Suppose we have two network en

Suppose we have two network entities, A and B. B has a supply of data messages that will be sent to A according to the following conventions. when A gets a request from the layer above to get the next data (D) message from B, A must send a request(R) message to B on the A-to-B channel. Only when B receives an R message can it send a data (D) message back to A on the B-to-A channel. A should deliver exactly one copy of each D message to the layer above. R messages can be lost(but not corrupted) in the A-to-B channel; D messages, once sent, are always delivered correctly. the delay along both channels is unknown and variable.
Design(give an FSM description of) a protocol that incorporates the appropriate mechanisms to compensate for the loss-prone A-to-B channel and implements message passing to the layer above at entity A, as discussed above. Use only those mechanisms that are absolutely necessary.