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The massive deployment of NAT/firewall devices in the Internet has greatly affected its end-to-end connectivity. Several applications, in particular Grid computing systems which span several Autonomous Domains require the communication among hosts behind NAT/firewall. Despite the existence of successful techniques for the establishment of UDP flows between hosts behind NAT/firewall, the same does not hold for TCP. Furthermore, existing techniques must be implemented individually by each application, possibly causing code duplication, or depend on relay servers, making it prone to performance problems. This work proposes a strategy that allows application processes behind NAT/firewall to communicate transparently, on top of any transport protocol. The system works by establishing IPv6-over-UDP tunnels between hosts, in which IPv6 packets are encapsulated within UDP data grams and are sent through a UDP hole punching session. A detailed description of the proposed system, case studies and experimental results are presented.