# On the Fidelity of NS-3 Simulations of Wireless Multipath TCP Connections
## 1 Introduction
We test the credibility of MPTCP simulation by comparing its performance with the experimental outcomes of a real hardware testbed replicating the exact simulation scenarios. The testbed consists of a real LTE evolved packet core (EPC) with a real eNodeB, real Wi-Fi networks, and an Ethernet-based backbone network. Specifically, we investigate the simulation performance of four congestion control algorithms when used by MPTCP in a multi-RAT environment of LTE and Wi-Fi. The research focuses mainly on two performance criteria, namely, data transfer throughput and round trip time (RTT). Using similar network setups in the hardware testbed and the ns-3 simulator, we compare the MPTCP performance for Cubic, OLIA, LIA, and wVegas congestion control algorithms for different flow sizes. We consider two setups, one with a bottleneck link and another without a bottleneck link, to assess the performance of MPTCP simulations. A bottleneck link refers to the network communication link used by both the subflows of an MPTCP connection. The bottleneck existence affects how the congestion control algorithm works. If congestion is detected in one subflow, this affects the amount of traffic pushed on the other by the congestion control algorithm, especially if they share the same link capacity. Although the design goals of MPTCP congestion control includes improved throughput, friendliness (not to harm other flows), and load balancing (balancing congestion), not all of them are satisfied by all the algorithms. For instance, LIA is normally able to achieve fairness and optimal balancing of congestion between subflows simultaneously. On the other hand, OLIA satisfies both load balancing and responsiveness to varying network conditions. However, wVegas is more responsive when compared to LIA and OLIA. For this reason, we study the performance of different algorithms using the two mentioned setups. This paper makes the following contributions. First, we analyze whether the uplink MPTCP throughput results obtained by the ns-3 simulator match the testbed throughput results. We analyze the difference in throughput performance of the aforementioned congestion control algorithms between simulation and hardware experiments for the two different setups. Second, the downlink MPTCP throughput achieved by simulation and hardware testbed is investigated. This also allows testing the fidelity of the LTE module of ns-3. Furthermore, we comparatively explore the impact of the flow lifetime on the throughput performance obtained by the simulator and testbed for the uplink and downlink. Third, the RTT performance of individual subflows is analyzed in different setups to estimate how faithful the MPTCP implementation of the simulator is with respect to the real hardware implementation. Fourth, the MPTCP throughput performance credibility is evaluated in ns-3 during the LTE handover. The rest of the paper is organized as follows. Section 2 introduces the most relevant research works in the literature. Section 3.4 provides a basic background of Cubic, LIA, OLIA, and wVegas algorithms. The experimental setups, including the used hardware and software tools, are described in Section 4.2. Section 5 presents the details of the simulation model. The testbed and simulation results are discussed in Section 6. Finally, Section 7 concludes this work.