Host Sending Algorithm

The process by which an IPv6 host sends a unicast IPv6 packet uses a combination of the local host's conceptual data structures and the ND protocol. An IPv6 host uses the following algorithm when sending a unicast packet to an arbitrary destination:

1. Check the destination cache for an entry matching the destination address.
2. If an entry matching the destination address is found in the destination cache, obtain the next-hop address from the destination cache entry. If the destination is a mobile IPv6 node, the destination cache entry might contain a pointer to a care-of destination cache entry. If so, the next-hop address is obtained from the care-of destination cache entry. For more information about IPv6 mobility support, see Chapter 12, "IPv6 Mobility." Go to step 4.

   If an entry matching the destination address is not found in the destination cache, determine if the destination address matches a prefix in the prefix list.

   If the destination address matches a prefix in the prefix list, the next-hop address is set to the destination address. Go to step 3.

   If the destination address does not match a prefix in the prefix list, check to see if there is a default router.

   If there is no default router (and there are no routers in the default router list), the next-hop address is set to the destination address.

3. Update the destination cache.
4. Check the neighbor cache for an entry matching the next-hop address.
5. If an entry matching the next-hop address is found in the neighbor cache, use the link-layer address of the matching entry.

   If an entry matching the next-hop address is not found in the neighbor cache, use address resolution to obtain the link-layer address for the next-hop address.

   If address resolution fails, indicate an error.

6. Send the packet by using the link-layer address of the neighbor cache entry.

Figure 6-30 shows the host sending algorithm in flowchart form.

Figure 6-30. The host sending algorithm

Because the IPv6 protocol for the Windows .NET Server 2003 family and Windows XP uses a routing table in place of a prefix list and default router list, the host-sending algorithm uses a different method to determine the next-hop address for the destination. For more information, see "End-to-End IPv6 Delivery Process" in Chapter 10, "IPv6 Routing."