RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                         G.Z. Papadopoulos
Expires: 25 April 2020                                    IMT Atlantique
                                                         23 October 2019

           Reliable and Available Wireless Problem Statement


   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
   forwarded 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

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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Use Cases and Requirements Served . . . . . . . . . . . . . .   5
   4.  Routing Time Scale vs. Forwarding Time Scale  . . . . . . . .   5
   5.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Related Work at The IETF  . . . . . . . . . . . . . . . . . .   7
   7.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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

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   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 DetNet architecture [DetNet-ARCH] is composed of three planes: a
   (User)Application Plane, a Controller Plane, and a Network Plane.
   Reliable and Available Wireless (RAW) extends DetNet to focus on
   issues that are mostly a concern on wireless links, and inherits the
   architecture and the planes.  A RAW Network Plane is thus a Network
   Plane inherited by RAW from DetNet.

   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.

   The wireless and wired media are fundamentally different at the
   physical level, and while the generic Problem Statement for DetNet
   applies to the wired as well as the wireless medium, the methods to
   achieve RAW will differ from those used to support time-sensitive
   networking over wires, as a RAW solution will need to address less
   consistent transmissions, energy conservation and shared spectrum

   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.

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   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 OAM and in-band control to
   improve the RAW operation at the Service and at the forwarding sub-
   layers, e.g., controlling whether to use packet replication, Hybrid
   ARQ and coding, with a constraint to limit the use of redundancy when
   it is really needed, e.g., when a spike of loss is observed.  This is
   discussed in more details in Section 4 and the next sections.

2.  Terminology

   RAW reuses terminology defined for DetNet in [DetNet-ARCH], e.g.,
   PREOF for Packet Replication, Elimination and Ordering Functions.

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

   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 wireles
      connectivity is interrupted for short transient times, typically
      of a subsecond duration.

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   This document reuses terms that are well-defined in the context of
   automation to networking and packet delivery, in particular for
   reliability and availability.  In the context of the RAW work, they
   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).

   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).

3.  Use Cases and Requirements Served

   [RFC8578] presents a number of wireless use cases including Wireless
   for Industrial Applications.  [RAW-USE-CASES] adds a number of use
   cases that demonstrate the need for RAW capabilities in Pro-Audio,
   gaming and robotics.

4.  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.

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

                           Figure 1: Time Scales

   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 forwarding 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

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   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.

5.  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 DetNet 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.

6.  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

   *  Operations, Administration and Maintenance (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

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      [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.

7.  Problem Statement

   Within a large routed topology, the routing 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.

   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 forwarding 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.

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   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

8.  Security Considerations

   This document is a problem statement and does not propose a solution
   that could yield security issues.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  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-27, 18 October 2019,

              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", Work in Progress,
              Internet-Draft, draft-ietf-detnet-architecture-13, 6 May
              2019, <https://tools.ietf.org/html/draft-ietf-detnet-

              Thubert, P., Cavalcanti, D., Vilajosana, X., and C.

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              Schmitt, "Reliable and Available Wireless Technologies",
              Work in Progress, Internet-Draft, draft-thubert-raw-
              technologies-03, 1 July 2019,

              Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-bernardos-raw-use-cases-00, 5 July 2019,

   [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,

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

10.2.  Informative References

   [BFD]      IETF, "Bidirectional Forwarding Detection", October 2019,

   [BIER]     IETF, "Bit Indexed Explicit Replication", October 2019,

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

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

   [DetNet]   IETF, "Deterministic Networking", October 2019,

              Varga, B., Farkas, J., Berger, L., Fedyk, D., Malis, A.,
              Bryant, S., and J. Korhonen, "DetNet Data Plane

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              Framework", Work in Progress, Internet-Draft, draft-ietf-
              detnet-data-plane-framework-02, 13 September 2019,

              Mirsky, G. and M. Chen, "Operations, Administration and
              Maintenance (OAM) for Deterministic Networks (DetNet) with
              IP Data Plane", Work in Progress, Internet-Draft, draft-
              mirsky-detnet-ip-oam-00, 8 July 2019,

   [MANET]    IETF, "Mobile Ad hoc Networking", October 2019,

   [SPRING]   IETF, "Source Packet Routing in Networking", October 2019,

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

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