Question

c) Assume that there are two parallel TCP transmissions in the network, one using TCP tahoe (regular TCP), the other using TCP reno(the one that considers fast recovery).

Assume the initial ssthresh for both TCP transmissions is set to 16.

Assume that a timeout (without dup Acks) is detected right after the 8th RTT, and another timeout (with 3 dup Acks) is detected right after the 12th RTT.

Display the results for the first 15 rounds (20 points).

• Indicate the size of congestion window in each transmission round (one round corresponds to one RTT).

• Indicate the value of ssthresh in each round.

Note:

In congestion control, when a timeout is detected (i.e., right after the 8th RTT), and the changes will be reflected at the next RTT (i.e., at 9th RTT).

Answer 1

TCP Tahoe:

Initial congestion window(cwnd) = 1 MSS

1st RTT:

cwnd = 2 MSS

2nd RTT:

cwnd = 4 MSS

3rd RTT:

cwnd = 8 MSS

4th RTT:

cwnd = 16 MSS

ssthresh reached.

5th RTT:

cwnd = 17 MSS

6th RTT:

cwnd = 18 MSS

7th RTT:

cwnd = 19 MSS

8th RTT:(timeout)

cwnd = 20 MSS

9th RTT:

cwnd = 1 MSS

ssthresh = 10 MSS

10th RTT:

cwnd = 2 MSS

11 th RTT:

cwnd = 4 MSS.

12 th RTT:(3 dup Acks)

cwnd = 8 MSS

13th RTT:

cwnd = 1 MSS

ssthresh = 4 MSS

14th RTT:

cwnd = 2 MSS

15th RTT:

cwnd = 4 MSS

TCP Reno:

Initial congestion window(cwnd) = 1 MSS

1st RTT:

cwnd = 2 MSS

2nd RTT:

cwnd = 4 MSS

3rd RTT:

cwnd = 8 MSS

4th RTT:

cwnd = 16 MSS

ssthresh reached.

5th RTT:

cwnd = 17 MSS

6th RTT:

cwnd = 18 MSS

7th RTT:

cwnd = 19 MSS

8th RTT:(timeout)

cwnd = 20 MSS

9th RTT:

cwnd = 1 MSS

ssthresh = 10 MSS

10th RTT:

cwnd = 2 MSS

11 th RTT:

cwnd = 4 MSS.

12 th RTT:(3 dup Acks)

cwnd = 8 MSS

13th RTT:

cwnd = 4 MSS

ssthresh = 4 MSS

14th RTT:

cwnd = 5 MSS

15th RTT:

cwnd = 6 MSS