You have seen the fundamental triad of Policy Routing and how these elements are implemented within the Linux kernel through the RPDB. You then traced through the logic of the packet paths for the packet filters and the RPDB action locations. Now you understand the logic of the traffic flow for Policy Routing in Linux.
Chapter 4, "IPROUTE2 Utility for Linux," will cover the usage and operation of the ip utility from Alexey Kuznetsov. This is the main utility implementing all of Policy Routing with the exception of TOS/QoS. Within the utility suite, however, is the tc utility, which performs all of the TOS/QoS functions of Policy Routing. After you have learned about the ip utility you will start to delve into hands-on experiences with Policy Routing by implementing a series of increasingly complex examples.