





Internet Engineering Task Force (IETF)                    A. Yourtchenko
Request for Comments: 7772                                         Cisco
BCP: 202                                                      L. Colitti
Category: Best Current Practice                                   Google
ISSN: 2070-1721                                            February 2016


          Reducing Energy Consumption of Router Advertisements

Abstract

   Frequent Router Advertisement messages can severely impact host power
   consumption.  This document recommends operational practices to avoid
   such impact.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7772.

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
   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.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Scenarios . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Solicited Multicast RAs on Large Networks . . . . . . . .   2
     2.2.  Frequent Periodic Router Advertisements . . . . . . . . .   3
   3.  Consequences  . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Router Advertisement Frequency  . . . . . . . . . . . . . . .   3
   5.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Network-Side Recommendations  . . . . . . . . . . . . . .   4
     5.2.  Device-Side Recommendations . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Routing information is communicated to IPv6 hosts by Router
   Advertisement (RA) messages [RFC4861].  If these messages are sent
   too frequently, they can severely impact power consumption on
   battery-powered hosts.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Problem Scenarios

2.1.  Solicited Multicast RAs on Large Networks

   On links with a large number of battery-powered devices, sending
   solicited multicast Router Advertisements can severely impact host
   power consumption.  This is because every time a device joins the
   network, all devices on the network receive a multicast Router
   Advertisement.  In the worst case, if devices are continually joining
   and leaving the network, and the network is large enough, then all
   devices on the network will receive solicited Router Advertisements
   at the maximum rate specified by Section 6.2.6 of [RFC4861], which is
   one every 3 seconds.









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2.2.  Frequent Periodic Router Advertisements

   Some networks send periodic multicast Router Advertisements very
   frequently (e.g., once every few seconds).  This may be due to a
   desire to minimize customer impact of network renumbering events,
   which in some large residential networks occur relatively frequently.
   In the presence of hosts that ignore RAs or even all IPv6 packets
   when in sleep mode, such networks may see a need to send RAs
   frequently in order to avoid leaving devices with non-functional IPv6
   configurations for extended periods of time.  Unfortunately, this has
   severe impact on battery life.

3.  Consequences

   Observed effects of frequently sending Router Advertisement messages
   to battery-powered devices include:

   o  Some hosts simply experience bad battery life on these networks
      and otherwise operate normally.  This is frustrating for users of
      these networks.

   o  Some hosts react by dropping all Router Advertisement messages
      when in power-saving mode on any network, e.g.,
      <https://code.google.com/p/android/issues/detail?id=32662>.  This
      causes devices to lose connectivity when in power-saving mode,
      potentially disrupting background network communications, because
      the device is no longer able to send packets or acknowledge
      received traffic.

   o  Some hosts react by dropping *all* IPv6 packets when in power-
      saving mode, <http://www.gossamer-threads.com/lists/nsp/
      ipv6/54641>.  This disrupts network communications.

   Compounding the problem, when dealing with devices that drop Router
   Advertisements when in power saving mode, some network administrators
   work around the problem by sending RAs even more frequently.  This
   causes devices to engage in even more aggressive filtering.

4.  Router Advertisement Frequency

   The appropriate frequency of periodic RAs depends on how constrained
   the network devices are.

   o  Laptop-class devices will likely experience no noticeable battery-
      life impact, even if RAs are sent every few seconds.






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   o  Tablets, phones, and watches experience it more noticeably.  At
      the time of writing, current-generation devices might consume on
      the order of 5 mA when the main processor is asleep.  Upon
      receiving a packet, they might consume on the order of 200 mA for
      250 ms, as the packet causes the main processor to wake up,
      process the RA, attend to other pending tasks, and then go back to
      sleep.  Thus, on such devices, the cost of receiving one RA will
      be approximately 0.014 mAh.

      In order to limit the amount of power used to receive Router
      Advertisements to, say, 2% of idle power (i.e., to impact idle
      battery life by no more than 2%), the average power budget for
      receiving RAs must be no more than 0.1 mA, or approximately 7 RAs
      per hour.  Due to background multicast loss and the tendency of
      current devices to rate-limit multicast when asleep, many of these
      RAs might not reach the device.  Thus, the minimum lifetimes for
      RA configuration parameters such as default router lifetime might
      reasonably be 5-10 times the RA period, or roughly 45-90 minutes.

      An impact of 2% relative to measured idle current is negligible,
      since on this sort of device average power consumption is
      typically much higher than idle power consumption.

   o  Specialized devices in non-general-purpose networks such as sensor
      networks might have tighter requirements.  In these environments,
      even longer RA intervals might be appropriate.

5.  Recommendations

5.1.  Network-Side Recommendations

   1.  Router manufacturers SHOULD allow network administrators to
       configure the routers to respond to Router Solicitations with
       unicast Router Advertisements if:

       *  The Router Solicitation's source address is not the
          unspecified address, and:

       *  The solicitation contains a valid Source Link-Layer Address
          option.

   2.  Administrators of networks that serve large numbers (tens or
       hundreds) of battery-powered devices SHOULD enable this behavior.

   3.  Networks that serve battery-powered devices SHOULD NOT send
       multicast RAs too frequently (see Section 4) unless the
       information in the RA packet has substantially changed.  If there
       is a desire to ensure that hosts pick up configuration changes



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       quickly, those networks MAY send frequent Router Advertisements
       for a limited period of time (e.g., not more than one minute)
       immediately after a configuration change.

   No protocol changes are required.  Responding to Router Solicitations
   with unicast Router Advertisements is already allowed by Section
   6.2.6 of [RFC4861], and Router Advertisement intervals are already
   configurable by the administrator to a wide range of values.

5.2.  Device-Side Recommendations

   1.  Maintaining IPv6 connectivity requires that hosts be able to
       receive periodic multicast RAs [RFC4861].  Therefore, hosts that
       process unicast packets sent while they are asleep MUST also
       process multicast RAs sent while they are asleep.  Battery-
       powered hosts MAY rate-limit identical RAs if they are sent too
       frequently.

   2.  Battery-powered devices that do not intend to maintain IPv6
       connectivity while asleep SHOULD either disconnect from the
       network, abandoning all IPv6 configuration on that network, or
       perform Detecting Network Attachment in IPv6 (DNAv6) procedures
       [RFC6059] when waking up.

6.  Security Considerations

   Misconfigured or malicious hosts sending rogue Router Advertisements
   [RFC6104] can also severely impact power consumption on battery-
   powered hosts if they send a significant number of such messages.
   Any IPv6 network where there is potential for misconfigured or
   malicious hosts should take appropriate countermeasures to mitigate
   the problem.

7.  References

7.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.





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   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,
              <http://www.rfc-editor.org/info/rfc6059>.

7.2.  Informative References

   [RFC6104]  Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
              Problem Statement", RFC 6104, DOI 10.17487/RFC6104,
              February 2011, <http://www.rfc-editor.org/info/rfc6104>.

Acknowledgements

   The authors wish to thank Steven Barth, Frank Bulk, David Farmer,
   Igor Gashinsky, Ray Hunter, Erik Kline, Erik Nordmark, Alexandru
   Petrescu, Libor Polcak, Mark Smith, Jinmei Tatuya, and James Woodyatt
   for feedback and helpful suggestions.

Authors' Addresses

   Andrew Yourtchenko
   Cisco
   7a de Kleetlaan
   Diegem, 1831
   Belgium

   Phone: +32 2 704 5494
   Email: ayourtch@cisco.com


   Lorenzo Colitti
   Google
   Roppongi 6-10-1
   Minato, Tokyo  106-6126
   Japan

   Email: lorenzo@google.com














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