Encapsulation of 6TiSCH Join and Enrollment Information Elements
RFC 9032

Document Type RFC - Proposed Standard (May 2021; No errata)
Authors Diego Dujovne  , Michael Richardson 
Last updated 2021-05-29
Replaces draft-richardson-6tisch-enrollment-enhanced-beacon
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Send notices to Pascal Thubert <pthubert@cisco.com>
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Internet Engineering Task Force (IETF)                   D. Dujovne, Ed.
Request for Comments: 9032                    Universidad Diego Portales
Category: Standards Track                                  M. Richardson
ISSN: 2070-1721                                 Sandelman Software Works
                                                                May 2021

    Encapsulation of 6TiSCH Join and Enrollment Information Elements

Abstract

   In the Time-Slotted Channel Hopping (TSCH) mode of IEEE Std 802.15.4,
   opportunities for broadcasts are limited to specific times and
   specific channels.  Routers in a 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 EB in order to conserve use of broadcast
   opportunities.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9032.

Copyright Notice

   Copyright (c) 2021 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   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
     1.1.  Terminology
     1.2.  Layer 2 Synchronization
     1.3.  Layer 3 Synchronization: IPv6 Router Solicitations and
           Advertisements
     1.4.  Layer 2 Selection
   2.  Protocol Definition
   3.  Security Considerations
   4.  Privacy Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   [RFC7554] describes the use of the Time-Slotted Channel Hopping
   (TSCH) mode of [IEEE.802.15.4].

   In TSCH mode of IEEE Std 802.15.4, opportunities for broadcasts are
   limited to specific times and specific channels.  Routers in a 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 EB to provide join and enrollment information to
   prospective pledges in a more efficient way.

   The following subsections explain the problem being solved, which
   justifies carrying the join and enrollment information in the EB.

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.

   Other terminology can be found in Section 2.1 of [RFC9030].

1.2.  Layer 2 Synchronization

   As explained in Section 4.5.2 of [RFC8180], the EB has a number of
   purposes: it carries synchronization information such as the Absolute
   Slot Number (ASN) and Join Metric and identifiers for the timeslot
   template and the channel hopping sequence, and it indicates the TSCH
   slotframe.

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

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

   There are a limited number of timeslots designated as broadcast slots
   by each router in the network.  Considering 10 ms slots and a
   slotframe 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 be 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 RAs.  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 for
   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 Appendix A.5
   of [RFC7554]) 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 timeslot.

   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 EBs before it can pick an
       appropriate network and/or closest Join Proxy to attach to.  If
       it must remain in the receive state for an RA as well as find the
       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 Network (LLN), multiple LLNs may be
   connected together by Backbone Routers (technology such as
   [RFC8929]), resulting in an area that is serviced by multiple,
   distinct Layer 2 instances.  These are called Personal Area Networks
   (PANs).  Each instance will have a separate Layer 2 security profile
   and will be distinguished by a different PANID.  The PANID is part of
   the Layer 2 header as defined in [IEEE.802.15.4]: it is a 16-bit
   value that is chosen to be unique, and it contributes context to the
   Layer 2 security mechanisms.  The PANID provides a context similar to
   the Extended Service Set ID (ESSID) in 802.11 networking and can be
   considered similar to the 802.3 Ethernet VLAN tag in that it provides
   context for all Layer 2 addresses.

   A device that is already enrolled in a network may find after a long
   sleep that it needs to resynchronize with the Layer 2 network.  The
   device's enrollment keys will be specific to a PANID, but the device
   may have more than one set of keys.  Such a device may wish to
   connect to a PAN that is experiencing less congestion or that has a
   shallower Routing Protocol for LLNs (RPL) tree [RFC6550].  It may
   even observe PANs for which it does not have keys, but for which it
   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 RPL networks [RFC6550], configuration of the network
   ID can be done with a 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 EB from a given PAN will
   likely have the same PAN priority.  Determination of the PAN priority
   is the subject of future work; but it is expected that it will be
   calculated by an algorithm in the 6LoWPAN Border Router (6LBR),
   possibly involving communication between 6LBRs over the backbone
   network.

   The parent selection process [RFC6550] 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 each announcer's rank.  Once the device synchronizes
   with a particular PAN's TSCH schedule, it may receive DIOs that are
   richer in their diversity than this value.  The use of this value 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 subtype 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |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.

   R:  The RA 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 RS message in order to receive an 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 that it 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 it has little security or privacy impact.

   proxy prio (proxy priority):  This field indicates the willingness of
      the sender to act as a Join Proxy.  Lower value indicates greater
      willingness to act as a Join Proxy as described in [RFC9031].
      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 Join 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 [NETWORK-ENROLLMENT]
      documents one way to set this field.

   rank priority:  The rank priority is set by the IPv6 LLN Router (6LR)
      that sent the beacon and is an indication of how willing this 6LR
      is to serve as a RPL parent [RFC6550] 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 the 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 that 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 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 it is used by devices that 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 more willingness to accept new nodes.

      An attacker can use this value, along with the observed PANID in
      the EB 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 per
      SLAAC [RFC4862].

      This field communicates the IID 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 Layer 3 address is not derived from
      the Layer 2 address.  An attacker will see both Layer 2 and Layer
      3 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 IE.

      In a 6TiSCH network, where RPL [RFC6550] is used as the mesh
      routing protocol, the network ID can be constructed from a
      truncated SHA-256 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 convenient way that results in a non-identifying
      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 network ID will not waste
      time attempting to enroll multiple times with the same network
      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 IE are transmitted in the clear.  The
   content of the EB 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
   ASN is needed.  The EB provides the ASN to new (and long-sleeping)
   nodes.

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

   The EB is authenticated at the Layer 2 level using 802.15.4
   mechanisms using the network-wide keying material.  Nodes that are
   enrolled will have the network-wide keying material and can validate
   the beacon.

   Pledges that 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 EB 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 SHA-256 hash of the DODAGID (see [RFC6550]), rather than
   using the DODAGID itself directly provides some privacy for 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 has assigned the following value in the "IEEE Std 802.15.4 IETF
   IE Subtype IDs" registry, as defined by [RFC8137].

                 +=======+==================+===========+
                 | Value | Subtype ID       | Reference |
                 +=======+==================+===========+
                 | 2     | 6tisch-Join-Info | RFC 9032  |
                 +-------+------------------+-----------+

                                 Table 1

6.  References

6.1.  Normative References

   [IEEE.802.15.4]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
              Standard 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875,
              April 2016,
              <https://ieeexplore.ieee.org/document/7460875>.

   [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>.

   [RFC9031]  Vučinić, M., Ed., Simon, J., Pister, K., and M.
              Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
              RFC 9031, DOI 10.17487/RFC9031, May 2021,
              <https://www.rfc-editor.org/info/rfc9031>.

6.2.  Informative References

   [NETWORK-ENROLLMENT]
              Richardson, M., Jadhav, R. A., Thubert, P., and H. She,
              "Controlling Secure Network Enrollment in RPL networks",
              Work in Progress, Internet-Draft, draft-ietf-roll-
              enrollment-priority-04, 7 February 2021,
              <https://tools.ietf.org/html/draft-ietf-roll-enrollment-
              priority-04>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [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>.

   [RFC8929]  Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
              "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
              November 2020, <https://www.rfc-editor.org/info/rfc8929>.

   [RFC9030]  Thubert, P., Ed., "An Architecture for IPv6 over the Time-
              Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
              RFC 9030, DOI 10.17487/RFC9030, May 2021,
              <https://www.rfc-editor.org/info/rfc9030>.

Acknowledgments

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

Authors' Addresses

   Diego Dujovne (editor)
   Universidad Diego Portales
   Escuela de Informática y Telecomunicaciones
   Av. Ejército 441
   Santiago
   Región Metropolitana
   Chile

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

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca