MBONED WG                                                     M. McBride
Internet-Draft                                                C. Perkins
Intended status: Standards Track                                  Huawei
Expires: July 28, 2018                                  January 24, 2018

                    Multicast WiFi Problem Statement


   There have been known issues with multicast, in an 802.11
   environment, which have prevented the deployment of multicast in
   these wifi environments.  IETF multicast experts have been meeting
   together to discuss these issues and provide IEEE updates.  The
   mboned working group is chartered to receive regular reports on the
   current state of the deployment of multicast technology, create
   "practice and experience" documents that capture the experience of
   those who have deployed and are deploying various multicast
   technologies, and provide feedback to other relevant working groups.
   As such, this document will gather the problems of wifi multicast
   into one problem statement document so as to offer the community
   guidance on current limitations.

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
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   This Internet-Draft will expire on July 28, 2018.

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   Copyright (c) 2018 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Multicast over WiFi Problems  . . . . . . . . . . . . . . . .   2
     2.1.  Low Reliability . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Low Data Rate . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  High Interference . . . . . . . . . . . . . . . . . . . .   4
     2.4.  High Power Consumption  . . . . . . . . . . . . . . . . .   4
   3.  Common remedies to multicast over wifi problems . . . . . . .   4
   4.  State of the Union  . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Multicast over wifi has been used to low levels of success, usually
   to a point of being so negative that multicast over wifi is not
   allowed.  In addition to protocol use of broadcast/multicast for
   control messages, more applications, such as push to talk in
   hospitals, video in enterprises and lectures in Universities, are
   streaming over wifi.  And many end devices are increasingly using
   wifi for their connectivity.  One of the primary problems multicast
   over wifi faces is that link local 802.11 doesn't necessarily support
   multicast, it supports broadcast.  To make make multicast over wifi
   work successfully we often need to modify the multicast to instead be
   sent as unicast in order for it to successfully transmit with useable
   quality.  Multicast over wifi experiences high packet error rates, no
   acknowledgements, and low data rate.  This draft reviews these
   problems found with multicast over wifi.  While this is not a
   solutions draft, common workarounds to some of the problems will be
   listed, along with the impact of the workarounds.

2.  Multicast over WiFi Problems

   802.11 is a wireless broadcast medium which works well for unicast
   and has become ubiquitous in its use.  With multicast, however,
   problems arise over wifi.  There are no ACKs for multicast packets,

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   for instance, so there can be a high level of packet error rate (PER)
   due to lack of retransmission and because the sender never backs off.
   It is not uncommon for there to be a packet loss rate of 5% which is
   particularly troublesome for video and other environments where high
   data rates and high reliability are required.  Multicast, over wifi,
   is typically sent on a low date rate which makes video negatively
   impacted.  Wifi loses many more packets then wired due to collisions
   and signal loss.  There are also problems because clients are unable
   to stay in sleep mode due to the multicast control packets continuing
   to unnecessarily wake up those clients which subsequently reduces
   energy savings.  Video is becoming the dominant content for end
   device applications, with multicast being the most natural method for
   applications to transmit video.  Unfortunately, multicast, even
   though it is a very natural choice for video, incurs a large penalty
   over wifi.

   One big difference between multicast over wired versus multicast over
   wired is that wired links are a fixed transmission rate.  Wifi, on
   the other hand, has a transmission rate which varies over time
   depending upon the clients proximity to the AP.  Throughput of video
   flows, and the capacity of the broader wifi network, will change and
   will impact the ability for QoS solutions to effectively reserve
   bandwidth and provide admission control.

   The main problems associated with multicast over WiFi are as follows:

   o  Low Reliability

   o  Lower Data Rate

   o  High interference

   o  High Power Consumption

   These points will be elaborated separately in the following

2.1.  Low Reliability

   Because of the lack of acknowledgement for packets from Access Point
   to the receivers, it is not possible for the Access Point to know
   whether or not a retransmission is needed.  Even in the wired
   Internet, this characteristic commonly causes undesirably high error
   rates, contributing to the relatively slow uptake of multicast
   applications even though the protocols have been available for
   decades.  The situation for wireless links is much worse, and is
   quite sensitive to the presence of background traffic.

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2.2.  Low Data Rate

   For wireless stations associated with an Access Points, the necessary
   power for good reception can vary from station to station.  For
   unicast, the goal is to minimize power requirements while maximizing
   the data rate to the destination.  For multicast, the goal is simply
   to maximize the number of receivers that will correctly receive the
   multicast packet.  For this purpose, generally the Access Point has
   to use a much lower data rate at a power level high enough for even
   the farthest station to receive the packet.  Consequently, the data
   rate of a video stream, for instance, would be constrained by the
   environmental considerations of the least reliable receiver
   associated with the Access Point.

2.3.  High Interference

   As mentioned in the previous subsection, multicast transmission to
   the stations associated to an Access Point typically proceeds at a
   much higher power level than is required for unicat to many of the
   receivers.  High power levels directly contribute to stronger
   interference.  The interference due to multicast may extend to
   effects inhibiting packet reception at more distant stations that
   might even be associated with other Access Points.  Moreover, the use
   of lower data rates implies that the physical medium will be occupied
   for a longer time to transmit a packet than would be required at high
   data rates.  Thus, the level of interference due to multicast will be
   not only higher, but longer in duration.

   Depending on the choice of 802.11 technology, and the configured
   choice for the base data rate for multicast transmission from the
   Access Point, the amount of additional interference can range from a
   factor of ten, to a factor thousands for 802.11ac.

2.4.  High Power Consumption

   One of the characteristics of multicast transmission is that every
   station has to be configured to wake up to receive the multicast,
   even though the received packet may ultimately be discarded.  This
   process has a relatively large impact on the power consumption by the
   multicast receiver station.

3.  Common remedies to multicast over wifi problems

   One common solution to the multicast over wifi problem is to convert
   the multicast traffic into unicast.  This is often referred to as
   multicast to unicast (MC2UC).  Converting the packets to unicast is
   beneficial because unicast packets are acknowledged and retransmitted
   as needed to prevent as much loss.  The Access Points (AP) is also

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   able to provide rate limiting as needed.  The drawback with this
   approach is that the benefit of using multicast is defeated.

   Using 802.11n helps provide a more reliable and higher level of
   signal-to-noise ratio in a wifi environment over which multicast
   (broadcast) packets can be sent.  This can provide higher throughput
   and reliability but the broadcast limitations remain.

4.  State of the Union

   In discussing these issues over email and, most recently, in a side
   meeting at IETF 99, it is generally agreed that these problems will
   not be fixed anytime soon primarily because it's expensive to do so
   and multicast is unreliable.  The problem of v6 neighbor discovery
   saturating the wifi link is only part of the problem.  A big problem
   is that the 802.11 multicast channel is an afterthought and only
   given 100th of the bandwidth.  Multicast is basically a second class
   citizen, to unicast, over wifi.  Unicast may have allocated 10mb
   while Multicast will be allocated 1mb.  There are many protocols
   using multicast and there needs to be something provided in order to
   make them more reliable.  Wifi traffic classes may help.  We need to
   determine what problem should be solved by the IETF and what problem
   should be solved by the IEEE.

   Apple's Bonjour protocol, for instance, provides service discovery
   (for printing) that utilizes multicast.  It's the first thing
   operators drop.  Even if multicast snooping is utilized, everyone
   registers at once using Bonjour and the network has serious
   degradation.  There is also a lot of work being developed to help
   save battery life such as the devices not waking up when receiving a
   multicast packet.  If an AP, for instance, expresses a DTIM of 3 then
   it will send a multicast packet every 3 packets.  But the reality is
   that most AP's will send a multicast every 30 packets.  For unicast
   there's a TIM.  But because multicast is going to everyone, the AP
   sends a broadcast to everyone.  DTIM does power management but
   clients can choose to wake up or not and whether to drop the packet
   or not.  Then they don't know why their bonjour doesn't work.  The
   IETF may just need to decide that broadcast is more expensive so
   multicast needs to be sent wired. 802.1ak works on ethernet and wifi.
   802.1ak has been pulled into 802.1Q as of 802.1Q-2011.  802.1Q-2014
   can be looked at here: http://www.ieee802.org/1/pages/802.1Q-
   2014.html . If we don't find a generic solution we need to establish
   guidelines for multicast over wifi within the mboned wg.  A multicast
   over wifi IETF mailing list is formed (mcast-wifi@ietf.org) and more
   discussion to be had there.  This draft will serve as the current
   state of affairs.

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   This is not a solutions draft, but to provide an idea going forward,
   a reliable registration to Layer-2 multicast groups and a reliable
   multicast operation at Layer-2 could provide a generic solution.
   There is no need to support 2^24 groups to get solicited node
   multicast working: it is possible to simply select a number of
   trailing bits that make sense for a given network size to limit the
   amount of unwanted deliveries to reasonable levels.  We need to
   encourage IEEE 802.1 and 802.11 to revisit L2 multicast issues.  In
   particular, Wi-Fi provides a broadcast service, not a multicast one.
   In fact all frames are broadcast at the PHY level unless we beamform.
   What comes with unicast is the property of being much faster (2
   orders of magnitude) and much more reliable (L2 ARQ).

5.  IANA Considerations


6.  Security Considerations


7.  Acknowledgments

   The following people have contributed information and discussion in
   the meetings and on the list which proved helpful for the development
   of the latest version this Internet Draft:

   Dave Taht, Donald Eastlake, Pascal Thubert, Juan Carlos Zuniga,
   Mikael Abrahamsson, Diego Dujovne, David Schinazi, Stig Venaas,
   Stuart Cheshire, Lorenzo, Greg Shephard, Mark Hamilton

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

Authors' Addresses

   Mike McBride
   2330 Central Expressway
   Santa Clara  CA 95055

   Email: michael.mcbride@huawei.com

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   Charlie Perkins
   2330 Central Expressway
   Santa Clara  CA 95055

   Email: charlie.perkins@huawei.com

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