Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 159
Neighbor Discovery and 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 and 1123 does specify some possible algorithms for Dead Gateway Detection (a subset of the problems that Neighbor Unreachability Detection tackles). Router Discovery is part of the base protocol set; there is no need for hosts 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; there is no need to have a separate mechanism to 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 (2ⁿ32) 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 may be configured to omit some or all prefixes from Router Advertise-ments. 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 (the same subnet as itself). 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 redirect facility. It is expected to be useful on nonbroadcast and shared media links in which it is undesirable or impossible for nodes to know all prefixes for on-link destinations.

Neighbor Unreachability Detection is part of the base significantly improving 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 due to stale ARP caches.

Unlike ARP, Neighbor Discovery detects half-link failures (using Neighbor Unreach-ability Detection) and avoids sending traffic to neighbors with which two-way connectivity is absent. 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 Detec-tion will detect dead routers and switch to a working one. The use of link-local addresses to uniquely identify routers (for Router Advertisement and Redirect messages) makes it possible for hosts to maintain the router associations in the event of the site renumbering to use new global prefixes.

Using the Hop-Limit-equal-to-255 trick, Neighbor Discovery is immune to off-link senders that accidentally or intentionally send ND messages. In IPv4, off-link senders can send both ICMP Redirects and Router Advertisement messages. Placing address resolution at the ICMP layer makes the protocol more media independent than ARP and makes it possible to use standard IP authentication and security mechanisms as appropriate.