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Unformatted text preview: e intermediate segments sent between the lost packet and the receipt of the first
duplicate ACK. This step is congestion avoidance, since we're slowing down to onehalf the rate we were at when the packet was lost.
We'll see what happens to the two variables cwnd and ssthresh in the calculations in the next
The fast retransmit algorithm first appeared in the 4.3BSD Tahoe release, but it was incorrectly followed by
slow start. The fast recovery algorithm appeared in the 4.3BSD Reno release. 21.8 Congestion Example (Continued)
Watching a connection using tcpdump and the socket debug option (which we described in
Section 21.4) we can see the values of cwnd and ssthresh as each segment is transmitted. If
the MSS is 256 bytes, the initial values of cwnd and ssthresh are 256 and 65535,
respectively. Each time an ACK is received we can see cwnd incremented by the MSS,
taking on the values 512, 768, 1024, 1280, and so on. Assuming congestion doesn't occur,
eventually the congestion window will exceed the receiver's advertised window, meaning the
advertised window will limit the data flow.
A more interesting example is to see what happens when congestion occurs. We'll use the
same example from Section 21.4. There were four occurrences of congestion while this
example was being run. There was a timeout on the transmission of the initial SYN to
establish the connection (Figure 21.5), followed by three lost packets during the data transfer
Figure 21.9 shows the values of the two variables cwnd and ssthresh when the initial SYN is
retransmitted, followed by the first seven data segments. (We showed the exchange of the
initial data segments and their ACKs in Figure 21.2.) We show the data bytes transmitted
using the tcpdump notation: 1:257(256) means bytes 1 through 256.
(Figure 21.2) Action
Send Receive Variable
ACK SYN, ACK
- cwnd ssthresh 256 65535 256 512 - - file:///D|/Documents%20and%20Settings/bigini/Docu.....
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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