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%.
I thing this article might be a good reading.
To manage the lease extension process, two timers are set at the time that a lease is allocated. The renewal timer (T1) goes off to tell the client it is time to try to renew the lease with the server that initially granted it. The rebinding timer (T2) goes off if the client is not successful in renewing with that server, and tells it to try any server to have the lease extended. If the lease is renewed or rebound, the client goes back to normal operation. If it cannot be rebound, it will expire and the client will need to seek a new lease.