Receiver Send Ack Short time to transmit ACK Fig 1.24
ACK Propagates First bit of ACK arrives at receiver -> Propagation delay… same as before Fig 1.24
Continuous RQ Sender continues sending, eventually acks arrive from the receiver At the same rate as they depart from the transmitter No Errors Idealistic Fig 1.26
Aggressive Sending Strategy but messages are lost Receiver recognizes Sender knows N+1 problem Sends in next slot What is the strategy being used by the receiver when receiving messages out of sequence? Fig 1.27
Idle 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 underutilized –Overall user delay may be excessive (why wait)
Selective repeat Aggressive sending Multiple outstanding messages (w/o ACK) If error at receiver –Receiver keeps later messages buffered at a lower level and only passes them up when the missing frames arrive –Sender only resends error messages –Extra management effort required at receiver
Go-Back-N Aggressive sending Multiple outstanding messages (w/o ACK) If error at receiver –Receiver REJECTs out of order messages –Sender only resends ALL messages from point of error –Simple management effort required at receiver
PiggyBacked 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
What if nothing in the other driection? Set a timer waiting for return travel If no return travel before the timer goes off, –Send a separate ACK when the timer expires. ELSE –Piggyback on return traffic.
What about reverse traffic Discussion only focuses on traffic in one direction Problem is symmetric. Software is tricky but the send and receive code are integrated into the same process.
Information Exchange Station AStation B SEND I Frames N(R)=0 N(S)=0 SEND I Frames N(R)=0 N(S)=0 N(R)=0 N(S)=1 Frame Damaged N(R)=1 N(S)=2 Frame Rejected Send REJ with N(R)=1 SEND I Frames N(R)=2 N(S)=1 N(R)=2 N(S)=2 N(R)=2 N(S)=3 (no activity) Send RRJ with N(R)=3