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Versions: 00

PAW                                                      P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Informational                          January 28, 2019
Expires: August 1, 2019


                              PAW for TSCH
                     draft-thubert-paw-for-tisch-00

Abstract

   This document builds on the 6TiSCH architecture that defines, among
   others, mechanisms to establish and maintain deterministic routing
   and scheduling in a centralized fashion.  The document details
   dependencies on DetNet and PCE controller to express topologies and
   capabilities, as well as abstract state that the controller must be
   able to program into the network devices to enable deterministic
   forwarding operations.

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 https://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 August 1, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://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.  6TiSCH Overview . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  SlotFrames and Priorities . . . . . . . . . . . . . . . .   7
     3.2.  Schedule Management by a PCE  . . . . . . . . . . . . . .   8
     3.3.  Track Scheduling Protocol . . . . . . . . . . . . . . . .   9
     3.4.  Track Forwarding  . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  Transport Mode  . . . . . . . . . . . . . . . . . . .  11
       3.4.2.  Tunnel Mode . . . . . . . . . . . . . . . . . . . . .  12
       3.4.3.  Tunnel Metadata . . . . . . . . . . . . . . . . . . .  13
       3.4.4.  OAM . . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Operations of Interest for DetNet and PCE . . . . . . . . . .  14
     4.1.  Packet Marking and Handling . . . . . . . . . . . . . . .  15
       4.1.1.  Tagging Packets for Flow Identification . . . . . . .  15
       4.1.2.  Replication, Retries and Elimination  . . . . . . . .  16
       4.1.3.  Differentiated Services Per-Hop-Behavior  . . . . . .  16
     4.2.  Topology and capabilities . . . . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The emergence of wireless technology has enabled a variety of new
   devices to get interconnected, at a very low marginal cost per
   device, at any distance ranging from Near Field to interplanetary,
   and in circumstances where wiring may not be practical, for instance
   on fast-moving or 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, quite
   sensitive to latency, and with a high degree of operational



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   criticality so that loss should be minimized at all times.  Such
   traffic is not limited to professional Audio/ Video networks, but is
   also found in command and control operations such as industrial
   automation and vehicular sensors and actuators.

   At IEEE802.1, the Audio/Video Task Group [IEEE802.1TSNTG] Time
   Sensitive Networking (TSN) to address Deterministic Ethernet.  The
   Medium access Control (MAC) of IEEE802.15.4 [IEEE802154] has evolved
   with the new TimeSlotted Channel Hopping (TSCH) [RFC7554] mode for
   deterministic industrial-type applications.  TSCH was introduced with
   the IEEE802.15.4e [IEEE802154e] amendment and will be wrapped up in
   the next revision of the IEEE802.15.4 standard.  For all practical
   purpose, this document is expected to be insensitive to the future
   versions of the IEEE802.15.4 standard, which is thus referenced
   undated.

   Though at a different time scale, both TSN and TSCH standards provide
   Deterministic capabilities to the point that a packet that pertains
   to a certain flow crosses the network from node to node following a
   very precise schedule, as a train that leaves intermediate stations
   at precise times along its path.  With TSCH, time is formatted into
   timeSlots, and an individual cell is allocated to unicast or
   broadcast communication at the MAC level.  The time-slotted operation
   reduces collisions, saves energy, and enables to more closely
   engineer the network for deterministic properties.  The channel
   hopping aspect is a simple and efficient technique to combat multi-
   path fading and co-channel interferences (for example by Wi-Fi
   emitters).

   The 6TiSCH Architecture [I-D.ietf-6tisch-architecture] defines a
   remote monitoring and scheduling management of a TSCH network by a
   Path Computation Element (PCE), which cooperates with an abstract
   Network Management Entity (NME) to manage timeSlots and device
   resources in a manner that minimizes the interaction with and the
   load placed on the constrained devices.

   This Architecture applies the concepts of Deterministic Networking on
   a TSCH network to enable the switching of timeSlots in a G-MPLS
   manner.  This document details the dependencies that the 6TiSCH
   Architecture has on PAW, PCE [PCE] and DetNet
   [I-D.ietf-detnet-architecture] to provide the necessary capabilities
   that may be specific to such networks.

2.  Terminology

   The draft uses terminology defined or referenced in
   [I-D.ietf-6tisch-architecture] and
   [I-D.ietf-roll-rpl-industrial-applicability].



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

3.  6TiSCH Overview

   The scope of the present work is a subnet that, in its basic
   configuration, is made of a TSCH [RFC7554] MAC Low Power Lossy
   Network (LLN).

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

             Figure 1: Basic Configuration of a 6TiSCH Network

   In the extended configuration, a Backbone Router (6BBR)
   [I-D.ietf-6lo-backbone-router] federates multiple 6TiSCH in a single
   subnet over a backbone.  PAW needs to ensure that 6BBRs synchronize
   with one another over the backbone, so that the multiple LLNs that
   form the IPv6 subnet stay tightly synchronized.





















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                  ---+-------- ............ ------------
                     |      External Network       |
                     |                          +-----+
                     |             +-----+      | NME |
                  +-----+          |  +-----+   |     |
                  |     | Router   |  | PCE |   +-----+
                  |     |          +--|     |
                  +-----+             +-----+
                     |                   |
                     | Subnet Backbone   |
               +--------------------+------------------+
               |                    |                  |
            +-----+             +-----+             +-----+
            |     | Backbone    |     | Backbone    |     | Backbone
       o    |     | router      |     | router      |     | router
            +-----+             +-----+             +-----+
       o                  o                   o                 o   o
           o    o   o         o   o  o   o         o  o   o    o
      o             o        o  LLN      o      o         o      o
         o   o    o      o      o o     o  o   o    o    o     o

           Figure 2: Extended Configuration of a 6TiSCH Network

   If the Backbone is Deterministic, then PAW must enable the 6BBRS to
   ensure that the end-to-end deterministic behavior is maintained
   between the LLN and the backbone.  This SHOULD be done in conformance
   to the DetNet Architecture [I-D.ietf-detnet-architecture] which
   studies Layer-3 aspects of Deterministic Networks, and covers
   networks that span multiple Layer-2 domains.  One particular
   requirement is that the PCE MUST be able to compute a deterministic
   path and to end across the TSCH network and an IEEE802.1 TSN Ethernet
   backbone, and DetNet MUST enable end-to-end deterministic forwarding.

   6TiSCH defines the concept of a Track, which is a complex form of a
   uni-directional Circuit ([I-D.ietf-6tisch-architecture]) but did not
   propose the methods to implement them: this would be a task for PAW.
   As opposed to a simple circuit that is a sequence of nodes and links,
   a Track is shaped as a directed acyclic graph towards a destination
   to support multi-path forwarding and route around failures.  A Track
   may also branch off and rejoin, for the purpose of the so-called
   Packet Replication and Elimination (PRE), over non congruent
   branches.  PRE may be used to complement layer-2 Automatic Repeat
   reQuest (ARQ) to meet industrial expectations in Packet Delivery
   Ratio (PDR), in particular when the Track extends beyond the 6TiSCH
   network.






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                     +-----+
                     | IoT |
                     | G/W |
                     +-----+
                        ^  <---- Elimination
                       | |
        Track branch   | |
               +-------+ +--------+ Subnet Backbone
               |                  |
            +--|--+            +--|--+
            |  |  | Backbone   |  |  | Backbone
       o    |  |  | router     |  |  | router
            +--/--+            +--|--+
       o     /    o     o---o----/       o
           o    o---o--/   o      o   o  o   o
      o     \  /     o               o   LLN    o
         o   v  <---- Replication
             o



                 Figure 3: End-to-End deterministic Track

   In the example above, a Track is laid out from a field device in a
   6TiSCH network to an IoT gateway that is located on a IEEE802.1 TSN
   backbone.

   The Replication function in the field device sends a copy of each
   packet over two different branches, and the PCE schedules each hop of
   both branches so that the two copies arrive in due time at the
   gateway.  In case of a loss on one branch, hopefully the other copy
   of the packet still makes it in due time.  If two copies make it to
   the IoT gateway, the Elimination function in the gateway ignores the
   extra packet and presents only one copy to upper layers.

   At each 6TiSCH hop along the Track, the PCE may schedule more than
   one timeSlot for a packet, so as to support Layer-2 retries (ARQ).
   It is also possible that the field device only uses the second branch
   if sending over the first branch fails.

   In current deployments, a TSCH Track does not necessarily support PRE
   but is systematically multi-path.  This means that a Track is
   scheduled so as to ensure that each hop has at least two forwarding
   solutions, and the forwarding decision is to try the preferred one
   and use the other in case of Layer-2 transmission failure as detected
   by ARQ.





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3.1.  SlotFrames and Priorities

   A slotFrame is the base object that the PCE needs to manipulate to
   program a schedule into an LLN node.  Elaboration on that concept can
   be fond in section "SlotFrames and Priorities" of
   [I-D.ietf-6tisch-architecture]

   IEEE802.15.4 TSCH avoids contention on the medium by formatting time
   and frequencies in cells of transmission of equal duration.  In order
   to describe that formatting of time and frequencies, the 6TiSCH
   architecture defines a global concept that is called a Channel
   Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of
   cells with an height equal to the number of available channels
   (indexed by ChannelOffsets) and a width (in timeSlots) that is the
   period of the network scheduling operation (indexed by slotOffsets)
   for that CDU matrix.  The size of a cell is a timeSlot duration, and
   values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to
   accommodate for the transmission of a frame and an ack, including the
   security validation on the receive side which may take up to a few
   milliseconds on some device architecture.

   The frequency used by a cell in the matrix rotates in a pseudo-random
   fashion, from an initial position at an epoch time, as the matrix
   iterates over and over.

   A CDU matrix is computed by the PCE, but unallocated timeSlots may be
   used opportunistically by the nodes for classical best effort IP
   traffic.  The PCE has precedence in the allocation in case of a
   conflict.

   In a given network, there might be multiple CDU matrices that operate
   with different width, so they have different durations and represent
   different periodic operations.  It is recommended that all CDU
   matrices in a 6TiSCH domain operate with the same cell duration and
   are aligned, so as to reduce the chances of interferences from
   slotted-aloha operations.  The PCE MUST compute the CDU matrices and
   shared that knowledge with all the nodes.  The matrices are used in
   particular to define slotFrames.

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

   Multiple slotFrames can coexist in a node schedule, i.e., a node can
   have multiple activities scheduled in different slotFrames, based on
   the precedence of the 6TiSCH topologies.  The slotFrames may be



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   aligned to different CDU matrices and thus have different width.
   There is typically one slotFrame for scheduled traffic that has the
   highest precedence and one or more slotFrame(s) for RPL traffic.  The
   timeSlots in the slotFrame are indexed by the SlotOffset; the first
   cell is at SlotOffset 0.

   The 6TiSCH architecture introduces the concept of chunks
   ([I-D.ietf-6tisch-architecture]) to operate such spectrum
   distribution for a whole group of cells at a time.  The CDU matrix is
   formatted into a set of chunks, each of them identified uniquely by a
   chunk-ID.  The PCE MUST compute the partitioning of CDU matrices into
   chunks and shared that knowledge with all the nodes in a 6TiSCH
   network.


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


                Figure 4: CDU matrix Partitioning in Chunks

   The appropriation of a chunk can be requested explicitly by the PCE
   to any node.  After a successful appropriation, the PCE owns the
   cells in that chunk, and may use them as hard cells to set up Tracks.
   Then again, 6TiSCH did not propose a method for chunk definition and
   a protocol for appropriation.  This is to be done at PAW.

3.2.  Schedule Management by a PCE

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

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





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

3.3.  Track Scheduling Protocol

   Section "Schedule Management Mechanisms" of the 6TiSCH architecture
   describes 4 paradigms to manage the TSCH schedule of the LLN nodes:
   Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring
   and scheduling management, and Hop-by-hop scheduling.  The Track
   operation for DetNet corresponds to a remote monitoring and
   scheduling management by a PCE.

   Early work at 6TiSCH on a data model and a protocol to program the
   schedule in the 6TiSCH device was never concluded as the group
   focussed on best effort traffic.  This work would be revived by PAW:

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

      [I-D.ietf-6tisch-coap] (to be reopened at PAW) was intended to
      define a mapping of the 6top set of commands, which is described
      in [I-D.ietf-6tisch-6top-interface], to CoAP resources.  This
      allows an entity to interact with the 6top layer of a node that is
      multiple hops away in a RESTful fashion.

      [I-D.ietf-6tisch-coap] also defined a basic set CoAP resources and
      associated RESTful access methods (GET/PUT/POST/DELETE).  The
      payload (body) of the CoAP messages is encoded using the CBOR
      format.  The PCE commands are expected to be issued directly as
      CoAP requests or to be mapped back and forth into CoAP by a
      gateway function at the edge of the 6TiSCH network.  For instance,
      it is possible that a mapping entity on the backbone transforms a
      non-CoAP protocol such as PCEP into the RESTful interfaces that
      the 6TiSCH devices support.








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3.4.  Track Forwarding

   By forwarding, this specification means the per-packet operation that
   allows to deliver a packet to a next hop or an upper layer in this
   node.  Forwarding is based on pre-existing state that was installed
   as a result of the routing computation of a Track by a PCE.  The
   6TiSCH architecture supports three different forwarding model, G-MPLS
   Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6
   Forwarding (6F) which is the classical IP operation.  The DetNet case
   relates to the Track Forwarding operation under the control of a PCE.

   A Track is a unidirectional path between a source and a destination.
   In a Track cell, the normal operation of IEEE802.15.4 Automatic
   Repeat-reQuest (ARQ) usually happens, though the acknowledgment may
   be omitted in some cases, for instance if there is no scheduled cell
   for a retry.

   Track Forwarding is the simplest and fastest.  A bundle of cells set
   to receive (RX-cells) is uniquely paired to a bundle of cells that
   are set to transmit (TX-cells), representing a layer-2 forwarding
   state that can be used regardless of the network layer protocol.
   This model can effectively be seen as a Generalized Multi-protocol
   Label Switching (G-MPLS) operation in that the information used to
   switch a frame is not an explicit label, but rather related to other
   properties of the way the packet was received, a particular cell in
   the case of 6TiSCH.  As a result, as long as the TSCH MAC (and
   Layer-2 security) accepts a frame, that frame can be switched
   regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN
   fragment, or a frame from an alternate protocol such as WirelessHART
   or ISA100.11a.

   A data frame that is forwarded along a Track normally has a
   destination MAC address that is set to broadcast - or a multicast
   address depending on MAC support.  This way, the MAC layer in the
   intermediate nodes accepts the incoming frame and 6top switches it
   without incurring a change in the MAC header.  In the case of
   IEEE802.15.4, this means effectively broadcast, so that along the
   Track the short address for the destination of the frame is set to
   0xFFFF.

   A Track is thus formed end-to-end as a succession of paired bundles,
   a receive bundle from the previous hop and a transmit bundle to the
   next hop along the Track, and a cell in such a bundle belongs to at
   most one Track.  For a given iteration of the device schedule, the
   effective channel of the cell is obtained by adding a pseudo-random
   number to the channelOffset of the cell, which results in a rotation
   of the frequency that used for transmission.  The bundles may be
   computed so as to accommodate both variable rates and



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   retransmissions, so they might not be fully used at a given iteration
   of the schedule.  The 6TiSCH architecture provides additional means
   to avoid waste of cells as well as overflows in the transmit bundle,
   as follows:

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

   On the other hand, it might happen that there are not enough TX-cells
   in the transmit bundle to accommodate the Track traffic, for instance
   if more retransmissions are needed than provisioned.  In that case,
   the frame can be placed for transmission in the bundle that is used
   for layer-3 traffic towards the next hop along the track as long as
   it can be routed by the upper layer, that is, typically, if the frame
   transports an IPv6 packet.  The MAC address should be set to the
   next-hop MAC address to avoid confusion.  It results that a frame
   that is received over a layer-3 bundle may be in fact associated to a
   Track.  In a classical IP link such as an Ethernet, off-track traffic
   is typically in excess over reservation to be routed along the non-
   reserved path based on its QoS setting.  But with 6TiSCH, since the
   use of the layer-3 bundle may be due to transmission failures, it
   makes sense for the receiver to recognize a frame that should be re-
   tracked, and to place it back on the appropriate bundle if possible.
   A frame should be re-tracked if the Per-Hop-Behavior group indicated
   in the Differentiated Services Field in the IPv6 header is set to
   Deterministic Forwarding, as discussed in Section 4.1.  A frame is
   re-tracked by scheduling it for transmission over the transmit bundle
   associated to the Track, with the destination MAC address set to
   broadcast.

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

3.4.1.  Transport Mode

   In transport mode, the Protocol Data Unit (PDU) is associated with
   flow-dependant meta-data that refers uniquely to the Track, so the
   6top sublayer can place the frame in the appropriate cell without



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   ambiguity.  In the case of IPv6 traffic, this flow identification is
   transported in the Flow Label of the IPv6 header.  Associated with
   the source IPv6 address, the Flow Label forms a globally unique
   identifier for that particular Track that is validated at egress
   before restoring the destination MAC address (DMAC) and punting to
   the upper layer.

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

                     Track Forwarding, Transport Mode

3.4.2.  Tunnel Mode

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

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
















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

                  Figure 5: Track Forwarding, Tunnel Mode

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

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

3.4.3.  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.

   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



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   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.

3.4.4.  OAM

   An Overview of Operations, Administration, and Maintenance (OAM)
   Tools [RFC7276] provides an overwiew of the existing tooling for OAM
   [RFC6291].  Tracks are complex paths and new tooling is necessary to
   manage them, with respect to load control, timing, and the Packet
   Replication and Elimination Functions (PREF).

   An example of such tooling can be found in the context of BIER
   [RFC8279] and more specifically BIER Traffic Engineering
   [I-D.ietf-bier-te-arch] (BIER-TE):
   [I-D.thubert-bier-replication-elimination] leverages BIER-TE to
   control the process of PREF, and to provide traceability of these
   operations, in the deterministic dataplane, along a complex Track.
   For the 6TiSCH type of constrained environment,
   [I-D.thubert-6lo-bier-dispatch] enables an efficient encoding of the
   BIER bitmap within the 6LoRH framework.

4.  Operations of Interest for DetNet and PCE

   In a classical system, the 6TiSCH device does not place the request
   for bandwidth between self and another device in the network.
   Rather, an Operation Control System invoked through an Human/Machine
   Interface (HMI) indicates the Traffic Specification, in particular in
   terms of latency and reliability, and the end nodes.  With this, the
   PCE must compute a Track between the end nodes and provision the
   network with per-flow state that describes the per-hop operation for
   a given packet, the corresponding timeSlots, and the flow
   identification that enables to recognize when a certain packet
   belongs to a certain Track, sort out duplicates, etc...

   For a static configuration that serves a certain purpose for a long
   period of time, it is expected that a node will be provisioned in one
   shot with a full schedule, which incorporates the aggregation of its
   behavior for multiple Tracks. 6TiSCH expects that the programing of
   the schedule will be done over COAP as discussed in 6TiSCH Resource
   Management and Interaction using CoAP [I-D.ietf-6tisch-coap].

   But an Hybrid mode may be required as well whereby a single Track is
   added, modified, or removed, for instance if it appears that a Track
   does not perform as expected for, say, PDR.  For that case, the
   expectation is that a protocol that flows along a Track (to be), in a
   fashion similar to classical Traffic Engineering (TE) [CCAMP], may be



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   used to update the state in the devices.  6TiSCH provides means for a
   device to negotiate a timeSlot with a neighbor, but in general that
   flow was not designed and no protocol was selected and it is expected
   that DetNet will determine the appropriate end-to-end protocols to be
   used in that case.

                         Stream Management Entity


                         Operational System and HMI

      -+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                PCE         PCE              PCE              PCE

      -+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

              --- 6TiSCH------6TiSCH------6TiSCH------6TiSCH--
     6TiSCH /     Device      Device      Device      Device   \
     Device-                                                    - 6TiSCH
            \     6TiSCH      6TiSCH      6TiSCH      6TiSCH   /  Device
              ----Device------Device------Device------Device--


                                 Figure 6

4.1.  Packet Marking and Handling

   Section "Packet Marking and Handling" of
   [I-D.ietf-6tisch-architecture] describes the packet tagging and
   marking that is expected in 6TiSCH networks.

4.1.1.  Tagging Packets for Flow Identification

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

   It results that the tagging that is used for a DetNet flow outside
   the 6TiSCH LLN MUST be swapped into 6TiSCH formats and back as the
   packet enters and then leaves the 6TiSCH network.

   Note: The method and format used for encoding the RPLInstanceID at
   6lo is generalized to all 6TiSCH topological Instances, which
   includes Tracks.





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4.1.2.  Replication, Retries and Elimination

   6TiSCH expects elimination and replication of packets along a complex
   Track, but has no position about how the sequence numbers would be
   tagged in the packet.

   As it goes, 6TiSCH expects that timeSlots corresponding to copies of
   a same packet along a Track are correlated by configuration, and does
   not need to process the sequence numbers.

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

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

   On the receive side, all timeSlots are programmed in a same 'OR'
   group.  Retries of a same copy as well as converging branches for
   elimination are converged, meaning that the first successful
   reception is enough and that all the other timeSlots can be ignored.

4.1.3.  Differentiated Services Per-Hop-Behavior

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







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4.2.  Topology and capabilities

   6TiSCH nodes are usually IoT devices, characterized by very limited
   amount of memory, just enough buffers to store one or a few IPv6
   packets, and limited bandwidth between peers.  It results that a node
   will maintain only a small number of peering information, and will
   not be able to store many packets waiting to be forwarded.  Peers can
   be identified through MAC or IPv6 addresses.

   Neighbors can be discovered over the radio using mechanism such as
   beacons, but, though the neighbor information is available in the
   6TiSCH interface data model, 6TiSCH does not describe a protocol to
   pro-actively push the neighborhood information to a PCE.  This
   protocol should be described and should operate over CoAP.  The
   protocol should be able to carry multiple metrics, in particular the
   same metrics as used for RPL operations [RFC6551]

   The energy that the device consumes in sleep, transmit and receive
   modes can be evaluated and reported.  So can the amount of energy
   that is stored in the device and the power that it can be scavenged
   from the environment.  The PCE SHOULD be able to compute Tracks that
   will implement policies on how the energy is consumed, for instance
   balance between nodes, ensure that the spent energy does not exceeded
   the scavenged energy over a period of time, etc...

5.  IANA Considerations

   This specification does not require IANA action.

6.  Security Considerations

   On top of the classical protection of control signaling that can be
   expected to support DetNet, it must be noted that 6TiSCH networks
   operate on limited resources that can be depleted rapidly if an
   attacker manages to operate a DoS attack on the system, for instance
   by placing a rogue device in the network, or by obtaining management
   control and to setup extra paths.

7.  Acknowledgments

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

   The authors wish to thank: Kris Pister, Thomas Watteyne, Xavier
   Vilajosana, Qin Wang, Tom Phinney, Robert Assimiti, Michael
   Richardson, Zhuo Chen, Malisa Vucinic, Alfredo Grieco, Martin Turon,



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   Dominique Barthel, Elvis Vogli, Guillaume Gaillard, Herman Storey,
   Maria Rita Palattella, Nicola Accettura, Patrick Wetterwald, Pouria
   Zand, Raghuram Sudhaakar, and Shitanshu Shah for their participation
   and various contributions.

8.  References

8.1.  Normative References

   [I-D.ietf-6tisch-6top-interface]
              Wang, Q. and X. Vilajosana, "6TiSCH Operation Sublayer
              (6top) Interface", draft-ietf-6tisch-6top-interface-04
              (work in progress), July 2015.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-19 (work
              in progress), December 2018.

   [I-D.ietf-6tisch-coap]
              Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
              Interaction using CoAP", draft-ietf-6tisch-coap-03 (work
              in progress), March 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

8.2.  Informative References

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

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

   [I-D.ietf-6lo-backbone-router]
              Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", draft-ietf-6lo-backbone-router-10 (work
              in progress), January 2019.

   [I-D.ietf-bier-te-arch]
              Eckert, T., Cauchie, G., Braun, W., and M. Menth, "Traffic
              Engineering for Bit Index Explicit Replication (BIER-TE)",
              draft-ietf-bier-te-arch-01 (work in progress), October
              2018.



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   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-10 (work in progress), December 2018.

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

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

   [I-D.thubert-6lo-bier-dispatch]
              Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
              6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-05
              (work in progress), July 2018.

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

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

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

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




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   [ISA100]   ISA/ANSI, "ISA100, Wireless Systems for Automation",
              <https://www.isa.org/isa100/>.

   [ISA100.11a]
              ISA/ANSI, "Wireless Systems for Industrial Automation:
              Process Control and Related Applications - ISA100.11a-2011
              - IEC 62734", 2011, <http://www.isa.org/Community/
              SP100WirelessSystemsforAutomation>.

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

   [RFC3444]  Pras, A. and J. Schoenwaelder, "On the Difference between
              Information Models and Data Models", RFC 3444,
              DOI 10.17487/RFC3444, January 2003,
              <https://www.rfc-editor.org/info/rfc3444>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291,
              DOI 10.17487/RFC6291, June 2011,
              <https://www.rfc-editor.org/info/rfc6291>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <https://www.rfc-editor.org/info/rfc6551>.

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,
              <https://www.rfc-editor.org/info/rfc7276>.






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

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

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

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

Author's Address

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

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




















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