Understanding TCP/IP Model Internals and Interfaces

1508 Understanding TCP/IP Model Internals and Interfaces
___________
4. 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.
5. How many segments were retransmitted? ___________
6. Were any segments retransmitted unnecessarily? ___________
7. When was the last segment sent? ___________
Detailed Analysis
Sequence Numbers and ACK Numbers
Note: Segments are identified by their segment sequence numbers (e.g., segment 1, 2…) In the
following text, the term “ACK for segment X” means that segment X has been received successfully
and the next expected segment is segment X + 1.
The server retransmits segment 23 after it has received a third duplicate ACK and set its congestion
window to one MSS. After the client receives the retransmitted segment, it also processes the
previously received segments 24, 25, and 26 and sends an ACK acknowledging segments 23 through
26. This allows the receiver to increase its congestion window to 2 and resend segments 27 and 28.
The ACK for segment 27 increases the server’s congestion window to 3 and segments 29 and 30 can
be resent. The ACK for segment 28 acknowledges all segments up through segment 46 (which has
sequence number 24,121). (These segments had been sent before the server received the third
duplicate ACK and cut the congestion window.) The server then continues sending unsent data, but
with the reduced congestion window.
Notice that segments 29 and 30 were unnecessarily retransmitted, and TCP Tahoe recovers from the
packet lost without retransmission timeout expiration.
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