6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                       November 10, 2018
Expires: May 14, 2019

      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
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   This Internet-Draft will expire on May 14, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  6TiSCH Terminology  . . . . . . . . . . . . . . . . . . .   4
     2.3.  References  . . . . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Subset of a 6LoWPAN Glossary  . . . . . . . . . . . . . .  10
   3.  High Level Architecture . . . . . . . . . . . . . . . . . . .  11
     3.1.  6TiSCH Stack  . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  TSCH: A Deterministic MAC Layer . . . . . . . . . . . . .  13
     3.3.  Scheduling TSCH . . . . . . . . . . . . . . . . . . . . .  14
     3.4.  Routing and Forwarding Over TSCH  . . . . . . . . . . . .  15
     3.5.  A Non-Broadcast Multi-Access Radio Mesh Network . . . . .  17
     3.6.  A Multi-Link Subnet Model . . . . . . . . . . . . . . . .  18
     3.7.  Join Process and Registration . . . . . . . . . . . . . .  20
   4.  Architecture Components . . . . . . . . . . . . . . . . . . .  22
     4.1.  6LoWPAN (and RPL) . . . . . . . . . . . . . . . . . . . .  22
       4.1.1.  RPL Leaf Support in 6LoWPAN ND  . . . . . . . . . . .  22
       4.1.2.  RPL Root And 6LBR . . . . . . . . . . . . . . . . . .  23
     4.2.  TSCH and 6top . . . . . . . . . . . . . . . . . . . . . .  24
       4.2.1.  6top  . . . . . . . . . . . . . . . . . . . . . . . .  24
       4.2.2.  Scheduling Functions and the 6P protocol  . . . . . .  24
       4.2.3.  6top and RPL Objective Function operations  . . . . .  25
       4.2.4.  Network Synchronization . . . . . . . . . . . . . . .  26
       4.2.5.  SlotFrames and Priorities . . . . . . . . . . . . . .  27
       4.2.6.  Distributing the reservation of cells . . . . . . . .  28
     4.3.  Communication Paradigms and Interaction Models  . . . . .  30
     4.4.  Schedule Management Mechanisms  . . . . . . . . . . . . .  31
       4.4.1.  Static Scheduling . . . . . . . . . . . . . . . . . .  31
       4.4.2.  Neighbor-to-neighbor Scheduling . . . . . . . . . . .  32
       4.4.3.  Remote Monitoring and Schedule Management . . . . . .  32
       4.4.4.  Hop-by-hop Scheduling . . . . . . . . . . . . . . . .  35
     4.5.  On Tracks . . . . . . . . . . . . . . . . . . . . . . . .  35
       4.5.1.  General Behavior of Tracks  . . . . . . . . . . . . .  35
       4.5.2.  Serial Track  . . . . . . . . . . . . . . . . . . . .  36
       4.5.3.  Complex Track with Replication and Elimination  . . .  37
       4.5.4.  DetNet End-to-end Path  . . . . . . . . . . . . . . .  37
       4.5.5.  Cell Reuse  . . . . . . . . . . . . . . . . . . . . .  38
     4.6.  Forwarding Models . . . . . . . . . . . . . . . . . . . .  39
       4.6.1.  Track Forwarding  . . . . . . . . . . . . . . . . . .  39
       4.6.2.  IPv6 Forwarding . . . . . . . . . . . . . . . . . . .  42
       4.6.3.  Fragment Forwarding . . . . . . . . . . . . . . . . .  42
     4.7.  Distributed vs. Centralized Routing . . . . . . . . . . .  44
       4.7.1.  Packet Marking and Handling . . . . . . . . . . . . .  44
       4.7.2.  Replication, Retries and Elimination  . . . . . . . .  45
       4.7.3.  Differentiated Services Per-Hop-Behavior  . . . . . .  46
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46

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   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  46
     6.1.  Join Process Highlights . . . . . . . . . . . . . . . . .  47
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  49
     7.1.  Contributors  . . . . . . . . . . . . . . . . . . . . . .  49
     7.2.  Special Thanks  . . . . . . . . . . . . . . . . . . . . .  50
     7.3.  And Do not Forget . . . . . . . . . . . . . . . . . . . .  50
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  51
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  52
     8.3.  Other Informative References  . . . . . . . . . . . . . .  58
   Appendix A.  Dependencies on Work In Progress . . . . . . . . . .  59
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  61

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
   distance ranging from Near Field to interplanetary, and in
   circumstances where wiring may be impractical, for instance on fast-
   moving or rotating devices.

   In the other hand, Deterministic Networks enable traffic that is
   highly sensitive to jitter, quite sensitive to latency, and with a
   high degree of operational criticality so that loss should be
   minimized at all times.  Applications that need such networks are
   presented in [I-D.ietf-detnet-use-cases].  They include Professional
   Media and Operation Technology (OT) Industrial Automation Control
   Systems (IACS).

   The Medium access Control (MAC) of IEEE Std 802.15.4 [IEEE802154] has
   evolved with the IEEE Std 802.15.4e timeslotted Channel Hopping
   (TSCH) [RFC7554] mode to provide deterministic properties on wireless
   networks.  TSCH was initially introduced with the IEEE Std 802.15.4e
   amendment [IEEE802154e] of the IEEE Std 802.15.4 standard and
   constituted a part of the standard from that day.  For all practical
   purpose, this document is expected to be insensitive to the revisions
   of the IEEE Std 802.15.4 standard, which is thus referenced undated.

   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.

   In order to enable the convergence of IT and OT in LLN environments,
   6TiSCH ports the IETF suite of protocols that are defined for such
   environments over the TSCH MAC. 6TiSCH also provides large scaling

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   capabilities, which, in a number of scenarios, require the addition
   of a high speed and reliable backbone and the use of IP version 6
   (IPv6).  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 attached and
   synchronized by Backbone Routers.

   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
   industrial control systems, building automation that leverage
   distributed routing to address multipath over a large number of hops,
   in-vehicle command and control that can be as demanding as industrial
   applications, commercial automation and asset Tracking with mobile
   scenarios, home automation and domotics which become more reliable
   and thus provide a better user experience, and resource management
   (energy, water, etc.).

2.  Terminology

2.1.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  6TiSCH Terminology

   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 802.15.4e):  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 highest layer of the IEEE
               Std 802.15.4 TSCH medium access control layer.  It
               implements and terminates 6P, and contains at least one

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   6P (6top Protocol):  Allows neighbor nodes to communicate to add/
               delete cells to one another in their TSCH schedule.

   6P Transaction:  Part of 6P, the action of two neighbors exchanging a
               6P request message and the corresponding 6P response

   ASN (Absolute Slot Number):  The total number of timeslots that have
               elapsed since the PAN coordinator has started the TSCH
               network.  Incremented by one at each timeslot.  It is
               wide enough to not roll over in practice.

   blacklist of frequencies:  A set of frequencies which should not be
               used for communication.

   broadcast cell:  A scheduled cell used for broadcast transmission.

   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.

   CCA (Clear Channel Assessment):  Mechanism defined in [IEEE802154],
               section  In a TSCH network, CCA can be used to
               detect other radio networks in vicinity.  Nodes listen
               the channel before sending, to detect other ongoing
               transmissions.  Because the network is synchronized, CCA
               cannot be used to detect colliding transmission within
               the same network.

   cell:       A single element in the TSCH schedule, identified by a
               slotOffset, a channelOffset, a slotframeHandle.  A cell
               can be scheduled or unscheduled.

   centralized cell reservation:  A reservation of a cell done by a
               centralized entity (e.g., a PCE) in the network.

   centralized track reservation:  A reservation of a track done by a
               centralized entity (e.g., a PCE) in the network.

   Channel Distribution/Usage (CDU) matrix:  : Matrix of cells (i,j)
               representing the spectrum (channel) distribution among

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               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.  It has to be noticed that such a
               matrix which includes all the cells grouped in chunks,
               belonging to different slotframes, is different from the
               TSCH schedule.

   channelOffset:  Identifies a row in the TSCH schedule.  The number of
               available channelOffset values is equal to the number of
               available frequencies.  The channelOffset translates into
               a frequency when the communication takes place, resulting
               in channel hopping.

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

   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 [I-D.ietf-detnet-architecture].  When
               applied to 6TiSCH, it refers to the reservation of tracks
               which guarantee an end-to-end latency and optimize the
               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 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.

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   hard cell:  A scheduled cell which the 6top sublayer cannot relocate.

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

   IE (Information Element):  Type-Length-Value containers placed at the
               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 [I-D.kivinen-802-15-ie].

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

   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

   link:       A communication facility or medium over which nodes can
               communicate at the link layer, the layer immediately
               below IP.  The IETF parlance for the term "Link" is
               adopted, as opposed to the IEEE Std 802.15.4 terminology.

   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

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

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   scheduled cell:  A cell which is assigned a neighbor MAC address
               (broadcast address is also possible), and one or more of
               the following flags: TX, RX, shared, timeskeeping.  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 cell.

   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 6P.

   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 slotframe.

   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

   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

   Track:      A determined sequence of cells along a multi-hop path.
               It is typically the result of a track reservation.  The
               node that initializes the process of establishing a track

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               is the owner of the track.  The latter assigns a unique
               identifier to the track, called TrackID.

   TrackID:    Unique identifier of a track.

   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.

2.3.  References

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

      "Neighbor Discovery Optimization for Low-power and Lossy Networks"

      "Registration Extensions for 6LoWPAN Neighbor Discovery"

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

      "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"

   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

   o  "Neighbor Discovery for IP version 6" [RFC4861],

   o  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

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

   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

   In addition, readers would benefit from reading:

   o  "Multi-Link Subnet Issues" [RFC4903],

   o  "Mobility Support in IPv6" [RFC6275],

   o  "RPL applicability in industrial networks"

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

   o  "Optimistic Duplicate Address Detection" [RFC4429],

   o  "Neighbor Discovery Proxies (ND Proxy)" [RFC4389],

   o  "FCFS SAVI: First-Come, First-Served Source Address Validation
      Improvement for Locally Assigned IPv6 Addresses" [RFC6620], and

   o  "Optimistic Duplicate Address Detection" [RFC4429]

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

2.4.  Subset of a 6LoWPAN Glossary

   This document often uses the following acronyms:

   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)

   6CIO: Capability Indication Option

   (E)ARO:  (Extended) Address Registration Option

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   (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

   NS:   Neighbor Solicitation

   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)

3.  High Level Architecture

3.1.  6TiSCH Stack

   The 6TiSCH architecture presents a reference stack that is
   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.  Now, 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.

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   The IETF proposes multiple techniques for implementing functions
   related to routing, transport or security.  In order to control the
   complexity of the possible deployments and device interactions, and
   to limit the size of the resulting object code, the architecture
   limits the possible variations of the stack and recommends a number
   of base elements for LLN applications.  In particular, UDP [RFC0768]
   [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 below:

      | CoAP/OSCORE     |  6LoWPAN ND  | RPL |
      |       UDP       |      ICMPv6        |
      |                 IPv6                 |
      |     6LoWPAN HC   /   6LoRH HC        | Scheduling Functions |
      |     6top (to be IEEE Std 802.15.12) inc. 6top protocol      |
      |                 IEEE Std 802.15.4 TSCH                      |

                      Figure 1: 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, 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.

   The Datagram Transport Layer Security (DTLS) [RFC6347] sitting either
   under CoAP or over CoAP so as to traverse proxies, as well as Object
   Security for Constrained RESTful Environments (OSCORE)
   [I-D.ietf-core-object-security], are examples of protocols that could

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   be used to protect application payload, and OSCORE is used in
   particular by the "Minimal Security Framework for 6TiSCH"
   [I-D.ietf-6tisch-minimal-security] for the the Join Process.

   An overview of the the initial steps of a device in a network can be
   found in Section 3.7; the security aspects of the join process are
   further detailed in Section 6.

   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].

3.2.  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 very 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
   [I-D.ietf-detnet-architecture], and IETF components such as the Path
   Computation Element (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.  RPL also

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   enables mobile use cases such as mobile workers and cranes, as
   presented in [I-D.ietf-roll-rpl-industrial-applicability].

3.3.  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 (see the
   Section 2.2 for more on these terms).  Scheduling effectively 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 transmit or receive on one channel at any given point of time.

   From the standpoint of a 6TiSCH node (at the MAC layer), its schedule
   is the collection of the times 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 a priority
   indicates which slotframe is actually activated.

   The 6top sublayer hides the complexity of the schedule from the upper
   layers.  The Link that IP may utilize between the 6TiSCH node and a
   peer may in fact be composed of a pair of 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.

   The 6TiSCH architecture identifies four ways a schedule can be
   managed and CDU cells can be allocated: Static Scheduling, Neighbor-
   to-Neighbor Scheduling, 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 fashion.  The static schedule is
      distributed through the native methods in the TSCH MAC layer.
      This operation leverages RPL to maintain a loopless graph for
      routing and time distribution.  It is specified in the Minimal
      6TiSCH Configuration [RFC8180] specification.  and does not
      preclude other scheduling operations to co-exist on a same 6TiSCH

   Neighbor-to-Neighbor Scheduling:  This refers to the dynamic
      adaptation of the bandwidth of the Links that are used for IPv6
      traffic between adjacent routers.  Scheduling Functions such as
      the "6TiSCH Minimal Scheduling Function (MSF)"
      [I-D.ietf-6tisch-msf] influence the operation of the MAC layer to

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      add, update and remove cells in peers schedule, using 6P [RFC8480]
      for the negotiation of the MAC resources.

   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 in order to minimize the interaction
      with and the load on the constrained device.  This model is the
      TSCH adaption of the "DetNet Architecture"
      [I-D.ietf-detnet-architecture], 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.

3.4.  Routing and Forwarding Over TSCH

   6TiSCH leverages the RPL routing protocol for interoperable
   distributed routing operations.  RPL 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 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 Scheduling.

   The 6TiSCH architecture supports three different forwarding models,
   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, G-MPLS Track Forwarding, which switches a frame received at a
   particular timeslot into another timeslot at Layer-2, and 6LoWPAN
   Fragment Forwarding, which allows to forward individual 6loWPAN
   fragments along the route set by the first fragment.

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   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.  On top of 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 shapes that are more complex than a simple direct
      sequence of nodes.

   6LoWPAN Fragment Forwarding:  This is an hybrid model that derives
      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.

   This can be broadly summarized in the following table:

|   Forwarding Model  |  Routing   |          Scheduling               |
|                     |            |   Static (Minimal Configuration)  |
+  classical IPv6     +     RPL    +-----------------------------------+
|         /           |            |   Neighbor-to-Neighbor (SF+6P)    |
+ 6LoWPAN Fragment F. +------------+-----------------------------------+
|                     |Reactive P2P|        Hop-by-Hop (TBD)           |
|G-MPLS Track Fwrding |     PCE    |Remote Monitoring and Schedule Mgt |

               Figure 2: Routing, Forwarding and Scheduling

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

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

   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 are not necessarily reachable from one another at
   Layer-2 and an LLN may span over multiple links.  This effectively
   forms an homogeneous non-broadcast multi-access (NBMA) subnet, which
   is beyond the scope of existing IPv6 ND methods.  Extensions to IPv6
   ND have to be introduced.

   Within that subnet, neighbor devices are discovered with 6LoWPAN
   Neighbor Discovery [RFC6775] (6LoWPAN ND), whereas RPL [RFC6550]
   enables routing in the so called Route Over fashion, either in
   storing (stateful) or non-storing (stateless, with routing headers)

               ---+-------- ............ ------------
                  |      External Network       |
                  |                          +-----+
               +-----+                       | NME |
               |     | LLN Border            |     |
               |     | router                +-----+
             o    o   o
         o     o   o     o    o
        o   o 6LoWPAN + RPL o    o
            o   o   o       o
                    o  o

             Figure 3: Basic Configuration of a 6TiSCH Network

   6TiSCH nodes join the mesh by attaching to nodes that are already
   members of the mesh.  Some nodes act as routers for 6LoWPAN ND and
   RPL operations, as detailed in Section 4.1.  Security aspects of the
   join process by which a device obtains access to the network are
   discussed in Section 6.

   With TSCH, devices are time-synchronized at the MAC level.  The use
   of a particular RPL Instance for time synchronization is discussed in

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   Section 4.2.4.  With this mechanism, the time synchronization starts
   at the RPL root and follows the RPL DODAGs with no timing loop.

   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 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

   A Backbone Router 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.  A Backbone Router may perform proxy IPv6 Neighbor Discovery
   (ND) [RFC4861] operations over the backbone on behalf of the 6TiSCH
   nodes so they can share a same IPv6 subnet and appear to be connected
   to the same backbone as classical devices.  A Backbone Router may
   alternatively redistribute the registration in a routing protocol
   such as OSPF [RFC5340] or BGP [RFC2545], or inject them in a mobility
   protocol such as MIPv6 [RFC6275], NEMO [RFC3963], or LISP [RFC6830].

   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 a host but the specifications for this to happen are not
   available at the time of this writing.

3.6.  A Multi-Link Subnet Model

   An extended configuration of the subnet comprises multiple LLNs.  The
   LLNs are interconnected and synchronized over a backbone, that can be
   wired or wireless.  The backbone can be a classical IPv6 network,
   with Neighbor Discovery operating as defined in [RFC4861] and

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   [RFC4862].  This architecture requires work to standardize the the
   registration of 6LoWPAN nodes to the Backbone Routers.

   In the extended configuration, a Backbone Router (6BBR) operates as
   described in [I-D.ietf-6lo-backbone-router].  The 6BBR performs ND
   proxy operations between the registered devices and the classical ND
   devices that are located over the backbone.  6TiSCH 6BBRs synchronize
   with one another over the backbone, so as to ensure that the multiple
   LLNs that form the IPv6 subnet stay tightly synchronized.

                  ---+-------- ............ ------------
                     |      External Network       |
                     |                          +-----+
                     |             +-----+      | NME |
                  +-----+          |  +-----+   |     |
                  |     | Router   |  | PCE |   +-----+
                  |     |          +--|     |
                  +-----+             +-----+
                     |                   |
                     | Subnet Backbone   |
               |                    |                  |
            +-----+             +-----+             +-----+
            |     | Backbone    |     | Backbone    |     | Backbone
       o    |     | router      |     | router      |     | router
            +-----+             +-----+             +-----+
       o                  o                   o                 o   o
           o    o   o         o   o  o   o         o  o   o    o
      o             o        o  LLN      o      o         o      o
         o   o    o      o      o o     o  o   o    o    o     o

           Figure 4: Extended Configuration of a 6TiSCH Network

   As detailed in Section 4.1 the 6LoWPAN ND 6LBR and the root of the
   RPL network need to be collocated and share information about the
   devices that is learned through either protocol but not both.  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
   [I-D.ietf-6lo-backbone-router] 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.

   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.  The DetNet Architecture
   [I-D.ietf-detnet-architecture] studies Layer-3 aspects of

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   Deterministic Networks, and covers networks that span multiple
   Layer-2 domains.

3.7.  Join Process and Registration

   As detailed in Section 6, a node that wishes to join the 6TiSCH
   network with a preshared key (PSK) performs the role of the pledge in
   the 6TiSCH Constrained Join Protocol (CoJP)
   [I-D.ietf-6tisch-minimal-security] In order to join, the pledge is
   helped by a Join Proxy (JP) that relays the link-scope 6JP Join
   request over the IP network to the Join Registrar/Coordinator (JRC)
   that can authenticate the pledge and validate that it is attached to
   the appropriate network.  As a result of this exchange the pledge is
   in possession of a Link-Layer material including a key and a short
   address, and all traffic is secured at the Link Layer .

   Figure 5 illustrates that very initial step.

    6LoWPAN Node       6LR              6LBR         Join Registrar
     (pledge)       (Join Proxy)       (root)      /Coordinator (JRC)
         |               |               |               |
         |  6LoWPAN ND   |6LoWPAN ND+RPL |  IPv6 network |
         |   LLN link    |Route-Over mesh| (the Internet)|
         |               |               |               |
         |   Layer-2     |               |               |
         |enhanced beacon|               |               |
         |<--------------|               |               |
       <-----------------|               |               |
         |  <------------|               |               |
         |               |               |               |
         | 6JP Join Req  |               |               |
         | Link Local @  |               |               |
         |-------------->|               |               |
         |               |       6JP Join Request        |
         |               |       Global Unicast @        |
         |               |------------------------------>|
         |               |               |               |
         |               |       6JP Join Response       |
         |               |       Global Unicast @        |
         |               |<------------------------------|
         | 6JP Join Resp |               |               |
         | Link Local @  |               |               |
         |<--------------|               |               |
         |               |               |               |

          Figure 5: (Re-)Registration Flow over Multi-Link Subnet

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   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 eables a 6LN to
   form a global address and register it to a 6LBR using 6LoWPAN ND
   [I-D.ietf-6lo-rfc6775-update], is then carried over RPL to the RPL
   root, and then to the 6BBR.

    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
         |               |               |               |
         |  IPv6 ND RS   |               |               |
         |-------------->|               |               |
         |----------->   |               |               |
         |------------------>            |               |
         |  IPv6 ND RA   |               |               |
         |<--------------|               |               |
         |               |    <once>     |               |
         |  NS(EARO)     |               |               |
         |-------------->|               |               |
         | 6LoWPAN ND    | Extended DAR  |               |
         |               |-------------->|               |
         |               |               |  NS(EARO)     |
         |               |               |-------------->|
         |               |               |               | DAD (once)
         |               |               |               |------>
         |               |               |               |
         |               |               |  NA(EARO)     |
         |               |               |<--------------|
         |               | Extended DAC  |               |
         |               |<--------------|               |
         |  NA(EARO)     |               |               |
         |<--------------|               |               |
         |               |               |               |

        Figure 6: Initial Registration Flow over Multi-Link Subnet

   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 [I-D.ietf-6lo-rfc6775-update], using 6LoWPAN ND ot the
   6LR, RPL to the RPL root, and then 6LoWPAN ND again to the 6BBR.

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    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
         |               |               |               |
         |               |  <periodic>   |               |
         |               |               |               |
         |  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.  Architecture Components

4.1.  6LoWPAN (and RPL)

4.1.1.  RPL Leaf Support in 6LoWPAN ND

   RPL needs a set of information in order to advertise a leaf node
   through a DAO message and establish reachability.

   At the bare minimum the leaf device must provide a sequence number
   that matches the RPL specification in section 7.  Section 5.3 of
   [I-D.ietf-6lo-backbone-router], on the Extended Address Registration
   Option (EARO), already incorporates that addition with a new field in
   the option called the Transaction ID.

   If for some reason the node is aware of RPL topologies, then
   providing the RPL InstanceID for the instances to which the node

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   wishes to participate would be a welcome addition.  In the absence of
   such information, the RPL router must infer the proper instanceID
   from external rules and policies.

   On the backbone, the InstanceID is expected to be mapped onto a an
   overlay that matches the instanceID, for instance a VLANID.

   This architecture leverages [I-D.ietf-6lo-backbone-router] that
   extends 6LoWPAN ND [RFC6775] to carry the counter as an abstract
   Transaction ID (TID).

4.1.2.  RPL Root And 6LBR

   With [RFC6775], information on the 6LBR is disseminated via an
   Authoritative Border Router Option (ABRO) in RA messages.  The
   discovery and liveliness of the RPL root are obtained through the RPL
   protocol [RFC6550].  The capability to support the update to RFC6775
   [I-D.ietf-6lo-rfc6775-update] is indicated in the 6LoWPAN Capability
   Indication Option (6CIO).

   "Routing for RPL Leaves" [I-D.thubert-roll-unaware-leaves] details
   the basic interaction of 6LoWPAN ND and RPL and enables a plain 6LN
   that supports [I-D.ietf-6lo-rfc6775-update] to obtain return
   connectivity via the RPL network as a non-RPL-aware leaf.  Though the
   above specification enables a model where the separation is possible,
   this architecture recommends to collocate the functions of 6LBR and
   RPL root.

   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 DAR/DAC exchange with the state that is maintained by RPL.  The
   DAR/DAC exchange becomes a preamble to the DAO messages that are used
   from then on to reconfirm the registration, thus eliminating a
   duplication of functionality between DAO and DAR messages.

   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

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   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 requires an
   alteration from [RFC6775] whereby the Target Address of the NS
   message is registered as opposed to the Source, which, in the case of
   a proxy registration, is that of the 6LBR / RPL root itself.

4.2.  TSCH and 6top

4.2.1.  6top

   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 operations are specified in [RFC8480].
   In particular, 6top provides a management interface that enables an
   external management entity to schedule cells and slotFrames, and
   allows the addition of complementary functionality, for instance to
   support a dynamic schedule management based on observed resource
   usage as discussed in Section 4.4.2.

   The 6top data model and management interfaces are further discussed
   in Section 4.4.3.  Hard Cells

   The architecture defines "soft" cells and "hard" cells.  "Hard" cells
   are owned and managed by an 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

   6top contains a monitoring process which monitors the performance of
   cells, and can move a cell in the TSCH schedule when it performs
   poorly.  This is only applicable to cells which are marked as "soft".
   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.

4.2.2.  Scheduling Functions and the 6P 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

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   the network.  The "6TiSCH Minimal Scheduling Function (MSF)"
   [I-D.ietf-6tisch-msf] provides a simple scheduling function that can
   be used by default by devices that support dynamic scheduling of soft

   The 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.

    +------------------------+          +------------------------+
    |  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.  The 6P
   protocol enables the peers to find an agreement in a transactional
   manner that ensures the final consistency of the nodes state.

4.2.3.  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.

   Most OFs require metrics about reachability, such as the ETX.  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 neighbor including the time when the last packet has
   been received from that neighbor, a set of cell quality metrics (e.g.

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   RSSI or LQI), the number of packets sent to the neighbor or the
   number of packets received from it.  This information can be obtained
   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 behaviour, 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 Enhance Beacons.
   Consideration was given towards finding a way to embed the Route
   Advertisements and the RPL DIO messages (both of which are multicast)
   into the IEEE Std 802.15.4 Enhanced Beacons.  It was determined that
   this produced undue timer coupling among layers, that the resulting
   packet size was potentially too large, and required it is not yet
   clear that there is any need for Enhanced Beacons in a production

4.2.4.  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.  In order 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

   Time distribution requires a loop-free structure.  Nodes taken in a
   synchronization loop will rapidly desynchronize from the network and
   become isolated.  It is expected that a RPL DAG with a dedicated
   global Instance is deployed 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

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   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

   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".

   A root is configured or obtains by some external 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).  We'll note that the Join
   Priority is now specified between 0 and 0x3F leaving 2 bits in the
   octet unused in the IEEE Std 802.15.4e specification.  After
   consultation with IEEE authors, it was asserted that 6TiSCH can make
   a full use of the octet to carry an integer value up to 0xFF.

   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.

4.2.5.  SlotFrames and Priorities

   6TiSCH enables in essence the capability to use IPv6 over a MAC layer
   that enables to schedule the transmissions.  In order to ensure that
   the medium is free of contending packets when time arrives for a
   scheduled transmission, a window of time is defined around the
   scheduled transmission time where the medium must be free of
   contending energy.

   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.

   In order to describe that formatting of time and frequencies, the
   6TiSCH architecture defines a global concept that is called a Channel

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   Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of
   cells with an 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.  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 slotted-aloha operations.
   The knowledge of the CDU matrices is shared between all the nodes and
   used in particular to define slotFrames.

   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, based on
   the precedence of the 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.

4.2.6.  Distributing the reservation of cells

   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

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   large and spatially distributed network, a 6TiSCH node is often in a
   good position to determine usage of spectrum in its vicinity.

   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.  With 6TiSCH, the abstraction
   of an IPv6 link is implemented as a pair of bundles of cells, one in
   each direction; the size of a bundle is optimal when both the energy
   wasted idle listening and the packet drops due to congestion loss are
   minimized.  This can be maintained if the number of cells in a bundle
   is adapted dynamically, and with enough reactivity, to match the
   variations of best-effort traffic.  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

   6TiSCH limits that interaction to RPL parents that will only
   negotiate with other RPL parents, and performs that negotiation by
   groups of cells as opposed to individual cells.  The 6TiSCH
   architecture allows RPL parents to adjust dynamically, and
   independently from the PCE, the amount of bandwidth that is used to
   communicate between themselves and their children, in both
   directions; to that effect, an allocation mechanism enables a RPL
   parent to obtain the exclusive use of a portion of a CDU matrix
   within its interference domain.  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.

   The 6TiSCH architecture introduces the concept of chunks Section 2.2)
   to operate such spectrum distribution for a whole group of cells at a
   time.  The CDU matrix is formatted into a set of chunks, each of them
   identified uniquely by a chunk-ID.  The knowledge of this formatting
   is shared between all the nodes in a 6TiSCH network. 6TiSCH also
   defines the process of chunk ownership appropriation whereby a RPL
   parent discovers a chunk that is not used in its interference domain
   (e.g lack of energy detected in reference cells in that chunk); then
   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.

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                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   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

                Figure 9: CDU matrix Partitioning in Chunks

   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
   chunk.  The RPL parent may thus orchestrate which transmissions occur
   in any of the cells in the chunk, by allocating cells from the chunk
   to any form of communication (unicast, multicast) in any direction
   between itself and its children.  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

   The appropriation of a chunk can also be requested explicitly by the
   PCE to any node.  In that case, the node still may need to perform
   the appropriation process to validate that no other node has claimed
   that chunk already.  After a successful appropriation, the PCE owns
   the cells in that chunk, and may use them as hard cells to set up

4.3.  Communication Paradigms and Interaction Models

   Section 2.2 provides the terms of Communication Paradigms and
   Interaction Models, which can be placed in parallel to the
   Information Models and Data Models that are defined in [RFC3444].

   A Communication Paradigms 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
   Models would be more refined and could point on 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.

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   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 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.

   This specification 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 (multithop 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.

4.4.  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.

   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].

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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.

   The 6top Protocol [RFC8480] specifies the exchanges between neighbor
   nodes to reserve soft cells to transmit to one another.  Because this
   reservation is done without global knowledge of the schedule of other
   nodes in the LLN, scheduling collisions are possible.  An optional
   Scheduling Function (SF) such as MSF [I-D.ietf-6tisch-msf] 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.

   Monitoring and relocation is done in the 6top layer.  For the upper
   layer, the connection between two neighbor nodes appears as an 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

   The work at the 6TiSCH WG is focused on non-deterministic traffic and
   does not provide the generic data model that would be necessary to
   monitor and manage resources of the 6top sublayer.  It is recognized
   that CoAP can be appropriate to interact with the 6top layer of a
   node that is multiple hops away across a 6TiSCH mesh.

   The entity issuing the CoAP requests can be a central scheduling
   entity (e.g. a PCE), a node multiple hops away with the authority to
   modify the TSCH schedule (e.g. the head of a local cluster), or a
   external device monitoring the overall state of the network (e.g.
   NME).  It is also possible that a mapping entity on the backbone
   transforms a non-CoAP protocol such as PCEP into the RESTful
   interfaces that the 6TiSCH devices support.

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   With respect to Centralized routing and scheduling, it is envisionned
   that the related component of the 6TiSCH Architecture would be an
   extension of the Deterministic Networking Architecture
   [I-D.ietf-detnet-architecture], which studies Layer-3 aspects of
   Deterministic Networks, and covers networks that span multiple
   Layer-2 domains.  The DetNet architecture is a form of SDN
   Architecture and is composed of three planes, a (User) Application
   Plane, a Controller Plane (where the PCE operates), and a Network
   Plane which in our case is the 6TiSCH LLN.  The generic SDN
   architecture is discussed in Software-Defined Networking (SDN):
   Layers and Architecture Terminology [RFC7426] and is represented

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           SDN Layers and Architecture Terminology per RFC 7426

                     |                                |
                     | +-------------+   +----------+ |
                     | | 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

   The PCE establishes end-to-end Tracks of hard cells, which are
   described in more details in Section 4.6.1.  The DetNet work is
   expected to enable end to end Deterministic Path across heterogeneous

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   network (e.g. 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 a destination node
   multiple hops away by installing soft cells at each intermediate
   node.  This forms a Track of soft cells.  It is the responsibility of
   the 6top sublayer of each node on the Track 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

4.5.1.  General Behavior of Tracks

   The architecture introduces the concept of a Track, which is a
   directed path from a source 6TiSCH node to a destination 6TiSCH node
   across a 6TiSCH LLN.  A Track is the 6TiSCH instantiation of the
   concept of a Deterministic Path as described in
   [I-D.ietf-detnet-architecture].  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.

   A Track is composed of 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 destination.
   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

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   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
       belongs 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.  For a given iteration of the device
   schedule, the effective channel of the cell is obtained by adding a
   pseudo-random number to 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 at a given
   iteration of the schedule.

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4.5.3.  Complex Track with Replication and Elimination

   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 a
   destination to support multi-path forwarding and route around

   A Complex Track may also branch off and rejoin, for the purpose of
   the DetNet Packet Replication and Elimination (PRE), over non
   congruent branches.  PRE may be used to complement Layer-2 ARQ to
   meet industrial expectations in Packet Delivery Ratio (PDR), in
   particular when the Track extends beyond the 6TiSCH network in a
   larger DetNet network.

   The art of Deterministic Networks already include PRE techniques.
   Example standards include the Parallel Redundancy Protocol (PRP) and
   the High-availability Seamless Redundancy (HSR) [IEC62439].

   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.

   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.

4.5.4.  DetNet End-to-end Path

   Ultimately, DetNet should enable to extend a Track beyond the 6TiSCH
   LLN.  Figure 11 illustrates 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.

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                     | 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: End-to-End deterministic Track

   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.

4.5.5.  Cell Reuse

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

      In one hand, 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 that a frame that is received in
      a RX-cell of a Track with a destination MAC address set to this
      node as opposed to broadcast must be extracted from the Track and
      delivered to the upper layer (a frame with an unrecognized

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      destination MAC address is dropped at the lower MAC layer and thus
      is not received at the 6top sublayer).

      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, the frame can be placed for transmission in the bundle
      that is used for Layer-3 traffic towards the next hop along the
      Track as long as it can be routed by the upper layer, that is,
      typically, if the frame transports an IPv6 packet.  The MAC
      address should be set to the next-hop MAC address to avoid
      confusion.  It results that 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 should be re-Tracked if the Per-Hop-Behavior
      group indicated in the Differentiated Services Field of the IPv6
      header is set to Deterministic Forwarding, as discussed in
      Section 4.7.1.  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 specification 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 (TF),
   6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).

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.

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   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

   There are 2 modes for a Track, transport mode and tunnel mode.  Transport Mode

   In transport mode, the Protocol Data Unit (PDU) is associated with
   flow-dependant 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 is
   transported in the Flow Label of the IPv6 header.  Associated with
   the source IPv6 address, the Flow Label forms a globally unique
   identifier for that particular Track that is validated at egress
   before restoring the destination MAC address (DMAC) and punting to
   the upper layer.

                          |                                    ^
      +--------------+    |                                    |
      |     IPv6     |    |                                    |
      +--------------+    |                                    |
      |  6LoWPAN HC  |    |                                    |
      +--------------+  ingress                              egress
      |     6top     |   sets     +----+          +----+     restores
      +--------------+  dmac to   |    |          |    |     dmac to
      |   TSCH MAC   |   brdcst   |    |          |    |      self
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+

                     Track Forwarding, Transport Mode  Tunnel Mode

   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.

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   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 to be transported transparently over the

      |     IPv6     |
      |  6LoWPAN HC  |
      +--------------+             set            restore
      |     6top     |            +dmac+          +dmac+
      +--------------+          to|brdcst       to|nexthop
      |   TSCH MAC   |            |    |          |    |
      +--------------+            |    |          |    |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
      +--------------+    |   ingress                 egress   |
                          |                                    |
      +--------------+    |                                    |
      |   LLN PHY    |    |                                    |
      +--------------+    |                                    |
      |   TSCH MAC   |    |                                    |
      +--------------+    | dmac =                             | dmac =
      |ISA100/WiHART |    | nexthop                            v nexthop

                 Figure 12: Track Forwarding, Tunnel Mode

   In that case, the flow information that identifies the Track at the
   ingress 6TiSCH router is derived from the RX-cell.  The dmac is set
   to this node but the flow information 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
   metadata associated to the Track, the frame is passed to the
   appropriate Link Layer with the destination MAC restored.  Tunnel Metadata

   Metadata coming with the Track configuration is expected to provide
   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.

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   In transport mode, if the final Layer-3 destination is 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.  It is thus mandatory at the ingress point to validate that
   the MAC address that was used at the 6LoWPAN sublayer for compression
   matches that of the tunnel egress point.  For that reason, the node
   that injects a packet on a Track checks that the destination is
   effectively 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 metadata.

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;
   the application of Differentiated Services is further discussed in

                          |                                    ^
      +--------------+    |                                    |
      |     IPv6     |    |       +-QoS+          +-QoS+       |
      +--------------+    |       |    |          |    |       |
      |  6LoWPAN HC  |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+

                         Figure 13: IP Forwarding

4.6.3.  Fragment Forwarding

   Considering that 6LoWPAN packets can be as large as 1280 bytes (the
   IPv6 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 in order to reform the packet and

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   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.

   [I-D.ietf-6lo-minimal-fragment] 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,
   [I-D.ietf-6lo-fragment-recovery] 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.

                          |                                    ^
      +--------------+    |                                    |
      |     IPv6     |    |       +----+          +----+       |
      +--------------+    |       |    |          |    |       |
      |  6LoWPAN HC  |    |       learn           learn        |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+

                   Figure 14: Forwarding First Fragment

   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.

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                          |                                    ^
      +--------------+    |                                    |
      |     IPv6     |    |                                    |
      +--------------+    |                                    |
      |  6LoWPAN HC  |    |       replay          replay       |
      +--------------+    |       |    |          |    |       |
      |     6top     |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   TSCH MAC   |    |       |    |          |    |       |
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+

                    Figure 15: 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.  Distributed vs. Centralized Routing

   6TiSCH enables a mixed model of centralized routes and distributed
   routes.  Centralized routes can for example be computed by a entity
   such as a PCE.  Distributed routes are computed by RPL.

   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 Instance ID, in a format that reuses the
   RPLInstanceID as defined in RPL [RFC6550].

   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.

4.7.1.  Packet Marking and Handling

   All packets inside a 6TiSCH domain must carry the Instance ID that
   identifies the 6TiSCH topology 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.

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   For packets that are routed by a PCE along a Track, the tuple formed
   by the IPv6 source address and a local RPLInstanceID in the packet
   identify uniquely the Track and associated transmit bundle.

   For packets that are routed by RPL, that information is the
   RPLInstanceID which is carried in the RPL Packet Information, as
   discussed in section 11.2 of [RFC6550], "Loop Avoidance and

   The RPL Packet Information (RPI) is carried in IPv6 packets as 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 expects elimination and replication of packets along a complex
   Track, but has no position about how the sequence numbers would be
   tagged in the packet.

   As it goes, 6TiSCH expects that timeslots corresponding to copies of
   a same packet along a Track are correlated by configuration, and does
   not need to process the sequence numbers.

   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.

   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 further
   transmissions should not be attempted for 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

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   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.

4.7.3.  Differentiated Services Per-Hop-Behavior

   Additionally, an IP packet that is sent along a Track uses the
   Differentiated Services Per-Hop-Behavior Group called Deterministic
   Forwarding, as described in

5.  IANA Considerations

   This specification does not require IANA action.

6.  Security Considerations

   This architecture operates on IEEE Std 802.15.4 and expects 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.

   The Minimal Security Framework for 6TiSCH
   [I-D.ietf-6tisch-minimal-security] describes the minimal mechanisms
   required to support secure enrollment of a pledge to a 6TiSCH network
   based on PSK.  The specification enables to establish of Link-Layer
   keys, typically used in combination with a variation of Counter with
   CBC-MAC (CCM) [RFC3610], and set up a secure end-to-end session
   between the joining node (called the pledge) and the join registrar/
   coordinator (JRC) in charge of authenticating the node via a Join
   Proxy (JP).  It can also be used to obtain a Link Layer short address
   as a side effect.  CoJP uses shared slots which are a constrained
   resource, so it is optimized to limit the number of messages to the
   strict minimum.  As an example, Neighbor Discovery between the pledge
   and the JP can be skipped when the IPv6 Link Local addresses that are
   used derive from the node's EUI-64 address.

   The "6tisch Zero-Touch Secure Join protocol"
   [I-D.ietf-6tisch-dtsecurity-zerotouch-join] wraps the minimal
   security draft with a flow inspired from ANIMA "Bootstrapping Remote

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   Secure Key Infrastructures (BRSKI)"

6.1.  Join Process Highlights

   The BRSKI architecture specifies three logical elements to describe
   the join process:

   Pledge:  Node that wishes to become part of the network;

   Join Registrar/Coordinator (JRC)  : An entity that arbitrates network
         access and hands out network parameters (such as keying

   Join Proxy (JP),  a one-hop (radio) neighbor of the joining node that
         acts as proxy network node and may provide connectivity with
         the JRC.

   The join protocol consists of three major activities:

   Device Authentication:  The Pledge and the JP mutually authenticate
         each other and establish a shared key, so as to ensure on-going
         authenticated communications.  This may involve a server as a
         third party.

   Authorization:  The JP decides on whether/how to authorize a Pledge
         (if denied, this may result in loss of bandwidth).  Conversely,
         the Pledge decides on whether/how to authorize the network (if
         denied, it will not join the network).  Authorization decisions
         may involve other nodes in the network.

   Configuration/Parameterization:  The JP distributes configuration
         information to the Pledge, such as scheduling information, IP
         address assignment information, and network policies.  This may
         originate from other network devices, for which the JP may act
         as proxy.  This step may also include distribution of
         information from the Pledge to the JP and other nodes in the
         network and, more generally, synchronization of information
         between these entities.

   The device joining process is depicted in Figure 16, where it is
   assumed that devices have access to certificates and where entities
   have access to the root CA keys of their communicating parties
   (initial set-up requirement).  Under these assumptions, the
   authentication step of the device joining process does not require
   online involvement of a third party.  Mutual authentication is
   performed between the Pledge and the JP using their certificates,
   which also results in a shared key between these two entities.

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   The JP assists the Pledge in mutual authentication with a remote
   server node (primarily via provision of a communication path with the
   server), which also results in a shared (end-to-end) key between
   those two entities.  The server node may be a JRC that arbitrages the
   network authorization of the Pledge (where the JP will deny bandwidth
   if authorization is not successful); it may distribute network-
   specific configuration parameters (including network-wide keys) to
   the Pledge.  In its turn, the Pledge may distribute and synchronize
   information (including, e.g., network statistics) to the server node
   and, if so desired, also to the JP.  The actual decision of the
   Pledge to become part of the network may depend on authorization of
   the network itself.

   The server functionality is a role which may be implemented with one
   (centralized) or multiple devices (distributed).  In either case,
   mutual authentication is established with each physical server entity
   with which a role is implemented.

   Note that in the above description, the JP does not solely act as a
   relay node, thereby allowing it to first filter traffic to be relayed
   based on cryptographic authentication criteria - this provides first-
   level access control and mitigates certain types of denial-of-service
   attacks on the network at large.

   Depending on more detailed insight in cost/benefit trade-offs, this
   process might be complemented by a more "relaxed" mechanism, where
   the JP acts as a relay node only.  The final architecture will
   provide mechanisms to also cover cases where the initial set-up
   requirements are not met or where some other out-of-sync behavior
   occurs; it will also suggest some optimizations in case JRC-related
   information is already available with the JP (via caching of

   When a device rejoins the network in the same authorization domain,
   the authorization step could be omitted if the server distributes the
   authorization state for the device to the JP when the device
   initially joined the network.  However, this generally still requires
   the exchange of updated configuration information, e.g., related to
   time schedules and bandwidth allocation.

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   {joining node}     {neighbor}               {server, etc.}   Example:
   +---------+        +---------+                 +---------+
   | Joining |        |  Join   |              +--|    CA   |certificate
   |  Node   |        |Assistant|              |  +---------+   issuance
   +---------+        +---------+              |  +---------+
      |                    |                   +--|Authoriz.| membership
      |<----Beaconing------|                   |  +---------+ test (JRC)
      |                    |                   |  +---------+
      |<--Authentication-->|                   +--| Routing | IP address
      |                    |<--Authorization-->|  +---------  assignment
      |<-------------------|                   |  +---------+
      |                    |                   +--| Gateway | backbone,
      |------------------->|                   |  +---------+    cloud
      |                    |<--Configuration-->|  +---------+
      |<-------------------|                   +--|Bandwidth|  PCE
                                                  +---------+  schedule
       .                    .                   .
       .                    .                   .

    Figure 16: Network joining, with only authorization by third party

7.  Acknowledgments

7.1.  Contributors

   The co-authors of this document are listed below:

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

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

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

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

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

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

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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;

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

   Thomas Watteyne  for his contribution to the whole design, in
         particular on TSCH and security.

7.2.  Special Thanks

   Special thanks to Tero Kivinen, Jonathan Simon, Giuseppe Piro, Subir
   Das and Yoshihiro Ohba for their deep contribution to the initial
   security work, 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 for his support in maintaining the
   connection active and the design in line with work happening at IEEE
   Std 802.15.4.

   Special thanks to Ted Lemon who was the INT Area A-D while this
   specification was developed for his great support and help

   Also special thanks to Ralph Droms who performed the first INT Area
   Directorate review, that was very deep and through and radically
   changed the orientations of this document.

7.3.  And Do not Forget

   This specification is the result of multiple interactions, in
   particular during the 6TiSCH (bi)Weekly Interim call, relayed through
   the 6TiSCH mailing list at the IETF.

   The authors wish to thank: Alaeddine Weslati, Chonggang Wang,
   Georgios Exarchakos, Zhuo Chen, Alfredo Grieco, Bert Greevenbosch,
   Cedric Adjih, Deji Chen, Martin Turon, Dominique Barthel, Elvis
   Vogli, Geraldine Texier, Malisa Vucinic, Guillaume Gaillard, Herman
   Storey, Kazushi Muraoka, Ken Bannister, Kuor Hsin Chang, Laurent
   Toutain, Maik Seewald, Maria Rita Palattella, 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,
   Tengfei Chang, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles
   and Samita Chakrabarti for their participation and various

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8.  References

8.1.  Normative References

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

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

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

   [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,

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   [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,

   [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,

   [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, <https://www.rfc-editor.org/info/rfc8138>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

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

8.2.  Informative References

              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", draft-ietf-6lo-ap-nd-08 (work in
              progress), October 2018.

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              Thubert, P. and C. Perkins, "IPv6 Backbone Router", draft-
              ietf-6lo-backbone-router-08 (work in progress), October

              Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
              ietf-6lo-fragment-recovery-00 (work in progress),
              September 2018.

              Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
              Fragment Forwarding", draft-ietf-6lo-minimal-fragment-00
              (work in progress), October 2018.

              Thubert, P., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for 6LoWPAN Neighbor
              Discovery", draft-ietf-6lo-rfc6775-update-21 (work in
              progress), June 2018.

              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in
              progress), October 2018.

              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-08 (work in progress), November

              Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
              D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
              draft-ietf-6tisch-msf-01 (work in progress), October 2018.

              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-17 (work in progress), November 2018.

              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-15 (work in
              progress), August 2018.

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              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-09 (work in progress), October 2018.

              Grossman, E., "Deterministic Networking Use Cases", draft-
              ietf-detnet-use-cases-19 (work in progress), October 2018.

              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00
              (work in progress), July 2018.

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

              Anamalamudi, S., Zhang, M., Perkins, C., Anand, S., and B.
              Liu, "Asymmetric AODV-P2P-RPL in Low-Power and Lossy
              Networks (LLNs)", draft-ietf-roll-aodv-rpl-05 (work in
              progress), October 2018.

              Phinney, T., Thubert, P., and R. Assimiti, "RPL
              applicability in industrial networks", draft-ietf-roll-
              rpl-industrial-applicability-02 (work in progress),
              October 2013.

              Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for IETF", draft-kivinen-802-15-ie-06 (work in
              progress), March 2017.

              Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
              draft-svshah-tsvwg-deterministic-forwarding-04 (work in
              progress), August 2015.

              Shah, S. and P. Thubert, "Differentiated Service Class
              Recommendations for LLN Traffic", draft-svshah-tsvwg-lln-
              diffserv-recommendations-04 (work in progress), February

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              Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
              6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-05
              (work in progress), July 2018.

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

              Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
              unaware-leaves-05 (work in progress), May 2018.

              Wang, Q. and X. Vilajosana, "6TiSCH Operation Sublayer
              (6top)", draft-wang-6tisch-6top-sublayer-04 (work in
              progress), November 2015.

   [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,

   [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,

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

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <https://www.rfc-editor.org/info/rfc3610>.

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

   [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,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <https://www.rfc-editor.org/info/rfc4389>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,

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

   [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,

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

   [RFC5889]  Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing
              Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
              September 2010, <https://www.rfc-editor.org/info/rfc5889>.

   [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,

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [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,

   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses",
              RFC 6620, DOI 10.17487/RFC6620, May 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,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <https://www.rfc-editor.org/info/rfc7102>.

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

   [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, <https://www.rfc-editor.org/info/rfc7426>.

   [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,

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

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8.3.  Other Informative References

   [ACE]      IETF, "Authentication and Authorization for Constrained

   [ANIMA]    IETF, "Autonomic Networking Integrated Model and

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

   [DETNET]   IETF, "Deterministic Networking",

   [DICE]     IETF, "DTLS In Constrained Environments",

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

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

              IEEE Standards Association, "IEEE 802.1 Time-Sensitive
              Networks Task Group", March 2013,

              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".

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              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

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

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

   [PCE]      IETF, "Path Computation Element",

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

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

Appendix A.  Dependencies on Work In Progress

   In order to control the complexity and the size of the 6TiSCH work,
   the architecture and the associated IETF work are staged and the WG
   is expected to recharter multiple times.  This document is 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:

   o  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.

   o  The operation of the Backbone Router
      [I-D.ietf-6lo-backbone-router] is stable but the RFC is not

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      published yet.  The protection of registered addresses against
      impersonation and take over will be guaranteed by Address
      Protected Neighbor Discovery for Low-power and Lossy Networks
      [I-D.ietf-6lo-ap-nd], which is not yet published either.

   o  The work on centralized Track computation is deferred to a
      subsequent work, not necessarily at 6TiSCH.  A Predicatable and
      Available Wireless (PAW) bar-BoF took place; PAW may form as a WG
      and take over that work.  The 6TiSCH Architecture should thus
      inherit from the DetNet [I-D.ietf-detnet-architecture]
      architecture and thus depends on it.  The Path Computation Element
      (PCE) should be a core component of that architecture.  Around the
      PCE, a protocol such as an extension to a TEAS [TEAS] protocol
      will be required to expose the 6TiSCH node capabilities and the
      network peers to the PCE, and a protocol such as a lightweight
      PCEP or an adaptation of CCAMP [CCAMP] G-MPLS formats and
      procedures will be used to publish the Tracks, as computed by the
      PCE, to the 6TiSCH nodes.

   o  BIER-TE-based OAM, Replication and Elimination
      [I-D.thubert-bier-replication-elimination] leverages Bit Index
      Explicit Replication - Traffic Engineering 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,
      whereas a 6loRH for BitStrings [I-D.thubert-6lo-bier-dispatch]
      proposes a 6LoWPAN compression for the BIER Bitstring based on
      6LoWPAN Routing Header [RFC8138].

   o  The security model and in particular the join process depends on
      the ANIMA [ANIMA] Bootstrapping Remote Secure Key Infrastructures
      (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra] in order to enable
      zero-touch security provisionning; for highly constrained nodes, a
      minimal model based on pre-shared keys (PSK) is also available.

   o  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.

   o  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.

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   o  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

Author's Address

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

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

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