rdt1.0
Assumptions: no error occurs in the transmission path, packet loss (reliable channel)
Process: pure sent by the sender, recipient receives
rdt2.0
Assumptions: the premise 1.0, adding the possibility of error (bit error), still no loss
Improvement: added to stop the other protocols, error detection, the receiving side will send ACK / NAK, a retransmission mechanisms resolve the error.
Drawback: ACK / NAK error may also occur
rdt2.1
Assumptions: Like 2.0
Improvement: plus sequence number (0,1) for each packet, see sender ACK / NAK is correct, while the recipient to see whether to repeat
rdt2.2
Assumptions: Like 2.0
Improvement: only ACK, cancel NAK. It was added last acknowledgment received in the ACK packet. Because there is stop and other agreements, or only 0 and 1 will suffice
rdt3.0
Assumptions: 2.0 based on adding packet loss
Improvement: adding a timer, after which time automatic retransmission.
Disadvantages: Due to the performance of other agreements to stop the very poor