Final answer:
The dad (Duplicate Address Detection) verification process uses a Neighbor Solicitation message as part of ICMPv6 to ensure a newly assigned global unicast address is unique.
Step-by-step explanation:
The dad verification process for the client's global unicast address refers to the Duplicate Address Detection (DAD) process used in IPv6 to ensure that each global unicast address is unique on the network. The specific packet used for DAD is a Neighbor Solicitation message, which is part of the ICMPv6 protocol suite. When a client device is assigned a new global unicast address, it performs DAD by sending out Neighbor Solicitation messages to confirm that no other device on the local link is using the same address. If a device with the same address is present, it will respond with a Neighbor Advertisement message, indicating a duplicate address is detected, and the client must select a new address.