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Unformatted text preview: e transmitted, and 18 RTT samples were collected. Figure 21.4
shows the measured RTT (taken from the tcpdump output) along with the RTO used by TCP for the file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (8 of 29) [12/09/2001 14.47.26] Chapter 21. TCP Timeout and Retransmission timeout (taken from the socket debug output). The x-axis starts at time 0 in Figure 21.2, when the first data
segment is transmitted, not when the first SYN is transmitted. Figure 21.4 Measured RTT and TCP's calculated RTO for example.
The first three data points for the measured RTT correspond to the 3 RTTs that we show in Figure 21.2. The
gaps in the RTT samples around times 10, 14, and 21 are caused by retransmissions that took place there
(which we'll show later in this chapter). Kam's algorithm prevents us from updating our estimators until
another segment is transmitted and acknowledged. Also note that for this implementation, TCP's calculated
RTO is always a multiple of 500 ms. RTT Estimator Calculations
Let's see how the RTT estimators (the smoothed RTT and the smoothed mean deviation) are initialized and
updated, and how each retransmission timeout is calculated.
The variables A and D are initialized to 0 and 3 seconds, respectively. The initial retransmission timeout is
calculated using the formula
RTO = A + 2D = 0 + 2x3 = 6 seconds (The factor 2D is used only for this initial calculation. After this 4D is added to A to calculate
RTO, as shown earlier.) This is the RTO for the transmission of the initial SYN.
It turns out that this initial SYN is lost, and we time out and retransmit. Figure 21.5 shows
the first four lines from the tcpdump output file. 1 0.0 slip.1024 > vangogh. discard: S
win 4096 <mss 256> file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (9 of 29) [12/09/2001 14.47.26] Chapter 21. TCP Timeout and Retransmission 2 5.802377
(5.8024) 3 6.269395
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12