building packets in libnet

Hi,

I am using libnet to create packets in a small network of two hosts back to back. Traffic I want is tcp/ip/ethernet. Is it necessary that the payload be attached to the packet using libnet_build_tcp() function itself? What happens if payload is left NULL in tcp build phase and attach it in build_ip() or build_ethernet() phase? I tried it, no error happens at any stage. But there is no traffic in the network. Is it possible at all in theory, to use the payload field in any of the build functions? The MAC address of src and destination are there accurately in the beginning of the packet. Libnet version is 1.1.1 

thanx
bf2