 sent, but no response received yet. After a response is received, the state is changed to REACHABLE. Introduction to IPv6 REACHABLE: Neighbor is reachable. The state will change to STALE if no communication takes place for a specified time, which is configured via Router Advertisement (see Section 3.4.3) and is link specific. STALE: More than a configured time has passed since the last positive confirmation of the neighbor's reachability, and hence reachability is unknown. The neighbor's reachability will be checked when the neighbor is to be contacted next time, in which the case state changes to DELAY. DELAY: Upper layer protocol has sent a packet, but confirmation of reachability is not yet received. This state lasts five seconds, and if no confirmation is received, the state changes to PROBE. PROBE: Reachability of the neighbor is being actively checked with periodic Neighbor Solicitation messages. Once the neighbor is reachable, the state is changed to REACH- ABLE. Attacks Against Neighbor Discovery It has been discovered that the Neighbor Discovery procedure can be used by attackers to perform denial-of-service attacks [72]. Effectively, attackers would try to get nodes to reserve memory for Neighbor Cache for addresses whose resolution will not complete (state remains INCOMPLETE), causing fixed sized Neighbor Caches to either fill up, or just to cause difficulties for setting up new legitimate IPv6 communication. This attack is based on remote nodes sending packets destined to addresses having the 64-bit prefix used on a link, for which the router will attempt to find a link-layer address before forwarding the packet. As the destinations are not there, the Neighbor Caches will fill up. The documented means for mitigation involve the following (and more): rate limiting of Neighbor Discovery messages, smart IPv6 packet filtering, prioritization of different Neighbor Discovery messages, and by smarter Neighbor Cache management (cleaning up of older entries in INCOMPLETE state and SO forth). 3.4.5 Neighbor Un