# A Multi-Parameter Comprehensive Optimized Algorithm for MPTCP Networks
## 1 Introduction  
MPTCP [1, 2] is an extension of the well-known Transmission Control Protocol (TCP) [3]. It realizes multi-path transmission on the transport layer. TCP only establishes a single-path connection for the communication between two hosts. When losing connectivity on the interface selection for a TCP connection, the connection is broken. It needs to be re-established over the other interface (with the other interface's IP address). However, MPTCP can dynamically conduct a multi-path connection consisting of multiple flows. Figure 1a illustrates the MPTCP protocol stack [2, 4]. MPTCP transfers data simultaneously over different subflows, as depicted in Figure 1a. MPTCP uses all the available subflows, including IPv4 and IPv6. Figure 1b shows the details of the MPTCP protocol functions, which consist of path management (PM) and packet scheduling (PS). PM [5] denotes the establishment, teardown and management of subflows, while PS is the scheduling of payload data onto the existing subflows. The main task of PM is to organize and manage those subflows that can participate in the end-to-end data transmission and make contributions. Through the PM algorithm, it is possible to dynamically add or delete subflows to participate in the concurrent transmission. MPTCP can improve the bandwidth and throughput as well as enhance the robustness and fast recovery of the network by using the existing network infrastructure. With the rapid development of various access technologies, mobile communications, satellite communications and Star Link technologies, multi-network convergence has become an inevitable direction of future network development. Therefore, heterogeneous fusion communication of the next generation Internet is a general case, while homogeneous fusion network communication is a special case. When MPTCP integrates the interconnection of different path attributes and path qualities into a single concurrent transport, many problems arise in multi-path transport systems, which will produce many issues. Issue 1: The FullMesh path management algorithm uses all available subflows to transmit concurrently, without considering the characteristics of subflows and traffic. The path characteristic data characteristic (PCDC) algorithm [6] introduces the impact factor (IF) of each subflow, only these subflows whose IF is greater than 0 can participate in concurrent transmission. Practice has proven that the concurrent use of all subflows does not necessarily improve the overall performance of the network, or even drag down the system, resulting in the reduction of throughput [6]. This is especially true in heterogeneous converged networks. However, it is not the best way to access only the subflows with IF greater than 0, because the network environment and load are changing rapidly. The IF at one moment is greater than 0, and at the next time it may be less than 0. Therefore, according to the transmission scenario, only a dynamic selection by the PM algorithm is in line with the actual scenario, but how to choose it remains to be solved. Issue 2: In heterogeneous networks, computing the buffer size \(B\) of a multi-path concurrent transmission with the traditional formula will greatly increase the buffer size requirements of devices at each end, because the calculation formula of buffer size is to sum the bandwidths \(BW\) of each subflow of the concurrent transmission, and then multiply by the sum of the maximum round-trip time (RTT) \(RTT\) and the maximum retransmission timeout (RTO) \(RTO\). Then, the formula for \(B\) is as follows [7]:  
\[B\geq\left(3\cdot\max_{1\leq i\leq N}\{RTT_{i}\}+\max_{1\leq i\leq N}\{RTO_{i }\}\right)\cdot\sum_{i=1}^{N}BW_{i} \tag{1}\]  
A much more detailed introduction of the underlying mechanisms is provided in [7]. However, a large number of test studies show that configuration according to the above formula in multi-path transmission will cause a lot of waste of buffer resources.