In selective retransmission protocol, as long as the receiver to receive their desired data frame immediately delivered to the network layer and send an ACK frames, so now understand the data frame will trigger an ACK is not received, but the data frame delivery events trigger an ACK, such understanding can explain the disorderly data frames arrive without an ACK, because there is no data delivery, if the data frame without any mistakes, I now understand to the receiving party receives a frame data is uploaded a frame data and trigger an ACK event, if there is a frame of data is missing, it will arrive after the disorder of cached data frames are waiting to send the frame data will be lost to send to come over, to be missing a copy of the frame arrived to meet a set of consecutive frames is delivered to the condition of the network layer and trigger an ACK event, and it is the ACK for more than a set of frames, so this analysis confirmed there are both one by one to confirm and accumulative total,