Exercises on TCP Congestion Control

Please try these yourself first before looking up solutions.

In the following, cwind stands for Congestion Window and ssthreshold stands for Slow Start Threshold.

1. 1. The difference between “Flow Control” and “Congestion Control” as applied to TCP transmission is: __________

2. When the retransmission timer expires at the sender, the value of ssthreshold is set to _______________________.

3. cwind parameter is taken from
   a) sender tcp header
   b) receiver tcp header
   c) sender tcp's window parameter
   e) receiver tcp's window
   e) None of the above

4. When TCP is in slow start, receipt of 2 ACKs before timeout results in cwind being increased by ___________

5. a) ssthreshold parameter determines ________________

6. In congestion avoidance phase, TCP's send rate increases _______

7. Why does TCP Reno get rid of slow start phase when a triple duplicate ACK is received?

8. What is “fast re-transmit”?

9. What is “fast recovery”?

10. The picture below shows the behavior of a TCP Reno.

a. Identify time intervals where TCP slow-start is operating.

b. Identify time intervals where TCP congestion-avoidance is operating

c. After the 16^th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout event?

d. After the 22^nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout event?

e. What is the ssthreshold value at the first transmission round?

f. What is the ssthreshold value at the 18^th transmission round?

g. What is the ssthreshold value at the 24^th transmission round?

h. What will be the values of cwind and ssthreshold if packet loss is detected after the 26^th round by receipt of triple duplicate ACKs?