## 2 Selfish Configuration in Unauthorized Wi-Fi Tethering
### Selfish Configuration in Wi-Fi Tethering  
In order to understand the impact of the selfish behavior of a Wi-Fi tethering system using CCA tuning, we performed extensive simulations with different transport-layer protocols (i.e., TCP and UDP, in multi-AP environments) while using a network simulator [12]. **Multi-AP Topology**: first, we study the impact when a selfish tethering launches within a well-planned multi-AP network with two representative topologies. The topologies used in the simulations are shown in Figure 4. We sampled the topologies from a popular Wi-Fi database _Wigle_[13]. For example, the topology that is shown in Figure 4a corresponds to the well-known FIM (Flow-In-the-Middle) topology [14], which can be easily found in real-world AP deployments [15]. An edge in the figure represents the neighboring interference, meaning that an AP is in the carrier sensing range of its connected APs. The vertex denoted by 'T' in the figure represents the tethering system launched within the network. We compare the performance for two cases while using UDP and TCP protocols: (i) Normal scenario, in which the host node uses the default CCA threshold and (ii) Selfish scenario, in which the host node manipulates the MAC protocol by increasing the CCA threshold as high as possible without losing the connectivity to the guest. We used the following settings in our simulations. Each Wi-Fi has an AP and two associated client nodes (i.e., 2 AP-clients flows per Wi-Fi), and all of the nodes operate on the same channel. We considered typical IEEE 802.11n MAC/PHY parameters with the maximum speed of 54 Mbps, and set the capacity \(B_{cel}\) of the cellular backhaul l  
Figure 3: (a) PHY capture effect allows a receiver to successfully capture the signal of interest (SoI) if its Tx power is sufficiently higher than the sum of interferences. (b) MIM (Message-In-Message) allows for a receiver to disengage from an ongoing packet reception, and engage in a new, stronger packet. and 20 Mbps. Traffic is generated by a constant bit rate (CBR) traffic generator for the UDP protocol (1 KB packet size, uplink direction), and an FTP download application is used to create TCP flows (1.5 KB packet size, downlink direction). For UDP flows, we assume 10 Mbps for flows in infrastructure APs and 20 Mbps for tethering flows. Figure 5 shows the throughput of the tethering link and APs. In Normal case, we can see a throughput imbalance among APs and, especially, the tethering flow is shown to experience starvation due to the FIM problem [14]. With the CCA cheating, on the other hand, we can see that the selfish tethering achieves a significant throughput gain at the cost of significant throughput degradation for other nearby well-behaving APs, for both UDP and TCP flows. Although we only manipulate the CCA threshold on the host side, i.e., the guest node is legitimate, the selfish tethering achieves a high throughput gain, even with the TCP protocol, which is a bi-directional, transfer-based protocol. This is attributed to the closed-loop TCP-ACK mechanism, i.e., the more data packets (or ACKs), the legitimate guest successfully receives from the misconfigured host, the more outstanding uplink ACKs (or data packets) the guest can transmit. The figure shows that the throughput gain increases proportional to the cellular backhaul link bandwidth, \(B_{cl}\), for both UDP and TCP flows. One can expect that selfish CCA manipulation would increase the collision probability of the tethering link, since the host node initiates packet transmission, even in the present of other nodes' transmissions. Nevertheless, we observe a significant gain of selfish behavior using CCA manipulation, as seen from the above results. We investigate the property of the successful receptions at the tethering receiver node, i.e., guest node in order to understand why the selfish tethering link can achieve a significant gain with CCA manipulation.