An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4Cisco Systems, IncBuilding D45 Allee des Ormes - BP1200 Mougins - Sophia Antipolis06254France+33 497 23 26 34pthubert@cisco.com
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6TiSCHDraft 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.
Introduction
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 range, and in circumstances where wiring may be impractical,
for instance on fast-moving or rotating devices.
On the other hand, Deterministic Networking maximizes the packet
delivery ratio within a bounded latency so as to enable
mission-critical machine-to-machine (M2M) operations.
Applications that need such networks are presented in
.
The considered applications include Professional Media, Industrial
Automation Control Systems (IACS), building
automation, in-vehicle command and control, commercial automation and
asset tracking with mobile scenarios, as well as gaming, drones and edge robotic control, and home automation applications.
The Timeslotted Channel Hopping (TSCH) mode
of the IEEE Std. 802.15.4 Medium Access
Control (MAC) was introduced with the IEEE Std. 802.15.4e
amendment and is now retrofitted in the
main standard. For all practical purposes, this document
is expected to be insensitive to the revisions of that standard,
which is thus referenced without a date.
TSCH is both a Time-Division Multiplexing and a Frequency-Division
Multiplexing technique whereby a different channel can be used for
each transmission, and that allows to schedule transmissions for
deterministic operations, and applies to the slower and most energy
constrained wireless use cases.
The scheduled operation provides for a more reliable experience which
can be used to monitor and manage resources, e.g., energy and water, in
a more efficient fashion.
Proven Deterministic Networking standards for use in Process Control,
including ISA100.11a and WirelessHART
, have demonstrated the capabilities
of the IEEE Std. 802.15.4 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.
To enable the convergence of Information Technology (IT) and
Operational Technology (OT) in Low-Power Lossy
Networks (LLNs), the 6TiSCH Architecture supports an IETF suite of
protocols over the IEEE Std. 802.15.4 TSCH MAC to provide
IP connectivity for energy and otherwise constrained wireless devices.
The 6TiSCH Architecture relies on IPv6 and the
use of routing to provide large scaling capabilities. The addition of a
high-speed federating backbone adds yet another degree of scalability
to the design. The backbone is typically a Layer-2 transit Link such as
an Ethernet bridged network, but it can also be a more complex routed
structure.
The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet model that
is composed of a federating backbone and a number of IEEE Std. 802.15.4
TSCH low-power wireless networks federated and synchronized by Backbone
Routers. If the backbone is a Layer-2 transit Link then the Backbone
Routers can operate as an IPv6 Neighbor Discovery (IPv6 ND)
proxy.
The 6TiSCH
Architecture leverages 6LoWPAN to adapt IPv6
to the constrained media and RPL for the
distributed routing operations.
Centralized routing refers to a model where routes are computed
and resources are allocated from a central controller. This is
particularly helpful to schedule deterministic multihop transmissions.
In contrast, Distributed Routing refers to a model that relies on
concurrent peer to peer protocol exchanges for TSCH resource allocation
and routing operations.
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 highly
scalable solutions such as used in Advanced Metering Infrastructure
solutions that leverage distributed routing to
enable multipath forwarding over large LLN meshes.
TerminologyNew Terms
The draft does not reuse terms from the
IEEE Std. 802.15.4 standard such as "path" or "link" which bear
a meaning that is quite different from classical IETF parlance.
This document adds the following terms:
6TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4):
6TiSCH defines an adaptation sublayer for IPv6 over TSCH called 6top,
a set of protocols for setting up a TSCH schedule in distributed
approach, and a security solution. 6TiSCH may be extended in the future for other
MAC/PHY pairs providing a service similar to TSCH.
6top (6TiSCH Operation Sublayer):
The next higher layer of the IEEE Std. 802.15.4 TSCH MAC layer.
6top provides the abstraction of an IP link over a TSCH MAC,
schedules packets over TSCH cells, and exposes a management
interface to schedule TSCH cells.
6P (6top Protocol):
The protocol defined in .
6P enables Layer-2 peers to allocate, move or deallocate
cells in their respective schedules to communicate.
6P operates at the 6top layer.
6P Transaction:
A 2-way or 3-way sequence of 6P messages used by Layer-2
peers to modify their communication schedule.
ASN (Absolute Slot Number):
Defined in , the ASN is the total
number of timeslots that have elapsed since the Epoch Time
when the TSCH network started.
Incremented by one at each timeslot.
It is wide enough to not roll over in practice.
bundle:
A group of equivalent scheduled cells, i.e., cells
identified by different [slotOffset, channelOffset],
which are scheduled for a same purpose, with the same
neighbor, with the same flags, and the same slotframe.
The size of the bundle refers to the number of cells it
contains.
For a given slotframe length, the size of the bundle
translates directly into bandwidth.
A bundle is a local abstraction that represents a
half-duplex link for either sending or receiving,
with bandwidth that amounts to the sum of the cells in the
bundle.
Layer-2 vs. Layer-3 bundle:
Bundles are associated for either Layer-2 (switching) or
Layer-3 (routing) forwarding operations. A pair of Layer-3
bundles (one for each direction) maps to an IP Link with a
neighbor, whereas a set of Layer-2 bundles (of an
"arbitrary" cardinality and direction) corresponds to the relation of one or more incoming bundle(s) from the
previous-hop neighbor(s) with one or more outgoing bundle(s)
to the next-hop neighbor(s) along a Track as part of the
switching role, which may include replication and elimination.
CCA (Clear Channel Assessment):
A mechanism defined in whereby
nodes listen to the channel before sending to
detect ongoing transmissions from other parties.
Because the network is synchronized, CCA cannot be used to
detect colliding transmissions within the same network, but
it can be used to detect other radio networks in vicinity.
cell:
A unit of transmission resource in the CDU matrix, a cell is
identified by a slotOffset and a channelOffset.
A cell can be scheduled or unscheduled.
Channel Distribution/Usage (CDU) matrix:
:
A matrix of cells (i,j) representing the spectrum (channel)
distribution among the different nodes in the 6TiSCH network.
The CDU matrix has width in timeslots, equal to the period
of the network scheduling operation, and height equal to
the number of available channels.
Every cell (i,j) in the CDU, identified by (slotOffset,
channelOffset), belongs to a specific chunk.
channelOffset:
Identifies a row in the TSCH schedule. The number of
channelOffset values is bounded by the number of available
frequencies. The channelOffset translates into a frequency
with a function that depends on the absolute time when the
communication takes place, resulting in a channel hopping
operation.
chunk:
A well-known list of cells, distributed in time and frequency, within a CDU matrix.
A chunk represents a portion of a CDU matrix.
The partition of the CDU matrix in chunks is globally known by all the nodes in the network to support the appropriation process, which is a negotiation between nodes within an interference domain.
A node that manages to appropriate a chunk gets to decide which transmissions will occur over the cells in the chunk within its interference domain, i.e., a parent node will decide when the cells within the appropriated chunk are used and by which node, among its children.
CoJP (Constrained Join Protocol):
The Constrained Join Protocol (CoJP) enables a pledge to
securely join a 6TiSCH network and obtain network parameters
over a secure channel.
Minimal Security Framework for 6TiSCH defines
the minimal CoJP setup with pre-shared keys defined. In that
mode, CoJP can operate with a single round trip exchange.
dedicated cell:
A cell that is reserved for a given node to transmit to a specific neighbor.
deterministic network:
The generic concept of deterministic network is defined in the "DetNet Architecture" document.
When applied to 6TiSCH, it refers to the reservation of Tracks which guarantees an end-to-end latency and optimizes the Packet Delivery Ratio (PDR) for well-characterized flows.
distributed cell reservation:
A reservation of a cell done by one or more in-network entities.
distributed Track reservation:
A reservation of a Track done by one or more in-network entities.
EB (Enhanced Beacon):
A special frame defined in
used by a node, including the JP, to announce the presence
of the network.
It contains enough information for a pledge to synchronize to the network.
hard cell:
A scheduled cell which the 6top sublayer may not relocate.
hopping sequence:
Ordered sequence of frequencies, identified by a Hopping_Sequence_ID, used for channel hopping when translating the channelOffset value into a frequency.
IE (Information Element):
Type-Length-Value containers placed at the end of the MAC header, used to pass data between layers or devices.
Some IE identifiers are managed by the IEEE .
Some IE identifiers are managed by the IETF , and uses one subtype to support the selection of the Join Proxy.
join process:
The overall process that includes the discovery of the network by pledge(s) and the execution of the join protocol.
join protocol:
The protocol that allows the pledge to join the network.
The join protocol encompasses authentication, authorization and parameter distribution.
The join protocol is executed between the pledge and the JRC.
joined node:
The new device, after having completed the join process, often just called a node.
JP (Join Proxy):
Node already part of the 6TiSCH network that serves as a relay to provide connectivity between the pledge and the JRC.
The JP announces the presence of the network by regularly sending EB frames.
JRC (Join Registrar/Coordinator):
Central entity responsible for the authentication, authorization and configuration of the pledge.
link:
A communication facility or medium over which nodes can communicate at the Link-Layer, the layer immediately below IP. In 6TiSCH, the concept is implemented as a collection
of Layer-3 bundles. Note:
the IETF parlance for the term "Link" is adopted, as opposed to the IEEE Std. 802.15.4 terminology.
Operational Technology:
OT refers to technology used in automation, for instance in
industrial control networks. The convergence of IT and OT is
the main object of the Industrial Internet of Things (IIOT).
pledge:
A new device that attempts to join a 6TiSCH network.
(to) relocate a cell:
The action operated by the 6top sublayer of changing the slotOffset and/or channelOffset of a soft cell.
(to) schedule a cell:
The action of turning an unscheduled cell into a scheduled cell.
scheduled cell:
A cell which is assigned a neighbor MAC address (broadcast address is also possible), and one or more of the following flags: TX, RX, Shared and Timekeeping.
A scheduled cell can be used by the IEEE Std. 802.15.4 TSCH implementation to communicate.
A scheduled cell can either be a hard or a soft cell.
SF (6top Scheduling Function):
The cell management entity that adds or deletes cells dynamically based on application networking requirements.
The cell negotiation with a neighbor is done using 6P.
SFID (6top Scheduling Function Identifier):
A 4-bit field identifying an SF.
shared cell:
A cell marked with both the "TX" and "shared" flags.
This cell can be used by more than one transmitter node.
A back-off algorithm is used to resolve contention.
slotframe:
A collection of timeslots repeating in time, analogous to a superframe in that it defines periods of communication opportunities.
It is characterized by a slotframe_ID, and a slotframe_size.
Multiple slotframes can coexist in a node's schedule, i.e., a node can have multiple activities scheduled in different slotframes, based on the priority of its packets/traffic flows.
The timeslots in the Slotframe are indexed by the SlotOffset; the first timeslot is at SlotOffset 0.
slotOffset:
A column in the TSCH schedule, i.e., the number of timeslots since the beginning of the current iteration of the slotframe.
soft cell:
A scheduled cell which the 6top sublayer can relocate.
time source neighbor:
A neighbor that a node uses as its time reference, and to which it needs to keep its clock synchronized.
timeslot:
A basic communication unit in TSCH which allows
a transmitter node to send a frame to a receiver neighbor, and
that receiver neighbor to optionally send back an acknowledgment.
Track:
A Track is a Directed Acyclic Graph (DAG) that is used as a
complex multi-hop path to the destination(s) of the path.
In the case of unicast traffic, the Track is a Destination
Oriented DAG (DODAG) where the Root of the DODAG is the
destination of the unicast traffic.
A Track enables replication, elimination and reordering functions on the way (more on those functions in
.
A Track reservation locks physical resources such as cells and buffers in every node along the DODAG.
A Track is associated with a owner that can be for instance the destination of the Track.
TrackID:
A TrackID is either globally unique, or locally unique to the Track owner,
in which case the identification of the owner must be provided together with the TrackID
to provide a full reference to the Track. If the Track owner is the destination of the
Track then the destination IP address of packets along the Track can be used as
identification of the owner and a local InstanceID
can be used as TrackID.
In that case, a RPL Packet Information in an IPv6 packet
can unambiguously identify the Track and can be expressed in a compressed form using
.
TSCH:
A medium access mode of the
IEEE Std. 802.15.4 standard which uses
time synchronization to achieve ultra-low-power operation, and
channel hopping to enable high reliability.
TSCH Schedule:
A matrix of cells, each cell indexed by a slotOffset and a channelOffset.
The TSCH schedule contains all the scheduled cells from all slotframes and is sufficient to qualify the communication in the TSCH network.
The number of channelOffset values (the "height" of the matrix) is equal to the number of available frequencies.
Unscheduled Cell:
A cell which is not used by the IEEE Std. 802.15.4 TSCH implementation.
Abbreviations This document uses the following abbreviations:
6BBR:
6LoWPAN Backbone Router (router with a proxy ND function)
6LBR:
6LoWPAN Border Router (authoritative on DAD)
6LN:
6LoWPAN Node
6LR:
6LoWPAN Router (relay to the registration process)
6CIO:
Capability Indication Option
(E)ARO:
(Extended) Address Registration Option
(E)DAR:
(Extended) Duplicate Address Request
(E)DAC:
(Extended) Duplicate Address Confirmation
DAD:
Duplicate Address Detection
DODAG:
Destination-Oriented Directed Acyclic Graph
LLN:
Low-Power and Lossy Network (a typical IoT network)
IPv6 Routing Protocol for LLNs (pronounced ripple)
RA:
Router Advertisement
RS:
Router Solicitation
TSCH:
timeslotted Channel Hopping
TID:
Transaction ID (a sequence counter in the EARO)
Related Documents
The draft also conforms to the terms and models described in
and and uses the
vocabulary and the concepts defined in for the
IPv6 Architecture and refers for reservation
The draft uses domain-specific terminology defined or referenced in:
6LoWPAN ND "Neighbor Discovery Optimization
for Low-power and Lossy Networks" and
"Registration Extensions for 6LoWPAN Neighbor Discovery",
"Terms Used in Routing for Low-Power
and Lossy Networks (LLNs)",
and RPL
"Objective Function Zero for the
Routing Protocol for Low-Power and Lossy Networks (RPL)"
, and
"RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks".
Other terms in use in LLNs are found in
"Terminology for Constrained-Node Networks".
Readers are expected to be familiar with all the terms and concepts
that are discussed in
"Neighbor Discovery for IP version 6"
, and
"IPv6 Stateless Address Autoconfiguration"
.
In addition, readers would benefit from reading:
"Problem Statement and Requirements for
IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing"
,
"Multi-Link Subnet Issues", and
"IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions,
Problem Statement, and Goals"
prior to this specification for a clear
understanding of the art in ND-proxying and binding.
High Level ArchitectureA Non-Broadcast Multi-Access Radio Mesh Network
A 6TiSCH network is an IPv6 subnet which, in
its basic configuration illustrated in , is a
single Low-Power Lossy Network (LLN) operating over a synchronized
TSCH-based mesh.
Basic Configuration of a 6TiSCH Network
Inside a 6TiSCH LLN, nodes rely on 6LoWPAN
Header Compression (6LoWPAN HC) to encode IPv6 packets.
From the perspective of the network layer, a single LLN interface
(typically an IEEE Std. 802.15.4-compliant radio) may be seen as a collection
of Links with different capabilities for unicast or multicast services.
6TiSCH nodes join a mesh network by attaching to nodes that are already
members of the mesh (see ). The security aspects
of the join process are further detailed in .
In a mesh network, 6TiSCH nodes are not necessarily reachable from one
another at Layer-2 and an LLN may span over multiple links.
This forms a homogeneous non-broadcast multi-access (NBMA) subnet,
which is beyond the scope of IPv6 Neighbor Discovery (IPv6 ND)
. 6LoWPAN Neighbor
Discovery (6LoWPAN ND)
specifies extensions to IPv6 ND that enable ND operations in this type
of subnet that can be protected against address theft and impersonation
with .
Once it has joined the 6TiSCH network, a node acquires IPv6 Addresses
and register them using 6LoWPAN ND. This guarantees that the addresses
are unique and protects the address ownership over the subnet, more in
.
Within the NBMA subnet, RPL enables
routing in the so-called Route Over fashion, either in storing
(stateful) or non-storing (stateless, with routing headers) mode.
From there, some nodes can act as routers for 6LoWPAN ND and RPL
operations, as detailed in .
With TSCH, devices are time-synchronized at the MAC level. The use of
a particular RPL Instance for time synchronization is discussed in
. With this mechanism, the time synchronization
starts at the RPL Root and follows the RPL loopless routing topology.
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
(6LBR), acts as RPL Root, 6LoWPAN HC terminator, and Border Router
for the LLN to the outside. The 6LBR is usually powered.
More on RPL Instances can be found in section 3.1 of
RPL, 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
6LoRH.
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 Path Computation
Element (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.
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 described in
.
A Multi-Link Subnet Model
An extended configuration of the subnet comprises multiple LLNs as
illustrated in .
In the extended configuration, a Routing Registrar
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. The Routing registrar
may perform IPv6 ND proxy operations, or redistribute the registration in
a routing protocol such as OSPF or
BGP, or inject a route in a mobility protocol
such as MIPv6, NEMO
, or LISP.
Multiple LLNs can be interconnected and possibly synchronized over a
backbone, which can be wired or wireless. The backbone can operate with
IPv6 ND procedures or an
hybrid of IPv6 ND and 6LoWPAN ND
.
Extended Configuration of a 6TiSCH Network
A Routing Registrar that performs proxy IPv6 ND operations over the
backbone on behalf of the 6TiSCH nodes is called a Backbone Router (6BBR)
. The 6BBRs are
placed along the wireless edge of a Backbone, and federate multiple
wireless links to form a single MultiLink Subnet. The 6BBRs synchronize
with one another over the backbone, so as to ensure that the multiple LLNs
that form the IPv6 subnet stay tightly synchronized.
The use of multicast can also be reduced on the backbone with a registrar
that would contribute to Duplicate Address Detection as well as Address
Lookup using only unicast request/response exchanges.
is a proposed method that
presents an example of how to this could be achieved with an extension of
, using an optional 6LBR as a SubNet-level registrar,
as illustrated in .
As detailed in the 6LBR that serves the LLN and
the Root of the RPL network need to share information about the devices
that are learned through either 6LoWPAN ND or RPL but not both.
The preferred way of achieving this is to collocate/combine them.
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 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.
The DetNet
Architecture studies Layer-3 aspects of Deterministic Networks, and
covers networks that span multiple Layer-2 domains.
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.
TSCH: A Deterministic MAC Layer
Though at a different time scale (several orders of magnitude),
both IEEE Std. 802.1TSN and IEEE Std. 802.15.4 TSCH
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 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 co-channel interference.
6TiSCH builds on the IEEE Std. 802.15.4 TSCH 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
DetNet Architecture,
and IETF components such as the PCE
, for the protocol aspects.
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 and mobile workers accessing
local devices.
Scheduling TSCHA 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. The CDU matrix can be formatted in
chunks that can be allocated exclusively to particular nodes to enable
distributed scheduling without collision.
More in .
From the standpoint of a 6TiSCH node (at the MAC layer), its schedule
is the collection of the timeslots 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 collide and require a device to
wake up at a same time, in which case the slotframe with the highest
priority is actionable.
The 6top sublayer (see for more) hides the
complexity of the schedule from the upper layers. The Link abstraction
that IP traffic utilizes is composed of a pair of Layer-3 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.
Scheduling 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 either transmit or receive on one channel at
any point of time.
Scheduled cells that fulfil the same role, e.g., receive IP packets from
a peer, are grouped in bundles.
The 6TiSCH architecture identifies four ways a schedule can be managed
and CDU cells can be allocated: Static Scheduling, Neighbor-to-Neighbor
Scheduling, Centralized (or Remote) Monitoring and Schedule Management,
and Hop-by-hop Scheduling.
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
and does not preclude other scheduling operations to co-exist on a same
6TiSCH network. A static schedule is
necessary for basic operations such as the join process and
for interoperability during the network formation, which is specified
as part of the Minimal 6TiSCH Configuration
.
Neighbor-to-Neighbor Scheduling:
This refers to the
dynamic adaptation of the bandwidth of the Links that are used for IPv6
traffic between adjacent peers. Scheduling Functions such as the
"6TiSCH Minimal Scheduling Function
(MSF)" influence the operation of the MAC layer to add, update
and remove cells in its own, and its peer's schedules using 6P
,
for the negotiation of the MAC resources.
Centralized (or 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 to minimize the interaction
with and the load on the constrained device.
This model is the TSCH adaption of the
DetNet Architecture,
and it enables Traffic Engineering with deterministic properties.
Hop-by-hop Scheduling:
This refers to the possibility to
reserves cells along a path for a particular flow using a distributed
mechanism.
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.
A deeper dive in those mechanisms can be found in .
Distributed vs. Centralized Routing
6TiSCH enables a mixed model of centralized routes and distributed routes.
Centralized routes can for example be computed by an entity such as a PCE.
6TiSCH leverages the RPL routing protocol
for interoperable distributed routing operations.
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 RPLInstanceID, in a format that reuses the RPLInstanceID as
defined in RPL.
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
Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks"
AODV-RPL), which derives from the
Ad Hoc On-demand Distance Vector Routing (AODV) will be
selected for Hop-by-hop Scheduling.
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.
Forwarding Over TSCH
The 6TiSCH architecture supports three different forwarding models.
One is 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. The second derives from Generic MPLS (G-MPLS) for so-called
Track Forwarding, whereby a frame received at a particular timeslot
can be switched into another timeslot at Layer-2 without regard to the
upper layer protocol. The third model is the
6LoWPAN Fragment Forwarding, which allows to forward individual 6loWPAN
fragments along a route that is setup by the first fragment.
In more details:
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. Besides 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.
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 DAG
shapes that are more complex than a simple direct sequence of nodes.
6LoWPAN Fragment Forwarding:
This is a 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.
A deeper dive on these operations can be found in
.
The following table summarizes how the forwarding models
apply to the various routing and scheduling possibilities:
6TiSCH Stack
The IETF proposes multiple techniques for implementing functions related
to routing, transport or security.
The 6TiSCH architecture limits the possible
variations of the stack and recommends a number of base elements for LLN
applications to control the complexity of
possible deployments and device interactions, and to limit the size of
the resulting object code. In particular, UDP ,
IPv6 and the Constrained
Application Protocol (CoAP) are used as the transport / binding of
choice for applications and management as opposed to TCP and HTTP.
The resulting protocol stack is represented in :
6TiSCH Protocol Stack
RPL is the routing protocol of choice for LLNs. So far, there was no
identified need to define a 6TiSCH specific Objective Function.
The Minimal 6TiSCH Configuration
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.
The 6LoWPAN Header Compression is used
to compress the IPv6 and UDP headers, whereas the
6LoWPAN Routing Header (6LoRH) 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.
"When to use RFC 6553, 6554
and IPv6-in-IPv6" provides the details on when headers or encapsulation are needed.
The
Object Security for Constrained RESTful Environments (OSCORE) ,
is leveraged by the Constrained Join Protocol (CoJP) and is expected to
be the primary protocol for the protection of the application payload
as well. The application payload may also be protected by
the Datagram Transport Layer Security (DTLS)
sitting either under CoAP or over CoAP so it can traverse
proxies.
The 6TiSCH Operation
sublayer (6top) 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, providing in particular dynamic cell allocation with the
6top Protocol (6P) .
The reference stack presented in this document was 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.
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.
Communication Paradigms and Interaction Models provides the terms
of Communication Paradigms and Interaction Models, in relation with
"On the Difference between Information Models
and Data Models".
A Communication Paradigm 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 Model would be more refined and could point to standard operation
such as a Representational state transfer (REST) "GET" operation and would match
a Data Model for the data that is provided over the protocol exchange.
Section 2.1.3 of
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 - one sender, two receivers for redundancy -
and its N-cast generalization are also provided.
Those paradigms are frequently used in industrial automation, which is
a major use case for IEEE Std. 802.15.4 TSCH wireless networks with
and , that
provides a wireless access to applications and
devices.
This document 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 (multihop along a Track), and Application-layer
(remote control) are discussed in .
Link-Layer frame forwarding interactions are discussed in , and
Network-layer Packet routing is addressed in .
Architecture Components6LoWPAN (and RPL)A RPL DODAG is formed of a Root, a collection of routers, and leaves that
are hosts. Hosts are nodes which do not forward packets that they did not generate.
RPL-aware leaves will participate to RPL to advertise their own
addresses, whereas RPL-unaware leaves depend on a connected RPL router to do
so. RPL interacts with 6LoWPAN ND at multiple levels, in particular at the
Root and in the RPL-unaware leaves.
RPL-Unaware Leaves and 6LoWPAN NDRPL needs a set of information to advertise
a leaf node through a Destination Advertisement Object (DAO) message and establish reachability.
"Routing for RPL Leaves"
details the basic interaction of 6LoWPAN ND and RPL and enables a plain 6LN
that supports to obtain return
connectivity via the RPL network as an RPL-unaware leaf.
The leaf indicates that it requires reachability services for the
Registered Address from a Routing Registrar by setting a 'R' flag in the
Extended Address Registration Option , and it
provides a TID that maps to a sequence number in section 7 of RPL .
also enables the leaf to signal
the RPL InstanceID that it wants to participate to using the
Opaque field of the EARO. On the backbone, the InstanceID is
expected to be mapped to an overlay that matches the RPL Instance, e.g.,
a Virtual LAN (VLAN) or a virtual routing and forwarding (VRF) instance.
Though at the time of this writing the above specification enables a model
where the separation is possible, this architecture recommends to
collocate the functions of 6LBR and RPL Root.
6LBR and RPL Root
With the 6LowPAN ND , information on the 6LBR is
disseminated via an Authoritative Border Router Option (ABRO) in RA messages.
extends to enable a
registration for routing and proxy ND.
The capability to support
is indicated in the 6LoWPAN Capability Indication Option (6CIO).
The discovery and liveliness of the RPL Root are obtained through RPL
itself.
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 EDAR/EDAC
exchange with the state that is maintained by RPL.
Section 7 of specifies how
the DAO messages are used to reconfirm the registration, thus eliminating a
duplication of functionality between DAO and EDAR/EDAC messages, as
illustrated in .
also provides the protocol
elements that are needed when the 6LBR and RPL Root functionalities are not
co-located.
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.
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
.
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 is why in the Registered Address is signaled
in the Target Address field of the NS message as opposed to the IPv6 Source
Address, which, in the case of a proxy registration, is that of the 6LBR /
RPL Root itself.
Network Access and AddressingJoin Process
A new device, called the pledge, undergoes the join protocol to become a node
in a 6TiSCH network. This usually occurs only once when the device is
first powered on. The pledge communicates with the Join Registrar/Coordinator
(JRC) of the network through a Join Proxy (JP), a radio neighbor of the pledge.
The JP is discovered though MAC layer beacons. When multiple JPs from possibly multiple networks are visible, trial and error till an acceptable position in the right network is obtained becomes ineffficient.
adds a new subtype in the Information Element that was delegated to the IETF and provides visibility on the network that can be joined and the willingness by the JP and the Root to be used by the pledge.
The join protocol provides the following functionality:
Mutual authentication
Authorization
Parameter distribution to the pledge over a secure channel
Minimal Security Framework for 6TiSCH
defines the minimal mechanisms required for this join process to occur in a secure
manner. The specification defines the Constrained Join Protocol (CoJP) that is used
to distribute the parameters to the pledge over a secure session established through
OSCORE , and a secure configuration of the network
stack. In the minimal setting with pre-shared keys (PSKs), CoJP allows the pledge to
join after a single round-trip exchange with the JRC. The provisioning of the PSK to
the pledge and the JRC needs to be done out of band, through a 'one-touch'
bootstrapping process, which effectively enrolls the pledge into the domain managed by
the JRC.
In certain use cases, the 'one touch' bootstrapping is not feasible due to the
operational constraints and the enrollment of the pledge into the domain needs to occur
in-band. This is handled through a 'zero-touch' extension of the Minimal Security Framework
for 6TiSCH. Zero touch extension leverages
the 'Bootstrapping Remote Secure Key Infrastructures (BRSKI)' [
work to establish a shared secret between a pledge and the JRC without necessarily having
them belong to a common (security) domain at join time. This happens through inter-domain
communication occurring between the JRC of the network and the domain of the pledge,
represented by a fourth entity, Manufacturer Authorized Signing Authority (MASA). Once
the zero-touch exchange completes, the CoJP exchange defined in
is carried over the secure session established between the pledge and the JRC.
depicts the join process and where a Link-Local
Address (LLA) is used, versus a Global Unicast Address (GUA).
Join process in a Multi-Link Subnet. Parentheses () denote optional exchanges.| | | |
| NA (EARO) | | | |
|<--------------| | | |
| | | | |
| (Zero-touch | | | |
| handshake) | (Zero-touch handshake) | (Zero-touch |
| using LLA | using GUA | handshake) |
|<------------->|<---------------------------->|<------------>|
| | | | |
| CoJP Join Req | | | | \
| using LLA | | | | |
|-------------->| | | | |
| | CoJP Join Request | | |
| | using GUA | | |
| |----------------------------->| | | C
| | | | | | o
| | CoJP Join Response | | | J
| | using GUA | | | P
| |<-----------------------------| | |
|CoJP Join Resp | | | | |
| using LLA | | | | |
|<--------------| | | | /
| | | | |
]]>Registration
Once the pledge successfully completes the CoJP protocol and becomes
a network node, it obtains the network prefix from neighboring routers
and registers its IPv6 addresses.
As detailed in , 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.
illustrates the initial IPv6 signaling that
enables a 6LN to form a global address and register it to a 6LBR
using 6LoWPAN ND , is then carried
over RPL to the RPL Root, and then to the 6BBR. This flow happens
just once when the address is created and first registered.
Initial Registration Flow over Multi-Link Subnet| | |
|-----------> | | |
|------------------> | |
| RA (unicast) | | |
|<--------------| | |
| | | |
| NS(EARO) | | |
|-------------->| | |
| 6LoWPAN ND | Extended DAR | |
| |-------------->| |
| | | NS(EARO) |
| | |-------------->|
| | | | NS-DAD
| | | |------>
| | | | (EARO)
| | | |
| | | NA(EARO) |
| | |<--------------|
| | Extended DAC | |
| |<--------------| |
| NA(EARO) | | |
|<--------------| | |
| | | |
]]> illustrates the repeating IPv6 signaling that
enables a 6LN to keep a global address alive and registered to its 6LBR
using 6LoWPAN ND to the 6LR, RPL to the RPL Root, and then 6LoWPAN ND
again
to the 6BBR, which avoids repeating the Extended DAR/DAC flow across
the network when RPL can suffice as a keep-alive mechanism.
Next Registration Flow over Multi-Link Subnet| | |
| NA(EARO) | | |
|<--------------| | |
| | DAO | |
| |-------------->| |
| | DAO-ACK | |
| |<--------------| |
| | | NS(EARO) |
| | |-------------->|
| | | NA(EARO) |
| | |<--------------|
| | | |
| | | |
]]>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.
TSCH and 6top6top
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 the spectrum in its
vicinity.
With 6TiSCH, the abstraction of an IPv6 link is implemented as a
pair of bundles of cells, one in each direction. IP Links are only
enabled between RPL parents and children. The 6TiSCH
operation is optimal when the size of a bundle is such that both
the energy wasted in idle listening and the packet drops due to
congestion loss are minimized, while packets are forwarded within
an acceptable latency.
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. The 6TiSCH operation can
remain optimal if RPL parents can adjust dynamically, and with enough reactivity to match the variations of best-effort traffic,
the amount of bandwidth that is used to communicate between themselves and their children, in both directions.
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.
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 protocol, 6P, which is specified in
, is one of the services provided by 6top.
In particular, the 6top services are available over a management
API that enables an external management entity to schedule cells
and slotframes, and allows the addition of complementary
functionality, for instance a Scheduling Function
that manages a dynamic schedule management based on
observed resource usage as discussed in .
For this purpose, the 6TiSCH architecture differentiates "soft"
cells and "hard" cells.
Hard Cells
"Hard" cells are cells that
are owned and managed by a 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.
Soft Cells
In contrast, "soft" cells are cells that 6top can manage locally.
6top contains a monitoring process which monitors the performance of
cells, and can add, remove soft cells in the TSCH schedule to adapt
to the traffic needs, or move one when it performs poorly.
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.
Scheduling Functions and the 6top protocolIn 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.
An 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 using the 6top protocol as illustrated in .
SF/6P stack in 6top | 6top cells negotiation |
+------------------------+ +------------------------+
Device A Device B
]]>
The SF relies on 6top services that implement the
6top Protocol (6P)
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. 6P enables the peers to find an agreement in a
transactional manner that ensures the final consistency of the nodes state.
MSF is one of the possible
scheduling functions. MSF uses the rendez-vous slot from
for network discovery, neighbor discovery, and any
other broadcast.
For basic unicast communication with any neighbor, each node uses a receive
cell at a well-known slotOffset/channelOffset, derived from a hash of their
own MAC address.
Nodes can reach any neighbor by installing a transmit (shared) cell with
slotOffset/channelOffset derived from the neighbor's MAC address.
For child-parent links, MSF continuously monitors the load to/from parents
and children. It then uses 6P to install/remove unicast cells whenever the
current schedule appears to be under-/over- provisioned.
6top and RPL Objective Function operations
An implementation of a RPL Objective Function
(OF), such as the RPL Objective Function Zero (OF0)
that is used in the Minimal
6TiSCH Configuration to support RPL over a static schedule, may
leverage, for its internal computation, the information maintained by 6top.
An OF may require metrics about reachability, such as the Expected
Transmission Count (ETX) metric .
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. A neighbor table entry may contain a set of statistics with
respect to that specific neighbor.
The neighbor information may include the time when the last
packet has been received from that neighbor, a set of cell quality
metrics, e.g., received signal strength indication (RSSI) or link
quality indicator (LQI), the number of packets sent to the
neighbor or the number of packets received from it. This
information can be made available through 6top management APIs
and used for instance to compute a Rank Increment that will
determine the selection of the preferred parent.
6top provides statistics about the underlying layer so the OF can be tuned
to the nature of the TSCH MAC layer. 6top also enables the RPL OF to
influence the MAC behavior, for instance by configuring the periodicity of
IEEE Std. 802.15.4 Extended Beacons (EBs). By augmenting the EB periodicity, it is
possible to change the network dynamics so as to improve the support of
devices that may change their point of attachment in the 6TiSCH network.
Some RPL control messages, such as the DODAG Information Object (DIO) are
ICMPv6 messages that are broadcast to all neighbor nodes.
With 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
Layer-2 Enhanced Beacons (EBs), which would produce undue timer
coupling among layers, packet size issues and could conflict with
the policy of production networks where EBs are mostly eliminated
to conserve energy.
Network Synchronization
Nodes in a TSCH network must be time synchronized.
A node keeps synchronized to its time source neighbor
through a combination of frame-based and acknowledgment-based synchronization.
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.
Time distribution requires a loop-free structure. Nodes taken in a synchronization loop will rapidly
desynchronize from the network and become isolated. 6TiSCH uses a RPL DAG with a dedicated global Instance for the purpose of time synchronization.
That Instance is referred to as the Time Synchronization Global Instance (TSGI).
The TSGI can be operated in either of the 3 modes that are detailed
in section 3.1.3 of RPL,
"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.
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 , "Rank Comparison".
The provisioning of a RPL Root is out of scope for both RPL and this Architecture, whereas RPL enables to propagate configuration information down the DODAG. This applies to the TSGI as well; a
Root is configured or obtains by unspecified 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 in
its IEEE Std. 802.15.4 Extended Beacons (EB).
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.
Slotframes and CDU matrix
6TiSCH enables IPv6 best effort (stochastic) transmissions over a MAC
layer that is also capable of scheduled (deterministic) transmissions.
A window of time is defined
around the scheduled transmission where the medium must, as much as
practically feasible, be free of contending energy to ensure that the
medium is free of contending packets when time comes for a scheduled
transmission.
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 IEEE Std. 802.15.4 TSCH as well as the Long
Term Evolution (LTE) of cellular networks.
The 6TiSCH architecture defines a global concept that is called a
Channel Distribution and Usage (CDU) matrix to describe that formatting
of time and frequencies,
A CDU matrix is defined centrally
as part of the network definition. It is a matrix of cells with a
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. There are different models for scheduling the usage of the
cells, which place the responsibility of avoiding collisions either on
a central controller or on the devices themselves, at an extra cost in
terms of energy to scan for free cells (more in ).
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.
A CDU matrix iterates over and over with a well-known channel rotation
called the hopping sequence.
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 the Slotted ALOHA operations.
The knowledge of the CDU matrices is shared
between all the nodes and used in particular to define slotframes.
A slotframe is a MAC-level abstraction that is common to all nodes and
contains a series of timeslots of equal length and precedence.
It is characterized by a slotframe_ID, and a slotframe_size.
A slotframe aligns to a CDU matrix for its parameters, such as number
and duration of timeslots.
Multiple slotframes can coexist in a node schedule, i.e., a node can
have multiple activities scheduled in different slotframes.
A slotframe is associated with a priority that may be related to
the precedence of different 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.
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
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.
Distributing the reservation of cells
The 6TiSCH architecture introduces the concept of chunks
() to distribute the allocation of
the spectrum for a whole group of cells at a time.
The CDU matrix is formatted into a set of chunks, possibly as
illustrated in , each of the chunks
identified uniquely by a chunk-ID. The knowledge of this
formatting is shared between all the nodes in a 6TiSCH network.
It could be conveyed during the join process, or codified into a profile document, or obtained using some other mechanism. This is as opposed
to static scheduling that refers to the pre-programmed mechanism that
is specified in and pre-exists to the
distribution of the chunk formatting.
CDU matrix Partitioning in Chunks
The 6TiSCH Architecture envisions a protocol that enables chunk
ownership appropriation whereby a RPL parent
discovers a chunk that is not used in its interference domain,
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.
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.
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.
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.
Schedule Management Mechanisms
6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes: Static Scheduling,
neighbor-to-neighbor Scheduling, remote monitoring and scheduling management, and Hop-by-hop scheduling.
Multiple mechanisms are defined that implement the associated Interaction Models,
and can be combined and used in the same LLN.
Which mechanism(s) to use depends on application requirements.
Static Scheduling
In the simplest instantiation of a 6TiSCH network, a common fixed
schedule may be shared by all nodes in the network. Cells are shared,
and nodes contend for slot access in a slotted ALOHA manner.
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 IEEE Std. 802.15.4 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
.
Neighbor-to-neighbor Scheduling
In the simplest instantiation of a 6TiSCH network described in
, nodes may expect a packet at any cell in
the schedule and will waste energy idle listening. In a more
complex instantiation of a 6TiSCH network, a matching portion of the
schedule is established between peers to reflect the observed amount
of transmissions between those nodes. The aggregation of the cells
between a node and a peer forms a bundle that the 6top layer uses to
implement the abstraction of a link for IP. The bandwidth on that
link is proportional to the number of cells in the bundle.
If the size of a bundle is configured to fit an average amount of
bandwidth, peak traffic is dropped. If the size is
configured to allow for peak emissions, energy is be wasted
idle listening.
As discussed in more details in , the
6top Protocol
specifies the exchanges between neighbor nodes to reserve soft cells
to transmit to one another, possibly under the control of a
Scheduling Function (SF). Because this reservation is done without
global knowledge of the schedule of other nodes in the LLN, scheduling
collisions are possible.
And as discussed in ,
an optional Scheduling Function (SF) 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. The
"6TiSCH Minimal Scheduling Function (MSF)" provides a simple
scheduling function that can be used by default by devices that
support dynamic scheduling of soft cells.
Monitoring and relocation is done in the 6top layer. For the upper
layer, the connection between two neighbor nodes appears as a 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.
Remote Monitoring and Schedule Management
Remote monitoring and Schedule Management refers to a DetNet/SDN model
whereby an NME and a scheduling entity, associated with a PCE, reside
in a central controller and interact with the 6top layer to control
IPv6 Links and Tracks () in a 6TiSCH network.
The composite centralized controller can assign physical resources
(e.g., buffers and hard cells) to a particular Track to optimize the
reliability within a bounded latency for a well-specified flow.
The work at the 6TiSCH WG focused on non-deterministic traffic and
did not provide the generic data model that is necessary for the
controller to monitor and manage resources of the 6top sublayer.
This is deferred to future work, see .
With respect to Centralized routing and scheduling, it is envisioned
that the related component of the 6TiSCH Architecture would be an
extension of the
DetNet Architecture,
which studies Layer-3 aspects of Deterministic Networks, and covers
networks that span multiple Layer-2 domains.
The DetNet architecture is a form of Software Defined Networking (SDN)
Architecture and is composed of three planes, a (User) Application
Plane, a Controller Plane (where the PCE operates), and a Network Plane
which can represent a 6TiSCH LLN.
Software-Defined Networking (SDN):
Layers and Architecture Terminology proposes a generic
representation of the SDN architecture that is reproduced in
.
SDN Layers and Architecture Terminology per RFC 7426The PCE establishes end-to-end Tracks of hard cells, which are described
in more details in .
The DetNet work is expected to enable end to end Deterministic Path
across heterogeneous network. This can be for instance 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.
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 IEEE Std. 802.1
TSN Ethernet backbone, and that of DetNet to enable end-to-end deterministic
forwarding.
Hop-by-hop Scheduling
A node can reserve a Track to one or more
destination(s) that are multiple hops away by installing soft cells at each
intermediate node.
This forms a Track of soft cells. A Track Scheduling Function above the 6top
sublayer of each node on the Track is needed to monitor these soft cells and
trigger relocation when needed.
This hop-by-hop reservation mechanism is expected to be similar in essence
to and/or /.
The protocol for a node to trigger hop-by-hop scheduling is not yet defined.
On Tracks
The architecture introduces the concept of a Track, which is a directed path
from a source 6TiSCH node to one or more destination 6TiSCH node(s)
across a 6TiSCH LLN.
A Track is the 6TiSCH instantiation of the concept of a Deterministic Path
as described in .
Constrained resources such as memory buffers are reserved for that Track in
intermediate 6TiSCH nodes to avoid loss related to limited capacity.
A 6TiSCH node along a Track not only knows which bundles of cells it should
use to receive packets from a previous hop, but also knows which bundle(s)
it should use to send packets to its next hop along the Track.
General Behavior of Tracks
A Track is associated with Layer-2 bundles of cells with related schedules
and logical relationships and that ensure that a packet that is injected in
a Track will progress in due time all the way to destination.
Multiple cells may be scheduled in a Track for the transmission of a single
packet, in which case the normal operation of IEEE Std. 802.15.4 Automatic
Repeat-reQuest (ARQ) can take place; the acknowledgment may be omitted in
some cases, for instance if there is no scheduled cell for a possible retry.
There are several benefits for using a Track to forward a packet from a
source node to the destination node.
Track forwarding, as further described in , 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.
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.
By knowing the scheduled time slots of incoming bundle(s) and outgoing
bundle(s), 6TiSCH nodes on a Track could save more energy by staying in
sleep state during in-active slots.
Tracks are protected from interfering with one another if a cell is scheduled to belong to at most one Track, and congestion loss is avoided if at most one
packet can be presented to the MAC to use that cell.
Tracks enhance the reliability of transmissions and thus further improve
the energy consumption in LLN Devices by reducing the chances of
retransmission.
Serial Track
A Serial (or simple) Track is the 6TiSCH version of a circuit; a bundle of
cells that are programmed 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 which can be used regardless of the network layer protocol.
A Serial 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.
For a given iteration of the device schedule, the effective channel of the
cell is obtained by following in a loop a well-known hopping sequence that
started at Epoch time at 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 in the iteration of the
schedule.
Complex Track with Replication and Elimination
The art of Deterministic Networks already include Packet Replication and
Elimination techniques. Example
standards include the Parallel Redundancy Protocol (PRP) and the
High-availability Seamless Redundancy (HSR) .
Similarly, and as opposed to a Serial Track that is a sequence of nodes
and links, a Complex Track is shaped as a directed acyclic graph towards one
or more destination(s) to support multi-path forwarding and route around
failures.
A Complex Track may branch off over non congruent branches for the purpose
of multicasting, and/or redundancy, in which case it reconverges later down
the path.
This enables the Packet Replication, Elimination and Ordering Functions (PREOF)
defined by Detnet. Packet ARQ, Replication, Elimination and Overhearing (PAREO)
adds radio-specific capabilities of Layer-2 ARQ and promiscuous listening to
redundant transmissions to compensate for the lossiness of the medium and meet
industrial expectations of a Reliable and Available Wireless network.
Combining PAREO and PREOF, a Track may extend beyond the 6TiSCH network in a larger DetNet network.
In the art of TSCH, a path does not necessarily support PRE but it is almost
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. Similarly,
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.
DetNet End-to-end Path
Ultimately, DetNet should
enable to extend a Track beyond the 6TiSCH LLN as illustrated in
. In that example, a Track that is laid out from a
field device in a 6TiSCH network to an IoT gateway that is located on an
802.1 Time-Sensitive Networking (TSN) backbone.
A 6TiSCH-Aware DetNet Service Layer handles the Packet Replication,
Elimination, and Ordering Functions over the DODAG that forms a Track.
The Replication function in the 6TiSCH Node 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.
Example End-to-End DetNet TrackCell Reuse
The 6TiSCH architecture provides means to avoid waste of cells as
well as overflows in the transmit bundle of a Track, as follows:
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 in a frame that is received in a RX-cell of a Track with a
destination MAC address set to this node as opposed to the broadcast MAC
address must be extracted from the Track and delivered to the upper layer.
Note that a frame with an unrecognized destination MAC address is dropped
at the lower MAC layer and thus is not received at the 6top sublayer.
On the other hand, it might happen that there are not enough TX-cells
in the transmit bundle to accommodate the Track traffic, for instance if
more retransmissions are needed than provisioned.
In that case, and if the frame transports an IPv6 packet, then it can be
placed for transmission in the bundle that is used for Layer-3 traffic
towards the next hop along the Track.
The MAC address should be set to the next-hop MAC address to avoid
confusion.
It results in 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 is re-Tracked by scheduling it for transmission over the
transmit bundle associated to the Track, with the destination MAC
address set to broadcast.
Forwarding Models
By forwarding, this document 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 .
6TiSCH supports three different forwarding model:(G-MPLS) Track
Forwarding, (classical) IPv6 Forwarding and (6LoWPAN) Fragment Forwarding.
Track Forwarding
Forwarding along a Track can be seen as a Generalized Multi-protocol
Label Switching (G-MPLS) operation in that the information used to
switch a frame is not an explicit label, but rather related to other
properties of the way the packet was received, a particular cell in
the case of 6TiSCH.
As a result, as long as the TSCH MAC (and Layer-2 security) accepts
a frame, that frame can be switched regardless of the protocol,
whether this is an IPv6 packet, a 6LoWPAN fragment, or a frame from
an alternate protocol such as WirelessHART or ISA100.11a.
A data frame that is forwarded along a Track normally has a
destination MAC address that is set to broadcast - or a multicast
address depending on MAC support.
This way, the MAC layer in the intermediate nodes accepts the
incoming frame and 6top switches it without incurring a change in
the MAC header.
In the case of IEEE Std. 802.15.4, this means effectively
broadcast, so that along the Track the short address for the
destination of the frame is set to 0xFFFF.
There are 2 modes for a Track, native mode and tunnel mode.
Native Mode
In native mode, the Protocol Data Unit (PDU) is associated
with flow-dependent 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 may be done using a 6-tuple as discussed in
. In particular,
implementations of this document should support identification of
DetNet flows based on the IPv6 Flow Label field.
The flow identification may also be done using a dedicated RPL
Instance (see section 3.1.3 of ),
signaled in a RPL Packet Information (more in section 11.2.2.1 of
).
The flow identification is validated at egress before restoring
the destination MAC address (DMAC) and punting to the upper layer.
illustrates the Track Forwarding operation
which happens at the 6top sublayer, below IP.
Track Forwarding, Native ModeTunnel Mode
In tunnel mode, the frames originate from an arbitrary protocol over a compatible MAC
that may or may not be synchronized with the 6TiSCH network. An example of
this would be a router with a dual radio that is capable of receiving and sending WirelessHART
or ISA100.11a frames with the second radio, by presenting itself as an access
Point or a Backbone Router, respectively.
In that mode, some entity (e.g., PCE) can coordinate with a
WirelessHART Network Manager or an ISA100.11a System Manager to
specify the flows that are transported.
Track Forwarding, Tunnel Mode
In that case, the flow information that identifies the Track at
the ingress 6TiSCH router is derived from the RX-cell.
The DMAC
is set to this node but the flow information indicates that the
frame must be tunneled over a particular Track so the frame is
not passed to the upper layer. Instead, the DMAC is forced to
broadcast and the frame is passed to the 6top sublayer for
switching.
At the egress 6TiSCH router, the reverse operation occurs. Based
on tunneling information of the Track, which may for instance
indicate that the tunneled datagram is an IP packet,
the datagram is passed to the appropriate Link-Layer with the
destination MAC restored.
Tunneling Information
Tunneling information coming with the Track configuration
provides 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.
If the Layer-3 destination address belongs to
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. Restoring the wrong MAC address at the egress
would then also result in the wrong IP address in the packet
after decompression.
For that reason, a packet can be injected in a Track only if
the destination MAC address is effectively that of the tunnel
egress point.
It is thus mandatory for the ingress router to validate that the
MAC address that was used at the 6LoWPAN
sublayer for compression matches 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
information.
IPv6 Forwarding
As the packets are routed at Layer-3, traditional QoS and Active
Queue Management (AQM) operations are expected to prioritize flows.
IP ForwardingFragment Forwarding
Considering that per section 4 of 6LoWPAN
packets can be as large as 1280 bytes (the IPv6 minimum MTU),
and that the non-storing mode of RPL implies Source Routing that requires space for routing
headers, and that a IEEE Std. 802.15.4 frame with security may carry in the order of 80 bytes of
effective payload, an IPv6 packet might be fragmented into more than 16 fragments at the
6LoWPAN sublayer.
This level of fragmentation is much higher than that traditionally experienced over the Internet
with IPv4 fragments, where fragmentation is already known as harmful.
In the case to a multihop route within a 6TiSCH network, Hop-by-Hop recomposition occurs at each
hop 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.
describes a framework for forwarding fragments end-to-end across a 6TiSCH route-over mesh.
Within that framework, details a virtual reassembly buffer mechanism whereby the datagram tag in the 6LoWPAN Fragment is used as a label for switching at the 6LoWPAN sublayer.
Building on this technique, introduces a new format for 6LoWPAN fragments that enables the selective recovery of individual fragments, and allows for a degree of flow control based on an Explicit Congestion Notification.
Forwarding First Fragment
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.
Forwarding Next Fragment
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.
Advanced 6TiSCH RoutingPacket Marking and Handling
All packets inside a 6TiSCH domain must carry the RPLInstanceID that
identifies the 6TiSCH topology that is to be used for
routing and forwarding that packet. The location of that information
must be the same for all packets forwarded inside the domain.
For packets that are routed by a PCE along a Track, the tuple formed by the
IPv6 source address and a local RPLInstanceID in the packet identify
uniquely the Track and associated transmit bundle.
For packets that are routed by RPL, that information is the RPLInstanceID
which is carried in the RPL Packet Information (RPI), as discussed in
section 11.2 of , "Loop Avoidance and Detection".
The RPI is transported by a RPL option in the IPv6 Hop-By-Hop Header
.
A compression mechanism for the RPL packet artifacts that integrates the
compression of IP-in-IP encapsulation and the Routing Header type 3
with that of the RPI in a 6LoWPAN dispatch/header type is specified in
and .
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.
Replication, Retries and Elimination
6TiSCH supports the PREOF operations of elimination and reordering of packets
along a complex Track, but has no requirement about whether a sequence number
is tagged in the packet for that purpose.
With 6TiSCH, the schedule can tell when multiple receive timeslots correspond
to copies of a same packet, in which case the receiver may avoid listening to
the extra copies once it had received one instance of the packet.
The semantics of the configuration will enable correlated timeslots to be
grouped for transmit (and respectively receive) with a 'OR' relations,
and then a 'AND' relation would be configurable between groups.
The semantics is that if the transmit (and respectively receive) operation
succeeded in one timeslot in a 'OR' group, then all the other timeslots in
the group are ignored.
Now, if there are at least two groups, the 'AND' relation between the groups
indicates that one operation must succeed in each of the groups.
On the transmit side, timeslots provisioned for retries along a same branch
of a Track are placed a same 'OR' group. The 'OR' relation indicates that if
a transmission is acknowledged, then retransmissions of that packet should
not be attempted for remaining timeslots in that group. There are as many
'OR' groups as there are branches of the Track departing from this node.
Different 'OR' groups are programmed for the purpose of replication, each
group corresponding to one branch of the Track. The 'AND' relation between the
groups indicates that transmission over any of branches must be attempted
regardless of whether a transmission succeeded in another branch. It is also
possible to place cells to different next-hop routers in a same 'OR' group.
This allows to route along multi-path Tracks, trying one next-hop and then
another only if sending to the first fails.
On the receive side, all timeslots are programmed in a same 'OR' group.
Retries of a same copy as well as converging branches for elimination
are converged, meaning that the first successful reception is enough and that
all the other timeslots can be ignored. A 'AND' group denotes different
packets that must all be received and transmitted over the associated
transmit groups within their respected 'AND' or 'OR' rules.
As an example say that we have a simple network as represented in
, and we want to enable PREOF between an ingress
node I and an egress node E.
Scheduling PREOF on a Simple Network
The assumption for this particular problem is
that a 6TiSCH node has a single radio, so it cannot perform 2 receive and/or
transmit operations at the same time, even on 2 different channels.
Say we have 6 possible channels, and at least 10 timeslots per slotframe.
shows a possible schedule whereby each transmission
is retried 2 or 3 times, and redundant copies are forwarded in parallel via
A and C on the one hand, and B and D on the other, providing time diversity,
spatial diversity though different physical paths, and frequency diversity.
Example Global Schedule
D| | | ...
+----+----+----+----+----+----+----+----+----+
channelOffset 1 | |I->A| |A->C|B->D| | | | | ...
+----+----+----+----+----+----+----+----+----+
channelOffset 2 |I->A| | |I->B| |C->E| |D->E| | ...
+----+----+----+----+----+----+----+----+----+
channelOffset 3 | | | | |A->C| | | | | ...
+----+----+----+----+----+----+----+----+----+
channelOffset 4 | | |I->B| | |B->D| | |D->E| ...
+----+----+----+----+----+----+----+----+----+
channelOffset 5 | | |A->C| | | |C->E| | | ...
+----+----+----+----+----+----+----+----+----+
]]>
This translates in a different slotframe for every node that provides the
waking and sleeping times, and the channelOffset to be used when awake.
shows the corresponding slotframe for node A.
Example Slotframe for Node A
The logical relationship between the timeslots is given
by the following table:
IANA Considerations
This document does not require IANA action.
Security Considerations
The "Minimal Security
Framework for 6TiSCH" was optimized for Low-Power and TSCH operations.
The reader is encouraged to review the Security Considerations section of
that document, which discusses 6TiSCH security issues in more details.
Availability of Remote Services
The operation of 6TiSCH Tracks inherits its high level operation from DetNet
and is subject to the observations in section 5 of
. The installation and the
maintenance of the 6TiSCH Tracks depends on the availability of a controller
with a PCE to compute and push them in the network. When that connectivity
is lost, existing Tracks may continue to operate until the end of their
lifetime, but cannot be removed or updated, and new Tracks cannot be
installed.
In a LLN, the communication with a remote PCE may be slow and unreactive to
rapid changes in the condition of the wireless communication. An attacker
may introduce extra delay by selectively jamming some packets or some flows.
The expectation is that the 6TiSCH Tracks enable enough redundancy to
maintain the critical traffic in operation while new routes are calculated
and programmed into the network.
As with DetNet in general, the communication with the PCE must be secured
and should be protected against DoS attacks, including delay injection and
blackholing attacks, and secured as discussed in the security considerations
defined for Abstraction and Control of Traffic Engineered Networks (ACTN) in
Section 9 of , which applies equally to DetNet and
6TiSCH. In a similar manner, the communication with the JRC must
be secured and should be protected against DoS attacks when possible.
Selective Jamming
The Hopping Sequence of a TSCH network is well-known, meaning that if a
rogue manages to identify a cell of a particular flow, then it may
to selectively jam that cell, without impacting any other traffic.
This attack can be performed at the PHY layer without any knowledge of the
Layer-2 keys, and is very hard to detect and diagnose because only one flow
is impacted.
proposes
a method to obfuscate the hopping sequence and make it harder to perpetrate
that particular attack.
MAC-Layer Security
This architecture operates on IEEE Std. 802.15.4 and expects the 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. In a typical deployment, all joined nodes use the same keys and
rekeying needs to be global.
The 6TISCH Architecture relies on the join process to deny authorization of
invalid nodes and preserve the integrity of the network keys. A rogue that
managed to access the network can perform a large variety of attacks from
DoS to injecting forged packets and routing information.
"Zero-trust" properties would be highly desirable but are mostly not
available at the time of this writing.
is a notable exception that protects the ownership of IPv6 addresses and
prevents a rogue node with L2 access from stealing and injecting traffic
on behalf of a legitimate node.
Time Synchronization
Time Synchronization in TSCH induces another event horizon whereby a node
will only communicate with another node if they are synchronized within a
guard time. The pledge discovers the synchronization of the network based
on the time of reception of the beacon. If an attacker synchronizes a pledge
outside of the guard time of the legitimate nodes then the pledge will never
see a legitimate beacon and may not discover the attack.
As discussed in , measures
must be taken to protect the time synchronization, and for 6TiSCH this
includes ensuring that the Absolute Slot Number (ASN), which is the node's
sense of time, is not compromised. Once installed and as long as the node is
synchronized to the network, ASN is implicit in the transmissions.
IEEE Std. 802.15.4 specifies that in a TSCH
network, the nonce that is used for the computation of the Message Integrity
Code (MIC) to secure Link-Layer frames is composed of the address
of the source of the frame and of the ASN. The standard assumes that the ASN
is distributed securely by other means. The ASN is not passed explicitly in
the data frames and does not constitute a complete anti-replay protection.
It results that upper layer protocols must provide a way to detect
duplicates and cope with them.
If the receiver and the sender have a different sense of ASN, the MIC will
not validate and the frame will be dropped. In that sense, TSCH induces an
event horizon whereby only nodes that have a common sense of ASN can talk to
one another in an authenticated manner. With 6TiSCH, the pledge discovers a
tentative ASN in beacons from nodes that have already joined the network.
But even if the beacon can be authenticated, the ASN cannot be trusted as it
could be a replay by an attacker and thus could announce an ASN that
represents a time in the past. If the pledge uses an ASN that is learned
from a replayed beacon for an encrypted transmission, a nonce-reuse attack
becomes possible and the network keys may be compromised.
Validating ASN
After obtaining the tentative ASN, a pledge that wishes to join the
6TiSCH network must use a join protocol to obtain its security keys.
The join protocol used in 6TiSCH is the Constrained Join Protocol (CoJP).
In the minimal setting defined in
, the authentication
requires a pre-shared key, based on which a secure session is derived.
The CoJP exchange may also be preceded with a zero-touch handshake
in order
to enable pledge joining based on certificates and/or inter-domain
communication.
As detailed in ,
a Join Proxy (JP) helps the pledge for the join procedure by relaying the
link-scope Join Request over the IP network to a Join Registrar/Coordinator
(JRC) that can authenticate the pledge and validate that it is attached to
the appropriate network. As a result of the CoJP exchange, the pledge is in
possession of a Link-Layer material including keys and a short address, and
if the ASN is known to be correct, all traffic can now be secured using CCM*
at the Link-Layer.
The authentication steps must be such that they cannot be replayed by an
attacker, and they must not depend on the tentative ASN being valid.
During the authentication, the keying material that the pledge obtains from
the JRC does not provide protection against spoofed ASN. Once the pledge has
obtained the keys to use in the network, it may still need to verify the ASN.
If the nonce used in the Layer-2 security derives from the extended (MAC-64)
address, then replaying the ASN alone cannot enable a nonce-reuse attack
unless the same node is lost its state with a previous ASN. But
if the nonce derives from the short address (e.g., assigned by the JRC) then
the JRC must ensure that it never assigns short addresses that were already
given to this or other nodes with the same keys. In other words, the network
must be rekeyed before the JRC runs out of short addresses.
Network Keying and Rekeying provides an overview of the CoJP process described in
by which an LLN
can be assembled in the field, having been provisioned in a lab.
is future
work that preceeds and then leverages the CoJP protocol using the
constrained profile
of (BRSKI).
This later work requires a yet-to-be standardized Lighweight Authenticated
Key Exchange protocol.
The CoJP protocol results in distribution of a network-wide key that
is to be used with security. The details of use are
described in sections
9.2 and 9.3.2.
The BRSKI mechanism may lead to the use of the CoJP protocol, in which case
it also results in distribution of a network-wide key. Alternatively
the BRSKI mechanism may be followed by use of
to enroll certificates for each device. In that case, the certificates
may be used with an key agreement protocol. The
description of this mechanism, while conceptually straight forward still
has significant standardization hurdles to pass.
section 9.2 describes
a mechanism to change (rekey) the network.
There are a number of reasons to initiate a network rekey: to remove
unwanted (corrupt/malicious) nodes, to recover unused 2-byte short
addresses, or due to limits in encryption algorithms.
For all of the mechanisms that distribute a network-wide key, rekeying
is also needed on a periodic basis. In more details:
The mechanism described in
section 9.2 requires
advance communication between the JRC and every one of the nodes before
the key change. Given that many nodes may be sleepy, this operation
may take a significant amount of time, and may consume a significant
portion of the available bandwidth. As such, network-wide rekeys in
order to exclude nodes that have become malicious will not be
particularly quick. If a rekey is already in progress, but the
unwanted node has not yet been updated, then it is possible to to just
continue the operation. If the unwanted node has already received the
update, then the rekey operation will need to be restarted.
The cryptographic mechanisms used by IEEE Std. 802.15.4 include the 2-byte
short address in the calculation of the context.
A nonce-reuse attack may become feasible if a short address is reassigned
to another node while the same network-wide keys are in operation.
A network that gains and loses nodes on a regular
basis is likely to reach the 65536 limit of the 2-byte (16-bit) short
addresses, even if the network has only a few thousand nodes. Network
planners should consider the need to rekey the network on a periodic
basis in order to recover 2-byte addresses. The rekey can update the
short addresses for active nodes if desired, but there is actually no
need to do this as long as the key has been changed.
With TSCH as it stands at the time of this writing, the ASN will wrap
after 2^40 timeslot durations, which means with the default values around
350 years. Wrapping ASN is not expected to happen within the lifetime of
most LLNs. Yet, should the ASN wrap, the network must be rekeyed to avoid
a nonce-reuse attack.
Many cipher algorithms have some suggested limits on how many bytes
should be encrypted with that algorithm before a new key is used.
These numbers are typically in the many to hundreds of gigabytes of
data. On very fast backbone networks this becomes an important
concern. On LLNs with typical data rates in the kilobits/second,
this concern is significantly less. With IEEE Std. 802.15.4 as it stands
at the time of this writing, the ASN will wrap before the limits of the
current L2 crypto (AES-CCM-128) are reached, so the problem should never
occur.
In any fashion, if the LLN is expected to operate continuously for decades
then the operators are advised to plan for the need to rekey.
Except for urgent rekeys caused by malicious nodes, the rekey operation
described in
can be done as a background task and can be done incrementally. It
is a make-before-break mechanism. The switch over to the new key is
not signaled by time, but rather by observation that the new key is in
use. As such, the update can take as long as needed, or occur in as
short a time as practical.
AcknowledgmentsContributorsThe co-authors of this document are listed below:
Thomas Watteyne
for his contribution to the whole design, in particular on TSCH and security,
and to the open source community with openWSN that he created.
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;
Kris Pister
for creating TSCH and his continuing guidance through the elaboration
of this design;
Malisa Vucinic
for the work on the one-touch join process and his contribution to the
Security Design Team;
Michael Richardson
for his leadership role in the Security Design Team and his
contribution throughout this document;
Tero Kivinen
for his contribution to the security work in general and the security
section in particular.
Maria Rita Palattella
for managing the Terminology document merged into this through the work of 6TiSCH;
Simon Duquennoy
for his contribution to the open source community with the 6TiSCH
implementaton of contiki, and for his contribution to MSF and
autonomous unicast cells.
Qin Wang
who lead the design of the 6top sublayer and contributed related text
that was moved and/or adapted in this document;
Rene Struik
for the security section and his contribution to the Security Design
Team;
Robert Assimiti
for his breakthrough work on RPL over TSCH and initial text and
guidance;
Special Thanks
Special thanks to Jonathan Simon, Giuseppe Piro, Subir Das
and Yoshihiro Ohba for their deep contribution to the initial security
work, to Yasuyuki Tanaka for his work on implementation and simulation
that tremendously helped build a robust system, to Diego Dujovne for
starting and leading the SF0 effort and to Tengfei Chang for evolving it
in the MSF.
Special thanks also to Pat Kinney, Charlie Perkins and Bob Heile for their
support in maintaining the connection active and the design in line with
work happening at IEEE 802.15.
Special thanks to Ted Lemon who was the INT Area A-D while this
document was initiated for his great support and help throughout,
and to Suresh Krishnan who took over with that kind efficiency of his till
publication.
Also special thanks to Ralph Droms who performed the first INT Area
Directorate review, that was very deep and thorough and radically changed
the orientations of this document, and then to Eliot Lear and Carlos
Pignataro who help finalize this document in preparation to the IESG
reviews, and to Gorry Fairhurst, David Mandelberg, Qin Wu, Francis Dupont,
Eric Vyncke, Mirja Kuhlewind, Roman Danyliw, Benjamin Kaduk
and Andrew Malis, who contributed to the final shaping of this document
through the IESG review procedure.
And Do not ForgetThis document is the result of multiple interactions, in
particular during the 6TiSCH (bi)Weekly Interim call, relayed through
the 6TiSCH mailing list at the IETF, over the course of more than 5 years.
The authors wish to thank in arbitrary order:
Alaeddine Weslati, Chonggang Wang, Georgios Exarchakos, Zhuo Chen,
Georgios Papadopoulos, Eric Levy-Abegnoli,
Alfredo Grieco, Bert Greevenbosch, Cedric Adjih, Deji Chen, Martin Turon,
Dominique Barthel, Elvis Vogli, Geraldine Texier,
Guillaume Gaillard, Herman Storey, Kazushi Muraoka, Ken Bannister,
Kuor Hsin Chang, Laurent Toutain, Maik Seewald,
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,
Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles and
Samita Chakrabarti for their participation and various contributions.
Normative ReferencesInformative ReferencesIEEE Std. 802.15.4, Part. 15.4: Wireless Medium Access
Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks
IEEE standard for Information Technology
Formal Specification of the CCM* Mode of Operation
IEEE standard for Information TechnologyIEEE 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
IEEE standard for Information TechnologyIndustrial Communication Networks - Wireless Communication Network and Communication Profiles - WirelessHART - IEC 62591www.hartcomm.orgHighway Addressable remote Transducer, a group of specifications for industrial process and control devices administered by the HART Foundationwww.hartcomm.orgWireless Systems for Industrial Automation: Process Control and Related Applications - ISA100.11a-2011 - IEC 62734ISA/ANSIISA100, Wireless Systems for AutomationISA/ANSITraffic Engineering Architecture and SignalingIETFAutonomic Networking Integrated Model and ApproachIETFPath Computation ElementIETFCommon Control and Measurement PlaneIETFAdvanced Metering Infrastructure and Customer Systems US Department of EnergyALOHA Packet System With and Without Slots and CaptureACM SIGCOMM Computer Communication ReviewIndustrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR) - IEC62439-3IECRelated Work In ProgressThis document has been incremented as the work progressed following the
evolution of the WG charter and the availability of dependent work.
The intent was to publish when the WG concludes on the covered items.
At the time of publishing the following specification are still in progress
and may affect the evolution of the stack in a 6TiSCH-aware node.
Unchartered IETF work items6TiSCH Zerotouch security
The security model and in particular the zerotouch join process
depends on
the ANIMA Bootstrapping Remote
Secure Key Infrastructures (BRSKI)
to enable zero-touch security provisionning; for highly
constrained nodes, a minimal model based on pre-shared keys (PSK)
is also available. As written to this day, it also depends on
a number of documents in progress as CORE, and on
"Ephemeral Diffie-Hellman Over
COSE (EDHOC)", which is being considered for adoption at the LAKE
WG.
6TiSCH Track Setup
ROLL is now standardizing a reactive routing protocol based on RPL
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.
At the time of this writing, there is new work planned in the IETF to provide
limited deterministic networking capabilities for wireless networks with a
focus on forwarding behaviors to react quickly and locally to the changes
as described in .
ROLL is also standardizing an extension to RPL to setup centrally-computed
routes
The 6TiSCH Architecture should thus inherit from the
DetNet architecture and
thus depends on it. The Path Computation Element (PCE) should be a
core component of that architecture.
An extension to RPL or to TEAS will be required to
expose the 6TiSCH node capabilities and the network peers to the PCE,
possibly in combination with .
A protocol such as a lightweight PCEP or an adaptation of CCAMP
G-MPLS formats and procedures could be used in
combination to to install
the Tracks, as computed by the PCE, to the 6TiSCH nodes.
Using BIER in a 6TiSCH Network ROLL is actively working on Bit Index
Explicit Replication (BIER) as a method to compress both the
dataplane packets and the routing tables in storing mode
.
BIER could also be used in the context of the DetNet service layer.
BIER-TE-based OAM, Replication and Elimination leverages BIER
Traffic Engineering (TE) to control in the data plane the
DetNet Replication and Elimination activities, and to provide traceability
on links where replication and loss happen, in a manner that is abstract to
the forwarding information.
a 6loRH for BitStrings
proposes a 6LoWPAN compression for the BIER Bitstring based on
6LoWPAN Routing Header.
External (non-IETF) work items
The current charter positions 6TiSCH on IEEE Std. 802.15.4 only.
Though most of the design should be portable on other link types,
6TiSCH has a strong dependency on IEEE Std. 802.15.4 and its evolution.
The impact of changes to TSCH on this Architecture should be minimal to
non-existent, but deeper work such as 6top and security may be impacted.
A 6TiSCH Interest Group at the IEEE maintains the synchronization
and helps foster work at the IEEE should 6TiSCH demand it.
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.
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 , 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.