# Source Anonymity against Global Adversary in WSNs Using Dummy Packet Injections: A Survey
## 3 Dummy Packets Techniques
### Proxy-Based Filtering (PFS)  
To overcome the issues of the _Baseline_ technique such as high overhead and poor delivery ratio, _PFS_[33] was introduced. The primary concept of _PFS_ is to hire some of the sensor nodes to act as designated proxies. These proxies filter the fake packets towards the sink, which reduces the overhead traffic while keeping the source anonymity.  
In _PFS_, some of the sensor nodes are selected to filter the fake packets from neighboring nodes, as shown in Figure 5, which reduces the overhead as many of the fake packets are dropped before reaching the sink. The proxy node filters the packets to decide which packets will be forwarded and which packets will be dropped. _PFS_ relies on the location of the proxy nodes; therefore, a proxy placement algorithm is performed to minimize the overhead of the network. Further, _PFS_ should select the values of the proxy parameters such as the buffer size. This selection is necessary to handle the transmission delay of the real event at the source node. [PERSON], [PERSON] et al. [33] claimed that the _PFS_ technique provides a nearly optimal proxy placement, high delivery ratio, and low bandwidth overhead.  
The _PFS_ technique divides the network into cells (groups of nodes) that allow every two nodes in neighboring cells to communicate directly with each other. When an event is detected, it belongs to the cell, not to the node. Each cell has a coordinator node that is responsible for all actions within the cell. A unique _ID_ is assigned to each cell in the network. A node recognizes its cell by using a _GPS_ or an attack-resilient localization scheme. The sink is assumed to be in the center of the network, and each event has a cell _ID_, event type, and event time.  
After proxies have been selected, they broadcast a \"hello message\" that includes Time-To-Live (_TTL_) value which has the ability to reach all cells in the network. Next, each cell records the nearest proxy based on the received \"hello message\" and assigns the selected proxy as the default one for future communications. Then, every cell responds back to the selected proxy to inform the proxy that it is the one selected by the cell. Each cell creates a pairwise key with its proxy using one of the keying schemes. In addition, each proxy has a shared key with the sink. When a cell has a message to transmit, this message is encrypted using the pairwise key and sent after encryption to the proxy using multi-hop routing protocol such as Greedy Perimeter Stateless Routing (_GPSR_). However, these messages follow the exponential distribution whether they are real or fake to avoid any _Time Correlation_ attacks by an adversary. Therefore, if a cell observed a real event, the real event is delayed until the cell finds the appropriate time interval that does not violate the exponential distribution. When a proxy receives a fake packet, it discards the packet immediately. However, if the received packet is a real packet, the proxy re-encrypts the packet using the key shared between itself and the sink. Then, the proxy forwards the packet after delaying it in its buffer for an appropriate time. In the case the proxy node did not receive a real packet for some time, it transmits a fake packet to the sink instead.  
Note that a proxy node is able to recognize the difference between real and fake packets. Moreover, if a proxy receives a packet from another proxy in the network whether the packet is real or fake, the proxy forwards the packet to the next hop without filtration. A message routes through multiple proxies on its way to the sink; however, it is only being filtered in the original proxy. Eventually, optimizing locations of the proxies is essential to avoid undesirable traffic overhead. A disadvantage of this technique is that the sink must be in the center of the network. Another disadvantage is the filtration delay as the packet has to be filtered by a proxy before arriving at the sink.