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Versions: 00 01 02 03 04 05                                             
RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                             D. Cavalcanti
Expires: December 8, 2019                                          Intel
                                                           X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                            June 6, 2019

              Reliable and Available Wireless Technologies


   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

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
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   This Internet-Draft will expire on December 8, 2019.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  On Scheduling . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Benefits of Scheduling on Wires . . . . . . . . . . . . .   4
     3.2.  Benefits of Scheduling on Wireless  . . . . . . . . . . .   4
   4.  IEEE 802 standards  . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . .   5
       4.1.1.  Provenance and Documents  . . . . . . . . . . . . . .   5
       4.1.2.  802.11ax High Efficiency (HE) . . . . . . . . . . . .   7
       4.1.3.  802.11be Extreme High Throughput (EHT)  . . . . . . .  10
       4.1.4.  802.11ad and 802.11ay (mmWave operation)  . . . . . .  11
     4.2.  IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . .  12
       4.2.1.  Provenance and Documents  . . . . . . . . . . . . . .  12
       4.2.2.  TimeSlotted Channel Hopping . . . . . . . . . . . . .  14
   5.  3GPP standards  . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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

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   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.

   Reliable:  That consistently performs as expected, the expectation
         for a network being to always deliver a packet in due time.

   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.

   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.

   Track:  A DODAG oriented to a destination, and that enables Packet
         ARQ, Replication, Elimination, and Ordering Functions.

   ARQ:  Automatic Repeat Request, enabling an acknowledged
         transmission, which is the typical model at Layer-2 on a
         wireless medium.

   HARQ: Forward error correction, sending redundant coded data to help
         the receiver recover transmission errors.

   HARQ: Hybrid ARQ, a combination of FEC and ARQ.

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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

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
   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

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   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

   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.

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

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   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
   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

   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

   Congestion Control:  IEEE802.11-2016 Admission Control; WFA Admission

   Security:  WFA Wi-Fi Protected Access, WPA2 and WPA3.

   Interoperating with IEEE802.1Q bridges:  IEEE802.11ak.

   Stream Reservation Protocol (part of IEEE802.1Qat):

       Scheduled channel access:  IEEE802.11ad Enhancements for very
              high throughput in the 60 GHz band [IEEE80211ad].

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       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

   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.

4.1.2.  802.11ax High Efficiency (HE)  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.  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

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   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.  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.

   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.  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.  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

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   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.  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.

   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.  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

   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.

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   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)  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,

   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.  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

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   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.  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].  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
   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.  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)  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

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   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]  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
   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 Timeslotted Channel Hopping
   mode, added to the 2015 revision of the IEEE802.15.4 standard
   [IEEE802154], is targeted at the embedded and industrial world, where
   reliability, energy consumption and cost drive the application space.

   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

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   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).

   IPv6 over TSCH is enabled by the work done at the 6TiSCH WG. 6TiSCH
   has enabled best effort 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 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 addressed by ISA100.11a and WirelessHART.  TODO

   The 6TiSCH architecture [I-D.ietf-6tisch-architecture] already
   identified different models to schedule resources along tracks
   exploiting the TSCH schedule structure however these models have not
   been standardized.  A Track, in the 6TiSCH architecture is considered
   a directed path from a source 6TiSCH node to one or more
   destination(s) 6TiSCH node(s) through the 6TiSCH network.  A Track in
   6TiSCH is the implementation of the deterministic path in the Detnet
   architecture [I-D.ietf-detnet-architecture] .  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.  More complex approaches are described
   in and complemented by extensions to the RPL routing protocol in
   [I-D.ietf-roll-nsa-extension].  Reliability measures are for example

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   achieved by exploiting concepts such as Replication and Elimination.
   In them, packets at origin are replicated and transmitted along
   disjoint tracks.  This redundancy measure exploiting track forwarding
   increases energy consumption of the network nodes but improves
   significantly the reliability of the network.

   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"

   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].

4.2.2.  TimeSlotted Channel Hopping  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.  For each timeslot, a set of available frequencies can be
   used, resulting in a matrix-like schedule (see Fig. Figure 1).

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                          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  |
        |    |    |    |    |    |    |    |    |    |    |   |     |
        +-------------------------------------------------+   +-----+

    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
   hopping sequence referred as macHopSeq in the IEEE802.15.4

   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.

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   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.  Applicability to Deterministic Flows

   Nodes in a TSCH network are tightly synchronized.  This enables to
   build the slotted structure an ensure efficient utilization of
   resources thranks to proper scheduling policies.  Scheduling is a key
   to orchestrate the resources that different nodes in a track or path
   are using.  Slotframes can be split in resource blocks reserving the
   needed capacity to certain needs.  Periodic and bursty traffic can be
   handled independently in the schedule, using active and reactive
   policies and taking advantage of certain cell overprovision.  Along a
   track, resource blocks can be chained so nodes in previous hops
   transmit their data before those that come later.  This provides a
   tight control to latency along a track.  Redundancy is achieved in a
   best effort manner by overprovision, giving time to the management
   plane of the network to request more resources if needed.  -time
   synchronization - scheduling capabilities, discuss such things as
   Resource Units, time slots or resource blocks.  Can we reserve
   periodic resources vs. ask each time, what precision can we get in
   latency control.  - diversity scenarios, what's available, - gap
   analysis, e.g. discuss multihop, or what's missing how to do PAREO

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.

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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

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
              in progress), March 2019.

              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,

   [RFC8480]  Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Protocol (6P)", RFC 8480,
              DOI 10.17487/RFC8480, November 2018,

9.2.  Informative References

              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.

              Jesica de Armas et al., "Determinism through path
              diversity: Why packet replication makes sense", September

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              Yasaman Ghasempour et al., "802.11ay: Next-Generation 60
              GHz Communications for 100 Gb/s Wi-Fi", December 2017.

              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.

              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.

              "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)

              "802.11ad: Enhancements for very high throughput in the 60
              GHz band".

              "802.11ak: Enhancements for Transit Links Within Bridged
              Networks", 2017.

              "802.11ax D4.0: Enhancements for High Efficiency WLAN".

              "802.11ay: Enhanced throughput for operation in license-
              exempt bands above 45 GHz".

              "802.11be: Extreme High Throughput".

              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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              "802.1Qat: Stream Reservation Protocol".

              "802.1Qcc: IEEE Standard for Local and Metropolitan Area
              Networks--Bridges and Bridged Networks -- Amendment 31:
              Stream Reservation Protocol (SRP) Enhancements and
              Performance Improvements".

              "802.11 Real-Time Applications (RTA) Topic Interest Group
              (TIG) Report", November 2018.

              Kevin Stanton et Al., "Time-Sensitive Applications Support
              in EHT", March 2019.

              Antoni Morell et al., "Label switching over IEEE802.15.4e
              networks", April 2013.

              Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz
              communication for multi-Gigabit-per-second Wi-Fi",
              December 2014.

              Xavier Vilajosana et al., "6TiSCH: Industrial Performance
              for IPv6 Internet-of-Things Networks", June 2019.

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

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   Dave Cavalcanti
   Intel Corporation
   2111 NE 25th Ave
   Hillsboro, OR  97124

   Phone: 503 712 5566
   Email: dave.cavalcanti@intel.com

   Xavier Vilajosana
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018

   Email: xvilajosana@uoc.edu

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