Reliable and Available Wireless Technologies
draft-thubert-raw-technologies-02
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draft-thubert-raw-technologies-02
RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational D. Cavalcanti
Expires: December 21, 2019 Intel
X. Vilajosana
Universitat Oberta de Catalunya
June 19, 2019
Reliable and Available Wireless Technologies
draft-thubert-raw-technologies-02
Abstract
This document presents a series of recent technologies that are
capable of time synchronization and scheduling of transmission,
making them suitable to carry time-sensitive flows with requirements
of both reliable delivery in bounded time, and availability at all
times, regardless of packet transmission or individual equipement
failures.
Status of This Memo
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. On Scheduling . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Benefits of Scheduling on Wires . . . . . . . . . . . . . 4
3.2. Benefits of Scheduling on Wireless . . . . . . . . . . . 5
4. IEEE 802 standards . . . . . . . . . . . . . . . . . . . . . 6
4.1. IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Provenance and Documents . . . . . . . . . . . . . . 6
4.1.2. 802.11ax High Efficiency (HE) . . . . . . . . . . . . 8
4.1.2.1. General Characteristics . . . . . . . . . . . . . 8
4.1.2.1.1. Multi-User OFDMA and Trigger-based Scheduled
Access . . . . . . . . . . . . . . . . . . . 8
4.1.2.1.2. Improved PHY Robustness . . . . . . . . . . . 8
4.1.2.1.3. Support for 6GHz band . . . . . . . . . . . . 9
4.1.2.2. Applicability to deterministic flows . . . . . . 9
4.1.2.2.1. 802.11 Managed network operation and
admission control . . . . . . . . . . . . . . 9
4.1.2.2.2. Scheduling for bounded latency and diversity 10
4.1.3. 802.11be Extreme High Throughput (EHT) . . . . . . . 10
4.1.3.1. General Characteristics . . . . . . . . . . . . . 10
4.1.3.2. Applicability to deterministic flows . . . . . . 11
4.1.3.2.1. Enhanced scheduled operation for bounded
latency . . . . . . . . . . . . . . . . . . . 11
4.1.3.2.2. Multi-AP coordination . . . . . . . . . . . . 11
4.1.3.2.3. Multi-band operation . . . . . . . . . . . . 12
4.1.4. 802.11ad and 802.11ay (mmWave operation) . . . . . . 12
4.1.4.1. General Characteristics . . . . . . . . . . . . . 12
4.1.4.2. Applicability to deterministic flows . . . . . . 12
4.2. IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Provenance and Documents . . . . . . . . . . . . . . 13
4.2.2. TimeSlotted Channel Hopping . . . . . . . . . . . . . 15
4.2.2.1. General Characteristics . . . . . . . . . . . . . 15
4.2.2.2. Applicability to Deterministic Flows . . . . . . 16
4.2.2.2.1. Centralized Path Computation . . . . . . . . 16
4.2.2.2.2. 6TiSCH Tracks . . . . . . . . . . . . . . . . 22
5. 3GPP standards . . . . . . . . . . . . . . . . . . . . . . . 29
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1. Normative References . . . . . . . . . . . . . . . . . . 29
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9.2. Informative References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
When used in math or philosophy, the term "deterministic" generally
refers to a perfection where all aspect are understood and
predictable. A perfectly Deterministic Network would ensure that
every packet reach its destination following a predetermined path
along a predefined schedule to be delivered at the exact due time.
In a real and imperfect world, a Deterministic Network must highly
predictable, which is a combination of reliability and availability.
On the one hand the network must be reliable, meaning that it will
perform as expected for all packets and in particular that it will
always deliver the packet at the destination in due time. On the
other hand, the network must be available, meaning that it is
resilient to any single outage, whether the cause is a software, a
hardware or a transmission issue.
RAW (Reliable and Available Wireless) is an effort to provide
Deterministic Networking on across a path that include a wireless
physical layer. Making Wireless Reliable and Available is even more
challenging than it is with wires, due to the numerous causes of loss
in transmission that add up to the congestion losses and the delays
caused by overbooked shared resources. In order to maintain a
similar quality of service along a multihop path that is composed of
wired and wireless hops, additional methods that are specific to
wireless must be leveraged to combat the sources of loss that are
also specific to wireless.
Such wireless-specific methods include per-hop retransmissions (HARQ)
and P2MP overhearing whereby multiple receivers are scheduled to
receive the same transmission, which balances the adverse effects of
the transmission losses that are experienced when a radio is used as
pure P2P.
2. Terminology
This specification uses several terms that are uncommon on protocols
that ensure bets effort transmissions for stochastics flows, such as
found in the traditional Internet and other statistically multiplexed
packet networks.
ARQ: Automatic Repeat Request, enabling an acknowledged transmission
and retries. ARQ is a typical model at Layer-2 on a wireless
medium. It is typically avoided end-to-end on deterministic
flows because it introduces excessive indetermination in
latency, but a limited number of retries within a bounded time
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may be used over a wireless link and yet respect end-to-end
constraints.
Available: That is exempt of unscheduled outage, the expectation for
a network being that the flow is maintained in the face of any
single breakage.
FEC: Forward error correction, sending redundant coded data to help
the receiver recover transmission errors without the delays
incurred with ARQ.
HARQ: Hybrid ARQ, a combination of FEC and ARQ.
PCE: Path Computation Element.
PAREO (functions): the wireless extension of DetNet PREOF. PAREO
functions include scheduled ARQ at selected hops, and expect
the use of new operations like overhearing where available.
Reliable: That consistently performs as expected, the expectation
for a network being to always deliver a packet in due time.
Track: A DODAG oriented to a destination, and that enables Packet
ARQ, Replication, Elimination, and Ordering Functions.
3. On Scheduling
The operations of a Deterministic Network often rely on precisely
applying a tight schedule, in order to avoid collision loss and
guarantee the worst-case time of delivery. To achieve this, there
must be a shared sense of time throughout the network. The sense of
time is usually provided by the lower layer and is not in scope for
RAW.
3.1. Benefits of Scheduling on Wires
A network is reliable when the statistical effects that affect the
packet transmission are eliminated. This involves maintaining at all
time the amount of critical packets within the physical capabilities
of the hardware and that of the radio medium. This is achieved by
controlling the use of time-shared resources such as CPUs and
buffers, by shaping the flows and by scheduling the time of
transmission of the packets that compose the flow at every hop.
Equipment failure, such as an access point rebooting, a broken radio
adapter, or a permanent obstacle to the transmission, is a secondary
source of packet loss. When a breakage occurs, multiple packets are
lost in a row before the flows are rerouted or the system may
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recover. This is not acceptable for critical applications such as
related to safety. A typical process control loop will tolerate an
occasional packet loss, but a loss of several packets in a row will
cause an emergency stop (e.g., after 4 packets lost, within a period
of 1 second).
Network Availability is obtained by making the transmission resilient
against hardware failures and radio transmission losses due to
uncontrolled events such as co-channel interferers, multipath fading
or moving obstacles. The best results are typically achieved by
pseudo randomly cumulating all forms of diversity, in the spatial
domain with replication and elimination, in the time domain with ARQ
and diverse scheduled transmissions, and in the frequency domain with
frequency hopping or channel hopping between frames.
3.2. Benefits of Scheduling on Wireless
In addition to the benefits listed in Section 3.1, scheduling
transmissions provides specific value to the wireless medium.
On the one hand, scheduling avoids collisions between scheduled
transmissions and can ensure both time and frequency diversity
between retries in order to defeat co-channel interference from un-
controlled transmitters as well as multipath fading. Transmissions
can be scheduled on multiple channels in parallel, which enables to
use the full available spectrum while avoiding the hidden terminal
problem, e.g., when the next packet in a same flow interferes on a
same channel with the previous one that progressed a few hops
farther.
On the other hand, scheduling optimizes the bandwidth usage: compared
to classical Collision Avoidance techniques, there is no blank time
related to inter-frame space (IFS) and exponential back-off in
scheduled operations. A minimal Clear Channel Assessment may be
needed to comply with the local regulations such as ETSI 300-328, but
that will not detect a collision when the senders are synchronized.
And because scheduling allows a time-sharing operation, there is no
limit to the ratio of isolated critical traffic.
Finally, scheduling plays a critical role to save energy. In IOT,
energy is the foremost concern, and synchronizing sender and listener
enables to always maintain them in deep sleep when there is no
scheduled transmission. This avoids idle listening and long
preambles and enables long sleep periods between traffic and
resynchronization, allowing battery-operated nodes to operate in a
mesh topology for multiple years.
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4. IEEE 802 standards
With an active portfolio of nearly 1,300 standards and projects under
development, IEEE is a leading developer of industry standards in a
broad range of technologies that drive the functionality,
capabilities, and interoperability of products and services,
transforming how people live, work, and communicate.
The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains
networking standards and recommended practices for local,
metropolitan, and other area networks, using an open and accredited
process, and advocates them on a global basis. The most widely used
standards are for Ethernet, Bridging and Virtual Bridged LANs
Wireless LAN, Wireless PAN, Wireless MAN, Wireless Coexistence, Media
Independent Handover Services, and Wireless RAN. An individual
Working Group provides the focus for each area. Standards produced
by the IEEE 802 SC are freely available from the IEEE GET Program
after they have been published in PDF for six months.
4.1. IEEE 802.11
4.1.1. Provenance and Documents
The IEEE 802.11 LAN standards define the underlying MAC and PHY
layers for the Wi-Fi technology. Wi-Fi/802.11 is one of the most
successful wireless technologies, supporting many application
domains. While previous 802.11 generations, such as 802.11n and
802.11ac, have focused mainly on improving peak throughput, more
recent generations are also considering other performance vectors,
such as efficiency enhancements for dense environments in 802.11ax,
and latency and support for Time-Sensitive Networking (TSN)
capabilities in 802.11be.
IEEE 802.11 already supports some 802.1 TSN standards and it is
undergoing efforts to support for other 802.1 TSN capabilities
required to address the use cases that require time synchronization
and timeliness (bounded latency) guarantees with high reliability and
availability. The IEEE 802.11 working group has been working in
collaboration with the IEEE 802.1 group for several years extending
802.1 features over 802.11. As with any wireless media, 802.11
imposes new constraints and restrictions to TSN-grade QoS, and
tradeoffs between latency and reliability guarantees must be
considered as well as managed deployment requirements. An overview
of 802.1 TSN capabilities and their extensions to 802.11 are
discussed in [Cavalcanti_2019].
Wi-Fi Alliance (WFA) is the worldwide network of companies that
drives global Wi-Fi adoption and evolution through thought
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leadership, spectrum advocacy, and industry-wide collaboration. The
WFA work helps ensure that Wi-Fi devices and networks provide users
the interoperability, security, and reliability they have come to
expect.
The following IEEE 802.11 specifications/certifications are relevant
in the context of reliable and available wireless services and
support for time-sensitive networking capabilities:
Time Synchronization: IEEE802.11-2016 with IEEE802.1AS; WFA TimeSync
Certification.
Congestion Control: IEEE802.11-2016 Admission Control; WFA Admission
Control.
Security: WFA Wi-Fi Protected Access, WPA2 and WPA3.
Interoperating with IEEE802.1Q bridges: IEEE802.11ak.
Stream Reservation Protocol (part of IEEE802.1Qat):
AIEEE802.11-2016.
Scheduled channel access: IEEE802.11ad Enhancements for very
high throughput in the 60 GHz band [IEEE80211ad].
802.11 Real-Time Applications: Topic Interest Group (TIG)
ReportDoc [IEEE_doc_11-18-2009-06].
In addition, major amendments being developed by the IEEE802.11
Working Group include capabilities that can be used as the basis for
providing more reliable and predictable wireless connectivity and
support time-sensitive applications:
IEEE 802.11ax D4.0: Enhancements for High Efficiency (HE). [IEEE802
11ax]
IEEE 802.11be Extreme High Throughput (EHT). [IEEE80211be]
IEE 802.11ay Enhanced throughput for operation in license-exempt
bands above 45 GHz. [IEEE80211ay]
The main 802.11ax and 802.11be capabilities and their relevance to
RAW are discussed in the remainder of this document.
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4.1.2. 802.11ax High Efficiency (HE)
4.1.2.1. General Characteristics
The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax
amendment [IEEE80211ax], which includes new capabilities to increase
efficiency, control and reduce latency. Some of the new features
include higher order 1024-QAM modulation, support for uplink multi-
user MIMO, OFDMA, trigger-based access and Target Wake time (TWT) for
enhanced power savings. The OFDMA mode and trigger-based access
enable scheduled operation, which is a key capability required to
support deterministic latency and reliability for time-sensitive
flows. 802.11ax can operate in up to 160 MHz channels and it includes
support for operation in the new 6 GHz band, which is expected to be
open to unlicensed use by the FCC and other regulatory agencies
worldwide.
4.1.2.1.1. Multi-User OFDMA and Trigger-based Scheduled Access
802.11ax introduced a new orthogonal frequency-division multiple
access (OFDMA) mode in which multiple users can be scheduled across
the frequency domain. In this mode, the Access Point (AP) can
initiate multi-user (MU) Uplink (UL) transmissions in the same PHY
Protocol Data Unit (PPDU) by sending a trigger frame. This
centralized scheduling capability gives the AP much more control of
the channel, and it can remove contention between devices for uplink
transmissions, therefore reducing the randomness caused by CSMA-based
access between stations. The AP can also transmit simultaneously to
multiple users in the downlink direction by using a Downlink (DL) MU
OFDMA PPDU. In order to initiate a contention free Transmission
Opportunity (TXOP) using the OFDMA mode, the AP still follows the
typical listen before talk procedure to acquire the medium, which
ensures interoperability and compliance with unlicensed band access
rules. However, 802.11ax also includes a multi-user Enhanced
Distributed Channel Access (MU-EDCA) capability, which allows the AP
to get higher channel access priority.
4.1.2.1.2. Improved PHY Robustness
The 802.11ax PHY can operate with 0.8, 1.6 or 3.2 microsecond guard
interval (GI). The larger GI options provide better protection
against multipath, which is expected to be a challenge in industrial
environments. The possibility to operate with smaller resource units
(e.g. 2 MHz) enabled by OFDMA also helps reduce noise power and
improve SNR, leading to better packet error rate (PER) performance.
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802.11ax supports beamforming as in 802.11ac, but introduces UL MU
MIMO, which helps improve reliability. The UL MU MIMO capability is
also enabled by the trigger based access operation in 802.11ax.
4.1.2.1.3. Support for 6GHz band
The 802.11ax specification [IEEE80211ax] includes support for
operation in the new 6 GHz band. Given the amount of new spectrum
available as well as the fact that no legacy 802.11 device (prior
802.11ax) will be able to operate in this new band, 802.11ax
operation in this new band can be even more efficient.
4.1.2.2. Applicability to deterministic flows
TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide
the underlying mechanism for supporting deterministic flows in a
Local Area Network (LAN). The 802.11 working group has already
incorporated support for several TSN capabilities, so that time-
sensitive flow can experience precise time synchronization and
timeliness when operating over 802.11 links. TSN capabilities
supported over 802.11 (which also extends to 802.11ax), include:
1. 802.1AS based Time Synchronization (other time synchronization
techniques may also be used)
2. Interoperating with IEEE802.1Q bridges
3. Time-sensitive Traffic Stream identification
The exiting 802.11 TSN capabilities listed above, and the 802.11ax
OFDMA and scheduled access provide a new set of tools to better
server time-sensitive flows. However, it is important to understand
the tradeoffs and constraints associated with such capabilities, as
well as redundancy and diversity mechanisms that can be used to
provide more predictable and reliable performance.
4.1.2.2.1. 802.11 Managed network operation and admission control
Time-sensitive applications and TSN standards are expected to operate
under a managed network (e.g. industrial/enterprise network). Thus,
the Wi-Fi operation must also be carefully managed and integrated
with the overall TSN management framework, as defined in the IEEE
Std. 802.1Qcc specification [IEEE8021Qcc].
Some of the random-access latency and interference from legacy/
unmanaged devices can be minimized under a centralized management
mode as defined in IEEE Std. 802.1Qcc, in which admission control
procedures are enforced.
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Existing traffic stream identification, configuration and admission
control procedures defined in IEEE Std. 802.11 QoS mechanism can be
re-used. However, given the high degree of determinism required by
many time-sensitive applications, additional capabilities to manage
interference and legacy devices within tight time-constraints need to
be explored.
4.1.2.2.2. Scheduling for bounded latency and diversity
As discussed earlier, the 802.11ax OFDMA mode introduces the
possibility of assigning different RUs (frequency resources) to users
within a PPDU. Several RU sizes are defined in the specification
(26, 52, 106, 242, 484, 996 subcarriers). In addition, the AP can
also decide on MCS and grouping of users within a given OFMDA PPDU.
Such flexibility can be leveraged to support time-sensitive
applications with bounded latency, especially in a managed network
where stations can be configured to operate under the control of the
AP.
As shown in [Cavalcanti_2019], it is possible to achieve latencies in
the order of 1msec with high reliability in an interference free
environment. Obviously, there are latency, reliability and capacity
tradeoffs to be considered. For instance, smaller Resource Units
(RU)s result in longer transmission durations, which may impact the
minimal latency that can be achieved, but the contention latency and
randomness elimination due to multi-user transmission is a major
benefit of the OFDMA mode.
The flexibility to dynamically assign RUs to each transmission also
enables the AP to provide frequency diversity, which can help
increase reliability.
4.1.3. 802.11be Extreme High Throughput (EHT)
4.1.3.1. General Characteristics
The 802.11be is the next major 802.11 amendment (after 802.11ax) for
operation in the 2.4, 5 and 6 GHz bands. 802.11be is expected to
include new PHY and MAC features and it is targeting extremely high
throughput (at least 30 Gbps), as well as enhancements to worst case
latency and jitter. It is also expected to improve the integration
with 802.1 TSN to support time-sensitive applications over Ethernet
and Wireless LANs.
The 802.11be Task Group started its operation in May 2019, therefore,
detailed information about specific features is not yet available.
Only high level candidate features have been discussed so far,
including:
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1. 320MHz bandwidth and more efficient utilization of non-
contiguous spectrum.
2. Multi-band/multi-channel aggregation and operation.
3. 16 spatial streams and related MIMO enhancements.
4. Multi-Access Point (AP) Coordination.
5. Enhanced link adaptation and retransmission protocol, e.g.
Hybrid Automatic Repeat Request (HARQ).
6. Any required adaptations to regulatory rules for the 6 GHz
spectrum.
4.1.3.2. Applicability to deterministic flows
The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG)
provided detailed information on use cases, issues and potential
solution directions to improve support for time-sensitive
applications in 802.11. The RTA TIG report [IEEE_doc_11-18-2009-06]
was used as input to the 802.11be project scope.
Improvements for worst-case latency, jitter and reliability were the
main topics identified in the RTA report, which were motivated by
applications in gaming, industrial automation, robotics, etc. The
RTA report also highlighted the need to support additional TSN
capabilities, such as time-aware (802.1Qbv) shaping and packet
replication and elimination as defined in 802.1CB.
802.11be is expected to build on and enhance 802.11ax capabilities to
improve worst case latency and jitter. Some of the enhancement areas
are discussed next.
4.1.3.2.1. Enhanced scheduled operation for bounded latency
In addition to the throughput enhancements, 802.11be will leverage
the trigger-based scheduled operation enabled by 802.11ax to provide
efficient and more predictable medium access. 802.11be is expected to
include enhancements to reduce overhead and enable more efficient
operation in managed network deployments [IEEE_doc_11-19-0373-00].
4.1.3.2.2. Multi-AP coordination
Multi-AP coordination is one of the main new candidate features in
802.11be. It can provide benefits in throughput and capacity and has
the potential to address some of the issues that impact worst case
latency and reliability. Multi-AP coordination is expected to
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address the contention due to overlapping Basic Service Sets (OBSS),
which is one of the main sources of random latency variations.
802.11be can define methods to enable better coordination between
APs, for instance, in a managed network scenario, in order to reduce
latency due to unmanaged contention.
Several multi-AP coordination approaches have been discussed with
different levels of complexities and benefits, but specific
coordination methods have not yet been defined.
4.1.3.2.3. Multi-band operation
802.11be will introduce new features to improve operation over
multiple bands and channels. By leveraging multiple bands/channels,
802.11be can isolate time-sensitive traffic from network congestion,
one of the main causes of large latency variations. In a managed
802.11be network, it should be possible to steer traffic to certain
bands/channels to isolate time-sensitive traffic from other traffic
and help achieve bounded latency.
4.1.4. 802.11ad and 802.11ay (mmWave operation)
4.1.4.1. General Characteristics
The IEEE 802.11ad amendment defines PHY and MAC capabilities to
enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave)
band. The standard addresses the adverse mmWave signal propagation
characteristics and provides directional communication capabilities
that take advantage of beamforming to cope with increased
attenuation. An overview of the 802.11ad standard can be found in
[Nitsche_2015] .
The IEEE 802.11ay is currently developing enhancements to the
802.11ad standard to enable the next generation mmWave operation
targeting 100 Gbps throughput. Some of the main enhancements in
802.11ay include MIMO, channel bonding, improved channel access and
beamforming training. An overview of the 802.11ay capabilities can
be found in [Ghasempour_2017]
4.1.4.2. Applicability to deterministic flows
The high data rates achievable with 802.11ad and 802.11ay can
significantly reduce latency down to microsecond levels. Limited
interference from legacy and other unlicensed devices in 60 GHz is
also a benefit. However, directionality and short range typical in
mmWave operation impose new challenges such as the overhead required
for beam training and blockage issues, which impact both latency and
reliability. Therefore, it is important to understand the use case
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and deployment conditions in order to properly apply and configure
802.11ad/ay networks for time sensitive applications.
The 802.11ad standard include a scheduled access mode in which
stations can be allocated contention-free service periods by a
central controller. This scheduling capability is also available in
802.11ay, and it is one of the mechanisms that can be used to provide
bounded latency to time-sensitive data flows. An analysis of the
theoretical latency bounds that can be achieved with 802.11ad service
periods is provided in [Cavalcanti_2019].
4.2. IEEE 802.15.4
4.2.1. Provenance and Documents
The IEEE802.15.4 Task Group has been driving the development of low-
power low-cost radio technology. The IEEE802.15.4 physical layer has
been designed to support demanding low-power scenarios targeting the
use of unlicensed bands, both the 2.4 GHz and sub GHz Industrial,
Scientific and Medical (ISM) bands. This has imposed requirements in
terms of frame size, data rate and bandwidth to achieve reduced
collision probability, reduced packet error rate, and acceptable
range with limited transmission power. The PHY layer supports frames
of up to 127 bytes. The Medium Access Control (MAC) sublayer
overhead is in the order of 10-20 bytes, leaving about 100 bytes to
the upper layers. IEEE802.15.4 uses spread spectrum modulation such
as the Direct Sequence Spread Spectrum (DSSS).
The Timeslotted Channel Hopping (TSCH) mode was added to the 2015
revision of the IEEE802.15.4 standard [IEEE802154]. TSCH is targeted
at the embedded and industrial world, where reliability, energy
consumption and cost drive the application space.
At the IETF, the 6TiSCH Working Group (WG) [TiSCH] deals with best
effort operation of IPv6 [RFC8200] over TSCH. 6TiSCH has enabled
distributed scheduling to exploit the deterministic access
capabilities provided by TSCH. The group designed the essential
mechanisms to enable the management plane operation while ensuring
IPv6 is supported. Yet the charter did not focus to providing a
solution to establish end to end Tracks while meeting quality of
service requirements. 6TiSCH, through the RFC8480 [RFC8480] defines
the 6P protocol which provides a pairwise negotiation mechanism to
the control plane operation. The protocol supports agreement on a
schedule between neighbors, enabling distributed scheduling. 6P goes
hand-in-hand with a Scheduling Function (SF), the policy that decides
how to maintain cells and trigger 6P transactions. The Minimal
Scheduling Function (MSF) [I-D.ietf-6tisch-msf] is the default SF
defined by the 6TiSCH WG; other standardized SFs can be defined in
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the future. MSF extends the minimal schedule configuration, and is
used to add child-parent links according to the traffic load.
Time sensitive networking on low power constrained wireless networks
have been partially addressed by ISA100.11a [ISA100.11a] and
WirelessHART [WirelessHART]. Both technologies involve a central
controller that computes redundant paths for industrial process
control traffic over a TSCH mesh. Moreover, ISA100.11a introduces
IPv6 capabilities with a Link-Local Address for the join process and
a global unicast addres for later exchanges, but the IPv6 traffic
typically ends at a local application gateway and the full power of
IPv6 for end-to-end communication is not enabled. Compared to that
state of the art, work at the IETF and in particular at RAW could
provide additional techniques such as optimized P2P routing, PAREO
functions, and end-to-end secured IPv6/CoAP connectivity.
The 6TiSCH architecture [I-D.ietf-6tisch-architecture] identifies
different models to schedule resources along so-called Tracks (see
Section 4.2.2.2.2) exploiting the TSCH schedule structure however the
focus at 6TiSCH is on best effort traffic and the group was never
chartered to produce standard work related to Tracks.
Useful References include:
1. IEEE Std 802.15.4: "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"
[IEEE802154]. The latest version at the time of this writing is
dated year 2015.
2. Morell, A. , Vilajosana, X. , Vicario, J. L. and Watteyne, T.
(2013), Label switching over IEEE802.15.4e networks. Trans.
Emerging Tel. Tech., 24: 458-475. doi:10.1002/ett.2650"
[morell13].
3. De Armas, J., Tuset, P., Chang, T., Adelantado, F., Watteyne,
T., Vilajosana, X. (2016, September). Determinism through path
diversity: Why packet replication makes sense. In 2016
International Conference on Intelligent Networking and
Collaborative Systems (INCoS) (pp. 150-154). IEEE. [dearmas16].
4. X. Vilajosana, T. Watteyne, M. Vucinic, T. Chang and K. S.
J. Pister, "6TiSCH: Industrial Performance for IPv6 Internet-
of-Things Networks," in Proceedings of the IEEE, vol. 107, no.
6, pp. 1153-1165, June 2019. [vilajosana19].
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4.2.2. TimeSlotted Channel Hopping
4.2.2.1. General Characteristics
As a core technique in IEEE802.15.4, TSCH splits time in multiple
time slots that repeat over time. The structure is referred as a
Slotframe (see Section 4.2.2.2.1.4). For each timeslot, a set of
available frequencies can be used, resulting in a matrix-like
schedule (see Fig. Figure 1).
timeslot offset
| 0 1 2 3 4 | 0 1 2 3 4 | Nodes
+------------------------+------------------------+ +-----+
| | | | | | | | | | | | C |
CH-1 | EB | | |C->B| | EB | | |C->B| | | |
| | | | | | | | | | | +-----+
+-------------------------------------------------+ |
| | | | | | | | | | | |
CH-2 | | |B->C| |B->A| | |B->C| |B->A| +-----+
| | | | | | | | | | | | B |
+-------------------------------------------------+ | |
... +-----+
|
+-------------------------------------------------+ |
| | | | | | | | | | | +-----+
CH-15| |A->B| | | | |A->B| | | | | A |
| | | | | | | | | | | | |
+-------------------------------------------------+ +-----+
ch.
offset
Figure 1: Slotframe example with scheduled cells between nodes A, B
and C
This schedule represents the possible communications of a node with
its neighbors, and is managed by a Scheduling Function such as The
Minimal Scheduling Function (MSF) [I-D.ietf-6tisch-msf]. Each cell
in the schedule is identified by its slotoffset and channeloffset
coordinates. A cell's timeslot offset indicates its position in
time, relative to the beginning of the slotframe. A cell's channel
offset is an index which maps to a frequency at each iteration of the
slotframe. Each packet exchanged between neighbors happens within
one cell. An Absolute Slot Number (ASN) indicates the number of
slots elapsed since the network started. It increments at every
slot. This is a 5 byte counter that can support networks running for
more than 300 years without wrapping (assuming a 10 ms timeslot).
Channel hopping provides increased reliability to multi-path fading
and external interference. It is handled by TSCH through a channel
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hopping sequence referred as macHopSeq in the IEEE802.15.4
specification.
The Time-Frequency Division Multiple Access provided by TSCH enables
the orchestration of traffic flows, spreading them in time and
frequency, and hence enabling an efficient management of the
bandwidth utilization. Such efficient bandwidth utilization can be
combined to OFDM modulations also supported by the IEEE802.15.4
standard [IEEE802154] since the 2015 version.
In the RAW context, low power reliable networks should address non-
critical control scenarios such as Class 2 and monitoring scenarios
such as Class 4 defined by the RFC5673 [RFC5673]. As a low power
technology targeting industrial scenarios radio transducers provide
low data rates (typically between 50kbps to 250kbps) and robust
modulations to trade-off performance to reliability. TSCH networks
are organized in mesh topologies and connected to a backbone.
Latency in the mesh network is mainly influenced by propagation
aspects such as interference. ARQ methods and redundancy techniques
such as replication and elimination should be studied to provide the
needed performance to address deterministic scenarios.
4.2.2.2. Applicability to Deterministic Flows
Nodes in a TSCH network are tightly synchronized. This enables to
build the slotted structure and ensure efficient utilization of
resources thanks to proper scheduling policies. Scheduling is a key
to orchestrate the resources that different nodes in a Track or a
path are using. Slotframes can be split in resource blocks reserving
the needed capacity to certain flows. Periodic and bursty traffic
can be handled independently in the schedule, using active and
reactive policies and taking advantage of overprovisionned cells to
measu reth excursion. Along a Track, resource blocks can be chained
so nodes in previous hops transmit their data before the next packet
comes. This provides a tight control to latency along a Track.
Collision loss is avoided for best effort traffic by
overprovisionning resources, giving time to the management plane of
the network to dedicate more resources if needed.
4.2.2.2.1. Centralized Path Computation
In a controlled environment, a 6TiSCH device usually does not place a
request for bandwidth between itself and another device in the
network. Rather, an Operation Control System (OCS) invoked through
an Human/Machine Interface (HMI) iprovides the Traffic Specification,
in particular in terms of latency and reliability, and the end nodes,
to a PCE. With this, the PCE computes a Track between the end nodes
and provisions every hop in the Track with per-flow state that
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describes the per-hop operation for a given packet, the corresponding
timeSlots, and the flow identification to recognize which packet is
placed in which Track, sort out duplicates, etc...
For a static configuration that serves a certain purpose for a long
period of time, it is expected that a node will be provisioned in one
shot with a full schedule, which incorporates the aggregation of its
behavior for multiple Tracks. The 6TiSCH Architecture expects that
the programing of the schedule is done over COAP as discussed in
"6TiSCH Resource Management and Interaction using CoAP"
[I-D.ietf-6tisch-coap].
But an Hybrid mode may be required as well whereby a single Track is
added, modified, or removed, for instance if it appears that a Track
does not perform as expected for, say, PDR. For that case, the
expectation is that a protocol that flows along a Track (to be), in a
fashion similar to classical Traffic Engineering (TE) [CCAMP], may be
used to update the state in the devices. 6TiSCH provides means for a
device to negotiate a timeSlot with a neighbor, but in general that
flow was not designed and no protocol was selected and it is expected
that DetNet will determine the appropriate end-to-end protocols to be
used in that case.
Stream Management Entity
Operational System and HMI
-+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
PCE PCE PCE PCE
-+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
--- 6TiSCH------6TiSCH------6TiSCH------6TiSCH--
6TiSCH / Device Device Device Device \
Device- - 6TiSCH
\ 6TiSCH 6TiSCH 6TiSCH 6TiSCH / Device
----Device------Device------Device------Device--
Figure 2
4.2.2.2.1.1. Packet Marking and Handling
Section "Packet Marking and Handling" of
[I-D.ietf-6tisch-architecture] describes the packet tagging and
marking that is expected in 6TiSCH networks.
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4.2.2.2.1.1.1. Tagging Packets for Flow Identification
For packets that are routed by a PCE along a Track, the tuple formed
by the IPv6 source address and a local RPLInstanceID is tagged in the
packets to identify uniquely the Track and associated transmit bundle
of timeSlots.
It results that the tagging that is used for a DetNet flow outside
the 6TiSCH LLN MUST be swapped into 6TiSCH formats and back as the
packet enters and then leaves the 6TiSCH network.
Note: The method and format used for encoding the RPLInstanceID at
6lo is generalized to all 6TiSCH topological Instances, which
includes Tracks.
4.2.2.2.1.1.2. Replication, Retries and Elimination
6TiSCH expects elimination and replication of packets along a complex
Track, but has no position about how the sequence numbers would be
tagged in the packet.
As it goes, 6TiSCH expects that timeSlots corresponding to copies of
a same packet along a Track are correlated by configuration, and does
not need to process the sequence numbers.
The semantics of the configuration MUST enable correlated timeSlots
to be grouped for transmit (and respectively receive) with a 'OR'
relations, and then a 'AND' relation MUST be configurable between
groups. The semantics is that if the transmit (and respectively
receive) operation succeeded in one timeSlot in a 'OR' group, then
all the other timeSLots in the group are ignored. Now, if there are
at least two groups, the 'AND' relation between the groups indicates
that one operation must succeed in each of the groups.
On the transmit side, timeSlots provisioned for retries along a same
branch of a Track are placed a same 'OR' group. The 'OR' relation
indicates that if a transmission is acknowledged, then further
transmissions SHOULD NOT be attempted for timeSlots in that group.
There are as many 'OR' groups as there are branches of the Track
departing from this node. Different 'OR' groups are programmed for
the purpose of replication, each group corresponding to one branch of
the Track. The 'AND' relation between the groups indicates that
transmission over any of branches MUST be attempted regardless of
whether a transmission succeeded in another branch. It is also
possible to place cells to different next-hop routers in a same 'OR'
group. This allows to route along multi-path Tracks, trying one
next-hop and then another only if sending to the first fails.
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On the receive side, all timeSlots are programmed in a same 'OR'
group. Retries of a same copy as well as converging branches for
elimination are converged, meaning that the first successful
reception is enough and that all the other timeSlots can be ignored.
4.2.2.2.1.1.3. Differentiated Services Per-Hop-Behavior
Additionally, an IP packet that is sent along a Track uses the
Differentiated Services Per-Hop-Behavior Group called Deterministic
Forwarding, as described in
[I-D.svshah-tsvwg-deterministic-forwarding].
4.2.2.2.1.2. Topology and capabilities
6TiSCH nodes are usually IoT devices, characterized by very limited
amount of memory, just enough buffers to store one or a few IPv6
packets, and limited bandwidth between peers. It results that a node
will maintain only a small number of peering information, and will
not be able to store many packets waiting to be forwarded. Peers can
be identified through MAC or IPv6 addresses.
Neighbors can be discovered over the radio using mechanism such as
beacons, but, though the neighbor information is available in the
6TiSCH interface data model, 6TiSCH does not describe a protocol to
pro-actively push the neighborhood information to a PCE. This
protocol should be described and should operate over CoAP. The
protocol should be able to carry multiple metrics, in particular the
same metrics as used for RPL operations [RFC6551]
The energy that the device consumes in sleep, transmit and receive
modes can be evaluated and reported. So can the amount of energy
that is stored in the device and the power that it can be scavenged
from the environment. The PCE SHOULD be able to compute Tracks that
will implement policies on how the energy is consumed, for instance
balance between nodes, ensure that the spent energy does not exceeded
the scavenged energy over a period of time, etc...
4.2.2.2.1.3. Schedule Management by a PCE
6TiSCH supports a mixed model of centralized routes and distributed
routes. Centralized routes can for example be computed by a entity
such as a Path Computation element (PCE) [PCE]. Distributed routes
are computed by RPL [RFC6550].
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
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topology is indexed by a Instance ID, in a format that reuses the
RPLInstanceID as defined in RPL.
Both RPL and PCE rely on shared sources such as policies to define
Global and Local RPLInstanceIDs that can be used by either method.
It is possible for centralized and distributed routing to share a
same topology. Generally they will operate in different slotFrames,
and centralized routes will be used for scheduled traffic and will
have precedence over distributed routes in case of conflict between
the slotFrames.
4.2.2.2.1.4. SlotFrames and Priorities
A slotFrame is the base object that a PCE needs to manipulate to
program a schedule into an LLN node. Elaboration on that concept can
be fond in section "SlotFrames and Priorities" of
[I-D.ietf-6tisch-architecture]
IEEE802.15.4 TSCH avoids contention on the medium by formatting time
and frequencies in cells of transmission of equal duration. In order
to describe that formatting of time and frequencies, the 6TiSCH
architecture defines a global concept that is called a Channel
Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of
cells with an height equal to the number of available channels
(indexed by ChannelOffsets) and a width (in timeSlots) that is the
period of the network scheduling operation (indexed by slotOffsets)
for that CDU matrix. The size of a cell is a timeSlot duration, and
values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to
accommodate for the transmission of a frame and an ack, including the
security validation on the receive side which may take up to a few
milliseconds on some device architecture.
The frequency used by a cell in the matrix rotates in a pseudo-random
fashion, from an initial position at an epoch time, as the matrix
iterates over and over.
A CDU matrix is computed by the PCE, but unallocated timeSlots may be
used opportunistically by the nodes for classical best effort IP
traffic. The PCE has precedence in the allocation in case of a
conflict.
In a given network, there might be multiple CDU matrices that operate
with different width, so they have different durations and represent
different periodic operations. It is recommended that all CDU
matrices in a 6TiSCH domain operate with the same cell duration and
are aligned, so as to reduce the chances of interferences from
slotted-aloha operations. The PCE MUST compute the CDU matrices and
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shared that knowledge with all the nodes. The matrices are used in
particular to define slotFrames.
A slotFrame is a MAC-level abstraction that is common to all nodes
and contains a series of timeSlots of equal length and precedence.
It is characterized by a slotFrame_ID, and a slotFrame_size. A
slotFrame aligns to a CDU matrix for its parameters, such as number
and duration of timeSlots.
Multiple slotFrames can coexist in a node schedule, i.e., a node can
have multiple activities scheduled in different slotFrames, based on
the precedence of the 6TiSCH topologies. The slotFrames may be
aligned to different CDU matrices and thus have different width.
There is typically one slotFrame for scheduled traffic that has the
highest precedence and one or more slotFrame(s) for RPL traffic. The
timeSlots in the slotFrame are indexed by the SlotOffset; the first
cell is at SlotOffset 0.
The 6TiSCH architecture introduces the concept of chunks
([I-D.ietf-6tisch-architecture]) to operate such spectrum
distribution for a whole group of cells at a time. The CDU matrix is
formatted into a set of chunks, each of them identified uniquely by a
chunk-ID. The PCE MUST compute the partitioning of CDU matrices into
chunks and shared that knowledge with all the nodes in a 6TiSCH
network.
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
...
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
0 1 2 3 4 5 6 M
Figure 3: CDU matrix Partitioning in Chunks
The appropriation of a chunk can be requested explicitly by the PCE
to any node. After a successful appropriation, the PCE owns the
cells in that chunk, and may use them as hard cells to set up Tracks.
Then again, 6TiSCH did not propose a method for chunk definition and
a protocol for appropriation. This is to be done at PAW.
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4.2.2.2.2. 6TiSCH Tracks
A Track at 6TiSCH is the application to wireless of the concept of a
path in the Detnet architecture [I-D.ietf-detnet-architecture]. A
Track can follow a simple sequence of relay nodes or can be
structured as a more complex destination oriented directed acyclic
graph (DODAG) to a unicast destination. Along a Track, 6TiSCH nodes
reserve the resources to enable the efficient transmission of packets
while aiming to optimize certain properties such as reliability and
ensure small jitter or bounded latency. The Track structure enables
Layer-2 forwarding schemes, reducing the overhead of taking routing
decisions at the Layer-3.
Serial Tracks can be understood as the concatenation of cells or
bundles along a routing path from a node towards a destination. The
serial Track concept is analogous to the circuit concept where
resources are chained through the multi-hop topology. For example, A
bundle of Tx Cells in a particular node is paired to a bundle of Rx
Cells in the next hop node following a routing path.
whereas scheduling ensures reliable delivery in bounded time along
any Track, high availability requires the application of PAREO
functions along a more complex DODAG Track structure. A DODAG has
forking and joining nodes where the concepts such as Replication and
Elimination can be exploited. Spatial redundancy increases the
oveall energy consumption in the network but improves significantly
the availability of the network as well as the packet delivery ratio.
A Track may also branch off and rejoin, for the purpose of the so-
called Packet Replication and Elimination (PRE), over non congruent
branches. PRE may be used to complement layer-2 Automatic Repeat
reQuest (ARQ) and and receiver-end Ordering to form the PAREO
functions. PAREO functions enable to meet industrial expectations in
Packet Delivery Ratio (PDR) within bounded delivery time over a Track
that includes wireless links, even when the Track extends beyond the
6TiSCH network.
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+-----+
| IoT |
| G/W |
+-----+
^ <---- Elimination
| |
Track branch | |
+-------+ +--------+ Subnet Backbone
| |
+--|--+ +--|--+
| | | Backbone | | | Backbone
o | | | router | | | router
+--/--+ +--|--+
o / o o---o----/ o
o o---o--/ o o o o o
o \ / o o LLN o
o v <---- Replication
o
Figure 4: End-to-End deterministic Track
In the example above, a Track is laid out from a field device in a
6TiSCH network to an IoT gateway that is located on a IEEE802.1 TSN
backbone.
The Replication function in the field device sends a copy of each
packet over two different branches, and a PCE schedules each hop of
both branches so that the two copies arrive in due time at the
gateway. In case of a loss on one branch, hopefully the other copy
of the packet still makes it in due time. If two copies make it to
the IoT gateway, the Elimination function in the gateway ignores the
extra packet and presents only one copy to upper layers.
At each 6TiSCH hop along the Track, the PCE may schedule more than
one timeSlot for a packet, so as to support Layer-2 retries (ARQ).
It is also possible that the field device only uses the second branch
if sending over the first branch fails.
In current deployments, a TSCH Track does not necessarily support PRE
but is systematically multi-path. This means that a Track is
scheduled so as to ensure that each hop has at least two forwarding
solutions, and the forwarding decision is to try the preferred one
and use the other in case of Layer-2 transmission failure as detected
by ARQ.
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Methods to implement complex Tracks are described in
[I-D.papadopoulos-paw-pre-reqs] and complemented by extensions to the
RPL routing protocol in [I-D.ietf-roll-nsa-extension] for best effort
traffic, but a centralized routing technique such as promoted in
DetNet is still missing.
4.2.2.2.2.1. Track Scheduling Protocol
Section "Schedule Management Mechanisms" of the 6TiSCH architecture
describes 4 paradigms to manage the TSCH schedule of the LLN nodes:
Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring
and scheduling management, and Hop-by-hop scheduling. The Track
operation for DetNet corresponds to a remote monitoring and
scheduling management by a PCE.
Early work at 6TiSCH on a data model and a protocol to program the
schedule in the 6TiSCH device was never concluded as the group
focussed on best effort traffic. This work would be revived by PAW:
The 6top interface document [RFC8480] (to be reopened at PAW) was
intended to specify the generic data model that can be used to
monitor and manage resources of the 6top sublayer. Abstract
methods were suggested for use by a management entity in the
device. The data model also enables remote control operations on
the 6top sublayer.
[I-D.ietf-6tisch-coap] (to be reopened at RAW) was intended to
define a mapping of the 6top set of commands, which is described
in RFC 8480, to CoAP resources. This allows an entity to interact
with the 6top layer of a node that is multiple hops away in a
RESTful fashion.
[I-D.ietf-6tisch-coap] also defined a basic set CoAP resources and
associated RESTful access methods (GET/PUT/POST/DELETE). The
payload (body) of the CoAP messages is encoded using the CBOR
format. The PCE commands are expected to be issued directly as
CoAP requests or to be mapped back and forth into CoAP by a
gateway function at the edge of the 6TiSCH network. For instance,
it is possible that a mapping entity on the backbone transforms a
non-CoAP protocol such as PCEP into the RESTful interfaces that
the 6TiSCH devices support.
4.2.2.2.2.2. Track Forwarding
By forwarding, this specification means the per-packet operation that
allows to deliver a packet to a next hop or an upper layer in this
node. Forwarding is based on pre-existing state that was installed
as a result of the routing computation of a Track by a PCE. The
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6TiSCH architecture supports three different forwarding model, G-MPLS
Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6
Forwarding (6F) which is the classical IP operation. The DetNet case
relates to the Track Forwarding operation under the control of a PCE.
A Track is a unidirectional path between a source and a destination.
In a Track cell, the normal operation of IEEE802.15.4 Automatic
Repeat-reQuest (ARQ) usually happens, though the acknowledgment may
be omitted in some cases, for instance if there is no scheduled cell
for a retry.
Track Forwarding is the simplest and fastest. A bundle of cells set
to receive (RX-cells) is uniquely paired to a bundle of cells that
are set to transmit (TX-cells), representing a layer-2 forwarding
state that can be used regardless of the network layer protocol.
This model can effectively be seen as a Generalized Multi-protocol
Label Switching (G-MPLS) operation in that the information used to
switch a frame is not an explicit label, but rather related to other
properties of the way the packet was received, a particular cell in
the case of 6TiSCH. As a result, as long as the TSCH MAC (and
Layer-2 security) accepts a frame, that frame can be switched
regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN
fragment, or a frame from an alternate protocol such as WirelessHART
or ISA100.11a.
A data frame that is forwarded along a Track normally has a
destination MAC address that is set to broadcast - or a multicast
address depending on MAC support. This way, the MAC layer in the
intermediate nodes accepts the incoming frame and 6top switches it
without incurring a change in the MAC header. In the case of
IEEE802.15.4, this means effectively broadcast, so that along the
Track the short address for the destination of the frame is set to
0xFFFF.
A Track is thus formed end-to-end as a succession of paired bundles,
a receive bundle from the previous hop and a transmit bundle to the
next hop along the Track, and a cell in such a bundle belongs to at
most one Track. For a given iteration of the device schedule, the
effective channel of the cell is obtained by adding a pseudo-random
number to the channelOffset of the cell, which results in a rotation
of the frequency that used for transmission. The bundles may be
computed so as to accommodate both variable rates and
retransmissions, so they might not be fully used at a given iteration
of the schedule. The 6TiSCH architecture provides additional means
to avoid waste of cells as well as overflows in the transmit bundle,
as follows:
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In one hand, a TX-cell that is not needed for the current iteration
may be reused opportunistically on a per-hop basis for routed
packets. When all of the frame that were received for a given Track
are effectively transmitted, any available TX-cell for that Track can
be reused for upper layer traffic for which the next-hop router
matches the next hop along the Track. In that case, the cell that is
being used is effectively a TX-cell from the Track, but the short
address for the destination is that of the next-hop router. It
results that a frame that is received in a RX-cell of a Track with a
destination MAC address set to this node as opposed to broadcast must
be extracted from the Track and delivered to the upper layer (a frame
with an unrecognized MAC address is dropped at the lower MAC layer
and thus is not received at the 6top sublayer).
On the other hand, it might happen that there are not enough TX-cells
in the transmit bundle to accommodate the Track traffic, for instance
if more retransmissions are needed than provisioned. In that case,
the frame can be placed for transmission in the bundle that is used
for layer-3 traffic towards the next hop along the Track as long as
it can be routed by the upper layer, that is, typically, if the frame
transports an IPv6 packet. The MAC address should be set to the
next-hop MAC address to avoid confusion. It results that a frame
that is received over a layer-3 bundle may be in fact associated to a
Track. In a classical IP link such as an Ethernet, off-Track traffic
is typically in excess over reservation to be routed along the non-
reserved path based on its QoS setting. But with 6TiSCH, since the
use of the layer-3 bundle may be due to transmission failures, it
makes sense for the receiver to recognize a frame that should be re-
Tracked, and to place it back on the appropriate bundle if possible.
A frame should be re-Tracked if the Per-Hop-Behavior group indicated
in the Differentiated Services Field in the IPv6 header is set to
Deterministic Forwarding, as discussed in Section 4.2.2.2.1.1. A
frame is re-Tracked by scheduling it for transmission over the
transmit bundle associated to the Track, with the destination MAC
address set to broadcast.
There are 2 modes for a Track, transport mode and tunnel mode.
4.2.2.2.2.2.1. 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
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before restoring the destination MAC address (DMAC) and punting to
the upper layer.
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | |
+--------------+ ingress egress
| 6top | sets +----+ +----+ restores
+--------------+ dmac to | | | | dmac to
| TSCH MAC | brdcst | | | | self
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
Track Forwarding, Transport Mode
4.2.2.2.2.2.2. Tunnel Mode
In tunnel mode, the frames originate from an arbitrary protocol over
a compatible MAC that may or may not be synchronized with the 6TiSCH
network. An example of this would be a router with a dual radio that
is capable of receiving and sending WirelessHART or ISA100.11a frames
with the second radio, by presenting itself as an Access Point or a
Backbone Router, respectively.
In that mode, some entity (e.g. PCE) can coordinate with a
WirelessHART Network Manager or an ISA100.11a System Manager to
specify the flows that are to be transported transparently over the
Track.
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+--------------+
| IPv6 |
+--------------+
| 6LoWPAN HC |
+--------------+ set restore
| 6top | +dmac+ +dmac+
+--------------+ to|brdcst to|nexthop
| TSCH MAC | | | | |
+--------------+ | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+ | ingress egress |
| |
+--------------+ | |
| LLN PHY | | |
+--------------+ | |
| TSCH MAC | | |
+--------------+ | dmac = | dmac =
|ISA100/WiHART | | nexthop v nexthop
+--------------+
Figure 5: Track Forwarding, Tunnel Mode
In that case, the flow information that identifies the Track at the
ingress 6TiSCH router is derived from the RX-cell. The dmac is set
to this node but the flow information indicates that the frame must
be tunneled over a particular Track so the frame is not passed to the
upper layer. Instead, the dmac is forced to broadcast and the frame
is passed to the 6top sublayer for switching.
At the egress 6TiSCH router, the reverse operation occurs. Based on
metadata associated to the Track, the frame is passed to the
appropriate link layer with the destination MAC restored.
4.2.2.2.2.2.3. Tunnel Metadata
Metadata coming with the Track configuration is expected to provide
the destination MAC address of the egress endpoint as well as the
tunnel mode and specific data depending on the mode, for instance a
service access point for frame delivery at egress. If the tunnel
egress point does not have a MAC address that matches the
configuration, the Track installation fails.
In transport mode, if the final layer-3 destination is the tunnel
termination, then it is possible that the IPv6 address of the
destination is compressed at the 6LoWPAN sublayer based on the MAC
address. It is thus mandatory at the ingress point to validate that
the MAC address that was used at the 6LoWPAN sublayer for compression
matches that of the tunnel egress point. For that reason, the node
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that injects a packet on a Track checks that the destination is
effectively that of the tunnel egress point before it overwrites it
to broadcast. The 6top sublayer at the tunnel egress point reverts
that operation to the MAC address obtained from the tunnel metadata.
4.2.2.2.2.2.4. OAM
An Overview of Operations, Administration, and Maintenance (OAM)
Tools [RFC7276] provides an overwiew of the existing tooling for OAM
[RFC6291]. Tracks are complex paths and new tooling is necessary to
manage them, with respect to load control, timing, and the Packet
Replication and Elimination Functions (PREF).
An example of such tooling can be found in the context of BIER
[RFC8279] and more specifically BIER Traffic Engineering
[I-D.ietf-bier-te-arch] (BIER-TE):
[I-D.thubert-bier-replication-elimination] leverages BIER-TE to
control the process of PREF, and to provide traceability of these
operations, in the deterministic dataplane, along a complex Track.
For the 6TiSCH type of constrained environment,
[I-D.thubert-6lo-bier-dispatch] enables an efficient encoding of the
BIER bitmap within the 6LoRH framework.
5. 3GPP standards
6. IANA Considerations
This specification does not require IANA action.
7. Security Considerations
Most RAW technologies integrate some authentication or encryption
mechanisms that were defined outside the IETF.
8. Acknowledgments
Many thanks to the participants of the RAW WG where a lot of the work
discussed here happened.
9. References
9.1. Normative References
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-21 (work
in progress), June 2019.
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[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-13 (work in progress), May 2019.
[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-Power and
Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
2009, <https://www.rfc-editor.org/info/rfc5673>.
[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>.
9.2. Informative References
[Cavalcanti_2019]
Dave Cavalcanti et al., "Extending Time Distribution and
Timeliness Capabilities over the Air to Enable Future
Wireless Industrial Automation Systems, the Proceedings of
IEEE", June 2019.
[CCAMP] IETF, "Common Control and Measurement Plane",
<https://dataTracker.ietf.org/doc/charter-ietf-ccamp/>.
[dearmas16]
Jesica de Armas et al., "Determinism through path
diversity: Why packet replication makes sense", September
2016.
[Ghasempour_2017]
Yasaman Ghasempour et al., "802.11ay: Next-Generation 60
GHz Communications for 100 Gb/s Wi-Fi", December 2017.
[I-D.ietf-6tisch-coap]
Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
Interaction using CoAP", draft-ietf-6tisch-coap-03 (work
in progress), March 2015.
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[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-03 (work in progress), April 2019.
[I-D.ietf-bier-te-arch]
Eckert, T., Cauchie, G., Braun, W., and M. Menth, "Traffic
Engineering for Bit Index Explicit Replication (BIER-TE)",
draft-ietf-bier-te-arch-02 (work in progress), May 2019.
[I-D.ietf-roll-nsa-extension]
Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P.
Thubert, "RPL DAG Metric Container Node State and
Attribute object type extension", draft-ietf-roll-nsa-
extension-01 (work in progress), March 2019.
[I-D.papadopoulos-paw-pre-reqs]
Papadopoulos, G., Koutsiamanis, R., Montavont, N., and P.
Thubert, "Exploiting Packet Replication and Elimination in
Complex Tracks in LLNs", draft-papadopoulos-paw-pre-
reqs-01 (work in progress), March 2019.
[I-D.svshah-tsvwg-deterministic-forwarding]
Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
draft-svshah-tsvwg-deterministic-forwarding-04 (work in
progress), August 2015.
[I-D.thubert-6lo-bier-dispatch]
Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-06
(work in progress), January 2019.
[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.
[IEEE80211]
"IEEE Standard 802.11 - IEEE Standard for Information
Technology - Telecommunications and information exchange
between systems Local and metropolitan area networks -
Specific requirements - Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY)
Specifications.".
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[IEEE80211ad]
"802.11ad: Enhancements for very high throughput in the 60
GHz band".
[IEEE80211ak]
"802.11ak: Enhancements for Transit Links Within Bridged
Networks", 2017.
[IEEE80211ax]
"802.11ax D4.0: Enhancements for High Efficiency WLAN".
[IEEE80211ay]
"802.11ay: Enhanced throughput for operation in license-
exempt bands above 45 GHz".
[IEEE80211be]
"802.11be: Extreme High Throughput".
[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".
[IEEE8021Qat]
"802.1Qat: Stream Reservation Protocol".
[IEEE8021Qcc]
"802.1Qcc: IEEE Standard for Local and Metropolitan Area
Networks--Bridges and Bridged Networks -- Amendment 31:
Stream Reservation Protocol (SRP) Enhancements and
Performance Improvements".
[IEEE_doc_11-18-2009-06]
"802.11 Real-Time Applications (RTA) Topic Interest Group
(TIG) Report", November 2018.
[IEEE_doc_11-19-0373-00]
Kevin Stanton et Al., "Time-Sensitive Applications Support
in EHT", March 2019.
[ISA100.11a]
ISA/IEC, "ISA100.11a, Wireless Systems for Automation,
also IEC 62734", 2011, < http://www.isa100wci.org/en-
US/Documents/PDF/3405-ISA100-WirelessSystems-Future-broch-
WEB-ETSI.aspx>.
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[morell13]
Antoni Morell et al., "Label switching over IEEE802.15.4e
networks", April 2013.
[Nitsche_2015]
Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz
communication for multi-Gigabit-per-second Wi-Fi",
December 2014.
[PCE] IETF, "Path Computation Element",
<https://dataTracker.ietf.org/doc/charter-ietf-pce/>.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291,
DOI 10.17487/RFC6291, June 2011,
<https://www.rfc-editor.org/info/rfc6291>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<https://www.rfc-editor.org/info/rfc6551>.
[RFC7276] Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
Weingarten, "An Overview of Operations, Administration,
and Maintenance (OAM) Tools", RFC 7276,
DOI 10.17487/RFC7276, June 2014,
<https://www.rfc-editor.org/info/rfc7276>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[TiSCH] IETF, "IPv6 over the TSCH mode over 802.15.4",
<https://dataTracker.ietf.org/doc/charter-ietf-6tisch/>.
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[vilajosana19]
Xavier Vilajosana et al., "6TiSCH: Industrial Performance
for IPv6 Internet-of-Things Networks", June 2019.
[WirelessHART]
www.hartcomm.org, "Industrial Communication Networks -
Wireless Communication Network and Communication Profiles
- WirelessHART - IEC 62591", 2010.
Authors' Addresses
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
Dave Cavalcanti
Intel Corporation
2111 NE 25th Ave
Hillsboro, OR 97124
USA
Phone: 503 712 5566
Email: dave.cavalcanti@intel.com
Xavier Vilajosana
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Email: xvilajosana@uoc.edu
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