Understanding TCP/IP Model Internals and Interfaces
Understanding TCP/IP Model Internals and Interfaces
Understanding TCP/IP Model Internals and Interfaces
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1508 <strong>Underst<strong>and</strong>ing</strong> <strong>TCP</strong>/<strong>IP</strong> <strong>Model</strong> <strong>Internals</strong> <strong>and</strong> <strong>Interfaces</strong><br />
3. How did the server recover from packet loss, by using fast retransmit or timeout retransmission?<br />
______________<br />
4. How many times was a slow start performed during the session lifetime? ___________<br />
Retransmission Pattern<br />
Examine sequence number of forwarded <strong>and</strong> dropped segments graph to answer the following.<br />
5. How many segments were retransmitted? ___________<br />
6. Were any segments retransmitted unnecessarily? ___________<br />
7. When is the last segment sent? ___________<br />
Detailed Analysis<br />
The size of the congestion window just before fast retransmission was 12,328 bytes. After fast<br />
retransmission, the congestion window is reset to<br />
12,864 / 2 + 3 * MSS = 6,432 + 3 * 536 = 8, 040<br />
Note: In this calculation, when the congestion window is halved, it is rounded down to an integral<br />
multiple of the MSS. In this example, 12,864 is 24 * 536, the result of the division is also an integral<br />
multiple of the MSS. If the window had been 23 * 536 = 12,328, then the result would be 11.5 *<br />
536 = 6,164, which must be rounded down to 11 * 536 = 5,895.<br />
Note: 12,328/2 is rounded down to the increments of MSS, rather than to the actual value (6,164).<br />
The process then continues receiving more duplicate ACKs <strong>and</strong> increases its congestion window by<br />
one MSS for each received ACK. Unlike in Reno, the ACK for the retransmitted segment 23 does<br />
not take New Reno out of the recovery process. New Reno will keep increasing its congestion<br />
window for each received ACK until the ACK for the last segment sent before retransmission is<br />
received (which is segment 46). When the client receives the retransmitted segment 28, then it will<br />
send an acknowledgement for all the outst<strong>and</strong>ing segments. When the sender gets this<br />
acknowledgement, fast recovery ends, <strong>and</strong> the sender enters slow start.<br />
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© 2010 OPNET Technologies, Inc.<br />
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