Internet Engineering Task Force (IETF)                          C. Gomez
Request for Comments: 8352                                           UPC
Category: Informational                                      M. Kovatsch
ISSN: 2070-1721                                               ETH Zurich
                                                                 H. Tian
                             China Academy of Telecommunication Research
                                                             Z. Cao, Ed.
                                                     Huawei Technologies
                                                              April 2018

       Energy-Efficient Features of Internet of Things Protocols


   This document describes the challenges for energy-efficient protocol
   operation on constrained devices and the current practices used to
   overcome those challenges.  It summarizes the main link-layer
   techniques used for energy-efficient networking, and it highlights
   the impact of such techniques on the upper-layer protocols so that
   they can together achieve an energy-efficient behavior.  The document
   also provides an overview of energy-efficient mechanisms available at
   each layer of the IETF protocol suite specified for constrained-node

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2018 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
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Medium Access Control and Radio Duty Cycling  . . . . . . . .   6
     3.1.  Techniques for Radio Duty Cycling . . . . . . . . . . . .   6
     3.2.  Latency and Buffering . . . . . . . . . . . . . . . . . .   7
     3.3.  Throughput  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Radio Interface Tuning  . . . . . . . . . . . . . . . . .   8
     3.5.  Packet Bundling . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  Power Save Services Available in Example Low-Power Radios   8
       3.6.1.  Power Save Services Provided by IEEE 802.11 . . . . .   8
       3.6.2.  Power Save Services Provided by Bluetooth LE  . . . .  10
       3.6.3.  Power Save Services in IEEE 802.15.4  . . . . . . . .  11
       3.6.4.  Power Save Services in DECT ULE . . . . . . . . . . .  12
   4.  IP Adaptation and Transport Layer . . . . . . . . . . . . . .  14
   5.  Routing Protocols . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Application Layer . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Energy-Efficient Features in CoAP . . . . . . . . . . . .  16
     6.2.  Sleepy Node Support . . . . . . . . . . . . . . . . . . .  17
     6.3.  CoAP Timers . . . . . . . . . . . . . . . . . . . . . . .  17
     6.4.  Data Compression  . . . . . . . . . . . . . . . . . . . .  18
   7.  Summary and Conclusions . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   Network systems for monitoring the physical world contain many
   battery-powered or energy-harvesting devices.  For example, in an
   environmental monitoring system or a temperature and humidity
   monitoring system, there may not be always on and sustained power
   supplies for the potentially large number of constrained devices.  In
   such deployment scenarios, it is necessary to optimize the energy
   consumption of the constrained devices.  In this document, we
   describe techniques that are in common use at Layer 2 and at Layer 3,
   and we indicate the need for higher-layer awareness of lower-layer

   Many research efforts have studied this "energy efficiency" problem.
   Most of this research has focused on how to optimize the system's
   power consumption in certain deployment scenarios or how an existing
   network function such as routing or security could be more energy
   efficient.  Only few efforts have focused on energy-efficient designs
   for IETF protocols and standardized network stacks for such
   constrained devices [CLASS1-CoAP].

   The IETF has developed a suite of Internet protocols suitable for
   such constrained devices, including IPv6 over Low-Power Wireless
   Personal Area Networks (6LoWPAN) [RFC6282] [RFC6775] [RFC4944], the
   IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL)
   [RFC6550], and the Constrained Application Protocol (CoAP) [RFC7252].
   This document tries to summarize the design considerations for making
   the IETF constrained 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.  In
   particular, it provides an overview of the techniques used by the
   lower layers to save energy and how these may impact on the upper
   layers.  Cross-layer interaction is therefore considered in this
   document from this specific point of view.  Providing further design
   recommendations that go beyond the layered protocol architecture is
   out of the scope of this document.

   After reviewing the energy-efficient designs of each layer, we
   summarize the document by presenting some overall conclusions.
   Though the lower-layer communication optimization is the key part of
   energy-efficient design, the protocol design at the upper layers is
   also important to make the device energy efficient.

1.1.  Terminology

   Terms used in this document are defined in [RFC7228] [CNN-TERMS].

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2.  Overview

   The IETF has developed protocols to enable end-to-end IP
   communication between constrained nodes and fully capable nodes.
   This work has expedited the evolution of the traditional Internet
   protocol stack to a lightweight Internet protocol stack.  As shown 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
   [IEEE802.15.4] and Bluetooth Low Energy (also referred to as
   Bluetooth LE and BTLE), with the support of routing by RPL and
   efficient neighbor discovery by 6LoWPAN Neighbor Discovery (6LoWPAN-
   ND). 6LoWPAN is currently being adapted by the 6lo Working Group to
   support IPv6 over various other technologies, such as ITU-T G.9959
   [G9959], Digital Enhanced Cordless Telecommunications Ultra Low
   Energy (DECT ULE) [TS102], Building Automation and Control Networks
   Master-Slave/Token-Passing (BACnet MS/TP) [MSTP], and Near Field
   Communication [NFC].

   +-----+   +-----+    +-----+                +------+
   |HTTP |   | FTP |    |SNMP |                | CoAP |
   +-----+   +-----+    +-----+                +------+
         \    /           /                   /        \
        +-----+     +-----+              +-----+      +-----+
        | TCP |     | UDP |              | TCP |      | UDP |
        +-----+     +-----+       ===>   +-----+      +-----+
               \   /                          \        /
    +-----+  +------+  +-------+               +------+   +-----+
    | RTG |--| IPv6 |--|ICMP/ND|               | IPv6 |---| RTG |
    +-----+  +------+  +-------+               +------+   +-----+
                 |                                 |
             +-------+                         +-------+  +----------+
             |MAC/PHY|                         |  6Lo  |--|6LoWPAN-ND|
             +-------+                         +-------+  +----------+

       Figure 1: Traditional and Lightweight Internet Protocol Stack

   There are numerous published studies reporting comprehensive
   measurements of wireless communication platforms [Powertrace].  As an
   example, below we list the energy-consumption profile of the most
   common operations involved in communication on a prevalent sensor
   node platform.  The measurement was based on the Tmote Sky with
   ContikiMAC [ContikiMAC] as the Radio Duty Cycling algorithm.  From
   this and many other measurement reports (e.g., [AN079]), we can see
   that the energy consumption of optimized transmission and reception

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   are in the same order.  For IEEE 802.15.4 [IEEE802.15.4] and Ultra
   WideBand (UWB) links, transmitting may actually be even cheaper than
   receiving.  It also shows that broadcast and non-synchronized
   communication transmissions are energy costly because they need to
   acquire the medium for a long time.

   | Activity                              | Energy        |
   |                                       | (microjoules) |
   | Broadcast reception                   |           178 |
   | Unicast reception                     |           222 |
   | Broadcast transmission                |          1790 |
   | Non-synchronized unicast transmission |          1090 |
   | Synchronized unicast transmission     |           120 |
   | Unicast TX to awake receiver          |            96 |
   | Listening (for 1000 ms)               |         63000 |

       Figure 2: Power Consumption of Common Operations Involved in
              Communication on the Tmote Sky with ContikiMAC

   At the Physical layer, one approach that may reduce the energy
   consumption of a device that uses a wireless interface is based on
   reducing the device transmit power level, as long as the intended
   next hop(s) is still within range of the device.  In some cases, if
   node A has to transmit a message to node B, a solution to reduce node
   A transmit power is to leverage an intermediate device, e.g., node C
   as a message forwarder.  Let d be the distance between node A and
   node B.  Assuming free-space propagation, where path loss is
   proportional to d^2, if node C is placed right in the middle of the
   path between A and B (that is, at a distance d/2 from both node A and
   node B), the minimum transmit power to be used by node A (and by node
   C) is reduced by a factor of 4.  However, this solution requires
   additional devices, it requires a routing solution, and it also
   increases transmission delay between A and B.

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3.  Medium Access Control and Radio Duty Cycling

   In networks, communication and power consumption are interdependent.
   The communication device is typically the most power-consuming
   component, but merely refraining from transmissions is not enough to
   achieve a low power consumption: the radio may consume as much power
   in listen mode as when actively transmitting.  This illustrates the
   key problem known as idle listening, whereby the radio of a device
   may be in receive mode (ready to receive any message), even if no
   message is being transmitted to that device.  Idle listening can
   consume a huge amount of energy unnecessarily.  To reduce power
   consumption, the radio must be switched completely off -- duty-cycled
   -- as much as possible.  By applying duty cycling, the lifetime of a
   device operating on a common button battery may be on the order of
   years, whereas otherwise the battery may be exhausted in a few days
   or even hours.  Duty cycling is a technique generally employed by
   devices that use the P1 strategy [RFC7228], which need to be able to
   communicate on a relatively frequent basis.  Note that a more
   aggressive approach to save energy relies on the P0 (Normally-off)
   strategy, whereby devices sleep for very long periods and communicate
   infrequently, even though they spend energy in network reattachment

   From the perspective of Medium Access Control (MAC) and Radio Duty
   Cycling (RDC), all upper-layer protocols, such as routing, RESTful
   communication, adaptation, and management flows, are applications.
   Since the duty-cycling algorithm is the key to energy efficiency of
   the wireless medium, it synchronizes transmission and/or reception
   requests from the higher layers.

   MAC and RDC are not in the scope of the IETF, yet lower-layer
   designers and chipset manufacturers take great care to save energy.
   By knowing the behaviors of these lower layers, engineers can design
   protocols that work well with them.  The IETF protocols to be
   discussed in the following sections are the customers of the lower

3.1.  Techniques for Radio Duty Cycling

   This subsection describes three main RDC techniques.  Note that more
   than one of these techniques may be available or can even be combined
   in a specific radio technology:

   a)  Channel sampling: In this solution, the radio interface of a
       device periodically monitors the channel for very short time
       intervals (i.e., with a low duty cycle) with the aim of detecting
       incoming transmissions.  In order to make sure that a receiver
       can correctly receive a transmitted data unit, the sender may

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       prepend a preamble of a duration at least the sampling period to
       the data unit to be sent.  Another option for the sender is to
       repeatedly transmit the data unit instead of sending a preamble
       before the data unit.  Once a transmission is detected by a
       receiver, the receiver may stay awake until the complete
       reception of the data unit.  Examples of radio technologies that
       use preamble sampling include ContikiMAC, the Coordinated Sampled
       Listening (CSL) mode of IEEE 802.15.4e [IEEE802.15.4], and the
       Frequently Listening (FL) mode of ITU-T G.9959 [G9959].

   b)  Scheduled transmissions: This approach allows a device to know
       the particular time at which it should be awake (during some time
       interval) in order to receive data.  Otherwise, the device may
       remain in sleep mode.  The decision on the times at which
       communication is attempted relies on some form of negotiation
       between the involved devices.  Such negotiation may be performed
       per transmission or per session/connection.  Bluetooth Low Energy
       (Bluetooth LE) is an example of a radio technology based on this

   c)  Listen after send: This technique allows a node to remain in
       sleep mode by default, then wake up and poll a sender (which must
       be ready to receive a poll message) for pending transmissions.
       After sending the poll message, the node remains in receive mode
       and is ready for a potential incoming transmission.  After a
       certain time interval, the node may go back to sleep.  For
       example, this technique is used in the Receiver Initiated
       Transmission (RIT) mode of IEEE 802.15.4e [IEEE802.15.4] and in
       the transmission of data between a coordinator and a device in
       the 2003 version of IEEE 802.15.4 [IEEE802.15.4].

3.2.  Latency and Buffering

   The latency of a data unit transmission to a duty-cycled device is
   equal to or greater than the latency of transmitting to an always-on
   device.  Therefore, duty cycling leads to a trade-off between energy
   consumption and latency.  Note that in addition to a latency
   increase, RDC may introduce latency variance since the latency
   increase is a random variable (which is uniformly distributed if duty
   cycling follows a periodic behavior).

   On the other hand, due to the latency increase introduced by duty
   cycling, a sender waiting for a transmission opportunity may need to
   store subsequent outgoing packets in a buffer.  This buffering would
   increase memory requirements and potentially incur queuing wait
   times.  Such wait times would in turn contribute to packet
   transmission delay and increase the probability of buffer overflow,
   leading to losses.

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3.3.  Throughput

   Although throughput is not typically a key concern in constrained-
   node network applications, it is indeed important in some services in
   such networks, such as over-the-air software updates or when off-line
   sensors accumulate measurements that have to be quickly transferred
   when there is an opportunity for connectivity.

   Since RDC introduces inactive intervals in energy-constrained
   devices, it reduces the throughput that can be achieved when
   communicating with such devices.  There exists a trade-off between
   the achievable throughput and energy consumption.

3.4.  Radio Interface Tuning

   The parameters controlling the radio duty cycle have to be carefully
   tuned to achieve the intended application and/or network
   requirements.  On the other hand, upper layers should take into
   account the expected latency and/or throughput behavior due to RDC.
   The next subsection provides details on key parameters controlling
   RDC mechanisms, and thus fundamental trade-offs, for various examples
   of relevant low-power radio technologies.

3.5.  Packet Bundling

   Another technique that may be useful to increase communication energy
   efficiency is packet bundling.  This technique, which is available in
   several radio interfaces (e.g., LTE and some 802.11 variants), allows
   for aggregation of several small packets into a single large packet.
   Header and communication overhead is therefore reduced.

3.6.  Power Save Services Available in Example Low-Power Radios

   This subsection presents power save services and techniques used in a
   few relevant examples of wireless low-power radios: IEEE 802.11
   [IEEE802.11], Bluetooth LE, and IEEE 802.15.4 [IEEE802.15.4].  For a
   more detailed overview of each technology, the reader may refer to
   the literature or to the corresponding specifications.

3.6.1.  Power Save Services Provided by IEEE 802.11

   IEEE 802.11 [IEEE802.11] defines the Power Save Mode (PSM) whereby a
   station may indicate to an Access Point (AP) that it will enter a
   sleep mode state.  While the station is sleeping, the AP buffers any
   frames that should be sent to the sleeping station.  The station
   wakes up every listen interval (which can be a multiple of the beacon
   interval) in order to receive beacons.  The AP signals, by means of a
   beacon field, whether there is data pending for the station or not.

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   If there are not frames to be sent to the station, the latter may get
   back to sleep mode.  Otherwise, the station may send a message
   requesting the transmission of the buffered data and stay awake in
   receive mode.

   IEEE 802.11v [IEEE802.11] further defines mechanisms and services for
   power save of stations/nodes that include Flexible Multicast Service
   (FMS), Proxy ARP advertisement, extended sleep modes, and traffic
   filtering.  Upper-layer protocol's knowledge of such capabilities,
   provided by the lower layer, enables better interworking.

   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 mode 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.

   FMS:  A service in which a non-AP 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 AP to a
      non-AP STA that can reduce the number of frames sent to the STA by
      dropping individually addressed frames that do not match traffic
      filters specified by the 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 Idle Period, and

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

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3.6.2.  Power Save Services Provided by Bluetooth LE

   Bluetooth LE is a wireless low-power communications technology that
   is the hallmark component of the Bluetooth 4.0, 4.1, and 4.2
   specifications [Bluetooth42].  BTLE has been designed for the goal of
   ultra-low power consumption.  IPv6 can be run IPv6 over Bluetooth LE
   networks by using a 6LoWPAN variant adapted to BTLE [RFC7668].

   Bluetooth LE networks comprise a master and one or more slaves, which
   are connected to the master.  The Bluetooth 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 Bluetooth LE is based on a Time-Division Multiple
   Access (TDMA) scheme that 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 from the end of its last connection
   event until the beginning of its next connection event.  Therefore,
   Bluetooth LE is duty-cycled by design.  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 that 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 Bluetooth 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).
   For example, assume connInterval = 4 seconds, connSlaveLatency =
   7 seconds, and connSupervisionTimeout = 32 seconds.  With these
   settings, communication opportunities between a master and a slave
   will occur during a given interval every 32 seconds.  Duration of the
   interval will depend on several factors, including number of

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   connected slaves, amount of data to be transmitted, etc.  In the
   worst case, only one data unit can be sent from master to slave (and
   vice versa) every 32 seconds.

3.6.3.  Power Save Services in IEEE 802.15.4

   IEEE 802.15.4 [IEEE802.15.4] is a family of standard radio interfaces
   for low-rate, low-power wireless networking.  Since the publication
   of its first version in 2003, IEEE 802.15.4 [IEEE802.15.4] has become
   the de facto choice for a wide range of constrained-node network
   application domains and has been a primary target technology of
   various IETF working groups such as 6LoWPAN [RFC6282] [RFC6775]
   [RFC4944] and 6TiSCH [ARCH-6TiSCH].  IEEE 802.15.4 [IEEE802.15.4]
   specifies a variety of related Physical layer (PHY) and MAC layer

   IEEE 802.15.4 [IEEE802.15.4] defines three roles called device,
   coordinator, and Personal Area Network (PAN) coordinator.  The device
   role is adequate for nodes that do not implement the complete IEEE
   802.15.4 [IEEE802.15.4] functionality and is mainly targeted for
   constrained nodes with a limited energy source.  The coordinator role
   includes synchronization capabilities and is suitable for nodes that
   do not suffer severe constraints (e.g., a mains-powered node).  The
   PAN coordinator is a special type of coordinator that acts as a
   principal controller in an IEEE 802.15.4 [IEEE802.15.4] network.

   IEEE 802.15.4 [IEEE802.15.4] defines two main types of networks
   depending on their configuration: beacon-enabled and non-beacon-
   enabled networks.  In the first network type, coordinators
   periodically transmit beacons.  The time between beacons is divided
   in three main parts: the Contention Access Period (CAP), the
   Contention Free Period (CFP), and an inactive period.  In the first
   period, nodes use slotted Carrier Sense Multiple Access with
   Collision Avoidance (CSMA/CA) for data communication.  In the second
   one, a TDMA scheme controls medium access.  During the idle period,
   communication does not take place, and thus the inactive period is a
   good opportunity for nodes to turn the radio off and save energy.
   The coordinator announces in each beacon the list of nodes for which
   data will be sent in the subsequent period.  Therefore, devices may
   remain in sleep mode by default and wake up periodically to listen to
   the beacons sent by their coordinator.  If a device wants to transmit
   data, or learns from a beacon that it is an intended destination,
   then it will exchange messages with the coordinator (and thus consume
   energy).  An underlying assumption is that when a message is sent to
   a coordinator, the radio of the coordinator will be ready to receive
   the message.

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   The beacon interval and the duration of the active portion of the
   beacon interval (i.e., the CAP and the CFP), and thus the duty cycle,
   can be configured.  The parameters that control these times are
   called macBeaconOrder and macSuperframeOrder, respectively.  As an
   example, when IEEE 802.15.4 [IEEE802.15.4] operates in the 2.4 GHz
   PHY, both times can be (independently) set to values in the range
   between 15.36 ms and 251.6 s.

   In the beaconless mode, nodes use unslotted CSMA/CA for data
   transmission.  The device may be in sleep mode by default and may
   activate its radio to either i) request to the coordinator whether
   there is pending data for the device, or to ii) transmit data to the
   coordinator.  The wake-up pattern of the device, if any, is out of
   the scope of IEEE 802.15.4 [IEEE802.15.4].

   Communication between the two ends of an IEEE 802.15.4 [IEEE802.15.4]
   link may also take place in a peer-to-peer configuration, whereby
   both link ends assume the same role.  In this case, data transmission
   can happen at any moment.  Nodes must have their radio in receive
   mode and be ready to listen to the medium by default (which for
   battery-enabled nodes may lead to a quick battery depletion) or apply
   synchronization techniques.  The latter are out of the scope of IEEE
   802.15.4 [IEEE802.15.4].

   The main MAC layer IEEE 802.15.4 [IEEE802.15.4] amendment to date is
   IEEE 802.15.4e.  This amendment includes various new MAC layer modes,
   some of which include mechanisms for low energy consumption.  Among
   these, the Time-Slotted Channel Hopping (TSCH) is an outstanding mode
   that offers robust features for industrial environments, among
   others.  In order to provide the functionality needed to enable IPv6
   over TSCH, the 6TiSCH Working Group was created.  TSCH is based on a
   TDMA schedule whereby a set of timeslots are used for frame
   transmission and reception, and other timeslots are unscheduled.  The
   latter timeslots may be used by a dynamic scheduling mechanism,
   otherwise, nodes may keep the radio off during the unscheduled
   timeslots, thus saving energy.  The minimal schedule configuration
   specified in [RFC8180] comprises 101 timeslots; 95 of these timeslots
   are unscheduled and the timeslot duration is 15 ms.

   The previously mentioned CSL and RIT are also 802.15.4e modes
   designed for low energy.

3.6.4.  Power Save Services in DECT ULE

   DECT Ultra Low Energy (DECT ULE) is a wireless technology building on
   the key fundamentals of traditional DECT / Cordless Advanced
   Technology - internet and quality (CAT-iq) [EN300] but with specific
   changes to significantly reduce the power consumption at the expense

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   of data throughput [TS102].  DECT ULE devices typically operate on
   special power-optimized silicon but can connect to a DECT Gateway
   supporting traditional DECT/CAT-iq for cordless telephony and data as
   well as the DECT ULE extensions.  IPv6 can be run over DECT ULE by
   using a 6LoWPAN variant [RFC8105].

   DECT defines two major roles: the Portable Part (PP) is the power
   constrained device while the Fixed Part (FP) is the Gateway or base
   station in a star topology.  Because TDMA/FDMA and Time-Division
   Duplex (TDD) using dynamic channel allocation for interference, DECT
   operates in license-free and reserved frequency bands.  It provides
   good indoor (~50 m) and outdoor (~300 m) coverage.  It uses a frame
   length of 10 ms divided into 24 timeslots, and it supports
   connection-oriented packet data and connection-less services.

   The FP usually transmits a so-called dummy bearer (beacon) that is
   used to broadcast synchronization, system, and paging information.
   The slot/carrier position of this dummy bearer can automatically be
   reallocated in order to avoid mutual interference with other DECT

   At the MAC level, DECT ULE communications between FP and PP are
   initiated by the PP.  An FP can initiate communication indirectly by
   sending a paging signal to a PP.  The PP determines the timeslot and
   frequency in which the communication between FP and PP takes place.
   The PP verifies the radio timeslot/frequency position is unoccupied
   before it initiates its transmitter.  An access-request message,
   which usually carries data, is sent to the FP.  The FP sends a
   confirm message, which also may carry data.  More data can be sent in
   subsequent frames.  A MAC-level automatic retransmission scheme
   significantly improves the reliability of data transfer.  A
   segmentation and reassembly scheme supports transfer of larger,
   higher-layer Service Data Units (SDUs) and provides data integrity
   checks.  The DECT ULE packet data service ensures data integrity,
   proper sequencing, and duplicate protection but not guaranteed
   delivery.  Higher-layer protocols have to take this into

   The FP may send paging information to PPs to trigger connection setup
   and indicate the required service type.  The interval between paging
   information to a specific PP can be defined in the range of 10 ms to
   327 s.  The PP may enter sleep mode to save power.  The listening
   interval is defined by the PP application.  For short sleep intervals
   (below ~10 seconds), the PP may be able to retain synchronization to
   the FP dummy bearer and only turn on the receiver during the expected
   timeslot.  For longer sleep intervals, the PP can't keep
   synchronization and has to search for, and resynchronize to, the FP
   dummy bearer.  Hence, longer sleep intervals reduce the average

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   energy consumption but add an energy consumption penalty for
   acquiring synchronization to the FP dummy bearer.  The PP can obtain
   all information to determine paging and acquire synchronization
   information in a single reception of one full timeslot.

   Packet data latency is normally 30 ms for short packets (below or
   equal to 32 octets), however, if retry and back-off scenarios occur,
   the latency is increased.  The latency can actually be reduced to
   about 10 ms by doing energy consuming Received Signal Strength
   Indication (RSSI) scanning in advance.  In the direction from FP to
   PP, the latency is usually increased by the used paging interval and
   the sleep interval.  The MAC layer can piggyback commands to improve
   efficiency (reduce latency) of higher-layer protocols.  Such commands
   can instruct the PP to initiate a new packet transfer in N frames
   without the need for resynchronization and can listen to paging or
   instruct the PP to stay in a higher duty-cycle paging detection mode.

   The DECT ULE technology allows a per-PP configuration of paging
   interval, MTU size, reassembly window size, and higher-layer service
   negotiation and protocol.

4.  IP Adaptation and Transport Layer

   6LoWPAN provides an adaptation layer designed to support IPv6 over
   IEEE 802.15.4 [IEEE802.15.4]. 6LoWPAN affects the energy-efficiency
   problem in three aspects, as follows.

   First, 6LoWPAN provides one fragmentation and reassembly mechanism,
   which is aimed at solving the packet size issue in IPv6 and could
   also affect energy efficiency.  IPv6 requires that every link in the
   Internet have an MTU of 1280 octets or greater.  On any link that
   cannot convey a 1280-octet packet in one piece, link-specific
   fragmentation and reassembly must be provided at a layer below IPv6
   [RFC8200].  6LoWPAN provides fragmentation and reassembly below the
   IP layer to solve the problem.  One of the benefits from placing
   fragmentation at a lower layer such as the 6LoWPAN layer is that it
   can avoid the presence of more IP headers because fragmentation at
   the IP layer will produce more IP packets, each one carrying its own
   IP header.  However, performance can be severely affected if, after
   IP layer fragmentation, then 6LoWPAN fragmentation happens as well
   (e.g., when the upper layer is not aware of the existence of the
   fragmentation at the 6LoWPAN layer).  One solution is to require that
   the higher layers have an awareness of the lower-layer features and
   generate small enough packets to avoid fragmentation.  In this
   regard, the Block option in CoAP can be useful when CoAP is used at
   the application layer [RFC7959].

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   Secondly, 6LoWPAN swaps computing with communication. 6LoWPAN applies
   compression of the IPv6 header.  Subject to the packet size limit of
   IEEE 802.15.4 [IEEE802.15.4], a 40-octet-long IPv6 header and an 8 or
   20-octet-long UDP and TCP header will consume even more packet space
   than the data itself. 6LoWPAN provides IPv6 and UDP header
   compression at the adaptation layer.  Therefore, a lower amount of
   data will be handled by the lower layers, whereas both the sender and
   receiver will spend more computing power on the compression and
   decompression of the packets over the air.  Compression can also be
   performed at higher layers (see Section 6.4).

   Finally, 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 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
   1 second apart).  NUD timeout settings should be tuned to take into
   account the latency that may be introduced by duty-cycled mechanisms
   at the link layer or the alternative, less impatient NUD algorithms
   should be considered [RFC7048].

   IPv6 underlies the higher-layer protocols, including both TCP/UDP
   transport and applications.  By design, the higher-layer protocols do
   not typically have specific information about the lower layers and
   thus cannot solve the energy-efficiency problem.

   The network stack can be designed to save 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 are out of the scope of the IETF and
   the LWIG Working Group.

5.  Routing Protocols

   RPL [RFC6550] is a routing protocol designed by the IETF for
   constrained environments.  RPL exchanges messages periodically and
   keeps 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.

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   The authors of the Powertrace tool [Powertrace] studied the power
   profile of RPL.  Their analysis divides the routing protocol into
   control and data traffic.  The control plane carries ICMP messages to
   establish and maintain the routing states.  The data plane carries
   any application that uses RPL for routing packets.  The study has
   shown that the power consumption of the control traffic goes down
   over time in a relatively stable network.  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 [RFC6206] algorithm
   parameters (i.e., Imin, Imax, and k) to appropriate values.  However,
   there exists a trade-off between energy consumption and other
   performance parameters such as network convergence time and

   RFC 6551 [RFC6551] defines routing metrics and constraints to be used
   by RPL in route computation.  Among others, RFC 6551 specifies a Node
   Energy object that allows to provide information related to node
   energy, such as the energy source type or the estimated percentage of
   remaining energy.  Appropriate use of energy-based routing metrics
   may help to balance energy consumption of network nodes, minimize
   network partitioning, and increase network lifetime.

6.  Application Layer

6.1.  Energy-Efficient Features in CoAP

   CoAP [RFC7252] is designed as a RESTful application protocol that
   connects 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.

   Energy efficiency is part of the CoAP protocol design.  CoAP uses a
   fixed-length binary header of only four bytes that may be followed by
   binary options.  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/
   response round trips while keeping information exchange an ubiquitous
   service; an energy-constrained server can remain in sleep mode during
   the period between observe notification transmissions.

   Furthermore, [RFC7252] defines CoAP proxies that can cache resource
   representations previously provided by sleepy CoAP servers.  The
   proxies themselves may respond to client requests if the

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   corresponding server is sleeping and the resource representation is
   recent enough.  Otherwise, a proxy may attempt to obtain the resource
   from the sleepy server.

   CoAP proxy and cache functionality may also be used to perform data
   aggregation.  This technique allows a node to receive data messages
   (e.g., carrying sensor readings) from other nodes in the network,
   perform an operation based on the content in those messages, and
   transmit the result of the operation.  Such operation may simply be
   intended to use one packet to carry the readings transported in
   several packets (which reduces header and transmission overhead), or
   it may be a more sophisticated operation, possibly based on
   mathematical, logical, or filtering principles (which reduces the
   payload size to be transmitted).

6.2.  Sleepy Node Support

   Beyond these features of CoAP, there have been a number of proposals
   to further support sleepy nodes at the application layer by
   leveraging CoAP mechanisms.  A good summary of such proposals can be
   found in [SLEEPY-DEVICES], while an example application (in the
   context of illustrating several security mechanisms) in a scenario
   with sleepy devices has been described [CRYPTO-SENSORS].  Approaches
   to support sleepy nodes include exploiting the use of proxies,
   leveraging the resource directory [CoRE-RD], or signaling when a node
   is awake to the interested nodes.  Recent work defines publish-
   subscribe and message queuing extensions to CoAP and the resource
   directory in order to support devices that spend most of their time
   asleep [CoAP-BROKER].  Notably, this work has been adopted by the
   CoRE Working Group.

   In addition to the work within the scope of CoAP to support sleepy
   nodes, other specifications define application-layer functionality
   for the same purpose.  The Lightweight Machine-to-Machine (LwM2M)
   specification from the Open Mobile Alliance (OMA) defines a queue
   mode whereby an LwM2M Server queues requests to an LwM2M Client until
   the latter (which may often stay in sleep mode) is online.  LwM2M
   functionality operates on top of CoAP.

   oneM2M defines a CoAP binding with an application-layer mechanism for
   sleepy nodes [oneM2M].

6.3.  CoAP Timers

   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

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   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 long latency (i.e., even greater than the initial
   RTO value), CoAP RTO parameters should be tuned accordingly in order
   to avoid spurious RTOs that would unnecessarily waste node energy and
   other resources.  On the other hand, note that CoAP can also run on
   top of TCP [RFC8323].  In that case, similar guidance applies to TCP
   timers, albeit with greater motivation to carefully configure TCP RTO
   parameters since [RFC6298] reduced the default initial TCP RTO to 1
   second, which may interact more negatively with duty-cycled links
   than default CoAP RTO values.

6.4.  Data Compression

   Another method intended to reduce the size of the data units to be
   communicated in constrained-node networks is data compression, which
   allows to encode data using fewer bits than the original data
   representation.  Data compression is more efficient at higher layers,
   particularly before encryption is used.  In fact, encryption
   mechanisms may generate an output that does not contain redundancy,
   making it almost impossible to reduce the data representation size.
   In CoAP, messages may be encrypted by using Datagram Transport Layer
   Security (DTLS) or TLS when CoAP over TCP is used, which is the
   default mechanism for securing CoAP exchanges.

7.  Summary and Conclusions

   We summarize the key takeaways of this document:

   a.  Internet protocols designed by the IETF can be considered the
       customer of the lower layers (PHY, MAC, and duty cycling).  To
       reduce power consumption, it is recommended that Layer 3 designs
       should operate based on awareness of lower-level parameters
       rather than treating the lower layer as a black box (see Sections
       4, 5, and 6).

   b.  It is always useful to compress the protocol headers in order to
       reduce the transmission/reception power.  This design principle
       has been employed by many protocols in the 6lo and CoRE Working
       Groups (see Sections 4 and 6).

   c.  Broadcast and non-synchronized transmissions consume more than
       other TX/RX operations.  If protocols must use these ways to
       collect information, reduction of their usage by aggregating
       similar messages together will be helpful in saving power (see
       Sections 2 and 6.1).

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   d.  Saving power by sleeping as much as possible is used widely
       (Section 3).

8.  IANA Considerations

   This document has no IANA actions.

9.  Security Considerations

   This document discusses energy-efficient protocol design and does not
   incur any changes or challenges on security issues besides what the
   protocol specifications have analyzed.

10.  References

10.1.  Normative References

              Bluetooth Special Interest Group, "Core Version 4.2",
              available from "Legacy Core Specifications", December
              2014, <

   [EN300]    ETSI, "Digital Enhanced Cordless Telecommunications
              (DECT); Common Interface (CI); Part 1: Overview", ETSI EN
              300 175-1 V2.6.1, July 2015,

   [G9959]    ITU-T, "Short range narrow-band digital radiocommunication
              transceivers - PHY, MAC, SAR and LLC layer
              specifications", ITU-T Recommendation G.9959, January
              2015, <>.

              IEEE, "IEEE Standard for Information technology--
              Telecommunications and information exchange between
              systems Local and metropolitan area networks--Specific
              requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications",
              IEEE 802.11, DOI 10.1109/IEEESTD.2016.7786995,

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              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE 802.15.4, DOI 10.1109/IEEESTD.2016.7460875,

   [MSTP]     ANSI/ASHRAE, "Addenda: BACnet -- A Data Communication
              Protocol for Building Automation and Control Networks
              ANSI/ASHRAE Addenda an, at, au, av, aw, ax, and az to
              ANSI/ASHRAE Standard 135-2012", July 2014,

   [NFC]      NFC Forum, "NFC Logical Link Control Protocol", Technical
              Specification, Version 1.3, March 2016.

   [oneM2M]   oneM2M, "oneM2M - Published Specifications",

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., 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,
              DOI 10.17487/RFC6550, March 2012,

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   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [TS102]    ETSI, "Digital Enhanced Cordless Telecommunications
              (DECT); Ultra Low Energy (ULE); Machine to Machine
              Communications; Part 2: Home Automation Network (phase 2",
              ETSI TS 102 939-2 V1.1.1, March 2015,

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10.2.  Informative References

   [AN079]    Kim, C., "Measuring Power Consumption of CC2530 With
              Z-Stack", Application Note AN079, SWRA292, September 2012,

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, draft-ietf-6tisch-
              architecture-13, November 2017.

              Kovatsch, M., "Implementing CoAP for Class 1 Devices",
              Work in Progress, draft-kovatsch-lwig-class1-coap-00,
              October 2012.

              Bormann, C., Ersue, M., Keranen, A., and C. Gomez,
              "Terminology for Constrained-Node Networks", Work in
              Progress, draft-bormann-lwig-7228bis-02, October 2017.

              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, draft-ietf-core-coap-pubsub-04,
              March 2018.

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

   [CoRE-RD]  Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, Ed., "CoRE Resource Directory", Work in
              Progress, draft-ietf-core-resource-directory-13, March

              Sethi, M., Arkko, J., Keranen, A., and H. Back, "Practical
              Considerations and Implementation Experiences in Securing
              Smart Object Networks", Work in Progress, draft-ietf-lwig-
              crypto-sensors-06, February 2018.

              Dunkels, A., Eriksson, J., Finne, N., and N. Tsiftes,
              "Powertrace: Network-level Power Profiling for Low-power
              Wireless Networks", SICS Technical Report T2011:05, March
              2011, <>.

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   [RFC7048]  Nordmark, E. and I. Gashinsky, "Neighbor Unreachability
              Detection Is Too Impatient", RFC 7048,
              DOI 10.17487/RFC7048, January 2014,

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,

   [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
              M., and D. Barthel, "Transmission of IPv6 Packets over
              Digital Enhanced Cordless Telecommunications (DECT) Ultra
              Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
              2017, <>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,

              Rahman, A., "Sleepy Devices: Do we need to Support them in
              CORE?", Work in Progress, draft-rahman-core-sleepy-nodes-
              do-we-need-01, February 2014.


   Carles Gomez has been supported by the Spanish Government, FEDER, and
   the ERDF through projects TEC2012-32531 and TEC2016-79988-P.

   The authors would like to give thanks for the review and feedback
   from a number of experts in this area: Carsten Bormann, Ari Keranen,
   Hannes Tschofenig, Dominique Barthel, Bernie Volz, and Charlie

   The text of this document was improved based on an IESG document
   editing session during IETF 87.  Thanks to Ted Lemon and Joel Jaeggli
   for initiating and facilitating this editing session.

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   Jens T. Petersen, RTX, contributed the section on power save services
   in DECT ULE.

Authors' Addresses

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


   Matthias Kovatsch
   ETH Zurich
   Universitaetstrasse 6
   Zurich, CH-8092


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


   Zhen Cao (editor)
   Huawei Technologies


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