IPv6 does not implement the ARP that is used in IPv4. Instead, IPv6 implements the ND protocol described in RFC 2461. Hosts use ND to implement plug-and-play functions that discover all other nodes in the same link, check for duplicate addresses, and find routers in the link. The protocol also searches for alternative routers if the primary fails.
The IPv6 ND protocol performs the following functions:
■ Address autoconfiguration—The host can determine its full IPv6 address without the use of DHCP.
■ Duplicate address detection—The host can determine whether the address it will use is already in use on the network.
■ Prefix discovery—The host finds out the link's IPv6 prefix.
■ Parameter discovery—The host finds out the link's MTU and hop count.
■ Address resolution—The host can determine the MAC address of other nodes without the use of ARP.
■ Router discovery—The host finds local routers without the use of DHCP.
■ Next-hop determination—The host can determine a destination's next hop.
■ Neighbor unreachability detection—The host can determine whether a neighbor is no longer reachable.
■ Redirect—The host can tell another host if a preferred next hop exists to reach a particular destination.
IPv6 ND uses ICMPv6 to implement some of its functions. These ICMPv6 messages are
■ Router Advertisement (RA)—Sent by routers to advertise their presence and link-specific parameters.
■ Router Solicitation (RS)—Sent by hosts to request RA from local routers.
■ Neighbor Solicitation (NS)—Sent by hosts to request link layer addresses of other hosts. Also used for duplicate address detection.
■ Neighbor Advertisement (NA)—Sent by hosts in response to an NS.
■ Redirect—Sent to a host to notify it of a better next hop to a destination.
The link address resolution process uses Neighbor Solicitation (NS) messages to obtain a neighbor's link layer address. Nodes respond with a Neighbor Advertisement (NA) message that contains the link layer address.
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