The IPv4 Routing Subsystem
Chapter 4 discussed the IPv4 subsystem. In this chapter and the next I discuss one of the most important Linux subsystems, the routing subsystem, and its implementation in Linux. The Linux routing subsystem is used in a wide range of routers—from home and small office routers, to enterprise routers (which connect organizations or ISPs) and core high speed routers on the Internet backbone. It is impossible to imagine the modern world without these devices. The discussion in these two chapters is limited to the IPv4 routing subsystem, which is very similar to the IPv6 implementation. This chapter is mainly an introduction and presents the main data structures that are used by the IPv4 routing subsystem, like the routing tables, the Forwarding Information Base (FIB) info and the FIB alias, the FIB TRIE and more. (TRIE is not an acronym, by the way, but it is derived from the word retrieval). The TRIE is a data structure, a special tree that replaced the FIB hash table. You will learn how a lookup in the routing subsystem is performed, how and when ICMP Redirect messages are generated, and about the removal of the routing cache code. Note that the discussion and the code examples in this chapter relate to kernel 3.9, except for two sections where a different kernel version is explicitly mentioned.