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

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Copyright Notice

   Copyright (C) The Internet Society (2006).

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

   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.  SNMPv3 Message Fields  securityStateReference . . . . . . . . . . . . . . . . . . 24
       4.1.1.  msgGlobalData  . . . . . . . . . . . . . . .
     4.2.  tmStateReference . . . . . 26
       4.1.2.  msgSecurityParameters . . . . . . . . . . . . . . . . 27 25
   5.  Cached Information and References  . . . . . . . . . . . . . . 27
     5.1.  tmSessionReference Cached Session Data . . . . . . . . . . 27
     5.2.  securityStateReference Cached Security Data  . . .  Abstract Service Interfaces  . . . . 27
   6.  Abstract Service Interfaces for TMSM . . . . . . . . . . . . . 28
     6.1. 25
     5.1.  Generating an Outgoing SNMP Message  . . . . . . . . . . . 29
     6.2.  TMSP 26
     5.2.  Processing for an Outgoing Message . . . . . . . . . . . . . . . 30
     6.3. 27
     5.3.  Processing an Incoming SNMP Message  . . . . . . . . . . . 30
       6.3.1.  TMSP for 28
       5.3.1.  Processing an Incoming Message . . . . . . . . . . . . . 30
       6.3.2. 28
       5.3.2.  Prepare Data Elements from Incoming Messages . . . . . 31
       6.3.3.  MPSP for 28
       5.3.3.  Processing an Incoming Message . . . . . . . . . . . . . 32
   7. 29
   6.  The TMSM MIB Transport-Subsystem-MIB Module . . . . . . . . . . . . . . . . . . . . . 33
     7.1. 30
     6.1.  Structure of the MIB Module  . . . . . . . . . . . . . . . 33
       7.1.1. 31
       6.1.1.  The tmsmStats Subtree  . . . . . . . . . . . . . . . . 33
     7.2. 31
     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
   10. 37
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   11. 37
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     11.1. 38
     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

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  [todo]

1.4.  Motivation

   There are multiple ways to secure one's home or business, in a
   continuum of alternatives.  Let's consider three general approaches.
   In the first approach, an individual could buy a gun, learn to use
   it, and sit on your front porch waiting for intruders.  In the second
   approach, one could hire an employee with a gun, schedule the
   employee, position the employee to guard what you want protected,
   hire a second guard to cover if the first gets sick, and so on.  In
   the third approach, you could hire a security company, tell them what
   you want protected, and they could hire employees, train them, buy
   the guns, position the guards, schedule the guards, send a
   replacement when a guard cannot make it, etc., thus providing the
   security you want, with no significant effort on your part other than
   identifying requirements and verifying the quality of the service
   being provided.

   The User-based Security Model (USM) as defined in [RFC3414] largely
   uses the first approach - it provides its own security.  It utilizes
   existing mechanisms (MD5=the gun), but provides all the coordination.
   USM provides for the authentication of a principal, message
   encryption, data integrity checking, timeliness checking, etc.

   USM was designed to be independent of other existing security
   infrastructures.  USM therefore requires a separate principal and key
   management infrastructure.  Operators have reported that deploying
   another principal and key management infrastructure in order to use
   SNMPv3 is a deterrent to deploying SNMPv3.  It is possible but
   difficult to define external mechanisms that handle the distribution
   of keys for use by the USM approach.

   A solution based on the second approach might use a USM-compliant
   architecture, but combine the authentication mechanism with an
   external mechanism, such as RADIUS [RFC2865], to provide the
   authentication service.  It might be possible to utilize an external
   protocol to encrypt a message, to check timeliness, to check data
   integrity, etc.  It is difficult to cobble together a number of
   subcontracted services and coordinate them however, because it is
   difficult to build solid security bindings between the various
   services, and potential for gaps in the security is significant.

   A solution based on the third approach might utilize one or more
   lower-layer security mechanisms to provide the message-oriented
   security services required.  These would include authentication of
   the sender, encryption, timeliness checking, and data integrity
   checking.  There are a number of IETF standards available or in
   development to address these problems through security layers at the
   transport layer or application layer, among them TLS [RFC4366], SASL
   [RFC4422], and SSH [RFC4251].

   From an operational perspective, it is highly desirable to use
   security mechanisms that can unify the administrative security
   management for SNMPv3, command line interfaces (CLIs) and other
   management interfaces.  The use of security services provided by
   lower layers is the approach commonly used for the CLI, and is also
   the approach being proposed for NETCONF [I-D.ietf-netconf-ssh].

   This document proposes 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.

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.

   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
   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
   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 security model is
   called by a message-processing model and will perform multiple
   security functions.  The TMSM approach performs
   similar functions but performs them in different places within the
   architecture, so we need to distinguish the two locations for
   security processing.

   Transport mapping security is by its very nature a security layer
   which is plugged into the RFC3411 architecture between the subsystem.  A transport
   layer and the message dispatcher.  Conceptually, model
   that supports a secure transport mapping protocol may perform similar
   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.

   +------------------------------+
   |           Network            |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v
   +-----+ +-----+       +-------+
   | UDP                 (traditional SNMP agent)
   +-------------------------------------------------------------------+
   | +--------------------------------------------------+              | TCP
   | . . . | other  Transport Subsystem                             |
   +-----+ +-----+       +-------+
      ^       ^              ^              |
   | |
      v       v              v +-----+ +-----+       +-------+
   | SSH | | TLS | . . . | other | +-----+ +-----+       +-------+            (traditional SNMP agent)
   +-------------------------------------------------------------------+
   |              ^                                                    |
   |              |  |              | Dispatcher   v
   | | +-------------------+                                             | | UDP | Transport |      +--------------+ TCP | | SSH | Mapping           |<---> | TMSM TLS | . . . | other |  | (e.g., RFC 3417)              |
   | TMSP | +-----+ +-----+ +-----+ +-----+       +-------+  |              |
   | +--------------------------------------------------+              |      +--------------+
   |              ^                                                    |
   |              |                                                    |
   | Dispatcher   v                                                    |
   | +-------------------+ +---------------------+  +----------------+ |
   | |                   | | Message Processing  |  | Security       | |
   | |                   | | Subsystem           |  | Subsystem      | |
   | |                   | |     +------------+  |  | +------------+ | |
   | |                   | |  +->| v1MP     * |<--->| +------------+ | USM      * | | |
   | |                   |  +------------+ |  |  +------------+  | Other  | +------------+ | |
   | |                   | |  |  +------------+  |  | | Security   | +------------+ | |
   | |                   | |  +->| v2cMP    * |<--->| | Model Transport* | | |
   | | Message           | |  |  +------------+  |  | +------------+ | Security   | | |
   | | Dispatcher  <--------->|  +------------+  |  | +------------+ | Model      | | |
   | |                   | |  +->| v3MP     * |<--->| +------------+ | TMSM |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | SMSP | | |
   | | PDU Dispatcher 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 |
   +-------------------------------------------------------------------+

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)
   3) decrypt the encrypted portions of the message (security model)
   4) translate parameters to model-independent parameters (security
      model)
   5) determine which application should get the decrypted portions
      (messaging model), and
   6) pass on the decrypted portions with model-independent parameters.

   The USM approach uses SNMP-specific message security and parameters.

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

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

2.2.1.2.  TMSM  Transport Subsystem and the RFC3411 Architecture

   In

   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)
   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).
   | -----------------------------------------------|
   |                            ------------------  |
   |   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.

   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.

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 isAccessAllowed() primitive is used for
   passing security-model independent parameters between the subsystems
   of the architecture.

   Mapping of (securityModel, securityName) to an access control policy
   should be handled within the access control subsystem, not the
   transport or security subsystem, subsystems, to be consistent with the
   modularity of the RFC3411 architecture.  This separation was a
   deliberate decision of the SNMPv3 WG, to allow support for
   authentication protocols which did not provide authorization
   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

   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.

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 (a transport model) and the
   dispatcher, and between the dispatcher and the message processing
   model.  If there is
   a need to have an SMSP called from the message processing model to,
   for example, verify that msgFlags and the transport security are
   consistent, then it will be necessary to pass the model-dependent
   security parameters from the TMSP through to the SMSP.

   This document describes a cache, cache mechanism, into which the TMSP transport
   model puts information about the transport and security parameters
   applied to an 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.

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
   receive a request may use that session to send an outgoing
   notification, and may use a session that was originally established
   to send a notification to send a request.  Responses are expected to
   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
   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 to another.  In addition sequence
   information MAY be maintained in the session which can be used to
   prevent the replay and reordering of messages within a session.

   A TMSM 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
   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 one session, multiple sessions could be established with different
   security levels, and for every packet the SNMP engine could select
   the appropriate session based on the requested securityLevel.  Some
   SNMP entities are resource-constrained.  Adding sessions increases
   the need for resources, but so does encrypting unnecessarily.
   Designers of security 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.

   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.

   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,
   there ASIs to indicate in a model-
   independent manner which cached information must flow between
   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].

   SNMPv3MessageSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN

          SNMPv3Message ::= SEQUENCE {
              -- identify a Local Configuration
   Datastore (LCD).  To simplify the layout elements of procedure, the SNMPv3Message
              -- this element is in same position as in SNMPv1
              -- and SNMPv2c, allowing recognition
              -- the value 3 is used for snmpv3
              msgVersion INTEGER ( 0 .. 2147483647 ),
              -- administrative parameters
              msgGlobalData HeaderData,
              -- security model-specific parameters
              -- format defined by Security Model
              msgSecurityParameters OCTET STRING,
              msgData  ScopedPduData
          }

          HeaderData ::= SEQUENCE {
              msgID      INTEGER (0..2147483647),
              msgMaxSize INTEGER (484..2147483647),

              msgFlags   OCTET STRING (SIZE(1)),
                         --  .... ...1   authFlag
                         --  .... ..1.   privFlag
                         --  .... .1..   reportableFlag
                         --              Please observe:
                         --  .... ..00 release
   of state information is OK, means noAuthNoPriv
                         --  .... ..01 not always explicitly specified.  As a
   general rule, if state information is OK, means authNoPriv
                         --  .... ..10   reserved, MUST NOT available when a message being
   processed gets discarded, the state related to that message should
   also be used.
                         --  .... ..11 discarded, and if state information 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 available when a
   relationship between engines is severed, such as defined in [RFC3416]
          }
      END

   The following describes how any TMSM model SHOULD treat certain
   fields in the message:

4.1.1.  msgGlobalData

   msgGlobalData is opaque to closing of a TMSM security model.  The values are set
   by
   transport session, the Message Processing model (e.g., SNMPv3 Message Processing),
   and SHOULD NOT be modified by a TMSM security model.

   The msgSecurityModel field should state information for that relationship might
   also be set by discarded.

   This document differentiates the Message Processing
   model to a value tmStateReference from the SnmpSecurityModel enumeration [RFC3411] to
   identify the specific TMSM model.  Each standards-track TMSM model
   should have
   securityStateReference.  This document does not specify an enumeration assigned by IANA.  Each enterprise-
   specific security model should have
   implementation strategy, only an enumeration assigned following
   instructions in abstract discussion of the description data that
   must flow between subsystems.  An implementation MAY use one cache
   and one reference to serve both functions, but an implementer must be
   aware of the SnmpSecurityModel TEXTUAL-
   CONVENTION from RFC3411.

   The msgFlags have cache-release issues to prevent the same values for cache from being
   released before a TMSM security or transport model as for the USM
   model.

4.1.1.1.  securityLevel and msgFlags

   For has had an outgoing message, msgFlags is opportunity
   to extract the requested security for information it needs.

4.1.  securityStateReference

   From RFC3411: "For each message received, the
   message; if Security Model caches
   the state information such that a TMSM cannot provide Response message can be generated
   using the requested securityLevel, same security information, even if the
   model MUST describe a standard behavior that Local Configuration
   Datastore is followed for that
   situation.  If the TMSM cannot provide at least altered between the requested level time of security, the TMSM MUST discard the incoming 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 response.

   A Message Processing Model has
   been applied to the actual message. responsibility for explicitly
   releasing the cached data if such data is no longer needed.  To avoid
   enable this, an abstract securityStateReference data element is
   passed from the need Security Model to mess with the ASN.1 encoding, Message Processing Model.  The
   cached security data may be implicitly released via the SNMPv3 message carries the requested
   msgFlags, not generation of
   a response, or explicitly released by using the actual securityLevel applied to stateRelease
   primitive, as described in RFC3411 section 4.5.1."

   The information saved should include the message.  If a
   message format model-independent parameters
   (transportType, transportAddress, securityName, securityModel, and
   securityLevel), related security parameters, and other than SNMPv3 is used, then information
   needed to imatch the new message may
   carry response with the more accurate securityLevel in request.  The Message
   Processing Model has the SNMP message.

   For an incoming message, responsibility for explicitly releasing the receiving TMSM knows what must
   securityStateReference when such data is no longer needed.  The
   securityStateReference cached data may be done
   to process implicitly released via the message based on
   generation of a response, or explicitly released by using the transport layer mechanisms.
   stateRelease primitive, as described in RFC 3411 section 4.5.1."

   If the underlying transport security mechanisms for the receiver cannot
   provide model connection is closed between the matching securityLevel, time a
   Request is received and a Response message is being prepared, then
   the Response message should follow
   the standard behaviors for the MAY be discarded.

4.2.  tmStateReference

   For each message or transport session, information about the message
   security mechanism, or is stored in the Local Configuration Datastore (LCD),
   supplemented with a cache, to pass model- and mechanism-specific
   parameters.  The state referenced by tmStateReference may be
   discarded silently.

   Part saved
   across multiple messages, as compared to securityStateReference which
   is only saved for the life of a request-response pair of messages.

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

   It is expected that the actual
   security provided by LCD will allow lookup based on the underlying transport layer security
   mechanisms is configured to meet or exceed
   combination of transportType, transportAddress, securityName,
   securityModel, and securityLevel.  It is expected that the securityLevel required
   by cache
   contain these values or contain pointers/references to entries in the msgFlags
   LCD.

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

5.  Abstract Service Interfaces

   [todo: the SMSP processes discussion of ASIs that are not directly related to the
   incoming message,
   transport or security models was added to the document because it should compare was
   difficult to understand what information was available at what
   points, and who provided the msgFlags field information.  The presence of this
   expository text can make it hard to find the
   securityLevel actually provided relevant ASIs for the message by the
   transport
   layer security.  If they differ, subsystem, and can be confusing because it talks about
   things that the SMSP transport subsystem should determine whether
   the changed securityLevel is acceptable.  If not, it not know about.  This text
   should discard be reduced.

   Abstract service interfaces have been defined by RFC 3411 to describe
   the message.  Depending on conceptual data flows between the model, various subsystems within an
   SNMP entity.

   To simplify the SMSP may issue a reportPDU
   with elements of procedure, the release of state
   information is not always explicitly specified.  As a model-specific counter.

4.1.2.  msgSecurityParameters

   The field msgSecurityParameters carries model-dependent security general rule,
   if state information between engines.  When is available when a security model does not utilize
   this field, its value MUST be message gets discarded, the BER serialization of a zero-length
   OCTET STRING, to prevent its being used in
   message-state information should also be released, and if state
   information is available when a manner that could session is closed, the session state
   information should also be
   damaging, such as released.

   An error indication may return an OID and value for carrying an incremented
   counter and a virus or worm.

   RFC3412 defines two primitives, generateRequestMsg() and
   processIncomingMsg() which require the specification of an
   authoritative SNMP entity.  The meaning of authoritative is model
   dependent.

5.  Cached Information and References

   he RFC3411 architecture uses caches to store dynamic model-specific
   information, and uses references in the ASIs to indicate in a model-
   independent manner which cached information must flow between
   subsystems.  For most TMSM models, there are two levels of state that
   need to be maintained: the session state, and the message security
   state.

5.1.  tmSessionReference Cached Session Data

   The tmSessionReference is used to pass references to the appropriate
   session information between the TMSP and SMSP through the ASIs.

   The TMSP may provide only some aspects of security, and leave some
   aspects to the SMSP. tmSessionReference should be used to pass any
   parameters, in a model- and mechanism-specific format, that will be
   needed to coordinate the activities of the TMSP and SMSP, plus the
   parameters subsequently passed in securityStateReference.

   The security model has the responsibility for explicitly releasing
   the complete tmSessionReference and possibly deleting the associated
   LCD information when the session is destroyed.

5.2.  securityStateReference Cached Security Data

   From RFC3411: "For each message received, the Security Model caches
   the state information such that a Response message can be generated
   using the same security information, even if the Local Configuration
   Datastore is altered between the time of the incoming request and the
   outgoing response.

   A Message Processing Model has the responsibility for explicitly
   releasing the cached data if such data is no longer needed.  To
   enable this, an abstract securityStateReference data element is
   passed from the Security Model to the Message Processing Model.  The
   cached security data may be implicitly released via the generation of
   a response, or explicitly released by using the stateRelease
   primitive, as described in RFC3411 section 4.5.1."

   For the TMSM approach, the TMSP may need to provide the information
   to be stored in the securityStateReference to the message processing
   model. such as the security-model-independent securityName,
   securityLevel, and securityModel parameters, and the transport
   address, and transport type.  For responses, the messaging model may
   need to pass the parameters back to the TMSP.

   This document will differentiate the tmSessionReference provided by
   the TMSP to the SMSP, from the securityStateReference provided by the
   SMSP to the Dispatcher.  This document does not specify an
   implementation strategy, only an abstract discussion of the data that
   must flow between subsystems.  An implementation MAY use one cache
   and one reference to serve both functions, but an implementer must be
   aware of the cache-release issues to prevent the cache from being
   released before the transport mapping has had an opportunity to
   extract the information it needs.

6.  Abstract Service Interfaces for TMSM

   Abstract service interfaces have been defined by RFC 3411 to describe
   the conceptual data flows between the various subsystems within an
   SNMP entity.  TMSM security models use some of these conceptual data
   flows when communicating between subsystems, such as the dispatcher
   and the Message Processing Subsystem.

   To simplify the elements of procedure, the release of state
   information is not always explicitly specified.  As a general rule,
   if state information is available when a message gets discarded, the
   message-state information should also be released, and if state
   information is available when a session is closed, the session state
   information should also be released.

   An error indication may return an OID and value for an incremented
   counter and a value for securityLevel, value for securityLevel, and values for contextEngineID
   and contextName for the counter, and the securityStateReference if
   the information is available at the point where the error is
   detected.

6.1.

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().

   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.

   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.

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 determined is model-specific.

   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:

   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.

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::

   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
   the processIncomingMsg primitive.

7.

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.

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

  IMPORTS
      MODULE-IDENTITY, OBJECT-TYPE,
      mib-2, Integer32, Unsigned32, Gauge32
        FROM SNMPv2-SMI
      TestAndIncr, StorageType, RowStatus
        FROM SNMPv2-TC
      MODULE-COMPLIANCE, OBJECT-GROUP
        FROM SNMPv2-CONF
      SnmpSecurityModel,
      SnmpAdminString,  SnmpSecurityLevel, SnmpEngineID
         FROM SNMP-FRAMEWORK-MIB
      TransportAddress, TransportAddressType
        FROM TRANSPORT-ADDRESS-MIB
      ;

   tmsmMIB

  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
                      "
         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.

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

  SnmpTransportModel ::= TEXTUAL-CONVENTION
      STATUS       current
      DESCRIPTION "An identifier that uniquely identifies a
                   Transport Model Security Model extension

   -- Statistics of 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
                     enterprise-specific Transport Models.  An
                     enterprise-specific transportModel value is defined
                     to be:

                     enterpriseID * 256 + transport model within
                     enterprise

                     For example, the fourth Transport Model Security Model extension

   tmsmStats         OBJECT IDENTIFIER ::= { tmsmObjects defined by
                     the enterprise whose enterpriseID is 1 }

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

                   This scheme for allocation of times an openSession() request
                  failed to open transportModel
                   values allows for a Session.
                   "
       ::= { tmsmStats 1 }

   tmsmSessionNoAvailableSessions  OBJECT-TYPE
       SYNTAX       Counter32
       MAX-ACCESS   read-only
       STATUS       current
       DESCRIPTION "The number maximum of times 255 standards-
                   based Transport Models, and for a Response message
                  was dropped because maximum of
                   256 Transport Models per enterprise.

                   It is believed that 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 assignment of open sessions.
                   "
       ::= { tmsmSession 1 }

   tmsmSessionMaxSupported  OBJECT-TYPE
       SYNTAX       Unsigned32
       MAX-ACCESS   read-only
       STATUS       current
       DESCRIPTION "The maximum 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 an openSession() request
                  failed to open a Session.
                   "
       ::= { tmsmSession 3 }

   tmsmSessionSecurityLevelNotAvailableErrors  OBJECT-TYPE
       SYNTAX       Counter32
       MAX-ACCESS   read-only
       STATUS       current
       DESCRIPTION "The be
                   insufficient over time, an enterprise number of times
                   can be allocated to obtain an outgoing message was
                  discarded because 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
                   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 requested securityLevel could not
                  provided. 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  SSH Transport Model
                  "
      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

8.

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.

9.

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.

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.

   [RFC2579]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Textual Conventions for SMIv2",
              STD 58, RFC 2579, April 1999.

   [RFC2580]  McCloghrie, K., Perkins, D., and J. Schoenwaelder,
              "Conformance Statements for SMIv2", STD 58, RFC 2580,
              April 1999.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3412]  Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
              "Message Processing and Dispatching for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3412,
              December 2002.

   [RFC3414]  Blumenthal, U. and B. Wijnen, "User-based Security Model
              (USM) for version 3 of the Simple Network Management
              Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.

   [RFC3416]  Presuhn, R., "Version 2 of the Protocol Operations for the
              Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3416, December 2002.

   [RFC3417]  Presuhn, R., "Transport Mappings for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3417,
              December 2002.

   [RFC3418]  Presuhn, R., "Management Information Base (MIB) for the
              Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3418, December 2002.

   [RFC3419]  Daniele, M. and J. Schoenwaelder, "Textual Conventions for
              Transport Addresses", RFC 3419, December 2002.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

11.2.

10.2.  Informative References

   [RFC3410]               Case, J., Mundy, R., Partain, D., and B.
                           Stewart, "Introduction and Applicability
                           Statements for Internet-Standard Management
                           Framework", RFC 3410, December 2002.

   [RFC3413]               Levi, D., Meyer, P., and B. Stewart, "Simple
                           Network Management Protocol (SNMP)
                           Applications", STD 62, RFC 3413,
                           December 2002.

   [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

   contextEngineID,In,In,In,In,In,In,Out,,,

   contextName,In,In,In,In,In,In,Out,,,

   expectResponse,In,,,,In,,,,,

   PDU,In,In,In,In,In,In,Out,,,

   pduVersion,In,In,In,In,In,In,Out,,,

   statusInfo,Out,In,,In,,In,Out,Out,Out,Out

   errorIndication,Out,Out,,,,,Out,,,

   sendPduHandle,Out,,,In,In,,Out,,,

   maxSizeResponsePDU,,In,In,,,In,Out,,Out,

   stateReference,,In,In,,,In,Out,,,

   wholeMessage,,,,,Out,Out,,Out,In,Out

   messageLength,,,,,Out,Out,,Out,In,Out

   maxMessageSize,,,,,,,,In,In,In

   globalData,,,,,,,,In,,In

   securityEngineID,,,,,,,,In,Out,In

   scopedPDU,,,,,,,,In,Out,In

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

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

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

   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 Transport Model and an SMSP. Security Model.

   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.

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 the architecture.

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

   Changes from revision -02- to -03-

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

   Changes from revision -01- to -02-

   o  wrote text for session establishment requirements section.
   o  wrote text for session maintenance requirements section.
   o  removed section on relation to SNMPv2-MIB
   o  updated MIB module to pass smilint
   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
   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

   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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