Energy Efficient Implementation of IETF Protocols on Constrained Devices
draft-hex-lwig-energy-efficient-00
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| Authors | Zehn Cao , Xuan He , Matthias Kovatsch | ||
| Last updated | 2013-02-18 | ||
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draft-hex-lwig-energy-efficient-00
Internet Engineering Task Force Z. Cao
Internet-Draft China Mobile
Intended status: Informational X. He
Expires: August 22, 2013 Hitachi (China) Research and
Development Corporation
M. Kovatsch
ETH Zurich
February 18, 2013
Energy Efficient Implementation of IETF Protocols on Constrained Devices
draft-hex-lwig-energy-efficient-00
Abstract
This document summarizes the problems and current practices of energy
efficient protocol implementation on constrained devices, mostly
about how to make the protocols within IETF scope behave energy
friendly.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 22, 2013.
Copyright Notice
Copyright (c) 2013 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. MAC and Radio Duty Cycling . . . . . . . . . . . . . . . . . . 5
4. IP Adaption and Transport Layer . . . . . . . . . . . . . . . . 6
5. Routing Protocols . . . . . . . . . . . . . . . . . . . . . . . 6
6. Application Layer . . . . . . . . . . . . . . . . . . . . . . . 7
7. Cross Layer Optimization . . . . . . . . . . . . . . . . . . . 7
8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
In many scenarios of embedded systems, the networked system is
composed of many battery-powered devices. For example, in an
environmental monitoring system or a temperature and humidity
monitoring system in the data center, there are no always-on and
handy sustained power supplies for the large number of small devices.
In such deployment environments, it is necessary to optimize the
energy consumption of the entire system, including computing,
application layer behavior, and lower layer communication.
Various research efforts have been spent on this "energy efficiency"
problem. Most of this research has focused on how to optimize the
system's power consumption regarding a certain deployment scenario or
how could an exisiting network function such as routing or security
be more energy-efficient. Only few efforts were spent on energy-
efficient designs for IETF protocols and standardized network stacks
for such constrained devices [I-D.kovatsch-lwig-class1-coap].
The IETF has developed a suite of Internet protocols suitable for
such small devices, including 6LoWPAN [RFC6282], 6LoWPAN-ND
[RFC6775], RPL[RFC6550], and CoAP[I-D.ietf-core-coap]. This document
tries to summarize the design considerations of making the IETF
protocol suite as energy-efficient as possible. While this document
does not provide detailed and systematic solutions to the energy
efficiency problem, it summarizes the design efforts and analyzes the
design space of this problem.
After reviewing the energy-efficient design of each layer, an overall
conclusion is summarized. Though the lower layer communication
optimization is the key part of energy efficient design, the protocol
design at the network and application layers is also important to
make the device battery-friendly.
1.1. Conventions used in this document
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]
1.2. Terminology
The terminologies used in this document can be referred to
[I-D.bormann-lwig-terms].
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2. Overview
The IETF has developed multiple protocols to enable end-to-end IP
communication between constrained nodes and fully capable nodes.
This work has witnessed the evolution of the traditional Internet
protocol stack to a light-weight Internet protocol stack. As show in
Figure 1 below, the IETF has developed CoAP as the application layer
and 6LoWPAN as the adaption layer to run IPv6 over IEEE 802.15.4 and
Bluetooth Low-Energy, with the support of routing by RPL and
efficient neighbor discovery by 6LoWPAN-ND.
+-----+ +-----+ +-----+ +------+
|http | | ftp | |SNMP | | COAP |
+-----+ +-----+ +-----+ +------+
\ / / / \
+-----+ +-----+ +-----+ +-----+
| tcp | | udp | | tcp | | udp |
+-----+ +-----+ ===> +-----+ +-----+
\ / \ /
+-----+ +------+ +-------+ +------+ +-----+
| RTG |--| ipv6 |--|ICMP/ND| | ipv6 |---| rpl |
+-----+ +------+ +-------+ +------+ +-----+
| |
+-------+ +-------+ +----------+
|MAC/PHY| |6lowpan|--|6lowpan-nd|
+-------+ +-------+ +----------+
|
+-------+
|MAC/PHY|
+-------+
Figure 1: Traditional and Lighweight Internet Protocol Stack
There are comprehensive measurements of wireless communication
[Powertrace]. Below we list the energy consumption profile of the
most common atom operations on a prevalent sensor node platform. The
measurement was based on the Tmote Sky with ContikiMAC as the radio
duty cycling algorithm. From the measurement, we can see that
optimized transmissions and reception consume almost the same amount
of energy. For IEEE 802.15.4 and UWB radios, transmitting is
actually even cheaper than receiving. Only for broadcast and non-
synchronized communication transmissions become costly in terms of
energy because they need to flood the medium for a long time.
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+---------------------------------------+---------------+
| Activity | Energy (uJ) |
+---------------------------------------+---------------+
| Broadcast reception | 178 |
+---------------------------------------+---------------+
| Unicast reception | 222 |
+---------------------------------------+---------------+
| Broadcast transmission | 1790 |
+---------------------------------------+---------------+
| Non-synchronized unicast transmission | 1090 |
+---------------------------------------+---------------+
| Synchronized unicast transmission | 120 |
+---------------------------------------+---------------+
| Unicast TX to awake receiver | 96 |
+---------------------------------------+---------------+
Figure 2: Power consumption of atom operations on the Tmote Sky with
ContikiMAC
3. MAC and Radio Duty Cycling
In low-power wireless networks, communication and power consumption
are intertwined. The communication device is typically the most
power-consuming component, but merely refraining from transmissions
is not enough to attain a low power consumption: the radio consumes
as much power in listen mode as when actively transmitting, as show
in Figure 2 . To reduce power consumption, the radio must be
switched completely off -- duty-cycled -- as much as possible.
ContikiMAC is a very typical Radio Duty Cycling protocol
[ContikiMAC].
From the perspective of MAC&RDC, all upper layer protocols, such as
routing, RESTful communication, adaption, and management flows, are
all applications. Since the duty cycling algorithm is the key to
energy-efficiency of the wireless medium, it synchronizes the TX/RX
request from the higher layer.
The MAC&RDC are not in the scope of the IETF, yet lower layer
designers and chipset manufactures take great care of the problem.
For the IETF protocol designers, however, it is good to know the
behaviors of lower layers so that the designed protocols can work
perfectly with them.
Once again, the IETF protocols we are going to talk about in the
following sections are the customers of the lower layer. If they
want to get better service in a cooperative way, they should be
considerative and understand each other.
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4. IP Adaption and Transport Layer
6LoWPAN is the adaption layer to run IPv6 over IEEE 802.15.4 MAC&PHY.
It was born to fill the gap that the IPv6 layer does not support
fragmentation and assembly of <1280-byte packets while IEEE 802.15.4
only supports a MTU of 127 bytes.
IPv6 is the basis for the higher layer protocols, including both TCP/
UDP transport and applications. So they are quite ignorant of the
transmission and reception behaviors, and are almost neutral to the
energy-efficiency problem.
What the network stack can optimize is to save the computing power.
For example the Contiki implementation has multiple cross layer
optimizations for buffers and energy management, e.g., the computing
and validation of UDP/TCP checksums without the need of reading IP
headers from a different layer. These optimizations are software
implementation techniques, and out of the scope of IETF and the LWIG
working group.
The 6LoWPAN contributes to the energy-efficiency problem in two ways.
First of all, it swaps computing with communication. 6LoWPAN applies
compression of the IPv6 header. This means less amount of data will
be handled by the lower layer, but both the sender and receiver
should spend more computing power on the compression and
decompression of the packets over the air. Secondly, the 6LoWPAN
working group developed the energy-efficient Neigbor Discovery called
6LoWPAN-ND, which is an energy efficient replacement of the IPv6 ND
in constrained environments. IPv6 Neighbor Discovery was not
designed for non-transitive wireless links, as its heavy use of
multicast makes it inefficient and sometimes impractical in a low-
power and lossy network. 6LoWPAN-ND describes simple optimizations to
IPv6 Neighbor Discovery, its addressing mechanisms, and duplicate
address detection for Low-power Wireless Personal Area Networks and
similar networks.
5. Routing Protocols
The routing protocol designed by the IETF for constrained
environments is called RPL [RFC6550]. As a routing protocol, RPL has
to exchange messages periodically and keep routing states for each
destination. RPL is optimized for the many-to-one commununication
pattern, where network nodes primarily send data towards the border
router, but has provisions for any-to-any routing as well.
The authors of the Powertrace tool studied the power profile of RPL.
It devides the routing protocol into control and data traffic. The
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control channel uses ICMP messages to establish and maintain the
routing states. The data channel is any application that uses RPL
for routing packets. The study has shown that the power consumption
of the control traffic goes down over time and data traffic stays
relatively constant. The study also reflects that the routing
protocol should keep the control traffic as low as possible to make
it energy-friendly.
6. Application Layer
CoAP [I-D.ietf-core-coap]was designed as a RESTful application
protocol, connecting the services of smart devices to the World Wide
Web. CoAP is not a chatty protocol, it provides basic communication
services such as service discovery and GET/POST/PUT/DELETE methods
with a binary header.
The energy-efficient design is implicitly included in the CoAP
protocol design. To reduce regular and frequent queries of the
resources, CoAP provides an observe mode, in which the requester
registers its interest of a certain resource and the responder will
report the value whenever it was updated. This reduces the request
reponse roundtrip while keeping information exchange a ubiquitous
service.
7. Cross Layer Optimization
The cross layer optimization is a technique used in many
scenarios.There are some technologies for power efficient
optimization via PHY to Routing cross layer design
[Cross-layer-Optimization]. In this research, cross-layer
optimization frameworks have been developed to minimize the total
power consumption or to maximize the utility-power tradeoff using
cooperative diversity.
Also a cross-layer design in multihop wireless networks is proposed
for congestion control, routing and scheduling--in transport, network
and link layers into a coherent framework [Cross-layer-design]. This
method and thinking could be applied to the implementation of energy
effective cross layer design.
8. Summary
We find a summary section necessary although most IETF documents do
not contain it. The points we would like to summarize are as
follows.
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a. All Internet protocols, which are in the scope of the IETF, are
customers of the lower layers (PHY, MAC, and Duty-cycling). In
order to get a better service, the designers of higher layers
should know them better.
b. The IETF has developed multiple protocols for constrained
networked devices. A lot of implicitly included design
principles have been used in these protocols.
c. The power trace analysis of different protocol operations showed
that for radio-duty-cycled networks broadcasts should be avoided.
Saving unnecessary states maintenance is also an effective method
to be energy-friendly.
9. IANA Considerations
This document has no IANA requests.
10. Security Considerations
This document discusses the energy efficient protocol design, and
does not incur any changes or challenges on security issues besides
what the protocol specifications have analyzed.
11. References
11.1. Normative References
[Announcementlayer]
Dunkels, A., "The Announcement Layer: Beacon Coordination
for the Sensornet Stack. In Proceedings of EWSN 2011".
[ContikiMAC]
Dunkels, A., "The ContikiMAC Radio Duty Cycling Protocol,
SICS Technical Report T2011:13", December 2011.
[Cross-layer-Optimization]
Le and Hossain, "Cross-Layer Optimization Frameworks for
Multihop Wireless Networks Using Cooperative Diversity",
July 2008.
[Cross-layer-design]
Chen, Low, and Doyle, "Cross-layer design in multihop
wireless networks", 2011.
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[I-D.bormann-lwig-terms]
Bormann, C. and M. Ersue, "Terminology for Constrained
Node Networks", draft-bormann-lwig-terms-00 (work in
progress), November 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-13 (work in progress), December 2012.
[I-D.kovatsch-lwig-class1-coap]
Kovatsch, M., "Implementing CoAP for Class 1 Devices",
draft-kovatsch-lwig-class1-coap-00 (work in progress),
October 2012.
[Powertrace]
Dunkels, Eriksson, Finne, and Tsiftes, "Powertrace:
Network-level Power Profiling for Low-power Wireless
Networks", March 2011.
11.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
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Authors' Addresses
Zhen Cao (Ed.)
China Mobile
Xuanwumenxi Ave. No.32
Beijing, 100871
P.R.China
Phone:
Email: zehn.cao@gmail.com, caozhen@chinamobile.com
Xuan He
Hitachi (China) Research and Development Corporation
301, Tower C North, Raycom, 2 Kexuyuan Nanlu, Haidian District
Beijing, 100190
P.R.China
Phone:
Fax:
Email: xhe@hitachi.cn
URI:
Matthias Kovatsch
ETH Zurich
Universitaetstrasse 6
Zurich, CH-8092
Switzerland
Phone:
Fax:
Email: kovatsch@inf.ethz.ch
URI:
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