One document matched: draft-ietf-6tisch-architecture-10.xml
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<rfc category="info" docName="draft-ietf-6tisch-architecture-10" ipr="trust200902">
<front>
<title abbrev="6tisch-architecture">An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4</title>
<author initials="P" surname="Thubert" fullname="Pascal Thubert" role="editor">
<organization abbrev="Cisco">Cisco Systems, Inc</organization>
<address>
<postal>
<street>Building D</street>
<street>45 Allee des Ormes - BP1200 </street>
<city>MOUGINS - Sophia Antipolis</city>
<code>06254</code>
<country>FRANCE</country>
</postal>
<phone>+33 497 23 26 34</phone>
<email>pthubert@cisco.com</email>
</address>
</author>
<!--
<author initials="T" surname="Watteyne" fullname="Thomas Watteyne">
<organization abbrev="Linear Technology">Linear Technology, Dust Networks Product Group</organization>
<address>
<postal>
<street>30695 Huntwood Avenue</street>
<city>Hayward</city>
<region>CA</region>
<code>94544</code>
<country>USA</country>
</postal>
<phone>+1 (510) 400-2978</phone>
<email>twatteyne@linear.com</email>
</address>
</author>
<author fullname="Rene Struik" initials="R" surname="Struik">
<organization abbrev="Struik Security Consultancy">Struik Security Consultancy</organization>
<address>
<email>rstruik.ext@gmail.com</email>
</address>
</author>
<author initials="M." surname="Richardson" fullname="Michael C. Richardson">
<organization abbrev="SSW">Sandelman Software Works</organization>
<address>
<postal>
<street>470 Dawson Avenue</street>
<city>Ottawa</city>
<region>ON</region>
<code>K1Z 5V7</code>
<country>CA</country>
</postal>
<email>mcr+ietf@sandelman.ca</email>
<uri>http://www.sandelman.ca/</uri>
</address>
</author>
<author initials="X" surname="Vilajosana" fullname="Xavier Vilajosana" >
<organization>Universitat Oberta de Catalunya</organization>
<address>
<postal>
<street>156 Rambla Poblenou</street>
<city>Barcelona</city>
<region>Catalonia</region>
<code>08018</code>
<country>Spain</country>
</postal>
<phone>+34 (646) 633 681</phone>
<email>xvilajosana@uoc.edu</email>
</address>
</author-->
<!--author initials="Q" surname="Wang" fullname="Qin Wang" role="editor">
<organization>Univ. of Sci. and Tech. Beijing </organization>
<address>
<postal>
<street>30 Xueyuan Road</street>
<city>Beijing</city>
<region>Hebei</region>
<code>100083</code>
<country>China</country>
</postal>
<phone>+86 (10) 6233 4781</phone>
<email>wangqin@ies.ustb.edu.cn</email>
</address>
</author-->
<!--author initials="RA" surname="Assimiti" fullname="Robert Assimiti">
<organization abbrev="Centero">Centero</organization>
<address>
<postal>
<street>961 Indian Hills Parkway</street>
<city>Marietta</city>
<region>GA</region>
<code>30068</code>
<country>USA</country>
</postal>
<phone>+1 404 461 9614</phone>
<email>robert.assimiti@centerotech.com</email>
</address>
</author-->
<date/>
<area>Internet Area</area>
<workgroup>6TiSCH</workgroup>
<keyword>Draft</keyword>
<abstract>
<t> This document describes a network architecture that provides
low-latency, low-jitter and high-reliability packet delivery. It
combines a high speed powered backbone and subnetworks using IEEE
802.15.4 time-slotted channel hopping (TSCH) to meet the
requirements of LowPower wireless deterministic applications.
<!--
This document presents the 6TiSCH architecture of an IPv6
Multi-Link subnet that is composed of a high speed powered backbone and
a number of IEEE802.15.4 TSCH low-power wireless networks attached and
synchronized by Backbone Routers. The architecture defines mechanisms
to establish and maintain routing and scheduling in a centralized,
distributed, or mixed fashion.
Backbone Routers perform proxy Neighbor Discovery operations over
the backbone on behalf of the wireless devices, so they can share a same
subnet and appear to be connected to the same backbone as classical devices.
-->
</t>
</abstract>
<!--note title="Requirements Language">
<t>
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 <xref target="RFC2119">RFC 2119</xref>.
</t>
</note-->
</front>
<middle>
<section title="Introduction">
<t>
Wireless Networks enable a wide variety of devices of any size
to get interconnected, often at a very low marginal cost per device,
at any distance ranging from Near Field to interplanetary, and in
circumstances where wiring may be impractical, for instance
on fast-moving or rotating devices.
</t>
<t>
In the other hand, Deterministic Networks enable traffic that
is highly sensitive to jitter, quite sensitive to latency,
and with a high degree of operational criticality so that
loss should be minimized at all times.
<!-- At IEEE802.1, the
<xref target="IEEE802.1TSNTG">Time Sensitive Networking</xref>(TSN)
task group was formed to provide deterministic properties at Layer-2
across multiple hops. -->
Applications that need such networks are presented in <xref target="I-D.ietf-detnet-use-cases"/>. They include Professional Media and
Operation Technology (OT) Industrial Automation Control Systems (IACS).
</t>
<t>
The Medium access Control (MAC) of IEEE802.15.4
<xref target="IEEE802154"/> has evolved with the
<xref target="RFC7554">
IEEE802.15.4e Timeslotted Channel Hopping (TSCH)</xref> mode
to provide deterministic properties on wireless networks.
TSCH was initially
introduced with the IEEE802.15.4e <xref target="IEEE802154e">amendment
</xref> of the IEEE802.15.4 standard and constituted a part of the
standard from that day. For all practical purpose, this document
is expected to be insensitive to the revisions of
the IEEE802.15.4 standard, which is thus referenced undated.
</t>
<t>
Proven Deterministic Networking standards for use in Process Control,
including ISA100.11a <xref target="ISA100.11a"/> and WirelessHART
<xref target="WirelessHART"/>, have demonstrated the capabilities
of the IEEE802.15.4 TSCH MAC for high reliability against interference,
low-power consumption on well-known flows, and its applicability for
Traffic Engineering (TE) from a central controller.
</t>
<t>In order to enable the convergence of IT and OT in LLN environments,
6TiSCH ports the IETF suite of protocol that are defined for such
environments over the TSCH MAC. 6TiSCH also provides large scaling
capabilities, which, in a number of scenarios, require the addition of
a high speed and reliable backbone and the use of IP version 6 (IPv6).
The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet model
that is composed of a federating backbone and a number of IEEE802.15.4
TSCH low-power wireless networks attached and synchronized by Backbone
Routers.
</t>
<t>
The architecture defines mechanisms
to establish and maintain routing and scheduling in a centralized,
distributed, or mixed fashion, for use in multiple OT environments.
It is applicable in particular to industrial control systems, building
automation that leverage distributed routing 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.).
</t>
</section>
<section title="Terminology">
<t>
The draft uses domain-specific terminology defined or referenced in
<xref target="I-D.ietf-6tisch-terminology"/>,
<xref target="I-D.ietf-6lo-backbone-router"/>, and
<xref target="I-D.ietf-roll-rpl-industrial-applicability"/>.
</t>
<t>
Readers are expected to be familiar with all the terms and concepts
that are discussed in <xref target="RFC4861">"Neighbor Discovery for
IP version 6"</xref>, <xref target="RFC4919">"IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions,
Problem Statement, and Goals"</xref>, and
<xref target="RFC6775">Neighbor Discovery Optimization
for Low-power and Lossy Networks</xref> where the 6LoWPAN Router
(6LR) and the 6LoWPAN Border Router (6LBR) are introduced.
<!--, and
<xref target="I-D.ietf-ipv6-Multi-Link-subnets">
"Multi-link Subnet Support in IPv6"</xref> -->
</t>
<t>
Readers may benefit from reading the <xref target="RFC6550"> "RPL:
IPv6 Routing Protocol for Low-Power and Lossy Networks" </xref> specification;
<xref target="RFC4903">"Multi-Link Subnet Issues"</xref>;
<xref target="RFC6275"> "Mobility Support in IPv6" </xref>;
<xref target="RFC4389"> "Neighbor Discovery Proxies (ND Proxy)" </xref>;
<xref target="RFC4862">"IPv6 Stateless Address Autoconfiguration"</xref>;
<xref target="RFC6620">"FCFS SAVI: First-Come, First-Served Source
Address Validation Improvement for Locally Assigned IPv6 Addresses"</xref>; and
<xref target="RFC4429">"Optimistic Duplicate Address Detection"</xref>
prior to this specification for a clear understanding of the art in ND-proxying and binding.
</t>
<t>
The draft also conforms to the terms and models described in
<xref target="RFC3444"/> and <xref target="RFC5889"/> and uses the vocabulary and the concepts
defined in <xref target="RFC4291"/> for the IPv6 Architecture and refers
<xref target="RFC4080"/> for reservation signaling and <xref target="RFC5191"/>
for authentication.
</t>
</section>
<section title="High Level Architecture">
<section anchor="fsixstac" title="6TiSCH Stack">
<t>
The 6TiSCH architecture presents a reference stack that is implemented
and interop tested by a conjunction of opensource, IETF and ETSI efforts.
One goal is to help other bodies to adopt the stack as a whole, making the
effort to move to an IPv6-based IOT stack easier. Now, for a particular,
environment, some of the choices that are made in this architecture may not
be relevant. For instance, RPL is not required for star topologies and
mesh-under layer-2 routed networks, and the 6LoWPAN compression may not be
sufficient for ultra-constrained cases such as some Low Power Wide Area
(LPWA) networks. In such cases, it is perfectly doable to adopt a subset
of the selection that is presented hereafter and then select alternate
components to complete the solution wherever needed.
</t>
<t>
The IETF proposes multiple techniques for implementing functions related
to routing, transport or security. In order to control the complexity of
the possible deployments and device interactions, and to limit the size of
the resulting object code, the architecture limits the possible variations
of the stack and recommends a number of base elements for LLN applications.
In particular, UDP <xref target="RFC0768"/> <xref target="RFC2460"/> and
the <xref target="RFC7252">Constrained Application Protocol</xref> (CoAP)
are used as the transport / binding of choice for applications and
management as opposed to TCP and HTTP.
</t>
<t>
The resulting stack is represented below:
</t>
<t>
<figure anchor="fig4" title="6TiSCH Protocol Stack">
<artwork><![CDATA[
+-----+-----+-----+------+-------+-----+
| (COMI) |(PANA)|6LoWPAN| RPL |
| CoAP / DTLS | | ND | |
+-----+-----+-----+------+-------+-----+
| UDP | ICMP |
+-----+-----+-----+-----+-------+------+-----+
| IPv6 |
+-------------------------------------------+
| 6LoWPAN adaptation and compression (HC) |
+-------------------------------------------+
| 6top |
+-------------------------------------------+
| IEEE802.15.4 TSCH |
+-------------------------------------------+
]]></artwork>
</figure>
</t>
<t>
RPL is the routing protocol of choice for LLNs. So far, there was no
identified need to define a 6TiSCH specific Objective Function.
The <xref target="I-D.ietf-6tisch-minimal">Minimal 6TiSCH Configuration
</xref> describes the operation of RPL over a static schedule used in
a slotted aloha fashion, whereby all active slots may be used for
emission or reception of both unicast and multicast frames.
</t>
<t>
The <xref target="RFC6282">6LoWPAN Header Compression</xref> is used
to compress the IPv6 and UDP headers, whereas the
<xref target="I-D.ietf-roll-routing-dispatch">
6LoWPAN Routing Header</xref> is used to compress the RPL artifacts in
the IPv6 data packets, including the RPL Packet Information (RPI),
the IP-in-IP encapsulation to/from the RPL root, and the Source Route
Header (SRH) in non-storing mode.
</t>
<t>
<!--The COMAN list 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.-->
6TiSCH has adopted the general direction of
<xref target="I-D.vanderstok-core-comi">
CoAP Management Interface (COMI)</xref> for the management of devices.
This is leveraged for instance for the implementation of the generic
data model for the 6top sublayer management interface
<xref target="I-D.ietf-6tisch-6top-interface"/>.
The proposed implementation is based on CoAP and CBOR,
and specified in <xref target="I-D.ietf-6tisch-coap">
6TiSCH Resource Management and Interaction using CoAP</xref>.
</t>
<t>
The <xref target="RFC6347">Datagram Transport Layer Security (DTLS)
</xref> is represented as an example of a protocol that could be used
to protect CoAP datagrams, but the exact stack is not determined at the
time of this writing..
</t>
<t>
Similarly, the <xref target="RFC5191">
Protocol for Carrying Authentication for Network access (PANA)</xref>
is represented as an example of a protocol that could be leveraged to
secure the join process, as a Layer-3 alternate to IEEE802.1x/EAP.
Regardless, the security model ensures that, prior to a join process,
packets from a untrusted device are controlled in volume and in
reachability. In particular, a PANA stack should be separated from
the main protocol stack to avoid attacks during the join process
that is introduced in <xref target='rflo'/>.
An overview of the security aspects of the join process can be found in
<xref target="sec"/>.
</t>
<t>
The <xref target="I-D.wang-6tisch-6top-sublayer">6TiSCH Operation
sublayer (6top)</xref> is a sublayer of a Logical Link Control (LLC)
that provides the abstraction of an IP link over a TSCH MAC and
schedules packets over TSCH cells,as further discussed in the next
sections.
</t>
</section>
<section title="TSCH: A Deterministic MAC Layer">
<t>
Though at a different time scale (several orders of magnitude),
both IEEE802.1TSN and IEEE802.15.4TSCH
standards provide Deterministic capabilities to the point that a packet
that pertains to a certain flow may traverse a network from node to node following
a very precise schedule, as a train that enters and then leaves intermediate stations
at precise times along its path. With TSCH, time is formatted into
timeslots, and individual communication cells are 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 Wi-Fi emitters).
</t>
<t>
6TiSCH builds on the IEEE802.15.4TSCH MAC and inherits its advanced
capabilities to enable them in multiple environments where they can
be leveraged to improve automated operations.
The 6TiSCH Architecture also inherits the capability to perform a
centralized route computation to achieve deterministic properties,
though it relies on the IETF
<xref target="I-D.finn-detnet-architecture">DetNet Architecture</xref>,
and IETF components such as the Path Computation Element (PCE)
<xref target="PCE"/>, for the protocol aspects.
</t>
<t>On top of this inheritance, 6TiSCH adds capabilities for distributed
routing and scheduling operations based on the RPL routing protocol
and capabilities to negotiate schedule adjustments between peers.
These distributed routing and scheduling operations simplify the
deployment of TSCH networks and enable wireless solutions in a larger
variety of use cases from operational technology in general. Examples
of such use-cases in industrial environments 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, as
presented in
<xref target="I-D.ietf-roll-rpl-industrial-applicability"/>.
</t>
</section>
<section title="Scheduling TSCH">
<t>A scheduling operation attributes cells in a Time-Division-Multiplexing
(TDM) / Frequency-Division Multiplexing (FDM) matrix called the Channel
distribution/usage (CDU) to either individual transmissions
or as multi-access shared resources (see the
<xref target="I-D.ietf-6tisch-terminology">6TiSCH Terminology</xref>
for more on these terms). Scheduling effectively enables
multiple communications at a same time in a same interference domain
using different channels; but a node equipped with a single radio can
only transmit or receive on one channel at any given point of time.
</t>
<t>
From the standpoint of a 6TiSCH node (at the MAC layer), its
schedule is the collection of the times at which it must wake up for
transmission, and the channels to which it should either send or listen
at those times. The schedule is expressed as one or more slotframes that
repeat over and over. Slotframes may collision and require a device to
wake at a same time, in which case a priority indicates which slotframe
is actually activated.
</t>
<t>The 6top sublayer hides the complexity of the schedule to the upper
layers. The Link that IP may utilize between the 6TiSCH node and a peer
may in fact be composed of a pair of cell bundles, one to receive and
one to transmit. Some of the cells may be shared, in which case the 6top
sublayer must perform some arbitration.
</t>
<t>The 6TiSCH architecture identifies four ways a schedule can be managed
and CDU cells can be allocated: Static Scheduling, Neighbor-to-Neighbor
Scheduling, Remote Monitoring and Schedule Management, and Hop-by-hop
Scheduling.
<list style="hanging">
<t hangText="Static Scheduling:">This refers to the minimal
6TiSCH operation whereby a static schedule is configured for the whole
network for use in a slotted-aloha fashion. The static schedule is
distributed through the native methods in the TSCH MAC layer.
This operation leverages RPL to maintain a loopless graph for routing
and time distribution. It is specified in the
<xref target="I-D.ietf-6tisch-minimal">Minimal 6TiSCH Configuration
</xref> specification.
and does not preclude other scheduling operations to co-exist on a same
6TiSCH network.</t>
<t hangText="Neighbor-to-Neighbor Scheduling:">This refers to the
dynamic adaptation of the bandwidth of the Links that are used for IPv6
traffic between adjacent routers. Scheduling Functions such as
<xref target="I-D.ietf-6tisch-6top-sf0">SF0</xref> influence the
operation of the
<xref target="I-D.wang-6tisch-6top-sublayer">6top sublayer</xref> to
add and remove cells in peers schedule, using the
<xref target="I-D.ietf-6tisch-6top-protocol">6top protocol</xref> for
the negotiation on the MAC resources.</t>
<t hangText="Remote Monitoring and Schedule Management:">This
refers to the central computation of a schedule and the capability
to forward a frame based on the cell of arrival. In that case,
the related portion of the device schedule as well as other device
resources are managed by an abstract Network Management Entity (NME),
which may cooperate with the PCE in order to minimize the interaction
with and the load on the constrained device.
This model is the TSCH adaption of the
<xref target="I-D.finn-detnet-architecture">DetNet Architecture</xref>,
and it enables Traffic Engineering with deterministic properties.
</t>
<t hangText="Hop-by-hop Scheduling:">This refers to the possibility to
reserves cells along a path for a particular flow using a distributed
mechanism.</t>
</list>
</t> <t>
It is not expected that all use cases will require all those mechanisms.
Static Scheduling with minimal configuration one is the only one that
is expected in all implementations, since it provides a simple and
solid basis for convergecast routing and time distribution.
</t><t>
A deeper dive in those mechanisms can be found in <xref target="schd"/>.
</t>
</section>
<section title="Routing and Forwarding Over TSCH">
<t>6TiSCH leverages the RPL routing protocol for interoperable distributed
routing operations. RPL is applicable to Static Scheduling and
Neighbor-to-Neighbor Scheduling. The architecture also supports a
centralized routing model for Remote Monitoring and Schedule Management.
It is expected that a routing protocol that is more optimized for
point-to-point routing than RPL, such as the <xref target="RFC6997">
Reactive Discovery of Point-to-Point Routes in Low-Power and Lossy
Networks</xref>(P2P RPL), or the <xref target="I-D.ietf-manet-aodvv2">
Ad Hoc On-demand Distance Vector Routing (AODV)</xref> will be
selected for Hop-by-hop Scheduling.
</t>
<t>
The 6TiSCH architecture supports three different forwarding models, the
classical IPv6 Forwarding, where the node selects a feasible successor
at Layer-3 on a per packet basis and based on its routing table,
G-MPLS Track Forwarding, which switches a frame received at a particular
Timeslot into another Timeslot at Layer-2, and
6LoWPAN Fragment Forwarding, which allows to forward individual 6loWPAN
fragments along the route set by the first fragment.
<list style="hanging">
<t hangText="IPv6 Forwarding:">This is the classical IP forwarding
model, with a Routing Information Based (RIB) that is installed by the
RPL routing protocol and used to select a feasible successor per packet.
The packet is placed on an outgoing Link, that the 6top layer maps into
a (Layer-3) bundle of cells, and scheduled for transmission based on QoS
parameters. On top of RPL, this model also applies to any routing
protocol which may be operated in the 6TiSCH network, and corresponds
to all the distributed scheduling models, Static, Neighbor-to-Neighbor
and Hop-by-Hop Scheduling.</t>
<t hangText="G-MPLS Track Forwarding:">This model corresponds to the
Remote Monitoring and Schedule Management. In this model, A central
controller (hosting a PCE) computes and installs the schedules in the
devices per flow. The incoming (Layer-2) bundle of cells from the
previous node along the path determines the outgoing (Layer-2) bundle
towards the next hop for that flow as determined by the PCE. The
programmed sequence for bundles is called a Track and can assume shapes
that are more complex than a simple direct sequence of nodes.</t>
<t hangText="6LoWPAN Fragment Forwarding:">This is an hybrid model
that derives from IPv6 forwarding for the case where packets must
be fragmented at the 6LoWPAN sublayer. The first fragment is forwarded
like any IPv6 packet and leaves a state in the intermediate hops to
enable forwarding of the next fragments that do not have a IP header
without the need to recompose the packet at every hop.</t>
</list>
</t>
<t> This can be broadly summarized in the following table:
<figure anchor="RaF" title="Routing, Forwarding and Scheduling">
<artwork>
<![CDATA[
+---------------------+------------+-----------------------------------+
| Forwarding Model | Routing | Scheduling |
+=====================+============+===================================+
|G-MPLS Track Fwrding | PCE |Remote Monitoring and Schedule Mgt |
+---------------------+------------+-----------------------------------+
| | | Static (Minimal Configuration) |
+ classical IPv6 + RPL +-----------------------------------+
| / | | Neighbor-to-Neighbor (SF0) |
+ 6LoWPAN Fragment F. +------------+-----------------------------------+
| |Reactive P2P| Hop-by-Hop (TBD) |
+---------------------+------------+-----------------------------------+
]]>
</artwork>
</figure>
</t>
</section>
<section title="A Non-Broadcast Multi-Access Radio Mesh Network">
<t>
A 6TiSCH network is an IPv6 <xref target="RFC2460"/> subnet which, in
its basic configuration, is a single Low Power Lossy Network (LLN)
operating over a synchronized TSCH-based mesh.
</t><t>
Inside a 6TiSCH LLN, nodes rely on <xref target="RFC6282">6LoWPAN
Header Compression (6LoWPAN HC)</xref> to encode IPv6 packets.
From the perspective of the network layer, 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.
</t><t>
6TiSCH nodes are not necessarily reachable from one another at Layer-2
and an LLN may span over multiple links. This effectively forms an
homogeneous non-broadcast multi-access (NBMA) subnet, which is beyond
the scope of existing IPv6 ND methods. Extensions to IPv6 ND have to be
introduced.
</t><t>
Within that subnet, neighbor devices are discovered with
<xref target="RFC6775"> 6LoWPAN Neighbor Discovery</xref> (6LoWPAN ND),
whereas <xref target="RFC6550">RPL</xref> enables routing
in the so called Route Over fashion, either in storing (stateful) or
non-storing (stateless, with routing headers) mode.
</t>
<t>
<figure anchor="fig1" title="Basic Configuration of a 6TiSCH Network">
<artwork><![CDATA[
---+-------- ............ ------------
| External Network |
| +-----+
+-----+ | NME |
| | LLN Border | |
| | router +-----+
+-----+
o o o
o o o o o
o o 6LoWPAN + RPL o o
o o o o
o o
]]></artwork>
</figure>
</t> <t>
6TiSCH nodes join the mesh by attaching to nodes that are already
members of the mesh. Some nodes act as routers for 6LoWPAN ND and RPL
operations, as detailed in <xref target="RPLvs6lo"/>.
Security aspects of the join process by which a device
obtains access to the network are discussed in <xref target="sec"/>.
</t><t>
With TSCH, devices are time-synchronized at the MAC level. The use of
a particular RPL Instance for time synchronization is discussed in
<xref target="sync"/>. With this mechanism, the time synchronization
starts at the RPL root and follows the RPL DODAGs with no timing loop.
</t><t>
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 6TiSCH node, the LLN Border Router
(LBR), acts as RPL root, 6LoWPAN HC terminator, and Border Router
for the LLN to the outside. The LBR is usually powered.
More on RPL Instances can be found in section 3.1 of
<xref target="RFC6550">RPL</xref>, in particular
"3.1.2. RPL Identifiers" and
"3.1.3. Instances, DODAGs, and DODAG Versions". RPL adds artifacts in
the data packets that are compressed with a 6LoWPAN addition
<xref target="I-D.ietf-roll-routing-dispatch">6LoRH</xref>.
</t><t>
Additional routing and scheduling protocols may be deployed to
establish on-demand Peer-to-Peer routes with particular characteristics
inside the 6TiSCH network.
This may be achieved in a centralized fashion by a PCE
<xref target="PCE"/> that programs both the routes and the schedules
inside the 6TiSCH nodes, or by in a distributed fashion using
a reactive routing protocol and a Hop-by-Hop scheduling protocol.
</t>
<t>
A Backbone Router may be connected to the node that acts as RPL root
and / or 6LoWPAN 6LBR and provides connectivity to the larger campus /
factory plant network over a high speed backbone or a back-haul link.
A Backbone Router may perform proxy
<xref target="RFC4861">IPv6 Neighbor Discovery (ND)</xref> operations
over the backbone on behalf of the 6TiSCH nodes
so they can share a same IPv6 subnet and appear to be
connected to the same backbone as classical devices. A Backbone
Router may alternatively redistribute the registration in a routing
protocol such as <xref target="RFC5340">OSPF</xref> or
<xref target="RFC2545">BGP</xref>, or inject them in a mobility
protocol such as <xref target="RFC6275">MIPv6</xref>,
<xref target="RFC3963">NEMO</xref>, or
<xref target="RFC6830">LISP</xref>.
</t>
<t>
This architecture expects that a 6LoWPAN node can connect as a
leaf to a RPL network, where the leaf support is the minimal
functionality to connect as a host to a RPL network without the need to
participate to the full routing protocol.
The architecture also expects that a 6LoWPAN node that is not aware
at all of the RPL protocol may also connect as a host but the
specifications for this to happen are not available at the time of this
writing.
</t>
</section>
<section title="A Multi-Link Subnet Model">
<t>
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
<xref target="RFC4861"/> and <xref target="RFC4862"/>.
This architecture requires work to standardize the
the registration of 6LoWPAN nodes to the Backbone Routers.
</t>
<t>
In the extended configuration, a Backbone Router (6BBR) operates
as described in
<xref target="I-D.ietf-6lo-backbone-router"/>.
The 6BBR performs ND proxy operations between the registered devices
and the classical ND devices that are located over the backbone.
6TiSCH 6BBRs synchronize with one another over the backbone, so as
to ensure that the multiple LLNs that form the IPv6 subnet stay
tightly synchronized.
</t>
<t>
<figure anchor="fig2" title="Extended Configuration of a 6TiSCH Network">
<artwork><![CDATA[
---+-------- ............ ------------
| 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
]]></artwork>
</figure>
</t>
<t>
As detailed in <xref target='RPLvs6lo'/> the 6LoWPAN ND 6LBR and
the root of the RPL network need to be collocated and share information
about the devices that is learned through either protocol but not both.
The combined RPL root and 6LBR may be collocated with the 6BBR, or
directly attached to the 6BBR. In the latter case, it leverages
the extended registration process defined in
<xref target="I-D.ietf-6lo-backbone-router"/> to proxy the 6LoWPAN ND
registration to the 6BBR on behalf of the LLN nodes, so that the 6BBR
may in turn perform proxy classical ND operations over the backbone.
</t>
<t>
If the Backbone is Deterministic (such as
defined by the Time Sensitive Networking WG at IEEE), then the
Backbone Router ensures that the end-to-end deterministic
behavior is maintained between the LLN and the backbone. The
<xref target="I-D.finn-detnet-architecture">DetNet Architecture</xref>
studies Layer-3 aspects of Deterministic Networks, and covers networks
that span multiple Layer-2 domains.
</t>
</section>
<section anchor='rflo' title="Join Process and Registration">
<t>
As detailed in <xref target='RPLvs6lo'/> the combined 6LoWPAN ND 6LBR
and root of the RPL network learn information such as the device Unique
ID (from 6LoWPAN ND) and the updated Sequence Number (from RPL), and
perform 6LoWPAN ND proxy registration to the 6BBR of behalf of the LLN
nodes.
<xref target='figReg'/> illustrates the periodic signaling that
starts at the leaf node with 6LoWPAN ND, is then carried
over RPL to the RPL root, and then to the 6BBR.
Efficient ND being an adaptation of 6LoWPAN ND, it makes sense to keep
those two homogeneous in the way they use the source and the target
addresses in the Neighbor Solicitation (NS) messages for registration,
as well as in the options that they use for that process.
<figure anchor='figReg' suppress-title='false'
title="(Re-)Registration Flow over Multi-Link Subnet">
<artwork><![CDATA[
6LoWPAN Node 6LR 6LBR 6BBR
(RPL leaf) (router) (root)
| | | |
| 6LoWPAN ND |6LoWPAN ND+RPL | Efficient ND | IPv6 ND
| LLN link |Route-Over mesh| IPv6 link | Backbone
| | | |
| NS(ARO) | | |
|-------------->| | |
| 6LoWPAN ND | DAR (then DAO)| |
| |-------------->| |
| | | NS(ARO) |
| | |-------------->|
| | | | DAD
| | | |------>
| | | |
| | | NA(ARO) |
| | |<--------------|
| | DAC | |
| |<--------------| |
| NA(ARO) | | |
|<--------------| | |
]]></artwork>
</figure>
</t><t>As the network builds up, a node should start as a
leaf to join the RPL network, and may later turn into both a RPL-capable
router and a 6LR, so as to accept leaf nodes
to recursively join the network.
</t>
</section>
<section title="Dependencies on Work In Progress">
<t>In order to control the complexity and the size of the 6TiSCH work,
the architecture and the associated IETF work are staged and the WG is
expected to recharter multiple times.
This document is incremented as the work progresses following the
evolution of the WG charter and the availability of dependent work.
The intent is to publish when the WG concludes.
</t>
<t>
At the time of this writing:
<list style='symbols'>
<t>
The architecture of the operation of RPL over a dynamic schedule is
being studied at 6TISCH as the second iteration of the charter.
</t>
<t>The need of a reactive routing protocol to establish on-demand
constraint-optimized routes and a reservation protocol to establish
Layer-3 Tracks is being discussed at 6TiSCH but not chartered for.
</t>
<t>
the components and protocols
that are required to implement this stage of architecture are not fully
available from the IETF. In particular, the requirements on an evolution
of 6LoWPAN Neighbor Discovery that are needed to implement the Backbone
Router as covered by this stage of the architecture are detailed in
<xref target="I-D.thubert-6lo-rfc6775-update-reqs"/>, and a number of
those requirements are fulfilled in
<xref target="I-D.ietf-6lo-backbone-router"/>.
</t> <t>
The work on centralized Track computation is deferred to a subsequent
iteration of the 6TiSCH charter. The idea at the time of this writing is
that 6TiSCH will apply the concepts of Deterministic Networking
on a Layer-3 network. The 6TiSCH Architecture should thus inherit from the
<xref target="I-D.finn-detnet-architecture">DetNet</xref> architecture and
thus depends on it. The Path Computation Element (PCE) should be a
core component of that architecture. Around the PCE, a protocol
such as an extension to a TEAS <xref target="TEAS"/> protocol
will be required to expose the 6TiSCH node capabilities and the network
peers to the PCE, and a protocol such as a lightweight PCEP or an
adaptation of CCAMP <xref target="CCAMP"/> G-MPLS formats and procedures
will be used to publish the Tracks, as computed by the PCE, to the 6TiSCH
nodes.
</t>
<t>The security model and in particular the join process are being
discussed at 6lo and 6TiSCH. PANA is presented in <xref target="fsixstac"/>
as a candidate of choice for the join process but alternatives are
discussed. Work resulting from <xref target="ACE"/> could be considered
as well.
<!--There is also a debate whether
the node should be able to send any unprotected packet on the medium.-->
Related contributions are presented in <xref target="cont"/>.
</t>
<t>
The current charter positions 6TiSCH on IEEE802.15.4 only.
Though most of the design should be portable on other link types,
6TiSCH has a strong dependency on IEEE802.15.4 and its evolution.
At the time of this writing, a revision of the IEEE802.15.4
standard is expected early 2016. That revision should
integrate TSCH as well as other amendments and fixes into the main
specification. The impact on this Architecture should be minimal to
non-existent, but deeper work such as 6top and security may be impacted.
A 6TiSCH Interest Group was formed at IEEE to maintain the synchronization
and help foster work at the IEEE should 6TiSCH demand it.
</t>
<t>
Work is being proposed at IEEE (802.15.12 PAR) for an LLC that would
logically include the 6top sublayer. The interaction with the 6top sublayer
and the Scheduling Functions described in this document are yet to be
defined.
</t>
<t>
ISA100 <xref target="ISA100"/> Common Network Management (CNM) is another
external work of interest for 6TiSCH. The group, referred to as ISA100.20,
defines a Common Network Management framework that should enable the
management of resources that are controlled by heterogeneous protocols
such as ISA100.11a <xref target="ISA100.11a"/>, WirelessHART
<xref target="WirelessHART"/>, and 6TiSCH. Interestingly, the
establishment of 6TiSCH Deterministic paths, called Tracks,
are also in scope, and ISA100.20 is working on requirements for DetNet.
</t>
</list>
</t>
</section>
</section>
<section anchor='dd' title="Deeper Dive">
<section anchor='RPLvs6lo' title="6LoWPAN (and RPL)">
<section anchor='leaf' title="RPL Leaf Support in 6LoWPAN ND">
<t>RPL needs a set of information in order to advertise
a leaf node through a DAO message and establish reachability.
</t><t>
At the bare minimum the leaf device must provide a sequence
number that matches the RPL specification in section 7.
Section 5.3 of
<xref target="I-D.ietf-6lo-backbone-router"/>,
on the Extended Address Registration Option (EARO),
already incorporates that addition with a new
field in the option called the Transaction ID.
</t><t>
If for some reason the node is aware of RPL topologies, then
providing the RPL InstanceID for the instances to which the
node wishes to participate would be a welcome addition.
In the absence of such information, the RPL router must
infer the proper instanceID from external rules and policies.
</t><t>
On the backbone, the InstanceID is expected to be mapped
onto a an overlay that matches the instanceID, for instance a VLANID.
</t><t>
This architecture leverages
<xref target="I-D.ietf-6lo-backbone-router"/>
that extends 6LoWPAN ND <xref target="RFC6775"/> to carry the counter
as an abstract Transaction ID (TID).
</t>
</section>
<section anchor='rpllbr' title="RPL Root And 6LBR">
<t>6LoWPAN ND is unclear on how the 6LBR is discovered, and how the liveliness
of the 6LBR is asserted over time. On the other hand, the discovery
and liveliness of the RPL root are obtained through the RPL protocol.
This architecture suggests to collocate these functions by default, in which
case the discovery of the 6LBR is automatic for RPL leaves.
</t>
<t>
When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL root functionalities
are co-located in order that the address of the 6LBR be indicated by RPL
DIO messages and to associate the unique ID from the DAR/DAC exchange with
the state that is maintained by RPL. The DAR/DAC exchange becomes a
preamble to the DAO messages that are used from then on to reconfirm the
registration, thus eliminating a duplication of functionality between DAO
and DAR messages.
</t>
<t>
Even though the root of the RPL network is integrated with the 6LBR,
it is logically separated from the Backbone Router (6BBR) that
is used to connect the 6TiSCH LLN to the backbone. This way,
the root has all information from 6LoWPAN ND and RPL about the LLN
devices attached to it.
</t><t>
This architecture also expects that the root of the RPL network
(proxy-)registers the 6TiSCH nodes on their behalf to the 6BBR,
for whatever operation the 6BBR performs on the backbone, such
as ND proxy, or redistribution in a routing protocol.
This relies on an extension of the 6LoWPAN ND registration described in
<xref target="I-D.ietf-6lo-backbone-router"/>.
</t><t>
This model supports
the movement of a 6TiSCH device across the Multi-Link Subnet, and
allows the proxy registration of 6TiSCH nodes deep into the 6TiSCH
LLN by the 6LBR / RPL root. This requires an alteration from
<xref target="RFC6775"/> whereby the Target Address of the NS message
is registered as opposed to the Source, which, in the case of a proxy
registration, is that of the 6LBR / RPL root itself.
</t>
</section>
<!--
<section anchor='gone' title="registration Failures Due to Movement">
<t>Registration to the 6LBR through DAR/DAC messages <xref target="RFC6775"/>
may percolate slowly through an LLN mesh, and it might happen that in
the meantime, the 6LoWPAN node moves and registers somewhere else. Both RPL
and 6LoWPAN ND lack the capability to indicate that the same node is
registered elsewhere, so as to invalidate states down the deprecated path.
</t><t> In its current expression and functionality,
6LoWPAN ND considers that the registration is used for the purpose of DAD
only as opposed to that of achieving reachability, and as long as the same
node registers the IPv6 address, the protocol is functional. In order to
act as a RPL leaf registration protocol and achieve reachability, the
device must use the same TID for all its concurrent registrations, and
registrations with a past TID should be declined. The state for an obsolete
registration in the 6LR, as well as the RPL routers on the way, should be
invalidated. This can only be achieved with the addition of a new Status in
the DAC message, and a new error/clean-up flow in RPL.
</t>
</section>
<section anchor='prox' title="Proxy registration">
<t>The 6BBR provides the capability to defend an address that is owned by
a 6LoWPAN Node, and attract packets to that address, whether it is done by
proxying ND over a Multi-Link Subnet, redistributing the address in a routing
protocol or advertising it through an alternate proxy registration such as
<xref target="RFC6830">the Locator/ID Separation Protocol</xref> (LISP) or
<xref target="RFC6275">Mobility Support in IPv6</xref> (MIPv6). In a LLN,
it makes sense to piggyback the request to proxy/defend an address with its
registration.
</t>
</section>
<section anchor='source' title="Target Registration">
<t>
In their current incarnations, both 6LoWPAN ND and Efficient ND expect
that the address being registered is the source of the NS(ARO) message and
thus impose that a Source Link-Layer Address (SLLA) option be present in the
message.
In a mesh scenario where the 6LBR is physically separated from the 6LoWPAN
Node, the 6LBR does not own the address being registered. This is why
<xref target="I-D.ietf-6lo-backbone-router"/>
registers the Target of the NS message as opposed to the Source Address.
From another perspective, it may happen, in the use case of a Star topology,
that the 6LR, 6LBR and 6BBR are effectively collapsed and should support
6LoWPAN ND clients. The convergence of efficient ND and 6LoWPAN ND into a
single protocol is thus highly desirable.
</t><t>
In any case, as long as the DAD process is not complete for the address
used as source of the packet, it is against the current practice to advertise
the SLLA, since this may corrupt the ND cache of the destination node, as
discussed in the <xref target="RFC4429">Optimistic DAD specification</xref>
with regards to the TENTATIVE state.
</t><t>
This may look like a chicken and an egg problem, but in fact 6LoWPAN ND
acknowledges that the Link-Local Address that is based on an EUI-64 address
of a LLN node may be autoconfigured without the need for DAD.
It results that a node could use that Address as source, with an SLLA
option in the message if required, to register any other addresses, either
Global or Unique-Local Addresses, which would be indicated in the Target.
</t>
<t>
The suggested change is to register the target of the NS message, and use
Target Link-Layer Address (TLLA) in the NS as opposed to the SLLA in order to
install a Neighbor Cache Entry. This would apply to both Efficient ND
and 6LoWPAN ND in a very same manner, with the caveat that depending on the
nature of the link between the 6LBR and the 6BBR, the 6LBR may resort to
classical ND or DHCPv6 to obtain the address that it uses to source the NS
registration messages, whether for itself or on behalf of LLN nodes.
</t>
</section>
<section anchor='Rroot' title="RPL root vs. 6LBR">
<t>6LoWPAN ND is unclear on how the 6LBR is discovered, and how the liveliness
of the 6LBR is asserted over time. On the other hand, the discovery
and liveliness of the RPL root are obtained through the RPL protocol.
</t><t>
When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL root functionalities
are co-located in order that the address of the 6LBR be indicated by RPL
DIO messages and to associate the unique ID from the DAR/DAC exchange with
the state that is maintained by RPL. The DAR/DAC exchange becomes a
preamble to the DAO messages that are used from then on to reconfirm the
registration, thus eliminating a duplication of functionality between DAO
and DAR messages.
</t>
</section>
<section anchor='Sec' title="Securing the Registration">
<t>
A typical attack against IPv6 ND is address spoofing, whereby a rogue node
claims the IPv6 Address of another node in and hijacks its traffic. The
threats against IPv6 ND as described in
<xref target="RFC3971">SEcure Neighbor Discovery (SEND)</xref>
are applicable to 6LoPWAN ND as well, but the solution can not work as the
route over network does not permit direct peer to peer communication.
</t><t>
Additionally SEND requires considerably enlarged ND messages to carry
cryptographic material, and requires that each protected address is generated
cryptographically, which implies the computation of a different key for
each Cryptographically Generated Address (CGA). SEND as defined in
<xref target="RFC3971"/> is thus largely unsuitable for application in a LLN.
</t><t>
With 6LoWPAN ND, as illustrated in <xref target='figReg'/>, it is
possible to leverage the registration state in the 6LBR, which may store
additional security information for later proof of ownership. If this
information proves the ownership independently of the address itself,
then a single proof may be used to protect multiple addresses.
</t><t>
Once an Address is registered,
the 6LBR maintains a state for that Address and is in position to bind
securely the first registration with the Node that placed it, whether the
Address is CGA or not. It should thus be possible to protect the ownership of
all the addresses of a 6LoWPAN Node with a single key, and there should not
be a need to carry the cryptographic material more than once to the 6LBR.
</t><t>
The energy constraint is usually a foremost factor, and attention should be
paid to minimize the burden on the CPU. Hardware-assisted support of variants
of the <xref target="RFC3610">Counter with CBC-MAC</xref> (CCM) authenticated
encryption block cipher mode such as CCM* are common in LowPower ship-set
implementations, and 6LoWPAN ND security mechanism should be capable to
reuse them when applicable.
</t><t>
Finally, the code footprint in the device being also an issue, the capability
to reuse not only hardware-assist mechanisms but also software across layers
has to be considered. For instance, if code has to be present for upper-layer
operations, e.g <xref target="RFC6655">AES-CCM Cipher Suites for Transport
Layer Security (TLS)</xref>, then the capability to reuse that code should be
considered.
</t>
-->
</section>
<section title="TSCH and 6top">
<section title="6top">
<t>
6top is a logical link control sitting between the IP layer and the
TSCH MAC layer, which provides the link abstraction that is required
for IP operations. The 6top operations are specified in
<xref target="I-D.ietf-6tisch-6top-protocol"/>. In particular, 6top
provides a management interface that enables an external
management entity to schedule cells and slotFrames, and allows the
addition of complementary functionality, for instance to support a
dynamic schedule management based on observed resource usage as
discussed in <xref target="dynsched"/>.
</t>
<t>
The 6top data model and management interfaces are further discussed
in <xref target='topint'/>.
</t>
<section title="Hard Cells">
<t>
The architecture defines "soft" cells and "hard" cells. "Hard" cells
are owned and managed by an separate scheduling entity (e.g. a PCE)
that specifies the slotOffset/channelOffset of the cells to be
added/moved/deleted, in which case 6top can only act as instructed,
and may not move hard cells in the TSCH schedule on its own.
</t>
</section>
<section title="Soft Cells">
<t>
6top contains a monitoring process which monitors the performance of
cells, and can move a cell in the TSCH schedule when it performs
poorly.
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 devices communicating over this
cell negotiate its new position in the TSCH schedule.
</t>
</section>
</section>
<section title="Scheduling Functions and the 6P protocol">
<t>In the case of soft cells, the cell management entity that controls the
dynamic attribution of cells to adapt to the dynamics of variable rate flows
is called a Scheduling Function (SF). There may be multiple SFs with more
or less aggressive reaction to the dynamics of the network. The
<xref target="I-D.ietf-6tisch-6top-sf0">6TiSCH 6top Scheduling Function Zero
(SF0)</xref> provides a simple scheduling function that can be used by
default by devices that support dynamic scheduling of soft cells.
</t>
<t>
The SF may be seen as divided between an upper bandwidth adaptation logic
that is not aware of the particular technology that is used to obtain and
release bandwidth, and an underlying service that maps those needs in the
actual technology, which means mapping the bandwidth onto cells in the case
of TSCH.
</t>
<figure anchor='fig6P' suppress-title='false'
title="SF/6P stack in 6top">
<artwork><![CDATA[
+------------------------+ +------------------------+
| Scheduling Function | | Scheduling Function |
| Bandwidth adaptation | | Bandwidth adaptation |
+------------------------+ +------------------------+
| Scheduling Function | | Scheduling Function |
| TSCH mapping to cells | | TSCH mapping to cells |
+------------------------+ +------------------------+
| 6top cells negotiation | <- 6P -> | 6top cells negotiation |
+------------------------+ +------------------------+
Device A Device B
]]></artwork>
</figure>
<t>
The SF relies on 6top services that implement the
<xref target="I-D.ietf-6tisch-6top-protocol"> 6top Protocol (6P) </xref>
to negotiate the precise cells that will be allocated or freed based on the
schedule of the peer. It may be for instance that a peer wants to use a
particular time slot that is free in its schedule, but that timeslot is
already in use by the other peer for a communication with a third party on a
different cell. The 6P protocol enables the peers to find an agreement in a
transactional manner that ensures the final consistency of the nodes state.
</t>
</section>
<section title="6top and RPL Objective Function operations">
<!-- 8.1.1. Support to RPL Neighbor Discovery and Parent Selection -->
<t>
An implementation of a <xref target="RFC6550">RPL</xref> Objective Function
(OF), such as the <xref target="RFC6552"> RPL Objective Function Zero (OF0)
</xref> that is used in the <xref target="I-D.ietf-6tisch-minimal"> Minimal
6TiSCH Configuration </xref> to support RPL over a static schedule, may
leverage, for its internal computation, the information maintained by 6top.
</t>
<t>Most OFs require metrics about reachability, such as the ETX.
6top creates and maintains an abstract neighbor table,
and this state may be leveraged to feed an OF and/or store OF information
as well.
In particular, 6top creates and maintains an abstract neighbor table. A neighbor
table entry contains a set of statistics with
respect to that specific neighbor including the time when the last packet has
been received from that neighbor, a set of cell quality metrics (e.g. RSSI or LQI),
the number of packets sent to the neighbor or the number of packets received
from it. This information can be obtained through 6top management APIs as
detailed in the <xref target="I-D.wang-6tisch-6top-sublayer">6top sublayer
specification</xref> and used for instance to compute a Rank Increment that will
determine the selection of the preferred parent.
</t>
<t>
6top provides statistics about the underlying layer so the OF can be tuned
to the nature of the TSCH MAC layer. 6top also enables the RPL OF to
influence the MAC behaviour, for instance by configuring the periodicity of
IEEE802.15.4 Extended Beacons (EB's). By augmenting the EB periodicity, it is
possible to change the network dynamics so as to improve the support of
devices that may change their point of attachment in the 6TiSCH network.
</t>
<!-- PT: I took of the text about time source; the way we do it is a bit reverse:
we have an Instance that is used for time sourcing, and the preferred parent
becomes the time source. If we change preferred parent we use the new one as
time source -->
<t>
Some RPL control messages, such as the DODAG Information Object (DIO) are
ICMPv6 messages that are broadcast to all neighbor nodes.
With 6TiSCH, the broadcast channel requirement is addressed by 6top
by configuring TSCH to provide a broadcast channel,
as opposed to, for instance, piggybacking the DIO messages in
Enhance Beacons. Consideration was given towards finding a way to
embed the Route Advertisements and the RPL DIO messages
(both of which are multicast) into the IEEE802.15.4 Enhanced Beacons.
It was determined that this produced undue timer coupling among
layers, that the resulting packet size was potentially too large,
and required it is not yet clear that there is any need for Enhanced
Beacons in a production network.
</t>
<!--t>
In the TSCH schedule, each cell has the IEEE802.15.4e LinkType attribute.
Setting the LinkType to ADVERTISING indicates that the cell MAY be used to send an
Enhanced Beacon. When a node forms its Enhanced Beacon, the cell,
with LinkType=ADVERTISING, SHOULD be included in the FrameAndLinkIE,
and its LinkOption field SHOULD be set to the combination of
"Receive" and "Timekeeping". The receiver of the Enhanced Beacon MAY
be listening at the cell to get the Enhanced Beacon ([IEEE802154e]).
6top takes this way to establish broadcast channel, which not only
allows TSCH to broadcast Enhanced Beacons, but also allows protocol
exchanges by an upper layer such as RPL.
</t>
<t>
To broadcast ICMPv6 control messages used by RPL such as DIO or DAO,
6top uses the payload of a Data frames. The message is inserted into the
queue associated with the cells which LinkType is set to ADVERTISING.
Then, taking advantage of the broadcast cell feature established with
FrameAndLinkIE (as described above), the RPL control message can be
received by neighbors, which enables the maintenance of RPL DODAGs.
</t>
<t>
A LinkOption combining "Receive" and "Timekeeping" bits indicates to
the receivers of the Enhanced Beacon that the cell MUST be used as a
broadcast cell. The frequency of sending Enhanced Beacons or other
broadcast messages by the upper layer is determined by the timers
associated with the messages. For example, the transmission of
Enhance Beacons is triggered by a timer in 6top; transmission of a
DIO message is triggered by the trickle timer of RPL.
</t-->
</section>
<section anchor="sync" title="Network Synchronization">
<t>
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 acknowledgment-based synchronization.
In order to maximize battery life and network throughput, it is advisable that RPL ICMP discovery
and maintenance traffic (governed by the trickle timer) be somehow coordinated with the
transmission of time synchronization packets (especially with enhanced beacons).
This could be achieved through an interaction of the 6top sublayer and the RPL objective Function,
or could be controlled by a management entity.
</t>
<!-- TW: Concept of TSGI developed in separate standards-Track draft? -->
<t>
Time distribution requires a loop-less structure. Nodes taken in a synchronization loop will rapidly
desynchronize from the network and become isolated. It is expected that a RPL DAG with
a dedicated global Instance is deployed for the purpose of time synchronization.
That Instance is referred to as the Time Synchronization Global Instance (TSGI).
The TSGI can be operated in either of the 3 modes that are detailed
in section 3.1.3 of <xref target="RFC6550">RPL</xref>,
"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.
Optionally, RPL's periodic operations may be used to
transport the network synchronization. This may
mean that 6top would need to trigger (override) the trickle timer if
no other traffic has occurred for such a time that nodes may get out
of synchronization.
</t>
<t>
A node that has not joined the TSGI advertises a MAC level Join Priority
of 0xFF to notify its neighbors that is not capable of serving as time parent.
A node that has joined the TSGI advertises a MAC level Join Priority set to
its DAGRank() in that Instance, where DAGRank() is the operation specified in
section 3.5.1 of <xref target="RFC6550"/>, "Rank Comparison".
</t>
<!-- TW: Official request made to move alter IEEE802.15.4e text. Maybe remove last sentence? -->
<t>
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 IEEE802.15.4 Extended Beacons (EB). We'll note that the
JP is now specified between 0 and 0x3F leaving 2 bits in the octet unused in the IEEE802.15.4e
specification. After consultation with IEEE authors, it was asserted that 6TiSCH can make
a full use of the octet to carry an integer value up to 0xFF.
</t>
<t>
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.
</t>
</section>
<section anchor="slotFrames" title="SlotFrames and Priorities">
<t>
6TiSCH enables in essence the capability to use IPv6 over a MAC
layer that enables to schedule some of the transmissions. In order
to ensure that the medium is free of contending packets when time
arrives for a scheduled transmission, a window of time is defined
around the scheduled transmission time where the medium must be free of
contending energy.
</t>
<t>
One simple way to obtain such a window is to format time and
frequencies in cells of transmission of equal duration. This is the
method that is adopted in IEEE802.15.4 TSCH as well as the Long Term
Evolution (LTE) of cellular networks.
</t>
<t>
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.
</t>
<t>
A CDU matrix iterates over and over with a pseudo-random rotation from
an epoch time.
In a given network, there might be multiple CDU matrices that operate
with different width, so they have different durations and represent
different periodic operations.
It is recommended that all CDU matrices in a 6TiSCH domain operate with
the same cell duration and are aligned, so as to reduce the
chances of interferences from slotted-aloha operations.
The knowledge of the CDU matrices is shared
between all the nodes and used in particular to define slotFrames.
</t>
<t>
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.
</t>
<t>
Multiple slotFrames can coexist in a node schedule, i.e., a node can
have multiple activities scheduled in different slotFrames, based on
the precedence of the 6TiSCH topologies. The slotFrames may be
aligned to different CDU matrices and thus have different width.
There is typically one slotFrame for scheduled traffic that has the
highest precedence and one or more slotFrame(s) for RPL traffic.
The timeslots in the slotFrame are indexed by the SlotOffset;
the first cell is at SlotOffset 0.
</t>
<t>
When a packet is received from a higher layer for transmission,
6top inserts that packet in the outgoing queue
which matches the packet best (Differentiated Services
<xref target="RFC2474"/> can therefore be used).
At each scheduled transmit slot, 6top looks for the frame
in all the outgoing queues that best matches the cells.
If a frame is found, it is given to the TSCH MAC for transmission.
</t>
</section>
<section anchor="DistRsvTS" title="Distributing the reservation of cells">
<t>
6TiSCH expects a high degree of scalability together with a distributed
routing functionality based on RPL. To achieve
this goal, the spectrum must be allocated in a way that allows for
spatial reuse between zones that will not interfere with one another.
In a large and spatially distributed network, a 6TiSCH node is often in a
good position to determine usage of spectrum in its vicinity.
</t>
<t>
Use cases for distributed routing are often associated with a
statistical distribution of best-effort traffic with variable needs
for bandwidth on each individual link. With 6TiSCH, the abstraction
of an IPv6 link is implemented as a pair of bundles of cells, one in
each direction; the size of a bundle is
optimal when both the energy wasted idle listening and the packet
drops due to congestion loss are minimized. This can be maintained if
the number of cells in a bundle is adapted dynamically, and with enough
reactivity, to match the variations of best-effort traffic. In turn,
the agility to fulfill the needs for additional cells improves when the
number of interactions with other devices and the protocol latencies
are minimized.
</t>
<t>
6TiSCH limits that interaction to RPL parents that will only
negotiate with other RPL parents, and performs that negotiation by
groups of cells as opposed to individual cells. The 6TiSCH architecture
allows RPL parents to adjust dynamically, and independently from
the PCE, the amount of bandwidth that is used to communicate between
themselves and their children, in both directions; to that effect,
an allocation mechanism enables a RPL parent to obtain the exclusive
use of a portion of a CDU matrix within its interference domain.
Note that a PCE is expected to have precedence in the allocation,
so that a RPL parent would only be able to obtain portions that are
not in-use by the PCE.
</t>
<t>
The 6TiSCH architecture introduces the concept of chunks
<xref target="I-D.ietf-6tisch-terminology"/>) to operate
such spectrum distribution for a whole group of cells at a time.
The CDU matrix is formatted into a set of chunks, each of them
identified uniquely by a chunk-ID. The knowledge of this
formatting is shared between all the nodes in a 6TiSCH network. 6TiSCH
also defines the process of chunk ownership appropriation whereby a
RPL parent discovers a chunk that is not used in its interference
domain (e.g lack of energy detected in reference cells in that chunk);
then claims the chunk, and then defends it in case another RPL parent
would attempt to appropriate it while it is in use.
The chunk is the basic unit of ownership that is used in that process.
</t>
<t>
<figure anchor="fig10" title="CDU matrix Partitioning in Chunks">
<artwork>
<![CDATA[
+-----+-----+-----+-----+-----+-----+-----+ +-----+
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
]]>
</artwork>
</figure>
</t>
<t>
As a result of the process of chunk ownership appropriation, the RPL
parent has exclusive authority to decide which cell in the appropriated
chunk can be used by which node in its interference domain. In other words, it is
implicitly delegated the right to manage the portion of the CDU matrix
that is represented by the chunk. The RPL parent may thus orchestrate
which transmissions occur in any of the cells in the chunk, by
allocating cells from the chunk to any form of communication (unicast,
multicast) in any direction between itself and its children.
Initially, those cells are added to the heap of free cells, then
dynamically placed into existing bundles, in new bundles, or allocated
opportunistically for one transmission.
</t>
<t>
The appropriation of a chunk can also be requested explicitly by the
PCE to any node. In that case, the node still may need to perform the
appropriation process to validate that no other node has claimed that
chunk already. After a successful appropriation, the PCE owns the cells
in that chunk, and may use them as hard cells to set up Tracks.
</t>
</section>
</section>
<!--
<section title="Functional Flows">
<t>
<list hangIndent="6" style="hanging">
<t hangText="Join:"></t>
<t hangText="Time Synchronization:"></t>
<t hangText="Setup for routing:"></t>
<t hangText="PCE reservation:"></t>
<t hangText="Distributed reservation:"></t>
<t hangText="Dynamic slot (de)allocation:"></t>
<t hangText="DSCP mapping:"></t>
</list>
</t>
</section>
-->
<section title="Communication Paradigms and Interaction Models">
<t>
<xref target="I-D.ietf-6tisch-terminology"/> defines the terms
of Communication Paradigms and Interaction Models, which can be placed
in parallel to the Information Models and Data Models that are defined in
<xref target="RFC3444"/>.
</t>
<t>
A Communication Paradigms would be an abstract view of a protocol exchange,
and would come with an Information Model for the information that is being exchanged.
In contrast, an Interaction Models would be more refined and could point on standard operation
such as a Representational state transfer (REST) "GET" operation and would match
a Data Model for the data that is provided over the protocol exchange.
</t>
<t>
section 2.1.3 of
<xref target="I-D.ietf-roll-rpl-industrial-applicability"/> and next
sections discuss application-layer paradigms, such as Source-sink (SS)
that is a Multipeer to Multipeer (MP2MP) model primarily used for
alarms and alerts, Publish-subscribe (PS, or pub/sub) that is typically
used for sensor data, as well as Peer-to-peer (P2P) and
Peer-to-multipeer (P2MP) communications.
Additional considerations on Duocast and its N-cast generalization are
also provided.
Those paradigms are frequently used in industrial automation, which is
a major use case for IEEE802.15.4 TSCH wireless networks with
<xref target="ISA100.11a"/> and <xref target="WirelessHART"/>, that
provides a wireless access to <xref target="HART"/> applications and
devices.
</t>
<t>
This specification focuses on Communication Paradigms and Interaction
Models for packet forwarding and TSCH resources (cells) management.
Management mechanisms for the TSCH schedule at Link-layer (one-hop),
Network-layer (multithop along a Track), and Application-layer
(remote control) are discussed in <xref target="schd"/>.
Link-layer frame forwarding interactions are discussed in <xref target="fwd"/>, and
Network-layer Packet routing is addressed in <xref target="rtg"/>.
</t>
</section>
<section anchor="schd" title="Schedule Management Mechanisms">
<t>
6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes: Static Scheduling,
neighbor-to-neighbor Scheduling, remote monitoring and scheduling management, and Hop-by-hop scheduling.
Multiple mechanisms are defined that implement the associated Interaction Models,
and can be combined and used in the same LLN.
Which mechanism(s) to use depends on application requirements.
</t>
<section anchor="mini" title="Static Scheduling">
<t>
In the simplest instantiation of a 6TiSCH network, a common fixed
schedule may be shared by all nodes in the network. Cells are shared,
and nodes contend for slot access in a slotted aloha manner.
</t>
<t>
A static TSCH schedule can be used to bootstrap a network, as an
initial phase during implementation, or as a fall-back mechanism in
case of network malfunction.
This schedule is pre-established, for instance decided by a network
administrator based on operational needs. It can be pre-configured
into the nodes, or, more commonly, learned by a node when joining
the network using standard IEEE802.15.4 Information Elements (IE).
Regardless, the schedule remains unchanged
after the node has joined a network.
RPL is used on the resulting network. This "minimal" scheduling
mechanism that implements this paradigm is detailed in
<xref target="I-D.ietf-6tisch-minimal"/>.
</t>
</section>
<section anchor="dynsched" title="Neighbor-to-neighbor Scheduling">
<t>
In the simplest instantiation of a 6TiSCH network described in
<xref target="mini"/>, nodes may expect a packet at any cell in
the schedule and will waste energy idle listening. In a more
complex instantiation of a 6TiSCH network, a matching portion of the
schedule is established between peers to reflect the observed amount
of transmissions between those nodes. The aggregation of the cells
between a node and a peer forms a bundle that the 6top layer uses to
implement the abstraction of a link for IP. The bandwidth on that
link is proportional to the number of cells in the bundle.
</t><t>
If the size of a bundle is configured to fit an average amount of
bandwidth, peak traffic is dropped. If the size is
configured to allow for peak emissions, energy is be wasted
idle listening.
</t><t>
The <xref target="I-D.wang-6tisch-6top-sublayer">6top sublayer </xref>
defines a protocol for neighbor nodes to reserve soft cells to
transmit 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 reallocation of a soft cell
in the schedule, using a negotiation protocol between the neighbors
nodes communicating over that cell.
</t><t>
In the most efficient instantiations of a 6TiSCH network, the size of
the bundles that implement the links may be changed dynamically
in order to adapt to the need of end-to-end flows routed by RPL.
An optional Scheduling Function (SF) such as
<xref target="I-D.ietf-6tisch-6top-sf0">SF0</xref> is used to
monitor bandwidth usage and perform requests for dynamic allocation
by the 6top sublayer.
The SF component is not part of the 6top sublayer. It may be
collocated on the same device or may be partially or fully offloaded
to an external system.
</t>
<t>
Monitoring and relocation is done in the 6top layer. For the upper layer,
the connection between two neighbor nodes appears as an number of cells.
Depending on traffic requirements, the upper layer can request 6top to add
or delete a number of cells scheduled to a particular neighbor, without
being responsible for choosing the exact slotOffset/channelOffset of those cells.
</t>
</section>
<section anchor="topint" title="Remote Monitoring and Schedule Management">
<t>
The 6top interface document
<xref target="I-D.ietf-6tisch-6top-interface"/>
specifies the generic data model that can be used to monitor and manage
resources of the 6top sublayer. Abstract methods are suggested for use
by a management entity in the device. The data model also enables
remote control operations on the 6top sublayer.
</t>
<t>
The capability to interact with the node 6top sublayer from multiple hops away
can be leveraged for monitoring, scheduling, or a combination of thereof.
The architecture supports variations on the deployment model, and
focuses on the flows rather than
whether there is a proxy or a translation operation en-route.
</t>
<t>
<xref target="I-D.ietf-6tisch-coap"/> defines an mapping of
the 6top set of commands, which is described in
<xref target="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.
</t>
<t>
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). It is also 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.
</t>
<!-- for later -->
<t>
With respect to Centralized routing and scheduling, the 6TiSCH
Architecture is (expected to be) be an extension of the detnet work
<xref target="I-D.finn-detnet-architecture">Deterministic Networking
Architecture</xref>,
which studies Layer-3 aspects of Deterministic Networks, and covers
networks that span multiple Layer-2 domains.
The DetNet architecture is a form of SDN Architecture and is composed
of three planes, a (User) Application Plane, a Controller Plane (where
the PCE operates), and a Network Plane which in our case is the 6TiSCH
LLN. The generic SDN architecture is discussed in
<xref target="RFC7426">Software-Defined Networking (SDN):
Layers and Architecture Terminology</xref> and is represented below:
</t>
<t>
<figure align="center" anchor="RFC7426archi">
<preamble>SDN Layers and Architecture Terminology per RFC 7426</preamble>
<artwork align="left"><![CDATA[
o--------------------------------o
| |
| +-------------+ +----------+ |
| | Application | | Service | |
| +-------------+ +----------+ |
| Application Plane |
o---------------Y----------------o
|
*-----------------------------Y---------------------------------*
| Network Services Abstraction Layer (NSAL) |
*------Y------------------------------------------------Y-------*
| |
| Service Interface |
| |
o------Y------------------o o---------------------Y------o
| | Control Plane | | Management Plane | |
| +----Y----+ +-----+ | | +-----+ +----Y----+ |
| | Service | | App | | | | App | | Service | |
| +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ |
| | | | | | | |
| *----Y-----------Y----* | | *---Y---------------Y----* |
| | Control Abstraction | | | | Management Abstraction | |
| | Layer (CAL) | | | | Layer (MAL) | |
| *----------Y----------* | | *----------Y-------------* |
| | | | | |
o------------|------------o o------------|---------------o
| |
| CP | MP
| Southbound | Southbound
| Interface | Interface
| |
*------------Y---------------------------------Y----------------*
| Device and resource Abstraction Layer (DAL) |
*------------Y---------------------------------Y----------------*
| | | |
| o-------Y----------o +-----+ o--------Y----------o |
| | Forwarding Plane | | App | | Operational Plane | |
| o------------------o +-----+ o-------------------o |
| Network Device |
+---------------------------------------------------------------+
]]></artwork>
</figure>
</t>
<t>The PCE establishes end-to-end Tracks of hard cells, which are described
in more details in <xref target="trkfwd"/>.
The DetNet work is expected to enable end to end Deterministic Path
across heterogeneous network (e.g. a 6TiSCH LLN and an Ethernet
Backbone). This model fits the 6TiSCH extended configuration, whereby a
6BBR federates
multiple 6TiSCH LLN in a single subnet over a backbone that can be,
for instance, Ethernet or Wi-Fi. In that model,
6TiSCH 6BBRs synchronize with one another over the backbone, so as
to ensure that the multiple LLNs that form the IPv6 subnet stay
tightly synchronized.
</t>
<t>
If the Backbone is Deterministic, then the
Backbone Router ensures that the end-to-end deterministic
behavior is maintained between the LLN and the backbone.
It is the responsibility of the PCE to compute a
deterministic path and to end across the TSCH network and an IEEE802.1
TSN Ethernet backbone, and that of DetNet to enable end-to-end deterministic
forwarding.
</t>
</section>
<section title="Hop-by-hop Scheduling">
<t>
A node can reserve a Track to a destination node multiple hops away by installing soft
cells at each intermediate node. This forms a Track of soft cells. It is the
responsibility of the 6top sublayer of each node on the Track to monitor these soft
cells and trigger relocation when needed.
</t>
<t>
This hop-by-hop reservation mechanism is expected to be similar
in essence to
<xref target="RFC3209"/> and/or <xref target="RFC4080"/>/<xref target="RFC5974"/>.
The protocol for a node to trigger hop-by-hop
scheduling is not yet defined.
</t>
</section>
</section>
<!-- for later
<section title="On Tracks">
<section title="General Behavior of Tracks">
<t>
A Track is a directional path from a source 6TiSCH node to a destination
6RiCH node across a 6TiSCH LLN. Resources such as timeslots and buffers
are reserved for that Track in intermediate 6TiSCH nodes. A 6TiSCH node
along a Track not only knows which bundles of cells it should use to
receive packets from its previous hop, but also knows which bundles it
should use to send packets to its next hop. There are several
benefits for using Track to forward a packet from the source node
to the destination node.
</t>
<t><list style="numbers">
<t>
Track forwarding, as further described in
<xref target="trkfwd"/>, is a layer-2 forwarding scheme,
which introduces less process delay and overhead than layer-3 forwarding
scheme. Therefore, LLN Devices can save more energy and resource,
which is critical for resource constrained devices.
</t>
<t>
Since channel resources, i.e. bundles of cells, have been reserved
for communications between 6TiSCH nodes of each hop on the Track, the
throughput and the maximum latency of the traffic along a Track are
guaranteed and the jitter is maintained small.
</t>
<t>
By knowing the scheduled time slots of incoming bundle and
outgoing bundle, 6TiSCH nodes on a Track could save more energy by
staying in sleep state during in-active slots. This is extreme
important for LLN Devices that are battery powered.
</t>
<t>
By allocating scheduled channel frequency, both inter-Track
and intra-Track interference can be reduced. This will enhance the
reliability of transmissions on a Track and reduce energy consumption
of LLN Devices by decreasing the number of retransmissions.
</t>
</list>
</t>
</section>
<section title="Retries vs. Replication and Elimination">
<t>
This specification details the concept of a Track, which is a complex form
of uni-directional Circuit (<xref target="I-D.ietf-6tisch-terminology"/>).
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.
The art of Deterministic Networks already include PRE techniques. For
instance, <xref target="IEC62439"/> standardizes the Parallel Redundancy
Protocol (PRP) and the High-availability Seamless Redundancy (HSR).
</t>
<t>
<figure anchor="elifig" title="End-to-End deterministic Track">
<artwork><![CDATA[
+-=-=-+
| 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
]]></artwork>
</figure>
</t>
<t>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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
In the art, a TSCH path 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.
</t>
</section>
<section anchor="topo" title="6TiSCH Device Capabilities">
<t>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, but a Cryptographically Generated Address <xref target="RFC3972"/>
(CGA) may also be used.
</t>
<t>
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 <xref target="RFC6551"/>.
</t>
<t>
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...
</t>
</section>
</section>
-->
<section anchor="fwd" title="Forwarding Models">
<!-- TW: Forwarding models should be formalized in a standards-Track draft? One should be MUST (IPv6?), the others SHOULD? -->
<t>
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 a routing computation <xref target="rtg"/>.
6TiSCH supports three different forwarding model, G-MPLS Track Forwarding (TF),
6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).
</t>
<section anchor="trkfwd" title="Track Forwarding">
<t>
A Track is a directional 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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
A Track is thus formed end-to-end as a succession of paired bundles,
a receive bundle from the previous hop and a transmit bundle to
the next hop along the Track, and a cell in such a bundle belongs to
at most one Track.
For a given iteration of the device schedule, the effective channel
of the cell is obtained by adding a pseudo-random number to the
channelOffset of the cell, which results in a rotation of the
frequency that used for transmission.
The bundles may be computed so as to accommodate both variable rates
and retransmissions, so they might not be fully used at a given
iteration of the schedule.
The 6TiSCH architecture provides additional means to avoid waste of
cells as well as overflows in the transmit bundle, as follows:
</t>
<t>
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).
</t>
<t>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
<xref target="pmh"/>.
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.
</t>
<t>
There are 2 modes for a Track, transport mode and tunnel mode.
</t>
<section title="Transport Mode">
<t>
In transport mode, the Protocol Data Unit (PDU) is associated
with flow-dependant meta-data that refers uniquely to the Track,
so the 6top sublayer can place the frame in the appropriate cell
without ambiguity. In the case of IPv6 traffic, this flow
identification is transported in the Flow Label of the IPv6
header.
Associated with the source IPv6 address, the Flow Label forms a
globally unique identifier for that particular Track that is
validated at egress before restoring
the destination MAC address (DMAC) and punting to the upper layer.
</t>
<t>
<figure title="Track Forwarding, Transport Mode">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | |
+--------------+ ingress egress
| 6top | sets +----+ +----+ restores
+--------------+ dmac to | | | | dmac to
| TSCH MAC | brdcst | | | | self
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
</section>
<section title="Tunnel Mode">
<t>
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.
</t>
<t>
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.
</t>
<t>
<figure anchor="fig6" title="Track Forwarding, Tunnel Mode">
<artwork><![CDATA[
+--------------+
| 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
+--------------+
]]></artwork>
</figure>
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Tunnel Metadata">
<t>
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.
</t>
<t>
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.
</t>
</section>
</section>
<section title="Fragment Forwarding">
<t>
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.
</t>
<t>
This level of fragmentation is much higher than that traditionally experienced over the Internet
with IPv4 fragments, where fragmentation is already known as harmful.
</t>
<t>
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.
</t>
<t>
<xref target="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 based on an Explicit Congestion Notification, as well as end-to-end individual fragment recovery.
</t>
<t>
<figure anchor="fig7" title="Forwarding First Fragment">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | +----+ +----+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | learn learn |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
<t>
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.
</t>
<t>
<figure anchor="fig8" title="Forwarding Next Fragment">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | replay replay |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
<t>
A bitmap and an ECN echo in the end-to-end acknowledgment 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.
</t>
</section>
<section title="IPv6 Forwarding">
<t>
As the packets are routed at Layer-3, traditional QoS and RED operations are expected to prioritize
flows; the application of Differentiated Services is further discussed in
<xref target="I-D.svshah-tsvwg-lln-diffserv-recommendations"/>.
</t>
<t>
<figure anchor="fig9" title="IP Forwarding">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | +-QoS+ +-QoS+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | | | | | |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="rtg" title="Centralized vs. Distributed Routing">
<t>
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.
</t>
<t>
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 <xref target="RFC6550">RPL</xref>.
</t>
<t>
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.
</t>
<section anchor="pmh" title="Packet Marking and Handling">
<t>
All packets inside a 6TiSCH domain must carry the Instance ID that
identifies the 6TiSCH topology that is to be used for
routing and forwarding that packet. The location of that information
must be the same for all packets forwarded inside the domain.
</t>
<t>
For packets that are routed by a PCE along a Track, the tuple formed by the
IPv6 source address and a local RPLInstanceID in the packet identify
uniquely the Track and associated transmit bundle.
Additionally, an IP packet that is sent along a Track uses the
Differentiated Services Per-Hop-Behavior Group called
Deterministic Forwarding, as described in
<xref target="I-D.svshah-tsvwg-deterministic-forwarding"/>.
</t>
<t>
For packets that are routed by RPL, that information is the RPLInstanceID
which is carried in the RPL Packet Information, as discussed in section 11.2
of <xref target="RFC6550"/>, "Loop Avoidance and Detection".
</t>
<t>The RPL Packet Information (RPI) is carried in IPv6 packets as a RPL
option in the IPv6 Hop-By-Hop Header <xref target="RFC6553"/>.
</t><t>
A compression mechanism for the RPL packet artifacts that integrates the
compression of IP-in-IP encapsulation and the Routing Header type 3
<xref target="RFC6554"/>
with that of the RPI in a 6LoWPAN dispatch/header type is concurrently
being evaluated as <xref target="I-D.ietf-roll-routing-dispatch"/>.</t>
<t>
<!--In a 6TiSCH network, the routing dispatch is the recommended encoding the
RPL Packet Information.-->
</t>
<t>
Either way, the method and format used for encoding the RPLInstanceID
is generalized to all 6TiSCH topological Instances, which include
both RPL Instances and Tracks.
</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>
This specification does not require IANA action.
</t>
</section>
<section anchor='sec' title="Security Considerations">
<t>
This architecture operates on IEEE802.15.4 and expects link-layer security to
be enabled at all times between connected devices, except for the very first
step of the device join process, where a joining device may need some initial,
unsecured exchanges so as to obtain its initial key material.
Work has already started at the 6TiSCH Security Design Team and an
overview of the current state of that work is presented in
<xref target="join"/>.
</t> <t>
Future work on 6TiSCH security and will examine in deeper detail how
to secure transactions end-to-end, and to maintain the
security posture of a device over its lifetime.
The result of that work will be described in a subsequent volume of this
architecture.
</t>
<section anchor='join' title="Join Process Highlights">
<t>The architecture specifies three logical elements to describe the join
process:
<list hangIndent="6" style="hanging">
<t hangText="Joining Node (JN):">
Node that wishes to become part of the network; </t>
<t hangText="Join Coordination Entity (JCE)">:
A Join Coordination Entity (JCE) that arbitrates network access and hands
out network parameters (such as keying material);</t>
<t hangText="Join Assistant (JA),">
a one-hop (radio) neighbor of the joining node
that acts as proxy network node and may provide connectivity
with the JCE.</t>
</list>
</t>
<t>The join protocol consists of three major activities:
<list hangIndent="6" style="hanging">
<t hangText="Device Authentication:">
The JN and the JA mutually authenticate each other
and establish a shared key, so as to ensure on-going authenticated
communications. This may involve a server as a third party.</t>
<t hangText="Authorization:">
The JA decides on whether/how to authorize a JN
(if denied, this may result in loss of bandwidth).
Conversely, the JN decides on whether/how to authorize the network
(if denied, it will not join the network).
Authorization decisions may involve other nodes in the network.</t>
<t hangText="Configuration/Parameterization:">
The JA distributes configuration information to the JN, such as scheduling
information, IP address assignment information, and network policies.
This may originate from other network devices, for which the JA may act as
proxy. This step may also include distribution of information
from the JN to the JA and other nodes in the network and, more generally,
synchronization of information between these entities.</t>
</list>
</t>
<t>The device joining process is depicted in <xref target='fig-first-example'/>,
where it is assumed that devices have access to certificates and where
entities have access to the root CA keys of their communicating parties
(initial set-up requirement).
Under these assumptions, the authentication step of the device joining
process does not require online involvement of a third party.
Mutual authentication is performed between the JN and the JA using their
certificates, which also results in a shared key between these two entities.
</t><t>
The JA assists the JN in mutual authentication with a remote server node
(primarily via provision of a communication path with the server), which
also results in a shared (end-to-end) key between those two entities.
The server node may be a JCE that arbitrages the network authorization of the
JN (where the JA will deny bandwidth if authorization is not successful);
it may distribute network-specific configuration parameters
(including network-wide keys) to the JN.
In its turn, the JN may distribute and synchronize information (including,
e.g., network statistics) to the server node and, if so desired, also to the
JA. The actual decision of the JN to become part of the network may
depend on authorization of the network itself.</t>
<t>The server functionality is a role which may be implemented with one
(centralized) or multiple devices (distributed).
In either case, mutual authentication is established
with each physical server entity with which a role is implemented. </t>
<t>
Note that in the above description, the JA does not solely act as a relay
node, thereby allowing it to first filter traffic to be relayed based on
cryptographic authentication criteria - this provides first-level access
control and mitigates certain types of denial-of-service attacks
on the network at large. </t>
<t>Depending on more detailed insight in cost/benefit trade-offs, this
process might be complemented by a more "relaxed" mechanism, where the
JA acts as a relay node only.
The final architecture will provide mechanisms to also cover cases where
the initial set-up requirements are not met or where some other
out-of-sync behavior occurs; it will also suggest some optimizations in
case JCE-related information is already available with the JA
(via caching of information).</t>
<t> When a device rejoins the network in the same authorization domain,
the authorization step could be omitted if the server distributes the
authorization state for the device to the JA when the device
initially joined the network. However, this generally still requires
the exchange of updated configuration information, e.g., related to time
schedules and bandwidth allocation.</t>
<figure title='Network joining, with only authorization by third party'
anchor='fig-first-example'>
<artwork><![CDATA[
{joining node} {neighbor} {server, etc.} Example:
+---------+ +---------+ +---------+
| Joining | | Join | +--| CA |certificate
| Node | |Assistant| | +---------+ issuance
+---------+ +---------+ | +---------+
| | +--|Authoriz.| membership
|<----Beaconing------| | +---------+ test (JCE)
| | | +---------+
|<--Authentication-->| +--| Routing | IP address
| |<--Authorization-->| +--------- assignment
|<-------------------| | +---------+
| | +--| Gateway | backbone,
|------------------->| | +---------+ cloud
| |<--Configuration-->| +---------+
|<-------------------| +--|Bandwidth| PCE
+---------+ schedule
. . .
. . .
]]></artwork>
</figure>
</section>
</section>
<section title="Acknowledgments">
<section title="Contributors">
<t>The co-authors of this document are listed below:
<list hangIndent="6" style="hanging">
<t hangText="Robert Assimiti">
for his breakthrough work on RPL over TSCH and initial text and
guidance.
</t>
<t hangText="Kris Pister">
for creating it all and his continuing guidance through the elaboration
of this design.
</t>
<t hangText="Michael Richardson">
for his leadership role in the Security Design Team and his
contribution throughout this document.
</t>
<t hangText="Rene Struik">
for the security section and his contribution to the Security Design
Team.
</t>
<t hangText="Xavier Vilajosana">
who lead the design of the minimal support with RPL and contributed
deeply to the 6top design and the G-MPLS operation of Track switching.
</t>
<t hangText="Qin Wang">
who lead the design of the 6top sublayer and contributed related text
that was moved and/or adapted in this document.
</t>
<t hangText="Thomas Watteyne">
for his contribution to the whole design, in
particular on TSCH and security.
</t>
</list>
</t>
</section>
<section title="Special Thanks"><t>
Special thanks to Tero Kivinen, Jonathan Simon, Giuseppe Piro, Subir Das
and Yoshihiro Ohba for their deep contribution to the initial security
work, and to Diego Dujovne for starting and leading the SF0 effort.
</t><t>
Special thanks also to Pat Kinney for his support in maintaining the
connection active and the design in line with work happening at
IEEE802.15.4.
</t> <t>
Special thanks to Ted Lemon who was the INT Area A-D while this
specification was developed for his great support and help throughout.
</t><t>
Also special thanks to Ralph Droms who performed the first INT Area
Directorate review, that was very deep and through and radically changed
the orientations of this document.
</t>
</section>
<section title="And Do not Forget">
<t>This specification is the result of multiple interactions, in
particular during the 6TiSCH (bi)Weekly Interim call, relayed through
the 6TiSCH mailing list at the IETF.
</t><t>
The authors wish to thank:
Alaeddine Weslati, Chonggang Wang, Georgios Exarchakos, Zhuo Chen,
Alfredo Grieco, Bert Greevenbosch, Cedric Adjih, Deji Chen, Martin Turon,
Dominique Barthel, Elvis Vogli, Geraldine Texier, Malisa Vucinic,
Guillaume Gaillard, Herman Storey, Kazushi Muraoka, Ken Bannister,
Kuor Hsin Chang, Laurent Toutain, Maik Seewald, Maria Rita Palattella,
Michael Behringer, Nancy Cam Winget, Nicola Accettura, Nicolas Montavont,
Oleg Hahm, Patrick Wetterwald, Paul Duffy, Peter van der Stock, Rahul Sen,
Pieter de Mil, Pouria Zand, Rouhollah Nabati, Rafa Marin-Lopez,
Raghuram Sudhaakar, Sedat Gormus, Shitanshu Shah, Steve Simlo,
Tengfei Chang, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles and
Samita Chakrabarti for their participation and various contributions.
</t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
<!-- 6TiSCH -->
<?rfc include='reference.I-D.ietf-6tisch-terminology'?>
<!-- others -->
<?rfc include="reference.RFC.0768"?> <!-- Internet Protocol, Version 6 (IPv6) Specification -->
<?rfc include="reference.RFC.2460"?> <!-- Internet Protocol, Version 6 (IPv6) Specification -->
<?rfc include="reference.RFC.4861"?> <!-- neighbor Discovery for IP version 6 (IPv6) -->
<?rfc include="reference.RFC.4862"?> <!-- IPv6 Stateless Address Autoconfiguration -->
<?rfc include="reference.RFC.6282"?> <!-- Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks -->
<?rfc include="reference.RFC.6550"?> <!-- RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks -->
<?rfc include="reference.RFC.6551"?> <!-- RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks -->
<?rfc include="reference.RFC.6552"?> <!-- RPL OF0: Objective Function Zero for RPL-->
<?rfc include="reference.RFC.6553"?> <!-- RPL Option for Carrying RPL Information in Data-Plane Datagrams -->
<?rfc include="reference.RFC.6554"?> <!-- An IPv6 Routing Header for Source Routes with RPL -->
<?rfc include="reference.RFC.6775"?> <!-- neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) -->
<?rfc include="reference.RFC.7252"?> <!-- CoAP -->
<?rfc include="reference.RFC.7554"?> <!-- 6TiSCH TSCH -->
<?rfc include='reference.I-D.finn-detnet-architecture'?>
<?rfc include='reference.I-D.ietf-6lo-backbone-router'?>
<?rfc include='reference.I-D.ietf-6tisch-minimal'?>
<?rfc include='reference.I-D.ietf-roll-routing-dispatch'?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.6620"?> <!-- FCFS SAVI: First-Come, First-Served Source Address Validation -->
<?rfc include="reference.RFC.6655"?> <!-- AES-CCM Cipher Suites for Transport Layer Security (TLS) -->
<?rfc include="reference.RFC.5191"?> <!-- Protocol for Carrying Authentication for Network Access (PANA) -->
<?rfc include="reference.RFC.5340"?> <!-- OSPF for IPv6 -->
<?rfc include="reference.RFC.6275"?> <!-- Mobility Support in IPv6 -->
<?rfc include="reference.RFC.2474"?> <!-- Differentiated Services Field -->
<?rfc include="reference.RFC.2545"?> <!-- BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing -->
<?rfc include="reference.RFC.3963"?> <!-- Network Mobility (NEMO) -->
<?rfc include="reference.RFC.3972"?> <!-- CGA -->
<?rfc include="reference.RFC.3209"?> <!-- RSVP TE -->
<?rfc include="reference.RFC.3971"?> <!-- SEcure Neighbor Discovery (SEND) -->
<?rfc include="reference.RFC.4291"?> <!-- IP Version 6 Addressing Architecture -->
<?rfc include="reference.RFC.4429"?> <!-- IP Version 6 Optimistic DAD -->
<?rfc include="reference.RFC.3444"?> <!-- On the Difference between Information Models and Data Models -->
<?rfc include="reference.RFC.3610"?> <!-- Counter with CBC-MAC (CCM) -->
<!-- 6TiSCH -->
<?rfc include="reference.RFC.4080"?> <!-- Next Steps in Signaling (NSIS): Framework -->
<?rfc include="reference.RFC.4389"?> <!-- IP Version 6 ND Proxy -->
<?rfc include="reference.RFC.4919"?> <!-- IPv6 over Low-Power Wireless Personal Area Networks -->
<?rfc include="reference.RFC.4903"?> <!-- IPv6 Multi-Link Subnet Issues -->
<?rfc include="reference.RFC.5889"?> <!-- IP Addressing Model in Ad Hoc Networks -->
<?rfc include="reference.RFC.5974"?> <!-- NSIS Signaling Layer Protocol (NSLP) for Quality-of-Service Signaling -->
<?rfc include="reference.RFC.6347"?> <!-- Datagram Transport Layer Security Version 1.2 -->
<?rfc include="reference.RFC.6830"?> <!-- The Locator/ID Separation Protocol (LISP) -->
<?rfc include="reference.RFC.6997"?> <!-- Reactive Discovery of Point-to-Point Routes in Low-Power and Lossy Networks -->
<?rfc include="reference.RFC.7426"?> <!-- Software-Defined Networking (SDN): Layers and Architecture Terminology -->
<?rfc include='reference.I-D.ietf-6tisch-6top-interface'?>
<?rfc include='reference.I-D.ietf-6tisch-coap'?>
<?rfc include='reference.I-D.ietf-6tisch-6top-sf0'?>
<?rfc include='reference.I-D.ietf-6tisch-6top-protocol'?>
<!-- others -->
<!--?rfc include='reference.I-D.ietf-ipv6-Multi-Link-subnets'?-->
<?rfc include='reference.I-D.ietf-roll-rpl-industrial-applicability'?>
<?rfc include='reference.I-D.thubert-roll-forwarding-frags'?>
<?rfc include='reference.I-D.svshah-tsvwg-lln-diffserv-recommendations'?>
<?rfc include='reference.I-D.svshah-tsvwg-deterministic-forwarding'?>
<?rfc include='reference.I-D.thubert-6lo-rfc6775-update-reqs'?>
<?rfc include='reference.I-D.vanderstok-core-comi'?>
<?rfc include='reference.I-D.wang-6tisch-6top-sublayer'?>
<?rfc include='reference.I-D.richardson-6tisch-security-architecture'?>
<?rfc include='reference.I-D.struik-6tisch-security-architecture-elements'?>
<?rfc include='reference.I-D.ietf-manet-aodvv2'?>
<?rfc include='reference.I-D.ietf-detnet-use-cases'?>
</references>
<references title="Other Informative References">
<reference anchor="IEEE802154">
<front>
<title>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
</title>
<author>
<organization>IEEE standard for Information Technology</organization>
</author>
<date/>
</front>
</reference>
<reference anchor="IEEE802154e">
<front>
<title>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
</title>
<author>
<organization>IEEE standard for Information Technology</organization>
</author>
<date month="April" year="2012"/>
</front>
</reference>
<reference anchor="IEEE802.1TSNTG" target="http://www.ieee802.org/1/pages/avbridges.html">
<front>
<title>IEEE 802.1 Time-Sensitive Networks Task Group</title>
<author>
<organization>IEEE Standards Association</organization>
</author>
<date day="08" month="March" year="2013" />
</front>
</reference>
<reference anchor="WirelessHART">
<front>
<title>Industrial Communication Networks - Wireless Communication Network and Communication Profiles - WirelessHART - IEC 62591</title>
<author>
<organization>www.hartcomm.org</organization>
</author>
<date year="2010" />
</front>
</reference>
<reference anchor="HART">
<front>
<title>Highway Addressable remote Transducer, a group of specifications for industrial process and control devices administered by the HART Foundation</title>
<author>
<organization>www.hartcomm.org</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="ISA100.11a" target="http://www.isa.org/Community/SP100WirelessSystemsforAutomation">
<front>
<title>Wireless Systems for Industrial Automation: Process Control and Related Applications - ISA100.11a-2011 - IEC 62734</title>
<author>
<organization>ISA/ANSI</organization>
</author>
<date year="2011" />
</front>
</reference>
<reference anchor="ISA100" target="https://www.isa.org/isa100/">
<front>
<title>ISA100, Wireless Systems for Automation</title>
<author>
<organization>ISA/ANSI</organization>
</author>
<date/>
</front>
</reference>
<reference anchor="TEAS" target="https://dataTracker.ietf.org/doc/charter-ietf-teas/">
<front>
<title>Traffic Engineering Architecture and Signaling</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="PCE" target="https://dataTracker.ietf.org/doc/charter-ietf-pce/">
<front>
<title>Path Computation Element</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="CCAMP" target="https://dataTracker.ietf.org/doc/charter-ietf-ccamp/">
<front>
<title>Common Control and Measurement Plane</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="DICE" target="https://dataTracker.ietf.org/doc/charter-ietf-dice/">
<front>
<title>DTLS In Constrained Environments</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="ACE" target="https://dataTracker.ietf.org/doc/charter-ietf-ace/">
<front>
<title>Authentication and Authorization for Constrained Environments</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="DETNET" target="https://datatracker.ietf.org/doc/charter-ietf-detnet/">
<front>
<title>Deterministic Networking</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="IEC62439" target="https://webstore.iec.ch/publication/7018">
<front>
<title>Industrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR) - IEC62439-3</title>
<author>
<organization>IEC</organization>
</author>
<date year="2012" />
</front>
</reference>
</references>
<section anchor="cont" title="Personal submissions relevant to upcoming work">
<t>This document covers a portion of the total work that is needed to
cover the full 6TiSCH architecture. Missing portions at this time
include Deterministic
Networking with Track Forwarding, Dynamic Scheduling, and Security.
</t>
<t>
<xref target="I-D.richardson-6tisch-security-architecture"/> elaborates on
the potential use of 802.1AR certificates, and some options for
the join process are presented in more details.
</t>
<t>
<xref target="I-D.struik-6tisch-security-architecture-elements"/>
describes 6TiSCH security architectural elements with
high level requirements and the security framework that are relevant
for the design of the 6TiSCH security solution.
</t>
</section>
</back>
</rfc>
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