TCP IP Illustrated

The reason for not performing slow start in this case

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Unformatted text preview: l be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost. (We saw this in Section 21.5.) We then perform a retransmission of what appears to be the missing segment, without waiting for a retransmission timer to expire. This is the fast retransmit algorithm. Next, congestion avoidance, but not slow start is performed. This is the fast recovery algorithm. In Figure 21.7 we saw that slow start was not performed after the three duplicate ACKs were received. Instead the sender did the retransmission, followed by three more segments with new data (segments 67, 69, and 71), before the acknowledgment of the retransmission was received (segment 72). The reason for not performing slow start in this case is that the receipt of the duplicate ACKs tells us more than just a packet has been lost. Since the receiver can only generate the duplicate ACK when another segment is received, that segment has left the network and is in the receiver's buffer. That is, there is still data flowing between the two ends, and we don't want to reduce the flow abruptly by going into slow start. This algorithms are usually implemented together as follows. 1. When the third duplicate ACK is received, set ssthresh to one-half the current congestion window, cwnd. Retransmit the missing segment. Set cwnd to ssthresh plus 3 times the segment size. 2. Each time another duplicate ACK arrives, increment cwnd by the segment size and transmit a packet (if allowed by the new value of cwnd). 3. When the next ACK arrives that acknowledges new data, set cwnd to ssthresh (the file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (17 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission value set in step 1). This should be the ACK of the retransmission from step 1, one round-trip time after the retransmission. Additionally, this ACK should acknowledge all th...
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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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