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IETF RFC 7772
Last modified on Thursday, February 11th, 2016
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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/RFC 7772.
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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RFC 7772 Reducing RA Energy Consumption February 2016
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 [RFC 4861]. 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 [RFC 2119].
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 [RFC 4861], which is
one every 3 seconds.
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RFC 7772 Reducing RA Energy Consumption February 2016
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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RFC 7772 Reducing RA Energy Consumption February 2016
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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RFC 7772 Reducing RA Energy Consumption February 2016
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 [RFC 4861], 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 [RFC 4861]. 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
[RFC 6059] when waking up.
6. Security Considerations
Misconfigured or malicious hosts sending rogue Router Advertisements
[RFC 6104] 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC 4861, September 2007,
<http://www.rfc-editor.org/info/RFC 4861>.
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RFC 7772 Reducing RA Energy Consumption February 2016
[RFC 6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
DOI 10.17487/RFC 6059, November 2010,
<http://www.rfc-editor.org/info/RFC 6059>.
7.2. Informative References
[RFC 6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, DOI 10.17487/RFC 6104,
February 2011, <http://www.rfc-editor.org/info/RFC 6104>.
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
Yourtchenko & Colitti Best Current Practice PAGE 6
RFC TOTAL SIZE: 12555 bytes
PUBLICATION DATE: Thursday, February 11th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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