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Versions: 00 01 draft-ietf-6tisch-architecture

6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                                     cisco
Intended status: Standards Track                            RA. Assimiti
Expires: April 11, 2014                                          Centero
                                                             T. Watteyne
                                       Linear Technology / Dust Networks
                                                        October 10, 2013

   An Architecture for IPv6 over the TSCH mode of IEEE IEEE802.15.4e
                  draft-thubert-6tisch-architecture-00

Abstract

   This document presents an architecture for an IPv6 multilink 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, in a distributed fashion using
   the Routing Protocol for Low Power and Lossy Networks, or in a mixed
   mode.  The Backbone Routers perform proxy Neighbor discovery
   operations over the backbone on behalf of the wireless device, 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 11, 2014.

Copyright Notice




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   Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Applications and Goals . . . . . . . . . . . . . . . . . . . .  3
   4.  Overview and Scope . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Centralized vs. Distributed Routing  . . . . . . . . . . . . .  7
   6.  Forwarding Models  . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Track Forwarding . . . . . . . . . . . . . . . . . . . . .  7
       6.1.1.  Transport Mode . . . . . . . . . . . . . . . . . . . .  8
       6.1.2.  Tunnel Mode  . . . . . . . . . . . . . . . . . . . . .  8
       6.1.3.  Tunnel Metadata  . . . . . . . . . . . . . . . . . . .  9
     6.2.  Fragment Forwarding  . . . . . . . . . . . . . . . . . . . 10
     6.3.  IPv6 Forwarding  . . . . . . . . . . . . . . . . . . . . . 11
   7.  Functional Flows . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Network Synchronization  . . . . . . . . . . . . . . . . . . . 12
   9.  TSCH and 6top  . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  6top . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.2.  Slotframes and Priorities  . . . . . . . . . . . . . . . . 13
     9.3.  Centralized Flow Reservation . . . . . . . . . . . . . . . 13
     9.4.  Distributed Flow Reservation . . . . . . . . . . . . . . . 14
     9.5.  Packet Marking and Handling  . . . . . . . . . . . . . . . 14
   10. Monitoring and Management  . . . . . . . . . . . . . . . . . . 14
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     14.1.  Normative References  . . . . . . . . . . . . . . . . . . 16
     14.2.  Informative References  . . . . . . . . . . . . . . . . . 16
     14.3.  External Informative References . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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 could be less than
   practical, for instance rotating devices.




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   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 will cross 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 timeslot is allocated to a unicast or a
   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 multilink 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

   The draft uses terminology defined in [I-D.palattella-6TiSCH-
   terminology], [I-D.chakrabarti-nordmark-6man-efficient-nd], [RFC5191]
   and [RFC4080].

   It conforms to the terms and models described for IPv6 in [RFC5889]
   and uses the vocabulary and the concepts defined in [RFC4291] for
   IPv6.

3.  Applications and Goals




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   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 and the
   centralized path computation element.  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.

   Additional industrial use cases are addressed with the addition of a
   more autonomic and distributed routing 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.

   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.watteyne-6TiSCH-tsch-lln-context] MAC Route-Over 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






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   The LLN devices communicate over IPv6 [RFC2460] using the 6LoWPAN
   Header Compression (6LoWPAN HC) [RFC6282].  From the Layer 3
   perspective, a single LLN interface (typically an IEEE802.15.4 radio)
   may be seen as a collection of Links with different capabilities for
   unicast or multicast services.  An IPv6 subnet will span over
   multiple links, effectively forming a multilink subnet.  Within that
   subnet, Neighbor Devices are discovered with 6LoWPAN Neighbor
   Discovery (6LoWPAN ND) [RFC6775].  The Routing Protocol for Low Power
   and Lossy Networks (RPL)  [RFC6550] enables routing within the LLN,
   typically within the multilink subnet in the so called Route Over
   fashion.  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 [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 is handled with the Protocol for Carrying
   Authentication for Network access (PANA) [RFC5191].

   The LLN devices are time-synchronized at 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





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   backbone.

   A Network Management Entity may participate to tre

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

   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)
   Objet system.  This standard includes DTLS, CoAP (core plus the 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 accomodate PCE routes and reservation.  Will probably look a
   lot like GMPLS


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   (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.

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

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

   IoT6 European Project: external, maintain consistency.

5.  Centralized vs.  Distributed Routing

   6TiSCH supports a mix model of centralized routes and distributed
   routes.  Centralized routes are typically computed by a entity such
   as the PCE.  Distributed routes are computed by the RPL routing
   protocol.

   Both RPL and the PCE may inject routes in the Routing Tables of the
   6TiSCH routers.  In either case, each route is associated with a
   topology that is indexed by an RPLInstanceID, as defined in RPL
   [RFC6550].  RPL and PCE rely on shared sources to define Global and
   Local RPLInstanceIDs.

   It is possible for RPL and PCE to share a same topology, in which
   case the PCE routes have precedence over RPL routes in case of a
   conflict.

   Inside the 6TiSCH domain, the flow label is used to indicate the
   topology that must be used for routing and the associated Routing
   Tables as discussed in [I-D.thubert-roll-flow-label].

6.  Forwarding Models

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

6.1.  Track Forwarding

   Track Forwarding is the simplest and fastest.  A set of input cells
   are uniquely bound to a set of output cells, representing a
   forwarding state that can be used regardless of the upper layer
   protocol.  This model can effectively be seen as a G-MPLS operation
   in that the information used to switch is not an explicit label but
   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 of ISA100.11a.


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   A Track is defined end-to-end as a succession of timeslots and a
   timeslot belongs to at most one Track.  For a given iteration of a
   Slotframe, the timeslot is associated uniquely with a cell which
   indicates the channel at which the timeslot operates for that
   iteration.

   A frame that is forwarded along a Track has a destination MAC address
   set to broadcast or a multicast address depending on the MAC support.
   This way, the MAC layer in the intermediate nodes will accept the
   incoming frame and 6top will switch 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 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 just ignored 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 PDU is associated with flow information that
   refers uniquely to the Track, so the 6top sublayer can place the
   frame in the appropriate timeslot without ambiguity.  In the case of
   IPv6 traffic, the identification of that flow information is
   transported in the Flow Label in 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    |    +-------+    +--...-----+    +-------+
         +--------------+


6.1.2.  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 perfectly 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, the PCE may coordinate with a WirelessHART Network
   Manager or an ISA100.11a System Manager in order 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 is
   uniquely derived from the information at the receiving end, for
   instance the incoming timeslots, or an ISA100.11a ContractId.  At the
   ingress 6TiSCH router, the packet destination is recognized as self
   but the flow information indicates that the frame must be tunnelled
   over a particular 6top Track so the packet is not punted to upper
   layer.  Instead, it is passed to the 6top sublayer for switching.
   The 6top sublayer in the ingress router overrides the destination MAC
   to broadcast and forwards.

   At the egress 6top 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.


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

   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, which
   is 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.  In that model, the first
   fragment is routed based on the IPv6 header that is present in that
   fragment.














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




   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.

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
   being defined to that effect in [I-D.svshah-tsvwg-lln-diffserv-
   recommendations].




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




7.  Functional Flows

8.  Network Synchronization

   Nodes in a TSCH are time synchronized.  A node keeps synchronized to
   its time source neighbor(s) 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 synch 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 the Device Management Entity.

   Time distribution requires a loopless structure.  Nodes taken in a
   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 mode 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 mean 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 bit sin the octet unused
   in the IEEE802.15.4e specification.  After concertation 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 is 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.

9.  TSCH and 6top

9.1.  6top

   6top is a sublayer which is the next higher layer to TSCH and which
   offers a set of commands defining data and management interfaces.
   6top is defined in [I-D.draft-wang-6TiSCH-6top]

   The management interface of 6top enables an upper layer to schedule
   cells and Slotframes in the TSCH schedule.

   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 typically 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
   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.

9.2.  Slotframes and Priorities

   6top uses priority queues to manage concurrent data flows of
   different priorities.  When a packet is received from an higher layer
   for transmission, the I-MUX module of 6top inserts that packet in the
   outgoing queue which matches the packet best (DSCP can therefore be
   used). At each scheduled transmit slot, the MUX module looks for the
   frame in all the outgoing queues that best matches the cells.  If a
   frame is found, it is given to TSCH for transmission.

9.3.  Centralized Flow Reservation



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   In a centralized setting, an entity (typically a PCE) is responsible
   for computing the TSCH schedule, and communicates with the different
   nodes in the network to configure their TSCH schedule.  Since it has
   full knowledge of the network's topology, the PCE can compute a
   collision-free schedule, which results in a high degree of
   communication determinism.

   The protocol for the PCE to communicate with the motes is not yet
   defined.  This protocol typically reserves hard cells on the
   transmitter side of a dedicated cell, and the negotiation protocol of
   6top takes care of reserving the same cell on the receiver node.

9.4.  Distributed Flow Reservation

   In a distributed setting, no central PCE is present in the network.
   Nodes use 6top to reserve soft cells with their neighbors.  Since no
   node has full knowledge of the network's topology and the traffic
   requirements, scheduling collisions are possible, for example because
   of a hidden terminal problem.

   A schedule collision can be detected if two motes have multiple
   dedicated cells schedule to one another.  The monitoring process of
   6top can be configured to continuously compute the packet delivery
   ratio of those cells, and it can declare a soft cell to perform bad
   when the statistics for that cell are significantly worse than for
   the other cells to the same neighbor.

   When this happens, the monitoring process of 6top moves the cell to
   another location in the 6TiSCH schedule, through a re-negotiation
   procedure with the neighbor.

   The entity that builds and maintains the schedule in a distributed
   fashion is not yet defined.

9.5.  Packet Marking and Handling

   reservation Deterministic flow allocation (hard reservation of
   timeslots) eg centralized RSVP? metrics?  Hop-by-hop interaction with
   6top.  Lazy reservation (use shared slots to transport extra burst
   and then dynamically (de)allocate) Classical QoS (dynamic based on
   observation)

10.  Monitoring and Management

   For the purpose of operations and management, a given LLN node
   interacts through the Backbone Router with an NME and optionally a
   PCE if centralized routing operations are enabled.  Both a PCE and an
   NME may require information about the LLN node and its link
   operations, and may control that operation for Instance by assigning
   new bundles or new tracks.  In order to avoid duplication, and
   simplify the interaction with the node, the Backbone Router may
   perform some proxy, publish/subscribe and/or translational operations
   on behalf of the LLN node.

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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

   The architecture supports variations on the deployment model, and
   focusses on the flows rather than the whether there is a proxy or a
   translational operation on the way.

   Discovery: PCE discovery; SCADA/DCS discovery; actuator discovery

   Request To ask the PCE to change schedule, typically directed to a
         PCE, from a LLN node or an NME.

   Action For the PCE to change the schedule of an LLN node, typically
         directed from the PCE, to a LLN node.

   Report For an LLN node to report periodic information or stats,
         eventually based on a profile, typically directed to a PCE or
         an NME from an LLN node.

   Event For an LLN node to report an exception, eventually based on a
         profile, typically directed to a PCE or an NME from an LLN
         node.

   Query For the PCE or an NME to ask for schedule information from an
         LLN node, typically directed from the PCE, to a LLN node.

11.  IANA Considerations

   This specification does not require IANA action.

12.  Security Considerations

   This specification is not found to introduce new security threat.

13.  Acknowledgements

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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.E. and R.M. Hinden, "Internet Protocol, Version
              6 (IPv6) Specification", RFC 2460, December 1998.

   [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.

   [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.

   [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.

   [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.

   [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

   [I-D.chakrabarti-nordmark-6man-efficient-nd]


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              Chakrabarti, S., Nordmark, E. and M. Wasserman,
              "Efficiency aware IPv6 Neighbor Discovery Optimizations",
              Internet-Draft draft-chakrabarti-nordmark-6man-efficient-
              nd-01, November 2012.

   [I-D.draft-wang-6TiSCH-6top]
              Wang, Q., Ed., Vilajosana, X. and T. Watteyne, "6TiSCH
              Operation Sublayer (6top). draft-wang-6TiSCH-6top-00 (work
              in progress) ", July 2013.

   [I-D.ohba-6tsch-security]
              Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P. and
              A. Yegin, "Security Framework and Key Management Protocol
              Requirements for 6TSCH", Internet-Draft draft-ohba-6tsch-
              security-01, July 2013.

   [I-D.palattella-6tsch-terminology]
              Palattella, M., Thubert, P., Watteyne, T. and Q. Wang,
              "Terminology in IPv6 over Time Slotted Channel Hopping",
              Internet-Draft draft-palattella-6tsch-terminology-00,
              March 2013.

   [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-00, February
              2013.

   [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-00, February
              2013.

   [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-flow-label]
              Thubert, P., "Use of the IPv6 Flow Label within an LLN",
              Internet-Draft draft-thubert-roll-flow-label-02, November
              2012.

   [I-D.thubert-roll-forwarding-frags]
              Thubert, P. and J. Hui, "LLN Fragment Forwarding and
              Recovery", Internet-Draft draft-thubert-roll-forwarding-
              frags-01, February 2013.

   [I-D.vilajosana-6tsch-basic]
              Vilajosana, X. and K. Pister, "Minimal 6TSCH
              Configuration", Internet-Draft draft-vilajosana-6tsch-
              basic-01, July 2013.

   [I-D.watteyne-6tsch-tsch-lln-context]

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              Watteyne, T., "Using IEEE802.15.4e TSCH in an LLN context:
              Overview, Problem Statement and Goals", Internet-Draft
              draft-watteyne-6tsch-tsch-lln-context-01, February 2013.

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", .

   [IEEE802.1TSNTG]
              IEEE Standards Association, "IEEE 802.1 Time-Sensitive
              Networks Task Group", March 2013, <http://www.ieee802.org/
              1/pages/avbridges.html>.

   [ISA100.11a]
              ISA, "ISA100, Wireless Systems for Automation", May 2008,
              <http://www.isa.org/Community/
              SP100WirelessSystemsforAutomation>.

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


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

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


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

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



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