Abstract: A data communication protocol is implemented to control data flow between a transport layer and a data link layer. The protocol provides two manners of data flow control. (1) In the first control manner, when the occupancy of packet buffers of the data link layer is smaller than a first threshold, the data link layer informs of the transport layer that the packet transport should be suspended, and thereafter when the occupancy is smaller than a second threshold smaller than the first one, the data link layer requests the transport layer to resume the packet transfer. (2) In the second control manner, when the amount of packets is smaller than a value determined based on the transfer rate and maximum burst size, the transport layer transfers a packet to a lower layer, and when the packet amount is larger than the value, a packet is transferred after a predetermined time passes to control the transfer rate to a predetermined one.