TCP IP Illustrated

This is the fast recovery algorithm figure 2111 is

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Unformatted text preview: isually between the exponential increase during slow start and the additive increase during congestion avoidance, because the slow start phase is so quick. We need to explain what is happening at the three points where a retransmission occurs. Recall that each of the retransmissions took place because three duplicate ACKs were received, indicating a packet had been lost. This is the fast retransmit algorithm from Section 21.7. ssthresh is immediately set to one-half the window size that was in effect when the retransmission took place, but cwnd is allowed to keep increasing while the duplicate ACKs are received, since each duplicate ACK means that a segment has left the network (the receiving TCP has buffered it, waiting for the missing hole in the data to arrive). This is the fast recovery algorithm. Figure 21.11 is similar to Figure 21.9, showing the values of cwnd and ssthresh. The segment numbers in the first column correspond to Figure 21.7. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (20 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission Segment# (Figure 21.7) 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 Action Send Variable Comment cwnd ssthresh ACK 6657 8705:8961(256) Receive ACK of new data 2426 512 ACK 6657 ACK 6657 ACK 6657 2426 2426 1792 512 512 1024 ACK 6657 ACK 6657 ACK 6657 duplicate ACK #1 duplicate ACK #2 duplicate ACK #3 retransmission duplicate ACK #4 duplicate ACK #5 duplicate ACK #6 2048 2304 2560 1024 1024 1024 ACK 6657 duplicate ACK #7 2816 1024 ACK 6657 duplicate ACK #8 3072 1024 ACK 8961 ACK of new data 1280 1024 6657:6913(256) 8961:9217(256) 9217:9473(256) 9473:9729(256) . Figure 21.11 Example of congestion avoidance (continued). The values for cwnd have been increasing continually, from the final value in Figure 21.9 for segment 12 (1089), to the first value in Figure 21.11 for segment 58 (2426). The value of ssthresh has remained the same (512), since there have been no retransmissions in this period. When the first two duplicate ACKs arrive (segments 60 and 61) they...
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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.

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