TCP hole punching

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TCP NAT traversal and TCP hole punching (sometimes NAT punch-through) in computer networking occurs when two hosts behind a network address translation (NAT) are trying to connect to each other with outbound TCP connections. Such a scenario is particularly important in the case of peer-to-peer communications, such as Voice-over-IP (VoIP), file sharing, teleconferencing, chat systems and similar applications.

TCP hole punching is an experimentally used NAT traversal technique for establishing a TCP connection between two peers on the Internet behind NAT devices. NAT traversal is a general term for techniques that establish and maintain TCP/IP network and/or TCP connections traversing NAT gateways.