The transport layer enables the applications to efficiently and reliably exchange data. Transport layer entities expect to be able to send segment to any destination without having to understand anything about the underlying subnetwork technologies. Many subnetwork technologies exist. Most of them differ in subtle details (frame size, addressing, ...). The network layer is the glue between these subnetworks and the transport layer. It hides to the transport layer all the complexity of the underlying subnetworks and ensures that information can be exchanged between hosts connected to different types of subnetworks.
In this chapter, we first explain the principles of the network layer. These principles include the datagram and virtual circuit modes, the separation between the data plane and the control plane and the algorithms used by routing protocols. Then, we explain, in more detail, the network layer in the Internet, starting with IPv4 and IPv6 and then moving to the routing protocols (RIP, OSPF and BGP).
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