| Chapter 8 introduced the Point-to-Point protocol (PPP). Today, it is most frequently used in access networks that use ADSL as the access technology. The ADSL (Asymmetric Digital Subscriber Line) access technology offers high-speed Internet access for private or commercial customers. From the technical viewpoint, this is a dedicated line (i.e., a permanent connection). Dedicated lines are normally billed on the basis of transmission volumes. In contrast, private Internet links are billed on a time basis. To enable ADSL to support time-specific billing as well, a new protocol, PPPoE, was developed. PPPoE is based on two accepted standards PPP and Ethernet. More specifically, an ADSL modem (NTBBA Network Termination Point Broad-Band Access), installed behind a so-called splitter, is connected to the computer over Ethernet. This means that the computer has to be equipped with an Ethernet network card. This dedicated Ethernet line between the PC of the home user and the dialup computer of the access network operator is used to establish a PPP connection, which allows the access network operator to identify the user and bill for the usage time between the PPP dialup and the termination of that PPP session. This PPP connection can be used to exchange IP packets. Figure 9-1 shows the resulting protocol stack. This chapter first introduces the PPPoE (PPP over Ethernet) protocol described in [MLEC+99]. Then, it introduces the implementation in the user space, which is used in kernel Versions 2.2 and 2.3. Finally, this chapter discusses the implementation in the kernel from kernel version 2.4 and up. Figure 9-1. Protocol stack for the use of PPP over Ethernet. 
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