The IPv6 neighbor discovery protocol corresponds to a combination of the IPv4 protocols Address Resolution Protocol (ARP), ICMP Router Discovery, and ICMP Redirect. IPv4 does not have a generally agreed on protocol or mechanism for neighbor unreachability detection. However, host requirements do specify some possible algorithms for dead gateway detection. Dead gateway detection is a subset of the problems that neighbor unreachability detection solves.
The neighbor discovery protocol provides a multitude of improvements over the IPv4 set of protocols.
Router discovery is part of the base protocol set. Hosts do not need to snoop the routing protocols.
Router advertisements carry link-layer addresses. No additional packet exchange is needed to resolve the router's link-layer address.
Router advertisements carry prefixes for a link. A separate mechanism is not needed to configure the netmask.
Router advertisements enable address autoconfiguration.
Routers can advertise an MTU for hosts to use on the link. Consequently, all nodes use the same MTU value on links that lack a well-defined MTU.
Address resolution multicasts are spread over 4 billion (2^32) multicast addresses, greatly reducing address-resolution-related interrupts on nodes other than the target. Moreover, non-IPv6 machines should not be interrupted at all.
Redirects contain the link-layer address of the new first hop. Separate address resolution is not needed on receiving a redirect.
Multiple prefixes can be associated with the same link. By default, hosts learn all on-link prefixes from router advertisements. However, routers can be configured to omit some or all prefixes from router advertisements. In such instances, hosts assume that destinations are off-link. Consequently, hosts send the traffic to routers. A router can then issue redirects as appropriate.
Unlike IPv4, the recipient of an IPv6 redirect assumes that the new next-hop is on-link. In IPv4, a host ignores redirects that specify a next-hop that is not on-link, according to the link's network mask. The IPv6 redirect mechanism is analogous to the XRedirect facility. The redirect mechanism is useful on non-broadcast and shared media links. On these links, it is undesirable or not possible for nodes to check for all prefixes for on-link destinations.
Neighbor unreachability detection improves packet delivery in the presence of failing routers. This capability improves packet delivery over partially failing or partitioned links. This capability also improves packet delivery over nodes that change their link-layer addresses. For instance, mobile nodes can move off-link without losing any connectivity because of stale ARP caches.
Unlike in IPv4 router discovery, the router advertisement messages do not contain a preference field. The preference field is not needed to handle routers of different stability. The neighbor unreachability detection detects dead routers and switches to a working router.
By using link-local addresses to uniquely identify routers, hosts can maintain the router associations. The ability to identify routers is required for router advertisements and is required for redirect messages. Hosts need to maintain router associations if the site uses new global prefixes.
Because neighbor discovery messages have a hop limit of 255 upon receipt, the protocol is immune to spoofing attacks originating from off-link nodes. In contrast, IPv4 off-link nodes can send Internet Control Message Protocol (ICMP) redirects and can send router advertisement messages.
By placing address resolution at the ICMP layer, the protocol becomes more media independent than ARP. Consequently, standard IP authentication and security mechanisms can be used.