UDP data size problem

I am currently using NIOS II + uCOS2 + Nichestack to run UDP sockets like everyone does.

My problem is :

when UDP data size is under 1500 Bytes, I can send and receive UDP packets correctly.

when UDP data szie is over 1500Bytes , the PC sends UDP packets to the Board and the UDP packet is fragmented by default.

Then the Board doesn't receive the righ packet. It seems that Nichestack doesn't perform IP fragments reassemble !!!

while I have alread enble this in system.h:# define IP FRAGMENTS .

What can I do to fix this problem?