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

Chapter 181
IPv6 Cache Entries

All of the following caches are built in part by the Neighbor Discovery process. Instead of the simplex ARP cache used with IPv4, IPv6 maintains four caches. (Actually, four caches may not be maintained. Implementers can integrate this information any way they wish, including simply using one large table or four linked tables in one database, but all the required information must be gathered and maintained. The entries are shown here separately for simplicity reasons.)

Destination cache. This cache contains information about destinations to which traffic has been recently sent. It includes both local and remote destinations and associates an IPv6 address of a destination with that of the neighbor toward which the packets are sent. This cache is updated with information learned from ICMP Redirect messages. Other information such as the Path MTU (PMTU) and round-trip timers maintained by transport protocols can be in this cache. Entries are created by the next-hop determination procedure.

Neighbor cache. A record that contains information about individual neighbors (host or a router may be an entry) to which traffic has been recently sent. It contains such information as the neighbor’s link-layer address, an indication of whether the neighbor is a host or a router, and a pointer to any queued packets waiting for address resolution to complete. This information is also used by the Neighbor Unreachability protocol.

Prefix List cache. Created from information received in Router Advertisements, this is a listing of the local prefixes and an individual expiration timer that defines a set of addresses that are on-link. Nodes receive and store this information that is transmitted from a router in this cache. This enables a node to determine a remote destination. A special “infinity” timer value specifies that a prefix remains valid forever, unless a new (finite) value is received in a subsequent advertisement. For example, the prefix of the local link to which a node is attached is considered to be on the prefix list with an infinite invalidation timer, regardless of whether routers are advertising a prefix for it. Received router advertisements cannot change this value.

Router List cache. Built from received router advertisements, this list contain information about those routers to which packets may be sent. Router List entries point to entries in the Neighbor cache; the algorithm for selecting a default router favors routers known to be reachable over those whose reachability is suspect. Each entry is mated with an associated expiration timer value (extracted from Router Advertisements). This timer is used to delete entries that the node has not received advertisements from.