3.1 Denial of Service (DoS)
It occurs by the unintentional failure of nodes or malicious
action. The simplest DoS attack tries to exhaust the resources
available to the victim node, by sending extra unnecessary
packets and thus prevents legitimate network users from
accessing services or resources to which they are entitled[1][2].
DoS attack is meant not only for the adversary‟s attempt to
subvert, disrupt, or destroy a network, but also for any event that
diminishes a network‟s capability to provide a service [2].
In wireless sensor networks, several types of DoS attacks in
different layers might be performed. At physical layer the DoS
attacks could be jamming and tampering, at link layer, collision,
exhaustion, unfairness, at network layer, neglect and gre ed,
homing, misdirection, black holes and at transport layer this
attack could be performed by malicious flooding and
desynchronization.
3.2 The Wormhole attack
One node in the network (sender) sends a message to the another
node in the network (receiver node)[10].Then the receiving node
attempts to send the message to its neighbors. The neighboring
nodes think the message was sent from the sender node(which is
usually out of range), so they attempt to send the message to the
originating node, but it never arrives since it is too far away.
Wormhole attack is a significant threat to wireless sensor
networks, because, this sort of attack does not require
compromising a sensor in the network rather, it could be
performed even at the initial phase when the sensors start to
discover neighboring information [12].
Wormhole attacks are difficult to counter because routing
information supplied by a node is difficult to verify.
3.3 The Sybil attack
In this attack, a single node i.e. a malicious node will appear to
be a set of nodes and will send incorrect information to a node in
the network.
The incorrect information can be a variety of things [10],
including position of nodes, signal strengths, making up nodes
that do not exist.
Authentication and encryption techniques can prevent an outsider
to launch a Sybil attack on the sensor network. However, an
insider cannot be prevented from participating in the network,
but he should only be able to do so using the identities of the
nodes he has compromised.
Public key cryptography can prevent such an insider attack, but it
is too expensive to be used in the resource constrained sensor