Session border controller and data transmission method

The invention discloses a session border controller and a data transmission method and belongs to the technical field of Internet. The method comprises the following steps that a data through layer receives a to-be-transmitted data packet, which is sent by a session initiation terminal through a first designated port when, under a restricted network and sends the information of the to-be-transmitted data packet and the first designated port to a data processing layer; the data processing layer receives the information of the to-be-transmitted data packet and the first designated port, stores a mapping relation between the information of the first designated port and the information of the data through layer and sends the to-be-transmitted data packet to a data transmission layer; and the data transmission layer receives the to-be-transmitted data packet and sends the to-be-transmitted data packet to an external network, and the external network sends the to-be-transmitted data packet to a session receiving te