# A Brief Review of Multipath TCP for Vehicular Networks
## 7 Open Research Problem  
The direct use of conventional TCP in vehicular networks faces some problems. Since TCP is connection-oriented, the constant movement of vehicles would result in frequent link disconnections which is the source of excessive retransmissions. MPTCP also faces a similar problem. Especially when a distributed vehicular communication technology, such as IEEE 802.11p, is used, vehicles face frequent link disconnections due to vehicular movement. One of the advantages of MPTCP is that it can transmit data through stable communication paths when a path is detected as vulnerable. Therefore, it is necessary to understand the performance difference between TCP and MPTCP under various conditions. This can shed a light for future studies. Vehicles can make an optimal decision by deciding how to utilize MPTCP functions based on prior knowledge. Likewise, the studies of congestion control and scheduler that we reviewed in this paper can also provide a reference to future studies on vehicular networks. Since the network environment in vehicular networks is constantly changing, there is a need to dynamically choose the congestion control and scheduling mechanism properly according to the network conditions. The knowledge can be utilized in the development of vehicular system applications. Although there are some studies that apply AI technology to dynamically adjust the congestion control and scheduling policies, efficiency and accuracy still need to be further improved. For further development, the design of congestion control and scheduling mechanism should consider vehicle mobility and communication capacity, thus optimizing the overall transmission performance. To achieve this goal, the underlying layer can be utilized to help in collecting information. On the other hand, technology such as SDN has been utilized in the vehicular networks to improve the performance [98]. The SDN technology can be also expected to be utilized for MPTCP in vehicular networks. In order to enable efficient transmissions and robust connectivity in the dynamic and complex vehicular environments, the limited communication resources are needed to be utilized efficiently by MPTCP and multipath routing approaches, which opens up many interesting research topics as the following. * MPTCP architecture for vehicular IoT applications: There is existing MPTCP architectural study. However, collaborative architecture design for MPTCP would be a continual research topic due to its capability to provide fine-grained control. New applications that cope with improved architecture need to be developed;
* MPTCP resource allocation with AI: The overall system transmission performance depends on the allocation of limited bandwidth resources. Conventional optimization approaches face challenges in solving the resource allocation problem since the optimization-based approach requires a precise prediction of future values of user demands. However, the vehicular environment is very complex and dynamic. Deep reinforcement learning and neural network approaches have the capability to solve the complex problem by dynamically adjusting their policy. A DRL-based MPTCP approach is considered to be a promising way to solve resource allocation problem in vehicular environments;
* Integration of MPTCP and multipath routing: In a highly dynamic environment, it is very difficult to find a robust communication path. MPTCP approaches that collaborate with multipath routing to provide more robust TCP paths that need to be further explored;
* Joint optimization of congestion control algorithm and scheduler: Scheduler and congestion control are the two main components of MPTCP. An efficient integration of these two mechanisms could achieve a better performance;
* Energy efficiency: Some user devices, such as mobile phones, are sensitive to energy consumption. In some studies, the energy efficiency is achieved by sacrificing some other network metrics.