# A Brief Review of Multipath TCP for Vehicular Networks
## 3 Congestion Control and Scheduler
### Congestion Control  
#### 3.1.1 Congestion Control with Predefined Rules  
Many fundamental MPTCP congestion control mechanisms have been proposed in the past few years, including linked increases algorithm (LIA) [46], opportunistic linked-increases congestion control algorithm (OLIA) [32], weighted vegas (wVegas) [49], and so forth. Among these MPTCP congestion control mechanisms, LIA is the standard coupled MPTCP congestion control mechanism. It couples the congestion windows of all the subflows and controls the total window size as one TCP connection, hence achieving the goal of TCP fairness. This mechanism also guarantees that the total throughput of all subflows is not less than the throughput as a TCP run on the best path. On the other hand, non-coupled is a simple extension of TCP to multiple paths. However, due to its aggressiveness usage of resources on the shared network, it causes a TCP unfairness problem. Congestion control mechanisms for MPTCP are mainly divided into loss-based congestion control (CC) and delay-based congestion control. The loss-based CC triggers an event to adjust the window size based on packet loss, while the delay-based CC adjusts window size based on the packet transmission delay. The LIA not only takes into account the packet loss rate for the congestion control like the classic Reno [50] does, but also considers the round trip time (RTT) for congestion window adjustment. However, bufferbloat [51] will occur in heterogeneous networks for LIA, which would result in delayed packet delivery to the application layer. [PERSON] et al. [49] proposed wVegas, which utilizes packet queuing delay to adjust congestion window size. By considering the delay more accurately in the congestion control, wVegas is more suitable for dynamic network environments. Moreover, it also occupies fewer link buffers and is able to reduce losses compared with loss-based algorithms. Smiliar to the way implemented by compound TCP, it is also being considered as a promising way by combining wVegas and LIA. As mentioned earlier in this paper, one of the main goals of MPTCP is to achieve TCP friendliness. One way to reach this goal is to couple different paths in a shared bottleneck. Many studies have been conducted to optimize congestion control on the premise of achieving TCP friendliness. [PERSON] et al. [52] presented a TCP-friendly approach for multipath transport protocol, namely bidimensional-probe multipath congestion control (BMC). It includes an aggressiveness manager and proportion manager. Aggressiveness manager is designed to ensure subflows throughput. In order to maximize the entire connection throughput, the proportion manager is for adjusting the weight of different subflows. In [53], [PERSON] et al. present a congestion control mechanism, called dynamic window coupling (DWC). DWC is able to detect shared bottlenecks by loss and delay signals. Then it couples or decouples the subflows depending on the detection results. Furthermore, DWC is able to detect the shifting of bottlenecks on various network environments. The DWC improves throughput by aggregating across distinct bottlenecks and achieves fairness among TCP flows. [PERSON] et al. [54] compared different congestion control variants of MPTCP, namely, NewReno, LIA, and DWC, in different scenarios. They further developed some DWC extensions to detect the subflows with the same bottleneck (multiple subflows are transmitting through the same network path that becomes the bottleneck of the network). Among the extensions, Delay-Delay variant has the highest efficiency in detecting bottlenecks because it can avoid active queue management problem. Delay-Delay variant utilizes a delay congestion event both in triggering congestion inspection, and grouping subflows with the same bottleneck. [PERSON] et al. [55] proposed DBCCA to optimize the transmission performance in heterogeneous networks. The DBCCA is a delay-based approach that consists of two phases.