reachability Detection IPv6 nodes keep track of reachability of their neighbors, including nodes and routers. The reachability status of neighbors can be indirectly updated based on successful IPv6 communications, but in the case of no communications or failing communications, a node may use Neighbor Unreachability Detection (NUD) procedures to check whether a given neighbor is still reachable. The explicit reachability check is performed by sending a Neighbor Solicitation message with a source address set to any of the configured unicast addresses on the used outbound interface and the destination set to the unicast address whose reachability is being checked. The neighbor is considered unreachable if it does not respond with a Neighbor Advertisement message and have the solicited 'S' bit set. The Neighbor Cache is updated when a node receives ICMPv6 Neighbor Solicitations, Neighbor Advertisements, Router Solicitations, or Router Advertisements with Source Link-Layer Address option indicating the link-layer address of the sender. The Neighbor Cache is not updated just based on MAC-addresses seen in use at the Ethernet layer. Deploying IPv6 in 3GPP Networks Additionally, reachability updates are made when upper layer protocols, such as TCP, have the ability to provide positive reachability information. 3.4.6 Next-hop Determination Whenever a node is sending an IPv6 packet, it must determine what is the IPv6 address of the next hop to which the packet is to be sent [41]. This process involves the determination of whether the destination is actually on the same link as the sending host, or further away. If the destination is on the same link, the packet can be sent directly to the destination. Otherwise the packet must be sent to a next-hop router for forwarding. The selection of the a router is based on selecting a router that is known to be reachable, or probably reachable (see description of states in Section 3.4.5). Alternatively, selection in a round-robin fashion from the default routers list. RFC 4191 furthe