6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                             T. Watteyne
Expires: December 18, 2014                             Linear Technology
                                                            RA. Assimiti
                                                           June 16, 2014

     An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4e


   This document presents an architecture for an IPv6 Multi-Link subnet
   that is composed of a high speed powered backbone and a number of
   IEEE802.15.4e TSCH wireless networks attached and synchronized by
   Backbone Routers.  The TSCH schedule can be static or dynamic.
   6TiSCH defines mechanisms to establish and maintain the routing and
   scheduling operations in a centralized, distributed, or mixed
   fashion.  Backbone Routers perform proxy Neighbor Discovery
   operations over the backbone on behalf of the wireless devices, so
   they can share a same subnet and appear to be connected to the same
   backbone as classical devices

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 18, 2014.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Applications and Goals  . . . . . . . . . . . . . . . . . . .   4
   4.  Overview and Scope  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Communication Paradigms and Interaction Models  . . . . . . .   8
   6.  Forwarding Models . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Track Forwarding  . . . . . . . . . . . . . . . . . . . .   9
       6.1.1.  Transport Mode  . . . . . . . . . . . . . . . . . . .  10
       6.1.2.  Tunnel Mode . . . . . . . . . . . . . . . . . . . . .  10
       6.1.3.  Tunnel Metadata . . . . . . . . . . . . . . . . . . .  11
     6.2.  Fragment Forwarding . . . . . . . . . . . . . . . . . . .  12
     6.3.  IPv6 Forwarding . . . . . . . . . . . . . . . . . . . . .  13
   7.  TSCH and 6top . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  6top  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  6top and RPL Objective Function operations  . . . . . . .  15
     7.3.  Network Synchronization . . . . . . . . . . . . . . . . .  16
     7.4.  SlotFrames and Priorities . . . . . . . . . . . . . . . .  17
     7.5.  Distributing the reservation of cells . . . . . . . . . .  18
   8.  Schedule Management Mechanisms  . . . . . . . . . . . . . . .  20
     8.1.  Minimal Static Scheduling . . . . . . . . . . . . . . . .  20
     8.2.  Neighbor-to-neighbor Scheduling . . . . . . . . . . . . .  20
     8.3.  Remote Monitoring and Schedule Management . . . . . . . .  21
     8.4.  Hop-by-hop Scheduling . . . . . . . . . . . . . . . . . .  22
   9.  Centralized vs. Distributed Routing . . . . . . . . . . . . .  22
     9.1.  Packet Marking and Handling . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  23
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  24

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     14.2.  Informative References . . . . . . . . . . . . . . . . .  26
     14.3.  External Informative References  . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   The emergence of radio technology enabled a large variety of new
   types of devices to be interconnected, at a very low marginal cost
   compared to wire, at any range from Near Field to interplanetary
   distances, and in circumstances where wiring would be less than
   practical, for instance rotating devices.

   At the same time, a new breed of Time Sensitive Networks is being
   developed to enable traffic that is highly sensitive to jitter and
   quite sensitive to latency.  Such traffic is not limited to voice and
   video, but also includes command and control operations such as found
   in industrial automation or in-vehicle sensors and actuators.

   At IEEE802.1, the "Audio/Video Task Group", was renamed TSN for Time
   Sensitive Networking to address Deterministic Ethernet.  The
   IEEE802.15.4 Medium access Control (MAC) has evolved with
   IEEE802.15.4e that provides in particular the timeSlotted Channel
   Hopping (TSCH) mode for industrial-type applications.

   Though at a different time scale, both standards provide
   Deterministic capabilities to the point that a packet that pertains
   to a certain flow crosses the network from node to node following a
   very precise schedule, as a train that leaves intermediate stations
   at precise times along its path.  With TSCH, time is formatted into
   timeSlots, and an individual cell is 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 external interference (for example by WiFi

   This document presents an architecture for an IPv6 Multi-Link subnet
   that is composed of a high speed powered backbone and a number of
   IEEE802.15.4e TSCH wireless networks attached and synchronized by
   backbone routers.  Route Computation may be achieved in a centralized
   fashion by a Path Computation Element (PCE), in a distributed fashion
   using the Routing Protocol for Low Power and Lossy Networks (RPL), or
   in a mixed mode.  The Backbone Routers perform proxy IPv6 neighbor
   Discovery (ND) operations over the backbone on behalf of the wireless
   devices, so they can share a same IPv6 subnet and appear to be
   connected to the same backbone as classical devices. timeSlots and
   other device resources are managed by an abstract Network Management

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   Entity (NME) that may cooperate with the PCE in order to minimize the
   interaction with and the load on the constrained device.

2.  Terminology

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in "neighbor Discovery for IP version 6"
   [RFC4861], "IPv6 over Low-Power Wireless Personal Area Networks
   (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals"
   [RFC4919], neighbor Discovery Optimization for Low-power and Lossy
   Networks [RFC6775] and "Multi-link Subnet Support in IPv6"

   Readers may benefit from reading the "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550] specification; "Multi-Link
   Subnet Issues" [RFC4903]; "Mobility Support in IPv6" [RFC6275];
   "neighbor Discovery Proxies (ND Proxy)" [RFC4389]; "IPv6 Stateless
   Address Autoconfiguration" [RFC4862]; "FCFS SAVI: First-Come, First-
   Served Source Address Validation Improvement for Locally Assigned
   IPv6 Addresses" [RFC6620]; and "Optimistic Duplicate Address
   Detection" [RFC4429] prior to this specification for a clear
   understanding of the art in ND-proxying and binding.

   The draft uses terminology defined or referenced in
   [I-D.ietf-roll-rpl-industrial-applicability], [RFC5191] and

   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.

3.  Applications and Goals

   The architecture derives from existing industrial standards for
   Process Control by its focus on Deterministic Networking, in
   particular with the use of the IEEE802.15.4e TSCH MAC [IEEE802154e]
   and the centralized PCE.  This approach leverages the TSCH MAC
   benefits for high reliability against interference, low-power
   consumption on deterministic traffic, and its Traffic Engineering
   capabilities.  Deterministic Networking applies in particular to open
   and closed control loops, as well as supervisory control flows and

   An incremental set of industrial requirements are addressed with the
   addition of an autonomic and distributed routing operation based on
   RPL.  These use cases include plant setup and decommissioning, as

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   well as monitoring of lots of lesser importance measurements such as
   corrosion and events.  RPL also enables mobile use cases such as
   mobile workers and cranes.

   A Backbone Router is included in order to scale the factory plant
   subnet to address large deployments, with proxy ND and time
   synchronization over a high speed backbone.

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

4.  Overview and Scope

   The scope of the present work is a subnet that, in its basic
   configuration, is made of a IEEE802.15.4e timeSlotted Channel Hopping
   (TSCH) [I-D.ietf-6tisch-tsch] MAC Low Power Lossy Network (LLN).

               ---+-------- ............ ------------
                  |      External Network       |
                  |                          +-----+
               +-----+                       | NME |
               |     | LLN Border            |     |
               |     | router                +-----+
             o    o   o
      o     o   o     o
         o   o LLN   o    o     o
            o   o   o       o

                       Figure 1: Basic Configuration

   The LLN devices communicate over IPv6 [RFC2460] using the 6LoWPAN
   Header Compression (6LoWPAN HC) [RFC6282].  From the perspective of
   Layer 3, a single LLN interface (typically an IEEE802.15.4-compliant
   radio) may be seen as a collection of Links with different
   capabilities for unicast or multicast services.  An IPv6 subnet spans
   over multiple links, effectively forming a Multi-Link subnet.  Within
   that subnet, neighbor Devices are discovered with 6LoWPAN neighbor
   Discovery (6LoWPAN ND) [RFC6775].  RPL [RFC6550] enables routing
   within the LLN, typically within the Multi-Link subnet in the so
   called Route Over fashion.

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   RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs)
   within Instances of the protocol, each Instance being associated with
   an Objective Function (OF) to form a routing topology.  A particular
   LLN device, the LLN Border Router (LBR), acts as RPL root, 6LoWPAN HC
   terminator, and LLN Border Router (LBR) to the outside.  The LBR is
   usually powered.  More on RPL Instances can be found in RPL
   [RFC6550], sections "3.1.2.  RPL Identifiers" and "3.1.3.  Instances,
   DODAGs, and DODAG Versions".

   An extended configuration of the subnet comprises multiple LLNs.  The
   LLNs are interconnected and synchronized over a backbone, that can be
   wired or wireless.  The backbone can be a classical IPv6 network,
   with neighbor Discovery operating as defined in [RFC4861] and
   [RFC4862].  The backbone can also support Efficiency-aware IPv6
   neighbor Discovery Optimizations
   [I-D.chakrabarti-nordmark-6man-efficient-nd] in mixed mode as
   described in [I-D.thubert-6lowpan-backbone-router].

   Security is often handled at layer 2 and Layer 4.  Authentication
   during the join process can be handled by the Protocol for Carrying
   Authentication for Network access (PANA) [RFC5191].

   The LLN devices are time-synchronized at the MAC level.  The LBR that
   serves as time source is a RPL parent in a particular RPL Instance
   that serves for time synchronization; this way, the time
   synchronization starts at the RPL root and follows the RPL DODAGs
   with no timing loop.

   In the extended configuration, the functionality of the LBR is
   enhanced to that of Backbone Router (BBR).  A BBR is an LBR, but also
   an Energy Aware Default Router (NEAR) as defined in
   [I-D.chakrabarti-nordmark-6man-efficient-nd].  The BBR performs ND
   proxy operations between the registered devices and the classical ND
   devices that are located over the backbone.  6TiSCH BBRs 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 (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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                  ---+-------- ............ ------------
                     |      External Network       |
                     |                          +-----+
                     |             +-----+      | NME |
                  +-----+          |  +-----+   |     |
                  |     | Router   |  | PCE |   +-----+
                  |     |          +--|     |
                  +-----+             +-----+
                     |                   |
                     | Subnet Backbone   |
               |                    |                  |
            +-----+             +-----+             +-----+
            |     | Backbone    |     | Backbone    |     | Backbone
       o    |     | router      |     | router      |     | router
            +-----+             +-----+             +-----+
       o                  o                   o                 o   o
           o    o   o         o   o  o   o         o  o   o    o
      o             o        o  LLN      o      o         o      o
         o   o    o      o      o o     o  o   o    o    o     o

                     Figure 2: Extended Configuration

   The main architectural blocks are arranged as follows:

      |PCEP |   CoAP    |PANA |6LoWPAN| RPL |
      | PCE |DTLS |     |     |   ND  |     |
      | TCP |       UDP       |    ICMP     |RSVP |
      |                 IPv6                      |
      |               6LoWPAN HC                  |
      |                   6top                    |
      |             IEEE802.15.4e   TSCH          |

                          Figure 3: 6TiSCH stack

   RPL is the routing protocol of choice for LLNs.  (TBD RPL) whether
   there is a need to define a 6TiSCH OF.

   (tbd NME) COMAN is working on network Management for LLN.  They are
   considering the Open Mobile Alliance (OMA) Lightweight M2M (LWM2M)

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   Object system.  This standard includes DTLS, CoAP (core plus Block
   and Observe patterns), SenML and CoAP Resource Directory.

   (tbd PCE) need to work with PCE WG to define flows to PCE, and define
   how to accommodate PCE routes and reservation.  Will probably look a
   lot like GMPLS.

   (tbd PANA) There is a debate whether PANA (layer 3) or IEEE802.1x
   (layer 2) should be used in the join process.  There is also a debate
   whether the node should be able to send any unprotected packet on the
   medium.  Regardless, the security model must ensure that, prior to a
   join process, packets from a untrusted device must be controlled in
   volume and in reachability.

   (tbd Backbone Router) need to work with 6MAN to define ND proxy.
   Also need BBR sync sync between deterministic Ethernet and 6TiSCH

   IEEE802.1TSN: external, maintain consistency.  See also AVnu.

   IEEE802.15.4: external, (tbd need updates?).

   ISA100.20 Common Network Management: external, maintain consistency.

   The 6TiSCH Operation sublayer (6top) [I-D.wang-6tisch-6top-sublayer]
   is an Logical Link Control (LLC) or a portion thereof that provides
   the abstraction of an IP link over a TSCH MAC.

5.  Communication Paradigms and Interaction Models

   [I-D.ietf-6tisch-terminology] defines 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.

   [I-D.ietf-roll-rpl-industrial-applicability] section 2.1.3. and next
   discusses appplication-layer paradigms, such as Source-sink (SS) that
   is a Multipeer to Multipeer (MP2MP) model that is 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

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   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 IEEE802.15.4e 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.
   L ink-layer and Network-layer Packet forwarding interactions are
   discussed in Section 6, whereas Link-layer (one-hop), Network-layer
   (multithop along a track), and Application-layer (remote control)
   management mechanisms for the TSCH schedule are discussed in
   Section 8.

6.  Forwarding Models

   6TiSCH supports three different forwarding model, G-MPLS Track
   Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding

6.1.  Track Forwarding

   Track Forwarding is the simplest and fastest.  A bundle of cells set
   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 that can be used regardless of the network layer protocol.
   This model can effectively 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

   A Track is defined end-to-end as a succession of paired bundles.  A
   cell in such a bundle belongs to at most one Track but it may be
   reused opportunistically on a per-hop basis for routed packets.  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.

   A data frame that is forwarded along a Track has a destination MAC
   address 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

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   the MAC header.  In the case of IEEE802.15.4e, this means effectively
   broadcast, so that along the Track the short address for the
   destination is set to 0xFFFF.

   Conversely, a frame that is received along a Track with a destination
   MAC address set to this node is extracted from the Track stream and
   delivered to the upper layer.  A frame with an unrecognized MAC
   address is dropped at the MAC layer and thus is not received at the
   6top sublayer.

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

6.1.1.  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.

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

                     Track Forwarding, Transport Mode

6.1.2.  Tunnel Mode

   In tunnel mode, the frames originate from an arbitrary protocol over
   a compatible MAC that may or may not be synchronized with the 6TiSCH
   network.  An example of this would be a router with a dual radio that
   is capable of receiving and sending WirelessHART or ISA100.11a frames
   with the second radio, by presenting itself as an access Point or a
   Backbone Router, respectively.

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

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

                  Figure 4: 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 tunnelled over a particular Track so the frame is not passed to
   the upper layer.  Instead, the dmac is forced to broadcast and the
   frame is passed to the 6top sublayer for switching.

   At the egress 6TiSCH router, the reverse operation occurs.  Based on
   metadata associated to the Track, the frame is passed to the
   appropriate link layer with the destination MAC restored.

6.1.3.  Tunnel Metadata

   Metadata coming with the Track configuration is expected to provide
   the destination MAC address of the egress endpoint as well as the
   tunnel mode and specific data depending on the mode, for instance a
   service access point for frame delivery at egress.  If the tunnel
   egress point does not have a MAC address that matches the
   configuration, the Track installation fails.

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

6.2.  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
   IEEE802.15.4 frame with security may carry in the order of 80 bytes
   of effective payload, an IPv6 packet might be fragmented into more
   than 16 fragments at the 6LoWPAN sublayer.

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

   In the case to a multihop route within a 6TiSCH network, Hop-by-Hop
   recomposition occurs at each hop in order to reform the packet and
   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.thubert-roll-forwarding-frags] describes a mechanism whereby the
   datagram tag in the 6LoWPAN Fragment is used as a label for switching
   at the 6LoWPAN sublayer.  The draft allows for a degree of flow
   control base on an Explicit Congestion Notification, as well as end-
   to-end individual fragment recovery.

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

                    Figure 5: 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.

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

                    Figure 6: Forwarding Next Fragment

   A bitmap and an ECN echo in the end-to-end acknowledgement 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.

6.3.  IPv6 Forwarding

   As the packets are routed at layer 3, traditional QoS and RED
   operations are expected to prioritize flows with differentiated
   services.  A new class of service for Deterministic Forwarding is

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   being defined to that effect in

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

                          Figure 7: IP Forwarding

7.  TSCH and 6top

7.1.  6top

   6top is a logical link control sitting between the IP layer and the
   TSCH MAC layer, which provides the link abstraction that is required
   for IP operations.  The 6top operations are specified in
   [I-D.wang-6tisch-6top-sublayer].  In particular, 6top provides a
   management interface that enables an external management entity to
   schedule cells and slotFrames, and allows the addition of
   complementary functionality, for instance to support a dynamic
   schedule management based on observed resource usage as discussed in
   section Section 8.2.  The 6top data model and management interfaces
   are further discussed in Section 8.3.

   If the scheduling entity explicitly specifies the slotOffset/
   channelOffset of the cells to be added/deleted, those cells are
   marked as "hard". 6top cannot move hard cells in the TSCH schedule.
   Hard cells are for example used by a central PCE.

   6top contains a monitoring process which monitors the performance of
   cells, and can move a cell in the TSCH schedule when it performs bad.
   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 motes communicating over this
   cell negotiate its new position in the TSCH schedule.

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7.2.  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 [I-D.ietf-6tisch-minimal] to support RPL
   over a static schedule, may leverage, for its internal computation,
   the information maintained by 6top.

   In particular, 6top creates and maintains an abstract neighbor table.
   A neighbor table entry contains a set of statistics with respect to
   that specific neighbor including the time when the last packet has
   been received from that neighbor, a set of cell quality metrics
   (RSSI, LQI), the number of packets sent to the neighbor or the number
   of packets received from it.  This information can be obtained
   through 6top management APIs as detailed in the 6top sublayer
   specification [I-D.wang-6tisch-6top-sublayer] and used to compute a
   Rank Increment that will determine the selection of the preferred

   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 IEEE802.15.4e Extended Beacons (EB's).  By augmenting
   the EB periodicity, it is possible to change the network dynamics so
   as to improve the support of devices that may change their point of
   attachment in the 6TiSCH network.

   Some RPL control messages, such as the DODAG Information Object (DIO)
   are ICMPv6 messages that are broadcast to all neighbor nodes.  With
   6TiSCH, the broadcast channel requirement is addressed by 6top by
   configuring TSCH to provide a broadcast channel, as opposed to, for
   instance, piggybacking the DIO messages in Enhance Beacons.

   In the TSCH schedule, each cell has the IEEE802.15.4e LinkType
   attribute.  Setting the LinkType to ADVERTISING indicates that the
   cell MAY be used to send an Enhanced Beacon.  When a node forms its
   Enhanced Beacon, the cell, with LinkType=ADVERTISING, SHOULD be
   included in the FrameAndLinkIE, and its LinkOption field SHOULD be
   set to the combination of "Receive" and "Timekeeping".  The receiver
   of the Enhanced Beacon MAY be listening at the cell to get the
   Enhanced Beacon ([IEEE802154e]).  6top takes this way to establish
   broadcast channel, which not only allows TSCH to broadcast Enhanced
   Beacons, but also allows an upper layer like RPL.

   To broadcast ICMPv6 control messages used by RPL such as DIO or DAO,
   6top uses the payload of a Data frames.  The message is inserted into
   the queue associated with the cells which LinkType is set to
   ADVERTISING.  Then, taking advantage of the broadcast cell feature

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   established with FrameAndLinkIE (as described above), the RPL control
   message can be received by neighbors, which enables the maintenance
   of RPL DODAGs.

   A LinkOption combining "Receive" and "Timekeeping" bits indicates to
   the receivers of the Enhanced Beacon that the cell MUST be used as a
   broadcast cell.  The frequency of sending Enhanced Beacons or other
   broadcast messages by the upper layer is determined by the timers
   associated with the messages.  For example, the transmission of
   Enhance Beacons is triggered by a timer in 6top; transmission of a
   DIO message is triggered by the trickle timer of RPL.

7.3.  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 acknowledgement-based synchronization.  In order to
   maximize battery life and network throughput, it is advisable that
   RPL ICMP discovery and maintenance traffic (governed by the trickle
   timer) be somehow coordinated with the transmission of time
   synchronization packets (especially with enhanced beacons).  This
   could be achieved through an interaction of the 6top sublayer and the
   RPL objective Function, or could be controlled by a management

   Time distribution requires a loop-less 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 RPL [RFC6550]  section "3.1.3.  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.

   A node that has not joined the TSGI advertises a MAC level Join
   Priority of 0xFF to notify its neighbors that is 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 [RFC6550], section "3.5.1.
   Rank Comparison".

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   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 (JP)
   in its IEEE802.15.4e Extended Beacons (EB).  We'll note that the JP
   is now specified between 0 and 0x3F leaving 2 bits in the octet
   unused in the IEEE802.15.4e 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.

7.4.  SlotFrames and Priorities

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

   One simple way to obtain such a window is to format time and
   frequencies in cells of transmission of equal duration.  This is the
   method that is adopted in IEEE802.15.4e 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 a matrix of
   cells with an height equal to the number of available channels
   (indexed by ChannelOffsets), a timeSlot duration (10-15 milliseconds
   are typical in 802.15.4e TSCH) and a width (in timeSlots) that is the
   period of the network scheduling operation (indexed by slotOffsets)
   for that CDU matrix.

   A CDU matrix iterates over and over with a pseudo-random rotation
   from an epoch time.  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 optimize 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.

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

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

   A 6TISCH Instance is associated to one slotFrame.  A slotFrame may be
   shared by multiple Instances of equal relative precedence.  Within an
   Instance, 6top uses priority queues to manage concurrent data flows
   of different priorities within an Instance and between Instances of a
   same precedence, associated to a given IPv6 link and a given bundle
   of TX-cells.  When a packet is received from an higher layer for
   transmission, 6top inserts that packet in the outgoing queue which
   matches the packet best (DSCP 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.

7.5.  Distributing the reservation of cells

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

   Use cases for distributed routing are often associated with a
   statistical distribution of best-effort traffic with variable needs
   for bandwidth on each individual link.  With 6TiSCH, the link
   abstraction is implemented as a bundle of cells; the size of a bundle
   is optimal when both the energy wasted idle listening and the packet
   drops due to congestion loss are minimized.  This can be maintained
   if the number of cells in a bundle is adapted dynamically, and with
   enough reactivity, to match the variations of best-effort traffic.
   In turn, the agility to fulfil the needs for additional cells
   improves when the number of interactions with other devices and the
   protocol latencies are minimized.

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

   The 6TiSCH architecture introduces the concept of chunks
   [I-D.ietf-6tisch-terminology]) to operate such spectrum distribution
   for a whole group of cells at a time.  The CDU matrix is formatted
   into a set of chunks, each of them identified uniquely by a chunk-ID.
   The knowledge of this formatting is shared between all the nodes in a
   6TiSCH network. 6TiSCH also defines the process of chunk ownership
   appropriation whereby a RPL parent discovers a chunk that is not used
   in its interference domain (e.g lack of energy detected in reference
   cells in that chunk); then claims the chunk, and then defends it in
   case another RPL parent would attempt to appropriate it while it is
   in use.  The chunks is the basic unit of ownership that is used in
   that process.

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

                Figure 8: CDU matrix Partitioning in Chunks

   As a result of the process of chunk ownership appropriation, the RPL
   parent has exclusive authority to decide which cell in the
   appropriated chunk can be used by which node in its interference
   domain.  In other words, it is implicitly delegated the right to
   manage the portion of the CDU matrix that is represented by the
   chunk.  The RPL parent may thus orchestrate which transmissions occur
   in any of the cells in the chunk, by allocating cells from the chunk
   to any form of communication (unicast, multicast) in any direction
   between itself and its children.  Initially, those cells are added to
   the heap of free cells, then dynamically placed into existing

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   bundles, in new bundles, or allocated opportunistically for one

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

8.  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.

8.1.  Minimal 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 scheduled can be preconfigured or
   learnt by a node when joining the network.  Regardless, the schedule
   remains unchanged after the node has joined a network.  The Routing
   Protocol for LLNs (RPL) is used on the resulting network.  This
   "minimal" scheduling mechanism that implements this paradigm is
   detailed in [I-D.ietf-6tisch-minimal].

8.2.  Neighbor-to-neighbor Scheduling

   In the simplest instantiation of a 6TiSCH network described in
   Section 8.1, 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 emissions will be destroyed.  If the size is

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   configured to allow for peak emissions, energy is be wasted idle

   In the most efficient instantiation of a 6TiSCH network, the size of
   the bundles that implement the links may be changed dynamically in
   order to adapt to the need of end-to-end flows routed by RPL.  An
   optional On-The-Fly (OTF) component may be used to monitor bandwidth
   usage and perform requests for dynamic allocation by the 6top
   sublayer.  The OTF 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 6top sublayer [I-D.wang-6tisch-6top-sublayer] defines a protocol
   for neighbor nodes to reserve soft cells to one another.  Because
   this reservation is done without global knowledge of the schedule of
   nodes in the LLN, scheduling collisions are possible. 6top defines a
   monitoring process which continuously tracks the packet delivery
   ratio of soft cells.  It uses these statistics to trigger the
   relocation of a soft cell in the schedule, using a negotiation
   protocol between the neighbors nodes communicating over that cell.

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

8.3.  Remote Monitoring and Schedule Management

   The 6top interface document [I-D.ietf-6tisch-6top-interface]
   specifies the generic data model that can be used to monitor and
   manage resources at the 6top sublayer.  Abstract methods are
   suggested for use by a management entity in the device.  The data
   model also enables remote control operations on the 6top sublayer.

   Being able to interact with the 6top sublayer of a node multiple hops
   away can be used for monitoring, scheduling, or a combination of
   both.  The architecture supports variations on the deployment model,
   and focuses on the flows rather than whether there is a proxy or a
   translational operation on the way.

   [I-D.ietf-6tisch-coap] defines an mapping of 6top's set of commands
   described in [I-D.ietf-6tisch-6top-interface] to CoAP resources.
   This allows an entity to interact with the 6top layer of a node that
   is multiple hops away in a RESTful fashion.

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   [I-D.ietf-6tisch-coap] defines a basic set CoAP resources and
   associated RESTful access methods (GET/PUT/POST/DELETE).  The payload
   (body) of the CoAP messages is encoded using the CBOR format.  The
   draft also defines the concept of "profiles" to allow for future or
   specific extensions, as well as a mechanism for a CoAP client to
   discover the profiles installed on a node.

   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).  The architecture allows for different types of interactions
   between this CoAP client and a node in the network:

8.4.  Hop-by-hop Scheduling

   A node can reserve a track to a destination node multiple hops away
   by installing soft cells at each intermediate node.  This forms a
   track of soft cells.  It is the responsibility of the 6top sublayer
   of each node on the track to monitor these soft cells and trigger
   relocation when needed.

   This hop-by-hop reservation mechanism is similar to [RFC2119] and
   [RFC5974].  The protocol for a node to trigger hop-by-hop scheduling
   is not yet defined.

9.  Centralized vs. Distributed Routing

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

   Both methods may inject routes in the Routing Tables of the 6TiSCH
   routers.  In either case, each route is associated with a 6TiSCH
   topology that can be a RPL Instance topology or a track.  The 6TiSCH
   topology is indexed by a Instance ID, in a format that reuses the
   RPLInstanceID as defined in RPL [RFC6550].

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

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9.1.  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 RPL [RFC6550], that information is the
   RPLInstanceID that is carried as part of the RPL Packet Information,
   which is defined in section 11.2 "Loop Avoidance and Detection".

   At the time of this writing, there are 2 methods to transport the RPL
   Packet Information in an IPv6 packet, either in a IPv6 Hop-By-Hop
   Header, or encoded in a compressed fashion in the IPv6 Flow Label.

   The former method places a RPL option [RFC6553] in the IPv6 Hop-By-
   Hop Header.  It MUST be used if at least one RPL Instance uses a
   MinHopRankIncrease that is less than DEFAULT_MIN_HOP_RANK_INCREASE
   (defined to 256 in [RFC6550]), which bars the capability to compress
   the SenderRank in the RPL Packet Information to a single octet.  If
   that is not the case, it is RECOMMENDED to use the latter method of
   encoding the RPL Packet Information in the Flow Label, which is
   specified in [I-D.thubert-6man-flow-label-for-rpl].

   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.

10.  IANA Considerations

   This specification does not require IANA action.

11.  Security Considerations

   This specification is not found to introduce new security threat.

12.  Contributors

   The editors and authors wish to recognize the contribution of

   Xavier Vilajosana  who lead the design of the minimal support with
         RPL and contributed deeply to the 6top design.

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

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13.  Acknowledgements

   This specification is the result interactions in particular during
   the 6TiSCH (bi)Weekly call.  The authors wish to thank: Alaeddine
   Weslati, Alfredo Grieco, Bert Greevenbosch, Cedric Adjih, Diego
   Dujovne, Dominique Barthel, Elvis Vogli, Geraldine Texier, Giuseppe
   Piro, Guillaume Gaillard, Herman Storey, Ines Robles, Jonathan Simon,
   Kazushi Muraoka, Ken Bannister, Kuor Hsin Chang, Laurent Toutain,
   Maik Seewald, Maria Rita Palattella, Michael Behringer, Michael
   Richardson, Nancy Cam Winget, Nicola Accettura, Nicolas Montavont,
   Oleg Hahm, Pat Kinney, Patrick Wetterwald, Paul Duffy, Peter van der
   Stock, Pieter de Mil, Pouria Zand, Rouhollah Nabati, Rafa Marin-
   Lopez, Raghuram Sudhaakar, Rene Struik, Sedat Gormus, Shitanshu Shah,
   Steve Simlo, Subir Das, Tengfei Chang, Tina Tsou, Tom Phinney, Xavier
   Lagrange and Yoshihiro Ohba for their various participation.

14.  References

14.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

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

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

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, April 2006.

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

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

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

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

   [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, August 2007.

   [RFC5191]  Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA)", RFC 5191, May 2008.

   [RFC5889]  Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
              Hoc Networks", RFC 5889, September 2010.

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

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

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

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

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

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

   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses", RFC
              6620, May 2012.

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   [RFC6775]  Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
              "Neighbor Discovery Optimization for IPv6 over Low-Power
              Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
              November 2012.

14.2.  Informative References

              Chakrabarti, S., Nordmark, E., Thubert, P., and M.
              Wasserman, "IPv6 Neighbor Discovery Optimizations for
              Wired and Wireless Networks", draft-chakrabarti-nordmark-
              6man-efficient-nd-05 (work in progress), February 2014.

              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Interface", draft-ietf-6tisch-
              6top-interface-00 (work in progress), March 2014.

              Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
              Interaction using CoAP", draft-ietf-6tisch-coap-00 (work
              in progress), May 2014.

              Vilajosana, X. and K. Pister, "Minimal 6TiSCH
              Configuration", draft-ietf-6tisch-minimal-00 (work in
              progress), November 2013.

              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-01 (work in
              progress), February 2014.

              Watteyne, T., Palattella, M., and L. Grieco, "Using
              IEEE802.15.4e TSCH in an LLN context: Overview, Problem
              Statement and Goals", draft-ietf-6tisch-tsch-00 (work in
              progress), November 2013.

              Thaler, D. and C. Huitema, "Multi-link Subnet Support in
              IPv6", draft-ietf-ipv6-multilink-subnets-00 (work in
              progress), July 2002.

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

              Shah, S. and P. Thubert, "Differentiated Service Class
              Recommendations for LLN Traffic", draft-svshah-tsvwg-lln-
              diffserv-recommendations-02 (work in progress), February

              Thubert, P., "6LoWPAN Backbone Router", draft-thubert-
              6lowpan-backbone-router-03 (work in progress), February

              Thubert, P., "The IPv6 Flow Label within a RPL domain",
              draft-thubert-6man-flow-label-for-rpl-03 (work in
              progress), May 2014.

              Thubert, P. and J. Hui, "LLN Fragment Forwarding and
              Recovery", draft-thubert-roll-forwarding-frags-02 (work in
              progress), September 2013.

              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top)", draft-wang-6tisch-6top-
              sublayer-00 (work in progress), February 2014.

14.3.  External Informative References

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

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

              IEEE standard for Information Technology, "IEEE std.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendament 1: MAC sublayer", April

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

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

Authors' Addresses

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

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

   Thomas Watteyne
   Linear Technology, Dust Networks Product Group
   30695 Huntwood Avenue
   Hayward, CA  94544

   Phone: +1 (510) 400-2978
   Email: twatteyne@linear.com

   Robert Assimiti
   961 Indian Hills Parkway
   Marietta, GA  30068

   Phone: +1 404 461 9614
   Email: robert.assimiti@centerotech.com

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