Understanding TCP/IP Model Internals and Interfaces

1508 Understanding TCP/IP Model Internals and Interfaces
Results Analysis
Congestion Window
Examine TCP Connection.Congestion Window Size (bytes) graph to answer the following.
To get an exact value, position your mouse in the desired region, and read the tooltip for the Nearest
Point:
1. What was the congestion window size before the packet loss was detected? ___________
2. What did the congestion window size value change to after detecting packet loss? _________
3. Why did the congestion window keep increasing after the loss was detected? ___________
4. Why did the congestion window stay flat for a large period? ___________
5. How did the server recover from packet loss—using fast retransmit or timeout retransmission?
______________
6. How many times was a slow start performed during the session lifetime? ___________
Retransmission Pattern
Examine sequence number of forwarded and dropped segments graph to answer the following.
To get an exact value, position your mouse in the desired region and read the tooltip for the Nearest
Point.
7. How many segments were retransmitted? ___________
8. Were any segments retransmitted unnecessarily? ___________
9. When was the last segment sent? ___________
Detailed Analysis (Homework)
The size of the congestion window just before fast retransmission was 12,864 bytes. The congestion
window after fast retransmission is reset to
12,864 / 2 + 3 * MSS = 6,432 + 3 * 536 = 8, 040
Note: In this calculation, when the congestion window is halved, it is rounded down to an integral
multiple of the MSS. In this example, 12,864 is 24 * 536, the result of the division is also an integral
multiple of the MSS. If the window had been 23 * 536 = 12,328, then the result would be 11.5 *
536 = 6,164, which must be rounded down to 11 * 536 = 5,895.
The sender then continues receiving more duplicate ACKs and increases its congestion window by
one MSS for each ACK. At time 1.086 seconds, the ACK for retransmitted segment 23 takes the
server out of fast recovery and the congestion window is set to half of the pre-fast retransmission
value. The sender then receives 3 duplicate ACKs for segment 26 and resends segment 27 using the
fast retransmission algorithm. However, because the server does not receive more ACKs, its
congestion window could not increase further. To be able to send more data, it has to wait for the
retransmission timer to expire. This increases application response time considerably.
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