In the following chapters, we will leave the hardware area and move on to the world of network protocols. Chapters 7 through 24 discuss the structure and implementation of network protocols in the Linux kernel.
The previous chapters introduced the most important basics of the Linux network architecture, including the general structure of communication systems and protocol instances (Chapter 3), representation of network packets in the Linux kernel (socket buffers, Chapter 4), and the abstraction of physical and logical network adapters (network devices, Chapter 5). Before we continue discussing the structure and implementation of network protocols in detail, this chapter gives a brief introduction to the structuring of the data-link layer, which represents the connecting layer between network devices and higher network protocols. Of primary interest is the background where network protocols run. Another important topic of this chapter is the interplay of different activities (hardware and software interrupts, tasklets) of the Linux network architecture.
The transition between the different activities in the data-link layer (layers 1 and 2 of the OSI model) occurs when packets are sent and received; these processes are described in detail in Sections 6.2.1 and 6.2.2. First, we will describe the path a packet takes from its arrival in a network adapter until it is handled by a protocol instance in the network layer; then we will describe how a packet is sent from the network layer until it is forwarded to the network adapter.
