Chapter 3: Transport Layer

Kurose and Keith W. Ross focuses on the transport layer, which bridges the gap between the network layer and application processes on different hosts. The chapter begins by clarifying the relationship between the transport and network layers, then explains the transport layer’s primary function—delivering data between application-layer processes via logical communication channels. It introduces the Internet’s two core transport protocols: UDP, a connectionless protocol offering minimal service, and TCP, a connection-oriented protocol that ensures reliable data transfer. The discussion of UDP covers multiplexing, demultiplexing, and its lightweight, low-overhead design, while the TCP section delves into reliable data transfer mechanisms, including sequence numbers, acknowledgments, retransmissions, and timers. The authors explain the fundamentals of flow control and congestion control, showing how TCP adapts its sending rate to network conditions to avoid overwhelming receivers or congesting the network. The chapter also addresses TCP’s three-way handshake for connection establishment, its connection termination process, and how these protocols fit within the broader Internet architecture. By alternating between transport-layer design principles and real-world implementations, the chapter provides a detailed, practical understanding of how reliability, efficiency, and fairness are achieved in modern networks.