# A Brief Review of Multipath TCP for Vehicular Networks
## 5 MPTCP in Vehicular Networks
### Lesson Learned  
In this section, we introduced some studies on MPTCP for vehicular networks. MPTCP is able to improve overall throughput and stability under certain network conditions in V2I. However, packet losses occur frequently when vehicles disconnect with infrastructure because of the frequent movement of vehicles. The application-level delay also increases due to retransmissions. In addition, the communication bottleneck problem is also pointed out in heterogeneous networks. It implies that although MPTCP has a potential to play a role in vehicular networks, some further improvements are required. The MPTCP in vehicular networks is expected to bring the following benefits.  
* Higher bandwidth utilization rate: MPTCP could realize better utilization of network bandwidth by conducting transmissions through multiple paths;
* Better performance: It is possible for MPTCP to achieve more stable transmissions by shifting between different subflows. When interruption incurs on one flow, the on-going transmission data can be shifted to other flows without reset or changing the IP address;
* More powerful applications: Since MPTCP can support a higher performance than the conventional single flow TCP, MPTCP could enable some new applications such as real time high definition video streaming.  
Existing studies show that we can improve the performance of MPTCP by leveraging other techniques. Network coding-based MPTCP approaches are able to maintain a high throughput in lossy networks by recovering lost packets. However, the network coding based approaches are only evaluated with theoretical analysis or simulations, and are not verified by real-world experiments. The Q-learning based approach is able to reduce buffer blocking in heterogeneous networks. However, the convergence of the model needs some time. It would be more beneficial if a pre-trained model is available.  
In addition to only considering the metrics of a transmission layer, other factors are also taken into consideration to collaboratively satisfy the design goals. For example, the message priority, device status, and user preference are utilized in some studies. However, these studies stay in the proposal stage or simple simulation stage. Performance metrics such as throughput and energy consumption need to be further investigated by real-world experiments. Other factors that influence network performance also need to be further investigated.  
Some important future research topics are as follows. First, it is urgent to design an efficient MPTCP architecture that addresses the characteristics of vehicular networks by considering more factors in order to satisfy user requirements in various scenarios. Second, AI-based solutions should be further investigated. Third, in order to enable an efficient MPTCP transmission, a support from the underlying routing layer is mandatory. Choosing proper routes to finish transmissions efficiently before topology change would be a promising way. Therefore, more research efforts are required to enable an efficient integration of MPTCP and multipath routing. Lastly, context-aware approaches should be further discussed as the scheduling of multipath transmissions should fully consider the contexts, including user requirements, available network resources, vehicle distributions, vehicle capability, and so forth.