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                         August 27,                        October 14, 2008
Expires: February 28, April 17, 2009

 Transport Subsystem for the Simple Network Management Protocol (SNMP)
                        draft-ietf-isms-tmsm-13
                        draft-ietf-isms-tmsm-14

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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 transports to include secure transports
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The Internet-Standard Management Framework . . . . . . . .  4
     1.2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Where this Extension Fits  . . . . . . . . . . . . . . . .  4
   2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Requirements of a Transport Model  . . . . . . . . . . . . . .  8
     3.1.  Message Security Requirements  . . . . . . . . . . . . . .  8
       3.1.1.  Security Protocol Requirements . . . . . . . . . . . .  8
     3.2.  SNMP Requirements  . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Architectural Modularity Requirements  . . . . . . . .  9
       3.2.2.  Access Control Requirements  . . . . . . . . . . . . . 12
       3.2.3.  Security Parameter Passing Requirements  . . . . . . . 12 13
       3.2.4.  Separation of Authentication and Authorization . . . . 13
     3.3.  Session Requirements . . . . . . . . . . . . . . . . . . . 14
       3.3.1.  Session Selection  . . . . . . . . . . . . . . . . . . 14
       3.3.2.  Session Establishment Requirements . . . . . . . . . . 15
       3.3.3.  Session Maintenance Requirements . . . . . . . . . . . 15 16
       3.3.4.  Message security versus session security . . . . . . . 16
   4.  Scenario Diagrams and the Transport Subsystem  . . . . . . . . 17
   5.  Cached Information and References  . . . . . . . . . . . . . . 17
     5.1.  securityStateReference . . . . . . . . . . . . . . . . . . 18
     5.2.  tmStateReference . . . . . . . . . . . . . . . . . . . . . 18
       5.2.1.  Transport information  . . . . . . . . . . . . . . . . 18 19
       5.2.2.  securityName . . . . . . . . . . . . . . . . . . . . . 19
       5.2.3.  securityLevel  . . . . . . . . . . . . . . . . . . . . 20
       5.2.4.  Session Information  . . . . . . . . . . . . . . . . . 20
   6.  Abstract Service Interfaces  . . . . . . . . . . . . . . . . . 20 21
     6.1.  sendMessage ASI  . . . . . . . . . . . . . . . . . . . . . 21
     6.2.  Changes to RFC3411 Outgoing ASIs . . . . . . . . . . . . . 22
       6.2.1.  Message Processing Subsystem Primitives  . . . . . . . 22
       6.2.2.  Security Subsystem Primitives  . . . . . . . . . . . . 23
     6.3.  The receiveMessage ASI . . . . . . . . . . . . . . . . . . 25
     6.4.  Changes to RFC3411 Incoming ASIs . . . . . . . . . . . . . 26
       6.4.1.  Message Processing Subsystem Primitive . . . . . . . . 26
       6.4.2.  Security Subsystem Primitive . . . . . . . . . . . . . 27
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
     7.1.  Coexistence, Security Parameters, and Access Control . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 31
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 30 31
     10.2. Informative References . . . . . . . . . . . . . . . . . . 31 32
   Appendix A.  Why tmStateReference? . . . . . . . . . . . . . . . . 32 33
     A.1.  Define an Abstract Service Interface . . . . . . . . . . . 33
     A.2.  Using an Encapsulating Header  . . . . . . . . . . . . . . 33 34
     A.3.  Modifying Existing Fields in an SNMP Message . . . . . . . 33 34
     A.4.  Using a Cache  . . . . . . . . . . . . . . . . . . . . . . 34
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 34 35
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 34 35

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.  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.

1.3.  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 modifying the ASIs that
   transport mappings (hereafter called transport models) can use to
   pass transport-specific security parameters to other subsystems,
   including transport-specific security parameters that are translated
   into the 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] utilize
   the Transport Subsystem.  The Transport Security Model is an
   alternative to the existing SNMPv1 Security Model [RFC3584], the
   SNMPv2c Security Model [RFC3584], and the User-based Security Model
   [RFC3414].  The Secure Shell Transport Model is an alternative to
   existing transport mappings as described in [RFC3417].

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 leave the details to
   them.  Considerations of hiring and training employees, positioning
   and scheduling the guards, arranging for cover, etc., are the
   responsibility of the security company.  The individual therefore
   achieves the desired security, with no significant effort...

   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
   existing user base infrastructures that were not specific to SNMP.

   A USM-compliant architecture might combine the authentication
   mechanism with an external mechanism, such as RADIUS [RFC2865] to
   provide the authentication service.  Similarly it might be possible
   to utilize an external protocol to encrypt a message, to check
   timeliness, to check data integrity, etc.  However this corresponds
   to the second approach - requiring the coordination of a number of
   differently subcontracted services.  Building solid security between
   the various services is difficult, and there is a significant
   potential for gaps in security.

   An alternative approach might be to 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.  This
   corresponds to the third approach described above.  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 [RFC5246], 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 other network management protocols,
   such as syslog [I-D.ietf-syslog-protocol] and 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) [RFC5246] 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:

   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 NOT require modifications to the underlying
   protocol.  Modifying the protocol might change its security
   characteristics in ways that could 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.

   Since multiple transport models can exist simultaneously within the
   transport subsystem, 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 Message Processing Subsystem
   permitting different message versions to be handled by a single
   engine, a Security Subsystem for enabling different methods of
   providing security services, 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 describes a Transport Subsystem that
   is compatible with the RFC3411 architecture.  As work is being done
   to use secure transports 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 permits Transport
   Models to be "plugged into" the RFC3411 architecture, supported by
   corresponding Transport Models (which may or may not be security-
   aware).  Such Transport Models would be independent of other modular
   SNMP components as much as possible.  This design also permits
   Transport Models to be advanced through the standards process
   independently of other Transport Models.

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

   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.  Changes to the RFC3411 Architecture

   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,
   including 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 on this path will be extended to pass security-model-
   independent values.  This document amends some of the ASIs defined in
   RFC 3411, and these changes are covered in section 6.

   New Security Models may be defined that understand how to work with
   these modified ASIs and the transport-information cache.  One such
   Security Model, the Transport Security Model, is defined in
   [I-D.ietf-isms-transport-security-model].

3.2.1.2.  Changes to RFC3411 processing

   The introduction of secure transports also affects the
   responsibilities and order of processing within the RFC3411
   architecture.  While the steps are the same, they may occur in a
   different order, and may be done by different subsystems.  With the
   existing RFC3411 architecture, security processing starts when the
   Message Processing Model decodes portions of the encoded message to
   extract parameters that identify which Security Model should handle
   the security-related tasks.

   A secure transport performs those security functions on the message,
   *before* the message is decoded.  Note that some of these functions
   might then be repeated by the selected Security Model.

3.2.1.3.  Passing Information between SNMP 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.
   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.

   Documents defining a new transport domain MUST define a prefix that
   will be prepended to all passed tmSecurityNames.  The prefix MUST
   include from one to four ASCII characters, not including a ":" (ASCII
   0x3a) character.  A tmSecurityName is constructed by concatenating
   the prefix and a ":" (ASCII 0x3a) character followed by a non-empty
   identity in an snmpAdminString compatible format.  Transport domains
   and their corresponding prefixes are coordinated via the IANA
   registry "SNMP Transport Domains".

   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.

   The introduction of a secure transport protocol means that the
   translation from a mechanism-specific identity to a tmSecurityName
   and tmSecurityLevel will be done by a Transport Model.  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 tmSecurityName mapping and tmSecurityLevel proposed
   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

   A Message Processing Model might unpack SNMP-specific security
   parameters from an incoming message before calling a specific
   Security Model to handle the security-related processing of the
   message.  When using a secure Transport Model, some security
   parameters might be extracted from the transport layer by the
   Security 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 between
   the Transport Subsystem, 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.

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 do not provide data access
   authorization capabilities, and to support data access authorization
   schemes, such as VACM, that do not perform their own authentication.

   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. msgSecurityModel
   field in 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 Security Model will be used for
   an incoming message, so cannot know how the securityName and
   securityLevel parameters will be determined.  It can propose an
   authenticated identity (via the tmSecurityName field), but there is
   no guarantee that this value will be used by the Security Model.  For
   example, non-transport-aware Security Models will typically determine
   the securityName (and securityLevel) based on the contents of the
   SNMP message itself.  Such Security Models will simply not know that
   the tmStateReference cache exists.. exists.

   Further, even if the Transport Model can influence the choice of
   securityName, it cannot directly determine the authorization allowed
   to this identity.  If two different Transport Model each authenticate
   a transport principal, that are then both mapped to the same
   securityName, then these two identities will typically be afforded
   exactly the same authorization by the Access Control Model.

   The only way for the Access Control Model to differentiate between
   identities based on the underlying Transport Model, would be for such
   transport-authenticated identities to be mapped to distinct
   securityNames.  How and if this is done is Security-Model-dependent.

3.3.  Session Requirements

   Some secure transports have a notion of sessions, while other secure
   transports provide channels or other session-like mechanism.
   Throughout this document, the term session is used in a broad sense
   to cover transport sessions, transport channels, and other transport-
   layer session-like mechanisms.  Transport-layer sessions that can
   secure multiple SNMP messages within the lifetime of the session are
   considered desirable because the cost of authentication can be
   amortized over potentially many transactions.  How a transport
   session is actually established, opened, closed, or maintained is
   specific to a particular Transport Model.

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

3.3.1.  Session Selection

   The architecture defined in [RFC3411] and the Transport Subsystem
   defined in this document do not support SNMP sessions or include a
   session selector in the Abstract Service Interfaces.  The Transport
   Subsystem does not have access to the pduType, so cannot select a
   given session for particular types of traffic.  However certain
   parameters of these ASIs might be used to guide the selection of the
   appropriate transport session to use for a given request.

   The transportDomain and transportAddress identify the transport
   connection to a remote network node.  Elements of the transport
   address (such as the port number) can be used to select different
   sessions for particular request types.  For example, UDP ports 161
   and 162 have typically been used to separate SNMP notifications from
   other request/response traffic.

   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).

   In summary, a unique combination of transportDomain,
   transportAddress, securityName, and securityLevel could serve to
   identify a given transport session.  Different values for any of
   these parameters would imply the use of a different session.

   However, because the handling of transport sessions is specific to
   each transport model, some transport models MAY restrict the
   applicability of these parameters for selecting an associated
   transport session.

   Implementations SHOULD be able to maintain some reasonable number of
   concurrent sessions, and MAY provide non-standard internal mechanisms
   to select sessions.

3.3.2.  Session Establishment Requirements

   SNMP applications provide the transportDomain, transportAddress,
   securityName, and securityLevel to be used to create a new session.

   If the Transport Model cannot provide at least the requested level of
   security, the Transport Model SHOULD discard the message and SHOULD
   notify the dispatcher that establishing a session and sending the
   message failed.  Similarly, if the session cannot be established,
   then the message should be discarded and the dispatcher notified.

   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.

3.3.3.  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.  How
   an implementation determines that an operation has completed is
   implementation-dependent.  While it is possible to tear down each
   transport session after processing for each message has completed,
   this is not recommended for performance reasons.

   The elements of procedure describe when cached information can be
   discarded, 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.

3.3.4.  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.  This would ensure that a cross-session
   replay attack would be unsuccessful; that is, an attacker could not
   take a message observed on one session, and successfully replay this
   on another session.

   A good security protocol would also protect against replay attacks
   within a session; that is, an attacker could not take a message
   observed on a session, and successfully replay this later in the same
   session.  One approach would be to use sequence information within
   the protocol, allowing the participants to detect if messages were
   replayed or reordered within a session.

   Note that 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.

   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 security level requested by a transport-aware security
   model, i.e. noAuthNoPriv and authNoPriv might 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, and a receiveMessage ASI to receive SNMP messages from the
   network, regardless of pduType.

5.  Cached Information and References

   When performing SNMP processing, there are two levels of state
   information that may need to be retained: the immediate state linking
   a request-response pair, and potentially longer-term state relating
   to transport and security.

   The RFC3411 architecture uses caches to maintain the short-term
   message state, and uses references in the ASIs to pass this
   information between subsystems.

   This document defines the requirements for a cache to handle the
   longer-term transport state information, using a tmStateReference
   parameter to pass this information between subsystems.

   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.  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 SHOULD also be
   discarded.

   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.

5.1.  securityStateReference

   The securityStateReference parameter is defined in RFC3411.  Its
   primary purpose is to provide a mapping between a request and the
   corresponding response.  This cache is not accessible to Transport
   Models, and an entry is typically only retained for the lifetime of a
   request-response pair of messages.

5.2.  tmStateReference

   For each transport session, information about the transport security
   is stored in a cache.  The tmStateReference parameter is used to pass
   model-specific and mechanism-specific parameters between the
   Transport subsystem and transport-aware Security Models.

   The tmStateReference cache will typically remain valid for the
   duration of the transport session, and hence may be used for several
   messages.

   Since this cache is only used within an implementation, and not on-
   the-wire, the precise contents and format are implementation-
   dependent.  However, for interoperability between Transport Models
   and transport-aware Security Models, entries in this cache must
   include at least the following fields:

      transportDomain

      transportAddress

      tmSecurityName

      tmRequestedSecurityLevel

      tmTransportSecurityLevel

      tmSameSecurity
      tmSessionID

5.2.1.  Transport information

   Information about the source of an incoming SNMP message is passed up
   from the Transport subsystem as far as the Message Processing
   subsystem.  However these parameters are not included in the
   processIncomingMsg ASI defined in RFC3411, and hence this information
   is not directly available to the Security Model.

   A transport-aware Security Model might wish to take account of the
   transport protocol and originating address when authenticating the
   request, and setting up the authorization parameters.  It is
   therefore necessary for the Transport Model to include this
   information in the tmStateReference cache, so that it is accessible
   to the Security Model.

   o  transportDomain: the transport protocol (and hence the Transport
      Model) used to receive the incoming message

   o  transportAddress: the source of the incoming message.

   Note that the ASIs used for processing an outgoing message all
   include explicit transportDomain and transportAddress parameters.
   These fields within the tmStateReference cache will typically not be
   used for outgoing messages.

5.2.2.  securityName

   There are actually three distinct "identities" that can be identified
   during the processing of an SNMP request over a secure transport:

   o  transport principal: the transport-authenticated identity, on
      whose behalf the secure transport connection was (or should be)
      established.  This value is transport-, mechanism- and
      implementation- specific, and is only used within a given
      Transport Model.

   o  tmSecurityName: a human-readable name (in snmpAdminString format)
      representing this transport identity.  This value is transport-
      and implementation-specific, and is only used (directly) by the
      Transport and Security Models.

   o  securityName: a human-readable name (in snmpAdminString format)
      representing the SNMP principal in a model-independent manner.

   o  Note that the transport principal may or may not be the same as
      the tmSecurityName.  Similarly, the tmSecurityName may or may not
      be the same as the securityName as seen by the Application and
      Access Control subsystems.  In particular, a non-transport-aware
      Security Model will ignore tmSecurityName completely when
      determining the SNMP securityName.

   o  However it is important that the mapping between the transport
      principal and the SNMP securityName (for transport-aware Security
      Models) is consistent and predictable, to allow configuration of
      suitable access control and the establishment of transport
      connections.

5.2.3.  securityLevel

   There are two distinct issues relating to security level as applied
   to secure transports.  For clarity, these are handled by separate
   fields in the tmStateReference cache:

   o  tmTransportSecurityLevel: an indication from the Transport Model
      of the level of security offered by this session.  The Security
      Model can use this to ensure that incoming messages were suitably
      protected before acting on them.

   o  tmRequestedSecurityLevel: an indication from the Security Model of
      the level of security required to be provided by the transport
      protocol.  The Transport Model can use this to ensure that
      outgoing messages will not be sent over an insufficiently secure
      session.

5.2.4.  Session Information

   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.

   When processing an outgoing message, if tmSameSecurity is true, then
   the tmSessionID MUST match the current transport session, otherwise
   the message MUST be discarded, and the dispatcher notified that
   sending the message failed.

   o  tmSameSecurity: this flag is used by a transport-aware Security
      Model to indicate whether the Transport Model MUST enforce this
      restriction.

   o  tmSessionID: in order to verify whether the session has changed,
      the Transport Model must be able to compare the session used to
      receive the original request with the one to be used to send the
      response.  This typically requires some form of session
      identifier.  This value is only ever used by the Transport Model,
      so the format and interpretation of this field are model-specific
      and implementation-dependent.

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 introduces a couple of new ASIs to define the interface
   between the Transport and Dispatcher Subsystems, and extends some of
   the ASIs defined in RFC3411 to include transport-related information.

   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 will typically include the
   OID and value for an incremented error counter.  This may be
   accompanied by values for contextEngineID and contextName for this
   counter, a value for securityLevel, and the appropriate state
   reference 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 defined in this
   document 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.  How the information
   in the cache is used is transport-model-dependent and 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.
   tmStateReference cache.

   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.  Changes to RFC3411 Outgoing ASIs

   [DISCUSS: this section has been significantly rewritten and
   reorganized.  This needs to be checked thoroughly to verify no
   technical changes have been introduced in the editorial changes.]

   Additional parameters have been added to the ASIs defined in RFC3411,
   concerned with communication between the Dispatcher and Message
   Processing subsystems, and between the Message Processing and
   Security Subsystems.

6.2.1.  Message Processing Subsystem Primitives

   A tmStateReference parameter has been added as an OUT parameter to
   the prepareOutgoingMessage and prepareResponseMessage ASIs.  This is
   passed from Message Processing Subsystem to the dispatcher, and from
   there to the Transport Subsystem.

   How or if the Message Processing Subsystem modifies or utilizes the
   contents of the cache is message-processing-model specific.

   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
               )

6.2.2.  Security Subsystem Primitives

   transportDomain and transportAddress parameters have been added as IN
   parameters to the generateOutgoingMessage and generateResponseMessage
   ASIs, and a tmStateReference parameter has been added as an OUT
   parameter.  The transportDomain and transportAddress parameters will
   have been passed into the Message Processing Subsystem from the
   dispatcher, and are passed on to the Security Subsystem.  The
   tmStateReference parameter will be passed from the Security Subsystem
   back to the Message Processing Subsystem, and on to the dispatcher
   and Transport subsystems.

   If a cache exists for a session identifiable from the
   transportDomain, transportAddress, tmSecurityName and requested
   securityLevel, then a transport-aware Security Model might create a
   tmStateReference parameter to this cache, and pass that as an OUT
   parameter.

   statusInformation =
   generateRequestMessage(
     IN   transportDomain         -- (NEW) destination transport domain
     IN   transportAddress        -- (NEW) destination transport address
     IN   messageProcessingModel  -- typically, SNMP version
     IN   globalData              -- message header, admin data
     IN   maxMessageSize          -- of the sending SNMP entity
     IN   securityModel           -- for the outgoing message
     IN   securityEngineID        -- authoritative SNMP entity
     IN   securityName            -- on behalf of this principal
     IN   securityLevel           -- Level of Security requested
     IN   scopedPDU               -- message (plaintext) payload
     OUT  securityParameters      -- filled in by Security Module
     OUT  wholeMsg                -- complete generated message
     OUT  wholeMsgLength          -- length of generated message
     OUT  tmStateReference        -- (NEW) reference to transport state
   )
              )
   statusInformation =
   generateResponseMessage(
     IN   transportDomain         -- (NEW) destination transport domain
     IN   transportAddress        -- (NEW) destination transport address
     IN   messageProcessingModel -- SNMPv3 Message Processing
                                       -- Model
     IN   globalData             -- msgGlobalData from step 7
     IN   maxMessageSize         -- from msgMaxSize (step 7c)
     IN   securityModel          -- as determined in step 7e
     IN   securityEngineID       -- the value of snmpEngineID
     IN   securityName           -- on behalf of this principal
     IN   securityLevel          -- for the outgoing message
     IN   scopedPDU              -- as prepared in step 6)
     IN   securityStateReference -- as determined in step 2
     OUT  securityParameters     -- filled in by Security Module
     OUT  wholeMsg               -- complete generated message
     OUT  wholeMsgLength         -- length of generated message
     OUT  tmStateReference        -- (NEW) reference to transport state
   )

              )

6.3.  The receiveMessage ASI

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

   When a message is received on a given transport session, if a cache
   does not already exist for that session, the Transport Model might
   create one, referenced by tmStateReference.  The contents of this
   cache are discussed in section 5.  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

   "Might createone" may sound underspecified, but a transport model
   might be something like SNMP over UDP over IPv6, where no transport
   security is not provided, so it might have no mechanisms for determining not create a
   securityName and securityLevel. cache.

   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.  Changes to RFC3411 Incoming ASIs

   The tmStateReference parameter has also been added to some of the
   incoming ASIs defined in RFC3411.  How or if a Message Processing
   Model or Security Model uses tmStateReference is message-processing-
   and 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.

6.4.1.  Message Processing Subsystem Primitive

   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.

   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
   )

6.4.2.  Security Subsystem Primitive

   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.

   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

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.  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.

   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 mapping 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

   IANA is requested to create a new registry in the Simple Network
   Management Protocol (SNMP) Number Spaces.  The new registry should be
   called "SNMP Transport Domains".  This document registry iwill contain ASCII
   strings of one to four characters to identify prefixes for
   corresponding SNMP transport domains.  Each transport domain requires no action by IANA.
   an OID assignment under snmpDomains [RFC2578] .  Values are to be
   assigned via RFC2434 "Specification Required".

   The registry should be populated with the following initial entries:

   Registry Name: SNMP Transport Domains
   Reference: [RFC2578] [RFC3417] [XXXX]
   Registration Procedures: Specification Required
   Each domain is assigned a MIB-defined OID under snmpDomains

   Prefix     snmpDomain                        Reference
   -------       -----------------------------  ---------
   udp           snmpUDPDomain                  RFC3417
   clns          snmpCLNSDomain                 RFC3417
   cons          snmpCONSDomain                 RFC3417
   ddp           snmpDDPDomain                  RFC3417
   ipx           snmpIPXDomain                  RFC3417
   prxy          rfc1157Domain                  RFC3417

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

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. Petch, Wes Hardaker, and Dave Shields.

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.

   [RFC2578]                                 McCloghrie, K., Ed.,
                                             Perkins, D., Ed., and J.
                                             Schoenwaelder, Ed.,
                                             "Structure of Management
                                             Information Version 2
                                             (SMIv2)", STD 58, RFC 2578,
                                             April 1999.

   [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.

   [RFC5246]                                 Dierks, T. and E. Rescorla,
                                             "The Transport Layer
                                             Security (TLS) Protocol
                                             Version 1.2", RFC 5246,
                                             August 2008.

   [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., D. and W.
                                             Hardaker, "Transport
                                             Security Model for SNMP", d
                                             raft-ietf-isms-transport-
                                             security-model-08
                                             security-model-09 (work in
                                             progress), July October 2008.

   [I-D.ietf-isms-secshell]                  Harrington, D. and J. D., Salowey,
                                             J., and W. Hardaker,
                                             "Secure Shell Transport
                                             Model for SNMP",
                                             draft-ietf-isms-secshell-11
                                             draft-ietf-isms-secshell-12
                                             (work in progress),
                                             July
                                             October 2008.

   [I-D.ietf-syslog-protocol]                Gerhards, R., "The syslog
                                             Protocol", draft-ietf-
                                             syslog-protocol-23 (work in
                                             progress), September 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 -12- to -13-

   o  moved conventions after Internet Standard framework, for
      consistency with related documents.

   o  editorial changes and reorganization

   Changes from -10- to -12-

   o  clarified relation to other documents.

   o  clarified relation to older security models.

   o  moved comparison of TSM and USM to TSM document

   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

   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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