Design, implementation and evaluation of an energy efficient RPL routing metric

Chapter 1
Internet of Things and Low power
and Lossy Networks
1.1 Introduction
What’s the meaning of Internet of Things (IoT) ? Technically, we can define IoT as a
wireless self-configuring sensors network, but the real sense is another. If every device in
the world was equipped with an identifier device, our life would be totally different. If
every object had a simple electronic intelligence, e.g., to understand where is located and
interacting with the enviroment, it could be connected one other to the Internet in order to
form an unique network to exchange informations.
This is the Internet of things: daily life’s objects interconnected by web or mobile
network speaking and basing on theirs collected informations may act accordingly. For
example, we can imagine a scenario where the cars talk together to avoid accidents, or the
appliances coordinate themselves to optimize the power consumption. Some years ago this
was impossible to achieve, due to lack of technology, but nowadays this is possible thanks
to the Smart Objects [15].
Smart Objects are the heart of IoT: they are small computers equipped with a sensor or
4
Chapter 1. Internet of Things and Low power and Lossy Networks 5
SMART 
  
OBJECTS
SMART 
CITIES
SMART 
HOME
SMART     SMART     
 CARS
SMART
SMART
LOGISTICS
METERING
HEALTH
Figure 1.1: Main areas of interesting of Smart Objects.
actuator, a microprocessor, a communication device and a power source. The smart object
interacts with the physical world through the sensor, data captured from the sensor are
elaborated by the microprocessor, while the communication device allows the smart object
to receive data from other objects and sent them to the outside world. They are embedded
in everyday objects, like cars, street lamps or industry machinery, and for this reason theirs
size cannot exceed a few cubic centimeters. The areas of application of smart objects are
wides, such as Smart Home and Smart Building (intrusion and surveillance), Smart City
(public transport localization, traffic monitoring), Smart Metering (intelligent electricity
meters to reduce the consumes), Smart Logistics (factory monitoring), Smart Cars (cars
that record the drive informations) and Smart Health (to monitor the patience remotely);
this activities are summarized in Figure 1.1.
Smart objects are wireless devices, generally are battery-powered nodes, and due to
small size they can’t have high power and high range of transmission; usually they can
Chapter 1. Internet of Things and Low power and Lossy Networks 6
reach a node forty/fifty meters away. Furthermore, smart object networks are potentially
very large scale, consisting of potentially hundreds of nodes operating over multiple hops
to cover a large geographical area, and they must operate unattended for years on modest
batteries. Thanktothisfeatures,asmartobjectsnetworkisaLow Power Wireless Personal
Area Network (LoWPAN). LoWPAN communication standard is IEEE 802.15.4, designed
specificallyforlong-livedapplicationthatrequirelow-costnodes,whichdefinesphysicallevel
and MAC level of stack OSI. The main requirements are throughput limited to 250 kbps in
the 2.4 GHz band, and frame length limited to 128 bytes to ensure acceptable low packet
error. Regarding to level three, paper [16] explains why using Internet Protocol (IP) for
smart objects is the best option, instead documents [17] and [18] explain how is possible to
extend IP over LoWPANs.
The rest of the chapter is structured as follows. In Section 1.2 is mentioned how an
efficient IPv6 communication over IEEE 802.15.4 is enabled, moreover in Section 1.3 we
introduce the Low Power and Lossy networks, and the main routing requirements in these
type of networks.
1.2 Supporting IPv6 for Smart Objects: 6LoWPAN
In 2005 the Internet Engineering Task Force (IETF) chartered the IPv6 over Low Power,
Wireless Networks (6LoWPAN) working group to standardize adaptations of IPv6 over
LoWPAN networks [1]. The IPv6 protocol is designed to replace IPv4 and to overcame
the lack of address space, expands the IP address space from 32 to 128 bits; furthermore
IPv6 increases the minimum maximum transmission unit (MTU) requirement from 576
to 1,280 bytes. IPv6 also reflects the progresses in link technologies: current WLAN tech-
nologies, such as Wi-Fi, can support similar sized MTUs and high link rates than Ethernet
capabilities. On the other hand, as mentioned before, WPAN techologies operate with low
power: throughput is limited to 250 kbps, the frame length is limited to 128 bytes and
Chapter 1. Internet of Things and Low power and Lossy Networks 7
communication range is short (tens of meters) because transmission power increases poli-
nomially with range. Due to this constraints, supporting IPv6 over 6LoWPANs present
several challenges:
• IPv6 datagrams aren’t naturalsuitable for LoWPANs; bufferwith limited capabilities
and frames that are one-tenth the size of the IPv6 minimum MTU, require make
datagram fragmentation and compression.
• due to 802.15.4 is low power and low throughput, it’s more affected to link failures
spurios interference. Such characteristics require the network layer to be adaptative
while remaining energy efficient.
• a LoWPAN’s topology is a mesh of short-range connections. This negates the as-
sumption that the link is a single broadcast domain on which a core of IP components
relies.
As explained in RFC 4944 [17], 6LoWPAN introduces an adaptation layer between the link
layerandnetworklayeroftheIPstack, toenableanefficienttransmissionofIPv6datagrams
over 802.15.4. The adaptation layer provides header compression to reduce transmission
overhead, fragmentation to support the IPv6 MTU requirement, and support for layer-
two forwarding to deliver an IPv6 datagram over multiple radio hops. The 6LoWPAN
protocol stack is depicted in Figure 1.2. The key concept applied in the adaptation layer
is that it uses stateless compression [2] to compress adaptation, network, and transport
layer header fields to a few bytes, combined. It’s possible to compress header fields when
they often carry common values, reserving an escape value for when less common ones
appear. Common values occur because of frequent use of a subset of IPv6 functionality,
i.e. UDP or ICMPv6, and simple assumptions of shared context, i.e. a common global
routing prefix for the entire network. Traditional IP header compression techniques are
stateful (the compressor maintains its state through all compressed records) [19] and focus
on optimizing individual flows over a highly constrained link; these methods assumes that
Chapter 1. Internet of Things and Low power and Lossy Networks 8
Application Application
Transport
Network
Data Link
Physical
UDP ICMP
IPv6
IEEE 802.15.4 MAC
IEEE 802.15.4 PHY
Adaptation Layer
Figure 1.2: 6LoWPAN Protocol Stack. An adaption layer between levels two and three is
introduced.
the compressor and decompressor are in direct communication, and compress network and
transport layer headers together. On the contrary, stateless compression in 6LoWPAn does
not require any per-flow state and lets routing protocols dinamically choose routes without
affecting compression efficiency.
The compression scheme for 6LowPAN is HC1, optimized for link-local IPv6 communi-
cation. Is possible to use HC1 if the devices are all in the same 6LoWPAN, so they share
the same prefix; in this mode HC1 works without algorithms and simply elides redundancy
informations. The most critical aspect during header reduction is represented by IPv6 ad-
dresses. The 64-bit network prefix for source and destination addresess are compressed to
a single bit each when they carry the link-local prefix, while the interface identifier (IID)
for source and destination addresess are elided if the destination can derive them from the
corresponding link-layer address. Fully compressed, the HC1 encoding reduces the IPv6
header to two bytes.
Chapter 1. Internet of Things and Low power and Lossy Networks 9
After the implementation of the adaptation layer, either in the network layer or the
adaptation layer is possible to take routing decision. If the routing decision is taken in the
network layer, it is named route-over, if the decision is taken in the adaptation layer, it
is named mesh-under. A mesh-under architecture defines the extent of an IPv6 link as all
nodes within the same multihop mesh, instead a route-over architecture defines the extent
of an IPv6 link as immediate neighbors reachable withina single link transmission. In other
words, in ruote-over a link-local address is sufficient to communicate with nodes in direct
radio communication, but a routable address is required to communicate with devices that
are multiple radio hops away. In this thesis RPL protocol has been used (see chapter 2),
which support route-over configuration.
To discover other nodes on the same link, to determine their link-layer addresses and
to find routers, a node uses IPv6 Neighbor Discovery (ND). But it was not designed for
non-transitive wireless links and multicast is not desirable in wireless low-power networks;
thus, a simple optimization was introduced to optimize Neighbor Discovery for 6LowPAN
networks [20]. ND bootstrapping process allows hosts to attach a LoWPAN without the
needtopartecipateinrouting,reducingcomplexity. 6lowpanRoutersrespondtoRouterSo-
licitation (RS) message from 6lowpan nodes (other hosts) with Router Advertisement (RA)
messages. RAs contain the prefix and context information for a node to discover the LoW-
PAN and autoconfigure its addresses. In a LoWPAN, neighbor information is mantained
by having nodes register with their default next hop routers. This is done using a unicast
Neighbor Solicitation/Neighbor Advertisement (NS/NA) carrying an Address Registration
Option.