Question-1 (25 points) Consider the graph below modeling behavior experienced by
ID: 3712121 • Letter: Q
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Question-1 (25 points) Consider the graph below modeling behavior experienced by a TCP protocol. The graph should be read to mean that Congwin-1 during round 1, Congwin-2 during round 2, Congwin-4 during round 3, etc. 25 o 20 15 1 2 3 45 6 7 8 9 1011 12 13 14 15 16 17 18 19 20 21 Transmission round Answer the following questions. In all cases give a short explanation as to how you derived your answer a) Is this TCP RENO or TCP Tahoe? b) A loss event occurs during the 11 th transmission round. What type of loss event occurs? c) A loss event occurs during the 17th transmission round. What type of loss event occurs? d) What is the initial value of Threshold during the first transmission round? e) Suppose there is no loss event during the 21st transmission round. What will the value of Congwin be during the 22nd round?Explanation / Answer
TCP basically provides a reliable transport service between two processes running on different hosts.
The main difference between TCP Tahoe and TCP Reno resides in the congestion estimation part. In TCP Tahoe each packet loss is considered as a serious congestion problem, resulting in setting the CWND size to minimum value after each packet loss. While in the Reno version, TCP can differentiate between a two cases:
• If packet loss was found by Retransmit Timeout then the network suffers from a
serious congestion problem. TCP here sets the window size to minimum value and
enters slow start phase.
• If packet loss was found by duplicate ACK's, then the congestion is not severe
because of the following:
1. At least 3 packets could arrive at the receiver after packet loss.
2. At least 3 packets left the network, so there may be a chance to transmit a packet.
Then,CWND(congestion window/Congwin) size is set to half the current CWND and TCP transits to congestion avoidance phase.
Answering your questions :
a) This is TCP Reno since In TCP Reno, when TCP does fast re-transmit instead of beginning a slow-start phase with cwind set to 1, it goes directly to congestion-avoidance phase, with cwind set to ½ its value as seen from the graph.
b) During the 11th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1.
c) During the 17th transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1 (remember that this means that TCP can send up to 1 MSS ).
d) The initial threshold value is 16, since it is at this window size that slow start stops and congestion avoidance begins.
e) Since there is no loss event during the 21st transmission round the value of Congwin will not be halved and hence will be 8.
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