1. Communication Networks Recitation 12: TCP Rehearsal

2. Comnet 20062 Observe the diagram 1 6 1617222326 1 21 26 32 42 8

3. Comnet 20063 Answer the following Identify Slow Start rounds.Identify Slow Start rounds. Identify Congestion Avoidance rounds.Identify Congestion Avoidance rounds. At 16 th round, is it timeout or triple duplicate ACK?At 16 th round, is it timeout or triple duplicate ACK? At 22 nd round, is it timeout or triple duplicate?At 22 nd round, is it timeout or triple duplicate? What is the Slow Start Threshold at round 1?What is the Slow Start Threshold at round 1? What is the Slow Start Threshold at round 24?What is the Slow Start Threshold at round 24? If a packet loss is detected after the 26 th round by the receipt of a triple duplicate ACK, what will be the value of the congestion-window size?If a packet loss is detected after the 26 th round by the receipt of a triple duplicate ACK, what will be the value of the congestion-window size? 1 6 1617222326 1 21 26 32 42 8

4. TCP Vegas The following parameters are given for a TCP Vegas connection  =50 KB/sec  =75 KB/sec  =50 KB/sec  =75 KB/sec RTT-Base=40 msecRTT-Base=40 msec RTT is always 50 msecRTT is always 50 msec Assuming packets are never lost:Assuming packets are never lost: What is the minimal and maximal equilibrium windows?What is the minimal and maximal equilibrium windows? What will happen if RTT will increase to 100ms?What will happen if RTT will increase to 100ms? 4

5. Comnet 20065 TCP Vegas - Reminder BaseRTT is the minimum of all measured RTTs (commonly the RTT of the first packet)BaseRTT is the minimum of all measured RTTs (commonly the RTT of the first packet) If not overflowing the connection, thenIf not overflowing the connection, then ExpectRate = CongestionWindow/BaseRTTExpectRate = CongestionWindow/BaseRTT Source calculates ActualRate once per RTTSource calculates ActualRate once per RTT Source compares ActualRate with ExpectRateSource compares ActualRate with ExpectRate –Diff = ExpectedRate - ActualRate –if Diff < a –increase CongestionWindow linearly –else if Diff > b –decrease CongestionWindow linearly –else –leave CongestionWindow unchanged

6. Comnet 20066 Fall 2002/3 Give an example to a case where the receiver of a TCP connection sends a packet where the receiver window size is zero

7. Comnet 20067 Zero Windows suppose the window size is reduced to zero: receiver ACKs, and says it has a window size of zero, since all buffers are fullsuppose the window size is reduced to zero: receiver ACKs, and says it has a window size of zero, since all buffers are full later the receiver reopens the window by sending a new ACK with a new windowlater the receiver reopens the window by sending a new ACK with a new window that ACK is lost!that ACK is lost! deadlock: sender is waiting for window update, receiver is waiting for new datadeadlock: sender is waiting for window update, receiver is waiting for new data

8. Comnet 20068 Fall 2002/3 cont. During slow start, TCP sender increases window by 1 for every received ACK. Explain how the linear increment results in an exponential growth in transmission rate

9. Comnet 20069 Fall 2002/3, cont. When routers run distance vector protocol for network routing, is it possible that a packet will go through the same router more than once?

10. Comnet 200610 Spring 2000-2001 B TCP connection uses flow control to pass a file with the following parameters file size is 51KBfile size is 51KB receiver window is always 12KBreceiver window is always 12KB congestion window is initially 1.5KBcongestion window is initially 1.5KB maximal segment size is 1.5KBmaximal segment size is 1.5KB timout is 50mstimout is 50ms ssthresh is 10KBssthresh is 10KB RTT is always 30msRTT is always 30ms

11. Comnet 200611 Spring 2001 B – cont. In every round, an entire window is sent, and after RTT all ACKs arriveIn every round, an entire window is sent, and after RTT all ACKs arrive fast retransmit is inactivefast retransmit is inactive Packet 4 is lostPacket 4 is lostAssumptions receiver does not accumulate packets not in orderreceiver does not accumulate packets not in order After timeout window is cut to half (with no slow start)After timeout window is cut to half (with no slow start)

12. Comnet 200612 Spring 2001 B - Solution sent bytes cwnd #round 1 2 3 4 5 6 7 8 9 10 1.5KB 1.5KB (tot. 1.5) 3KB 3KB (tot. 4.5) 6KB 6KB (no ACKs!) 3KB (timeout) 3KB (tot. 7.5) 4.5KB 4.5KB (tot. 12) 6KB 6KB (tot. 18) 7.5KB 7.5KB (tot. 25.5) 9KB 9KB (tot. 34.5) 10.5KB 10.5KB (tot. 45) 12KB 6KB (tot. 51)