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RFC 5953


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

Part 1 of 3, p. 1 to 20
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Internet Engineering Task Force (IETF)                       W. Hardaker
Request for Comments: 5953                                  SPARTA, Inc.
Category: Standards Track                                    August 2010
ISSN: 2070-1721


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

Abstract

   This document describes a Transport Model for the Simple Network
   Management Protocol (SNMP), that uses either the Transport Layer
   Security protocol or the Datagram Transport Layer Security (DTLS)
   protocol.  The TLS and DTLS protocols provide authentication and
   privacy services for SNMP applications.  This document describes how
   the TLS Transport Model (TLSTM) implements the needed features of a
   SNMP Transport Subsystem to make this protection possible in an
   interoperable way.

   This Transport Model is designed to meet the security and operational
   needs of network administrators.  It supports the sending of SNMP
   messages over TLS/TCP and DTLS/UDP.  The TLS mode can make use of
   TCP's improved support for larger packet sizes and the DTLS mode
   provides potentially superior operation in environments where a
   connectionless (e.g., UDP) transport is preferred.  Both TLS and DTLS
   integrate well into existing public keying infrastructures.

   This document also defines a portion of the Management Information
   Base (MIB) for use with network management protocols.  In particular,
   it defines objects for managing the TLS Transport Model for SNMP.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5953.

Page 2 
Copyright Notice

   Copyright (c) 2010 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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.

Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions ................................................7
   2. The Transport Layer Security Protocol ...........................8
   3. How the TLSTM Fits into the Transport Subsystem .................8
      3.1. Security Capabilities of this Model .......................10
           3.1.1. Threats ............................................10
           3.1.2. Message Protection .................................11
           3.1.3. (D)TLS Connections .................................12
      3.2. Security Parameter Passing ................................13
      3.3. Notifications and Proxy ...................................13
   4. Elements of the Model ..........................................14
      4.1. X.509 Certificates ........................................14
           4.1.1. Provisioning for the Certificate ...................14
      4.2. (D)TLS Usage ..............................................16
      4.3. SNMP Services .............................................17
           4.3.1. SNMP Services for an Outgoing Message ..............17
           4.3.2. SNMP Services for an Incoming Message ..............18
      4.4. Cached Information and References .........................19
           4.4.1. TLS Transport Model Cached Information .............19

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                  4.4.1.1. tmSecurityName ............................19
                  4.4.1.2. tmSessionID ...............................20
                  4.4.1.3. Session State .............................20
   5. Elements of Procedure ..........................................20
      5.1. Procedures for an Incoming Message ........................20
           5.1.1. DTLS over UDP Processing for Incoming Messages .....21
           5.1.2. Transport Processing for Incoming SNMP Messages ....22
      5.2. Procedures for an Outgoing SNMP Message ...................24
      5.3. Establishing or Accepting a Session .......................25
           5.3.1. Establishing a Session as a Client .................25
           5.3.2. Accepting a Session as a Server ....................27
      5.4. Closing a Session .........................................28
   6. MIB Module Overview ............................................29
      6.1. Structure of the MIB Module ...............................29
      6.2. Textual Conventions .......................................29
      6.3. Statistical Counters ......................................29
      6.4. Configuration Tables ......................................29
           6.4.1. Notifications ......................................30
      6.5. Relationship to Other MIB Modules .........................30
           6.5.1. MIB Modules Required for IMPORTS ...................30
   7. MIB Module Definition ..........................................30
   8. Operational Considerations .....................................53
      8.1. Sessions ..................................................53
      8.2. Notification Receiver Credential Selection ................54
      8.3. contextEngineID Discovery .................................54
      8.4. Transport Considerations ..................................55
   9. Security Considerations ........................................55
      9.1. Certificates, Authentication, and Authorization ...........55
      9.2. (D)TLS Security Considerations ............................56
           9.2.1. TLS Version Requirements ...........................56
           9.2.2. Perfect Forward Secrecy ............................56
      9.3. Use with SNMPv1/SNMPv2c Messages ..........................56
      9.4. MIB Module Security .......................................57
   10. IANA Considerations ...........................................58
   11. Acknowledgements ..............................................59
   12. References ....................................................60
      12.1. Normative References .....................................60
      12.2. Informative References ...................................61
   Appendix A.  Target and Notification Configuration Example ........63
     A.1.  Configuring a Notification Originator .....................63
     A.2.  Configuring TLSTM to Utilize a Simple Derivation of
           tmSecurityName ............................................64
     A.3.  Configuring TLSTM to Utilize Table-Driven Certificate
           Mapping ...................................................64

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1.  Introduction

   It is important to understand the modular SNMPv3 architecture as
   defined by [RFC3411] and enhanced by the Transport Subsystem
   [RFC5590].  It is also important to understand the terminology of the
   SNMPv3 architecture in order to understand where the Transport Model
   described in this document fits into the architecture and how it
   interacts with the other architecture subsystems.  For a detailed
   overview of the documents that describe the current Internet-Standard
   Management Framework, please refer to Section 7 of [RFC3410].

   This document describes a Transport Model that makes use of the
   Transport Layer Security (TLS) [RFC5246] and the Datagram Transport
   Layer Security (DTLS) Protocol [RFC4347], within a Transport
   Subsystem [RFC5590].  DTLS is the datagram variant of the Transport
   Layer Security (TLS) protocol [RFC5246].  The Transport Model in this
   document is referred to as the Transport Layer Security Transport
   Model (TLSTM).  TLS and DTLS take advantage of the X.509 public
   keying infrastructure [RFC5280].  While (D)TLS supports multiple
   authentication mechanisms, this document only discusses X.509
   certificate-based authentication.  Although other forms of
   authentication are possible, they are outside the scope of this
   specification.  This transport model is designed to meet the security
   and operational needs of network administrators, operating in both
   environments where a connectionless (e.g., UDP) transport is
   preferred and in environments where large quantities of data need to
   be sent (e.g., over a TCP-based stream).  Both TLS and DTLS integrate
   well into existing public keying infrastructures.  This document
   supports sending of SNMP messages over TLS/TCP and DTLS/UDP.

   This document also defines a portion of the Management Information
   Base (MIB) for use with network management protocols.  In particular,
   it defines objects for managing the TLS Transport Model for SNMP.

   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:
   [RFC2578], [RFC2579], and [RFC2580].

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   The diagram shown below gives a conceptual overview of two SNMP
   entities communicating using the TLS Transport Model (shown as
   "TLSTM").  One entity contains a command responder and notification
   originator application, and the other a command generator and
   notification receiver application.  It should be understood that this
   particular mix of application types is an example only and other
   combinations are equally valid.

   Note: this diagram shows the Transport Security Model (TSM) being
   used as the security model that is defined in [RFC5591].

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 +---------------------------------------------------------------------+
 |                              Network                                |
 +---------------------------------------------------------------------+
     ^                     |            ^               |
     |Notifications        |Commands    |Commands       |Notifications
 +---|---------------------|-------+ +--|---------------|--------------+
 |   |                     V       | |  |               V              |
 | +------------+  +------------+  | | +-----------+   +----------+    |
 | |  (D)TLS    |  |  (D)TLS    |  | | | (D)TLS    |   | (D)TLS   |    |
 | |  (Client)  |  |  (Server)  |  | | | (Client)  |   | (Server) |    |
 | +------------+  +------------+  | | +-----------+   +----------+    |
 |       ^             ^           | |       ^              ^          |
 |       |             |           | |       |              |          |
 |       +-------------+           | |       +--------------+          |
 | +-----|------------+            | | +-----|------------+            |
 | |     V            |            | | |     V            |            |
 | | +--------+       |   +-----+  | | | +--------+       |   +-----+  |
 | | | TLS TM |<--------->|Cache|  | | | | TLS TM |<--------->|Cache|  |
 | | +--------+       |   +-----+  | | | +--------+       |   +-----+  |
 | |Transport Subsys. |      ^     | | |Transport Subsys. |      ^     |
 | +------------------+      |     | | +------------------+      |     |
 |    ^                      |     | |    ^                      |     |
 |    |                      +--+  | |    |                      +--+  |
 |    v                         |  | |    V                         |  |
 | +-----+ +--------+ +-------+ |  | | +-----+ +--------+ +-------+ |  |
 | |     | |Message | |Securi.| |  | | |     | |Message | |Securi.| |  |
 | |Disp.| |Proc.   | |Subsys.| |  | | |Disp.| |Proc.   | |Subsys.| |  |
 | |     | |Subsys. | |       | |  | | |     | |Subsys. | |       | |  |
 | |     | |        | |       | |  | | |     | |        | |       | |  |
 | |     | | +----+ | | +---+ | |  | | |     | | +----+ | | +---+ | |  |
 | |    <--->|v3MP|<--> |TSM|<--+  | | |    <--->|v3MP|<--->|TSM|<--+  |
 | |     | | +----+ | | +---+ |    | | |     | | +----+ | | +---+ |    |
 | |     | |        | |       |    | | |     | |        | |       |    |
 | +-----+ +--------+ +-------+    | | +-----+ +--------+ +-------+    |
 |    ^                            | |    ^                            |
 |    |                            | |    |                            |
 |    +-+------------+             | |    +-+----------+               |
 |      |            |             | |      |          |               |
 |      v            v             | |      v          V               |
 | +-------------+ +-------------+ | | +-------------+ +-------------+ |
 | |   COMMAND   | | NOTIFICAT.  | | | |  COMMAND    | | NOTIFICAT.  | |
 | |  RESPONDER  | | ORIGINATOR  | | | | GENERATOR   | | RECEIVER    | |
 | | application | | application | | | | application | | application | |
 | +-------------+ +-------------+ | | +-------------+ +-------------+ |
 |                     SNMP entity | |                     SNMP entity |
 +---------------------------------+ +---------------------------------+

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1.1.  Conventions

   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.  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 a 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 [RFC3411] architecture, all SNMP entities have the
   capability of acting as manager, agent, or both depending on the SNMP
   application types supported in the implementation.  Where distinction
   is required, the application names of command generator, command
   responder, notification originator, notification receiver, and proxy
   forwarder are used.  See "SNMP Applications" [RFC3413] for further
   information.

   Large portions of this document simultaneously refer to both TLS and
   DTLS when discussing TLSTM components that function equally with
   either protocol.  "(D)TLS" is used in these places to indicate that
   the statement applies to either or both protocols as appropriate.
   When a distinction between the protocols is needed, they are referred
   to independently through the use of "TLS" or "DTLS".  The Transport
   Model, however, is named "TLS Transport Model" and refers not to the
   TLS or DTLS protocol but to the specification in this document, which
   includes support for both TLS and DTLS.

   Throughout this document, the terms "client" and "server" are used to
   refer to the two ends of the (D)TLS transport connection.  The client
   actively opens the (D)TLS connection, and the server passively
   listens for the incoming (D)TLS connection.  An SNMP entity may act
   as a (D)TLS client or server or both, depending on the SNMP
   applications supported.

   The User-Based Security Model (USM) [RFC3414] is a mandatory-to-
   implement Security Model in STD 62.  While (D)TLS and USM frequently
   refer to a user, the terminology preferred in RFC 3411 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, with each acting in a particular role; an application
   or a set of applications, or a combination of these within an
   administrative domain.

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   Throughout this document, the term "session" is used to refer to a
   secure association between two TLS Transport Models that permits the
   transmission of one or more SNMP messages within the lifetime of the
   session.  The (D)TLS protocols also have an internal notion of a
   session and although these two concepts of a session are related,
   when the term "session" is used this document is referring to the
   TLSTM's specific session and not directly to the (D)TLS protocol's
   session.

   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].

2.  The Transport Layer Security Protocol

   (D)TLS provides authentication, data message integrity, and privacy
   at the transport layer (see [RFC4347]).

   The primary goals of the TLS Transport Model are to provide privacy,
   peer identity authentication and data integrity between two
   communicating SNMP entities.  The TLS and DTLS protocols provide a
   secure transport upon which the TLSTM is based.  Please refer to
   [RFC5246] and [RFC4347] for complete descriptions of the protocols.

3.  How the TLSTM Fits into the Transport Subsystem

   A transport model is a component of the Transport Subsystem.  The TLS
   Transport Model thus fits between the underlying (D)TLS transport
   layer and the Message Dispatcher [RFC3411] component of the SNMP
   engine.

   The TLS Transport Model will establish a session between itself and
   the TLS Transport Model of another SNMP engine.  The sending
   transport model passes unencrypted and unauthenticated messages from
   the Dispatcher to (D)TLS to be encrypted and authenticated, and the
   receiving transport model accepts decrypted and authenticated/
   integrity-checked incoming messages from (D)TLS and passes them to
   the Dispatcher.

   After a TLS Transport Model session is established, SNMP messages can
   conceptually be sent through the session from one SNMP message
   Dispatcher to another SNMP Message Dispatcher.  If multiple SNMP
   messages are needed to be passed between two SNMP applications they
   MAY be passed through the same session.  A TLSTM implementation
   engine MAY choose to close the session to conserve resources.

Top      ToC       Page 9 
   The TLS Transport Model of an SNMP engine will perform the
   translation between (D)TLS-specific security parameters and SNMP-
   specific, model-independent parameters.

   The diagram below depicts where the TLS Transport Model (shown as
   "(D)TLS TM") fits into the architecture described in RFC 3411 and the
   Transport Subsystem:

   +------------------------------+
   |    Network                   |
   +------------------------------+
      ^       ^              ^
      |       |              |
      v       v              v
   +-------------------------------------------------------------------+
   | +--------------------------------------------------+              |
   | |  Transport Subsystem                             |  +--------+  |
   | | +-----+ +-----+ +-------+             +-------+  |  |        |  |
   | | | UDP | | SSH | |(D)TLS |    . . .    | other |<--->| Cache  |  |
   | | |     | | TM  | | TM    |             |       |  |  |        |  |
   | | +-----+ +-----+ +-------+             +-------+  |  +--------+  |
   | +--------------------------------------------------+         ^    |
   |              ^                                               |    |
   |              |                                               |    |
   | 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          |

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   | +-------------+   +---------+   +--------------+  +-------------+ |
   | |   COMMAND   |   | ACCESS  |   | NOTIFICATION |  |    PROXY    | |
   | |  RESPONDER  |<->| CONTROL |<->|  ORIGINATOR  |  |  FORWARDER  | |
   | | application |   |         |   | applications |  | application | |
   | +-------------+   +---------+   +--------------+  +-------------+ |
   |      ^                                 ^                          |
   |      |                                 |                          |
   |      v                                 v                          |
   | +----------------------------------------------+                  |
   | |             MIB instrumentation              |      SNMP entity |
   +-------------------------------------------------------------------+

3.1.  Security Capabilities of this Model

3.1.1.  Threats

   The TLS Transport Model provides protection against the threats
   identified by the RFC 3411 architecture [RFC3411]:

   1.  Modification of Information - The modification threat is the
       danger that an unauthorized entity may alter in-transit SNMP
       messages generated on behalf of an authorized principal in such a
       way as to effect unauthorized management operations, including
       falsifying the value of an object.

       (D)TLS provides verification that the content of each received
       message has not been modified during its transmission through the
       network, data has not been altered or destroyed in an
       unauthorized manner, and data sequences have not been altered to
       an extent greater than can occur non-maliciously.

   2.  Masquerade - The masquerade threat is the danger that management
       operations unauthorized for a given principal may be attempted by
       assuming the identity of another principal that has the
       appropriate authorizations.

       The TLSTM verifies the identity of the (D)TLS server through the
       use of the (D)TLS protocol and X.509 certificates.  A TLS
       Transport Model implementation MUST support the authentication of
       both the server and the client.

   3.  Message stream modification - The re-ordering, delay, or replay
       of messages can and does occur through the natural operation of
       many connectionless transport services.  The message stream
       modification threat is the danger that messages may be
       maliciously re-ordered, delayed or replayed to an extent that is

Top      ToC       Page 11 
       greater than can occur through the natural operation of
       connectionless transport services, in order to effect
       unauthorized management operations.

       (D)TLS provides replay protection with a Message Authentication
       Code (MAC) that includes a sequence number.  Since UDP provides
       no sequencing ability, DTLS uses a sliding window protocol with
       the sequence number used for replay protection (see [RFC4347]).

   4.  Disclosure - The disclosure threat is the danger of eavesdropping
       on the exchanges between SNMP engines.

       (D)TLS provides protection against the disclosure of information
       to unauthorized recipients or eavesdroppers by allowing for
       encryption of all traffic between SNMP engines.  A TLS Transport
       Model implementation MUST support message encryption to protect
       sensitive data from eavesdropping attacks.

   5.  Denial of Service - the RFC 3411 architecture [RFC3411] states
       that denial-of-service (DoS) attacks need not be addressed by an
       SNMP security protocol.  However, connectionless transports (like
       DTLS over UDP) are susceptible to a variety of DoS attacks
       because they are more vulnerable to spoofed IP addresses.  See
       Section 4.2 for details on how the cookie mechanism is used.
       Note, however, that this mechanism does not provide any defense
       against DoS attacks mounted from valid IP addresses.

   See Section 9 for more detail on the security considerations
   associated with the TLSTM and these security threats.

3.1.2.  Message Protection

   The RFC 3411 architecture recognizes three levels of security:

   o  without authentication and without privacy (noAuthNoPriv)

   o  with authentication but without privacy (authNoPriv)

   o  with authentication and with privacy (authPriv)

   The TLS Transport Model determines from (D)TLS the identity of the
   authenticated principal, the transport type and the transport address
   associated with an incoming message.  The TLS Transport Model
   provides the identity and destination type and address to (D)TLS for
   outgoing messages.

Top      ToC       Page 12 
   When an application requests a session for a message, it also
   requests a security level for that session.  The TLS Transport Model
   MUST ensure that the (D)TLS connection provides security at least as
   high as the requested level of security.  How the security level is
   translated into the algorithms used to provide data integrity and
   privacy is implementation dependent.  However, the NULL integrity and
   encryption algorithms MUST NOT be used to fulfill security level
   requests for authentication or privacy.  Implementations MAY choose
   to force (D)TLS to only allow cipher_suites that provide both
   authentication and privacy to guarantee this assertion.

   If a suitable interface between the TLS Transport Model and the
   (D)TLS Handshake Protocol is implemented to allow the selection of
   security-level-dependent algorithms (for example, a security level to
   cipher_suites mapping table), then different security levels may be
   utilized by the application.

   The authentication, integrity, and privacy algorithms used by the
   (D)TLS Protocols may vary over time as the science of cryptography
   continues to evolve and the development of (D)TLS continues over
   time.  Implementers are encouraged to plan for changes in operator
   trust of particular algorithms.  Implementations SHOULD offer
   configuration settings for mapping algorithms to SNMPv3 security
   levels.

3.1.3.  (D)TLS Connections

   (D)TLS connections are opened by the TLS Transport Model during the
   elements of procedure for an outgoing SNMP message.  Since the sender
   of a message initiates the creation of a (D)TLS connection if needed,
   the (D)TLS connection will already exist for an incoming message.

   Implementations MAY choose to instantiate (D)TLS connections in
   anticipation of outgoing messages.  This approach might be useful to
   ensure that a (D)TLS connection to a given target can be established
   before it becomes important to send a message over the (D)TLS
   connection.  Of course, there is no guarantee that a pre-established
   session will still be valid when needed.

   DTLS connections, when used over UDP, are uniquely identified within
   the TLS Transport Model by the combination of transportDomain,
   transportAddress, tmSecurityName, and requestedSecurityLevel
   associated with each session.  Each unique combination of these
   parameters MUST have a locally chosen unique tlstmSessionID for each
   active session.  For further information, see Section 5.  TLS over
   TCP sessions, on the other hand, do not require a unique pairing of

Top      ToC       Page 13 
   address and port attributes since their lower-layer protocols (TCP)
   already provide adequate session framing.  But they must still
   provide a unique tlstmSessionID for referencing the session.

   The tlstmSessionID MUST NOT change during the entire duration of the
   session from the TLSTM's perspective, and MUST uniquely identify a
   single session.  As an implementation hint: note that the (D)TLS
   internal SessionID does not meet these requirements, since it can
   change over the life of the connection as seen by the TLSTM (for
   example, during renegotiation), and does not necessarily uniquely
   identify a TLSTM session (there can be multiple TLSTM sessions
   sharing the same D(TLS) internal SessionID).

3.2.  Security Parameter Passing

   For the (D)TLS server-side, (D)TLS-specific security parameters
   (i.e., cipher_suites, X.509 certificate fields, IP addresses, and
   ports) are translated by the TLS Transport Model into security
   parameters for the TLS Transport Model and security model (e.g.,
   tmSecurityLevel, tmSecurityName, transportDomain, transportAddress).
   The transport-related and (D)TLS-security-related information,
   including the authenticated identity, are stored in a cache
   referenced by tmStateReference.

   For the (D)TLS client side, the TLS Transport Model takes input
   provided by the Dispatcher in the sendMessage() Abstract Service
   Interface (ASI) and input from the tmStateReference cache.  The
   (D)TLS Transport Model converts that information into suitable
   security parameters for (D)TLS and establishes sessions as needed.

   The elements of procedure in Section 5 discuss these concepts in much
   greater detail.

3.3.  Notifications and Proxy

   (D)TLS connections may be initiated by (D)TLS clients on behalf of
   SNMP applications that initiate communications, such as command
   generators, notification originators, proxy forwarders.  Command
   generators are frequently operated by a human, but notification
   originators and proxy forwarders are usually unmanned automated
   processes.  The targets to whom notifications and proxied requests
   should be sent is typically determined and configured by a network
   administrator.

   The SNMP-TARGET-MIB module [RFC3413] contains objects for defining
   management targets, including transportDomain, transportAddress,
   securityName, securityModel, and securityLevel parameters, for
   notification originator, proxy forwarder, and SNMP-controllable

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   command generator applications.  Transport domains and transport
   addresses are configured in the snmpTargetAddrTable, and the
   securityModel, securityName, and securityLevel parameters are
   configured in the snmpTargetParamsTable.  This document defines a MIB
   module that extends the SNMP-TARGET-MIB's snmpTargetParamsTable to
   specify a (D)TLS client-side certificate to use for the connection.

   When configuring a (D)TLS target, the snmpTargetAddrTDomain and
   snmpTargetAddrTAddress parameters in snmpTargetAddrTable SHOULD be
   set to the snmpTLSTCPDomain or snmpDTLSUDPDomain object and an
   appropriate snmpTLSAddress value.  When used with the SNMPv3 message
   processing model, the snmpTargetParamsMPModel column of the
   snmpTargetParamsTable SHOULD be set to a value of 3.  The
   snmpTargetParamsSecurityName SHOULD be set to an appropriate
   securityName value and the snmpTlstmParamsClientFingerprint parameter
   of the snmpTlstmParamsTable SHOULD be set a value that refers to a
   locally held certificate (and the corresponding private key) to be
   used.  Other parameters, for example, cryptographic configuration
   such as which cipher_suites to use, must come from configuration
   mechanisms not defined in this document.

   The securityName defined in the snmpTargetParamsSecurityName column
   will be used by the access control model to authorize any
   notifications that need to be sent.

4.  Elements of the Model

   This section contains definitions required to realize the (D)TLS
   Transport Model defined by this document.

4.1.  X.509 Certificates

   (D)TLS can make use of X.509 certificates for authentication of both
   sides of the transport.  This section discusses the use of X.509
   certificates in the TLSTM.

   While (D)TLS supports multiple authentication mechanisms, this
   document only discusses X.509-certificate-based authentication; other
   forms of authentication are outside the scope of this specification.
   TLSTM implementations are REQUIRED to support X.509 certificates.

4.1.1.  Provisioning for the Certificate

   Authentication using (D)TLS will require that SNMP entities have
   certificates, either signed by trusted Certification Authorities
   (CAs), or self signed.  Furthermore, SNMP entities will most commonly
   need to be provisioned with root certificates that represent the list
   of trusted CAs that an SNMP entity can use for certificate

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   verification.  SNMP entities SHOULD also be provisioned with a X.509
   certificate revocation mechanism which can be used to verify that a
   certificate has not been revoked.  Trusted public keys from either CA
   certificates and/or self-signed certificates MUST be installed into
   the server through a trusted out-of-band mechanism and their
   authenticity MUST be verified before access is granted.

   Having received a certificate from a connecting TLSTM client, the
   authenticated tmSecurityName of the principal is derived using the
   snmpTlstmCertToTSNTable.  This table allows mapping of incoming
   connections to tmSecurityNames through defined transformations.  The
   transformations defined in the SNMP-TLS-TM-MIB include:

   o  Mapping a certificate's subjectAltName or CommonName components to
      a tmSecurityName, or

   o  Mapping a certificate's fingerprint value to a directly specified
      tmSecurityName

   As an implementation hint: implementations may choose to discard any
   connections for which no potential snmpTlstmCertToTSNTable mapping
   exists before performing certificate verification to avoid expending
   computational resources associated with certificate verification.

   Deployments SHOULD map the "subjectAltName" component of X.509
   certificates to the TLSTM specific tmSecurityNames.  The
   authenticated identity can be obtained by the TLS Transport Model by
   extracting the subjectAltName(s) from the peer's certificate.  The
   receiving application will then have an appropriate tmSecurityName
   for use by other SNMPv3 components like an access control model.

   An example of this type of mapping setup can be found in Appendix A.

   This tmSecurityName may be later translated from a TLSTM specific
   tmSecurityName to a SNMP engine securityName by the security model.
   A security model, like the TSM security model [RFC5591], may perform
   an identity mapping or a more complex mapping to derive the
   securityName from the tmSecurityName offered by the TLS Transport
   Model.

   The standard View-Based Access Control Model (VACM) access control
   model constrains securityNames to be 32 octets or less in length.  A
   TLSTM generated tmSecurityName, possibly in combination with a
   messaging or security model that increases the length of the
   securityName, might cause the securityName length to exceed 32
   octets.  For example, a 32-octet tmSecurityName derived from an IPv6
   address, paired with a TSM prefix, will generate a 36-octet

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   securityName.  Such a securityName will not be able to be used with
   standard VACM or TARGET MIB modules.  Operators should be careful to
   select algorithms and subjectAltNames to avoid this situation.

   A pictorial view of the complete transformation process (using the
   TSM security model for the example) is shown below:

    +-------------+     +-------+                   +-----+
    | Certificate |     |       |                   |     |
    |    Path     |     | TLSTM |  tmSecurityName   | TSM |
    | Validation  | --> |       | ----------------->|     |
    +-------------+     +-------+                   +-----+
                                                        |
                                                        | securityName
                                                        V
                                                    +-------------+
                                                    | application |
                                                    +-------------+

4.2.  (D)TLS Usage

   (D)TLS MUST negotiate a cipher_suite that uses X.509 certificates for
   authentication, and MUST authenticate both the client and the server.
   The mandatory-to-implement cipher_suite is specified in the TLS
   specification [RFC5246].

   TLSTM verifies the certificates when the connection is opened (see
   Section 5.3).  For this reason, TLS renegotiation with different
   certificates MUST NOT be done.  That is, implementations MUST either
   disable renegotiation completely (RECOMMENDED), or they MUST present
   the same certificate during renegotiation (and MUST verify that the
   other end presented the same certificate).

   For DTLS over UDP, each SNMP message MUST be placed in a single UDP
   datagram; it MAY be split to multiple DTLS records.  In other words,
   if a single datagram contains multiple DTLS application_data records,
   they are concatenated when received.  The TLSTM implementation SHOULD
   return an error if the SNMP message does not fit in the UDP datagram,
   and thus cannot be sent.

   For DTLS over UDP, the DTLS server implementation MUST support DTLS
   cookies ([RFC4347] already requires that clients support DTLS
   cookies).  Implementations are not required to perform the cookie
   exchange for every DTLS handshake; however, enabling it by default is
   RECOMMENDED.

   For DTLS, replay protection MUST be used.

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4.3.  SNMP Services

   This section describes the services provided by the TLS Transport
   Model with their inputs and outputs.  The services are between the
   Transport Model and the Dispatcher.

   The services are described as primitives of an abstract service
   interface (ASI) and the inputs and outputs are described as abstract
   data elements as they are passed in these abstract service
   primitives.

4.3.1.  SNMP Services for an Outgoing Message

   The Dispatcher passes the information to the TLS Transport Model
   using the ASI defined in the Transport Subsystem:

      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
       )

   The abstract data elements returned from or passed as parameters into
   the abstract service primitives are as follows:

   statusInformation:  An indication of whether the sending of the
      message was successful.  If not, it is an indication of the
      problem.

   destTransportDomain:  The transport domain for the associated
      destTransportAddress.  The Transport Model uses this parameter to
      determine the transport type of the associated
      destTransportAddress.  This document specifies the
      snmpTLSTCPDomain and the snmpDTLSUDPDomain transport domains.

   destTransportAddress:  The transport address of the destination TLS
      Transport Model in a format specified by the SnmpTLSAddress
      TEXTUAL-CONVENTION.

   outgoingMessage:  The outgoing message to send to (D)TLS for
      encapsulation and transmission.

   outgoingMessageLength:  The length of the outgoingMessage.

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   tmStateReference:  A reference used to pass model-specific and
      mechanism-specific parameters between the Transport Subsystem and
      transport-aware Security Models.

4.3.2.  SNMP Services for an Incoming Message

   The TLS Transport Model processes the received message from the
   network using the (D)TLS service and then passes it to the Dispatcher
   using the following ASI:

      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
       )

   The abstract data elements returned from or passed as parameters into
   the abstract service primitives are as follows:

   statusInformation:  An indication of whether the passing of the
      message was successful.  If not, it is an indication of the
      problem.

   transportDomain:  The transport domain for the associated
      transportAddress.  This document specifies the snmpTLSTCPDomain
      and the snmpDTLSUDPDomain transport domains.

   transportAddress:  The transport address of the source of the
      received message in a format specified by the SnmpTLSAddress
      TEXTUAL-CONVENTION.

   incomingMessage:  The whole SNMP message after being processed by
      (D)TLS.

   incomingMessageLength:  The length of the incomingMessage.

   tmStateReference:  A reference used to pass model-specific and
      mechanism-specific parameters between the Transport Subsystem and
      transport-aware Security Models.

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4.4.  Cached Information and References

   When performing SNMP processing, there are two levels of state
   information that may need to be retained: the immediate state linking
   a request-response pair, and potentially longer-term state relating
   to transport and security.  "Transport Subsystem for the Simple
   Network Management Protocol (SNMP)" [RFC5590] defines general
   requirements for caches and references.

4.4.1.  TLS Transport Model Cached Information

   The TLS Transport Model has specific responsibilities regarding the
   cached information.  See the Elements of Procedure in Section 5 for
   detailed processing instructions on the use of the tmStateReference
   fields by the TLS Transport Model.

4.4.1.1.  tmSecurityName

   The tmSecurityName MUST be a human-readable name (in snmpAdminString
   format) representing the identity that has been set according to the
   procedures in Section 5.  The tmSecurityName MUST be constant for all
   traffic passing through a single TLSTM session.  Messages MUST NOT be
   sent through an existing (D)TLS connection that was established using
   a different tmSecurityName.

   On the (D)TLS server side of a connection, the tmSecurityName is
   derived using the procedures described in Section 5.3.2 and the SNMP-
   TLS-TM-MIB's snmpTlstmCertToTSNTable DESCRIPTION clause.

   On the (D)TLS client side of a connection, the tmSecurityName is
   presented to the TLS Transport Model by the security model through
   the tmStateReference.  This tmSecurityName is typically a copy of or
   is derived from the securityName that was passed by application
   (possibly because of configuration specified in the SNMP-TARGET-MIB).
   The Security Model likely derived the tmSecurityName from the
   securityName presented to the Security Model by the application
   (possibly because of configuration specified in the SNMP-TARGET-MIB).

   Transport-Model-aware security models derive tmSecurityName from a
   securityName, possibly configured in MIB modules for notifications
   and access controls.  Transport Models SHOULD use predictable
   tmSecurityNames so operators will know what to use when configuring
   MIB modules that use securityNames derived from tmSecurityNames.  The
   TLSTM generates predictable tmSecurityNames based on the
   configuration found in the SNMP-TLS-TM-MIB's snmpTlstmCertToTSNTable
   and relies on the network operators to have configured this table
   appropriately.

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4.4.1.2.  tmSessionID

   The tmSessionID MUST be recorded per message at the time of receipt.
   When tmSameSecurity is set, the recorded tmSessionID can be used to
   determine whether the (D)TLS connection available for sending a
   corresponding outgoing message is the same (D)TLS connection as was
   used when receiving the incoming message (e.g., a response to a
   request).

4.4.1.3.  Session State

   The per-session state that is referenced by tmStateReference may be
   saved across multiple messages in a Local Configuration Datastore.
   Additional session/connection state information might also be stored
   in a Local Configuration Datastore.



(page 20 continued on part 2)

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