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.
Wireless Networks enable a wide variety of devices of any size
to get interconnected, often at a very low marginal cost per device,
at any distance ranging from Near Field to interplanetary, and in
circumstances where wiring may be impractical, for instance
on fast-moving or rotating devices.
In the other hand, Deterministic Networks enable traffic that
is highly sensitive to jitter, quite sensitive to latency,
and with a high degree of operational criticality so that
loss should be minimized at all times.
Applications that need such networks are presented in . They include Professional Media and
Operation Technology (OT) Industrial Automation Control Systems (IACS).
The Medium access Control (MAC) of IEEE Std 802.15.4
has evolved with the
IEEE Std 802.15.4e Timeslotted Channel Hopping (TSCH) mode
to provide deterministic properties on wireless networks.
TSCH was initially
introduced with the IEEE Std 802.15.4e amendment
of the IEEE Std 802.15.4 standard and constituted a part of the
standard from that day. For all practical purpose, this document
is expected to be insensitive to the revisions of
the IEEE Std 802.15.4 standard, which is thus referenced undated.
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.
In order to enable the convergence of IT and OT in LLN environments,
6TiSCH ports the IETF suite of protocol that are defined for such
environments over the TSCH MAC. 6TiSCH also provides large scaling
capabilities, which, in a number of scenarios, require the addition of
a high speed and reliable backbone and the use of IP version 6 (IPv6).
The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet model
that is composed of a federating backbone and a number of IEEE Std 802.15.4
TSCH low-power wireless networks attached and synchronized by Backbone
Routers.
The architecture defines mechanisms
to establish and maintain routing and scheduling in a centralized,
distributed, or mixed fashion, for use in multiple OT environments.
It is applicable in particular to industrial control systems, building
automation that leverage distributed routing to address multipath over
a large number of hops, in-vehicle command and control that can be as
demanding as industrial applications, commercial automation and asset
Tracking with mobile scenarios, home automation and domotics which
become more reliable and thus provide a better user experience, and
resource management (energy, water, etc.).
The draft uses domain-specific terminology defined or referenced in
,
, and
.
Readers are expected to be familiar with all the terms and concepts
that are discussed in "Neighbor Discovery for
IP version 6", "IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions,
Problem Statement, and Goals", and
Neighbor Discovery Optimization
for Low-power and Lossy Networks where the 6LoWPAN Router
(6LR) and the 6LoWPAN Border Router (6LBR) are introduced.
Readers may benefit from reading the "RPL:
IPv6 Routing Protocol for Low-Power and Lossy Networks" specification;
"Multi-Link Subnet Issues";
"Mobility Support in IPv6" ;
"Neighbor Discovery Proxies (ND Proxy)" ;
"IPv6 Stateless Address Autoconfiguration";
"FCFS SAVI: First-Come, First-Served Source
Address Validation Improvement for Locally Assigned IPv6 Addresses"; and
"Optimistic Duplicate Address Detection"
prior to this specification for a clear understanding of the art in ND-proxying and binding.
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 signaling and
for authentication.
The 6TiSCH architecture presents a reference stack that is implemented
and interop tested by a conjunction of opensource, IETF and ETSI efforts.
One goal is to help other bodies to adopt the stack as a whole, making the
effort to move to an IPv6-based IOT stack easier. Now, for a particular,
environment, some of the choices that are made in this architecture may not
be relevant. For instance, RPL is not required for star topologies and
mesh-under Layer-2 routed networks, and the 6LoWPAN compression may not be
sufficient for ultra-constrained cases such as some Low Power Wide Area
(LPWA) networks. In such cases, it is perfectly doable to adopt a subset
of the selection that is presented hereafter and then select alternate
components to complete the solution wherever needed.
The IETF proposes multiple techniques for implementing functions related
to routing, transport or security. In order to control the complexity of
the possible deployments and device interactions, and to limit the size of
the resulting object code, the architecture limits the possible variations
of the stack and recommends a number of base elements for LLN applications.
In particular, UDP 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 below:
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.
6TiSCH has adopted the general direction of
CoAP Management Interface (COMI) for the management of devices.
This is leveraged for instance for the implementation of the generic
data model for the 6top sublayer management interface
.
The proposed implementation is based on CoAP and CBOR,
and specified in
6TiSCH Resource Management and Interaction using CoAP.
The Datagram Transport Layer Security (DTLS)
sitting either under CoAP or over CoAP so as to traverse proxies,
as well as
Ephemeral Diffie-Hellman Over COSE (EDHOC),
are examples of protocol that could be used
to protect CoAP datagrams, but the exact stack is not determined at the
time of this writing.
An overview of the the join process can be found in ;
the security aspects of the join process are further detailed in
.
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.
Though at a different time scale (several orders of magnitude),
both IEEE Std 802.1TSN and IEEE Std 802.15.4TSCH
standards provide Deterministic capabilities to the point that a packet
that pertains to a certain flow may traverse a network from node to node following
a very precise schedule, as a train that enters and then leaves intermediate stations
at precise times along its path. With TSCH, time is formatted into
timeslots, and individual communication cells are allocated to unicast or
broadcast communication at the MAC level. The time-slotted operation
reduces collisions, saves energy, and enables to more closely engineer
the network for deterministic properties.
The channel hopping aspect is a simple and efficient technique to combat
multipath fading and external interference (for example by Wi-Fi emitters).
6TiSCH builds on the IEEE Std 802.15.4TSCH MAC and inherits its advanced
capabilities to enable them in multiple environments where they can
be leveraged to improve automated operations.
The 6TiSCH Architecture also inherits the capability to perform a
centralized route computation to achieve deterministic properties,
though it relies on the IETF
DetNet Architecture,
and IETF components such as the Path Computation Element (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.
RPL also enables mobile use cases such as mobile workers and cranes, as
presented in
.
A scheduling operation attributes cells in a Time-Division-Multiplexing
(TDM) / Frequency-Division Multiplexing (FDM) matrix called the Channel
distribution/usage (CDU) to either individual transmissions
or as multi-access shared resources (see the
6TiSCH Terminology
for more on these terms). Scheduling effectively enables
multiple communications at a same time in a same interference domain
using different channels; but a node equipped with a single radio can
only transmit or receive on one channel at any given point of time.
From the standpoint of a 6TiSCH node (at the MAC layer), its
schedule is the collection of the times at which it must wake up for
transmission, and the channels to which it should either send or listen
at those times. The schedule is expressed as one or more slotframes that
repeat over and over. Slotframes may collide and require a device to
wake at a same time, in which case a priority indicates which slotframe
is actually activated.
The 6top sublayer hides the complexity of the schedule to the upper
layers. The Link that IP may utilize between the 6TiSCH node and a peer
may in fact be composed of a pair of cell bundles, one to receive and
one to transmit. Some of the cells may be shared, in which case the 6top
sublayer must perform some arbitration.
The 6TiSCH architecture identifies four ways a schedule can be managed
and CDU cells can be allocated: Static Scheduling, Neighbor-to-Neighbor
Scheduling, Remote Monitoring and Schedule Management, and Hop-by-hop
Scheduling.
This refers to the minimal
6TiSCH operation whereby a static schedule is configured for the whole
network for use in a slotted-aloha fashion. The static schedule is
distributed through the native methods in the TSCH MAC layer.
This operation leverages RPL to maintain a loopless graph for routing
and time distribution. It is specified in the
Minimal 6TiSCH Configuration
specification.
and does not preclude other scheduling operations to co-exist on a same
6TiSCH network.This refers to the
dynamic adaptation of the bandwidth of the Links that are used for IPv6
traffic between adjacent routers. Scheduling Functions such as the
"6TiSCH Minimal Scheduling Function
(MSF)" influence the operation of the
"6TiSCH Operation Sublayer
(6top)" to add and remove cells in peers schedule, using the
"6top Protocol (6P)"
for the negotiation on the MAC resources.This
refers to the central computation of a schedule and the capability
to forward a frame based on the cell of arrival. In that case,
the related portion of the device schedule as well as other device
resources are managed by an abstract Network Management Entity (NME),
which may cooperate with the PCE in order to minimize the interaction
with and the load on the constrained device.
This model is the TSCH adaption of the
"DetNet Architecture",
and it enables Traffic Engineering with deterministic properties.
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 .
6TiSCH leverages the RPL routing protocol for interoperable distributed
routing operations. RPL is applicable to Static Scheduling and
Neighbor-to-Neighbor Scheduling. The architecture also supports a
centralized routing model for Remote Monitoring and Schedule Management.
It is expected that a routing protocol that is more optimized for
point-to-point routing than RPL, such as
the
"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.
The 6TiSCH architecture supports three different forwarding models, the
classical IPv6 Forwarding, where the node selects a feasible successor
at Layer-3 on a per packet basis and based on its routing table,
G-MPLS Track Forwarding, which switches a frame received at a particular
Timeslot into another Timeslot at Layer-2, and
6LoWPAN Fragment Forwarding, which allows to forward individual 6loWPAN
fragments along the route set by the first fragment.
This is the classical IP forwarding
model, with a Routing Information Based (RIB) that is installed by the
RPL routing protocol and used to select a feasible successor per packet.
The packet is placed on an outgoing Link, that the 6top layer maps into
a (Layer-3) bundle of cells, and scheduled for transmission based on QoS
parameters. On top of RPL, this model also applies to any routing
protocol which may be operated in the 6TiSCH network, and corresponds
to all the distributed scheduling models, Static, Neighbor-to-Neighbor
and Hop-by-Hop Scheduling.This model corresponds to the
Remote Monitoring and Schedule Management. In this model, A central
controller (hosting a PCE) computes and installs the schedules in the
devices per flow. The incoming (Layer-2) bundle of cells from the
previous node along the path determines the outgoing (Layer-2) bundle
towards the next hop for that flow as determined by the PCE. The
programmed sequence for bundles is called a Track and can assume shapes
that are more complex than a simple direct sequence of nodes.This is an hybrid model
that derives from IPv6 forwarding for the case where packets must
be fragmented at the 6LoWPAN sublayer. The first fragment is forwarded
like any IPv6 packet and leaves a state in the intermediate hops to
enable forwarding of the next fragments that do not have a IP header
without the need to recompose the packet at every hop. This can be broadly summarized in the following table:
A 6TiSCH network is an IPv6 subnet which, in
its basic configuration, is a single Low Power Lossy Network (LLN)
operating over a synchronized TSCH-based mesh.
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 are not necessarily reachable from one another at Layer-2
and an LLN may span over multiple links. This effectively forms an
homogeneous non-broadcast multi-access (NBMA) subnet, which is beyond
the scope of existing IPv6 ND methods. Extensions to IPv6 ND have to be
introduced.
Within that subnet, neighbor devices are discovered with
6LoWPAN Neighbor Discovery (6LoWPAN ND),
whereas RPL enables routing
in the so called Route Over fashion, either in storing (stateful) or
non-storing (stateless, with routing headers) mode.
6TiSCH nodes join the mesh by attaching to nodes that are already
members of the mesh. Some nodes act as routers for 6LoWPAN ND and RPL
operations, as detailed in .
Security aspects of the join process by which a device
obtains access to the network are discussed 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 DODAGs with no timing loop.
RPL forms Destination Oriented
Directed Acyclic Graphs (DODAGs) within Instances of the protocol,
each Instance being associated with an Objective Function (OF) to
form a routing topology. A particular 6TiSCH node, the LLN Border Router
(LBR), acts as RPL root, 6LoWPAN HC terminator, and Border Router
for the LLN to the outside. The LBR is usually powered.
More on RPL Instances can be found in section 3.1 of
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 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.
A Backbone Router may be connected to the node that acts as RPL root
and / or 6LoWPAN 6LBR and provides connectivity to the larger campus /
factory plant network over a high speed backbone or a back-haul link.
A Backbone Router may perform proxy
IPv6 Neighbor Discovery (ND) operations
over the backbone on behalf of the 6TiSCH nodes
so they can share a same IPv6 subnet and appear to be
connected to the same backbone as classical devices. A Backbone
Router may alternatively redistribute the registration in a routing
protocol such as OSPF or
BGP, or inject them in a mobility
protocol such as MIPv6,
NEMO, or
LISP.
This architecture expects that a 6LoWPAN node can connect as a
leaf to a RPL network, where the leaf support is the minimal
functionality to connect as a host to a RPL network without the need to
participate to the full routing protocol.
The architecture also expects that a 6LoWPAN node that is not aware
at all of the RPL protocol may also connect as a host but the
specifications for this to happen are not available at the time of this
writing.
An extended configuration of the subnet comprises multiple LLNs.
The LLNs are interconnected and synchronized over a backbone, that
can be wired or wireless. The backbone can be a classical IPv6
network, with Neighbor Discovery operating as defined in
and .
This architecture requires work to standardize the
the registration of 6LoWPAN nodes to the Backbone Routers.
In the extended configuration, a Backbone Router (6BBR) operates
as described in
.
The 6BBR performs ND proxy operations between the registered devices
and the classical ND devices that are located over the backbone.
6TiSCH 6BBRs synchronize with one another over the backbone, so as
to ensure that the multiple LLNs that form the IPv6 subnet stay
tightly synchronized.
As detailed in the 6LoWPAN ND 6LBR and
the root of the RPL network need to be collocated and share information
about the devices that is learned through either protocol but not both.
The combined RPL root and 6LBR may be collocated with the 6BBR, or
directly attached to the 6BBR. In the latter case, it leverages
the extended registration process defined in
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.
If the Backbone is Deterministic (such as
defined by the Time Sensitive Networking WG at IEEE), then the
Backbone Router ensures that the end-to-end deterministic
behavior is maintained between the LLN and the backbone. The
DetNet Architecture
studies Layer-3 aspects of Deterministic Networks, and covers networks
that span multiple Layer-2 domains.
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 periodic signaling that
starts at the leaf node with 6LoWPAN ND, is then carried
over RPL to the RPL root, and then to the 6BBR.
Efficient ND being an adaptation of 6LoWPAN ND, it makes sense to keep
those two homogeneous in the way they use the source and the target
addresses in the Neighbor Solicitation (NS) messages for registration,
as well as in the options that they use for that process.
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.
In order to control the complexity and the size of the 6TiSCH work,
the architecture and the associated IETF work are staged and the WG is
expected to recharter multiple times.
This document is incremented as the work progresses following the
evolution of the WG charter and the availability of dependent work.
The intent is to publish when the WG concludes.
At the time of this writing:
The architecture of the operation of RPL over a dynamic schedule is
being studied at 6TISCH as the second iteration of the charter.
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.
The components and protocols
that are required to implement this stage of architecture are being
standardized at the IETF.
An Update to 6LoWPAN ND covers the evolution of 6LoWPAN Neighbor
Discovery that is needed to implement the Backbone Router
. In addition
the protection of registered addresses against impersonation and take over
can be guaranteed by Address
Protected Neighbor Discovery for Low-power and Lossy Networks.
The work on centralized Track computation is deferred to a subsequent
iteration of the 6TiSCH charter. The idea at the time of this writing is
that 6TiSCH will apply the concepts of Deterministic Networking
on a Layer-3 network. The 6TiSCH Architecture should thus inherit from the
DetNet architecture and
thus depends on it. The Path Computation Element (PCE) should be a
core component of that architecture. Around the PCE, a protocol
such as an extension to a TEAS protocol
will be required to expose the 6TiSCH node capabilities and the network
peers to the PCE, and a protocol such as a lightweight PCEP or an
adaptation of CCAMP G-MPLS formats and procedures
will be used to publish the Tracks, as computed by the PCE, to the 6TiSCH
nodes.
BIER-TE-based OAM, Replication and Elimination leverages Bit Index
Explicit Replication - Traffic Engineering 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, whereas
a 6loRH for BitStrings
proposes a 6LoWPAN compression for the BIER Bitstring based on
6LoWPAN Routing Header.
The security model and in particular the join process depends on the ANIMA
Bootstrapping Remote
Secure Key Infrastructures (BRSKI)
in order to enable zero-touch security provisionning; for highly
constrained nodes, a minimal model based on pre-shared keys (PSK)
is also available.
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.
At the time of this writing, a revision of the IEEE Std 802.15.4
standard is expected early 2016. That revision should
integrate TSCH as well as other amendments and fixes into the main
specification. The impact on this Architecture should be minimal to
non-existent, but deeper work such as 6top and security may be impacted.
A 6TiSCH Interest Group was formed at IEEE to maintain the synchronization
and help foster work at the IEEE should 6TiSCH demand it.
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.
RPL needs a set of information in order to advertise
a leaf node through a DAO message and establish reachability.
At the bare minimum the leaf device must provide a sequence
number that matches the RPL specification in section 7.
Section 5.3 of
,
on the Extended Address Registration Option (EARO),
already incorporates that addition with a new
field in the option called the Transaction ID.
If for some reason the node is aware of RPL topologies, then
providing the RPL InstanceID for the instances to which the
node wishes to participate would be a welcome addition.
In the absence of such information, the RPL router must
infer the proper instanceID from external rules and policies.
On the backbone, the InstanceID is expected to be mapped
onto a an overlay that matches the instanceID, for instance a VLANID.
This architecture leverages
that extends 6LoWPAN ND to carry the counter
as an abstract Transaction ID (TID).
6LoWPAN ND is unclear on how the 6LBR is discovered, and how the liveliness
of the 6LBR is asserted over time. On the other hand, the discovery
and liveliness of the RPL root are obtained through the RPL protocol.
This architecture suggests to collocate these functions by default, in which
case the discovery of the 6LBR is automatic for RPL leaves.
When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL root functionalities
are co-located in order that the address of the 6LBR be indicated by RPL
DIO messages and to associate the unique ID from the DAR/DAC exchange with
the state that is maintained by RPL. The DAR/DAC exchange becomes a
preamble to the DAO messages that are used from then on to reconfirm the
registration, thus eliminating a duplication of functionality between DAO
and DAR messages.
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 requires an alteration from
whereby the Target Address of the NS message
is registered as opposed to the Source, which, in the case of a proxy
registration, is that of the 6LBR / RPL root itself.
6top is a logical link control sitting between the IP layer and the
TSCH MAC layer, which provides the link abstraction that is required
for IP operations. The 6top operations are specified in
. In particular, 6top
provides a management interface that enables an external
management entity to schedule cells and slotFrames, and allows the
addition of complementary functionality, for instance to support a
dynamic schedule management based on observed resource usage as
discussed in .
The 6top data model and management interfaces are further discussed
in .
The architecture defines "soft" cells and "hard" cells. "Hard" cells
are owned and managed by an separate scheduling entity (e.g. a PCE)
that specifies the slotOffset/channelOffset of the cells to be
added/moved/deleted, in which case 6top can only act as instructed,
and may not move hard cells in the TSCH schedule on its own.
6top contains a monitoring process which monitors the performance of
cells, and can move a cell in the TSCH schedule when it performs
poorly.
This is only applicable to cells which are marked as "soft".
To reserve a soft cell, the higher layer does not indicate the exact
slotOffset/channelOffset of the cell to add, but rather the resulting
bandwidth and QoS requirements. When the monitoring process triggers
a cell reallocation, the two neighbor devices communicating over this
cell negotiate its new position in the TSCH schedule.
In the case of soft cells, the cell management entity that controls the
dynamic attribution of cells to adapt to the dynamics of variable rate flows
is called a Scheduling Function (SF). There may be multiple SFs with more
or less aggressive reaction to the dynamics of the network. The
"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.
The SF may be seen as divided between an upper bandwidth adaptation logic
that is not aware of the particular technology that is used to obtain and
release bandwidth, and an underlying service that maps those needs in the
actual technology, which means mapping the bandwidth onto cells in the case
of TSCH.
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. The 6P protocol enables the peers to find an agreement in a
transactional manner that ensures the final consistency of the nodes state.
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.
Most OFs require metrics about reachability, such as the ETX.
6top creates and maintains an abstract neighbor table,
and this state may be leveraged to feed an OF and/or store OF information
as well.
In particular, 6top creates and maintains an abstract neighbor table. A neighbor
table entry contains a set of statistics with
respect to that specific neighbor including the time when the last packet has
been received from that neighbor, a set of cell quality metrics (e.g. RSSI or LQI),
the number of packets sent to the neighbor or the number of packets received
from it. This information can be obtained through 6top management APIs as
detailed in the 6top sublayer
specification 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 behaviour, for instance by configuring the periodicity of
IEEE Std 802.15.4 Extended Beacons (EB's). By augmenting the EB periodicity, it is
possible to change the network dynamics so as to improve the support of
devices that may change their point of attachment in the 6TiSCH network.
Some RPL control messages, such as the DODAG Information Object (DIO) are
ICMPv6 messages that are broadcast to all neighbor nodes.
With 6TiSCH, the broadcast channel requirement is addressed by 6top
by configuring TSCH to provide a broadcast channel,
as opposed to, for instance, piggybacking the DIO messages in
Enhance Beacons. Consideration was given towards finding a way to
embed the Route Advertisements and the RPL DIO messages
(both of which are multicast) into the IEEE Std 802.15.4 Enhanced Beacons.
It was determined that this produced undue timer coupling among
layers, that the resulting packet size was potentially too large,
and required it is not yet clear that there is any need for Enhanced
Beacons in a production network.
Nodes in a TSCH network must be time synchronized.
A node keeps synchronized to its time source neighbor
through a combination of frame-based and acknowledgment-based synchronization.
In order to maximize battery life and network throughput, it is advisable that RPL ICMP discovery
and maintenance traffic (governed by the trickle timer) be somehow coordinated with the
transmission of time synchronization packets (especially with enhanced beacons).
This could be achieved through an interaction of the 6top sublayer and the RPL objective Function,
or could be controlled by a management entity.
Time distribution requires a loop-less structure. Nodes taken in a synchronization loop will rapidly
desynchronize from the network and become isolated. It is expected that a RPL DAG with
a dedicated global Instance is deployed for the purpose of time synchronization.
That Instance is referred to as the Time Synchronization Global Instance (TSGI).
The TSGI can be operated in either of the 3 modes that are detailed
in section 3.1.3 of 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".
A root is configured or obtains by some external means the knowledge of the RPLInstanceID
for the TSGI. The root advertises its DagRank in the TSGI, that must be less than 0xFF,
as its Join Priority (JP) in its IEEE Std 802.15.4 Extended Beacons (EB). We'll note that the
JP is now specified between 0 and 0x3F leaving 2 bits in the octet unused in the IEEE Std 802.15.4e
specification. After consultation with IEEE authors, it was asserted that 6TiSCH can make
a full use of the octet to carry an integer value up to 0xFF.
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.
6TiSCH enables in essence the capability to use IPv6 over a MAC
layer that enables to schedule some of the transmissions. In order
to ensure that the medium is free of contending packets when time
arrives for a scheduled transmission, a window of time is defined
around the scheduled transmission time where the medium must be free of
contending energy.
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.
In order to describe that formatting of time and frequencies, the
6TiSCH architecture defines a global concept that is called a Channel
Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of
cells with an height equal to the number of available channels
(indexed by ChannelOffsets) and a width (in timeslots) that is the
period of the network scheduling operation (indexed by slotOffsets) for
that CDU matrix. The size of a cell is a timeslot duration, and
values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to
accommodate for the transmission of a frame and an ack, including the
security validation on the receive side which may take up to a few
milliseconds on some device architecture.
A CDU matrix iterates over and over with a pseudo-random rotation from
an epoch time.
In a given network, there might be multiple CDU matrices that operate
with different width, so they have different durations and represent
different periodic operations.
It is recommended that all CDU matrices in a 6TiSCH domain operate with
the same cell duration and are aligned, so as to reduce the
chances of interferences from slotted-aloha operations.
The knowledge of the CDU matrices is shared
between all the nodes and used in particular to define slotFrames.
A slotFrame is a MAC-level abstraction that is common to all nodes and
contains a series of timeslots of equal length and precedence.
It is characterized by a slotFrame_ID, and a slotFrame_size.
A slotFrame aligns to a CDU matrix for its parameters, such as number
and duration of timeslots.
Multiple slotFrames can coexist in a node schedule, i.e., a node can
have multiple activities scheduled in different slotFrames, based on
the precedence of the 6TiSCH topologies. The slotFrames may be
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.
6TiSCH expects a high degree of scalability together with a distributed
routing functionality based on RPL. To achieve
this goal, the spectrum must be allocated in a way that allows for
spatial reuse between zones that will not interfere with one another.
In a large and spatially distributed network, a 6TiSCH node is often in a
good position to determine usage of spectrum in its vicinity.
Use cases for distributed routing are often associated with a
statistical distribution of best-effort traffic with variable needs
for bandwidth on each individual link. With 6TiSCH, the abstraction
of an IPv6 link is implemented as a pair of bundles of cells, one in
each direction; the size of a bundle is
optimal when both the energy wasted idle listening and the packet
drops due to congestion loss are minimized. This can be maintained if
the number of cells in a bundle is adapted dynamically, and with enough
reactivity, to match the variations of best-effort traffic. In turn,
the agility to fulfill the needs for additional cells improves when the
number of interactions with other devices and the protocol latencies
are minimized.
6TiSCH limits that interaction to RPL parents that will only
negotiate with other RPL parents, and performs that negotiation by
groups of cells as opposed to individual cells. The 6TiSCH architecture
allows RPL parents to adjust dynamically, and independently from
the PCE, the amount of bandwidth that is used to communicate between
themselves and their children, in both directions; to that effect,
an allocation mechanism enables a RPL parent to obtain the exclusive
use of a portion of a CDU matrix within its interference domain.
Note that a PCE is expected to have precedence in the allocation,
so that a RPL parent would only be able to obtain portions that are
not in-use by the PCE.
The 6TiSCH architecture introduces the concept of chunks
) to operate
such spectrum distribution for a whole group of cells at a time.
The CDU matrix is formatted into a set of chunks, each of them
identified uniquely by a chunk-ID. The knowledge of this
formatting is shared between all the nodes in a 6TiSCH network. 6TiSCH
also defines the process of chunk ownership appropriation whereby a
RPL parent discovers a chunk that is not used in its interference
domain (e.g lack of energy detected in reference cells in that chunk);
then claims the chunk, and then defends it in case another RPL parent
would attempt to appropriate it while it is in use.
The chunk is the basic unit of ownership that is used in that process.
As a result of the process of chunk ownership appropriation, the RPL
parent has exclusive authority to decide which cell in the appropriated
chunk can be used by which node in its interference domain. In other words, it is
implicitly delegated the right to manage the portion of the CDU matrix
that is represented by the chunk. The RPL parent may thus orchestrate
which transmissions occur in any of the cells in the chunk, by
allocating cells from the chunk to any form of communication (unicast,
multicast) in any direction between itself and its children.
Initially, those cells are added to the heap of free cells, then
dynamically placed into existing bundles, in new bundles, or allocated
opportunistically for one transmission.
The appropriation of a chunk can also be requested explicitly by the
PCE to any node. In that case, the node still may need to perform the
appropriation process to validate that no other node has claimed that
chunk already. After a successful appropriation, the PCE owns the cells
in that chunk, and may use them as hard cells to set up Tracks.
defines the terms
of Communication Paradigms and Interaction Models, which can be placed
in parallel to the Information Models and Data Models that are defined in
.
A Communication Paradigms would be an abstract view of a protocol exchange,
and would come with an Information Model for the information that is being exchanged.
In contrast, an Interaction Models would be more refined and could point on standard operation
such as a Representational state transfer (REST) "GET" operation and would match
a Data Model for the data that is provided over the protocol exchange.
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 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 specification focuses on Communication Paradigms and Interaction
Models for packet forwarding and TSCH resources (cells) management.
Management mechanisms for the TSCH schedule at Link-layer (one-hop),
Network-layer (multithop along a Track), and Application-layer
(remote control) are discussed in .
Link-layer frame forwarding interactions are discussed in , and
Network-layer Packet routing is addressed in .
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.
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
.
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.
The 6top sublayer
defines a protocol for neighbor nodes to reserve soft cells to
transmit to one another. Because this reservation is done without
global knowledge of the schedule of nodes in the LLN, scheduling
collisions are possible. 6top defines a monitoring process which
continuously Tracks the packet delivery ratio of soft cells.
It uses these statistics to trigger the reallocation of a soft cell
in the schedule, using a negotiation protocol between the neighbors
nodes communicating over that cell.
In the most efficient instantiations of a 6TiSCH network, the size of
the bundles that implement the links may be changed dynamically
in order to adapt to the need of end-to-end flows routed by RPL.
An optional Scheduling Function (SF) such as
MSF 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.
Monitoring and relocation is done in the 6top layer. For the upper layer,
the connection between two neighbor nodes appears as an number of cells.
Depending on traffic requirements, the upper layer can request 6top to add
or delete a number of cells scheduled to a particular neighbor, without
being responsible for choosing the exact slotOffset/channelOffset of those cells.
The 6top interface document
specifies the generic data model that can be used to monitor and manage
resources of the 6top sublayer. Abstract methods are suggested for use
by a management entity in the device. The data model also enables
remote control operations on the 6top sublayer.
The capability to interact with the node 6top sublayer from multiple hops away
can be leveraged for monitoring, scheduling, or a combination of thereof.
The architecture supports variations on the deployment model, and
focuses on the flows rather than
whether there is a proxy or a translation operation en-route.
defines an mapping of
the 6top set of commands, which is described in
, to CoAP resources.
This allows an entity to interact with the 6top layer of a node that
is multiple hops away in a RESTful fashion.
The entity issuing the CoAP requests can be a central scheduling entity
(e.g. a PCE), a node multiple hops away with the authority to modify the TSCH
schedule (e.g. the head of a local cluster), or a external device monitoring the
overall state of the network (e.g. NME). It is also possible that a
mapping entity on the backbone transforms a non-CoAP protocol such
as PCEP into the RESTful interfaces that the 6TiSCH devices support.
With respect to Centralized routing and scheduling, the 6TiSCH
Architecture is (expected to be) be an extension of the detnet work
Deterministic Networking
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 SDN Architecture and is composed
of three planes, a (User) Application Plane, a Controller Plane (where
the PCE operates), and a Network Plane which in our case is the 6TiSCH
LLN. The generic SDN architecture is discussed in
Software-Defined Networking (SDN):
Layers and Architecture Terminology and is represented below:
The 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 (e.g. a 6TiSCH LLN and an Ethernet
Backbone). This model fits the 6TiSCH extended configuration, whereby a
6BBR federates
multiple 6TiSCH LLN in a single subnet over a backbone that can be,
for instance, Ethernet or Wi-Fi. In that model,
6TiSCH 6BBRs synchronize with one another over the backbone, so as
to ensure that the multiple LLNs that form the IPv6 subnet stay
tightly synchronized.
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.
A node can reserve a Track to a destination
node multiple hops away by installing soft cells at each intermediate node.
This forms a Track of soft cells. It is the responsibility of the 6top
sublayer of each node on the Track to monitor these soft cells and trigger
relocation when needed.
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.
The architecture introduces the concept of a Track, which is a directed path
from a source 6TiSCH node to a destination 6TiSCH node 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.
A Track is composed of 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 belongs
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.
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 adding a pseudo-random number to the channelOffset of
the cell, which results in a rotation of the frequency that used for
transmission.
The bundles may be computed so as to accommodate both variable rates and
retransmissions, so they might not be fully used at a given iteration of the
schedule.
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 a destination to
support multi-path forwarding and route around failures.
A Complex Track may also branch off and rejoin, for the purpose of the
DetNet Packet Replication and Elimination (PRE), over non congruent branches.
PRE may be used to complement Layer-2 ARQ to meet industrial expectations in
Packet Delivery Ratio (PDR), in particular when the Track extends beyond the
6TiSCH network in a larger DetNet network.
The art of Deterministic Networks already include PRE techniques. Example
standards include the Parallel Redundancy Protocol (PRP) and the
High-availability Seamless Redundancy (HSR) .
At each 6TiSCH hop along the Track, the PCE may schedule more than one
timeslot for a packet, so as to support Layer-2 retries (ARQ). It is also
possible that the field device only uses the second branch if sending over
the first branch fails.
In 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.
Ultimately, DetNet should enable to extend a Track beyond the 6TiSCH LLN.
illustrates 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.
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.
The 6TiSCH architecture provides means to avoid waste of cells as
well as overflows in the transmit bundle pof a Track, as follows:
In one hand, a TX-cell that is not needed for the current iteration may
be reused opportunistically on a per-hop basis for routed packets.
When all of the frame that were received for a given Track are
effectively transmitted, any available TX-cell for that Track can be
reused for upper layer traffic for which the next-hop router matches the
next hop along the Track.
In that case, the cell that is being used is effectively a TX-cell from
the Track, but the short address for the destination is that of the
next-hop router.
It results that a frame that is received in a RX-cell of a Track with a
destination MAC address set to this node as opposed to broadcast must be
extracted from the Track and delivered to the upper layer (a frame with
an unrecognized 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, the frame can be placed for transmission in the bundle
that is used for Layer-3 traffic towards the next hop along the Track as
long as it can be routed by the upper layer, that is, typically, if the
frame transports an IPv6 packet.
The MAC address should be set to the next-hop MAC address to avoid
confusion.
It results that a frame that is received over a Layer-3 bundle may be in
fact associated to a Track. In a classical IP link such as an Ethernet,
off-Track traffic is typically in excess over reservation to be routed
along the non-reserved path based on its QoS setting.
But with 6TiSCH, since the use of the Layer-3 bundle may be due to
transmission failures, it makes sense for the receiver to recognize a
frame that should be re-Tracked, and to place it back on the appropriate
bundle if possible.
A frame should be re-Tracked if the Per-Hop-Behavior group indicated in
the Differentiated Services Field of the IPv6 header is set to
Deterministic Forwarding, as discussed in .
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.
By forwarding, this specification means the per-packet operation that
allows to deliver a packet to a next hop or an upper layer in this node.
Forwarding is based on pre-existing state that was installed as a
result of a routing computation .
6TiSCH supports three different forwarding model, G-MPLS Track Forwarding (TF),
6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).
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, transport mode and tunnel mode.
In transport mode, the Protocol Data Unit (PDU) is associated
with flow-dependant meta-data that refers uniquely to the Track,
so the 6top sublayer can place the frame in the appropriate cell
without ambiguity. In the case of IPv6 traffic, this flow
identification is transported in the Flow Label of the IPv6
header.
Associated with the source IPv6 address, the Flow Label forms a
globally unique identifier for that particular Track that is
validated at egress before restoring
the destination MAC address (DMAC) and punting to the upper layer.
In tunnel mode, the frames originate from an arbitrary protocol over a compatible MAC
that may or may not be synchronized with the 6TiSCH network. An example of
this would be a router with a dual radio that is capable of receiving and sending WirelessHART
or ISA100.11a frames with the second radio, by presenting itself as an access
Point or a Backbone Router, respectively.
In that mode, some entity (e.g. PCE) can coordinate with a
WirelessHART Network Manager or an ISA100.11a System Manager to
specify the flows that are to be transported transparently
over the Track.
In that case, the flow information that identifies the Track at
the ingress 6TiSCH router is derived from the RX-cell. The dmac
is set to this node but the flow information indicates that the
frame must be tunneled over a particular Track so the frame is
not passed to the upper layer. Instead, the dmac is forced to
broadcast and the frame is passed to the 6top sublayer for switching.
At the egress 6TiSCH router, the reverse operation occurs. Based
on metadata associated to the Track, the frame is passed to the
appropriate link layer with the destination MAC restored.
Metadata coming with the Track configuration is expected to provide the destination MAC address
of the egress endpoint as well as the tunnel mode and specific data depending on the mode,
for instance a service access point for frame delivery at egress.
If the tunnel egress point does not have a MAC address that matches the configuration,
the Track installation fails.
In transport mode, if the final Layer-3 destination is the tunnel termination, then it is possible
that the IPv6 address of the destination is compressed at the 6LoWPAN sublayer based on the MAC address.
It is thus mandatory at the ingress point to validate that the MAC address that was used at the 6LoWPAN
sublayer for compression matches that of the tunnel egress point. For that reason, the node that injects
a packet on a Track checks that the destination is effectively that of the tunnel egress point
before it overwrites it to broadcast.
The 6top sublayer at the tunnel egress point reverts that operation to the MAC address obtained
from the tunnel metadata.
Considering that 6LoWPAN packets can be as large as 1280 bytes (the IPv6 MTU),
and that the non-storing mode of RPL implies Source Routing that requires space for routing
headers, and that a 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 in order to reform the packet and route it. This creates additional latency and forces intermediate
nodes to store a portion of a packet for an undetermined time, thus impacting critical resources such
as memory and battery.
describes a mechanism whereby the datagram tag in the
6LoWPAN Fragment is used as a label for switching at the 6LoWPAN sublayer. The draft allows for a degree of
flow control based on an Explicit Congestion Notification, as well as end-to-end individual fragment recovery.
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.
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.
As the packets are routed at Layer-3, traditional QoS and Active
Queue Management (AQM) operations are expected to prioritize flows;
the application of Differentiated Services is further discussed in
.
6TiSCH supports a mixed model of centralized routes and distributed routes.
Centralized routes can for example be computed by a entity such as a PCE.
Distributed routes are computed by RPL.
Both methods may inject routes in the Routing Tables of the 6TiSCH routers.
In either case, each route is associated with a 6TiSCH topology that can
be a RPL Instance topology or a Track. The 6TiSCH topology is
indexed by a Instance ID, in a format that reuses the RPLInstanceID as
defined in RPL.
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.
All packets inside a 6TiSCH domain must carry the Instance ID that
identifies the 6TiSCH topology that is to be used for
routing and forwarding that packet. The location of that information
must be the same for all packets forwarded inside the domain.
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, as discussed in section 11.2
of , "Loop Avoidance and Detection".
The RPL Packet Information (RPI) is carried in IPv6 packets as 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.
6TiSCH expects elimination and replication of packets along a complex
Track, but has no position about how the sequence numbers would be tagged in
the packet.
As it goes, 6TiSCH expects that timeSlots corresponding to copies
of a same packet along a Track are correlated by configuration, and does not
need to process the sequence numbers.
The semantics of the configuration 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 further transmissions should not be
attempted for timeSlots in that group. There are as many 'OR' groups as
there are branches of the Track departing from this node. Different 'OR' groups
are programmed for the purpose of replication, each group corresponding to
one branch of the Track. The 'AND' relation between the groups indicates that
transmission over any of branches must be attempted regardless of whether a
transmission succeeded in another branch. It is also possible to place cells
to different next-hop routers in a same 'OR' group. This allows to route along
multi-path tracks, trying one next-hop and then another only if sending to the
first fails.
On the receive side, all timeSlots are programmed in a same 'OR' group.
Retries of a same copy as well as converging branches for elimination
are converged, meaning that the first successful reception is enough and that
all the other timeSlots can be ignored.
Additionally, an IP packet that is sent along a Track uses the
Differentiated Services Per-Hop-Behavior Group called
Deterministic Forwarding, as described in
.
This specification does not require IANA action.
This architecture operates on IEEE Std 802.15.4 and expects link-layer security to
be enabled at all times between connected devices, except for the very first
step of the device join process, where a joining device may need some initial,
unsecured exchanges so as to obtain its initial key material.
The
Minimal Security Framework for 6TiSCH describes the minimal
mechanisms required to support secure enrollment of a pledge to a 6TiSCH
network based on PSK. The specification enables to
establish of link-layer keys, typically used in combination with a
variation of Counter with CBC-MAC (CCM),
and set up a secure end-to-end session between
the pledge and the join registrar via a Join Proxy. It can also be used to
obtain a link layer short address.
The
"6tisch Zero-Touch Secure Join protocol" wraps the minimal security
draft with a flow inspired from ANIMA
"Bootstrapping Remote Secure Key Infrastructures (BRSKI)".
The BRSKI architecture specifies three logical elements to describe the join
process:
Node that wishes to become part of the network; :
An entity that arbitrates network access and hands
out network parameters (such as keying material);
a one-hop (radio) neighbor of the joining node
that acts as proxy network node and may provide connectivity
with the JRC.The join protocol consists of three major activities:
The Pledge and the JP mutually authenticate each other
and establish a shared key, so as to ensure on-going authenticated
communications. This may involve a server as a third party.
The JP decides on whether/how to authorize a Pledge
(if denied, this may result in loss of bandwidth).
Conversely, the Pledge decides on whether/how to authorize the network
(if denied, it will not join the network).
Authorization decisions may involve other nodes in the network.
The JP distributes configuration information to the Pledge, such as scheduling
information, IP address assignment information, and network policies.
This may originate from other network devices, for which the JP may act as
proxy. This step may also include distribution of information
from the Pledge to the JP and other nodes in the network and, more generally,
synchronization of information between these entities.The device joining process is depicted in ,
where it is assumed that devices have access to certificates and where
entities have access to the root CA keys of their communicating parties
(initial set-up requirement).
Under these assumptions, the authentication step of the device joining
process does not require online involvement of a third party.
Mutual authentication is performed between the Pledge and the JP using their
certificates, which also results in a shared key between these two entities.
The JP assists the Pledge in mutual authentication with a remote server node
(primarily via provision of a communication path with the server), which
also results in a shared (end-to-end) key between those two entities.
The server node may be a JRC that arbitrages the network authorization of the
Pledge (where the JP will deny bandwidth if authorization is not successful);
it may distribute network-specific configuration parameters
(including network-wide keys) to the Pledge.
In its turn, the Pledge may distribute and synchronize information (including,
e.g., network statistics) to the server node and, if so desired, also to the
JP. The actual decision of the Pledge to become part of the network may
depend on authorization of the network itself.The server functionality is a role which may be implemented with one
(centralized) or multiple devices (distributed).
In either case, mutual authentication is established
with each physical server entity with which a role is implemented.
Note that in the above description, the JP does not solely act as a relay
node, thereby allowing it to first filter traffic to be relayed based on
cryptographic authentication criteria - this provides first-level access
control and mitigates certain types of denial-of-service attacks
on the network at large. Depending on more detailed insight in cost/benefit trade-offs, this
process might be complemented by a more "relaxed" mechanism, where the
JP acts as a relay node only.
The final architecture will provide mechanisms to also cover cases where
the initial set-up requirements are not met or where some other
out-of-sync behavior occurs; it will also suggest some optimizations in
case JRC-related information is already available with the JP
(via caching of information). When a device rejoins the network in the same authorization domain,
the authorization step could be omitted if the server distributes the
authorization state for the device to the JP when the device
initially joined the network. However, this generally still requires
the exchange of updated configuration information, e.g., related to time
schedules and bandwidth allocation.The co-authors of this document are listed below:
for his breakthrough work on RPL over TSCH and initial text and
guidance.
for creating it all and his continuing guidance through the elaboration
of this design.
for his leadership role in the Security Design Team and his
contribution throughout this document.
for the security section and his contribution to the Security Design
Team.
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.
who lead the design of the 6top sublayer and contributed related text
that was moved and/or adapted in this document.
for his contribution to the whole design, in
particular on TSCH and security.
Special thanks to Tero Kivinen, Jonathan Simon, Giuseppe Piro, Subir Das
and Yoshihiro Ohba for their deep contribution to the initial security
work, 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 for his support in maintaining the
connection active and the design in line with work happening at
IEEE Std 802.15.4.
Special thanks to Ted Lemon who was the INT Area A-D while this
specification was developed for his great support and help throughout.
Also special thanks to Ralph Droms who performed the first INT Area
Directorate review, that was very deep and through and radically changed
the orientations of this document.
This specification is the result of multiple interactions, in
particular during the 6TiSCH (bi)Weekly Interim call, relayed through
the 6TiSCH mailing list at the IETF.
The authors wish to thank:
Alaeddine Weslati, Chonggang Wang, Georgios Exarchakos, Zhuo Chen,
Alfredo Grieco, Bert Greevenbosch, Cedric Adjih, Deji Chen, Martin Turon,
Dominique Barthel, Elvis Vogli, Geraldine Texier, Malisa Vucinic,
Guillaume Gaillard, Herman Storey, Kazushi Muraoka, Ken Bannister,
Kuor Hsin Chang, Laurent Toutain, Maik Seewald, Maria Rita Palattella,
Michael Behringer, Nancy Cam Winget, Nicola Accettura, Nicolas Montavont,
Oleg Hahm, Patrick Wetterwald, Paul Duffy, Peter van der Stock, Rahul Sen,
Pieter de Mil, Pouria Zand, Rouhollah Nabati, Rafa Marin-Lopez,
Raghuram Sudhaakar, Sedat Gormus, Shitanshu Shah, Steve Simlo,
Tengfei Chang, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles and
Samita Chakrabarti for their participation and various contributions.
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
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 TechnologyIEEE 802.1 Time-Sensitive Networks Task GroupIEEE Standards AssociationIndustrial 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 PlaneIETFDTLS In Constrained EnvironmentsIETFAuthentication and Authorization for Constrained EnvironmentsIETFDeterministic NetworkingIETFIndustrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR) - IEC62439-3IEC