A data transmission method in software-defined networking (SDN) environment using transmission control protocol (TCP). An application server sends a transmission resource allocation request to a controller. The request carries an address of a sending device and an address of a receiving device. The controller determines a first transmission path and a second transmission path between the sending device and the receiving device according to the request. The first transmission path is used by the sending device to transmit data to the receiving device, and the second transmission path is used by the receiving device to transmit an acknowledgment to the sending device. The controller sends a transmission resource allocation response to the application server. The application server determines, according to the response, an initial value of a transmit window for transmitting data. The sending device transmits the data to the receiving device according to the transmit window.