This preview shows page 1. Sign up to view the full content.
Unformatted text preview: data segment arrives (segment 5 in Figure 21.2), 1 clock tick
is counted (0.5 seconds) and our estimators are updated as Err = M - A = 0.5 - 2 = -1.5
A = A + gErr = 2 - 0.125x1.5 = 1.8125
D = D + h(|Err| - D) = 1 + 0.25x(1.5 - 1) = 1.125
RTO = A + 4D = 1.8125 + 4x1.125 = 6.3125
There are some subtleties in the fixed-point representations of Err, A, and D, and the fixedpoint calculations that are actually used (which we've shown in floating-point for simplicity).
These differences yield an RTO of 6 seconds (not 6.3125), which is what we plot in Figure
21.4 for time 1.871.
We described the slow start algorithm in Section 20.6. We can see it in action again in Figure
Only one segment is initially transmitted on the connection, and its acknowledgment must be
received before another segment is transmitted. When segment 2 is received, two more
segments are transmitted. 21.5 Congestion Example
Now let's look at the transmission of the data segments. Figure 21.6 is a plot of the starting
sequence number in a segment versus the time that the segment was sent. This provides a
nice way to visualize the data transmission. Normally the data points should move up and to
the right, with the slope of the points being the transfer rate. Retransmissions will appear as
motion down and to the right.
At the beginning of Section 21.4 we said the total time for the transfer was about 45 seconds,
but we show only 35 seconds in this figure. These 35 seconds account for sending the data
segments only. The first data segment was not transmitted until 6.3 seconds after the first
SYN was sent, because the first SYN appears to have been lost and was retransmitted (Figure
21.5). Also, after the final data segment and the FIN were sent (at time 34.1 in Figure 21.6) it
took another 4.0 seconds to receive the final 14 ACKs from the receiver, before the receiver's
FIN was received. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (11 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission Figure 21.6 Sending of 32768 bytes of data from sl...
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