IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and Enrollment Information
draft-ietf-6tisch-enrollment-enhanced-beacon-11

6tisch Working Group                                          D. Dujovne
Internet-Draft                                Universidad Diego Portales
Intended status: Standards Track                           M. Richardson
Expires: 17 August 2020                         Sandelman Software Works
                                                        14 February 2020

   IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and
                         Enrollment Information
            draft-ietf-6tisch-enrollment-enhanced-beacon-11

Abstract

   In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are
   limited to specific times and specific channels.  Nodes in a TSCH
   network typically frequently transmit Enhanced Beacon (EB) frames to
   announce the presence of the network.  This document provides a
   mechanism by which information critical for new nodes (pledges) and
   long sleeping nodes may be carried within the Enhanced Beacon.

Status of This Memo

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   This Internet-Draft will expire on 17 August 2020.

Copyright Notice

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

1.  Introduction

   [RFC7554] describes the use of the time-slotted channel hopping
   (TSCH) mode of [ieee802154].  As further detailed in [RFC8180], an
   Enhanced Beacon (EB) is transmitted during a slot designated as a
   broadcast slot.

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.

   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 ASN and Join Metric,
   carrying timeslot template identifier, carrying the channel hopping
   sequence identifier, and indicating the TSCH SlotFrame.

   The EB is used by nodes already part of a TSCH network to announce
   their existence.  Receiving an EB allows a Joining Node (pledge) to
   learn about the network and synchronize to it.  The EB may also be

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   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, or Neighbor
   Discovery 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 RS.  This is an expensive
   operation for two reasons: First, there are few multicast timeslots
   for unsolicited RAs; and second, if a pledge node does not receive an
   RA, and decides to transmit an RS, a broadcast aloha slot is consumed
   with unencrypted traffic.  In this case, a unicast RS may be
   transmitted in response.

   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.

   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 a very long time.

   This document defines a new IETF IE subtype to provide join and
   enrollment information to prospective pledges in a more efficient
   way.

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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 lower-64                 |
   +                        (present if P=1)                       +
   |                                                               |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                           network ID                          |
   +                   variable length, up to 16 bytes             +
   ~                                                               ~
   +                                                               +
   |                                                               |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

                       Figure 1: IE subtype structure

   R:  The Router Advertisement R-flag is set if the sending node will
      act as a Router for host-only nodes that need addressing via
      unicast Router Solicitation messages.

      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 indicates 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 lower-64
      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.

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   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 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.

   rank priority:  The rank "priority" is set by the 6LR which sent the
      beacon and is an indication of how willing this 6LR is to serve as
      an RPL parent within a particular network ID.  This is a local
      value to be determined in other work.  It might be calculated from
      RPL rank, and it may include some modifications based upon current
      number of children, or number of neighbor cache entries available.
      This value MUST be ignored by pledges, it is for enrolled devices
      only.  Lower values are better.

      An attacker can use this value to determine which nodes are
      potentially more interesting.  Nodes which are less willing to 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 DODAG root to
      indicate the relative priority of this LLN compared to those with
      different PANIDs.  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.
      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.

      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 lower-64:  If the P bit is set, then 64 bits (8 bytes) of
      address are present.  This field provides the suffix (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.

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      This field communicates a lower-64 bits that should be used for
      this nodes' 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 SHA256
      hash of the prefix (/64) of the network.  That is just a
      suggestion for a default value.  In some LLNs where multiple
      PANIDs may lead to the same management device (the JRC), then a
      common value that is the same across all PANs MUST be configured.

      If the the network ID is derived as suggested, then it will an
      opaque, seemingly random value, and will reveal nothing in of
      itself.  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 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.

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   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.

4.  Privacy Considerations

   The use of a network ID may reveal information about the network.
   The use of a SHA256 hash of the DODAGID, rather than using the
   DODAGID (which is usually derived from the LLN prefix) directly
   provides some privacy for the the addresses used within the network.
   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

   Allocate a new number TBD-XXX from Registry IETF Information Element
   (IE) Sub-type ID, as defined by [RFC8137].  This entry should be
   called 6tisch-Join-Info, and should refer to 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/

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              internet-drafts/draft-ietf-6tisch-minimal-security-
              15.txt>.

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

   [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,

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              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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