Internet Engineering Task Force C. Gomez
Internet-Draft Universitat Politecnica de Catalunya
Intended status: Informational M. Kovatsch
Expires: April 24, 2018 ETH Zurich
H. Tian
China Academy of Telecommunication Research
Z. Cao, Ed.
Huawei Technologies
October 21, 2017
Energy-Efficient Features of Internet of Things Protocols
draft-ietf-lwig-energy-efficient-08
Abstract
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
networks.
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2018.
Copyright Notice
Copyright (c) 2017 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Medium Access Control 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 . . . . . . . . . . . . . . . . . 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 . . . . 9
3.6.3. Power Save Services in IEEE 802.15.4 . . . . . . . . 10
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 . . . . . . . . . . . . . . . . . . . 16
6.3. CoAP timers . . . . . . . . . . . . . . . . . . . . . . . 17
6.4. Data compression . . . . . . . . . . . . . . . . . . . . 17
7. Summary and Conclusions . . . . . . . . . . . . . . . . . . . 18
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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
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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 features.
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 [I-D.kovatsch-lwig-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]
[I-D.bormann-lwig-7228bis].
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
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protocol stack to a light-weight 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 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 [G9959], DECT ULE [TS102],
MS/TP-BACnet [MSTP], and Near Field Communication (NFC) [NFC].
+-----+ +-----+ +-----+ +------+
|HTTP | | FTP | |SNMP | | CoAP |
+-----+ +-----+ +-----+ +------+
\ / / / \
+-----+ +-----+ +-----+ +-----+
| TCP | | UDP | | TCP | | UDP |
+-----+ +-----+ ===> +-----+ +-----+
\ / \ /
+-----+ +------+ +-------+ +------+ +-----+
| RTG |--| IPv6 |--|ICMP/ND| | IPv6 |---| RTG |
+-----+ +------+ +-------+ +------+ +-----+
| |
+-------+ +-------+ +----------+
|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 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
are in the same order. For IEEE 802.15.4 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.
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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 |
+---------------------------------------+---------------+
| 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 allow reducing 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) are 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.
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
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-- 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
procedures.
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, IETF 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
layers.
3.1. Radio Duty Cycling techniques
This subsection describes three main three 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 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 [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
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in sleep mode. The decision on the times at which communication is
attempted relies on 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 Initiated
Transmission (RIT) mode of 802.15.4e, and the transmission of data
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's 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
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.
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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
to aggregate 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,
Bluetooth LE and IEEE 802.15.4. 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 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, and traffic
filtering. Upper layer protocols knowledge of such capabilities
provided by the lower layer enables better interworking.
These services include:
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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) 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 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 Idel Period and FMS).
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.
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]. BT-LE 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 BT-LE [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 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
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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 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 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,
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 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 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 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 a variety of related PHY and MAC layer functionalites.
IEEE 802.15.4 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
functionality, and is mainly targeted for constrained nodes with a
limited energy source. The coordinator role includes synchronization
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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 defines two main 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 Carrier Sense
Multiple Access / 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,
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.
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
seconds.
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
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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 was 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; 95 of these time
slots are unscheduled and the time slot 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 / CAT-iq [EN300] but with
specific changes to significantly reduce the power consumption at the
expense 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 [I-D.ietf-6lo-dect-ule].
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. DECT operates 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 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
signals.
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At the 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
significantly improves data transfer reliability. 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
not guaranteed delivery. Higher layers protocols have to take this
into consideration.
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 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
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.
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4. IP Adaptation and Transport Layer
6LoWPAN provides an adaptation layer designed to support IPv6 over
IEEE 802.15.4. 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
[RFC2460]. 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
higher layers awareness of 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 [RFC 7959].
Secondly, 6LoWPAN swaps computing with communication. 6LoWPAN applies
compression of the IPv6 header. Subject to the packet size limit of
IEEE 802.15.4, 40 octets long IPv6 header and 8 octets or 20 octets
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
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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].
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 out of the scope of 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.
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
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
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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,
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.
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 a ubiquitous
service; an energy-constrained server can 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.
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 [I-D.rahman-core-sleepy-nodes-do-we-need], while an example
application (in the context of illustrating several security
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mechanisms) in a scenario with sleepy devices has been described
[I-D.ietf-lwig-crypto-sensors]. Approaches to support sleepy nodes
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. 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 asleep [I-D.ietf-core-coap-pubsub]. 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
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. On the other hand, note that CoAP can also run
on top of TCP [I-D.ietf-core-coap-tcp-tls]. 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 less bits than the original data
representation. Data compression is more efficient at higher layers,
particularly before encryption is used. In fact, encryption
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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 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 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
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 (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 6Lo and CoRE working group
(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
(Sections 2 and 6.1).
d. Saving power by sleeping as much as possible is used widely
(Section 3).
8. Contributors
Jens T. Petersen, RTX, contributed the section on power save
services in DECT ULE.
9. Acknowledgments
Carles Gomez has been supported by the Spanish Government, FEDER and
the ERDF through projects TEC2012-32531 and TEC2016-79988-P.
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, Bernie Volz and Charlie Perkins.
The text of this document was improved based on IESG Document Editing
session during IETF87. Thanks to Ted Lemon and Joel Jaeglli for
initiating and facilitating this editing session.
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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 Special Interest Group, "Bluetooth Core
Specification Version 4.2", December 2014,
<https://www.bluetooth.org/en-us/specification/
adopted-specifications>.
[EN300] ETSI, "Digital Enhanced Cordless Telecommunications
(DECT); Common Interface (CI)", March 2015,
<https://www.etsi.org/deliver/
etsi_en/300100_300199/30017501/02.06.01_60/
en_30017501v020601p.pdf>.
[fifteendotfour]
IEEE Computer Society, "IEEE Std. 802.15.4-2015 IEEE
Standard for Local and metropolitan area networks--Part
15.4: Low-Rate Wireless Personal Area Networks (LR-
WPANs)", 2015, <https://standards.ieee.org/findstds/
standard/802.15.4-2015.html>.
[G9959] International Telecommunication Union, "Short range
narrow-band digital radiocommunication transceivers - PHY
and MAC layer specifications, ITU-T Recommendation
G.9959", January 2015,
<http://www.itu.int/rec/T-REC-G.9959>.
[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.
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[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,
<https://www.ashrae.org/File%20Library/docLib/StdsAddenda/
07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>.
[NFC] NFC Forum, "NFC Logical Link Control Protocol version 1.3,
NFC Forum Technical Specification", March 2016.
[oneM2M] oneM2M, "oneM2M specifications",
<http://www.onem2m.org/technical/published-documents>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[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,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
March 2011, <https://www.rfc-editor.org/info/rfc6206>.
[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,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[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,
<https://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,
<https://www.rfc-editor.org/info/rfc6551>.
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[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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7252>.
[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,
<https://www.rfc-editor.org/info/rfc7668>.
[TS102] ETSI, "Digital Enhanced Cordless Telecommunications
(DECT); Ultra Low Energy (ULE); Machine to Machine
Communications; Part 2: Home Automation Network (phase 2",
March 2015, <https://www.etsi.org/deliver/
etsi_ts/102900_102999/10293902/01.01.01_60/
ts_10293902v010101p.pdf>.
12.2. Informative References
[AN079] Kim, C., "Measuring Power Consumption of CC2530 With
Z-Stack", September 2012,
<http://www.ti.com/lit/an/swra292/swra292.pdf>.
[ContikiMAC]
Dunkels, A., "The ContikiMAC Radio Duty Cycling Protocol,
SICS Technical Report T2011:13", December 2011,
<https://www.mysciencework.com/publication/download/2f406d
3c4cc1eda32a234f7a1ad2cc3b/7eb199e4f8b00857e21af2b7d2b31c0
d>.
[I-D.bormann-lwig-7228bis]
Bormann, C., Ersue, M., Keranen, A., and C. Gomez,
"Terminology for Constrained-Node Networks", draft-
bormann-lwig-7228bis-01 (work in progress), May 2017.
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[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-09 (work in progress),
December 2016.
[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-12 (work
in progress), August 2017.
[I-D.ietf-6tisch-minimal]
Vilajosana, X., Pister, K., and T. Watteyne, "Minimal
6TiSCH Configuration", draft-ietf-6tisch-minimal-21 (work
in progress), February 2017.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-02 (work in
progress), July 2017.
[I-D.ietf-core-coap-tcp-tls]
Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
draft-ietf-core-coap-tcp-tls-09 (work in progress), May
2017.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-11 (work in progress), July 2017.
[I-D.ietf-lwig-crypto-sensors]
Sethi, M., Arkko, J., Keranen, A., and H. Back, "Practical
Considerations and Implementation Experiences in Securing
Smart Object Networks", draft-ietf-lwig-crypto-sensors-04
(work in progress), August 2017.
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[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, <https://core.ac.uk/download/
pdf/11435067.pdf?repositoryId=362>.
Authors' Addresses
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
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@ritt.cn
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Zhen Cao (editor)
Huawei Technologies
China
Email: zhencao.ietf@gmail.com
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