path: root/doc/rfc/rfc7416.txt

Internet Engineering Task Force (IETF)                           T. Tsao
Request for Comments: 7416                                  R. Alexander
Category: Informational            Eaton's Cooper Power Systems Business
ISSN: 2070-1721                                                M. Dohler
                                                                    CTTC
                                                                 V. Daza
                                                               A. Lozano
                                                Universitat Pompeu Fabra
                                                      M. Richardson, Ed.
                                                Sandelman Software Works
                                                            January 2015


                     A Security Threat Analysis for
      the Routing Protocol for Low-Power and Lossy Networks (RPLs)

Abstract

   This document presents a security threat analysis for the Routing
   Protocol for Low-Power and Lossy Networks (RPLs).  The development
   builds upon previous work on routing security and adapts the
   assessments to the issues and constraints specific to low-power and
   lossy networks.  A systematic approach is used in defining and
   evaluating the security threats.  Applicable countermeasures are
   application specific and are addressed in relevant applicability
   statements.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7416.









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Copyright Notice

   Copyright (c) 2015 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
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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Relationship to Other Documents . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Considerations on RPL Security  . . . . . . . . . . . . . . .   5
     4.1.  Routing Assets and Points of Access . . . . . . . . . . .   6
     4.2.  The ISO 7498-2 Security Reference Model . . . . . . . . .   8
     4.3.  Issues Specific to or Amplified in LLNs . . . . . . . . .  10
     4.4.  RPL Security Objectives . . . . . . . . . . . . . . . . .  12
   5.  Threat Sources  . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Threats and Attacks . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Threats Due to Failures to Authenticate . . . . . . . . .  14
       6.1.1.  Node Impersonation  . . . . . . . . . . . . . . . . .  14
       6.1.2.  Dummy Node  . . . . . . . . . . . . . . . . . . . . .  14
       6.1.3.  Node Resource Spam  . . . . . . . . . . . . . . . . .  15
     6.2.  Threats Due to Failure to Keep Routing Information
           Confidential  . . . . . . . . . . . . . . . . . . . . . .  15
       6.2.1.  Routing Exchange Exposure . . . . . . . . . . . . . .  15
       6.2.2.  Routing Information (Routes and Network Topology)
               Exposure  . . . . . . . . . . . . . . . . . . . . . .  15
     6.3.  Threats and Attacks on Integrity  . . . . . . . . . . . .  16
       6.3.1.  Routing Information Manipulation  . . . . . . . . . .  16
       6.3.2.  Node Identity Misappropriation  . . . . . . . . . . .  17
     6.4.  Threats and Attacks on Availability . . . . . . . . . . .  18
       6.4.1.  Routing Exchange Interference or Disruption . . . . .  18
       6.4.2.  Network Traffic Forwarding Disruption . . . . . . . .  18
       6.4.3.  Communications Resource Disruption  . . . . . . . . .  20
       6.4.4.  Node Resource Exhaustion  . . . . . . . . . . . . . .  20







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   7.  Countermeasures . . . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  Confidentiality Attack Countermeasures  . . . . . . . . .  21
       7.1.1.  Countering Deliberate Exposure Attacks  . . . . . . .  21
       7.1.2.  Countering Passive Wiretapping Attacks  . . . . . . .  22
       7.1.3.  Countering Traffic Analysis . . . . . . . . . . . . .  22
       7.1.4.  Countering Remote Device Access Attacks . . . . . . .  23
     7.2.  Integrity Attack Countermeasures  . . . . . . . . . . . .  24
       7.2.1.  Countering Unauthorized Modification Attacks  . . . .  24
       7.2.2.  Countering Overclaiming and Misclaiming Attacks . . .  24
       7.2.3.  Countering Identity (including Sybil) Attacks . . . .  25
       7.2.4.  Countering Routing Information Replay Attacks . . . .  25
       7.2.5.  Countering Byzantine Routing Information Attacks  . .  26
     7.3.  Availability Attack Countermeasures . . . . . . . . . . .  26
       7.3.1.  Countering HELLO Flood Attacks and ACK Spoofing
               Attacks . . . . . . . . . . . . . . . . . . . . . . .  27
       7.3.2.  Countering Overload Attacks . . . . . . . . . . . . .  27
       7.3.3.  Countering Selective Forwarding Attacks . . . . . . .  29
       7.3.4.  Countering Sinkhole Attacks . . . . . . . . . . . . .  29
       7.3.5.  Countering Wormhole Attacks . . . . . . . . . . . . .  30
   8.  RPL Security Features . . . . . . . . . . . . . . . . . . . .  31
     8.1.  Confidentiality Features  . . . . . . . . . . . . . . . .  32
     8.2.  Integrity Features  . . . . . . . . . . . . . . . . . . .  32
     8.3.  Availability Features . . . . . . . . . . . . . . . . . .  33
     8.4.  Key Management  . . . . . . . . . . . . . . . . . . . . .  34
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     10.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Acknowledgments  . . . . . .  . . . . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   In recent times, networked electronic devices have found an
   increasing number of applications in various fields.  Yet, for
   reasons ranging from operational application to economics, these
   wired and wireless devices are often supplied with minimum physical
   resources; the constraints include those on computational resources
   (RAM, clock speed, and storage) and communication resources (duty
   cycle, packet size, etc.) but also form factors that may rule out
   user-access interfaces (e.g., the housing of a small stick-on switch)
   or simply safety considerations (e.g., with gas meters).  As a
   consequence, the resulting networks are more prone to loss of traffic
   and other vulnerabilities.  The proliferation of these Low-Power and
   Lossy Networks (LLNs), however, are drawing efforts to examine and
   address their potential networking challenges.  Securing the
   establishment and maintenance of network connectivity among these
   deployed devices becomes one of these key challenges.



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   This document presents a threat analysis for securing the Routing
   Protocol for LLNs (RPL).  The process requires two steps.  First, the
   analysis will be used to identify pertinent security issues.  The
   second step is to identify necessary countermeasures to secure RPL.
   As there are multiple ways to solve the problem and the specific
   trade-offs are deployment specific, the specific countermeasure to be
   used is detailed in applicability statements.

   This document uses a model based on [ISO.7498-2.1989], which
   describes authentication, access control, data confidentiality, data
   integrity, and non-repudiation security services.  This document
   expands the model to include the concept of availability.  As
   explained below, non-repudiation does not apply to routing protocols.

   Many of the issues in this document were also covered in the IAB
   Smart Object Workshop [RFC6574] and the IAB Smart Object Security
   Workshop [RFC7397].

   This document concerns itself with securing the control-plane
   traffic.  As such, it does not address authorization or
   authentication of application traffic.  RPL uses multicast as part of
   its protocol; therefore, mechanisms that RPL uses to secure this
   traffic might also be applicable to the Multicast Protocol for Low-
   Power and Lossy Networks (MPL) control traffic as well: the important
   part is that the threats are similar.

2.  Relationship to Other Documents

   Routing Over Low-Power and Lossy (ROLL) networks has specified a set
   of routing protocols for LLNs [RFC6550].  A number of applicability
   texts describe a subset of these protocols and the conditions that
   make the subset the correct choice.  The text recommends and
   motivates the accompanying parameter value ranges.  Multiple
   applicability domains are recognized, including Building and Home and
   Advanced Metering Infrastructure.  The applicability domains
   distinguish themselves in the way they are operated, by their
   performance requirements, and by the most probable network
   structures.  Each applicability statement identifies the
   distinguishing properties according to a common set of subjects
   described in as many sections.

   The common set of security threats herein are referred to by the
   applicability statements, and that series of documents describes the
   preferred security settings and solutions within the applicability
   statement conditions.  This applicability statement may recommend
   more lightweight security solutions and specify the conditions under
   which these solutions are appropriate.




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3.  Terminology

   This document adopts the terminology defined in [RFC6550], [RFC4949],
   and [RFC7102].

   The terms "control plane" and "forwarding plane" are used in a manner
   consistent with Section 1 of [RFC6192].

   The term "Destination-Oriented DAG (DODAG)" is from [RFC6550].

   Extensible Authentication Protocol - Transport Layer Security
   (EAP-TLS) is defined in [RFC5216].

   The Protocol for Carrying Authentication for Network Access (PANA) is
   defined in [RFC5191].

   Counter with CBC-MAC (CCM) mode is defined in [RFC3610].

   The term "sleepy node", introduced in [RFC7102], refers to a node
   that may sometimes go into a low-power state, suspending protocol
   communications.

   The terms Service Set Identifier (SSID), Extended Service Set
   Identifier (ESSID), and Personal Area Network (PAN) refer to network
   identifiers, defined in [IEEE.802.11] and [IEEE.802.15.4].

   Although this is not a protocol specification, the key words "MUST",
   "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] in order to
   clarify and emphasize the guidance and directions to implementers and
   deployers of LLN nodes that utilize RPL.

4.  Considerations on RPL Security

   Routing security, in essence, ensures that the routing protocol
   operates correctly.  It entails implementing measures to ensure
   controlled state changes on devices and network elements, both based
   on external inputs (received via communications) or internal inputs
   (physical security of the device itself and parameters maintained by
   the device, including, e.g., clock).  State changes would thereby
   involve not only authorization of the injector's actions,
   authentication of injectors, and potentially confidentiality of
   routing data, but also proper order of state changes through
   timeliness, since seriously delayed state changes, such as commands
   or updates of routing tables, may negatively impact system operation.
   A security assessment can, therefore, begin with a focus on the
   assets [RFC4949] that may be the target of the state changes and the



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   access points in terms of interfaces and protocol exchanges through
   which such changes may occur.  In the case of routing security, the
   focus is directed towards the elements associated with the
   establishment and maintenance of network connectivity.

   This section sets the stage for the development of the analysis by
   applying the systematic approach proposed in [Myagmar2005] to the
   routing security, while also drawing references from other reviews
   and assessments found in the literature, particularly [RFC4593] and
   [Karlof2003] (i.e., selective forwarding, wormhole, and sinkhole
   attacks).  The subsequent subsections begin with a focus on the
   elements of a generic routing process that is used to establish
   routing assets and points of access to the routing functionality.
   Next, the security model based on [ISO.7498-2.1989] is briefly
   described.  Then, consideration is given to issues specific to or
   amplified in LLNs.  This section concludes with the formulation of a
   set of security objectives for RPL.

4.1.  Routing Assets and Points of Access

   An asset is an important system resource (including information,
   process, or physical resource); the access to and corruption or loss
   of an asset adversely affects the system.  In the control-plane
   context, an asset is information about the network, processes used to
   manage and manipulate this data, and the physical devices on which
   this data is stored and manipulated.  The corruption or loss of these
   assets may adversely impact the control plane of the network.  Within
   the same context, a point of access is an interface or protocol that
   facilitates interaction between control-plane assets.  Identifying
   these assets and points of access will provide a basis for
   enumerating the attack surface of the control plane.

   A level-0 data flow diagram [Yourdon1979] is used here to identify
   the assets and points of access within a generic routing process.
   The use of a data flow diagram allows for a clear and concise model
   of the way in which routing nodes interact and process information;
   hence, it provides a context for threats and attacks.  The goal of
   the model is to be as detailed as possible so that corresponding
   assets, points of access, and processes in an individual routing
   protocol can be readily identified.

   Figure 1 shows that nodes participating in the routing process
   transmit messages to discover neighbors and to exchange routing
   information; routes are then generated and stored, which may be
   maintained in the form of the protocol forwarding table.  The nodes
   use the derived routes for making forwarding decisions.





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                    ...................................................
                    :                                                 :
                    :                                                 :
        |Node_i|<------->(Routing Neighbor       _________________    :
                    :     Discovery)------------>Neighbor Topology    :
                    :                            -------+---------    :
                    :                                   |             :
        |Node_j|<------->(Route/Topology       +--------+             :
                    :     Exchange)            |                      :
                    :           |              V            ______    :
                    :           +---->(Route Generation)--->Routes    :
                    :                                       ---+--    :
                    :                                          |      :
                    : Routing on Node_k                        |      :
                    ...................................................
                                                               |
        |Forwarding                                            |
        |on Node_l|<-------------------------------------------+

   Notation:

   (Proc)     A process Proc

   ________
   topology   A structure storing neighbor adjacency (parent/child)
   --------
   ________
    routes    A structure storing the forwarding information base (FIB)
   --------

   |Node_n|   An external entity Node_n

   ------->   Data flow


         Figure 1: Data Flow Diagram of a Generic Routing Process

   Figure 1 shows the following:

   o  Assets include

      *  routing and/or topology information;

      *  route generation process;

      *  communication channel resources (bandwidth);





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      *  node resources (computing capacity, memory, and remaining
         energy); and

      *  node identifiers (including node identity and ascribed
         attributes such as relative or absolute node location).

   o  Points of access include

      *  neighbor discovery;

      *  route/topology exchange; and

      *  node physical interfaces (including access to data storage).

   A focus on the above list of assets and points of access enables a
   more directed assessment of routing security; for example, it is
   readily understood that some routing attacks are in the form of
   attempts to misrepresent routing topology.  Indeed, the intention of
   the security threat analysis is to be comprehensive.  Hence, some of
   the discussion that follows is associated with assets and points of
   access that are not directly related to routing protocol design but
   are nonetheless provided for reference since they do have direct
   consequences on the security of routing.

4.2.  The ISO 7498-2 Security Reference Model

   At the conceptual level, security within an information system, in
   general, and applied to RPL in particular is concerned with the
   primary issues of authentication, access control, data
   confidentiality, data integrity, and non-repudiation.  In the context
   of RPL:

   Authentication
         Authentication involves the mutual authentication of the
         routing peers prior to exchanging route information (i.e., peer
         authentication) as well as ensuring that the source of the
         route data is from the peer (i.e., data origin authentication).
         LLNs can be drained by unauthenticated peers before
         configuration per [RFC5548].  Availability of open and
         untrusted side channels for new joiners is required by
         [RFC5673], and strong and automated authentication is required
         so that networks can automatically accept or reject new
         joiners.

   Access Control
         Access Control provides protection against unauthorized use of
         the asset and deals with the authorization of a node.




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   Confidentiality
         Confidentiality involves the protection of routing information
         as well as routing neighbor maintenance exchanges so that only
         authorized and intended network entities may view or access it.
         Because LLNs are most commonly found on a publicly accessible
         shared medium, e.g., air or wiring in a building, and are
         sometimes formed ad hoc, confidentiality also extends to the
         neighbor state and database information within the routing
         device since the deployment of the network creates the
         potential for unauthorized access to the physical devices
         themselves.

   Integrity
         Integrity entails the protection of routing information and
         routing neighbor maintenance exchanges, as well as derived
         information maintained in the database, from unauthorized
         modifications, insertions, deletions, or replays to be
         addressed beyond the routing protocol.

   Non-repudiation
         Non-repudiation is the assurance that the transmission and/or
         reception of a message cannot later be denied.  The service of
         non-repudiation applies after the fact; thus, it relies on the
         logging or other capture of ongoing message exchanges and
         signatures.  Routing protocols typically do not have a notion
         of repudiation, so non-repudiation services are not required.
         Further, with the LLN application domains as described in
         [RFC5867] and [RFC5548], proactive measures are much more
         critical than retrospective protections.  Finally, given the
         significant practical limits to ongoing routing transaction
         logging and storage and individual device digital signature
         verification for each exchange, non-repudiation in the context
         of routing is an unsupportable burden that bears no further
         consideration as an RPL security issue.

   It is recognized that, besides those security issues captured in the
   ISO 7498-2 model, availability is a security requirement:

   Availability
         Availability ensures that routing information exchanges and
         forwarding services are available when they are required for
         the functioning of the serving network.  Availability will
         apply to maintaining efficient and correct operation of routing
         and neighbor discovery exchanges (including needed information)
         and forwarding services so as not to impair or limit the
         network's central traffic flow function.





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   It should be emphasized here that for RPL security, the above
   requirements must be complemented by the proper security policies and
   enforcement mechanisms to ensure that security objectives are met by
   a given RPL implementation.

4.3.  Issues Specific to or Amplified in LLNs

   The requirements work detailed in Urban Requirements [RFC5548],
   Industrial Requirements [RFC5673], Home Automation [RFC5826], and
   Building Automation [RFC5867] have identified specific issues and
   constraints of routing in LLNs.  The following is a list of
   observations from those requirements and evaluations of their impact
   on routing security considerations.

   Limited energy, memory, and processing node resources
         As a consequence of these constraints, the need to evaluate the
         kinds of security that can be provided needs careful study.
         For instance, security provided at one level could be very
         memory efficient yet might also be very energy costly for the
         network (as a whole) if it requires significant effort to
         synchronize the security state.  Synchronization of security
         states with sleepy nodes [RFC7102] is a complex issue.  A non-
         rechargeable battery-powered node may well be limited in energy
         for it's lifetime: once exhausted, it may well never function
         again.

   Large scale of rolled out network
         The possibly numerous nodes to be deployed make manual on-site
         configuration unlikely.  For example, an urban deployment can
         see several hundreds of thousands of nodes being installed by
         many installers with a low level of expertise.  Nodes may be
         installed and not activated for many years, and additional
         nodes may be added later on, which may be from old inventory.
         The lifetime of the network is measured in decades, and this
         complicates the operation of key management.

   Autonomous operations
         Self-forming and self-organizing are commonly prescribed
         requirements of LLNs.  In other words, a routing protocol
         designed for LLNs needs to contain elements of ad hoc
         networking and, in most cases, cannot rely on manual
         configuration for initialization or local filtering rules.
         Network topology/ownership changes, partitioning or merging,
         and node replacement can all contribute to complicating the
         operations of key management.






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   Highly directional traffic
         Some types of LLNs see a high percentage of their total traffic
         traverse between the nodes and the LLN Border Routers (LBRs)
         where the LLNs connect to non-LLNs.  The special routing status
         of and the greater volume of traffic near the LBRs have routing
         security consequences as a higher-valued attack target.  In
         fact, when Point-to-MultiPoint (P2MP) and MultiPoint-to-Point
         (MP2P) traffic represents a majority of the traffic, routing
         attacks consisting of advertising incorrect preferred routes
         can cause serious damage.

         While it might seem that nodes higher up in the acyclic graph
         (i.e., those with lower rank) should be secured in a stronger
         fashion, it is not, in general, easy to predict which nodes
         will occupy those positions until after deployment.  Issues of
         redundancy and inventory control suggest that any node might
         wind up in such a sensitive attack position, so all nodes are
         to be capable of being fully secured.

         In addition, even if it were possible to predict which nodes
         will occupy positions of lower rank and provision them with
         stronger security mechanisms, in the absence of a strong
         authorization model, any node could advertise an incorrect
         preferred route.

   Unattended locations and limited physical security
         In many applications, the nodes are deployed in unattended or
         remote locations; furthermore, the nodes themselves are often
         built with minimal physical protection.  These constraints
         lower the barrier of accessing the data or security material
         stored on the nodes through physical means.

   Support for mobility
         On the one hand, only a limited number of applications require
         the support of mobile nodes, e.g., a home LLN that includes
         nodes on wearable health care devices or an industry LLN that
         includes nodes on cranes and vehicles.  On the other hand, if a
         routing protocol is indeed used in such applications, it will
         clearly need to have corresponding security mechanisms.

         Additionally, nodes may appear to move from one side of a wall
         to another without any actual motion involved, which is the
         result of changes to electromagnetic properties, such as the
         opening and closing of a metal door.







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   Support for multicast and anycast
         Support for multicast and anycast is called out chiefly for
         large-scale networks.  Since application of these routing
         mechanisms in autonomous operations of many nodes is new, the
         consequence on security requires careful consideration.

   The above list considers how an LLN's physical constraints, size,
   operations, and variety of application areas may impact security.
   However, it is the combinations of these factors that particularly
   stress the security concerns.  For instance, securing routing for a
   large number of autonomous devices that are left in unattended
   locations with limited physical security presents challenges that are
   not found in the common circumstance of administered networked
   routers.  The following subsection sets up the security objectives
   for the routing protocol designed by the ROLL WG.

4.4.  RPL Security Objectives

   This subsection applies the ISO 7498-2 model to routing assets and
   access points, taking into account the LLN issues, to develop a set
   of RPL security objectives.

   Since the fundamental function of a routing protocol is to build
   routes for forwarding packets, it is essential to ensure that:

   o  routing/topology information integrity remains intact during
      transfer and in storage;

   o  routing/topology information is used by authorized entities; and

   o  routing/topology information is available when needed.

   In conjunction, it is necessary to be assured that:

   o  Authorized peers authenticate themselves during the routing
      neighbor discovery process.

   o  The routing/topology information received is generated according
      to the protocol design.

   However, when trust cannot be fully vested through authentication of
   the principals alone, i.e., concerns of an insider attack, assurance
   of the truthfulness and timeliness of the received routing/topology
   information is necessary.  With regard to confidentiality, protecting
   the routing/topology information from unauthorized exposure may be
   desirable in certain cases but is in itself less pertinent, in
   general, to the routing function.




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   One of the main problems of synchronizing security states of sleepy
   nodes, as listed in the last subsection, lies in difficulties in
   authentication; these nodes may not have received the most recent
   update of security material in time.  Similarly, the issues of
   minimal manual configuration, prolonged rollout and delayed addition
   of nodes, and network topology changes also complicate key
   management.  Hence, routing in LLNs needs to bootstrap the
   authentication process and allow for a flexible expiration scheme of
   authentication credentials.

   The vulnerability brought forth by some special-function nodes, e.g.,
   LBRs, requires the assurance, particularly in a security context, of
   the following:

   o  The availability of communication channels and node resources.

   o  The neighbor discovery process operates without undermining
      routing availability.

   There are other factors that are not part of RPL but directly affect
   its function.  These factors include a weaker barrier of accessing
   the data or security material stored on the nodes through physical
   means; therefore, the internal and external interfaces of a node need
   to be adequate for guarding the integrity, and possibly the
   confidentiality, of stored information, as well as the integrity of
   routing and route generation processes.

   Each individual system's use and environment will dictate how the
   above objectives are applied, including the choices of security
   services as well as the strengths of the mechanisms that must be
   implemented.  The next two sections take a closer look at how the RPL
   security objectives may be compromised and how those potential
   compromises can be countered.

5.  Threat Sources

   [RFC4593] provides a detailed review of the threat sources: outsiders
   and Byzantine.  RPL has the same threat sources.

6.  Threats and Attacks

   This section outlines general categories of threats under the ISO
   7498-2 model and highlights the specific attacks in each of these
   categories for RPL.  As defined in [RFC4949], a threat is "a
   potential for violation of security, which exists when there is a
   circumstance, capability, action, or event that could breach security
   and cause harm."




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   Per [RFC3067], an attack is "an assault on system security that
   derives from an intelligent threat, i.e., an intelligent act that is
   a deliberate attempt (especially in the sense of a method or
   technique) to evade security services and violate the security policy
   of a system."

   The subsequent subsections consider the threats and the attacks that
   can cause security breaches under the ISO 7498-2 model to the routing
   assets and via the routing points of access identified in
   Section 4.1.  The assessment reviews the security concerns of each
   routing asset and looks at the attacks that can exploit routing
   points of access.  The threats and attacks identified are based on
   the routing model analysis and associated review of the existing
   literature.  The source of the attacks is assumed to be from either
   inside or outside attackers.  While some attackers inside the network
   will be using compromised nodes and, therefore, are only able to do
   what an ordinary node can ("node-equivalent"), other attacks may not
   be limited in memory, CPU, power consumption, or long-term storage.
   Moore's law favors the attacker with access to the latest
   capabilities, while the defenders will remain in place for years to
   decades.

6.1.  Threats Due to Failures to Authenticate

6.1.1.  Node Impersonation

   If an attacker can join a network using any identity, then it may be
   able to assume the role of a legitimate (and existing node).  It may
   be able to report false readings (in metering applications) or
   provide inappropriate control messages (in control systems involving
   actuators) if the security of the application is implied by the
   security of the routing system.

   Even in systems where there is application-layer security, the
   ability to impersonate a node would permit an attacker to direct
   traffic to itself.  This may permit various on-path attacks that
   would otherwise be difficult, such as replaying, delaying, or
   duplicating (application) control messages.

6.1.2.  Dummy Node

   If an attacker can join a network using any identify, then it can
   pretend to be a legitimate node, receiving any service legitimate
   nodes receive.  It may also be able to report false readings (in
   metering applications), provide inappropriate authorizations (in
   control systems involving actuators), or perform any other attacks
   that are facilitated by being able to direct traffic towards itself.




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6.1.3.  Node Resource Spam

   If an attacker can join a network with any identity, then it can
   continuously do so with new (random) identities.  This act may drain
   down the resources of the network (battery, RAM, bandwidth).  This
   may cause legitimate nodes of the network to be unable to
   communicate.

6.2.  Threats Due to Failure to Keep Routing Information Confidential

   The assessment in Section 4.2 indicates that there are attacks
   against the confidentiality of routing information at all points of
   access.  This threat may result in disclosure, as described in
   Section 3.1.2 of [RFC4593], and may involve a disclosure of routing
   information.

6.2.1.  Routing Exchange Exposure

   Routing exchanges include both routing information as well as
   information associated with the establishment and maintenance of
   neighbor state information.  As indicated in Section 4.1, the
   associated routing information assets may also include device-
   specific resource information, such as available memory, remaining
   power, etc., that may be metrics of the routing protocol.

   The routing exchanges will contain reachability information, which
   would identify the relative importance of different nodes in the
   network.  Nodes higher up in the DODAG, to which more streams of
   information flow, would be more interesting targets for other
   attacks, and routing exchange exposures could identify them.

6.2.2.  Routing Information (Routes and Network Topology) Exposure

   Routes (which may be maintained in the form of the protocol
   forwarding table) and neighbor topology information are the products
   of the routing process that are stored within the node device
   databases.

   The exposure of this information will allow attackers to gain direct
   access to the configuration and connectivity of the network, thereby
   exposing routing to targeted attacks on key nodes or links.  Since
   routes and neighbor topology information are stored within the node
   device, attacks on the confidentiality of the information will apply
   to the physical device, including specified and unspecified internal
   and external interfaces.






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   The forms of attack that allow unauthorized access or disclosure of
   the routing information will include:

   o  Physical device compromise.

   o  Remote device access attacks (including those occurring through
      remote network management or software/field upgrade interfaces).

   Both of these attack vectors are considered a device-specific issue
   and are out of scope for RPL to defend against.  In some
   applications, physical device compromise may be a real threat, and it
   may be necessary to provide for other devices to securely detect a
   compromised device and react quickly to exclude it.

6.3.  Threats and Attacks on Integrity

   The assessment in Section 4.2 indicates that information and identity
   assets are exposed to integrity threats from all points of access.
   In other words, the integrity threat space is defined by the
   potential for exploitation introduced by access to assets available
   through routing exchanges and the on-device storage.

6.3.1.  Routing Information Manipulation

   Manipulation of routing information that ranges from neighbor states
   to derived routes will allow unauthorized sources to influence the
   operation and convergence of the routing protocols and ultimately
   impact the forwarding decisions made in the network.

   Manipulation of topology and reachability information will allow
   unauthorized sources to influence the nodes with which routing
   information is exchanged and updated.  The consequence of
   manipulating routing exchanges can thus lead to suboptimality and
   fragmentation or partitioning of the network by restricting the
   universe of routers with which associations can be established and
   maintained.

   A suboptimal network may use too much power and/or may congest some
   routes leading to premature failure of a node and a denial of service
   (DoS) on the entire network.

   In addition, being able to attract network traffic can make a black-
   hole attack more damaging.








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   The forms of attack that allow manipulation to compromise the content
   and validity of routing information include:

   o  falsification, including overclaiming and misclaiming (claiming
      routes to devices that the device cannot in fact reach);

   o  routing information replay;

   o  Byzantine (internal) attacks that permit corruption of routing
      information in the node even when the node continues to be a
      validated entity within the network (see, for example, [RFC4593]
      for further discussions on Byzantine attacks); and

   o  physical device compromise or remote device access attacks.

6.3.2.  Node Identity Misappropriation

   Falsification or misappropriation of node identity between routing
   participants opens the door for other attacks; it can also cause
   incorrect routing relationships to form and/or topologies to emerge.
   Routing attacks may also be mounted through less-sophisticated node
   identity misappropriation in which the valid information broadcasted
   or exchanged by a node is replayed without modification.  The receipt
   of seemingly valid information that is, however, no longer current
   can result in routing disruption and instability (including failure
   to converge).  Without measures to authenticate the routing
   participants and to ensure the freshness and validity of the received
   information, the protocol operation can be compromised.  The forms of
   attack that misuse node identity include:

   o  Identity attacks, including Sybil attacks (see [Sybil2002]) in
      which a malicious node illegitimately assumes multiple identities.

   o  Routing information replay.

















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6.4.  Threats and Attacks on Availability

   The assessment in Section 4.2 indicates that the process and resource
   assets are exposed to threats against availability; attacks in this
   category may exploit directly or indirectly information exchange or
   forwarding (see [RFC4732] for a general discussion).

6.4.1.  Routing Exchange Interference or Disruption

   Interference is the threat action and disruption is the threat
   consequence that allows attackers to influence the operation and
   convergence of the routing protocols by impeding the routing
   information exchange.

   The forms of attack that allow interference or disruption of routing
   exchange include:

   o  routing information replay;

   o  ACK spoofing; and

   o  overload attacks (Section 7.3.2).

   In addition, attacks may also be directly conducted at the physical
   layer in the form of jamming or interfering.

6.4.2.  Network Traffic Forwarding Disruption

   The disruption of the network traffic forwarding capability will
   undermine the central function of network routers and the ability to
   handle user traffic.  This affects the availability of the network
   because of the potential to impair the primary capability of the
   network.


















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   In addition to physical-layer obstructions, the forms of attack that
   allow disruption of network traffic forwarding include [Karlof2003]:

   o  selective forwarding attacks;

         |Node_1|--(msg1|msg2|msg3)-->|Attacker|--(msg1|msg3)-->|Node_2|

                  Figure 2: Selective Forwarding Example

   o  wormhole attacks; and

               |Node_1|-------------Unreachable---------x|Node_2|
                  |                                         ^
                  |               Private Link              |
                  '-->|Attacker_1|===========>|Attacker_2|--'

                        Figure 3: Wormhole Attacks

   o  sinkhole attacks.

                |Node_1|     |Node_4|
                    |            |
                    `--------.   |
                Falsify as    \  |
                Good Link \   |  |
                to Node_5  \  |  |
                            \ V  V
                |Node_2|-->|Attacker|--Not Forwarded---x|Node_5|
                              ^  ^ \
                              |  |  \ Falsify as
                              |  |   \Good Link
                              /  |    to Node_5
                     ,-------'   |
                     |           |
                |Node_3|     |Node_i|

                     Figure 4: Sinkhole Attack Example

   These attacks are generally done to both control- and forwarding-
   plane traffic.  A system that prevents control-plane traffic (RPL
   messages) from being diverted in these ways will also prevent actual
   data from being diverted.









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6.4.3.  Communications Resource Disruption

   Attacks mounted against the communication channel resource assets
   needed by the routing protocol can be used as a means of disrupting
   its operation.  However, while various forms of DoS attacks on the
   underlying transport subsystem will affect routing protocol exchanges
   and operation (for example, physical-layer Radio Frequency (RF)
   jamming in a wireless network or link-layer attacks), these attacks
   cannot be countered by the routing protocol.  As such, the threats to
   the underlying transport network that supports routing is considered
   beyond the scope of the current document.  Nonetheless, attacks on
   the subsystem will affect routing operation and must be directly
   addressed within the underlying subsystem and its implemented
   protocol layers.

6.4.4.  Node Resource Exhaustion

   A potential threat consequence can arise from attempts to overload
   the node resource asset by initiating exchanges that can lead to the
   exhaustion of processing, memory, or energy resources.  The
   establishment and maintenance of routing neighbors opens the routing
   process to engagement and potential acceptance of multiple
   neighboring peers.  Association information must be stored for each
   peer entity and for the wireless network operation provisions made to
   periodically update and reassess the associations.  An introduced
   proliferation of apparent routing peers can, therefore, have a
   negative impact on node resources.

   Node resources may also be unduly consumed by attackers attempting
   uncontrolled topology peering or routing exchanges, routing replays,
   or the generating of other data-traffic floods.  Beyond the
   disruption of communications channel resources, these consequences
   may be able to exhaust node resources only where the engagements are
   able to proceed with the peer routing entities.  Routing operation
   and network forwarding functions can thus be adversely impacted by
   node resources exhaustion that stems from attacks that include:

   o  identity (including Sybil) attacks (see [Sybil2002]);

   o  routing information replay attacks;

   o  HELLO-type flood attacks; and

   o  overload attacks (Section 7.3.2).







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7.  Countermeasures

   By recognizing the characteristics of LLNs that may impact routing,
   this analysis provides the basis for understanding the capabilities
   within RPL used to deter the identified attacks and mitigate the
   threats.  The following subsections consider such countermeasures by
   grouping the attacks according to the classification of the ISO
   7498-2 model so that associations with the necessary security
   services are more readily visible.

7.1.  Confidentiality Attack Countermeasures

   Attacks to disclosure routing information may be mounted at the level
   of the routing information assets, at the points of access associated
   with routing exchanges between nodes, or through device interface
   access.  To gain access to routing/topology information, the attacker
   may rely on a compromised node that deliberately exposes the
   information during the routing exchange process, on passive
   wiretapping or traffic analysis, or on attempting access through a
   component or device interface of a tampered routing node.

7.1.1.  Countering Deliberate Exposure Attacks

   A deliberate exposure attack is one in which an entity that is party
   to the routing process or topology exchange allows the routing/
   topology information or generated route information to be exposed to
   an unauthorized entity.

   For instance, due to misconfiguration or inappropriate enabling of a
   diagnostic interface, an entity might be copying ("bridging") traffic
   from a secured ESSID/PAN to an unsecured interface.

   A prerequisite to countering this attack is to ensure that the
   communicating nodes are authenticated prior to data encryption
   applied in the routing exchange.  The authentication ensures that the
   LLN starts with trusted nodes, but it does not provide an indication
   of whether the node has been compromised.

   Reputation systems could be used to help when some nodes may sleep
   for extended periods of time.  It is also unclear if resulting
   datasets would even fit into constrained devices.

   To mitigate the risk of deliberate exposure, the process that
   communicating nodes use to establish session keys must be
   peer-to-peer (i.e., between the routing initiating and responding
   nodes).  As is pointed out in [RFC4107], automatic key management is
   critical for good security.  This helps ensure that neither node is
   exchanging routing information with another peer without the



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   knowledge of both communicating peers.  For a deliberate exposure
   attack to succeed, the comprised node will need to be more overt and
   take independent actions in order to disclose the routing information
   to a third party.

   Note that the same measures that apply to securing routing/topology
   exchanges between operational nodes must also extend to field tools
   and other devices used in a deployed network where such devices can
   be configured to participate in routing exchanges.

7.1.2.  Countering Passive Wiretapping Attacks

   A passive wiretap attack seeks to breach routing confidentiality
   through passive, direct analysis and processing of the information
   exchanges between nodes.

   Passive wiretap attacks can be directly countered through the use of
   data encryption for all routing exchanges.  Only when a validated and
   authenticated node association is completed will routing exchange be
   allowed to proceed using established session keys and an agreed
   encryption algorithm.  The mandatory-to-implement CCM mode AES-128
   method, described in [RFC3610], is believed to be secure against a
   brute-force attack by even the most well-equipped adversary.

   The significant challenge for RPL is in the provisioning of the key,
   which in some modes of RFC 6550 is used network wide.  This problem
   is not solved in RFC 6550, and it is the subject of significant
   future work: see, for instance, [AceCharterProposal],
   [SolaceProposal], and [SmartObjectSecurityWorkshop].

   A number of deployments, such as [ZigBeeIP] specify no Layer 3 (L3) /
   RPL encryption or authentication and rely upon similar security at
   Layer 2 (L2).  These networks are immune to outside wiretapping
   attacks but are vulnerable to passive (and active) routing attacks
   through compromises of nodes (see Section 8.2).

   Section 10.9 of [RFC6550] specifies AES-128 in CCM mode with a 32-bit
   Message Authentication Code (MAC).

   Section 5.6 of ZigBee IP [ZigBeeIP] specifies use of CCM, with PANA
   and EAP-TLS for key management.

7.1.3.  Countering Traffic Analysis

   Traffic analysis provides an indirect means of subverting
   confidentiality and gaining access to routing information by allowing
   an attacker to indirectly map the connectivity or flow patterns
   (including link load) of the network from which other attacks can be



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   mounted.  The traffic-analysis attack on an LLN, especially one
   founded on a shared medium, is passive and relies on the ability to
   read the immutable source/destination L2 and/or L3 routing
   information that must remain unencrypted to permit network routing.

   One way in which passive traffic-analysis attacks can be muted is
   through the support of load balancing that allows traffic to a given
   destination to be sent along diverse routing paths.  RPL does not
   generally support multipath routing within a single DODAG.  Multiple
   DODAGs are supported in the protocol, and an implementation could
   make use of that.  RPL does not have any inherent or standard way to
   guarantee that the different DODAGs would have significantly diverse
   paths.  Having the diverse DODAGs routed at different border routers
   might work in some instances, and this could be combined with a
   multipath technology like Multipath TCP (MPTCP) [RFC6824].  It is
   unlikely that it will be affordable in many LLNs, as few deployments
   will have memory space for more than a few sets of DODAG tables.

   Another approach to countering passive traffic analysis could be for
   nodes to maintain a constant amount of traffic to different
   destinations through the generation of arbitrary traffic flows; the
   drawback of course would be the consequent overhead and energy
   expenditure.

   The only means of fully countering a traffic-analysis attack is
   through the use of tunneling (encapsulation) where encryption is
   applied across the entirety of the original packet source/destination
   addresses.  Deployments that use L2 security that includes encryption
   already do this for all traffic.

7.1.4.  Countering Remote Device Access Attacks

   Where LLN nodes are deployed in the field, measures are introduced to
   allow for remote retrieval of routing data and for software or field
   upgrades.  These paths create the potential for a device to be
   remotely accessed across the network or through a provided field
   tool.  In the case of network management, a node can be directly
   requested to provide routing tables and neighbor information.

   To ensure confidentiality of the node routing information against
   attacks through remote access, any local or remote device requesting
   routing information must be authenticated and must be authorized for
   that access.  Since remote access is not invoked as part of a routing
   protocol, security of routing information stored on the node against
   remote access will not be addressable as part of the routing
   protocol.





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7.2.  Integrity Attack Countermeasures

   Integrity attack countermeasures address routing information
   manipulation, as well as node identity and routing information
   misuse.  Manipulation can occur in the form of a falsification attack
   and physical compromise.  To be effective, the following development
   considers the two aspects of falsification, namely, the unauthorized
   modifications and the overclaiming and misclaiming content.  The
   countering of physical compromise was considered in the previous
   section and is not repeated here.  With regard to misuse, there are
   two types of attacks to be deterred: identity attacks and replay
   attacks.

7.2.1.  Countering Unauthorized Modification Attacks

   Unauthorized modifications may occur in the form of altering the
   message being transferred or the data stored.  Therefore, it is
   necessary to ensure that only authorized nodes can change the portion
   of the information that is allowed to be mutable, while the integrity
   of the rest of the information is protected, e.g., through well-
   studied cryptographic mechanisms.

   Unauthorized modifications may also occur in the form of insertion or
   deletion of messages during protocol changes.  Therefore, the
   protocol needs to ensure the integrity of the sequence of the
   exchange sequence.

   The countermeasure to unauthorized modifications needs to:

   o  implement access control on storage;

   o  provide data integrity service to transferred messages and stored
      data; and

   o  include a sequence number under integrity protection.

7.2.2.  Countering Overclaiming and Misclaiming Attacks

   Both overclaiming and misclaiming aim to introduce false routes or a
   false topology that would not occur otherwise, while there are not
   necessarily unauthorized modifications to the routing messages or
   information.  In order to counter overclaiming, the capability to
   determine unreasonable routes or topology is required.

   The counter to overclaiming and misclaiming may employ:

   o  Comparison with historical routing/topology data.




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   o  Designs that restrict realizable network topologies.

   RPL includes no specific mechanisms in the protocol to counter
   overclaims or misclaims.  An implementation could have specific
   heuristics implemented locally.

7.2.3.  Countering Identity (including Sybil) Attacks

   Identity attacks, sometimes simply called spoofing, seek to gain or
   damage assets whose access is controlled through identity.  In
   routing, an identity attacker can illegitimately participate in
   routing exchanges, distribute false routing information, or cause an
   invalid outcome of a routing process.

   A perpetrator of Sybil attacks assumes multiple identities.  The
   result is not only an amplification of the damage to routing but
   extension to new areas, e.g., where geographic distribution is
   explicitly or implicitly an asset to an application running on the
   LLN, for example, the LBR in a P2MP or MP2P LLN.

   RPL includes specific public key-based authentication at L3 that
   provides for authorization.  Many deployments use L2 security that
   includes admission controls at L2 using mechanisms such as PANA.

7.2.4.  Countering Routing Information Replay Attacks

   In many routing protocols, message replay can result in false
   topology and/or routes.  This is often counted with some kind of
   counter to ensure the freshness of the message.  Replay of a current,
   literal RPL message is, in general, idempotent to the topology.  If
   replayed, an older (lower DODAGVersionNumber) message would be
   rejected as being stale.  If the trickle algorithm further dampens
   the effect of any such replay, as if the message was current, then it
   would contain the same information as before, and it would cause no
   network changes.

   Replays may well occur in some radio technologies (though not very
   likely; see [IEEE.802.15.4]) as a result of echos or reflections, so
   some replays must be assumed to occur naturally.

   Note that for there to be no effect at all, the replay must be done
   with the same apparent power for all nodes receiving the replay.  A
   change in apparent power might change the metrics through changes to
   the Expected Transmission Count (ETX); therefore, it might affect the
   routing even though the contents of the packet were never changed.
   Any replay that appears to be different should be analyzed as a
   selective forwarding attack, sinkhole attack, or wormhole attack.




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7.2.5.  Countering Byzantine Routing Information Attacks

   Where a node is captured or compromised but continues to operate for
   a period with valid network security credentials, the potential
   exists for routing information to be manipulated.  This compromise of
   the routing information could thus exist in spite of security
   countermeasures that operate between the peer routing devices.

   Consistent with the end-to-end principle of communications, such an
   attack can only be fully addressed through measures operating
   directly between the routing entities themselves or by means of
   external entities accessing and independently analyzing the routing
   information.  Verification of the authenticity and liveliness of the
   routing entities can, therefore, only provide a limited counter
   against internal (Byzantine) node attacks.

   For link-state routing protocols where information is flooded with,
   for example, areas (OSPF [RFC2328]) or levels (IS-IS [RFC7142]),
   countermeasures can be directly applied by the routing entities
   through the processing and comparison of link-state information
   received from different peers.  By comparing the link information
   from multiple sources, decisions can be made by a routing node or
   external entity with regard to routing information validity; see
   Chapter 2 of [Perlman1988] for a discussion on flooding attacks.

   For distance vector protocols, such as RPL, where information is
   aggregated at each routing node, it is not possible for nodes to
   directly detect Byzantine information manipulation attacks from the
   routing information exchange.  In such cases, the routing protocol
   must include and support indirect communications exchanges between
   non-adjacent routing peers to provide a secondary channel for
   performing routing information validation.  S-RIP [Wan2004] is an
   example of the implementation of this type of dedicated routing
   protocol security where the correctness of aggregate distance vector
   information can only be validated by initiating confirmation
   exchanges directly between nodes that are not routing neighbors.

   RPL does not provide any direct mechanisms like S-RIP.  It does
   listen to multiple parents and may switch parents if it begins to
   suspect that it is being lied to.

7.3.  Availability Attack Countermeasures

   As alluded to before, availability requires that routing information
   exchanges and forwarding mechanisms be available when needed so as to
   guarantee proper functioning of the network.  This may, e.g., include
   the correct operation of routing information and neighbor state
   information exchanges, among others.  We will highlight the key



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   features of the security threats along with typical countermeasures
   to prevent or at least mitigate them.  We will also note that an
   availability attack may be facilitated by an identity attack as well
   as a replay attack, as was addressed in Sections 7.2.3 and 7.2.4,
   respectively.

7.3.1.  Countering HELLO Flood Attacks and ACK Spoofing Attacks

   HELLO Flood [Karlof2003], [HELLO], and ACK spoofing attacks are
   different but highly related forms of attacking an LLN.  They
   essentially lead nodes to believe that suitable routes are available
   even though they are not and hence constitute a serious availability
   attack.

   A HELLO attack mounted against RPL would involve sending out (or
   replaying) DODAG Information Object (DIO) messages by the attacker.
   Lower-power LLN nodes might then attempt to join the DODAG at a lower
   rank than they would otherwise.

   The most effective method from [HELLO] is bidirectional verification.
   A number of L2 links are arranged in controller/spoke arrangements
   and are continuously validating connectivity at layer 2.

   In addition, in order to calculate metrics, the ETX must be computed,
   and this involves, in general, sending a number of messages between
   nodes that are believed to be adjacent.  One such protocol is
   [MESH-LINK].

   In order to join the DODAG, a Destination Advertisement Object (DAO)
   message is sent upwards.  In RPL, the DAO is acknowledged by the
   DAO-ACK message.  This clearly checks bidirectionality at the control
   plane.

   As discussed in Section 5.1 of [HELLO], a receiver with a sensitive
   receiver could well hear the DAOs and even send DAO-ACKs as well.
   Such a node is a form of wormhole attack.

   These attacks are also all easily defended against using either L2 or
   L3 authentication.  Such an attack could only be made against a
   completely open network (such as might be used for provisioning new
   nodes) or by a compromised node.

7.3.2.  Countering Overload Attacks

   Overload attacks are a form of DoS attack in that a malicious node
   overloads the network with irrelevant traffic, thereby draining the
   nodes' energy store more quickly when the nodes rely on batteries or
   energy scavenging.  Thus, it significantly shortens the lifetime of



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   networks of energy-constrained nodes and constitutes another serious
   availability attack.

   With energy being one of the most precious assets of LLNs, targeting
   its availability is a fairly obvious attack.  Another way of
   depleting the energy of an LLN node is to have the malicious node
   overload the network with irrelevant traffic.  This impacts
   availability since certain routes get congested, which:

   o  renders them useless for affected nodes; hence, data cannot be
      delivered;

   o  makes routes longer as the shortest path algorithms work with the
      congested network; and

   o  depletes battery and energy scavenging nodes more quickly and thus
      shortens the network's availability at large.

   Overload attacks can be countered by deploying a series of mutually
   non-exclusive security measures that:

   o  introduce quotas on the traffic rate each node is allowed to send;

   o  isolate nodes that send traffic above a certain threshold based on
      system operation characteristics; and

   o  allow only trusted data to be received and forwarded.

   As for the first one, a simple approach to minimize the harmful
   impact of an overload attack is to introduce traffic quotas.  This
   prevents a malicious node from injecting a large amount of traffic
   into the network, even though it does not prevent the said node from
   injecting irrelevant traffic at all.  Another method is to isolate
   nodes from the network at the network layer once it has been detected
   that more traffic is injected into the network than allowed by a
   prior set or dynamically adjusted threshold.  Finally, if
   communication is sufficiently secured, only trusted nodes can receive
   and forward traffic, which also lowers the risk of an overload
   attack.

   Receiving nodes that validate signatures and sending nodes that
   encrypt messages need to be cautious of cryptographic processing
   usage when validating signatures and encrypting messages.  Where
   feasible, certificates should be validated prior to use of the
   associated keys to counter potential resource overloading attacks.
   The associated design decision needs to also consider that the
   validation process requires resources; thus, it could be exploited
   for attacks.  Alternatively, resource management limits can be placed



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   on routing security processing events (see the comment in Section 6,
   paragraph 4, of [RFC5751]).

7.3.3.  Countering Selective Forwarding Attacks

   Selective forwarding attacks are a form of DoS attack that impacts
   the availability of the generated routing paths.

   A selective forwarding attack may be done by a node involved with the
   routing process, or it may be done by what otherwise appears to be a
   passive antenna or other RF feature or device, but is in fact an
   active (and selective) device.  An RF antenna/repeater that is not
   selective is not a threat.

   An insider malicious node basically blends in neatly with the network
   but then may decide to forward and/or manipulate certain packets.  If
   all packets are dropped, then this attacker is also often referred to
   as a "black hole".  Such a form of attack is particularly dangerous
   if coupled with sinkhole attacks since inherently a large amount of
   traffic is attracted to the malicious node, thereby causing
   significant damage.  In a shared medium, an outside malicious node
   would selectively jam overheard data flows, where the thus caused
   collisions incur selective forwarding.

   Selective forwarding attacks can be countered by deploying a series
   of mutually non-exclusive security measures:

   o  Multipath routing of the same message over disjoint paths.

   o  Dynamically selecting the next hop from a set of candidates.

   The first measure basically guarantees that if a message gets lost on
   a particular routing path due to a malicious selective forwarding
   attack, there will be another route that successfully delivers the
   data.  Such a method is inherently suboptimal from an energy
   consumption point of view; it is also suboptimal from a network
   utilization perspective.  The second method basically involves a
   constantly changing routing topology in that next-hop routers are
   chosen from a dynamic set in the hope that the number of malicious
   nodes in this set is negligible.  A routing protocol that allows for
   disjoint routing paths may also be useful.

7.3.4.  Countering Sinkhole Attacks

   In sinkhole attacks, the malicious node manages to attract a lot of
   traffic mainly by advertising the availability of high-quality links
   even though there are none [Karlof2003].  Hence, it constitutes a
   serious attack on availability.



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   The malicious node creates a sinkhole by attracting a large amount
   of, if not all, traffic from surrounding neighbors by advertising in
   and outwards links of superior quality.  Hence, affected nodes
   eagerly route their traffic via the malicious node that, if coupled
   with other attacks such as selective forwarding, may lead to serious
   availability and security breaches.  Such an attack can only be
   executed by an inside malicious node and is generally very difficult
   to detect.  An ongoing attack has a profound impact on the network
   topology and essentially becomes a problem of flow control.

   Sinkhole attacks can be countered by deploying a series of mutually
   non-exclusive security measures to:

   o  use geographical insights for flow control;

   o  isolate nodes that receive traffic above a certain threshold;

   o  dynamically pick up the next hop from a set of candidates; and

   o  allow only trusted data to be received and forwarded.

   A canary node could periodically call home (using a cryptographic
   process) with the home system, noting if it fails to call in.  This
   provides detection of a problem, but does not mitigate it, and it may
   have significant energy consequences for the LLN.

   Some LLNs may provide for geolocation services, often derived from
   solving triangulation equations from radio delay calculation; such
   calculations could in theory be subverted by a sinkhole that
   transmitted at precisely the right power in a node-to-node fashion.

   While geographic knowledge could help assure that traffic always goes
   in the physical direction desired, it would not assure that the
   traffic is taking the most efficient route, as the lowest cost real
   route might match the physical topology, such as when different parts
   of an LLN are connected by high-speed wired networks.

7.3.5.  Countering Wormhole Attacks

   In wormhole attacks, at least two malicious nodes claim to have a
   short path between themselves [Karlof2003].  This changes the
   availability of certain routing paths and hence constitutes a serious
   security breach.

   Essentially, two malicious insider nodes use another, more powerful,
   transmitter to communicate with each other and thereby distort the
   would-be-agreed routing path.  This distortion could involve
   shortcutting and hence paralyzing a large part of the network; it



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   could also involve tunneling the information to another region of the
   network where there are, e.g., more malicious nodes available to aid
   the intrusion or where messages are replayed, etc.

   In conjunction with selective forwarding, wormhole attacks can create
   race conditions that impact topology maintenance and routing
   protocols as well as any security suits built on "time of check" and
   "time of use".

   A pure wormhole attack is nearly impossible to detect.  A wormhole
   that is used in order to subsequently mount another kind of attack
   would be defeated by defeating the other attack.  A perfect wormhole,
   in which there is nothing adverse that occurs to the traffic, would
   be difficult to call an attack.  The worst thing that a benign
   wormhole can do in such a situation is to cease to operate (become
   unstable), causing the network to have to recalculate routes.

   A highly unstable wormhole is no different than a radio opaque (i.e.,
   metal) door that opens and closes a lot.  RPL includes hysteresis in
   its objective functions [RFC6719] in an attempt to deal with frequent
   changes to the ETX between nodes.

8.  RPL Security Features

   The assessments and analysis in Section 6 examined all areas of
   threats and attacks that could impact routing, and the
   countermeasures presented in Section 7 were reached without confining
   the consideration to means only available to routing.  This section
   puts the results into perspective, dealing with those threats that
   are endemic to this field, that have been mitigated through RPL
   protocol design, and that require specific decisions to be made as
   part of provisioning a network.

   The first part of this section, Sections 8.1 to 8.3, presents a
   description of RPL security features that address specific threats.
   The second part of this section, Section 8.4, discusses issues of the
   provisioning of security aspects that may impact routing but that
   also require considerations beyond the routing protocol, as well as
   potential approaches.

   RPL employs multicast, so these alternative communications modes MUST
   be secured with the same routing security services specified in this
   section.  Furthermore, irrespective of the modes of communication,
   nodes MUST provide adequate physical tamper resistance commensurate
   with the particular application-domain environment to ensure the
   confidentiality, integrity, and availability of stored routing
   information.




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8.1.  Confidentiality Features

   With regard to confidentiality, protecting the routing/topology
   information from unauthorized disclosure is not directly essential to
   maintaining the routing function.  Breaches of confidentiality may
   lead to other attacks or the focusing of an attacker's resources (see
   Section 6.2) but does not of itself directly undermine the operation
   of the routing function.  However, to protect against and reduce
   consequences from other more direct attacks, routing information
   should be protected.  Thus, to secure RPL:

   o  Implement payload encryption using L3 mechanisms described in
      [RFC6550] or

   o  Implement L2 confidentiality

   Where confidentiality is incorporated into the routing exchanges,
   encryption algorithms and key lengths need to be specified in
   accordance with the level of protection dictated by the routing
   protocol and the associated application-domain transport network.
   For most networks, this means use of AES-128 in CCM mode, but this
   needs to be specified clearly in the applicability statement.

   In terms of the lifetime of the keys, the opportunity to periodically
   change the encryption key increases the offered level of security for
   any given implementation.  However, where strong cryptography is
   employed, physical, procedural, and logical data access protection
   considerations may have a more significant impact on cryptoperiod
   selection than algorithm and key size factors.  Nevertheless, in
   general, shorter cryptoperiods, during which a single key is applied,
   will enhance security.

   Given the mandatory protocol requirement to implement routing node
   authentication as part of routing integrity (see Section 8.2), key
   exchanges may be coordinated as part of the integrity verification
   process.  This provides an opportunity to increase the frequency of
   key exchange and shorten the cryptoperiod as a complement to the key
   length and encryption algorithm required for a given application
   domain.

8.2.  Integrity Features

   The integrity of routing information provides the basis for ensuring
   that the function of the routing protocol is achieved and maintained.
   To protect integrity, RPL must run either using only the secure
   versions of the messages or over a L2 that uses channel binding
   between node identity and transmissions.




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   Some L2 security mechanisms use a single key for the entire network,
   and these networks cannot provide a significant amount of integrity
   protection, as any node that has that key may impersonate any other
   node.  This mode of operation is likely acceptable when an entire
   deployment is under the control of a single administrative entity.

   Other L2 security mechanisms form a unique session key for every pair
   of nodes that needs to communicate; this is often called a per-link
   key.  Such networks can provide a strong degree of origin
   authentication and integrity on unicast messages.

   However, some RPL messages are broadcast, and even when per-node L2
   security mechanisms are used, the integrity and origin authentication
   of broadcast messages cannot be as trusted due to the proliferation
   of the key used to secure them.

   RPL has two specific options that are broadcast in RPL Control
   Messages: the DIO and the DODAG Information Solicitation (DIS).  The
   purpose of the DIS is to cause potential parents to reply with a DIO,
   so the integrity of the DIS is not of great concern.  The DIS may
   also be unicast.

   The DIO is a critical piece of routing and carries many critical
   parameters.  RPL provides for asymmetric authentication at L3 of the
   RPL Control Message carrying the DIO, and this may be warranted in
   some deployments.  A node could, if it felt that the DIO that it had
   received was suspicious, send a unicast DIS message to the node in
   question, and that node would reply with a unicast DIS.  Those
   messages could be protected with the per-link key.

8.3.  Availability Features

   Availability of routing information is linked to system and network
   availability, which in the case of LLNs require a broader security
   view beyond the requirements of the routing entities.  Where
   availability of the network is compromised, routing information
   availability will be accordingly affected.  However, to specifically
   assist in protecting routing availability, nodes MAY:

   o  restrict neighborhood cardinality;

   o  use multiple paths;

   o  use multiple destinations;

   o  choose randomly if multiple paths are available;

   o  set quotas to limit transmit or receive volume; and



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   o  use geographic information for flow control.

8.4.  Key Management

   The functioning of the routing security services requires keys and
   credentials.  Therefore, even though it's not directly an RPL
   security requirement, an LLN MUST have a process for initial key and
   credential configuration, as well as secure storage within the
   associated devices.  Anti-tampering SHOULD be a consideration in
   physical design.  Beyond initial credential configuration, an LLN is
   also encouraged to have automatic procedures for the revocation and
   replacement of the maintained security credentials.

   While RPL has secure modes, some modes are impractical without the
   use of public key cryptography, which is believed to be too expensive
   by many.  RPL L3 security will often depend upon existing LLN L2
   security mechanisms, which provide for node authentication but little
   in the way of node authorization.

9.  Security Considerations

   The analysis presented in this document provides security analysis
   and design guidelines with a scope limited to RPL.  Security services
   are identified as requirements for securing RPL.  The specific
   mechanisms to be used to deal with each threat is specified in link-
   Land deployment-specific applicability statements.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005,
              <http://www.rfc-editor.org/info/rfc4107>.

   [RFC6550]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", RFC 6550, March 2012,
              <http://www.rfc-editor.org/info/rfc6550>.

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719, September 2012,
              <http://www.rfc-editor.org/info/rfc6719>.



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   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, January 2014,
              <http://www.rfc-editor.org/info/rfc7102>.

   [ZigBeeIP] ZigBee Alliance, "ZigBee IP Specification", Public
              Document 15-002r00, March 2013.

10.2.  Informative References

   [AceCharterProposal]
              Li, Kepeng., Ed., "Draft Charter V0.9c - Authentication
              and Authorization for Constrained Environment Charter",
              Work in Progress, December 2013,
              <http://trac.tools.ietf.org/wg/core/trac/wiki/
              ACE_charter>.

   [HELLO]    Park, S., "Routing Security in Sensor Network: HELLO Flood
              Attack and Defense", Work in Progress, draft-suhopark-
              hello-wsn-00, December 2005.

   [IEEE.802.11]
              IEEE, "IEEE Standard for Information Technology -
              Telecommunications and information exchange between
              systems - Local and metropolitan area networks - Specific
              requirements Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE Std
              802.11-2012, March 2012,
              <http://standards.ieee.org/about/get/802/802.11.html>.

   [IEEE.802.15.4]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks - Specific requirements - Part 15.4: Low-Rate
              Wireless Personal Area Networks (LR-WPANs)", IEEE Std
              802.15.4-2011, September 2011,
              <http://standards.ieee.org/getieee802/802.15.html>.

   [ISO.7498-2.1989]
              International Organization for Standardization,
              "Information processing systems - Open Systems
              Interconnection -- Basic Reference Model - Part 2:
              Security Architecture", ISO Standard 7498-2, 1989.










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   [Karlof2003]
              Karlof, C. and D. Wagner, "Secure Routing in Wireless
              Sensor Networks: Attacks and Countermeasures", Elsevier Ad
              Hoc Networks Journal, Special Issue on Sensor Network
              Applications and Protocols, 1(2):293-315, September 2003,
              <http://nest.cs.berkeley.edu/papers/
              sensor-route-security.pdf>.

   [MESH-LINK]
              Kelsey, R., "Mesh Link Establishment", Work in Progress,
              draft-kelsey-intarea-mesh-link-establishment-06, May 2014.

   [Myagmar2005]
              Myagmar, S., Lee, AJ., and W. Yurcik, "Threat Modeling as
              a Basis for Security Requirements", in Proceedings of the
              Symposium on Requirements Engineering for Information
              Security (SREIS'05), Paris, France pp. 94-102, August
              2005.

   [Perlman1988]
              Perlman, R., "Network Layer Protocols with Byzantine
              Robustness", MIT LCS Tech Report, 429, August 1988.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC3067]  Arvidsson, J., Cormack, A., Demchenko, Y., and J. Meijer,
              "TERENA'S Incident Object Description and Exchange Format
              Requirements", RFC 3067, February 2001,
              <http://www.rfc-editor.org/info/rfc3067>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, September 2003,
              <http://www.rfc-editor.org/info/rfc3610>.

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, October 2006,
              <http://www.rfc-editor.org/info/rfc4593>.

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006,
              <http://www.rfc-editor.org/info/rfc4732>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.





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   [RFC5191]  Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA)", RFC 5191, May 2008,
              <http://www.rfc-editor.org/info/rfc5191>.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, March 2008,
              <http://www.rfc-editor.org/info/rfc5216>.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
              "Routing Requirements for Urban Low-Power and Lossy
              Networks", RFC 5548, May 2009,
              <http://www.rfc-editor.org/info/rfc5548>.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009,
              <http://www.rfc-editor.org/info/rfc5673>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010,
              <http://www.rfc-editor.org/info/rfc5751>.

   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks", RFC
              5826, April 2010,
              <http://www.rfc-editor.org/info/rfc5826>.

   [RFC5867]  Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, June 2010,
              <http://www.rfc-editor.org/info/rfc5867>.

   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
              Router Control Plane", RFC 6192, March 2011,
              <http://www.rfc-editor.org/info/rfc6192>.

   [RFC6574]  Tschofenig, H. and J. Arkko, "Report from the Smart Object
              Workshop", RFC 6574, April 2012,
              <http://www.rfc-editor.org/info/rfc6574>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013,
              <http://www.rfc-editor.org/info/rfc6824>.





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   [RFC7142]  Shand, M. and L. Ginsberg, "Reclassification of RFC 1142
              to Historic", RFC 7142, February 2014,
              <http://www.rfc-editor.org/info/rfc7142>.

   [RFC7397]  Gilger, J. and H. Tschofenig, "Report from the Smart
              Object Security Workshop", RFC 7397, November 2014,
              <http://www.rfc-editor.org/info/rfc7397>.

   [SmartObjectSecurityWorkshop]
              Klausen, T., Ed., "Workshop on Smart Object Security",
              March 2012, <http://www.lix.polytechnique.fr/hipercom/
              SmartObjectSecurity>.

   [SolaceProposal]
              Bormann, C., Ed., "Notes from the SOLACE ad hoc at IETF
              85", November 2012, <http://www.ietf.org/
              mail-archive/web/solace/current/msg00015.html>.

   [Sybil2002]
              Douceur, J., "The Sybil Attack", First International
              Workshop on Peer-to-Peer Systems, March 2002.

   [Wan2004]  Wan, T., Kranakis, E., and PC. van Oorschot, "S-RIP: A
              Secure Distance Vector Routing Protocol", in Proceedings
              of the 2nd International Conference on Applied
              Cryptography and Network Security, pp. 103-119, June 2004.

   [Yourdon1979]
              Yourdon, E. and L. Constantine, "Structured Design:
              Fundamentals of a Discipline of Computer Program and
              Systems Design", Yourdon Press, New York, Chapter 10, pp.
              187-222, 1979.



















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Acknowledgments

   The authors would like to acknowledge the review and comments from
   Rene Struik and JP Vasseur.  The authors would also like to
   acknowledge the guidance and input provided by the ROLL Chairs, David
   Culler and JP Vasseur, and Area Director Adrian Farrel.

   This document started out as a combined threat and solutions
   document.  As a result of a series of security reviews performed by
   Steve Kent, the document was split up by ROLL Co-Chair Michael
   Richardson and Security Area Director Sean Turner as it went through
   the IETF publication process.  The solutions to the threats are
   application and L2 specific and have, therefore, been moved to the
   relevant applicability statements.

   Ines Robles and Robert Cragie kept track of the many issues that were
   raised during the development of this document.


































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Authors' Addresses

   Tzeta Tsao
   Eaton's Cooper Power Systems Business
   910 Clopper Rd., Suite 201S
   Gaithersburg, Maryland  20878
   United States
   EMail: tzetatsao@eaton.com

   Roger K. Alexander
   Eaton's Cooper Power Systems Business
   910 Clopper Rd., Suite 201S
   Gaithersburg, Maryland  20878
   United States
   EMail: rogeralexander@eaton.com

   Mischa Dohler
   CTTC
   Parc Mediterrani de la Tecnologia, Av. Canal Olimpic S/N
   Castelldefels, Barcelona  08860
   Spain
   EMail: mischa.dohler@kcl.ac.uk

   Vanesa Daza
   Universitat Pompeu Fabra
   P/ Circumval.lacio 8, Oficina 308
   Barcelona  08003
   Spain
   EMail: vanesa.daza@upf.edu

   Angel Lozano
   Universitat Pompeu Fabra
   P/ Circumval.lacio 8, Oficina 309
   Barcelona  08003
   Spain
   EMail: angel.lozano@upf.edu

   Michael Richardson (editor)
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z5V7
   Canada
   EMail: mcr+ietf@sandelman.ca








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