TCP IP Illustrated

The values of a and d are not updated because of

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Unformatted text preview: 6.270796 (0.0014) slip.1024 > vangogh.discard: S 35648001:35648001(0) win 4096 <mss 256> vangogh.discard > slip.1024: S 1365512705:1365512705(0) ack 35648002 win 8192 <mss 512> slip. 1024 > vangogh.discard: . ack 1 win 4096 Figure 21.5 Timeout and retransmission of initial SYN. When the timeout occurs after 5.802 seconds, the current RTO is calculated as RTO = A + 4D = 0 + 4x3 = 12 seconds The exponential backoff is then applied to the RTO of 12. Since this is the first timeout we use a multiplier of 2, giving the next timeout value as 24 seconds. The next timeout is calculated using a multiplier of 4, giving a value of 48 seconds: 12 x 4. (These initial RTOs for the first SYN on a connection, 6 seconds and then 24 seconds, are what we saw in Figure 4.5.) The ACK arrives 467 ms after the retransmission. The values of A and D are not updated, because of Karn's algorithm dealing with the retransmission ambiguity. The next segment sent is the ACK on line 4, but it is not timed since it is only an ACK. (Only segments containing data are timed.) When the first data segment is sent (segment 1 in Figure 21.2) the RTO is not changed, again owing to Karn's algorithm. The current value of 24 seconds is reused until an RTT measurement is made. This means the RTO for time 0 in Figure 21.4 is really 24, but we didn't plot that point. When the ACK for the first data segment arrives (segment 2 in Figure 21.2), three clock ticks were counted and our estimators are initialized as A = M + 0.5 = 1.5 + 0.5 = 2 D = A/2 = 1 (The value 1.5 for M is for 3 clock ticks.) The previous initialization of A and D to 0 and 3 was for the initial RTO calculation. This initialization is for the first calculation of the estimators using the first RTT measurement M. The RTO is calculated as RTO = A + 4D = 2 + 4x1 = 6 seconds file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (10 of 29) [12/09/2001 14.47.26] Chapter 21. TCP Timeout and Retransmission When the ACK for the second...
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