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rfc:rfc5590

Network Working Group D. Harrington Request for Comments: 5590 Huawei Technologies (USA) Updates: 3411, 3412, 3414, 3417 J. Schoenwaelder Category: Standards Track Jacobs University Bremen

                                                             June 2009

Transport Subsystem for the Simple Network Management Protocol (SNMP)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

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 that is
 comparable to other subsystems in the RFC 3411 architecture.  As work
 is being done to expand the transports to include secure transports,
 such as the Secure Shell (SSH) Protocol and Transport Layer Security

Harrington & Schoenwaelder Standards Track [Page 1] RFC 5590 SNMP Transport Subsystem June 2009

 (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.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  3
   1.3.  Where This Extension Fits  . . . . . . . . . . . . . . . .  4
 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  . . . . . . . . . . . . . 11
     3.2.3.  Security Parameter Passing Requirements  . . . . . . . 12
     3.2.4.  Separation of Authentication and Authorization . . . . 12
   3.3.  Session Requirements . . . . . . . . . . . . . . . . . . . 13
     3.3.1.  No SNMP Sessions . . . . . . . . . . . . . . . . . . . 13
     3.3.2.  Session Establishment Requirements . . . . . . . . . . 14
     3.3.3.  Session Maintenance Requirements . . . . . . . . . . . 15
     3.3.4.  Message Security versus Session Security . . . . . . . 15
 4.  Scenario Diagrams and the Transport Subsystem  . . . . . . . . 16
 5.  Cached Information and References  . . . . . . . . . . . . . . 17
   5.1.  securityStateReference . . . . . . . . . . . . . . . . . . 17
   5.2.  tmStateReference . . . . . . . . . . . . . . . . . . . . . 17
     5.2.1.  Transport Information  . . . . . . . . . . . . . . . . 18
     5.2.2.  securityName . . . . . . . . . . . . . . . . . . . . . 19
     5.2.3.  securityLevel  . . . . . . . . . . . . . . . . . . . . 20
     5.2.4.  Session Information  . . . . . . . . . . . . . . . . . 20
 6.  Abstract Service Interfaces  . . . . . . . . . . . . . . . . . 21
   6.1.  sendMessage ASI  . . . . . . . . . . . . . . . . . . . . . 21
   6.2.  Changes to RFC 3411 Outgoing ASIs  . . . . . . . . . . . . 22
     6.2.1.  Message Processing Subsystem Primitives  . . . . . . . 22
     6.2.2.  Security Subsystem Primitives  . . . . . . . . . . . . 23
   6.3.  The receiveMessage ASI . . . . . . . . . . . . . . . . . . 24
   6.4.  Changes to RFC 3411 Incoming ASIs  . . . . . . . . . . . . 25
     6.4.1.  Message Processing Subsystem Primitive . . . . . . . . 25
     6.4.2.  Security Subsystem Primitive . . . . . . . . . . . . . 26
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   7.1.  Coexistence, Security Parameters, and Access Control . . . 27
 8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
 9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 30

Harrington & Schoenwaelder Standards Track [Page 2] RFC 5590 SNMP Transport Subsystem June 2009

   10.2. Informative References . . . . . . . . . . . . . . . . . . 30
 Appendix A.  Why tmStateReference? . . . . . . . . . . . . . . . . 32
   A.1.  Define an Abstract Service Interface . . . . . . . . . . . 32
   A.2.  Using an Encapsulating Header  . . . . . . . . . . . . . . 32
   A.3.  Modifying Existing Fields in an SNMP Message . . . . . . . 32
   A.4.  Using a Cache  . . . . . . . . . . . . . . . . . . . . . . 33

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].
 Lowercase versions of the keywords should be read as in normal
 English.  They will usually, but not always, be used in a context
 that relates to compatibility with the RFC 3411 architecture or the
 subsystem defined here but that 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 RFC 3411
 subsystems and Abstract Service Interfaces (ASIs).  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 STD 62, 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.
 This document discusses an extension to the modular RFC 3411
 architecture; this is not a protocol document.  An architectural
 "MUST" is a really sharp constraint; to allow for the evolution of
 technology and to not unnecessarily constrain future models, often a

Harrington & Schoenwaelder Standards Track [Page 3] RFC 5590 SNMP Transport Subsystem June 2009

 "SHOULD" or a "should" is more appropriate than a "MUST" in an
 architecture.  Future models MAY express tighter requirements for
 their own model-specific processing.

1.3. Where This Extension Fits

 It is expected that readers of this document will have read RFCs 3410
 and 3411, 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 RFC 3411, Section 3.1.
 The following diagram depicts its place in the RFC 3411 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   |  |              |        |  |
 |  |  +-------------+  +--------------+  +--------------+        |  |
 |  +-------------------------------------------------------------+  |
 |                                                                   |
 +-------------------------------------------------------------------+

Harrington & Schoenwaelder Standards Track [Page 4] RFC 5590 SNMP Transport Subsystem June 2009

 The transport mappings defined in RFC 3417 do not provide lower-layer
 security functionality, and thus do not provide transport-specific
 security parameters.  This document updates RFC 3411 and RFC 3417 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 [RFC5591] and the Secure Shell Transport
 Model [RFC5592] 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 significantly less effort on his
 part except for identifying requirements and verifying the quality of
 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 uses a separate principal and key
 management infrastructure.  Operators have reported that deploying
 another principal and key management infrastructure in order to use

Harrington & Schoenwaelder Standards Track [Page 5] RFC 5590 SNMP Transport Subsystem June 2009

 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 management 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 are TLS [RFC5246], Simple
 Authentication and Security Layer (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 [RFC5424] and NETCONF [RFC4741].
 This document defines a Transport Subsystem extension to the RFC 3411
 architecture that is 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 RFC 3411 architecture.

Harrington & Schoenwaelder Standards Track [Page 6] RFC 5590 SNMP Transport Subsystem June 2009

 There are a number of Internet security protocols and mechanisms that
 are in widespread 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].  The
 security requirements outlined there do not require protection
 against denial of service or traffic analysis; however, transport
 security protocols 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.

Harrington & Schoenwaelder Standards Track [Page 7] RFC 5590 SNMP Transport Subsystem June 2009

 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 specification
 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 Section 3 of [RFC3411]) to allow the evolution of the SNMP
 protocol standards over time and to minimize the side effects between
 subsystems when changes are made.
 The RFC 3411 architecture includes a Message Processing Subsystem for
 permitting different message versions to be handled by a single
 engine, a Security Subsystem for enabling different methods of
 providing security services, Applications to support different types
 of Application processors, and an Access Control Subsystem for
 allowing multiple approaches to access control.  The RFC 3411
 architecture does not include a subsystem for Transport Models,
 despite the fact there are multiple transport mappings already
 defined for SNMP [RFC3417].  This document describes a Transport
 Subsystem that is compatible with the RFC 3411 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 RFC
 3411 architecture that are defined in Section 1.5 of [RFC3411].  This
 Transport Subsystem uses a modular design that permits Transport
 Models (which might or might not be security-aware) to be "plugged
 into" the RFC 3411 architecture.  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.
 The following diagram depicts the SNMPv3 architecture, including the
 new Transport Subsystem defined in this document and a new Transport
 Security Model defined in [RFC5591].

Harrington & Schoenwaelder Standards Track [Page 8] RFC 5590 SNMP Transport Subsystem June 2009

 +------------------------------+
 |    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 |
 +-------------------------------------------------------------------+

Harrington & Schoenwaelder Standards Track [Page 9] RFC 5590 SNMP Transport Subsystem June 2009

3.2.1.1. Changes to the RFC 3411 Architecture

 The RFC 3411 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; these changes are covered in Section 6 of this document.
 New Security Models might 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
 [RFC5591].

3.2.1.2. Changes to RFC 3411 Processing

 The introduction of secure transports affects the responsibilities
 and order of processing within the RFC 3411 architecture.  While the
 steps are the same, they might occur in a different order, and might
 be done by different subsystems.  With the existing RFC 3411
 architecture, security processing starts when the Message Processing
 Model decodes portions of the encoded message to extract parameters
 that identify which Security Model MUST handle the security-related
 tasks.
 A secure transport performs those security functions on the message,
 before the message is decoded.  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 possibly
 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.  While
 the Community Security Models [RFC3584] and the User-based Security
 Model establish a security association for each SNMP message, newer
 Transport Models MAY support sending multiple SNMP messages through
 the same tunnel to amortize the costs of establishing a security
 association.

Harrington & Schoenwaelder Standards Track [Page 10] RFC 5590 SNMP Transport Subsystem June 2009

3.2.2. Access Control Requirements

 RFC 3411 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".
 A Security Model plays 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 that 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 encrypted, and to ensure that outgoing messages are
 authenticated and encrypted based on the value of securityLevel.
 A Transport Model MAY provide suggested values for securityName and
 securityLevel.  A Security Model might have multiple sources for
 determining the principal and desired security services, and a
 particular Security Model might or might not utilize the values
 proposed by a Transport Model when deciding the value of securityName
 and securityLevel.
 Documents defining a new transport domain MUST define a prefix that
 MAY be prepended to all securityNames passed by the Security Model.
 The prefix MUST include one to four US-ASCII alpha-numeric
 characters, not including a ":" (US-ASCII 0x3a) character.  If a
 prefix is used, a securityName is constructed by concatenating the
 prefix and a ":" (US-ASCII 0x3a) character, followed by a non-empty
 identity in an snmpAdminString-compatible format.  The prefix can be
 used by SNMP Applications to distinguish "alice" authenticated by SSH

Harrington & Schoenwaelder Standards Track [Page 11] RFC 5590 SNMP Transport Subsystem June 2009

 from "alice" authenticated by TLS.  Transport domains and their
 corresponding prefixes are coordinated via the IANA registry "SNMP
 Transport Domains".

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
 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;
 a Security Model might extract that information from the cache.  A
 tmStateReference is passed as an extra parameter in the ASIs between
 the Transport Subsystem and the Message Processing and Security
 Subsystems in order 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 RFC
 3411 ASIs.

3.2.4. Separation of Authentication and Authorization

 The RFC 3411 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, in order
 to allow support for authentication protocols that do not provide
 data-access authorization capabilities, and in order to support data-
 access authorization schemes, such as the View-based access Control
 Model (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) or
 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.

Harrington & Schoenwaelder Standards Track [Page 12] RFC 5590 SNMP Transport Subsystem June 2009

 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, and 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.
 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 Models 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 mechanisms.
 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. No SNMP Sessions

 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.

Harrington & Schoenwaelder Standards Track [Page 13] RFC 5590 SNMP Transport Subsystem June 2009

 The Transport Subsystem might support transport sessions.  However,
 the Transport Subsystem does not have access to the pduType (i.e.,
 the SNMP operation type), and so cannot select a given transport
 session for particular types of traffic.
 Certain parameters of the Abstract Service Interfaces might be used
 to guide the selection of an appropriate transport session to use for
 a given request by an Application.
 The transportDomain and transportAddress identify the transport
 connection to a remote network node.  Elements of the transport
 address (such as the port number) might be used by an Application to
 send a particular PDU type to a particular transport address.  For
 example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are
 used to configure notification originators with the destination port
 to which SNMPv2-Trap PDUs or Inform PDUs are to be sent, but the
 Transport Subsystem never looks inside the PDU.
 The securityName identifies which security principal to communicate
 with at that address (e.g., different Network Management System (NMS)
 applications), and the securityLevel might permit selection of
 different sets of security properties for different purposes (e.g.,
 encrypted SET vs. non-encrypted GET operations).
 However, because the handling of transport sessions is specific to
 each Transport Model, some Transport Models MAY restrict selecting a
 particular transport session.  A user application might use a unique
 combination of transportDomain, transportAddress, securityModel,
 securityName, and securityLevel to try to force the selection of a
 given transport session.  This usage is NOT RECOMMENDED because it is
 not guaranteed to be interoperable across implementations and across
 models.
 Implementations SHOULD be able to maintain some reasonable number of
 concurrent transport sessions, and MAY provide non-standard internal
 mechanisms to select transport 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.

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 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 happens
 to the objects when a related session is torn down, since this will
 impact the 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 it 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 it 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.

Harrington & Schoenwaelder Standards Track [Page 15] RFC 5590 SNMP Transport Subsystem June 2009

 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.  The architecture does not mandate
 this requirement in order to allow for future Security Models where
 this might make sense; however, not requiring this could lead to
 added complexity and security vulnerabilities, so most Security
 Models SHOULD 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
 command generator to poll for potentially non-sensitive performance
 data on thousands of interfaces every ten minutes; such 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.  A Transport Model MUST NOT
 downgrade the security level requested by a transport-aware Security
 Model, and SHOULD discard any message where this would occur.  This
 is a SHOULD rather than a MUST only to permit the potential
 development of models that can perform error-handling in a manner
 that is less severe than discarding the message.  However, any model
 that does not discard the message in this circumstance should have a
 clear justification for why not discarding will not create a security
 vulnerability.

4. Scenario Diagrams and the Transport Subsystem

 Sections 4.6.1 and 4.6.2 of RFC 3411 provide scenario diagrams to
 illustrate how an outgoing message is created and how an incoming
 message is processed.  RFC 3411 does not define ASIs for the "Send
 SNMP Request Message to Network", "Receive SNMP Response Message from
 Network", "Receive SNMP Message from Network" and "Send SNMP message
 to Network" arrows in these diagrams.
 This document defines two ASIs corresponding to these arrows: a
 sendMessage ASI to send SNMP messages to the network and a
 receiveMessage ASI to receive SNMP messages from the network.  These
 ASIs are used for all SNMP messages, regardless of pduType.

Harrington & Schoenwaelder Standards Track [Page 16] RFC 5590 SNMP Transport Subsystem June 2009

5. Cached Information and References

 When performing SNMP processing, there are two levels of state
 information that might need to be retained: the immediate state
 linking a request-response pair and a potentially longer-term state
 relating to transport and security.
 The RFC 3411 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
 additional short-term message state and 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 is
 implementation-specific.

5.1. securityStateReference

 The securityStateReference parameter is defined in RFC 3411.  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 tmState cache or datastore that is referenced by a
 tmStateReference.  The tmStateReference parameter is used to pass
 model-specific and mechanism-specific parameters between the
 Transport Subsystem and transport-aware Security Models.
 In general, when necessary, the tmState is populated by the Security
 Model for outgoing messages and by the Transport Model for incoming
 messages.  However, in both cases, the model populating the tmState

Harrington & Schoenwaelder Standards Track [Page 17] RFC 5590 SNMP Transport Subsystem June 2009

 might have incomplete information, and the missing information might
 be populated by the other model when the information becomes
 available.
 The tmState might contain both long-term and short-term information.
 The session information typically remains valid for the duration of
 the transport session, might be used for several messages, and might
 be stored in a local configuration datastore.  Some information has a
 shorter lifespan, such as tmSameSecurity and
 tmRequestedSecurityLevel, which are associated with a specific
 message.
 Since this cache is only used within an implementation, and not on-
 the-wire, the precise contents and format of the cache are
 implementation-dependent.  For architectural modularity between
 Transport Models and transport-aware Security Models, a fully-defined
 tmState MUST conceptually include at least the following fields:
    tmTransportDomain
    tmTransportAddress
    tmSecurityName
    tmRequestedSecurityLevel
    tmTransportSecurityLevel
    tmSameSecurity
    tmSessionID
 The details of these fields are described in the following
 subsections.

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 RFC 3411; 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

Harrington & Schoenwaelder Standards Track [Page 18] RFC 5590 SNMP Transport Subsystem June 2009

 necessary for the Transport Model to include this information in the
 tmStateReference cache so that it is accessible to the Security
 Model.
 o  tmTransportDomain: the transport protocol (and hence the Transport
    Model) used to receive the incoming message.
 o  tmTransportAddress: the source of the incoming message.
 The ASIs used for processing an outgoing message all include explicit
 transportDomain and transportAddress parameters.  The values within
 the securityStateReference cache might override these parameters 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.
    This value is used directly by SNMP Applications, the Access
    Control Subsystem, the Message Processing Subsystem, and the
    Security Subsystem.
 The transport principal might or might not be the same as the
 tmSecurityName.  Similarly, the tmSecurityName might or might 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.
 However, it is important that the mapping between the transport
 principal and the SNMP securityName (for transport-aware Security
 Models) is consistent and predictable in order to allow configuration
 of suitable access control and the establishment of transport
 connections.

Harrington & Schoenwaelder Standards Track [Page 19] RFC 5590 SNMP Transport Subsystem June 2009

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.
 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 needs 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.
 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.

Harrington & Schoenwaelder Standards Track [Page 20] RFC 5590 SNMP Transport Subsystem June 2009

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; it also extends some
 of the ASIs defined in RFC 3411 to include transport-related
 information.
 This document follows the example of RFC 3411 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.  Keeping sensitive security information
 longer than necessary might introduce potential vulnerabilities to an
 implementation.
 2)An error indication in statusInformation will typically include the
 Object Identifier (OID) and value for an incremented error counter.
 This might 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.  The sendMessageASI
 defined in this document replaces the text "Send SNMP Request Message
 to Network" that appears in the diagram in Section 4.6.1 of RFC 3411
 and the text "Send SNMP Message to Network" that appears in Section
 4.6.2 of RFC 3411.
 If present and valid, the tmStateReference refers to a cache
 containing Transport-Model-specific parameters for the transport and
 transport security.  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.

Harrington & Schoenwaelder Standards Track [Page 21] RFC 5590 SNMP Transport Subsystem June 2009

 This might 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
 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 RFC 3411 Outgoing ASIs

 Additional parameters have been added to the ASIs defined in RFC 3411
 that are 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 the 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

Harrington & Schoenwaelder Standards Track [Page 22] RFC 5590 SNMP Transport Subsystem June 2009

 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 generateRequestMsg and generateResponseMsg 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
 tmTransportDomain, tmTransportAddress, 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.

Harrington & Schoenwaelder Standards Track [Page 23] RFC 5590 SNMP Transport Subsystem June 2009

 statusInformation =
 generateRequestMsg(
   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 =
 generateResponseMsg(
   IN   transportDomain         -- (NEW) destination transport domain
   IN   transportAddress        -- (NEW) destination transport address
   IN   messageProcessingModel -- Message Processing Model
   IN   globalData             -- msgGlobalData
   IN   maxMessageSize         -- from msgMaxSize
   IN   securityModel          -- as determined by MPM
   IN   securityEngineID       -- the value of snmpEngineID
   IN   securityName           -- on behalf of this principal
   IN   securityLevel          -- for the outgoing message
   IN   scopedPDU              -- as provided by MPM
   IN   securityStateReference -- as provided by MPM
   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.  The receiveMessage ASI replaces the
 text "Receive SNMP Response Message from Network" that appears in the
 diagram in Section 4.6.1 of RFC 3411 and the text "Receive SNMP
 Message from Network" from Section 4.6.2 of RFC3411.
 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

Harrington & Schoenwaelder Standards Track [Page 24] RFC 5590 SNMP Transport Subsystem June 2009

 cache are discussed in Section 5.  How this information is determined
 is implementation- and Transport-Model-specific.
 "Might create one" might sound underspecified, but a Transport Model
 might be something like SNMP over UDP over IPv6, where transport
 security is not provided, so it might not create a 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.
 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 RFC 3411 Incoming ASIs

 The tmStateReference parameter has also been added to some of the
 incoming ASIs defined in RFC 3411.  How or if a Message Processing
 Model or Security Model uses tmStateReference is message-processing-
 and Security-Model-specific.
 This might 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

Harrington & Schoenwaelder Standards Track [Page 25] RFC 5590 SNMP Transport Subsystem June 2009

 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
 )

Harrington & Schoenwaelder Standards Track [Page 26] RFC 5590 SNMP Transport Subsystem June 2009

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 that Transport
 Model.
 It is considered desirable by some industry segments that SNMP
 Transport Models 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 secrecy is appropriate for
 that Transport Model.
 The denial-of-service characteristics of various Transport Models and
 security protocols will vary and should be evaluated when determining
 the applicability of a Transport Model to a particular deployment
 situation.
 Since the cache 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 an 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 might 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 RFC 3411 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.

Harrington & Schoenwaelder Standards Track [Page 27] RFC 5590 SNMP Transport Subsystem June 2009

 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.
 Using a non-transport-aware Security Model with a secure Transport
 Model is NOT RECOMMENDED for the following reasons.
 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 combining older Security Models with Secure Transport
 Models are known.  The securityName used for access control decisions
 is based on the message-driven identity, which might be
 unauthenticated, and not on the transport-driven, authenticated
 identity:
 o  An SNMPv1 message will always be paired with an SNMPv1 Security
    Model (per RFC 3584), regardless of the transport mapping or
    Transport Model used, and access controls will be based on the
    unauthenticated community name.
 o  An SNMPv2c message will always be paired with an SNMPv2c Security
    Model (per RFC 3584), regardless of the transport mapping or
    Transport Model used, and access controls will be based on the
    unauthenticated community name.
 o  An SNMPv3 message will always be paired with the securityModel
    specified in the msgSecurityParameters field of the message (per
    RFC 3412), regardless of the transport mapping or Transport Model
    used.  If the SNMPv3 message specifies the User-based Security
    Model (USM) with noAuthNoPriv, then the access controls will be
    based on the unauthenticated USM user.
 o  For outgoing messages, if a Secure Transport Model is selected in
    combination with a Security Model that does not populate a
    tmStateReference, the Secure Transport Model SHOULD detect the
    lack of a valid tmStateReference and fail.

Harrington & Schoenwaelder Standards Track [Page 28] RFC 5590 SNMP Transport Subsystem June 2009

 In times of network stress, a Secure Transport Model might not work
 properly if its underlying security mechanisms (e.g., Network Time
 Protocol (NTP) or Authentication, Authorization, and Accounting (AAA)
 protocols or certificate authorities) are not reachable.  The User-
 based Security Model was explicitly designed to not depend upon
 external network services, and provides its own security services.
 It is RECOMMENDED that operators provision authPriv USM as a fallback
 mechanism to supplement any Security Model or Transport Model that
 has external dependencies, so that secure SNMP communications can
 continue when the external network service is not available.

8. IANA Considerations

 IANA has created a new registry in the Simple Network Management
 Protocol (SNMP) Number Spaces.  The new registry is called "SNMP
 Transport Domains".  This registry contains US-ASCII alpha-numeric
 strings of one to four characters to identify prefixes for
 corresponding SNMP transport domains.  Each transport domain MUST
 have an OID assignment under snmpDomains [RFC2578].  Values are to be
 assigned via [RFC5226] "Specification Required".
 The registry has been populated with the following initial entries:
 Registry Name: SNMP Transport Domains
 Reference: [RFC2578] [RFC3417] [RFC5590]
 Registration Procedures: Specification Required
 Each domain is assigned a MIB-defined OID under snmpDomains
 Prefix        snmpDomains                    Reference
 -------       -----------------------------  ---------
 udp           snmpUDPDomain                  [RFC3417] [RFC5590]
 clns          snmpCLNSDomain                 [RFC3417] [RFC5590]
 cons          snmpCONSDomain                 [RFC3417] [RFC5590]
 ddp           snmpDDPDomain                  [RFC3417] [RFC5590]
 ipx           snmpIPXDomain                  [RFC3417] [RFC5590]
 prxy          rfc1157Domain                  [RFC3417] [RFC5590]

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.

Harrington & Schoenwaelder Standards Track [Page 29] RFC 5590 SNMP Transport Subsystem June 2009

 WG members who performed detailed reviews: Wes Hardaker, Jeffrey
 Hutzelman, Tom Petch, Dave Shield, and Bert Wijnen.

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

Harrington & Schoenwaelder Standards Track [Page 30] RFC 5590 SNMP Transport Subsystem June 2009

 [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.
 [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
            Protocol Architecture", RFC 4251, January 2006.
 [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
            Security Layer (SASL)", RFC 4422, June 2006.
 [RFC4741]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
            December 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
 [RFC5591]  Harrington, D. and W. Hardaker, "Transport Security Model
            for the Simple Network Management Protocol (SNMP)",
            RFC 5591, June 2009.
 [RFC5592]  Harrington, D., Salowey, J., and W. Hardaker, "Secure
            Shell Transport Model for the Simple Network Management
            Protocol (SNMP)", RFC 5592, June 2009.

Harrington & Schoenwaelder Standards Track [Page 31] RFC 5590 SNMP Transport Subsystem June 2009

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.
 2.  One could add a header to encapsulate the SNMP message.
 3.  One could utilize fields already defined in the existing SNMPv3
     message.
 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 the Security Subsystem has the advantage
 of being consistent with existing RFC 3411/3412 practice; it also
 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 models or 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) Message 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

Harrington & Schoenwaelder Standards Track [Page 32] RFC 5590 SNMP Transport Subsystem June 2009

 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 in order for the Transport Model to know 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 (TM)
 puts information about the security applied to an incoming message; 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 that 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 RFC 3411 ASIs.

Harrington & Schoenwaelder Standards Track [Page 33] RFC 5590 SNMP Transport Subsystem June 2009

Authors' Addresses

 David Harrington
 Huawei Technologies (USA)
 1700 Alma Dr. Suite 100
 Plano, TX 75075
 USA
 Phone: +1 603 436 8634
 EMail: ietfdbh@comcast.net
 Juergen Schoenwaelder
 Jacobs University Bremen
 Campus Ring 1
 28725 Bremen
 Germany
 Phone: +49 421 200-3587
 EMail: j.schoenwaelder@jacobs-university.de

Harrington & Schoenwaelder Standards Track [Page 34]

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