NAT Traversal

NAT traversal is a general term for techniques that establish and maintain Internet protocol connections traversing network address translation (NAT) gateways. Network address translation breaks end-to-end connectivity. Intercepting and modifying traffic can only be performed transparently in the absence of secure encryption and authentication. NAT traversal techniques are typically required for client-to-client networking applications, especially peer-to-peer and Voice over IP (VoIP) deployments. Many techniques exist, but no single method works in every situation since NAT behavior is not standardized. Many NAT traversal techniques require assistance from a server at a publicly routable IP address. Some methods use the server only when establishing the connection, while others are based on relaying all data through it, which adds bandwidth costs and increases latency, detrimental to real-time voice and video communications.

Most NAT behavior-based techniques bypass enterprise security policies. Enterprise security experts prefer techniques that explicitly cooperate with NAT and firewalls, allowing NAT traversal while still enabling marshalling at the NAT to enforce enterprise security policies. From this point of view, the most promising IETF standards are Realm-Specific IP (RSIP) and Middlebox Communications (MIDCOM).

SOCKS, the oldest NAT traversal protocol, is still widely available. In home or small office settings, Universal Plug and Play (UPnP) is supported by most small NAT gateways. NAT-T is commonly used by IPsec virtual private network clients in order to have Encapsulating Security Payload packets traverse NAT.