Network Working Group                                     Y. Desmouceaux
Internet-Draft                                                     Cisco
Intended status: Informational                            August 1, 2014
Expires: January 31, 2015

        Power consumption due to IPv6 multicast on WiFi devices


   IPv6 networks make a consequent use of multicast for several
   purposes, including mandatory functions such as Neighbor Discovery.
   Although this use of multicast does not create real difficulties on
   wired networks, it can become painful on wireless ones, notably in
   terms of power consumption.  There might be little effect on home
   networks, however, such effects become more important on large-scale
   networks.  This memo provides statistics about the multicast traffic
   rate in a large IPv6 wireless network and the induced device power
   consumption, in response to a call emitted at IETF 89.

Status of this Memo

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   This Internet-Draft will expire on January 31, 2015.

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   and restrictions with respect to this document.  Code Components
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  IPv6 typical multicast traffic . . . . . . . . . . . . . . . .  3
     2.1.  Behavior when joining a network  . . . . . . . . . . . . .  3
     2.2.  Behavior once connected  . . . . . . . . . . . . . . . . .  3
   3.  Power consumption induced by multicast traffic . . . . . . . .  4
   4.  Large-scale wireless networks  . . . . . . . . . . . . . . . .  5
     4.1.  Typical orders of magnitude  . . . . . . . . . . . . . . .  5
       4.1.1.  Arrival rates  . . . . . . . . . . . . . . . . . . . .  5
       4.1.2.  Connection durations . . . . . . . . . . . . . . . . .  6
     4.2.  Influence of such networks over devices  . . . . . . . . .  7
     4.3.  Roaming  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Possible solutions . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Optimizing Router Solicitations retransmissions  . . . . .  8
     5.2.  Proxying multicast traffic . . . . . . . . . . . . . . . .  8
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   Multicast is widely used in IPv6 for configuration purposes through
   the Neighbor Discovery protocol [RFC4861].  On IEEE 802.11 wireless
   links, this can have a negative impact on low-energy devices such as
   smartphones, as stated in [I-D.vyncke-6man-mcast-not-efficient].  The
   802.11 protocol [IEEE80211] includes a power-save mode for such
   devices, allowing them to be in a sleep state in which they will be
   notified when packets addressed to them are pending.  However, there
   is no known powerful equivalent of wired-links Multicast Listener
   Discovery (MLD) snooping [RFC4541], hence all devices will be woken
   up when multicast traffic is pending.

   In this document, we provide data illustrating the issue at 3 levels:

   o  typical multicast traffic emitted by a device when joining an IPv6
      network, and 'background multicast noise' generated once

   o  power consumption statistics for wireless devices when confronted
      to multicast traffic;

   o  orders of magnitude of connections frequencies and durations in

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      large-scale wireless networks.

   Confronting these 3 levels makes it possible to model the power
   consumption overhead of multicast traffic on typical large wireless

   We finally discuss possible solutions at the IP and the 802.11 level.

2.  IPv6 typical multicast traffic

2.1.  Behavior when joining a network

   When joining an IPv6 network, devices usually emit the following
   multicast traffic:

   1.  duplicate address detection for the link-local address;

   2.  router solicitation;

   3.  duplicate address detection for the SLAAC address;

   4.  duplicate address detection for the SLAAC privacy address.

   In addition, MLDv2 [RFC3810] frames are emitted for registration to
   the solicited-node multicast addresses needed for Duplicate Address
   Detection.  Their number is not deterministic since other multicast
   protocols may interfere, but is typically at least 4.

   Depending on the configuration of the router, the Router
   Advertisement sent by the router in response to the node's Router
   Solicitation may also be multicast.

   On Linux and Apple MacOS X clients that were tested, joining the
   network also induces Multicast Domain Name System (mDNS) [RFC6762]
   traffic.  Once more, the number of packets emitted depends on the
   network environment, but is around 20. Likewise, Microsoft Windows
   clients generate Link-local Multicast Name Resolution (LLMNR)
   [RFC4795] multicast traffic when they connect to the network.

2.2.  Behavior once connected

   Once connected, some devices keep on sending IPv6 multicast frames.
   Protocols inducing such traffic include Neighbor Discovery, MLD, and
   local discovery or name-resolution protocols, such as mDNS, LLMNR,
   Simple Service Discovery Protocol (SSDP), Web Services Dynamic
   Discovery (WS-D).

   On a typical middle-scale enterprise network, IPv6 multicast traffic
   induced by these protocols has a noticeable impact.  Our measures on
   a 160-hosts network show that the average rate of multicast traffic
   transiting through the link is 4.5 frames per second.  The following
   table shows the repartition of the multicast traffic between these

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   protocols, as it was measured:

   Protocol            ICMPv6  mDNS    LLMNR   WS-D

   Ratio               0.53    0.33    0.12    0.03

   Based on the previous data, we can deduce how much multicast traffic
   is generated by a "representative" device.  Doing a simple cross-
   multiplication, we obtain a rate of 0.028 multicast packets/s for a
   single device, or 101 frames/hour.  (This result is confirmed on a
   small isolated test network of two hosts.) The following table shows
   the indicative number of multicast packets emitted by a such a
   representative device on our test network in an hour:

   Protocol            ICMPv6  mDNS    LLMNR   WS-D

   Frames per hour     53      33      12      3

3.  Power consumption induced by multicast traffic

   While multicast traffic has no significant influence on actively
   connected devices, it might have a poor impact on the ones that are
   in power-save mode.

   In power-save mode, the hardware of such a device needs to regularly
   wake up in order to check whether pending frames are buffered for it.
   To do so, it wakes up every DTIM_PERIOD  beacons (DTIM_PERIOD usually
   being set to 1 or 2) and retrieves the beacon emitted by the Access
   Point (AP). If multicast traffic is pending, this will be indicated
   by the AP by setting a specific bit in the beacon's Time Information
   Management (TIM) information element to 1.

   If a device sees that this bit is set, it will stay awake in order to
   retrieve the frames.  The device CPU will then be awakened and frames
   will be transmitted by the wireless firmware to the IP stack.  (Note
   that this might not happen if the firmware implements a multicast
   filter, but even in this case, current will be drawn to retrieve the

   Power consumption measurements on smartphone devices confirm the
   negative impact of multicast traffic on sleeping devices.  The
   measurement was performed on a Samsung i9195 by attaching an ammeter
   between the battery and the phone.  When idle (screen off, GSM off,
   WiFi on, 802.11 power-save enabled by the device), current drawn is
   10 mA in average.  When a multicast frame is received and the CPU
   awakened to process it, the current draw goes to between 100 and 150
   mA during a small peak of time.

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   Assuming that the duration of the current peak induced by processing
   a multicast frame is 0.1 s, a device would hence use K times more
   energy when it receives RATE multicast packets per second, than when
   it receives none, where K = (10*(1 - RATE*0.1) + 150*RATE*0.1)/10.
   The following table gives K as a function of RATE.

   RATE (pkts/s)   0.01    0.1     0.5     1       2       4       >10

   K               1.014   1.14    1.7     2.4     3.8     6.6     15

   A possible workaround to this issue is to implement a multicast
   filter at the firmware level, which will only forward multicast
   packets to the IP stack if their destination address matches one of
   those registered by the device.  Although this would have a positive
   impact on battery consumption, the following measurements show that
   this is not entirely satisfying.

   Indeed, current drawn to retrieve pending multicast frames at L2
   without waking up the main CPU is around 40 mA, which is still 4
   times more than idle consumption.  In order to simulate this, one can
   tweak a router so that it always advertises the presence of multicast
   frames by setting the TIM multicast bit to 1. The device's radio will
   then always try to retrieve frames following this beacon.

4.  Large-scale wireless networks

   Previous sections provide data about the battery usage induced by
   individual multicast frames, and the number of multicast frames
   generated by a single host when connecting and once connected.

   In order to model the battery usage of real-life networks, the
   following section provides data about connection arrival rates and
   connection durations in usual large-scale wireless networks.

4.1.  Typical orders of magnitude

   The following results are based on anonymized data from a dual-stack
   WiFi network in a conference setup.  These data provide interesting
   statistics about user habits in a network characterized by mobility,
   where users move much from one room to another.  Plus, it is a good
   example of a large wireless network, since the user count during
   working hours is between 600 and 700.

4.1.1.  Arrival rates

   Arrival rates in this test network follow a probability distribution
   that is very close to an exponential law (the error rate being only
   6%). The observed parameter for this law is such that 1/lambda = 6

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   We are therefore in a scheme expected by the classical queuing
   theory, where arrivals are memoryless and have the Markov property of
   independence of the past.  The smallness of the parameter is also an
   indicator of the great mobility of such networks: in average, a
   client joined the network every 6 seconds.

   There is a small bias in previous measures due to people arriving en
   masse at the beginning of the day and going to lunch at noon.  This
   bias does not affect the exponential nature of the law.  However, it
   has an non-negligible effect over the parameter of the distribution.
   Computing the probability distribution over a shorter unbiased amount
   of time leads to a greater arrival rate: 1/lambda becomes 4.5

   From these data it appears that arrival rates in wireless networks
   characterized by mobility are high.  Since arrivals mean multicast
   traffic issued by the client's IPv6 stack, this already provides a
   rough intuition that large wireless networks feature high rates of
   multicast frames.

4.1.2.  Connection durations

   Once clients are connected, duration of their connections follow a
   less typical probability distribution, whose form is represented

   dur. distr.
   |    ||
   |    ||
   |    |\
   |    | |
   |    | |
   |    | |
   |    |  \
   |    |   \_
   |    |     \__
   |    |        \_____
   |____|              \______
   |                          \_________
   +--------------------------------------> t

   From these data it appears that there is a peak of connections that
   last 5 minutes, whereas almost no connection lasts less than this.  A
   plausible explanation for this is that there are two kinds of
   behaviors: devices that automatically join the network without human
   intervention and disconnect after an idle timeout, and users who
   consciously connect to the network.  After this peak, the
   distribution is roughly an exponential law, which correspond to the
   latter case.

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   An average connection time of 55 minutes has been measured: at least,
   clients do not seem to be eager to leave the network too soon once
   they have joined it.

   In our test network, arrivals follow an exponential law, and service
   times do not follow a close-form probability distribution.  We can
   therefore model it as an M/G/oo queue.  Once stabilized, the number
   of hosts in such a system follows a Poisson law of parameter S/T,
   where S is the expected service time and T=1/lambda the expected time
   between two arrivals.

4.2.  Influence of such networks over devices

   In this section, we model the influence of a large wireless network
   over a device, in terms of power consumption.

   Let us assume that in average, N devices are present in the network,
   and that the arrival rate is lambda.  (See Section 4.1 for typical
   numerical values). Then, Section 2 implies that the rate of multicast
   frames in the network is RATE = 0.025*N + 4*lambda.  The following
   table gives RATE for a few values of N and lambda:

   N               5       10      50      100     500     500

   1/lambda (s)    600     600     60      60      60      5

   RATE (pkts/s)   0.13    0.26    1.32    2.57    12.6    13.3

   Finally, using Section 3, we can compute what is the energy overhead
   induced by the multicast traffic on a device.  When comparing to idle
   mode, K more times energy will be used, where K = (10*(1 - (0.025*N +
   4*lambda)*0.1) + 150*(0.025*N + 4*lambda)*0.1)/10. The following
   table gives K as a function of N and lambda:

   N               5       10      50      100     500     500

   1/lambda (s)    600     600     60      60      60      5

   K               1.18    1.36    2.84    4.59    15      15

   Thus, battery consumption induced by multicast traffic will be
   doubled in a network of 30 nodes where the arrival rate is 10

4.3.  Roaming

   Depending on the configuration of the wireless network, roaming might
   have different consequences on multicast traffic emission.

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   When a host moves from one access point to another, roaming is
   supposed to be seamless.  If a device detects a drop in signal
   quality, it will probe for a nearer access point with the same SSID.
   If it finds one, it will send an Authentication frame and a
   Reassociation Request.  This will seem transparent to L3, except that
   some timers may time out, causing retransmissions.

   However, if access points are not properly geographically distributed
   within the network, a device might lose connection to one before it
   can reconnect to another.  In such a case, seamless roaming cannot
   happen and the device will have to go through the whole process of
   connection at the IPv6 level.  This will generate multicast traffic
   as discussed in Section 2.1.

5.  Possible solutions

   [I-D.yourtchenko-colitti-nd-reduce-multicast] already provides
   solutions at the IP layer.  These include:

   o  increasing intervals between Router Advertisements, and possibly
      remove the limit of 9000 s set by [RFC4861];

   o  increasing the timer value for Neighbor Unreachability Detection;

   o  unicasting Router Advertisements;

   o  multicast filtering at the infrastructure level (MLD snooping).

   At the L2 layer, it suggests using an efficient on-device multicast
   filter which would send frames to the IP layer only if their
   destination address is registered.

   We explore other possible solutions in the next sections.

5.1.  Optimizing Router Solicitations retransmissions

   Some wireless systems retransmit all multicast packets received, so
   that hosts have a better chance to obtain them.  This causes a
   problem when retransmitted packets are not desirable for hosts.

   Without having to implementing a full multicast snooping mechanism,
   which can be costly in terms of resources and complexity for small
   systems like home boxes, a simple fix can be applied to APs in order
   to reduce the amount of multicast traffic retransmitted.  APs should
   refrain from retransmitting packets destined to the all routers
   address (ff02::2), since routers should not be present on the
   wireless side.  Essentially, Router Solicitations will be concerned.

5.2.  Proxying multicast traffic

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   Some multicast traffic is only relevant to routers (for instance, MLD
   reports) or can be proxied by them (mDNS, ND). For instance,
   [RFC4389] specifies a means to proxy Neighbor Discovery traffic.
   Hosts are informed of the router proxying Neighbor Discovery traffic
   thanks to a bit in Router Advertisements.

   On the same model, a more general option in Router Advertisements
   could indicate which multicast addresses a router is able to proxy.
   When a host receives such a Router Advertisement, it will record
   those addresses.  It will then use the router L2 address instead of a
   33:33:XX:XX:XX:XX multicast L2 address as destination for
   corresponding packets, thus reducing multicast traffic emission.  The
   router will still be able to know that the final destination is
   multicast, since the destination IP address remains multicast (a
   behavior permitted by [RFC6085]). A disadvantage of this solution,
   though, is that it requires changes to the hosts.

6.  Acknowledgements

   The author would like to thank Andrew Yourtchenko, Eric Vyncke, Ole
   Troan, Brian Hart and Mark Townsley for their precious opinions on
   the subject.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations


9.  References

9.1.  Normative References

              Institute of Electrical and Electronics Engineers,
              "Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications", IEEE Standard 802.11, 2012.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W. and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6085]  Gundavelli, S., Townsley, M., Troan, O. and W. Dec,
              "Address Mapping of IPv6 Multicast Packets on Ethernet",
              RFC 6085, January 2011.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

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9.2.  Informative References

              Vyncke, E., Thubert, P., Levy-Abegnoli, E. and A.
              Yourtchenko, "Why Network-Layer Multicast is Not Always
              Efficient At Datalink Layer", Internet-Draft draft-vyncke-
              6man-mcast-not-efficient-01, February 2014.

              Yourtchenko, A. and L. Colitti, "Reducing Multicast in
              IPv6 Neighbor Discovery", Internet-Draft draft-
              yourtchenko-colitti-nd-reduce-multicast-00, February 2014.

   [RFC4389]  Thaler, D., Talwar, M. and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, April 2006.

   [RFC4541]  Christensen, M., Kimball, K. and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, May 2006.

   [RFC4795]  Aboba, B., Thaler, D. and L. Esibov, "Link-local Multicast
              Name Resolution (LLMNR)", RFC 4795, January 2007.

Author's Address

   Yoann Desmouceaux
   Issy Les Moulineaux, 92130


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