Chapter 33. Routing: The Routing Cache

The routing cache is used to reduce the lookup time on the routing tables. The center of the routing cache is the Protocol Independent Destination Cache, which is simply called DST . Even if policy routing is in effect—creating multiple routing tables—a single routing cache is always shared by all the routing tables.

The main job of the cache is to store information that allows the routing subsystem to find destinations for packets, and to offer this information through a set of functions to higher layers. The cache also offers some functions to manage cleanup. The cache stores the information about the routing table cache entries that applies to all L3 protocols and can therefore be included in any data structure used to represent a routing table cache entry.

In this chapter, we will see:

  • How the cache is implemented

  • How new elements are inserted and existing ones are deleted

  • How ingress and egress lookups are implemented, and where they differ

  • How external subsystems can interact with the cache via an interface provided by the DST

  • How different kinds of garbage collection keep the size of the cache under control

  • How the DST provides a rate-limiting mechanism for egress ICMP REDIRECT messages

Routing Cache Initialization

The routing cache is implemented as a hash table. It is initialized in ip_rt_init, which is the initialization function of the routing subsystem and is described in the section "Routing Subsystem Initialization" in Chapter 32.

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