# A Brief Review of Multipath TCP for Vehicular Networks
## 2 TCP and MPTCP
### Comparison of Single Path TCP and MPTCP  
To prove the advantage of MPTCP, many works were conducted to compare the performance of single path TCP (SPTCP) and MPTCP. At the very beginning, MPTCP is designed for improving the efficiency of data center communications. In [39], [PERSON] et al. proposedan approach to replace TCP with MPTCP in datacenter networks. They investigate how each parameter of MPTCP influences the network performance and compare MPTCP performance with TCP. The influence metrics include the network topology, number of subflows, traffic matrix, and congestion control. Firstly, the results show that MPTCP can achieve a 90 percent bandwidth utilization with two subflows and eight subflows, for VL2 and FatTree network topology, respectively. Second, the experiment shows that the number of subflows also has an effect on the throughput performance. At least for the permutation traffic matrix, eight subflows are sufficient to achieve 90 percent throughput utilization. The effect of different congestion control mechanisms on network performance are also studied. They compared MPTCP's linked congestion control mechanism with uncoupled TCP and equally weighted TCP (EWTCP). Lastly, they evaluated the performance of MPTCP in dual-homed FatTree (DHFT) scenario, a dual-homed variant of FatTree topology. The results show that MPTCP not only improves throughput, but also improves robustness with the same cost. [PERSON] et al. [40] conducted a web traffic measurement to compare the performance of single path TCP and MPTCP over Wi-Fi and cellular networks. Since different web traffics can have different traffic loads, they put their efforts to analyze the relationships of network factors and application level performance. They consider file downloading traffics with different file sizes, specifically 8 KB, 64 KB, 512 KB, 4 MB, 8 MB, 16 MB, and 32 MB. When the Wi-Fi is the default path, the MPTCP download performance has no obvious improvement compared with the SPTCP. For small size file download (such as 64 KB size) scenarios, the single path over Wi-Fi provides the best performance. Instead, for slightly longer traffic flows, single path over LTE becomes the best choice. The reason is that the Wi-Fi has a short RTT and high loss rate, while the cellular network has a low loss rate. The small size files can be completed very quickly by Wi-Fi. However, when the file size becomes larger, the retransmission rate will increase due to the high loss rate of Wi-Fi packets. In this case, the cellular network will result in a better performance due to its low loss rate of data packets. For 4-path MPTCP, the small size files can be quickly completed by two Wi-Fi subflows without utilizing cellular communications. Likewise, the cellular will be playing a leading role when the file size becomes larger. The effects of many factors on latencies are also evaluated. The factors include the number of subflows, congestion controllers, background traffic, and the starting time of each flow. IEEE 802.11p, a well-known communication approach for vehicle-to-vehicle communication, is similar to Wi-Fi. On the other hand, vehicles can also communicate with each other by cellular networks. Therefore, this findings from this study can be used in designing a new MPTCP approach for file transfer and video streaming in vehicular environments. In [41], [PERSON] et al. compared transmission performances of Wi-Fi and LTE by conducting a real world experiment. A crowed sourced mobile application was used by 750 users over 180 days from 16 different countries in the experiment. The results show that at a considerable amount of time, LTE outperforms Wi-Fi, since Wi-Fi performance is affected by signal quality or user density easily. They also compared the MPTCP performance with TCP performance over Wi-Fi or LTE from 20 different locations of seven cities in USA. For short flows, the faster link of regular TCP outperforms MPTCP performance.