the transport session SHOULD be
used to provide authentication, integrity checking, and encryption
services, as needed, 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. This would ensure that a cross-session
replay attack would be unsuccessful; that is, an attacker could not
take a message observed on one session and successfully replay it on
another session.
A good security protocol would also protect against replay attacks
within a session; that is, an attacker could not take a message
observed on a session and successfully replay it later in the same
session. One approach would be to use sequence information within
the protocol, allowing the participants to detect if messages were
replayed or reordered within a session.
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If a secure transport session is closed between the time a request
message is received and the corresponding response message is sent,
then the response message SHOULD be discarded, even if a new session
has been established. The SNMPv3 WG decided that this should be a
"SHOULD" architecturally, and it is a Security-Model-specific
decision whether to REQUIRE this. The architecture does not mandate
this requirement in order to allow for future Security Models where
this might make sense; however, not requiring this could lead to
added complexity and security vulnerabilities, so most Security
Models SHOULD require this.
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
command generator to poll for potentially non-sensitive performance
data on thousands of interfaces every ten minutes; such 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 security level requested by a transport-aware Security
Model, i.e., noAuthNoPriv and authNoPriv might be sent over an
authenticated and encrypted session. A Transport Model MUST NOT
downgrade the security level requested by a transport-aware Security
Model, and SHOULD discard any message where this would occur. This
is a SHOULD rather than a MUST only to permit the potential
development of models that can perform error-handling in a manner
that is less severe than discarding the message. However, any model
that does not discard the message in this circumstance should have a
clear justification for why not discarding will not create a security
vulnerability.
4. Scenario Diagrams and the Transport Subsystem
Sections 4.6.1 and 4.6.2 of RFC 3411 provide scenario diagrams to
illustrate how an outgoing message is created and how an incoming
message is processed. RFC 3411 does not define ASIs for the "Send
SNMP Request Message to Network", "Receive SNMP Response Message from
Network", "Receive SNMP Message from Network" and "Send SNMP message
to Network" arrows in these diagrams.
This document defines two ASIs corresponding to these arrows: a
sendMessage ASI to send SNMP messages to the network and a
receiveMessage ASI to receive SNMP messages from the network. These
ASIs are used for all SNMP messages, regardless of pduType.
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5. Cached Information and References
When performing SNMP processing, there are two levels of state
information that might need to be retained: the immediate state
linking a request-response pair and a potentially longer-term state
relating to transport and security.
The RFC 3411 architecture uses caches to maintain the short-term
message state, and uses references in the ASIs to pass this
information between subsystems.
This document defines the requirements for a cache to handle
additional short-term message state and longer-term transport state
information, using a tmStateReference parameter to pass this
information between subsystems.
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. 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 should also be
discarded.
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.
5.1. securityStateReference
The securityStateReference parameter is defined in RFC 3411. Its
primary purpose is to provide a mapping between a request and the
corresponding response. This cache is not accessible to Transport
Models, and an entry is typically only retained for the lifetime of a
request-response pair of messages.
5.2. tmStateReference
For each transport session, information about the transport security
is stored in a tmState cache or datastore that is referenced by a
tmStateReference. The tmStateReference parameter is used to pass
model-specific and mechanism-specific parameters between the
Transport Subsystem and transport-aware Security Models.
In general, when necessary, the tmState is populated by the Security
Model for outgoing messages and by the Transport Model for incoming
messages. However, in both cases, the model populating the tmState
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might have incomplete information, and the missing information might
be populated by the other model when the information becomes
available.
The tmState might contain both long-term and short-term information.
The session information typically remains valid for the duration of
the transport session, might be used for several messages, and might
be stored in a local configuration datastore. Some information has a
shorter lifespan, such as tmSameSecurity and
tmRequestedSecurityLevel, which are associated with a specific
message.
Since this cache is only used within an implementation, and not on-
the-wire, the precise contents and format of the cache are
implementation-dependent. For architectural modularity between
Transport Models and transport-aware Security Models, a fully-defined
tmState MUST conceptually include at least the following fields:
tmTransportDomain
tmTransportAddress
tmSecurityName
tmRequestedSecurityLevel
tmTransportSecurityLevel
tmSameSecurity
tmSessionID
The details of these fields are described in the following
subsections.
5.2.1. Transport Information
Information about the source of an incoming SNMP message is passed up
from the Transport Subsystem as far as the Message Processing
Subsystem. However, these parameters are not included in the
processIncomingMsg ASI defined in RFC 3411; hence, this information
is not directly available to the Security Model.
A transport-aware Security Model might wish to take account of the
transport protocol and originating address when authenticating the
request and setting up the authorization parameters. It is therefore
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necessary for the Transport Model to include this information in the
tmStateReference cache so that it is accessible to the Security
Model.
o tmTransportDomain: the transport protocol (and hence the Transport
Model) used to receive the incoming message.
o tmTransportAddress: the source of the incoming message.
The ASIs used for processing an outgoing message all include explicit
transportDomain and transportAddress parameters. The values within
the securityStateReference cache might override these parameters for
outgoing messages.
5.2.2. securityName
There are actually three distinct "identities" that can be identified
during the processing of an SNMP request over a secure transport:
o transport principal: the transport-authenticated identity on whose
behalf the secure transport connection was (or should be)
established. This value is transport-, mechanism-, and
implementation-specific, and is only used within a given Transport
Model.
o tmSecurityName: a human-readable name (in snmpAdminString format)
representing this transport identity. This value is transport-
and implementation-specific, and is only used (directly) by the
Transport and Security Models.
o securityName: a human-readable name (in snmpAdminString format)
representing the SNMP principal in a model-independent manner.
This value is used directly by SNMP Applications, the Access
Control Subsystem, the Message Processing Subsystem, and the
Security Subsystem.
The transport principal might or might not be the same as the
tmSecurityName. Similarly, the tmSecurityName might or might not be
the same as the securityName as seen by the Application and Access
Control Subsystems. In particular, a non-transport-aware Security
Model will ignore tmSecurityName completely when determining the SNMP
securityName.
However, it is important that the mapping between the transport
principal and the SNMP securityName (for transport-aware Security
Models) is consistent and predictable in order to allow configuration
of suitable access control and the establishment of transport
connections.
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5.2.3. securityLevel
There are two distinct issues relating to security level as applied
to secure transports. For clarity, these are handled by separate
fields in the tmStateReference cache:
o tmTransportSecurityLevel: an indication from the Transport Model
of the level of security offered by this session. The Security
Model can use this to ensure that incoming messages were suitably
protected before acting on them.
o tmRequestedSecurityLevel: an indication from the Security Model of
the level of security required to be provided by the transport
protocol. The Transport Model can use this to ensure that
outgoing messages will not be sent over an insufficiently secure
session.
5.2.4. Session Information
For security reasons, if a secure transport session is closed between
the time a request message is received and the corresponding response
message is sent, then the response message SHOULD be discarded, even
if a new session has been established. The SNMPv3 WG decided that
this should be a "SHOULD" architecturally, and it is a Security-
Model-specific decision whether to REQUIRE this.
o tmSameSecurity: this flag is used by a transport-aware Security
Model to indicate whether the Transport Model MUST enforce this
restriction.
o tmSessionID: in order to verify whether the session has changed,
the Transport Model must be able to compare the session used to
receive the original request with the one to be used to send the
response. This typically needs some form of session identifier.
This value is only ever used by the Transport Model, so the format
and interpretation of this field are model-specific and
implementation-dependent.
When processing an outgoing message, if tmSameSecurity is true, then
the tmSessionID MUST match the current transport session; otherwise,
the message MUST be discarded and the Dispatcher notified that
sending the message failed.
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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 introduces a couple of new ASIs to define the interface
between the Transport and Dispatcher Subsystems; it also extends some
of the ASIs defined in RFC 3411 to include transport-related
information.
This document follows the example of RFC 3411 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. Keeping sensitive security information
longer than necessary might introduce potential vulnerabilities to an
implementation.
2)An error indication in statusInformation will typically include the
Object Identifier (OID) and value for an incremented error counter.
This might be accompanied by values for contextEngineID and
contextName for this counter, a value for securityLevel, and the
appropriate state reference 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. The sendMessageASI
defined in this document replaces the text "Send SNMP Request Message
to Network" that appears in the diagram in Section 4.6.1 of RFC 3411
and the text "Send SNMP Message to Network" that appears in Section
4.6.2 of RFC 3411.
If present and valid, the tmStateReference refers to a cache
containing Transport-Model-specific parameters for the transport and
transport security. How a tmStateReference is determined to be
present and valid is implementation-dependent. How the information
in the cache is used is Transport-Model-dependent and implementation-
dependent.
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This might sound underspecified, but 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
tmStateReference cache.
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. Changes to RFC 3411 Outgoing ASIs
Additional parameters have been added to the ASIs defined in RFC 3411
that are concerned with communication between the Dispatcher and
Message Processing Subsystems, and between the Message Processing and
Security Subsystems.
6.2.1. Message Processing Subsystem Primitives
A tmStateReference parameter has been added as an OUT parameter to
the prepareOutgoingMessage and prepareResponseMessage ASIs. This is
passed from the Message Processing Subsystem to the Dispatcher, and
from there to the Transport Subsystem.
How or if the Message Processing Subsystem modifies or utilizes the
contents of the cache is Message-Processing-Model specific.
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
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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
)
statusInformation = -- success or errorIndication
prepareResponseMessage(
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 maxSizeResponseScopedPDU -- maximum size able to accept
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
OUT tmStateReference -- (NEW) reference to transport state
)
6.2.2. Security Subsystem Primitives
transportDomain and transportAddress parameters have been added as IN
parameters to the generateRequestMsg and generateResponseMsg ASIs,
and a tmStateReference parameter has been added as an OUT parameter.
The transportDomain and transportAddress parameters will have been
passed into the Message Processing Subsystem from the Dispatcher and
are passed on to the Security Subsystem. The tmStateReference
parameter will be passed from the Security Subsystem back to the
Message Processing Subsystem, and on to the Dispatcher and Transport
Subsystems.
If a cache exists for a session identifiable from the
tmTransportDomain, tmTransportAddress, tmSecurityName, and requested
securityLevel, then a transport-aware Security Model might create a
tmStateReference parameter to this cache and pass that as an OUT
parameter.
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statusInformation =
generateRequestMsg(
IN transportDomain -- (NEW) destination transport domain
IN transportAddress -- (NEW) destination transport address
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 generated message
OUT tmStateReference -- (NEW) reference to transport state
)
statusInformation =
generateResponseMsg(
IN transportDomain -- (NEW) destination transport domain
IN transportAddress -- (NEW) destination transport address
IN messageProcessingModel -- Message Processing Model
IN globalData -- msgGlobalData
IN maxMessageSize -- from msgMaxSize
IN securityModel -- as determined by MPM
IN securityEngineID -- the value of snmpEngineID
IN securityName -- on behalf of this principal
IN securityLevel -- for the outgoing message
IN scopedPDU -- as provided by MPM
IN securityStateReference -- as provided by MPM
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) reference to transport state
)
6.3. The receiveMessage ASI
The receiveMessage ASI is used to pass a message from the Transport
Subsystem to the Dispatcher. The receiveMessage ASI replaces the
text "Receive SNMP Response Message from Network" that appears in the
diagram in Section 4.6.1 of RFC 3411 and the text "Receive SNMP
Message from Network" from Section 4.6.2 of RFC3411.
When a message is received on a given transport session, if a cache
does not already exist for that session, the Transport Model might
create one, referenced by tmStateReference. The contents of this
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cache are discussed in Section 5. How this information is determined
is implementation- and Transport-Model-specific.
"Might create one" might sound underspecified, but a Transport Model
might be something like SNMP over UDP over IPv6, where transport
security is not provided, so it might not create a cache.
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.
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. Changes to RFC 3411 Incoming ASIs
The tmStateReference parameter has also been added to some of the
incoming ASIs defined in RFC 3411. How or if a Message Processing
Model or Security Model uses tmStateReference is message-processing-
and Security-Model-specific.
This might sound underspecified, but a Message Processing Model might
have access to all the information from the cache and from the
message. 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 specified to use a secure transport such as that provided by the
SSH Transport Model to send the message to its destination.
6.4.1. Message Processing Subsystem Primitive
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.
result = -- SUCCESS or errorIndication
prepareDataElements(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN wholeMsg -- as received from the network
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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
)
6.4.2. Security Subsystem Primitive
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.
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
)
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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 that Transport
Model.
It is considered desirable by some industry segments that SNMP
Transport Models 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 secrecy is appropriate for
that Transport Model.
The denial-of-service characteristics of various Transport Models and
security protocols will vary and should be evaluated when determining
the applicability of a Transport Model to a particular deployment
situation.
Since the cache 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 an 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.
A Security Model might have multiple sources from which to define the
securityName and securityLevel. The use of a secure Transport Model
does not imply that the securityName and securityLevel chosen by the
Security Model represent the transport-authenticated identity or the
transport-provided security services. The securityModel,
securityName, and securityLevel parameters are a related set, and an
administrator should understand how the specified securityModel
selects the corresponding securityName and securityLevel.
7.1. Coexistence, Security Parameters, and Access Control
In the RFC 3411 architecture, the Message Processing Model makes the
decision about which Security Model to use. The architectural change
described by this document does not alter that.
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The architecture change described by this document does, however,
allow SNMP to support two different approaches to security --
message-driven security and transport-driven security. With message-
driven security, SNMP provides its own security and passes security
parameters within the SNMP message; with transport-driven security,
SNMP depends on an external entity to provide security during
transport by "wrapping" the SNMP message.
Using a non-transport-aware Security Model with a secure Transport
Model is NOT RECOMMENDED for the following reasons.
Security Models defined before the Transport Security Model (i.e.,
SNMPv1, SNMPv2c, and USM) do not support transport-based security and
only have access to the security parameters contained within the SNMP
message. They do not know about the security parameters associated
with a secure transport. As a result, the Access Control Subsystem
bases its decisions on the security parameters extracted from the
SNMP message, not on transport-based security parameters.
Implications of combining older Security Models with Secure Transport
Models are known. The securityName used for access control decisions
is based on the message-driven identity, which might be
unauthenticated, and not on the transport-driven, authenticated
identity:
o An SNMPv1 message will always be paired with an SNMPv1 Security
Model (per RFC 3584), regardless of the transport mapping or
Transport Model used, and access controls will be based on the
unauthenticated community name.
o An SNMPv2c message will always be paired with an SNMPv2c Security
Model (per RFC 3584), regardless of the transport mapping or
Transport Model used, and access controls will be based on the
unauthenticated community name.
o An SNMPv3 message will always be paired with the securityModel
specified in the msgSecurityParameters field of the message (per
RFC 3412), regardless of the transport mapping or Transport Model
used. If the SNMPv3 message specifies the User-based Security
Model (USM) with noAuthNoPriv, then the access controls will be
based on the unauthenticated USM user.
o For outgoing messages, if a Secure Transport Model is selected in
combination with a Security Model that does not populate a
tmStateReference, the Secure Transport Model SHOULD detect the
lack of a valid tmStateReference and fail.
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In times of network stress, a Secure Transport Model might not work
properly if its underlying security mechanisms (e.g., Network Time
Protocol (NTP) or Authentication, Authorization, and Accounting (AAA)
protocols or certificate authorities) are not reachable. The User-
based Security Model was explicitly designed to not depend upon
external network services, and provides its own security services.
It is RECOMMENDED that operators provision authPriv USM as a fallback
mechanism to supplement any Security Model or Transport Model that
has external dependencies, so that secure SNMP communications can
continue when the external network service is not available.
8. IANA Considerations
IANA has created a new registry in the Simple Network Management
Protocol (SNMP) Number Spaces. The new registry is called "SNMP
Transport Domains". This registry contains US-ASCII alpha-numeric
strings of one to four characters to identify prefixes for
corresponding SNMP transport domains. Each transport domain MUST
have an OID assignment under snmpDomains [RFC2578]. Values are to be
assigned via [RFC5226] "Specification Required".
The registry has been populated with the following initial entries:
Registry Name: SNMP Transport Domains
Reference: [RFC2578] [RFC3417] [RFC5590]
Registration Procedures: Specification Required
Each domain is assigned a MIB-defined OID under snmpDomains
Prefix snmpDomains Reference
------- ----------------------------- ---------
udp snmpUDPDomain [RFC3417] [RFC5590]
clns snmpCLNSDomain [RFC3417] [RFC5590]
cons snmpCONSDomain [RFC3417] [RFC5590]
ddp snmpDDPDomain [RFC3417] [RFC5590]
ipx snmpIPXDomain [RFC3417] [RFC5590]
prxy rfc1157Domain [RFC3417] [RFC5590]
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.
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RFC 5590 SNMP Transport Subsystem June 2009
WG members who performed detailed reviews: Wes Hardaker, Jeffrey
Hutzelman, Tom Petch, Dave Shield, and Bert Wijnen.
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.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[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.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, 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.
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RFC 5590 SNMP Transport Subsystem June 2009
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen,
"Coexistence between Version 1, Version 2, and Version 3
of the Internet-standard Network Management Framework",
BCP 74, RFC 3584, August 2003.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5591] Harrington, D. and W. Hardaker, "Transport Security Model
for the Simple Network Management Protocol (SNMP)",
RFC 5591, June 2009.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
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RFC 5590 SNMP Transport Subsystem June 2009
Appendix A. 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.
2. One could add a header to encapsulate the SNMP message.
3. One could utilize fields already defined in the existing SNMPv3
message.
4. One could pass the information in an implementation-specific
cache or via a MIB module.
A.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 the Security Subsystem has the advantage
of being consistent with existing RFC 3411/3412 practice; it also
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 models or subsystems other than those
that are clearly defined by an ASI.
A.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
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) Message 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.
A.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
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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 in order for the Transport Model to know which fields could
be modified. This would seriously violate the modularity of the
architecture.
A.4. Using a Cache
This document describes a cache into which the Transport Model (TM)
puts information about the security applied to an incoming message; 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 that 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 RFC 3411 ASIs.
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Authors' Addresses
David Harrington
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: ietfdbh@comcast.net
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
EMail: j.schoenwaelder@jacobs-university.de
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