Network Working Group D. Harrington
Internet-Draft Futurewei Technologies
Expires: September 5, 2006 J. Schoenwaelder
International University Bremen
March 4, 2006
Transport Mapping Security Model (TMSM) Architectural Extension for the
Simple Network Management Protocol (SNMP)
draft-ietf-isms-tmsm-01.txt
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Copyright (C) The Internet Society (2006).
Abstract
This document describes a Transport Mapping Security Model (TMSM)
subsystem for the Simple Network Management Protocol (SNMP)
architecture defined in RFC 3411. This document identifies and
discusses some key aspects that need to be considered for any
transport-mapping-based security model for SNMP.
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This memo also defines a portion of the Management Information Base
(MIB) for managing the Transport Mapping Security Model Subsystem.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements of a Transport Mapping Security Model . . . . . . 6
2.1. Security Requirements . . . . . . . . . . . . . . . . . . 6
2.1.1. Security Protocol Requirements . . . . . . . . . . . . 6
2.2. Session Requirements . . . . . . . . . . . . . . . . . . . 7
2.2.1. Session Establishment Requirements . . . . . . . . . . 8
2.2.2. Session Maintenance Requirements . . . . . . . . . . . 8
2.2.3. Message security versus session security . . . . . . . 8
2.3. Architectural Modularity Requirements . . . . . . . . . . 9
2.3.1. USM and the RFC3411 Architecture . . . . . . . . . . . 12
2.3.2. TMSM and the RFC3411 Architecture . . . . . . . . . . 13
2.4. Passing Messages between Subsystems . . . . . . . . . . . 15
2.5. Security Parameter Passing Requirement . . . . . . . . . . 16
2.5.1. Define an Abstract Service Interface . . . . . . . . . 17
2.5.2. Using an Encapsulating Header . . . . . . . . . . . . 17
2.5.3. Modifying Existing Fields in an SNMP Message . . . . . 17
2.5.4. Using a Cache . . . . . . . . . . . . . . . . . . . . 18
2.6. Architectural Requirements for Access Control . . . . . . 18
2.6.1. securityName Binding . . . . . . . . . . . . . . . . . 18
2.6.2. Separation of Authentication and Authorization . . . . 19
2.7. Requirements for Notifications . . . . . . . . . . . . . . 20
3. Scenario Diagrams . . . . . . . . . . . . . . . . . . . . . . 21
3.1. Command Generator or Notification Originator . . . . . . . 21
3.2. Command Responder . . . . . . . . . . . . . . . . . . . . 22
4. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 23
5. TMSM Abstract Service Interfaces . . . . . . . . . . . . . . . 24
6. Integration with the SNMPv3 Message Format . . . . . . . . . . 26
6.1. msgVersion . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2. msgGlobalData . . . . . . . . . . . . . . . . . . . . . . 27
6.3. securityLevel and msgFlags . . . . . . . . . . . . . . . . 27
7. The tmStateReference for Passing Security Parameters . . . . . 28
8. securityStateReference Cached Security Data . . . . . . . . . 29
9. Prepare an Outgoing SNMP Message . . . . . . . . . . . . . . . 29
10. Prepare Data Elements from an Incoming SNMP Message . . . . . 30
11. Notifications . . . . . . . . . . . . . . . . . . . . . . . . 31
12. Transport Mapping Security Model Samples . . . . . . . . . . . 31
12.1. TLS/TCP Transport Mapping Security Model . . . . . . . . . 31
12.1.1. tmStateReference for TLS . . . . . . . . . . . . . . . 32
12.1.2. MPSP for TLS TM-Security Model . . . . . . . . . . . . 32
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12.1.3. MIB Module for TLS Security . . . . . . . . . . . . . 32
12.2. DTLS/UDP Transport Mapping Security Model . . . . . . . . 32
12.2.1. tmStateReference for DTLS . . . . . . . . . . . . . . 33
12.3. SASL Transport Mapping Security Model . . . . . . . . . . 34
12.3.1. tmStateReference for SASL DIGEST-MD5 . . . . . . . . 34
13. The TMSM MIB Module . . . . . . . . . . . . . . . . . . . . . 35
13.1. Structure of the MIB Module . . . . . . . . . . . . . . . 35
13.1.1. Textual Conventions . . . . . . . . . . . . . . . . . 35
13.1.2. The tmsmStats Subtree . . . . . . . . . . . . . . . . 35
13.1.3. The tmsmsSession Subtree . . . . . . . . . . . . . . . 35
13.1.4. The Notifications Subtree . . . . . . . . . . . . . . 35
13.2. Relationship to Other MIB Modules . . . . . . . . . . . . 36
13.2.1. Relationship to the SNMPv2-MIB . . . . . . . . . . . . 36
13.2.2. MIB Modules Required for IMPORTS . . . . . . . . . . . 36
14. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 36
15. Implementation Considerations . . . . . . . . . . . . . . . . 42
15.1. Applications that Benefit from Sessions . . . . . . . . . 42
15.2. Applications that Suffer from Sessions . . . . . . . . . . 43
15.2.1. Troubleshooting . . . . . . . . . . . . . . . . . . . 43
16. Security Considerations . . . . . . . . . . . . . . . . . . . 43
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45
19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
19.1. Normative References . . . . . . . . . . . . . . . . . . . 45
19.2. Informative References . . . . . . . . . . . . . . . . . . 47
Appendix A. Questions about msgFlags: . . . . . . . . . . . . . . 47
A.1. msgFlags versus actual security . . . . . . . . . . . . . 48
Appendix B. Parameter Table . . . . . . . . . . . . . . . . . . . 49
B.1. ParameterList.csv . . . . . . . . . . . . . . . . . . . . 49
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 50
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 51
Intellectual Property and Copyright Statements . . . . . . . . . . 51
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1. Introduction
This document describes a Transport Mapping Security Model (TMSM)
subsystem for the Simple Network Management Protocol (SNMP)
architecture defined in [RFC3411]. This document identifies and
discusses some key aspects that need to be considered for any
transport-mapping-based security 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].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. MIB objects are generally
accessed through the Simple Network Management Protocol (SNMP).
Objects in the MIB are defined using the mechanisms defined in the
Structure of Management Information (SMI). This memo specifies a MIB
module that is compliant to the SMIv2, which is described in STD 58,
RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
[RFC2580].
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].
Some points requiring further WG research and discussion are
identified by [discuss] markers in the text. Some points requiring
further editing by the editors are marked [todo] in the text.
1.3. Motivation
There are multiple ways to secure one's home or business, but they
largely boil down to a continuum of alternatives. Let's consider
three general approaches. In the first approach, an individual could
buy a gun, learn to use it, and sit on your front porch waiting for
intruders. In the second approach, one could hire an employee with a
gun, schedule the employee, position the employee to guard what you
want protected, hire a second guard to cover if the first gets sick,
and so on. In the third approach, you could hire a security company,
tell them what you want protected, and they could hire employees,
train them, buy the guns, position the guards, schedule the guards,
send a replacement when a guard cannot make it, etc., thus providing
the security you want, with no significant effort on your part other
than identifying requirements and verifying the quality of the
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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 (MD5=the gun), 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 user and key
management infrastructure. Operators have reported that deploying
another user and key management infrastructure in order to use SNMPv3
is a deterrent to deploying SNMPv3. It is possible but difficult to
define external mechanisms that handle the distribution of keys for
use by the USM approach.
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 [RFC2246], SASL
[RFC2222], 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 [I-D.ietf-netconf-ssh].
This document proposes a Transport Mapping Security Model (TMSM)
subsystem, as an extension of the RFC3411 architecture, that allows
security to be provided by an external protocol connected to the SNMP
engine through an SNMP transport-mapping. Such a TMSM would then
enable the use of existing security mechanisms such as (TLS)
[RFC2246] or SSH [RFC4251] within the RFC3411 architecture.
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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 TMSM 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 work without any surprises. These challenges are
discussed in detail in this document. For some key issues, design
choices are discussed that may be made to provide a workable solution
that meets operational requirements and fits into the SNMP
architecture defined in [RFC3411] .
2. Requirements of a Transport Mapping Security Model
2.1. Security Requirements
Transport mapping security protocols SHOULD ideally provide the
protection against the following message-oriented threats [RFC3411]:
1. modification of information
2. masquerade
3. message stream modification
4. disclosure
According to [RFC3411], it is not required to protect against denial
of service or traffic analysis.
2.1.1. Security Protocol Requirements
There are a number of standard protocols that could be proposed as
possible solutions within the TMSM framework. Some factors should be
considered when selecting a protocol for use within this framework.
Using a protocol in a manner for which it was not designed has
numerous problems. The advertised security characteristics of a
protocol may depend on its being used as designed; when used in other
ways, it may not deliver the expected security characteristics. It
is recommended that any proposed model include a discussion of the
applicability statement of the protocols to be used.
A protocol used for the TMSM framework should ideally require no
modifications to the protocol. Modifying the protocol may 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. It is recommended that
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any proposed model include a discussion of potential impact on other
usages of the protocol.
It has been a long-standing requirement that SNMP be able to work
when the network is unstable, to enable network troubleshooting and
repair. The UDP approach has been considered to meet that need well,
with an assumption that getting small messages through, even if out
of order, is better than getting no messages through. There has been
a long debate about whether UDP actually offers better support than
TCP when the underlying IP or lower layers are unstable. There has
been recent discussion of whether operators actually use SNMP to
troubleshoot and repair unstable networks.
There has been discussion of ways SNMP could be extended to better
support management/monitoring needs when a network is running just
fine. Use of a TCP transport, for example, could enable larger
message sizes and more efficient table retrievals.
TMSM models MUST be able to coexist with other protocol models, and
may be designed to utilize either TCP or UDP, depending on the
transport.
2.2. Session Requirements
Throughout this document, the term session is used. Some underlying
secure transports will have a notion of session. Some underlying
secure transports might enable the use of channels or other session-
like thing. In this document the term session refers to an
association between two SNMP engines, that permits the secure
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 security model.
Sessions are not part of the SNMP architecture described in
[RFC3411], but are considered desirable because the cost of
authentication can be amortized over potentially many transactions.
It is important to note that the architecture described in [RFC3411]
does not include a session selector in the Abstract Service
Interfaces, and neither is that done for this architectural
extension, so an SNMP application cannot select the session except by
passing a unique combination of securityName, securityModel, and
securityLevel.
All TMSM-based security models should discuss the impact of sessions
on SNMP usage, including how to establish/open a TMSM session (i.e.
how it maps to the concepts of session-like things of the underlying
protocol), how to behave when a TMSM session cannot be established,
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how to close a TMSM session (and the underlying protocol equivalent)
properly, how to behave when a TMSM session is closed improperly, the
session security properties, session establishment overhead, and
session maintenance overhead.
To reduce redundancy, this document will discuss aspects that are
expected to be common to all TMSM-based security model sessions.
2.2.1. Session Establishment Requirements
[todo] contributions welcome.
2.2.2. Session Maintenance Requirements
[todo] contributions welcome.
2.2.3. Message security versus session security
A TMSM session is associated with state information that is
maintained for its lifetime. This state information allows for the
application of various security services to TMSM-based security
models. 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
TMSM-based security model session SHOULD ensure that fresh new
session keys are generated for each session. If each session uses
new session keys, then messages cannot be replayed from one session
to another. In addition sequence information MAY be maintained in
the session which can be used to prevent the replay and reordering of
messages within a session.
A TMSM session will typically have a single securityName and
securityLevel associated with it. If an exchange between
communicating engines would require a different securityLevel or
would be on behalf of a different securityName, 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 TMSM models, securityName is typically 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
there is not much value in using encryption for a Commander Generator
to poll for non-sensitive performance data on thousands of interfaces
every ten minutes; the encryption adds significant overhead to
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processing of the messages.
Some TMSM-based security models MAY 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.
Some security models may use an underlying transport that provides a
per-message requested level of authentication and encryption
services. For example, if a session is created as 'authPriv', then
keys for encryption could still be negotiated once at the beginning
of the session. But if a message is presented to the session with a
security level of authNoPriv, then that message could simply be
authenticated and not encrypted within the same transport session.
Whether this is possible depends on the security model and the secure
transport used.
If the underlying transport layer security was configurable on a per-
message basis, a TMSM-based security model could have a security-
model-specific MIB module with configurable maxSecurityLevel and a
minSecurityLevel objects to identify the range of possible levels. A
session's maxSecurityLevel would identify the maximum security it
could provide, and a session created with a minSecurityLevel of
authPriv would reject an attempt to send an authNoPriv message. The
elements of procedure of the security model would need to describe
the procedures to enable this determination.
For security models that do not support variable security services in
one session, multiple sessions could be established, with different
security levels, and for every packet the SNMP engine could select
the appropriate session based on the requested securityLevel. Some
SNMP entities are resource-constrained. Adding sessions increases
the need for resources, but so does encrypting unnecessarily.
Designers of security models should consider the tradeoffs for
resource-constrained devices.
2.3. Architectural Modularity Requirements
SNMP version 3 (SNMPv3) is based on a modular architecture (described
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. This architecture includes a Security
Subsystem which is responsible for realizing security services.
In SNMPv2, there were many problems of side effects between
subsystems caused by the manipulation of MIB objects, especially
those related to authentication and authorization, because many of
the parameters were stored in shared MIB objects, and different
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models and protocols could assign different values to the objects.
Contributors assumed slightly different shades of meaning depending
on the models and protocols being used. As the shared MIB module
design was modified to accommodate a specific model, other models
which used the same MIB objects were broken.
Abstract Service Interfaces (ASIs) were developed to pass model-
independent parameters. The models were required to translate from
their model-dependent formats into a model-independent format,
defined using model-independent semantics, which would not impact
other models.
Parameters have been provided in the ASIs to pass model-independent
information about the authentication that has been provided. These
parameters include a model-independent identifier of the security
"principal", the security model used to perform the authentication,
and which SNMP-specific security features were applied to the message
(authentication and/or privacy).
The design of a transport mapping security model must abide the goals
of the RFC3411 architecture defined in [RFC3411]. To that end, this
transport mapping security model proposal focuses on a modular
subsystem that can be advanced through the standards process
independently of other proposals, and independent of other subsystems
as much as possible.
There has been some discussion of maintaining multiple sessions for
different security levels or for different applications. The ability
to have an application select different sessions or connections on a
per-message basis would likely require a modification to the SNMP
architecture to provide new ASIs, which is out of scope for this
document.
[discuss] I am not sure whether the previous paragraph is still
correct - I think we need to solve at least some of the session
problem space.
IETF standards typically require one mandatory-to-implement solution,
with the capability of adding new security mechanisms in the future.
Any transport mapping security model should define one minimum-
compliance mechanism, preferably one which is already widely deployed
within the transport layer security protocol used.
The TMSM subsystem is designed as an architectural extension that
permits additional transport security protocols to be "plugged into"
the RFC3411 architecture, supported by corresponding transport-
security-aware transport mapping models.
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The RFC3411 architecture, and the USM approach, assume that a
security model is called by a message-processing model and will
perform multiple security functions. The TMSM approach performs
similar functions but performs them in different places within the
architecture, so we need to distinguish the two locations for
security processing.
Transport mapping security is by its very nature a security layer
which is plugged into the RFC3411 architecture between the transport
layer and the message dispatcher. Conceptually, transport mapping
security processing will be called from within the Transport Mapping
functionality of an SNMP engine dispatcher to perform the translation
of transport security parameters to/from security-model-independent
parameters. This transport mapping security processor will be
referred to in this document as TMSP.
Additional functionality may be performed as part of the message
processing function, i.e. in the security subsystem of the RFC3411
architecture. This document will refer to message processor's
security processor as the MPSP.
Thus a TMSM is composed of both a TPSP and an MPSP.
+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-----+ +-----+ +-------+
| UDP | | TCP | . . . | other |
+-----+ +-----+ +-------+
^ ^ ^
| | |
v v v
+-----+ +-----+ +-------+
| SSH | | TLS | . . . | other |
+-----+ +-----+ +-------+ (traditional SNMP agent)
+-------------------------------------------------------------------+
| ^ |
| | |
| Dispatcher v |
| +-------------------+ |
| | Transport | +--------------+ |
| | Mapping |<---> | TMSM | |
| | (e.g., RFC 3417) | | TMSP | |
| | | +--------------+ |
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| | | |
| | | +---------------------+ +----------------+ |
| | | | Message Processing | | Security | |
| | | | Subsystem | | Subsystem | |
| | | | +------------+ | | | |
| | | | +->| v1MP * |<--->| +------------+ | |
| | | | | +------------+ | | | Other | | |
| | | | | +------------+ | | | Security | | |
| | | | +->| v2cMP * |<--->| | Model | | |
| | Message | | | +------------+ | | +------------+ | |
| | Dispatcher <--------->| +------------+ | | +------------+ | |
| | | | +->| v3MP * |<--->| | TMSM | | |
| | | | | +------------+ | | | MPSP | | |
| | PDU Dispatcher | | | +------------+ | | | | | |
| +-------------------+ | +->| otherMP * |<--->| +------------+ | |
| ^ | +------------+ | | | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
2.3.1. USM and the RFC3411 Architecture
The following diagrams illustrate the difference in the security
processing done by the USM model and the security processing done by
a TMSM model.
The USM security model is encapsulated by the messaging model,
because the messaging model needs to perform the following steps (for
incoming messages)
1) decode the ASN.1 (messaging model)
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2) determine the SNMP security model and parameters (messaging model)
3) decrypt the encrypted portions of the message (security model)
4) translate parameters to model-independent parameters (security
model)
5) determine which application should get the decrypted portions
(messaging model), and
6) pass on the decrypted portions with model-independent parameters.
The USM approach uses SNMP-specific message security and parameters.
| -----------------------------------------------|
| transport layer |
| -----------------------------------------------|
^
|
v
--------------------------------------------------
| -----------------------------------------------|
| | transport mapping |
| -----------------------------------------------|
| ^
| |
| v
| --------------------------------------------- |
| --------------------- ------------------ |
| SNMP messaging <--> | decryption + | |
| | translation | |
| --------------------- ------------------ |
| ^
| |
| v
| --------------------- ------------------ |
| | SNMP applications | <--> | access control | |
| --------------------- ------------------ |
| --------------------------------------------- |
2.3.2. TMSM and the RFC3411 Architecture
In the TMSM approach, the order of the steps differ and may be
handled by different subsystems:
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1) decrypt the encrypted portions of the message (transport layer)
2) determine the SNMP security model and parameters (transport
mapping)
3*) translate parameters to model-independent parameters (transport
mapping)
4) decode the ASN.1 (messaging model)
5) determine which application should get the decrypted portions
(messaging model)
6*) translate parameters to model-independent parameters (security
model)
7) pass on the decrypted portions with model-independent security
parameters
This is largely based on having non-SNMP-specific message security
and parameters. The transport mapping model might provide the
translation from e.g., an SSH user name to the securityName in step
3, OR the SSH user might be passed to the messaging model to pass to
a TMSM security model to do the translation in step 6, if the WG
decides all translations should use the same translation table (e.g.,
the USM MIB).
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| -----------------------------------------------|
| ------------------ |
| transport layer <--> | decryption | |
| ------------------ |
| -----------------------------------------------|
^
|
v
--------------------------------------------------
| -----------------------------------------------|
| ------------------ |
| transport mapping <--> | translation* | |
| ------------------ |
| -----------------------------------------------|
| ^
| |
| v
| --------------------------------------------- |
| ------------------ |
| SNMP messaging <--> | translation* | |
| ------------------ |
| --------------------- ------------------ |
| ^
| |
| v
| --------------------- ------------------ |
| | SNMP applications | <--> | access control | |
| --------------------- ------------------ |
| --------------------------------------------- |
2.4. Passing Messages between Subsystems
RFC3411 defines ASIs that describe the passing of messages between
subsystem within an engine, and the parameters which are expected to
be passed between the subsystems. The ASIs generally pass model-
independent information.
A TMSM model will establish an encrypted tunnel between the transport
mappings of two SNMP engines. One transport mapping security model
instance encrypts all messages, and the other transport mapping
security model instance decrypts the messages.
After the transport layer tunnel is established, then SNMP messages
can conceptually be sent through the tunnel from one SNMP message
dispatcher to another SNMP message dispatcher. Once the tunnel is
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established, multiple SNMP messages may be able to be passed through
the same tunnel.
Within an engine, outgoing SNMP messages are passed unencrypted from
the message dispatcher to the transport mapping, and incoming
messages are passed unencrypted from the transport mapping to the
message dispatcher.
2.5. Security Parameter Passing Requirement
RFC3411 section 4 describes primitives to describe the abstract
service interfaces used to conceptually pass information between the
various subsystems, models and applications within the architecture.
The security parameters include a model-independent identifier of the
security "principal", the security model used to perform the
authentication, and which SNMP-specific security services were
(should be) applied to the message (authentication and/or privacy).
In the RFC3411 architecture, the messaging model must unpack SNMP-
specific security parameters from the message before calling a
security model to authenticate and decrypt an incoming message,
perform integrity checking, and translate model-specific security
parameters into model-independent parameters. In the TMSM approach,
the security-model specific parameters are not all carried in the
SNMP message, and can be determined from the transport layer by the
transport mapping, before the message processing begins.
[discuss] For outgoing messages, it is necessary to have an MPSP
because it is the MPSP that actually creates the message from its
component parts. Does the MPSP need to know the transport address or
the actual transport security capabilities, or can this be handled in
the TMSP, given the model-independent (and message-version-
independent) parameters? Are there any security services provided by
the MPSP for an outgoing message?
[discuss] For incoming messages, is there security functionality that
can only be handled after the message version is known, such as the
comparison of transport security capabilities and msgFlags? Does
that functionality need to know the transport address and session or
just the model-independent security parameters (securityName, model,
level)? Are there any SNMP-specific parameters that need to be
unpacked from the message for MPSP handling? msgFlags, securityLevel,
etc.?
The RFC3411 architecture has no ASI parameters for passing security
information between the transport mapping and the dispatcher, and
between the dispatcher and the message processing model. If there is
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a need to have an MPSP called from the message processing model to,
for example, verify that msgFlags and the transport security are
consistent, then it will be necessary to pass the model-independent
security parameters from the TPSP through to the MPSP.
There are four approaches that could be used for passing information
between the TMSP and an MPSP.
1. one could define an ASI to supplement the existing ASIs, or
2. the TMSM could add a header to encapsulate the SNMP message,
3. the TMSM could utilize fields already defined in the existing
SNMPv3 message, or
4. the TMSM could pass the information in an implementation-specific
cache or via a MIB module.
2.5.1. Define an Abstract Service Interface
Abstract Service Interfaces (ASIs) [RFC3411] 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 mapping to a messaging security model
has the advantage of being consistent with existing RFC3411/3412
practice, and helps to ensure that any TMSM 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.
2.5.2. Using an Encapsulating Header
A header could encapsulate the SNMP message to pass necessary
information from the TMSP to the dispatcher and then to a messaging
security model. The message header would be included in the
wholeMessage ASI parameter, and would be removed by a corresponding
messaging 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 MPSP.
2.5.3. Modifying Existing Fields in an SNMP Message
[RFC3412] describes the SNMPv3 message, which contains fields to pass
security related parameters. The TMSM could use these fields in an
SNMPv3 message, or comparable fields in other message formats to pass
information between transport mapping security models in different
SNMP engines, and to pass information between a transport mapping
security model and a corresponding messaging security model.
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If the fields in an incoming SNMPv3 message are changed by the TMSP
before passing it to the MPSP, then the TMSP 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 TMSP knew which fields
could be modified. This would seriously violate the modularity of
the architecture.
2.5.4. Using a Cache
A cache mechanism could be used, into which the TMSP puts information
about the security applied to an incoming message, and an MPSP
extracts that information from the cache. Given that there may be
multiple TM-security caches, a cache ID would need to be passed
through an ASI so the MPSP knows which cache of information to
consult.
The cache reference could be thought of as an additional parameter in
the ASIs between the transport mapping and the messaging security
model. The RFC3411 ASIs would not need to be changed since the
SNMPv3 WG expected that additional parameters could be passed for
value-add features of specific implementations.
This approach does create dependencies between a model-specific TPSP
and a corresponding specific MPSP. If a TMSM-model-independent ASI
parameter is passed, this approach would be consistent with the
securityStateReference cache already being passed around in the ASI.
This document will describe a cache-based approach.
2.6. Architectural Requirements for Access Control
2.6.1. securityName Binding
For SNMP access control to function properly, the security mechanism
must establish a securityModel identifier, a securityLevel, and a
securityName, which is the security model independent identifier for
a principal. The SNMPv3 message processing architecture 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 USM-specific principal to a security-model independent
securityName which can be used for subsequent processing, such as for
access control.
The TMSM is a two-stage security model, with a transport mapping
security process (TMSP) and a message processing security process
(MPSP). Depending on the design of the specific TMSM model, i.e.
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which transport layer protocol is used, different features might be
provided by the TMSP or by the MPSP. For example, the translation
from a mechanism-specific authenticated identity to a securityName
might be done by the TMSP or by the MPSP.
[discuss] It may be possible to define a consistent division of
stages regardless of the transport layer protocol used, and a
consistent division of functionality would be preferred.
The SNMP architecture distinguishes between messages with no
authentication and no privacy (noAuthNoPriv), authentication without
privacy (authNoPriv) and authentication with privacy (authPriv).
Hence, the authentication of a transport layer identity plays an
important role and must be considered by any TMSM, and user
authentication must be available via the transport layer security
protocol.
If the type of authentication provided by the transport layer (e.g.
host-based or anonymous) is considered adequate to secure and/or
encrypt the message, but inadequate to provide the desired
granularity of access control (e.g. user-based), a second
authentication, e.g. one provided by a AAA server, may be used to
provide the authentication identity which is bound to the
securityName. This approach would require a good analysis of the
potential for man-in-the-middle attacks or masquerade possibilities.
2.6.2. Separation of Authentication and Authorization
A TMSM security model should take care to not violate the separation
of authentication and authorization in the RFC3411 architecture..
The isAccessAllowed() primitive is used for passing security-model
independent parameters between the subsystems of the architecture.
Mapping of (securityModel, securityName) to an access control policy
should be handled within the access control subsystem, not the
security subsystem, 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 capabilities, and to support
authorization schemes, such as VACM, that do not perform their own
authentication.
An authorization model MAY require authentication by certain
securityModels and a minimum securityLevel to allow access to the
data.
TMSM is an enhancement for the SNMPv3 privacy and authentication
provisions, but it is not a significant improvement for the
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authorization needs of SNMPv3. TMSM provides all the model-
independent parameters for the isAccessAllowed() primitive [RFC3411].
TMSM does not specify how the securityModel and securityName could be
dynamically mapped to a VACM-style groupName. The mapping of
(securityModel, securityName) to a groupName is a VACM-specific
mechanism for naming an access control policy, and for tying the
named policy to the addressing capabilities of the data modeling
language (e.g. SMIv2 [RFC2578]), the operations supported, and other
factors. 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,
Windows domains, XML hierarchies, or task-based controls. The
preferred approach is to pass the model-independent security
parameters via the isAccessAllowed() ASI, and perform the mapping
within the access control model.
To provide support for protocols which simultaneously send
information for authentication and authorization, such as RADIUS
[RFC2865], model-specific authorization information MAY be cached or
otherwise made available to the access control subsystem, e.g. via a
MIB table similar to the vacmSecurityToGroupTable, so the access
control subsystem can create an appropriate binding between the
model-independent securityModel and securityName and a model-specific
access control policy. This may be highly undesirable, however, if
it creates a dependency between a security model and an access
control model, just as it is undesirable that the TMSM approach
creates a dependency between a TMSP and an MPSP.
2.7. Requirements for Notifications
[todo] cleanup this section
RFC 3430 (SNMP over TCP) suggests that TCP connections are initiated
by notification originators in case there is no currently established
connection that can be used to send the notification. Following this
approach with SSH would require to provision authentication
credentials on the agent so that agents can successfully authenticate
to a notification receiver. There might be other approaches, like
the reuse of manager initiated secure transport connections for
notifications. There is some text in Appendix A in RFC 3430 which
captures some of these discussions when RFC 3430 was written.
[todo] merge this text and text from RFC 3430 into the section
dealing with sessions? This seems to be the right place for this
discussion.
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3. Scenario Diagrams
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 the ASI for sending an SNMP request to the
network or receiving an SNMP response message from the network. In
section 4.6.2, the diagram doesn't illustrate the interfaces required
to receive an SNMP message from the network, or to send an SNMP
message to the network.
3.1. 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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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 | | |
|<-------------------| | |
| | | |
3.2. 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) send back to the network.
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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 | |
4. Abstract Service Interfaces
The OUT parameters of the prepareOutgoingMessage() ASI are used to
pass information from the message processing model to the dispatcher
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and on to the transport mapping:
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
)
5. TMSM Abstract Service Interfaces
A set of abstract service interfaces have been defined within this
document to describe the conceptual data flows between the Transport
Mapping Security Models and adjacent components in the system..
The SendMessage ASI is used to pass a message from the Dispatcher to
the transport mapping security model subsystem for sending.
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 --
OUT sessionID
)
The RecvMessage ASI is used to pass a message from the transport
mapping security model subsystem to the Dispatcher.
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statusInformation RecvMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN incomingMessage -- the message received
IN incomingMessageLength -- its length
OUT tmStateReference --
OUT sessionID
)
The Transport Mapping Security Model provides the following
primitives to pass data back and forth between the TMSM and specific
TMSM-based security models, which provide the interface to the
underlying secure transport service. Each TMSM-based security model
should define the security-model-specific elements of procedure for
the establishSession(), closeSession(), TxMessage(), and RxMessage()
interfaces.
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statusInformation TxMessage(
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 --
OUT sessionID
)
statusInformation RxMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN incomingMessage -- the message to send
IN incomingMessageLength -- its length
OUT tmStateReference --
)
statusInformation establishSession(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN tmStateReference --
OUT sessionID
)
statusInformation closeSession(
IN sessionID
)
6. Integration with the SNMPv3 Message Format
TMSM proposals can use the SNMPv3 message format, defined in RFC3412,
section 6. This section discusses how the fields could be reused.
6.1. msgVersion
For proposals that reuse the SNMPv3 message format, this field should
contain the value 3.
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6.2. msgGlobalData
The fields msgID and msgMaxSize are used identically for the TMSM
models as for the USM model.
The msgSecurityModel field should be set to a value from the
SnmpSecurityModel enumeration [RFC3411] to identify the specific TMSM
model. Each standards-track TMSM model should have an enumeration
assigned by IANA. Each enterprise-specific security model should
have an enumeration assigned following instructions in the
description of the SnmpSecurityModel TEXTUAL-CONVENTION from RFC3411.
The msgSecurityParameters field would carry security information
required for message security processing. It is unclear whether this
field would be useful or what parameters would be carried to support
security, since message security is provided by an external process,
and msgSecurityParameters are not used by the access control
subsystem.
RFC3412 defines two primitives, generateRequestMsg() and
processIncomingMsg() which require the specification of an
authoritative SNMP entity. [discuss] We need to discuss what the
meaning of authoritative would be in a TMSM environment, whether the
specific services provided in USM security from msgSecurityParameters
still are needed, and how the Message Processing model provides this
information to the security model via generateRequestMsg() and
processIncomingMsg() primitives. RFC3412 specifies that "The data in
the msgSecurityParameters field is used exclusively by the Security
Model, and the contents and format of the data is defined by the
Security Model. This OCTET STRING is not interpreted by the v3MP,
but is passed to the local implementation of the Security Model
indicated by the msgSecurityModel field in the message."
The msgFlags have the same values for the TMSM models as for the USM
model. "The authFlag and privFlag fields indicate the securityLevel
that was applied to the message before it was sent on the wire."
6.3. securityLevel and msgFlags
For an outgoing message, msgFlags is the requested security for the
message; if a TMSM cannot provide the requested securityLevel, the
model MUST describe a standard behavior that is followed for that
situation. If the TMSM cannot provide at least the requested level
of security, the TMSM MUST discard the request and SHOULD notify the
message processing model that the request failed.
[discuss] how is yet to be determined, and may be model-specific or
implementation-specific.
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For an outgoing message, if the TMSM is able to provide stronger than
requested security, that may be acceptable. The transport layer
protocol would need to indicate to the receiver what security has
been applied to the actual message. To avoid the need to mess with
the ASN.1 encoding, the SNMPv3 message carries the requested
msgFlags, not the actual securityLevel applied to the message. If a
message format other than SNMPv3 is used, then the new message may
carry the more accurate securityLevel in the SNMP message.
For an incoming message, the receiving TMSM knows what must be done
to process the message based on the transport layer mechanisms. If
the underlying transport security mechanisms for the receiver cannot
provide the matching securityLevel, then the message should follow
the standard behaviors for the transport security mechanism, or be
discarded silently.
Part of the responsibility of the TMSM is to ensure that the actual
security provided by the underlying transport layer security
mechanisms is configured to meet or exceed the securityLevel required
by the msgFlags in the SNMP message. When the MPSP processes the
incoming message, it should compare the msgFlags field to the
securityLevel actually provided for the message by the transport
layer security. If they differ, the MPSP should determine whether
the changed securityLevel is acceptable. If not, it should discard
the message. Depending on the model, the MPSP may issue a reportPDU
with the XXXXXXX model-specific counter.
7. The tmStateReference for Passing Security Parameters
A tmStateReference is used to pass data between the TMSP and the
MPSP, similar to the securityStateReference described in RFC3412.
This can be envisioned as being appended to the ASIs between the TM
and the MP or as being passed in an encapsulating header.
The TMSP may provide only some aspects of security, and leave some
aspects to the MPSP. tmStateReference should be used to pass any
parameters, in a model- and mechanism-specific format, that will be
needed to coordinate the activities of the TMSP and MPSP, and the
parameters subsequently passed in securityStateReference. For
example, the TMSP may provide privacy and data integrity and
authentication and authorization policy retrievals, or some subset of
these features, depending on the features available in the transport
mechanisms. A field in tmStateReference should identify which
services were provided for each received message by the TMSP, the
securityLevel applied to the received message, the model-specific
security identity, the session identifier for session based transport
security, and so on.
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8. securityStateReference Cached Security Data
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.
A Message Processing Model has the responsibility for explicitly
releasing the cached data if such data is no longer needed. To
enable this, an abstract securityStateReference data element is
passed from the Security Model to the Message Processing Model. The
cached security data may be implicitly released via the generation of
a response, or explicitly released by using the stateRelease
primitive, as described in RFC3411 section 4.5.1."
For the TMSM approach, the TMSP may need to provide information to
the message processing model, such as the security-model-independent
securityName, securityLevel, and securityModel parameters, and for
responses, the messaging model may need to pass the parameters back
to the TMSP. To differentiate what information needs to be provided
to the message processing model by the TMSP, and vice-versa, this
document will differentiate the tmStateReference provide by the TMSP
from the securityStateReference provided by the MPSP. An
implementation MAY 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 the transport
mapping has had an opportunity to extract the information it needs.
9. Prepare an Outgoing SNMP Message
Following RFC3412, section 7.1, the SNMPv3 message processing model
uses the generateResponseMsg() or generateRequestMsg() primitives, to
call the MPSP. The message processing model, or the MPSP it calls,
may need to put information into the tmStateReference cache for use
by the TMSP, such as:
tmSecurityStateReference - the unique identifier for the cached
information
tmTransportDomain
tmTransportAddress
tmSecurityModel - an indicator of which mechanisms to use
tmSecurityName - a model-specific identifier of the security
principal
tmSecurityLevel - an indicator of which security services are
requested
and may contain additional information such as
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tmSessionID
tmSessionKey
tmSessionMsgID
According to RFC3411, section 4.1.1, the application provides the
transportDomain and transportAddress to the PDU dispatcher via the
sendPDU() primitive. If we permit multiple sessions per
transportAddress, then we would need to define how session
identifiers get passed from the application to the PDU dispatcher
(and then to the MP model).
The SNMP over TCP Transport Mapping document [RFC3430] says that TCP
connections can be recreated dynamically or kept for future use and
actually leaves all that to the transport mapping.
[discuss] we might define a new transportDomain and transportAddress,
which includes the address and session identifier. For situations
where a session has not yet been established, we could pass a 0x0000
session identifier (or whatever) to indicate that a session should be
established. Well, this won't work with the current TAddress
definitions and is probably too ugly to do.
We might have a MIB module that records the session information for
subsequent use by the applications and other subsystems, or it might
be passed in the tmStateReference cache. For notifications, I assume
the SNMPv3 notification tables would be a place to find the address,
but I'm not sure how to identify the presumably-dynamic session
identifiers. The MIB module could identify whether the session was
initiated by the remote engine or initiated by the current engine,
and possibly assigned a purpose (incoming request/response or
outgoing notifications). First we need to decide whether to handle
notifications and requests in one or two (or more) sessions, which
might depend on the transport protocol we choose (the same problem
netconf faced).
10. Prepare Data Elements from an Incoming SNMP Message
For an incoming message, the TMSP will need to put information from
the transport mechanisms used into the tmStateReference so the MPSP
can extract the information and add it conceptually to the
securityStateReference.
The tmStateReference cache will likely contain at least the following
information:
tmStateReference - a unique identifier for the cached information
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tmSecurityStateReference - the unique identifier for the cached
information
tmTransportDomain
tmTransportAddress
tmSecurityModel - an indicator of which mechanisms to use
tmSecurityName - a model-specific identifier of the security
principal
tmSecurityLevel - an indicator of which security services are
requested
tmAuthProtocol
tmPrivProtocol
and may contain additional information such as
tmSessionID
tmSessionKey
tmSessionMsgID
11. Notifications
For notifications, if the cache has been released and then session
closed, then the MPSP will request the TMSP to establish a session,
populate the cache, and pass the securityStateReference to the MPSP.
[discuss] We need to determine what state needs to be saved here.
12. Transport Mapping Security Model Samples
There are a number of standard protocols that could be proposed as
possible solutions within the TMSM framework. Some factors should be
considered when selecting a protocol for use within this framework.
Using a protocol in a manner for which is was not designed has
numerous problems. The advertised security characteristics of a
protocol may depend on its being used as designed; when used in other
ways, it may not deliver the expected security characteristics. It
is recommended that any proposed model include a discussion of the
applicability statement of the protocols to be used.
12.1. TLS/TCP Transport Mapping Security Model
SNMP supports multiple transports. The preferred transport for SNMP
over IP is UDP [RFC3417]. An experimental transport for SNMP over
TCP is defined in [RFC3430].
TLS/TCP will create an association between the TMSM of one SNMP
entity and the TMSM of another SNMP entity. The created "tunnel" may
provide encryption and data integrity. Both encryption and data
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integrity are optional features in TLS. The TLS TMSP MUST provide
authentication if auth is requested in the securityLevel of the SNMP
message request (RFC3412 4.1.1). The TLS TM-security model MUST
specify that the messages be encrypted if priv is requested in the
securityLevel parameter of the SNMP message request (RFC3412 4.1.1).
The TLS TM-security model MUST support the TLS Handshake Protocol
with mutual authentication.
12.1.1. tmStateReference for TLS
Upon establishment of a TLS session, the TMSP will cache the state
information. A unique tmStateReference will be passed to the
corresponding MPSP. The MPSP will pass the securityStateReference to
the Message Processing Model for memory management.
The tmStateReference cache:
tmStateReference
tmSecurityStateReference
tmTransportDomain = TCP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - TLS TMSM
tmSecurityName = "dbharrington"
tmSecurityLevel = "authPriv"
12.1.2. MPSP for TLS TM-Security Model
messageProcessingModel = SNMPv3
securityModel = TLS TMSM
securityName = tmSecurityName
securityLevel = msgSecurityLevel
12.1.3. MIB Module for TLS Security
Each security model should use its own MIB module, rather than
utilizing the USM MIB, to eliminate dependencies on a model that
could be replaced some day. See RFC3411 section 4.1.1.
The TLS MIB module needs to provide the mapping from model-specific
identity to a model independent securityName.
[todo] Module needs to be worked out once things become stable...
12.2. DTLS/UDP Transport Mapping Security Model
DTLS has been proposed as a UDP-based TLS. Transport Layer Security
(TLS) [RFC2246] traditionally requires a connection-oriented
transport and is usually used over TCP. Datagram Transport Layer
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Security (DTLS) [I-D.rescorla-dtls] provides security services
equivalent to TLS for connection-less transports such as UDP.
DTLS provides all the security services needed from an SNMP
architectural point of view. Although it is possible to derive a
securityName from the public key certificates (e.g. the subject
field), this approach requires installing certificates on all SNMP
entities, leading to a certificate management problem which does not
integrate well with established AAA systems. [discuss] why does this
not integrate well with existing AAA systems?
Another option is to run an authentication exchange which is
integrated with TLS, such as Secure Remote Password with TLS
[I-D.ietf-tls-srp]. A similar option would be to use Kerberos
authentication with TLS as defined in [RFC2712].
It is important to stress that the authentication exchange must be
integrated into the TLS mechanism to prevent man-in-the-middle
attacks. While SASL [RFC2222] is often used on top of a TLS
encrypted channel to authenticate users, this choice seems to be
problematic until the mechanism to cryptographically bind SASL into
the TLS mechanism has been defined.
DTLS will create an association between the TMSM of one SNMP entity
and the TMSM of another SNMP entity. The created "tunnel" may
provide encryption and data integrity. Both encryption and data
integrity are optional features in DTLS. The DTLS TM-security model
MUST provide authentication if auth is requested in the securityLevel
of the SNMP message request (RFC3412 4.1.1). The TLS TM-security
model MUST specify that the messages be encrypted if priv is
requested in the securityLevel parameter of the SNMP message request
(RFC3412 4.1.1).
The DTLS TM-security model MUST support the TLS Handshake Protocol
with mutual authentication.
12.2.1. tmStateReference for DTLS
DTLS has been suggested as a possible secure transport. It is not
clear whether DTLS is a reasonable choice for SNMP interactions. It
is mentioned here only as an example.
Upon establishment of a DTLS session, the TMSP will cache the state
information. A unique tmStateReference will be passed to the
corresponding MPSP. The MPSP will pass the securityStateReference to
the Message Processing Model for memory management.
The tmStateReference cache:
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tmStateReference
tmSecurityStateReference
tmTransportDomain = UDP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - DTLS TMSM
tmSecurityName = "dbharrington"
tmSecurityLevel = "authPriv"
12.3. SASL Transport Mapping Security Model
The Simple Authentication and Security Layer (SASL) [RFC2222]
provides a hook for authentication and security mechanisms to be used
in application protocols. SASL supports a number of authentication
and security mechanisms, among them Kerberos via the GSSAPI mechanism
[RFC4121].
This sample will use DIGEST-MD5 because it supports authentication,
integrity checking, and confidentiality.
DIGEST-MD5 supports auth, auth with integrity, and auth with
confidentiality. Since SNMPv3 assumes integrity checking is part of
authentication, if msgFlags is set to authNoPriv, the qop-value
should be set to auth-int; if msgFlags is authPriv, then qop-value
should be auth-conf.
Realm is optional, but can be utilized by the securityModel if
desired. SNMP does not use this value, but a TMSM could map the
realm into SNMP processing in various ways. For example, realm and
username could be concatenated to be the securityName value, e.g.
helpdesk::username", or the realm could be used to specify a
groupName to use in the VACM access control. This would be similar
to having the securityName-to-group mapping done by the external AAA
server.
12.3.1. tmStateReference for SASL DIGEST-MD5
The tmStateReference cache:
tmStateReference
tmSecurityStateReference
tmTransportDomain = TCP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - SASL TMSM
tmSecurityName = username
tmSecurityLevel = [auth-conf]
tmAuthProtocol = md5-sess
tmPrivProtocol = 3des
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tmServicesProvided mutual authentication,
reauthentication,
integrity,
encryption
tmParameters = "realm=helpdesk, serv-type=SNMP
13. The TMSM MIB Module
This memo defines a portion of the Management Information Base (MIB)
for managing the Transport Mapping Security Model Subsystem.
13.1. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree
is organized as a set of related objects. The overall structure and
assignment of objects to their subtrees, and the intended purpose of
each subtree, is shown below.
13.1.1. Textual Conventions
Generic and Common Textual Conventions used in this document can be
found summarized at http://www.ops.ietf.org/mib-common-tcs.html
13.1.2. The tmsmStats Subtree
This subtree contains security-model-independent counters which are
applicable to all security models based on the .Transport Mapping
Security Model Subsystem.
This subtree provides information for identifying fault conditions
and performance degradation.
13.1.3. The tmsmsSession Subtree
This subtree contains security-model-independent information about
sessions which are applicable to all security models based on the
Transport Mapping Security Model Subsystem.
This subtree provides information for managing sessions for any
security model based on the Transport Mapping Security Model
Subsystem.
13.1.4. The Notifications Subtree
This subtree contains notifications to alert other entities to events
which could alter the operational behavior of the entity in a network
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utilizing the SAMPLE Protocol.
13.2. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing this MIB. In particular, it is assumed
that an entity implementing the TMSM-MIB module will also implement
the SNMPv2-MIB [RFC3418].
This MIB module is expected to be used with the MIB modules defined
for managing specific security models that are based on the TMSM
subsystem. This MIB module is designed to be security-model
independent, and conatins objects useful for managing common aspects
of any TMSM-based security model. Specific security models may
define a MIB module to contain security-model-dependent information.
13.2.1. Relationship to the SNMPv2-MIB
The 'system' subtree in the SNMPv2-MIB [RFC3418] is defined as being
mandatory for all systems, and the objects apply to the entity as a
whole. The 'system' subtree provides identification of the
management entity and certain other system-wide data. The TMSM-MIB
utilizes, but does not dupicate, some of those objects. [todo] do we
actually use any of the objects, since we don't have any elements of
procedure?
13.2.2. MIB Modules Required for IMPORTS
The following MIB module imports items from [RFC2578], [RFC2579],
[RFC2580], [RFC3411], and [RFC3419]
14. Definitions
TMSM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
mib-2, Integer32, Unsigned32, Gauge32
FROM SNMPv2-SMI
TestAndIncr
FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
SnmpSecurityModel,
SnmpAdminString, SnmpSecurityLevel, SnmpEngineID
FROM SNMP-FRAMEWORK-MIB
TransportAddress, TransportAddressType
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FROM TRANSPORT-ADDRESS-MIB
;
tmsmMIB MODULE-IDENTITY
LAST-UPDATED "200602270000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
+49 6221 90511-15
quittek@netlab.nec.de
Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@iu-bremen.de
Editor:
David Harrington
Effective Software
50 Harding Rd
Portsmouth, New Hampshire 03801
USA
+1 603-436-8634
ietfdbh@comcast.net
"
DESCRIPTION "The Transport Mapping Security Model
Subsystem MIB
Copyright (C) The Internet Society (2006). This
version of this MIB module is part of RFC XXXX;
see the RFC itself for full legal notices.
-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
"
REVISION "200602270000Z" -- 27 February 2006
DESCRIPTION "The initial version, published in RFC XXXX.
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-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
"
::= { mib-2 xxxx }
-- RFC Ed.: replace xxxx with IANA-assigned number and
-- remove this note
-- ---------------------------------------------------------- --
-- subtrees in the TMSM-MIB
-- ---------------------------------------------------------- --
tmsmNotifications OBJECT IDENTIFIER ::= { tmsmMIB 0 }
tmsmObjects OBJECT IDENTIFIER ::= { tmsmMIB 1 }
tmsmConformance OBJECT IDENTIFIER ::= { tmsmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
-- Statistics for the Transport Model Security Model Subsystem
tmsmStats OBJECT IDENTIFIER ::= { tmsmObjects 1 }
-- [discuss] do we need any tmsm stats?
-- these should be for interoperability, not local debug.
-- we could probably track session establishment failures
-- although this really belongs in an SSH-MIB, not TMSM-MIB
-- The tmsmSession Group
tmsmSession OBJECT IDENTIFIER ::= { tmsmObjects 2 }
tmsmSessionSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
TMSM security models to coordinate their
use of facilities to create sessions in the
tmsmSessionTable.
"
::= { tmsmSession 1 }
tmsmSessionCurrent OBJECT-TYPE
SYNTAX Gauge32
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MAX-ACCESS read-only
STATUS current
DESCRIPTION "The current number of established sessions.
"
::= { tmsmSession 2 }
tmsmSessionMaxSupported OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The maximum number of open sessions allowed.
"
::= { tmsmSession 3 }
tmsmSessionTable OBJECT-TYPE
SYNTAX SEQUENCE OF TmsmSessionEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of currently available sessions configured
in the SNMP engine's Local Configuration Datastore
(LCD).
Sessions are created as needed, and do not persist
across network management system reboots.
"
::= { tmsmSession 4 }
tmsmSessionEntry OBJECT-TYPE
SYNTAX TmsmSessionEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A session configured in the SNMP engine's Local
Configuration Datastore (LCD) for Transport Mapping
Security Models.
"
INDEX { tmsmSessionID }
::= { tmsmSessionTable 1 }
TmsmSessionEntry ::= SEQUENCE
{
tmsmSessionID Integer32,
tmsmSessionTransport TransportAddressType,
tmsmSessionAddress TransportAddress,
tmsmSessionSecurityModel SnmpSecurityModel,
tmsmSessionSecurityName SnmpAdminString,
tmsmSessionSecurityLevel SnmpSecurityLevel,
tmsmSessionEngineID SnmpEngineID
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}
tmsmSessionID OBJECT-TYPE
SYNTAX Integer32 (1..65535)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A locally-unique identifier for a session.
"
::= { tmsmSessionEntry 1 }
tmsmSessionTransport OBJECT-TYPE
SYNTAX TransportAddressType
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The transport domain associated with this session.
"
::= { tmsmSessionEntry 2 }
tmsmSessionAddress OBJECT-TYPE
SYNTAX TransportAddress
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The transport address associated with this session.
"
::= { tmsmSessionEntry 3 }
tmsmSessionSecurityModel OBJECT-TYPE
SYNTAX SnmpSecurityModel
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The Security Model associated with this session."
::= { tmsmSessionEntry 4 }
tmsmSessionSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A human readable string representing the principal
in Security Model independent format.
The default transformation of the Secure Shell
Security Model dependent security ID to the
securityName
and vice versa is the identity function so that the
securityName is the same as the SSH user name.
"
::= { tmsmSessionEntry 5 }
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tmsmSessionSecurityLevel OBJECT-TYPE
SYNTAX SnmpSecurityLevel
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The Level of Security at which SNMP messages can be
sent using this session, in particular, one of:
noAuthNoPriv - without authentication and
without privacy,
authNoPriv - with authentication but
without privacy,
authPriv - with authentication and
with privacy.
"
DEFVAL { authPriv }
::= { tmsmSessionEntry 6 }
tmsmSessionEngineID OBJECT-TYPE
SYNTAX SnmpEngineID
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The administratively-unique identifier for the
remote SNMP engine associated with this session.
"
::= { tmsmSessionEntry 7 }
-- -------------------------------------------------------------
-- tmsmMIB - Conformance Information
-- -------------------------------------------------------------
tmsmGroups OBJECT IDENTIFIER ::= { tmsmConformance 1 }
tmsmCompliances OBJECT IDENTIFIER ::= { tmsmConformance 2 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
tmsmGroup OBJECT-GROUP
OBJECTS {
tmsmSessionCurrent,
tmsmSessionMaxSupported,
tmsmSessionTransport,
tmsmSessionAddress,
tmsmSessionSecurityModel,
tmsmSessionSecurityName,
tmsmSessionSecurityLevel,
tmsmSessionEngineID,
tmsmSessionSpinLock
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}
STATUS current
DESCRIPTION "A collection of objects for maintaining session
information of an SNMP engine which implements the
SNMP Secure Shell Security Model.
"
::= { tmsmGroups 2 }
-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------
tmsmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
TMSM-MIB"
MODULE
MANDATORY-GROUPS { tmsmGroup }
::= { tmsmCompliances 1 }
END
15. Implementation Considerations
15.1. Applications that Benefit from Sessions
[todo] contributions welcome.
There has been discussion of ways SNMP could be extended to better
support management/monitoring needs when a network is running just
fine. Use of a TCP transport, for example, could enable larger
message sizes and more efficient table retrievals.
Discussing how to improve SNMP once you have less strict message size
constraints is beyond the scope of this document, or that of TMSM-
based security models. Applications utilizing TMSM-based security
models may want to take advantage of the increased message sizes by
sending larger requests and utilizing existing SNMP operations (e.g.
getbulk) effectively. However, doing so might have negative impacts
on existing SNMP management and the networks that contain them.
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15.2. Applications that Suffer from Sessions
[todo] contributions welcome.
15.2.1. Troubleshooting
It has been a long-standing requirement that SNMP be able to work
when the network is unstable, to enable network troubleshooting and
repair. The UDP approach has been considered to meet that need well,
with an assumption that getting small messages through, even if out
of order, is better than gettting no messages through. There has
been a long debate about whether UDP actually offers better support
than TCP when the underlying IP or lower layers are unstable. There
has been recent discussion of whether operators actually use SNMP to
troubleshoot and repair unstable networks.
The need to establish a session before using SNMP to troubleshoot a
device may prove problematic in practice. TMSM-based security models
should include discussion of how troubleshooting applications might
be impacted by the use of the specific security model, and recommend
workarounds.
This document RECOMMENDS that all TMSM-based security models include
a fallback approach, triggered by multiple failed attempts to
establish sessions. The default fallback should be to utilize the
IETF-Standard USM security model to send a notification, so an
administrator can attempt to manually correct the problem.
16. Security Considerations
This document describes an architectural approach and multiple
proposed configurations that would permit SNMP to utilize transport
layer security services. Each section containing a proposal should
discuss the security considerations of that approach. [discuss]
expand as needed.
It is considered desirable by some industry segments that SNMP
security 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.
There are a number of management objects defined in this MIB module
with a MAX-ACCESS clause of read-write and/or read-create. Such
objects may be considered sensitive or vulnerable in some network
environments. The support for SET operations in a non-secure
environment without proper protection can have a negative effect on
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network operations. These are the tables and objects and their
sensitivity/vulnerability:
o [todo] list the tables and objects and state why they are
sensitive.
There are no management objects defined in this MIB module that have
a MAX-ACCESS clause of read-write and/or read-create. So, if this
MIB module is implemented correctly, then there is no risk that an
intruder can alter or create any management objects of this MIB
module via direct SNMP SET operations.
Some of the readable objects in this MIB module (i.e., objects with a
MAX-ACCESS other than not-accessible) may be considered sensitive or
vulnerable in some network environments. It is thus important to
control even GET and/or NOTIFY access to these objects and possibly
to even encrypt the values of these objects when sending them over
the network via SNMP. These are the tables and objects and their
sensitivity/vulnerability:
o [todo] list the tables and objects and state why they are
sensitive.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPSec),
even then, there is no control as to who on the secure network is
allowed to access and GET/SET (read/change/create/delete) the objects
in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410], section 8),
including full support for the SNMPv3 cryptographic mechanisms (for
authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
17. IANA Considerations
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The MIB module in this document uses the following IANA-assigned
OBJECT IDENTIFIER values recorded in the SMI Numbers registry:
Descriptor OBJECT IDENTIFIER value
---------- -----------------------
tmsmMIB { mib-2 XXXX }
Editor's Note (to be removed prior to publication): the IANA is
requested to assign a value for "XXXX" under the 'mib-2' subtree
and to record the assignment in the SMI Numbers registry. When
the assignment has been made, the RFC Editor is asked to replace
"XXXX" (here and in the MIB module) with the assigned value and to
remove this note.
[discuss] How do we add a new TransportType?
18. 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, Dave Harrington, Keith McCloghrie, Kaushik Narayan, Dave
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
19. References
19.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2222] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
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[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[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.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC3419] Daniele, M. and J. Schoenwaelder, "Textual Conventions for
Transport Addresses", RFC 3419, December 2002.
[RFC3430] Schoenwaelder, J., "Simple Network Management Protocol
Over Transmission Control Protocol Transport Mapping",
RFC 3430, December 2002.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
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Internet-Draft SNMP Transport Mapping Security Model March 2006
[I-D.rescorla-dtls]
Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", draft-rescorla-dtls-05 (work in progress),
June 2005.
19.2. Informative References
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.
[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
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[I-D.ietf-netconf-ssh]
Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure Shell (SSH)",
draft-ietf-netconf-ssh-05 (work in progress),
October 2005.
[I-D.ietf-tls-srp]
Taylor, D., "Using SRP for TLS Authentication",
draft-ietf-tls-srp-10 (work in progress), October 2005.
Appendix A. Questions about msgFlags:
[discuss] many of these questions can be resolved by deciding whether
the TMSP or MPSP provides the service of comparing msgFlags (from
inside the message) to actual capabilities of the transport layer
security (external to the message). It may however be necessary to
provide this service for two slightly different purposes depending on
whether the message is outgoing (and may need to be checked by the
TMSP when a new transport session might be created) or the message is
incoming ( the capabilities of the transport layer session are
already known, but msgFlags has not been unpacked yet at the TMSP, so
the comparison must be done at the MPSP). Of course, we really only
need to compare the authflag and the privflag, i.e. the
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securityLevel, so if we pass the securityLevel between the two
stages, then they each have the info they need to do their respective
comparisons.
There have been a large number of questions about msgFlags in the
TMSM approach, mostly concerning the msgFlags value and the actual
security provided, and whether msgFlags can be used to initiate per-
message or per-session security.
A.1. msgFlags versus actual security
Using IPSEC, SSH, or SSL/TLS to provide security services "below" the
SNMP message, the use of securityName and securityLevel will differ
from the USM/VACM approach to SNMP access control. VACM uses the
"securityName" and the "securityLevel" to determine if access is
allowed. With the SNMPv3 message and USM security model, both
securityLevel and securityName are contained in every SNMPv3 message.
Any proposal for a security model using IPSEC, SSH, or SSL/TLS needs
to specify how this info is made available to the SNMPv3 message
processing, and how it is used.
One specific case to consider is the relationship between the
msgFlags of an SNMPv3 message, and the actual services provided by
the lower layer security. For example, if a session is set up with
encryption, is the priv bit always (or never) set in the msgFlags
field, and is the PDU never (or always) encrypted? Do msgFlags have
to match the security services provided by the lower layer, or are
the msgFlags ignored and the values from the lower layer used?
Is the securityLevel looked at before the security model gets to
it.? No. the security model has two parts - the TMSP and the
MPSP. The securityLevel is looked at by the TMSP before it gets
to the MPSP, but both are parts of the same security model.
Would it be legal for the security model to ignore the incoming
flags and change them before passing them back up? If it changed
them, it wouldn't necessarily be ignoring them. The TMSP should
pass both an actual securityLevel applied to the message, and the
msgFlags in the SNMP message to the MPSP for consideration related
to access control.. The msgFlags parameter in the SNMP message is
never changed when processing an incoming message.
Would it be legal for the security model to ignore the outgoing
flags and change them before passing them out? no; because the two
stages are parts of the same security model, either the MPSP
should recognize that a securityLevel cannot be met or exceeded,
and reject the message during the message-build phase, or the TMSP
should determine if it is possible to honor the request. It is
possible to apply an increased securityLevel for an outgoing
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request, but the procedure to do so must be spelled out clearly in
the model design.
The security model MUST check the incoming security level flags to
make sure they matched the transport session setup. and if not
drop the message. Yes, mostly. Depending on the model, either
the TMSP or the MPSP MUST verify that the actual processing met or
exceeded the securityLevel requested by the msgFlags and that it
is acceptable to the specific-model processing (or operator
configuration) for this different securityLevel to be applied to
the message. This is also true (especially) for outgoing
messages.
You might legally be able to have a authNoPriv message that is
actually encrypted via the transport (but not the other way around
of course). Yes, a TMSM could define that as the behavior (or
permit an operator to specify that is acceptable behavior) when a
requested securityLevel cannot be provided, but a stronger
securityLevel can be provided.
Appendix B. Parameter Table
Following is a CSV-formatted matrix useful for tracking data flows
into and out of the dispatcher, message, and security subsystems.
Import this into your favorite spreadsheet or other CSV-compatible
application. You wil need to remove lines feeds from the second and
thrid lines, which needed to be wrapped to fit into RFC limits.
B.1. ParameterList.csv
,Dispatcher,,,,Messaging,,,Security,,
,sendPDU,returnResponse,processPDU,processResponse
,prepareOutgoingMessage,prepareResponseMessage,prepareDataElements
,generateRequest,processIncoming,generateResponse
transportDomain,In,,,,In,,In,,,
transportAddress,In,,,,In,,In,,,
destTransportDomain,,,,,Out,Out,,,,
destTransportAddress,,,,,Out,Out,,,,
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
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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,,Out,In,Out
messageLength,,,,,Out,Out,,Out,In,Out
maxMessageSize,,,,,,,,In,In,In
globalData,,,,,,,,In,,In
securityEngineID,,,,,,,,In,Out,In
scopedPDU,,,,,,,,In,Out,In
securityParameters,,,,,,,,Out,,Out
securityStateReference,,,,,,,,,Out,In
pduType,,,,,,,Out,,,
tmStateReference,,,,,,Out,In,,In,
Appendix C. Open Issues
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Appendix D. Change Log
NOTE to RFC editor: Please remove this change log before publishing
this document as an RFC.
Changes from revison -00-
changed SSH references from I-Ds to RFCs
removed parameters from tmState Reference for DTLS that revealed
lower layer info.
Added TMSM-MIB module
Added Internet-Standard Management Framework boilerplate
Added Structure of the MIB Module
Added MIB security considerations boilerplate (to be completed)
Added IANA Considerations
Added ASI Parameter table
Added discussion of Sessions
Added Open issues and Change Log
Rearranged sections
Authors' Addresses
David Harrington
Futurewei Technologies
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: dharrington@huawei.com
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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