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Internet Area WG                                              C. Perkins
Internet-Draft                                                 Futurewei
Intended status: Informational                                D. Stanley
Expires: September 14, 2017                                          HPE
                                                               W. Kumari
                                                                  Google
                                                              JC. Zuniga
                                                                  SIGFOX
                                                          March 13, 2017


         Multicast Considerations over IEEE 802 Wireless Media
               draft-perkins-intarea-multicast-ieee802-02

Abstract

   Some performance issues have been observed when multicast packet
   transmissions of IETF protocols are used over IEEE 802 wireless
   media.  Even though enhamcements for multicast transmissions have
   been designed at both IETF and IEEE 802, there seems to exist a
   disconnect between specifications, implementations and configuration
   choices.

   This draft describes the different issues that have been observed,
   the multicast enhancement features that have been specified at IETF
   and IEEE 802 for wireless media, as well as the operational chioces
   that can be taken to improve the performace of the network.  Finally,
   it provides some recommendations about the usage and combination of
   these features and operational choices.

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 http://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 September 14, 2017.





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Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Identified mulitcast issues . . . . . . . . . . . . . . . . .   4
     3.1.  Issues at Layer 2 and below . . . . . . . . . . . . . . .   4
       3.1.1.  Multicast reliability . . . . . . . . . . . . . . . .   4
       3.1.2.  Lower data rate . . . . . . . . . . . . . . . . . . .   4
       3.1.3.  Power-save effects on multicast . . . . . . . . . . .   5
     3.2.  Issues at Layer 3 and above . . . . . . . . . . . . . . .   5
       3.2.1.  IPv4 issues . . . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  IPv6 issues . . . . . . . . . . . . . . . . . . . . .   5
       3.2.3.  MLD issues  . . . . . . . . . . . . . . . . . . . . .   6
       3.2.4.  Spurious Neighbor Discovery . . . . . . . . . . . . .   6
   4.  Multicast protocol optimizations  . . . . . . . . . . . . . .   7
     4.1.  Proxy ARP in 802.11-2012  . . . . . . . . . . . . . . . .   7
     4.2.  Buffering to improve Power-Save . . . . . . . . . . . . .   8
     4.3.  IPv6 support in 802.11-2012 . . . . . . . . . . . . . . .   8
     4.4.  Conversion of multicast to unicast  . . . . . . . . . . .   8
     4.5.  Directed Multicast Service (DMS)  . . . . . . . . . . . .   8
     4.6.  GroupCast with Retries (GCR)  . . . . . . . . . . . . . .   9
   5.  Operational optimizations . . . . . . . . . . . . . . . . . .  10
     5.1.  Mitigating Problems from Spurious Neighbor Discovery  . .  10
   6.  Multicast Considerations for Other Wireless Media . . . . . .  12
   7.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13







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1.  Introduction

   Many IETF protocols depend on multicast/broadcast for delivery of
   control messages to multiple receivers.  Multicast is used for
   various purposes such as neighborhood discovery, network flooding,
   address resolution, as well as for reduction in media access for the
   transmission of data that is intended for multiple receivers.

   IETF protocols typically rely on network protocol layering in order
   to reduce or eliminate any dependence of higher level protocols on
   the specific nature of the MAC layer protocols or the physical media.
   In the case of multicast transmissions, higher level protocols have
   traditionally been designed as if transmitting a packet to an IP
   address had the same cost in interference and network media access,
   regardless of whether the destination IP address is a unicast address
   or a multicast or broadcast address.  This model was reasonable for
   networks where the physical medium was usually wired, like Ethernet.
   Unfortunately, for many wireless media, the costs to access the
   medium can be quite different.  Some enhancements have been designed
   in IETF protocols that are assumed to work primarily over wilress
   media.  However, these enhancements are usually implemented in
   limited deployments and not widely spread on most wireless networks.

   IEEE 802 wireless protocols have been designed with certain fetures
   to support multicat traffic.  For instance, lower modulations are
   used to transmit multicast frames, so that these can be received by
   all stations in the cell, regardless of the distance or path
   attenuation from the base station or access point.  However, these
   lower modulation transmissions take longer on the medium and
   therefore they reduce the capabilities to transmit more high
   efficiency traffic with higher order modulations to stations that may
   be in closer vicinity.  Due to these and other reasons, some IEEE 802
   working groups like 802.11 have designed several features to improve
   the performance of multicast transmissions at Layer 2 [REF
   11-15-1261-03].  Besides protocol design features, some operational
   and configuration enhancements can also be applied to overcome the
   network performance issues created by multicast traffic.

   This Internet Draft identifies the problems created by the usage of
   multicast traffic over wireless networks.  It also highlights the
   different enhancements that have been designed at IETF and IEEE 802,
   as well as the operational choices that can be taken, to ameliorate
   the effects of multicast traffic.  Some recommendations about the
   usage and combinations of these enhancements are also provided.







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2.  Terminology

   This document uses the following definitions:

   AP
      IEEE 802.11 Access Point.

   STA
      IEEE 802.11 station.

   basic rate
      The "lowest common denominator" data rate at which multicast and
      broadcast traffic is generally transmitted.

   MCS
      Modulation and Coding Scheme.


3.  Identified mulitcast issues

3.1.  Issues at Layer 2 and below

   In this section we list some of the issues related to the use of
   multicast transmissions over IEEE 802 wireless technologies.

3.1.1.  Multicast reliability

   Multicast traffic is typically much less reliable than unicast
   traffic.  Since multicast makes point-to-multipoint communications,
   multiple acknowledgements would be needed to guarantee the reception
   on all recepients.

3.1.2.  Lower data rate

   Because lower MCS have longer range but also lower data rate,
   multicast / broadcast traffic is generally transmitted at the lowest
   common denominator rate, also known as a basic rate.  On IEEE 802.11
   networks (aka Wi-Fi), this rate might be as low as 6 Mbps, when some
   unicast links in the same cell can be operating at rates up to 600
   Mbps.  Transmissions at a lower rate require more occupancy of the
   wireless medium and thus restrict the airtime for all other medium
   communications and degrade the overall capacity.

   Wired multicast affects wireless LANs because the AP extends the
   wired segment and multicast / broadcast frames on the wired LAN side
   are copied to WLAN.  Since broadcast messages are transmitted at the
   most robust MCS, this implies that large frames sent at slow rate
   over the air.



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3.1.3.  Power-save effects on multicast

   Multicast can work poorly with the power-save mechanisms defined in
   IEEE 802.11.

   o  Both unicast and multicast traffic can be delayed by power-saving
      mechanisms.
   o  Unicast is delayed until a STA wakes up and asks for it.
      Additionally, unicast traffic may be delayed to improve power
      save, efficiency and increase probability of aggregation.
   o  Multicast traffic is delayed in a wireless network if any of the
      STAs in that network are power savers.  All STAs have to be awake
      at a known time to receive multicast traffic.
   o  Packets can also be discarded due to buffer limitations in the AP
      and non-AP STA.

3.2.  Issues at Layer 3 and above

   In this section we mention a few representative IETF protocols, and
   describe some possible negative effects due to performance
   degradation when using multicast transmissions for control messages.
   Common uses of multicast include:

   o  Control plane for IPv4 and IPv6
   o  ARP and Neighbor Discovery
   o  Service discovery
   o  Applications (video delivery, stock data etc)
   o  Other L3 protocols (non-IP)

3.2.1.  IPv4 issues

   The following list contains a few representative IPv4 protocols using
   multicast.

   o  ARP
   o  DHCP
   o  mDNS

   After initial configuration, ARP and DHCP occur much less commonly.

3.2.2.  IPv6 issues

   The following list contains a few representative IPv6 protocols using
   multicast.  IPv6 makes much more extensive use of multicast.

   o  DHCPv6
   o  Liveness detection (NUD)




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   o  Some control plane protocols are not very tolerant of packet loss,
      especially neighbor discovery.
   o  Services may be considered lost if several consecutive packets
      fail.

   Address Resolution

   Service Discovery

   Route Discovery

   Decentralized Address Assignment

   Geographic routing

3.2.3.  MLD issues

   Multicast Listener Discovery(MLD) [RFC4541] is often used to identify
   members of a multicast group that are connected to the ports of a
   switch.  Forwarding multicast frames into a WiFi-enabled area can use
   such switch support for hardware forwarding state information.
   However, since IPv6 makes heavy use of multicast, each STA with an
   IPv6 address will require state on the switch for several and
   possibly many multicast solicited-node addresses.  Multicast
   addresses that do not have forwarding state installed (perhaps due to
   hardware memory limitations on the switch) cause frames to be flooded
   on all ports of the switch.

3.2.4.  Spurious Neighbor Discovery

   On the Internet there is a "background radiation" of scanning traffic
   (people scanning for vulnerable machines) and backscatter (responses
   from spoofed traffic, etc).  This means that the router is constantly
   getting packets destined for machines whose IP addresses may or may
   not be in use.  In the cases where the IP is assigned to a machine,
   the router broadcasts an ARP request, gets back an ARP reply, caches
   this and then can deliver traffic to the host.  In the cases where
   the IP address is not in use, the router broadcasts one (or more) ARP
   requests, and never gets a reply.  This means that it does not
   populate the ARP cache, and the next time there is traffic for that
   IP address it will broadcast ARP requests again.

   The rate of these ARP requests is proportional to the size of the
   subnets, the rate of scanning and backscatter, and how long the
   router keeps state on non-responding ARPs.  As it turns out, this
   rate is inversely proportional to how occupied the subnet is (valid
   ARPs end up in a cache, stopping the broadcasting; unused IPs never
   respond, and so cause more broadcasts).  Depending on the address



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   space in use, the time of day, how occupied the subnet is, and other
   unknown factors, on the order of 2000 broadcasts per second have been
   observed at the IETF NOCs.

   On a wired network, there is not a huge difference amongst unicast,
   multicast and broadcast traffic; but this is not true in the wireless
   realm.  Wireless equipment often is unable to send this amount of
   broadcast and multicast traffic.  Consequently, on the wireless
   networks, we observe a significant amount of dropped broadcast and
   multicast packets.  This, in turn, means that when a host connects it
   is often not able to complete DHCP, and IPv6 RAs get dropped, leading
   to users being unable to use the network.

4.  Multicast protocol optimizations

   This section lists some optimizations that have been specified in
   IEEE 802 and IETF that are aimed at reducing or eliminating the
   issues discussed in Section 3.

4.1.  Proxy ARP in 802.11-2012

   The AP knows all associated STAs MAC address and IP address; in other
   words, the AP acts as the central "manager" for all the 802.11 STAs
   in its BSS.  Proxy ARP is easy to implement at the AP, and offers the
   following advantages:

   o  Reduced broadcast traffic (transmitted at low MCS) on the wireless
      medium
   o  STA benefits from extended power save in sleep mode, as ARP
      requests are replied to by AP.
   o  Keeps ARP frames off the wireless medium.
   o  Changes are not needed to STA implementation.

   Here is the specification language from clause 10.23.13 in [2] as
   described in [dot11-proxyarp]:

      When the AP supports Proxy ARP "[...] the AP shall maintain a
      Hardware Address to Internet Address mapping for each associated
      station, and shall update the mapping when the Internet Address of
      the associated station changes.  When the IPv4 address being
      resolved in the ARP request packet is used by a non-AP STA
      currently associated to the BSS, the proxy ARP service shall
      respond on behalf of the non-AP STA"








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4.2.  Buffering to improve Power-Save

   The AP acts on behalf of STAs in various ways.  In order to improve
   the power-saving feature for STAs in its BSS, the AP buffers frames
   for delivery to the STA at the time when the STA is scheduled for
   reception.

4.3.  IPv6 support in 802.11-2012

   IPv6 uses Neighbor Discovery Protocol (NDP) instead Every IPv6 node
   subscribes to special multicast address Neighbor-Solicitation message
   replaces ARP

   Here is the specification language from-10.23.13 in [2]:

      "When an IPv6 address is being resolved, the Proxy Neighbor
      Discovery service shall respond with a Neighbor Advertisement
      message [...] on behalf of an associated STA to an [ICMPv6]
      Neighbor Solicitation message [...].  When MAC address mappings
      change, the AP may send unsolicited Neighbor Advertisement
      Messages on behalf of a STA."

   NDP may be used to request additional information

   o  Maximum Transmission Unit
   o  Router Solicitation
   o  Router Advertisement, etc.

   NDP messages are sent as group addressed (broadcast) frames in
   802.11.  Using the proxy operation helps to keep NDP messages off the
   wireless medium.

4.4.  Conversion of multicast to unicast

   It is often possible to transmit control and data messages by using
   unicast transmissions to each station individually.

4.5.  Directed Multicast Service (DMS)

   There are situations where more is needed than simply converting
   multicast to unicast [Editor's note: citation needed].  For these
   purposes, DMS enables a client to request that the AP transmit
   multicast group addressed frames destined to the requesting clients
   as individually addressed frames [i.e., convert multicast to
   unicast].

   o  DMS Requires 802.11n A-MSDUs




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   o  Individually addressed frames are acknowledged and are buffered
      for power save clients
   o  Requesting STA may specify traffic characteristics for DMS traffic
   o  DMS was defined in IEEE Std 802.11v-2011

   DMS is not currently implemented in products.  DMS does require
   changes to both AP and STA implementation.

4.6.  GroupCast with Retries (GCR)

   GCR (defined in [dot11aa]) provides greater reliability by using
   either unsolicited retries or a block acknowledgement mechanism.  GCR
   increases probability of broadcast frame reception success, but still
   does not guarantee success.

   For the block acknowledgement mechanism, the AP transmits each group
   addressed frame as conventional group addressed transmission.
   Retransmissions are group addressed, but hidden from non-11aa
   clients.  A directed block acknowledgement scheme is used to harvest
   reception status from receivers; retransmissions are based upon these
   responses.

   GCR is suitable for all group sizes including medium to large groups.
   As the number of devices in the group increases, GCR can send block
   acknowledgement requests to only a small subset of the group.  GCR
   does require changes to both AP and STA implementation.

   GCR may introduce unacceptable latency.  After sending a group of
   data frames to the group, the AP has do the following:

   o  unicast a Block Ack Request (BAR) to a subset of members.
   o  wait for the corresponding Block Ack (BA).
   o  retransmit any missed frames.
   o  resume other operations which may have been delayed.

   This latency may not be acceptable for some traffic.

   There are ongoing extensions in 802.11 to improve GCR performance.

   o  BAR is sent using downlink MU-MIMO (note that downlink MU-MIMO is
      already specified in 802.11-REVmc 4.3).
   o  BA is sent using uplink MU-MIMO (which is a .11ax feature).
   o  Additional 802.11ax extensions are under consideration; see
      [mc-ack-mux]
   o  Latency may also be reduced by simultaneously receiving BA
      information from multiple clients.





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5.  Operational optimizations

   This section lists some operational optimizations that can be
   implemented when deploying wireless IEEE 802 networks to mitigate the
   issues discussed in Section 3.

5.1.  Mitigating Problems from Spurious Neighbor Discovery

   ARP Sponges

         An ARP Sponge sits on a network and learn which IPs addresses
         are actually in use.  It also listen for ARP requests, and, if
         it sees an ARP for an IP address which it believes is not used,
         it will reply with its own MAC address.  This means that the
         router now has an IP to MAC mapping, which it caches.  If that
         IP is later assigned to an machine (e.g using DHCP), the ARP
         sponge will see this, and will stop replying for that address.
         Gratuitous ARPs (or the machine ARPing for its gateway) will
         replace the sponged address in the router ARP table.  This
         technique is quite effective; but, unfortunately, the ARP
         sponge daemons were not really designed for this use (the
         standard one [arpsponge], was designed to deal with the
         disappearance of participants from an IXP) and so are not
         optimized for this purpose.  We have to run one daemon per
         subnet, the tuning is tricky (the scanning rate versus the
         population rate versus retires, etc.) and sometimes the daemons
         just seem to stop, requiring a restart of the daemon and
         causing disruption.

   Router mitigations

         Some routers (often those based on Linux) implement a "negative
         ARP cache" daemon.  Simply put, if the router does not see a
         reply to an ARP it can be configured to cache this information
         for some interval.  Unfortunately, the core routers which we
         are using do not support this.  When a host connects to network
         and gets an IP address, it will ARP for its default gateway
         (the router).  The router will update its cache with the IP to
         host MAC mapping learnt from the request (passive ARP
         learning).

   Firewall unused space

         The distribution of users on wireless networks / subnets
         changes from meeting to meeting (e.g the "IETF-secure" SSID was
         renamed to "IETF", fewer users use "IETF-legacy", etc).  This
         utilization is difficult to predict ahead of time, but we can
         monitor the usage as attendees use the different networks.  By



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         configuring multiple DHCP pools per subnet, and enabling them
         sequentially, we can have a large subnet, but only assign
         addresses from the lower portions of it.  This means that we
         can apply input IP access lists, which deny traffic to the
         upper, unused portions.  This means that the router does not
         attempt to forward packets to the unused portions of the
         subnets, and so does not ARP for it.  This method has proven to
         be very effective, but is somewhat of a blunt axe, is fairly
         labor intensive, and requires coordination.

   Disabling/filtering ARP requests

         In general, the router does not need to ARP for hosts; when a
         host connects, the router can learn the IP to MAC mapping from
         the ARP request sent by that host.  This means that we should
         be able to disable and / or filter ARP requests from the
         router.  Unfortunately, ARP is a very low level / fundamental
         part of the IP stack, and is often offloaded from the normal
         control plane.  While many routers can filter layer-2 traffic,
         this is usually implemented as an input filter and / or has
         limited ability to filter output broadcast traffic.  This means
         that the simple "just disable ARP or filter it outbound" seems
         like a really simple (and obvious) solution, but
         implementations / architectural issues make this difficult or
         awkward in practice.

   NAT

         The broadcasts are overwhelmingly being caused by outside
         scanning / backscatter traffic.  This means that, if we were to
         NAT the entire (or a large portion) of the attendee networks,
         there would be no NAT translation entries for unused addresses,
         and so the router would never ARP for them.  The IETF NOC has
         discussed NATing the entire (or large portions) attendee
         address space, but a: elegance and b: flaming torches and
         pitchfork concerns means we have not attempted this yet.

   Stateful firewalls

         Another obvious solution would be to put a stateful firewall
         between the wireless network and the Internet.  This firewall
         would block incoming traffic not associated with an outbound
         request.  The IETF philosophy has been to have the network as
         open as possible / honor the end-to-end principle.  An attendee
         on the meeting network should be an Internet host, and should
         be able to receive unsolicited requests.  Unfortunately,
         keeping the network working and stable is the first priority




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         and a stateful firewall may be required in order to achieve
         this.

6.  Multicast Considerations for Other Wireless Media

   Many of the causes of performance degradation described in earlier
   sections are also observable for wireless media other than 802.11.

   For instance, problems with power save, excess media occupancy, and
   poor reliability will also affect 802.15.3 and 802.15.4.  However,
   802.15 media specifications do not include similar mechanisms of the
   type that have been developed for 802.11.  In fact, the design
   philosophy for 802.15 is more oriented towards minimality, with the
   result that many such functions would more likely be relegated to
   operation within higher layer protocols.  This leads to a patchwork
   of non-interoperable and vendor-specific solutions.  See [uli] for
   some additional discussion, and a proposal for a task group to
   resolve similar issues, in which the multicast problems might be
   considered for mitigation.

7.  Recommendations

   This section provides some recommendations about the usage and
   combinations of the multicast enhancements described in Section 4 and
   Section 5.

   (FFS)

8.  Security Considerations

   This document does not introduce any security mechanisms, and does
   not have any impact on existing security mechanisms.

9.  IANA Considerations

   This document does not specify any IANA actions.

10.  Informative References

   [arpsponge]
              Arien Vijn, Steven Bakker, , "Arp Sponge", March 2015.

   [dot11]    P802.11, , "Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", March
              2012.

   [dot11-proxyarp]
              P802.11, , "Proxy ARP in 802.11ax", September 2015.



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   [dot11aa]  P802.11, , "Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications Amendment 2:
              MAC Enhancements for Robust Audio Video Streaming", March
              2012.

   [mc-ack-mux]
              Yusuke Tanaka et al., , "Multiplexing of Acknowledgements
              for Multicast Transmission", July 2015.

   [mc-prob-stmt]
              Mikael Abrahamsson and Adrian Stephens, , "Multicast on
              802.11", March 2015.

   [mc-props]
              Adrian Stephens, , "IEEE 802.11 multicast properties",
              March 2015.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <http://www.rfc-editor.org/info/rfc4541>.

   [uli]      Pat Kinney, , "LLC Proposal for 802.15.4", Nov 2015.

Authors' Addresses

   Charles E. Perkins
   Futurewei Inc.
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Phone: +1-408-330-4586
   Email: charliep@computer.org


   Dorothy Stanley
   Hewlett Packard Enterprise
   2000 North Naperville Rd.
   Naperville, IL  60566
   USA

   Phone: +1 630 979 1572
   Email: dstanley@arubanetworks.com






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   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   Email: warren@kumari.net


   Juan Carlos Zuniga
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: j.c.zuniga@ieee.org



































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