WDIFF

draft-ietf-isms-tmsm-03.txt --> draft-ietf-isms-tmsm-04.txt

Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Intended status: Informational J. Schoenwaelder
Expires: December 25, 2006 April 14, 2007 International University Bremen
June 23,
October 11, 2006

Transport Mapping Security Model (TMSM) Architectural Extension Subsystem for the Simple Network Management Protocol (SNMP)
draft-ietf-isms-tmsm-03.txt
draft-ietf-isms-tmsm-04.txt

Status of This Memo

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This Internet-Draft will expire on December 25, 2006. April 14, 2007.

Abstract

This document describes a Transport Mapping Security Model (TMSM)
extension for Subsystem, extending the Simple
Network Management Protocol (SNMP) architecture defined in RFC 3411.
This document describes a subsystem to contain transport models,
comparable to other subsystems in the RFC3411 architecture. As work
is being done to expand the transport to include secure transport
such as SSH and TLS, using a subsystem will enable consistent design
and modularity of such transport models. This document identifies

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and discusses some key aspects that need to be considered for any
transport-mapping-based security
transport model for SNMP.

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This memo also defines a portion of the Management Information Base
(MIB) for managing sessions models in the Transport Mapping Security Model
extension. Subsystem.

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 4
2. Requirements of a Transport Mapping Security Model . . . . . . . . . . . . . . 6
2.1. Message Security Requirements . . . . . . . . . . . . . . 6
2.1.1. Security Protocol Requirements . . . . . . . . . . . . 7 6
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 . . . . . . . . . . the Transport Subsystem . . . . . . . . 21
3.1. Command Generator or Notification Originator . . . . . . . 21
3.2. Command Responder . . . . . . . . . . . . . . . . . . . . 22
4. Message Formats Cached Information and References . . . . . . . . . . . . . . 23
4.1. securityStateReference . . . . . . . . . 23
4.1. SNMPv3 Message Fields . . . . . . . . . 24
4.2. tmStateReference . . . . . . . . . 24
4.1.1. msgGlobalData . . . . . . . . . . . . 25
5. Abstract Service Interfaces . . . . . . . . 26
4.1.2. msgSecurityParameters . . . . . . . . . 25
5.1. Generating an Outgoing SNMP Message . . . . . . . 27
5. Cached Information and References . . . . 26
5.2. Processing for an Outgoing Message . . . . . . . . . . 27
5.1. tmSessionReference Cached Session Data . . 27
5.3. Processing an Incoming SNMP Message . . . . . . . . 27
5.2. securityStateReference Cached Security Data . . . 28
5.3.1. Processing an Incoming Message . . . . 27
6. Abstract Service Interfaces for TMSM . . . . . . . . 28
5.3.2. Prepare Data Elements from Incoming Messages . . . . . 28
6.1. Generating
5.3.3. Processing an Outgoing SNMP Incoming Message . . . . . . . . . . . 29
6.2. TMSP for an Outgoing Message . 29
6. The Transport-Subsystem-MIB Module . . . . . . . . . . . . . . 30
6.3. Processing an Incoming SNMP Message
6.1. Structure of the MIB Module . . . . . . . . . . . 30
6.3.1. TMSP for an Incoming Message . . . . 31
6.1.1. The tmsmStats Subtree . . . . . . . . . . . 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.
6.2. Relationship to Other MIB Modules . . . . . . . . . . . . 33
7.2.1. 31
6.2.1. Textual Conventions . . . . . . . . . . . . . . . . . 33
7.2.2. 31
6.2.2. MIB Modules Required for IMPORTS . . . . . . . . . . . 33
7.3. 31
6.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 33
8. 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 38
9. 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39

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10. 37
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
11. 37
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11.1. 38

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10.1. Normative References . . . . . . . . . . . . . . . . . . . 39
11.2. 38
10.2. Informative References . . . . . . . . . . . . . . . . . . 40 39
Appendix A. Parameter Table . . . . . . . . . . . . . . . . . . . 41 39
A.1. ParameterList.csv . . . . . . . . . . . . . . . . . . . . 41 39
Appendix B. Why tmSessionReference? tmStateReference? . . . . . . . . . . . . . . . 42 . 41
B.1. Define an Abstract Service Interface . . . . . . . . . . . 43 41
B.2. Using an Encapsulating Header . . . . . . . . . . . . . . 43 41
B.3. Modifying Existing Fields in an SNMP Message . . . . . . . 43 42
B.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 44 42
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 44 42
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 44 42

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1. Introduction

This document describes a Transport Mapping Security Model (TMSM)
extension for Subsystem, extending 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 transport model for SNMP.

1.1. The Internet-Standard Management Framework

For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [RFC3410].

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

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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].

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This document proposes describes a Transport Mapping Security Model (TMSM) Subsystem 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
transport-model [RFC3417]. Such a TMSM transport model would then enable
the use of existing security mechanisms such as (TLS) [RFC4366] or
SSH [RFC4251] within the RFC3411 architecture.

There are a number of Internet security protocols and mechanisms that
are in wide spread use. Many of them try to provide a generic
infrastructure to be used by many different application layer
protocols. The motivation behind TMSM the transport subsystem is to
leverage these protocols where it seems useful.

There are a number of challenges to be addressed to map the security
provided by a secure transport into the SNMP architecture so that
SNMP continues to 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. transport subsystem. Some factors
should be considered when selecting a protocol for use within this framework. protocol.

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.

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A protocol used for the TMSM framework transport model should ideally require no modifications to the underlying
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 transport
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

Transport models MUST be able to coexist with other protocol transport models,
and may be designed to utilize either TCP or UDP, depending on the
transport. UDP or SCTP.

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 RFC3411 architecture includes a Security

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Subsystem which is responsible security subsystem for realizing enabling
different methods of providing security services. services, a messaging
subsystem permitting different message versions to be handled by a
single engine, an application subsystem to support different types of
application processors, and an access control subsystem for allowing
multiple approaches to access control. The RFC3411 architecture does
not include a subsystem for transport models, despite the fact there
are multiple transport mappings already defined for SNMP. This
document addresses the need for a transport subsystem compatible with
the RFC3411 architecture.

In SNMPv2, there were many problems of side effects between
subsystems caused by the manipulation of MIB objects, especially

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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 would be 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 subsystem must abide the goals of the
RFC3411 architecture defined in [RFC3411]. To that end, this
transport mapping security model subsystem proposal uses a modular design that
can will permit
transport models to be advanced through the standards process
independently of other
proposals, transport models, and independent of other
modular SNMP 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. Part of
the motivstion of developing transport models is to develop support
for secure transport protocols, such as a transport model that
utilizes the Secure Shell protocol. Any transport mapping security model should
define one minimum-
compliance minimum-compliance security mechanism, preferably one
which is already widely deployed
within used to secure the transport layer security protocol used. protocol.

The TMSM architectural extension Transport Subsystem permits additional multiple transport
security protocols to be
"plugged into" the RFC3411 architecture, supported by corresponding transport-security-aware
transport mapping
models. models, including models that are security-aware.

The RFC3411 architecture, and architecture,and the USM approach, assume that a

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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 subsystem. A transport
layer and the message dispatcher. Conceptually, model
that supports a secure transport mapping protocol may perform similar

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security processing will be called from functions within the Transport Mapping
functionality of an SNMP engine dispatcher to transport subsystem. A transport model
may perform the translation of transport security parameters to/from
security-model-independent parameters. This To accommodate this, the ASIs
for the transport mapping security processor will be
referred to in this document as TMSP.

Additional functionality may be performed as part of subsystem, the message
processing function, i.e., in messaging subsystem, and the
security subsystem of the RFC3411
architecture. This document will refer be extended to security model's security
processor as the SMSP.

Thus pass security-model-
independent values, and a TMSM is composed cache of both a TMSP and an SMSP. transport-specific information.

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+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-----+ +-----+ +-------+ (traditional SNMP agent)
+-------------------------------------------------------------------+
| UDP +--------------------------------------------------+ |
| TCP | . . . Transport Subsystem | | other
| | +-----+ +-----+ +-----+ +-----+ +-------+
^ ^ ^ | |
|
v v v
+-----+ +-----+ +-------+ | | UDP | | TCP | | SSH | | TLS | . . . | other | | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ (traditional SNMP agent)
+-------------------------------------------------------------------+ | |
| +--------------------------------------------------+ |
| ^ |
| | |
| Dispatcher v |
| +-------------------+ +---------------------+ +----------------+ |
| | Transport | +--------------+ | Message Processing | | Mapping |<---> | TMSM | |
| | (e.g., RFC 3417) | | TMSP | |
| | | +--------------+ |
| | | |

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| | | +---------------------+ +----------------+ |
| | | | Message Processing | | Security Security | |
| | | | Subsystem | | Subsystem | |
| | | | +------------+ | | +------------+ | |
| | | | +->| v1MP * |<--->| +------------+ | USM * | | |
| | | +------------+ | | +------------+ | Other | +------------+ | |
| | | | | +------------+ | | | Security | +------------+ | |
| | | | +->| v2cMP * |<--->| | Model Transport* | | |
| | Message | | | +------------+ | | +------------+ | Security | | |
| | Dispatcher <--------->| +------------+ | | +------------+ | Model | | |
| | | | +->| v3MP * |<--->| | TMSM | +------------+ | |
| | | | | +------------+ | | | SMSP | +------------+ | |
| | PDU Dispatcher | | | +------------+ | | | Other * | | |
| +-------------------+ | +->| otherMP * |<--->| +------------+ | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+

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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
potentially done by a TMSM transport 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.

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| -----------------------------------------------|
| transport layer |
| -----------------------------------------------|
^
|
v
--------------------------------------------------
| -----------------------------------------------|
| | transport mapping |
| -----------------------------------------------|
| ^
| |
| v
| --------------------------------------------- |
| --------------------- ------------------ |
| SNMP messaging <--> | decryption + | |
| | translation | |
| --------------------- ------------------ |
| ^
| |
| v
| --------------------- ------------------ |
| | SNMP applications | <--> | access control | |
| --------------------- ------------------ |

| --------------------------------------------- |

2.2.1.2. TMSM Transport Subsystem and the RFC3411 Architecture

With the TMSM approach, Transport Subsystem, the order of the steps may differ and
may be handled by different subsystems:
1) decrypt the encrypted portions of the message (transport layer)

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2)
2*) translate parameters to model-independent parameters (transport
model)
3) determine the SNMP security model and parameters (transport
mapping)
3*) translate parameters to model-independent parameters (transport
mapping) (messaging model)
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

If a message is largely based on having secured using non-SNMP-specific message security and parameters. The
parameters, then the transport mapping model might should 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 model <--> | translation* translation | |
| ------------------ |
| -----------------------------------------------|
| ^
| |
| v
| --------------------------------------------- |
| ------------------ |
| SNMP messaging <--> message model | translation*
| |
| ------------------ -----------------------------------------------|
| ^
| --------------------- ------------------ |
| ^ v
| --------------------------------------------- |
| |
| security model |
| |
| -----------------------------------------------|
| ^
| |
| v
| --------------------- ------------------ |
| | SNMP applications | <--> | access control | |
| --------------------- ------------------ |

| --------------------------------------------- |

2.2.1.3. Passing Information between Engines

A TMSM secure transport model will establish an encrypted tunnel between
the transport
mappings models of two SNMP engines. One transport mapping security model
instance encrypts all messages, and the other transport mapping
security model
instance decrypts the messages.

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After the a transport layer tunnel is established, then SNMP messages can
conceptually be sent through the tunnel from one SNMP message
dispatcher engine to
another SNMP message dispatcher. engine. 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 processing
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 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 for incoming
messages before the SMSP
portion of the security 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 This translation
from a transport layer mechanism-specific authenticated identity plays an
important role and must to a securityName
MAY be considered done by any TMSM, and principal
authentication must be available via the transport layer model, and the securityname is then
provided to the security
protocol. model to be passed to the message processing
model..

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 via a
RADIUS server) may 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 transport model that provides security services should take care to
not violate the separation of authentication and authorization in the
RFC3411 architecture. The

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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
transport or security subsystem, subsystems, to be consistent with the
modularity of the RFC3411 architecture. This separation was a

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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 (in the access control subsystem) MAY require
authentication by certain securityModels and a minimum securityLevel
to allow access to the data.

TMSM is an enhancement

Transport models that provide secure transport are an enhancement for
the SNMPv3 privacy and authentication
provisions, authentication, but it is they are not a significant
improvement for the authorization (access control) needs of SNMPv3. TMSM provides all
Only the model-
independent model-independent parameters for the isAccessAllowed()
primitive [RFC3411].

TMSM does [RFC3411] are provided by the transport and security
subsystems.

A transport model must not specify how the securityModel and
securityName could be dynamically mapped to an access control
mechanism, such as a VACM-style groupName. The 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 from the model-independent security parameters to
an authorization-model-dependent access policy 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 transport model or a security model
and an access control model, just as it is undesirable that the TMSM approach
creates for a
transport model to create a dependency between an SNMP message
version and the security provided by a transport mapping. model.

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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 transport model, outgoing SNMP messages are
passed unencrypted from the message dispatcher to the transport mapping,
model, and incoming messages are passed unencrypted from the
transport mapping model 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

When using a secure transport model, security parameters are not
carried MAY be
provided through means other than carrying them in the SNMP message.
The parameters are MAY be provided by SNMP applications for outgoing
messages, and the parameters for incoming messages are MAY be extracted
from the transport layer by the security-
model-specific transport mapping model before the message is
passed to the message processing subsystem.

For outgoing messages, even when a secure transport model will
provide the security services, it is necessary to have an SMSP security
model because it is the SMSP security model that actually creates the
message from its component parts. Whether there are any security
services provided by the SMSP security model for an outgoing message is
model-dependent.

For incoming messages, even when a secure transport model provides
security services, a security model is necessary because there might
be some security functionality that can only be handled provided after the
message version is known. The message version is determined by the
Message Processing model and passed to the SMSP security model via the
processIncoming() ASI.

The RFC3411 architecture has no ASI parameters for passing security
information between the a 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 (a transport security are
consistent, then it will be necessary to pass model) and the model-dependent

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security parameters from

dispatcher, and between the TMSP through to dispatcher and the SMSP. message processing
model.

This document describes a cache, cache mechanism, into which the TMSP transport
model puts information about the transport and security parameters
applied to a transport connection or an incoming message, and an SMSP
extracts a
security model MAY extract that information from the cache. Given that there may be
multiple TM-security caches, a tmSessionReference A
tmStateReference is passed as an extra parameter in the ASIs between of the
transport mapping subsystem and the messaging and security model, so the SMSP knows which cache of
information subsystems, to consult.

This approach does create dependencies between a model-specific TMSP
and a corresponding specific SMSP.
identify the relevant cache.

This approach of passing a model-
independent model-independent reference is consistent
with the securityStateReference cache already being passed around in
the RFC3411 ASIs. [todo: can we avoid dependencies?]

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 transport 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, the transport subsystem, so
an SNMP application cannot select the session except by passing a
unique combination of transport type, transport address,
securityName, securityModel, and securityLevel.

All TMSM-based security transport models should discuss the impact of sessions on SNMP
usage, including how to establish/open a TMSM transport 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 transport model sessions.

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2.3.1. Session Establishment Requirements

SNMP applications must provide the transport type, 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 transport model designer wants to
differentiate a session established for different purposes, such as a
notification session versus a request-response session, the
application can use different securityNames or transport addresses
(e.g., port 161 vs. port 162) for different purposes.

An SNMP engine containing an application that initiates
communication, e.g., a Command Generator or Notification Originator,
MUST be able to attempt to establish a session for delivery if a
session does not yet exist. If a session cannot be established then
the message is discarded.

Sessions are usually established by the transport mapping security
processor model 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.
notifications. How sessions are established in advance is beyond the
scope of this document.

Sessions are initiated by notification originators when there is no
currently established connection that can be used to send the
notification. For a client-server security protocol, this 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 transport model must be able to determine whether a session does or
does not exist, and must be able to determine which session has the
appropriate security characteristics (transport type, transport
address, securityName, securityModel, and securityLevel) for an
outgoing message. [discuss: does the transport model have insight
into the securityModel?]

A TMSM security transport model implementation MAY reuse an already established
session with the appropriate transport type, 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

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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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be returned using the same session that carried the corresponding
request message. Reuse of sessions is not required for conformance.

If a session can be reused for a different 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. receiver. This
may strongly affect the usefulness of session reuse.

2.3.2. Session Maintenance Requirements

A TMSM-based security transport 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 transport models should be sure to
describe the expected behavior when no session exists for a response.
[todo: do we already say that the message should be discarded, or is
that just in the ssh transport model?]

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 transport model defines MIB module objects to maintain session
state information, then the security transport 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 transport model session is associated with state information that
is maintained for its lifetime. This state information allows for
the application of various security services to TMSM-based security
models. multiple messages.
Cryptographic keys established at the beginning of the session SHOULD
be used to provide authentication, integrity checking, and encryption
services for data that is communicated during the session. The

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cryptographic protocols used to establish keys for a
TMSM-based security transport 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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information MAY be maintained in the session which can be used to
prevent the replay and reordering of messages within a session.

A TMSM transport model session will typically have a single transport
type, ransport address, securityModel, 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, or to use a different securityModel, then
another session would be needed. An immediate consequence of this is
that implementations should be able to maintain some reasonable
number of concurrent sessions.

For TMSM transport 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 transport 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 transport 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 transport model and the
secure transport used.

If the underlying transport layer security was is configurable on a per-
message basis, a TMSM-based security transport model could have a security-
model-specific transport-model 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

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of procedure of the security transport model would need to describe the
procedures to enable this determination. [discuss: this is a feature
I find questionable. It can be developed as a feature of a specific
transport model. Do we need this discussion here?]

For security transport models that do not support variable security services
in

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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 transport models should consider the trade offs for
resource-constrained devices.

3. Scenario Diagrams for TMSM the Transport Subsystem

RFC3411 section 4.6 provides scenario diagrams to illustrate how an
outgoing message is created, and how an incoming message is
processed. Both diagrams are incomplete, however. In section 4.6.1,
the diagram doesn't show 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 Cached Information and References

The syntax of an SNMP message using this Security Model adheres RFC3411 architecture uses caches to
the message format defined store dynamic model-specific
information, and uses references in the version-specific Message Processing
Model document (for example [RFC3412]). At the time of this writing, ASIs to indicate in a model-
independent manner which cached information must flow between

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there

subsystems.

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(), two levels of state that show the data
flow between may need to be maintained: the Message Processor
security state in a request-response pair, and the SMSP.

4.1. SNMPv3 Message Fields

The SNMPv3Message SEQUENCE potentially long-term
state relating to transport and security.

This state is defined maintained in [RFC3412] caches and [RFC3416].

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SNMPv3MessageSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN

SNMPv3Message ::= SEQUENCE {
-- identify a Local Configuration
Datastore (LCD). To simplify the layout elements of procedure, the SNMPv3Message
-- this element release
of state information is in same position as in SNMPv1
-- and SNMPv2c, allowing recognition
-- not always explicitly specified. As a
general rule, if state information is available when a message being
processed gets discarded, the value 3 state related to that message should
also be discarded, and if state information 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 available when a model-specific counter.

4.1.2. msgSecurityParameters

The field msgSecurityParameters carries model-dependent security
information
relationship 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 engines 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 severed, such as the closing of a
transport session, the state information for that
need to relationship might
also 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 discarded.

This document differentiates the TMSP and SMSP through tmStateReference from the ASIs.

The TMSP may provide
securityStateReference. This document does not specify an
implementation strategy, only some aspects an abstract discussion of security, and leave some
aspects to the SMSP. tmSessionReference should be used to pass any
parameters, in a model- and mechanism-specific format, data that will be
needed
must flow between subsystems. An implementation MAY use one cache
and one reference to coordinate the activities serve both functions, but an implementer must be
aware of the TMSP and SMSP, plus cache-release issues to prevent the
parameters subsequently passed in securityStateReference.

The cache from being
released before a security or transport model has had an opportunity
to extract the responsibility for explicitly releasing
the complete tmSessionReference and possibly deleting the associated
LCD information when the session is destroyed.

5.2. it needs.

4.1. 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 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."

The information saved should include the model-independent parameters
(transportType, transportAddress, securityName, securityModel, and
securityLevel), related security parameters, and other information
needed to imatch the response with the request. The Message
Processing Model has the responsibility for explicitly releasing the
securityStateReference when 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
securityStateReference cached security data may be implicitly released via the

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generation of a response, or explicitly released by using the
stateRelease primitive, as described in RFC3411 RFC 3411 section 4.5.1."

For

If the TMSM approach, transport model connection is closed between the TMSP may need to provide time a
Request is received and a Response message is being prepared, then
the information
to Response message MAY be discarded.

4.2. tmStateReference

For each message or transport session, information about the message
security is stored in the securityStateReference Local Configuration Datastore (LCD),
supplemented with a cache, to the message processing
model. such as the security-model-independent securityName,
securityLevel, and securityModel parameters, and the transport
address, pass model- and transport type. For responses, the messaging model mechanism-specific
parameters. The state referenced by tmStateReference may
need to pass the parameters back be saved
across multiple messages, as compared to securityStateReference which
is only saved for the TMSP.

This document will differentiate life of a request-response pair of messages.

The format of the tmSessionReference provided by cache and the TMSP LCD are implementation-specific. For
ease of explanation, this document defines a MIB module to
conceptually represent the SMSP, LCD, but this is not meant to contrain
implementations from doing it differently.

It is expected that the securityStateReference provided by the
SMSP to LCD will allow lookup based on the Dispatcher. This document does not specify an
implementation strategy, only an abstract discussion
combination of the data that
must flow between subsystems. An implementation MAY use one cache transportType, transportAddress, securityName,
securityModel, and one reference to serve both functions, but an implementer must be
aware of securityLevel. It is expected that the cache-release issues cache
contain these values or contain pointers/references to prevent entries in the cache from being
released before
LCD.

It is expected that a transport model may store transport-specific
parameters in the LCD for subsequent usage.

5. Abstract Service Interfaces

[todo: the discussion of ASIs that are not directly related to the
transport mapping has had an opportunity or security models was added to
extract the document because it was
difficult to understand what information was available at what
points, and who provided the information. The presence of this
expository text can make it needs.

6. Abstract Service Interfaces hard to find the relevant ASIs for TMSM the
transport subsystem, and can be confusing because it talks about
things that the transport subsystem should not know about. This text
should be reduced.

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

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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.

5.1. 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 tmStateReference
)

Note that tmSessionReference tmStateReference 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().

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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 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 model:

5.2. Processing for an Outgoing Message

The sendMessage ASI is used to pass a message from the Dispatcher to
the appropriate transport mapping model 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 tmStateReference
)

The Transport Mapping Security Model Subsystem provides the following primitives to pass
data back and forth between the TMSM dispatcher and specific
TMSM-based security transport
models, which provide the interface to the underlying secure
transport service. Each TMSM-based security transport 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 tmStateReference
)

statusInformation =
closeSession(
IN tmSessionReference tmStateReference
)

6.3.

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5.3. Processing an Incoming SNMP Message

6.3.1. TMSP for

5.3.1. Processing an Incoming Message

If one does not exist, the TMSP Transport Model will need to create an
entry in a Local Configuration Datastore referenced by tmSessionReference.
tmStateReference. 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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The recvMessage ASI is used to pass a message from the transport
mapping
subsystem 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 tmStateReference
)

6.3.2.

5.3.2. Prepare Data Elements from Incoming Messages

The abstract service primitive from the Dispatcher to a Message
Processing Model for a received message is:

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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 tmStateReference -- from the transport mapping model
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- the version of the PDU
OUT PDU -- SNMP Protocol Data Unit
OUT pduType -- SNMP PDU type
OUT sendPduHandle -- handle for matched request
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for possible Response
)

Note that tmSessionReference tmStateReference has been added to this ASI.

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6.3.3. MPSP for

5.3.3. Processing an Incoming Message

This section describes the procedure followed by the SMSP Security Model
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::

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statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
IN tmSessionReference tmStateReference -- from the transport mapping model
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
) -- information, needed for response

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. tmStateReference.

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.

6. The TMSM MIB Transport-Subsystem-MIB Module

This memo defines a portion of the Management Information Base (MIB)
for statistics in the Transport Mapping Security Model extension.

7.1. Subsystem.

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6.1. 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.

6.1.1. 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. Subsystem.
This subtree provides information for identifying fault conditions
and performance degradation.

7.2.

6.2. 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 Transport-Subsystem-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 transport models that are based on within the TMSM
extension. transport
subsystem. This MIB module is designed to be transport-model
independent and security-model independent, and contains objects
useful for managing common aspects of any TMSM-based security transport model. Specific security
transport models may define a MIB module to contain security-model-dependent transport-model
dependent information.

7.2.1.

6.2.1. 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.

6.2.2. MIB Modules Required for IMPORTS

The. following MIB module imports items from [RFC2578], [RFC2579],
[RFC2580], [RFC3411], and [RFC3419]

7.3.

6.3. Definitions

TMSM-MIB

Transport-Subsystem-MIB DEFINITIONS ::= BEGIN

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IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
mib-2, Integer32, Unsigned32, Gauge32
FROM SNMPv2-SMI
TestAndIncr, StorageType, RowStatus

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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

tmsMIB MODULE-IDENTITY
LAST-UPDATED "200604200000Z" "200610060000Z"
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 Subsystem 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.

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-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
"

REVISION "200604200000Z" "200610060000Z" -- 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 Transport-Subsystem-MIB
-- ---------------------------------------------------------- --

tmsmNotifications

tmsNotifications OBJECT IDENTIFIER ::= { tmsmMIB tmsMIB 0 }
tmsmObjects
tmsObjects OBJECT IDENTIFIER ::= { tmsmMIB tmsMIB 1 }
tmsmConformance
tmsConformance OBJECT IDENTIFIER ::= { tmsmMIB tmsMIB 2 }

-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------

-- Textual Conventions

-- Notifications for the Transport Model Security Model extension

-- Statistics for the Transport Model Security Model extension

tmsmStats OBJECT IDENTIFIER

SnmpTransportModel ::= { tmsmObjects 1 }

tmsmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only TEXTUAL-CONVENTION
STATUS current
DESCRIPTION "The number "An identifier that uniquely identifies a
Transport Model of times an openSession() request
failed the Transport Subsystem within
the SNMP Management Architecture.

The values for transportModel are allocated as
follows:

- The zero value does not identify any particular
transport model.

- Values between 1 and 255, inclusive, are reserved
for standards-track Transport Models and are
managed by the Internet Assigned Numbers Authority
(IANA).
- Values greater than 255 are allocated to open a Session.
"
enterprise-specific Transport Models. An
enterprise-specific transportModel value is defined
to be:

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::= { tmsmStats

enterpriseID * 256 + transport model within
enterprise

For example, the fourth Transport Model defined by
the enterprise whose enterpriseID is 1 }

tmsmSessionNoAvailableSessions OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number would be
260.

This scheme for allocation of times transportModel
values allows for 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 maximum of open sessions.
"
::= { tmsmSession 1 }

tmsmSessionMaxSupported OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The 255 standards-
based Transport Models, and for a maximum of
256 Transport Models per enterprise.

It is believed that the assignment of new
transportModel values will be rare in practice
because the larger the number of open sessions supported.
The value zero indicates simultaneously
utilized Transport Models, the maximum larger the
chance that interoperability will suffer.
Consequently, it is dynamic.
"
::= { tmsmSession 2 }

tmsmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The believed that such a range
will be sufficient. In the unlikely event that
the standards committee finds this number of times to be
insufficient over time, an openSession() request
failed enterprise number
can be allocated to obtain an additional 256
possible values.

Note that the most significant bit must be zero;
hence, there are 23 bits allocated for various
organizations to design and define non-standard
transportModels. This limits the ability to open
define new proprietary implementations of Transport
Models to the first 8,388,608 enterprises.

It is worthwhile to note that, in its encoded
form, the transportModel value will normally
require only a Session.
"
::= { tmsmSession single byte since, in practice,
the leftmost bits will be zero for most messages
and sign extension is suppressed by the encoding
rules.

As of this writing, there are several values
of transportModel defined for use with SNMP or
reserved for use with supporting MIB objects.
They are as follows:

0 reserved for 'any'
1 reserved for UDP
2 reserved for TCP
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 SSH Transport Model
"

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provided.
"

SYNTAX INTEGER(0 .. 2147483647)

-- Notifications for the Transport Subsystem

-- Statistics for the Transport Subsystem

tmsStats OBJECT IDENTIFIER ::= { tmsmSession 4 tmsObjects 1 }

-- -------------------------------------------------------------
-- tmsmMIB - Conformance Information
-- -------------------------------------------------------------

tmsmGroups

tmsGroups OBJECT IDENTIFIER ::= { tmsmConformance tmsConformance 1 }

tmsmCompliances

tmsCompliances OBJECT IDENTIFIER ::= { tmsmConformance tmsConformance 2 }

-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
tmsmGroup
tmsGroup 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.
Transport subsystem.
"

::= { tmsmGroups tmsGroups 2 }

-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------

tmsmCompliance

tmsCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
TMSM-MIB"
Transport-Subsystem-MIB"
MODULE
MANDATORY-GROUPS { tmsmGroup tmsGroup }
::= { tmsmCompliances tmsCompliances 1 }

END

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7. 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. considerations.

It is considered desirable by some industry segments that SNMP
security
transport models should utilize transport layer security that
addresses perfect forward secrecy at least for encryption keys.
Perfect forward secrecy guarantees that compromise of long term
secret keys does not result in disclosure of past session keys.

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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8. IANA Considerations

IANA is requested to create a new registry in the Simple Network
Management Protocol (SNMP) Number Spaces for SnmpTransportModels, as
described in the Transport-Subsystem-MIB defined in this document.
Values 0 through 255 are IANA-assigned by Standards Action, as
defined in RFC2434. Values above 255 are assigned by Hierarchical
allocation, using the algorithm defined in the definition of the
SnmpTransportModels TEXTUAL-CONVENTION in the Transport-Subsystem-MIB
in this document.

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

Transport-Subsystem-MIB { 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.

9. Acknowledgments

The Integrated Security for SNMP WG would like to thank the following
people for their contributions to the process:

The authors of submitted security model proposals: Chris Elliot, Wes
Hardaker, 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.

10. References

11.1.

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10.1. 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

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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.

10.2. 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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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

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

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securityStateReference,,,,,,,,,Out,In

pduType,,,,,,,Out,,,

tmSessionReference,,,,,,Out,In,,In,

tmStateReference,,,,,,Out,In,,In,

Appendix B. Why tmSessionReference? tmStateReference?

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 one could add a header to encapsulate the SNMP message,
3. the TMSM one could utilize fields already defined in the existing SNMPv3
message, or
4. the TMSM one could pass the information in an implementation-specific
cache or via a MIB module.

B.1. Define an Abstract Service Interface

Abstract Service Interfaces (ASIs) [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 subsystem to a messaging security model subsystem has
the advantage of being consistent with existing RFC3411/3412
practice, and helps to ensure that any TMSM transport model proposals pass
the necessary data, and do not cause side effects by creating model-
specific dependencies between itself and other models or other
subsystems other than those that are clearly defined by an ASI.

B.2. Using an Encapsulating Header

A header could encapsulate the SNMP message to pass necessary
information from the TMSP Transport Model 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. Security Model.

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B.3. Modifying Existing Fields in an SNMP Message

[RFC3412] describes the SNMPv3 message, which contains fields to pass
security related parameters. The TMSM transport subsystem 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
Transport Model before passing it to the SMSP, Security Model, then the TMSP
Transport Model will need to decode the ASN.1 message, modify the
fields, and re-encode the message in ASN.1 before passing the message
on to the message dispatcher or to the transport layer. This would
require an intimate knowledge of the message format and message
versions so the TMSP Transport Model knew which fields could be modified.
This would seriously violate the modularity of

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B.4. Using a Cache

This document describes a cache, into which the TMSP Transport Model puts
information about the security applied to an incoming message, and an SMSP
Security Model extracts that information from the cache. Given that
there may be multiple TM-security caches, a tmSessionReference tmStateReference is
passed as an extra parameter in the ASIs between the transport mapping
subsystem and the
messaging security model, subsystem, so the SMSP Security Model knows
which cache of information to consult.

This approach does create dependencies between a model-specific TMSP specific Transport
Model and a corresponding specific SMSP. Security Model. This approach of
passing a model-
independent 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 -03- to -04-

changed title from Transport Mapping Security Model Architectural
Extension to Transport Subsystem
modified the abstract and introduction

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changed TMSM to TMS
changed MPSP to simply Security Model
changed SMSP to simply Security Model
changed TMSP to Transport Model
removed MPSP and TMSP and SMSP from Acronyms section
modified diagrams
removed most references to dispatcher functionality
worked to remove dependencies between transport and security
models.
defined snmpTransportModel enumeration similar to
snmpSecurityModel, etc.
eliminated all reference to SNMPv3 msgXXXX fields
changed tmSessionReference back to tmStateReference

Changes from revision -02- to -03-

o removed session table from MIB module
o removed sessionID from ASIs
o reorganized to put ASI discussions in EOP section, as was done in
SSHSM
o changed user auth to client auth
o changed tmStateReference to tmSessionReference
o modified document to meet consensus positions published by JS
o
* authoritative is model-specific
* msgSecurityParameters usage is model-specific
* msgFlags vs. securityLevel is model/implementation-specific
* notifications must be able to cause creation of a session
* security considerations must be model-specific
* TDomain and TAddress are model-specific
* MPSP changed to SMSP (Security model security processing)

Changes from revision -01- to -02-

o wrote text for session establishment requirements section.
o wrote text for session maintenance requirements section.

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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 TMS and MIB
security issues

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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 TMS-MIB module
o Added Internet-Standard Management Framework boilerplate
o Added Structure of the MIB Module
o Added MIB security considerations boilerplate (to be completed)
o Added IANA Considerations
o Added ASI Parameter table
o Added discussion of Sessions
o Added Open issues and Change Log
o Rearranged sections

Authors' Addresses

David Harrington
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA

Phone: +1 603 436 8634
EMail: dharrington@huawei.com

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Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany

Phone: +49 421 200-3587
EMail: j.schoenwaelder@iu-bremen.de

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