An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4
draft-ietf-6tisch-architecture-18
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6TiSCH P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track December 7, 2018
Expires: June 10, 2019
An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4
draft-ietf-6tisch-architecture-18
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 10, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. 6TiSCH Terminology . . . . . . . . . . . . . . . . . . . 4
2.3. References . . . . . . . . . . . . . . . . . . . . . . . 10
2.4. Subset of a 6LoWPAN Glossary . . . . . . . . . . . . . . 11
3. High Level Architecture . . . . . . . . . . . . . . . . . . . 12
3.1. 6TiSCH Stack . . . . . . . . . . . . . . . . . . . . . . 12
3.2. TSCH: A Deterministic MAC Layer . . . . . . . . . . . . . 14
3.3. Scheduling TSCH . . . . . . . . . . . . . . . . . . . . . 14
3.4. Routing and Forwarding Over TSCH . . . . . . . . . . . . 16
3.5. A Non-Broadcast Multi-Access Radio Mesh Network . . . . . 17
3.6. A Multi-Link Subnet Model . . . . . . . . . . . . . . . . 19
3.7. Join Process and Registration . . . . . . . . . . . . . . 20
4. Architecture Components . . . . . . . . . . . . . . . . . . . 24
4.1. 6LoWPAN (and RPL) . . . . . . . . . . . . . . . . . . . . 24
4.1.1. RPL-Unaware Leaves and 6LoWPAN ND . . . . . . . . . . 24
4.1.2. RPL Root And 6LBR . . . . . . . . . . . . . . . . . . 25
4.2. TSCH and 6top . . . . . . . . . . . . . . . . . . . . . . 26
4.2.1. 6top . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2.2. Scheduling Functions and the 6top protocol . . . . . 26
4.2.3. 6top and RPL Objective Function operations . . . . . 27
4.2.4. Network Synchronization . . . . . . . . . . . . . . . 28
4.2.5. SlotFrames and CDU matrix . . . . . . . . . . . . . . 29
4.2.6. Distributing the reservation of cells . . . . . . . . 30
4.3. Communication Paradigms and Interaction Models . . . . . 32
4.4. Schedule Management Mechanisms . . . . . . . . . . . . . 33
4.4.1. Static Scheduling . . . . . . . . . . . . . . . . . . 33
4.4.2. Neighbor-to-neighbor Scheduling . . . . . . . . . . . 34
4.4.3. Remote Monitoring and Schedule Management . . . . . . 35
4.4.4. Hop-by-hop Scheduling . . . . . . . . . . . . . . . . 37
4.5. On Tracks . . . . . . . . . . . . . . . . . . . . . . . . 37
4.5.1. General Behavior of Tracks . . . . . . . . . . . . . 37
4.5.2. Serial Track . . . . . . . . . . . . . . . . . . . . 38
4.5.3. Complex Track with Replication and Elimination . . . 39
4.5.4. DetNet End-to-end Path . . . . . . . . . . . . . . . 39
4.5.5. Cell Reuse . . . . . . . . . . . . . . . . . . . . . 40
4.6. Forwarding Models . . . . . . . . . . . . . . . . . . . . 41
4.6.1. Track Forwarding . . . . . . . . . . . . . . . . . . 41
4.6.2. IPv6 Forwarding . . . . . . . . . . . . . . . . . . . 44
4.6.3. Fragment Forwarding . . . . . . . . . . . . . . . . . 44
4.7. Distributed vs. Centralized Routing . . . . . . . . . . . 46
4.7.1. Packet Marking and Handling . . . . . . . . . . . . . 46
4.7.2. Replication, Retries and Elimination . . . . . . . . 47
4.7.3. Differentiated Services Per-Hop-Behavior . . . . . . 48
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 48
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49
7.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 49
7.2. Special Thanks . . . . . . . . . . . . . . . . . . . . . 50
7.3. And Do not Forget . . . . . . . . . . . . . . . . . . . . 50
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.1. Normative References . . . . . . . . . . . . . . . . . . 50
8.2. Informative References . . . . . . . . . . . . . . . . . 53
8.3. Other Informative References . . . . . . . . . . . . . . 58
Appendix A. Join Process Highlights . . . . . . . . . . . . . . 59
Appendix B. Dependencies on Work In Progress . . . . . . . . . . 61
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 63
1. 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
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 [I-D.ietf-detnet-use-cases]. They include Professional
Media and Operation Technology (OT) Industrial Automation Control
Systems (IACS).
The Timeslotted Channel Hopping (TSCH) [RFC7554] mode of the IEEE Std
802.15.4 [IEEE802154] Medium Access Control (MAC) was introduced with
the IEEE Std 802.15.4e [IEEE802154e] amendment and is now retrofitted
in the main standard. For all practical purpose, this document is
expected to be insensitive to the revisions of that standard, which
is thus referenced undated. 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.
Proven Deterministic Networking standards for use in Process Control,
including ISA100.11a [ISA100.11a] and WirelessHART [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.
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In order to enable the convergence of IT and OT in LLN environments,
6TiSCH ports the IETF suite of protocols 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, e.g., an Ethernet bridged network- 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.).
2. Terminology
2.1. BCP 14
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. 6TiSCH Terminology
The draft does not reuse terms from the IEEE Std 802.15.4
[IEEE802154] 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 802.15.4e): 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.
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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 [RFC8480]. 6P enables
Layer-2 peers to allocate, move or deallocate cells in
their respective schedules in order 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): The total number of timeslots that have
elapsed since the PAN coordinator has started the TSCH
network. 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 Layer-3 bundle pair maps to an IP Link
with a neighbor, whereas a Layer-2 bundle set 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.
CCA (Clear Channel Assessment): A mechanism defined in [IEEE802154]
whereby nodes listen to the channel before sending, in
order 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.
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cell: A single element in the TSCH schedule, identified by a
slotOffset, a channelOffset, a slotframeHandle. A cell
can be scheduled or unscheduled.
Channel Distribution/Usage (CDU) matrix: : 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. It has to be noticed that such a
matrix which includes all the cells grouped in chunks,
belonging to different slotframes, is different from the
TSCH schedule.
channelOffset: Identifies a row in the TSCH schedule. The number of
available channelOffset values is equal to the number of
available frequencies. The channelOffset translates into
a frequency when the communication takes place, resulting
in channel hopping.
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): CoJP is a one-touch join protocol
defined in the Minimal Security Framework for 6TiSCH
[I-D.ietf-6tisch-minimal-security]. CoJP requires the
distribution of preshared keys (PSK), and enables a node
to join with a single round trip to the JRC via the JP.
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 [I-D.ietf-detnet-architecture]. When
applied to 6TiSCH, it refers to the reservation of Tracks
which guarantee an end-to-end latency and optimize the
PDR for well-characterized flows.
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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 [IEEE802154] 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 cannot relocate.
hopping sequence: Ordered sequence of frequencies, identified by a
Hopping_Sequence_ID, used for channel hopping when
translating the channel offset 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
[IEEE802154]. Some IE identifiers are managed by the
IETF [I-D.kivinen-802-15-ie].
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
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for the term "Link" is adopted, as opposed to the IEEE
Std 802.15.4 terminology.
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, timeskeeping. 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.
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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 complex multi-hop path that is structured as
a DODAG to the destination of the path, enabling
replication, elimination and reordering functions on the
way (more on those functions in the Deterministic
Networking Architecture [I-D.ietf-detnet-architecture]).
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 [RFC6550] can be used as TrackID. In that
case, a RPL Packet Information [RFC6550] in an IPv6
packet can unambiguously identify the Track and can be
expressed in a compressed form using [RFC8138].
TSCH: A medium access mode of the IEEE Std 802.15.4
[IEEE802154] 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.
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2.3. References
The draft uses domain-specific terminology defined or referenced in:
"Neighbor Discovery Optimization for Low-power and Lossy Networks"
[RFC6775],
"Registration Extensions for 6LoWPAN Neighbor Discovery"
[RFC8505],
"Terms Used in Routing for Low-Power and Lossy Networks (LLNs)"
[RFC7102],
"Objective Function Zero for the Routing Protocol for Low-Power
and Lossy Networks (RPL)" [RFC6552], and
"RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks"
[RFC6550].
Other terms in use in LLNs are found in "Terminology for Constrained-
Node Networks" [RFC7228].
Readers are expected to be familiar with all the terms and concepts
that are discussed in
o "Neighbor Discovery for IP version 6" [RFC4861],
o "IPv6 Stateless Address Autoconfiguration" [RFC4862],
o "Problem Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606].
The draft also conforms to the terms and models described in
[RFC3444] and [RFC5889] and uses the vocabulary and the concepts
defined in [RFC4291] for the IPv6 Architecture and refers [RFC4080]
for reservation
In addition, readers would benefit from reading:
o "Multi-Link Subnet Issues" [RFC4903],
o "Mobility Support in IPv6" [RFC6275],
o "RPL applicability in industrial networks"
[I-D.ietf-roll-rpl-industrial-applicability],
o "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals" [RFC4919].
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o "Optimistic Duplicate Address Detection" [RFC4429],
o "Neighbor Discovery Proxies (ND Proxy)" [RFC4389],
o "FCFS SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses" [RFC6620], and
o "Optimistic Duplicate Address Detection" [RFC4429]
prior to this specification for a clear understanding of the art in
ND-proxying and binding.
2.4. Subset of a 6LoWPAN Glossary
This document often uses the following acronyms:
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)
NA: Neighbor Advertisement
NCE: Neighbor Cache Entry
ND: Neighbor Discovery
NDP: Neighbor Discovery Protocol
NS: Neighbor Solicitation
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ROVR: Registration Ownership Verifier (pronounced rover)
RPL: 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)
3. High Level Architecture
3.1. 6TiSCH Stack
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 [RFC0768]
[RFC8200] and the Constrained Application Protocol [RFC7252] (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 Figure 1:
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+--------+--------+
| CoJP | Applis |
+--------+--------+--------------+-----+
| CoAP / OSCORE | 6LoWPAN ND | RPL |
+-----------------+--------------+-----+
| UDP | ICMPv6 |
+-----------------+--------------------+
| IPv6 |
+--------------------------------------+----------------------+
| 6LoWPAN HC / 6LoRH HC | Scheduling Functions |
+--------------------------------------+----------------------+
| 6top (to be IEEE Std 802.15.12) inc. 6top protocol |
+-------------------------------------------------------------+
| IEEE Std 802.15.4 TSCH |
+-------------------------------------------------------------+
Figure 1: 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 [RFC8180] 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 [RFC6282] is used to compress the IPv6
and UDP headers, whereas the 6LoWPAN Routing Header (6LoRH) [RFC8138]
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.
The Datagram Transport Layer Security (DTLS) [RFC6347] sitting either
under CoAP or over CoAP so as to traverse proxies, as well as Object
Security for Constrained RESTful Environments (OSCORE)
[I-D.ietf-core-object-security], are examples of protocols that could
be used to protect application payload. OSCORE is used in particular
by the Constrained Join Protocol (CoJP) defined in the "Minimal
Security Framework for 6TiSCH" [I-D.ietf-6tisch-minimal-security].
An overview of the the initial steps of a device in a network can be
found in Section 3.7; the security aspects of the join process are
further detailed in Section 6.
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
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the next sections, providing in particular dynamic cell allocation
with the 6top Protocol (6P) [RFC8480].
3.2. 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 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 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
[I-D.ietf-detnet-architecture], and IETF components such as the Path
Computation Element (PCE) [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 [I-D.ietf-roll-rpl-industrial-applicability].
3.3. Scheduling TSCH
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
Section 2.2 for more on these terms).
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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 up at a same time, in which case a the
slotframe with the highest priority is actually activated.
The 6top sublayer hides the complexity of the schedule from the upper
layers. The Link that IP may utilize between the 6TiSCH node and a
peer 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.
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.
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 [RFC8180].
Neighbor-to-Neighbor Scheduling: This refers to the dynamic
adaptation of the bandwidth of the Links that are used for IPv6
traffic between adjacent routers. Scheduling Functions such as
the "6TiSCH Minimal Scheduling Function (MSF)"
[I-D.ietf-6tisch-msf] influence the operation of the MAC layer to
add, update and remove cells in its own, and its peers schedules
using 6P [RFC8480], 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 in order to minimize the
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interaction with and the load on the constrained device. This
model is the TSCH adaption of the "DetNet Architecture"
[I-D.ietf-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 Section 4.4.
3.4. Routing and Forwarding Over TSCH
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 [RFC6550], such as
the "Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks"
[I-D.ietf-roll-aodv-rpl] (AODV-RPL), which derives from the Ad Hoc
On-demand Distance Vector Routing (AODV) [I-D.ietf-manet-aodvv2] 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.
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.
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G-MPLS Track Forwarding: This model corresponds to the Remote
Monitoring and Schedule Management. In this model, A central
controller (hosting a PCE) computes and installs the schedules in
the devices per flow. The incoming (Layer-2) bundle of cells from
the previous node along the path determines the outgoing (Layer-2)
bundle towards the next hop for that flow as determined by the
PCE. The programmed sequence for bundles is called a Track and
can assume shapes that are more complex than a simple direct
sequence of nodes.
6LoWPAN Fragment Forwarding: This is an hybrid model that derives
from IPv6 forwarding for the case where packets must be fragmented
at the 6LoWPAN sublayer. The first fragment is forwarded like any
IPv6 packet and leaves a state in the intermediate hops to enable
forwarding of the next fragments that do not have a IP header
without the need to recompose the packet at every hop.
This can be broadly summarized in the following table:
+---------------------+------------+-----------------------------------+
| Forwarding Model | Routing | Scheduling |
+=====================+============+===================================+
| | | Static (Minimal Configuration) |
+ classical IPv6 + RPL +-----------------------------------+
| / | | Neighbor-to-Neighbor (SF+6P) |
+ 6LoWPAN Fragment F. +------------+-----------------------------------+
| |Reactive P2P| Hop-by-Hop (TBD) |
+---------------------+------------+-----------------------------------+
|G-MPLS Track Fwrding | PCE |Remote Monitoring and Schedule Mgt |
+---------------------+------------+-----------------------------------+
Figure 2: Routing, Forwarding and Scheduling
3.5. A Non-Broadcast Multi-Access Radio Mesh Network
A 6TiSCH network is an IPv6 [RFC8200] 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) [RFC6282] 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.
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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 [RFC6775] (6LoWPAN ND), whereas RPL [RFC6550]
enables routing in the so called Route Over fashion, either in
storing (stateful) or non-storing (stateless, with routing headers)
mode.
---+-------- ............ ------------
| External Network |
| +-----+
+-----+ | NME |
| | LLN Border | |
| | router +-----+
+-----+
o o o
o o o o o
o o 6LoWPAN + RPL o o
o o o o
o o
Figure 3: Basic Configuration of a 6TiSCH Network
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 Section 4.1. Security aspects of the
join process by which a device obtains access to the network are
discussed in Section 6.
With TSCH, devices are time-synchronized at the MAC level. The use
of a particular RPL Instance for time synchronization is discussed in
Section 4.2.4. 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 (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 [RFC6550], 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 [RFC8138].
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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 [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) [RFC4861] 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 [RFC5340] or BGP [RFC2545], or inject them in a mobility
protocol such as MIPv6 [RFC6275], NEMO [RFC3963], or LISP [RFC6830].
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.
3.6. A Multi-Link Subnet Model
An extended configuration of the subnet comprises multiple LLNs. The
LLNs are interconnected and synchronized over a backbone, that can be
wired or wireless. The backbone can be a classical IPv6 network,
with Neighbor Discovery operating as defined in [RFC4861] and
[RFC4862]. 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 [I-D.ietf-6lo-backbone-router]. The 6BBR performs ND
proxy operations between the registered devices and the classical ND
devices that are located over the backbone. 6TiSCH 6BBRs synchronize
with one another over the backbone, so as to ensure that the multiple
LLNs that form the IPv6 subnet stay tightly synchronized.
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---+-------- ............ ------------
| External Network |
| +-----+
| +-----+ | NME |
+-----+ | +-----+ | |
| | Router | | PCE | +-----+
| | +--| |
+-----+ +-----+
| |
| Subnet Backbone |
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
o | | router | | router | | router
+-----+ +-----+ +-----+
o o o o o
o o o o o o o o o o o
o o o LLN o o o o
o o o o o o o o o o o o
Figure 4: Extended Configuration of a 6TiSCH Network
As detailed in Section 4.1 the 6LoWPAN ND 6LBR and the root of the
RPL network need share information about the devices that is learned
through either protocol but not both. One way af 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 6LoWPAN ND [RFC8505] 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
[I-D.ietf-detnet-architecture] studies Layer-3 aspects of
Deterministic Networks, and covers networks that span multiple
Layer-2 domains.
3.7. Join Process and Registration
The Minimal Security Framework for 6TiSCH
[I-D.ietf-6tisch-minimal-security] specifies the CoJP protocol that
provides the minimal-security mechanisms required to enable a joining
node to securely join a 6TiSCH network based on a PSK.
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CoJP allows to establish Link-Layer keys, typically used in
combination with a variation of Counter with CBC-MAC (CCM) [RFC3610],
and set up a secure end-to-end session between the joining node
(called the pledge) and the join registrar/coordinator (JRC) in
charge of authenticating the node via a Join Proxy (JP) that acts as
a relay. It can also be used to obtain a Link-Layer short address as
a side effect. It is optimized to limit the number of messages to
the strict minimum. CoJP messages on the first hop are transmitted
on shared slots, a consequence of CoJP being executed when a pledge
is not yet part of the network.
The "6tisch Zero-Touch Secure Join protocol"
[I-D.ietf-6tisch-dtsecurity-zerotouch-join] wraps the minimal-
security mechanisms within a flow inspired from ANIMA "Bootstrapping
Remote Secure Key Infrastructures (BRSKI)"
[I-D.ietf-anima-bootstrapping-keyinfra]. The zero-touch operation
precedes minimal-security flow by defining the establishment of a
shared secret based on (manufacturer-installed) certificate. The
shared secret obtained by zero touch is then used to provide a secure
OSCORE [I-D.ietf-core-object-security] session to CoJP.
The following flows illustrate the steps that are needed to provide
reachability for a device in a secure fashion.
Figure 5 illustrates that very initial joining phase.
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6LoWPAN Node 6LR 6LBR Join Registrar
(pledge) (Join Proxy) (root) /Coordinator (JRC)
| | | |
| 6LoWPAN ND |6LoWPAN ND+RPL | IPv6 network |
| LLN link |Route-Over mesh| (the Internet)|
| | | |
| Layer-2 | | |
|enhanced beacon| | |
|<--------------| | |
<-----------------| | |
| <------------| | |
| | | |
| NS(EARO) | | |
|(for the LL @) | | |
|-------------->| | |
| NA(EARO) | | |
|<--------------| | |
| | | |
| CoJP Join Req | | |
| Link Local @ | | |
|-------------->| | |
| | CoJP Join Request |
| | Global Unicast @ |
| |------------------------------>|
| | | |
| | CoJP Join Response |
| | Global Unicast @ |
| |<------------------------------|
|CoJP Join Resp | | |
| Link Local @ | | |
|<--------------| | |
| | | |
Figure 5: CoJP join process in a Multi-Link Subnet
As detailed in Section 4.1, 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.
Figure 6 illustrates the initial IPv6 signaling that enables a 6LN to
form a global address and register it to a 6LBR using 6LoWPAN ND
[RFC8505], is then carried over RPL to the RPL root, and then to the
6BBR.
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6LoWPAN Node 6LR 6LBR 6BBR
(RPL leaf) (router) (root)
| | | |
| 6LoWPAN ND |6LoWPAN ND+RPL | 6LoWPAN ND | IPv6 ND
| LLN link |Route-Over mesh|Ethernet/serial| Backbone
| | | |
| IPv6 ND RS | | |
|-------------->| | |
|-----------> | | |
|------------------> | |
| IPv6 ND RA | | |
|<--------------| | |
| | <once> | |
| NS(EARO) | | |
|-------------->| | |
| 6LoWPAN ND | Extended DAR | |
| |-------------->| |
| | | NS(EARO) |
| | |-------------->|
| | | | NS-DAD
| | | |------>
| | | | (EARO)
| | | |
| | | NA(EARO) |<timeout>
| | |<--------------|
| | Extended DAC | |
| |<--------------| |
| NA(EARO) | | |
|<--------------| | |
| | | |
Figure 6: Initial Registration Flow over Multi-Link Subnet
Figure 7 illustrates the repeating IPv6 signaling that enables a 6LN
to keep a global address alive and registered to its 6LBR using
6LoWPAN ND [RFC8505], using 6LoWPAN ND ot the 6LR, RPL to the RPL
root, and then 6LoWPAN ND again to the 6BBR.
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6LoWPAN Node 6LR 6LBR 6BBR
(RPL leaf) (router) (root)
| | | |
| 6LoWPAN ND |6LoWPAN ND+RPL | 6LoWPAN ND | IPv6 ND
| LLN link |Route-Over mesh| ant IPv6 link | Backbone
| | | |
| | <periodic> | |
| | | |
| NS(EARO) | | |
|-------------->| | |
| NA(EARO) | | |
|<--------------| | |
| | DAO | |
| |-------------->| |
| | DAO-ACK | |
| |<--------------| |
| | | NS(EARO) |
| | |-------------->|
| | | NA(EARO) |
| | |<--------------|
| | | |
| | | |
Figure 7: Next Registration Flow over Multi-Link Subnet
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.
4. Architecture Components
4.1. 6LoWPAN (and RPL)
A RPL DODAG is formed of a Root, a collection of routers, and leaves
that are hosts, in other words do not forward packets that they did
not generate. RPL-aware leaves will participate to RPL in order to
advertise their own addresses, whereas RPL-unware 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-unware
leaves.
4.1.1. RPL-Unaware Leaves and 6LoWPAN ND
RPL needs a set of information in order to advertise a leaf node
through a DAO message and establish reachability.
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At the bare minimum the leaf device must provide a sequence number
that matches the RPL specification in section 7 of [RFC6550].
Section 4.1 of [RFC8505], on the Extended Address Registration Option
(EARO), already incorporates that addition with a new field in the
option called the Transaction ID.
"Routing for RPL Leaves" [I-D.thubert-roll-unaware-leaves] details
the basic interaction of 6LoWPAN ND and RPL and enables a plain 6LN
that supports [RFC8505] to obtain return connectivity via the RPL
network as a non-RPL-aware leaf. Though the above specification
enables a model where the separation is possible, this architecture
recommends to collocate the functions of 6LBR and RPL root.
On the backbone, the InstanceID is expected to be mapped onto a an
overlay that matches the instanceID, for instance a VLANID.
4.1.2. RPL Root And 6LBR
With [RFC6775], information on the 6LBR is disseminated via an
Authoritative Border Router Option (ABRO) in RA messages. [RFC8505]
extends 6LowPAN ND [RFC6775] to enable a registration for routing or
proxy services. The discovery and liveliness of the RPL root are
obtained through the RPL protocol [RFC6550]. The capability to
support the update to RFC6775 [RFC8505] is indicated in the 6LoWPAN
Capability Indication Option (6CIO).
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
[I-D.ietf-6lo-backbone-router].
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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 [RFC6775] whereby the Target Address of the NS
message is registered as opposed to the Source, which, in the case of
a proxy registration, is that of the 6LBR / RPL root itself.
4.2. TSCH and 6top
4.2.1. 6top
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
[RFC8480], 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 Section 4.4.2.
4.2.1.1. Hard Cells
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.
4.2.1.2. Soft Cells
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.
4.2.2. Scheduling Functions and the 6top protocol
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.
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The "6TiSCH Minimal Scheduling Function (MSF)" [I-D.ietf-6tisch-msf]
provides a simple scheduling function that can be used by default by
devices that support dynamic scheduling of soft cells.
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.
+------------------------+ +------------------------+
| Scheduling Function | | Scheduling Function |
| Bandwidth adaptation | | Bandwidth adaptation |
+------------------------+ +------------------------+
| Scheduling Function | | Scheduling Function |
| TSCH mapping to cells | | TSCH mapping to cells |
+------------------------+ +------------------------+
| 6top cells negotiation | <- 6P -> | 6top cells negotiation |
+------------------------+ +------------------------+
Device A Device B
Figure 8: SF/6P stack in 6top
The SF relies on 6top services that implement the 6top Protocol (6P)
[RFC8480] 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.
4.2.3. 6top and RPL Objective Function operations
An implementation of a RPL [RFC6550] Objective Function (OF), such as
the RPL Objective Function Zero (OF0) [RFC6552] that is used in the
Minimal 6TiSCH Configuration [RFC8180] 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. A
neighbor table entry may contain 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
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number of packets received from it. This information can be obtained
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 behaviour, 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 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.
4.2.4. 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. 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-free 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 [RFC6550], "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).
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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 [RFC6550],
"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 in its
IEEE Std 802.15.4 Extended Beacons (EB). We'll note that the Join
Priority 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.
4.2.5. SlotFrames and CDU matrix
6TiSCH enables in essence the capability to use IPv6 over a MAC layer
that enables to schedule 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.
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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 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 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 [RFC2474] can therefore be used). At
each scheduled transmit slot, 6top looks for the frame in all the
outgoing queues that best matches the cells. If a frame is found, it
is given to the TSCH MAC for transmission.
4.2.6. Distributing the reservation of cells
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.
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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 in 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 Section 2.2)
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.
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+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
...
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
0 1 2 3 4 5 6 M
Figure 9: CDU matrix Partitioning in Chunks
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.
4.3. Communication Paradigms and Interaction Models
Section 2.2 provides the terms of Communication Paradigms and
Interaction Models, which can be placed in parallel to the
Information Models and Data Models that are defined in [RFC3444].
A Communication Paradigms would be an abstract view of a protocol
exchange, and would come with an Information Model for the
information that is being exchanged. In contrast, an Interaction
Models would be more refined and could point on standard operation
such as a Representational state transfer (REST) "GET" operation and
would match a Data Model for the data that is provided over the
protocol exchange.
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Section 2.1.3 of [I-D.ietf-roll-rpl-industrial-applicability] and
next sections discuss application-layer paradigms, such as Source-
sink (SS) that is a Multipeer to Multipeer (MP2MP) model primarily
used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that
is typically used for sensor data, as well as Peer-to-peer (P2P) and
Peer-to-multipeer (P2MP) communications. Additional considerations
on Duocast and its N-cast generalization are also provided. Those
paradigms are frequently used in industrial automation, which is a
major use case for IEEE Std 802.15.4 TSCH wireless networks with
[ISA100.11a] and [WirelessHART], that provides a wireless access to
[HART] 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 Section 4.4. Link-Layer frame
forwarding interactions are discussed in Section 4.6, and Network-
layer Packet routing is addressed in Section 4.7.
4.4. 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.
4.4.1. 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 [RFC8180].
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4.4.2. Neighbor-to-neighbor Scheduling
In the simplest instantiation of a 6TiSCH network described in
Section 4.4.1, 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 Protocol [RFC8480] specifies the exchanges between neighbor
nodes to reserve soft cells to transmit to one another. Because this
reservation is done without global knowledge of the schedule of other
nodes in the LLN, scheduling collisions are possible. 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.
MSF [I-D.ietf-6tisch-msf] is one of the possible scheduling
functions. MSF uses the rendez-vous slot from [RFC8180] 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.
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.
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4.4.3. Remote Monitoring and Schedule Management
The work at the 6TiSCH WG is focused on non-deterministic traffic and
does not provide the generic data model that would be necessary to
monitor and manage resources of the 6top sublayer. It is recognized
that CoAP can be appropriate to interact with the 6top layer of a
node that is multiple hops away across a 6TiSCH mesh.
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, it is envisionned
that the related component of the 6TiSCH Architecture would be an
extension of the Deterministic Networking Architecture
[I-D.ietf-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 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 [RFC7426] and is represented
below:
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SDN Layers and Architecture Terminology per RFC 7426
o--------------------------------o
| |
| +-------------+ +----------+ |
| | Application | | Service | |
| +-------------+ +----------+ |
| Application Plane |
o---------------Y----------------o
|
*-----------------------------Y---------------------------------*
| Network Services Abstraction Layer (NSAL) |
*------Y------------------------------------------------Y-------*
| |
| Service Interface |
| |
o------Y------------------o o---------------------Y------o
| | Control Plane | | Management Plane | |
| +----Y----+ +-----+ | | +-----+ +----Y----+ |
| | Service | | App | | | | App | | Service | |
| +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ |
| | | | | | | |
| *----Y-----------Y----* | | *---Y---------------Y----* |
| | Control Abstraction | | | | Management Abstraction | |
| | Layer (CAL) | | | | Layer (MAL) | |
| *----------Y----------* | | *----------Y-------------* |
| | | | | |
o------------|------------o o------------|---------------o
| |
| CP | MP
| Southbound | Southbound
| Interface | Interface
| |
*------------Y---------------------------------Y----------------*
| Device and resource Abstraction Layer (DAL) |
*------------Y---------------------------------Y----------------*
| | | |
| o-------Y----------o +-----+ o--------Y----------o |
| | Forwarding Plane | | App | | Operational Plane | |
| o------------------o +-----+ o-------------------o |
| Network Device |
+---------------------------------------------------------------+
Figure 10
The PCE establishes end-to-end Tracks of hard cells, which are
described in more details in Section 4.6.1. The DetNet work is
expected to enable end to end Deterministic Path across heterogeneous
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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.
4.4.4. Hop-by-hop Scheduling
A node can reserve a Track (Section 4.5) to a destination node
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 [RFC3209] and/or [RFC4080]/[RFC5974]. The protocol for a
node to trigger hop-by-hop scheduling is not yet defined.
4.5. On Tracks
4.5.1. General Behavior of Tracks
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
[I-D.ietf-detnet-architecture]. 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 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;
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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.
1. Track forwarding, as further described in Section 4.6.1, 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.
2. 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.
3. 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.
4. 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.
4.5.2. 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 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.
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4.5.3. Complex Track with Replication and Elimination
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) [IEC62439].
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.
4.5.4. DetNet End-to-end Path
Ultimately, DetNet [I-D.ietf-detnet-architecture] should enable to
extend a Track beyond the 6TiSCH LLN. Figure 11 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. A 6TiSCH-Aware DetNet Service Layer handles the Packet
Replication, Elimination, and Ordering Functions over the DODAG that
forms a Track.
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+-=-=-+
| IoT |
| G/W |
+-=-=-+
^ <=== Elimination
| |
Track branch | |
+-=-=-=-+ +-=-=-=-=+ Subnet Backbone
| |
+-=|-=+ +-=|-=+
| | | Backbone | | | Backbone
o | | | router | | | router
+-=/-=+ +-=|-=+
o / o o-=-o-=-=/ o
o o-=-o-=/ o o o o o
o \ / o o LLN o
o v <=== Replication
o
Figure 11: Example End-to-End DetNet 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.
4.5.5. Cell Reuse
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
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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
Section 4.7.1. A frame is re-Tracked by scheduling it for
transmission over the transmit bundle associated to the Track,
with the destination MAC address set to broadcast.
4.6. Forwarding Models
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 Section 4.7. 6TiSCH supports
three different forwarding model, G-MPLS Track Forwarding (TF),
6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).
4.6.1. 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.
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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.
4.6.1.1. Transport Mode
In transport mode, the Protocol Data Unit (PDU) is associated with
flow-dependant meta-data that refers uniquely to the Track, so the
6top sublayer can place the frame in the appropriate cell without
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.
Figure 12 illustrates the Track Forwarding operation which happens at
the 6top sublayer, below IP.
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | |
+--------------+ ingress egress
| 6top | sets +----+ +----+ restores
+--------------+ dmac to | | | | dmac to
| TSCH MAC | brdcst | | | | self
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
Figure 12: Track Forwarding, Transport Mode
4.6.1.2. Tunnel Mode
In tunnel mode, the frames originate from an arbitrary protocol over
a compatible MAC that may or may not be synchronized with the 6TiSCH
network. An example of this would be a router with a dual radio that
is capable of receiving and sending WirelessHART or ISA100.11a frames
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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.
+--------------+
| IPv6 |
+--------------+
| 6LoWPAN HC |
+--------------+ set restore
| 6top | +dmac+ +dmac+
+--------------+ to|brdcst to|nexthop
| TSCH MAC | | | | |
+--------------+ | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+ | ingress egress |
| |
+--------------+ | |
| LLN PHY | | |
+--------------+ | |
| TSCH MAC | | |
+--------------+ | dmac = | dmac =
|ISA100/WiHART | | nexthop v nexthop
+--------------+
Figure 13: Track Forwarding, Tunnel Mode
In that case, the flow information that identifies the Track at the
ingress 6TiSCH router is derived from the RX-cell. The dmac is set
to this node but the flow information indicates that the frame must
be tunneled over a particular Track so the frame is not passed to the
upper layer. Instead, the dmac is forced to broadcast and the frame
is passed to the 6top sublayer for switching.
At the egress 6TiSCH router, the reverse operation occurs. Based on
metadata associated to the Track, the frame is passed to the
appropriate Link-Layer with the destination MAC restored.
4.6.1.3. Tunnel Metadata
Metadata coming with the Track configuration is expected to provide
the destination MAC address of the egress endpoint as well as the
tunnel mode and specific data depending on the mode, for instance a
service access point for frame delivery at egress. If the tunnel
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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.
4.6.2. IPv6 Forwarding
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
[I-D.svshah-tsvwg-lln-diffserv-recommendations].
| ^
+--------------+ | |
| IPv6 | | +-QoS+ +-QoS+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | | | | | |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
Figure 14: IP Forwarding
4.6.3. Fragment Forwarding
Considering that 6LoWPAN packets can be as large as 1280 bytes (the
IPv6 MTU), and that the non-storing mode of RPL implies Source
Routing that requires space for routing headers, and that a 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.
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In the case to a multihop route within a 6TiSCH network, Hop-by-Hop
recomposition occurs at each hop in order to reform the packet and
route it. This creates additional latency and forces intermediate
nodes to store a portion of a packet for an undetermined time, thus
impacting critical resources such as memory and battery.
[I-D.ietf-6lo-minimal-fragment] describes a framework for forwarding
fragments end-to-end across a 6TiSCH route-over mesh. Within that
framework, [I-D.ietf-lwig-6lowpan-virtual-reassembly] 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,
[I-D.ietf-6lo-fragment-recovery] 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.
| ^
+--------------+ | |
| IPv6 | | +----+ +----+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | learn learn |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
Figure 15: 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.
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| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | replay replay |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
Figure 16: 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.
4.7. Distributed vs. Centralized Routing
6TiSCH enables a mixed model of centralized routes and distributed
routes. Centralized routes can for example be computed by a entity
such as a PCE. Distributed routes are computed by RPL.
Both methods may inject routes in the Routing Tables of the 6TiSCH
routers. In either case, each route is associated with a 6TiSCH
topology that can be a RPL Instance topology or a Track. The 6TiSCH
topology is indexed by a Instance ID, in a format that reuses the
RPLInstanceID as defined in RPL [RFC6550].
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.
4.7.1. Packet Marking and Handling
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.
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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 [RFC6550], "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 [RFC6553].
A compression mechanism for the RPL packet artifacts that integrates
the compression of IP-in-IP encapsulation and the Routing Header type
3 [RFC6554] with that of the RPI in a 6LoWPAN dispatch/header type is
specified in [RFC8025] and [RFC8138].
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.
4.7.2. Replication, Retries and Elimination
6TiSCH expects elimination and replication of packets along a complex
Track, but has no position about how the sequence numbers would be
tagged in the packet.
As it goes, 6TiSCH expects that timeslots corresponding to copies of
a same packet along a Track are correlated by configuration, and does
not need to process the sequence numbers.
The semantics of the configuration 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
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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.
4.7.3. Differentiated Services Per-Hop-Behavior
Additionally, an IP packet that is sent along a Track uses the
Differentiated Services Per-Hop-Behavior Group called Deterministic
Forwarding, as described in
[I-D.svshah-tsvwg-deterministic-forwarding].
5. IANA Considerations
This specification does not require IANA action.
6. Security Considerations
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.
As detailed in Section 3.7, a pledge that wishes to join the 6TiSCH
network must participate to a join process to obtain its security
keys.
The join process can be zero-touch and leverage ANIMA procedures, as
detailed in the 6tisch Zero-Touch Secure Join protocol
[I-D.ietf-6tisch-dtsecurity-zerotouch-join].
Alternatively, the join process can be one-touch, in which case the
pledge is provisioned with a preshared key (PSK), and uses CoJP as
specified in [I-D.ietf-6tisch-minimal-security].
In order to join, the pledge is helped by a Join Proxy (JP) that
relays the link-scope Join Request over the IP network to the Join
Registrar/Coordinator (JRC) that can authenticate the pledge and
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validate that it is attached to the appropriate network. As a result
of this exchange the pledge is in possession of a Link-Layer material
including a key and a short address, and all traffic is secured at
the Link-Layer.
7. Acknowledgments
7.1. Contributors
The co-authors of this document are listed below:
Robert Assimiti for his breakthrough work on RPL over TSCH and
initial text and guidance;
Kris Pister for creating it all and his continuing guidance through
the elaboration of this design;
Maria Rita Palattella for managing the Terminology document merged
into this through the work of 6TiSCH;
Michael Richardson for his leadership role in the Security Design
Team and his contribution throughout this document;
Rene Struik for the security section and his contribution to the
Security Design Team;
Malisa Vucinic for the work on the one-touch join process and his
contribution to the Security Design Team;
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;
Qin Wang who lead the design of the 6top sublayer and contributed
related text that was moved and/or adapted in this document;
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.
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.
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7.2. Special Thanks
Special thanks to Tero Kivinen, 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 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.
7.3. And Do not Forget
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.
8. References
8.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
<https://www.rfc-editor.org/info/rfc6552>.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553,
DOI 10.17487/RFC6553, March 2012,
<https://www.rfc-editor.org/info/rfc6553>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.
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[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
RFC 8025, DOI 10.17487/RFC8025, November 2016,
<https://www.rfc-editor.org/info/rfc8025>.
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/info/rfc8180>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC8480, November 2018,
<https://www.rfc-editor.org/info/rfc8480>.
[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.
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8.2. Informative References
[I-D.ietf-6lo-ap-nd]
Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
"Address Protected Neighbor Discovery for Low-power and
Lossy Networks", draft-ietf-6lo-ap-nd-08 (work in
progress), October 2018.
[I-D.ietf-6lo-backbone-router]
Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
Backbone Router", draft-ietf-6lo-backbone-router-09 (work
in progress), December 2018.
[I-D.ietf-6lo-fragment-recovery]
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
ietf-6lo-fragment-recovery-00 (work in progress),
September 2018.
[I-D.ietf-6lo-minimal-fragment]
Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
Fragment Forwarding", draft-ietf-6lo-minimal-fragment-00
(work in progress), October 2018.
[I-D.ietf-6tisch-dtsecurity-zerotouch-join]
Richardson, M., "6tisch Zero-Touch Secure Join protocol",
draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in
progress), October 2018.
[I-D.ietf-6tisch-minimal-security]
Vucinic, M., Simon, J., Pister, K., and M. Richardson,
"Minimal Security Framework for 6TiSCH", draft-ietf-
6tisch-minimal-security-09 (work in progress), November
2018.
[I-D.ietf-6tisch-msf]
Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
draft-ietf-6tisch-msf-01 (work in progress), October 2018.
[I-D.ietf-anima-bootstrapping-keyinfra]
Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
S., and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
keyinfra-17 (work in progress), November 2018.
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[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-15 (work in
progress), August 2018.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-09 (work in progress), October 2018.
[I-D.ietf-detnet-use-cases]
Grossman, E., "Deterministic Networking Use Cases", draft-
ietf-detnet-use-cases-19 (work in progress), October 2018.
[I-D.ietf-lwig-6lowpan-virtual-reassembly]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00
(work in progress), July 2018.
[I-D.ietf-manet-aodvv2]
Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and
V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2
(AODVv2) Routing", draft-ietf-manet-aodvv2-16 (work in
progress), May 2016.
[I-D.ietf-roll-aodv-rpl]
Anamalamudi, S., Zhang, M., Perkins, C., Anand, S., and B.
Liu, "Asymmetric AODV-P2P-RPL in Low-Power and Lossy
Networks (LLNs)", draft-ietf-roll-aodv-rpl-05 (work in
progress), October 2018.
[I-D.ietf-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P., and R. Assimiti, "RPL
applicability in industrial networks", draft-ietf-roll-
rpl-industrial-applicability-02 (work in progress),
October 2013.
[I-D.kivinen-802-15-ie]
Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
Element for IETF", draft-kivinen-802-15-ie-06 (work in
progress), March 2017.
[I-D.svshah-tsvwg-deterministic-forwarding]
Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
draft-svshah-tsvwg-deterministic-forwarding-04 (work in
progress), August 2015.
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[I-D.svshah-tsvwg-lln-diffserv-recommendations]
Shah, S. and P. Thubert, "Differentiated Service Class
Recommendations for LLN Traffic", draft-svshah-tsvwg-lln-
diffserv-recommendations-04 (work in progress), February
2015.
[I-D.thubert-6lo-bier-dispatch]
Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-05
(work in progress), July 2018.
[I-D.thubert-bier-replication-elimination]
Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
TE extensions for Packet Replication and Elimination
Function (PREF) and OAM", draft-thubert-bier-replication-
elimination-03 (work in progress), March 2018.
[I-D.thubert-roll-unaware-leaves]
Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
unaware-leaves-06 (work in progress), November 2018.
[I-D.wang-6tisch-6top-sublayer]
Wang, Q. and X. Vilajosana, "6TiSCH Operation Sublayer
(6top)", draft-wang-6tisch-6top-sublayer-04 (work in
progress), November 2015.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444,
DOI 10.17487/RFC3444, January 2003,
<https://www.rfc-editor.org/info/rfc3444>.
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[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <https://www.rfc-editor.org/info/rfc3610>.
[RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol",
RFC 3963, DOI 10.17487/RFC3963, January 2005,
<https://www.rfc-editor.org/info/rfc3963>.
[RFC4080] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework",
RFC 4080, DOI 10.17487/RFC4080, June 2005,
<https://www.rfc-editor.org/info/rfc4080>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
2006, <https://www.rfc-editor.org/info/rfc4389>.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
<https://www.rfc-editor.org/info/rfc4429>.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
DOI 10.17487/RFC4903, June 2007,
<https://www.rfc-editor.org/info/rfc4903>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, DOI 10.17487/RFC4919, August 2007,
<https://www.rfc-editor.org/info/rfc4919>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5889] Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing
Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
September 2010, <https://www.rfc-editor.org/info/rfc5889>.
[RFC5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling", RFC 5974, DOI 10.17487/RFC5974, October 2010,
<https://www.rfc-editor.org/info/rfc5974>.
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[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
2011, <https://www.rfc-editor.org/info/rfc6275>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing",
RFC 6606, DOI 10.17487/RFC6606, May 2012,
<https://www.rfc-editor.org/info/rfc6606>.
[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, DOI 10.17487/RFC6620, May 2012,
<https://www.rfc-editor.org/info/rfc6620>.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
DOI 10.17487/RFC6830, January 2013,
<https://www.rfc-editor.org/info/rfc6830>.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <https://www.rfc-editor.org/info/rfc7102>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
Defined Networking (SDN): Layers and Architecture
Terminology", RFC 7426, DOI 10.17487/RFC7426, January
2015, <https://www.rfc-editor.org/info/rfc7426>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
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8.3. Other Informative References
[ACE] IETF, "Authentication and Authorization for Constrained
Environments",
<https://dataTracker.ietf.org/doc/charter-ietf-ace/>.
[ANIMA] IETF, "Autonomic Networking Integrated Model and
Approach",
<https://dataTracker.ietf.org/doc/charter-ietf-anima/>.
[CCAMP] IETF, "Common Control and Measurement Plane",
<https://dataTracker.ietf.org/doc/charter-ietf-ccamp/>.
[DETNET] IETF, "Deterministic Networking",
<https://dataTracker.ietf.org/doc/charter-ietf-detnet/>.
[DICE] IETF, "DTLS In Constrained Environments",
<https://dataTracker.ietf.org/doc/charter-ietf-dice/>.
[HART] www.hartcomm.org, "Highway Addressable remote Transducer,
a group of specifications for industrial process and
control devices administered by the HART Foundation".
[IEC62439]
IEC, "Industrial communication networks - High
availability automation networks - Part 3: Parallel
Redundancy Protocol (PRP) and High-availability Seamless
Redundancy (HSR) - IEC62439-3", 2012,
<https://webstore.iec.ch/publication/7018>.
[IEEE802.1TSNTG]
IEEE Standards Association, "IEEE 802.1 Time-Sensitive
Networks Task Group", March 2013,
<http://www.ieee802.org/1/pages/avbridges.html>.
[IEEE802154]
IEEE standard for Information Technology, "IEEE Std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks".
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[IEEE802154e]
IEEE standard for Information Technology, "IEEE standard
for Information Technology, IEEE Std. 802.15.4, Part.
15.4: Wireless Medium Access Control (MAC) and Physical
Layer (PHY) Specifications for Low-Rate Wireless Personal
Area Networks, June 2011 as amended by IEEE Std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendment 1: MAC sublayer", April
2012.
[ISA100] ISA/ANSI, "ISA100, Wireless Systems for Automation",
<https://www.isa.org/isa100/>.
[ISA100.11a]
ISA/ANSI, "Wireless Systems for Industrial Automation:
Process Control and Related Applications - ISA100.11a-2011
- IEC 62734", 2011, <http://www.isa.org/Community/
SP100WirelessSystemsforAutomation>.
[PCE] IETF, "Path Computation Element",
<https://dataTracker.ietf.org/doc/charter-ietf-pce/>.
[TEAS] IETF, "Traffic Engineering Architecture and Signaling",
<https://dataTracker.ietf.org/doc/charter-ietf-teas/>.
[WirelessHART]
www.hartcomm.org, "Industrial Communication Networks -
Wireless Communication Network and Communication Profiles
- WirelessHART - IEC 62591", 2010.
Appendix A. Join Process Highlights
The joining of a node consists of three major activities:
Device Authentication: The pledge and the JP authenticate each other
and establish a shared key, so as to ensure ongoing
authenticated communications. This may involve a server as a
third party.
Authorization: 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.
Configuration/Parameterization: The JP distributes configuration
information to the pledge, such as scheduling information, IP
address assignment information, and network policies. This may
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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 Figure 17, 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).
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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.
{joining node} {neighbor} {server, etc.} Example:
+---------+ +---------+ +---------+
| Joining | | Join | +--| CA |certificate
| Node | |Assistant| | +---------+ issuance
+---------+ +---------+ | +---------+
| | +--|Authoriz.| membership
|<----Beaconing------| | +---------+ test (JRC)
| | | +---------+
|<--Authentication-->| +--| Routing | IP address
| |<--Authorization-->| +--------- assignment
|<-------------------| | +---------+
| | +--| Gateway | backbone,
|------------------->| | +---------+ cloud
| |<--Configuration-->| +---------+
|<-------------------| +--|Bandwidth| PCE
+---------+ schedule
. . .
. . .
Figure 17: Network joining, with only authorization by third party
Appendix B. Dependencies on Work In Progress
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 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:
o 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.
o The operation of the Backbone Router
[I-D.ietf-6lo-backbone-router] is stable but the RFC is not
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published yet. The protection of registered addresses against
impersonation and take over will be guaranteed by Address
Protected Neighbor Discovery for Low-power and Lossy Networks
[I-D.ietf-6lo-ap-nd], which is not yet published either.
o The work on centralized Track computation is deferred to a
subsequent work, not necessarily at 6TiSCH. A Predicatable and
Available Wireless (PAW) bar-BoF took place; PAW may form as a WG
and take over that work. The 6TiSCH Architecture should thus
inherit from the DetNet [I-D.ietf-detnet-architecture]
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 [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 [CCAMP] G-MPLS formats and
procedures will be used to publish the Tracks, as computed by the
PCE, to the 6TiSCH nodes.
o BIER-TE-based OAM, Replication and Elimination
[I-D.thubert-bier-replication-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 [I-D.thubert-6lo-bier-dispatch]
proposes a 6LoWPAN compression for the BIER Bitstring based on
6LoWPAN Routing Header [RFC8138].
o The security model and in particular the join process depends on
the ANIMA [ANIMA] Bootstrapping Remote Secure Key Infrastructures
(BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra] in order to enable
zero-touch security provisionning; for highly constrained nodes, a
minimal model based on pre-shared keys (PSK) is also available.
o 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.
o 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.
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o ISA100 [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 [ISA100.11a],
WirelessHART [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.
Author's Address
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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