6TiSCH                                                   P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                             T. Watteyne
Expires: April 22, 2014                                Linear Technology
                                                            RA. Assimiti
                                                       November 18, 2013

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


   This document presents an architecture for an IPv6 Multi-Link subnet
   that is composed of a high speed powered backbone and a number of
   IEEE802.15.4e TSCH wireless networks attached and synchronized by
   Backbone Routers.  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",
   "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
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   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 22, 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 . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview and Scope . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Communication Paradigms and Interaction Models . . . . . . . .  7
   6.  Forwarding Models  . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Track Forwarding . . . . . . . . . . . . . . . . . . . . .  8
       6.1.1.  Transport Mode . . . . . . . . . . . . . . . . . . . .  9
       6.1.2.  Tunnel Mode  . . . . . . . . . . . . . . . . . . . . .  9
       6.1.3.  Tunnel Metadata  . . . . . . . . . . . . . . . . . . . 10
     6.2.  Fragment Forwarding  . . . . . . . . . . . . . . . . . . . 11
     6.3.  IPv6 Forwarding  . . . . . . . . . . . . . . . . . . . . . 12
   7.  TSCH and 6top  . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  6top . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.2.  Network Synchronization  . . . . . . . . . . . . . . . . . 13
     7.3.  Slotframes and Priorities  . . . . . . . . . . . . . . . . 14
     7.4.  Packet Marking and Handling  . . . . . . . . . . . . . . . 14
   8.  Schedule Management Mechanisms . . . . . . . . . . . . . . . . 14
     8.1.  Minimal Static Scheduling  . . . . . . . . . . . . . . . . 14
     8.2.  Neighbor-to-Neighbor Scheduling  . . . . . . . . . . . . . 15
     8.3.  Remote Monitoring and Schedule Management  . . . . . . . . 15
     8.4.  Hop-by-hop Scheduling  . . . . . . . . . . . . . . . . . . 16
   9.  Centralized vs. Distributed Routing  . . . . . . . . . . . . . 16
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 17
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 18
     13.3.  External Informative References . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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

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   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 timeslot is allocated to unicast or
   broadcast communication at the MAC level.  The time slotted operation
   reduces collisions, saves energy, and enables to more closely
   engineer the network for deterministic properties.  The channel
   hopping aspect is a simple and efficient technique to combat
   multipath fading and external interference (for example by WiFi

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

2.  Terminology

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

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   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.palattella-
   6tisch-terminology], [I-D.chakrabarti-nordmark-6man-efficient-nd],
   [I-D.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

   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.

   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

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   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] 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

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

   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].

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

   In the extended configuration, the functionality of the LBR is
   enhanced to that of Backbone Router (BBR). A BBR is an LBR, but also
   an Energy Aware Default Router (NEAR) as defined in [I-D.chakrabarti-
   nordmark-6man-efficient-nd].  The BBR performs ND proxy operations
   between the registered devices and the classical ND devices that are
   located over the backbone.  6TiSCH BBRs synchronize with one another
   over the backbone, so as to ensure that the multiple LLNs that form
   the IPv6 subnet stay tightly synchronized.  If the Backbone is
   Deterministic (such as defined by the Time Sensitive Networking WG at
   IEEE), then the Backbone Router ensures that the end-to-end
   deterministic behavior is maintained between the LLN and the

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

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

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

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

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

   IoT6 European Project: external, maintain consistency.

5.  Communication Paradigms and Interaction Models

   [I-D.palattella-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

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   would match a Data Model for the data that is provided over the
   protocol exchange.

   [I-D.roll-rpl-industrial-applicability] section 2.1.3. and next
   discusses appplication-layer paradigms, such as Source-sink (SS) that
   is a Multipeer to Multipeer (MP2MP) model that is primarily used for
   alarms and alerts, Publish-subscribe (PS, or pub/sub) that is
   typically used for sensor data, as well as Peer-to-peer (P2P) and
   Peer-to-multipeer (P2MP) communications.  Additional considerations
   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 [HART].

   This specification focusses on Communication Paradigms and
   Interaction Models for packet forwarding and TSCH resources (cells)
   management.  L ink-layer and Network-layer Packet forwarding
   interactions are discussed in Section 6, whereas Link-layer (one-
   hop), Network-layer (multithop along a track), and Application-layer
   (remote control) management mechanisms for the TSCH schedule are
   discussed in Section 8.

6.  Forwarding Models

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

6.1.  Track Forwarding

   Track Forwarding is the simplest and fastest.  A 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
   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 of ISA100.11a.

   A Track is defined end-to-end as a succession of timeslots.  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

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   A data frame that is forwarded along a Track has a destination MAC
   address set to broadcast or a multicast address depending on MAC
   support.  This way, the MAC layer in the intermediate nodes accepts
   the incoming frame and 6top switches it without incurring a change in
   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 unrecognised MAC
   address is 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 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, 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    |    +-------+    +--...-----+    +-------+

6.1.2.  Tunnel Mode

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

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

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   specify the flows that are to be transported transparently over the

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

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

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

6.2.  Fragment Forwarding

   Considering that 6LoWPAN packets can be as large as 1280 bytes (the
   IPv6 MTU), and that the non-storing mode of RPL implies Source
   Routing that requires space for routing headers, and that a
   IEEE802.15.4 frame with security may carry in the order of 80 bytes
   of effective payload, an IPv6 packet might be fragmented into more
   than 16 fragments at the 6LoWPAN sublayer.

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

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

   [I-D.thubert-roll-forwarding-frags] describes a mechanism whereby the
   datagram tag in the 6LoWPAN Fragment is used as a label for switching
   at the 6LoWPAN sublayer.  The draft allows for a degree of flow
   control base on an Explicit Congestion Notification, as well as end-
   to-end individual fragment recovery.

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

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-

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

7.  TSCH and 6top

7.1.  6top

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   6top is a sublayer which is the next higher layer to TSCH and which
   offers a set of commands defining data and management interfaces.
   The management interface of 6top enables an upper layer to schedule
   cells and Slotframes in the TSCH schedule.  6top is defined in [I-D

   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.  Network Synchronization

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

   Time distribution requires a loop-less structure.  Nodes taken in a
   synchronization loop will rapidly desynchronize from the network and
   become isolated.  It is expected that a RPL DAG with a dedicated
   global Instance is  deployed for the purpose of time synchronization.
   That Instance is referred to as the Time Synchronization Global
   Instance (TSGI).  The TSGI can be operated in either of the 3 modes
   that are detailed in RPL [RFC6550]  section "3.1.3. Instances,
   DODAGs, and DODAG Versions".  Multiple uncoordinated DODAGs with
   independent roots may be used if all the roots share a common time
   source such as the Global Positioning System (GPS). In the absence of
   a common time source, the TSGI should form a single DODAG with a
   virtual root.  A backbone network is then used to synchronize and
   coordinate RPL operations between the backbone routers that act as
   sinks for the LLN.

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   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".

   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 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 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.

7.3.  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.

7.4.  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

8.  Schedule Management Mechanisms

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

8.1.  Minimal Static Scheduling

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   A static TSCH schedule can be used to bootstrap a network, as a
   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, but it 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

8.2.  Neighbor-to-Neighbor Scheduling

   The 6top sublayer [I-D.wang-6tisch-6top] 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 node appears as an number
   of cells.  Depending on the 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

   [I-D.sudhaakar-6tisch-coap] defines an mapping of 6top's set of
   commands to CoAP resources.  This allows an entity to interact with
   the 6top layer of a node that is multiple hops away.  [I-D.sudhaakar-
   6tisch-coap] defines the CoAP resources and associated methods (GET/
   PUT/POST/DELETE). The payload of those signalling packets use CBOR to
   encode the different fields sent and received.

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

   The entity issuing the CoAP requests can be a central scheduling
   entity (e.g.  a PCE), a node multiple hops away with the authority to
   modify the TSCH schedule (e.g.  the head of a local cluster), or a
   external device monitoring the overall state of the network (e.g.
   NME). The architecture allows for different types of interactions
   between this CoAP client and a node in the network:

   Query The CoAP client may retrieve information from a specific node
         in the network.  This is typically a CoAP GET request issued on
         the appropriate resource on the node.

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   Report The CoAP client may register for periodic updates from a
         resource, for example to monitor the state of some statistics
         maintained by the node.  This is typically done through CoAP

   Action The CoAP client may request the node to take some action, for
         example add a cell to its TSCH schedule.  This is typically a
         CoAP PUT/POST/DELETE request issued on the appropriate resource
         on the node.

   Request The node may issue a request to the client to trigger some
         action, for example the calculation of a multi-hop route.  This
         is typically a CoAP POST request issued by the node on the
         appropriate resource on the CoAP client.

   Event The node may indicate the occurrence of a specific event to the
         CoAP client, for example the discovery of a new neighbor.  This
         is typically a CoAP PUT request issued by the node on the
         appropriate resource on the CoAP client.

   [I-D.sudhaakar-6tisch-coap] defines the a basic set of CoAP
   resources.  For cases where extra functionality is needed, the draft
   also defines the concept of "profiles", as well as a mechanism for a
   CoAP client to discover the profiles installed on a node.

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
   reallocations 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 defined yet.

9.  Centralized vs.  Distributed Routing

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

   Both 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 centralized and distributed routing to share a
   same topology.  In this case, centralizes routes have precedence over
   distributed routes in case of a conflict.

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

10.  IANA Considerations

   This specification does not require IANA action.

11.  Security Considerations

   This specification is not found to introduce new security threat.

12.  Acknowledgements

13.  References

13.1.  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

   [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.

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   [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.

13.2.  Informative References

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

              Phinney, T., Thubert, P. and R. Assimiti, "RPL
              applicability in industrial networks", Internet-Draft
              draft-ietf-roll-rpl-industrial-applicability-01, September

              Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P. and
              A. Yegin, "Security Framework and Key Management Protocol
              Requirements for 6TiSCH", Internet-Draft draft-ohba-
              6tisch-security-00, October 2013.

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

              Watteyne, T., "6TiSCH Data Model for CoAP", Internet-Draft
              draft-sudhaakar-6tisch-coap-00, October 2013.

              Shah, S. and P. Thubert, "Differentiated Service Class
              Recommendations for LLN Traffic", Internet-Draft draft-
              svshah-tsvwg-lln-diffserv-recommendations-00, February

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

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

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

              Vilajosana, X. and K. Pister, "Minimal 6TiSCH
              Configuration", Internet-Draft draft-vilajosana-6tisch-
              minimal-00, October 2013.

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

              Watteyne, T., "Using IEEE802.15.4e TSCH in an LLN context:
              Overview, Problem Statement and Goals", Internet-Draft
              draft-watteyne-6tisch-tsch-00, October 2013.

13.3.  External Informative References

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

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

              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

              ISA, "ISA100, Wireless Systems for Automation", May 2008,

Authors' Addresses

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

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

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

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

   Robert Assimiti
   961 Indian Hills Parkway
   Marietta, GA 30068

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

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