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Versions: 00 01 02 03 draft-ietf-mboned-ieee802-mcast-problems

Internet Area [intarea]                                       C. Perkins
Internet-Draft                                                 Futurewei
Expires: September 22, 2016                                   D. Stanley
                                                               W. Kumari
                                                              JC. Zuniga
                                                          March 21, 2016

         Multicast Considerations over IEEE 802 Wireless Media


   This document describes some performance issues that have been
   observed when multicast packet transmission is attempted over IEEE
   802 wireless media.  Multicast features specified for IEEE 802
   wireless media related to multicast are also described, along with
   explanations about how these features can help ameliorate the
   observed performance issues.  IETF protocols that are likely to be
   affected by the observed performance issues are identified, and
   workarounds are proposed in some cases.  The performance of multicast
   over wireless media often can be quite different than the performance
   of unicast.  This draft describes the nature of the differences and
   the effects on representative IETF protocols.  We also describe some
   efforts that have been made by IEEE 802 Wireless groups to ameliorate
   the performance differences.

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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 September 22, 2016.

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Copyright Notice

   Copyright (c) 2016 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
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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Identified Issues at Layer 2  . . . . . . . . . . . . . . . .   3
   4.  Some Possible Effects on Representative IETF protocols  . . .   4
     4.1.  IPv4 uses . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  IPv6 uses . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Disabling Multicast on WiFi . . . . . . . . . . . . . . .   5
     4.4.  Spurious Neighbor Discovery . . . . . . . . . . . . . . .   5
   5.  Layer 2 optimizations . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Proxy ARP in 802.11-2012  . . . . . . . . . . . . . . . .   6
     5.2.  Buffering to improve Power-Save . . . . . . . . . . . . .   7
     5.3.  IPv6 support in 802.11-2012 . . . . . . . . . . . . . . .   7
     5.4.  Directed Multicast Service (DMS)  . . . . . . . . . . . .   7
     5.5.  GroupCast with Retries (GCR)  . . . . . . . . . . . . . .   8
   6.  Higher Layer Optimizations and Mitigations  . . . . . . . . .   8
     6.1.  Mitigating Problems from Spurious Neighbor Discovery  . .   9
   7.  Multicast Considerations for Other Wireless Media . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Many IETF protocol designs depend upon multicast or 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 reduction in media access
   for data traffic.

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   IETF protocols typically expect to 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 transmission, higher level
   protocols may be designed as if transmitting a packet to an IP
   address has 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 of operation was
   reasonable for networks where the physical medium was like an

   Unfortunately, for many wireless media, the costs can be quite
   different.  It is the purpose of this Internet Draft to identify the
   ways in which the costs can be different.  Using this information, we
   then proceed to identify some possible effects on the actual
   operation of IETF protocols over wireless media.

   IEEE 802 Wireless working groups, especially 802.11, have made a
   number of attempts to improve the performance of multicast
   transmissions at layer 2.  In this draft we also include a
   description of some of these efforts.  This information is closely
   related to material presented at IETF 94 [cite 11-15-1261-03]

2.  Terminology

   This document defines the following terminology:

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

      Modulation and Coding Scheme.

3.  Identified Issues at Layer 2

   In this section we list some of the issues arising at layer 2
   surrounding the use of multicast in IETF protocols over wireless

   o  Multicast traffic is typically much less reliable than unicast
   o  Multicast / broadcast traffic is generally sent at a lowest common
      denominator rate, known as a basic rate.  This might be as low as
      6 Mbps, when unicast links are operating at 600 Mbps.
      Transmission at a lower rate requires more occupancy of the
      wireless medium and thus less airtime for everything else.

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   o  Wireless multicast affects wired LANs because the AP extends the
      wired segment.

      *  All broadcast frames on LAN side are copied to WLAN.
      *  In WLAN, broadcast messages transmitted at most robust MCS.
      *  Most robust MCS implies large frames sent at slow rate.
   o  Multicast can work poorly with the power-save mechanisms in

      *  Both unicast and multicast traffic can be delayed by power-
         saving mechanisms.
      *  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.
      *  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.
      *  Packets can also be discarded due to buffer limitations in the
         AP and non-AP STA.

4.  Some Possible Effects on Representative IETF protocols

   In this section we list some of the issues arising at layer 3
   surrounding the use of multicast in IETF protocols over wireless
   media.  We mention a few representative IETF protocols, and describe
   some possible effects due to performance degradation when using
   multicast transmissions for control messages.  Common uses 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)

4.1.  IPv4 uses

   The following list contains a few representative IPv4 protocols using

   o  ARP
   o  DHCP
   o  mDNS

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

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4.2.  IPv6 uses

   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)
   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

   Address Resolution

   Service Discovery

   Route Discovery

   Decentralized Address Assignment

   Geographic routing

4.3.  Disabling Multicast on WiFi

   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.

4.4.  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

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   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 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

   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.

5.  Layer 2 optimizations

   This section lists some optimizations that have been specified for
   use with 802.11 that are aimed at reducing or eliminating the causes
   of performance loss discussed in section Section 3.

5.1.  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
   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.

   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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5.2.  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

5.3.  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.

5.4.  Directed Multicast Service (DMS)

   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
   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.

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5.5.  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

   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 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
   o  Latency may also be reduced by simultaneously receiving BA
      information from multiple clients.

6.  Higher Layer Optimizations and Mitigations

   This section lists some optimizations that have been specified for
   use with 802.11 that are aimed at reducing or eliminating the causes
   of performance loss discussed in section Section 6.

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6.1.  Mitigating Problems from Spurious Neighbor Discovery

   ARP Sponges

         ARP Sponges sit on a network and learn what IPs addresses are
         actually in use.  They 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

   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
         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

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         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.


         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
         and a stateful firewall may be required in order to achieve

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7.  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.

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

              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

              P802.11, , "Proxy ARP in 802.11ax", September 2015.

   [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

              Yusuke Tanaka et al., , "Multiplexing of Acknowledgements
              for Multicast Transmission", July 2015.

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              Mikael Abrahamsson and Adrian Stephens, , "Multicast on
              802.11", March 2015.

              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,

   [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

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

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

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

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: warren@kumari.net

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   Juan Carlos Zuniga
   1000 Sherbrooke W, 10th Floor
   Montreal, QC  H3A 3G4

   Email: j.c.zuniga@ieee.org

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