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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP IPv6 Neighbor Discovery Protocol (ND)

IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration 1 2 3 IPv6 ND Redirect Function

Address Resolution

If a host determines that the destination of a datagram is in fact local, it will then need to send the datagram to that device. The actual transmission will occur using whatever physical layer and data link layer technology has been used to implement the local network. This requires that we know the layer two address of the destination, even though we only generally have the layer three address from the datagram. Getting from the layer three address to the layer two address is known as the address resolution problem.

In IPv6, the ND protocol is responsible for address resolution. When a host wants to get the layer two address of a datagram destination it sends a Neighbor Solicitation ICMPv6 message containing the IP address of the device whose layer two address it wishes to determine. That device responds back with a Neighbor Advertisement message that contains its layer two address. Instead of using a broadcast that would disrupt each device on the local network, the solicitation is sent using a special multicast to the destination device's solicited-node address. A more complete description of address resolution in IPv6 can be found in the general section on address resolution.

Note also that even though this discussion does concentrate on communication between hosts, address resolution may also be done when a host needs to send a datagram to a local router and has no entry for it in its destination cache. In the context of address resolution, a destination device is “just a neighbor”. Whether it is a host or a router only matters in terms of what happens after the datagram has been sent and received. In other words, these “host-to-host” functions are so named only because they are not specific to the communication between hosts and routers like the tasks in the preceding topic.

Devices do not routinely send Neighbor Advertisements the way routers send Router Advertisements. There really isn't any need for this: neighbors don't change much over time, and resolution will occur naturally over time as devices send datagrams to each other. In addition, having advertisements sent regularly by so many devices on a network would be wasteful.

A host may, however, send an unsolicited Neighbor Advertisement under certain conditions where it feels it is necessary to immediately provide updated information to other neighbors on the local network. A good example of this is a hardware failure—in particular, the failure of a network interface card. When the card is replaced, the device's layer two (MAC) address will change. Assuming this can be detected by the device's IP layer, it can send out an unsolicited Neighbor Advertisement message to tell other devices to update their resolution caches with the new MAC address.

IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration 1 2 3 IPv6 ND Redirect Function