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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
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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