# On the Fidelity of NS-3 Simulations of Wireless Multipath TCP Connections
## 1 Introduction  
There has been tremendous growth in wireless data traffic and the number of wirelessly connected devices in recent years. The main goals of 5G networks, such as high data rates and low latency, can be supported by millimeter wave (mmWave) technology. However, mmWave communication signals lack strong diffraction and are more prone to blockages by the physical objects in the environment, which may result in the disruption of communication links [1; 2]. Even the human body is considered a strong blocker of the mmWave communication system. In fact, there can be self-body blockages that are caused by the users of the communication device themselves [3]. The authors of [4] showed that the human body can bring down the mmWave signal strength by around 20 dB and block the link for approximately 500 ms. The frequent blockages of mmWave connections and long blockage duration can significantly impact 5G communication reliability. Hence, it is essential to support redundancy in network communication links. The availability of multiple network connections provides a failover solution to compensate for a blockage-affected mmWave link by using another communication link. Luckily, the concept of wireless multihoming is gaining higher traction, driven byemerging heterogeneous wireless networks and the increased availability of wireless devices equipped with multiple wireless interfaces. The simultaneous connection to two networks is standardized by 3 GPP as Dual Connectivity in Release 12 [5]. The 5G networks are designed to integrate multiple radio access technologies, such as cellular and Wi-Fi, resulting in multiple radio access technologies (RAT) environment. Multihoming is therefore considered to be one of the key features of the 5G networks. Multipath TCP is a transport layer protocol designed for networks with multihomed devices. Since the legacy transport layer protocol of the Internet, TCP (transmission control protocol), supports only single-path connections, the Internet engineering task force (IETF) introduced MPTCP (backward compatible with TCP) to support multipath connections [6]. The extension of the standard TCP to support multiple paths simultaneously is expected to increase the overall resource utilization and, in turn, the network throughput. Moreover, the availability of multiple paths to MPTCP provides resilience to network failure. Furthermore, 3 GPP recently standardized MPTCP as an integral feature of 5G in Release 16 [5]. Thus, multipath TCP is gaining research focus, and investigating the performance of MPTCP with multi-RAT is of paramount importance. However, studying the performance of MPTCP in a controlled cellular environment requires real testbed deployment of 4G long-term evolution (LTE)/5G systems, which incur high costs. In general, real hardware equipment is challenging to modify, upgrade, and scale, and it requires storage space. Therefore, many researchers tend to study the performance of MPTCP using computer simulations. Network Simulator version 3 (ns-3) is one of the most widely used simulators for cellular networks. It is an open-source discrete-event network simulator licensed under the GNU GPLv2 license [7]. Ns-3 offers multi-RAT simulation capabilities, with Wi-Fi, LTE (LTE-Advanced (LTE-A), License Assisted Access (LAA), LTE-Unlicensed (LTE-U)), and Wireless Gigabit (WiGig). LTE-EPC Network Simulator (LENA) is the LTE module of ns-3 developed by Centre Tecnologic de Telecomunicacions de Catalunya (CTC) [8]. 5G-LENA is developed as an extension to LENA module [9]. However, MPTCP is still not available as a module in any stable version of ns-3. Alternatively, the Direct Code Execution (DCE) feature of ns3 enables the use of the Linux kernel implementation of MPTCP. DCE is a module of ns-3 that allows the execution of existing implementations of userspace and kernel space network protocols/applications within ns-3 [10]. The objective of this research is to assess the MPTCP simulation fidelity of ns3-DCE.