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Unformatted text preview: Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (27 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission first example was a lost SYN to establish a connection and we saw how an exponential
backoff is applied to successive retransmission timeout values.
TCP calculates the round-trip time and then uses these measurements to keep track of a
smoothed RTT estimator and a smoothed mean deviation estimator. These two estimators are
then used to calculate the next retransmission timeout value. Many implementations only
measure a single RTT per window. Karn's algorithm removes the retransmission ambiguity
problem by preventing us from measuring the RTT when a packet is lost.
Our detailed example, which included three lost packets, let us see many of TCP's algorithms
in action: slow start, congestion avoidance, fast retransmit, and fast recovery. We were also
able to hand calculate TCP RTT estimators along with the congestion window and slow-start
threshold, and verify the values with the actual values from the trace output.
We finished the chapter by looking at the effect various ICMP errors have on a TCP
connection and how TCP is allowed to repacketize its data. We saw how the "soft" ICMP
errors don't cause a connection to be terminated, but are remembered so that if the connection
terminates abnormally, the soft error can be reported.
21.1 In Figure 21.5 the first timeout was calculated as 6 seconds and the next as 12 seconds.
If the ACK for the initial SYN had not arrived after the 12-second timeout expired, when
would the next timeout occur?
21.2 In the discussion following Figure 21.5 we said that the timeout intervals are calculated
as 6, 24, and then 48 seconds, as we saw in Figure 4.5. But if we watch a TCP connection to
a nonexistent host from an SVR4 system, the timeout intervals are 6, 12, 24, and 48 seconds.
What's going on?
21.3 Compare the performance of TCP's sliding window versus TFTP's stop-and-wait
protocol as follows....
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