SYNI - TCP Hole Punching Based on SYN Injection

The shortage of IPv4 addresses and the very slow transition to IPv6 leads to pragmatic solutions in the Internet: today many hosts are still using IPv4 and are connected to the Internet over a Network Address Translation (NAT) router. However, there are many applications, which need inbound connections, like e.g. peer-to-peer-based systems or voice-over-IP. For such NATed hosts inbound connections usually pose a problem, since without additional measures the router filters the incoming connection attempts. These additional measures are usually referred to as NAT traversal mechanisms and hole punching is one of those techniques. In this paper we propose a new protocol for a TCP-based hole punching mechanism based on self-injecting SYN-packets in the local network stack.