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6TiSCH Working Group                                           M. Tiloca
Internet-Draft                                              S. Duquennoy
Intended status: Standards Track                               RISE SICS
Expires: December 31, 2018                                       G. Dini
                                                      University of Pisa
                                                           June 29, 2018


     Robust scheduling against selective jamming in 6TiSCH networks
                draft-tiloca-6tisch-robust-scheduling-00

Abstract

   This document defines a method to generate robust TSCH schedules in a
   6TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4-2015) network, so as
   to protect network nodes against selective jamming attack.  Network
   nodes independently compute the new schedule at each slotframe, by
   altering the one originally available from 6top or alternative
   protocols, while preserving a consistent and collision-free
   communication pattern.  This method can be added on top of the
   minimal security framework for 6TiSCH.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 31, 2018.

Copyright Notice

   Copyright (c) 2018 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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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Properties of TSCH that simplify selective jamming  . . . . .   3
   3.  Attack example  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Building robust schedules . . . . . . . . . . . . . . . . . .   6
   5.  Adaptation to the 6TiSCH minimal security framework . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Effectiveness of schedule shuffling . . . . . . . . . . .   9
     6.2.  Renewal of key material . . . . . . . . . . . . . . . . .   9
     6.3.  Static timeslot allocations . . . . . . . . . . . . . . .   9
     6.4.  Network joining through randez-vous cells . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Nodes in a 6TiSCH network communicate using the IEEE 802.15.4-2015
   standard and its Timeslotted Channel Hopping (TSCH) mode.  Some
   properties of TSCH make schedule units, i.e. cells, and their usage
   predictable, even if security services are used at the MAC layer.

   This allows an external adversary to easy derive the communication
   pattern of a victim node.  After that, the adversary can perform a
   selective jamming attack, by efficiently and effectively transmitting
   over the only exact cell(s) in the victim's schedule.

   This document describes a method to counteract such an attack.  At
   each slotframe, every node autonomously computes a TSCH schedule, as
   a pseudo-random permutation of the one originally available from 6top
   [I-D.ietf-6tisch-6top-protocol] or alternative protocols.

   The resulting schedule is provided to TSCH and used to communicate
   during the next slotframe.  In particular, the new communication
   pattern results unpredictable for an external adversary.  Besides,
   since all nodes compute the same pseudo-random permutation, the new
   communication pattern remains consistent and collision-free.



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   Furthermore, this document specifies how this method can be added on
   top of the minimal security framework for 6TiSCH described in
   [I-D.ietf-6tisch-minimal-security].

1.1.  Terminology

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with terms and concepts defined
   in [I-D.ietf-6tisch-minimal-security], [I-D.ietf-6tisch-terminology]
   and [RFC8152].

   This document refers also to the following terminology:

   o  Permutation key.  A cryptographic key shared by network nodes and
      used to permute schedules.  Different keys are used to permute the
      utilization pattern of timeslots and of channelOffsets.

2.  Properties of TSCH that simplify selective jamming

   This section highlights a number of properties of the TSCH cell usage
   that greatly simplify the performance of the selective jamming attack
   described in Section 3.

   Given:

   o  The channel 'f' to communicate at timeslot 's' with ASN and
      channelOffset 'chOff' computed as f = F[(ASN + chOff) mod N_C];

   And assuming for simplicity that:

   o  N_S and N_C are coprime values;

   o  The channel hopping sequence is N_C in size and equal to {0, 1,
      ..., N_C - 1};

   Then, the following properties hold:

   o  Periodicity property.  The sequence of channels used for
      communication by a certain cell repeats with period (N_C x N_S)
      timeslots.

   o  Usage property.  Within a period, every cell uses all the
      available channels, each of which only once.



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   o  Offset property.  All cells follow the same sequence of channels
      with a certain offset.

   o  Predictability property.  For each cell, the sequence of channels
      is predictable.  That is, by knowing the channel used by a cell in
      a given timeslot, it is possible to compute the remaining channel
      hopping sub-sequence.

      In fact, given a cell active on channel 'f' and timeslot 's' on
      slotframe 'T', and since ASN = (s + T x N_S), it holds that

      f = [(s + T x N_S + c) mod N_C]  (Eq. 1)

      By solving this equation in 'c', one can predict the channels used
      by the cell in the next sloframes.  Note that, in order to do
      that, one does not need to know the absolute number 'T' of the
      slotframe (and thus the exact ASN) in which timeslot 's' uses a
      certain channel 'f'.  In fact, one can re-number slotframes
      starting from any arbitrarily assumed "starting-slotframe".

3.  Attack example

   This section describes how an external adversary can exploit the
   proterties in Section 2, and determine the full schedule of a victim
   node, even if security services at the MAC layer are used.  It is
   also assumed that the victim node actually transmits/receives during
   all its allocated cells at each slotframe.

   The example considers Figure 1, where N_S = 3, N_C = 4, and the
   channel hopping sequence is {0,1,2,3}. The shown schedule refers to a
   network node that uses three cells 'L_1', 'L_2' and 'L_3', with
   {0,3}, {1,1} and {2,0} as pairs {timeslot, channelOffset},
   respectively.


















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|==|===================================================================|
|Ch|                                ASN                                |
|  |===================================================================|
|Of| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 |11 |12 |13 |14 |15 |16 |
|==|===================================================================|
|0 |   |   |f=2|   |   |f=1|   |   |f=0|   |   |f=3|   |   |f=2|   |   |
|--|-------------------------------------------------------------------|
|1 |   |f=2|   |   |f=1|   |   |f=0|   |   |f=3|   |   |f=2|   |   |f=1|
|--|-------------------------------------------------------------------|
|2 |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
|--|-------------------------------------------------------------------|
|3 |f=3|   |   |f=2|   |   |f=1|   |   |f=0|   |   |f=3|   |   |f=2|   |
|==|===================================================================|
   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
   |s=0|s=1|s=2|s=0|s=1|s=2|s=0|s=1|s=2|s=0|s=1|s=2|s=0|s=1|s=2|s=0|s=1
   |           |           |           |           |           |
   |   T = 0   |   T = 1   |   T = 2   |   T = 3   |   T = 4   |  T = 5
               |
               \__ t = 0

           Figure 1: Attack example with slotframe re-numbering

   1.  The adversary starts the attack at absolute slotframe T = 1,
       which is assumed as "starting-slotframe" and thus renamed as
       slotframe t = 0.  The renaming is possible due to the offset and
       predictability properties.

   2.  The adversary picks a channel 'f*' at random, and monitors it for
       N_C consecutive slotframes to determine the timeslots in which
       the victim node communicates on that channel.  Due to the usage
       property, the number of such timeslots is equal to the number of
       cells assigned to the victim node.

       With reference to Figure 1, if, for example, f* = 1, the
       adversary determines that the victim node uses channel 'f*' in
       timeslots s = 1 and s = 2 of slotframe t = 0 and in timeslot s =
       0 of slotframe t = 1.  The adversary can then deduce that the
       victim node uses three different cells 'L_1', 'L_2' and 'L_3', in
       timeslots 0, 1 and 2, respectively.

   3.  The adversary determines the channels on which the victim node is
       going to transmit in the next slotframes, by exploiting the
       predictability property.

       That is, by instantiating Equation 1 for cell L_1, timeslot s = 0
       and slotframe t = 1, one gets [1 = (3 + c_1) mod 4], which has
       solution for c_1 = 2.  Hence, the function to predict the channel
       'f_1' to be used by cell 'L_1' in a slotframe 't', t >= 1, is f_1



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       = [(2 + 3 x t) mod 4], which produces the correct periodic
       sequence of channels {1, 0, 3, 2}. Similarly, one can instantiate
       Equation 1 for cells 'L_2' and 'L_3', so producing the respective
       periodic sequence of channels {1,0,3,2} and {1,0,3,2}.

   4.  The adversary has discovered the full schedule of the victim node
       and can proceed with the actual selective jamming attack.  That
       is, according to the found schedule, the adversary transmits over
       the exact cells used by the victim node for transmission/
       reception, while staying quiet and saving energy otherwise.  This
       results in a highly effective, highly efficient and hard to
       detect attack against communications of network nodes.

4.  Building robust schedules

   This section defines a method to protect network nodes against the
   selective jamming attack described in Section 3.  The proposed method
   alters the communication pattern of all network nodes at every
   slotframe, in a way unpredictable for an external adversary.

   At each slotframe 'T', network nodes autonomously compute the
   communication pattern for the next slotframe 'T+1' as a pseudo-random
   permutation of the one originally available.  In order to ensure that
   the new communication pattern remains consistent and collision-free,
   all nodes compute the same permutation of the original one.  In
   particular, at every slotframe, each node separately and
   independently permutes its timeslot utilization pattern (optionally)
   as well as its channelOffset utilization pattern.

   To perform the required permutations, all network nodes rely on a
   same secure pseudo-random number generator (SPRNG) as shown in
   Figure 2, where E(x,y) denotes a cipher which encrypts a plaintext
   'y' by means of a key 'x'.  Network nodes MUST support the AES-CCM-
   16-64-128 algorithm from [RFC8152].

   unsigned random(unsigned K, unsigned z) {
       unsigned val = E(K,z);
       return val;
   }

              Figure 2: Secure Pseudo-Random Number Generator

   All network nodes share the same following pieces of information.

   o  K_s, a permutation key used to permute the timeslot utilization
      pattern, and used as input to the random() function in Figure 2.
      K_s is provided upon joining the network, and MAY be provided as
      described in Section 5.



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   o  K_c, a permutation key used to permute the channelOffset
      utilization pattern, and used as input to the random() function in
      Figure 2.  K_c is provided upon joining the network, and MAY be
      provided as described in Section 5.

   o  z_s, a counter used to permute the timeslot utilization pattern,
      and used as input to the random() function in Figure 2.  At the
      beginning of each slotframe, z_s is equal to the ASN value of the
      first timeslot of that slotframe.  Then, z_s grows by N_S from the
      beginning of a slotframe to the beginning of the next one.

   o  z_c, a counter used to permute the channelOffset utilization
      pattern, and used as input to the random() function in Figure 2.
      At the beginning of each slotframe, z_c is equal to [N_C x
      floor(ASN* / N_S)], where ASN* is the ASN value of the first
      timeslot of that slotframe.  Then, z_c grows by N_C from the
      beginning of a slotframe to the beginning of the next one.

   Then, at every slotframe, each network node takes the following
   steps, and generates its own permuted communication schedule to be
   used at the following slotframe.  The actual permutation of cells
   relies on the well-known Fisher-Yates algorithm, that requires to
   generate n pseudo-random numbers in order to pseudo-randomly shuffle
   a vector of n elements.

   1.  First, a pseudo-random permutation is performed on the timeslot
       dimension of the slotframe.  This requires N_S invocations of
       random(K,z), consistently with the Fisher-Yates algorithm.  In
       particular, K = K_s, while z_s is passed as second argument and
       is incremented by 1 after each invocation.  The result of this
       step is a permuted timeslot utilization pattern, while the
       channelOffset utilization pattern is not permuted yet.

   2.  Second, a pseudo-random permutation is performed on the
       channelOffset dimension of the slotframe.  This requires N_C
       invocations of random(K,z), consistently with the Fisher-Yates
       algorithm.  In particular, K = K_c, while z_c is passed as second
       argument and is incremented by 1 after each invocation.  The
       result of this step is a fully shuffled communication pattern.

   The resulting schedule is then provided to TSCH and considered for
   sending/receiving traffic during the next slotframe.

   As further discussed in Section 6.3, it is possible, although NOT
   RECOMMENDED, to skip step 1 above, and hence permute only the
   channeOffset utilization pattern, while keeping a static timeslot
   utilization pattern.




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   Note for implementation: the process described above can be
   practically implemented by using two vectors, i.e. one for shuffling
   the timeslot utilization pattern and one for shuffling the
   channelOffset utilization pattern.

5.  Adaptation to the 6TiSCH minimal security framework

   The security mechanism described in this specification can be added
   on top of the minimal security framework for 6TiSCH
   [I-D.ietf-6tisch-minimal-security].

   That is, the two permutation keys K_s and K_c can be provided to a
   pledge when performing 6TiSCH Join Protocol (6JP).  To this end, the
   payload of the Join Response defined in Section 9.2 of
   [I-D.ietf-6tisch-minimal-security] is extended with a further
   COSE_KeySet specified in [RFC8152].

   Specifically, this COSE_KeySet contains one or two permutations keys
   and is interpreted as follows.  If two keys are present, they are
   used as K_s and K_c to permute the timeslot utilization pattern and
   the channelOffset utilization pattern, respectively, as per
   Section 4.  Instead, if only one key is present, it is used as K_c to
   permute the channelOffset utilization pattern as per Section 4.

   The resulting payload of Join Responses becomes as follows:

   response_payload = [
      COSE_KeySet,
      short_address,
      ? JRC_address : bstr,
      ? COSE_KeySet,
   ]

6.  Security Considerations

   With reference to Section 3.9 of [RFC7554], this specification
   achieves an additional "Secure Communication" objective, namely it
   defines a mechanism to build and enforce a TSCH schedule which is
   robust against selective jamming attack, while at the same time
   consistent and collision-free.

   Furthermore, the same security considerations from the minimal
   security framework for 6TiSCH [I-D.ietf-6tisch-minimal-security] hold
   for this document.  The rest of this section discusses a number of
   additional security considerations.






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6.1.  Effectiveness of schedule shuffling

   The countermeasure defined in Section 4 practically makes each node's
   schedule look random to an external observer.  Hence, it prevents the
   adversary from performing the attack described in Section 3.

   Then, a still available strategy for the adversary is to jam a number
   of cells selected at random, possibly on a per-slotframe basis.  This
   considerably reduces the attack effectiveness in successfully
   jeopardizing victims' communications.

   At the same time, nodes using different cells than the intended
   victims' would experience an overall slightly higher fraction of
   corrupted messages.  In fact, the communications of such accidental
   victims might be corrupted by the adversary, when they occur during a
   jammed timeslot and exactly over the channelOffset chosen at random.

6.2.  Renewal of key material

   It is RECOMMENDED that the two permutation keys K_s and K_c are
   revoked and renewed every time a node leaves the network.  This
   prevents a leaving node to keep the permutation keys, which may be
   exploited to selectively jam communications in the network.

   This rekeying operation is supposed to be performed anyway upon every
   change of network membership, in order to preserve backward and
   forward security.  In particular, new IEEE 802.15.4 link-layer keys
   are expected to be distributed before a new pledge can join the
   network, or after one ore more nodes have left the network.

   The specific approach to renew the two permutation keys, possibly
   together with other security material, is out of the scope of this
   specification.

6.3.  Static timeslot allocations

   As mentioned in Section 4 and Section 5, it is possible to permute
   only the channelOffset utilization pattern, while preserving the
   originally scheduled timeslot utilization pattern.  This can be
   desirable, or even unavoidable in some scenarios, in order to
   guarantee end-to-end latencies in multi-hop networks, as per
   accordingly designed schedules.

   However, it is NOT RECOMMENDED to preserve a static timeslot
   utilization pattern, as this would considerably increase the attack
   surface for a random jammer adversary.  That is, the adversary would
   immediately learn the timeslot utilization pattern of a victim node,




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   and would have a chance to successfully jam a victim's cell equal to
   (1/N_C), where N_C is the number of available channelOffsets.

6.4.  Network joining through randez-vous cells

   As described in [I-D.ietf-6tisch-minimal-security], a pledge joins a
   6TiSCH network through a Join Proxy (JP), according to 6TiSCH Join
   Protocol (6JP) and based on the information conveyed in Enhanced
   Beacons (EBs).  In particular, the pledge will communicate with the
   JP over indicated randez-vous cells.  In practice, such cells are
   typically part of a dedicate slotframe sequence, which is different
   from the slotframe sequence used for EB and data transmission.

   In order to keep the join process deterministic, the solution
   described in this specification can not be applied to the slotframe
   sequence with the randez-vous cells.  That is, an adversary would
   remain able to selectively jam the randez-vous cells, so potentially
   jeopardizing the 6JP and preventing pledges to join altogether.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Acknowledgments

   The authors sincerely thank Malisa Vucinic for the initial discussion
   about this document.

   The work on this document has been partly supported by the EIT-
   Digital High Impact Initiative ACTIVE.

9.  References

9.1.  Normative References

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-06 (work in progress), May 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.



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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer Protocol (6P)", draft-ietf-6tisch-6top-
              protocol-12 (work in progress), June 2018.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terms Used in IPv6 over the TSCH mode of IEEE 802.15.4e",
              draft-ietf-6tisch-terminology-10 (work in progress), March
              2018.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015, <https://www.rfc-
              editor.org/info/rfc7554>.

Authors' Addresses

   Marco Tiloca
   RISE SICS
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se


   Simon Duquennoy
   RISE SICS
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: simon.duquennoy@ri.se










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   Gianluca Dini
   University of Pisa
   Largo L. Lazzarino 2
   Pisa  56122
   Italy

   Email: gianluca.dini@unipi.it












































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