Reducing Energy Consumption of Router Advertisements
Why sending IPv6 Router Advertisements too often drains batteries, and what networks and devices should do about it.
Yourtchenko, A. and L. Colitti, "Reducing Energy Consumption of Router Advertisements", BCP 202, RFC 7772, DOI 10.17487/RFC7772, February 2016, <https://www.rfc-editor.org/info/rfc7772>.
@misc{rfc7772,
series = {Request for Comments},
number = 7772,
howpublished = {RFC 7772},
publisher = {RFC Editor},
doi = {10.17487/RFC7772},
url = {https://www.rfc-editor.org/info/rfc7772},
author = {A. Yourtchenko and L. Colitti},
title = {{Reducing Energy Consumption of Router Advertisements}},
pagetotal = 6,
year = 2016,
month = feb,
} RFC text
Contents
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.
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.
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.
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
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>.
[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
AI ELI5
On this page
§1Introduction
IPv6 hosts learn routing information from Router Advertisement (RA) messages defined in RFC 4861. If those messages are sent too frequently, they can severely hurt power consumption on battery-powered hosts. This RFC recommends operational practices to avoid that.
Routers shout "I am here and this is how to reach the internet!" Shouting too often tires out the gadgets listening.
§2The Two Problem Scenarios
The first scenario is solicited multicast RAs on large networks. Every time a device joins, all devices on the link receive a multicast RA. If devices keep joining and leaving on a large enough network, everyone receives solicited RAs at the maximum rate allowed by Section 6.2.6 of RFC 4861, which is one every 3 seconds.
The second is frequent periodic RAs, sometimes sent every few seconds. Operators may do this to limit customer impact of renumbering events, which in some large residential networks happen relatively often. Some hosts ignore RAs or even all IPv6 packets in sleep mode, so networks send RAs often to avoid leaving devices with non-functional IPv6 configurations. This has a severe impact on battery life.
Imagine a classroom where every time a new kid walks in, the teacher announces the rules to everyone again. If kids keep coming and going, nobody gets any peace.
§3Consequences and the Right Frequency
The observed effects include bad battery life on otherwise normal hosts, hosts that drop all RAs in power-saving mode (losing connectivity), and hosts that drop all IPv6 packets in power-saving mode. It gets worse: some administrators respond to RA-dropping devices by sending RAs even more often, which causes even more aggressive filtering.
The right frequency depends on how constrained the devices are. Laptop-class devices likely see no noticeable impact even with RAs every few seconds. Tablets, phones and watches feel it more. The RFC's example figures are about 5 mA with the main processor asleep and about 200 mA for 250 ms on receiving a packet, so one RA costs roughly 0.014 mAh.
To keep RA reception to 2% of idle power, the budget is no more than 0.1 mA, or approximately 7 RAs per hour. Because multicast may be lost or rate-limited while asleep, lifetimes such as the default router lifetime might reasonably be 5 to 10 times the RA period, roughly 45 to 90 minutes. Sensor networks and similar non-general-purpose networks may have tighter needs and even longer intervals.
A big laptop barely notices a tap on the shoulder. A tiny watch with a small battery notices every single one, and they add up.
§4Recommendations for Networks
Router manufacturers SHOULD let administrators configure routers to answer Router Solicitations with unicast RAs when the solicitation's source address is not the unspecified address and it contains a valid Source Link-Layer Address option. Administrators of networks serving large numbers (tens or hundreds) of battery-powered devices SHOULD enable this.
Such networks SHOULD NOT send multicast RAs too frequently unless the RA's information has substantially changed. To get hosts to notice a change quickly, they MAY send frequent RAs for a limited time (for example not more than one minute) right after a configuration change.
No protocol changes are required: unicast replies to solicitations are already allowed by Section 6.2.6 of RFC 4861, and RA intervals are already configurable over a wide range.
When one new kid arrives, tell only that kid the rules instead of announcing them to the whole class. Only make a big announcement when something really changes.
§5Recommendations for Devices
Staying connected over IPv6 requires receiving periodic multicast RAs. So hosts that process unicast packets sent while they are asleep MUST also process multicast RAs sent while asleep. Battery-powered hosts MAY rate-limit identical RAs that arrive too frequently.
Battery-powered devices that do not intend to keep IPv6 connectivity while asleep SHOULD either disconnect from the network, abandoning all IPv6 configuration there, or run Detecting Network Attachment in IPv6 (DNAv6) procedures from RFC 6059 on waking.
A gadget that listens while napping must listen to the router's announcements too. If it does not want to listen, it should say goodbye and ask again when it wakes up.
§6Security Considerations
Misconfigured or malicious hosts that send rogue Router Advertisements (see RFC 6104) can also severely hurt battery-powered hosts' power consumption if they send many such messages. Any IPv6 network where such hosts are possible should take appropriate countermeasures.
A prankster who keeps ringing the doorbell can drain a sleeping gadget too, so networks should watch out for pranksters.