Question
Consider the network described in the previous problem. Now suppose that the two TCP connections, C1 and C2, have the same RTT of 100 msec. Suppose that at time t 0 , C1’s congestion window size is 15 segments but C2’s congestion window size is 10 segments.
a. What are their congestion window sizes after 2200msec?
b. In the long run, will these two connections get about the same share of the bandwidth of the congested link?
c. We say that two connections are synchronized, if both connections reach their maximum window sizes at the same time and reach their minimum window sizes at the same time. In the long run, will these two connections get synchronized eventually? If so, what are their maximum window sizes?
d. Will this synchronization help to improve the utilization of the shared link? Why? Sketch some idea to break this synchronization.
Answer
a)
Compute the window sizes over the time as shown in the table. The TCP connections C1 and C2 have same window size 2 after 2200msec
The RTT for TCP connections is 100msec.
Congestion window size of C1’s is 15 segments and congestion window size of C2’s is 10 segments.
|
|
|
|
|
|
|
Time is sec |
Window size=No.of segments sent in next 100msec |
Data sending speed=segments per second(window size/0.1) |
Window size=No.of segments sent in next 100msec |
Data sending speed=segments per second |
|
0 |
15 |
150 |
10 |
100 |
|
100 |
7 |
70 |
5 |
50 |
|
200 |
3 |
30 |
2 |
20 |
|
300 |
1 |
10 |
1 |
10 |
|
400 |
2 |
20 |
2 |
20 |
|
500 |
1 |
10 |
1 |
10 |
|
600 |
2 |
20 |
2 |
20 |
|
700 |
1 |
10 |
1 |
10 |
|
800 |
2 |
20 |
2 |
20 |
|
900 |
1 |
10 |
1 |
10 |
|
1000 |
2 |
20 |
2 |
20 |
|
1100 |
1 |
10 |
1 |
10 |
|
1200 |
2 |
20 |
2 |
20 |
|
1300 |
1 |
10 |
1 |
10 |
|
1400 |
2 |
20 |
2 |
20 |
|
1500 |
1 |
10 |
1 |
10 |
|
1600 |
2 |
20 |
2 |
20 |
|
1700 |
1 |
10 |
1 |
10 |
|
1800 |
2 |
20 |
2 |
20 |
|
1900 |
1 |
10 |
1 |
10 |
|
2000 |
2 |
20 |
2 |
20 |
|
2100 |
1 |
10 |
1 |
10 |
|
2200 |
2 |
20 |
2 |
20 |
b)
These two TCP connections will not get the same share of the bandwidth of the congested link in the long run.
c) In the above table, the maximum window size is 2. So, these two TCP connections will get synchronized at the window sizes is 2.
d) This synchronization will not help to improve the utilization of the shared link because two TCP connections will act as a single link. The window size is varying between minimum and maximum.
An idea to break the synchronization is to add finite buffer to the link and randomly drop the packets in the buffer before buffer overflow.This is caused to different connections cut their window sizes at different times.