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RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                        September 16, 2019
Expires: March 19, 2020


           Reliable and Available Wireless Problem Statement
                draft-pthubert-raw-problem-statement-00

Abstract

   This document describes the problem space for Reliable and Available
   Wireless at the IETF.

Status of This Memo

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

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   This Internet-Draft will expire on March 19, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Use Cases and Requirements Served . . . . . . . . . . . . . .   4
   3.  Routing Scale vs. Forwarding Scale  . . . . . . . . . . . . .   4
   4.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Functional Gaps . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   IP networks become more predictable when the effects of statistical
   multiplexing (jitter and collision loss) are eliminated.  This
   requires a tight control of the physical resources to maintain the
   amount of traffic within the physical capabilities of the underlying
   technology, e.g., by the use of time-shared resources (bandwidth and
   buffers) per circuit, and/or by shaping and/or scheduling the packets
   at every hop.

   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.  It is getting traction in various industries
   including manufacturing, online gaming, professional A/V, cellular
   radio and others, making possible many cost and performance
   optimizations.

   This innovation is enabled by recent developments in technologies
   including IEEE 802.1 TSN (for Ethernet LANs) and IETF DetNet (for
   wired IP networks).  Reliable and Available Wireless (RAW) networking
   services extend DetNet services 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.

   Wireless networks operate on a shared medium, and thus transmissions
   cannot be fully deterministic due to uncontrolled interferences,
   including the self-induced multipath fading.  However, scheduling of
   transmissions can alleviate those effects by leveraging diversity in
   the spatial, time and frequency domains, providing a more predictable
   and available service.

   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



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   achieve RAW will differ from those used to support time-sensitive
   networking over wires, and 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 establishment of the path is out of scope, and may inherit from a
   centralized Architecture as described for DetNet and 6TiSCH, with a
   primary focus on scheduled wireless operations.  As opposed to wire,
   the action of setting up a path on a wireless network may be slow
   compared to the speed at which the transmission conditions vary, and
   the extra medium used for redundancy may be expensive.  So in
   wireless, it makes sense for a centralized router to provide multiple
   forwarding solutions and leave it to the data plane to select which
   of those solutions are used fir a given packet based on the current
   conditions.

   The scope of the RAW WG will be protocol elements such as OAM to
   improve the forwarding decision along a path where intermediate nodes
   are capable of transmission redundancy, e.g., using packet
   replication and elimination, Hybrid ARQ and coding, but is
   constrained so as not to overuse this methods, eg., because energy
   and spectrum are limited.

   RAW should stay abstract to the radio layer (keep a layered
   approach).  How the PHY is programmed, 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.

   RAW distinguishes the time scale at which routes are computed that we
   qualify as slow from the forwarding time scale where per-packet
   decisions are made.  RAW operates at the forwarding time scale on one
   DetNet flow over one Track that is preestablished and installed by



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   means outside of the scope of RAW.  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-
   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



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   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.

5.  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.

   The RAW forwarding decisions include the selection of points of
   replication and elimination, how many retries can take place, and
   cccccckehblnlcbljtkbcdkrhrjgiibvcidbklbglndf a limit of validity for




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   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 withing 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 inband 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 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.

6.  References

6.1.  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.

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

6.2.  Informative References

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

   [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.

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

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

Author's Address

   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








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