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Unformatted text preview: segments, but they're really maintained in bytes. Figure 21.8 Visualization of slow start and congestion avoidance.
In this figure we assume that congestion occurred when cwnd had a value of 32 segments.
ssthresh is then set to 16 segments and cwnd is set to 1 segment. One segment is then sent at
time 0 and assuming its ACK is returned at time 1, cwnd is incremented to 2 segments. Two
segments are then sent and assuming their ACKs return by time 2, cwnd is incremented to 4
segments (once for each ACK). This exponential increase continues until cwnd equals
ssthresh, after 8 ACKs are received between times 3 and 4. From this point on the increase in
cwnd is linear, with a maximum increase of one segment per round-trip time.
As we can see in this figure, the term "slow start" is not completely correct. It is a slower
transmission of packets than what caused the congestion, but the rate of increase in the file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (16 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission number of packets injected into the network increases during slow start. The rate of increase
doesn't slow down until ssthresh is reached, when congestion avoidance takes over. 21.7 Fast Retransmit and Fast Recovery Algorithms
Modifications to the congestion avoidance algorithm were proposed in 1990 [Jacobson
1990b]. We've already seen these modifications in action in our congestion example (Section
Before describing the change, realize that TCP is required to generate an immediate
acknowledgment (a duplicate ACK) when an out-of-order segment is received. This
duplicate ACK should not be delayed. The purpose of this duplicate ACK is to let the other
end know that a segment was received out of order, and to tell it what sequence number is
Since we don't know whether a duplicate ACK is caused by a lost segment or just a
reordering of segments, we wait for a small number of duplicate ACKs to be received. It is
assumed that if there is just a reordering of the segments, there wil...
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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