Data send thru ethernet

Well I am on a crossroad then. I still want to generate UDP checksum in hardware even if it is not necessary, however now I have two possible solutions:

1) Generate payload data + add UDP header (including checksum) in hardware. Then forward this packet to Nios cpu which is running InterNiche, so the stack should add IP header by itself. Then I have another problem: where to find documentation how to calculate and add IP header to incoming packet from the subsystem? I suppose InterNiche has functions for IP header calculation, so I need only pass the data to the stack.

2) Generate payload data + UDP header + IP header in hardware and pass it to Nios+InterNiche. However I still need some documentation how to pass these packets to InterNiche and then to the network.

Btw: I am now trying to do packet checksum by hand. I have a real packet sniffed from network with all the parameters available, but no matter how I try to calculate the checksum, I get it different from the correct one. Maybe there are checksum calculators? I think I may do wrong calculations because of wrong byte order or something else. I've taken the shortest packet, which consists only 18 byte data, so in this case I should get no carry bits and do calculations without carry bit mistake. Maybe someone have a good reference how to calculate the UDP checksum correctly in hardware?