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An Architecture for IPv6 over the Time-Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9030.
Author Pascal Thubert
Last updated 2021-05-29 (Latest revision 2020-11-26)
Replaces draft-thubert-6tisch-architecture
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Shwetha Bhandari
Shepherd write-up Show Last changed 2019-07-05
IESG IESG state Became RFC 9030 (Informational)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Suresh Krishnan
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                          26 November 2020
Expires: 30 May 2021

      An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4


   This document describes a network architecture that provides low-
   latency, low-jitter and high-reliability packet delivery.  It
   combines a high-speed powered backbone and subnetworks using IEEE
   802.15.4 time-slotted channel hopping (TSCH) to meet the requirements
   of LowPower wireless deterministic applications.

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

   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 30 May 2021.

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
   Provisions Relating to IETF Documents (
   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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .  10
     2.3.  Related Documents . . . . . . . . . . . . . . . . . . . .  11
   3.  High Level Architecture . . . . . . . . . . . . . . . . . . .  12
     3.1.  A Non-Broadcast Multi-Access Radio Mesh Network . . . . .  12
     3.2.  A Multi-Link Subnet Model . . . . . . . . . . . . . . . .  14
     3.3.  TSCH: A Deterministic MAC Layer . . . . . . . . . . . . .  16
     3.4.  Scheduling TSCH . . . . . . . . . . . . . . . . . . . . .  17
     3.5.  Distributed vs. Centralized Routing . . . . . . . . . . .  18
     3.6.  Forwarding Over TSCH  . . . . . . . . . . . . . . . . . .  19
     3.7.  6TiSCH Stack  . . . . . . . . . . . . . . . . . . . . . .  20
     3.8.  Communication Paradigms and Interaction Models  . . . . .  22
   4.  Architecture Components . . . . . . . . . . . . . . . . . . .  23
     4.1.  6LoWPAN (and RPL) . . . . . . . . . . . . . . . . . . . .  23
       4.1.1.  RPL-Unaware Leaves and 6LoWPAN ND . . . . . . . . . .  23
       4.1.2.  6LBR and RPL Root . . . . . . . . . . . . . . . . . .  24
     4.2.  Network Access and Addressing . . . . . . . . . . . . . .  24
       4.2.1.  Join Process  . . . . . . . . . . . . . . . . . . . .  25
       4.2.2.  Registration  . . . . . . . . . . . . . . . . . . . .  27
     4.3.  TSCH and 6top . . . . . . . . . . . . . . . . . . . . . .  28
       4.3.1.  6top  . . . . . . . . . . . . . . . . . . . . . . . .  28
       4.3.2.  Scheduling Functions and the 6top protocol  . . . . .  30
       4.3.3.  6top and RPL Objective Function operations  . . . . .  31
       4.3.4.  Network Synchronization . . . . . . . . . . . . . . .  32
       4.3.5.  Slotframes and CDU matrix . . . . . . . . . . . . . .  33
       4.3.6.  Distributing the reservation of cells . . . . . . . .  34
     4.4.  Schedule Management Mechanisms  . . . . . . . . . . . . .  35
       4.4.1.  Static Scheduling . . . . . . . . . . . . . . . . . .  35
       4.4.2.  Neighbor-to-neighbor Scheduling . . . . . . . . . . .  36
       4.4.3.  Remote Monitoring and Schedule Management . . . . . .  37
       4.4.4.  Hop-by-hop Scheduling . . . . . . . . . . . . . . . .  39
     4.5.  On Tracks . . . . . . . . . . . . . . . . . . . . . . . .  39
       4.5.1.  General Behavior of Tracks  . . . . . . . . . . . . .  40
       4.5.2.  Serial Track  . . . . . . . . . . . . . . . . . . . .  40
       4.5.3.  Complex Track with Replication and Elimination  . . .  41
       4.5.4.  DetNet End-to-end Path  . . . . . . . . . . . . . . .  41
       4.5.5.  Cell Reuse  . . . . . . . . . . . . . . . . . . . . .  42
     4.6.  Forwarding Models . . . . . . . . . . . . . . . . . . . .  43
       4.6.1.  Track Forwarding  . . . . . . . . . . . . . . . . . .  43
       4.6.2.  IPv6 Forwarding . . . . . . . . . . . . . . . . . . .  46
       4.6.3.  Fragment Forwarding . . . . . . . . . . . . . . . . .  47
     4.7.  Advanced 6TiSCH Routing . . . . . . . . . . . . . . . . .  48
       4.7.1.  Packet Marking and Handling . . . . . . . . . . . . .  48
       4.7.2.  Replication, Retries and Elimination  . . . . . . . .  49

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   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  52
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  52
     6.1.  Availability of Remote Services . . . . . . . . . . . . .  52
     6.2.  Selective Jamming . . . . . . . . . . . . . . . . . . . .  52
     6.3.  MAC-Layer Security  . . . . . . . . . . . . . . . . . . .  53
     6.4.  Time Synchronization  . . . . . . . . . . . . . . . . . .  53
     6.5.  Validating ASN  . . . . . . . . . . . . . . . . . . . . .  54
     6.6.  Network Keying and Rekeying . . . . . . . . . . . . . . .  55
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  56
     7.1.  Contributors  . . . . . . . . . . . . . . . . . . . . . .  56
     7.2.  Special Thanks  . . . . . . . . . . . . . . . . . . . . .  57
     7.3.  And Do not Forget . . . . . . . . . . . . . . . . . . . .  58
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  58
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  62
   Appendix A.  Related Work In Progress . . . . . . . . . . . . . .  69
     A.1.  Unchartered IETF work items . . . . . . . . . . . . . . .  69
       A.1.1.  6TiSCH Zerotouch security . . . . . . . . . . . . . .  69
       A.1.2.  6TiSCH Track Setup  . . . . . . . . . . . . . . . . .  69
       A.1.3.  Using BIER in a 6TiSCH Network  . . . . . . . . . . .  70
     A.2.  External (non-IETF) work items  . . . . . . . . . . . . .  70
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  71

1.  Introduction

   Wireless Networks enable a wide variety of devices of any size to get
   interconnected, often at a very low marginal cost per device, at any
   range, and in circumstances where wiring may be impractical, for
   instance on fast-moving or rotating devices.

   On the other hand, Deterministic Networking maximizes the packet
   delivery ratio within a bounded latency so as to enable mission-
   critical machine-to-machine (M2M) operations.  Applications that need
   such networks are presented in [RFC8578].  The considered
   applications include Professional Media, Industrial Automation
   Control Systems (IACS), building automation, in-vehicle command and
   control, commercial automation and asset tracking with mobile
   scenarios, as well as gaming, drones and edge robotic control, and
   home automation applications.

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   The Timeslotted Channel Hopping (TSCH) [RFC7554] mode of the IEEE
   Std. 802.15.4 [IEEE802154] Medium Access Control (MAC) was introduced
   with the IEEE Std. 802.15.4e [IEEE802154e] amendment and is now
   retrofitted in the main standard.  For all practical purposes, this
   document is expected to be insensitive to the revisions of that
   standard, which is thus referenced without a date.  TSCH is both a
   Time-Division Multiplexing and a Frequency-Division Multiplexing
   technique whereby a different channel can be used for each
   transmission, and that allows to schedule transmissions for
   deterministic operations, and applies to the slower and most energy
   constrained wireless use cases.

   The scheduled operation provides for a more reliable experience which
   can be used to monitor and manage resources, e.g., energy and water,
   in a more efficient fashion.

   Proven Deterministic Networking standards for use in Process Control,
   including ISA100.11a [ISA100.11a] and WirelessHART [WirelessHART],
   have demonstrated the capabilities of the IEEE Std. 802.15.4 TSCH MAC
   for high reliability against interference, low-power consumption on
   well-known flows, and its applicability for Traffic Engineering (TE)
   from a central controller.

   To enable the convergence of Information Technology (IT) and
   Operational Technology (OT) in Low-Power Lossy Networks (LLNs), the
   6TiSCH Architecture supports an IETF suite of protocols over the IEEE
   Std. 802.15.4 TSCH MAC to provide IP connectivity for energy and
   otherwise constrained wireless devices.

   The 6TiSCH Architecture relies on IPv6 [RFC8200] and the use of
   routing to provide large scaling capabilities.  The addition of a
   high-speed federating backbone adds yet another degree of scalability
   to the design.  The backbone is typically a Layer-2 transit Link such
   as an Ethernet bridged network, but it can also be a more complex
   routed structure.

   The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet model
   that is composed of a federating backbone and a number of IEEE Std.
   802.15.4 TSCH low-power wireless networks federated and synchronized
   by Backbone Routers.  If the backbone is a Layer-2 transit Link then
   the Backbone Routers can operate as an IPv6 Neighbor Discovery (IPv6
   ND) [RFC4861] proxy.

   The 6TiSCH Architecture leverages 6LoWPAN [RFC4944] to adapt IPv6 to
   the constrained media and RPL [RFC6550] for the distributed routing

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   Centralized routing refers to a model where routes are computed and
   resources are allocated from a central controller.  This is
   particularly helpful to schedule deterministic multihop
   transmissions.  In contrast, Distributed Routing refers to a model
   that relies on concurrent peer to peer protocol exchanges for TSCH
   resource allocation and routing operations.

   The architecture defines mechanisms to establish and maintain routing
   and scheduling in a centralized, distributed, or mixed fashion, for
   use in multiple OT environments.  It is applicable in particular to
   highly scalable solutions such as used in Advanced Metering
   Infrastructure [AMI] solutions that leverage distributed routing to
   enable multipath forwarding over large LLN meshes.

2.  Terminology

2.1.  New Terms

   The draft does not reuse terms from the IEEE Std. 802.15.4
   [IEEE802154] standard such as "path" or "link" which bear a meaning
   that is quite different from classical IETF parlance.

   This document adds the following terms:

   6TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4):  6TiSCH defines an
      adaptation sublayer for IPv6 over TSCH called 6top, a set of
      protocols for setting up a TSCH schedule in distributed approach,
      and a security solution. 6TiSCH may be extended in the future for
      other MAC/PHY pairs providing a service similar to TSCH.

   6top (6TiSCH Operation Sublayer):  The next higher layer of the IEEE
      Std. 802.15.4 TSCH MAC layer.  6top provides the abstraction of an
      IP link over a TSCH MAC, schedules packets over TSCH cells, and
      exposes a management interface to schedule TSCH cells.

   6P (6top Protocol):  The protocol defined in [RFC8480].  6P enables
      Layer-2 peers to allocate, move or deallocate cells in their
      respective schedules to communicate.  6P operates at the 6top

   6P Transaction:  A 2-way or 3-way sequence of 6P messages used by
      Layer-2 peers to modify their communication schedule.

   ASN (Absolute Slot Number):  Defined in [IEEE802154], the ASN is the
      total number of timeslots that have elapsed since the Epoch Time
      when the TSCH network started.  Incremented by one at each
      timeslot.  It is wide enough to not roll over in practice.

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   bundle:  A group of equivalent scheduled cells, i.e., cells
      identified by different [slotOffset, channelOffset], which are
      scheduled for a same purpose, with the same neighbor, with the
      same flags, and the same slotframe.  The size of the bundle refers
      to the number of cells it contains.  For a given slotframe length,
      the size of the bundle translates directly into bandwidth.  A
      bundle is a local abstraction that represents a half-duplex link
      for either sending or receiving, with bandwidth that amounts to
      the sum of the cells in the bundle.

   Layer-2 vs. Layer-3 bundle:  Bundles are associated for either
      Layer-2 (switching) or Layer-3 (routing) forwarding operations.  A
      pair of Layer-3 bundles (one for each direction) maps to an IP
      Link with a neighbor, whereas a set of Layer-2 bundles (of an
      "arbitrary" cardinality and direction) corresponds to the relation
      of one or more incoming bundle(s) from the previous-hop
      neighbor(s) with one or more outgoing bundle(s) to the next-hop
      neighbor(s) along a Track as part of the switching role, which may
      include replication and elimination.

   CCA (Clear Channel Assessment):  A mechanism defined in [IEEE802154]
      whereby nodes listen to the channel before sending to detect
      ongoing transmissions from other parties.  Because the network is
      synchronized, CCA cannot be used to detect colliding transmissions
      within the same network, but it can be used to detect other radio
      networks in vicinity.

   cell:  A unit of transmission resource in the CDU matrix, a cell is
      identified by a slotOffset and a channelOffset.  A cell can be
      scheduled or unscheduled.

   Channel Distribution/Usage (CDU) matrix:  : A matrix of cells (i,j)
      representing the spectrum (channel) distribution among the
      different nodes in the 6TiSCH network.  The CDU matrix has width
      in timeslots, equal to the period of the network scheduling
      operation, and height equal to the number of available channels.
      Every cell (i,j) in the CDU, identified by (slotOffset,
      channelOffset), belongs to a specific chunk.

   channelOffset:  Identifies a row in the TSCH schedule.  The number of
      channelOffset values is bounded by the number of available
      frequencies.  The channelOffset translates into a frequency with a
      function that depends on the absolute time when the communication
      takes place, resulting in a channel hopping operation.

   chunk:  A well-known list of cells, distributed in time and

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      frequency, within a CDU matrix.  A chunk represents a portion of a
      CDU matrix.  The partition of the CDU matrix in chunks is globally
      known by all the nodes in the network to support the appropriation
      process, which is a negotiation between nodes within an
      interference domain.  A node that manages to appropriate a chunk
      gets to decide which transmissions will occur over the cells in
      the chunk within its interference domain, i.e., a parent node will
      decide when the cells within the appropriated chunk are used and
      by which node, among its children.

   CoJP (Constrained Join Protocol):  The Constrained Join Protocol
      (CoJP) enables a pledge to securely join a 6TiSCH network and
      obtain network parameters over a secure channel.  Minimal Security
      Framework for 6TiSCH [MIN-SECURITY] defines the minimal CoJP setup
      with pre-shared keys defined.  In that mode, CoJP can operate with
      a single round trip exchange.

   dedicated cell:  A cell that is reserved for a given node to transmit
      to a specific neighbor.

   deterministic network:  The generic concept of deterministic network
      is defined in the "DetNet Architecture" [RFC8655] document.  When
      applied to 6TiSCH, it refers to the reservation of Tracks which
      guarantees an end-to-end latency and optimizes the Packet Delivery
      Ratio (PDR) for well-characterized flows.

   distributed cell reservation:  A reservation of a cell done by one or
      more in-network entities.

   distributed Track reservation:  A reservation of a Track done by one
      or more in-network entities.

   EB (Enhanced Beacon):  A special frame defined in [IEEE802154] used
      by a node, including the JP, to announce the presence of the
      network.  It contains enough information for a pledge to
      synchronize to the network.

   hard cell:  A scheduled cell which the 6top sublayer may not

   hopping sequence:  Ordered sequence of frequencies, identified by a
      Hopping_Sequence_ID, used for channel hopping when translating the
      channelOffset value into a frequency.

   IE (Information Element):  Type-Length-Value containers placed at the

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      end of the MAC header, used to pass data between layers or
      devices.  Some IE identifiers are managed by the IEEE
      [IEEE802154].  Some IE identifiers are managed by the IETF
      [RFC8137], and [ENH-BEACON] uses one subtype to support the
      selection of the Join Proxy.

   join process:  The overall process that includes the discovery of the
      network by pledge(s) and the execution of the join protocol.

   join protocol:  The protocol that allows the pledge to join the
      network.  The join protocol encompasses authentication,
      authorization and parameter distribution.  The join protocol is
      executed between the pledge and the JRC.

   joined node:  The new device, after having completed the join
      process, often just called a node.

   JP (Join Proxy):  Node already part of the 6TiSCH network that serves
      as a relay to provide connectivity between the pledge and the JRC.
      The JP announces the presence of the network by regularly sending
      EB frames.

   JRC (Join Registrar/Coordinator):  Central entity responsible for the
      authentication, authorization and configuration of the pledge.

   link:  A communication facility or medium over which nodes can
      communicate at the Link-Layer, the layer immediately below IP.  In
      6TiSCH, the concept is implemented as a collection of Layer-3
      bundles.  Note: the IETF parlance for the term "Link" is adopted,
      as opposed to the IEEE Std. 802.15.4 terminology.

   Operational Technology:  OT refers to technology used in automation,
      for instance in industrial control networks.  The convergence of
      IT and OT is the main object of the Industrial Internet of Things

   pledge:  A new device that attempts to join a 6TiSCH network.

   (to) relocate a cell:  The action operated by the 6top sublayer of
      changing the slotOffset and/or channelOffset of a soft cell.

   (to) schedule a cell:  The action of turning an unscheduled cell into
      a scheduled cell.

   scheduled cell:  A cell which is assigned a neighbor MAC address

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      (broadcast address is also possible), and one or more of the
      following flags: TX, RX, Shared and Timekeeping.  A scheduled cell
      can be used by the IEEE Std. 802.15.4 TSCH implementation to
      communicate.  A scheduled cell can either be a hard or a soft

   SF (6top Scheduling Function):  The cell management entity that adds
      or deletes cells dynamically based on application networking
      requirements.  The cell negotiation with a neighbor is done using

   SFID (6top Scheduling Function Identifier):  A 4-bit field
      identifying an SF.

   shared cell:  A cell marked with both the "TX" and "shared" flags.
      This cell can be used by more than one transmitter node.  A back-
      off algorithm is used to resolve contention.

   slotframe:  A collection of timeslots repeating in time, analogous to
      a superframe in that it defines periods of communication
      opportunities.  It is characterized by a slotframe_ID, and a
      slotframe_size.  Multiple slotframes can coexist in a node's
      schedule, i.e., a node can have multiple activities scheduled in
      different slotframes, based on the priority of its packets/traffic
      flows.  The timeslots in the Slotframe are indexed by the
      SlotOffset; the first timeslot is at SlotOffset 0.

   slotOffset:  A column in the TSCH schedule, i.e., the number of
      timeslots since the beginning of the current iteration of the

   soft cell:  A scheduled cell which the 6top sublayer can relocate.

   time source neighbor:  A neighbor that a node uses as its time
      reference, and to which it needs to keep its clock synchronized.

   timeslot:  A basic communication unit in TSCH which allows a
      transmitter node to send a frame to a receiver neighbor, and that
      receiver neighbor to optionally send back an acknowledgment.

   Track:  A Track is a Directed Acyclic Graph (DAG) that is used as a

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      complex multi-hop path to the destination(s) of the path.  In the
      case of unicast traffic, the Track is a Destination Oriented DAG
      (DODAG) where the Root of the DODAG is the destination of the
      unicast traffic.  A Track enables replication, elimination and
      reordering functions on the way (more on those functions in
      [RFC8655].  A Track reservation locks physical resources such as
      cells and buffers in every node along the DODAG.  A Track is
      associated with a owner that can be for instance the destination
      of the Track.

   TrackID:  A TrackID is either globally unique, or locally unique to
      the Track owner, in which case the identification of the owner
      must be provided together with the TrackID to provide a full
      reference to the Track. typically, the Track owner is the ingress
      of the Track then the IPv6 source address of packets along the
      Track can be used as identification of the owner and a local
      InstanceID [RFC6550] in the namespace of that owner can be used as
      TrackID.  If the Track is reversible, then the owner is found in
      the IPv6 destination address of a packet coming back along the
      Track.  In that case, a RPL Packet Information [RFC6550] in an
      IPv6 packet can unambiguously identify the Track and can be
      expressed in a compressed form using [RFC8138].

   TSCH:  A medium access mode of the IEEE Std. 802.15.4 [IEEE802154]
      standard which uses time synchronization to achieve ultra-low-
      power operation, and channel hopping to enable high reliability.

   TSCH Schedule:  A matrix of cells, each cell indexed by a slotOffset
      and a channelOffset.  The TSCH schedule contains all the scheduled
      cells from all slotframes and is sufficient to qualify the
      communication in the TSCH network.  The number of channelOffset
      values (the "height" of the matrix) is equal to the number of
      available frequencies.

   Unscheduled Cell:  A cell which is not used by the IEEE Std. 802.15.4
      TSCH implementation.

2.2.  Abbreviations

   This document uses the following abbreviations:

   6BBR:  6LoWPAN Backbone Router (router with a proxy ND function)

   6LBR:  6LoWPAN Border Router (authoritative on DAD)

   6LN:  6LoWPAN Node

   6LR:  6LoWPAN Router (relay to the registration process)

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   6CIO:  Capability Indication Option

   (E)ARO:  (Extended) Address Registration Option

   (E)DAR:  (Extended) Duplicate Address Request

   (E)DAC:  (Extended) Duplicate Address Confirmation

   DAD:  Duplicate Address Detection

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network (a typical IoT network)

   NA:  Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:  Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   PCE:  Path Computation Element

   NME:  Network Management Entity

   ROVR:  Registration Ownership Verifier (pronounced rover)

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple)

   RA:  Router Advertisement

   RS:  Router Solicitation

   TSCH:  timeslotted Channel Hopping

   TID:  Transaction ID (a sequence counter in the EARO)

2.3.  Related Documents

   The draft also conforms to the terms and models described in
   [RFC3444] and [RFC5889] and uses the vocabulary and the concepts
   defined in [RFC4291] for the IPv6 Architecture and refers [RFC4080]
   for reservation

   The draft uses domain-specific terminology defined or referenced in:

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      6LoWPAN ND "Neighbor Discovery Optimization for Low-power and
      Lossy Networks" [RFC6775] and "Registration Extensions for 6LoWPAN
      Neighbor Discovery" [RFC8505],

      "Terms Used in Routing for Low-Power and Lossy Networks (LLNs)"

      and RPL "Objective Function Zero for the Routing Protocol for
      Low-Power and Lossy Networks (RPL)" [RFC6552], and "RPL: IPv6
      Routing Protocol for Low-Power and Lossy Networks" [RFC6550].

   Other terms in use in LLNs are found in "Terminology for
   Constrained-Node Networks" [RFC7228].

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in

   *  "Neighbor Discovery for IP version 6" [RFC4861], and "IPv6
      Stateless Address Autoconfiguration" [RFC4862].

   In addition, readers would benefit from reading:

   *  "Problem Statement and Requirements for IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606],

   *  "Multi-Link Subnet Issues" [RFC4903], and

   *  "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
      Overview, Assumptions, Problem Statement, and Goals" [RFC4919]

   prior to this specification for a clear understanding of the art in
   ND-proxying and binding.

3.  High Level Architecture

3.1.  A Non-Broadcast Multi-Access Radio Mesh Network

   A 6TiSCH network is an IPv6 [RFC8200] subnet which, in its basic
   configuration illustrated in Figure 1, is a single Low-Power Lossy
   Network (LLN) operating over a synchronized TSCH-based mesh.

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               ---+-------- ............ ------------
                  |      External Network       |
                  |                          +-----+
               +-----+                       | NME |
               |     | LLN Border            | PCE |
               |     | router (6LBR)         +-----+
             o    o   o
         o     o   o     o    o
        o   o 6LoWPAN + RPL o    o
            o   o   o       o

             Figure 1: Basic Configuration of a 6TiSCH Network

   Inside a 6TiSCH LLN, nodes rely on 6LoWPAN Header Compression
   (6LoWPAN HC) [RFC6282] to encode IPv6 packets.  From the perspective
   of the network layer, a single LLN interface (typically an IEEE Std.
   802.15.4-compliant radio) may be seen as a collection of Links with
   different capabilities for unicast or multicast services.

   6TiSCH nodes join a mesh network by attaching to nodes that are
   already members of the mesh (see Section 4.2.1).  The security
   aspects of the join process are further detailed in Section 6.  In a
   mesh network, 6TiSCH nodes are not necessarily reachable from one
   another at Layer-2 and an LLN may span over multiple links.

   This forms a homogeneous non-broadcast multi-access (NBMA) subnet,
   which is beyond the scope of IPv6 Neighbor Discovery (IPv6 ND)
   [RFC4861][RFC4862]. 6LoWPAN Neighbor Discovery (6LoWPAN ND)
   [RFC6775][RFC8505] specifies extensions to IPv6 ND that enable ND
   operations in this type of subnet that can be protected against
   address theft and impersonation with [AP-ND].

   Once it has joined the 6TiSCH network, a node acquires IPv6 Addresses
   and register them using 6LoWPAN ND.  This guarantees that the
   addresses are unique and protects the address ownership over the
   subnet, more in Section 4.2.2.

   Within the NBMA subnet, RPL [RFC6550] enables routing in the so-
   called Route Over fashion, either in storing (stateful) or non-
   storing (stateless, with routing headers) mode.  From there, some
   nodes can act as routers for 6LoWPAN ND and RPL operations, as
   detailed in Section 4.1.

   With TSCH, devices are time-synchronized at the MAC level.  The use
   of a particular RPL Instance for time synchronization is discussed in
   Section 4.3.4.  With this mechanism, the time synchronization starts
   at the RPL Root and follows the RPL loopless routing topology.

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   RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs)
   within Instances of the protocol, each Instance being associated with
   an Objective Function (OF) to form a routing topology.  A particular
   6TiSCH node, the LLN Border Router (6LBR), acts as RPL Root, 6LoWPAN
   HC terminator, and Border Router for the LLN to the outside.  The
   6LBR is usually powered.  More on RPL Instances can be found in
   section 3.1 of RPL [RFC6550], in particular "3.1.2.  RPL Identifiers"
   and "3.1.3.  Instances, DODAGs, and DODAG Versions".  RPL adds
   artifacts in the data packets that are compressed with a 6LoWPAN
   addition 6LoRH [RFC8138].

   Additional routing and scheduling protocols may be deployed to
   establish on-demand Peer-to-Peer routes with particular
   characteristics inside the 6TiSCH network.  This may be achieved in a
   centralized fashion by a Path Computation Element (PCE) [PCE] that
   programs both the routes and the schedules inside the 6TiSCH nodes,
   or by in a distributed fashion using a reactive routing protocol and
   a Hop-by-Hop scheduling protocol.

   This architecture expects that a 6LoWPAN node can connect as a leaf
   to a RPL network, where the leaf support is the minimal functionality
   to connect as a host to a RPL network without the need to participate
   to the full routing protocol.  The architecture also expects that a
   6LoWPAN node that is not aware at all of the RPL protocol may also
   connect as described in [RUL-DRAFT].

3.2.  A Multi-Link Subnet Model

   An extended configuration of the subnet comprises multiple LLNs as
   illustrated in Figure 2.  In the extended configuration, a Routing
   Registrar [RFC8505] may be connected to the node that acts as RPL
   Root and / or 6LoWPAN 6LBR and provides connectivity to the larger
   campus / factory plant network over a high-speed backbone or a back-
   haul link.  The Routing registrar may perform IPv6 ND proxy
   operations, or redistribute the registration in a routing protocol
   such as OSPF [RFC5340] or BGP [RFC2545], or inject a route in a
   mobility protocol such as MIPv6 [RFC6275], NEMO [RFC3963], or LISP

   Multiple LLNs can be interconnected and possibly synchronized over a
   backbone, which can be wired or wireless.  The backbone can operate
   with IPv6 ND [RFC4861][RFC4862] procedures or an hybrid of IPv6 ND
   and 6LoWPAN ND [RFC6775][RFC8505][AP-ND].

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                +-----+                +-----+         +-----+
      (default) |     |     (Optional) |     |         |     | IPv6
         Router |     |           6LBR |     |         |     | Node
                +-----+                +-----+         +-----+
                   |  Backbone side       |               |
               |                        |                        |
         +-----------+            +-----------+            +-----------+
         | Routing   |            | Routing   |            | Routing   |
         | Registrar |            | Registrar |            | Registrar |
         +-----------+            +-----------+            +-----------+
           o     Wireless side       o  o                     o o
       o o   o  o                o o   o  o  o          o  o  o  o o
     o   6TiSCH                o   6TiSCH   o  o          o o  6TiSCH o
     o   o LLN     o o           o o LLN   o               o     LLN   o
     o   o  o  o  o            o  o  o o o            o  o    o        o

            Figure 2: Extended Configuration of a 6TiSCH Network

   A Routing Registrar that performs proxy IPv6 ND operations over the
   backbone on behalf of the 6TiSCH nodes is called a Backbone Router
   (6BBR) [6BBR-DRAFT].  The 6BBRs are placed along the wireless edge of
   a Backbone, and federate multiple wireless links to form a single
   MultiLink Subnet.  The 6BBRs synchronize with one another over the
   backbone, so as to ensure that the multiple LLNs that form the IPv6
   subnet stay tightly synchronized.

   The use of multicast can also be reduced on the backbone with a
   registrar that would contribute to Duplicate Address Detection as
   well as Address Lookup using only unicast request/response exchanges.
   [I-D.thubert-6man-unicast-lookup] is a proposed method that presents
   an example of how to this could be achieved with an extension of
   [RFC8505], using an optional 6LBR as a SubNet-level registrar, as
   illustrated in Figure 2.

   As detailed in Section 4.1 the 6LBR that serves the LLN and the Root
   of the RPL network need to share information about the devices that
   are learned through either 6LoWPAN ND or RPL but not both.  The
   preferred way of achieving this is to collocate/combine them.  The
   combined RPL Root and 6LBR may be collocated with the 6BBR, or
   directly attached to the 6BBR.  In the latter case, it leverages the
   extended registration process defined in [RFC8505] to proxy the
   6LoWPAN ND registration to the 6BBR on behalf of the LLN nodes, so
   that the 6BBR may in turn perform proxy classical ND operations over
   the backbone.

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   The DetNet Architecture [RFC8655] studies Layer-3 aspects of
   Deterministic Networks, and covers networks that span multiple
   Layer-2 domains.  If the Backbone is Deterministic (such as defined
   by the Time Sensitive Networking WG at IEEE), then the Backbone
   Router ensures that the end-to-end deterministic behavior is
   maintained between the LLN and the backbone.

3.3.  TSCH: A Deterministic MAC Layer

   Though at a different time scale (several orders of magnitude), both
   IEEE Std. 802.1TSN and IEEE Std. 802.15.4 TSCH standards provide
   Deterministic capabilities to the point that a packet that pertains
   to a certain flow may traverse a network from node to node following
   a precise schedule, as a train that enters and then leaves
   intermediate stations at precise times along its path.

   With TSCH, time is formatted into timeslots, and individual
   communication cells are allocated to unicast or broadcast
   communication at the MAC level.  The time-slotted operation reduces
   collisions, saves energy, and enables to more closely engineer the
   network for deterministic properties.  The channel hopping aspect is
   a simple and efficient technique to combat multipath fading and co-
   channel interference.

   6TiSCH builds on the IEEE Std. 802.15.4 TSCH MAC and inherits its
   advanced capabilities to enable them in multiple environments where
   they can be leveraged to improve automated operations.  The 6TiSCH
   Architecture also inherits the capability to perform a centralized
   route computation to achieve deterministic properties, though it
   relies on the IETF DetNet Architecture [RFC8655], and IETF components
   such as the PCE [PCE], for the protocol aspects.

   On top of this inheritance, 6TiSCH adds capabilities for distributed
   routing and scheduling operations based on the RPL routing protocol
   and capabilities to negotiate schedule adjustments between peers.
   These distributed routing and scheduling operations simplify the
   deployment of TSCH networks and enable wireless solutions in a larger
   variety of use cases from operational technology in general.
   Examples of such use-cases in industrial environments include plant
   setup and decommissioning, as well as monitoring of lots of lesser
   importance measurements such as corrosion and events and mobile
   workers accessing local devices.

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3.4.  Scheduling TSCH

   A scheduling operation attributes cells in a Time-Division-
   Multiplexing (TDM) / Frequency-Division Multiplexing (FDM) matrix
   called the Channel distribution/usage (CDU) to either individual
   transmissions or as multi-access shared resources.  The CDU matrix
   can be formatted in chunks that can be allocated exclusively to
   particular nodes to enable distributed scheduling without collision.
   More in Section 4.3.5.

   From the standpoint of a 6TiSCH node (at the MAC layer), its schedule
   is the collection of the timeslots at which it must wake up for
   transmission, and the channels to which it should either send or
   listen at those times.  The schedule is expressed as one or more
   slotframes that repeat over and over.  Slotframes may collide and
   require a device to wake up at a same time, in which case the
   slotframe with the highest priority is actionable.

   The 6top sublayer (see Section 4.3 for more) hides the complexity of
   the schedule from the upper layers.  The Link abstraction that IP
   traffic utilizes is composed of a pair of Layer-3 cell bundles, one
   to receive and one to transmit.  Some of the cells may be shared, in
   which case the 6top sublayer must perform some arbitration.

   Scheduling enables multiple communications at a same time in a same
   interference domain using different channels; but a node equipped
   with a single radio can only either transmit or receive on one
   channel at any point of time.  Scheduled cells that fulfil the same
   role, e.g., receive IP packets from a peer, are grouped in bundles.

   The 6TiSCH architecture identifies four ways a schedule can be
   managed and CDU cells can be allocated: Static Scheduling, Neighbor-
   to-Neighbor Scheduling, Centralized (or Remote) Monitoring and
   Schedule Management, and Hop-by-hop Scheduling.

   Static Scheduling:  This refers to the minimal 6TiSCH operation
      whereby a static schedule is configured for the whole network for
      use in a Slotted ALOHA [S-ALOHA] fashion.  The static schedule is
      distributed through the native methods in the TSCH MAC layer and
      does not preclude other scheduling operations to co-exist on a
      same 6TiSCH network.  A static schedule is necessary for basic
      operations such as the join process and for interoperability
      during the network formation, which is specified as part of the
      Minimal 6TiSCH Configuration [RFC8180].

   Neighbor-to-Neighbor Scheduling:  This refers to the dynamic

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      adaptation of the bandwidth of the Links that are used for IPv6
      traffic between adjacent peers.  Scheduling Functions such as the
      "6TiSCH Minimal Scheduling Function (MSF)" [MSF] influence the
      operation of the MAC layer to add, update and remove cells in its
      own, and its peer's schedules using 6P [RFC8480], for the
      negotiation of the MAC resources.

   Centralized (or Remote) Monitoring and Schedule Management:  This
      refers to the central computation of a schedule and the capability
      to forward a frame based on the cell of arrival.  In that case,
      the related portion of the device schedule as well as other device
      resources are managed by an abstract Network Management Entity
      (NME), which may cooperate with the PCE to minimize the
      interaction with and the load on the constrained device.  This
      model is the TSCH adaption of the DetNet Architecture [RFC8655],
      and it enables Traffic Engineering with deterministic properties.

   Hop-by-hop Scheduling:  This refers to the possibility to reserves
      cells along a path for a particular flow using a distributed

   It is not expected that all use cases will require all those
   mechanisms.  Static Scheduling with minimal configuration one is the
   only one that is expected in all implementations, since it provides a
   simple and solid basis for convergecast routing and time

   A deeper dive in those mechanisms can be found in Section 4.4.

3.5.  Distributed vs. Centralized Routing

   6TiSCH enables a mixed model of centralized routes and distributed
   routes.  Centralized routes can for example be computed by an entity
   such as a PCE.  6TiSCH leverages the RPL [RFC6550] routing protocol
   for interoperable distributed routing operations.

   Both methods may inject routes in the Routing Tables of the 6TiSCH
   routers.  In either case, each route is associated with a 6TiSCH
   topology that can be a RPL Instance topology or a Track.  The 6TiSCH
   topology is indexed by a RPLInstanceID, in a format that reuses the
   RPLInstanceID as defined in RPL.

   RPL [RFC6550] is applicable to Static Scheduling and Neighbor-to-
   Neighbor Scheduling.  The architecture also supports a centralized
   routing model for Remote Monitoring and Schedule Management.  It is
   expected that a routing protocol that is more optimized for point-to-
   point routing than RPL [RFC6550], such as the Asymmetric AODV-P2P-RPL

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   in Low-Power and Lossy Networks" [I-D.ietf-roll-aodv-rpl] AODV-RPL),
   which derives from the Ad Hoc On-demand Distance Vector Routing
   (AODV) [I-D.ietf-manet-aodvv2] will be selected for Hop-by-hop

   Both RPL and PCE rely on shared sources such as policies to define
   Global and Local RPLInstanceIDs that can be used by either method.
   It is possible for centralized and distributed routing to share a
   same topology.  Generally they will operate in different slotframes,
   and centralized routes will be used for scheduled traffic and will
   have precedence over distributed routes in case of conflict between
   the slotframes.

3.6.  Forwarding Over TSCH

   The 6TiSCH architecture supports three different forwarding models.
   One is the classical IPv6 Forwarding, where the node selects a
   feasible successor at Layer-3 on a per packet basis and based on its
   routing table.  The second derives from Generic MPLS (G-MPLS) for so-
   called Track Forwarding, whereby a frame received at a particular
   timeslot can be switched into another timeslot at Layer-2 without
   regard to the upper layer protocol.  The third model is the 6LoWPAN
   Fragment Forwarding, which allows to forward individual 6loWPAN
   fragments along a route that is setup by the first fragment.

   In more details:

   IPv6 Forwarding:  This is the classical IP forwarding model, with a
      Routing Information Based (RIB) that is installed by the RPL
      routing protocol and used to select a feasible successor per
      packet.  The packet is placed on an outgoing Link, that the 6top
      layer maps into a (Layer-3) bundle of cells, and scheduled for
      transmission based on QoS parameters.  Besides RPL, this model
      also applies to any routing protocol which may be operated in the
      6TiSCH network, and corresponds to all the distributed scheduling
      models, Static, Neighbor-to-Neighbor and Hop-by-Hop Scheduling.

   G-MPLS Track Forwarding:  This model corresponds to the Remote
      Monitoring and Schedule Management.  In this model, a central
      controller (hosting a PCE) computes and installs the schedules in
      the devices per flow.  The incoming (Layer-2) bundle of cells from
      the previous node along the path determines the outgoing (Layer-2)
      bundle towards the next hop for that flow as determined by the
      PCE.  The programmed sequence for bundles is called a Track and
      can assume DAG shapes that are more complex than a simple direct
      sequence of nodes.

   6LoWPAN Fragment Forwarding:  This is a hybrid model that derives

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      from IPv6 forwarding for the case where packets must be fragmented
      at the 6LoWPAN sublayer.  The first fragment is forwarded like any
      IPv6 packet and leaves a state in the intermediate hops to enable
      forwarding of the next fragments that do not have a IP header
      without the need to recompose the packet at every hop.

   A deeper dive on these operations can be found in Section 4.6.

   The following table summarizes how the forwarding models apply to the
   various routing and scheduling possibilities:

   |  Forwarding Model |  Routing   |          Scheduling              |
   |                   |            |   Static (Minimal Configuration) |
   +  classical IPv6   +     RPL    +----------------------------------+
   |         /         |            |   Neighbor-to-Neighbor (SF+6P)   |
   + 6LoWPAN Fragment  +------------+----------------------------------+
   |                   |  Reactive  |     Hop-by-Hop (AODV-RPL)        |
   |G-MPLS Track Fwding|     PCE    |Remote Monitoring and Schedule Mgt|

                                  Figure 3

3.7.  6TiSCH Stack

   The IETF proposes multiple techniques for implementing functions
   related to routing, transport or security.

   The 6TiSCH architecture limits the possible variations of the stack
   and recommends a number of base elements for LLN applications to
   control the complexity of possible deployments and device
   interactions, and to limit the size of the resulting object code.  In
   particular, UDP [RFC0768], IPv6 [RFC8200] and the Constrained
   Application Protocol [RFC7252] (CoAP) are used as the transport /
   binding of choice for applications and management as opposed to TCP
   and HTTP.

   The resulting protocol stack is represented in Figure 4:

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      | Applis |  CoJP  |
      | CoAP / OSCORE   |  6LoWPAN ND  | RPL |
      |       UDP       |      ICMPv6        |
      |                 IPv6                 |
      |     6LoWPAN HC   /   6LoRH HC        | Scheduling Functions |
      |               6top inc. 6top protocol                       |
      |                 IEEE Std. 802.15.4 TSCH                     |

                      Figure 4: 6TiSCH Protocol Stack

   RPL is the routing protocol of choice for LLNs.  So far, there was no
   identified need to define a 6TiSCH specific Objective Function.  The
   Minimal 6TiSCH Configuration [RFC8180] describes the operation of RPL
   over a static schedule used in a Slotted ALOHA fashion [S-ALOHA],
   whereby all active slots may be used for emission or reception of
   both unicast and multicast frames.

   The 6LoWPAN Header Compression [RFC6282] is used to compress the IPv6
   and UDP headers, whereas the 6LoWPAN Routing Header (6LoRH) [RFC8138]
   is used to compress the RPL artifacts in the IPv6 data packets,
   including the RPL Packet Information (RPI), the IP-in-IP
   encapsulation to/from the RPL Root, and the Source Route Header (SRH)
   in non-storing mode.  "When to use RFC 6553, 6554 and IPv6-in-IPv6"
   [USEofRPLinfo] provides the details on when headers or encapsulation
   are needed.

   The Object Security for Constrained RESTful Environments (OSCORE)
   [I-D.ietf-core-object-security], is leveraged by the Constrained Join
   Protocol (CoJP) and is expected to be the primary protocol for the
   protection of the application payload as well.  The application
   payload may also be protected by the Datagram Transport Layer
   Security (DTLS) [RFC6347] sitting either under CoAP or over CoAP so
   it can traverse proxies.

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   The 6TiSCH Operation sublayer (6top) is a sublayer of a Logical Link
   Control (LLC) that provides the abstraction of an IP link over a TSCH
   MAC and schedules packets over TSCH cells, as further discussed in
   the next sections, providing in particular dynamic cell allocation
   with the 6top Protocol (6P) [RFC8480].

   The reference stack presented in this document was implemented and
   interop-tested by a conjunction of opensource, IETF and ETSI efforts.
   One goal is to help other bodies to adopt the stack as a whole,
   making the effort to move to an IPv6-based IoT stack easier.

   For a particular environment, some of the choices that are made in
   this architecture may not be relevant.  For instance, RPL is not
   required for star topologies and mesh-under Layer-2 routed networks,
   and the 6LoWPAN compression may not be sufficient for ultra-
   constrained cases such as some Low-Power Wide Area (LPWA) networks.
   In such cases, it is perfectly doable to adopt a subset of the
   selection that is presented hereafter and then select alternate
   components to complete the solution wherever needed.

3.8.  Communication Paradigms and Interaction Models

   Section 2.1 provides the terms of Communication Paradigms and
   Interaction Models, in relation with "On the Difference between
   Information Models and Data Models" [RFC3444].  A Communication
   Paradigm would be an abstract view of a protocol exchange, and would
   come with an Information Model for the information that is being
   exchanged.  In contrast, an Interaction Model would be more refined
   and could point to standard operation such as a Representational
   state transfer (REST) "GET" operation and would match a Data Model
   for the data that is provided over the protocol exchange.

   Section 2.1.3 of [I-D.ietf-roll-rpl-industrial-applicability] and
   next sections discuss application-layer paradigms, such as Source-
   sink (SS) that is a Multipeer to Multipeer (MP2MP) model primarily
   used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that
   is typically used for sensor data, as well as Peer-to-peer (P2P) and
   Peer-to-multipeer (P2MP) communications.

   Additional considerations on Duocast - one sender, two receivers for
   redundancy - and its N-cast generalization are also provided.  Those
   paradigms are frequently used in industrial automation, which is a
   major use case for IEEE Std. 802.15.4 TSCH wireless networks with
   [ISA100.11a] and [WirelessHART], that provides a wireless access to
   [HART] applications and devices.

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   This document focuses on Communication Paradigms and Interaction
   Models for packet forwarding and TSCH resources (cells) management.
   Management mechanisms for the TSCH schedule at Link-Layer (one-hop),
   Network-layer (multihop along a Track), and Application-layer (remote
   control) are discussed in Section 4.4.  Link-Layer frame forwarding
   interactions are discussed in Section 4.6, and Network-layer Packet
   routing is addressed in Section 4.7.

4.  Architecture Components

4.1.  6LoWPAN (and RPL)

   A RPL DODAG is formed of a Root, a collection of routers, and leaves
   that are hosts.  Hosts are nodes which do not forward packets that
   they did not generate.  RPL-aware leaves will participate to RPL to
   advertise their own addresses, whereas RPL-unaware leaves depend on a
   connected RPL router to do so.  RPL interacts with 6LoWPAN ND at
   multiple levels, in particular at the Root and in the RPL-unaware

4.1.1.  RPL-Unaware Leaves and 6LoWPAN ND

   RPL needs a set of information to advertise a leaf node through a
   Destination Advertisement Object (DAO) message and establish

   "Routing for RPL Leaves" [RUL-DRAFT] details the basic interaction of
   6LoWPAN ND and RPL and enables a plain 6LN that supports [RFC8505] to
   obtain return connectivity via the RPL network as an RPL-unaware
   leaf.  The leaf indicates that it requires reachability services for
   the Registered Address from a Routing Registrar by setting a 'R' flag
   in the Extended Address Registration Option [RFC8505], and it
   provides a TID that maps to a sequence number in section 7 of RPL

   [RUL-DRAFT] also enables the leaf to signal the RPL InstanceID that
   it wants to participate to using the Opaque field of the EARO.  On
   the backbone, the InstanceID is expected to be mapped to an overlay
   that matches the RPL Instance, e.g., a Virtual LAN (VLAN) or a
   virtual routing and forwarding (VRF) instance.

   Though at the time of this writing the above specification enables a
   model where the separation is possible, this architecture recommends
   to collocate the functions of 6LBR and RPL Root.

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4.1.2.  6LBR and RPL Root

   With the 6LowPAN ND [RFC6775], information on the 6LBR is
   disseminated via an Authoritative Border Router Option (ABRO) in RA
   messages.  [RFC8505] extends [RFC6775] to enable a registration for
   routing and proxy ND.  The capability to support [RFC8505] is
   indicated in the 6LoWPAN Capability Indication Option (6CIO).  The
   discovery and liveliness of the RPL Root are obtained through RPL
   [RFC6550] itself.

   When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL Root
   functionalities are co-located in order that the address of the 6LBR
   be indicated by RPL DIO messages and to associate the unique ID from
   the EDAR/EDAC [RFC8505] exchange with the state that is maintained by

   Section 7 of [RUL-DRAFT] specifies how the DAO messages are used to
   reconfirm the registration, thus eliminating a duplication of
   functionality between DAO and EDAR/EDAC messages, as illustrated in
   Figure 7.  [RUL-DRAFT] also provides the protocol elements that are
   needed when the 6LBR and RPL Root functionalities are not co-located.

   Even though the Root of the RPL network is integrated with the 6LBR,
   it is logically separated from the Backbone Router (6BBR) that is
   used to connect the 6TiSCH LLN to the backbone.  This way, the Root
   has all information from 6LoWPAN ND and RPL about the LLN devices
   attached to it.

   This architecture also expects that the Root of the RPL network
   (proxy-)registers the 6TiSCH nodes on their behalf to the 6BBR, for
   whatever operation the 6BBR performs on the backbone, such as ND
   proxy, or redistribution in a routing protocol.  This relies on an
   extension of the 6LoWPAN ND registration described in [6BBR-DRAFT].

   This model supports the movement of a 6TiSCH device across the Multi-
   Link Subnet, and allows the proxy registration of 6TiSCH nodes deep
   into the 6TiSCH LLN by the 6LBR / RPL Root.  This is why in [RFC8505]
   the Registered Address is signaled in the Target Address field of the
   NS message as opposed to the IPv6 Source Address, which, in the case
   of a proxy registration, is that of the 6LBR / RPL Root itself.

4.2.  Network Access and Addressing

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4.2.1.  Join Process

   A new device, called the pledge, undergoes the join protocol to
   become a node in a 6TiSCH network.  This usually occurs only once
   when the device is first powered on.  The pledge communicates with
   the Join Registrar/Coordinator (JRC) of the network through a Join
   Proxy (JP), a radio neighbor of the pledge.

   The JP is discovered though MAC layer beacons.  When multiple JPs
   from possibly multiple networks are visible, trial and error till an
   acceptable position in the right network is obtained becomes
   ineffficient.  [ENH-BEACON] adds a new subtype in the Information
   Element that was delegated to the IETF [RFC8137] and provides
   visibility on the network that can be joined and the willingness by
   the JP and the Root to be used by the pledge.

   The join protocol provides the following functionality:

   *  Mutual authentication

   *  Authorization

   *  Parameter distribution to the pledge over a secure channel

   Minimal Security Framework for 6TiSCH [MIN-SECURITY] defines the
   minimal mechanisms required for this join process to occur in a
   secure manner.  The specification defines the Constrained Join
   Protocol (CoJP) that is used to distribute the parameters to the
   pledge over a secure session established through OSCORE
   [I-D.ietf-core-object-security], and a secure configuration of the
   network stack.  In the minimal setting with pre-shared keys (PSKs),
   CoJP allows the pledge to join after a single round-trip exchange
   with the JRC.  The provisioning of the PSK to the pledge and the JRC
   needs to be done out of band, through a 'one-touch' bootstrapping
   process, which effectively enrolls the pledge into the domain managed
   by the JRC.

   In certain use cases, the 'one touch' bootstrapping is not feasible
   due to the operational constraints and the enrollment of the pledge
   into the domain needs to occur in-band.  This is handled through a
   'zero-touch' extension of the Minimal Security Framework for 6TiSCH.
   Zero touch [I-D.ietf-6tisch-dtsecurity-zerotouch-join] extension
   leverages the 'Bootstrapping Remote Secure Key Infrastructures
   (BRSKI)' [[I-D.ietf-anima-bootstrapping-keyinfra] work to establish a
   shared secret between a pledge and the JRC without necessarily having
   them belong to a common (security) domain at join time.  This happens
   through inter-domain communication occurring between the JRC of the

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   network and the domain of the pledge, represented by a fourth entity,
   Manufacturer Authorized Signing Authority (MASA).  Once the zero-
   touch exchange completes, the CoJP exchange defined in [MIN-SECURITY]
   is carried over the secure session established between the pledge and
   the JRC.

   Figure 5 depicts the join process and where a Link-Local Address
   (LLA) is used, versus a Global Unicast Address (GUA).

   6LoWPAN Node       6LR           6LBR      Join Registrar     MASA
    (pledge)       (Join Proxy)     (Root)    /Coordinator (JRC)
     |               |               |              |              |
     |  6LoWPAN ND   |6LoWPAN ND+RPL | IPv6 network |IPv6 network  |
     |   LLN link    |Route-Over mesh|(the Internet)|(the Internet)|
     |               |               |              |              |
     |   Layer-2     |               |              |              |
     |enhanced beacon|               |              |              |
     |<--------------|               |              |              |
     |               |               |              |              |
     |    NS (EARO)  |               |              |              |
     | (for the LLA) |               |              |              |
     |-------------->|               |              |              |
     |    NA (EARO)  |               |              |              |
     |<--------------|               |              |              |
     |               |               |              |              |
     |  (Zero-touch  |               |              |              |
     |   handshake)  |     (Zero-touch handshake)   | (Zero-touch  |
     |   using LLA   |           using GUA          |  handshake)  |
     |               |               |              |              |
     | CoJP Join Req |               |              |              | \
     |  using LLA    |               |              |              | |
     |-------------->|               |              |              | |
     |               |       CoJP Join Request      |              | |
     |               |           using GUA          |              | |
     |               |----------------------------->|              | | C
     |               |               |              |              | | o
     |               |       CoJP Join Response     |              | | J
     |               |           using GUA          |              | | P
     |               |<-----------------------------|              | |
     |CoJP Join Resp |               |              |              | |
     |  using LLA    |               |              |              | |
     |<--------------|               |              |              | /
     |               |               |              |              |

       Figure 5: Join process in a Multi-Link Subnet.  Parentheses ()
                         denote optional exchanges.

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4.2.2.  Registration

   Once the pledge successfully completes the CoJP protocol and becomes
   a network node, it obtains the network prefix from neighboring
   routers and registers its IPv6 addresses.  As detailed in
   Section 4.1, the combined 6LoWPAN ND 6LBR and Root of the RPL network
   learn information such as the device Unique ID (from 6LoWPAN ND) and
   the updated Sequence Number (from RPL), and perform 6LoWPAN ND proxy
   registration to the 6BBR of behalf of the LLN nodes.

   Figure 6 illustrates the initial IPv6 signaling that enables a 6LN to
   form a global address and register it to a 6LBR using 6LoWPAN ND
   [RFC8505], is then carried over RPL to the RPL Root, and then to the
   6BBR.  This flow happens just once when the address is created and
   first registered.

       6LoWPAN Node        6LR             6LBR            6BBR
        (RPL leaf)       (router)         (Root)
            |               |               |               |
            |  6LoWPAN ND   |6LoWPAN ND+RPL | 6LoWPAN ND    | IPv6 ND
            |   LLN link    |Route-Over mesh|Ethernet/serial| Backbone
            |               |               |               |
            |  RS (mcast)   |               |               |
            |-------------->|               |               |
            |----------->   |               |               |
            |------------------>            |               |
            |  RA (unicast) |               |               |
            |<--------------|               |               |
            |               |               |               |
            |  NS(EARO)     |               |               |
            |-------------->|               |               |
            | 6LoWPAN ND    | Extended DAR  |               |
            |               |-------------->|               |
            |               |               |  NS(EARO)     |
            |               |               |-------------->|
            |               |               |               | NS-DAD
            |               |               |               |------>
            |               |               |               | (EARO)
            |               |               |               |
            |               |               |  NA(EARO)     |<timeout>
            |               |               |<--------------|
            |               | Extended DAC  |               |
            |               |<--------------|               |
            |  NA(EARO)     |               |               |
            |<--------------|               |               |
            |               |               |               |

         Figure 6: Initial Registration Flow over Multi-Link Subnet

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   Figure 7 illustrates the repeating IPv6 signaling that enables a 6LN
   to keep a global address alive and registered to its 6LBR using
   6LoWPAN ND to the 6LR, RPL to the RPL Root, and then 6LoWPAN ND again
   to the 6BBR, which avoids repeating the Extended DAR/DAC flow across
   the network when RPL can suffice as a keep-alive mechanism.

    6LoWPAN Node        6LR             6LBR            6BBR
     (RPL leaf)       (router)         (Root)
         |               |               |               |
         |  6LoWPAN ND   |6LoWPAN ND+RPL | 6LoWPAN ND    | IPv6 ND
         |   LLN link    |Route-Over mesh| ant IPv6 link | Backbone
         |               |               |
         |               |               |               |
         |  NS(EARO)     |               |               |
         |-------------->|               |               |
         |  NA(EARO)     |               |               |
         |<--------------|               |               |
         |               | DAO           |               |
         |               |-------------->|               |
         |               | DAO-ACK       |               |
         |               |<--------------|               |
         |               |               |  NS(EARO)     |
         |               |               |-------------->|
         |               |               |  NA(EARO)     |
         |               |               |<--------------|
         |               |               |               |
         |               |               |               |

          Figure 7: Next Registration Flow over Multi-Link Subnet

   As the network builds up, a node should start as a leaf to join the
   RPL network, and may later turn into both a RPL-capable router and a
   6LR, so as to accept leaf nodes to recursively join the network.

4.3.  TSCH and 6top

4.3.1.  6top

   6TiSCH expects a high degree of scalability together with a
   distributed routing functionality based on RPL.  To achieve this
   goal, the spectrum must be allocated in a way that allows for spatial
   reuse between zones that will not interfere with one another.  In a
   large and spatially distributed network, a 6TiSCH node is often in a
   good position to determine usage of the spectrum in its vicinity.

   With 6TiSCH, the abstraction of an IPv6 link is implemented as a pair
   of bundles of cells, one in each direction.  IP Links are only
   enabled between RPL parents and children.  The 6TiSCH operation is

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   optimal when the size of a bundle is such that both the energy wasted
   in idle listening and the packet drops due to congestion loss are
   minimized, while packets are forwarded within an acceptable latency.

   Use cases for distributed routing are often associated with a
   statistical distribution of best-effort traffic with variable needs
   for bandwidth on each individual link.  The 6TiSCH operation can
   remain optimal if RPL parents can adjust dynamically, and with enough
   reactivity to match the variations of best-effort traffic, the amount
   of bandwidth that is used to communicate between themselves and their
   children, in both directions.  In turn, the agility to fulfill the
   needs for additional cells improves when the number of interactions
   with other devices and the protocol latencies are minimized.

   6top is a logical link control sitting between the IP layer and the
   TSCH MAC layer, which provides the link abstraction that is required
   for IP operations.  The 6top protocol, 6P, which is specified in
   [RFC8480], is one of the services provided by 6top.  In particular,
   the 6top services are available over a management API that enables an
   external management entity to schedule cells and slotframes, and
   allows the addition of complementary functionality, for instance a
   Scheduling Function that manages a dynamic schedule management based
   on observed resource usage as discussed in Section 4.4.2.  For this
   purpose, the 6TiSCH architecture differentiates "soft" cells and
   "hard" cells.  Hard Cells

   "Hard" cells are cells that are owned and managed by a separate
   scheduling entity (e.g., a PCE) that specifies the slotOffset/
   channelOffset of the cells to be added/moved/deleted, in which case
   6top can only act as instructed, and may not move hard cells in the
   TSCH schedule on its own.  Soft Cells

   In contrast, "soft" cells are cells that 6top can manage locally.
   6top contains a monitoring process which monitors the performance of
   cells, and can add, remove soft cells in the TSCH schedule to adapt
   to the traffic needs, or move one when it performs poorly.  To
   reserve a soft cell, the higher layer does not indicate the exact
   slotOffset/channelOffset of the cell to add, but rather the resulting
   bandwidth and QoS requirements.  When the monitoring process triggers
   a cell reallocation, the two neighbor devices communicating over this
   cell negotiate its new position in the TSCH schedule.

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4.3.2.  Scheduling Functions and the 6top protocol

   In the case of soft cells, the cell management entity that controls
   the dynamic attribution of cells to adapt to the dynamics of variable
   rate flows is called a Scheduling Function (SF).

   There may be multiple SFs with more or less aggressive reaction to
   the dynamics of the network.

   An SF may be seen as divided between an upper bandwidth adaptation
   logic that is not aware of the particular technology that is used to
   obtain and release bandwidth, and an underlying service that maps
   those needs in the actual technology, which means mapping the
   bandwidth onto cells in the case of TSCH using the 6top protocol as
   illustrated in Figure 8.

    +------------------------+          +------------------------+
    |  Scheduling Function   |          |  Scheduling Function   |
    |  Bandwidth adaptation  |          |  Bandwidth adaptation  |
    +------------------------+          +------------------------+
    |  Scheduling Function   |          |  Scheduling Function   |
    | TSCH mapping to cells  |          | TSCH mapping to cells  |
    +------------------------+          +------------------------+
    | 6top cells negotiation | <- 6P -> | 6top cells negotiation |
    +------------------------+          +------------------------+
            Device A                             Device B

                       Figure 8: SF/6P stack in 6top

   The SF relies on 6top services that implement the 6top Protocol (6P)
   [RFC8480] to negotiate the precise cells that will be allocated or
   freed based on the schedule of the peer.  It may be for instance that
   a peer wants to use a particular time slot that is free in its
   schedule, but that timeslot is already in use by the other peer for a
   communication with a third party on a different cell. 6P enables the
   peers to find an agreement in a transactional manner that ensures the
   final consistency of the nodes state.

   [MSF] is one of the possible scheduling functions.  MSF uses the
   rendez-vous slot from [RFC8180] for network discovery, neighbor
   discovery, and any other broadcast.

   For basic unicast communication with any neighbor, each node uses a
   receive cell at a well-known slotOffset/channelOffset, derived from a
   hash of their own MAC address.  Nodes can reach any neighbor by
   installing a transmit (shared) cell with slotOffset/channelOffset
   derived from the neighbor's MAC address.

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   For child-parent links, MSF continuously monitors the load to/from
   parents and children.  It then uses 6P to install/remove unicast
   cells whenever the current schedule appears to be under-/over-

4.3.3.  6top and RPL Objective Function operations

   An implementation of a RPL [RFC6550] Objective Function (OF), such as
   the RPL Objective Function Zero (OF0) [RFC6552] that is used in the
   Minimal 6TiSCH Configuration [RFC8180] to support RPL over a static
   schedule, may leverage, for its internal computation, the information
   maintained by 6top.

   An OF may require metrics about reachability, such as the Expected
   Transmission Count (ETX) metric [RFC6551].  6top creates and
   maintains an abstract neighbor table, and this state may be leveraged
   to feed an OF and/or store OF information as well.  A neighbor table
   entry may contain a set of statistics with respect to that specific

   The neighbor information may include the time when the last packet
   has been received from that neighbor, a set of cell quality metrics,
   e.g., received signal strength indication (RSSI) or link quality
   indicator (LQI), the number of packets sent to the neighbor or the
   number of packets received from it.  This information can be made
   available through 6top management APIs and used for instance to
   compute a Rank Increment that will determine the selection of the
   preferred parent.

   6top provides statistics about the underlying layer so the OF can be
   tuned to the nature of the TSCH MAC layer. 6top also enables the RPL
   OF to influence the MAC behavior, for instance by configuring the
   periodicity of IEEE Std. 802.15.4 Extended Beacons (EBs).  By
   augmenting the EB periodicity, it is possible to change the network
   dynamics so as to improve the support of devices that may change
   their point of attachment in the 6TiSCH network.

   Some RPL control messages, such as the DODAG Information Object (DIO)
   are ICMPv6 messages that are broadcast to all neighbor nodes.  With
   6TiSCH, the broadcast channel requirement is addressed by 6top by
   configuring TSCH to provide a broadcast channel, as opposed to, for
   instance, piggybacking the DIO messages in Layer-2 Enhanced Beacons
   (EBs), which would produce undue timer coupling among layers, packet
   size issues and could conflict with the policy of production networks
   where EBs are mostly eliminated to conserve energy.

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4.3.4.  Network Synchronization

   Nodes in a TSCH network must be time synchronized.  A node keeps
   synchronized to its time source neighbor through a combination of
   frame-based and acknowledgment-based synchronization.  To maximize
   battery life and network throughput, it is advisable that RPL ICMP
   discovery and maintenance traffic (governed by the trickle timer) be
   somehow coordinated with the transmission of time synchronization
   packets (especially with enhanced beacons).

   This could be achieved through an interaction of the 6top sublayer
   and the RPL objective Function, or could be controlled by a
   management entity.

   Time distribution requires a loop-free structure.  Nodes taken in a
   synchronization loop will rapidly desynchronize from the network and
   become isolated. 6TiSCH uses a RPL DAG with a dedicated global
   Instance for the purpose of time synchronization.  That Instance is
   referred to as the Time Synchronization Global Instance (TSGI).  The
   TSGI can be operated in either of the 3 modes that are detailed in
   section 3.1.3 of RPL [RFC6550], "Instances, DODAGs, and DODAG
   Versions".  Multiple uncoordinated DODAGs with independent Roots may
   be used if all the Roots share a common time source such as the
   Global Positioning System (GPS).

   In the absence of a common time source, the TSGI should form a single
   DODAG with a virtual Root.  A backbone network is then used to
   synchronize and coordinate RPL operations between the backbone
   routers that act as sinks for the LLN.  Optionally, RPL's periodic
   operations may be used to transport the network synchronization.
   This may mean that 6top would need to trigger (override) the trickle
   timer if no other traffic has occurred for such a time that nodes may
   get out of synchronization.

   A node that has not joined the TSGI advertises a MAC level Join
   Priority of 0xFF to notify its neighbors that is not capable of
   serving as time parent.  A node that has joined the TSGI advertises a
   MAC level Join Priority set to its DAGRank() in that Instance, where
   DAGRank() is the operation specified in section 3.5.1 of [RFC6550],
   "Rank Comparison".

   The provisioning of a RPL Root is out of scope for both RPL and this
   Architecture, whereas RPL enables to propagate configuration
   information down the DODAG.  This applies to the TSGI as well; a Root
   is configured or obtains by unspecified means the knowledge of the
   RPLInstanceID for the TSGI.  The Root advertises its DagRank in the
   TSGI, that must be less than 0xFF, as its Join Priority in its IEEE
   Std. 802.15.4 Extended Beacons (EB).

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   A node that reads a Join Priority of less than 0xFF should join the
   neighbor with the lesser Join Priority and use it as time parent.  If
   the node is configured to serve as time parent, then the node should
   join the TSGI, obtain a Rank in that Instance and start advertising
   its own DagRank in the TSGI as its Join Priority in its EBs.

4.3.5.  Slotframes and CDU matrix

   6TiSCH enables IPv6 best effort (stochastic) transmissions over a MAC
   layer that is also capable of scheduled (deterministic)
   transmissions.  A window of time is defined around the scheduled
   transmission where the medium must, as much as practically feasible,
   be free of contending energy to ensure that the medium is free of
   contending packets when time comes for a scheduled transmission.  One
   simple way to obtain such a window is to format time and frequencies
   in cells of transmission of equal duration.  This is the method that
   is adopted in IEEE Std. 802.15.4 TSCH as well as the Long Term
   Evolution (LTE) of cellular networks.

   The 6TiSCH architecture defines a global concept that is called a
   Channel Distribution and Usage (CDU) matrix to describe that
   formatting of time and frequencies,

   A CDU matrix is defined centrally as part of the network definition.
   It is a matrix of cells with a height equal to the number of
   available channels (indexed by ChannelOffsets) and a width (in
   timeslots) that is the period of the network scheduling operation
   (indexed by slotOffsets) for that CDU matrix.  There are different
   models for scheduling the usage of the cells, which place the
   responsibility of avoiding collisions either on a central controller
   or on the devices themselves, at an extra cost in terms of energy to
   scan for free cells (more in Section 4.4).

   The size of a cell is a timeslot duration, and values of 10 to 15
   milliseconds are typical in 802.15.4 TSCH to accommodate for the
   transmission of a frame and an ack, including the security validation
   on the receive side which may take up to a few milliseconds on some
   device architecture.

   A CDU matrix iterates over and over with a well-known channel
   rotation called the hopping sequence.  In a given network, there
   might be multiple CDU matrices that operate with different width, so
   they have different durations and represent different periodic
   operations.  It is recommended that all CDU matrices in a 6TiSCH
   domain operate with the same cell duration and are aligned, so as to
   reduce the chances of interferences from the Slotted ALOHA
   operations.  The knowledge of the CDU matrices is shared between all
   the nodes and used in particular to define slotframes.

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   A slotframe is a MAC-level abstraction that is common to all nodes
   and contains a series of timeslots of equal length and precedence.
   It is characterized by a slotframe_ID, and a slotframe_size.  A
   slotframe aligns to a CDU matrix for its parameters, such as number
   and duration of timeslots.

   Multiple slotframes can coexist in a node schedule, i.e., a node can
   have multiple activities scheduled in different slotframes.  A
   slotframe is associated with a priority that may be related to the
   precedence of different 6TiSCH topologies.  The slotframes may be
   aligned to different CDU matrices and thus have different width.
   There is typically one slotframe for scheduled traffic that has the
   highest precedence and one or more slotframe(s) for RPL traffic.  The
   timeslots in the slotframe are indexed by the SlotOffset; the first
   cell is at SlotOffset 0.

   When a packet is received from a higher layer for transmission, 6top
   inserts that packet in the outgoing queue which matches the packet
   best (Differentiated Services [RFC2474] can therefore be used).  At
   each scheduled transmit slot, 6top looks for the frame in all the
   outgoing queues that best matches the cells.  If a frame is found, it
   is given to the TSCH MAC for transmission.

4.3.6.  Distributing the reservation of cells

   The 6TiSCH architecture introduces the concept of chunks
   (Section 2.1) to distribute the allocation of the spectrum for a
   whole group of cells at a time.  The CDU matrix is formatted into a
   set of chunks, possibly as illustrated in Figure 9, each of the
   chunks identified uniquely by a chunk-ID.  The knowledge of this
   formatting is shared between all the nodes in a 6TiSCH network.  It
   could be conveyed during the join process, or codified into a profile
   document, or obtained using some other mechanism.  This is as opposed
   to static scheduling that refers to the pre-programmed mechanism that
   is specified in [RFC8180] and pre-exists to the distribution of the
   chunk formatting.

                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 0  |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 1  |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                   0     1     2     3     4     5     6          M

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                Figure 9: CDU matrix Partitioning in Chunks

   The 6TiSCH Architecture envisions a protocol that enables chunk
   ownership appropriation whereby a RPL parent discovers a chunk that
   is not used in its interference domain, claims the chunk, and then
   defends it in case another RPL parent would attempt to appropriate it
   while it is in use.  The chunk is the basic unit of ownership that is
   used in that process.

   As a result of the process of chunk ownership appropriation, the RPL
   parent has exclusive authority to decide which cell in the
   appropriated chunk can be used by which node in its interference
   domain.  In other words, it is implicitly delegated the right to
   manage the portion of the CDU matrix that is represented by the

   Initially, those cells are added to the heap of free cells, then
   dynamically placed into existing bundles, in new bundles, or
   allocated opportunistically for one transmission.

   Note that a PCE is expected to have precedence in the allocation, so
   that a RPL parent would only be able to obtain portions that are not
   in-use by the PCE.

4.4.  Schedule Management Mechanisms

   6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes:
   Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring
   and scheduling management, and Hop-by-hop scheduling.  Multiple
   mechanisms are defined that implement the associated Interaction
   Models, and can be combined and used in the same LLN.  Which
   mechanism(s) to use depends on application requirements.

4.4.1.  Static Scheduling

   In the simplest instantiation of a 6TiSCH network, a common fixed
   schedule may be shared by all nodes in the network.  Cells are
   shared, and nodes contend for slot access in a slotted ALOHA manner.

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   A static TSCH schedule can be used to bootstrap a network, as an
   initial phase during implementation, or as a fall-back mechanism in
   case of network malfunction.  This schedule is pre-established, for
   instance decided by a network administrator based on operational
   needs.  It can be pre-configured into the nodes, or, more commonly,
   learned by a node when joining the network using standard IEEE Std.
   802.15.4 Information Elements (IE).  Regardless, the schedule remains
   unchanged after the node has joined a network.  RPL is used on the
   resulting network.  This "minimal" scheduling mechanism that
   implements this paradigm is detailed in [RFC8180].

4.4.2.  Neighbor-to-neighbor Scheduling

   In the simplest instantiation of a 6TiSCH network described in
   Section 4.4.1, nodes may expect a packet at any cell in the schedule
   and will waste energy idle listening.  In a more complex
   instantiation of a 6TiSCH network, a matching portion of the schedule
   is established between peers to reflect the observed amount of
   transmissions between those nodes.  The aggregation of the cells
   between a node and a peer forms a bundle that the 6top layer uses to
   implement the abstraction of a link for IP.  The bandwidth on that
   link is proportional to the number of cells in the bundle.

   If the size of a bundle is configured to fit an average amount of
   bandwidth, peak traffic is dropped.  If the size is configured to
   allow for peak emissions, energy is be wasted idle listening.

   As discussed in more details in Section 4.3, the 6top Protocol
   [RFC8480] specifies the exchanges between neighbor nodes to reserve
   soft cells to transmit to one another, possibly under the control of
   a Scheduling Function (SF).  Because this reservation is done without
   global knowledge of the schedule of other nodes in the LLN,
   scheduling collisions are possible.

   And as discussed in Section 4.3.2, an optional Scheduling Function
   (SF) is used to monitor bandwidth usage and perform requests for
   dynamic allocation by the 6top sublayer.  The SF component is not
   part of the 6top sublayer.  It may be collocated on the same device
   or may be partially or fully offloaded to an external system.  The
   "6TiSCH Minimal Scheduling Function (MSF)" [MSF] provides a simple
   scheduling function that can be used by default by devices that
   support dynamic scheduling of soft cells.

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   Monitoring and relocation is done in the 6top layer.  For the upper
   layer, the connection between two neighbor nodes appears as a number
   of cells.  Depending on traffic requirements, the upper layer can
   request 6top to add or delete a number of cells scheduled to a
   particular neighbor, without being responsible for choosing the exact
   slotOffset/channelOffset of those cells.

4.4.3.  Remote Monitoring and Schedule Management

   Remote monitoring and Schedule Management refers to a DetNet/SDN
   model whereby an NME and a scheduling entity, associated with a PCE,
   reside in a central controller and interact with the 6top layer to
   control IPv6 Links and Tracks (Section 4.5) in a 6TiSCH network.  The
   composite centralized controller can assign physical resources (e.g.,
   buffers and hard cells) to a particular Track to optimize the
   reliability within a bounded latency for a well-specified flow.

   The work at the 6TiSCH WG focused on non-deterministic traffic and
   did not provide the generic data model that is necessary for the
   controller to monitor and manage resources of the 6top sublayer.
   This is deferred to future work, see Appendix A.1.2.

   With respect to Centralized routing and scheduling, it is envisioned
   that the related component of the 6TiSCH Architecture would be an
   extension of the DetNet Architecture [RFC8655], which studies Layer-3
   aspects of Deterministic Networks, and covers networks that span
   multiple Layer-2 domains.

   The DetNet architecture is a form of Software Defined Networking
   (SDN) Architecture and is composed of three planes, a (User)
   Application Plane, a Controller Plane (where the PCE operates), and a
   Network Plane which can represent a 6TiSCH LLN.

   Software-Defined Networking (SDN): Layers and Architecture
   Terminology [RFC7426] proposes a generic representation of the SDN
   architecture that is reproduced in Figure 10.

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                     |                                |
                     | +-------------+   +----------+ |
                     | | Application |   |  Service | |
                     | +-------------+   +----------+ |
                     |       Application Plane        |
       |           Network Services Abstraction Layer (NSAL)           |
              |                                                |
              |               Service Interface                |
              |                                                |
       o------Y------------------o       o---------------------Y------o
       |      |    Control Plane |       | Management Plane    |      |
       | +----Y----+   +-----+   |       |  +-----+       +----Y----+ |
       | | Service |   | App |   |       |  | App |       | Service | |
       | +----Y----+   +--Y--+   |       |  +--Y--+       +----Y----+ |
       |      |           |      |       |     |               |      |
       | *----Y-----------Y----* |       | *---Y---------------Y----* |
       | | Control Abstraction | |       | | Management Abstraction | |
       | |     Layer (CAL)     | |       | |      Layer (MAL)       | |
       | *----------Y----------* |       | *----------Y-------------* |
       |            |            |       |            |               |
       o------------|------------o       o------------|---------------o
                    |                                 |
                    | CP                              | MP
                    | Southbound                      | Southbound
                    | Interface                       | Interface
                    |                                 |
       |         Device and resource Abstraction Layer (DAL)           |
       |            |                                 |                |
       |    o-------Y----------o   +-----+   o--------Y----------o     |
       |    | Forwarding Plane |   | App |   | Operational Plane |     |
       |    o------------------o   +-----+   o-------------------o     |
       |                       Network Device                          |

      Figure 10: SDN Layers and Architecture Terminology per RFC 7426

   The PCE establishes end-to-end Tracks of hard cells, which are
   described in more details in Section 4.6.1.

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   The DetNet work is expected to enable end to end Deterministic Path
   across heterogeneous network.  This can be for instance a 6TiSCH LLN
   and an Ethernet Backbone.

   This model fits the 6TiSCH extended configuration, whereby a 6BBR
   federates multiple 6TiSCH LLN in a single subnet over a backbone that
   can be, for instance, Ethernet or Wi-Fi.  In that model, 6TiSCH 6BBRs
   synchronize with one another over the backbone, so as to ensure that
   the multiple LLNs that form the IPv6 subnet stay tightly

   If the Backbone is Deterministic, then the Backbone Router ensures
   that the end-to-end deterministic behavior is maintained between the
   LLN and the backbone.  It is the responsibility of the PCE to compute
   a deterministic path and to end across the TSCH network and an IEEE
   Std. 802.1 TSN Ethernet backbone, and that of DetNet to enable end-
   to-end deterministic forwarding.

4.4.4.  Hop-by-hop Scheduling

   A node can reserve a Track (Section 4.5) to one or more
   destination(s) that are multiple hops away by installing soft cells
   at each intermediate node.  This forms a Track of soft cells.  A
   Track Scheduling Function above the 6top sublayer of each node on the
   Track is needed to monitor these soft cells and trigger relocation
   when needed.

   This hop-by-hop reservation mechanism is expected to be similar in
   essence to [RFC3209] and/or [RFC4080]/[RFC5974].  The protocol for a
   node to trigger hop-by-hop scheduling is not yet defined.

4.5.  On Tracks

   The architecture introduces the concept of a Track, which is a
   directed path from a source 6TiSCH node to one or more destination
   6TiSCH node(s) across a 6TiSCH LLN.

   A Track is the 6TiSCH instantiation of the concept of a Deterministic
   Path as described in [RFC8655].  Constrained resources such as memory
   buffers are reserved for that Track in intermediate 6TiSCH nodes to
   avoid loss related to limited capacity.  A 6TiSCH node along a Track
   not only knows which bundles of cells it should use to receive
   packets from a previous hop, but also knows which bundle(s) it should
   use to send packets to its next hop along the Track.

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4.5.1.  General Behavior of Tracks

   A Track is associated with Layer-2 bundles of cells with related
   schedules and logical relationships and that ensure that a packet
   that is injected in a Track will progress in due time all the way to

   Multiple cells may be scheduled in a Track for the transmission of a
   single packet, in which case the normal operation of IEEE Std.
   802.15.4 Automatic Repeat-reQuest (ARQ) can take place; the
   acknowledgment may be omitted in some cases, for instance if there is
   no scheduled cell for a possible retry.

   There are several benefits for using a Track to forward a packet from
   a source node to the destination node.

   1.  Track forwarding, as further described in Section 4.6.1, is a
       Layer-2 forwarding scheme, which introduces less process delay
       and overhead than Layer-3 forwarding scheme.  Therefore, LLN
       Devices can save more energy and resource, which is critical for
       resource constrained devices.

   2.  Since channel resources, i.e., bundles of cells, have been
       reserved for communications between 6TiSCH nodes of each hop on
       the Track, the throughput and the maximum latency of the traffic
       along a Track are guaranteed and the jitter is maintained small.

   3.  By knowing the scheduled time slots of incoming bundle(s) and
       outgoing bundle(s), 6TiSCH nodes on a Track could save more
       energy by staying in sleep state during in-active slots.

   4.  Tracks are protected from interfering with one another if a cell
       is scheduled to belong to at most one Track, and congestion loss
       is avoided if at most one packet can be presented to the MAC to
       use that cell.  Tracks enhance the reliability of transmissions
       and thus further improve the energy consumption in LLN Devices by
       reducing the chances of retransmission.

4.5.2.  Serial Track

   A Serial (or simple) Track is the 6TiSCH version of a circuit; a
   bundle of cells that are programmed to receive (RX-cells) is uniquely
   paired to a bundle of cells that are set to transmit (TX-cells),
   representing a Layer-2 forwarding state which can be used regardless
   of the network layer protocol.  A Serial Track is thus formed end-to-
   end as a succession of paired bundles, a receive bundle from the
   previous hop and a transmit bundle to the next hop along the Track.

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   For a given iteration of the device schedule, the effective channel
   of the cell is obtained by following in a loop a well-known hopping
   sequence that started at Epoch time at the channelOffset of the cell,
   which results in a rotation of the frequency that used for
   transmission.  The bundles may be computed so as to accommodate both
   variable rates and retransmissions, so they might not be fully used
   in the iteration of the schedule.

4.5.3.  Complex Track with Replication and Elimination

   The art of Deterministic Networks already include Packet Replication
   and Elimination techniques.  Example standards include the Parallel
   Redundancy Protocol (PRP) and the High-availability Seamless
   Redundancy (HSR) [IEC62439].  Similarly, and as opposed to a Serial
   Track that is a sequence of nodes and links, a Complex Track is
   shaped as a directed acyclic graph towards one or more destination(s)
   to support multi-path forwarding and route around failures.

   A Complex Track may branch off over non congruent branches for the
   purpose of multicasting, and/or redundancy, in which case it
   reconverges later down the path.  This enables the Packet
   Replication, Elimination and Ordering Functions (PREOF) defined by
   Detnet.  Packet ARQ, Replication, Elimination and Overhearing (PAREO)
   adds radio-specific capabilities of Layer-2 ARQ and promiscuous
   listening to redundant transmissions to compensate for the lossiness
   of the medium and meet industrial expectations of a Reliable and
   Available Wireless network.  Combining PAREO and PREOF, a Track may
   extend beyond the 6TiSCH network in a larger DetNet network.

   In the art of TSCH, a path does not necessarily support PRE but it is
   almost systematically multi-path.  This means that a Track is
   scheduled so as to ensure that each hop has at least two forwarding
   solutions, and the forwarding decision is to try the preferred one
   and use the other in case of Layer-2 transmission failure as detected
   by ARQ.  Similarly, at each 6TiSCH hop along the Track, the PCE may
   schedule more than one timeslot for a packet, so as to support
   Layer-2 retries (ARQ).  It is also possible that the field device
   only uses the second branch if sending over the first branch fails.

4.5.4.  DetNet End-to-end Path

   Ultimately, DetNet should enable to extend a Track beyond the 6TiSCH
   LLN as illustrated in Figure 11.  In that example, a Track that is
   laid out from a field device in a 6TiSCH network to an IoT gateway
   that is located on an 802.1 Time-Sensitive Networking (TSN) backbone.
   A 6TiSCH-Aware DetNet Service Layer handles the Packet Replication,
   Elimination, and Ordering Functions over the DODAG that forms a

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   The Replication function in the 6TiSCH Node sends a copy of each
   packet over two different branches, and the PCE schedules each hop of
   both branches so that the two copies arrive in due time at the
   gateway.  In case of a loss on one branch, hopefully the other copy
   of the packet still makes it in due time.  If two copies make it to
   the IoT gateway, the Elimination function in the gateway ignores the
   extra packet and presents only one copy to upper layers.

                     | IoT |
                     | G/W |
                        ^  <=== Elimination
        Track branch   | |
               +-=-=-=-+ +-=-=-=-=+ Subnet Backbone
               |                  |
            +-=|-=+            +-=|-=+
            |  |  | Backbone   |  |  | Backbone
       o    |  |  | router     |  |  | router
            +-=/-=+            +-=|-=+
       o     /    o     o-=-o-=-=/       o
           o    o-=-o-=/   o      o   o  o   o
      o     \  /     o               o   LLN    o
         o   v  <=== Replication

                 Figure 11: Example End-to-End DetNet Track

4.5.5.  Cell Reuse

   The 6TiSCH architecture provides means to avoid waste of cells as
   well as overflows in the transmit bundle of a Track, as follows:

   A TX-cell that is not needed for the current iteration may be reused
   opportunistically on a per-hop basis for routed packets.  When all of
   the frame that were received for a given Track are effectively
   transmitted, any available TX-cell for that Track can be reused for
   upper layer traffic for which the next-hop router matches the next
   hop along the Track.  In that case, the cell that is being used is
   effectively a TX-cell from the Track, but the short address for the
   destination is that of the next-hop router.

   It results in a frame that is received in a RX-cell of a Track with a
   destination MAC address set to this node as opposed to the broadcast
   MAC address must be extracted from the Track and delivered to the
   upper layer.  Note that a frame with an unrecognized destination MAC
   address is dropped at the lower MAC layer and thus is not received at
   the 6top sublayer.

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   On the other hand, it might happen that there are not enough TX-cells
   in the transmit bundle to accommodate the Track traffic, for instance
   if more retransmissions are needed than provisioned.  In that case,
   and if the frame transports an IPv6 packet, then it can be placed for
   transmission in the bundle that is used for Layer-3 traffic towards
   the next hop along the Track.  The MAC address should be set to the
   next-hop MAC address to avoid confusion.

   It results in a frame that is received over a Layer-3 bundle may be
   in fact associated to a Track.  In a classical IP link such as an
   Ethernet, off-Track traffic is typically in excess over reservation
   to be routed along the non-reserved path based on its QoS setting.
   But with 6TiSCH, since the use of the Layer-3 bundle may be due to
   transmission failures, it makes sense for the receiver to recognize a
   frame that should be re-Tracked, and to place it back on the
   appropriate bundle if possible.  .  A frame is re-Tracked by
   scheduling it for transmission over the transmit bundle associated to
   the Track, with the destination MAC address set to broadcast.

4.6.  Forwarding Models

   By forwarding, this document means the per-packet operation that
   allows to deliver a packet to a next hop or an upper layer in this
   node.  Forwarding is based on pre-existing state that was installed
   as a result of a routing computation Section 4.7.  6TiSCH supports
   three different forwarding model:(G-MPLS) Track Forwarding,
   (classical) IPv6 Forwarding and (6LoWPAN) Fragment Forwarding.

4.6.1.  Track Forwarding

   Forwarding along a Track can be seen as a Generalized Multi-protocol
   Label Switching (G-MPLS) operation in that the information used to
   switch a frame is not an explicit label, but rather related to other
   properties of the way the packet was received, a particular cell in
   the case of 6TiSCH.  As a result, as long as the TSCH MAC (and
   Layer-2 security) accepts a frame, that frame can be switched
   regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN
   fragment, or a frame from an alternate protocol such as WirelessHART
   or ISA100.11a.

   A data frame that is forwarded along a Track normally has a
   destination MAC address that is set to broadcast - or a multicast
   address depending on MAC support.  This way, the MAC layer in the
   intermediate nodes accepts the incoming frame and 6top switches it
   without incurring a change in the MAC header.  In the case of IEEE
   Std. 802.15.4, this means effectively broadcast, so that along the
   Track the short address for the destination of the frame is set to

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   There are 2 modes for a Track, an IPv6 native mode and a protocol-
   independant tunnel mode.  Native Mode

   In native mode, the Protocol Data Unit (PDU) is associated with flow-
   dependent meta-data that refers uniquely to the Track, so the 6top
   sublayer can place the frame in the appropriate cell without
   ambiguity.  In the case of IPv6 traffic, this flow identification may
   be done using a 6-tuple as discussed in [I-D.ietf-detnet-ip].  In
   particular, implementations of this document should support
   identification of DetNet flows based on the IPv6 Flow Label field.

   The flow follows a Track which identification is done using a RPL
   Instance (see section 3.1.3 of [RFC6550]), signaled in a RPL Packet
   Information (more in section of [RFC6550]) and the
   destination address in the case of a local instance.  One or more
   flows may be placed in a same Track and the Track identification
   (TrackID + owner) may be placed in an IP-in-IP encapsulation.  The
   forwarding operation is based on the Track and does not depend on the
   flow therein.

   The Track identification is validated at egress before restoring the
   destination MAC address (DMAC) and punting to the upper layer.

   Figure 12 illustrates the Track Forwarding operation which happens at
   the 6top sublayer, below IP.

                          | Packet flowing across the network  ^
      +--------------+    |                                    |
      |     IPv6     |    |                                    |
      +--------------+    |                                    |
      |  6LoWPAN HC  |    |                                    |
      +--------------+  ingress                              egress
      |     6top     |   sets     +----+          +----+    restores
      +--------------+  DMAC to   |    |          |    |    DMAC to
      |   TSCH MAC   |   brdcst   |    |          |    |     dest
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
                        Ingress   Relay            Relay     Egress
         Stack Layer     Node     Node             Node       Node

                  Figure 12: Track Forwarding, Native Mode

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   In tunnel mode, the frames originate from an arbitrary protocol over
   a compatible MAC that may or may not be synchronized with the 6TiSCH
   network.  An example of this would be a router with a dual radio that
   is capable of receiving and sending WirelessHART or ISA100.11a frames
   with the second radio, by presenting itself as an access Point or a
   Backbone Router, respectively.  In that mode, some entity (e.g., PCE)
   can coordinate with a WirelessHART Network Manager or an ISA100.11a
   System Manager to specify the flows that are transported.

      |     IPv6     |
      |  6LoWPAN HC  |
      +--------------+             set            restore
      |     6top     |            +DMAC+          +DMAC+
      +--------------+          to|brdcst       to|nexthop
      |   TSCH MAC   |            |    |          |    |
      +--------------+            |    |          |    |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
      +--------------+    |   ingress                 egress   |
                          |                                    |
      +--------------+    |                                    |
      |   LLN PHY    |    |                                    |
      +--------------+    |  Packet flowing across the network |
      |   TSCH MAC   |    |                                    |
      +--------------+    | DMAC =                             | DMAC =
      |ISA100/WiHART |    | nexthop                            v nexthop
                        Source   Ingress          Egress   Destination
         Stack Layer     Node     Node             Node       Node

                  Figure 13: Track Forwarding, Tunnel Mode

   In that case, the TrackID that identifies the Track at the ingress
   6TiSCH router is derived from the RX-cell.  The DMAC is set to this
   node but the TrackID indicates that the frame must be tunneled over a
   particular Track so the frame is not passed to the upper layer.
   Instead, the DMAC is forced to broadcast and the frame is passed to
   the 6top sublayer for switching.

   At the egress 6TiSCH router, the reverse operation occurs.  Based on
   tunneling information of the Track, which may for instance indicate
   that the tunneled datagram is an IP packet, the datagram is passed to
   the appropriate Link-Layer with the destination MAC restored.

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   Tunneling information coming with the Track configuration provides
   the destination MAC address of the egress endpoint as well as the
   tunnel mode and specific data depending on the mode, for instance a
   service access point for frame delivery at egress.

   If the tunnel egress point does not have a MAC address that matches
   the configuration, the Track installation fails.

   If the Layer-3 destination address belongs to the tunnel termination,
   then it is possible that the IPv6 address of the destination is
   compressed at the 6LoWPAN sublayer based on the MAC address.
   Restoring the wrong MAC address at the egress would then also result
   in the wrong IP address in the packet after decompression.  For that
   reason, a packet can be injected in a Track only if the destination
   MAC address is effectively that of the tunnel egress point.  It is
   thus mandatory for the ingress router to validate that the MAC
   address that was used at the 6LoWPAN sublayer for compression matches
   that of the tunnel egress point before it overwrites it to broadcast.
   The 6top sublayer at the tunnel egress point reverts that operation
   to the MAC address obtained from the tunnel information.

4.6.2.  IPv6 Forwarding

   As the packets are routed at Layer-3, traditional QoS and Active
   Queue Management (AQM) operations are expected to prioritize flows.

                          | Packet flowing across the network  ^
      +--------------+    |                                    |
      |     IPv6     |    |       +-QoS+          +-QoS+       |
      +--------------+    |       |    |          |    |       |
      |  6LoWPAN HC  |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
                        Source   Ingress          Egress   Destination
         Stack Layer     Node    Router           Router      Node

                          Figure 14: IP Forwarding

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4.6.3.  Fragment Forwarding

   Considering that per section 4 of [RFC4944] 6LoWPAN packets can be as
   large as 1280 bytes (the IPv6 minimum MTU), and that the non-storing
   mode of RPL implies Source Routing that requires space for routing
   headers, and that a IEEE Std. 802.15.4 frame with security may carry
   in the order of 80 bytes of effective payload, an IPv6 packet might
   be fragmented into more than 16 fragments at the 6LoWPAN sublayer.

   This level of fragmentation is much higher than that traditionally
   experienced over the Internet with IPv4 fragments, where
   fragmentation is already known as harmful.

   In the case to a multihop route within a 6TiSCH network, Hop-by-Hop
   recomposition occurs at each hop to reform the packet and route it.
   This creates additional latency and forces intermediate nodes to
   store a portion of a packet for an undetermined time, thus impacting
   critical resources such as memory and battery.

   [MIN-FRAG] describes a framework for forwarding fragments end-to-end
   across a 6TiSCH route-over mesh.  Within that framework,
   [I-D.ietf-lwig-6lowpan-virtual-reassembly] details a virtual
   reassembly buffer mechanism whereby the datagram tag in the 6LoWPAN
   Fragment is used as a label for switching at the 6LoWPAN sublayer.

   Building on this technique, [RECOV-FRAG] introduces a new format for
   6LoWPAN fragments that enables the selective recovery of individual
   fragments, and allows for a degree of flow control based on an
   Explicit Congestion Notification.

                          | Packet flowing across the network  ^
      +--------------+    |                                    |
      |     IPv6     |    |       +----+          +----+       |
      +--------------+    |       |    |          |    |       |
      |  6LoWPAN HC  |    |       learn           learn        |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
                        Source   Ingress          Egress   Destination
         Stack Layer     Node    Router           Router      Node

                    Figure 15: Forwarding First Fragment

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   In that model, the first fragment is routed based on the IPv6 header
   that is present in that fragment.  The 6LoWPAN sublayer learns the
   next hop selection, generates a new datagram tag for transmission to
   the next hop, and stores that information indexed by the incoming MAC
   address and datagram tag.  The next fragments are then switched based
   on that stored state.

                          | Packet flowing across the network  ^
      +--------------+    |                                    |
      |     IPv6     |    |                                    |
      +--------------+    |                                    |
      |  6LoWPAN HC  |    |       replay          replay       |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
                        Source   Ingress          Egress   Destination
         Stack Layer     Node    Router           Router      Node

                    Figure 16: Forwarding Next Fragment

   A bitmap and an ECN echo in the end-to-end acknowledgment enable the
   source to resend the missing fragments selectively.  The first
   fragment may be resent to carve a new path in case of a path failure.
   The ECN echo set indicates that the number of outstanding fragments
   should be reduced.

4.7.  Advanced 6TiSCH Routing

4.7.1.  Packet Marking and Handling

   All packets inside a 6TiSCH domain must carry the RPLInstanceID that
   identifies the 6TiSCH topology (e.g., a Track) that is to be used for
   routing and forwarding that packet.  The location of that information
   must be the same for all packets forwarded inside the domain.

   For packets that are routed by a PCE along a Track, the tuple formed
   by 1) (typically) the IPv6 source or (possibly) destination address
   in the IPv6 Header and 2) a local RPLInstanceID in the RPI that
   serves as TrackID, identify uniquely the Track and associated
   transmit bundle.

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   For packets that are routed by RPL, that information is the
   RPLInstanceID which is carried in the RPL Packet Information (RPI),
   as discussed in section 11.2 of [RFC6550], "Loop Avoidance and
   Detection".  The RPI is transported by a RPL option in the IPv6 Hop-
   By-Hop Header [RFC6553].

   A compression mechanism for the RPL packet artifacts that integrates
   the compression of IP-in-IP encapsulation and the Routing Header type
   3 [RFC6554] with that of the RPI in a 6LoWPAN dispatch/header type is
   specified in [RFC8025] and [RFC8138].

   Either way, the method and format used for encoding the RPLInstanceID
   is generalized to all 6TiSCH topological Instances, which include
   both RPL Instances and Tracks.

4.7.2.  Replication, Retries and Elimination

   6TiSCH supports the PREOF operations of elimination and reordering of
   packets along a complex Track, but has no requirement about whether a
   sequence number is tagged in the packet for that purpose.  With
   6TiSCH, the schedule can tell when multiple receive timeslots
   correspond to copies of a same packet, in which case the receiver may
   avoid listening to the extra copies once it had received one instance
   of the packet.

   The semantics of the configuration will enable correlated timeslots
   to be grouped for transmit (and respectively receive) with a 'OR'
   relations, and then a 'AND' relation would be configurable between
   groups.  The semantics is that if the transmit (and respectively
   receive) operation succeeded in one timeslot in a 'OR' group, then
   all the other timeslots in the group are ignored.  Now, if there are
   at least two groups, the 'AND' relation between the groups indicates
   that one operation must succeed in each of the groups.

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   On the transmit side, timeslots provisioned for retries along a same
   branch of a Track are placed a same 'OR' group.  The 'OR' relation
   indicates that if a transmission is acknowledged, then
   retransmissions of that packet should not be attempted for remaining
   timeslots in that group.  There are as many 'OR' groups as there are
   branches of the Track departing from this node.  Different 'OR'
   groups are programmed for the purpose of replication, each group
   corresponding to one branch of the Track.  The 'AND' relation between
   the groups indicates that transmission over any of branches must be
   attempted regardless of whether a transmission succeeded in another
   branch.  It is also possible to place cells to different next-hop
   routers in a same 'OR' group.  This allows to route along multi-path
   Tracks, trying one next-hop and then another only if sending to the
   first fails.

   On the receive side, all timeslots are programmed in a same 'OR'
   group.  Retries of a same copy as well as converging branches for
   elimination are converged, meaning that the first successful
   reception is enough and that all the other timeslots can be ignored.
   A 'AND' group denotes different packets that must all be received and
   transmitted over the associated transmit groups within their
   respected 'AND' or 'OR' rules.

   As an example say that we have a simple network as represented in
   Figure 17, and we want to enable PREOF between an ingress node I and
   an egress node E.

                              +-+         +-+
                           -- |A|  ------ |C| --
                         /    +-+         +-+    \
                       /                           \
                  +-+                                +-+
                  |I|                                |E|
                  +-+                                +-+
                       \                           /
                         \    +-+         +-+    /
                           -- |B| ------- |D| --
                              +-+         +-+

              Figure 17: Scheduling PREOF on a Simple Network

   The assumption for this particular problem is that a 6TiSCH node has
   a single radio, so it cannot perform 2 receive and/or transmit
   operations at the same time, even on 2 different channels.

   Say we have 6 possible channels, and at least 10 timeslots per
   slotframe.  Figure 18 shows a possible schedule whereby each
   transmission is retried 2 or 3 times, and redundant copies are

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   forwarded in parallel via A and C on the one hand, and B and D on the
   other, providing time diversity, spatial diversity though different
   physical paths, and frequency diversity.

       slotOffset      0    1    2    3    4    5    6    7    9
    channelOffset 0 |    |    |    |    |    |    |B->D|    |    | ...
    channelOffset 1 |    |I->A|    |A->C|B->D|    |    |    |    | ...
    channelOffset 2 |I->A|    |    |I->B|    |C->E|    |D->E|    | ...
    channelOffset 3 |    |    |    |    |A->C|    |    |    |    | ...
    channelOffset 4 |    |    |I->B|    |    |B->D|    |    |D->E| ...
    channelOffset 5 |    |    |A->C|    |    |    |C->E|    |    | ...

                     Figure 18: Example Global Schedule

   This translates in a different slotframe for every node that provides
   the waking and sleeping times, and the channelOffset to be used when
   awake.  Figure 19 shows the corresponding slotframe for node A.

       slotOffset      0    1    2    3    4    5    6    7    9
    operation       |rcv |rcv |xmit|xmit|xmit|none|none|none|none| ...
    channelOffset   |  2 |  1 |  5 |  1 |  3 |N/A |N/A |N/A |N/A | ...

                  Figure 19: Example Slotframe for Node A

   The logical relationship between the timeslots is given by the
   following table:

               | Node |    rcv slotOffset   |    xmit slotOffset     |
               | I    |         N/A         | (0 OR 1) AND (2 OR 3)  |
               | A    |       (0 OR 1)      |     (2 OR 3 OR 4)      |
               | B    |       (2 OR 3)      |     (4 OR 5 OR 6)      |
               | C    |    (2 OR 3 OR 4)    |        (5 OR 6)        |
               | D    |    (4 OR 5 OR 6)    |        (7 OR 8)        |
               | E    |  (5 OR 6 OR 7 OR 8) |          N/A           |

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                                 Figure 20

5.  IANA Considerations

   This document does not require IANA action.

6.  Security Considerations

   The "Minimal Security Framework for 6TiSCH" [MIN-SECURITY] was
   optimized for Low-Power and TSCH operations.  The reader is
   encouraged to review the Security Considerations section of that
   document, which discusses 6TiSCH security issues in more details.

6.1.  Availability of Remote Services

   The operation of 6TiSCH Tracks inherits its high level operation from
   DetNet and is subject to the observations in section 5 of [RFC8655].
   The installation and the maintenance of the 6TiSCH Tracks depends on
   the availability of a controller with a PCE to compute and push them
   in the network.  When that connectivity is lost, existing Tracks may
   continue to operate until the end of their lifetime, but cannot be
   removed or updated, and new Tracks cannot be installed.

   In a LLN, the communication with a remote PCE may be slow and
   unreactive to rapid changes in the condition of the wireless
   communication.  An attacker may introduce extra delay by selectively
   jamming some packets or some flows.  The expectation is that the
   6TiSCH Tracks enable enough redundancy to maintain the critical
   traffic in operation while new routes are calculated and programmed
   into the network.

   As with DetNet in general, the communication with the PCE must be
   secured and should be protected against DoS attacks, including delay
   injection and blackholing attacks, and secured as discussed in the
   security considerations defined for Abstraction and Control of
   Traffic Engineered Networks (ACTN) in Section 9 of [RFC8453], which
   applies equally to DetNet and 6TiSCH.  In a similar manner, the
   communication with the JRC must be secured and should be protected
   against DoS attacks when possible.

6.2.  Selective Jamming

   The Hopping Sequence of a TSCH network is well-known, meaning that if
   a rogue manages to identify a cell of a particular flow, then it may
   to selectively jam that cell, without impacting any other traffic.
   This attack can be performed at the PHY layer without any knowledge
   of the Layer-2 keys, and is very hard to detect and diagnose because
   only one flow is impacted.

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   [I-D.tiloca-6tisch-robust-scheduling] proposes a method to obfuscate
   the hopping sequence and make it harder to perpetrate that particular

6.3.  MAC-Layer Security

   This architecture operates on IEEE Std. 802.15.4 and expects the
   Link-Layer security to be enabled at all times between connected
   devices, except for the very first step of the device join process,
   where a joining device may need some initial, unsecured exchanges so
   as to obtain its initial key material.  In a typical deployment, all
   joined nodes use the same keys and rekeying needs to be global.

   The 6TISCH Architecture relies on the join process to deny
   authorization of invalid nodes and preserve the integrity of the
   network keys.  A rogue that managed to access the network can perform
   a large variety of attacks from DoS to injecting forged packets and
   routing information.  "Zero-trust" properties would be highly
   desirable but are mostly not available at the time of this writing.
   [AP-ND] is a notable exception that protects the ownership of IPv6
   addresses and prevents a rogue node with L2 access from stealing and
   injecting traffic on behalf of a legitimate node.

6.4.  Time Synchronization

   Time Synchronization in TSCH induces another event horizon whereby a
   node will only communicate with another node if they are synchronized
   within a guard time.  The pledge discovers the synchronization of the
   network based on the time of reception of the beacon.  If an attacker
   synchronizes a pledge outside of the guard time of the legitimate
   nodes then the pledge will never see a legitimate beacon and may not
   discover the attack.

   As discussed in [RFC8655], measures must be taken to protect the time
   synchronization, and for 6TiSCH this includes ensuring that the
   Absolute Slot Number (ASN), which is the node's sense of time, is not
   compromised.  Once installed and as long as the node is synchronized
   to the network, ASN is implicit in the transmissions.

   IEEE Std. 802.15.4 [IEEE802154] specifies that in a TSCH network, the
   nonce that is used for the computation of the Message Integrity Code
   (MIC) to secure Link-Layer frames is composed of the address of the
   source of the frame and of the ASN.  The standard assumes that the
   ASN is distributed securely by other means.  The ASN is not passed
   explicitly in the data frames and does not constitute a complete
   anti-replay protection.  It results that upper layer protocols must
   provide a way to detect duplicates and cope with them.

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   If the receiver and the sender have a different sense of ASN, the MIC
   will not validate and the frame will be dropped.  In that sense, TSCH
   induces an event horizon whereby only nodes that have a common sense
   of ASN can talk to one another in an authenticated manner.  With
   6TiSCH, the pledge discovers a tentative ASN in beacons from nodes
   that have already joined the network.  But even if the beacon can be
   authenticated, the ASN cannot be trusted as it could be a replay by
   an attacker and thus could announce an ASN that represents a time in
   the past.  If the pledge uses an ASN that is learned from a replayed
   beacon for an encrypted transmission, a nonce-reuse attack becomes
   possible and the network keys may be compromised.

6.5.  Validating ASN

   After obtaining the tentative ASN, a pledge that wishes to join the
   6TiSCH network must use a join protocol to obtain its security keys.
   The join protocol used in 6TiSCH is the Constrained Join Protocol
   (CoJP).  In the minimal setting defined in [MIN-SECURITY], the
   authentication requires a pre-shared key, based on which a secure
   session is derived.  The CoJP exchange may also be preceded with a
   zero-touch handshake [I-D.ietf-6tisch-dtsecurity-zerotouch-join] in
   order to enable pledge joining based on certificates and/or inter-
   domain communication.

   As detailed in Section 4.2.1, a Join Proxy (JP) helps the pledge for
   the join procedure by relaying the link-scope Join Request over the
   IP network to a Join Registrar/Coordinator (JRC) that can
   authenticate the pledge and validate that it is attached to the
   appropriate network.  As a result of the CoJP exchange, the pledge is
   in possession of a Link-Layer material including keys and a short
   address, and if the ASN is known to be correct, all traffic can now
   be secured using CCM* [CCMstar] at the Link-Layer.

   The authentication steps must be such that they cannot be replayed by
   an attacker, and they must not depend on the tentative ASN being
   valid.  During the authentication, the keying material that the
   pledge obtains from the JRC does not provide protection against
   spoofed ASN.  Once the pledge has obtained the keys to use in the
   network, it may still need to verify the ASN.  If the nonce used in
   the Layer-2 security derives from the extended (MAC-64) address, then
   replaying the ASN alone cannot enable a nonce-reuse attack unless the
   same node is lost its state with a previous ASN.  But if the nonce
   derives from the short address (e.g., assigned by the JRC) then the
   JRC must ensure that it never assigns short addresses that were
   already given to this or other nodes with the same keys.  In other
   words, the network must be rekeyed before the JRC runs out of short

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6.6.  Network Keying and Rekeying

   Section 4.2.1 provides an overview of the CoJP process described in
   [MIN-SECURITY] by which an LLN can be assembled in the field, having
   been provisioned in a lab.
   [I-D.ietf-6tisch-dtsecurity-zerotouch-join] is future work that
   preceeds and then leverages the CoJP protocol using the
   [I-D.ietf-anima-constrained-voucher] constrained profile of
   [I-D.ietf-anima-bootstrapping-keyinfra] (BRSKI).  This later work
   requires a yet-to-be standardized Lighweight Authenticated Key
   Exchange protocol.

   The CoJP protocol results in distribution of a network-wide key that
   is to be used with [IEEE802154] security.  The details of use are
   described in [MIN-SECURITY] sections 9.2 and 9.3.2.

   The BRSKI mechanism may lead to the use of the CoJP protocol, in
   which case it also results in distribution of a network-wide key.
   Alternatively the BRSKI mechanism may be followed by use of
   [I-D.ietf-ace-coap-est] to enroll certificates for each device.  In
   that case, the certificates may be used with an [IEEE802154] key
   agreement protocol.  The description of this mechanism, while
   conceptually straight forward still has significant standardization
   hurdles to pass.

   [MIN-SECURITY] section 9.2 describes a mechanism to change (rekey)
   the network.  There are a number of reasons to initiate a network
   rekey: to remove unwanted (corrupt/malicious) nodes, to recover
   unused 2-byte short addresses, or due to limits in encryption
   algorithms.  For all of the mechanisms that distribute a network-wide
   key, rekeying is also needed on a periodic basis.  In more details:

   *  The mechanism described in [MIN-SECURITY] section 9.2 requires
      advance communication between the JRC and every one of the nodes
      before the key change.  Given that many nodes may be sleepy, this
      operation may take a significant amount of time, and may consume a
      significant portion of the available bandwidth.  As such, network-
      wide rekeys in order to exclude nodes that have become malicious
      will not be particularly quick.  If a rekey is already in
      progress, but the unwanted node has not yet been updated, then it
      is possible to to just continue the operation.  If the unwanted
      node has already received the update, then the rekey operation
      will need to be restarted.

   *  The cryptographic mechanisms used by IEEE Std. 802.15.4 include
      the 2-byte short address in the calculation of the context.  A
      nonce-reuse attack may become feasible if a short address is

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      reassigned to another node while the same network-wide keys are in
      operation.  A network that gains and loses nodes on a regular
      basis is likely to reach the 65536 limit of the 2-byte (16-bit)
      short addresses, even if the network has only a few thousand
      nodes.  Network planners should consider the need to rekey the
      network on a periodic basis in order to recover 2-byte addresses.
      The rekey can update the short addresses for active nodes if
      desired, but there is actually no need to do this as long as the
      key has been changed.

   *  With TSCH as it stands at the time of this writing, the ASN will
      wrap after 2^40 timeslot durations, which means with the default
      values around 350 years.  Wrapping ASN is not expected to happen
      within the lifetime of most LLNs.  Yet, should the ASN wrap, the
      network must be rekeyed to avoid a nonce-reuse attack.

   *  Many cipher algorithms have some suggested limits on how many
      bytes should be encrypted with that algorithm before a new key is
      used.  These numbers are typically in the many to hundreds of
      gigabytes of data.  On very fast backbone networks this becomes an
      important concern.  On LLNs with typical data rates in the
      kilobits/second, this concern is significantly less.  With IEEE
      Std. 802.15.4 as it stands at the time of this writing, the ASN
      will wrap before the limits of the current L2 crypto (AES-CCM-128)
      are reached, so the problem should never occur.

   *  In any fashion, if the LLN is expected to operate continuously for
      decades then the operators are advised to plan for the need to

   Except for urgent rekeys caused by malicious nodes, the rekey
   operation described in [MIN-SECURITY] can be done as a background
   task and can be done incrementally.  It is a make-before-break
   mechanism.  The switch over to the new key is not signaled by time,
   but rather by observation that the new key is in use.  As such, the
   update can take as long as needed, or occur in as short a time as

7.  Acknowledgments

7.1.  Contributors

   The co-authors of this document are listed below:

   Thomas Watteyne  for his contribution to the whole design, in
      particular on TSCH and security, and to the open source community
      with openWSN that he created.

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   Xavier Vilajosana  who lead the design of the minimal support with
      RPL and contributed deeply to the 6top design and the G-MPLS
      operation of Track switching;

   Kris Pister  for creating TSCH and his continuing guidance through
      the elaboration of this design;

   Malisa Vucinic  for the work on the one-touch join process and his
      contribution to the Security Design Team;

   Michael Richardson  for his leadership role in the Security Design
      Team and his contribution throughout this document;

   Tero Kivinen  for his contribution to the security work in general
      and the security section in particular.

   Maria Rita Palattella  for managing the Terminology document merged
      into this through the work of 6TiSCH;

   Simon Duquennoy  for his contribution to the open source community
      with the 6TiSCH implementaton of contiki, and for his contribution
      to MSF and autonomous unicast cells.

   Qin Wang  who lead the design of the 6top sublayer and contributed
      related text that was moved and/or adapted in this document;

   Rene Struik  for the security section and his contribution to the
      Security Design Team;

   Robert Assimiti  for his breakthrough work on RPL over TSCH and
      initial text and guidance;

7.2.  Special Thanks

   Special thanks to Jonathan Simon, Giuseppe Piro, Subir Das and
   Yoshihiro Ohba for their deep contribution to the initial security
   work, to Yasuyuki Tanaka for his work on implementation and
   simulation that tremendously helped build a robust system, to Diego
   Dujovne for starting and leading the SF0 effort and to Tengfei Chang
   for evolving it in the MSF.

   Special thanks also to Pat Kinney, Charlie Perkins and Bob Heile for
   their support in maintaining the connection active and the design in
   line with work happening at IEEE 802.15.

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   Special thanks to Ted Lemon who was the INT Area A-D while this
   document was initiated for his great support and help throughout, and
   to Suresh Krishnan who took over with that kind efficiency of his
   till publication.

   Also special thanks to Ralph Droms who performed the first INT Area
   Directorate review, that was very deep and thorough and radically
   changed the orientations of this document, and then to Eliot Lear and
   Carlos Pignataro who help finalize this document in preparation to
   the IESG reviews, and to Gorry Fairhurst, David Mandelberg, Qin Wu,
   Francis Dupont, Eric Vyncke, Mirja Kuhlewind, Roman Danyliw, Benjamin
   Kaduk and Andrew Malis, who contributed to the final shaping of this
   document through the IESG review procedure.

7.3.  And Do not Forget

   This document is the result of multiple interactions, in particular
   during the 6TiSCH (bi)Weekly Interim call, relayed through the 6TiSCH
   mailing list at the IETF, over the course of more than 5 years.

   The authors wish to thank in arbitrary order: Alaeddine Weslati,
   Chonggang Wang, Georgios Exarchakos, Zhuo Chen, Georgios
   Papadopoulos, Eric Levy-Abegnoli, Alfredo Grieco, Bert Greevenbosch,
   Cedric Adjih, Deji Chen, Martin Turon, Dominique Barthel, Elvis
   Vogli, Geraldine Texier, Guillaume Gaillard, Herman Storey, Kazushi
   Muraoka, Ken Bannister, Kuor Hsin Chang, Laurent Toutain, Maik
   Seewald, Michael Behringer, Nancy Cam Winget, Nicola Accettura,
   Nicolas Montavont, Oleg Hahm, Patrick Wetterwald, Paul Duffy, Peter
   van der Stock, Rahul Sen, Pieter de Mil, Pouria Zand, Rouhollah
   Nabati, Rafa Marin-Lopez, Raghuram Sudhaakar, Sedat Gormus, Shitanshu
   Shah, Steve Simlo, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines
   Robles and Samita Chakrabarti for their participation and various

8.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [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,

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   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [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,

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,

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   [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,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,

   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
              2017, <>.

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <>.

   [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, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC8480]  Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Protocol (6P)", RFC 8480,
              DOI 10.17487/RFC8480, November 2018,

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,

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   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <>.

   [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,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC5889]  Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing
              Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
              September 2010, <>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,

              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,

              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", Work in Progress, Internet-Draft, draft-
              ietf-6lo-backbone-router-20, 23 March 2020,

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              Thubert, P., "6LoWPAN Selective Fragment Recovery", Work
              in Progress, Internet-Draft, draft-ietf-6lo-fragment-
              recovery-21, 23 March 2020, <

   [MIN-FRAG] Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
              6LoWPAN Fragments over a Multihop IPv6 Network", Work in
              Progress, Internet-Draft, draft-ietf-6lo-minimal-fragment-
              15, 23 March 2020, <

   [AP-ND]    Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", Work in Progress, Internet-Draft, draft-
              ietf-6lo-ap-nd-23, 30 April 2020,

              Robles, I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", Work in
              Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-42,
              12 November 2020, <

              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              Work in Progress, Internet-Draft, draft-ietf-roll-unaware-
              leaves-23, 10 November 2020, <

              Dujovne, D. and M. Richardson, "IEEE 802.15.4 Information
              Element encapsulation of 6TiSCH Join and Enrollment
              Information", Work in Progress, Internet-Draft, draft-
              ietf-6tisch-enrollment-enhanced-beacon-14, 21 February
              2020, <

   [MSF]      Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
              D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-msf-
              18, 12 September 2020,

9.  Informative References

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   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, DOI 10.17487/RFC3963, January 2005,

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

   [RFC3444]  Pras, A. and J. Schoenwaelder, "On the Difference between
              Information Models and Data Models", RFC 3444,
              DOI 10.17487/RFC3444, January 2003,

   [RFC4080]  Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
              Bosch, "Next Steps in Signaling (NSIS): Framework",
              RFC 4080, DOI 10.17487/RFC4080, June 2005,

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,

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   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,

   [RFC5974]  Manner, J., Karagiannis, G., and A. McDonald, "NSIS
              Signaling Layer Protocol (NSLP) for Quality-of-Service
              Signaling", RFC 5974, DOI 10.17487/RFC5974, October 2010,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <>.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,

              Phinney, T., Thubert, P., and R. Assimiti, "RPL
              applicability in industrial networks", Work in Progress,
              Internet-Draft, draft-ietf-roll-rpl-industrial-
              applicability-02, 21 October 2013,

              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-
              dtsecurity-zerotouch-join-04, 8 July 2019,

              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments

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              (OSCORE)", Work in Progress, Internet-Draft, draft-ietf-
              core-object-security-16, 6 March 2019,

              Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and
              V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2
              (AODVv2) Routing", Work in Progress, Internet-Draft,
              draft-ietf-manet-aodvv2-16, 4 May 2016,

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,

              Varga, B., Farkas, J., Berger, L., Fedyk, D., and S.
              Bryant, "DetNet Data Plane: IP", Work in Progress,
              Internet-Draft, draft-ietf-detnet-ip-07, 3 July 2020,

              Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", Work in Progress, Internet-
              Draft, draft-ietf-anima-bootstrapping-keyinfra-45, 11
              November 2020, <

              Anamalamudi, S., Zhang, M., Perkins, C., Anand, S., and B.
              Liu, "AODV based RPL Extensions for Supporting Asymmetric
              P2P Links in Low-Power and Lossy Networks", Work in
              Progress, Internet-Draft, draft-ietf-roll-aodv-rpl-08, 7
              May 2020,

              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
              lwig-6lowpan-virtual-reassembly-02, 9 March 2020,

              Thubert, P., Jadhav, R., and M. Gillmore, "Root initiated
              routing state in RPL", Work in Progress, Internet-Draft,

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              draft-ietf-roll-dao-projection-14, 2 October 2020,

              Jadhav, R. and P. Thubert, "RPL Mode of Operation
              extension", Work in Progress, Internet-Draft, draft-rahul-
              roll-mop-ext-01, 9 June 2019,

              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-selander-ace-cose-ecdhe-14, 11
              September 2019, <

              Thubert, P., "RPL-BIER", Work in Progress, Internet-Draft,
              draft-thubert-roll-bier-02, 24 July 2018,

              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", Work in Progress, Internet-
              Draft, draft-thubert-bier-replication-elimination-03, 3
              March 2018, <

              Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
              6loRH for BitStrings", Work in Progress, Internet-Draft,
              draft-thubert-6lo-bier-dispatch-06, 28 January 2019,

              Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
              Unicast Lookup", Work in Progress, Internet-Draft, draft-
              thubert-6man-unicast-lookup-00, 29 July 2019,

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              Thubert, P. and G. Papadopoulos, "Reliable and Available
              Wireless Problem Statement", Work in Progress, Internet-
              Draft, draft-pthubert-raw-problem-statement-04, 23 October
              2019, <

              Tiloca, M., Duquennoy, S., and G. Dini, "Robust Scheduling
              against Selective Jamming in 6TiSCH Networks", Work in
              Progress, Internet-Draft, draft-tiloca-6tisch-robust-
              scheduling-02, 10 June 2019, <

              Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
              "EST over secure CoAP (EST-coaps)", Work in Progress,
              Internet-Draft, draft-ietf-ace-coap-est-18, 6 January

              Richardson, M., Stok, P., and P. Kampanakis, "Constrained
              Voucher Artifacts for Bootstrapping Protocols", Work in
              Progress, Internet-Draft, draft-ietf-anima-constrained-
              voucher-09, 2 November 2020, <

              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".

   [CCMstar]  Struik, R., "Formal Specification of the CCM* Mode of
              Operation", September 2004, <

              IEEE standard for Information Technology, "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, June 2011 as amended by IEEE Std.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April

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    , "Industrial Communication Networks -
              Wireless Communication Network and Communication Profiles
              - WirelessHART - IEC 62591", 2010.

   [HART], "Highway Addressable remote Transducer,
              a group of specifications for industrial process and
              control devices administered by the HART Foundation".

              ISA/ANSI, "Wireless Systems for Industrial Automation:
              Process Control and Related Applications - ISA100.11a-2011
              - IEC 62734", 2011, <

   [ISA100]   ISA/ANSI, "ISA100, Wireless Systems for Automation",

   [TEAS]     IETF, "Traffic Engineering Architecture and Signaling",

   [ANIMA]    IETF, "Autonomic Networking Integrated Model and

   [PCE]      IETF, "Path Computation Element",

   [CCAMP]    IETF, "Common Control and Measurement Plane",

   [AMI]      US Department of Energy, "Advanced Metering Infrastructure
              and Customer Systems", 2006,

   [S-ALOHA]  Roberts, L. G., "ALOHA Packet System With and Without
              Slots and Capture", doi 10.1145/1024916.1024920, April
              1975, <>.

   [IEC62439] IEC, "Industrial communication networks - High
              availability automation networks - Part 3: Parallel
              Redundancy Protocol (PRP) and High-availability Seamless
              Redundancy (HSR) - IEC62439-3", 2012,

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Appendix A.  Related Work In Progress

   This document has been incremented as the work progressed following
   the evolution of the WG charter and the availability of dependent
   work.  The intent was to publish when the WG concludes on the covered
   items.  At the time of publishing the following specification are
   still in progress and may affect the evolution of the stack in a
   6TiSCH-aware node.

A.1.  Unchartered IETF work items

A.1.1.  6TiSCH Zerotouch security

   The security model and in particular the zerotouch join process
   [I-D.ietf-6tisch-dtsecurity-zerotouch-join] depends on the ANIMA
   [ANIMA] Bootstrapping Remote Secure Key Infrastructures (BRSKI)
   [I-D.ietf-anima-bootstrapping-keyinfra] to enable zero-touch security
   provisionning; for highly constrained nodes, a minimal model based on
   pre-shared keys (PSK) is also available.  As written to this day, it
   also depends on a number of documents in progress as CORE, and on
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)"
   [I-D.selander-ace-cose-ecdhe], which is being considered for adoption
   at the LAKE WG.

A.1.2.  6TiSCH Track Setup

   ROLL is now standardizing a reactive routing protocol based on RPL
   [I-D.ietf-roll-aodv-rpl] The need of a reactive routing protocol to
   establish on-demand constraint-optimized routes and a reservation
   protocol to establish Layer-3 Tracks is being discussed at 6TiSCH but
   not chartered for.

   At the time of this writing, there is new work planned in the IETF to
   provide limited deterministic networking capabilities for wireless
   networks with a focus on forwarding behaviors to react quickly and
   locally to the changes as described in

   ROLL is also standardizing an extension to RPL to setup centrally-
   computed routes [I-D.ietf-roll-dao-projection]

   The 6TiSCH Architecture should thus inherit from the DetNet [RFC8655]
   architecture and thus depends on it.  The Path Computation Element
   (PCE) should be a core component of that architecture.  An extension
   to RPL or to TEAS [TEAS] will be required to expose the 6TiSCH node
   capabilities and the network peers to the PCE, possibly in
   combination with [I-D.rahul-roll-mop-ext].  A protocol such as a
   lightweight PCEP or an adaptation of CCAMP [CCAMP] G-MPLS formats and

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   procedures could be used in combination to
   [I-D.ietf-roll-dao-projection] to install the Tracks, as computed by
   the PCE, to the 6TiSCH nodes.

A.1.3.  Using BIER in a 6TiSCH Network

   ROLL is actively working on Bit Index Explicit Replication (BIER) as
   a method to compress both the dataplane packets and the routing
   tables in storing mode [I-D.thubert-roll-bier].

   BIER could also be used in the context of the DetNet service layer.
   BIER-TE-based OAM, Replication and Elimination
   [I-D.thubert-bier-replication-elimination] leverages BIER Traffic
   Engineering (TE) to control in the data plane the DetNet Replication
   and Elimination activities, and to provide traceability on links
   where replication and loss happen, in a manner that is abstract to
   the forwarding information.

   a 6loRH for BitStrings [I-D.thubert-6lo-bier-dispatch] proposes a
   6LoWPAN compression for the BIER Bitstring based on 6LoWPAN Routing
   Header [RFC8138].

A.2.  External (non-IETF) work items

   The current charter positions 6TiSCH on IEEE Std. 802.15.4 only.
   Though most of the design should be portable on other link types,
   6TiSCH has a strong dependency on IEEE Std. 802.15.4 and its
   evolution.  The impact of changes to TSCH on this Architecture should
   be minimal to non-existent, but deeper work such as 6top and security
   may be impacted.  A 6TiSCH Interest Group at the IEEE maintains the
   synchronization and helps foster work at the IEEE should 6TiSCH
   demand it.

   Work is being proposed at IEEE (802.15.12 PAR) for an LLC that would
   logically include the 6top sublayer.  The interaction with the 6top
   sublayer and the Scheduling Functions described in this document are
   yet to be defined.

   ISA100 [ISA100] Common Network Management (CNM) is another external
   work of interest for 6TiSCH.  The group, referred to as ISA100.20,
   defines a Common Network Management framework that should enable the
   management of resources that are controlled by heterogeneous
   protocols such as ISA100.11a [ISA100.11a], WirelessHART
   [WirelessHART], and 6TiSCH.  Interestingly, the establishment of
   6TiSCH Deterministic paths, called Tracks, are also in scope, and
   ISA100.20 is working on requirements for DetNet.

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Author's Address

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 Mougins - Sophia Antipolis

   Phone: +33 497 23 26 34

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