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6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                           March 1, 2019
Expires: September 2, 2019


      An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4
                   draft-ietf-6tisch-architecture-20

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 LowPower wireless deterministic applications.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 2, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terms and References  . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .  10
     2.3.  References  . . . . . . . . . . . . . . . . . . . . . . .  11
   3.  High Level Architecture . . . . . . . . . . . . . . . . . . .  12
     3.1.  A Non-Broadcast Multi-Access Radio Mesh Network . . . . .  12
     3.2.  A Multi-Link Subnet Model . . . . . . . . . . . . . . . .  13
     3.3.  TSCH: A Deterministic MAC Layer . . . . . . . . . . . . .  15
     3.4.  Scheduling TSCH . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Distributed vs. Centralized Routing . . . . . . . . . . .  17
     3.6.  Forwarding Over TSCH  . . . . . . . . . . . . . . . . . .  18
     3.7.  6TiSCH Stack  . . . . . . . . . . . . . . . . . . . . . .  19
   4.  Architecture Components . . . . . . . . . . . . . . . . . . .  21
     4.1.  6LoWPAN (and RPL) . . . . . . . . . . . . . . . . . . . .  21
       4.1.1.  RPL-Unaware Leaves and 6LoWPAN ND . . . . . . . . . .  21
       4.1.2.  RPL Root And 6LBR . . . . . . . . . . . . . . . . . .  21
     4.2.  Network Access and Addressing . . . . . . . . . . . . . .  22
       4.2.1.  Join Process  . . . . . . . . . . . . . . . . . . . .  22
       4.2.2.  Registration  . . . . . . . . . . . . . . . . . . . .  24
     4.3.  TSCH and 6top . . . . . . . . . . . . . . . . . . . . . .  26
       4.3.1.  6top  . . . . . . . . . . . . . . . . . . . . . . . .  26
       4.3.2.  Scheduling Functions and the 6top protocol  . . . . .  27
       4.3.3.  6top and RPL Objective Function operations  . . . . .  29
       4.3.4.  Network Synchronization . . . . . . . . . . . . . . .  29
       4.3.5.  SlotFrames and CDU matrix . . . . . . . . . . . . . .  31
       4.3.6.  Distributing the reservation of cells . . . . . . . .  32
     4.4.  Communication Paradigms and Interaction Models  . . . . .  33
     4.5.  Schedule Management Mechanisms  . . . . . . . . . . . . .  34
       4.5.1.  Static Scheduling . . . . . . . . . . . . . . . . . .  34
       4.5.2.  Neighbor-to-neighbor Scheduling . . . . . . . . . . .  34
       4.5.3.  Remote Monitoring and Schedule Management . . . . . .  35
       4.5.4.  Hop-by-hop Scheduling . . . . . . . . . . . . . . . .  37
     4.6.  On Tracks . . . . . . . . . . . . . . . . . . . . . . . .  37
       4.6.1.  General Behavior of Tracks  . . . . . . . . . . . . .  38
       4.6.2.  Serial Track  . . . . . . . . . . . . . . . . . . . .  38
       4.6.3.  Complex Track with Replication and Elimination  . . .  39
       4.6.4.  DetNet End-to-end Path  . . . . . . . . . . . . . . .  39
       4.6.5.  Cell Reuse  . . . . . . . . . . . . . . . . . . . . .  40
     4.7.  Forwarding Models . . . . . . . . . . . . . . . . . . . .  41
       4.7.1.  Track Forwarding  . . . . . . . . . . . . . . . . . .  41
       4.7.2.  IPv6 Forwarding . . . . . . . . . . . . . . . . . . .  44
       4.7.3.  Fragment Forwarding . . . . . . . . . . . . . . . . .  44
     4.8.  Advanced 6TiSCH Routing . . . . . . . . . . . . . . . . .  46
       4.8.1.  Packet Marking and Handling . . . . . . . . . . . . .  46
       4.8.2.  Replication, Retries and Elimination  . . . . . . . .  47



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       4.8.3.  Differentiated Services Per-Hop-Behavior  . . . . . .  49
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  49
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  50
     7.1.  Contributors  . . . . . . . . . . . . . . . . . . . . . .  50
     7.2.  Special Thanks  . . . . . . . . . . . . . . . . . . . . .  51
     7.3.  And Do not Forget . . . . . . . . . . . . . . . . . . . .  51
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  52
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  54
   Appendix A.  Dependencies on Work In Progress . . . . . . . . . .  60
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  61

1.  Introduction

   Wireless Networks enable a wide variety of devices of any size to get
   interconnected, often at a very low marginal cost per device, at any
   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 [I-D.ietf-detnet-use-cases].  They
   include Professional Media and Operation Technology (OT) Industrial
   Automation Control Systems (IACS).

   The Timeslotted Channel Hopping (TSCH) [RFC7554] mode of the IEEE Std
   802.15.4 [IEEE802154] Medium Access Control (MAC) was introduced with
   the IEEE Std 802.15.4e [IEEE802154e] amendment and is now retrofitted
   in the main standard.  For all practical purposes, this document is
   expected to be insensitive to the revisions of that standard, which
   is thus referenced undated.  TSCH is both a Time-Division
   Multiplexing and a Frequency-Division Multiplexing technique whereby
   a different channel can be used for each transmission, and that
   allows to schedule transmissions for deterministic operations.

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

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



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   6TiSCH provides large scaling capabilities, which, in a number of
   scenarios, require the addition of a high-speed and reliable backbone
   and the use of IP version 6 (IPv6).

   The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet model
   that is composed of a federating backbone, e.g., an Ethernet bridged
   network, and a number of IEEE Std 802.15.4 TSCH low-power wireless
   networks federated and synchronized by Backbone Routers.

   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.  Distributed is a concurrent model that relies in more
   classical peer to peer protocols 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 a Advance metering that leverage
   distributed routing to address multipath over a large number of hops
   and nodes.

   Other use cases includes industrial control systems, building
   automation, in-vehicle command and control, commercial automation and
   asset tracking with mobile scenarios, and home automation
   applications.  The determinism 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.

2.  Terms and References

2.1.  Terminology

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

   This document adds the following terms:

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





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

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

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

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

   Layer-2 vs. Layer-3 bundle:  Bundles are associated for either
               Layer-2 (switching) or Layer-3 (routing) forwarding
               operations.  A pair of Layer-3 bundles (one for each
               direction) maps to an IP Link with a neighbor, whereas a
               set of Layer-2 bundles (a number per neighbor, either
               from or to the neighbor) 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.

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




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   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.  In
               the minimal setup with pre-shared keys defined in
               [I-D.ietf-6tisch-minimal-security], CoJP can operate with
               a single round trip exchange.

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

   deterministic network:  The generic concept of deterministic network
               is defined in [I-D.ietf-detnet-architecture].  When
               applied to 6TiSCH, it refers to the reservation of Tracks
               which guarantee an end-to-end latency and optimize the
               PDR for well-characterized flows.




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

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

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

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

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

   IE (Information Element):  Type-Length-Value containers placed at the
               end of the MAC header, used to pass data between layers
               or devices.  Some IE identifiers are managed by the IEEE
               [IEEE802154].  Some IE identifiers are managed by the
               IETF [RFC8137].

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

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

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

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

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

   link:       A communication facility or medium over which nodes can
               communicate at the Link-Layer, the layer immediately
               below IP.  In 6TiSCH, the concept is implemented as a



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               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.

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

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

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

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

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

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

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

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

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

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




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

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

   Track:      A Track is a Directed Acyclic Graph (DAG) that is used as
               a complex multi-hop path to the destination(s) of the
               path.  In the case of unicast traffic, the Track is a
               Destination Oriented DAG (DODAG) where the root of the
               DODAG is the destination of the unicast traffic.  A Track
               enables replication, elimination and reordering functions
               on the way (more on those functions in the Deterministic
               Networking Architecture [I-D.ietf-detnet-architecture]).
               A Track reservation locks physical resources such as
               cells and buffers in every node along the DODAG.  A Track
               is associated with a owner that can be for instance the
               destination of the Track.

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

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

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

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



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2.2.  Abbreviations

   This document uses the following abbreviations:

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

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

   6LN:  6LoWPAN Node

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

   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

   NS:   Neighbor Solicitation

   ROVR: Registration Ownership Verifier (pronounced rover)

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple)

   RA:   Router Advertisement

   RS:   Router Solicitation

   TSCH: timeslotted Channel Hopping

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




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2.3.  References

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

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

      "Neighbor Discovery Optimization for Low-power and Lossy Networks"
      [RFC6775],

      "Registration Extensions for 6LoWPAN Neighbor Discovery"
      [RFC8505],

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

      "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

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

   o  "IPv6 Stateless Address Autoconfiguration" [RFC4862].

   In addition, readers would benefit from reading:

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

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

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

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





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3.  High Level Architecture

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

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


               ---+-------- ............ ------------
                  |      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
                    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.

   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 an homogeneous non-broadcast multi-access (NBMA) subnet,
   which is beyond the scope of IPv6 Neighbor Discovery (IPv6 ND)
   [RFC4861][RFC4862].  Extensions to IPv6 ND were introduced with
   6LoWPAN Neighbor Discovery (6LoWPAN ND) [RFC6775][RFC8505].  Within
   an NBMA subnet, 6LoWPAN ND enables an address registration to avoid
   address duplication, and RPL [RFC6550] enables routing in the so-
   called Route Over fashion, either in storing (stateful) or non-
   storing (stateless, with routing headers) mode.

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



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   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
   addition 6LoRH [RFC8138].

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

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

3.2.  A Multi-Link Subnet Model

   An extended configuration of the subnet comprises multiple LLNs as
   illustrated in Figure 2.  In the extended configuration, a Routing
   Registrar [RFC8505] may be connected to the node that acts as RPL
   root and / or 6LoWPAN 6LBR and provides connectivity to the larger
   campus / factory plant network over a high-speed backbone or a back-
   haul link.  The Routing registrar may perform IPv6 ND proxy
   operations, or redistribute the registration in a routing protocol
   such as OSPF [RFC5340] or BGP [RFC2545], or inject a route in a
   mobility protocol such as MIPv6 [RFC6275], NEMO [RFC3963], or LISP
   [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 [RFC4861][RFC4862] procedures or an hybrid of IPv6 ND
   and 6LoWPAN ND [RFC6775] [RFC8505].



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                  |
               +-----+                +-----+         +-----+
     (default) |     |     (Optional) |     |         |     | IPv6
        Router |     |           6LBR |     |         |     | Node
               +-----+                +-----+         +-----+
                  |  Backbone side       |               |
      --------+---+--------------------+-+---------------+------+---
              |                        |                        |
        +-----------+            +-----------+            +-----------+
        | Routing   |            | Routing   |            | Routing   |
        | Registrar |            | Registrar |            | Registrar |
        +-----------+            +-----------+            +-----------+
          o     Wireless side       o  o                     o o
      o o   o  o                o o   o  o  o          o  o  o  o o
    o   6TiSCH                o   6TiSCH   o  o          o o  6TiSCH o o
    o   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) [I-D.ietf-6lo-backbone-router].  The 6BBRs are placed along
   the wireless edge of a Backbone, and federate multiple wireless links
   to form a single MultiLink Subnet.  The 6BBRs synchronize with one
   another over the backbone, so as to ensure that the multiple LLNs
   that form the IPv6 subnet stay tightly synchronized.

   A 6LBR located on the backbone may contribute to Duplicate Address
   Detection as well as Address Lookup and save multicast operations
   [I-D.thubert-6lo-unicast-lookup].

   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 protocol but not both.  The preferred way
   of achieving this is to collocate/combine them.  The combined RPL
   root and 6LBR may be collocated with the 6BBR, or directly attached
   to the 6BBR.  In the latter case, it leverages the extended
   registration process defined in [RFC8505] to proxy the 6LoWPAN ND
   registration to the 6BBR on behalf of the LLN nodes, so that the 6BBR
   may in turn perform proxy classical ND operations over the backbone.

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



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3.3.  TSCH: A Deterministic MAC Layer

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

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

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

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

3.4.  Scheduling TSCH

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





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

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

   Scheduling enables multiple communications at a same time in a same
   interference domain using different channels; but a node equipped
   with a single radio can only either transmit or receive on one
   channel at any point of time.  Scheduled cells that play an equal
   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, Remote Monitoring and Schedule Management,
   and Hop-by-hop Scheduling.

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

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



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      (NME), which may cooperate with the PCE in order to minimize the
      interaction with and the load on the constrained device.  This
      model is the TSCH adaption of the "DetNet Architecture"
      [I-D.ietf-detnet-architecture], and it enables Traffic Engineering
      with deterministic properties.

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

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

   A deeper dive in those mechanisms can be found in Section 4.5.

3.5.  Distributed vs. Centralized Routing

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

   Both methods may inject routes in the Routing Tables of the 6TiSCH
   routers.  In either case, each route is associated with a 6TiSCH
   topology that can be a RPL Instance topology or a Track.  The 6TiSCH
   topology is indexed by a Instance ID, 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" [I-D.ietf-roll-aodv-rpl] (AODV-
   RPL), which derives from the Ad Hoc On-demand Distance Vector Routing
   (AODV) [I-D.ietf-manet-aodvv2] will be selected for Hop-by-hop
   Scheduling.

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



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3.6.  Forwarding Over TSCH

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

   In more details:

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

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

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

   A deeper dive on these operations can be found in Section 4.7.

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






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


3.7.  6TiSCH Stack

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

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

   The resulting protocol stack is represented in Figure 4:


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


                      Figure 4: 6TiSCH Protocol Stack



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

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

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

   An overview of the initial steps of a device in a network can be
   found in Section 4.2.1; the security aspects of the join process are
   further detailed in Section 6.

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

   The reference stack that the 6TiSCH architecture presents was
   implemented and interop tested by a conjunction of opensource, IETF
   and ETSI efforts.  One goal is to help other bodies to adopt the
   stack as a whole, making the effort to move to an IPv6-based IoT
   stack easier.

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



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4.  Architecture Components

4.1.  6LoWPAN (and RPL)

   A RPL DODAG is formed of a root, a collection of routers, and leaves
   that are hosts.  Hosts are nodes which do not forward packets that
   they did not generate.  RPL-aware leaves will participate to RPL in
   order 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 in order to advertise a leaf node
   through a DAO message and establish reachability.

   "Routing for RPL Leaves" [I-D.thubert-roll-unaware-leaves] details
   the basic interaction of 6LoWPAN ND and RPL and enables a plain 6LN
   that supports [RFC8505] to obtain return connectivity via the RPL
   network as an RPL-unaware leaf.  The leaf indicates that it requires
   reachability services for the Registered Address from a Routing
   Registrar by settig a 'R' flag in the Extended Address Registration
   Option [RFC8505], and it provides a TID that maps to a sequence
   number in section 7 of RPL [RFC6550].

   The RPL InstanceID that the leaf wants to participate to may be
   signaled in the Opaque field of the EARO.  On the backbone, the
   InstanceID is expected to be mapped to an overlay that matches the
   RPL Instance, e.g., a Virtual LAN (VLAN) or a virtual routing and
   forwarding (VRF) instance.

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

4.1.2.  RPL Root And 6LBR

   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



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   be indicated by RPL DIO messages and to associate the unique ID from
   the EDAR/EDAC [RFC8505] exchange with the state that is maintained by
   RPL.

   Section 5 of [I-D.thubert-roll-unaware-leaves] details how the DAO
   messages are used to reconfirm the registration, thus eliminating a
   duplication of functionality between DAO and EDAR/EDAC messages.

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

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

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

4.2.  Network Access and Addressing

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 join protocol provides the following functionality:

   o  Mutual authentication

   o  Authorization

   o  Parameter distribution to the pledge over a secure channel

   Minimal Security Framework for 6TiSCH
   [I-D.ietf-6tisch-minimal-security] defines the minimal mechanisms



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

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

   Figure 5 depicts the join process.























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  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 LL @)|               |              |              |
     |-------------->|               |              |              |
     |    NA (EARO)  |               |              |              |
     |<--------------|               |              |              |
     |               |               |              |              |
     |  (Zero-touch  |               |              |              |
     |   handshake)  |     (Zero-touch handshake)   | (Zero-touch  |
     | Link Local @  |        Global Unicast @      |  handshake)  |
     |<------------->|<---------------------------->|<------------>|
     |               |               |              |              |
     | CoJP Join Req |               |              |              | \
     | Link Local @  |               |              |              | |
     |-------------->|               |              |              | |
     |               |       CoJP Join Request      |              | |
     |               |       Global Unicast @       |              | |
     |               |----------------------------->|              | | C
     |               |               |              |              | | o
     |               |       CoJP Join Response     |              | | J
     |               |       Global Unicast @       |              | | P
     |               |<-----------------------------|              | |
     |CoJP Join Resp |               |              |              | |
     | Link Local @  |               |              |              | |
     |<--------------|               |              |              | /
     |               |               |              |              |

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

4.2.2.  Registration

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



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   Figure 6 illustrates the initial IPv6 signaling that enables a 6LN to
   form a global address and register it to a 6LBR using 6LoWPAN ND
   [RFC8505], is then carried over RPL to the RPL root, and then to the
   6BBR.


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

        Figure 6: Initial Registration Flow over Multi-Link Subnet

   Figure 7 illustrates the repeating IPv6 signaling that enables a 6LN
   to keep a global address alive and registered to its 6LBR using
   6LoWPAN ND [RFC8505], using 6LoWPAN ND ot the 6LR, RPL to the RPL
   root, and then 6LoWPAN ND again to the 6BBR.








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



          Figure 7: Next Registration Flow over Multi-Link Subnet

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

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



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

   6top is a logical link control sitting between the IP layer and the
   TSCH MAC layer, which provides the link abstraction that is required
   for IP operations.  The 6top protocol, 6P, which is specified in
   [RFC8480], is one of the services provided by 6top.  In particular,
   the 6top services are available over a management API that enables an
   external management entity to schedule cells and slotFrames, and
   allows the addition of complementary functionality, for instance a
   Scheduling Function that manages a dynamic schedule management based
   on observed resource usage as discussed in Section 4.5.2.

4.3.1.1.  Hard Cells

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

4.3.1.2.  Soft Cells

   6top contains a monitoring process which monitors the performance of
   cells, and can move a cell in the TSCH schedule when it performs
   poorly.  This is only applicable to cells which are marked as "soft".
   To reserve a soft cell, the higher layer does not indicate the exact
   slotOffset/channelOffset of the cell to add, but rather the resulting
   bandwidth and QoS requirements.  When the monitoring process triggers
   a cell reallocation, the two neighbor devices communicating over this
   cell negotiate its new position in the TSCH schedule.

4.3.2.  Scheduling Functions and the 6top protocol

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

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




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


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


                       Figure 8: SF/6P stack in 6top

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

   MSF [I-D.ietf-6tisch-msf] is one of the possible scheduling
   functions.  MSF uses the rendez-vous slot from [RFC8180] for network
   discovery, neighbor discovery, and any other broadcast.

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

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






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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 ETX.  6top
   creates and maintains an abstract neighbor table, and this state may
   be leveraged to feed an OF and/or store OF information as well.  A
   neighbor table entry may contain a set of statistics with respect to
   that specific neighbor.

   The neighbor infoirmation may include the time when the last packet
   has been received from that neighbor, a set of cell quality metrics
   (e.g.  RSSI or 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 behaviour, for instance by configuring the
   periodicity of IEEE Std 802.15.4 Extended Beacons (EBs).  By
   augmenting the EB periodicity, it is possible to change the network
   dynamics so as to improve the support of devices that may change
   their point of attachment in the 6TiSCH network.

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

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.  In order to
   maximize battery life and network throughput, it is advisable that
   RPL ICMP discovery and maintenance traffic (governed by the trickle
   timer) be somehow coordinated with the transmission of time
   synchronization packets (especially with enhanced beacons).



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

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

   A root is configured or obtains by some external means the knowledge
   of the RPLInstanceID for the TSGI.  The root advertises its DagRank
   in the TSGI, that must be less than 0xFF, as its Join Priority in its
   IEEE Std 802.15.4 Extended Beacons (EB).  We'll note that the Join
   Priority is now specified between 0 and 0x3F leaving 2 bits in the
   octet unused in the IEEE Std 802.15.4e specification.  After
   consultation with IEEE authors, it was asserted that 6TiSCH can make
   a full use of the octet to carry an integer value up to 0xFF.

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






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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.  In order to ensure that the medium is free of
   contending packets when time comes for a scheduled transmission, a
   window of time is defined around the scheduled transmission where the
   medium must, as much as practically feasible, be free of contending
   energy.  One simple way to obtain such a window is to format time and
   frequencies in cells of transmission of equal duration.  This is the
   method that is adopted in IEEE Std 802.15.4 TSCH as well as the Long
   Term Evolution (LTE) of cellular networks.

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

   A CDU matrix is defined centrally as part of the network definition.
   It is a matrix of cells with an height equal to the number of
   available channels (indexed by ChannelOffsets) and a width (in
   timeslots) that is the period of the network scheduling operation
   (indexed by slotOffsets) for that CDU matrix.  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 4.5).

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

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

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



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

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

4.3.6.  Distributing the reservation of cells

   The 6TiSCH architecture introduces the concept of chunks
   (Section 2.1) to distribute the allocation of the spectrum for a
   whole group of cells at a time.  The CDU matrix is formatted into a
   set of chunks, possibly as illustrated in Figure 9, each of the
   chunks identified uniquely by a chunk-ID.  The knowledge of this
   formatting is shared between all the nodes in a 6TiSCH network.


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


                Figure 9: CDU matrix Partitioning in Chunks

   The 6TiSCH Architecture expects that a future protocol will enable a
   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.




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   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, in new bundles, or
   allocated opportunistically for one transmission.

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

4.4.  Communication Paradigms and Interaction Models

   Section 2.1 provides the terms of Communication Paradigms and
   Interaction Models, which can be placed in parallel to the
   Information Models and Data Models that are defined in [RFC3444].

   A Communication Paradigms would be an abstract view of a protocol
   exchange, and would come with an Information Model for the
   information that is being exchanged.  In contrast, an Interaction
   Models would be more refined and could point on standard operation
   such as a Representational state transfer (REST) "GET" operation and
   would match a Data Model for the data that is provided over the
   protocol exchange.

   Section 2.1.3 of [I-D.ietf-roll-rpl-industrial-applicability] and
   next sections discuss application-layer paradigms, such as Source-
   sink (SS) that is a Multipeer to Multipeer (MP2MP) model primarily
   used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that
   is typically used for sensor data, as well as Peer-to-peer (P2P) and
   Peer-to-multipeer (P2MP) communications.  Additional considerations
   on Duocast and its N-cast generalization are also provided.  Those
   paradigms are frequently used in industrial automation, which is a
   major use case for IEEE Std 802.15.4 TSCH wireless networks with
   [ISA100.11a] and [WirelessHART], that provides a wireless access to
   [HART] applications and devices.

   This specification focuses on Communication Paradigms and Interaction
   Models for packet forwarding and TSCH resources (cells) management.
   Management mechanisms for the TSCH schedule at Link-Layer (one-hop),
   Network-layer (multithop along a Track), and Application-layer
   (remote control) are discussed in Section 4.5.  Link-Layer frame
   forwarding interactions are discussed in Section 4.7, and Network-
   layer Packet routing is addressed in Section 4.8.



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4.5.  Schedule Management Mechanisms

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

4.5.1.  Static Scheduling

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

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

4.5.2.  Neighbor-to-neighbor Scheduling

   In the simplest instantiation of a 6TiSCH network described in
   Section 4.5.1, nodes may expect a packet at any cell in the schedule
   and will waste energy idle listening.  In a more complex
   instantiation of a 6TiSCH network, a matching portion of the schedule
   is established between peers to reflect the observed amount of
   transmissions between those nodes.  The aggregation of the cells
   between a node and a peer forms a bundle that the 6top layer uses to
   implement the abstraction of a link for IP.  The bandwidth on that
   link is proportional to the number of cells in the bundle.

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

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



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

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

4.5.3.  Remote Monitoring and Schedule Management

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

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

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

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

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




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                     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 10: SDN Layers and Architecture Terminology per RFC 7426

   The PCE establishes end-to-end Tracks of hard cells, which are
   described in more details in Section 4.7.1.






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

   This model fits the 6TiSCH extended configuration, whereby a 6BBR
   federates multiple 6TiSCH LLN in a single subnet over a backbone that
   can be, for instance, Ethernet or Wi-Fi.  In that model, 6TiSCH 6BBRs
   synchronize with one another over the backbone, so as to ensure that
   the multiple LLNs that form the IPv6 subnet stay tightly
   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 and to end across the TSCH network and an IEEE
   Std 802.1 TSN Ethernet backbone, and that of DetNet to enable end-to-
   end deterministic forwarding.

4.5.4.  Hop-by-hop Scheduling

   A node can reserve a Track (Section 4.6) to one or more
   destination(s) that are multiple hops away by installing soft cells
   at each intermediate node.  This forms a Track of soft cells.  A
   Track Scheduling Function above the 6top sublayer of each node on the
   Track is needed to monitor these soft cells and trigger relocation
   when needed.

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

4.6.  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(s)
   6TiSCH node(s) across a 6TiSCH LLN.

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






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4.6.1.  General Behavior of Tracks

   A Track is associated with Layer-2 bundles of cells with related
   schedules and logical relationships and that ensure that a packet
   that is injected in a Track will progress in due time all the way to
   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 4.7.1, is a
       Layer-2 forwarding scheme, which introduces less process delay
       and overhead than Layer-3 forwarding scheme.  Therefore, LLN
       Devices can save more energy and resource, which is critical for
       resource constrained devices.

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

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

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

4.6.2.  Serial Track

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




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   For a given iteration of the device schedule, the effective channel
   of the cell is obtained by adding a pseudo-random number to the
   channelOffset of the cell, which results in a rotation of the
   frequency that used for transmission.  The bundles may be computed so
   as to accommodate both variable rates and retransmissions, so they
   might not be fully used in the iteration of the schedule.

4.6.3.  Complex Track with Replication and Elimination

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

   A Complex Track may branch off over non congruent branches for the
   purpose of multicasting, and/or redundancy, in which case it
   reconverges later down the path.  This enables the DetNet Packet
   Replication, Elimination and Ordering Functions (PREOF).  PRE may be
   used to complement Layer-2 ARQ to meet industrial expectations in
   Packet Delivery Ratio (PDR), in particular when the Track extends
   beyond the 6TiSCH network in a larger DetNet network.

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

4.6.4.  DetNet End-to-end Path

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



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   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 11: Example End-to-End DetNet Track

4.6.5.  Cell Reuse

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

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

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



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   and if the frame transports an IPv6 packet, then it can be placed for
   transmission in the bundle that is used for Layer-3 traffic towards
   the next hop along the Track.  The MAC address should be set to the
   next-hop MAC address to avoid confusion.

   It results in a frame that is received over a Layer-3 bundle may be
   in fact associated to a Track.  In a classical IP link such as an
   Ethernet, off-Track traffic is typically in excess over reservation
   to be routed along the non-reserved path based on its QoS setting.
   But with 6TiSCH, since the use of the Layer-3 bundle may be due to
   transmission failures, it makes sense for the receiver to recognize a
   frame that should be re-Tracked, and to place it back on the
   appropriate bundle if possible.  A frame should be re-Tracked if the
   Per-Hop-Behavior group indicated in the Differentiated Services Field
   of the IPv6 header is set to Deterministic Forwarding, as discussed
   in Section 4.8.1.  A frame is re-Tracked by scheduling it for
   transmission over the transmit bundle associated to the Track, with
   the destination MAC address set to broadcast.

4.7.  Forwarding Models

   By forwarding, this specification means the per-packet operation that
   allows to deliver a packet to a next hop or an upper layer in this
   node.  Forwarding is based on pre-existing state that was installed
   as a result of a routing computation Section 4.8.  6TiSCH supports
   three different forwarding model, G-MPLS Track Forwarding, 6LoWPAN
   Fragment Forwarding and classical IPv6 Forwarding.

4.7.1.  Track Forwarding

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

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



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

4.7.1.1.  Transport Mode

   In transport mode, the Protocol Data Unit (PDU) is associated with
   flow-dependant meta-data that refers uniquely to the Track, so the
   6top sublayer can place the frame in the appropriate cell without
   ambiguity.  In the case of IPv6 traffic, this flow identification is
   transported in the Flow Label of the IPv6 header.  Associated with
   the source IPv6 address, the Flow Label forms a globally unique
   identifier for that particular Track that is validated at egress
   before restoring the destination MAC address (DMAC) and punting to
   the upper layer.

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


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


                Figure 12: Track Forwarding, Transport Mode

4.7.1.2.  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 to be transported
   transparently over the Track.




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


                 Figure 13: Track Forwarding, Tunnel Mode

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

   At the egress 6TiSCH router, the reverse operation occurs.  Based on
   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.

4.7.1.3.  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.





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   In transport mode, if the final Layer-3 destination is the tunnel
   termination, then it is possible that the IPv6 address of the
   destination is compressed at the 6LoWPAN sublayer based on the MAC
   address.  It is thus mandatory at the ingress point to validate that
   the MAC address that was used at the 6LoWPAN sublayer for compression
   matches that of the tunnel egress point.  For that reason, the node
   that injects a packet on a Track checks that the destination is
   effectively that of the tunnel egress point before it overwrites it
   to broadcast.  The 6top sublayer at the tunnel egress point reverts
   that operation to the MAC address obtained from the tunnel
   information.

4.7.2.  IPv6 Forwarding

   As the packets are routed at Layer-3, traditional QoS and Active
   Queue Management (AQM) operations are expected to prioritize flows;
   the application of Differentiated Services is further discussed in
   [I-D.svshah-tsvwg-lln-diffserv-recommendations].


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


                         Figure 14: IP Forwarding

4.7.3.  Fragment Forwarding

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





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

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

   [I-D.ietf-6lo-minimal-fragment] describes a framework for forwarding
   fragments end-to-end across a 6TiSCH route-over mesh.  Within that
   framework, [I-D.ietf-lwig-6lowpan-virtual-reassembly] details a
   virtual reassembly buffer mechanism whereby the datagram tag in the
   6LoWPAN Fragment is used as a label for switching at the 6LoWPAN
   sublayer.  Building on this technique,
   [I-D.ietf-6lo-fragment-recovery] introduces a new format for 6LoWPAN
   fragments that enables the selective recovery of individual
   fragments, and allows for a degree of flow control based on an
   Explicit Congestion Notification.


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


                   Figure 15: Forwarding First Fragment

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





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


                    Figure 16: Forwarding Next Fragment

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

4.8.  Advanced 6TiSCH Routing

4.8.1.  Packet Marking and Handling

   All packets inside a 6TiSCH domain must carry the Instance ID that
   identifies the 6TiSCH topology that is to be used for routing and
   forwarding that packet.  The location of that information must be the
   same for all packets forwarded inside the domain.

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

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

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




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   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.

4.8.2.  Replication, Retries and Elimination

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

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

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

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






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

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

              Figure 17: Scheduling PREOF on a Simple Network

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

   Say we have 6 possible channels, and at least 10 timeslots per
   slotframe.  Figure 18 shows a possible schedule whereby each
   transmission is retried 2 or 3 times, and redundant copies are
   forwarded in parallel via A and C on the one hand, and B and D on the
   other, providing time diversity, spatial diversity though different
   physical paths, and frequency diversity.


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


                    Figure 18: Example Global Schedule




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


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


                  Figure 19: Example Slotframe for Node A

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


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


4.8.3.  Differentiated Services Per-Hop-Behavior

   Additionally, an IP packet that is sent along a Track uses the
   Differentiated Services Per-Hop-Behavior Group called Deterministic
   Forwarding, as described in
   [I-D.svshah-tsvwg-deterministic-forwarding].

5.  IANA Considerations

   This specification does not require IANA action.

6.  Security Considerations

   This architecture operates on IEEE Std 802.15.4 and expects Link-
   Layer security to be enabled at all times between connected devices,
   except for the very first step of the device join process, where a




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   joining device may need some initial, unsecured exchanges so as to
   obtain its initial key material.

   As detailed in Section 4.2.1, 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 [I-D.ietf-6tisch-minimal-security],
   the authentication requires a pre-shared key, based on which a secure
   session is derived.  The CoJP exchange may also be preceded with a
   zero-touch handshake [I-D.ietf-6tisch-dtsecurity-zerotouch-join] in
   order to enable pledge joining based on certificates and/or inter-
   domain communication.

   In order to join, the pledge is helped by a Join Proxy (JP) that
   relays the link-scope Join Request over the IP network to the Join
   Registrar/Coordinator (JRC) that can authenticate the pledge and
   validate that it is attached to the appropriate network.  As a result
   of this exchange the pledge is in possession of a Link-Layer material
   including a key and a short address, and all traffic is secured at
   the Link-Layer.

7.  Acknowledgments

7.1.  Contributors

   The co-authors of this document are listed below:

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

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

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

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

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

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

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



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

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

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

7.2.  Special Thanks

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

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

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

7.3.  And Do not Forget

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

   The authors wish to thank: Alaeddine Weslati, Chonggang Wang,
   Georgios Exarchakos, Zhuo Chen, Alfredo Grieco, Bert Greevenbosch,
   Cedric Adjih, Deji Chen, Martin Turon, Dominique Barthel, Elvis
   Vogli, Geraldine Texier, Malisa Vucinic, Guillaume Gaillard, Herman
   Storey, Kazushi Muraoka, Ken Bannister, Kuor Hsin Chang, Laurent
   Toutain, Maik Seewald, Maria Rita Palattella, Michael Behringer,
   Nancy Cam Winget, Nicola Accettura, Nicolas Montavont, Oleg Hahm,
   Patrick Wetterwald, Paul Duffy, Peter van der Stock, Rahul Sen,
   Pieter de Mil, Pouria Zand, Rouhollah Nabati, Rafa Marin-Lopez,
   Raghuram Sudhaakar, Sedat Gormus, Shitanshu Shah, Steve Simlo,



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   Tengfei Chang, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles
   and Samita Chakrabarti for their participation and various
   contributions.

8.  References

8.1.  Normative References

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-11 (work in progress), February 2019.

   [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>.

   [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>.

   [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>.








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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.




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

8.2.  Informative References

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

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

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

   [I-D.ietf-6lo-ap-nd]
              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", draft-ietf-6lo-ap-nd-11 (work in
              progress), February 2019.

   [I-D.ietf-6lo-backbone-router]
              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", draft-ietf-6lo-backbone-router-11 (work
              in progress), February 2019.

   [I-D.ietf-6lo-fragment-recovery]
              Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
              ietf-6lo-fragment-recovery-02 (work in progress), January
              2019.

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

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







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   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-09 (work in progress), November
              2018.

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

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-18 (work in progress), January 2019.

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

   [I-D.ietf-detnet-use-cases]
              Grossman, E., "Deterministic Networking Use Cases", draft-
              ietf-detnet-use-cases-20 (work in progress), December
              2018.

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

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

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







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

   [I-D.ietf-roll-useofrplinfo]
              Robles, I., Richardson, M., and P. Thubert, "Using RPL
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", draft-ietf-
              roll-useofrplinfo-24 (work in progress), January 2019.

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

   [I-D.svshah-tsvwg-lln-diffserv-recommendations]
              Shah, S. and P. Thubert, "Differentiated Service Class
              Recommendations for LLN Traffic", draft-svshah-tsvwg-lln-
              diffserv-recommendations-04 (work in progress), February
              2015.

   [I-D.thubert-6lo-bier-dispatch]
              Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
              6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-06
              (work in progress), January 2019.

   [I-D.thubert-6lo-unicast-lookup]
              Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery
              Unicast Lookup", draft-thubert-6lo-unicast-lookup-00 (work
              in progress), January 2019.

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

   [I-D.thubert-roll-unaware-leaves]
              Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
              unaware-leaves-06 (work in progress), November 2018.









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   [IEC62439]
              IEC, "Industrial communication networks - High
              availability automation networks - Part 3: Parallel
              Redundancy Protocol (PRP) and High-availability Seamless
              Redundancy (HSR) - IEC62439-3", 2012,
              <https://webstore.iec.ch/publication/7018>.

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE Std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks".

   [IEEE802154e]
              IEEE standard for Information Technology, "IEEE standard
              for Information Technology, IEEE Std.  802.15.4, Part.
              15.4: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Low-Rate Wireless Personal
              Area Networks, June 2011 as amended by IEEE Std.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April
              2012.

   [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", 2011, <http://www.isa.org/Community/
              SP100WirelessSystemsforAutomation>.

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

   [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>.






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   [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>.

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

   [RFC5974]  Manner, J., Karagiannis, G., and A. McDonald, "NSIS
              Signaling Layer Protocol (NSLP) for Quality-of-Service
              Signaling", RFC 5974, DOI 10.17487/RFC5974, October 2010,
              <https://www.rfc-editor.org/info/rfc5974>.





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   [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>.

   [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>.

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <https://www.rfc-editor.org/info/rfc7426>.

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

   [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>.

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





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

Appendix A.  Dependencies on Work In Progress

   In order to control the complexity and the size of the 6TiSCH work,
   the architecture and the associated IETF work are staged and the WG
   is expected to recharter multiple times.  This document is been
   incremented as the work progressed following the evolution of the WG
   charter and the availability of dependent work.  The intent was to
   publish when the WG concludes on the covered items.

   At the time of publishing:

   o  The need of a reactive routing protocol to establish on-demand
      constraint-optimized routes and a reservation protocol to
      establish Layer-3 Tracks is being discussed at 6TiSCH but not
      chartered for.

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

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

   o  BIER-TE-based OAM, Replication and Elimination
      [I-D.thubert-bier-replication-elimination] leverages Bit Index
      Explicit Replication - Traffic Engineering to control in the data
      plane the DetNet Replication and Elimination activities, and to
      provide traceability on links where replication and loss happen,
      in a manner that is abstract to the forwarding information,



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

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

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

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

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

Author's Address

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

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






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