Patent ID: 8406242

Claim:
A communication system for performing communication based on Real-time Transport Protocol/RTP Control Protocol (RTP/RTCP) comprising: a communication control device configured to be connected to an external network; a caller terminal device configured to be connected to an internal network that is subordinate to a first NAT device, the first NAT device configured to be connected to the external network and provided with a NAT function; and a callee terminal device configured to be connected to another internal network that is subordinate to a second NAT device, the second NAT device configured to be connected to the external network and provided with the NAT function, wherein the caller terminal device includes: a first transmission unit that transmits a first packet through the first NAT device to the communication control device; a first receiving unit that receives from the communication control device a second packet that contains a first port number and a second port number, the first port number being stored in the first NAT device in a case where the first NAT device received the first packet that was transmitted by the caller terminal device, the second port number being stored in the second NAT device in a case where the second NAT device received a third packet that was transmitted by the callee terminal device; a first determination unit that determines whether the second port number that is contained in the second packet that was received by the first receiving unit is one of an odd number and an even number; and a second transmission unit that transmits, in a case where it has been determined that the second port number is an odd number, an RTP packet to a port number that is computed by adding 2n−1 (n being an integer that is not less than 1) to the second port number, then transmits an RTCP packet to a port number that is computed by adding 2n to the second port number, and transmits in a case where it has been determined that the second port number is an even number, the RTP packet to a port number that is computed by adding 2n to the second port number, then transmits the RTCP packet to a port number that is computed by adding 2n−1 to the second port number, the callee terminal device includes: a second receiving unit that receives the first packet transmitted from the communication control device; and a third transmission unit that transmits the third packet through the second NAT device to the communication control device, the communication control device includes: a third receiving unit that receives the first packet that is transmitted from the caller terminal device; a fourth transmission unit that transmits the received first packet to the callee terminal device; a fourth receiving unit that receives the third packet that is transmitted from the callee terminal device; and a fifth transmission unit that transmits the second packet containing the first port number and the second port number to the caller terminal device.