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Unformatted text preview: ablishment from this time line.) file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (12 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow Figure 20.8 Example of slow start.
We see the sender transmit one segment with 512 bytes of data and then wait for its ACK.
The ACK is received 716 ms later, which is an indicator of the round-trip time. The
congestion window is then increased to two segments, and two segments are sent. When the
ACK in segment 5 is received, the congestion window is increased to three segments.
Although three more could be sent, only two are sent before another ACK is received.
We'll return to slow start in Section 21.6 and see how it's normally implemented with
another technique called congestion avoidance. 20.7 Bulk Data Throughput
Let's look at the interaction of the window size, the windowed flow control, and slow start
on the throughput of a TCP connection carrying bulk data.
Figure 20.9 shows the steps over time of a connection between a sender on the left and a
receiver on the right. Sixteen units of time are shown. We show only discrete units of time in
this figure, for simplicity. We show segments carrying data going from the left to right in the
top half of each picture, numbered 1, 2, 3, and so on. The ACKs go in the other direction in
the bottom half of each picture. We draw the ACKs smaller, and show the segment number
being acknowledged. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (13 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow Figure 20.9 Times 0-15 for bulk data throughput example.
At time 0 the sender transmits one segment. Since the sender is in slow start (its congestion
window is one segment), it must wait for the acknowledgment of this segment before
At times 1, 2, and 3 the segment moves one unit of time to the right. At time 4 the receiver
reads the segment and generates the acknowledgment. At times 5, 6, and 7 the ACK moves
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