Chapter 4: Network Layer: Data Plane

Kurose and Keith W. Ross introduces the network layer’s data plane, the set of per-router functions that forward datagrams from incoming input links to appropriate output links. Unlike the application and transport layers, the network layer exists in every host and router, making it one of the most complex parts of the protocol stack. The chapter explains how the network layer can be divided into the data plane (responsible for local forwarding) and the control plane (responsible for global routing decisions, covered in Chapter 5). Key topics include the forwarding function, switching fabrics inside routers, and different architectures such as memory-based, bus-based, and interconnection network designs. The authors describe how input port processing performs lookup, buffering, and scheduling, while output port processing handles packet queuing, scheduling disciplines (FIFO, priority queuing, round robin, weighted fair queuing), and packet dropping policies. The chapter also covers destination-based forwarding, generalized forwarding (matching on multiple header fields), and the role of a router’s forwarding table. Practical examples illustrate IP datagram format, fragmentation, and how routers use Longest Prefix Matching for efficient lookups. By combining theoretical principles with real-world hardware considerations, this chapter gives readers a detailed understanding of how the network layer data plane ensures fast, reliable packet forwarding within the Internet.