This preview shows page 1. Sign up to view the full content.
Unformatted text preview: .homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (18 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission 1
ACK 257 slow start 512 512 ACK 513 slow start 768 512 ACK 769 cong. avoid 885 512 ACK 1025 cong. avoid 991 512 ACK 1281 cong. avoid 1089 512 257:513(256)
- Figure 21.9 Example of congestion avoidance.
When the timeout of the SYN occurs, ssthresh is set to its minimum value (512 bytes. which
is two segments for this example), cwnd is set to one segment (256 bytes, which it was
already at) to enter the slow start phase.
When the SYN and ACK are received, nothing happens to the two variables, since new data
is not being acknowledged.
When the ACK 257 arrives, we are still in slow start since cwnd is less than or equal to
ssthresh, so cwnd in incremented by 256. The same thing happens when the ACK 512
When the ACK 769 arrives we are no longer in slow start, but enter congestior avoidance.
The new value for cwnd is calculated as cwnd <- cwnd + segsize x segsize/cwnd + segsize/8
This is the 1/cwnd increase that we mentioned earlier, taking into account that cwnd is really
maintained in bytes and not segments. For this example we calculate cwnd <- 768 + 256 x 256 / 768 + 256 / 8
which equals 885 (using integer arithmetic). When the next ACK 1025 arrives we calculate cwnd <- 885 + 256x256/885 + 256/8
which equals 991. (In these expressions we include the incorrect 256/8 term to match the
values calculated by the implementation, as we noted in Section 21.6) file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (19 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission This additive increase in cwnd continues until the first retransmission, around the 10-second
mark in Figure 21.6. Figure 21.10 is a plot of the same data as in Figure 21.6, with the value
of cwnd added. Figure 21.10 Value of cwnd and send sequence number while data is being transmitted.
The first six values for cwnd in this figure are the values we calculated for Figure 21.9. It is
impossible in this figure to tell the difference v...
View Full Document
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