Understanding Linux Network Internals

General Tasks of the Neighboring Infrastructure

This section describes a few general concepts that you should be familiar with before delving into specific functions within the neighboring infrastructure: caching , reference counting, and timers.

Caching

The neighboring layer implements two kinds of caching:

Neighbor mappings

As with any other kind of data that can be used multiple times, it makes sense to cache the results of the L3-to-L2 mappings. Negative results (where an attempt to resolve the address failed) are not cached. But the neighbour structures associated with failed mappings are set to the NUD_FAILED state so that the garbage collection timer can clean them up (see the section "Garbage Collection").

L2 headers

The neighboring infrastructure caches L2 headers to speed up the time required to encapsulate an L3 packet into an L2 frame. Otherwise, the infrastructure would have to initialize each field of the L2 header one by one.

Because the caching of neighbor mappings is central to the operation of the neighboring subsystem , this section describes it in detail. (The later section "L2 Header Caching" describes L2 header caching.) The contents of a neighbour structure are described in the section "neighbour Structure" in Chapter 29, and the structure's creation and deletion are described in later sections in this chapter. Here we will stay at a higher level, describing how those structures are organized and accessed by the neighboring infrastructure.

The neighboring infrastructure ...