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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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   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
   (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



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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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