9Receiver Send AckShort time to transmit ACKFig 1.24
10ACK Propagates First bit of ACK arrives at receiver -> Propagation delay… same as beforeFig 1.24
11Continuous RQ Sender continues sending, eventually acks arrive from the receiver At thesame rate as they departfrom the transmitterNo ErrorsIdealisticFig 1.26
12Aggressive Sending Strategy but messages are lost What is the strategybeing used by thereceiver whenreceiving messagesout of sequence?Receiver recognizesSender knows N+1 problemSends in next slotFig 1.27
17Idle RQ Send a message Do not send the next until ACK is received Messages and ACKS numbered 0/1/0/1/….If “a” is large,capacity of link may be severely underutilizedOverall user delay may be excessive (why wait)
18Selective repeat Aggressive sending Multiple outstanding messages (w/o ACK)If error at receiverReceiver keeps later messages buffered at a lower level and only passes them up when the missing frames arriveSender only resends error messagesExtra management effort required at receiver
19Go-Back-N Aggressive sending Multiple outstanding messages (w/o ACK) If error at receiverReceiver REJECTs out of order messagesSender only resends ALL messages from point of errorSimple management effort required at receiver
29PiggyBacked ACKs Traffic frequently going in the reverse direction No send to send an entire new frame with additional overhead.Typically, a message contains the name of the message in transit AND the name of the ack message, message expected in the other direction
30What if nothing in the other driection? Set a timer waiting for return travelIf no return travel before the timer goes off,Send a separate ACK when the timer expires.ELSEPiggyback on return traffic.
31What about reverse traffic Discussion only focuses on traffic in one directionProblem is symmetric.Software is tricky but the send and receive code are integrated into the same process.
54Information Exchange Station A Station B SEND I Frames N(R)=0 N(S)=0 Frame DamagedN(R)=1 N(S)=2Frame RejectedSend REJ with N(R)=1SEND I FramesN(R)=2 N(S)=1(no activity)N(R)=2 N(S)=2Send RRJ with N(R)=3N(R)=2 N(S)=3