238Understanding IPv6, Second Edition

Table 10-2 Example Routing Table Entries

End-to-End IPv6 Delivery Process

The following sections describe the process of forwarding an IPv6 packet from the sending host, across one or more IPv6 routers, and its receipt at the final destination. This example assumes that the Hop-by-Hop Options, Destination Options, and Routing extension headers are not present.

IPv6 on the Sending Host

An IPv6 host uses the following algorithm when sending a packet to an arbitrary unicast destination address:

1.Set the Hop Limit field value to either a default or application-specified value.

2.Check the destination cache for an entry matching the destination address. For computers running Windows Server 2008 or Windows Vista, you can view the destination cache with the netsh interface ipv6 show destinationcache command.

3.If an entry matching the destination address is found in the destination cache, obtain the next-hop address and interface from the destination cache entry. Go to step 9.

4.If there is no source address for the packet, perform an unconstrained route lookup.

5.If there is a source address for the packet and strong host send behavior is enabled on the source interface, perform a constrained route lookup.

6.If there is a source address for the packet and strong host send behavior is disabled on the source interface, perform an unconstrained route lookup.

7.If there is a longest matching usable route, determine the next-hop interface and address used for forwarding the packet from the route.

If there is not a longest matching usable route, indicate an error.

8.Update the destination cache.

9.Check the neighbor cache for an entry matching the next-hop address. For computers running Windows Server 2008 or Windows Vista, you can view the neighbor cache with the netsh interface ipv6 show neighbors command.

10.If an entry matching the next-hop address is found in the neighbor cache, obtain the link-layer address. Go to step 12.

11.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 and add an entry to the neighbor cache.

If address resolution is not successful, indicate an error.

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

Note For unicast destination addresses that are assigned to the sending host, IPv6 skips steps 9 through 11.

Figure 10-1 shows the sending host process.

IPv6 on the Router

An IPv6 router uses the following algorithm when receiving and forwarding a packet to an arbitrary unicast or anycast destination:

1.Perform optional header error checks, such as ensuring that the Version field is set to 6 and that the source address is not the loopback address (::1) or a multicast address.

2.Verify whether the destination address in the IPv6 packet corresponds to an address assigned to a router interface.

If so, process the IPv6 packet as the destination host. (See step 3 in the “IPv6 on the Destination Host” section in this chapter.)

3.Decrement the value of the Hop Limit field by 1.

If the value of the Hop Limit field is less than 1, send an ICMPv6 Time Exceeded–Hop Limit Exceeded in Transit message to the sender and discard the packet.

4.If the value of the Hop Limit field is greater than 0, update the Hop Limit field in the IPv6 header of the packet.

5.Check the destination cache for an entry matching the destination address.

If an entry matching the destination address is found in the destination cache, obtain the next-hop interface and address from the destination cache entry. Go to step 9.

6.Check the local IPv6 routing table, and select the longest matching usable route to the destination IPv6 address with the lowest metric.

8.Update the destination cache.

9.If the interface on which the packet was received is the same as the interface on which the packet is being forwarded, the interface is a point-to-point link, and the Destination Address field matches a prefix assigned to the interface, send an ICMPv6 Destination Unreachable—Address Unreachable message to the sending host and discard the packet. This prevents the needless “ping-pong” forwarding of IPv6 packets between the two interfaces on a point-to-point link for a packet whose destination matches the prefix of the point-to-point link but does not match the address of either interface.

10.If the interface on which the packet was received is the same as the interface on which the packet is being forwarded, and the Source Address field matches a prefix assigned to the interface, send a Redirect message to the sending host.

11.Compare the link maximum transmission unit (MTU) of the next-hop interface to the size of the IPv6 packet being forwarded.

If the link MTU is smaller than the packet size, send an ICMPv6 Packet Too Big message to the sending host and discard the packet.

12.Check the neighbor cache for an entry matching the next-hop address.

13.If an entry matching the next-hop address is found in the neighbor cache, obtain the link-layer address. Go to step 15.

14.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 and add an entry to the neighbor cache.

If address resolution is not successful, send an ICMPv6 Destination Unreachable— Address Unreachable message to the sending host and discard the packet.

15.Send the packet with the link-layer address of the neighbor cache entry.

Figures 10-2 and 10-3 show the router forwarding process.

This entire process is repeated at each router in the path between the source and destination hosts.

IPv6 on the Destination Host

An IPv6 host uses the following algorithm when receiving an IPv6 packet:

1.Perform optional header error checks, such as ensuring that the Version field is set to 6 and that the source address is not the loopback address (::1) or a multicast address.

2.Verify whether the destination address in the IPv6 packet corresponds to an IPv6 address assigned to a local host interface.

If the destination address is not assigned to a local host interface, silently discard the IPv6 packet.