recvfrom a large packet

if i do a recvfrom (sk, buf, 10000, 0, &addr, &len), shall i recv all the data i mean the 10000 bytes? since fragmentation happen in the ip layer and assembled happen in the ip layer it doesnt matter for the upper layer about the packet size. i wrote a test code and it seems to be working. is there any problem will come if i turn on firewall. any ideas?