An Architecture for IPv6 over the Time-Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)
RFC 9030

Document Type RFC - Informational (May 2021; No errata)
Author Pascal Thubert 
Last updated 2021-05-29
Replaces draft-thubert-6tisch-architecture
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Internet Engineering Task Force (IETF)                   P. Thubert, Ed.
Request for Comments: 9030                                 Cisco Systems
Category: Informational                                         May 2021
ISSN: 2070-1721

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

Abstract

   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 low-power wireless deterministic applications.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9030.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  New Terms
     2.2.  Abbreviations
     2.3.  Related Documents
   3.  High-Level Architecture
     3.1.  A Non-broadcast Multi-access Radio Mesh Network
     3.2.  A Multi-Link Subnet Model
     3.3.  TSCH: a Deterministic MAC Layer
     3.4.  Scheduling TSCH
     3.5.  Distributed vs. Centralized Routing
     3.6.  Forwarding over TSCH
     3.7.  6TiSCH Stack
     3.8.  Communication Paradigms and Interaction Models
   4.  Architecture Components
     4.1.  6LoWPAN (and RPL)
       4.1.1.  RPL-Unaware Leaves and 6LoWPAN ND
       4.1.2.  6LBR and RPL Root
     4.2.  Network Access and Addressing
       4.2.1.  Join Process
       4.2.2.  Registration
     4.3.  TSCH and 6top
       4.3.1.  6top
       4.3.2.  Scheduling Functions and the 6top Protocol
       4.3.3.  6top and RPL Objective Function Operations
       4.3.4.  Network Synchronization
       4.3.5.  Slotframes and CDU Matrix
       4.3.6.  Distributing the Reservation of Cells
     4.4.  Schedule Management Mechanisms
       4.4.1.  Static Scheduling
       4.4.2.  Neighbor-to-Neighbor Scheduling
       4.4.3.  Remote Monitoring and Schedule Management
       4.4.4.  Hop-by-Hop Scheduling
     4.5.  On Tracks
       4.5.1.  General Behavior of Tracks
       4.5.2.  Serial Track
       4.5.3.  Complex Track with Replication and Elimination
       4.5.4.  DetNet End-to-End Path
       4.5.5.  Cell Reuse
     4.6.  Forwarding Models
       4.6.1.  Track Forwarding
       4.6.2.  IPv6 Forwarding
       4.6.3.  Fragment Forwarding
     4.7.  Advanced 6TiSCH Routing
       4.7.1.  Packet Marking and Handling
       4.7.2.  Replication, Retries, and Elimination
   5.  IANA Considerations
   6.  Security Considerations
     6.1.  Availability of Remote Services
     6.2.  Selective Jamming
     6.3.  MAC-Layer Security
     6.4.  Time Synchronization
     6.5.  Validating ASN
     6.6.  Network Keying and Rekeying
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Related Work in Progress
     A.1.  Unchartered IETF Work Items
       A.1.1.  6TiSCH Zero-Touch Security
       A.1.2.  6TiSCH Track Setup
       A.1.3.  Using BIER in a 6TiSCH Network
     A.2.  External (Non-IETF) Work Items
   Acknowledgments
   Contributors
   Author's Address

1.  Introduction

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

   On the other hand, Deterministic Networking maximizes the packet
   delivery ratio within a bounded latency so as to enable mission-
   critical machine-to-machine (M2M) operations.  Applications that need
   such networks are presented in [RFC8578] and [RAW-USE-CASES], which
   presents a number of additional use cases for Reliable and Available
   Wireless networks (RAW).  The considered applications include
   professional media, Industrial Automation and Control Systems (IACS),
   building automation, in-vehicle command and control, commercial
   automation and asset tracking with mobile scenarios, as well as
   gaming, drones and edge robotic control, and home automation
   applications.

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

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

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

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

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

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

   The 6TiSCH architecture leverages 6LoWPAN [RFC4944] to adapt IPv6 to
   the constrained media and the Routing Protocol for Low-Power and
   Lossy Networks (RPL) [RFC6550] for the distributed routing
   operations.

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

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

2.  Terminology

2.1.  New Terms

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

   This document adds the following terms:

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

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

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

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

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

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

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

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

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

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

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

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

   CoJP (Constrained Join Protocol):  The Constrained Join Protocol
      (CoJP) enables a pledge to securely join a 6TiSCH network and
      obtain network parameters over a secure channel.  "Constrained
      Join Protocol (CoJP) for 6TiSCH" [RFC9031] defines the minimal
      CoJP setup with pre-shared keys defined.  In that mode, CoJP can
      operate with a single round-trip exchange.

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

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

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

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

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

   hard cell:  A scheduled cell that the 6top sublayer may not relocate.

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

   IE (Information Element):  Type-Length-Value containers placed at the
      end of the MAC header and used to pass data between layers or
      devices.  Some IE identifiers are managed by the IEEE
      [IEEE802154].  Some IE identifiers are managed by the IETF
      [RFC8137].  [RFC9032] uses one subtype to support the selection of
      the Join Proxy.

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

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

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

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

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

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

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

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

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

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

   scheduled cell:  A cell that is assigned a neighbor MAC address
      (broadcast address is also possible) and one or more of the
      following flags: TX, RX, Shared, and Timekeeping.  A scheduled
      cell can be used by the IEEE Std 802.15.4 TSCH implementation to
      communicate.  A scheduled cell can either be a hard or a soft
      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 that the 6top sublayer can relocate.

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

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

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

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

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

   TSCH Schedule:  A matrix of cells, with 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 that is not used by the IEEE Std 802.15.4
      TSCH implementation.

2.2.  Abbreviations

   This document uses the following abbreviations:

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

   6LBR:  6LoWPAN Border Router (authoritative on Duplicate Address
      Detection (DAD))

   6LN:  6LoWPAN Node

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

   6CIO:  Capability Indication Option

   (E)ARO:  (Extended) Address Registration Option

   (E)DAR:  (Extended) Duplicate Address Request

   (E)DAC:  (Extended) Duplicate Address Confirmation

   DAD:  Duplicate Address Detection

   DODAG:  Destination-Oriented Directed Acyclic Graph

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

   NA:  Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:  Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   PCE:  Path Computation Element

   NME:  Network Management Entity

   ROVR:  Registration Ownership Verifier (pronounced rover)

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple)

   RA:  Router Advertisement

   RS:  Router Solicitation

   TSCH:  Time-Slotted Channel Hopping

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

2.3.  Related Documents

   The document conforms to the terms and models described in [RFC3444]
   and [RFC5889], uses the vocabulary and the concepts defined in
   [RFC4291] for the IPv6 architecture, and refers to [RFC4080] for
   reservation.

   The document uses domain-specific terminology defined or referenced
   in the following:

   *  6LoWPAN ND: "Neighbor Discovery Optimization for IPv6 over
      Low-Power Wireless Personal Area Networks (6LoWPANs)" [RFC6775]
      and "Registration Extensions for IPv6 over Low-Power Wireless
      Personal Area Network (6LoWPAN) Neighbor Discovery" [RFC8505],

   *  "Terms Used in Routing for Low-Power and Lossy Networks"
      [RFC7102], and

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

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

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

   *  "Neighbor Discovery for IP version 6 (IPv6)" [RFC4861] and

   *  "IPv6 Stateless Address Autoconfiguration" [RFC4862].

   In addition, readers would benefit from reading the following prior
   to this specification for a clear understanding of the art in ND-
   proxying and binding:

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

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

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

3.  High-Level Architecture

3.1.  A Non-broadcast Multi-access Radio Mesh Network

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

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

             Figure 1: Basic Configuration of a 6TiSCH Network

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

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

   This forms a homogeneous non-broadcast multi-access (NBMA) subnet,
   which is beyond the scope of IPv6 Neighbor Discovery (IPv6 ND)
   [RFC4861] [RFC4862]. 6LoWPAN Neighbor Discovery (6LoWPAN ND)
   [RFC6775] [RFC8505] specifies extensions to IPv6 ND that enable ND
   operations in this type of subnet that can be protected against
   address theft and impersonation with [RFC8928].

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

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

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

   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
   Routing Header (6LoRH) [RFC8138].  In a preexisting network, the
   compression can be globally turned on in a DODAG once all nodes are
   migrated to support [RFC8138] using [RFC9035].

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

   This architecture expects that a 6LoWPAN node can connect as a leaf
   to a RPL network, where the leaf support is the minimal functionality
   to connect as a host to a RPL network without the need to participate
   in the full routing protocol.  The architecture also expects that a
   6LoWPAN node that is unaware of RPL may also connect as described in
   [RFC9010].

3.2.  A Multi-Link Subnet Model

   An extended configuration of the subnet comprises multiple LLNs as
   illustrated in Figure 2.  In the extended configuration, a Routing
   Registrar [RFC8505] may be connected to the node that acts as the RPL
   Root and/or 6LoWPAN 6LBR and provides connectivity to the larger
   campus or factory plant network over a high-speed backbone or a back-
   haul link.  The Routing Registrar may perform IPv6 ND proxy
   operations; redistribute the registration in a routing protocol such
   as OSPF [RFC5340] or BGP [RFC2545]; or inject a route in a mobility
   protocol such as Mobile IPv6 (MIPv6) [RFC6275], Network Mobility
   (NEMO) [RFC3963], or Locator/ID Separation Protocol (LISP) [RFC6830].

   Multiple LLNs can be interconnected and possibly synchronized over a
   backbone, which can be wired or wireless.  The backbone can operate
   with IPv6 ND procedures [RFC4861] [RFC4862] or a hybrid of IPv6 ND
   and 6LoWPAN ND [RFC6775] [RFC8505] [RFC8928].

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

            Figure 2: Extended Configuration of a 6TiSCH Network

   A Routing Registrar that performs proxy IPv6 ND operations over the
   backbone on behalf of the 6TiSCH nodes is called a Backbone Router
   (6BBR) [RFC8929].  The 6BBRs are placed along the wireless edge of a
   backbone and federate multiple wireless links to form a single multi-
   link subnet.  The 6BBRs synchronize with one another over the
   backbone, so as to ensure that the multiple LLNs that form the IPv6
   subnet stay tightly synchronized.

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

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

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

3.3.  TSCH: a Deterministic MAC Layer

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

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

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

   On top of this inheritance, 6TiSCH adds capabilities for distributed
   routing and scheduling operations based on RPL and capabilities for
   negotiating 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 a multiplicity of minor
   notifications such as corrosion measurements, events, and access of
   local devices by mobile workers.

3.4.  Scheduling TSCH

   A scheduling operation allocates cells in a TDM/FDM matrix called a
   CDU either to individual transmissions or as multi-access shared
   resources.  The CDU matrix can be formatted in chunks that can be
   allocated exclusively to particular nodes to enable distributed
   scheduling without collision.  More in Section 4.3.5.

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

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

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

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

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

   Neighbor-to-Neighbor Scheduling:  This refers to the dynamic
      adaptation of the bandwidth of the links that are used for IPv6
      traffic between adjacent peers.  Scheduling Functions such as the
      "6TiSCH Minimal Scheduling Function (MSF)" [RFC9033] influence the
      operation of the MAC layer to add, update, and remove cells in its
      own and its peer's schedules using 6P [RFC8480] for the
      negotiation of the MAC resources.

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

   Hop-by-Hop Scheduling:  This refers to the possibility of reserving
      cells along a path for a particular flow using a distributed
      mechanism.

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

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

3.5.  Distributed vs. Centralized Routing

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

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

   RPL [RFC6550] is applicable to Static Scheduling and Neighbor-to-
   Neighbor Scheduling.  The architecture also supports a centralized
   routing model for Remote Monitoring and Schedule Management.  It is
   expected that a routing protocol that is more optimized for point-to-
   point routing than RPL [RFC6550], such as the "Asymmetric
   AODV-P2P-RPL in Low-Power and Lossy Networks" (AODV-RPL) [AODV-RPL],
   which derives from the "Ad Hoc On-demand Distance Vector (AODVv2)
   Routing" [AODVv2], will be selected for Hop-by-Hop Scheduling.

   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 the
   same topology.  Generally they will operate in different slotframes,
   and centralized routes will be used for scheduled traffic and will
   have precedence over distributed routes in case of conflict between
   the slotframes.

3.6.  Forwarding over TSCH

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

   In more detail:

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

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

   6LoWPAN Fragment Forwarding:  This is a hybrid model that derives
      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 an IP header
      without the need to recompose the packet at every hop.

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

   Table 1 summarizes how the forwarding models apply to the various
   routing and scheduling possibilities:

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

                                 Table 1

3.7.  6TiSCH Stack

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

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

   The resulting protocol stack is represented in Figure 3:

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

                      Figure 3: 6TiSCH Protocol Stack

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

   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 Routing Header
   (SRH) in non-storing mode.  "Using RPI Option Type, Routing Header
   for Source Routes, and IPv6-in-IPv6 Encapsulation in the RPL Data
   Plane" [RFC9008] provides the details on when headers or
   encapsulation are needed.

   The Object Security for Constrained RESTful Environments (OSCORE)
   [RFC8613] is leveraged by the Constrained Join Protocol (CoJP) and is
   expected to be the primary protocol for the protection of the
   application payload as well.  The application payload may also be
   protected by the Datagram Transport Layer Security (DTLS) [RFC6347]
   sitting either under CoAP or over CoAP so it can traverse proxies.

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

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

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

3.8.  Communication Paradigms and Interaction Models

   Section 2.1 provides the terms of Communication Paradigms and
   Interaction Models in combination with "On the Difference between
   Information Models and Data Models" [RFC3444].  A Communication
   Paradigm is an abstract view of a protocol exchange and has an
   Information Model for the information that is being exchanged.  In
   contrast, an Interaction Model is more refined and points to standard
   operation such as a Representational State Transfer (REST) "GET"
   operation and matches a Data Model for the data that is provided over
   the protocol exchange.

   Section 2.1.3 of [RPL-APPLICABILITY] and its following sections
   discuss application-layer paradigms such as source-sink, which is a
   multipeer-to-multipeer model primarily used for alarms and alerts,
   publish-subscribe, which is typically used for sensor data, as well
   as peer-to-peer and peer-to-multipeer communications.

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

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

4.  Architecture Components

4.1.  6LoWPAN (and RPL)

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

4.1.1.  RPL-Unaware Leaves and 6LoWPAN ND

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

   "Routing for RPL Leaves" [RFC9010] details the basic interaction of
   6LoWPAN ND and RPL and enables a plain 6LN that supports [RFC8505] to
   obtain return connectivity via the RPL network as a RPL-unaware leaf.
   The leaf indicates that it requires reachability services for the
   Registered Address from a Routing Registrar by setting an 'R' flag in
   the Extended Address Registration Option [RFC8505], and it provides a
   TID that maps to the "Path Sequence" defined in Section 6.7.8 of
   [RFC6550], and its operation is defined in Section 7.2 of [RFC6550].

   [RFC9010] also enables the leaf to signal with the RPLInstanceID that
   it wants to participate by using the Opaque field of the EARO.  On
   the backbone, the RPLInstanceID is expected to be mapped to an
   overlay that matches the RPL Instance, e.g., a Virtual LAN (VLAN) or
   a virtual routing and forwarding (VRF) instance.

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

4.1.2.  6LBR and RPL Root

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

   When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL Root
   functionalities are co-located in order that the address of the 6LBR
   is indicated by RPL DODAG Information Object (DIO) messages and to
   associate the ROVR from the Extended Duplicate Address Request/
   Confirmation (EDAR/EDAC) exchange [RFC8505] with the state that is
   maintained by RPL.

   Section 7 of [RFC9010] specifies how the DAO messages are used to
   reconfirm the registration, thus eliminating a duplication of
   functionality between DAO and EDAR/EDAC messages, as illustrated in
   Figure 6.  [RFC9010] also provides the protocol elements that are
   needed when the 6LBR and RPL Root functionalities are not co-located.

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

   This architecture also expects that the Root of the RPL network
   (proxy-)registers the 6TiSCH nodes on their behalf to the 6BBR, for
   whatever operation the 6BBR performs on the backbone, such as ND
   proxy or redistribution in a routing protocol.  This relies on an
   extension of the 6LoWPAN ND registration described in [RFC8929].

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

4.2.  Network Access and Addressing

4.2.1.  Join Process

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

   The JP is discovered though MAC-layer beacons.  When multiple JPs
   from possibly multiple networks are visible, using trial and error
   until an acceptable position in the right network is obtained becomes
   inefficient.  [RFC9032] adds a new subtype in the Information Element
   that was delegated to the IETF [RFC8137] and provides visibility into
   the network that can be joined and the willingness of the JP and the
   Root to be used by the pledge.

   The join protocol provides the following functionality:

   *  Mutual authentication

   *  Authorization

   *  Parameter distribution to the pledge over a secure channel

   The Minimal Security Framework for 6TiSCH [RFC9031] defines the
   minimal mechanisms required for this join process to occur in a
   secure manner.  The specification defines the Constrained Join
   Protocol (CoJP), which is used to distribute the parameters to the
   pledge over a secure session established through OSCORE [RFC8613] and
   which describes the secure configuration of the network stack.  In
   the minimal setting with pre-shared keys (PSKs), CoJP allows the
   pledge to join after a single round-trip exchange with the JRC.  The
   provisioning of the PSK to the pledge and the JRC needs to be done
   out of band, through a 'one-touch' bootstrapping process, which
   effectively enrolls the pledge into the domain managed by the JRC.

   In certain use cases, the 'one-touch' bootstrapping is not feasible
   due to the operational constraints, and the enrollment of the pledge
   into the domain needs to occur in-band.  This is handled through a
   'zero-touch' extension of the Minimal Security Framework for 6TiSCH.
   The zero-touch extension [ZEROTOUCH-JOIN] leverages the
   "Bootstrapping Remote Secure Key Infrastructure (BRSKI)" [RFC8995]
   work to establish a shared secret between a pledge and the JRC
   without necessarily having them belong to a common (security) domain
   at join time.  This happens through inter-domain communication
   occurring between the JRC of the network and the domain of the
   pledge, represented by a fourth entity, Manufacturer Authorized
   Signing Authority (MASA).  Once the zero-touch exchange completes,
   the CoJP exchange defined in [RFC9031] is carried over the secure
   session established between the pledge and the JRC.

   Figure 4 depicts the join process and where a Link-Local Address
   (LLA) is used, versus a Global Unicast Address (GUA).

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

       Figure 4: Join Process in a Multi-Link Subnet.  Parentheses ()
                         denote optional exchanges.

4.2.2.  Registration

   Once the pledge successfully completes the CoJP exchange and becomes
   a network node, it obtains the network prefix from neighboring
   routers and registers its IPv6 addresses.  As detailed in
   Section 4.1, the combined 6LoWPAN ND 6LBR and Root of the RPL network
   learn information such as an identifier (device EUI-64 [RFC6775] or a
   ROVR [RFC8505] (from 6LoWPAN ND)) and the updated Sequence Number
   (from RPL), and perform 6LoWPAN ND proxy registration to the 6BBR on
   behalf of the LLN nodes.

   Figure 5 illustrates the initial IPv6 signaling that enables a 6LN to
   form a global address and register it to a 6LBR using 6LoWPAN ND
   [RFC8505].  It is then carried over RPL to the RPL Root and then to
   the 6BBR.  This flow happens just once when the address is created
   and first registered.

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

         Figure 5: Initial Registration Flow over Multi-Link Subnet

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

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

          Figure 6: 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 recursively joining the network.

4.3.  TSCH and 6top

4.3.1.  6top

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

   With 6TiSCH, the abstraction of an IPv6 link is implemented as a pair
   of bundles of cells, one in each direction.  IP links are only
   enabled between RPL parents and children.  The 6TiSCH operation is
   optimal when the size of a bundle minimizes both the energy wasted in
   idle listening and the packet drops due to congestion loss, while
   packets are forwarded within an acceptable latency.

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

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

4.3.1.1.  Hard Cells

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

4.3.1.2.  Soft Cells

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

4.3.2.  Scheduling Functions and the 6top Protocol

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

   There may be multiple SFs that react more or less aggressively to the
   dynamics of the network.

   An SF may be seen as divided between an upper bandwidth-adaptation
   logic that is unaware of the particular technology used to obtain and
   release bandwidth and an underlying service that maps those needs in
   the actual technology.  In the case of TSCH using the 6top Protocol
   as illustrated in Figure 7, this means mapping the bandwidth onto
   cells.

    +------------------------+          +------------------------+
    |  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 7: 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.  For instance, it may be
   that a peer wants to use a particular timeslot that is free in its
   schedule, but that timeslot is already in use by the other peer to
   communicate with a third party on a different cell. 6P enables the
   peers to find an agreement in a transactional manner that ensures the
   final consistency of the nodes' state.

   MSF [RFC9033] is one of the possible Scheduling Functions.  MSF uses
   the rendezvous slot from [RFC8180] for network discovery, neighbor
   discovery, and any other broadcast.

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

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

4.3.3.  6top and RPL Objective Function Operations

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

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

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

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

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

4.3.4.  Network Synchronization

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

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

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

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

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

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

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

4.3.5.  Slotframes and CDU Matrix

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

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

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

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

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

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

   Multiple slotframes can coexist in a node schedule, i.e., a node can
   have multiple activities scheduled in different slotframes.  A
   slotframe is associated with a priority that may be related to the
   precedence of different 6TiSCH topologies.  The slotframes may be
   aligned to different CDU matrices and thus have different widths.
   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 that matches the packet
   best (Differentiated Services [RFC2474] can therefore be used).  At
   each scheduled transmit slot, 6top looks for the frame in all the
   outgoing queues that best matches the cells.  If a frame is found, it
   is given to the TSCH MAC for transmission.

4.3.6.  Distributing the Reservation of Cells

   The 6TiSCH architecture introduces the concept of chunks
   (Section 2.1) to distribute the allocation of the spectrum for a
   whole group of cells at a time.  The CDU matrix is formatted into a
   set of chunks, possibly as illustrated in Figure 8, each of the
   chunks identified uniquely by a chunk-ID.  The knowledge of this
   formatting is shared between all the nodes in a 6TiSCH network.  It
   could be conveyed during the join process, codified into a profile
   document, or obtained using some other mechanism.  This is as opposed
   to Static Scheduling, which refers to the preprogrammed mechanism
   specified in [RFC8180] and which existed before the distribution of
   the chunk formatting.

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

                Figure 8: CDU Matrix Partitioning in Chunks

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

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

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

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

4.4.  Schedule Management Mechanisms

   6TiSCH uses four 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 they 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 preestablished, for
   instance, decided by a network administrator based on operational
   needs.  It can be preconfigured into the nodes, or, more commonly,
   learned by a node when joining the network using standard IEEE Std
   802.15.4 Information Elements (IE).  Regardless, the schedule remains
   unchanged after the node has joined a network.  RPL is used on the
   resulting network.  This "minimal" scheduling mechanism that
   implements this paradigm is detailed in [RFC8180].

4.4.2.  Neighbor-to-Neighbor Scheduling

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

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

   And as discussed in Section 4.3.2, an optional SF is used to monitor
   bandwidth usage and to perform requests for dynamic allocation by the
   6top sublayer.  The SF component is not part of the 6top sublayer.
   It may be co-located on the same device or may be partially or fully
   offloaded to an external system.  The "6TiSCH Minimal Scheduling
   Function (MSF)" [RFC9033] provides a simple SF that can be used by
   default by devices that support dynamic scheduling of soft cells.

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

4.4.3.  Remote Monitoring and Schedule Management

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

   The work in the 6TiSCH Working Group focused on nondeterministic
   traffic and did not provide the generic data model necessary for the
   controller to monitor and manage resources of the 6top sublayer.
   This is deferred to future work, see Appendix A.1.2.

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

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

   "Software-Defined Networking (SDN): Layers and Architecture
   Terminology" [RFC7426] proposes a generic representation of the SDN
   architecture that is reproduced in Figure 9.

                     o--------------------------------o
                     |                                |
                     | +-------------+   +----------+ |
                     | | Application |   |  Service | |
                     | +-------------+   +----------+ |
                     |       Application Plane        |
                     o---------------Y----------------o
                                     |
       *-----------------------------Y---------------------------------*
       |           Network Services Abstraction Layer (NSAL)           |
       *------Y------------------------------------------------Y-------*
              |                                                |
              |               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
                    |                                 |
       *------------Y---------------------------------Y----------------*
       |         Device and resource Abstraction Layer (DAL)           |
       *------------Y---------------------------------Y----------------*
       |            |                                 |                |
       |    o-------Y----------o   +-----+   o--------Y----------o     |
       |    | Forwarding Plane |   | App |   | Operational Plane |     |
       |    o------------------o   +-----+   o-------------------o     |
       |                       Network Device                          |
       +---------------------------------------------------------------+

       Figure 9: SDN Layers and Architecture Terminology per RFC 7426

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

   The DetNet work is expected to enable end-to-end deterministic paths
   across heterogeneous networks.  This can be, for instance, a 6TiSCH
   LLN and an Ethernet backbone.

   This model fits the 6TiSCH extended configuration, whereby a 6BBR
   federates multiple 6TiSCH LLNs 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
   synchronized.

   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 end to end across the TSCH network and an IEEE
   Std 802.1 TSN Ethernet backbone, and it is the responsibility of
   DetNet to enable end-to-end deterministic forwarding.

4.4.4.  Hop-by-Hop Scheduling

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

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

4.5.  On Tracks

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

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

4.5.1.  General Behavior of Tracks

   A Track is associated with Layer 2 bundles of cells with related
   schedules and logical relationships 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
   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 a Layer 3 forwarding scheme.  Therefore, LLN
       devices can save more energy and resources, 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 minimized.

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

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

4.5.2.  Serial Track

   A Serial (or simple) Track is the 6TiSCH version of a circuit: a
   bundle of cells that are programmed to receive (RX-cells) is uniquely
   paired with a bundle of cells that are set to transmit (TX-cells),
   representing a Layer 2 forwarding state that 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 looping through a well-known hopping
   sequence beginning at Epoch time and starting at the cell's
   channelOffset, which results in a rotation of the frequency that is
   used for transmission.  The bundles may be computed so as to
   accommodate both variable rates and retransmissions, so they might
   not be fully used in the iteration of the schedule.

4.5.3.  Complex Track with Replication and Elimination

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

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

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

4.5.4.  DetNet End-to-End Path

   Ultimately, DetNet should enable extending a Track beyond the 6TiSCH
   LLN as illustrated in Figure 10.  In that example, a Track is laid
   out from a field device in a 6TiSCH network to an IoT gateway that is
   located on an 802.1 Time-Sensitive Networking (TSN) backbone.  A
   6TiSCH-aware DetNet service layer handles the Packet Replication,
   Elimination, and Ordering Functions over the DODAG that forms a
   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.

                     +-=-=-+
                     | 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
             o

                 Figure 10: Example End-to-End DetNet Track

4.5.5.  Cell Reuse

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

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

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

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

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

4.6.  Forwarding Models

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

4.6.1.  Track Forwarding

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

   A data frame that is forwarded along a Track normally has a
   destination MAC address that is set to broadcast or a multicast
   address depending on MAC support.  This way, the MAC layer in the
   intermediate nodes accepts the incoming frame and 6top switches it
   without incurring a change in the MAC header.  In the case of IEEE
   Std 802.15.4, this means effectively to broadcast, so that along the
   Track the short address for the destination of the frame is set to
   0xFFFF.

   There are two modes for a Track: an IPv6 native mode and a protocol-
   independent tunnel mode.

4.6.1.1.  Native Mode

   In native mode, the Protocol Data Unit (PDU) is associated with flow-
   dependent metadata 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 may be identified
   using a 6-tuple as discussed in [RFC8939].  In particular,
   implementations of this document should support identification of
   DetNet flows based on the IPv6 Flow Label field.

   The flow follows a Track that is identified using a RPL Instance (see
   Section 3.1.3 of [RFC6550]), signaled in a RPL Packet Information
   (more in Section 11.2.2.1 of [RFC6550]) and the source address of a
   packet going down the DODAG formed by a local instance.  One or more
   flows may be placed in a same Track and the Track identification
   (TrackID plus owner) may be placed in an IP-in-IP encapsulation.  The
   forwarding operation is based on the Track and does not depend on the
   flow therein.

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

   Figure 11 illustrates the Track Forwarding operation that happens at
   the 6top sublayer, below IP.

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

                  Figure 11: Track Forwarding, Native Mode

4.6.1.2.  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.  In that mode, some entity (e.g., PCE)
   can coordinate with a WirelessHART Network Manager or an ISA100.11a
   System Manager to specify the flows that are transported.

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

                  Figure 12: Track Forwarding, Tunnel Mode

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

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

4.6.1.3.  Tunneling Information

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

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

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

4.6.2.  IPv6 Forwarding

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

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

                          Figure 13: IP Forwarding

4.6.3.  Fragment Forwarding

   Considering that, per Section 4 of [RFC4944], 6LoWPAN packets can be
   as large as 1280 bytes (the IPv6 minimum MTU) and that the non-
   storing mode of RPL implies source routing, which requires space for
   routing headers, and that an 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 of a multihop route within a 6TiSCH network, hop-by-hop
   recomposition occurs at each hop to reform the packet and route it.
   This creates additional latency and forces intermediate nodes to
   store a portion of a packet for an undetermined time, thus impacting
   critical resources such as memory and battery.

   [RFC8930] describes a framework for forwarding fragments end-to-end
   across a 6TiSCH route-over mesh.  Within that framework,
   [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, [RFC8931] 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 (ECN).

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

                    Figure 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.

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

                    Figure 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.  Advanced 6TiSCH Routing

4.7.1.  Packet Marking and Handling

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

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

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

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

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

4.7.2.  Replication, Retries, and Elimination

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

   The semantics of the configuration enable correlated timeslots to be
   grouped for transmit (and receive, respectively) with 'OR' relations,
   and then an 'AND' relation can be configurable between groups.  The
   semantics are such that if the transmit (and receive, respectively)
   operation succeeded in one timeslot in an '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 in the same 'OR' group.  The 'OR'
   relation indicates that if a transmission is acknowledged, then
   retransmissions of that packet should not be attempted for the
   remaining timeslots in that group.  There are as many 'OR' groups as
   there are branches of the Track departing from this node.  Different
   'OR' groups are programmed for the purpose of replication, each group
   corresponding to one branch of the Track.  The 'AND' relation between
   the groups indicates that transmission over any of branches must be
   attempted regardless of whether a transmission succeeded in another
   branch.  It is also possible to place cells to different next-hop
   routers in the same 'OR' group.  This allows routing along multipath
   Tracks, trying one next hop and then another only if sending to the
   first fails.

   On the receive side, all timeslots are programmed in the same 'OR'
   group.  Retries of the 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.
   An 'AND' group denotes different packets that must all be received
   and transmitted over the associated transmit groups within their
   respected 'AND' or 'OR' rules.

   As an example, say that we have a simple network as represented in
   Figure 16, and we want to enable PREOF between an ingress node I and
   an egress node E.

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

              Figure 16: Scheduling PREOF on a Simple Network

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

   Say we have six possible channels, and at least ten timeslots per
   slotframe.  Figure 17 shows a possible schedule whereby each
   transmission is retried two or three times, and redundant copies are
   forwarded in parallel via A and C on the one hand, and B and D on the
   other, providing time diversity, spatial diversity though different
   physical paths, and frequency diversity.

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

                     Figure 17: Example Global Schedule

   This translates into a different slotframe that provides the waking
   and sleeping times for every node, and the channelOffset to be used
   when awake.  Figure 18 shows the corresponding slotframe for node A.

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

                  Figure 18: Example Slotframe for Node A

   The logical relationship between the timeslots is given by Table 2:

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

                                  Table 2

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   The "Minimal Security Framework for 6TiSCH" [RFC9031] was optimized
   for Low-Power and TSCH operations.  The reader is encouraged to
   review the Security Considerations section of that document
   (Section 9), which discusses 6TiSCH security issues in more details.

6.1.  Availability of Remote Services

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

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

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

6.2.  Selective Jamming

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

   [ROBUST-SCHEDULING] proposes a method to obfuscate the hopping
   sequence and make it harder to perpetrate that particular attack.

6.3.  MAC-Layer Security

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

   The 6TISCH architecture relies on the join process to deny
   authorization of invalid nodes and to preserve the integrity of the
   network keys.  A rogue that managed to access the network can perform
   a large variety of attacks from DoS to injecting forged packets and
   routing information.  "Zero-trust" properties would be highly
   desirable but are mostly not available at the time of this writing.
   [RFC8928] is a notable exception that protects the ownership of IPv6
   addresses and prevents a rogue node with L2 access from stealing and
   injecting traffic on behalf of a legitimate node.

6.4.  Time Synchronization

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

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

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

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

6.5.  Validating ASN

   After obtaining the tentative ASN, a pledge that wishes to join the
   6TiSCH network must use a join protocol to obtain its security keys.
   The join protocol used in 6TiSCH is the Constrained Join Protocol
   (CoJP).  In the minimal setting defined in [RFC9031], the
   authentication requires a pre-shared key, based on which a secure
   session is derived.  The CoJP exchange may also be preceded by a
   zero-touch handshake [ZEROTOUCH-JOIN] in order to enable pledge
   joining based on certificates and/or inter-domain communication.

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

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

6.6.  Network Keying and Rekeying

   Section 4.2.1 provides an overview of the CoJP process described in
   [RFC9031] by which an LLN can be assembled in the field, having been
   provisioned in a lab.  [ZEROTOUCH-JOIN] is future work that precedes
   and then leverages CoJP using the [CONSTRAINED-VOUCHER] constrained
   profile of [RFC8995].  This later work requires a yet-to-be
   standardized Lightweight Authenticated Key Exchange protocol.

   CoJP results in distribution of a network-wide key that is to be used
   with [IEEE802154] security.  The details of use are described in
   [RFC9031], Sections 9.2 and 9.3.2.

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

   Section 8.2 of [RFC9031] describes a mechanism to change (rekey) the
   network.  There are a number of reasons to initiate a network rekey:
   to remove unwanted (corrupt/malicious) nodes, to recover unused
   2-byte short addresses, or due to limits in encryption algorithms.
   For all of the mechanisms that distribute a network-wide key,
   rekeying is also needed on a periodic basis.  In more detail:

   *  The mechanism described in Section 8.2 of [RFC9031] requires
      advance communication between the JRC and every one of the nodes
      before the key change.  Given that many nodes may be sleepy, this
      operation may take a significant amount of time and may consume a
      significant portion of the available bandwidth.  As such, network-
      wide rekeys to exclude nodes that have become malicious will not
      be particularly quick.  If a rekey is already in progress, but the
      unwanted node has not yet been updated, then it is possible to
      just continue the operation.  If the unwanted node has already
      received the update, then the rekey operation will need to be
      restarted.

   *  The cryptographic mechanisms used by IEEE Std 802.15.4 include the
      2-byte short address in the calculation of the context.  A nonce-
      reuse attack may become feasible if a short address is reassigned
      to another node while the same network-wide keys are in operation.
      A network that gains and loses nodes on a regular basis is likely
      to reach the 65536 limit of the 2-byte (16-bit) short addresses,
      even if the network has only a few thousand nodes.  Network
      planners should consider the need to rekey the network on a
      periodic basis in order to recover 2-byte addresses.  The rekey
      can update the short addresses for active nodes if desired, but
      there is actually no need to do this as long as the key has been
      changed.

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

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

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

   Except for urgent rekeys caused by malicious nodes, the rekey
   operation described in [RFC9031] can be done as a background task and
   can be done incrementally.  It is a make-before-break mechanism.  The
   switch over to the new key is not signaled by time, but rather by
   observation that the new key is in use.  As such, the update can take
   as long as needed, or occur in as short a time as practical.

7.  References

7.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

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

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <https://www.rfc-editor.org/info/rfc6551>.

   [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,
              <https://www.rfc-editor.org/info/rfc6552>.

   [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,
              <https://www.rfc-editor.org/info/rfc6553>.

   [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,
              <https://www.rfc-editor.org/info/rfc6554>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [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,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <https://www.rfc-editor.org/info/rfc7554>.

   [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,
              <https://www.rfc-editor.org/info/rfc8025>.

   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
              2017, <https://www.rfc-editor.org/info/rfc8137>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [RFC8480]  Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Protocol (6P)", RFC 8480,
              DOI 10.17487/RFC8480, November 2018,
              <https://www.rfc-editor.org/info/rfc8480>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [RFC8928]  Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
              "Address-Protected Neighbor Discovery for Low-Power and
              Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
              2020, <https://www.rfc-editor.org/info/rfc8928>.

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

   [RFC8930]  Watteyne, T., Ed., Thubert, P., Ed., and C. Bormann, "On
              Forwarding 6LoWPAN Fragments over a Multi-Hop IPv6
              Network", RFC 8930, DOI 10.17487/RFC8930, November 2020,
              <https://www.rfc-editor.org/info/rfc8930>.

   [RFC8931]  Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal
              Area Network (6LoWPAN) Selective Fragment Recovery",
              RFC 8931, DOI 10.17487/RFC8931, November 2020,
              <https://www.rfc-editor.org/info/rfc8931>.

   [RFC9008]  Robles, M.I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes, and IPv6-
              in-IPv6 Encapsulation in the RPL Data Plane", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,
              <https://www.rfc-editor.org/info/rfc9008>.

   [RFC9010]  Thubert, P., Ed. and M. Richardson, "Routing for RPL
              (Routing Protocol for Low-Power and Lossy Networks)
              Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
              <https://www.rfc-editor.org/info/rfc9010>.

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

   [RFC9032]  Dujovne, D., Ed. and M. Richardson, "Encapsulation of
              6TiSCH Join and Enrollment Information Elements",
              RFC 9032, DOI 10.17487/RFC9032, May 2021,
              <https://www.rfc-editor.org/info/rfc9032>.

   [RFC9033]  Chang, T., Ed., Vučinić, M., Vilajosana, X., Duquennoy,
              S., and D. Dujovne, "6TiSCH Minimal Scheduling Function
              (MSF)", RFC 9033, DOI 10.17487/RFC9033, May 2021,
              <https://www.rfc-editor.org/info/rfc9033>.

7.2.  Informative References

   [AMI]      U.S. Department of Energy, "Advanced Metering
              Infrastructure and Customer Systems", 2006,
              <https://www.energy.gov/sites/prod/files/2016/12/f34/
              AMI%20Summary%20Report_09-26-16.pdf>.

   [ANIMA]    IETF, "Autonomic Networking Integrated Model and Approach
              (anima)",
              <https://datatracker.ietf.org/doc/charter-ietf-anima/>.

   [AODV-RPL] Anamalamudi, S., Zhang, M., Perkins, C. E., Anand, S., and
              B. Liu, "Supporting Asymmetric Links in Low Power
              Networks: AODV-RPL", Work in Progress, Internet-Draft,
              draft-ietf-roll-aodv-rpl-10, 4 April 2021,
              <https://tools.ietf.org/html/draft-ietf-roll-aodv-rpl-10>.

   [AODVv2]   Perkins, C. E., Ratliff, S., Dowdell, J., Steenbrink, L.,
              and V. Mercieca, "Ad Hoc On-demand Distance Vector Version
              2 (AODVv2) Routing", Work in Progress, Internet-Draft,
              draft-ietf-manet-aodvv2-16, 4 May 2016,
              <https://tools.ietf.org/html/draft-ietf-manet-aodvv2-16>.

   [BITSTRINGS-6LORH]
              Thubert, P., Ed., Brodard, Z., Jiang, H., and G. Texier,
              "A 6loRH for BitStrings", Work in Progress, Internet-
              Draft, draft-thubert-6lo-bier-dispatch-06, 28 January
              2019, <https://tools.ietf.org/html/draft-thubert-6lo-bier-
              dispatch-06>.

   [CCAMP]    IETF, "Common Control and Measurement Plane (ccamp)",
              <https://datatracker.ietf.org/doc/charter-ietf-ccamp/>.

   [CCMstar]  Struik, R., "Formal Specification of the CCM* Mode of
              Operation", September 2004, <http://www.ieee802.org/15/
              pub/2004/15-04-0537-00-004b-formal-specification-ccm-star-
              mode-operation.doc>.

   [CONSTRAINED-VOUCHER]
              Richardson, M., van der Stok, P., and P. Kampanakis,
              "Constrained Voucher Artifacts for Bootstrapping
              Protocols", Work in Progress, Internet-Draft, draft-ietf-
              anima-constrained-voucher-10, 21 February 2021,
              <https://tools.ietf.org/html/draft-ietf-anima-constrained-
              voucher-10>.

   [DAO-PROJECTION]
              Thubert, P., Jadhav, R. A., and M. Gillmore, "Root
              initiated routing state in RPL", Work in Progress,
              Internet-Draft, draft-ietf-roll-dao-projection-16, 15
              January 2021, <https://tools.ietf.org/html/draft-ietf-
              roll-dao-projection-16>.

   [EDHOC]    Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-selander-ace-cose-ecdhe-14, 11
              September 2019, <https://tools.ietf.org/html/draft-
              selander-ace-cose-ecdhe-14>.

   [EST-COAPS]
              van der Stok, P., Kampanakis, P., Richardson, M., and S.
              Raza, "EST over secure CoAP (EST-coaps)", Work in
              Progress, Internet-Draft, draft-ietf-ace-coap-est-18, 6
              January 2020,
              <https://tools.ietf.org/html/draft-ietf-ace-coap-est-18>.

   [HART]     FieldComm Group, "HART",
              <https://fieldcommgroup.org/technologies/hart>.

   [IEC62439] IEC, "Industrial communication networks - High
              availability automation networks - Part 3: Parallel
              Redundancy Protocol (PRP) and High-availability Seamless
              Redundancy (HSR)", IEC 62439-3:2016, 2016,
              <https://webstore.iec.ch/publication/24438>.

   [IEEE802154]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
              Standard 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875,
              April 2016,
              <https://ieeexplore.ieee.org/document/7460875>.

   [IEEE802154e]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", IEEE
              Standard 802.15.4e-2012, DOI 10.1109/IEEESTD.2012.6185525,
              April 2012,
              <https://ieeexplore.ieee.org/document/6185525>.

   [ISA100]   ISA/ANSI, "ISA100, Wireless Systems for Automation",
              <https://www.isa.org/isa100/>.

   [ISA100.11a]
              ISA/ANSI, "Wireless Systems for Industrial Automation:
              Process Control and Related Applications - ISA100.11a-
              2011", IEC 62734:2014, 2011,
              <https://webstore.iec.ch/publication/65581>.

   [ND-UNICAST-LOOKUP]
              Thubert, P., Ed. and E. Levy-Abegnoli, "IPv6 Neighbor
              Discovery Unicast Lookup", Work in Progress, Internet-
              Draft, draft-thubert-6man-unicast-lookup-00, 29 July 2019,
              <https://tools.ietf.org/html/draft-thubert-6man-unicast-
              lookup-00>.

   [PCE]      IETF, "Path Computation Element (pce)",
              <https://datatracker.ietf.org/doc/charter-ietf-pce/>.

   [RAW-ARCHITECTURE]
              Thubert, P., Ed. and G. Z. Papadopoulos, "Reliable and
              Available Wireless Problem Statement", Work in Progress,
              Internet-Draft, draft-pthubert-raw-architecture-05, 15
              November 2020, <https://tools.ietf.org/html/draft-
              pthubert-raw-architecture-05>.

   [RAW-USE-CASES]
              Papadopoulos, G. Z., Thubert, P., Theoleyre, F., and C. J.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-ietf-raw-use-cases-01, 21 February 2021,
              <https://tools.ietf.org/html/draft-ietf-raw-use-cases-01>.

   [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,
              <https://www.rfc-editor.org/info/rfc2474>.

   [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,
              <https://www.rfc-editor.org/info/rfc2545>.

   [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,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC3444]  Pras, A. and J. Schoenwaelder, "On the Difference between
              Information Models and Data Models", RFC 3444,
              DOI 10.17487/RFC3444, January 2003,
              <https://www.rfc-editor.org/info/rfc3444>.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, DOI 10.17487/RFC3963, January 2005,
              <https://www.rfc-editor.org/info/rfc3963>.

   [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,
              <https://www.rfc-editor.org/info/rfc4080>.

   [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>.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,
              <https://www.rfc-editor.org/info/rfc4903>.

   [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,
              <https://www.rfc-editor.org/info/rfc4919>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [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,
              <https://www.rfc-editor.org/info/rfc5974>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc6606>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [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>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,
              <https://www.rfc-editor.org/info/rfc8578>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC8939]  Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
              Bryant, "Deterministic Networking (DetNet) Data Plane:
              IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,
              <https://www.rfc-editor.org/info/rfc8939>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

   [RFC9035]  Thubert, P., Ed. and L. Zhao, "A Routing Protocol for Low-
              Power and Lossy Networks (RPL) Destination-Oriented
              Directed Acyclic Graph (DODAG) Configuration Option for
              the 6LoWPAN Routing Header", RFC 9035,
              DOI 10.17487/RFC9035, April 2021,
              <https://www.rfc-editor.org/info/rfc9035>.

   [ROBUST-SCHEDULING]
              Tiloca, M., Duquennoy, S., and G. Dini, "Robust Scheduling
              against Selective Jamming in 6TiSCH Networks", Work in
              Progress, Internet-Draft, draft-tiloca-6tisch-robust-
              scheduling-02, 10 June 2019, <https://tools.ietf.org/html/
              draft-tiloca-6tisch-robust-scheduling-02>.

   [RPL-APPLICABILITY]
              Phinney, T., Ed., Thubert, P., and R. Assimiti, "RPL
              applicability in industrial networks", Work in Progress,
              Internet-Draft, draft-ietf-roll-rpl-industrial-
              applicability-02, 21 October 2013,
              <https://tools.ietf.org/html/draft-ietf-roll-rpl-
              industrial-applicability-02>.

   [RPL-BIER] Thubert, P., Ed., "RPL-BIER", Work in Progress, Internet-
              Draft, draft-thubert-roll-bier-02, 24 July 2018,
              <https://tools.ietf.org/html/draft-thubert-roll-bier-02>.

   [RPL-MOP]  Jadhav, R., Ed., Thubert, P., Richardson, M., and R.
              Sahoo, "RPL Capabilities", Work in Progress, Internet-
              Draft, draft-ietf-roll-capabilities-08, 17 March 2021,
              <https://tools.ietf.org/html/draft-ietf-roll-capabilities-
              08>.

   [S-ALOHA]  Roberts, L. G., "ALOHA packet system with and without
              slots and capture", ACM SIGCOMM Computer Communication
              Review, DOI 10.1145/1024916.1024920, April 1975,
              <https://dl.acm.org/citation.cfm?id=1024920>.

   [TE-PREF]  Thubert, P., Ed., Eckert, T., Brodard, Z., and H. Jiang,
              "BIER-TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", Work in Progress, Internet-
              Draft, draft-thubert-bier-replication-elimination-03, 3
              March 2018, <https://tools.ietf.org/html/draft-thubert-
              bier-replication-elimination-03>.

   [TEAS]     IETF, "Traffic Engineering Architecture and Signaling
              (teas)",
              <https://datatracker.ietf.org/doc/charter-ietf-teas/>.

   [VIRTUAL-REASSEMBLY]
              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
              lwig-6lowpan-virtual-reassembly-02, 9 March 2020,
              <https://tools.ietf.org/html/draft-ietf-lwig-6lowpan-
              virtual-reassembly-02>.

   [WirelessHART]
              International Electrotechnical Commission, "Industrial
              networks - Wireless communication network and
              communication profiles - WirelessHART(TM)",
              IEC 62591:2016, March 2016,
              <https://webstore.iec.ch/publication/24433>.

   [ZEROTOUCH-JOIN]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-
              dtsecurity-zerotouch-join-04, 8 July 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-dtsecurity-
              zerotouch-join-04>.

Appendix A.  Related Work in Progress

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

A.1.  Unchartered IETF Work Items

A.1.1.  6TiSCH Zero-Touch Security

   The security model and in particular the zero-touch join process
   [ZEROTOUCH-JOIN] depend on the ANIMA (Autonomic Networking Integrated
   Model and Approach) [ANIMA] "Bootstrapping Remote Secure Key
   Infrastructure (BRSKI)" [RFC8995] to enable zero-touch security
   provisioning; for highly constrained nodes, a minimal model based on
   pre-shared keys (PSK) is also available.  As currently written, it
   also depends on a number of documents in progress in the CORE
   (Constrained RESTful Environments) WG and on "Ephemeral
   Diffie-Hellman Over COSE (EDHOC)" [EDHOC], which is being considered
   for adoption by the LAKE (Lightweight Authenticated Key Exchange) WG.

A.1.2.  6TiSCH Track Setup

   ROLL (Routing Over Low power and Lossy networks) is now standardizing
   a reactive routing protocol based on RPL [AODV-RPL].  The need of a
   reactive routing protocol to establish on-demand, constraint-
   optimized routes and a reservation protocol to establish Layer 3
   Tracks is being discussed in 6TiSCH but not yet chartered.

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

   ROLL is also standardizing an extension to RPL to set up centrally
   computed routes [DAO-PROJECTION].

   The 6TiSCH architecture should thus inherit from the DetNet
   architecture [RFC8655] and thus depends on it.  The PCE should be a
   core component of that architecture.  An extension to RPL or to TEAS
   (Traffic Engineering Architecture and Signaling) [TEAS] will be
   required to expose the 6TiSCH node capabilities and the network peers
   to the PCE, possibly in combination with [RPL-MOP].  A protocol such
   as a lightweight Path Computation Element Communication Protocol
   (PCEP) or an adaptation of Common Control and Measurement Plane
   (CCAMP) [CCAMP] GMPLS formats and procedures could be used in
   combination to [DAO-PROJECTION] to install the Tracks, as computed by
   the PCE, to the 6TiSCH nodes.

A.1.3.  Using BIER in a 6TiSCH Network

   ROLL is actively working on Bit Index Explicit Replication (BIER) as
   a method to compress both the data-plane packets and the routing
   tables in storing mode [RPL-BIER].

   BIER could also be used in the context of the DetNet service layer.
   "BIER-TE extensions for Packet Replication and Elimination Function
   (PREF) and OAM" [TE-PREF] leverages BIER Traffic Engineering (TE) to
   control the DetNet Replication and Elimination activities in the data
   plane, and to provide traceability on links where replication and
   loss happen, in a manner that is abstract to the forwarding
   information.

   "A 6loRH for BitStrings" [BITSTRINGS-6LORH] proposes a 6LoWPAN
   compression for the BIER BitString based on 6LoWPAN Routing Header
   [RFC8138].

A.2.  External (Non-IETF) Work Items

   The current charter positions 6TiSCH on IEEE Std 802.15.4 only.
   Though most of the design should be portable to 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 nonexistent, but deeper work such as 6top and security
   may be impacted.  A 6TiSCH Interest Group at the IEEE maintains the
   synchronization and helps foster work at the IEEE should 6TiSCH
   demand it.

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

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

Acknowledgments

Special Thanks

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

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

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

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

And Do Not Forget

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

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

Contributors

   The co-authors of this document are listed below:

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

      Xavier Vilajosana, who led the design of the minimal support with
      RPL and contributed deeply to the 6top design and the GMPLS
      operation of Track switching;

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

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

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

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

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

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

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

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

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

Author's Address

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

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