rfc3489bis Procedures

If the Binding Request contains the appropriate cookie in its Transaction ID, the server constructs a Binding Response populated with the XOR-MAPPED-ADDRESS attribute. That attribute will contain the source IP address and port from which the server saw the request come. Then the server encodes and sends the message to the client from the IP address and port on which the request was received. The message is sent to the IP address and port from which the request came.