6tisch Working Group                                          D. Dujovne
Internet-Draft                                Universidad Diego Portales
Intended status: Standards Track                           M. Richardson
Expires: 19 July 2020                           Sandelman Software Works
                                                         16 January 2020

   IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and
                         Enrollment Information


   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 send Enhanced Beacon (EB) frames to
   announce the presence of the network.  This document provides a
   mechanism by which small details 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 19 July 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 a
   broadcast slot.

1.1.  Use of BCP 14 Terminology

   ::boilerplate bcp14

   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 annouce its
   existence.  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

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   There is a limited number of timeslots designated as a broadcast slot
   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/s for a broadcast, 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

   At layer 3, [RFC4861] defines a mechanism by which nodes learn about
   routers by receiving multicast Router Advertisements (RA).  If no RA
   is heard within a set time, then a Router Solicitation (RS) may be
   sent as 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 hear an
   RA, and decides to send a RS, a broadcast aloha slot is consumed with
   unencrypted traffic.  In this case, a unicast RS may be sent in

   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 hears
       a Router Advertisement (RA) that it can use.

   3.  a new pledge may listen to many Enhanced Beacons (EB) before it
       can pick an appropriate network and/or closest Join Assistant to
       attach to.  If it must listen for a 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

2.  Protocol Definition

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

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   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.

   P  if the Proxy Address P-flag is set, then the lower 64-bits of the
      Join Proxy's link-local address follows the network ID.  If the
      Proxy Address bit is not set, then the Link Layer address of the
      Join Proxy is identical to the Layer-2 8-byte address used to
      originate this enhanced beacon.  In either case, the destination
      layer-2 address of this beacon may use the layer-2 address which
      was used to originate the beacon.

   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.

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   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

   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.

   Join Proxy lower-64  if the P bit is set, then 64 bits (8 bytes) of
      address are present.  This field provides the suffix of the Link-
      Local address of the Join Proxy.  The associated prefix is well-
      known as fe80::/64.

   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.

3.  Security Considerations

   All of the contents of this Information Element are sent in the
   clear.  The containing Enhanced Beacon is not encrypted.

   The Enhanced Beagon 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

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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, rather than using the
   DODAGID directly provides some cover 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 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

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>.

              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/

              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,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

   [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>.

7.2.  Informative References

              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,

   [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,

   [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,

   [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>.

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Authors' Addresses

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

   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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