 address' (see Section 3.2.4) of the tentative address node has (see more about address states in Section 3.5.1). Uniqueness of a tentative address is tested by sending an ICMPv6 Neighbor Solicitation [41] message, sourced from an undefined address : : if the node does not have address yet, to the Solicited-node multicast address [4] of the tentative address. The number of times the Neighbor Solicitation is sent may be configurable via nodes' DupAddrDetectTransmits configuration variable [73]. Introduction to IPv6 The default value is one indicating no retransmissions (configuration variable value zero would skip DAD). If there is no reply after the last (and by default the only) Neighbor Solicitation message, the node can start using the new address. If there is a Neighbor Advertisement reply indicating that the tentative address is already used by another node, the node behavior depends on how the IID was configured. If the IID was based on hardware address, the failed DAD means there is a conflict of hardware addresses and hence IPv6 operations on a link should be stopped. If the IID was randomly selected, the node can just randomize a new identifier and repeat the DAD procedure with it. The DAD procedure can be optimized by using a technology called Optimistic DAD. Optimistic DAD makes the address configuration faster in the successful case. It essentially assumes no conflicts, and in case of one, it will back off fast [74]. This is a very effective improvement as address conflicts are very unlikely in real life scenarios where unique hardware-based or pseudorandom IIDs are used. Thus, the Optimistic DAD allows the node to proceed faster to actual data transmission phase. 3.4.8 Redirect A router can inform a sending node with ICMPv6 Redirect message about a better next-hop destination than itself. This is a useful property if a router knows another router that is more preferred to route packets destined to certain addresses. Furthermore, a router can use ICMPv6 Redirect to inform a sending node that the desti