Why I have to increase MAXBIGPKTS to make the UDP transfer

No the problem is in fact the other way round. The TCP/IP stack waits until a packet has been sent before preparing a new one, so you won't run out of buffers when sending a lot of UDP packets. It will just be slower.

On the receive side, it is another story. Each time an ethernet packet arrives, the LAN chip wakes up the CPU and call an interrupt routine. This routine will then allocate one buffer from the pool and put the data in it. Then the TCP/IP stack will wake up, read and process the packet contents, and frees the buffer. If the Nios system is receiving more packets that it can handle (either because the incoming bitrate is too high or because the CPU is too busy doing something else) then you will run out of buffers.

So what you must check is:

• What kind of packets are transmitted to your NIOS system? From what you are telling you are mostly sending packets, so it's strange that you are receiving so many packets. You can use a software such as Wireshark on your PC to see all the packets that are received/transmitted

• Do you have high priority tasks that eat all your CPU power, leaving not enough cycles to receive packets properly?