According to various DHCP description after reaching 50% client will enter RENEW state and will start sending DHCPREQUEST messages to the server in order to renew the lease. It will continue to send the requests until either getting reponse or reaching 87.5% of time. After reaching that time, it will change the state to REBOUND and will continue to send requests (until response or 100%).
So, whats the difference between RENEW and REBOUND state? Whats the point of this mark timer, if there is no change in the behavior on the client side? To the outside world it looks like the server will just send requests from after passing 50% of the lease until either getting the response or reaching 100%.