Network Working Group                                      D. Harrington
Internet-Draft                                 Huawei Technologies (USA)
Updates: 3411,3412,3414,3417                            J. Schoenwaelder
(if approved)                                   Jacobs University Bremen
Intended status: Standards Track                      September 15,                       November 18, 2007
Expires: March 18, May 21, 2008

 Transport Subsystem for the Simple Network Management Protocol (SNMP)
                        draft-ietf-isms-tmsm-10
                        draft-ietf-isms-tmsm-11

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document defines a Transport Subsystem, extending the Simple
   Network Management Protocol (SNMP) architecture defined in RFC 3411.
   This document defines a subsystem to contain Transport Models,
   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 describes some key aspects that need to be considered for any
   Transport Model for SNMP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  The Internet-Standard Management Framework . . . . . . . .  3
     1.2.  Where this Extension Fits  . . . . . . . . . . . . . . . .  3
     1.3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Requirements of a Transport Model  . . . . . . . . . . . . . .  7
     3.1.  Message Security Requirements  . . . . . . . . . . . . . .  7
       3.1.1.  Security Protocol Requirements . . . . . . . . . . . .  7
     3.2.  SNMP Requirements  . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Architectural Modularity Requirements  . . . . . . . .  8
       3.2.2.  Access Control Requirements  . . . . . . . . . . . . . 12 11
       3.2.3.  Security Parameter Passing Requirements  . . . . . . . 13 12
       3.2.4.  Separation of Authentication and Authorization . . . . 14 13
     3.3.  Session Requirements . . . . . . . . . . . . . . . . . . . 15 14
       3.3.1.  Session Establishment Requirements . . . . . . . . . . 15 14
       3.3.2.  Session Maintenance Requirements . . . . . . . . . . . 16 15
       3.3.3.  Message security versus session security . . . . . . . 17 16
   4.  Scenario Diagrams and the Transport Subsystem  . . . . . . . . 18 17
   5.  Cached Information and References  . . . . . . . . . . . . . . 18 17
     5.1.  securityStateReference . . . . . . . . . . . . . . . . . . 19 18
     5.2.  tmStateReference . . . . . . . . . . . . . . . . . . . . . 19 18
   6.  Abstract Service Interfaces  . . . . . . . . . . . . . . . . . 20 19
     6.1.  sendMessage ASI  . . . . . . . . . . . . . . . . . . . . . 20 19
     6.2.  Other Outgoing ASIs  . . . . . . . . . . . . . . . . . . . 21 20
     6.3.  The receiveMessage ASI . . . . . . . . . . . . . . . . . . 23 22
     6.4.  Other Incoming ASIs  . . . . . . . . . . . . . . . . . . . 23 22
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25 24
     7.1.  Coexistence, Security Parameters, and Access Control . . . 26 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27 26
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 27 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 28 27
   Appendix A.  Why tmStateReference? . . . . . . . . . . . . . . . . 29 28
     A.1.  Define an Abstract Service Interface . . . . . . . . . . . 29 28
     A.2.  Using an Encapsulating Header  . . . . . . . . . . . . . . 29
     A.3.  Modifying Existing Fields in an SNMP Message . . . . . . . 30 29
     A.4.  Using a Cache  . . . . . . . . . . . . . . . . . . . . . . 30 29
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 30
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 31 30

1.  Introduction

   This document defines a Transport Subsystem, extending the Simple
   Network Management Protocol (SNMP) architecture defined in [RFC3411].
   This document identifies and describes some key aspects that need to
   be considered for any Transport Model for SNMP.

1.1.  The Internet-Standard Management Framework

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

1.2.  Where this Extension Fits

   It is expected that readers of this document will have read RFC3410
   and RFC3411, and have a general understanding of the functionality
   defined in RFCs 3412-3418.

   The "Transport Subsystem" is an additional component for the SNMP
   Engine depicted in RFC3411, section 3.1.

   The following diagram depicts its place in the RFC3411 architecture.:

   +-------------------------------------------------------------------+
   |  SNMP entity                                                      |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  SNMP engine (identified by snmpEngineID)                   |  |
   |  |                                                             |  |
   |  |  +------------+                                             |  |
   |  |  | Transport  |                                             |  |
   |  |  | Subsystem  |                                             |  |
   |  |  +------------+                                             |  |
   |  |                                                             |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  |  | Dispatcher | | Message    | | Security  | | Access    |  |  |
   |  |  |            | | Processing | | Subsystem | | Control   |  |  |
   |  |  |            | | Subsystem  | |           | | Subsystem |  |  |
   |  |  +------------+ +------------+ +-----------+ +-----------+  |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   |  +-------------------------------------------------------------+  |
   |  |  Application(s)                                             |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Proxy        |        |  |
   |  |  | Generator   |  | Receiver     |  | Forwarder    |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |                                                             |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  |  | Command     |  | Notification |  | Other        |        |  |
   |  |  | Responder   |  | Originator   |  |              |        |  |
   |  |  +-------------+  +--------------+  +--------------+        |  |
   |  +-------------------------------------------------------------+  |
   |                                                                   |
   +-------------------------------------------------------------------+

   The transport mappings defined in RFC3417 do not provide lower-layer
   security functionality, and thus do not provide transport-specific
   security parameters.  This document updates RFC3411 and RFC3417 by
   defining an architectural extension and ASIs that transport mappings
   (models) can use to pass transport-specific security parameters to
   other subsystems, including transport-specific security parameters
   translated into transport-independent securityName and securityLevel
   parameters

   The Transport Security Model [I-D.ietf-isms-transport-security-model]
   and the Secure Shell Transport Model [I-D.ietf-isms-secshell] and utilize
   the Transport Subsystem.  The Transport Security Model [I-D.ietf-isms-transport-security-model]
   utilize is an
   alternative to the existing SNMPv1 Security Model [RFC3584], the
   SNMPv2c Security Model [RFC3584], and the User-based Secutiry Model
   [RFC3414].  The Secure Shell Transport Subsystem. Model is an alternative to
   existing transport mappings (or models) as described in [RFC3417].

1.3.  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].

   Non uppercased versions of the keywords should be read as in normal
   English.  They will usually, but not always, be used in a context
   relating to compatibility with the RFC3411 architecture or the
   subsystem defined here, but which might have no impact on on-the-wire
   compatibility.  These terms are used as guidance for designers of
   proposed IETF models to make the designs compatible with RFC3411
   subsystems and Abstract Service Interfaces (see section 3.2).
   Implementers are free to implement differently.  Some usages of these
   lowercase terms are simply normal English usage.

   For consistency with SNMP-related specifications, this document
   favors terminology as defined in STD62 rather than favoring
   terminology that is consistent with non-SNMP specifications that use
   different variations of the same terminology.  This is consistent
   with the IESG decision to not require the SNMPv3 terminology be
   modified to match the usage of other non-SNMP specifications when
   SNMPv3 was advanced to Full Standard.

2.  Motivation

   Just as there are multiple ways to secure one's home or business, in
   a continuum of alternatives, there are multiple ways to secure a
   network management protocol.  Let's consider three general
   approaches.

   In the first approach, an individual could sit on his front porch
   waiting for intruders.  In the second approach, he could hire an
   employee , schedule the employee, position the employee to guard what
   he wants protected, hire a second guard to cover if the first gets
   sick, and so on.  In the third approach, he could hire a security
   company, tell them what he wants protected, and they could hire
   employees, train them, position the guards, schedule the guards, send
   a replacement when a guard cannot make it, etc., thus providing the
   desired security, with no significant effort on his 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 (e.g., SHA), but provides all the coordination.
   USM provides for the authentication of a principal, message
   encryption, data integrity checking, timeliness checking, etc.

   USM was designed to be independent of other existing security
   infrastructures.  USM therefore 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 to use
   external mechanisms to handle the distribution of keys for use by
   USM.  The more important issue is that operators wanted to leverage a
   single user base that wasn't specific to SNMP.

   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 [RFC4346], 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 [RFC4741].

   This document defines a Transport Subsystem extension to the RFC3411
   architecture based on the third approach.  This extension specifies
   how other lower layer protocols with common security infrastructures
   can be used underneath the SNMP protocol and the desired goal of
   unified administrative security can be met.

   This extension allows security to be provided by an external protocol
   connected to the SNMP engine through an SNMP Transport Model
   [RFC3417].  Such a Transport Model would then enable the use of
   existing security mechanisms such as (TLS) [RFC4346] 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 the Transport Subsystem is to
   leverage these protocols where it seems useful.

   There are a number of challenges to be addressed to map the security
   provided by a secure transport into the SNMP architecture so that
   SNMP continues to provide interoperability with existing
   implementations.  These challenges are described in detail in this
   document.  For some key issues, design choices are described that
   might be made to provide a workable solution that meets operational
   requirements and fits into the SNMP architecture defined in
   [RFC3411].

3.  Requirements of a Transport Model

3.1.  Message Security Requirements

   Transport security protocols SHOULD provide protection against the
   following message-oriented threats [RFC3411]:

   1.  modification of information
   2.  masquerade
   3.  message stream modification
   4.  disclosure

   These threats are described in section 1.4 of [RFC3411].  It is not
   required to protect against denial of service or traffic analysis,
   but it should not make those threats significantly worse.

3.1.1.  Security Protocol Requirements

   There are a number of standard protocols that could be proposed as
   possible solutions within the Transport Subsystem.  Some factors
   SHOULD be considered when selecting a protocol.

   Using a protocol in a manner for which it was not designed has
   numerous problems.  The advertised security characteristics of a
   protocol might depend on it being used as designed; when used in
   other ways, it might not deliver the expected security
   characteristics.  It is recommended that any proposed model include a
   description of the applicability of the Transport Model.

   A Transport Model SHOULD require no modifications to the underlying
   protocol.  Modifying the protocol might 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.  Any Transport Model SHOULD include a
   description of potential impact on other usages of the protocol.

   Transport Models MUST be able to coexist with each other.

3.2.  SNMP Requirements

3.2.1.  Architectural Modularity Requirements

   SNMP version 3 (SNMPv3) is based on a modular architecture (defined
   in [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 for enabling
   different methods of providing security services, a Message
   Processing Subsystem permitting different message versions to be
   handled by a single engine, Applications(s) 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.  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.

   The design of this Transport Subsystem accepts the goals of the
   RFC3411 architecture defined in section 1.5 of [RFC3411].  This
   Transport Subsystem uses a modular design that will permit Transport
   Models to be advanced through the standards process independently of
   other Transport Models, and independent of other modular SNMP
   components as much as possible.

   Parameters have been added to the ASIs to pass model-independent
   transport address information.

   IETF standards typically require one mandatory to implement solution,
   with the capability of adding new mechanisms in the future.  Part of
   the motivation 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 Model SHOULD
   define one minimum-compliance security mechanism, such as
   certificates, to ensure a basic level of interoperability, but should
   also be able to support additional existing and new mechanisms.

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

   The RFC3411 architecture and the Security Subsystem assume that a
   Security Model is called by a Message Processing Model and will
   perform multiple security functions within the Security Subsystem.  A
   Transport Model that supports a secure transport protocol might
   perform similar security functions within the Transport Subsystem.  A
   Transport Model might perform the translation of transport security
   parameters to/from security-model-independent parameters.

   To accommodate this, an implementation-specific cache of transport-
   specific information will be described (not shown), and the data
   flows between the Transport Subsystem and the Transport Dispatch,
   between the Message Dispatch and the Message Processing Subsystem,
   and between the Message Processing Subsystem and the Security
   Subsystem will be extended to pass security-model-independent values.
   New Security Models may also be defined that understand how to work
   with the modified ASIs and the cache.  One such Security Model, the
   Transport Security Model, is defined in
   [I-D.ietf-isms-transport-security-model]

   The following diagram depicts the SNMPv3 architecture including the
   new Transport Subsystem defined in this document, and a new Transport
   Security Model defined in [I-D.ietf-isms-transport-security-model].

   +------------------------------+
   |    Network                   |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v
   +-------------------------------------------------------------------+
   | +--------------------------------------------------+              |
   | |  Transport Subsystem                             |              |
   | | +-----+ +-----+ +-----+ +-----+       +-------+  |              |
   | | | UDP | | TCP | | SSH | | TLS | . . . | other |  |              |
   | | +-----+ +-----+ +-----+ +-----+       +-------+  |              |
   | +--------------------------------------------------+              |
   |              ^                                                    |
   |              |                                                    |
   | Dispatcher   v                                                    |
   | +-------------------+ +---------------------+  +----------------+ |
   | | Transport         | | Message Processing  |  | Security       | |
   | | Dispatch          | | Subsystem           |  | Subsystem      | |
   | |                   | |     +------------+  |  | +------------+ | |
   | |                   | |  +->| v1MP       |<--->| | USM        | | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | |                   | |  +->| v2cMP      |<--->| | Transport  | | |
   | | Message           | |  |  +------------+  |  | | Security   | | |
   | | Dispatch    <--------->|  +------------+  |  | | Model      | | |
   | |                   | |  +->| v3MP       |<--->| +------------+ | |
   | |                   | |  |  +------------+  |  | +------------+ | |
   | | PDU Dispatch      | |  |  +------------+  |  | | Other      | | |
   | +-------------------+ |  +->| otherMP    |<--->| | Model(s)   | | |
   |              ^        |     +------------+  |  | +------------+ | |
   |              |        +---------------------+  +----------------+ |
   |              v                                                    |
   |      +-------+-------------------------+---------------+          |
   |      ^                                 ^               ^          |
   |      |                                 |               |          |
   |      v                                 v               v          |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   | |   COMMAND   |   | ACCESS  |   | NOTIFICATION |  |    PROXY    | |
   | |  RESPONDER  |<->| CONTROL |<->|  ORIGINATOR  |  |  FORWARDER  | |
   | | application |   |         |   | applications |  | application | |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   |      ^                                 ^                          |
   |      |                                 |                          |
   |      v                                 v                          |
   | +----------------------------------------------+                  |
   | |             MIB instrumentation              |      SNMP entity |
   +-------------------------------------------------------------------+

3.2.1.1.  Processing Differences between USM and Secure Transport

   USM and secure transports differ in the processing order and
   responsibilities within the RFC3411 architecture.  While the steps
   are the same, they occur in a different order, and may be done by
   different subsystems.  The following lists illustrate the difference
   in the flow and the responsibility for different processing steps for
   incoming messages when using USM and when using a secure transport.
   (Note that these lists are simplified for illustrative purposes, and
   do not represent all details of processing.  Transport Models must
   provide the detailed elements of procedure.)  With USM and some other Security Models,
   security processing starts when the Message Processing Model decodes
   portions of the ASN.1 encoded message to extract an opaque block of security parameters and
   header parameters that identify which Security Model should process
   the message to perform authentication, decryption, timeliness
   checking, integrity checking, and translation of parameters to model-independent model-
   independent parameters.  A secure transport performs those security
   functions on the message, before the ASN.1 message is decoded.

   Step 6 cannot occur until after decryption occurs.  Step 6 and beyond
   are the same for USM and a secure transport.

3.2.1.1.1.  USM and the RFC3411 Architecture

   1) decode the ASN.1 header (Message Processing Model)
   2) determine the SNMP Security Model and parameters (Message
      Processing Model)
   3) verify securityLevel.  [Security Model]
   4) translate parameters to model-independent parameters (Security
      Model)
   5) authenticate the principal, check message integrity and
      timeliness, and decrypt the message.  [Security Model]
   6) determine the pduType in the decrypted portions (Message
      Processing Model), and
   7) pass on the decrypted portions with model-independent parameters.

3.2.1.2.  Transport Subsystem and the RFC3411 Architecture

   1) authenticate the principal, check integrity and timeliness of the
      message, and decrypt the message.  [Transport Model]
   2) translate parameters to model-independent parameters (Transport
      Model)
   3) decode the ASN.1 header (Message Processing Model)
   4) determine the SNMP Security Model and parameters (Message
      Processing Model)
   5) verify securityLevel [Security Model]
   6) determine the pduType in the decrypted portions (Message
      Processing Model), and
   7) pass on the decrypted portions with model-independent security
      parameters

   If a message is secured using a secure transport layer, then the
   Transport Model should provide the translation from the authenticated
   identity (e.g., an SSH user name) to a model-independent securityName
   in step 2.

3.2.1.3.  Passing Information between Engines

   A secure Transport Model will establish an authenticated and/or
   encrypted tunnel between the Transport Models of two SNMP engines.
   After a transport layer tunnel is established, then SNMP messages can
   be sent through the tunnel from one SNMP engine to the other SNMP
   engine.  Transport Models MAY support sending multiple SNMP messages
   through the same tunnel.

3.2.2.  Access Control Requirements

   RFC3411 made some design decisions related to the support of an
   Access Control Subsystem.  These include establishing and passing in
   a model-independent manner the securityModel, securityName and
   securityLevel parameters, and separating message authentication from
   data access authorization.

3.2.2.1.  securityName and securityLevel Mapping

   SNMP data access controls are expected to work on the basis of who
   can perform what operations on which subsets of data, and based on
   the security services that will be provided to secure the data in
   transit.  The securityModel and securityLevel parameters establish
   the protections for transit - whether authentication and privacy
   services will be or have been applied to the message.  The
   securityName is a model-independent identifier of the security
   "principal",

   The Message Processing 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 security-model-specific
   principal for an incoming message to a security-model independent
   securityName which can be used for subsequent processing, such as for
   access control.  The securityName is mapped from a mechanism-specific
   identity, and this mapping must be done for incoming messages by the
   Security Model before it passes securityName to the Message
   Processing Model via the processIncoming ASI.

   A Security Model is also responsible to specify, via the
   securityLevel parameter, whether incoming messages have been
   authenticated and/or encrypted, and to ensure that outgoing messages
   are authenticated and/or encrypted based on the value of
   securityLevel.

   A translation from a mechanism-specific identity to a securityName
   might be done by a Transport Model, and the proposed securityName and
   a proposed securityLevel might then be made available to a Security
   Model via the tmStateReference.  A Security Model may have multiple
   sources for determining the principal and desired security services,
   and a particular Security Model may or may not utilize the
   securityName mapping and securityLevel made available by the
   Transport Model when deciding the value of the securityName and
   securityLevel to be passed to the Message Processing Model.

3.2.3.  Security Parameter Passing Requirements

   RFC3411 section 4 describes abstract data flows between the
   subsystems, models and applications within the architecture.
   Abstract Service Interfaces describe the flow of data, passing model-
   independent information between subsystems within an engine.  The
   RFC3411 architecture has no ASI parameters for passing security
   information between the Transport Subsystem and the dispatcher, or
   between the dispatcher and the Message Processing Model.  This
   document defines or modifies ASIs for this purpose.

   A Message Processing Model might 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 security-model-specific
   parameters into model-independent parameters.  When using a secure
   Transport Model, some security parameters might be provided through
   means other than carrying them in the SNMP message; some of the
   parameters for incoming messages might be extracted from the
   transport layer by the Transport Model before the message is passed
   to the Message Processing Subsystem.

   This document describes a cache mechanism (see Section 5), into which
   the Transport Model puts information about the transport and security
   parameters applied to a transport connection or an incoming message,
   and a Security Model may extract that information from the cache.  A
   tmStateReference is passed as an extra parameter in the ASIs of the
   Transport Subsystem and the Message Processing and Security
   Subsystems, to identify the relevant cache.  This approach of passing
   a model-independent reference is consistent with the
   securityStateReference cache already being passed around in the
   RFC3411 ASIs.

   For outgoing messages, even when a secure Transport Model will
   provide the security services, a Message Processing Model might have
   a Security Model actually create the message from its component
   parts.  Whether there are any security services provided by the
   Security Model for an outgoing message is security-model-dependent.
   For incoming messages, even when a secure Transport Model provides
   security services, a Security Model might provide some security
   functionality that can only be provided after the message version or
   other parameters are extracted from the message.

3.2.4.  Separation of Authentication and Authorization

   The RFC3411 architecture defines a separation of authentication and
   the authorization to access and/or modify MIB data.  A set of model-
   independent parameters (securityModel, securityName, and
   securityLevel) are passed between the Security Subsystem, the
   applications, and the Access Control Subsystem.

   This separation was a deliberate decision of the SNMPv3 WG, to allow
   support for authentication protocols which did not provide data
   access authorization capabilities, and to support data access
   authorization schemes, such as VACM, that do not perform their own
   authentication.  This decision also permits different types of data
   access policies, such as one built on UNIX groups or Windows domains.
   The VACM approach is based on administrator-defined groups of users.

   A Message Processing Model determines which Security Model is used,
   either based on the message version, e.g., SNMPv1 and SNMPv2c, and
   possibly by a value specified in the message, e.g., SNMPv3.

   The Security Model makes the decision which securityName and
   securityLevel values are passed as model-independent parameters to an
   application, which then passes them via the isAccessAllowed ASI to
   the Access Control Subsystem.

   An Access Control Model performs the mapping from the model-
   independent security parameters to a policy within the Access Control
   Model that is access-control-model-dependent.

   A Transport Model does not know which securityModel will be used for
   an incoming message, so a Transport Model cannot know how the
   securityName and securityLevel parameters are determined.  A
   Transport Model can provide a mapping from a transport-specific
   identity and provide candidate values for the securityName and
   securityLevel, but there is no guarantee the transport-provided
   values will be used by the Security Model.

   For example, the SNMPv1 Message Processing Model described in RFC3584
   always selects the SNMPv1 Security Model.  This is true even if the
   SNMPv1 message was protected in transit using a secure Transport
   Model, such as one based on SSH or TLS.  The SNMPv1 Security Model
   does not know the tmStateReference exists.

3.3.  Session Requirements

   Some secure transports might have a notion of sessions, while other
   secure transports might provide channels or other session-like
   mechanism.  Throughout this document, the term session is used in a
   broad sense to cover sessions, channels, and session-like mechanisms.
   Session refers to an association between two SNMP engines that
   permits the 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 Transport
   Model.

   Sessions are not part of the SNMP architecture defined in [RFC3411],
   but are considered desirable because the cost of authentication can
   be amortized over potentially many transactions.

   The architecture defined in [RFC3411] does not include a session
   selector in the Abstract Service Interfaces, and neither is that done
   for the Transport Subsystem, so an SNMP application has no mechanism
   to select a session using the ASIs except by passing a unique
   combination of transportDomain, transportAddress, securityName, and
   securityLevel.  Implementers, of course, might provide non-standard
   mechanisms to select sessions.  The transportDomain and
   transportAddress identify the transport connection to a remote
   network node; the securityName identifies which security principal to
   communicate with at that address (e.g., different NMS applications),
   and the securityLevel might permit selection of different sets of
   security properties for different purposes (e.g., encrypted SETs vs.
   non-encrypted GETs).

   To reduce redundancy, this document describes aspects that are
   expected to be common to all Transport Model sessions.

3.3.1.  Session Establishment Requirements

   SNMP has no mechanism to specify a transport session using the ASIs
   except by passing a unique combination transportDomain,
   transportAddress, securityName, and securityLevel to be used to
   identify a session in a transport-independent manner.  SNMP
   applications provide the transportDomain, transportAddress,
   securityName, and securityLevel to be used to create a session.

   For an outgoing message, securityLevel is the requested security for
   the message, passed in the ASIs.  If the Transport Model cannot
   provide at least the requested level of security, the Transport Model
   SHOULD discard the message and notify the dispatcher that
   establishing a session and sending the message failed.

   A Transport Model determines whether an appropriate session exists
   (transportDomain, transportAddress, securityName, and securityLevel)
   for an outgoing message.  If an appropriate session does not yet
   exist, the Transport Model attempts to establish a session for
   delivery .  If a session cannot be established then the message is
   discarded and the dispatcher should be notified that sending the
   message failed.

   Transport session establishment might require provisioning
   authentication credentials at an engine, either statically or
   dynamically.  How this is done is dependent on the transport model
   and the implementation.

   The Transport Subsystem has no knowledge of pduType, so cannot
   distinguish between a session created to carry different pduTypes.
   To differentiate a session established for different purposes, such
   as a notification session versus a request-response session, an
   application can use different securityNames or transport addresses.
   For example, in SNMPv1, UDP ports 161 and 162 were used to
   differentiate types of traffic.  New transport models may define a
   single well-known port for all traffic types.  Administrators might
   choose to define one port for SNMP request-response traffic, but
   configure notifications to be sent to a different port.

3.3.2.  Session Maintenance Requirements

   A Transport Model can tear down sessions as needed.  It might be
   necessary for some implementations to tear down sessions as the
   result of resource constraints, for example.

   The decision to tear down a session is implementation-dependent.
   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.

   The elements of procedure describe when cached information can be
   discarded, in some circumstances, and the timing of cache cleanup
   might have security implications, but cache memory management is an
   implementation issue.

   If a Transport Model defines MIB module objects to maintain session
   state information, then the Transport Model MUST define what SHOULD
   happen to the objects when a related session is torn down, since this
   will impact interoperability of the MIB module.

3.3.3.  Message security versus session security

   A Transport Model session is associated with state information that
   is maintained for its lifetime.  This state information allows for
   the application of various security services to multiple messages.
   Cryptographic keys associated with the transport session SHOULD be
   used to provide authentication, integrity checking, and encryption
   services, as needed, for data that is communicated during the
   session.  The cryptographic protocols used to establish keys for a
   Transport Model session SHOULD ensure that fresh new session keys are
   generated for each session.  In addition sequence information might
   be maintained in the session which can be used to prevent the replay
   and reordering of messages within a session.  If each session uses
   new keys, then a cross-session replay attack will be unsuccessful;
   that is, an attacker cannot successfully replay on one session a
   message he observed from another session.  A good security protocol
   will also protect against replay attacks _within_ a session; that is,
   an attacker cannot successfully replay a message observed earlier in
   the same session.

   A Transport Model session will have a single transportDomain,
   transportAddress, securityName and securityLevel associated with it.
   If an exchange between communicating engines requires a different
   securityLevel or is on behalf of a different securityName, then
   another session would be needed.  An immediate consequence of this is
   that implementations SHOULD be able to maintain some reasonable
   number of concurrent sessions.

   For Transport Models, securityName should be 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,
   for example, there is not much value in using encryption for a
   Commander Generator to poll for potentially non-sensitive performance
   data on thousands of interfaces every ten minutes; the encryption
   might add significant overhead to processing of the messages.

   Some Transport Models might support only specific authentication and
   encryption services, such as requiring all messages to be carried
   using both authentication and encryption, regardless of the security
   level requested by an SNMP application.  A Transport Model may
   upgrade the requested security level, i.e. noAuthNoPriv and
   authNoPriv MAY be sent over an authenticated and encrypted session.

4.  Scenario Diagrams and the Transport Subsystem

   RFC3411 section 4.6.1 and 4.6.2 provide scenario diagrams to
   illustrate how an outgoing message is created, and how an incoming
   message is processed.  RFC3411 does not define ASIs for "Send SNMP
   Request Message to Network" or "Receive SNMP Response Message from
   Network", and does not define ASIs for "Receive SNMP Message from
   Network" or "Send SNMP message to Network".

   This document defines a sendMessage ASI to send SNMP messages to the
   network, regardless of pduType, and a receiveMessage ASI to receive
   SNMP messages from the network, regardless of pduType.

5.  Cached Information and References

   The 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 flows between subsystems.

   There are two levels of state that might need to be maintained: the
   security state in a request-response pair, and potentially long-term
   state relating to transport and security.

   This state is maintained in caches.  To simplify the elements of
   procedure, the release of state information is not always explicitly
   specified.  As a general rule, if state information is available when
   a message being processed gets discarded, the state related to that
   message should also be discarded, and if state information is
   available when a relationship between engines is severed, such as the
   closing of a transport session, the state information for that
   relationship might also be discarded.

   This document differentiates the tmStateReference from the
   securityStateReference.  This document does not specify an
   implementation strategy, only an abstract description of the data
   that flows between subsystems.  An implementation might 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 a security or Transport Model has had an opportunity
   to extract the information it needs.

5.1.  securityStateReference

   The securityStateReference parameter is defined in RFC3411.
   securityStateReference is not accessible to models of the Transport
   Subsystem.

5.2.  tmStateReference

   For each transport session, information about the message security is
   stored in a cache to pass model- and mechanism-specific parameters.
   The state referenced by tmStateReference may be saved across multiple
   messages, in a Local Configuration Datastore (LCD), as compared to
   securityStateReference which is usually only saved for the life of a
   request-response pair of messages.

   For security reasons, if a secure transport session is closed between
   the time a request message is received and the corresponding response
   message is sent, then the response message SHOULD be discarded, even
   if a new session has been established.  The SNMPv3 WG decided that
   this should be a SHOULD architecturally, and it is a security-model-
   specific decision whether to REQUIRE this.

   Since a transport model does not know whether a message contains a
   response, and transport session information is transport-model-
   specific, the tmStateReference contains two pieces of information for
   performing the request-response transport session pairing.

   Each transport model that supports sessions and supports the
   tmStateReference cache SHOULD include a transport-specific session
   identifier in the cache for an incoming message, so that if a
   security model requests the same session, the transport model can
   determine whether the current existing session is the same as the
   session used for the incoming request.

   Each Security Model that supports the tmStateReference cache SHOULD
   pass a tmSameSession parameter in the tmStateReference cache for
   outgoing messages to indicate whether the same session MUST be used
   for the outgoing message as was used for the corresponding incoming
   message.

   If the same session requirement is indicated by the security model,
   but the session identified in the tmStateReference does not match the
   current established transport session, i.e., it is not the same
   session, then the message MUST be discarded, and the dispatcher
   should be notified the sending of the message failed.

   Since the contents of a cache are meaningful only within an
   implementation, and not on-the-wire, the format of the cache and the
   LCD are implementation-specific.

6.  Abstract Service Interfaces

   Abstract service interfaces have been defined by RFC 3411 to describe
   the conceptual data flows between the various subsystems within an
   SNMP entity, and to help keep the subsystems independent of each
   other except for the common parameters.

   This document follows the example of RFC3411 regarding the release of
   state information, and regarding error indications.

   1) The release of state information is not always explicitly
   specified in a transport model.  As a general rule, if state
   information is available when a message gets discarded, the message-
   state information should also be released, and if state information
   is available when a session is closed, the session state information
   should also be released.  Note that keeping sensitive security
   information longer than necessary might introduce potential
   vulnerabilities to an implementation.

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

6.1.  sendMessage ASI

   The sendMessage ASI is used to pass a message from the Dispatcher to
   the appropriate Transport Model for sending.

   In the diagram in section 4.6.1 of RFC 3411, the sendMessage ASI
   replaces the text "Send SNMP Request Message to Network".  In section
   4.6.2, the sendMessage ASI replaces the text "Send SNMP Message to
   Network"

   If present and valid, the tmStateReference refers to a cache
   containing transport-model-specific parameters for the transport and
   transport security.  How the information in the cache is used is
   transport-model-dependent and implementation-dependent.  How a
   tmStateReference is determined to be present and valid is
   implementation-dependent.

   This may sound underspecified, but a transport model might be
   something like SNMP over UDP over IPv6, where no security is
   provided, so it might have no mechanisms for utilizing a securityName
   and securityLevel.

   statusInformation =
   sendMessage(
   IN   destTransportDomain           -- transport domain to be used
   IN   destTransportAddress          -- transport address to be used
   IN   outgoingMessage               -- the message to send
   IN   outgoingMessageLength         -- its length
   IN   tmStateReference              -- reference to transport state
    )

6.2.  Other Outgoing ASIs

   A tmStateReference parameter has been added to the
   prepareOutgoingMessage, prepareResponseMessage, generateRequestMsg,
   and generateResponseMsg ASIs as an OUT parameter.  The
   transportDomain and transportAddress parameters have been added to
   the generateRequestMsg, and generateResponseMsg ASIs as IN parameters
   (not shown).

   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  tmStateReference        -- (NEW) reference to transport state
               )
   statusInformation =          -- success or errorIndication
   prepareResponseMessage(
   IN  messageProcessingModel   -- typically, SNMP version
   IN  securityModel            -- Security Model to use
   IN  securityName             -- on behalf of this principal
   IN  securityLevel            -- Level of Security requested
   IN  contextEngineID          -- data from/at this entity
   IN  contextName              -- data from/in this context
   IN  pduVersion               -- the version of the PDU
   IN  PDU                      -- SNMP Protocol Data Unit
   IN  maxSizeResponseScopedPDU -- maximum size able to accept
   IN  stateReference           -- reference to state information
                                -- as presented with the request
   IN  statusInformation        -- success or errorIndication
                                -- error counter OID/value if error
   OUT destTransportDomain      -- destination transport domain
   OUT destTransportAddress     -- destination transport address
   OUT outgoingMessage          -- the message to send
   OUT outgoingMessageLength    -- its length
   OUT tmStateReference         -- (NEW) reference to transport state
               )

   The tmStateReference parameter of generateRequestMsg or
   generateResponseMsg is passed in the OUT parameters of the Security
   Subsystem to the Message Processing Subsystem.  If a cache exists for
   a session identifiable from transportDomain, transportAddress,
   securityModel, securityName, and securityLevel, then an appropriate
   Security Model might create a tmStateReference to the cache and pass
   that as an OUT parameter.

   If one does not exist, the Security Model might create a cache
   referenced by tmStateReference.  This information might include
   transportDomain, transportAddress, the securityLevel, and the
   securityName, plus any model or mechanism-specific details.  The
   contents of the cache may be incomplete until the Transport Model has
   established a session.  What information is passed, and how this
   information is determined, is implementation and security-model-
   specific.

   The prepareOutgoingMessage ASI passes tmStateReference from the
   Message Processing Subsystem to the dispatcher.  How or if the
   Message Processing Subsystem modifies or utilizes the contents of the
   cache is message-processing-model-specific.

   This may sound underspecified, but a message processing model might
   have access to all the information from the cache and from the
   message, and an application might specify a Security Model such as
   USM to authenticate and secure the SNMP message, but also specify a
   secure transport such as that provided by the SSH Transport Model to
   send the message to its destination.

6.3.  The receiveMessage ASI

   If one does not exist, the Transport Model might create a cache
   referenced by tmStateReference.  If present, this information might
   include transportDomain, transportAddress, securityLevel, and
   securityName, plus model or mechanism-specific details.  How this
   information is determined is implementation and transport-model-
   specific.

   In the diagram in section 4.6.1 of RFC 3411, the receiveMessage ASI
   replaces the text "Receive SNMP Response Message from Network".  In
   section 4.6.2, the receiveMessage ASI replaces the text "Receive SNMP
   Message from Network"

   This may sound underspecified, but a transport model might be
   something like SNMP over UDP over IPv6, where no security is
   provided, so it might have no mechanisms for determining a
   securityName and securityLevel.

   The Transport Model does not know the securityModel for an incoming
   message; this will be determined by the Message Processing Model in a
   message-processing-model-dependent manner.

   The receiveMessage ASI is used to pass a message from the Transport
   Subsystem to the Dispatcher.

   statusInformation =
   receiveMessage(
   IN   transportDomain               -- origin transport domain
   IN   transportAddress              -- origin transport address
   IN   incomingMessage               -- the message received
   IN   incomingMessageLength         -- its length
   IN   tmStateReference              -- reference to transport state
    )

6.4.  Other Incoming ASIs

   To support the Transport Subsystem, the tmStateReference is added to
   the prepareDataElements ASI (from the Dispatcher to the Message
   Processing Subsystem), and to the processIncomingMsg ASI (from the
   Message Processing Subsystem to the Security Model Subsystem).  How
   or if a Message Processing Model or Security Model uses
   tmStateReference is message-processing-model-dependent and security-
   model-dependent.

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

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

   The tmStateReference parameter of prepareDataElements is passed from
   the dispatcher to the Message Processing Subsystem.  How or if the
   Message Processing Subsystem modifies or utilizes the contents of the
   cache is message-processing-model-specific.

   The processIncomingMessage ASI passes tmStateReference from the
   Message Processing Subsystem to the Security Subsystem.

   If tmStateReference is present and valid, an appropriate Security
   Model might utilize the information in the cache.  How or if the
   Security Subsystem utilizes the information in the cache is security-
   model-specific.

   This may sound underspecified, but a message processing model might
   have access to all the information from the cache and from the
   message.  The Message Processing Model might determine that the USM
   Security Model is specified in an SNMPv3 message header; the USM
   Security Model has no need of values in the tmStateReference cache to
   authenticate and secure the SNMP message, but an application might
   have specified to use a secure transport such as that provided by the
   SSH Transport Model to send the message to its destination.

7.  Security Considerations

   This document defines an architectural approach that permits SNMP to
   utilize transport layer security services.  Each proposed Transport
   Model should discuss the security considerations of the Transport
   Model.

   It is considered desirable by some industry segments that SNMP
   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.  Each
   proposed Transport Model should include a discussion in its security
   considerations of whether perfect forward security is appropriate for
   the Transport Model.

   Since the cache and LCD will contain security-related parameters,
   implementers should store this information (in memory or in
   persistent storage) in a manner to protect it from unauthorized
   disclosure and/or modification.

   Care must be taken to ensure that a SNMP engine is sending packets
   out over a transport using credentials that are legal for that engine
   to use on behalf of that user.  Otherwise an engine that has multiple
   transports open might be "tricked" into sending a message through the
   wrong transport.

   A Security Model may have multiple sources from which to define the
   securityName and securityLevel.  The use of a secure Transport Model
   does not imply that the securityName and securityLevel chosen by the
   Security Model represent the transport-authenticated identity or the
   transport-provided security services.  The securityModel,
   securityName, and securityLevel parameters are a related set, and an
   administrator should understand how the specified securityModel
   selects the corresponding securityName and securityLevel.

7.1.  Coexistence, Security Parameters, and Access Control

   In the RFC3411 architecture, the Message Processing Model makes the
   decision about which Security Model to use.  The architectural change
   described by this document does not alter that.

   The architecture change described by this document does however,
   allow SNMP to support two different approaches to security - message-
   driven security and transport-driven security.  With message-driven
   security, SNMP provides its own security, and passes security
   parameters within the SNMP message; with transport-driven security,
   SNMP depends on an external entity to provide security during
   transport by "wrapping" the SNMP message.

   Security models defined before the Transport Security Model (i.e.,
   SNMPv1, SNMPv2c, and USM) do not support transport-based security,
   and only have access to the security parameters contained within the
   SNMP message.  They do not know about the security parameters
   associated with a secure transport.  As a result, the Access Control
   Subsystem bases its decisions on the security parameters extracted
   from the SNMP message, not on transport-based security parameters.

   Implications of coexistence of older security models with secure
   transport models are known: known.  The securityName used for access control
   decisions represents an SNMP-authenticated identity, not the
   transport-authenticated identity.  (I can transport-authenticate as
   guest and then simply use a community name for root, or a USM non-
   authenticated identity.)
   o  An SNMPv1 message will always be paired with an SNMPv1 Security
      Model (per RFC3584), regardless of the transport mapping or
      transport model used, and access controls will be based on the
      community name.
   o  An SNMPv2c message will always be paired with an SNMPv2c Security
      Model (per RFC3584), regardless of the transport mapping or
      transport model used, and access controls will be based on the
      community name.. name.
   o  An SNMPv3 message will always be paired with the securityModel
      specified in the msgSecurityParameters field of the message (per
      RFC3412), regardless of the transport mappng or transport model
      used.  If the SNMPv3 message specifies the User-based Security
      Model (USM), access controls will be based on the USM user.If the
      SNMPv3 message specifies the Transport Security Model (TSM),
      access controls will be based on the principal authenticated by
      the transport.

8.  IANA Considerations

   This document requires no action by IANA.

9.  Acknowledgments

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

   The authors of submitted Security Model proposals: Chris Elliot, Wes
   Hardaker, David Harrington, Keith McCloghrie, Kaushik Narayan, David
   Perkins, Joseph Salowey, and Juergen Schoenwaelder.

   The members of the Protocol Evaluation Team: Uri Blumenthal,
   Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla.

   WG members who performed detailed reviews: Jeffrey Hutzelman, Bert
   Wijnen, Tom Petch.

10.  References

10.1.  Normative References

   [RFC2119]                                 Bradner, S., "Key words for
                                             use in RFCs to Indicate
                                             Requirement Levels",
                                             BCP 14, RFC 2119,
                                             March 1997.

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

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

   [RFC3414]                                 Blumenthal, U. and B.

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

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

10.2.  Informative References

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

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

   [RFC3584]                                 Frye, R., Levi, D.,
                                             Routhier, S., and B.
                                             Wijnen, "Coexistence
                                             between Version 1, Version
                                             2, and Version 3 of the
                                             Internet-standard Network
                                             Management Framework",
                                             BCP 74, RFC 3584,
                                             August 2003.

   [RFC4346]                                 Dierks, T. and E. Rescorla,
                                             "The Transport Layer
                                             Security (TLS) Protocol
                                             Version 1.1", RFC 4346,
                                             April 2006.

   [RFC4422]                                 Melnikov, A. and K.
                                             Zeilenga, "Simple
                                             Authentication and Security
                                             Layer (SASL)", RFC 4422,
                                             June 2006.

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

   [RFC4741]                                 Enns, R., "NETCONF
                                             Configuration Protocol",
                                             RFC 4741, December 2006.

   [I-D.ietf-isms-transport-security-model]  Harrington, D., "Transport
                                             Security Model for SNMP", d
                                             raft-ietf-isms-transport-
                                             security-model-05
                                             security-model-06 (work in
                                             progress), July September 2007.

   [I-D.ietf-isms-secshell]                  Salowey, J. and D.                  Harrington, D. and J.
                                             Salowey, "Secure Shell
                                             Transport Model for SNMP",
                                             draft-ietf-isms-secshell-08
                                             draft-ietf-isms-secshell-09
                                             (work in progress),
                                             July
                                             November 2007.

Appendix A.  Why tmStateReference?

   This appendix considers why a cache-based approach was selected for
   passing parameters.

   There are four approaches that could be used for passing information
   between the Transport Model and a Security Model.

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

A.1.  Define an Abstract Service Interface

   Abstract Service Interfaces (ASIs) are defined by a set of primitives
   that specify the services provided and the abstract data elements
   that are to be passed when the services are invoked.  Defining
   additional ASIs to pass the security and transport information from
   the Transport Subsystem to Security Subsystem has the advantage of
   being consistent with existing RFC3411/3412 practice, and helps to
   ensure that any Transport Model proposals pass the necessary data,
   and do not cause side effects by creating model-specific dependencies
   between itself and other models or other subsystems other than those
   that are clearly defined by an ASI.

A.2.  Using an Encapsulating Header

   A header could encapsulate the SNMP message to pass necessary
   information from the Transport Model to the dispatcher and then to a
   Message Processing Model.  The message header would be included in
   the wholeMessage ASI parameter, and would be removed by a
   corresponding Message Processing Model.  This would imply the (one
   and only) 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 Security Model.

A.3.  Modifying Existing Fields in an SNMP Message

   [RFC3412] defines the SNMPv3 message, which contains fields to pass
   security related parameters.  The Transport Subsystem could use these
   fields in an SNMPv3 message, or comparable fields in other message
   formats to pass information between Transport Models in different
   SNMP engines, and to pass information between a Transport Model and a
   corresponding Message Processing Model.

   If the fields in an incoming SNMPv3 message are changed by the
   Transport Model before passing it to the Security Model, then the
   Transport Model will need to decode the ASN.1 message, modify the
   fields, and re-encode the message in ASN.1 before passing the message
   on to the message dispatcher or to the transport layer.  This would
   require an intimate knowledge of the message format and message
   versions so the Transport Model knew which fields could be modified.
   This would seriously violate the modularity of the architecture.

A.4.  Using a Cache

   This document describes a cache, into which the Transport Model puts
   information about the security applied to an incoming message, and a
   Security Model can extract that information from the cache.  Given
   that there might be multiple TM-security caches, a tmStateReference
   is passed as an extra parameter in the ASIs between the Transport
   Subsystem and the Security Subsystem, so the Security Model knows
   which cache of information to consult.

   This approach does create dependencies between a specific Transport
   Model and a corresponding specific Security Model.  However, the
   approach of passing a model-independent reference to a model-
   dependent cache is consistent with the securityStateReference already
   being passed around in the RFC3411 ASIs.

Appendix B.  Open Issues

   NOTE to RFC editor: If this section is empty, then please remove this
   open issues section before publishing this document as an RFC.  (If
   it is not empty, please send it back to the editor to resolve.
   o

Appendix C.  Change Log

   NOTE to RFC editor: Please remove this change log before publishing
   this document as an RFC.

   Changes from -09- to -10-

   o  Pointed to companion documents
   o  Wordsmithed extensively
   o  Modified the note about SNMPv3-consistent terminology
   o  Modified the note about RFC2119 terminology.
   o  Modified discussion of cryptographic key generation.
   o  Added security considerations about coexistence with older
      security models
   o  Expanded discussion of same session functionality
   o  Described how sendMessage and receiveMessage fit into RFC3411
      diagrams
   o  Modified prepareResponseMessage ASI
   o

   Changes from -08- to -09-

   o  A question was raised that notifications would not work properly,
      but we could never find the circumstances where this was true.
   o  removed appendix with parameter matrix
   o  Added a note about terminology, for consistency with SNMPv3 rather
      than with RFC2828.

   Changes from -07- to -08-

   o  Identified new parameters in ASIs.
   o  Added discussion about well-known ports.

   Changes from -06- to -07-

   o  Removed discussion of double authentication
   o  Removed all direct and indirect references to pduType by Transport
      Subsystem
   o  Added warning regarding keeping sensitive security information
      available longer than needed.
   o  Removed knowledge of securityStateReference from Transport
      Subsystem.
   o  Changed transport session identifier to not include securityModel,
      since this is not known for incoming messages until the message
      processing model.

   Changes from revision -05- to -06-

      mostly editorial changes
      removed some paragraphs considered unnecessary
      added Updates to header
      modified some text to get the security details right
      modified text re: ASIs so they are not API-like
      cleaned up some diagrams
      cleaned up RFC2119 language
      added section numbers to citations to RFC3411
      removed gun for political correctness

   Changes from revision -04- to -05-

      removed all objects from the MIB module.
      changed document status to "Standard" rather than the xml2rfc
      default of informational.

      changed mention of MD5 to SHA
      moved addressing style to TDomain and TAddress
      modified the diagrams as requested
      removed the "layered stack" diagrams that compared USM and a
      Transport Model processing
      removed discussion of speculative features that might exist in
      future Transport Models
      removed openSession and closeSession ASIs, since those are model-
      dependent
      removed the MIB module
      removed the MIB boilerplate intro (this memo defines a SMIv2 MIB
      ...)
      removed IANA considerations related to the now-gone MIB module
      removed security considerations related to the MIB module
      removed references needed for the MIB module
      changed receiveMessage ASI to use origin transport domain/address
      updated Parameter CSV appendix
   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
      *  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 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 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
   Jacobs University Bremen
   Campus Ring 1
   28725 Bremen
   Germany

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

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