TCP/IPv6 Configuration and Management Manual (G06.24+)

IPv6 Fundamentals
HP NonStop TCP/IPv6 Configuration and Management Manual—524523-008
A-17
Neighbor Discovery Protocol
Comparison With IPv4
The IPv6 neighbor discovery protocol corresponds to a combination of the IPv4
protocols ARP, ICMP Router Discovery, and ICMP Redirect. In IPv4 there is no
generally agreed upon protocol or mechanism for neighbor unreachability detection,
although Hosts Requirements [RFC 1122] does specify some possible algorithms for
Dead Gateway Detection (a subset of the problems neighbor unreachability detection
tackles).
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 need not 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 need not
configure the netmask.
•
Router advertisements enable address autoconfiguration.
•
Routers can advertise an MTU for hosts to use on the link, ensuring that all nodes
use the same MTU value on links lacking 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 upon 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 cases hosts assume
that destinations are off-link and send 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 specifying 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 specified in [RFC 1620]. The IPv6 redirect mechanism is
expected to be useful on non-broadcast and shared media links in which nodes
should not or cannot know all prefixes for on-link destinations.
•
Neighbor unreachability detection is part of the base which significantly improves
the robustness of packet delivery in the presence of failing routers, partially failing
or partitioned links, and nodes that change their link-layer addresses. For instance,
mobile nodes can move off-link without losing any connectivity because of stale
ARP caches.