# Source Anonymity against Global Adversary in WSNs Using Dummy Packet Injections: A Survey
## 2 Background  
Security in WSNs has been a challenge because of the unique aspects they have. It is unsatisfactory to use the ordinary security mechanisms in WSNs due to the limitation of resources such as processing power and battery life. Sensors are independent and do not normally follow a central control entity because of their large scale and frequent topology changes. Therefore, traditional security solutions are inapplicable since they require significant overhead and sufficient memory. One of the basic defenses against security attacks is to prevent the access to network nodes physically. This is impractical in WSNs as many applications require sensor nodes to be deployed in remote and open locations that are difficult to reach, control, manage and protect from unauthorized physical accesses. Packets in WSNs are vulnerable. They can be lost or corrupted because of routing failures or collisions. Such challenges must be taken into consideration when developing security protocols for WSNs [37]. There are several kinds of security attacks against WSNs such as _DoS_, routing attacks, transport layer attacks, data aggregation attacks, and privacy attacks, as depicted in Figure 3. _DoS_ occurs when an adversary prevents the network from functioning or providing the expected services to applications. _DoS_ could attack the physical layer and link layer. One of the attacks on the physical layer is the _Jamming_ attack where an attacker interferes with radio frequencies of the sensor nodes to prevent them from transmitting and receiving meaningful data. _Tampering_ attack is also a physical layer attack that attempts to destroy or modify the sensor node physically. This attack might lead the adversary to obtain sensitive information which could compromise the entire network. A _Collision_ attack is a link layer attack that interferes with packet transmissions to make the network re-transmit the same packets repeatedly, which increases the overhead and power consumption [37]. A variety of routing attacks can be performed by an adversary. For example, a malicious node in the _Blackhole_ attack and _Sinkhole_ attack tries to convince the sensor nodes that it is a legitimate data forwarder in the network. Once the malicious node receives the packets, this node drops them right away. Another attack is the _Selective Forwarding_ attack, which is very similar to the _Blackhole_ attack and _Sinkhole_ attack. The only difference is that _Selective Forwarding_ attack discards a certain number of packets based on specific criteria rather than dropping all incoming packets. Some other attacks target the on-demand routing protocols such as the _Rushing_ attack. In this attack, the malicious node forwards all incoming route requests to nearby nodes without using the actual routing protocol policies, which involves more nodes in the route. Location-based routing protocols are weak against the _Sybil_ attack that provides the adversary with multiple identities at different locations in the network. Legitimate nodes will think that the malicious node is one of its trusted neighboring nodes and start forwarding packets to the malicious node. _Wormhole_ is another attack on routing. The _Wormhole_ attack occurs when the malicious node has more capabilities than other nodes in the network, such as bandwidth. This increase in capabilities might attract authorized nodes to forward their data to the unauthorized node since it has a high-speed connection. The _Wormhole_ attack could help other attacks such as _Blackhole_ to take place [37]. Transport layer protocols such as Transmission Control Protocol (_TCP_) and User Datagram Protocol (_ILDP_) are vulnerable to the _Flooding_ attack. A protocol such as _TCP_ keeps state information allowing the adversary to send multiple connection requests that waste memory and reject any future connection requests even from authorized nodes.