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6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                             T. Watteyne
Expires: April 28, 2015                                Linear Technology
                                                            RA. Assimiti
                                                                 Centero
                                                        October 27, 2014

     An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4e
                   draft-ietf-6tisch-architecture-04

Abstract

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 April 28, 2015.

Copyright Notice




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   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 carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Applications and Goals . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview and Scope . . . . . . . . . . . . . . . . . . . . . .  5
   5.  6LoWPAN (and RPL)  . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  RPL Leaf Support in 6LoWPAN ND . . . . . . . . . . . . . .  9
     5.2.  registration Failures Due to Movement  . . . . . . . . . .  9
     5.3.  Proxy registration . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Target Registration  . . . . . . . . . . . . . . . . . . . 10
     5.5.  RPL root vs. 6LBR  . . . . . . . . . . . . . . . . . . . . 10
     5.6.  Securing the Registration  . . . . . . . . . . . . . . . . 11
   6.  Communication Paradigms and Interaction Models . . . . . . . . 11
   7.  TSCH and 6top  . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  6top . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.2.  6top and RPL Objective Function operations . . . . . . . . 13
     7.3.  Network Synchronization  . . . . . . . . . . . . . . . . . 14
     7.4.  SlotFrames and Priorities  . . . . . . . . . . . . . . . . 15
     7.5.  Distributing the reservation of cells  . . . . . . . . . . 16
   8.  Schedule Management Mechanisms . . . . . . . . . . . . . . . . 18
     8.1.  Minimal Static Scheduling  . . . . . . . . . . . . . . . . 18
     8.2.  Neighbor-to-neighbor Scheduling  . . . . . . . . . . . . . 19
     8.3.  Remote Monitoring and Schedule Management  . . . . . . . . 19
     8.4.  Hop-by-hop Scheduling  . . . . . . . . . . . . . . . . . . 20
   9.  Forwarding Models  . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Track Forwarding . . . . . . . . . . . . . . . . . . . . . 21
       9.1.1.  Transport Mode . . . . . . . . . . . . . . . . . . . . 22
       9.1.2.  Tunnel Mode  . . . . . . . . . . . . . . . . . . . . . 22
       9.1.3.  Tunnel Metadata  . . . . . . . . . . . . . . . . . . . 23
     9.2.  Fragment Forwarding  . . . . . . . . . . . . . . . . . . . 24
     9.3.  IPv6 Forwarding  . . . . . . . . . . . . . . . . . . . . . 25
   10. Centralized vs. Distributed Routing  . . . . . . . . . . . . . 25
     10.1.  Packet Marking and Handling . . . . . . . . . . . . . . . 26
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 27
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     15.1.  Normative References  . . . . . . . . . . . . . . . . . . 27
     15.2.  Informative References  . . . . . . . . . . . . . . . . . 29
     15.3.  External Informative References . . . . . . . . . . . . . 30

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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31

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
   emitters).

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

2.  Terminology







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   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" [I-D.ietf-
   ipv6-multilink-subnets].

   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-6tisch-
   terminology], [I-D.chakrabarti-nordmark-6man-efficient-nd], [I-D
   .ietf-roll-rpl-industrial-applicability], [RFC5191] and [RFC4080].

   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
   management.

   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 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.






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   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
                 o

   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 [RFC6775] (6LoWPAN ND).  RPL [RFC6550] enables routing
   within the LLN, in the so called Route Over fashion, either in
   storing (stateful) or non-storing (stateless, with routing headers)
   mode.

   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 section 3.1
   of RPL [RFC6550], in particular "3.1.2. RPL Identifiers" and "3.1.3.
   Instances, DODAGs, and DODAG Versions".







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   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.

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

   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




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   deterministic behavior is maintained between the LLN and the
   backbone.

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

   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)
   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
   LLNs.

   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]





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   is an Logical Link Control (LLC) or a portion thereof that provides
   the abstraction of an IP link over a TSCH MAC.

5.  6LoWPAN (and RPL)

   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 support of leaf can be implemented
   as a minor increment to 6LoWPAN ND, with the additional capability to
   carry a sequence number that is used to track the movements of the
   device, and optionally some information about the RPL topology that
   this device will join.

   The root of the RPL topology is logically separated from the 6BBR
   that is used to connect the RPL topology to the backbone.  The RPL
   root can use Efficient ND as the interface to register an LLN node in
   its topology to the 6BBR for whatever operation the 6BBR performs,
   such as ND proxy operations, or injection in a routing protocol.  It
   results that, as illustrated in Figure 4, the periodic signaling
   could start at the leaf node with 6LoWPAN ND, then would be carried
   over RPL to the RPL root, and then with Efficient-ND to the 6BBR.
   Efficient ND being an adaptation of 6LoWPAN ND, it makes sense to
   keep those two homogeneous in the way they use the source and the
   target addresses in the Neighbor Solicitation (NS) messages for
   registration, as well as in the options that they use for that
   process.


    6LoWPAN Node        6LR             6LBR            6BBR
     (RPL leaf)       (router)         (root)
         |               |               |               |
         |  6LoWPAN ND   |6LoWPAN ND+RPL | Efficient ND  | IPv6 ND
         |   LLN link    |Route-Over mesh|  IPv6 link    | Backbone
         |               |               |               |
         |  NS(ARO)      |               |               |
         |-------------->|               |               |
         | 6LoWPAN ND    | DAR (then DAO)|               |
         |               |-------------->|               |
         |               |               |  NS(ARO)      |
         |               |               |-------------->|
         |               |               |               | DAD
         |               |               |               |------>
         |               |               |               |
         |               |               |  NA(ARO)      |
         |               |               |<--------------|
         |               | DAC           |               |
         |               |<--------------|               |
         |  NA(ARO)      |               |               |
         |<--------------|               |               |





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   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.

5.1.  RPL Leaf Support in 6LoWPAN ND

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

   At the bare minimum the leaf device must provide a sequence number
   that matches the RPL specification in section 7.  Section 4.1 of [I-D
   .chakrabarti-nordmark-6man-efficient-nd], on the Address Registration
   Option (ARO), already incorporates that addition with a new field in
   the option called the Transaction ID.

   If for some reason the node is aware of RPL topologies, then
   providing the RPL InstanceID for the instances to which the node
   wishes to participate would be a welcome addition.  In the absence of
   such information, the RPL router must infer the proper instanceID
   from external rules and policies.

   On the backbone, the InstanceID is expected to be mapped onto a
   VLANID. Neither WiFi nor Efficient ND do provide a mapping to
   VLANIDs, and it is unclear, when a wireless node attaches to a
   backbone where VLANs are defined, which VLAN the wireless device
   attaches to.  Considering that a VLAN is effectively the IP link on
   the backbone, adding the InstanceID to both specifications could be a
   welcome addition.

5.2.  registration Failures Due to Movement

   Registration to the 6LBR through DAR/DAC messages [RFC6775] may
   percolate slowly through an LLN mesh, and it might happen that in the
   meantime, the 6LoWPAN node moves and registers somewhere else.  Both
   RPL and 6LoWPAN ND lack the capability to indicate that the same node
   is registered elsewhere, so as to invalidate states down the
   deprecated path.

   In its current expression and functionality, 6LoWPAN ND considers
   that the registration is used for the purpose of DAD only as opposed
   to that of achieving reachability, and as long as the same node
   registers the IPv6 address, the protocol is functional.  In order to
   act as a RPL leaf registration protocol and achieve reachability, the
   device must use the same TID for all its concurrent registrations,
   and registrations with a past TID should be declined.  The state for
   an obsolete registration in the 6LR, as well as the RPL routers on
   the way, should be invalidated.  This can only be achieved with the
   addition of a new Status in the DAC message, and a new error/clean-up
   flow in RPL.

5.3.  Proxy registration


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   The 6BBR provides the capability to defend an address that is owned
   by a 6LoWPAN Node, and attract packets to that address, whether it is
   done by proxying ND over a MultiLink Subnet, redistributing the
   address in a routing protocol or advertising it through an alternate
   proxy registration such as the Locator/ID Separation Protocol
   [RFC6830] (LISP) or Mobility Support in IPv6 [RFC6275] (MIPv6). In a
   LLN, it makes sense to piggyback the request to proxy/defend an
   address with its registration.

5.4.  Target Registration

   In their current incarnations, both 6LoWPAN ND and Efficient ND
   expect that the address being registered is the source of the NS(ARO)
   message and thus impose that a Source Link-Layer Address (SLLA)
   option be present in the message.  In a mesh scenario where the 6LBR
   is physically separated from the 6LoWPAN Node, the 6LBR does not own
   the address being registered.  This suggests that [I-D.chakrabarti-
   nordmark-6man-efficient-nd] should evolve to register the Target of
   the NS message as opposed to the Source Address.  From another
   perspective, it may happen, in the use case of a Star topology, that
   the 6LR, 6LBR and 6BBR are effectively collapsed and should support
   6LoWPAN ND clients.  The convergence of efficient ND and 6LoWPAN ND
   into a single protocol is thus highly desirable.

   In any case, as long as the DAD process is not complete for the
   address used as source of the packet, it is against the current
   practice to advertise the SLLA, since this may corrupt the ND cache
   of the destination node, as discussed in the Optimistic DAD
   specification [RFC4429] with regards to the TENTATIVE state.

   This may look like a chicken and an egg problem, but in fact 6LoWPAN
   ND acknowledges that the Link-Local Address that is based on an
   EUI-64 address of a LLN node may be autoconfigured without the need
   for DAD.  It results that a node could use that Address as source,
   with an SSLA option in the message if required, to register any other
   addresses, either Global or Unique-Local Addresses, which would be
   indicated in the Target.

   The suggested change is to register the target of the NS message, and
   use Target Link-Layer Address (TLLA) in the NS as opposed to the SLLA
   in order to install a Neighbor Cache Entry.  This would apply to both
   Efficient ND and 6LoWPAN ND in a very same manner, with the caveat
   that depending on the nature of the link between the 6LBR and the
   6BBR, the 6LBR may resort to classical ND or DHCPv6 to obtain the
   address that it uses to source the NS registration messages, whether
   for itself or on behalf of LLN nodes.

5.5.  RPL root vs.  6LBR

   6LoWPAN ND is unclear on how the 6LBR is discovered, and how the




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   liveliness of the 6LBR is asserted over time.  On the other hand, the
   discovery and liveliness of the RPL root are obtained through the RPL
   protocol.

   When 6LoWPAN ND is coupled with RPL, it makes sense to collocate the
   6LBR and the RPL root functionalities.  The DAR/DAC exchange becomes
   a preamble to the DAO messages that are used from then on to
   reconfirm the registration, thus eliminating a duplication of
   functionality between DAO and DAR messages.

5.6.  Securing the Registration

   A typical attack against IPv6 ND is address spoofing, whereby a rogue
   node claims the IPv6 Address of another node in and hijacks its
   traffic.

   SEcure Neighbor Discovery (SEND) [RFC3971] is designed to protect
   each individual ND lookup/advertisement in a peer to peer model where
   each lookup may be between different parties.  This is not the case
   in a 6LoWPAN ND LLN where, as illustrated in Figure 4, the 6LBR
   terminates all the flows and may store security information for later
   validation.

   Additionally SEND requires considerably enlarged ND messages to carry
   cryptographic material, and requires that each protected address is
   generated cryptographically, which implies the computation of a
   different key for each Cryptographically Generated Address (CGA).
   SEND as defined in [RFC3971] is thus largely unsuitable for
   application in a LLN.

   Once an Address is registered, the 6LBR maintains a state for that
   Address and is in position to bind securely the first registration
   with the Node that placed it, whether the Address is CGA or not.  It
   should thus be possible to protect the ownership of all the addresses
   of a 6LoWPAN Node with a single key, and there should not be a need
   to carry the cryptographic material more than once to the 6LBR.

   The energy constraint is usually a foremost factor, and attention
   should be paid to minimize the burden on the CPU. Hardware-assisted
   support of variants of the Counter with CBC-MAC [RFC3610] (CCM)
   authenticated encryption block cipher mode such as CCM* are common in
   LowPower ship-set implementations, and 6LoWPAN ND security mechanism
   should be capable to reuse them when applicable.

   Finally, the code footprint in the device being also an issue, the
   capability to reuse not only hardware-assist mechanisms but also
   software across layers has to be considered.  For instance, if code
   has to be present for upper-layer operations, e.g AES-CCM Cipher
   Suites for Transport Layer Security (TLS) [RFC6655], then the
   capability to reuse that code should be considered.

6.  Communication Paradigms and Interaction Models



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   [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.

   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 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.
   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 8.  Link-layer frame
   forwarding interactions are discussed in Section 9, and Network-layer
   Packet routing is addressed in Section 10.

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 8.2.

   The 6top data model and management interfaces are further discussed
   in Section 8.3.






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   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.

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
   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 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












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   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
   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
   entity.












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   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 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.

   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 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 (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 is free of contending packets when time
   arrives for a scheduled transmission, a window of time is defined
   around the scheduled transmission time where the medium must be free
   of contending energy.

   One simple way to obtain such a window is to format time and
   frequencies in cells of transmission of equal duration.  This is the
   method that is adopted in IEEE802.15.4e TSCH as well as the Long Term





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   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 reduce the chances of
   interferences from slotted-aloha operations.  The knowledge of the
   CDU matrices is shared between all the nodes and used in particular
   to define slotFrames.

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

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

   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






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   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 fulfill the needs for additional cells
   improves when the number of interactions with other devices and the
   protocol latencies are minimized.

   6TiSCH limits that interaction to RPL parents that will only
   negotiate with other RPL parents, and performs that negotiation by
   groups of cells as opposed to individual cells.  The 6TiSCH
   architecture allows RPL parents to adjust dynamically, and
   independently from the PCE, the amount of bandwidth that is used to
   communicate between themselves and their children, in both
   directions; to that effect, an allocation mechanism enables a RPL
   parent to obtain the exclusive use of a portion of a CDU matrix
   within its interference domain.  Note that a PCE is expected to have
   precedence in the allocation, so that a RPL parent would only be able
   to obtain portions that are not in-use by the PCE.

   The 6TiSCH architecture introduces the concept of chunks [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 chunk is the basic unit of ownership that is used in
   that process.











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


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

   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
   tracks.

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.







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   A static TSCH schedule can be used to bootstrap a network, as an
   initial phase during implementation, or as a fall-back mechanism in
   case of network malfunction.  This 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
   configured to allow for peak emissions, energy is be wasted idle
   listening.

   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


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   The 6top interface document [I-D.ietf-6tisch-6top-interface]
   specifies the generic data model that can be used to monitor and
   manage resources of 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.

   The capability to interact with the node 6top sublayer from multiple
   hops away can be leveraged for monitoring, scheduling, or a
   combination of thereof.  The architecture supports variations on the
   deployment model, and focuses on the flows rather than whether there
   is a proxy or a translation operation en-route.

   [I-D.ietf-6tisch-coap] defines an mapping of the 6top set of
   commands, which is 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.

   [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).

   At the time of this writing, a Deterministic Networking (DetNet) [I-D
   .finn-detnet-problem-statement] effort as started at the IETF to
   provide homogeneous flows and services across layers.  This
   architecture will be refined to comply with DetNet when the work is
   formalized.

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.  Forwarding Models

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


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   (6F).

9.1.  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
   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
   IEEE802.15.4e, this means effectively broadcast, so that along the
   Track the short address for the destination of the frame is set to
   0xFFFF.

   A 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, and a cell in such a bundle belongs to at
   most one Track.  For a given iteration of the device schedule, the
   effective channel of the cell is obtained by adding a pseudo-random
   number to the channelOffset of the cell, which results in a rotation
   of the frequency that used for transmission.  The bundles may be
   computed so as to accommodate both variable rates and
   retransmissions, so they might not be fully used at a given iteration
   of the schedule.  The 6TiSCH architecture provides additional means
   to avoid waste of cells as well as overflows in the transmit bundle,
   as follows:

   In one hand, a TX-cell that is not needed for the current iteration
   may be reused opportunistically on a per-hop basis for routed
   packets.  When all of the frame that were received for a given Track
   are effectively transmitted, any available TX-cell for that Track can
   be reused for upper layer traffic for which the next-hop router
   matches the next hop along the Track.  In that case, the cell that is
   being used is effectively a TX-cell from the Track, but the short
   address for the destination is that of the next-hop router.  It
   results that a frame that is received in a RX-cell of a Track with a
   destination MAC address set to this node as opposed to broadcast must
   be extracted from the Track and delivered to the upper layer (a frame
   with an unrecognized MAC address is dropped at the lower MAC layer
   and thus is not received at the 6top sublayer).


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   On the other hand, it might happen that there are not enough TX-cells
   in the transmit bundle to accommodate the Track traffic, for instance
   if more retransmissions are needed than provisioned.  In that case,
   the frame can be placed for transmission in the bundle that is used
   for layer-3 traffic towards the next hop along the track as long as
   it can be routed by the upper layer, that is, typically, if the frame
   transports an IPv6 packet.  The MAC address should be set to the
   next-hop MAC address to avoid confusion.  It results that a frame
   that is received over a layer-3 bundle may be in fact associated to a
   Track.  In a classical IP link such as an Ethernet, off-track traffic
   is typically in excess over reservation to be routed along the non-
   reserved path based on its QoS setting.  But with 6TiSCH, since the
   use of the layer-3 bundle may be due to transmission failures, it
   makes sense for the receiver to recognize a frame that should be re-
   tracked, and to place it back on the appropriate bundle if possible.
   A frame should be re-tracked if the Per-Hop-Behavior group indicated
   in the Differentiated Services Field in the IPv6 header is set to
   Deterministic Forwarding, as discussed in Section 10.1.  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.

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

9.1.1.  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    |    +-------+    +--...-----+    +-------+
   +--------------+

9.1.2.  Tunnel Mode




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

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

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

   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.

9.1.3.  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.

9.2.  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.

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






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

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

   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.

9.3.  IPv6 Forwarding

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

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

10.  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.



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   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.

10.1.  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.
   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].

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

   The RPL Packet Information (RPI) is carried in IPv6 packets as a RPL
   option in the IPv6 Hop-By-Hop Header [RFC6553].  6LoWPAN provides a
   Next Header Compression (NHC) for the RPI (RPI-NHC).  The RPI-NHC is
   specified in [I-D.thubert-6lo-rpl-nhc], and is the compressed
   equivalent to the whole HbH header with the RPL option.  In a 6LoWPAN
   network, the RPI-NHC is the recommended encoding the RPL Packet
   Information.

   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.

11.  IANA Considerations

   This specification does not require IANA action.

12.  Security Considerations

   This specification is not found to introduce new security threat.

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13.  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.

14.  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.

15.  References

15.1.  Normative References

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

   [RFC2460]  Deering, S.E. and R.M. 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
              2003.

   [RFC3610]  Whiting, D., Housley, R. and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, September 2003.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [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.


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   [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.

   [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
              2007.

   [RFC4919]  Kushalnagar, N., Montenegro, G. and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals", RFC
              4919, 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
              2012.


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   [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.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.

   [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.

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

15.2.  Informative References

   [I-D.chakrabarti-nordmark-6man-efficient-nd]
              Chakrabarti, S., Nordmark, E., Thubert, P. and M.
              Wasserman, "Wired and Wireless IPv6 Neighbor Discovery
              Optimizations", Internet-Draft draft-chakrabarti-nordmark-
              6man-efficient-nd-04, October 2013.

   [I-D.finn-detnet-problem-statement]
              Finn, N. and P. Thubert, "Deterministic Networking Problem
              Statement", Internet-Draft draft-finn-detnet-problem-
              statement-01, October 2014.

   [I-D.ietf-6tisch-6top-interface]
              Wang, Q., Vilajosana, X. and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Interface", Internet-Draft
              draft-ietf-6tisch-6top-interface-00, March 2014.

   [I-D.ietf-6tisch-coap]
              Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
              Interaction using CoAP", Internet-Draft draft-ietf-6tisch-
              coap-00, May 2014.

   [I-D.ietf-6tisch-minimal]
              Vilajosana, X. and K. Pister, "Minimal 6TiSCH
              Configuration", Internet-Draft draft-ietf-6tisch-
              minimal-00, November 2013.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T. and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", Internet-Draft draft-ietf-6tisch-
              terminology-00, November 2013.

   [I-D.ietf-6tisch-tsch]


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

   [I-D.ietf-ipv6-multilink-subnets]
              Thaler, D. and C. Huitema, "Multi-link Subnet Support in
              IPv6", Internet-Draft draft-ietf-ipv6-multilink-
              subnets-00, July 2002.

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

   [I-D.svshah-tsvwg-deterministic-forwarding]
              Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
              Internet-Draft draft-svshah-tsvwg-deterministic-
              forwarding-01, March 2014.

   [I-D.svshah-tsvwg-lln-diffserv-recommendations]
              Shah, S. and P. Thubert, "Differentiated Service Class
              Recommendations for LLN Traffic", Internet-Draft draft-
              svshah-tsvwg-lln-diffserv-recommendations-01, August 2013.

   [I-D.thubert-6lo-rpl-nhc]
              Thubert, P. and C. Bormann, "A compression mechanism for
              the RPL option", Internet-Draft draft-thubert-6lo-rpl-
              nhc-02, October 2014.

   [I-D.thubert-6lowpan-backbone-router]
              Thubert, P., "6LoWPAN Backbone Router", Internet-Draft
              draft-thubert-6lowpan-backbone-router-03, February 2013.

   [I-D.thubert-roll-forwarding-frags]
              Thubert, P. and J. Hui, "LLN Fragment Forwarding and
              Recovery", Internet-Draft draft-thubert-roll-forwarding-
              frags-02, September 2013.

   [I-D.wang-6tisch-6top-sublayer]
              Wang, Q., Vilajosana, X. and T. Watteyne, "6TiSCH
              Operation Sublayer (6top)", Internet-Draft draft-wang-
              6tisch-6top-00, October 2013.

15.3.  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", .

   [IEEE802.1TSNTG]



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              IEEE Standards Association, "IEEE 802.1 Time-Sensitive
              Networks Task Group", March 2013, <http://www.ieee802.org/
              1/pages/avbridges.html>.

   [IEEE802154e]
              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
              2012.

   [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>.

   [WirelessHART]
              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
   FRANCE

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


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

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


   Robert Assimiti
   Centero
   961 Indian Hills Parkway
   Marietta, GA 30068
   USA

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


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