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RAW                                                         F. Theoleyre
Internet-Draft                                                      CNRS
Intended status: Standards Track                         G. Papadopoulos
Expires: April 28, 2021                                   IMT Atlantique
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        October 25, 2020


   Operations, Administration and Maintenance (OAM) features for RAW
                   draft-theoleyre-raw-oam-support-04

Abstract

   Some critical applications may use a wireless infrastructure.
   However, wireless networks exhibit a bandwidth of several orders of
   magnitude lower than wired networks.  Besides, wireless transmissions
   are lossy by nature; the probability that a packet cannot be decoded
   correctly by the receiver may be quite high.  In these conditions,
   guaranteeing the network infrastructure works properly is
   particularly challenging, since we need to address some issues
   specific to wireless networks.  This document lists the requirements
   of the Operation, Administration, and Maintenance (OAM) features
   recommended to construct a predictable communication infrastructure
   on top of a collection of wireless segments.  This document describes
   the benefits, problems, and trade-offs for using OAM in wireless
   networks to achieve Service Level Objectives (SLO).

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 28, 2021.







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Copyright Notice

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

   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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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Role of OAM in RAW  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Link concept and quality  . . . . . . . . . . . . . . . .   6
     2.2.  Broadcast Transmissions . . . . . . . . . . . . . . . . .   6
     2.3.  Complex Layer 2 Forwarding  . . . . . . . . . . . . . . .   7
   3.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Information Collection  . . . . . . . . . . . . . . . . .   7
     3.2.  Continuity Check  . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Connectivity Verification . . . . . . . . . . . . . . . .   7
     3.4.  Route Tracing . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Fault Verification/detection  . . . . . . . . . . . . . .   8
     3.6.  Fault Isolation/identification  . . . . . . . . . . . . .   8
   4.  Administration  . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Worst-case metrics  . . . . . . . . . . . . . . . . . . .   9
     4.2.  Efficient data retrieval  . . . . . . . . . . . . . . . .  10
   5.  Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Dynamic Resource Reservation  . . . . . . . . . . . . . .  11
     5.2.  Reliable Reconfiguration  . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13








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1.  Introduction

   Reliable and Available Wireless (RAW) is an effort that extends
   DetNet to approach end-to-end deterministic performances over a
   network that includes scheduled wireless segments.  In wired
   networks, many approaches try to enable Quality of Service (QoS) by
   implementing traffic differentiation so that routers handle each type
   of packets differently.  However, this differentiated treatment was
   expensive for most applications.

   Deterministic Networking (DetNet) [RFC8655] has proposed to provide a
   bounded end-to-end latency on top of the network infrastructure,
   comprising both Layer 2 bridged and Layer 3 routed segments.  Their
   work encompasses the data plane, OAM, time synchronization,
   management, control, and security aspects.

   However, wireless networks create specific challenges.  First of all,
   radio bandwidth is significantly lower than for wired networks.  In
   these conditions, the volume of signaling messages has to be very
   limited.  Even worse, wireless links are lossy: a layer 2
   transmission may or may not be decoded correctly by the receiver,
   depending on a broad set of parameters.  Thus, providing high
   reliability through wireless segments is particularly challenging.

   Wired networks rely on the concept of _links_. All the devices
   attached to a link receive any transmission.  The concept of a link
   in wireless networks is somewhat different from what many are used to
   in wireline networks.  A receiver may or may not receive a
   transmission, depending on the presence of a colliding transmission,
   the radio channel's quality, and the external interference.  Besides,
   a wireless transmission is broadcast by nature: any _neighboring_
   device may be able to decode it.  The document includes detailed
   information on what the implications for the OAM features are.

   Last but not least, radio links present volatile characteristics.  If
   the wireless networks use an unlicensed band, packet losses are not
   anymore temporally and spatially independent.  Typically, links may
   exhibit a very bursty characteristic, where several consecutive
   packets may be dropped.  Thus, providing availability and reliability
   on top of the wireless infrastructure requires specific Layer 3
   mechanisms to counteract these bursty losses.

   Operations, Administration, and Maintenance (OAM) Tools are of
   primary importance for IP networks [RFC7276].  It defines a toolset
   for fault detection, isolation, and performance measurement.

   The primary purpose of this document is to detail the specific
   requirements of the OAM features recommended to construct a



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   predictable communication infrastructure on top of a collection of
   wireless segments.  This document describes the benefits, problems,
   and trade-offs for using OAM in wireless networks to provide
   availability and predictability.

   In this document, the term OAM will be used according to its
   definition specified in [RFC6291].  We expect to implement an OAM
   framework in RAW networks to maintain a real-time view of the network
   infrastructure, and its ability to respect the Service Level
   Objectives (SLO), such as delay and reliability, assigned to each
   data flow.

1.1.  Terminology

   We re-use here the same terminology as [detnet-oam]:

   o  OAM entity: a data flow to be controlled;

   o  Maintenance End Point (MEP): OAM devices crossed when entering/
      exiting the network.  In RAW, it corresponds mostly to the source
      or destination of a data flow.  OAM message can be exchanges
      between two MEPs;

   o  Maintenance Intermediate endPoint (MIP): OAM devices along the
      flow; OAM messages can be exchanged between a MEP and a MIP;

   o  control/data plane: while the control plane expects to configure
      and control the network (long-term), the data plane takes the
      individual decision;

   o  passive / active methods (as defined in [RFC7799]): active methods
      send additionnal control information (inserting novel fields,
      generating novel control packets).  Passive methods infer
      information just by observing unmodified existing flows.

   o  active methods may implement one of these two strategies:

      *  In-band: control information follows the same path as the data
         packets.  In other words, a failure in the data plane may
         prevent the control information to reach the destination (e.g.,
         end-device or controller).

      *  out-of-band: control information is sent separately from the
         data packets.  Thus, the behavior of control vs. data packets
         may differ;

   We also adopt the following terminology, which is particularly
   relevant for RAW segments.



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   o  piggybacking vs. dedicated control packets: control information
      may be encapsulated in specific (dedicated) control packets.
      Alternatively, it may be piggybacked in existing data packets,
      when the MTU is larger than the actual packet length.
      Piggybacking makes specifically sense in wireless networks: the
      cost (bandwidth and energy) is not linear with the packet size.

   o  router-over vs. mesh under: a control packet is either forwarded
      directly to the layer-3 next hop (mesh under) or handled hop-by-
      hop by each router.  While the latter option consumes more
      resource, it allows to collect additionnal intermediary
      information, particularly relevant in wireless networks.

   o  Defect: a temporary change in the network (e.g., a radio link
      which is broken due to a mobile obstacle);

   o  Fault: a definite change which may affect the network performance,
      e.g., a node runs out of energy.

1.2.  Acronyms

   OAM Operations, Administration, and Maintenance

   DetNet Deterministic Networking

   SLO Service Level Objective

   QoS Quality of Service

   SNMP Simple Network Management Protocol

   SDN Software-Defined Network

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Role of OAM in RAW

   RAW networks expect to make the communications reliable and
   predictable on top of a wireless network infrastructure.  Most
   critical applications will define an SLO to be required for the data
   flows it generates.  RAW considers network plane protocol elements




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   such as OAM to improve the RAW operation at the service and the
   forwarding sub-layers.

   To respect strict guarantees, RAW relies on an orchestrator able to
   monitor and maintain the network.  Typically, a Software-Defined
   Network (SDN) controller is in charge of scheduling the transmissions
   in the deployed network, based on the radio link characteristics, SLO
   of the flows, the number of packets to forward.  Thus, resources have
   to be provisioned a priori to handle any defect.  OAM represents the
   core of the pre-provisioning process and maintains the network
   operational by updating the schedule dynamically.

   Fault-tolerance also assumes that multiple paths have to be
   provisioned so that an end-to-end circuit keeps on existing whatever
   the conditions.  The Packet Replication and Elimination Function
   ([PREF-draft]) on a node is typically controlled by a central
   controller/orchestrator.  OAM mechanisms can be used to monitor that
   PREOF is working correctly on a node and within the domain.

   To be energy-efficient, reserving some dedicated out-of-band
   resources for OAM seems idealistic, and only in-band solutions are
   considered here.

   RAW supports both proactive and on-demand troubleshooting.

   The specific characteristics of RAW are discussed below.

2.1.  Link concept and quality

   In wireless networks, a _link_ does not exist physically.  A common
   convention is to define a wireless link as a pair of devices that
   have a non-null probability of exchanging a packet that the receiver
   can decode.  Similarly, we designate as *neighbor* any device with a
   radio link with a specific transmitter.

   Each wireless link is associated with a link quality, often measured
   as the Packet Delivery Ratio (PDR), i.e., the probability that the
   receiver can decode the packet correctly.  It is worth noting that
   this link quality depends on many criteria, such as the level of
   external interference, the presence of concurrent transmissions, or
   the radio channel state.  This link quality is even time-variant.

2.2.  Broadcast Transmissions

   In modern switching networks, the unicast transmission is delivered
   uniquely to the destination.  Wireless networks are much closer to
   the ancient *shared access* networks.  Practically, unicast and
   broadcast frames are handled similarly at the physical layer.  The



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   link layer is just in charge of filtering the frames to discard
   irrelevant receptions (e.g., different unicast MAC address).

   However, contrary to wired networks, we cannot be sure that a packet
   is received by *all* the devices attached to the layer-2 segment.  It
   depends on the radio channel state between the transmitter(s) and the
   receiver(s).  In particular, concurrent transmissions may be possible
   or not, depending on the radio conditions (e.g., do the different
   transmitters use a different radio channel or are they sufficiently
   spatially separated?)

2.3.  Complex Layer 2 Forwarding

   Multiple neighbors may receive a transmission.  Thus, anycast layer-2
   forwarding helps to maximize the reliability by assigning multiple
   receivers to a single transmission.  That way, the packet is lost
   only if *none* of the receivers decode it.  Practically, it has been
   proven that different neighbors may exhibit very different radio
   conditions, and that reception independency may hold for some of them
   [anycast-property].

3.  Operation

   OAM features will enable RAW with robust operation both for
   forwarding and routing purposes.

3.1.  Information Collection

   The model to exchange information should be the same as for detnet
   network, for the sake of inter-operability.  YANG may typically
   fulfill this objective.

   However, RAW networks imply specific constraints (e.g., low
   bandwidth, packet losses, cost of medium access) that may require to
   minimize the volume of information to collect.  Thus, we discuss in
   Section 4.2 the different ways to collect information, i.e., transfer
   physically the OAM information from the emitter to the receiver.

3.2.  Continuity Check

   Similarly to detnet, we need to verify that the source and the
   destination are connected (at least one valid path exists)

3.3.  Connectivity Verification

   As in detnet, we have to verify the absence of misconnection.  We
   will focus here on the RAW specificities.




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   Because of radio transmissions' broadcast nature, several receivers
   may be active at the same time to enable anycast Layer 2 forwarding.
   Thus, the connectivity verification must test any combination.  We
   also consider priority-based mechanisms for anycast forwarding, i.e.,
   all the receivers have different probabilities of forwarding a
   packet.  To verify a delay SLO for a given flow, we must also
   consider all the possible combinations, leading to a probability
   distribution function for end-to-end transmissions.  If this
   verification is implemented naively, the number of combinations to
   test may be exponential and too costly for wireless networks with low
   bandwidth.

3.4.  Route Tracing

   Wireless networks are meshed by nature: we have many redundant radio
   links.  These meshed networks are both an asset and a drawback: while
   several paths exist between two endpoints, and we should choose the
   most efficient one(s), concerning specifically the reliability, and
   the delay.

   Thus, multipath routing can be considered to make the network fault-
   tolerant.  Even better, we can exploit the broadcast nature of
   wireless networks to exploit meshed multipath routing: we may have
   multiple Maintenance Intermediate Endpoints (MIE) for each hop in the
   path.  In that way, each Maintenance Intermediate Endpoint has
   several possible next hops in the forwarding plane.  Thus, all the
   possible paths between two maintenance endpoints should be retrieved,
   which may quickly become untractable if we apply a naive approach.

3.5.  Fault Verification/detection

   Wired networks tend to present stable performances.  On the contrary,
   wireless networks are time-variant.  We must consequently make a
   distinction between _normal_ evolutions and malfunction.

3.6.  Fault Isolation/identification

   The network has isolated and identified the cause of the fault.
   While detnet already expects to identify malfunctions, some problems
   are specific to wireless networks.  We must consequently collect
   metrics and implement algorithms tailored for wireless networking.

   For instance, the decrease in the link quality may be caused by
   several factors: external interference, obstacles, multipath fading,
   mobility.  It it fundamental to be able to discriminate the different
   causes to make the right decision.





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4.  Administration

   The RAW network has to expose a collection of metrics to support an
   operator making proper decisions, including:

   o  Packet losses: the time-window average and maximum values of the
      number of packet losses have to be measured.  Many critical
      applications stop to work if a few consecutive packets are
      dropped;

   o  Received Signal Strength Indicator (RSSI) is a very common metric
      in wireless to denote the link quality.  The radio chipset is in
      charge of translating a received signal strength into a normalized
      quality indicator;

   o  Delay: the time elapsed between a packet generation / enqueuing
      and its reception by the next hop;

   o  Buffer occupancy: the number of packets present in the buffer, for
      each of the existing flows.

   These metrics should be collected per device, virtual circuit, and
   path, as detnet already does.  However, we have to face in RAW to a
   finer granularity:

   o  per radio channel to measure, e.g., the level of external
      interference, and to be able to apply counter-measures (e.g.,
      blacklisting).

   o  per link to detect misbehaving link (assymetrical link,
      fluctuating quality).

   o  per resource block: a collision in the schedule is particularly
      challenging to identify in radio networks with spectrum reuse.  In
      particular, a collision may not be systematic (depending on the
      radio characteristics and the traffic profile)

4.1.  Worst-case metrics

   RAW inherits the same requirements as detnet: we need to know the
   distribution of a collection of metrics.  However, wireless networks
   are know to be highly variable.  Changes may be frequent, and may
   exhibit a periodical pattern.  Collecting and analyzing this amount
   of measurements is challenging.

   Wireless networks are known to be lossy, and RAW has to implement
   strategies to improve reliability on top of unreliable links.  Hybrid
   Automatic Repeat reQuest (ARQ) has typically to enable



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   retransmissions based on the end-to-end reliability and latency
   requirements.

4.2.  Efficient data retrieval

   We have to minimize the number of statistics / measurements to
   exchange:

   o  energy efficiency: low-power devices have to limit the volume of
      monitoring information since every bit consumes energy.

   o  bandwidth: wireless networks exhibit a bandwidth significantly
      lower than wired, best-effort networks.

   o  per-packet cost: it is often more expensive to send several
      packets instead of combining them in a single link-layer frame.

   In conclusion, we have to take care of power and bandwidth
   consumption.  The following techniques aim to reduce the cost of such
   maintenance:

      on-path collection: some control information is inserted in the
      data packets if they do not fragment the packet (i.e., the MTU is
      not exceeded).  Information Elements represent a standardized way
      to handle such information;

      flags/fields: we have to set-up flags in the packets to monitor to
      be able to monitor the forwarding process accurately.  A sequence
      number field may help to detect packet losses.  Similarly, path
      inference tools such as [ipath] insert additional information in
      the headers to identify the path followed by a packet a
      posteriori.

      hierarchical monitoring; localized and centralized mechanisms have
      to be combined together.  Typically, a local mechanism should
      contiuously monitor a set of metrics and trigger distant OAM
      exchances only when a fault is detected (but possibly not
      identified).  For instance, local temporary defects must not
      trigger expensive OAM transmissions.

5.  Maintenance

   RAW needs to implement a self-healing and self-optimization approach.
   The network must continuously retrieve the state of the network, to
   judge about the relevance of a reconfiguration, quantifying:

      the cost of the sub-optimality: resources may not be used
      optimally (e.g., a better path exists);



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      the reconfiguration cost: the controller needs to trigger some
      reconfigurations.  For this transient period, resources may be
      twice reserved, and control packets have to be transmitted.

   Thus, reconfiguration may only be triggered if the gain is
   significant.

5.1.  Dynamic Resource Reservation

   Wireless networks exhibit time-variant characteristics.  Thus, the
   network has to provide additional resources along the path to fit the
   worst-case performance.  This time-variant characteristics make the
   resource reservation very challenging: over-reaction waste radio and
   energy resources.  Inversely, under-reaction jeopardize the network
   operations, and some SLO may be violated.

5.2.  Reliable Reconfiguration

   Wireless networks are known to be lossy.  Thus, commands may be
   received or not by the node to reconfigure.  Unfortunately,
   inconsistent states may create critical misconfigurations, where
   packets may be lost along a path because it has not been properly
   configured.

   We have to propose mechanisms to guarantee that the network state is
   always consistent, even if some control packets are lost.  Timeouts
   and retransmissions are not sufficient since the reconfiguration
   duration would be, in that case, unbounded.

6.  IANA Considerations

   This document has no actionable requirements for IANA.  This section
   can be removed before the publication.

7.  Security Considerations

   This section will be expanded in future versions of the draft.

8.  Acknowledgments

   TBD

9.  Informative References








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   [anycast-property]
              Teles Hermeto, R., Gallais, A., and F. Theoleyre, "Is
              Link-Layer Anycast Scheduling Relevant for IEEE
              802.15.4-TSCH Networks?", 2019,
              <https://doi.org/10.1109/LCNSymposium47956.2019.9000679>.

   [detnet-oam]
              Theoleyre, F., Papadopoulos, G. Z., Mirsky, G., and C. J.
              Bernardos, "Operations, Administration and Maintenance
              (OAM) features for detnet", 2020,
              <https://tools.ietf.org/html/draft-theoleyre-detnet-oam-
              support>.

   [ipath]    Gao, Y., Dong, W., Chen, C., Bu, J., Wu, W., and X. Liu,
              "iPath: path inference in wireless sensor networks.",
              2016, <https://doi.org/10.1109/TNET.2014.2371459>.

   [PREF-draft]
              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", 2018,
              <https://tools.ietf.org/html/draft-thubert-bier-
              replication-elimination>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc6291>.

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,
              <https://www.rfc-editor.org/info/rfc7276>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.



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   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

Authors' Addresses

   Fabrice Theoleyre
   CNRS
   Building B
   300 boulevard Sebastien Brant - CS 10413
   Illkirch - Strasbourg  67400
   FRANCE

   Phone: +33 368 85 45 33
   Email: theoleyre@unistra.fr
   URI:   http://www.theoleyre.eu


   Georgios Z. Papadopoulos
   IMT Atlantique
   Office B00 - 102A
   2 Rue de la Chataigneraie
   Cesson-Sevigne - Rennes  35510
   FRANCE

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


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com

















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