



6tisch Working Group                                          D. Dujovne
Internet-Draft                                Universidad Diego Portales
Intended status: Standards Track                           M. Richardson
Expires: 24 August 2020                         Sandelman Software Works
                                                        21 February 2020


   IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and
                         Enrollment Information
            draft-ietf-6tisch-enrollment-enhanced-beacon-14

Abstract

   In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are
   limited to specific times and specific channels.  Routers in a Time-
   Slotted Channel Hopping (TSCH) network transmit Enhanced Beacon (EB)
   frames to announce the presence of the network.  This document
   provides a mechanism by which additional information critical for new
   nodes (pledges) and long sleeping nodes may be carried within the
   Enhanced Beacon in order to conserve use of broadcast opportunities.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 24 August 2020.

Copyright Notice

   Copyright (c) 2020 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
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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Use of BCP 14 Terminology . . . . . . . . . . . . . . . .   2
     1.2.  Layer-2 Synchronization . . . . . . . . . . . . . . . . .   3
     1.3.  Layer-3 synchronization: IPv6 Router Solicitations and
           Advertisements  . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Layer-2 Selection . . . . . . . . . . . . . . . . . . . .   4
   2.  Protocol Definition . . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [RFC7554] describes the use of the Time-Slotted Channel Hopping
   (TSCH) mode of [ieee802154].

   In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are
   limited to specific times and specific channels.  Routers in a Time-
   Slotted Channel Hopping (TSCH) network transmit Enhanced Beacon (EB)
   frames during broadcast slots in order to announce the time and
   channel schedule.

   This document defines a new IETF Information Element (IE) subtype to
   place into the Enhanced Beacon (EB) to provide join and enrollment
   information to prospective pledges in a more efficient way.

   The following sub-sections explain the problem being solved, which
   justify carrying the join and enrollement information in the EB.

1.1.  Use of BCP 14 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.




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   Other terminology can be found in [I-D.ietf-6tisch-architecture] in
   section 2.1.

1.2.  Layer-2 Synchronization

   As explained in section 6 of [RFC8180], the Enhanced Beacon (EB) has
   a number of purposes: synchronization of the Absolute Slot Number
   (ASN) and Join Metric, carrying the timeslot template identifier,
   carrying the channel hopping sequence identifier, and indicating the
   TSCH SlotFrame.

   An EB announces the existence of a TSCH network, and of the nodes
   already joined to that network.  Receiving an EB allows a Joining
   Node (pledge) to learn about the network and synchronize to it.

   The EB may also be used as a means for a node already part of the
   network to re-synchronize [RFC7554].

   There are a limited number of timeslots designated as broadcast slots
   by each router in the network.  Considering 10ms slots and a slot-
   frame length of 100, these slots are rare and could result in only 1
   slot per second for broadcasts, which needs to be used for the
   beacon.  Additional broadcasts for Router Advertisements (RA), or
   Neighbor Discovery (ND) could even more scarce.

1.3.  Layer-3 synchronization: IPv6 Router Solicitations and
      Advertisements

   At layer 3, [RFC4861] defines a mechanism by which nodes learn about
   routers by receiving multicast Router Advertisements (RA).  If no RA
   is received within a set time, then a Router Solicitation (RS) may be
   transmitted as a multicast, to which an RA will be received, usually
   unicast.

   Although [RFC6775] reduces the amount of multicast necessary to do
   address resolution via Neighbor Solicitation (NS) messages, it still
   requires multicast of either RAs or RSes.  This is an expensive
   operation for two reasons: there are few multicast timeslots for
   unsolicited RAs; and if a pledge node does not receive an RA, and
   decides to transmit an RS, a broadcast aloha slot (see [RFC7554]
   section A.5) is consumed with unencrypted traffic.  [RFC6775] already
   allows for a unicast reply to such an RS.

   This is a particularly acute issue for the join process for the
   following reasons:

   1.  Use of a multicast slot by even a non-malicious unauthenticated
       node for a Router Solicitation (RS) may overwhelm that time slot.



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   2.  It may require many seconds of on-time before a new pledge
       receives a Router Advertisement (RA) that it can use.

   3.  A new pledge may have to receive many Enhanced Beacons (EB)
       before it can pick an appropriate network and/or closest Join
       Assistant to attach to.  If it must remain in the receive state
       for an RA as well as find the Enhanced Beacon (EB), then the
       process may take dozens of seconds, even minutes for each
       enrollment attempt that it needs to make.

1.4.  Layer-2 Selection

   In a complex Low-power and Lossy Networks (LLN), multiple LLNs may be
   connected together by backbone routers ( technology such as
   [I-D.ietf-6lo-backbone-router]), resulting in an area that is
   serviced by multiple distinct Layer-2 instances.  These are called
   Personal Area Networks (PAN).  Each instance will have a separate
   Layer-2 security profile, and will be distinguished by a different
   PANID.  The PANID is part of the [ieee802154] layer-2 header: it is a
   16-bit value which is chosen to be unique, and it contributes context
   to the layer-2 security mechanisms.  The PANID provides a context
   similar to the ESSID does in 802.11 networking, and can be conceived
   of in a similar fashion as the 802.3 ethernet VLAN tag in that it
   provides context for all layer-2 addresses.

   A device which is already enrolled in a network may find after a long
   sleep that it needs to resynchronize to the Layer 2 network.  The
   enrollment keys that it has will be specific to a PANID, but it may
   have more than one set of keys.  Such a device may wish to connect to
   a PAN that is experiencing less congestion, or which has a shalower
   ([RFC6550]) Routing Protocol for LLNs (RPL) tree.  It may even
   observe PANs for which it does not have keys, but which is believes
   it may have credentials that would allow it to join.

   In order to identify which PANs are part of the same backbone
   network, the network ID is introduced in this extension.  PANs that
   are part of the same backbone will be configured to use the same
   network ID.  For [RFC6550] RPL networks, configuration of the network
   ID can be done with an configuration option, which is the subject of
   future work.

   In order to provide some input to the choice of which PAN to use, the
   PAN priority field has been added.  This lists the relative priority
   for the PAN among different PANs.  Every Enhanced Beacon from a given
   PAN will likely have the same PAN priority.  Determination of the the
   PAN priority is the subject of future work; but it is expected that
   it will be calculated by an algorithm in the 6LBR, possibly involving
   communication between 6LBRs over the backbone network.



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   The [RFC6550] parent selection process can only operate within a
   single PAN, because it depends upon receiving RPL DIO messages from
   all available parents.  As part of the PAN selection process, the
   device may wish to know how deep in the LLN mesh it will be if it
   joins a particular PAN, and the rank priority field provides an
   estimation of what the rank of each announcer is.  Once the device
   synchronizes to a particular PAN's TSCH schedule then it may receive
   DIOs that are richer in their diversity than this value.  How this
   value will be used in practice is the subject of future research, and
   the interpretation of this value of the structure is considered
   experimental.

2.  Protocol Definition

   [RFC8137] creates a registry for new IETF IE subtypes.  This document
   allocates a new subtype.

   The new IE subtype structure is as follows.  As explained in
   [RFC8137] the length of the Sub-Type Content can be calculated from
   the container, so no length information is necessary.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   TBD-XXX     |R|P| res |  proxy prio |    rank priority      |
   +-+-+-+-+-+-+-+-+-+-------------+-------------+-----------------+
   | pan priority  |                                               |
   +---------------+                                               +
   |                     Join Proxy Interface-ID                   |
   +                        (present if P=1)                       +
   |                                                               |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                           network ID                          |
   +                   variable length, up to 16 bytes             +
   ~                                                               ~
   +                                                               +
   |                                                               |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

                       Figure 1: IE subtype structure

   res:  reserved bits MUST be ignored upon receipt, and SHOULD be set
      to 0 when sending.




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   R:  The Router Advertisement R-flag is set if the sending node will
      act as a Router for host-only nodes relying on stateless address
      auto-configuration (SLAAC) to get their global IPv6 address.
      Those hosts MUST send a unicast Router Solicitation message in
      order to receive a RA with the Prefix Information Option.

      In most cases, every node sending a beacon will set this flag, and
      in a typical mesh, this will be every single node.  When this bit
      is not set, it might indicate that this node may be under
      provisioned, or may have no additional slots for additional nodes.
      This could make this node more interesting to an attacker.

   P:  If the Proxy Address P-flag is set, then the Join Proxy Interface
      ID bit field is present.  Otherwise, it is not provided.

      This bit only indicates if another part of the structure is
      present, and has little security or privacy impact.

   proxy priority (proxy prio):  This field indicates the willingness of
      the sender to act as join proxy.  Lower value indicates greater
      willingness to act as a Join Proxy as described in
      [I-D.ietf-6tisch-minimal-security].  Values range from 0x00 (most
      willing) to 0x7e (least willing).  A priority of 0x7f indicates
      that the announcer should never be considered as a viable
      enrollment proxy.  Only unenrolled pledges look at this value.

      Lower values in this field indicate that the transmitter may have
      more capacity to handle unencrypted traffic.  A higher value may
      indicate that the transmitter is low on neighbor cache entries, or
      other resources.  Ongoing work such as
      [I-D.ietf-roll-enrollment-priority] documents one way to set this
      field.

   rank priority:  The rank "priority" is set by the IPv6 LLN Router
      (6LR) which sent the beacon and is an indication of how willing
      this 6LR is to serve as an RPL [RFC6550] parent within a
      particular network ID.  Lower values indicate more willingness,
      and higher values indicate less willingness.  This value is
      calculated by each 6LR according to algorithms specific to the
      routing metrics used by the RPL ([RFC6550]).  The exact process is
      a subject of significant research work.  It will typically be
      calculated from the RPL rank, and it may include some
      modifications based upon current number of children, or number of
      neighbor cache entries available.  Pledges MUST ignore this value.
      It helps enrolled devices only to compare connection points.

      An attacker can use this value to determine which nodes are
      potentially more interesting.  Nodes which are less willingness to



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      be parents likely have more traffic, and an attacker could use
      this information to determine which nodes would be more
      interesting to attack or disrupt.

   pan priority:  The pan priority is a value set by the Destination-
      Oriented Directed Acyclic Graph (DODAG) root (see [RFC6550],
      typically, the 6LBR) to indicate the relative priority of this LLN
      compared to those with different PANIDs that the operator might
      control.  This value may be used as part of the enrollment
      priority, but typically is used by devices which have already
      enrolled, and need to determine which PAN to pick when resuming
      from a long sleep.  Unenrolled pledges MAY consider this value
      when selecting a PAN to join.  Enrolled devices MAY consider this
      value when looking for an eligible parent device.  Lower values
      indicate a higher willingness to accept new nodes.

      An attacker can use this value, along with the observed PANID in
      the Beacon to determine which PANIDs have more network resources,
      and may have more interesting traffic.

   Join Proxy Interface ID:  If the P bit is set, then 64 bits (8 bytes)
      of address are present.  This field provides the Interface ID
      (IID) of the Link-Local address of the Join Proxy.  The associated
      prefix is well-known as fe80::/64.  If this field is not present,
      then IID is derived from the layer-2 address of the sender as per
      SLAAC ([RFC4662]).

      This field communicates the Interface ID bits that should be used
      for this node's layer-3 address, if it should not be derived from
      the layer-2 address.  Communication with the Join Proxy occurs in
      the clear.  This field avoids the need for an additional service-
      discovery process for the case where the L3 address is not derived
      from the L2 address.  An attacker will see both L2 and L3
      addresses, so this field provides no new information.

   network ID:  This is a variable length field, up to 16-bytes in size
      that uniquely identifies this network, potentially among many
      networks that are operating in the same frequencies in overlapping
      physical space.  The length of this field can be calculated as
      being whatever is left in the Information Element.

      In a 6tisch network, where RPL [RFC6550] is used as the mesh
      routing protocol, the network ID can be constructed from a
      truncated SHA256 hash of the prefix (/64) of the network.  This
      will be done by the RPL DODAG root and communicated by the RPL
      Configuration Option payloads, so it is not calculated more than
      once.  This is just a suggestion for a default algorithm: it may
      be set in any convenience way that results in a non-identifing



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      value.  In some LLNs where multiple PANIDs may lead to the same
      management device (the Join Registrar/Coordinator - JRC), then a
      common value that is the same across all the PANs MUST be
      configured.  Pledges that see the same networkID will not waste
      time attempting to enroll multiple times with the same network
      that when the network has multiple attachment points.

      If the network ID is derived as suggested, then it will be an
      opaque, seemingly random value, and will not directly reveal any
      information about the network.  An attacker can match this value
      across many transmissions to map the extent of a network beyond
      what the PANID might already provide.

3.  Security Considerations

   All of the contents of this Information Element are transmitted in
   the clear.  The content of the Enhanced Beacon is not encrypted.
   This is a restriction in the cryptographic architecture of the
   802.15.4 mechanism.  In order to decrypt or do integrity checking of
   layer-2 frames in TSCH, the TSCH Absolute Slot Number (ASN) is
   needed.  The Enhanced Beacon provides the ASN to new (and long-
   sleeping) nodes.

   The sensitivity of each field is described within the description of
   each field.

   The Enhanced Beacon is authenticated at the layer-2 level using
   802.15.4 mechanisms using the network-wide keying material.  Nodes
   which are enrolled will have the network-wide keying material and can
   validate the beacon.

   Pledges which have not yet enrolled are unable to authenticate the
   beacons, and will be forced to temporarily take the contents on
   faith.  After enrollment, a newly enrolled node will be able to
   return to the beacon and validate it.

   In addition to the enrollment and join information described in this
   document, the Enhanced Beacon contains a description of the TSCH
   schedule to be used by the transmitter of this packet.  The schedule
   can provide an attacker with a list of channels and frequencies on
   which communication will occur.  Knowledge of this can help an
   attacker to more efficiently jam communications, although there is
   future work being considered to make some of the schedule less
   visible.  Encrypting the schedule does not prevent an attacker from
   jamming, but rather increases the energy cost of doing that jamming.






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4.  Privacy Considerations

   The use of a network ID may reveal information about the network.
   The use of a SHA256 hash of the DODAGID (see [RFC6550]), rather than
   using the DODAGID itself directly provides some privacy for the the
   addresses used within the network, as the DODAGID is usually the IPv6
   address of the root of the RPL mesh.

   An interloper with a radio sniffer would be able to use the network
   ID to map out the extent of the mesh network.

5.  IANA Considerations

   IANA is asked to assign a new number TBD-XXX from Registry "IEEE Std
   802.15.4 IETF IE Subtype IDs" as defined by [RFC8137].

   This entry should be called 6tisch-Join-Info, and should refer to
   this document.

          Value   Subtype-ID          Reference
          ----    ----------          -----------
          TBD-XXX 6tisch-Join-Inbfo   [this document]

6.  Acknowledgements

   Thomas Watteyne provided extensive editorial comments on the
   document.  Carles Gomez Montenegro generated a detailed review of the
   document at WGLC.  Tim Evens provided a number of useful editorial
   suggestions.

7.  References

7.1.  Normative References

   [BCP14]    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>.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, 10 December 2019, <http://www.ietf.org/
              internet-drafts/draft-ietf-6tisch-minimal-security-
              15.txt>.

   [ieee802154]
              IEEE standard for Information Technology, ., "IEEE Std.



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              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", 2015,
              <http://standards.ieee.org/findstds/
              standard/802.15.4-2015.html>.

   [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>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
              2017, <https://www.rfc-editor.org/info/rfc8137>.

   [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>.

7.2.  Informative References

   [I-D.ietf-6lo-backbone-router]
              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", Work in Progress, Internet-Draft, draft-
              ietf-6lo-backbone-router-17, 20 February 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-6lo-
              backbone-router-17.txt>.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-6tisch-
              architecture-28.txt>.

   [I-D.ietf-roll-enrollment-priority]
              Richardson, M., "Enabling secure network enrollment in RPL



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              networks", Work in Progress, Internet-Draft, draft-ietf-
              roll-enrollment-priority-00, 16 September 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-roll-
              enrollment-priority-00.txt>.

   [RFC4662]  Roach, A. B., Campbell, B., and J. Rosenberg, "A Session
              Initiation Protocol (SIP) Event Notification Extension for
              Resource Lists", RFC 4662, DOI 10.17487/RFC4662, August
              2006, <https://www.rfc-editor.org/info/rfc4662>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., 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,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [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>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

Authors' Addresses

   Diego Dujovne (editor)
   Universidad Diego Portales
   Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441
   Santiago, Region Metropolitana
   Chile

   Phone: +56 (2) 676-8121
   Email: diego.dujovne@mail.udp.cl


   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca







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