A system allows data transfer in which the most appropriate packet processing is executed according to the performance of a terminal. A data transmission side stores scalable-encoded data in a packet as a payload, assigns priority-level information corresponding to a layer level of the encoded data stored in the packet to a packet header, and transmits the packet. A data receiving side performs processing by referring to the priority-level information. A level of priority dependent on an application is specified in an RTP payload header according to the degree of importance of scalable-encoded data, and a level of priority is further specified in an IP header. The most appropriate packet processing based on the performance of a terminal is allowed. Re-transmission control based on the layer level of encoded data is also allowed.