1. Fairness
Suppose there are two connections, A and B, sharing a 10 Kbit link. Let us simplify the TCP congestion control protocol to include the following assumptions:
• 1 MSS = 1000 bits
• the two connections increase their window sizes by 1 MSS per RTT, in lock-step, whenever their aggregate bandwidth is less than or equal to 10 Kbits
• when the aggregate bandwidth is greater than 10 Kbits, both connections simultaneously decrease their window size to 1/2 of the previous size. The window is rounded up to the next MSS size.
(a) Suppose A's window is currently 9000 bits and B's window is 1000 bits, and both their RTTs are 1s. Show how the window sizes change after 20s
(b) Suppose A and B both have a window of 1000 bits, A has RTT of 1s and B has RTT of 2s. Show how the window sizes change after 20s. (Assume that whenever the aggregate bandwidth exceeds 10 Kbit, the next window of each of the connection will be cut in half.)
Time
A’s window
B’s window
0
9000
1000
1
10000
2000
2
5000 (half)
1000(half)
3
6000
4
7000
3000
5
8000
4000
6
4000(half)
2000(half)
7
5000
8
9
10
3000(half)
11
12
13
14
15
16
17
18
19
20
5000(half)
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