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Versions: 00 01 02 draft-ietf-lwig-energy-efficient

Internet Engineering Task Force                                   Z. Cao
Internet-Draft                                              China Mobile
Intended status: Informational                                     X. He
Expires: April 24, 2014Hitachi (China) Research and Development Corporat
                                                             M. Kovatsch
                                                              ETH Zurich
                                                                 H. Tian
                             China Academy of Telecommunication Research
                                                                C. Gomez
                              Universitat Politecnica de Catalunya/i2CAT
                                                        October 21, 2013

   Energy Efficient Implementation of IETF Constrained Protocol Suite


   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.  This document also summarizes the impact of link layer
   protocol power saving behaviors to the upper layer protocols, so that
   they can coordinately make the system energy efficient.

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 April 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  MAC and Radio Duty Cycling  . . . . . . . . . . . . . . . . .   4
     3.1.  Power Save Services Provided by IEEE 802.11v  . . . . . .   5
     3.2.  Power Save Services Provided by Bluetooth Low Energy  . .   6
     3.3.  Power Save Services in IEEE 802.15.4  . . . . . . . . . .   7
   4.  IP Adaptation and Transport Layer . . . . . . . . . . . . . .   7
   5.  Routing Protocols . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Application Layer . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Cross Layer Optimization  . . . . . . . . . . . . . . . . . .   9
   8.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   In many scenarios, the network systems comprises many battery-powered
   or energy-harvesting 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 existing network function such as routing or security be

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   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],[RFC6775],[RFC4944]
   ), 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",
   document are to be interpreted as described in [RFC2119]

1.2.  Terminology

   The terminologies used in this document can be referred to

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 |
        +-----+     +-----+       ===>   +-----+      +-----+

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               \   /                          \        /
    +-----+  +------+  +-------+               +------+   +-----+
    | RTG |--| ipv6 |--|ICMP/ND|               | ipv6 |---| rpl |
    +-----+  +------+  +-------+               +------+   +-----+
                 |                                 |
             +-------+                         +-------+  +----------+
             |MAC/PHY|                         |6lowpan|--|6lowpan-nd|
             +-------+                         +-------+  +----------+

       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.

   | 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

3.  MAC and Radio Duty Cycling

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   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 (RDC) protocol [ContikiMAC].

   From the perspective of MAC&RDC, all upper layer protocols, such as
   routing, RESTful communication, adaptation, 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
   considerate and understand each other.

3.1.  Power Save Services Provided by IEEE 802.11v

   IEEE 802.11v [IEEE80211v] defines mechanisms and services for power
   save of stations/nodes that include flexible multicast service (FMS),
   proxy ARP advertisement, extended sleep modes, traffic filtering.  It
   would be useful if upper layer protocols knows such capabilities
   provided by the lower layer, so that they can coordinate with each

   These services include:

   Proxy ARP: The Proxy ARP capability enables an Access Point (AP) to
   indicate that the non-AP station (STA) will not receive ARP frames.
   The Proxy ARP capability enables the non-AP STA to remain in power-
   save for longer periods of time.

   Basic Service Set (BSS) Max Idle Period management enables an AP to
   indicate a time period during which the AP does not disassociate a
   STA due to non-receipt of frames from the STA.  This supports
   improved STA power saving and AP resource management.

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   FMS: A service in which a non-access point (non-AP) station (STA) can
   request a multicast delivery interval longer than the delivery
   traffic indication message (DTIM) interval for the purposes of
   lengthening the period of time a STA may be in a power save state.

   Traffic Filtering Service (TFS): A service provided by an access
   point (AP) to a non-AP station (STA) that can reduce the number of
   frames sent to the non-AP STA by not forwarding individually
   addressed frames addressed to the non-AP STA that do not match
   traffic filters specified by the non-AP STA.

   Using the above services provided by the lower layer, the constrained
   nodes can achieve either client initiated power save (via TFS) or
   network assisted power save (Proxy-ARP, BSS Max Idel Period and FMS).

   Upper layer protocols would better synchronize with the parameters
   such as FMS interval and BSS MAX Idle Period, so that the wireless
   transmissions are not triggered periodically.

3.2.  Power Save Services Provided by Bluetooth Low Energy

   Bluetooth Low Energy (BT-LE) is a wireless low-power communications
   technology that is the hallmark component of the Bluetooth 4.0
   specification.  BT-LE has been designed for the goal of ultra-low-
   power consumption.  Currently, it is possible to run IPv6 over BT-LE
   networks by using a 6LoWPAN variant adapted to BT-LE

   BT-LE networks comprise a master and one or more slaves which are
   connected to the master.  The BT-LE master is assumed to be a
   relatively powerful device, whereas a slave is typically a
   constrained device (e.g. a class 1 device).

   Medium access in BT-LE is based on a TDMA scheme which is coordinated
   by the master.  This device determines the start of connection
   events, in which communication between the master and a slave takes
   place.  At the beginning of a connection event, the master sends a
   poll message, which may encapsulate data, to the slave.  The latter
   must send a response, which may also contain data.  The master and
   the slave may continue exchanging data until the end of the
   connection event.  The next opportunity for communication between the
   master and the slave will be in the next connection event scheduled
   for the slave.

   The time between consecutive connection events is defined by the
   connInterval parameter, which may range between 7.5 ms and 4 s. The
   slave may remain in sleep mode since the end of its last connection
   event until the beginning of its next connection event.  Therefore,

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   BT-LE is duty-cycled by nature.  Furthermore, after having replied to
   the master, a slave is not required to listen to the master (and thus
   may keep the radio in sleep mode) for connSlaveLatency consecutive
   connection events. connSlaveLatency is an integer parameter between 0
   and 499 which should not cause link inactivity for more than
   connSupervisionTimeout time.  The connSupervisionTimeout parameter is
   in the range between 100 ms and 32 s.

   Upper layer protocols should take into account the medium access and
   duty-cycling behavior of BT-LE.  In particular, connInterval,
   connSlaveLatency and connSupervisionTimeout determine the time
   between two consecutive connection events for a given slave.  The
   upper layer packet generation pattern and rate should be consistent
   with the settings of the aforementioned parameters (and vice versa).

3.3.  Power Save Services in IEEE 802.15.4

   To be added.

4.  IP Adaptation 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
   lower layers, and are almost neutral to the energy-efficiency

   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 Neighbor Discovery
   called 6LoWPAN-ND, which is an energy efficient replacement of the
   IPv6 ND in constrained environments.  IPv6 Neighbor Discovery was not

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   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.  However, 6LoWPAN ND does not modify Neighbor
   Unreachability Detection (NUD) timeouts, which are very short (by
   default three transmissions spaced one second apart).  NUD timeout
   settings should be tuned taking into account the latency that may be
   introduced by duty-cycled mechanisms at the link layer, or
   alternative, less impatient NUD algorithms should be considered

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 communication
   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 [Powertrace] studied the power
   profile of RPL.  It divides the routing protocol into control and
   data traffic.  The 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.  The amount of RPL control traffic can be
   tuned by setting the Trickle algorithm parameters (i.e. Imin, Imax
   and k) to adequate values.  However, there exists a trade-off between
   energy consumption and other performance parameters such as network
   convergence time and robustness.

   Todo: more discussion of energy efficient routing.

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

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   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
   response roundtrip while keeping information exchange a ubiquitous

   CoAP offers mechanisms for reliable communication between two CoAP
   endpoints.  A CoAP message may be signaled as a confirmable (CON)
   message, and an acknowledgment (ACK) is issued by the receiver if the
   CON message is correctly received.  The sender starts a
   Retransmission TimeOut (RTO) for every CON message sent.  The initial
   RTO value is chosen randomly between 2 and 3 s. If an RTO expires,
   the new RTO value is doubled (unless a limit on the number of
   retransmissions has been reached).  Since duty-cycling at the link
   layer may lead to large latencies (i.e. even greater than the initial
   RTO value), CoAP RTO parameters should be tuned accordingly in order
   to avoid spurious RTOs which would unnecessarily waste node energy
   and other resources.

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 trade-off 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.

   Todo: more discussion of Cross layer issues.

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

   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.

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   b.  The IETF has developed multiple protocols for constrained
       networked devices.  A lot of implicit energy efficient 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.  Acknowledgments

   Carles Gomez has been supported by Ministerio de Economia y
   Competitividad and FEDER through project TEC2012-32531.

10.  IANA Considerations

   This document has no IANA requests.

11.  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.

12.  References

12.1.  Normative References

              Dunkels, A., "The Announcement Layer: Beacon Coordination
              for the Sensornet Stack. In Proceedings of EWSN 2011", .

              Dunkels, A., "The ContikiMAC Radio Duty Cycling Protocol,
              SICS Technical Report T2011:13", December 2011.

              Le, . and . Hossain, "Cross-Layer Optimization Frameworks
              for Multihop Wireless Networks Using Cooperative
              Diversity", July 2008.

              Chen, ., Low, ., and . Doyle, "Cross-layer design in
              multihop wireless networks", 2011.

              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets

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              over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
              (work in progress), February 2013.

              Gashinsky, I. and E. Nordmark, "Neighbor Unreachability
              Detection is too impatient", draft-ietf-6man-impatient-
              nud-06 (work in progress), April 2013.

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

              Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained Node Networks", draft-ietf-lwig-terminology-05
              (work in progress), July 2013.

              Kovatsch, M., "Implementing CoAP for Class 1 Devices",
              draft-kovatsch-lwig-class1-coap-00 (work in progress),
              October 2012.

              IEEE, ., "Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) specifications, Amendment
              8: IEEE 802.11 Wireless Network Management.", February

              Dunkels, ., Eriksson, ., Finne, ., and . Tsiftes,
              "Powertrace: Network-level Power Profiling for Low-power
              Wireless Networks", March 2011.

12.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

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   [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.

Authors' Addresses

   Zhen Cao (Ed.)
   China Mobile
   Xuanwumenxi Ave. No.32
   Beijing  100871

   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

   Email: xhe@hitachi.cn

   Matthias Kovatsch
   ETH Zurich
   Universitaetstrasse 6
   Zurich, CH-8092

   Email: kovatsch@inf.ethz.ch

   Hui Tian
   China Academy of Telecommunication Research
   Huayuanbeilu No.52
   Beijing, Haidian District  100191

   Email: tianhui@mail.ritt.com.cn

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   Carles Gomez
   Universitat Politecnica de Catalunya/i2CAT
   C/Esteve Terradas, 7
   Castelldefels  08860

   Email: carlesgo@entel.upc.edu

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