Implement custom network protocol for local processes

Hi, How can one implement custom header network protocol. I mean header that wraps regular network packets like ethernet, udp, tcp, etc... The sending of this custom packets is done between processes running locally. ie. I get a Ethernet packet, inspect it, then add my custom header and forward it to another process that listens to a socket, and expect to get data wrapped with this custom header. What is a good way to go about and implement this thing? Thanks, Kfir