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RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                           G. Papadopoulos
Expires: April 5, 2020                                    IMT Atlantique
                                                         October 3, 2019


           Reliable and Available Wireless Problem Statement
                draft-pthubert-raw-problem-statement-02

Abstract

   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 5, 2020.

Copyright Notice

   Copyright (c) 2019 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Use Cases and Requirements Served . . . . . . . . . . . . . .   4
   3.  Routing Scale vs. Forwarding Scale  . . . . . . . . . . . . .   4
   4.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Related Work at The IETF  . . . . . . . . . . . . . . . . . .   6
   6.  Functional Gaps . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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



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   industries including manufacturing, online gaming, professional A/V,
   cellular radio and others, making possible many cost and performance
   optimizations.

   Reliable and Available Wireless (RAW) networking services extend
   DetNet to approach end-to-end deterministic performances in a network
   with scheduled wireless segments, possibly combined with wired
   segments, and possibly sharing physical resources with non-
   deterministic traffic.  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 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 [I-D.thubert-raw-technologies]
   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 RAW Network Plane to select which of those forwarding
   solutions are to be used for a given packet based on the current
   conditions.



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   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 3 and the next sections.

2.  Use Cases and Requirements Served

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

3.  Routing Scale vs. Forwarding Scale

   RAW extends DetNet to focus on issues that are mostly a concern on
   wireless links.  See [I-D.ietf-detnet-architecture] for more on
   DetNet.  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 may be slow and expensive in terms of
   critical resources such as air time and energy.

   Reaching to the routing computation can 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,
   routing 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 a
   shade of gray as opposed to either up or down.

   In the case of wireless, the changes that affect the forwarding
   decision can happen frequently and often for shot 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-



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   equal cost multipath (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 routing time scale that is order
   of magnitude longer than the forwarding 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.

4.  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
   [I-D.ietf-6tisch-architecture].  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.







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5.  Related Work at The IETF

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

   o  The Dynamic Link Exchange Protocol [RFC8175] (DLEP) 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
      hops

   o  Operations, Administration and Maintenance (OAM) work at [DetNet]
      such as [I-D.mirsky-detnet-ip-oam] for the case of the IP Data
      Plane observes the state of DetNet paths, typically MPLS and IPv6
      pseudowires [I-D.ietf-detnet-data-plane-framework], 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 and may find a home at RAW.

   o  [BFD] detect faults in the path between an ingress and an egress
      forwarding engines, but is aware 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.

   o  [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
      [I-D.thubert-bier-replication-elimination] 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 be met.

   o  [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.

6.  Functional Gaps

   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 follows 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 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.

   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
   [I-D.thubert-bier-replication-elimination].

   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
   policies.

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

7.  References

7.1.  Normative References

   [I-D.bernardos-raw-use-cases]
              Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
              Bernardos, "RAW use cases", draft-bernardos-raw-use-
              cases-00 (work in progress), July 2019.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-26 (work
              in progress), August 2019.




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   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-13 (work in progress), May 2019.

   [I-D.thubert-raw-technologies]
              Thubert, P., Cavalcanti, D., Vilajosana, X., and C.
              Schmitt, "Reliable and Available Wireless Technologies",
              draft-thubert-raw-technologies-03 (work in progress), 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,
              <https://www.rfc-editor.org/info/rfc8175>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,
              <https://www.rfc-editor.org/info/rfc8578>.

7.2.  Informative References

   [BFD]      IETF, "Bidirectional Forwarding Detection",
              <https://dataTracker.ietf.org/doc/charter-ietf-bfd/>.

   [BIER]     IETF, "Bit Indexed Explicit Replication",
              <https://dataTracker.ietf.org/doc/charter-ietf-bier/>.

   [CCAMP]    IETF, "Common Control and Measurement Plane",
              <https://dataTracker.ietf.org/doc/charter-ietf-ccamp/>.

   [DetNet]   IETF, "Deterministic Networking",
              <https://dataTracker.ietf.org/doc/charter-ietf-detnet/>.

   [I-D.ietf-detnet-data-plane-framework]
              Varga, B., Farkas, J., Berger, L., Fedyk, D., Malis, A.,
              Bryant, S., and J. Korhonen, "DetNet Data Plane
              Framework", draft-ietf-detnet-data-plane-framework-02
              (work in progress), September 2019.

   [I-D.mirsky-detnet-ip-oam]
              Mirsky, G. and M. Chen, "Operations, Administration and
              Maintenance (OAM) for Deterministic Networks (DetNet) with
              IP Data Plane", draft-mirsky-detnet-ip-oam-00 (work in
              progress), July 2019.






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   [I-D.thubert-bier-replication-elimination]
              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", draft-thubert-bier-replication-
              elimination-03 (work in progress), March 2018.

   [MANET]    IETF, "Mobile Ad hoc Networking",
              <https://dataTracker.ietf.org/doc/charter-ietf-manet/>.

   [PCE]      IETF, "Path Computation Element",
              <https://dataTracker.ietf.org/doc/charter-ietf-pce/>.

   [SPRING]   IETF, "Source Packet Routing in Networking",
              <https://dataTracker.ietf.org/doc/charter-ietf-spring/>.

   [TEAS]     IETF, "Traffic Engineering Architecture and Signaling",
              <https://dataTracker.ietf.org/doc/charter-ietf-teas/>.

Authors' Addresses

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

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


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

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











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