Understanding Linux Network Internals

Neighbor States and Network Unreachability Detection (NUD)

Figure 26-11 is a simplified summary of the steps the kernel has to go through when transmitting a packet to a given L3 address.

Figure 26-12 is a simplified model that shows the states a neighbor can go through.

The two simple models in Figures 26-11 and 26-12 would work in most cases, but the Linux kernel uses a more sophisticated model to handle all possible states. The next section will expand the model in Figure 26-12, and later sections will focus on the details in Figure 26-11.

As you can see, an important part of managing neighbors is to know whether they are reachable.

Figure 26-11. L3-to-L2 address resolution steps

Figure 26-12. States of an L3-to-L2 mapping

Reachability

Reachability, from the neighboring subsystem's perspective, can be described through a real-life analogy. Suppose you are in a dark room with other people, including me. If you say "Everybody out of the room!" everybody will leave the room because they all can hear you. But if you want only me to go out, you will need one more piece of information: my name.

Thus, a solicitation reply sent to a broadcast destination address does not carry the same amount of information as one with a unicast destination address: anyone can receive a broadcast, but you need the ...