# A Brief Review of Multipath TCP for Vehicular Networks
## 1 Introduction  
In the past, people have mostly relied on newspapers, magazines, or books to acquire the information they need. After that, with the development of electromagnetic technologies, people began to utilize some extra options of electric tools such as television and radio to acquire information. The communication world is constantly and quickly changing due to the evolvement of information and communication technology and it gradually changes and affects our life. Nowadays, we can chat with people from most places in the world via a smartphone. There are many transmission protocols, such as user datagram protocol (UDP) [1], transmission control protocol (TCP) [2], and stream control transmission protocol (SCTP) [3], to support communications. TCP and SCTP can provide reliable transmissions. However, their transmission rates are very limited. Therefore, we need a solution to solve this challenge. In the current era, many smart phones and mobile devices are equipped with dual interfaces, even with multiple interfaces. Cellular network technology and Wi-Fi are per-vasively utilized. In January 2013, to overcome the limitation of conventional single path transmission rate, the Internet Engineering Task Force (IETF) published the multipath TCP (MPTCP) specification for TCP extensions [4]. Multipath TCP allows an end host to transmit data across a number of TCP subflows simultaneously on one TCP connection. MTCP is designed by considering the following points:  
* Improving throughput: MPTCP's throughput is larger than a single path TCP by sending the data through multiple paths simultaneously;
* TCP friendliness: MPTCP does not incur a negative effect on the conventional single path TCP;* Balancing congestion and improving resiliency: MPTCP utilizes the least congested path, and therefore is able to shift the flows from congested or failure path to the least congested path. These advantages have attracted attention from academia and the industry. Many studies in regards to MPTCP were conducted under various topics, including data center communications, smart phone applications, energy efficiency, vehicular networks, and so on. On September 2013, Apple released iOS7 which included the commercial deployment of MPTCP [5]. At IETF93 in Prague, [PERSON] first presented that Korean Telecom would provide a service that enables smartphone users to reach a bandwidth of up to 1 Gbps on smartphones [6]. They provide the service by combining fast long term evolution (LTE) and fast Wi-Fi. The vehicular Internet-of-Things (IoT) systems are equipped with many sensors, including a camera and radar, and hence generate a large amount of data. Meanwhile, emerging cooperative autonomous cars and intelligent transport systems demand unprecedented high transmission speed and a high degree of robustness. Most of the current vehicular IoT systems utilize single path to transmit data between vehicles, which hardly reaches the demand of data transmission speed of autonomous driving. Moreover, in the context of mobility, the IP address changes when users move out of an access point's coverage range or is disconnected when using single path TCP. The unfinished data will be stopped or the whole transmission will be reset. Meanwhile, vehicular networks have a feature of complex and dynamic topology. Therefore, in order to achieve a more efficient and robust vehicular IoT system, MPTCP is considered to be a promising approach to reach this goal since it can transmit data across multiple paths to improve transmission throughput. In addition, it is able to shift the flows to other paths when the transmitting path is interrupted. Furthermore, it increases resilience to respond to constant topology changes. On the other hand, because vehicular networks conduct wireless communication, it may cause communication interferences. Therefore, it is pointed out that selecting the proper transmission paths is also very important [7]. Studies regarding multipath routing [8; 9] were widely conducted to provide efficient and robust transmission paths.