# Adaptive TCP Transmission Adjustment for UAV Network Infrastructure
## 1 Introduction  
With the development of wireless network devices, it has become possible to collect a wide range of information and form flexible topologies through various small network devices. However, due to the wide range of activities or mobility, it is difficult for all equipment to be directly connected to the Internet's backbone. In environments where it is difficult to connect directly to the Internet, many devices can connect to the Internet with a wireless ad hoc network. There are various techniques based on wireless multi-hop environments; for example, Unmanned aerial vehicle (UAV)-based network system [1; 2; 3], multiple Internet of Things (IoT) devices [4; 5], 5G network with small mesh networks for fast data transmission speed [6], VANET technology for vehicles [7; 8], and MANET technology [9; 10]. However, various wireless multi-hop networks including UAV network (e.g., flying ad hoc network) often suffers from segment loss [11]. UAVs have an unstable network because they are difficult to load high-performance device due to limited payload size and moves around frequently to carry out various tasks. As a result, UAV networks often suffer from segment losses due to unstable networks. In this situation, since the formation information and the control message necessary for UAV must be received, the transmission control protocol (TCP), which is a reliable communication protocol, is used to recover the segment loss [12]. Based on this observation, this paper suggests an improvement technique of TCP to enhance the performance of the overall UAV network. If the segment loss occurs, TCP considers the network to be congested and decreases the congestion window (CWND) to reduce the transmission speed. The reason for reducing the transmission speed is that segment losses are caused by heavy traffic exceeding link capacity in most of wired environment. However, in wireless multi-hop environments such as UAV networks, segment losses are caused by not only buffer overflow but also transient link instability. For example, if powerdrained UAV is dropped from the network, a routing change occurs. In this case, data transmission is disconnected until the transmission path is reconstructed. Also, the mobility of UAV in most wireless multi-hop networks causes connection loss, resulting in time delay to find an alternative path to handle packet delivery. The delay misleads the end nodes to understand the phenomenon as a buffer overflow due to heavy traffic and will make the sender reduce traffic. Indeed, we frequently experienced the aforementioned phenomena in UAV network performance experiments based on ad hoc networks. However, aforementioned transient link interference should not be a reason to reduce the transmission throughput of TCP link because a path that recovered from the transient problems may have or tends to have enough capacity to support existing transmission throughput. Therefore, it is necessary to adjust TCP, which is characterized by a decrease in throughput when segment loss occurs. End-to-end controls including TCP cannot distinguish between whether segment loss occurs due to excessive traffic or transient link instability [13]. TCP therefore should have an additional solution for congestion incurred by reasons other than buffer overflow. To propose the solution, this paper suggests Adaptive Sethresh Reviser for flying Ad hoc Network (_ASRAN_) that is designed to quickly recover the reduced throughput caused by transient link instability, which is suitable for UAV network. The contribution of _ASRAN_ is listed as follows:  
* The _ASRAN_ modifies the TCP to be suitable for UAV network. * The _ASRAN_ deals with the transient link instability, separated from heavy traffic, which is a factor of reduced transmission performance in a UAV network. * In transient link instability situation, _ASRAN_ quickly recovers unnecessarily reduced throughput. This paper is organized as follows. Section 2 shows related work on enhancing performance and analyzing TCP in wireless network or ad hoc network.