Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Intended status: Informational J. Schoenwaelder
Expires: December 25, 2006 International University Bremen
June 23, 2006
Transport Mapping Security Model (TMSM) Architectural Extension for the
Simple Network Management Protocol (SNMP)
draft-ietf-isms-tmsm-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a Transport Mapping Security Model (TMSM)
extension 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 sessions in the Transport Mapping Security Model
extension.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements of a Transport Mapping Security Model . . . . . . 6
2.1. Message Security Requirements . . . . . . . . . . . . . . 6
2.1.1. Security Protocol Requirements . . . . . . . . . . . . 7
2.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Architectural Modularity Requirements . . . . . . . . 7
2.2.2. Access Control Requirements . . . . . . . . . . . . . 14
2.2.3. Security Parameter Passing Requirements . . . . . . . 16
2.3. Session Requirements . . . . . . . . . . . . . . . . . . . 17
2.3.1. Session Establishment Requirements . . . . . . . . . . 18
2.3.2. Session Maintenance Requirements . . . . . . . . . . . 19
2.3.3. Message security versus session security . . . . . . . 19
3. Scenario Diagrams for TMSM . . . . . . . . . . . . . . . . . . 21
3.1. Command Generator or Notification Originator . . . . . . . 21
3.2. Command Responder . . . . . . . . . . . . . . . . . . . . 22
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 23
4.1. SNMPv3 Message Fields . . . . . . . . . . . . . . . . . . 24
4.1.1. msgGlobalData . . . . . . . . . . . . . . . . . . . . 26
4.1.2. msgSecurityParameters . . . . . . . . . . . . . . . . 27
5. Cached Information and References . . . . . . . . . . . . . . 27
5.1. tmSessionReference Cached Session Data . . . . . . . . . . 27
5.2. securityStateReference Cached Security Data . . . . . . . 27
6. Abstract Service Interfaces for TMSM . . . . . . . . . . . . . 28
6.1. Generating an Outgoing SNMP Message . . . . . . . . . . . 29
6.2. TMSP for an Outgoing Message . . . . . . . . . . . . . . . 30
6.3. Processing an Incoming SNMP Message . . . . . . . . . . . 30
6.3.1. TMSP for an Incoming Message . . . . . . . . . . . . . 30
6.3.2. Prepare Data Elements from Incoming Messages . . . . . 31
6.3.3. MPSP for an Incoming Message . . . . . . . . . . . . . 32
7. The TMSM MIB Module . . . . . . . . . . . . . . . . . . . . . 33
7.1. Structure of the MIB Module . . . . . . . . . . . . . . . 33
7.1.1. The tmsmStats Subtree . . . . . . . . . . . . . . . . 33
7.2. Relationship to Other MIB Modules . . . . . . . . . . . . 33
7.2.1. Textual Conventions . . . . . . . . . . . . . . . . . 33
7.2.2. MIB Modules Required for IMPORTS . . . . . . . . . . . 33
7.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
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10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11.1. Normative References . . . . . . . . . . . . . . . . . . . 39
11.2. Informative References . . . . . . . . . . . . . . . . . . 40
Appendix A. Parameter Table . . . . . . . . . . . . . . . . . . . 41
A.1. ParameterList.csv . . . . . . . . . . . . . . . . . . . . 41
Appendix B. Why tmSessionReference? . . . . . . . . . . . . . . . 42
B.1. Define an Abstract Service Interface . . . . . . . . . . . 43
B.2. Using an Encapsulating Header . . . . . . . . . . . . . . 43
B.3. Modifying Existing Fields in an SNMP Message . . . . . . . 43
B.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 44
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 44
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
This document describes a Transport Mapping Security Model (TMSM)
extension 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. Acronyms
This section contains a list of acronyms used within the document and
references to where in the document the acronym is defined, for easy
lookup.
o TMSM - a Transport Mapping Security Model
o SMSP - a Security Model Security Processor, the portion of a TMSM
security model that resides in the Message Processing subsystem of
an SNMPv3 engine. See Section 2.2.1
o TMSP - the Transport Mapping Security Processor, the portion of a
TMSM security model that resides in the Transport Mapping section
of the Dispatcher of an SNMPv3 engine. See Section 2.2.1
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1.4. Motivation
There are multiple ways to secure one's home or business, in 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 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 principal and key
management infrastructure. Operators have reported that deploying
another principal and key management infrastructure in order to use
SNMPv3 is a deterrent to deploying SNMPv3. It is possible 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 [RFC4366], SASL
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[RFC4422], and SSH [RFC4251].
From an operational perspective, it is highly desirable to use
security mechanisms that can unify the administrative security
management for SNMPv3, command line interfaces (CLIs) and other
management interfaces. The use of security services provided by
lower layers is the approach commonly used for the CLI, and is also
the approach being proposed for NETCONF [I-D.ietf-netconf-ssh].
This document proposes a Transport Mapping Security Model (TMSM)
extension to the RFC3411 architecture, that allows security to be
provided by an external protocol connected to the SNMP engine through
an SNMP transport-mapping [RFC3417]. Such a TMSM would then enable
the use of existing security mechanisms such as (TLS) [RFC4366] or
SSH [RFC4251] within the RFC3411 architecture.
There are a number of Internet security protocols and mechanisms that
are in wide spread use. Many of them try to provide a generic
infrastructure to be used by many different application layer
protocols. The motivation behind 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. Message 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.
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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
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. SNMP Requirements
2.2.1. 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. The architecture includes a Security
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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
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).
Parameters have been provided in the ASIs to pass model-independent
transport address information. These parameters utilize the
TransportType and TransportAddress
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 uses a modular design that
can be advanced through the standards process independently of other
proposals, and independent of other modular components as much as
possible.
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 architectural extension permits additional transport
security protocols to be "plugged into" the RFC3411 architecture,
supported by corresponding transport-security-aware transport mapping
models.
The RFC3411 architecture, and the USM approach, assume that a
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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 security model's security
processor as the SMSP.
Thus a TMSM is composed of both a TMSP and an SMSP.
+------------------------------+
| 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 | | |
| | | | | +------------+ | | | SMSP | | |
| | PDU Dispatcher | | | +------------+ | | | | | |
| +-------------------+ | +->| otherMP * |<--->| +------------+ | |
| ^ | +------------+ | | | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
2.2.1.1. 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)
2) determine the SNMP security model and parameters (messaging model)
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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 | |
| --------------------- ------------------ |
| --------------------------------------------- |
2.2.1.2. TMSM and the RFC3411 Architecture
In the TMSM approach, the order of the steps differ and may be
handled by different subsystems:
1) decrypt the encrypted portions of the message (transport layer)
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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 | |
| --------------------- ------------------ |
| --------------------------------------------- |
2.2.1.3. Passing Information between Engines
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
established, multiple SNMP messages may be able to be passed through
the same tunnel.
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2.2.2. Access Control Requirements
2.2.2.1. 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 security model security process (SMSP).
Depending on the design of the specific TMSM model, i.e., which
transport layer protocol is used, different features might be
provided by the TMSP or by the SMSP. For example, the translation
from a mechanism-specific authenticated identity to a securityName
might be done by the TMSP or by the SMSP.
The securityName MUST be bound to the mechanism-specific
authenticated identity, and this mapping MUST be done before the SMSP
portion of the model passes securityName to the message processing
model via the processIncoming() ASI.
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 principal
authentication must be available via the transport layer security
protocol.
If the type of authentication provided by the transport layer (e.g.
TLS) is considered adequate to secure and/or encrypt the message, but
inadequate to provide the desired granularity of access control (e.g.
user-based), then a second authentication (e.g., one provided by a
RADIUS 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.
2.2.2.2. 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
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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
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
or Windows domains. 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 an SNMP message version and the security
provided by a transport mapping.
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2.2.3. Security Parameter Passing Requirements
RFC3411 section 4 describes primitives to describe the abstract data
flows between the various subsystems, models and applications within
the architecture. Abstract Service Interfaces describe the flow of
data between subsystems within an engine. The ASIs generally pass
model-independent information.
Within an engine using a TMSM-based security model, 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.
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, which reflects the USM security model
design, the messaging model must unpack SNMP-specific security
parameters from an incoming message before calling a specific
security model to authenticate and decrypt an incoming message,
perform integrity checking, and translate model-specific security
parameters into model-independent parameters.
In the TMSM approach, the security-model specific parameters are not
carried in the SNMP message. The parameters are provided by SNMP
applications for outgoing messages, and the parameters for incoming
messages are extracted from the transport layer by the security-
model-specific transport mapping before the message is passed to the
message processing subsystem.
For outgoing messages, it is necessary to have an SMSP because it is
the SMSP that actually creates the message from its component parts.
Whether there are any security services provided by the SMSP for an
outgoing message is model-dependent.
For incoming messages, there might be security functionality that can
only be handled after the message version is known. The message
version is determined by the Message Processing model and passed to
the SMSP via the processIncoming() ASI.
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
a need to have an SMSP 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-dependent
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security parameters from the TMSP through to the SMSP.
This document describes a cache, into which the TMSP puts information
about the security applied to an incoming message, and an SMSP
extracts that information from the cache. Given that there may be
multiple TM-security caches, a tmSessionReference is passed as an
extra parameter in the ASIs between the transport mapping and the
messaging security model, so the SMSP knows which cache of
information to consult.
This approach does create dependencies between a model-specific TMSP
and a corresponding specific SMSP. This approach of passing a model-
independent reference is consistent with the securityStateReference
cache already being passed around in the RFC3411 ASIs.
2.3. 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 transport address, 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,
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.
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2.3.1. Session Establishment Requirements
SNMP applications must provide the transport address, securityName,
securityModel, and securityLevel to be used for a session.
SNMP Applications typically have no knowledge of whether the session
that will be used to carry commands was initially established as a
notification session, or a request-response session, and SHOULD NOT
make any assumptions based on knowing the direction of the session.
If an administrator or security model designer wants to differentiate
a session established for different purposes, such as a notification
session versus a request-response session, the application can use
different securityNames or transport addresses (e.g., port 161 vs.
port 162) for different purposes.
An SNMP engine containing an application that initiates
communication, e.g., a Command Generator or Notification Originator,
MUST be able to attempt to establish a session for delivery if a
session does not yet exist. If a session cannot be established then
the message is discarded.
Sessions are usually established by the transport mapping security
processor when no appropriate session is found for an outgoing
message, but sessions may be established in advance to support
features such as notifications and call-home. How sessions are
established in advance is beyond the scope of this document.
Sessions are initiated by notification originators when there is no
currently established connection that can be used to send the
notification. For a client-server security protocol, this may
require provisioning authentication credentials on the agent, either
statically or dynamically, so the client/agent can successfully
authenticate to a notification receiver.
A TMSM-based security model must be able to determine whether a
session does or does not exist, and must be able to determine which
session has the appropriate security characteristics (transport
address, securityName, securityModel, and securityLevel) for an
outgoing message.
A TMSM security model implementation MAY reuse an already established
session with the appropriate transport address, securityName,
securityModel, and securityLevel characteristics for delivery of a
message originated by a different type of application than originally
caused the session to be created. For example, an implementation
that has an existing session originally established to receive a
request may use that session to send an outgoing notification, and
may use a session that was originally established to send a
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notification to send a request. Responses are expected to be
returned using the same session that carried the corresponding
request message. Reuse is not required for conformance.
If a session can be reused for a different type of message, but a
receiver is not prepared to accept different message types over the
same session, then the message MAY be dropped by the manager.
2.3.2. Session Maintenance Requirements
A TMSM-based security model can tear down sessions as needed. It may
be necessary for some implementations to tear down sessions as the
result of resource constraints, for example.
The decision to tear down a session is implementation-dependent.
While it is possible for an implementation to automatically tear down
each session once an operation has completed, this is not recommended
for anticipated performance reasons. How an implementation
determines that an operation has completed, including all potential
error paths, is implementation-dependent.
Implementations should be careful to not tear down a session between
the time a request is received and the time the response is sent.
The elements of procedure for TMSM-based security models should be
sure to describe the expected behavior when no session exists for a
response.
The elements of procedure may discuss when cached information can be
discarded, and the timing of cache cleanup may have security
implications, but cache memory management is an implementation issue.
If a security model defines MIB module objects to maintain session
state information, then the security model MUST describe what happens
to the objects when a related session is torn down, since this will
impact interoperability of the MIB module.
2.3.3. 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
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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 transport address,
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
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
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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 trade offs for
resource-constrained devices.
3. Scenario Diagrams for TMSM
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. Message Formats
The syntax of an SNMP message using this Security Model adheres to
the message format defined in the version-specific Message Processing
Model document (for example [RFC3412]). At the time of this writing,
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there are three defined message formats - SNMPv1, SNMPv2c, and
SNMPv3. SNMPv1 and SNMPv2c have been declared Historic, so this memo
only deals with SNMPv3 messages.
The processing is compatible with the RFC 3412 primitives,
generateRequestMsg() and processIncomingMsg(), that show the data
flow between the Message Processor and the SMSP.
4.1. SNMPv3 Message Fields
The SNMPv3Message SEQUENCE is defined in [RFC3412] and [RFC3416].
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SNMPv3MessageSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
SNMPv3Message ::= SEQUENCE {
-- identify the layout of the SNMPv3Message
-- this element is in same position as in SNMPv1
-- and SNMPv2c, allowing recognition
-- the value 3 is used for snmpv3
msgVersion INTEGER ( 0 .. 2147483647 ),
-- administrative parameters
msgGlobalData HeaderData,
-- security model-specific parameters
-- format defined by Security Model
msgSecurityParameters OCTET STRING,
msgData ScopedPduData
}
HeaderData ::= SEQUENCE {
msgID INTEGER (0..2147483647),
msgMaxSize INTEGER (484..2147483647),
msgFlags OCTET STRING (SIZE(1)),
-- .... ...1 authFlag
-- .... ..1. privFlag
-- .... .1.. reportableFlag
-- Please observe:
-- .... ..00 is OK, means noAuthNoPriv
-- .... ..01 is OK, means authNoPriv
-- .... ..10 reserved, MUST NOT be used.
-- .... ..11 is OK, means authPriv
msgSecurityModel INTEGER (1..2147483647)
}
ScopedPduData ::= CHOICE {
plaintext ScopedPDU,
encryptedPDU OCTET STRING -- encrypted scopedPDU value
}
ScopedPDU ::= SEQUENCE {
contextEngineID OCTET STRING,
contextName OCTET STRING,
data ANY -- e.g., PDUs as defined in [RFC3416]
}
END
The following describes how any TMSM model SHOULD treat certain
fields in the message:
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4.1.1. msgGlobalData
msgGlobalData is opaque to a TMSM security model. The values are set
by the Message Processing model (e.g., SNMPv3 Message Processing),
and SHOULD NOT be modified by a TMSM security model.
The msgSecurityModel field should be set by the Message Processing
model 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 msgFlags have the same values for a TMSM model as for the USM
model.
4.1.1.1. 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.
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 SMSP processes the
incoming message, it should compare the msgFlags field to the
securityLevel actually provided for the message by the transport
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layer security. If they differ, the SMSP should determine whether
the changed securityLevel is acceptable. If not, it should discard
the message. Depending on the model, the SMSP may issue a reportPDU
with a model-specific counter.
4.1.2. msgSecurityParameters
The field msgSecurityParameters carries model-dependent security
information between engines. When a security model does not utilize
this field, its value MUST be the BER serialization of a zero-length
OCTET STRING, to prevent its being used in a manner that could be
damaging, such as for carrying a virus or worm.
RFC3412 defines two primitives, generateRequestMsg() and
processIncomingMsg() which require the specification of an
authoritative SNMP entity. The meaning of authoritative is model
dependent.
5. Cached Information and References
he RFC3411 architecture uses caches to store dynamic model-specific
information, and uses references in the ASIs to indicate in a model-
independent manner which cached information must flow between
subsystems. For most TMSM models, there are two levels of state that
need to be maintained: the session state, and the message security
state.
5.1. tmSessionReference Cached Session Data
The tmSessionReference is used to pass references to the appropriate
session information between the TMSP and SMSP through the ASIs.
The TMSP may provide only some aspects of security, and leave some
aspects to the SMSP. tmSessionReference 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 SMSP, plus the
parameters subsequently passed in securityStateReference.
The security model has the responsibility for explicitly releasing
the complete tmSessionReference and possibly deleting the associated
LCD information when the session is destroyed.
5.2. 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
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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 the information
to be stored in the securityStateReference to the message processing
model. such as the security-model-independent securityName,
securityLevel, and securityModel parameters, and the transport
address, and transport type. For responses, the messaging model may
need to pass the parameters back to the TMSP.
This document will differentiate the tmSessionReference provided by
the TMSP to the SMSP, from the securityStateReference provided by the
SMSP to the Dispatcher. This document does not specify an
implementation strategy, only an abstract discussion of the data that
must flow between subsystems. 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.
6. Abstract Service Interfaces for TMSM
Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an
SNMP entity. TMSM security models use some of these conceptual data
flows when communicating between subsystems, such as the dispatcher
and the Message Processing Subsystem.
To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule,
if state information is available when a message gets discarded, the
message-state information should also be released, and if state
information is available when a session is closed, the session state
information should also be released.
An error indication may return an OID and value for an incremented
counter and a value for securityLevel, and values for contextEngineID
and contextName for the counter, and the securityStateReference if
the information is available at the point where the error is
detected.
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6.1. Generating an Outgoing SNMP Message
This section describes the procedure followed by an RFC3411-
compatible system whenever it generates a message containing a
management operation (such as a request, a response, a notification,
or a report) on behalf of a user.
statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
IN sendPduHandle -- the handle for matching
incoming responses
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
OUT tmSessionReference
)
Note that tmSessionReference has been added to this ASI.
The IN parameters of the prepareOutgoingMessage() ASI are used to
pass information from the dispatcher (for the application subsystem)
to the message processing subsystem.
The abstract service primitive from a Message Processing Model to a
Security Model to generate the components of a Request message is
generateRequestMsg().
The abstract service primitive from a Message Processing Model to a
Security Model to generate the components of a Response message is
generateResponseMsg().
Upon completion of the SMSP processing, the Security model returns
statusInformation. If the process was successful, the completed
message is returned. If the process was not successful, then an
errorIndication is returned.
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The OUT parameters of the prepareOutgoingMessage() ASI are used to
pass information from the message processing model to the dispatcher
and on to the transport mapping:
6.2. TMSP for an Outgoing Message
The sendMessage ASI is used to pass a message from the Dispatcher to
the appropriate transport mapping 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 tmSessionReference
)
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 openSession() and closeSession() interfaces.
statusInformation =
openSession(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN tmSessionReference
)
statusInformation =
closeSession(
IN tmSessionReference
)
6.3. Processing an Incoming SNMP Message
6.3.1. TMSP for an Incoming Message
If one does not exist, the TMSP will need to create an entry in a
Local Configuration Datastore referenced by tmSessionReference. This
information will include transportDomain, transportAddress, the
securityModel, the securityLevel, and the securityName, plus any
model or mechanism-specific details. How this information is
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determined is model-specific.
The recvMessage ASI is used to pass a message from the transport
mapping to the Dispatcher.
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
IN tmSessionReference
)
6.3.2. Prepare Data Elements from Incoming Messages
The abstract service primitive from the Dispatcher to a Message
Processing Model for a received message is:
result = -- SUCCESS or errorIndication
prepareDataElements(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN wholeMsg -- as received from the network
IN wholeMsgLength -- as received from the network
IN tmSessionReference -- from the transport mapping
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- the version of the PDU
OUT PDU -- SNMP Protocol Data Unit
OUT pduType -- SNMP PDU type
OUT sendPduHandle -- handle for matched request
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for possible Response
)
Note that tmSessionReference has been added to this ASI.
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6.3.3. MPSP for an Incoming Message
This section describes the procedure followed by the SMSP whenever it
receives an incoming message containing a management operation on
behalf of a user from a Message Processing model.
The Message Processing Model extracts some information from the
wholeMsg. The abstract service primitive from a Message Processing
Model to the Security Subsystem for a received message is::
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
IN tmSessionReference -- from the transport mapping
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
) -- information, needed for response
1) The securityEngineID is set to a value in a model-specific manner.
If the securityEngineID is not utilized by the specific model, then
it should be set to the local snmpEngineID, to satisfy the SNMPv3
message processing model in RFC 3412 section 7.2 13a).
2) Extract the value of securityName from the Local Configuration
Datastore entry referenced by tmSessionReference.
3) The scopedPDU component is extracted from the wholeMsg.
4) The maxSizeResponseScopedPDU is calculated. This is the maximum
size allowed for a scopedPDU for a possible Response message.
5)The security data is cached as cachedSecurityData, so that a
possible response to this message can and will use the same security
parameters. Then securityStateReference is set for subsequent
reference to this cached data.
4) The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified in
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the processIncomingMsg primitive.
7. The TMSM MIB Module
This memo defines a portion of the Management Information Base (MIB)
for statistics in the Transport Mapping Security Model extension.
7.1. 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.
7.1.1. The tmsmStats Subtree
This subtree contains security-model-independent counters which are
applicable to all security models based on the .Transport Mapping
Security Model extension. This subtree provides information for
identifying fault conditions and performance degradation.
7.2. 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
extension. This MIB module is designed to be security-model
independent, and contains 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.
7.2.1. 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
7.2.2. MIB Modules Required for IMPORTS
The. following MIB module imports items from [RFC2578], [RFC2579],
[RFC2580], [RFC3411], and [RFC3419]
7.3. Definitions
TMSM-MIB DEFINITIONS ::= BEGIN
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IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
mib-2, Integer32, Unsigned32, Gauge32
FROM SNMPv2-SMI
TestAndIncr, StorageType, RowStatus
FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
SnmpSecurityModel,
SnmpAdminString, SnmpSecurityLevel, SnmpEngineID
FROM SNMP-FRAMEWORK-MIB
TransportAddress, TransportAddressType
FROM TRANSPORT-ADDRESS-MIB
;
tmsmMIB MODULE-IDENTITY
LAST-UPDATED "200604200000Z"
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
FutureWei Technologies
1700 Alma Drive, Suite 100
Plano, Texas 75075
USA
+1 603-436-8634
dharrington@huawei.com
"
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DESCRIPTION "The Transport Mapping Security Model
MIB Module
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 "200604200000Z" -- 20 April 2006
DESCRIPTION "The initial version, published in RFC XXXX.
-- 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
-- -------------------------------------------------------------
-- Textual Conventions
-- Notifications for the Transport Model Security Model extension
-- Statistics for the Transport Model Security Model extension
tmsmStats OBJECT IDENTIFIER ::= { tmsmObjects 1 }
tmsmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request
failed to open a Session.
"
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::= { tmsmStats 1 }
tmsmSessionNoAvailableSessions OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times a Response message
was dropped because the corresponding
session was no longer available.
"
::= { tmsmStats 2 }
-- The tmsmSession Group
tmsmSession OBJECT IDENTIFIER ::= { tmsmObjects 2 }
tmsmSessionCurrent OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The current number of open sessions.
"
::= { tmsmSession 1 }
tmsmSessionMaxSupported OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The maximum number of open sessions supported.
The value zero indicates the maximum is dynamic.
"
::= { tmsmSession 2 }
tmsmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request
failed to open a Session.
"
::= { tmsmSession 3 }
tmsmSessionSecurityLevelNotAvailableErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an outgoing message was
discarded because a requested securityLevel could not
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provided.
"
::= { tmsmSession 4 }
-- -------------------------------------------------------------
-- tmsmMIB - Conformance Information
-- -------------------------------------------------------------
tmsmGroups OBJECT IDENTIFIER ::= { tmsmConformance 1 }
tmsmCompliances OBJECT IDENTIFIER ::= { tmsmConformance 2 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
tmsmGroup OBJECT-GROUP
OBJECTS {
tmsmSessionOpenErrors,
tmsmSessionSecurityLevelNotAvailableErrors,
tmsmSessionCurrent,
tmsmSessionMaxSupported,
}
STATUS current
DESCRIPTION "A collection of objects for maintaining session
information of an SNMP engine which implements the
TMSM architectural extension.
"
::= { 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
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8. 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.
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 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.
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9. IANA Considerations
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.
10. 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
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
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[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.
[RFC3416] Presuhn, R., "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3416, 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.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
11.2. Informative References
[RFC3410] Case, J., Mundy, R., Partain, D., and B.
Stewart, "Introduction and Applicability
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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.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple
Authentication and Security Layer (SASL)",
RFC 4422, June 2006.
[I-D.ietf-netconf-ssh] Wasserman, M. and T. Goddard, "Using the
NETCONF Configuration Protocol over Secure
Shell (SSH)", draft-ietf-netconf-ssh-06 (work
in progress), March 2006.
Appendix A. 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 will need to remove lines feeds from the second and
third lines, which needed to be wrapped to fit into RFC limits.
A.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
securityLevel,In,In,In,In,In,In,Out,In,In,In
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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,,,
tmSessionReference,,,,,,Out,In,,In,
Appendix B. Why tmSessionReference?
This appendix considers why a cache-based approach was selected for
passing parameters. This section may be removed from subsequent
revisions of the document.
There are four approaches that could be used for passing information
between the TMSP and an SMSP.
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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.
B.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.
B.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 SMSP.
B.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.
If the fields in an incoming SNMPv3 message are changed by the TMSP
before passing it to the SMSP, 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
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the architecture.
B.4. Using a Cache
This document describes a cache, into which the TMSP puts information
about the security applied to an incoming message, and an SMSP
extracts that information from the cache. Given that there may be
multiple TM-security caches, a tmSessionReference is passed as an
extra parameter in the ASIs between the transport mapping and the
messaging security model, so the SMSP knows which cache of
information to consult.
This approach does create dependencies between a model-specific TMSP
and a corresponding specific SMSP. This approach of passing a model-
independent reference is consistent with the securityStateReference
cache already being passed around in the RFC3411 ASIs.
Appendix C. Open Issues
Appendix D. Change Log
NOTE to RFC editor: Please remove this change log before publishing
this document as an RFC.
Changes from revision -02- to -03-
o removed session table from MIB module
o removed sessionID from ASIs
o reorganized to put ASI discussions in EOP section, as was done in
SSHSM
o changed user auth to client auth
o changed tmStateReference to tmSessionReference
o modified document to meet consensus positions published by JS
o
* authoritative is model-specific
* msgSecurityParameters usage is model-specific
* msgFlags vs. securityLevel is model/implementation-specific
* notifications must be able to cause creation of a session
* security considerations must be model-specific
* TDomain and TAddress are model-specific
* MPSP changed to SMSP (Security model security processing)
Changes from revision -01- to -02-
o wrote text for session establishment requirements section.
o wrote text for session maintenance requirements section.
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o removed section on relation to SNMPv2-MIB
o updated MIB module to pass smilint
o Added Structure of the MIB module, and other expected MIB-related
sections.
o updated author address
o corrected spelling
o removed msgFlags appendix
o Removed section on implementation considerations.
o started modifying the security boilerplate to address TMSM and MIB
security issues
o reorganized slightly to better separate requirements from proposed
solution. This probably needs additional work.
o removed section with sample protocols and sample
tmSessionReference.
o Added section for acronyms
o moved section comparing parameter passing techniques to appendix.
o Removed section on notification requirements.
Changes from revision -00-
o changed SSH references from I-Ds to RFCs
o removed parameters from tmSessionReference for DTLS that revealed
lower layer info.
o Added TMSM-MIB module
o Added Internet-Standard Management Framework boilerplate
o Added Structure of the MIB Module
o Added MIB security considerations boilerplate (to be completed)
o Added IANA Considerations
o Added ASI Parameter table
o Added discussion of Sessions
o Added Open issues and Change Log
o Rearranged sections
Authors' Addresses
David Harrington
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: dharrington@huawei.com
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Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
EMail: j.schoenwaelder@iu-bremen.de
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on an
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Acknowledgement
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