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Unformatted text preview: are counted, and cwnd
is left alone. (This is the flat portion of Figure 21.10 preceding the retransmission.) When the
third one arrives, however, ssthresh is set to one-half cwnd (rounded down to the next
multiple of the segment size), cwnd is set to ssthresh plus the number of duplicate ACKs
times the segment size (i.e., 1024 plus 3 times 256). The retransmission is then sent.
Five more duplicate ACKs arrive (segments 64-66, 68, and 70) and cwnd is incremented by
the segment size each time. Finally a new ACK arrives (segment 72) and cwnd is set to
ssthresh (1024) and the normal congestion avoidance takes over. Since cwnd is less than or
equal to ssthresh (they are equal), the segment size is added to cwnd, giving a value of 1280.
When the next new ACK is received (which isn't shown in Figure 21.11), cwnd is greater
than ssthresh, so cwnd is set to 1363.
During the fast retransmit and fast recovery phase, we transmit new data after receiving the
duplicate ACKs in segments 66, 68, and 70, but not after receiving the duplicate ACKs in
segments 64 and 65. The reason is the value of cwnd, versus the number of unacknowledged
bytes of data. When segment 64 is received, cwnd equals 2048, but we have 2304
unacknowledged bytes (nine segments: 46, 48, 50, 52, 54, 55, 57, 59, and 63). We can't send
anything. When segment 65 arrives, cwnd equals 2304, so we still can't send anything. But file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (21 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission when segment 66 arrives, cwnd equals 2560, so we can send a new data segment. Similarly
when segment 68 arrives, cwnd equals 2816, which is greater than the 2560 bytes of
unacknowledged data, so we can send another new data segment. The same scenario happens
when segment 70 is received.
W^hen the next retransmission takes place at time 14.3 in Figure 21.10, it is also triggered by
the reception of three duplicate ACKs, so we see the same increase in cwnd as one more
duplicate ACK arrives, followed by a decrease to 1024.
The retransmission at time 21.1 in Figure 21.10...
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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