RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                         G.Z. Papadopoulos
Expires: 19 November 2020                                 IMT Atlantique
                                                           R. Buddenberg
                                                             18 May 2020

         Reliable and Available Wireless Architecture/Framework


   Due to uncontrolled interferences, including the self-induced
   multipath fading, deterministic networking can only be approached on
   wireless links.  The radio conditions may change -way- faster than a
   centralized routing can adapt and reprogram, in particular when the
   controller is distant and connectivity is slow and limited.  RAW
   separates the routing time scale at which a complex path is
   recomputed from the forwarding time scale at which the forwarding
   decision is taken for an individual packet.  RAW operates at the
   forwarding time scale.  The RAW problem is to decide, within the
   redundant solutions that are proposed by the routing, which will be
   used for each individual packet to provide a DetNet service while
   minimizing the waste of resources.

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 19 November 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Related Work at The IETF  . . . . . . . . . . . . . . . . . .   6
   4.  Use Cases and Requirements Served . . . . . . . . . . . . . .   6
     4.1.  Radio Access Protection . . . . . . . . . . . . . . . . .   7
     4.2.  End-to-End Protection in a Wireless Mesh  . . . . . . . .   7
   5.  RAW Considerations  . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Reliability and Availability  . . . . . . . . . . . . . .   8
       5.1.1.  High Availability Engineering Principles  . . . . . .   8
       5.1.2.  Applying Reliability Concepts to Networking . . . . .  10
       5.1.3.  Reliability in the Context of RAW . . . . . . . . . .  11
     5.2.  RAW Prerequisites . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Routing Time Scale vs. Forwarding Time Scale  . . . . . .  13
   6.  RAW Architecture Elements . . . . . . . . . . . . . . . . . .  14
     6.1.  PAREO Functions . . . . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Packet Replication  . . . . . . . . . . . . . . . . .  15
       6.1.2.  Packet Elimination  . . . . . . . . . . . . . . . . .  16
       6.1.3.  Promiscuous Overhearing . . . . . . . . . . . . . . .  16
       6.1.4.  Constructive Interference . . . . . . . . . . . . . .  17
     6.2.  Wireless Tracks . . . . . . . . . . . . . . . . . . . . .  17
   7.  RAW Architecture  . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  PCE vs. PSE . . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  RAW OAM . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.3.  Source-Routed vs. Distributed Forwarding Decision . . . .  20
     7.4.  Flow Identification . . . . . . . . . . . . . . . . . . .  21
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     12.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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1.  Introduction

   Bringing determinism in a packet network means eliminating the
   statistical effects of multiplexing that result in probabilistic
   jitter and loss.  This can be approached with a tight control of the
   physical resources to maintain the amount of traffic within a
   budgetted volume of data per unit of time that fits the physical
   capabilities of the underlying technology, and the use of time-shared
   resources (bandwidth and buffers) per circuit, and/or by shaping and/
   or scheduling the packets at every hop.

   Wireless networks operate on a shared medium where uncontrolled
   interference, including the self-induced multipath fading, adds
   another dimension to the statistical effects that affect the
   delivery.  Scheduling transmissions can alleviate those effects by
   leveraging diversity in the spatial, time, code, and frequency
   domains, and provide a Reliable and Available service while
   preserving energy and optimizing the use of the shared spectrum.

   Deterministic Networking is an attempt to mostly eliminate packet
   loss for a committed bandwidth with a guaranteed worst-case end-to-
   end latency, even when co-existing with best-effort traffic in a
   shared network.  This innovation is enabled by recent developments in
   technologies including IEEE 802.1 TSN (for Ethernet LANs) and IETF
   DetNet (for wired IP networks).  It is getting traction in various
   industries including manufacturing, online gaming, professional A/V,
   cellular radio and others, making possible many cost and performance

   The "Deterministic Networking Architecture" [RFC8655] is composed of
   three planes: the Application (User) Plane, the Controller Plane, and
   the Network Plane.  Reliable and Available Wireless (RAW) extends RAW
   to focus on issues that are mostly a co"ern on wireless links, and
   inherits the architecture and the planes.  A RAW Network Plane is
   thus a Network Plane inherited by RAW from DetNet, composed of one or
   multiple hops of homogeneous or heterogeneous technologies, e.g. a
   Wi-Fi6 Mesh or one-hop CBRS access links federated by a 5G backhaul.

   RAW networking aims at providing highly available and reliable end-
   to-end performances in a network with scheduled wireless segments.
   Uncontrolled interference and transmission obstacles may impede the
   transmission, and techniques such as beamforming with Multi-User MIMO
   can only alleviate some of those issues, so the term "deterministic"
   is usually not associated with short range radios, in particular in
   the ISM band.  This uncertainty places limits to the amount of
   traffic that can be transmitted on a link while conforming to a RAW
   Service Level Agreement (SLA) that may vary rapidly.

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   The wireless and wired media are fundamentally different at the
   physical level, and while the generic "Deterministic Networking
   Problem Statement" [RFC8557] applies to both the wired and the
   wireless media, the methods to achieve RAW must extend those used to
   support time-sensitive networking over wires, as a RAW solution has
   to address less consistent transmissions, energy conservation and
   shared spectrum efficiency.

   The development of RAW technologies has been lagging behind
   deterministic efforts for wired systems both at the IEEE and the
   IETF.  But recent efforts at the IEEE and 3GPP indicate that wireless
   is finally catching up at the lower layer and that it is now possible
   for the IETF to extend DetNet for wireless segments that are capable
   of scheduled wireless transmissions.

   The intent for RAW is to provide DetNet elements that are specialized
   for short range radios.  From this inheritance, RAW stays agnostic to
   the radio layer underneath though the capability to schedule
   transmissions is assumed.  How the PHY is programmed to do so, and
   whether the radio is single-hop or meshed, are unknown at the IP
   layer and not part of the RAW abstraction.

   Still, in order to focus on real-worlds issues and assert the
   feasibility of the proposed capabilities, RAW will focus on selected
   technologies that can be scheduled at the lower layers: IEEE Std.
   802.15.4 timeslotted channel hopping (TSCH), 3GPP 5G ultra-reliable
   low latency communications (URLLC), IEEE 802.11ax/be where 802.11be
   is extreme high throughput (EHT), and L-band Digital Aeronautical
   Communications System (LDACS).  See [RAW-TECHNOS] for more.

   The establishment of a path is not in-scope for RAW.  It may be the
   product of a centralized Controller Plane as described for DetNet.
   As opposed to wired networks, the action of installing a path over a
   set of wireless links may be very slow relative to the speed at which
   the radio conditions vary, and it makes sense in the wireless case to
   provide redundant forwarding solutions along a complex path and to
   leave it to the Network Plane to select which of those forwarding
   solutions are to be used for a given packet based on the current

   RAW distinguishes the longer time scale at which routes are computed
   from the the shorter forwarding time scale where per-packet decisions
   are made.  RAW operates at the forwarding time scale on one DetNet
   flow over one path that is preestablished and installed by means
   outside of the scope of RAW.  The scope of the RAW WG comprises
   Network plane protocol elements such as Operations, Administration
   and Maintenance (OAM) and in-band control to improve the RAW
   operation at the Service and at the forwarding sub-layers.  RAW

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   controls whether to use packet replication, Automatic Repeat reQuest
   (ARQ), Hybrid ARQ (HARQ) that includes Forward Error Correction (FEC)
   and coding, with a constraint to limit the use of redundancy as is
   really needed, e.g., when a spike of loss is observed.  This is
   discussed in more details in Section 5.3 and the next sections.

2.  Terminology

   RAW reuses terminology defined for DetNet in the "Deterministic
   Networking Architecture" [RFC8655], e.g., PREOF for Packet
   Replication, Elimination and Ordering Functions.

   RAW also reuses terminology defined for 6TiSCH in [6TiSCH-ARCH] such
   as the term Track.  6TiSCH defined a Track as a complex path with
   associated PAREO operations.

   RAW uses the term OAM as defined in [RFC6291].

   RAW defines the following terms:

   PAREO:  Packet (hybrid) ARQ, Replication, Elimination and Ordering.
      PAREO is a superset Of DetNet's PREOF that includes radio-specific
      techniques such as short range broadcast, MUMIMO, constructive
      interference and overhearing, which can be leveraged separately or
      combined to increase the reliability.

   Flapping:  In the context of RAW, a link flaps when the wireless
      connectivity is interrupted for short transient times, typically
      of a subsecond duration.

   In the context of the RAW work, Reliability and Availability are
   defined as follows:

   Reliability:  Reliability is a measure of the probability that an
      item will perform its intended function for a specified interval
      under stated conditions.  For RAW, the service that is expected is
      delivery within a bounded latency and a failure is when the packet
      is either lost or delivered too late.  RAW expresses reliability
      in terms of Mean Time Between Failure (MTBF) and Maximum
      Consecutive Failures (MCF).  More in [NASA].

   Availability:  Availability is a measure of the relative amount of
      time where a path operates in stated condition, in other words
      (uptime)/(uptime+downtime).  Because a serial wireless path may
      not be good enough to provide the required availability, and even
      2 parallel paths may not be over a longer period of time, the RAW
      availability implies a path that is a lot more complex than what
      DetNet typically envisages (a Track).

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3.  Related Work at The IETF

   RAW intersects with protocols or practices in development at the IETF
   as follows:

   *  The Dynamic Link Exchange Protocol (DLEP) [RFC8175] from [MANET]
      can be leveraged at each hop to derive generic radio metrics
      (e.g., based on LQI, RSSI, queueing delays and ETX) on individual

   *  OAM work at [detnet] such as [DetNet-IP-OAM] for the case of the
      IP Data Plane observes the state of DetNet paths, typically MPLS
      and IPv6 pseudowires [DetNet-DP-FW], in the direction of the
      traffic.  RAW needs feedback that flows on the reverse path and
      gathers instantaneous values from the radio receivers at each hop
      to inform back the source and replicating relays so they can make
      optimized forwarding decisions.  The work named ICAN may be
      related as well.

   *  [BFD] detect faults in the path between an ingress and an egress
      forwarding engines, but is unaware of the complexity of a path
      with replication, and expects bidirectionality.  BFD considers
      delivery as success whereas with RAW the bounded latency can be as
      important as the delivery itself.

   *  [SPRING] and [BIER] define in-band signaling that influences the
      routing when decided at the head-end on the path.  There's already
      one RAW-related draft at BIER [BIER-PREF] more may follow.  RAW
      will need new in-band signaling when the decision is distributed,
      e.g., required chances of reliable delivery to destination within
      latency.  This signaling enables relays to tune retries and
      replication to meet the required SLA.

   *  [CCAMP] defines protocol-independent metrics and parameters
      (measurement attributes) for describing links and paths that are
      required for routing and signaling in technology-specific
      networks.  RAW would be a source of requirements for CCAMP to
      define metrics that are significant to the focus radios.

4.  Use Cases and Requirements Served

   [RFC8578] presents a number of wireless use cases including Wireless
   for Industrial Applications, Pro-Audio and SmartGrid.
   [RAW-USE-CASES] adds a number of use cases that demonstrate the need
   for RAW capabilities for new applications such as Pro-Gaming and
   drones.  The use cases can be abstracted in two families, Loose
   Tracks, e.g., for first op Radio Access Protection and Strict Tracks,
   e.g., for End-to-End Protection in a wireless mesh.

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4.1.  Radio Access Protection

   To maintain the committed reliability at all times, a wireless host
   may use more than one Radio Access Network (RAN) in parallel.

                                              ***   **
                           RAN 1  -----  ***     **  ***
                        /              *    **         ****
              +----+  /              *           **      ****
              |    |-                *                    *****
              |Host|--zzz- RAN 2 -- *      Internet       *****
              |    |-                 *                  *****
              +----+  $$รน              *              *******
                        \               ***   ***    *****
                           RAN n  --------  ***  *****

           zzz = flapping now  $$$ expensive

                     Figure 1: Radio Access Protection

   The RANs may be heterogeneous, e.g., 5G [I-D.farkas-raw-5g] and Wi-Fi
   [RAW-TECHNOS] for high-speed communication, in which case a Layer-3
   abstraction becomes useful to select which of the RANs are used at a
   particular point of time, and the amount of traffic that is
   distributed over each RAN.

   The idea is that the rest of the path to the destination(s) is
   protected separately (e.g., uses non-congruent paths) and/or is a lot
   more reliable, e.g., wired.  In that case, RAW observes reliability
   of the path through each of the RANs but only operates on the first

4.2.  End-to-End Protection in a Wireless Mesh

   In radio technologies that support mesh networking (e.g., Wi-Fi and
   TSCH), a Track is a complex path with distributed PAREO capabilities.
   In that case, RAW operates through the multipath and makes decisions
   either at the Ingress or at every hop (more in Section 6.2).

                          /  \   /       /        \
                   Ingress ----M-------N--zzzzz--- Egress
                          \      \   /            /

                  zzz = flapping now

                      Figure 2: End-to-End Protection

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   The Protection may be imposed by the source based on end-to-end OAM,
   or performed hop-by-hop, in which case the OAM must enables the
   intermediate Nodes to estimate the quality of the rest of the
   feasible paths in the sub-Track to the destination.

5.  RAW Considerations

5.1.  Reliability and Availability

5.1.1.  High Availability Engineering Principles

   The reliability criteria of a critical system pervade through its
   elements, and if the system comprises a data network then the data
   network is also subject to the inherited reliability and availability
   criteria.  It is only natural to consider the art of high
   availability engineering and apply it to wireless communicaitons in
   the context of RAW.

   There are three principles [pillars] of high availability

   1.  elimination of single points of failure
   2.  reliable crossover
   3.  prompt detection of failures as they occur.

   These principles are common to all high availability systems, not
   just ones with Internet technology at the center.  Examples of both
   non-Internet and Internet are included.  Elimination of Single Points of Failure

   Physical and logical components in a system happen to fail, either as
   the effect of wear and tear, when used beyond acceptable limits, or
   due to a software bug.  It is necessary to decouple component failure
   from system failure to avoid the latter.  This allows failed
   components to be restored while the rest of the system continues to

   A non-Internet example is a standby generator available to power the
   system on failure of grid power.  An Internet example is more than
   one communication several non-congruent link/path between Nodes in a
   routable network.

   There is a rather open-ended issue over alternate routes -- for
   example, when links are cabled through the same conduit, they form a
   shared risk link group (SRLG), and will share the same fate if the
   bundle is cut.  Just how distributed the infrastructure is a matter
   of discussion; there is no single right answer.  It should be noted

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   that intermediate Nodes such as routers, switches, and the air medium
   itself can become single points of failure; this must be avoided,
   using link- and Node-disjoint paths, and, for RAW, a high degree of
   diversity in the transmissions over the air.

   From an economics standpoint, executing this principle properly
   generally increases capitalization expense because of the redundant
   equipment.  In a constrained network where the waste of energy and
   bandwidth should be minimized, an excessive use of redundant links
   must be avoided; for RAW this means that the extra bandwidth must
   only be used as a replacement of that lost due to a failure.  Reliable Crossover

   Having a backup equipment has a limited value unless it can be
   reliably switched into use within the down-time parameters.

   Using the backup generator example: one that does not automatically
   sense grid power failure, start itself, and place itself on line does
   not represent reliable crossover.

   Routers and IGPs execute reliable crossover continuously because the
   routers will use any alternate routes that are available [RFC0791].
   This is due to the stateless nature of IP datagrams and the
   dissociation of the datagrams from the forwarding routes they take.
   The "IP Fast Reroute Framework" [FRR] analyzes mechanisms for fast
   failure detection and path repair for IP Fast-Reroute, and discusses
   the case of multiple failures and SRLG.  Examples of FRR techniques
   include Remote Loop-Free Alternate [RLFA-FRR] and backup label-
   switched path (LSP) tunnels for the local repair of LSP tunnels using
   RSVP-TE [RFC4090].

   The DetNet PREOF leverages 1+1 redundancy whereby a packet is sent
   twice, over non-congruent paths.  This avoids the gap during the fast
   reroute operation, but doubles the traffic in the network.  In the
   case of RAW, the expectation is that multiple transient faults may
   happen in overlapping time windows, in which case the 1+1 redundancy
   with delayed reestablishment of the second path will not provide the
   required guarantees.  The Data Plane must be configured with a
   sufficient degree of redundancy to select an alternate redundat path
   immediately upon a fault, without the need for a slow intervention
   from the controller plane.  Prompt Notification of Failures

   The execution of the two above principles is likely to render a
   system where the user will rarely see a failure.  But someone needs
   to in order to direct maintenance.

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   There are many reasons for system monitoring (FCAPS for fault,
   configuration, accounting, performance, security is a handy mental
   checklist) but fault monitoring is sufficient reason [STD 62]
   describes how to use SNMP to observe and correct long-term faults.
   "Overview and Principles of Internet Traffic Engineering" [TE]
   discusses the importance of measurement for network protection, and
   provides abstract an method for network survivability with the
   analysis of a traffic matrix as observed by SNMP, probing techniques,
   FTP, IGP link state advertisements, and more.

   Using the art of SNMP, the above described backup generator would
   include an SNMP agent that can report the status of the generator
   (get messages) on demand, and report changes in status (e.g. startup,
   amount of fuel in the tank) (trap messages).

   Those measurements are needed in the context of RAW to inform the
   controller and make the long term reactive decision to rebuild a
   complex path.  But RAW itself operates in the Network Plane at a
   faster time scale.  To act on the Data Plane, RAW needs live
   information from the Operational Plane , e.g., using Bidirectional
   Forwarding Detection [BFD] and its variants (bidirectional and remote
   BFD) to protect a link, and OAM techniques to protect a path.

5.1.2.  Applying Reliability Concepts to Networking

   The terms Reliaility and Availability are defined for use in RAW in
   Section 2 and the reader is invited to read [NASA] for more details
   on the general definition of Reliability.  Practically speaking a
   number of nines is often used to indicate the reliability of a data
   link, e.g., 5 nines indicate a Packet Delivery Ratio (PDR) of

   This number is typical in a wired environment where the loss is due
   to a random event such as a solar particle that affects the
   transmission of a particular frame, but does not affect the previous
   or next frame, nor frames transmitted on other links.  Note that the
   QoS requirements in RAW may include a bounded latency, and a packet
   that arrives too late is a fault and not considered as delivered.

   For a periodic pattern such as an automation control loop, this
   number is proportional to the Mean Time Between Failures (MTBF).  If
   a single fault can have dramatic consequences, then the MTBF is the
   expression of the chances that an unwanted event occurs.  In data
   networks, this is rarely the case.  Packet loss cannot never be fully
   avoided and the systems are built to resist to one loss, e.g., using
   redundancy with Retries (HARQ) or Packet Replication and Elimination
   (PRE), or, in a typical control loop, by linear interpolation from
   the previous measuremnents.

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   But the linear interpolation method can not resist to multiple
   consecutive losses, and a high MTBF is desired as a guarantee that
   this will not happen, IOW that the losses-in-a-row can be bounded.
   In that case, what's really desired is a Maximum Consecutive Failures
   (MCF).  If the number of losses in a row passes the MCF, the control
   loop has to abort.  Engineers that build automated processes may use
   the network reliability expressed in nines or as an MTBF to provide
   an MCF, e.g., as described in section 7.4 of [RFC8578].

5.1.3.  Reliability in the Context of RAW

   In contrast with wired networks, errors in transmission are the
   predominent source of packet loss in wireless networks.  The root
   cause may be of multiple origins:

   Multipath Fading:  A destructive interference by a reflection of the
      original signal.

      A radio signal may be received directly (line-of-sight) and/or as
      a reflection on a physical structure (echo).  The reflections take
      a longer path and are delayed by the extra distance divided by the
      speed of light in the medium.  Depending on the frequency, the
      echo lands with a different phase which may add up to
      (constructive interference) or destroy the signal (destructive

      The affected frequencies depend on the relative position of the
      sender, the receiver, and all the reflecting objects in the
      environment.  A given hop will suffer from multipath fading for
      multiple packets in a row till the something moves that changes
      the reflection patterns.

   Co-channel Interference:  Energy in the spectrum used for the
      transmission confuses the receiver.

      The wireless medium itself is a Shared Risk Link Group (SRLG) for
      nearby users of the same spectrum, as an interference may affect
      multiple co-channel transmissions between different peers within
      the interference domain of the interferer, possibly even when they
      use different technologies.

   Obstacle in Fresnel Zone:  The optimal transmission happens when the
      Fresnel Zone between the sender and the receiver is free of

      As long as a physical object (e.g., a metallic trolley between
      peers) that affects the transmission is not removed, the quality
      of the link is affected.

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   In an environment that is rich of metallic structures and mobile
   objects, a single radio link will provide a fuzzy service, meaning
   that it cannot be trusted to transport the traffic reliably over a
   long period of time.

   Transmission errors are typically not independent, and their nature
   and duration are unpredictable; as long as a physical object (e.g., a
   metallic trolley between peers) that affects the transmission is not
   removed, or as long as the interferer (e.g., a radar) keeps
   transmitting, a continuous stream of packets will be affected.

   The key word to combat losses is diversity.  A single packet may be
   sent at different times over different paths that rely on different
   radio frequencies and different PHY technologies, e.g., narrowband
   vs. spread spectrum.  It is typically retried a number of times in
   case of a loss, and if possible the retries should again vary all
   possible parameters.  Each form of diversity combats a particular
   cause of loss and use of diversity must be maximised to optimize the

5.2.  RAW Prerequisites

   A prerequisite to the RAW work is that an end-to-end routing function
   computes a complex sub-topology along which forwarding can happen
   between a source and one or more destinations.  For 6TiSCH, this is a
   Track.  The concept of Track is specified in the 6TiSCH Architecture
   [6TiSCH-ARCH].  Tracks provide a high degree of redundancy and
   diversity and enable RAW PREOF, end-to-end network coding, and
   possibly radio-specific abstracted techniques such as ARQ,
   overhearing, frequency diversity, time slotting, and possibly others.

   How the routing operation computes the Track is out of scope for RAW.
   The scope of the RAW operation is one Track, and the goal of the RAW
   operation is to optimize the use of the Track at the forwarding
   timescale to maintain the expected service while optimizing the usage
   of constrained resources such as energy and spectrum.

   Another prerequisite is that an IP link can be established over the
   radio with some guarantees in terms of service reliability, e.g., it
   can be relied upon to transmit a packet within a bounded latency and
   provides a guaranteed BER/PDR outside rare but existing transient
   outage windows that can last from split seconds to minutes.  The
   radio layer can be programmed with abstract parameters, and can
   return an abstract view of the state of the Link to help forwarding
   decision (think DLEP from MANET).  In the layered approach, how the
   radio manages its PHY layer is out of control and out of scope.
   Whether it is single hop or meshed is also unknown and out of scope.

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5.3.  Routing Time Scale vs. Forwarding Time Scale

   With DetNet, the end-to-end routing can be centralized and can reside
   outside the network.  In wireless, and in particular in a wireless
   mesh, the path to the controller that performs the route computation
   and maintenance expensive in terms of critical resources such as air
   time and energy.

   Reaching to the routing computation can also be slow in regards to
   the speed of events that affect the forwarding operation at the radio
   layer.  Due to the cost and latency to perform a route computation,
   the controller plane is not expected to be sensitive/reactive to
   transient changes.  The abstraction of a link at the routing level is
   expected to use statistical operational metrics that aggregate the
   behavior of a link over long periods of time, and represent its
   availability as shades of gray as opposed to either up or down.

                     |  Controller    |
                     |    (PCE)       |
                     |  [Routing ]    |
                     |  [Function]    |
         _-._-._-._-._-._-.  |  ._-._-._-._-._-._-._-._-._-._-._-._-
       _-._-._-._-._-._-._-. | _-._-._-._-._-._-._-._-._-._-._-._-
                      ....   |  .......
                  ....    .  | .       .....
               ....          v             ...
             ..   A-------B-------C---D     ..
          ...    /  \   /       /      \     ..
         .      I ----M-------N--zzz-- E  ..
         ..      \      \   /         /     .
           ..     P--zzz--Q----------R   ..
             ..                         ..
               .......               ...
        zzz = flapping now

                           Figure 3: Time Scales

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   In the case of wireless, the changes that affect the forwarding
   decision can happen frequently and often for short durations, e.g., a
   mobile object moves between a transmitter and a receiver, and will
   cancel the line of sight transmission for a few seconds, or a radar
   measures the depth of a pool and interferes on a particular channel
   for a split second.

   There is thus a desire to separate the long term computation of the
   route and the short term forwarding decision.  In such a model, the
   routing operation computes a complex Track that enables multiple Non-
   Equal Cost Multi-Path (N-ECMP) forwarding solutions, and leaves it to
   the Data Plane to make the per-packet decision of which of these
   possibilities should be used.

   In the case of wires, the concept is known in traffic engineering
   where an alternate path can be used upon the detection of a failure
   in the main path, e.g., using OAM in MPLS-TP or BFD over a collection
   of SD-WAN tunnels.  RAW formalizes a forwarding time scale that is an
   order(s) of magnitude shorter than the controler plane routing time
   scale, and separates the protocols and metrics that are used at both
   scales.  Routing can operate on long term statistics such as delivery
   ratio over minutes to hours, but as a first approximation can ignore
   flapping.  On the other hand, the RAW forwarding decision is made at
   packet speed, and uses information that must be pertinent at the
   present time for the current transmission.

6.  RAW Architecture Elements

6.1.  PAREO Functions

   In a nutshell, PRE establishes several paths in a network to provide
   redundancy and parallel transmissions to bound the end-to-end delay
   to traverse the network.  Optionally, promiscuous listening between
   paths is possible, such that the Nodes on one path may overhear
   transmissions along the other path.  Considering the scenario shown
   in Figure 4, many different paths are possible for S to reach R.  A
   simple way to benefit from this topology could be to use the two
   independent paths via Nodes A, C, E and via B, D, F.  But more
   complex paths are possible by interleaving transmissions from the
   lower level of the path to the upper level.

   PRE may also take advantage of the shared properties of the wireless
   medium to compensate for the potential loss that is incurred with
   radio transmissions.  For instance, when the source sends to A, B may
   listen also and get a second chance to receive the frame without an
   additional transmission.  Note that B would not have to listen if it
   already received that particular frame at an earlier timeslot in a
   dedicated transmission towards B.

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                               (A)   (C)   (E)

                 source (S)                       (R) (root)

                               (B)   (D)   (F)

      Figure 4: A Typical Ladder Shape with Two Parallel Paths Toward
                              the Destination

   The PRE model can be implemented in both centralized and distributed
   scheduling approaches.  In the centralized approach, a Path
   Computation Element (PCE) scheduler calculates the routes and
   schedules the communication among the Nodes along a circuit such as a
   Label switched path.  In the distributed approach, each Node selects
   its route to the destination, typically using a source routing
   header.  In both cases, at each Node in the paths, a default parent
   and alternative parent(s) should be selected to set up complex

   In the following Subsections, all the required operations defined by
   PRE, namely, Alternative Path Selection, Packet Replication, Packet
   Elimination and Promiscuous Overhearing, are described.

6.1.1.  Packet Replication

   The objective of PRE is to provide deterministic networking
   properties: high reliability and bounded latency.  To achieve this
   goal, determinism in every hop of the forwarding paths MUST be
   guaranteed.  By employing a Packet Replication procedure, each Node
   forwards a copy of each data packet to multiple parents: its Default
   Parent (DP) and multiple Alternative Parents (APs).  To do so, each
   Node (i.e., source and intermediate Node) transmits the data packet
   multiple times in unicast to each parent.  For instance, in Figure 5,
   the source Node S is transmitting the packet to both parents, Nodes A
   and B, at two different times.  An example schedule is shown in
   Table 1.  Thus, the packet can use non-congruent paths to the

                          ===> (A) => (C) => (E) ===
                        //        \\//   \\//       \\
              source (S)          //\\   //\\         (R) (root)
                        \\       //  \\ //  \\      //
                          ===> (B) => (D) => (F) ===

       Figure 5: Packet Replication: S transmits twice the same data
                  packet, to its DP (A) and to its AP (B).

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       | Channel |  0   |  1   |  2   |  3   |  4   |  5   |  6   |
       | 0       | S->A | S->B | B->C | B->D | C->F | E->R | F->R |
       | 1       |      | A->C | A->D | C->E | D->E | D->F |      |

               Table 1: Packet Replication: Sample schedule

6.1.2.  Packet Elimination

   The replication operation increases the traffic load in the network,
   due to packet duplications.  Thus, a Packet Elimination operation
   SHOULD be applied at each RPL DODAG level to reduce the unnecessary
   traffic.  To this aim, once a Node receives the first copy of a data
   packet, it discards the subsequent copies.  Because the first copy
   that reaches a Node is the one that matters, it is the only copy that
   will be forwarded upward.  Then, once a Node performs the Packet
   Elimination operation, it will proceed with the Packet Replication
   operation to forward the packet toward the RPL DODAG Root.

6.1.3.  Promiscuous Overhearing

   Considering that the wireless medium is broadcast by nature, any
   neighbor of a transmitter may overhear a transmission.  By employing
   the Promiscuous Overhearing operation, a DP and some AP(s) eventually
   have more chances to receive the data packets.  In Figure 6, when
   Node A is transmitting to its DP (Node C), the AP (Node D) and its
   sibling (Node B) may decode this data packet as well.  As a result,
   by employing corellated paths, a Node may have multiple opportunities
   to receive a given data packet.  This feature not only enhances the
   end-to-end reliability but also it reduces the end-to-end delay and
   increases energy efficiency.

                      ===> (A) ====> (C) ====> (E) ====
                    //     ^ | \\                      \\
          source (S)       | |   \\                      (R) (root)
                    \\     | v     \\                  //
                      ===> (B) ====> (D) ====> (F) ====

           Figure 6: Unicast to DP with Overhearing: by employing
      Promiscuous Overhearing, DP, AP and the sibling Nodes have more
               opportunities to receive the same data packet.

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6.1.4.  Constructive Interference

   Constructive Interference can be seen as the reverse of Promiscuous
   Overhearing, and refers to the case where two senders transmit the
   exact same signal in a fashion that the emitted symbols add up at the
   receiver and permit a reception that would not be possible with a
   single sender at the same PHY mode and the same power level.

   Constructive Interference was proposed on 5G, Wi-Fi7 and even tested
   on IEEE 802.14.5.  The hard piece is to synchronize the senders to
   the point that the signals are emitted at slightly different time to
   offset the difference of propagation delay that corresponds to the
   difference of distance of the transmitters to the receiver at the
   speed of light to the point that the symbols are superposed long
   enough to be recognizable.

6.2.  Wireless Tracks

   The "6TiSCH Architecture" [6TiSCH-ARCH] introduces the concept of
   Track a a possibly complex path with the PAREO functions operated

   A simple track is composed of a direct sequence of reserved hops to
   ensure the transmission of a single packet from a source Node to a
   destination Node across a multihop path.

   A Complex Track is designed as a directed acyclic graph from a source
   Node towards a destination Node to support multi-path forwarding, as
   introduced in "6TiSCH Architecture" [6TiSCH-ARCH].  By employing PRE
   functions [RFC8655], several paths may be computed, and these paths
   may be more or less independent.  For example, a complex Track may
   branch off and rejoin over non-congruent paths (branches).

   Some more details for Deterministic Network PRE techniques are
   presented in the following Section.

7.  RAW Architecture

   RAW inherits the conceptual model described in section 4 of the
   DetNet Architecture [RFC8655].

   A Controller Plane Function (CPF) called the Path Computation
   Element(PCE) [RFC4655] interacts with RAW Nodes over a Southbound
   API.  The RAW Nodes are DetNet relays that are capable of additional
   diversity mechanisms and measurement functions related to the radio
   interface, in particular the PAREO redundancy mechanisms.

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   The PCE defines a complex path between an Ingress End System and an
   Egress End System, and indicates to the RAW Nodes where the PAREO
   operations may be actioned in the Network Plane.  The path may be
   loosely expressed in order to traverse a non-RAW subnetwork.  In that
   case, the expectation is that the non-RAW subnetwork can be neglected
   in the RAW computation, that is, considered infinitely fast, reliable
   and/or available in comparison with the links between RAW nodes.

                   CPF         CPF              CPF              CPF

         -+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                       RAW  --z   RAW  --z   RAW  --z   RAW
                   z-- Node  z--  Node  z--  Node  z--  Node --z
        Ingress --z    /          /                     /     z-- Egress
        End           Z          Z                     Z          End
        Node   ---z   /          /                     /      z-- Node
                 z-- RAW  --z   RAW   ( non-RAW ) --- RAW ---z
                     Node  z--  Node --- ( Nodes  )   Node

           --z   radio                      wired
            z--  link                   --- link

                            Figure 7: RAW Nodes

   The Link-Layer metrics are reported to the PCE in a time-aggregated,
   e.g., statistical fashion.  Example Link-Layer metrics include
   typical Link bandwidth (the medium speed depends dynamically on the
   PHY mode and the number of users sharing the spectrum) and average
   availability and reliability figures.

   Based on those metrics, the PCE installs a complex path with enough
   redundant forwarding solutions to ensure that the Network Plane can
   reliably deliver the packets within a System Level Agreement (SLA)
   associated to the flow.  The SLA defines end-to-end reliability and
   availability figures, where reliability may be expressed a successful
   delivery within a bounded delay.  One a path is established, end-to-
   end subpath and overall reliability and availability metrics are also
   reported to the PCE to assure that the SLA is continuously served and
   recompute the path if not.

   Depending on the SLA, the path or a leg of the path may include non-
   RAW Nodes, either interleaved inside the path, or more typically till
   the Egress End Node.  RAW observes the Lower-Layer Links between RAW
   nodes (typically, radio links) and the end-to-end Network Layer
   subpath to decide at all times which of the PAREO redundancy is
   actioned by which RAW Nodes.

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7.1.  PCE vs. PSE

   Section 5.3 shows that the time scale at which RAW needs to operate
   is not that of the Controller Plane that needs to deal with a
   possibly large whole network and make global optimization across
   multiple flows that may contend for limited resources.

   RAW separates the path computation time scale at which a complex path
   is recomputed from the path selection time scale at which the
   forwarding decision is taken for one or a few packets.  RAW operates
   at the path selection time scale.  The RAW problem is to decide,
   within the redundant solutions that are proposed by the PCE, which
   will be used for each packet to provide a Reliable and Available
   service while minimizing the waste of resources.

   To that effect, RAW defines the Path Selection Engine (PSE) that is
   the counter-part of the PCE to perform rapid local adjustments of the
   forwarding tables to avoid excessive use of the resource diversity
   that the PCE selects.  The PSE enables to exploit the richer
   forwarding capabilities with PAREO and scheduled transmissions at a
   faster time scale over the smaller domain that is the Track, either
   Loose or Strict.

      |               |   PCE (Not in Scope)   |   PSE (In Scope)  |
      | Operation     |      Centralized       |  Source-Routed or |
      |               |                        |    Distributed    |
      | Communication |    Slow, expensive     |    Fast, local    |
      | Time Scale    |   Long (hours, days)   |  Short (seconds,  |
      |               |                        |    sub-second)    |
      | Network Size  | Large, many Tracks to  | Small, within one |
      |               |   optimize globally    |       Track       |
      | Considered    | Averaged, Statistical, |  Instant values / |
      | Metrics       |     Shade of grey      | boolean condition |

                           Table 2: PCE vs. PSE

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7.2.  RAW OAM

   The RAW OAM operation in the Network Plane observes a subset of the
   links along that redundant path and the RAW PSE makes the decision on
   which PAREO function in actioned at which RAW Node, for a packet or a
   small collection of packets.

   In the case of a End-to-End Protection in a Wireless Mesh, the Track
   is strict and congruent with the path so all links are observed.
   Conversely, in the case of Radio Access Protection, the Track is
   Loose and in that case only the first hop is observed; the rest of
   the path is abstracted and considered infinitely reliable, meaning
   that the loss of a packet that was sent over one of the possible
   first hops is attributed to that first hop, even what a particular
   loss effectively happens farther down the path.

                                         ***   **
                      RAN 1  -----  ***     **  ***
                   /              *    **         ****
      +-------+  /              *           **      ****    +------+
      |Ingress|-                *                    *****  |Egress|
      |  End  |------ RAN 2 -- *      Internet       ****---| End  |
      |System |-                 *                  *****   |System|
      +-------+  \              *              *******      +------+
                   \               ***   ***    *****
                      RAN n  --------  ***  *****

              <------------------> <-------------------->
                 Observed by OAM       Opaque to OAM

            Figure 8: Observed Links in Radio Access Protection

   The Links that are not observed by OAM are opaque to it, meaning that
   the OAM information is carried and possibly echoed as data.  In the
   example above, the Internet is opaque and not controlled by RAW, but
   RAW measures the end-to-end latency and delivery ratio for packets
   sent over each if RAN 1, RAN 2 and RAN 3, and determines whether a
   packet should be sent over either or a collection of those access

7.3.  Source-Routed vs. Distributed Forwarding Decision

   Within a large routed topology, the route-over mesh operation builds
   a particular complex Track with one source and one or more
   destinations; within the Track, packets may follow different paths
   and may be subject to RAW forwarding operations that include
   replication, elimination, retries, overhearing and reordering.

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   The RAW forwarding decisions include the selection of points of
   replication and elimination, how many retries can take place, and a
   limit of validity for the packet beyond which the packet should be
   destroyed rather than forwarded uselessly further down the Track.

   The decision to apply the RAW techniques must be done quickly, and
   depends on a very recent and precise knowledge of the forwarding
   conditions within the complex Track.  There is a need for an
   observation method to provide the RAW Data Plane with the specific
   knowledge of the state of the Track for the type of flow of interest
   (e.g., for a QoS level of interest).  To observe the whole Track in
   quasi real time, RAW will consider existing tools such as
   L2-triggers, DLEP, BFD and in-band and out-of-band OAM.

   One possible way of making the RAW forwarding decisions is to make
   them all at the ingress and express them in-band in the packet, which
   requires new loose or strict Hop-by-hop signaling.  To control the
   RAW forwarding operation along a Track for the individual packets,
   RAW may leverage and extend known techniques such as DetNet tagging,
   Segment Routing (SRv6) or BIER-TE such as done with [BIER-PREF].

   An alternate way is to enable each forwarding Node to make the RAW
   forwarding decisions for a packet on its own, based on its knowledge
   of the expectation (timeliness and reliability) for that packet and a
   recent observation of the rest of the way across the possible paths
   within the Track.  Information about the service should be placed in
   the packet and matched with the forwarding Node's capabilities and

   In either case, a per-flow state is installed in all intermediate
   Nodes to recognize the flow and determine the forwarding policy to be

7.4.  Flow Identification

   Section 4.7 of the DetNet Architecture [RFC8655] ties the app-flow
   identification which is an appliation layer concept with the network
   path identification that depends on the networking technology by
   "exporting of flow identification", e.g., to a MPLS label.

   With RAW, this exporting operation is injective but not bijective.
   e.g., a flow is fully placed within one RAW Track, but not all
   packets along that Track are necessarily part of the same flow.  For
   instance, out-of-band OAM packets must circulate in the exact same
   fashion as the flows that they observe.  It results that the flow
   identification that maps to to app-flow at the network layer must be
   separate from the path identification that is used to forward a

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                    Flow 1 (6-tuple) ----+
               Flow 2 (6-tuple)  ---+    |
                                    |    |
            OAM     -----------+    |    |
                               |    |    |
                               |    |    |
                          |    |    |    |    |
                          |    v    v    v    |
                          |                   |
                                    +------------> Track 1
                                           (IP address, instanceId)

                          Figure 9: Flow Injection

   Section 3.4 of the DetNet data-plane framework [DetNet-DP-FW]
   indicates that for a DetNet IP Data Plane, a flow is identified by an
   IPv6 6-tuple.  With RAW, that 6-tuple is not what indicates the
   Track, in other words, the flow ID is not the Track ID.

   For instance, the 6TiSCH Architecture [6TiSCH-ARCH] uses a
   combination of the address of the Ingress End System and an instance
   identifier in a Hop-by-hop option to indicate a Track.  Packets that
   are tagged with the same (address, instance ID) tuple will experience
   the same forwarding behavior regardless of the IPv6 6-tuple, and
   regardless of whether they transport application flows or OAM.

8.  Security Considerations

9.  IANA Considerations

   This document has no IANA actions.

10.  Contributors

   Xavi Vilajosana:  Wireless Networks Research Lab, Universitat Oberta
      de Catalunya

   Rex Buddenberg:

   Remous-Aris Koutsiamanis:  IMT Atlantique

   Nicolas Montavont:  IMT Atlantique

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11.  Acknowledgments


12.  References

12.1.  Normative References

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,

              Thubert, P., Cavalcanti, D., Vilajosana, X., and C.
              Schmitt, "Reliable and Available Wireless Technologies",
              Work in Progress, Internet-Draft, draft-thubert-raw-
              technologies-04, 6 January 2020,

              Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-bernardos-raw-use-cases-03, 8 March 2020,

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [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,

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,

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   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,

   [RFC8557]  Finn, N. and P. Thubert, "Deterministic Networking Problem
              Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019,

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,

12.2.  Informative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [TE]       Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
              Xiao, "Overview and Principles of Internet Traffic
              Engineering", RFC 3272, DOI 10.17487/RFC3272, May 2002,

   [STD 62]   Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              DOI 10.17487/RFC3411, December 2002,

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,

   [FRR]      Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,

   [RLFA-FRR] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,

              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination

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              Function (PREF) and OAM", Work in Progress, Internet-
              Draft, draft-thubert-bier-replication-elimination-03, 3
              March 2018, <https://tools.ietf.org/html/draft-thubert-

              Mirsky, G., Chen, M., and D. Black, "Operations,
              Administration and Maintenance (OAM) for Deterministic
              Networks (DetNet) with IP Data Plane", Work in Progress,
              Internet-Draft, draft-mirsky-detnet-ip-oam-02, 23 March
              2020, <https://tools.ietf.org/html/draft-mirsky-detnet-ip-

              Varga, B., Farkas, J., Berger, L., Malis, A., and S.
              Bryant, "DetNet Data Plane Framework", Work in Progress,
              Internet-Draft, draft-ietf-detnet-data-plane-framework-06,
              6 May 2020, <https://tools.ietf.org/html/draft-ietf-

              Farkas, J., Dudda, T., Shapin, A., and S. Sandberg, "5G -
              Ultra-Reliable Wireless Technology with Low Latency", Work
              in Progress, Internet-Draft, draft-farkas-raw-5g-00, 1
              April 2020,

   [NASA]     Adams, T., "RELIABILITY: Definition & Quantitative
              Illustration", <https://kscddms.ksc.nasa.gov/Reliability/

   [MANET]    IETF, "Mobile Ad hoc Networking",

   [detnet]   IETF, "Deterministic Networking",

   [SPRING]   IETF, "Source Packet Routing in Networking",

   [BIER]     IETF, "Bit Indexed Explicit Replication",

   [BFD]      IETF, "Bidirectional Forwarding Detection",

   [CCAMP]    IETF, "Common Control and Measurement Plane",

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Authors' Addresses

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

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

   Georgios Z. Papadopoulos
   IMT Atlantique
   Office B00 - 114A
   2 Rue de la Chataigneraie
   35510 Cesson-Sevigne - Rennes

   Phone: +33 299 12 70 04
   Email: georgios.papadopoulos@imt-atlantique.fr

   Rex Buddenberg
   United States of America

   Email: buddenbergr@gmail.com

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