Internet Engineering Task Force C. Gomez
Internet-Draft Universitat Politecnica de Catalunya/i2CAT
Intended status: Informational M. Kovatsch
Expires: April 16, 2017 ETH Zurich
H. Tian
China Academy of Telecommunication Research
Z. Cao, Ed.
Huawei Technologies
October 13, 2016
Energy-Efficient Features of Internet of Things Protocols
draft-ietf-lwig-energy-efficient-05
Abstract
This document describes the problems and current practices of energy
efficient protocol operation on constrained devices. It summarizes
the main link layer techniques for energy efficient networking, and
it highlights the impact of such techniques on the upper layer
protocols, so that they can coordinately achieve an energy efficient
behavior. The document also provides an overview of energy efficient
mechanisms available at each layer of the constrained node network
IETF protocol suite.
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
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on April 16, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . 5
3.1. Radio Duty Cycling techniques . . . . . . . . . . . . . . 6
3.2. Latency and buffering . . . . . . . . . . . . . . . . . . 7
3.3. Throughput . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Radio interface tuning . . . . . . . . . . . . . . . . . 7
3.5. Power save services available in example low-power radios 7
3.5.1. Power Save Services Provided by IEEE 802.11 . . . . . 8
3.5.2. Power Save Services Provided by Bluetooth LE . . . . 9
3.5.3. Power Save Services in IEEE 802.15.4 . . . . . . . . 10
3.5.4. Power Save Services in DECT ULE . . . . . . . . . . . 11
4. IP Adaptation and Transport Layer . . . . . . . . . . . . . . 13
5. Routing Protocols . . . . . . . . . . . . . . . . . . . . . . 14
6. Application Layer . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Energy efficient features in CoAP . . . . . . . . . . . . 15
6.2. Sleepy node support . . . . . . . . . . . . . . . . . . . 15
6.3. CoAP timers . . . . . . . . . . . . . . . . . . . . . . . 16
7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Network systems for physical world monitoring contain many battery-
powered or energy-harvesting devices. For example, in an
environmental monitoring system, or a temperature and humidity
monitoring system, there are no always-on and sustained power
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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.
A large body of research efforts have been put 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 more energy-efficient. Only few efforts
focused 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 constrained 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 contrained 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 a comprehensive overview of
the techniques used by the lower layers to save energy and how these
may impact on the upper layers.
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 upper layers is also important to make the device
energy-efficient.
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 [RFC7228].
2. Overview
The IETF has developed 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 shown
in Figure 1 below, the IETF has developed CoAP as the application
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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. 6LoWPAN is currently
being adapted by the 6lo working group to support IPv6 over various
other technologies, such as ITU-T G.9959, DECT ULE, MS/TP-BACnet and
NFC.
+-----+ +-----+ +-----+ +------+
|HTTP | | FTP | |SNMP | | CoAP |
+-----+ +-----+ +-----+ +------+
\ / / / \
+-----+ +-----+ +-----+ +-----+
| TCP | | UDP | | TCP | | UDP |
+-----+ +-----+ ===> +-----+ +-----+
\ / \ /
+-----+ +------+ +-------+ +------+ +-----+
| RTG |--| IPv6 |--|ICMP/ND| | IPv6 |---| RPL |
+-----+ +------+ +-------+ +------+ +-----+
| |
+-------+ +-------+ +----------+
|MAC/PHY| | 6Lo |--|6LoWPAN-ND|
+-------+ +-------+ +----------+
|
+-------+
|MAC/PHY|
+-------+
Figure 1: Traditional and Light-weight 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 atom operations 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. [AN053]), we can see that the energy
consumption of optimized transmission and reception are in the same
order. For IEEE 802.15.4 and 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.
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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 may
consume as much power in listen mode as when actively transmitting.
This augments 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 consumes 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 in
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 procedures.
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
transmission and/or reception request from the higher layer.
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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 layers. If the
different protocol layers want to get better service in a cooperative
way, they should be considerate and understand each other.
3.1. Radio Duty Cycling techniques
This subsection describes the main three RDC techniques. Note that
more than one of the presented 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 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, and the
Frequently Listening (FL) mode of ITU-T G.9959.
b) Scheduled transmissions. This approach allows a device to know
the instants in which it should be awake (during some time interval)
in order to receive data units. Otherwise, the device may remain in
sleep mode. The decision on the instants that will be used for
communication is reached by means of some form of negotation 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
mechanism.
c) Listen after send. This technique allows a node to remain in
sleep mode by default, 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, ready for a
potential incoming transmission. After a certain time interval, the
node may go back to sleep. For example, the Receiver Initated
Transmission (RIT) mode of 802.15.4e, and the transmission of data
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between a coordinator and a device in IEEE 802.15.4-2003 use this
technique.
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 periodical behavior).
On the other hand, due to the latency increase of duty-cycling, a
sender waiting for a transmission opportunity may need to store
subsequent outgoing packets in a buffer, increasing memory
requirements and potentially incurring queuing waiting time that
contributes to the packet overall delay and increases the probability
of buffer overflow, leading to losses.
3.3. Throughput
Although throughput is not typically a key concern in constrained
node network applications, it is indeed important in some services in
this kind of networks, such as over-the-air software updates or when
off-line sensors accumulate measurements that have to be quickly
transferred when there is a connectivity opportunity.
Since RDC introduces inactive intervals in energy-constrained
devices, it reduces the throughput that can 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. 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,
Bluetooth LE and IEEE 802.15.4. For a more detailed overview of each
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technology, the reader may refer to the literature or to the
corresponding specifications.
3.5.1. Power Save Services Provided by IEEE 802.11
IEEE 802.11 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 in the beacon whether there
is data pending for the station or not. 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 [IEEE80211v] further 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 other.
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.
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.
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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.
3.5.2. 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]. BT-LE has been designed for the goal
of ultra-low-power consumption. Currently, it is possible to run
IPv6 over Bluetooth LE networks by using a 6LoWPAN variant adapted to
BT-LE [I-D.ietf-6lowpan-btle].
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 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,
Bluetooth 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 Bluetooth LE. In particular, connInterval,
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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).
3.5.3. Power Save Services in IEEE 802.15.4
IEEE 802.15.4 is a family of standard radio interfaces for low-rate,
low-power wireless networking [fifteendotfour]. Since the
publication of its first version in 2003, IEEE 802.15.4 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
[I-D.ietf-6tisch-architecture]. IEEE 802.15.4 specifies PHY and MAC
layer functionality.
IEEE 802.15.4 defines three roles called device, coordinator and PAN
coordinator. The device role is adequate for nodes that do not
implement the complete IEEE 802.15.4 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
network.
IEEE 802.15.4 has mainly defined two types of networks depending on
their configuration: beacon-enabled and nonbeacon-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 CSMA/
CA for data communication. In the second one, a TDMA scheme controls
medium access. During the idle period, communication does not take
place, 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 will thus consume energy.
An underlying assumption is that when a message is sent to a
coordinator, the radio of the latter will be ready to receive the
message.
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The beacon interval and the duration of the beacon interval active
portion (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 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 ii) to transmit data to the
coordinator. The wake-up pattern of the device, if any, is out of
the scope of IEEE 802.15.4.
Communication between the two ends of an IEEE 802.15.4 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.
The main MAC layer IEEE 802.15.4 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 which
offers robust features for industrial environments, among others. In
order to provide the functionality needed to enable IPv6 over TSCH,
the 6TiSCH working group has been recently created. TSCH is based on
a TDMA schedule whereby a set of time slots are used for frame
transmission and reception, and other time slots are unscheduled.
The latter time slots may be used by a dynamic scheduling mechanism,
otherwise nodes may keep the radio off during the unscheduled time
slots, thus saving energy. The minimal schedule configuration
specified in [I-D.ietf-6tisch-minimal] comprises 101 time slots,
whereby 95 of these time slots are unscheduled and the time slot
duration is 15 ms.
Other 802.15.4e modes, which are in fact designed for low energy, are
the previously mentioned CSL and RIT.
3.5.4. Power Save Services in DECT ULE
DECT Ultra Low Energy (DECT ULE) is a wireless technology building on
the key fundamentals of traditional DECT / CAT-iq [EN300] but with
specific changes to significantly reduce the power consumption on the
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expense of data throughput as specified in [TS102]. DECT ULE devices
typically operates 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. It
is possible to run IPv6 over DECT ULE by using a 6LoWPAN variant
adapted for DECT ULE [I-D.ietf-6lo-dect-ule].
DECT terminology operates with two major role definitions: The
Portable Part (PP) is the power constrained device, while the Fixed
Part (FP) is the Gateway or base station in a star topology. DECT is
operating in license free and reserved frequency bands based on TDMA/
FDMA and TDD using dynamic channel allocation for interference
avoidance. It provides good indoor (~50 m) and outdoor (~300 m)
coverage. It is using a frame length of 10 ms, which is divided into
24 timeslots and it is supporting 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
signals.
At MAC level DECT ULE communications between FP and PP are initiated
by the PP. A FP can initiate communication indirectly by sending
paging signal to a PP. The PP determines the timeslot and frequency
on 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
improves data transfer reliability significant. A segmentation and
reassembly scheme supports transfer of larger higher layer SDUs and
provides data integrity check. The DECT ULE packet data service
ensures data integrity, proper sequencing, duplicate protection, but
does not guaranteed delivery. Higher layers protocols have to take
this into considerations.
The FP may send paging information to PPs to trigger connection setup
and indicate required service type. The interval between paging
information to a specific PP can be defined in range 10 ms to 327
seconds. 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
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dummybearer. Hence, longer sleep interval reduces the average energy
consumption, but adds a 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 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 listening to paging or instruct the PP to stay in a higher duty
cycle paging detection mode.
The DECT ULE technology allows 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 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
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.
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
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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 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
[I-D.ietf-6man-impatient-nud].
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 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 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.
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.
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6. Application Layer
6.1. Energy efficient features in CoAP
CoAP [RFC7252] is 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. 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 a ubiquitous service and, most importantly, it allows an
energy-constrained server to remain in sleep mode during the period
between observe notification transmissions.
Furthermore, [RFC7252] defines CoAP proxies which can cache resource
representations previously provided by sleepy CoAP servers. The
proxies themselves may respond to client requests if the
corresponding server is sleeping and the resource representation is
recent enough. Otherwise, a proxy may attempt to obtain the resource
from the sleepy server.
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 [I-D.rahman-core-sleepy-nodes-do-we-need]. The different
approaches include exploiting the use of proxies, leveraging the
Resource Directory [I-D.ietf-core-resource-directory] or signaling
when a node is awake to the interested nodes. A more 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 in a sleeping state [I-D.koster-core-coap-pubsub]. As of
the writing, none of these proposals 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
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the latter (which may often stay in sleep mode) is online. LWM2M
functionality operates on top of CoAP.
On the other hand, oneM2M defines a CoAP binding with an application
layer mechanism for sleepy nodes.
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
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 which would unnecessarily waste node energy
and other resources.
7. Summary
We summarize the key takeaways in this document:
a. Internet protocols designed by IETF can be considered as the
customer of the lower layers (PHY, MAC, and Duty-cycling). To
save power consumption, it is recommended to synergize with the
lower layer other than treating the lower layer as a black box.
b. It is always useful to compresss the protocol headers in order to
reduce the transmission/reception power. This design principles
have been employed by many protocols in 6Lo and CoRE working
group.
c. Broadcast and non-synchronzed 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.
d. Saving power by sleeping occasionally is used widely. Reduction
of states is also an effective method to be energy efficient.
8. Contributors
Jens T. Petersen, RTX, contributed the section on power save
services in DECT ULE.
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9. Acknowledgments
Carles Gomez has been supported by Ministerio de Economia y
Competitividad and FEDER through project TEC2012-32531.
Authors would like to thank the review and feedback from a number of
experts in this area: Carsten Bormann, Ari Keranen, Hannes
Tschofenig, Dominique Barthel.
The text of this document was improved based on IESG Document Editing
session during IETF87. Thank Ted Lemon, Joel Jaeggli, and efforts to
initiate this facilities.
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
[Bluetooth42]
"Bluetooth Core Specification Version 4.2", 2014.
[EN300] ""Digital Enhanced Cordless Telecommunications (DECT);
Common Interface (CI);"", 2013.
[fifteendotfour]
"802.15.4-2011", 2011.
[IEEE80211v]
IEEE, , "Part 11: Wireless LAN Medium Access Control (MAC)
and Physical Layer (PHY) specifications, Amendment 8: IEEE
802.11 Wireless Network Management.", February 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[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,
<http://www.rfc-editor.org/info/rfc4944>.
[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,
<http://www.rfc-editor.org/info/rfc6282>.
[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,
<http://www.rfc-editor.org/info/rfc6550>.
[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,
<http://www.rfc-editor.org/info/rfc6551>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[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,
<http://www.rfc-editor.org/info/rfc6775>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[TS102] ""Digital Enhanced Cordless Telecommunications (DECT);
Ultra Low Energy (ULE); Machine to Machine Communications;
Part 1: Home Automation Network (phase 1)"", 2013.
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12.2. Informative References
[AN053] Selvig, B., "Measuring power consumption with CC2430 and
Z-Stack".
[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.
[I-D.ietf-6lo-dect-ule]
Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D.
Barthel, "Transmission of IPv6 Packets over DECT Ultra Low
Energy", draft-ietf-6lo-dect-ule-06 (work in progress),
October 2016.
[I-D.ietf-6lowpan-btle]
Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
(work in progress), February 2013.
[I-D.ietf-6man-impatient-nud]
Nordmark, E. and I. Gashinsky, "Neighbor Unreachability
Detection is too impatient", draft-ietf-6man-impatient-
nud-07 (work in progress), October 2013.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-10 (work
in progress), June 2016.
[I-D.ietf-6tisch-minimal]
Vilajosana, X. and K. Pister, "Minimal 6TiSCH
Configuration", draft-ietf-6tisch-minimal-16 (work in
progress), June 2016.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
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[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-08
(work in progress), July 2016.
[I-D.ietf-lwig-terminology]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", draft-ietf-lwig-terminology-07
(work in progress), February 2014.
[I-D.koster-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-koster-core-coap-pubsub-05 (work in
progress), July 2016.
[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.
[I-D.rahman-core-sleepy-nodes-do-we-need]
Rahman, A., "Sleepy Devices: Do we need to Support them in
CORE?", draft-rahman-core-sleepy-nodes-do-we-need-01 (work
in progress), February 2014.
[Powertrace]
Dunkels, , Eriksson, , Finne, , and Tsiftes, "Powertrace:
Network-level Power Profiling for Low-power Wireless
Networks", March 2011.
Authors' Addresses
Carles Gomez
Universitat Politecnica de Catalunya/i2CAT
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
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Matthias Kovatsch
ETH Zurich
Universitaetstrasse 6
Zurich, CH-8092
Switzerland
Email: kovatsch@inf.ethz.ch
Hui Tian
China Academy of Telecommunication Research
Huayuanbeilu No.52
Beijing, Haidian District 100191
China
Email: tianhui@mail.ritt.com.cn
Zhen Cao (editor)
Huawei Technologies
China
Email: zhencao.ietf@gmail.com
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