# Source Anonymity against Global Adversary in WSNs Using Dummy Packet Injections: A Survey
## 2 Background
The _Desynchronization_ attack attempts to block the transmission between two nodes by sending fake messages using a modified sequence number to both parties making each node believe that its packet has not been delivered. Therefore, a re-transmission is needed causing unnecessary overhead [37]. Data aggregation works by combining duplicated data from multiple sensor nodes to reduce the overhead and redundant information. There are many aggregation functions such as _Sum_, _Average_, _Count_, _Max_, and _Min_ that can be easily modified by the adversary to make the network act differently [37]. Privacy attacks focus on analyzing the traffic of the network [15; 16; 17; 38]. An adversary can obtain critical information by snooping on the network. The nature of WSNs facilitates attackers to monitor and capture the traffic between sensor nodes. Traffic analysis allows the adversary to identify the most important nodes in the network such as source and sink nodes or gain information about the hot-spot and high traffic regions in the network, known as the _Rate Monitoring_ attack. _Time Correlation_ is another privacy attack that monitors the difference between transmitting times of packets and if the network packets follow a specific distribution type in trying to find the relationship, e.g., transmitting times between real and fake packets. This could lead to exposing the location of the source node. Another attack is the _Size/Structure Correlation_, which focuses on the size and payload structure of packets to observe any differences. This work focuses on providing techniques that prevent privacy attacks against source nodes. There are many protocols in the literature that counter global adversary models. Dummy packet injections is one of the most effective methods against the global adversary model. The global adversary has unlimited power, sufficient resources, and the ability to analyze the entire network, which makes the adversary difficult to be defeated. Other techniques such as Separate Path Routing (_SPR_), Network Location Anonymization, and Network Coding try to modifying the routing protocol, transmitting the packets using different paths, or use pseudonyms to hide the identity of the source node. These techniques would work defending against local adversaries who have a partial view of the network, limited power, and resources. Nevertheless, they are not effective against the global adversaries [1]. Global adversaries need more sophisticated schemes to be encountered. Letting some nodes transmit fake packets along with real one could create a confusion from the global adversary perspective. Even if the global adversary has a full view of the network, it is not straightforward to trace back the origin and detect the real event because they have multiple paths and combinations to calculate. They need to apply complex rate monitoring and time correlation attacks to get near the source node. Dummy packet injection techniques have a massive impact on defending against global adversary models. They might be the most effective solution to counter a well-equipped attacker. Dummy packet injection techniques use a mix of real and fake packets to mislead the adversary away from the source node, unlike other methods that use only real packets. Dealing only with the packets of the real event can limit the countering options since the adversary knows the occurrence of the real event in advance. The general notion behind such techniques is to inject the network with fake packets. Fake packets are identical to the real ones regarding the size and structure to avoid any size/structure correlation attacks. Usually, fake packets are transmitted from several sensor nodes in different places within the network whenever a real event is detected. In some techniques, they generate and transmit fake packets even if there are no real events to add more uncertainty from the attacker perspective. The injection of dummy traffic is according to a specific set of rules.