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RFC 5591

Transport Security Model for the Simple Network Management Protocol (SNMP)

Pages: 28
Internet Standard: 78
STD 78 is also:  534355906353

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Network Working Group                                      D. Harrington
Request for Comments: 5591                     Huawei Technologies (USA)
Category: Standards Track                                    W. Hardaker
                                               Cobham Analytic Solutions
                                                               June 2009


                    Transport Security Model 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.
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Abstract

This memo describes a Transport Security Model for the Simple Network Management Protocol (SNMP). This memo also defines a portion of the Management Information Base (MIB) for monitoring and managing the Transport Security Model for SNMP.

Table of Contents

1. Introduction ....................................................3 1.1. The Internet-Standard Management Framework .................3 1.2. Conventions ................................................3 1.3. Modularity .................................................4 1.4. Motivation .................................................5 1.5. Constraints ................................................5 2. How the Transport Security Model Fits in the Architecture .......6 2.1. Security Capabilities of this Model ........................6 2.1.1. Threats .............................................6 2.1.2. Security Levels .....................................7 2.2. Transport Sessions .........................................7 2.3. Coexistence ................................................7 2.3.1. Coexistence with Message Processing Models ..........7 2.3.2. Coexistence with Other Security Models ..............8 2.3.3. Coexistence with Transport Models ...................8 3. Cached Information and References ...............................8 3.1. Transport Security Model Cached Information ................9 3.1.1. securityStateReference ..............................9 3.1.2. tmStateReference ....................................9 3.1.3. Prefixes and securityNames ..........................9 4. Processing an Outgoing Message .................................10 4.1. Security Processing for an Outgoing Message ...............10 4.2. Elements of Procedure for Outgoing Messages ...............11 5. Processing an Incoming SNMP Message ............................12 5.1. Security Processing for an Incoming Message ...............12 5.2. Elements of Procedure for Incoming Messages ...............13 6. MIB Module Overview ............................................14 6.1. Structure of the MIB Module ...............................14 6.1.1. The snmpTsmStats Subtree ...........................14 6.1.2. The snmpTsmConfiguration Subtree ...................14 6.2. Relationship to Other MIB Modules .........................14 6.2.1. MIB Modules Required for IMPORTS ...................15 7. MIB Module Definition ..........................................15 8. Security Considerations ........................................20 8.1. MIB Module Security .......................................20 9. IANA Considerations ............................................21 10. Acknowledgments ...............................................22
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   11. References ....................................................22
      11.1. Normative References .....................................22
      11.2. Informative References ...................................23
   Appendix A.  Notification Tables Configuration ....................24
     A.1.  Transport Security Model Processing for Notifications .....25
   Appendix B.  Processing Differences between USM and Secure
                Transport ............................................26
     B.1.  USM and the RFC 3411 Architecture .........................26
     B.2.  Transport Subsystem and the RFC 3411 Architecture .........27

1. Introduction

This memo describes a Transport Security Model for the Simple Network Management Protocol for use with secure Transport Models in the Transport Subsystem [RFC5590]. This memo also defines a portion of the Management Information Base (MIB) for monitoring and managing the Transport Security Model for SNMP. It is important to understand the SNMP architecture and the terminology of the architecture to understand where the Transport Security Model described in this memo fits into the architecture and interacts with other subsystems and models within the architecture. It is expected that readers will have also read and understood [RFC3411], [RFC3412], [RFC3413], and [RFC3418].

1.1. The Internet-Standard Management Framework

For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of RFC 3410 [RFC3410]. Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58, RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580 [RFC2580].

1.2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
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   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.

   Authentication in this document typically refers to the English
   meaning of "serving to prove the authenticity of" the message, not
   data source authentication or peer identity authentication.

   The terms "manager" and "agent" are not used in this document
   because, in the RFC 3411 architecture, all SNMP entities have the
   capability of acting as manager, agent, or both depending on the SNMP
   applications included in the engine.  Where distinction is needed,
   the application names of command generator, command responder,
   notification originator, notification receiver, and proxy forwarder
   are used.  See "Simple Network Management Protocol (SNMP)
   Applications" [RFC3413] for further information.

   While security protocols frequently refer to a user, the terminology
   used in [RFC3411] and in this memo is "principal".  A principal is
   the "who" on whose behalf services are provided or processing takes
   place.  A principal can be, among other things, an individual acting
   in a particular role, a set of individuals each acting in a
   particular role, an application or a set of applications, or a
   combination of these within an administrative domain.

1.3. Modularity

The reader is expected to have read and understood the description of the SNMP architecture, as defined in [RFC3411], and the architecture extension specified in "Transport Subsystem for the Simple Network Management Protocol (SNMP)" [RFC5590], which enables the use of external "lower-layer transport" protocols to provide message
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   security.  Transport Models are tied into the SNMP architecture
   through the Transport Subsystem.  The Transport Security Model is
   designed to work with such lower-layer, secure Transport Models.

   In keeping with the RFC 3411 design decisions to use self-contained
   documents, this memo includes the elements of procedure plus
   associated MIB objects that are needed for processing the Transport
   Security Model for SNMP.  These MIB objects SHOULD NOT be referenced
   in other documents.  This allows the Transport Security Model to be
   designed and documented as independent and self-contained, having no
   direct impact on other modules.  It also allows this module to be
   upgraded and supplemented as the need arises, and to move along the
   standards track on different time-lines from other modules.

   This modularity of specification is not meant to be interpreted as
   imposing any specific requirements on implementation.

1.4. Motivation

This memo describes a Security Model to make use of Transport Models that use lower-layer, secure transports and existing and commonly deployed security infrastructures. This Security Model is designed to meet the security and operational needs of network administrators, maximize usability in operational environments to achieve high deployment success, and at the same time minimize implementation and deployment costs to minimize the time until deployment is possible.

1.5. Constraints

The design of this SNMP Security Model is also influenced by the following constraints: 1. In times of network stress, the security protocol and its underlying security mechanisms SHOULD NOT depend solely upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or Authentication, Authorization, and Accounting (AAA) protocols). 2. When the network is not under stress, the Security Model and its underlying security mechanisms MAY depend upon the ready availability of other network services. 3. It might not be possible for the Security Model to determine when the network is under stress. 4. A Security Model SHOULD NOT require changes to the SNMP architecture.
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   5.  A Security Model SHOULD NOT require changes to the underlying
       security protocol.

2. How the Transport Security Model Fits in the Architecture

The Transport Security Model is designed to fit into the RFC 3411 architecture as a Security Model in the Security Subsystem and to utilize the services of a secure Transport Model. For incoming messages, a secure Transport Model will pass a tmStateReference cache, described in [RFC5590]. To maintain RFC 3411 modularity, the Transport Model will not know which securityModel will process the incoming message; the Message Processing Model will determine this. If the Transport Security Model is used with a non- secure Transport Model, then the cache will not exist or will not be populated with security parameters, which will cause the Transport Security Model to return an error (see Section 5.2). The Transport Security Model will create the securityName and securityLevel to be passed to applications, and will verify that the tmTransportSecurityLevel reported by the Transport Model is at least as strong as the securityLevel requested by the Message Processing Model. For outgoing messages, the Transport Security Model will create a tmStateReference cache (or use an existing one), and will pass the tmStateReference to the specified Transport Model.

2.1. Security Capabilities of this Model

2.1.1. Threats

The Transport Security Model is compatible with the RFC 3411 architecture and provides protection against the threats identified by the RFC 3411 architecture. However, the Transport Security Model does not provide security mechanisms such as authentication and encryption itself. Which threats are addressed and how they are mitigated depends on the Transport Model used. To avoid creating potential security vulnerabilities, operators should configure their system so this Security Model is always used with a Transport Model that provides appropriate security, where "appropriate" for a particular deployment is an administrative decision.
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2.1.2. Security Levels

The RFC 3411 architecture recognizes three levels of security: - without authentication and without privacy (noAuthNoPriv) - with authentication but without privacy (authNoPriv) - with authentication and with privacy (authPriv) The model-independent securityLevel parameter is used to request specific levels of security for outgoing messages and to assert that specific levels of security were applied during the transport and processing of incoming messages. The transport-layer algorithms used to provide security should not be exposed to the Transport Security Model, as the Transport Security Model has no mechanisms by which it can test whether an assertion made by a Transport Model is accurate. The Transport Security Model trusts that the underlying secure transport connection has been properly configured to support security characteristics at least as strong as reported in tmTransportSecurityLevel.

2.2. Transport Sessions

The Transport Security Model does not work with transport sessions directly. Instead the transport-related state is associated with a unique combination of transportDomain, transportAddress, securityName, and securityLevel, and is referenced via the tmStateReference parameter. How and if this is mapped to a particular transport or channel is the responsibility of the Transport Subsystem.

2.3. Coexistence

In the RFC 3411 architecture, a Message Processing Model determines which Security Model SHALL be called. As of this writing, IANA has registered four Message Processing Models (SNMPv1, SNMPv2c, SNMPv2u/ SNMPv2*, and SNMPv3) and three other Security Models (SNMPv1, SNMPv2c, and the User-based Security Model).

2.3.1. Coexistence with Message Processing Models

The SNMPv1 and SNMPv2c message processing described in BCP 74 [RFC3584] always selects the SNMPv1(1) and SNMPv2c(2) Security Models. Since there is no mechanism defined in RFC 3584 to select an
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   alternative Security Model, SNMPv1 and SNMPv2c messages cannot use
   the Transport Security Model.  Messages might still be able to be
   conveyed over a secure transport protocol, but the Transport Security
   Model will not be invoked.

   The SNMPv2u/SNMPv2* Message Processing Model is an historic artifact
   for which there is no existing IETF specification.

   The SNMPv3 message processing defined in [RFC3412] extracts the
   securityModel from the msgSecurityModel field of an incoming
   SNMPv3Message.  When this value is transportSecurityModel(4),
   security processing is directed to the Transport Security Model.  For
   an outgoing message to be secured using the Transport Security Model,
   the application MUST specify a securityModel parameter value of
   transportSecurityModel(4) in the sendPdu Abstract Service Interface
   (ASI).

2.3.2. Coexistence with Other Security Models

The Transport Security Model uses its own MIB module for processing to maintain independence from other Security Models. This allows the Transport Security Model to coexist with other Security Models, such as the User-based Security Model (USM) [RFC3414].

2.3.3. Coexistence with Transport Models

The Transport Security Model (TSM) MAY work with multiple Transport Models, but the RFC 3411 Abstract Service Interfaces (ASIs) do not carry a value for the Transport Model. The MIB module defined in this memo allows an administrator to configure whether or not TSM prepends a Transport Model prefix to the securityName. This will allow SNMP applications to consider Transport Model as a factor when making decisions, such as access control, notification generation, and proxy forwarding. To have SNMP properly utilize the security services coordinated by the Transport Security Model, this Security Model MUST only be used with Transport Models that know how to process a tmStateReference, such as the Secure Shell Transport Model [RFC5592].

3. 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. "Transport Subsystem for the Simple Network Management Protocol (SNMP)" [RFC5590] defines general requirements for caches and references.
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   This document defines additional cache requirements related to the
   Transport Security Model.

3.1. Transport Security Model Cached Information

The Transport Security Model has specific responsibilities regarding the cached information.

3.1.1. securityStateReference

The Transport Security Model adds the tmStateReference received from the processIncomingMsg ASI to the securityStateReference. This tmStateReference can then be retrieved during the generateResponseMsg ASI so that it can be passed back to the Transport Model.

3.1.2. tmStateReference

For outgoing messages, the Transport Security Model uses parameters provided by the SNMP application to look up or create a tmStateReference. For the Transport Security Model, the security parameters used for a response MUST be the same as those used for the corresponding request. This Security Model uses the tmStateReference stored as part of the securityStateReference when appropriate. For responses and reports, this Security Model sets the tmSameSecurity flag to true in the tmStateReference before passing it to a Transport Model. For incoming messages, the Transport Security Model uses parameters provided in the tmStateReference cache to establish a securityName, and to verify adequate security levels.

3.1.3. Prefixes and securityNames

The SNMP-VIEW-BASED-ACM-MIB module [RFC3415], the SNMP-TARGET-MIB module [RFC3413], and other MIB modules contain objects to configure security parameters for use by applications such as access control, notification generation, and proxy forwarding. Transport domains and their corresponding prefixes are coordinated via the IANA registry "SNMP Transport Domains". If snmpTsmConfigurationUsePrefix is set to true, then all securityNames provided by, or provided to, the Transport Security Model MUST include a valid transport domain prefix.
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   If snmpTsmConfigurationUsePrefix is set to false, then all
   securityNames provided by, or provided to, the Transport Security
   Model MUST NOT include a transport domain prefix.

   The tmSecurityName in the tmStateReference stored as part of the
   securityStateReference does not contain a prefix.

4. Processing an Outgoing Message

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

4.1. Security Processing for an Outgoing Message

This section describes the procedure followed by the Transport Security Model. The parameters needed for generating a message are supplied to the Security Model by the Message Processing Model via the generateRequestMsg() or the generateResponseMsg() ASI. The Transport Subsystem architectural extension has added the transportDomain, transportAddress, and tmStateReference parameters to the original RFC 3411 ASIs. statusInformation = -- success or errorIndication generateRequestMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN transportDomain -- (NEW) specified by application IN transportAddress -- (NEW) specified by application 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) transport info ) statusInformation = -- success or errorIndication generateResponseMsg( IN messageProcessingModel -- typically, SNMP version
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          IN   globalData              -- message header, admin data
          IN   maxMessageSize          -- of the sending SNMP entity
          IN   transportDomain         -- (NEW) specified by application
          IN   transportAddress        -- (NEW) specified by application
          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
          IN   securityStateReference  -- reference to security state
                                       -- information from original
                                       -- request
          OUT  securityParameters      -- filled in by Security Module
          OUT  wholeMsg                -- complete generated message
          OUT  wholeMsgLength          -- length of generated message
          OUT  tmStateReference        -- (NEW) transport info
               )

4.2. Elements of Procedure for Outgoing Messages

1. If there is a securityStateReference (Response or Report message), then this Security Model uses the cached information rather than the information provided by the ASI. Extract the tmStateReference from the securityStateReference cache. Set the tmRequestedSecurityLevel to the value of the extracted tmTransportSecurityLevel. Set the tmSameSecurity parameter in the tmStateReference cache to true. The cachedSecurityData for this message can now be discarded. 2. If there is no securityStateReference (e.g., a Request-type or Notification message), then create a tmStateReference cache. Set tmTransportDomain to the value of transportDomain, tmTransportAddress to the value of transportAddress, and tmRequestedSecurityLevel to the value of securityLevel. (Implementers might optimize by pointing to saved copies of these session-specific values.) Set the transaction-specific tmSameSecurity parameter to false. If the snmpTsmConfigurationUsePrefix object is set to false, then set tmSecurityName to the value of securityName. If the snmpTsmConfigurationUsePrefix object is set to true, then use the transportDomain to look up the corresponding prefix. (Since the securityStateReference stores the tmStateReference with the tmSecurityName for the incoming message, and since tmSecurityName never has a prefix, the prefix-stripping step only occurs when we are not using the securityStateReference).
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          If the prefix lookup fails for any reason, then the
          snmpTsmUnknownPrefixes counter is incremented, an error
          indication is returned to the calling module, and message
          processing stops.

          If the lookup succeeds, but there is no prefix in the
          securityName, or the prefix returned does not match the prefix
          in the securityName, or the length of the prefix is less than
          1 or greater than 4 US-ASCII alpha-numeric characters, then
          the snmpTsmInvalidPrefixes counter is incremented, an error
          indication is returned to the calling module, and message
          processing stops.

          Strip the transport-specific prefix and trailing ':' character
          (US-ASCII 0x3a) from the securityName.  Set tmSecurityName to
          the value of securityName.

   3.  Set securityParameters to a zero-length OCTET STRING ('0400').

   4.  Combine the message parts into a wholeMsg and calculate
       wholeMsgLength.

   5.  The wholeMsg, wholeMsgLength, securityParameters, and
       tmStateReference are returned to the calling Message Processing
       Model with the statusInformation set to success.

5. Processing an Incoming SNMP Message

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

5.1. Security Processing for an Incoming Message

This section describes the procedure followed by the Transport Security Model whenever it receives an incoming message from a Message Processing Model. The ASI from a Message Processing Model to the Security Subsystem for a received message is: statusInformation = -- errorIndication or success -- error counter OID/value if error processIncomingMsg( IN messageProcessingModel -- typically, SNMP version IN maxMessageSize -- from the received message IN securityParameters -- from the received message IN securityModel -- from the received message IN securityLevel -- from the received message
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   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

5.2. Elements of Procedure for Incoming Messages

1. Set the securityEngineID to the local snmpEngineID. 2. If tmStateReference does not refer to a cache containing values for tmTransportDomain, tmTransportAddress, tmSecurityName, and tmTransportSecurityLevel, then the snmpTsmInvalidCaches counter is incremented, an error indication is returned to the calling module, and Security Model processing stops for this message. 3. Copy the tmSecurityName to securityName. If the snmpTsmConfigurationUsePrefix object is set to true, then use the tmTransportDomain to look up the corresponding prefix. If the prefix lookup fails for any reason, then the snmpTsmUnknownPrefixes counter is incremented, an error indication is returned to the calling module, and message processing stops. If the lookup succeeds but the prefix length is less than 1 or greater than 4 octets, then the snmpTsmInvalidPrefixes counter is incremented, an error indication is returned to the calling module, and message processing stops. Set the securityName to be the concatenation of the prefix, a ':' character (US-ASCII 0x3a), and the tmSecurityName. 4. Compare the value of tmTransportSecurityLevel in the tmStateReference cache to the value of the securityLevel parameter passed in the processIncomingMsg ASI. If securityLevel specifies privacy (Priv) and tmTransportSecurityLevel specifies no privacy (noPriv), or if securityLevel specifies authentication (auth) and tmTransportSecurityLevel specifies no authentication (noAuth) was provided by the Transport Model, then the snmpTsmInadequateSecurityLevels counter is incremented, an error indication (unsupportedSecurityLevel) together with the OID and
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       value of the incremented counter is returned to the calling
       module, and Transport Security Model processing stops for this
       message.

   5.  The tmStateReference is cached as cachedSecurityData so that a
       possible response to this message will use the same security
       parameters.  Then securityStateReference is set for subsequent
       references to this cached data.

   6.  The scopedPDU component is extracted from the wholeMsg.

   7.  The maxSizeResponseScopedPDU is calculated.  This is the maximum
       size allowed for a scopedPDU for a possible Response message.

   8.  The statusInformation is set to success and a return is made to
       the calling module passing back the OUT parameters as specified
       in the processIncomingMsg ASI.

6. MIB Module Overview

This MIB module provides objects for use only by the Transport Security Model. It defines a configuration scalar and related error counters.

6.1. Structure of the MIB Module

Objects in this MIB module are arranged into subtrees. Each subtree is organized as a set of related objects. The overall structure and assignment of objects to their subtrees, and the intended purpose of each subtree, is shown below.

6.1.1. The snmpTsmStats Subtree

This subtree contains error counters specific to the Transport Security Model.

6.1.2. The snmpTsmConfiguration Subtree

This subtree contains a configuration object that enables administrators to specify if they want a transport domain prefix prepended to securityNames for use by applications.

6.2. Relationship to Other MIB Modules

Some management objects defined in other MIB modules are applicable to an entity implementing the Transport Security Model. In particular, it is assumed that an entity implementing the Transport Security Model will implement the SNMP-FRAMEWORK-MIB [RFC3411], the
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   SNMP-TARGET-MIB [RFC3413], the SNMP-VIEW-BASED-ACM-MIB [RFC3415], and
   the SNMPv2-MIB [RFC3418].  These are not needed to implement the
   SNMP-TSM-MIB.

6.2.1. MIB Modules Required for IMPORTS

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

7. MIB Module Definition

SNMP-TSM-MIB DEFINITIONS ::= BEGIN IMPORTS MODULE-IDENTITY, OBJECT-TYPE, mib-2, Counter32 FROM SNMPv2-SMI -- RFC2578 MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF -- RFC2580 TruthValue FROM SNMPv2-TC -- RFC2579 ; snmpTsmMIB MODULE-IDENTITY LAST-UPDATED "200906090000Z" ORGANIZATION "ISMS Working Group" CONTACT-INFO "WG-EMail: isms@lists.ietf.org Subscribe: isms-request@lists.ietf.org Chairs: Juergen Quittek NEC Europe Ltd. Network Laboratories Kurfuersten-Anlage 36 69115 Heidelberg Germany +49 6221 90511-15 quittek@netlab.nec.de Juergen Schoenwaelder Jacobs University Bremen Campus Ring 1 28725 Bremen Germany +49 421 200-3587 j.schoenwaelder@jacobs-university.de
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                  Editor:
                    David Harrington
                    Huawei Technologies USA
                    1700 Alma Dr.
                    Plano TX 75075
                    USA
                    +1 603-436-8634
                    ietfdbh@comcast.net

                    Wes Hardaker
                    Cobham Analytic Solutions
                    P.O. Box 382
                    Davis, CA  95617
                    USA
                    +1 530 792 1913
                    ietf@hardakers.net
                 "
    DESCRIPTION
       "The Transport Security Model MIB.

        In keeping with the RFC 3411 design decisions to use
        self-contained documents, the RFC that contains the definition
        of this MIB module also includes the elements of procedure
        that are needed for processing the Transport Security Model
        for SNMP.  These MIB objects SHOULD NOT be modified via other
        subsystems or models defined in other documents.  This allows
        the Transport Security Model for SNMP to be designed and
        documented as independent and self-contained, having no direct
        impact on other modules, and this allows this module to be
        upgraded and supplemented as the need arises, and to move
        along the standards track on different time-lines from other
        modules.

        Copyright (c) 2009 IETF Trust and the persons
        identified as authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, are permitted provided that the
        following conditions are met:

        - Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        - Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials
          provided with the distribution.
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        - Neither the name of Internet Society, IETF or IETF Trust,
          nor the names of specific contributors, may be used to endorse
          or promote products derived from this software without
          specific prior written permission.

        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
        CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES,
        INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
        MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
        DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
        CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
        SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
        NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
        LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
        HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
        CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
        OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
        EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

        This version of this MIB module is part of RFC 5591;
        see the RFC itself for full legal notices."

    REVISION    "200906090000Z"
    DESCRIPTION "The initial version, published in RFC 5591."

    ::= { mib-2 190 }

-- ---------------------------------------------------------- --
-- subtrees in the SNMP-TSM-MIB
-- ---------------------------------------------------------- --

snmpTsmNotifications OBJECT IDENTIFIER ::= { snmpTsmMIB 0 }
snmpTsmMIBObjects    OBJECT IDENTIFIER ::= { snmpTsmMIB 1 }
snmpTsmConformance   OBJECT IDENTIFIER ::= { snmpTsmMIB 2 }

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

-- Statistics for the Transport Security Model

snmpTsmStats         OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 1 }

snmpTsmInvalidCaches OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of incoming messages dropped because the
Top   ToC   RFC5591 - Page 18
                 tmStateReference referred to an invalid cache.
                "
    ::= { snmpTsmStats 1 }

snmpTsmInadequateSecurityLevels OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of incoming messages dropped because
                 the securityLevel asserted by the Transport Model was
                 less than the securityLevel requested by the
                 application.
                "
    ::= { snmpTsmStats 2 }

snmpTsmUnknownPrefixes OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of messages dropped because
                 snmpTsmConfigurationUsePrefix was set to true and
                 there is no known prefix for the specified transport
                 domain.
                "
    ::= { snmpTsmStats 3 }

snmpTsmInvalidPrefixes OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of messages dropped because
                 the securityName associated with an outgoing message
                 did not contain a valid transport domain prefix.
                "
    ::= { snmpTsmStats 4 }

-- -------------------------------------------------------------
-- Configuration
-- -------------------------------------------------------------

-- Configuration for the Transport Security Model

snmpTsmConfiguration   OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 2 }

snmpTsmConfigurationUsePrefix OBJECT-TYPE
    SYNTAX      TruthValue
    MAX-ACCESS  read-write
    STATUS      current
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    DESCRIPTION "If this object is set to true, then securityNames
                 passing to and from the application are expected to
                 contain a transport-domain-specific prefix.  If this
                 object is set to true, then a domain-specific prefix
                 will be added by the TSM to the securityName for
                 incoming messages and removed from the securityName
                 when processing outgoing messages.  Transport domains
                 and prefixes are maintained in a registry by IANA.
                 This object SHOULD persist across system reboots.
                "
    DEFVAL { false }
    ::= { snmpTsmConfiguration 1 }

-- -------------------------------------------------------------
-- snmpTsmMIB - Conformance Information
-- -------------------------------------------------------------

snmpTsmCompliances OBJECT IDENTIFIER ::= { snmpTsmConformance 1 }

snmpTsmGroups      OBJECT IDENTIFIER ::= { snmpTsmConformance 2 }

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

snmpTsmCompliance MODULE-COMPLIANCE
    STATUS      current
    DESCRIPTION "The compliance statement for SNMP engines that support
                 the SNMP-TSM-MIB.
                "
    MODULE
        MANDATORY-GROUPS { snmpTsmGroup }
    ::= { snmpTsmCompliances 1 }

-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
snmpTsmGroup OBJECT-GROUP
    OBJECTS {
        snmpTsmInvalidCaches,
        snmpTsmInadequateSecurityLevels,
        snmpTsmUnknownPrefixes,
        snmpTsmInvalidPrefixes,
        snmpTsmConfigurationUsePrefix
    }
    STATUS      current
    DESCRIPTION "A collection of objects for maintaining
                 information of an SNMP engine that implements
Top   ToC   RFC5591 - Page 20
                 the SNMP Transport Security Model.
                "

    ::= { snmpTsmGroups 2 }

END

8. Security Considerations

This document describes a Security Model, compatible with the RFC 3411 architecture, that permits SNMP to utilize security services provided through an SNMP Transport Model. The Transport Security Model relies on Transport Models for mutual authentication, binding of keys, confidentiality, and integrity. The Transport Security Model relies on secure Transport Models to provide an authenticated principal identifier and an assertion of whether authentication and privacy are used during transport. This Security Model SHOULD always be used with Transport Models that provide adequate security, but "adequate security" is a configuration and/or run-time decision of the operator or management application. The security threats and how these threats are mitigated should be covered in detail in the specifications of the Transport Models and the underlying secure transports. An authenticated principal identifier (securityName) is used in SNMP applications for purposes such as access control, notification generation, and proxy forwarding. This Security Model supports multiple Transport Models. Operators might judge some transports to be more secure than others, so this Security Model can be configured to prepend a prefix to the securityName to indicate the Transport Model used to authenticate the principal. Operators can use the prefixed securityName when making application decisions about levels of access.

8.1. MIB Module Security

There are a number of management objects defined in this MIB module with a MAX-ACCESS clause of read-write and/or read-create. Such objects may be considered sensitive or vulnerable in some network environments. The support for SET operations in a non-secure environment without proper protection can have a negative effect on network operations. These are the tables and objects and their sensitivity/vulnerability:
Top   ToC   RFC5591 - Page 21
   o  The snmpTsmConfigurationUsePrefix object could be modified,
      creating a denial of service or authorizing SNMP messages that
      would not have previously been authorized by an Access Control
      Model (e.g., the View-based Access Control Model (VACM)).

   Some of the readable objects in this MIB module (i.e., objects with a
   MAX-ACCESS other than not-accessible) may be considered sensitive or
   vulnerable in some network environments.  It is thus important to
   control even GET and/or NOTIFY access to these objects and possibly
   to even encrypt the values of these objects when sending them over
   the network via SNMP.  These are the tables and objects and their
   sensitivity/vulnerability:

   o  All the counters in this module refer to configuration errors and
      do not expose sensitive information.

   SNMP versions prior to SNMPv3 did not include adequate security.
   Even if the network itself is secure (for example by using IPsec),
   even then, there is no control as to who on the secure network is
   allowed to access and GET/SET (read/change/create/delete) the objects
   in this MIB module.

   It is RECOMMENDED that implementers consider the security features as
   provided by the SNMPv3 framework (see [RFC3410], section 8),
   including full support for the USM and Transport Security Model
   cryptographic mechanisms (for authentication and privacy).

   Further, deployment of SNMP versions prior to SNMPv3 is NOT
   RECOMMENDED.  Instead, it is RECOMMENDED to deploy SNMPv3 and to
   enable cryptographic security.  It is then a customer/operator
   responsibility to ensure that the SNMP entity giving access to an
   instance of this MIB module is properly configured to give access to
   the objects only to those principals (users) that have legitimate
   rights to indeed GET or SET (change/create/delete) them.

9. IANA Considerations

IANA has assigned: 1. An SMI number (190) with a prefix of mib-2 in the MIB module registry for the MIB module in this document. 2. A value (4) to identify the Transport Security Model, in the Security Models registry of the SNMP Number Spaces registry. This results in the following table of values:
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   Value   Description                         References
   -----   -----------                         ----------
     0     reserved for 'any'                  [RFC3411]
     1     reserved for SNMPv1                 [RFC3411]
     2     reserved for SNMPv2c                [RFC3411]
     3     User-Based Security Model (USM)     [RFC3411]
     4     Transport Security Model (TSM)      [RFC5591]

10. Acknowledgments

The editors would like to thank Jeffrey Hutzelman for sharing his SSH insights and Dave Shield for an outstanding job wordsmithing the existing document to improve organization and clarity. Additionally, helpful document reviews were received from Juergen Schoenwaelder.

11. References

11.1. 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. [RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., "Textual Conventions for SMIv2", STD 58, RFC 2579, April 1999. [RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder, "Conformance Statements for SMIv2", STD 58, RFC 2580, April 1999. [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.
Top   ToC   RFC5591 - Page 23
   [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.

   [RFC5590]  Harrington, D. and J. Schoenwaelder, "Transport Subsystem
              for the Simple Network Management Protocol (SNMP)",
              RFC 5590, June 2009.

11.2. Informative References

[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction and Applicability Statements for Internet- Standard Management Framework", RFC 3410, December 2002. [RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3415, December 2002. [RFC3418] Presuhn, R., "Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3418, 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. [RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)", RFC 5592, June 2009.
Top   ToC   RFC5591 - Page 24

Appendix A. Notification Tables Configuration

The SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are used to configure notification originators with the destinations to which notifications should be sent. Most of the configuration is Security-Model-independent and Transport-Model-independent. The values we will use in the examples for the five model-independent security and transport parameters are: transportDomain = snmpSSHDomain transportAddress = 192.0.2.1:5162 securityModel = Transport Security Model securityName = alice securityLevel = authPriv The following example will configure the notification originator to send informs to a notification receiver at 192.0.2.1:5162 using the securityName "alice". "alice" is the name for the recipient from the standpoint of the notification originator and is used for processing access controls before sending a notification. The columns marked with an "*" are the items that are Security-Model- specific or Transport-Model-specific. The configuration for the "alice" settings in the SNMP-VIEW-BASED- ACM-MIB objects are not shown here for brevity. First, we configure which type of notification will be sent for this taglist (toCRTag). In this example, we choose to send an Inform. snmpNotifyTable row: snmpNotifyName CRNotif snmpNotifyTag toCRTag snmpNotifyType inform snmpNotifyStorageType nonVolatile snmpNotifyColumnStatus createAndGo Then we configure a transport address to which notifications associated with this taglist will be sent, and we specify which snmpTargetParamsEntry will be used (toCR) when sending to this transport address.
Top   ToC   RFC5591 - Page 25
          snmpTargetAddrTable row:
             snmpTargetAddrName              toCRAddr
         *   snmpTargetAddrTDomain           snmpSSHDomain
         *   snmpTargetAddrTAddress          192.0.2.1:5162
             snmpTargetAddrTimeout           1500
             snmpTargetAddrRetryCount        3
             snmpTargetAddrTagList           toCRTag
             snmpTargetAddrParams            toCR   (MUST match below)
             snmpTargetAddrStorageType       nonVolatile
             snmpTargetAddrColumnStatus      createAndGo

   Then we configure which principal at the host will receive the
   notifications associated with this taglist.  Here, we choose "alice",
   who uses the Transport Security Model.
         snmpTargetParamsTable row:
             snmpTargetParamsName            toCR
             snmpTargetParamsMPModel         SNMPv3
         *   snmpTargetParamsSecurityModel   TransportSecurityModel
             snmpTargetParamsSecurityName    "alice"
             snmpTargetParamsSecurityLevel   authPriv
             snmpTargetParamsStorageType     nonVolatile
             snmpTargetParamsRowStatus       createAndGo


A.1. Transport Security Model Processing for Notifications

The Transport Security Model is called using the generateRequestMsg() ASI, with the following parameters (those with an * are from the above tables): statusInformation = -- success or errorIndication generateRequestMsg( IN messageProcessingModel -- *snmpTargetParamsMPModel IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN transportDomain -- *snmpTargetAddrTDomain IN transportAddress -- *snmpTargetAddrTAddress IN securityModel -- *snmpTargetParamsSecurityModel IN securityEngineID -- immaterial; TSM will ignore. IN securityName -- snmpTargetParamsSecurityName IN securityLevel -- *snmpTargetParamsSecurityLevel 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 -- reference to transport info )
Top   ToC   RFC5591 - Page 26
   The Transport Security Model will determine the Transport Model based
   on the snmpTargetAddrTDomain.  The selected Transport Model will
   select the appropriate transport connection using the
   tmStateReference cache created from the values of
   snmpTargetAddrTAddress, snmpTargetParamsSecurityName, and
   snmpTargetParamsSecurityLevel.

Appendix B. Processing Differences between USM and Secure Transport

USM and secure transports differ in the processing order and responsibilities within the RFC 3411 architecture. While the steps are the same, they occur in a different order and might be done by different subsystems. The following lists illustrate the difference in the flow and the responsibility for different processing steps for incoming messages when using USM and when using a secure transport. (These lists are simplified for illustrative purposes, and do not represent all details of processing. Transport Models MUST provide the detailed elements of procedure.) With USM, SNMPv1, and SNMPv2c Security Models, security processing starts when the Message Processing Model decodes portions of the ASN.1 message to extract header fields that are used to determine which Security Model will process the message to perform authentication, decryption, timeliness checking, integrity checking, and translation of parameters to model-independent parameters. By comparison, a secure transport performs those security functions on the message, before the ASN.1 is decoded. Step 6 cannot occur until after decryption occurs. Steps 6 and beyond are the same for USM and a secure transport.

B.1. USM and the RFC 3411 Architecture

1) Decode the ASN.1 header (Message Processing Model). 2) Determine the SNMP Security Model and parameters (Message Processing Model). 3) Verify securityLevel (Security Model). 4) Translate parameters to model-independent parameters (Security Model). 5) Authenticate the principal, check message integrity and timeliness, and decrypt the message (Security Model).
Top   ToC   RFC5591 - Page 27
   6) Determine the pduType in the decrypted portions (Message
      Processing Model).

   7) Pass on the decrypted portions with model-independent parameters.

B.2. Transport Subsystem and the RFC 3411 Architecture

1) Authenticate the principal, check integrity and timeliness of the message, and decrypt the message (Transport Model). 2) Translate parameters to model-independent parameters (Transport Model). 3) Decode the ASN.1 header (Message Processing Model). 4) Determine the SNMP Security Model and parameters (Message Processing Model). 5) Verify securityLevel (Security Model). 6) Determine the pduType in the decrypted portions (Message Processing Model). 7) Pass on the decrypted portions with model-independent security parameters. If a message is secured using a secure transport layer, then the Transport Model will provide the translation from the authenticated identity (e.g., an SSH user name) to a human-friendly identifier (tmSecurityName) in step 2. The Security Model will provide a mapping from that identifier to a model-independent securityName.
Top   ToC   RFC5591 - Page 28

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 Wes Hardaker Cobham Analytic Solutions P.O. Box 382 Davis, CA 95617 US Phone: +1 530 792 1913 EMail: ietf@hardakers.net