Internet Area WG                                              M. McBride
Internet-Draft                                                C. Perkins
Intended status: Standards Track                                  Huawei
Expires: January 7, 2016                                    July 6, 2015

                    Multicast Wifi Problem Statement


   There have been known issues with IP Multicast, in an 802.11
   environment, which have prevented the deployment of multicast in
   these wifi environments.  The mboned working group would like to
   gather the problems of wifi multicast into one problem statement
   document so as to offer the community guidance on current

Status of This Memo

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   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 January 7, 2016.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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.  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.  To make make multicast over wifi work successfully
   they 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.
   With multicast, however, there are no ACKs for multicast packets 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.  Additionally, multicast is
   typically sent on a low date rate which makes video particularly
   troublesome.  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 and subsequently
   reduce energy savings.  Video is becoming the dominant content for

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   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.

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

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   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

   802.11 multicast is somewhat incompatible with radio sleep schedules.
   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
   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
   packets can be sent.  This can provide higher throughput and

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4.  IANA Considerations


5.  Security Considerations


6.  Acknowledgments

   The following people have contributed information helpful for the
   development of this Internet Draft:

   o  Dave Taht

   o  Donald Eastlake

   o  Marc Mosko

7.  Normative References

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

Authors' Addresses

   Mike McBride
   2330 Central Expressway
   Santa Clara  CA 95055


   Charlie Perkins
   2330 Central Expressway
   Santa Clara  CA 95055


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