Understanding Linux Network Internals

Interaction Between Neighboring Protocols and L3 Transmission Functions

We saw in Chapter 21 that packet transmission in the IPv4 subsystem ends with a call to ip_finish_output2, which passes the packet down to the L2 layer. In this section, we'll see how this function interacts with the neighboring subsystem. The function that has a similar name and task within the IPv6 subsystem behaves the same way, except that it calls the ND protocol instead of IPv4's ARP protocol.

The input skb buffer, ip_finish_output2, includes the packet data (but without an L2 header), along with information such as the device to use for transmission and the routing table cache entry (dst) that was used by the kernel to make the forwarding decision. As we saw in Figure 27-1, that dst entry includes a pointer to the neighbour entry associated with the next hop (which can be either a router or the final destination itself). The decisions made by ip_finish_output2 that are of interest to us in this chapter are summarized in Figure 27-12.

If a cached L2 header is available (hh is not NULL), it is copied into the skb buffer. (skb->data points to the start of the user data, which is where the L2 header should be placed.) Finally, hh_output is invoked.

If no cached L2 header is available, ip_finish_output2 invokes the neigh->output method. As explained earlier in this chapter, the precise function associated with neigh->output depends on the state of the neighbour entry. If the L2 address is ready, the function ...