# A Brief Review of Multipath TCP for Vehicular Networks
## 5 MPTCP in Vehicular Networks
### Existing Studies on MPTCP in Vehicular Environment  
[PERSON] et al. [100] conducted some experiments and showed that MPTCP can improve the overall communication throughput in vehicle-to-infrastructure (V2I) environments when the multiple paths have similar characteristics. MPTCP-enabled vehicles create multiple flows by triggering a link with road side unit (RSU) with IEEE 802.11p while connected to the base station with cellular communications. The IEEE 802.11p link will break when vehicle moves out of the communication range of the RSU. Despite retransmission being required when the link disruption occurs, the performance of using MPTCP is still larger than the conventional single path TCP. In [101], [PERSON] et al. made efforts on improving the stability of multipath TCP transmissions. A vehicle detects whether it is in the communication range of RSU. When the vehicle is in the communication range of RSU, it communicates with the server through both the RSUs and cellular base station concurrently. The vehicle transmits data only with the stable interface (here is the cellular interface) when it moves out of the transmission range of the RSU. In order to improve the transmission performance in heterogeneous networks, [PERSON] et al. [102] first conducted an empirical measurement for the simultaneous use of three different communication approaches in heterogeneous network environments. Then they propose a MPTCP with networking coding (MPTCP/NC) protocol. The advantage of MPTCP/NC is maintaining throughput in lossy networks by recovering lost packets with some redundancies. In [103], [PERSON] et al. also proposed a network coding-based MPTCP (NCMPTCP)  
Figure 4: MPTCP in vehicular networks. for vehicular ad hoc networks (VANETs) to handle the communication bottle neck incurred from vehicle mobility. [PERSON] et al. [104] introduced Q-Learning based MPTCP to schedule the path and reduce the buffer blocking in heterogeneous networks. In [105], [PERSON] et al. proposed PTLIA, a congestion control algorithm based on message priority and throughput estimation, to improve the transmission performance. For example, the security messages have a high priority, and therefore they are needed to be sent prior to other messages. [PERSON] et al. [106] introduced a concurrent multipath transmission scheme for high speed trains. They implemented an experiment that aggregated six available links to provide bandwidth to the onboard devices. [PERSON] et al. [107] discussed a collaborative MPTCP architecture to achieve fine-grained control. In addition to protocol layer information, such as RTT, and congestion window, the device status and user preference are also considered to achieve an efficient transmission. For example, the decision of whether to turn on or off the MPTCP when commuting or reading in library based on user preference. In [108], [PERSON] et al. proposed a mechanism that combines MPTCP with deep packet inspection (DPI). The mechanism utilizes selective discarding on the sender side and partial reliability on the receiver side to reduce the waiting time at buffer. There are also studies that apply MPTCP in unmanned aerial vehicles (UAVs). [PERSON] et al. in [109] proposed an unmanned aerial system (UAS), which utilizes multiple cooperating UAVs. UAS also faces problems of frequent disconnection and high packet loss rates due to the mobility of UAVs. To improve performance, the UAS utilizes MPTCP to ensure robust connection and seamless handover on heterogeneous paths. They set up a mathematical model to match the UAV interface type to transmission requirements, and thus achieve optimal traffic allocation. For example, they set larger window openings to appropriate interfaces. [PERSON] et al. in [110] proposed an adaptive offloading architecture with MPTCP for UAVs. The architecture consists of three modules, namely, the response