IPv6 ND protocol

The IPv6 Neighbor Discovery (ND) protocol uses the following ICMPv6 messages:

This function is similar to ARP in IPv4. An IPv6 node acquires the link-layer addresses of neighboring nodes on the same link through NS and NA messages. Figure 60 shows how Host A acquires the link-layer address of Host B on the same link.

The address resolution procedure is as follows:

1. Host A multicasts an NS message. The source address of the NS message is the IPv6 address of the sending interface of Host A. The destination address is the solicited-node multicast address of Host B. The NS message body contains the link-layer address of Host A and the target IPv6 address.

2. After receiving the NS message, Host B determines whether the target address of the packet is its IPv6 address. If it is, Host B learns the link-layer address of Host A, and then unicasts an NA message containing its link-layer address.

3. Host A acquires the link-layer address of Host B from the NA message.

After Host A acquires the link-layer address of its neighbor Host B, Host A can use NS and NA messages to test reachability of Host B as follows:

1. Host A sends an NS message whose destination address is the IPv6 address of Host B.

2. If Host A receives an NA message from Host B, Host A decides that Host B is reachable. Otherwise, Host B is unreachable.

After Host A acquires an IPv6 address, it performs Duplicate Address Detection (DAD) to check whether the address is being used by any other node. This is similar to gratuitous ARP in IPv4. DAD is accomplished through NS and NA messages.

1. Host A sends an NS message. The source address is the unspecified address and the destination address is the corresponding solicited-node multicast address of the IPv6 address to be detected. The NS message body contains the detected IPv6 address.

2. If Host B uses this IPv6 address, Host B returns an NA message that contains its IPv6 address.

3. Host A knows that the IPv6 address is being used by Host B after receiving the NA message from Host B. If receiving no NA message, Host A decides that the IPv6 address is not in use and uses this address.

A node performs router/prefix discovery and stateless address autoconfiguration as follows:

1. At startup, a node sends an RS message to request configuration information from a router.

2. The router returns an RA message containing the Prefix Information option and other configuration information. (The router also periodically sends an RA message.)

3. The node automatically generates an IPv6 address and other configuration parameters according to the configuration information in the RA message.

The Prefix Information option contains an address prefix and the preferred lifetime and valid lifetime of the address prefix. A node updates the preferred lifetime and valid lifetime upon receiving a periodic RA message.

The generated IPv6 address is valid within the valid lifetime and becomes invalid when the valid lifetime expires.

After the preferred lifetime expires, the node cannot use the generated IPv6 address to establish new connections, but can receive packets destined for the IPv6 address. The preferred lifetime cannot be greater than the valid lifetime.

Upon receiving a packet from a host, the gateway sends an ICMPv6 redirect message to inform the host of a better next hop when the following conditions are met:

• The interface receiving the packet is the same as the interface forwarding the packet.

• The selected route is not created or modified by an ICMPv6 redirect message.

• The selected route is not a default route on the device.

• The forwarded IPv6 packet does not contain the routing extension header.