Communication system and server

A method for informing a port number, the method applied to a server included in a communication system which is formed of a first terminal located behind a first NAT router, a second terminal located behind a second NAT router, and the server which is not located behind any NAT routers, and in which the first terminal and the second terminal communicate with each other through the first NAT router and the second NAT router by use of a source address, a source port number, a destination address and a destination port number, the method comprising the steps of:

• identifying the first NAT router as a NAT router of a certain type which translates the source port number into a different source port number if the destination port number is changed;
• defining, in response to reception of data transmitted from the first terminal through the first NAT router in order to inform the second terminal of a source port number which the first terminal is to use for communication between the first terminal and the second terminal in a case where the first NAT router has been identified as the certain type, a port number obtained by adding a value to the source port number used for transmission of the received data or by subtracting a value from the source port number used for transmission of the received data as a guessed port number;
• and
• informing the second terminal of the guessed port number defined by the port number guessing step.

A computer program for informing a port number, the program executed by a server included in a communication system which is formed of a first terminal located behind a first NAT router, a second terminal located behind a second NAT router, and the server which is not located behind any NAT routers, and in which the first terminal and the second terminal communicate with each other through the first NAT router and the second NAT router by use of a source address, a source port number, a destination address and a destination port number, the program comprising the steps of:

• a NAT type identifying step of identifying the first NAT router as a NAT router of a certain type which translates the source port number into a different source port number if the destination port number is changed;
• a port number guessing step of defining, in response to reception of data transmitted from the first terminal through the first NAT router in order to inform the second terminal of a source port number which the first terminal is to use for communication between the first terminal and the second terminal in a case where the first NAT router has been identified as the certain type by the NAT type identifying step, a port number obtained by adding a value to the source port number used for transmission of the received data or by subtracting a value from the source port number used for transmission of the received data as a guessed port number;
• and
• a guessed port informing step of informing the second terminal of the guessed port number defined by the port number guessing step.

A communication system which is formed of a first terminal located behind a first NAT router, a second terminal located behind a second NAT router, and a server which is not located behind any NAT routers, and in which the first terminal and the second terminal communicate with each other through the first NAT router and the second NAT router by use of a source address, a source port number, a destination address and a destination port number, wherein

• the server comprises:
• a NAT type identifying portion for identifying the first NAT router as a NAT router of a certain type which translates the source port number into a different source port number if the destination port number is changed;
• a port number guessing portion for defining, in response to reception of data transmitted from the first terminal through the first NAT router in order to inform the second terminal of a source port number which the first terminal is to use for communication between the first terminal and the second terminal in a case where the first NAT router has been identified as the certain type by the NAT type identifying portion, a port number obtained by adding a value to the source port number used for transmission of the received data or by subtracting a value from the source port number used for transmission of the received data as a guessed port number;
• and
• a guessed port informing portion for informing the second terminal of the guessed port number defined by the port number guessing portion;
• the first terminal comprises a first data transmitting portion for transmitting data for informing the second terminal of the source port number which the first terminal is to use for communication between the first terminal and the second terminal to the server through the first NAT router;
• and
• the second terminal communicates directly with the first terminal through the second NAT router and the first NAT router, using the guessed port number informed by the guessed port informing portion as the destination port number.

A communication method applied to a communication system which is formed of a first terminal located behind a first NAT router, a second terminal located behind a second NAT router, and a server which is not located behind any NAT routers, and in which the first terminal and the second terminal communicate with each other through the first NAT router and the second NAT router by use of a source address, a source port number, a destination address and a destination port number, wherein

• the server identifies the first NAT router as a NAT router of a certain type which translates the source port number into a different source port number if the destination port number is changed;
• in response to reception of data transmitted from the first terminal through the first NAT router in order to inform the second terminal of a source port number which the first terminal is to use for communication between the first terminal and the second terminal in a case where the first NAT router has been identified as the certain type, the server defines a port number obtained by adding a value to the source port number used for transmission of the received data or by subtracting a value from the source port number used for transmission of the received data as a guessed port number;
• the server informs the second terminal of the guessed port number;
• the first terminal transmits data for informing the second terminal of a source port number which the first terminal is to use for communication between the first terminal and the second terminal to the server through the first NAT router;
• and
• the second terminal communicates directly with the first terminal through the second NAT router and the first NAT router, using the guessed port number informed by the server as the destination port number.