There are some following general problems
1)Verification with Lease Extension
2)Availability of Fault-tolerant Lease Extension
3)Clarification of DhcpRequest
4)Acknowledgement of DhcpRelease
5)Reduction of DhcpRequest Congestion
6)Active Notification from Server to Client
If you want to find more details about this just go to the following link
https://www.isoc.org/HMP/PAPER/127/html/paper.html#CH4