3.1.5.3 proxy=replace Extension for Firewall Traversal
Session Initiation Protocol Extensions introduces a new header parameter, proxy=replace, to enable firewall traversal for the SIP channel. This parameter tells the outbound proxy to replace the contact information in the contact header with its own, enabling other clients/servers to reach the client using the proxy's IP address, even if the client is behind a firewall. The client SHOULD include a header parameter proxy=replace in the "Contact" header if it wants to enable this extension. The client SHOULD also include a maddr URI parameter containing its IP address. The outbound proxy SHOULD replace the IP address in the maddr parameter with its own IP address if the proxy=replace header parameter is present. Any entity receiving this contact header SHOULD send any new requests to the new IP address in the maddr parameter, which is the IP address of the proxy. The proxy SHOULD then route this request to the client.