A transmitting apparatus comprising: a user input unit configured to receive a user input; a communication interface configured to transmit a check packet to a receiving apparatus using a Transmission Control Protocol; and circuitry (21, 29) configured to: receive an acknowledgement message to the check packet transmitted using TCP from the receiving apparatus, the acknowledgement message including acknowledgement information based on shared data shared with the data receiving apparatus; compute expected acknowledgment information based on the shared data shared with the data receiving apparatus; produce a comparison result for the data receiving apparatus based on the acknowledgment information and the expected acknowledgment information; determine whether a response time of the acknowledgement message is less than a predetermined time; and prohibit transmission of content data to the data receiving apparatus when the comparison result indicates that acknowledgment information and the expected acknowledgment information do not match or when the determining determines that the response time of the acknowledgment message is larger than the predetermined time. a user input unit configured to receive a user input; a communication interface configured to transmit a check packet to a receiving apparatus using a Transmission Control Protocol; and circuitry (21, 29) configured to: receive an acknowledgement message to the check packet transmitted using TCP from the receiving apparatus, the acknowledgement message including acknowledgement information based on shared data shared with the data receiving apparatus; compute expected acknowledgment information based on the shared data shared with the data receiving apparatus; produce a comparison result for the data receiving apparatus based on the acknowledgment information and the expected acknowledgment information; determine whether a response time of the acknowledgement message is less than a predetermined time; and prohibit transmission of content data to the data receiving apparatus when the comparison result indicates that acknowledgment information and the expected acknowledgment information do not match or when the determining determines that the response time of the acknowledgment message is larger than the predetermined time. receive an acknowledgement message to the check packet transmitted using TCP from the receiving apparatus, the acknowledgement message including acknowledgement information based on shared data shared with the data receiving apparatus; compute expected acknowledgment information based on the shared data shared with the data receiving apparatus; produce a comparison result for the data receiving apparatus based on the acknowledgment information and the expected acknowledgment information; determine whether a response time of the acknowledgement message is less than a predetermined time; and prohibit transmission of content data to the data receiving apparatus when the comparison result indicates that acknowledgment information and the expected acknowledgment information do not match or when the determining determines that the response time of the acknowledgment message is larger than the predetermined time.