# Challenges, Threats, Security Issues and New Trends of Underwater Wireless Sensor Networks
## 3 Threats and Challenges in Underwater Wireless Sensor Networks (UWSNs)
### DoS Attacks
#### 3.3.2 Datalink Layer  
Due to the limited bandwidth of the underwater acoustic channel, MAC protocols are utilized to enable multiple devices to share the acoustic channel in an efficient and fair way. But unfortunately, there are several kinds of DoS attacks that can be launched at this layer to consume the battery of legitimate nodes or disturb normal MAC operation, including the _Jamming Attack_, _Collision Attack_, _Exhaustion Attack_, _Denial-of-sleep Attack_, and _Unfairness_.  
A fatalink layer jamming attack is similar to that in the physical layer, but in a smarter and more efficient way. The malicious attacker can achieve the goal by just repeatedly sending request to send (RTS) packets. The legitimate nodes are deprived of the chance of accessing the channel. As a consequence, this disrupts the cooperation between nodes and communication of the network. In contention-based MAC protocols, the malicious attacker can give itself the highest priority and occupy the channel all the time. Therefore, scheduled MAC protocols are capable of defending against the attack. Anti-replay protection and link-layer authentication can mitigate these attacks. Unfortunately, receiving large quantities of RTS packets still consume a node's energy and occupy channels.  
For the RTS/CTS handshake mechanism, in which any node detected either an RTS or a CTS packet from the channel, it should not transmit packets during the time period. However, a malicious node may violate this mechanism, and transmits packets even after detecting a CTS packet destined for a legitimate node, which will cause a collision at the receiver. It is obvious that a change will occur in the collisional data, and the packet will become invalid. Colluding in a collision attack interrupts packets during communication, and a mitigating colluding collision technique can defend against a colluding collision attack [25]. In a manner, an error correcting code is a feasible technique to avoid collision.  
An exhaustion attack refers to keeping the channel busy and exhausting the node's battery by introducing a malicious node in the network. It can be launched by the intruder, or by a compromised node on which the internal program code was modified by an intruder. Another form of exhaustion attack is that the intruder node sends numerous join-requests or RTS/CTS packets to force the target node to send or receive. The feasible solution is rate limitation on each node in the network. A fuzzy logic based solution against distributed node exhaustion attack is proposed in [26].  
A denial-of-sleep attack is to prevent the node from going into sleep mode [27]. An attacker might choose to execute a denial-of-sleep attack over a simple jamming-based DoS attack. To permanently disable the network, it may take months to exhaust the targeted node's batteries. A smarter denial-of-sleep attack that keeps the sensor nodes on working mode would drain the batteries in a very short period (maybe only a few weeks or days). Worse, some denial-of-sleep attackers are not required to constantly send signals, making it more difficult to locate the attacker via its emitted transmissions.  
Unfairness  
This is a weak form of DoS attack which is performed by attacker to degrade the network performance instead of completely preventing legitimate nodes from accessing the channel. A small frames technique can be used to reduce the amount of time. This technique lessens the effect at theexpense of efficiency. Moreover, it is susceptible to further unfairness. For example, an attacker may retransmit at a higher speed instead of randomly delaying.  
The majority of datalink layer DoS attacks discussed above can be prevented by using error detection code, rate limitation, and by dividing the packets into small frames.