Introducing a New TCP Variant for UAV Networks Following Comparative Simulations George Amponis a,b , Thomas Lagkas b , Konstantinos Tsiknas b , Panagiotis Radoglou-Grammatikis a and Panagiotis Sarigiannidis c,∗ a Department of Research and Development, K3Y Ltd., Sofia, 1612, Bulgaria b Department of Computer Science, International Hellenic University, Kavala Campus, 65404, Greece c Department of Electrical and Computer Engineering, University of Western Macedonia, Kozani, 50100, Greece ARTICLE INFO Keywords: TCP Congestion Control Drone Swarms Flying Ad Hoc Networks ABSTRACT The Transmission Control Protocol (TCP) is a reliable, connection oriented, congestion control mechanism, currently utilized the majority of both wired and wireless networks at the transport layer. An important function of TCP is the network congestion control mechanism; it governs the packet transmission rate and us enables the protocol to respond to congestion signals. Considering the wide spectrum of requirements stemming from unique network and channel characteristics, there exist numerous variants of TCP. While is commonly utilized in applications requiring reliable and ordered reception of packets, the standard TCP congestion control mechanism demonstrates poor performance in high-mobility wireless networking scenarios, as high mobility implies unreliable radio links and consecutive re-transmissions. In this paper we performed a comparative analysis of a spectrum of TCP variants, with the ultimate goal of deriving best practices to support real-time, yet reliable communication in high-mobility scenarios, with a special focus on aerial mobile ad hoc networks composed of Unmanned Aerial Vehicles (UAVs). Following the findings of this simulation evaluation, we introduce a new TCP variant for Flying Ad hoc Networks (FANETs), named Swarm HTCP (S-HTCP), which is shown to outperform the other variants in such network conditions. 1. Introduction Ad hoc networks, and especially the ones composed of aerial nodes such as UAVs require timely transmission of (usually) real-time data and minimal overhead. For that purpose, in most use cases, developers resort to the User Datagram Protocol (UDP). UDP offers minimal transmission delay, and requires no connection setup process. When necessary, flow control or re-transmission can also be applied, (when necessary) at the application layer (1). As such, UDP is well-suited for use in real time applications, namely video and audio (surveillance/remote sensing/monitoring). However, UDP in ad hoc networks suffers from performance degradation, which can be attributed to several factors directly associated with the core functionality of the protocol. The most important factor on which this paper focuses, is lack of a congestion control mechanism. This is particularly true for Flying Ad Hoc Networks (FANET), where node mobility is highlighted, and link states are extremely volatile. UDP is effectively blind to the state of the overall network and is only concerned with transmitting a packet. TCP inherently implements congestion avoidance in order to address the issue of channel resource miss-allocation and reliable transmission. To do this, TCP implements a slow start phase described by Algorithm 1, a congestion avoidance phase described by Algorithm 2, and a fast recovery phase described by Algorithm 3, where applicable (2). While TCP’s congestion avoidance and control mechanisms are indeed valuable for reliable data transfer, it is primarily designed for wired networks; as such, it faces performance degradation when applied to the wireless ad hoc scenario (3), where re-transmissions due to high node mobility introduce a high degree of packet loss. Given that TCP was mainly designed to recover only from congestion situations, losses of TCP segments due to errors in the transmission link are This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101016941. ∗ Corresponding author gamponis@k3y.bg (G. Amponis); geaboni@cs.ihu.gr (G. Amponis); tlagkas@cs.ihu.gr (T. Lagkas); ktsik@emt.ihu.gr (K. Tsiknas); pradoglou@k3y.bg (P. Radoglou-Grammatikis); psarigiannidis@uowm.gr (P. Sarigiannidis) ORCID(s): 0000-0001-6411-0485 (G. Amponis); 0000-0002-0749-9794 (T. Lagkas); 0000-0001-9698-1285 (K. Tsiknas); 0000-0003-1605-9413 (P. Radoglou-Grammatikis); 0000-0001-6042-0355 (P. Sarigiannidis) G. Amponis et al.: Preprint submitted to Elsevier Page 1 of 22