TCP-based Congestion Control Algorithms Subjects: Telecommunications | Engineering, Electrical & Electronic | Computer Science, Information Systems Contributors: Josip Lorincz, Zvonimir Klarin  Submitted by: Josip Lorincz Definition 1. Introduction At the transport layer of the Open System Interconnection (OSI) model, the transmission control protocol (TCP) is a major protocol used in today’s communication over the Internet. The TCP is designed to provide a reliable end-to-end connection over unreliable links. Besides affecting the physical and media access control (MAC) layers of the OSI model, ensuring the high data rates in environments characterised with fluctuations in the quality of the wired links or wireless channels deteriorate the performance of the transport layer of the network . Ensuring reliable end-to-end connections over versatile wired and wireless networks which become complex and heterogeneous, presents a demanding challenge in terms of its practical implementation. This challenge arose during the last decades as the subject of the researchers’ interest . One of the mechanisms that plays a critical role in TCP performance is the congestion control (CC) mechanism. One of the main purposes of the CC mechanism is to probe the network for available capacity by dynamically increasing or decreasing the amount of data (also known as window size) that can be sent over a communication link. This approach enables avoiding the congestion of the network that can be caused by a large burst of data during transmission . Node or link congestion in wired networks and blockage and beam misalignment in wireless networks can seriously affect the TCP CC mechanism. This further results in poor end-to-end transmission performance. Conventional TCP CC algorithms are not able to differentiate between the potential causes of packet loss. They can occur due to a lack of wired link capacity or poor wireless channel quality . In today’s data networks, the main protocol used to ensure reliable communications is the transmission control protocol (TCP). The TCP performance is largely determined by the used congestion control (CC) algorithm, which main purpose is to avoid the congestion of the network that can be caused by a large burst of data traffic. TCP CC algorithms have evolved over the past three decades and a large number of CC algorithm variations have been developed to accommodate various communication network environments. Considering the importance of CC in communication networks, the fundamentals of the TCP as the main transport layer protocol and CC process have been explained in detail. Also, an encyclopedic overview of the most popular single-flow and multi-flow TCP CC algorithms with corresponding alternatives has been present. Future directions in the possible improvement of CC algorithms for implementation in complex heterogeneous networks composed of wired and wireless elements are lastly discussed in this encyclopedic work. [1 ] [2 ] [3 ] [4 ]