Explanation
DHCPv6 can be implemented in two ways : Rapid-Commit and Normal Commit mode. In Rapid-Commit mode , the DHCP client obtain configuration parameters from the server through a rapid two message exchange (solicit and reply).
The "solicit" message is sent out by the DHCP Client to verify that there is a DHCP Server available to handle its requests.
The "reply" message is sent out by the DHCP Server to the DHCP Client, and it contains the "configurable information" that the DHCP Client requested.
Just for your information, in Normal-Commit mode, the DHCP client uses four message exchanges (solicit, advertise, request and reply). By default normal-commit is used.