Understanding Linux Network Internals

Reasons That Neighboring Protocols Are Needed

In this section, we'll look at the basic reasons for the neighboring subsystem. They stem from the fundamental division of networks into layers, and the existence of shared media such as Ethernet.

When L3 Addresses Need to Be Translated to L2 Addresses

The reason for the distinction between the network Layer two (Ethernet, 802.11 wireless, Token Ring, point-to-point, etc.) and Layer three (IP or proprietary) protocols is that many different L2 protocols exist to take data between neighbors, whereas the routing L3 layer should not have to worry what medium is being used for transmission. The higher layer should be able to employ the same software to send packets between two systems whether they're on an Ethernet or a point-to-point connection.

Figure 26-3 shows the different situations that require different responses by the neighboring subsystem.

Figure 26-3. Point-to-point connection versus shared medium

Figure 26-3(a) shows a point-to-point connection, such as a dial-up line. The L2 protocol is fairly simple, handling such issues as error checking and taking turns if it's running on a half-duplex medium. The neighboring protocol is minimal, because it simply has to invoke the L2 protocol. There is no choice of which neighbor to send a packet to.

Figure 26-3(b) shows a more complicated situation: a host on an Ethernet or other shared medium ...